摘要:

438 Date 1964 1964 1965 1965 1966 1966 1967 1967 1968 1968 1969 1969 1970 1970 1971 1971 1972 1972 1973 1973 1974 1974 1975 1975 1976 1977 1977 1977 1978 1978 1979 1979 1980 GENERAL DISCUSSIONS OF THE FARADAY SOCIETY Subject Chemical Reactions in the Atmosphere Dislocations in Solids The Kinetics of Proton Transfer Processes Intermolecular Forces The Role of the Adsorbed State in Heterogeneous Catalysis Colloid Stability in Aqueous and Non-Aqueous Media The Structure and Properties of Liquids Molecular Dynamics of the Chemical Reactions of Gases Electrode Reactions of Organic Compounds Homogeneous Catalysis with Special Reference to Hydrogenation and Bonding in Metallo-Organic Compounds Motions in Molecular Crystals Polymer Solutions The Vitreous State Electrical Conduction in Organic Solids Surface Chemistry of Oxides Reactions of Small Molecules in Excited States The Photoelectron Spectroscopy of Molecules Molecular Beam Scattering Intermediates in Electrochemical Reactions Gels and Gelling Processes Photo-effects in Adsorbed Species Physical Adsorption in Condensed Phases Electron Spectroscopy of Solids and Surfaces Precipitation Potential Energy Surfaces Radiation Effects in Liquids and Solids Ion-Ion and Ion-Solvent Interactions Colloid Stability Structure and Motion in Molecular Liquids Kinetics of State Selected Species Organization of Macromolecules in the Condensed Phase Phase Transitions in Molecular Solids Oxidation Vo Iume 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 * 66 67 68 69 * Not available; for current injormation on prices, etc, of available volumes, please contact the Marketing Oficer, Royal Society of Chemistry, Burlington House, London WI V OBN stating whether or not you are a member of the Society.438 Date 1964 1964 1965 1965 1966 1966 1967 1967 1968 1968 1969 1969 1970 1970 1971 1971 1972 1972 1973 1973 1974 1974 1975 1975 1976 1977 1977 1977 1978 1978 1979 1979 1980 GENERAL DISCUSSIONS OF THE FARADAY SOCIETY Subject Chemical Reactions in the Atmosphere Dislocations in Solids The Kinetics of Proton Transfer Processes Intermolecular Forces The Role of the Adsorbed State in Heterogeneous Catalysis Colloid Stability in Aqueous and Non-Aqueous Media The Structure and Properties of Liquids Molecular Dynamics of the Chemical Reactions of Gases Electrode Reactions of Organic Compounds Homogeneous Catalysis with Special Reference to Hydrogenation and Bonding in Metallo-Organic Compounds Motions in Molecular Crystals Polymer Solutions The Vitreous State Electrical Conduction in Organic Solids Surface Chemistry of Oxides Reactions of Small Molecules in Excited States The Photoelectron Spectroscopy of Molecules Molecular Beam Scattering Intermediates in Electrochemical Reactions Gels and Gelling Processes Photo-effects in Adsorbed Species Physical Adsorption in Condensed Phases Electron Spectroscopy of Solids and Surfaces Precipitation Potential Energy Surfaces Radiation Effects in Liquids and Solids Ion-Ion and Ion-Solvent Interactions Colloid Stability Structure and Motion in Molecular Liquids Kinetics of State Selected Species Organization of Macromolecules in the Condensed Phase Phase Transitions in Molecular Solids Oxidation Vo Iume 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 * 66 67 68 69 * Not available; for current injormation on prices, etc, of available volumes, please contact the Marketing Oficer, Royal Society of Chemistry, Burlington House, London WI V OBN stating whether or not you are a member of the Society.

ISSN:0301-7249    

DOI:10.1039/DC98070FX001

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

Introductory Lecture : Pho toelectrochemistry BY ARTHUR J. NOZIK Photoconversion Research Branch, Solar Energy Research Institute, Golden, Colorado 80401, U.S.A. Received 8th September, 1980 The topic for this Faraday Discussion is called " Photoelectrochemistry ". This term undoubtedly has a different meaning for various types of scientists. However, for a majority of the scientists familiar with the term, it has come to be specifically associated, in recent years, with phenomena resulting from the illumination of semi- conductor materials in contact with liquid electrolytes. The scope of this Discussion, however, is much broader than just photoexcited semiconductor electrochemistry. The underlying theme (and impetus) for the research discussed here is that it is connected with approaches to solar-energy conversion that are (1) new and potentially better (operationally or economically) than other proposed or existing schemes and (2) more intimately related to chemistry and photochemistry than other solar-energy systems.The latter relationship is very appealing since the most venerable solar- energy conversion system is, of course, photosynthesis-the solar photochemical pro- cess that sustains life on earth. The basic features of both photosynthesis and the various photo-processes dis- cussed here are the following: (1) an interface is involved; i.e., the physical systems are heterogeneous; (2) charges, in the form of electrons and positive holes, are created by the absorp- tion of light in the system, and these negative and positive charges are separ- ated and/or stabilized by the interface; and (3) redox chemistry ensues after item (2).In light of these facts, a more comprehensive description of the Discussion might read: " Interfacial Photoinduced Charge Separation and Redox Chemistry ". If one studies the Discussion programme, five general approaches to interfacial photoinduced charge separation and redox chemistry become apparent. The ap- roaches are listed in table 1, together with a description of the absorbing element, the nature of the interface and the output from the system. In photoelectrosynthesis and in electrochemical photovoltaic cells, the light is absorbed by a semiconductor, the interface is between a semiconductor and a liquid, and the output is chemical and electrical energy, respectively.In photogalvanic cells, as described by Prof. Albery, the light is passed through a transparent electrode and absorbed by dye molecules in solution. A redox reaction then occurs between the excited dye and a second reactant such that oxidized and reduced species are produced in solution. The oxidized species diffuses to the illuminated (or dark) electrode and is reduced back to its original state. The reduced species diffuses to the dark (or illu- minated) electrode of the cell and is oxidized back to its original state. The netPHOTOELECTROCHEMISTRY TABLE 1 .-FIVE APPROACHES TO INTERFACIAL PHOTOINDUCED CHARGE SEPARATION AND REDOX CHEMISTRY system light absorption interface output photoelectrosynthesis semiconductor electrochemical semiconductor photovoltaic cells photogalvanic cells molecular pigment in solution microheterogeneous molecular pigments redox chemistry in solution synthetic chloroplasts molecular pigments on membranes semiconductor- chemical elect r ol y t e semiconductor- electrical electrolyte metal (or degenerate electrical semiconductor)- electrolyte micelles-liquid- chemical metal-liquid semiconductor-liquid membranes-liquid chemical effect is the circulation of charge from one electrode to the other to satisfy the oxida- tion-reduction reactions, and electrical power can be withdrawn from the cell.Micro-heterogeneous redox systems as described by Prof. Gratzel consist of colloi- dal dispersions of micellular or semiconductor particles that permit charge separation and stabilization through the interfacial potential created at the particle-liquid inter- face.The light is absorbed by dye molecules that are attached to or intimately asso- ciated with the particle surface. Catalytic agents, such as metallic Pt and Ru02, have also been introduced into the particle system to assist in the reduction and oxidation reactions, respectively. The final approach, presented by Prof. Calvin, uses synthetic chloroplasts that consist of molecular pigments associated with artificial membranes. Photoexcitation FIG. 1 .-Inter-relationships between five approaches to interfacial photo-induced charge separation and redox chemistry resulting from common research areas.A . J . NOZIK 9 of the pigments results in charge separation across the membrane, and oxidation and reduction reactions then occur on opposite sides of the membrane.The pigments can be structured to yield two photosystems, as exists in natural photosynthesis, to produce enhanced photopotentials for driving high-energy chemical reactions. A fascinating and significant aspect of the five approaches discussed above is that they have many common elements that closely link the areas to each other. This is represented in fig, 1, where the multiple connections between research topics and ap- proaches produce a web-like framework that overall constitutes the field of interfacial photoinduced charge separation and redox chemistry. A very interesting example of these inter-relationships is that specific microheterogeneous systems described by Prof.Gratzel bear extremely close relationships to systems derived from semiconductor electrochemistry. In fact, the latest configuration of these microheterogeneous systems is a dye-sensitized, Ru0,-catalysed photochemical diode. More about this interesting interconnection will be forthcoming during the discussion of Prof. Gratzel’s paper. In general, it is clear that much benefit can be derived from good communication and interaction between the scientists working in the areas outlined in fig. 1. CRITICAL TOPICS IN PHOTOELECTROCHEMISTRY The remainder of this introductory paper will focus on selected topics in photo- electrochemistry as defined above, i.e. photoexcited semiconductor electrochemistry. In table 2, the most important topics and problems in photoelectrochemistry are TABLE 2.-FARADAY DISCUSSION ON PHOTOELECTROCHEMISTRY CRITICAL TOPICS topic authors conversion efficiency - energetics , kinetics, surface states surface modification Miller, Dare-Edwards catalysis - new materials and structures novel chemistry Calvin new techniques and phenomena organic photoconductors and semiconductors Renschler, Ahuja photogalvanic cells Albery microheterogeneous redox chemistry Gratzel synthetic chloroplasts Singh, Calvin Bard, Gautron, Gerischer, Cardon Davidson, Tributsch, Ang, Ayers, Dare- Edwards, Chamberlain, Parkinson Deutscher, Sprunken, McAleer, Ellis, Ayers listed, together with the related papers in this Discussion. Also included in table 2 are the topics covered in this Discussion that are not included in the above definition of photoelectrochemistry.Because of time limitations, only the first six items in table 2 will be discussed further. CONVERSION EFFICIENCIES A critical factor for all solar-energy conversion systems is the attainable conversion efficiency. The higher the efficiency, the more economically feasible will be the con- version scheme. Indeed, one recent analysis suggests that in order for solar-energy systems to make a major impact on the energy budget, conversion efficiencies must be10 PHOTOELECTROCHEMISTRY >25 % for systems using solar concentrators, and 20-25 % for systems without concen- trators. The first problem that must be considered is to establish the thermodynamic limits on conversion efficiency. The systems under investigation here are all based on quan- tum conversion, and the thermodynamics of quantum conversion have been analysed by several authors.2-6 It is generally agreed that the thermodynamic limit on conver- sion efficiency for single threshold absorbers (i.e.all photons absorbed having energies above the threshold energy, no photons absorbed having energies below the threshold) is 31%.2-6 Concentration of the sunlight can increase this eficiency up to 40% with a concentration factor of lo4. If instead of a single threshold absorber, the system consists of two or more quan- tum absorbers in series with decreasing band gaps, the conversion efficiency can be considerably higher.2-8 In the limit of an infinite number of quantum absorbers the thermodynamic efficiency is 68.3 %.395*7 However, the efficiency increases very rapidly with the first few sequential absorbers.Hence, two absorbers yield 42%, three ab- sorbers yield 49% and four absorbers yield 53%.5*7 In order to obtain the maximum efficiency benefit from multiple, cascaded absor- bers, it is necessary to tap the excited charge from each absorber independently. For solid-state devices, this would result in a very complicated and impractical wiring scheme. For photoelectrochemical cells the problem is simplified in principle in that different redox couples can be used to tap the photopotential of each absorber at its optimum value. The scheme for an ideal 3-absorber system is shown in fig. 2. I e- FIG. 2.-Ideal photoelectrochemical system for generating the maximum conversion efficiency from a cascaded, 3-absorber cell.The redox couples are selected to tap the photopotential at each absorber at its maximum value. Another way to increase the upper limit on conversion efficiency is to operate photoelectrochemical cells with hot-carrier-injection proces~es.~*~~ These processes occur when electrons or holes are injected into the electrolyte before they become thermally equilibrated with the semiconductor l a t t i ~ e . ~ ~ l ~ If the hot carriers drive redox reactions that are not possible with a given semiconductor with thermalized charge carriers, then the conversion efficiency can be substantially increased. Initial estimatesll suggest that the efficiency limit for hot carriers can be about twice that from thermalized carriers. All the previous thermodynamic analyses cited above assume thermal equilibration of photoexcited carriers. A final important consideration for the conversion efficiency of quantum systems is the question of the thermodynamic cost of chemical storage.Since the output from photoelectrosynthetic cells is in the form of stored chemical energy, the question has been raised4 whether chemical storage requires an intrinsic thermodynamic loss.A . J . NOZIK 11 For quantum conversion in general, where thermal equilibrium obtains, it appears to be generally agreed6 that ca. 0.4 eV must be lost in converting visible photons into available work. For a single absorber, this loss represents the difference between the minimum absorbed photon energy (the band gap) and the resultant chemical potential (the chemical free energy per electron).This loss has also been identified6 with the entropy increase produced by the absorption of light (loss = T'ASmix, where T is temperature, and AS,,, is the entropy of mixing of excited-state and ground-state species). Further analyses6 seem to show that chemical storage does not require an additional thermodynamic loss ; however, this question has not yet been unequivo- cally settled. In the context of the above discussion, the effect of hot-carrier processes would be to significantly reduce the difference between the photon energy and the potential available for chemical work, and hence to increase the attainable conversion efficiency. ENERGETICS OF SEMICONDUCTOR-ELECTROLYTE INTERFACES Important new considerations have recently been introduced concerning the rela- tionships between the energy levels of the semiconductor and the redox levels of the electrolyte.In the previously accepted general framework for photoelectrochemical phenomena, it was assumed that the positions of the semiconductor band edges at the lbl I - + FIG. 3.-Energy-level diagrams for situation where semiconductor bands edges at the electrolyte interface (EsE and Evs) are pinned (a) and unpinned (b). In the pinned case, changes in potential appear only across the semiconductor space-charge layer and produce equivalent changes in the band bending ( VB). In the unpinned case, potential changes appear across the Helmholtz layer and shift the band edges with respect to the electrolyte redox potentials.Ur, is the flat-band potential, and UE is the electrode potential. electrolyte interface were fixed (i.e. pinned) with respect to the redox potentials of the electrolyte, and independent of electrode potential. This assumption is indeed valid if the charge density and corresponding capacitance of the Helmholtz layer in the elec- trolyte is significantly greater than that of the semiconductor space-charge layer, such that changes in potential applied to the semiconductor electrode occur only across the semiconductor space-charge layer. The voltage drop in the Helmholtz layer is thus constant and independent of potential. The potential changes in the semiconductor space-charge layer produce proportional changes in the band bending (see fig. 3). This model thus leads to the following important situation: (a) the band bending and photovoltage are equal to the difference between the electrode flat-band potential and12 PHOTOELECTROCHEMISTRY the electrolyte redox potential; and (b) photoeffects can only be expected with redox couples lying within the band gap.It is becoming increasingly apparent 12-17 that the situation described above is fre- quently not observed in semiconductor electrochemistry, and that it may prove to be the exception rather than the rule. Recent experiments show that open-circuit photopotentials can be independent of the electrolyte redox p~tential,'~*'~ and that photoeffects can be obtained with redox couples lying outside the band gap.12-15 These results can be explained by the condi- tion wherein the charge density and capitance of the semiconductor space-charge region is greater than that of the Helmholtz layer. Under these conditions, changes in potential applied to the system will appear across the Helmholtz layer rather than the space-charge layer, and this will shift or unpin the position of the semiconductor band edges with respect to the redox levels in solution (see fig.3). The semiconductor thus behaves similarly to a metal in that the energy bands at the surface move with applied potential. Several mechanisms have recently been proposed that could explain how higher charge densities could be produced in the semiconductor compared with the Helm- holtz layer. Bard and Wrighton and co-workers'2 propose that a high density of surface states could be present that pins the Fermi level of the semiconductor to the surface-state energy.Prof. Bard will expound on this idea, which is well-known in solid-state Schottky barriers, later in the Discussion. Another mechanism 1 4 p 1 5 invokes the well-known effect of inversion at semiconductor surfaces. Inversion occurs when the band-bending in the semiconductor becomes sufficiently large such that the Fermi level at the surface lies closer to the minority carrier band than to the majority carrier band.'* In inversion, the charge density and capacitance of the inver- sion layer near the surface rises drastically with increased reverse bias potential. This effect is pronounced for smaller-band-gap semiconductors, and the classical behaviour of capacitance-voltage-illumination-intensity data that is indicative of inversion effects has been observed with p-Si in contact with non-aqueous electrolyte^.^^*^^ Finally, Prof.Gerischer and coworkers 16917 propose that surface states exist that can be charged and discharged with applied potential. This mechanism is also discussed further in Prof. Gerischer's paper. SURFACE MODIFICATION A very important and promising area of photoelectrochemistry involves modifica- tion of the semiconductor surface either through chemical derivatization or ionic adsorption. In the former approach, introduced by Prof. Wrighton, molecular redox species are covalently bonded to the semiconductor surface through oxygen bridges to form a facile redox couple on the ~ u r f a c e . ' ~ ~ ~ ~ This surface-attached (organo- metallic) redox couple then acts preferentially to trap photogenerated minority car- riers from the semiconductor, and prevents photodecomposition of the semiconductor by destructive redox reactions involving the semiconductor itself.In a second step, the oxidized or reduced surface species then transfers the trapped charge to the electro- lyte to drive a desirable redox reaction in solution. The overall effect is to stabilize the semiconductor against photocorrosion and preferentially drive a beneficial photo- redox reaction. The ultimate objective of this approach would be to stabilize a low band gap semi- conductor against photodecomposition and permit the oxidation of water to oxygen (see fig. 4). The control of the kinetics of charge transfer across semiconductor- electrolyte interfaces through chemical derivatization of the semiconductor surfaceA .J . NOZIK 13 appears to be analogous to the preferential oxidation of water to oxygen in Photo- system I1 during photosynthesis. Here also, the photogenerated hole in Photosystem I1 should prefer (thermodynamically) to oxidize chlorophyll itself, but instead the hole is selectively and efficiently channelled to the water oxidation reaction uia a manganese complex (of unknown structure). In this analogy, the surface-bonded organometallic species plays the role of the Mn complex in controlling the selective oxidation pathways for the photogenerated holes. -j= bare n-AB AB/B H,O/Oz AB+hi-t A++ B ------4. I n-AB derrvatrzed w i t h R-D/D' AB/ B HZO/O, 2 h 4 + 2 D -2D* 2 D T + H ~ 0 - Z D + ~ O ~ + 2H* 2h'+ H,O - \ 0 , + 2 H * FIG.4.-Chemical derivatization of semiconductor surfaces to control kinetics of interfacial charge transfer. With bare surface (top), photogenerated hole oxidizes semiconductor (n-AB) in thermo dynamically favoured reaction. With surface derivatized with D (bottom), photogenerated hole is first preferentially trapped by surface-bonded D, and then D+ oxidizes HzO to O2 in a second step. The overall effect is electrode stabilization and photo-oxidation of H 2 0 to Oz. In addition to chemical derivatization of the surface, beneficial effects can also be obtained by surface modification with absorbed Heller, Miller, Parkinson and other co-workers at Bell Laboratories have shown that certain adsorbed ions, par- ticularly Ru3+, can dramatically reduce electron-hole recombination velocities at sur- face states and at grain boundaries.This results in greatly improved conversion effi- ciencies in electrochemical photovoltaic ~ e l l s . ~ l - ~ ~ This effect is also discussed here further in the paper by Miller, Heller, Menezes and Lewerenz. CATALYSIS It has been recently shown by WrightonZ4 that surface derivatization can be used to affect the catalytic properties of semiconductor surfaces dramatically. Experi- ments with p-type Si that has been chemically modified with a methyl viologen deriva- tive:14 PHOTOELECTROCHEMISTRY show that H2 can be photogenerated from aqueous solution at a much more positive potential compared to the bare p-Si surface.This catalytic effect is enhanced if Pt metal particles are also associated with the p-Si surface.24 The prospects for improving the catalytic properties of semiconductor surfaces is very encouraging. Again, the work by Gratzel using Pt and Ru02 as catalysts on Ti02 for H2 and O2 evolution, respectively, is a good example of what can be done. Future progress in photoelectrochemistry will undoubtedly involve significant advances in improving the catalytic behaviour of semiconductor surfaces. NEW MATERIALS Another very important and active area of research in photoelectrochemistry is the study of new semiconductor electrode materials and structures. This is especially critical for photoelectrosynthesis, where n-type semiconductor electrodes must be stable under very harsh oxidizing conditions.For electrochemical photovoltaic cells, electrode stability is also the major problem but there is more opportunity for amelior- ation since the electrolyte redox couple can be manipulated to optimize electrode stability. In photoelectrosynthesis, the redox reactions are dictated by the desired chemical output from the system, and hence cannot be adjusted for the sake of elec- trode stability. A critical problem here is to simultaneously minimize the band gap, maximize the stability, and optimize the flat-band potential. Experimental work to date has failed to uncover a single semiconductor material in which all three parameters are optimized. It is not yet established whether this empirical trend is an intrinsic characteristic of semiconductor photoelectrodes. Therefore, research is in progress in many labora- tories on new semiconductor compositions for photoelectrosynthetic applications.In addition to the search for new semiconductor materials, several other approaches are being pursued to solve the materials problem. These involve surface modification (previously discussed), composite structures and dye sensitization. With respect to new semiconductor compositions, much interest has been focused on layered compounds because the top of the valence band is comprised of non-bond- ing metal d,z orbitals rather than anion 4p orbital^.^^-^^ This means that optical transitions near the band-gap energy involve only metal to metal transitions and do not disrupt metal-anion bonds. to be less susceptible to photocorrosion than are the usual transitions involving anion-derived orbitals in the valence band.Dr. Tributsch has been a pioneer in this field, and his paper in this Discussion reviews the many significant and interesting results that have been obtained from layered transition-metal dichalcogenides. Although enhanced resistance to photocorrosion is indeed observed for faces per- pendicular to the c-axis of the crystals (planar face) photocorrosion readily occurs at planes parallel to the c-axis.16*29,30 This presents problems for the planar face since atomic-sized step dislocations in the surface become photocorrosion sites. The paper by Dr, Parkinson and coworkers deals further with the deleterious effects of edge sites on crystal surfaces. Another class of semiconductor compound that continues to receive attention is the oxides.The oxide semiconductors are generally the most stable materials in the presence of water oxidation to 02, but their band gaps are too large. Attempts are being made31-34 to reduce the band gap by creating a d-band above the oxygen 2p band. One major problem with this approach is the low hole mobilities expected from the system. Results on a new electrode material, LuRh03, having a metal- This type of optical transition isA . J . NOZIK 15 derived d-band on top of the oxygen p-band are being presented during the discussion by Dr. Jarrett. Work on dye sensitization is directed toward efficient absorption of sunlight in a dye that is intimately associated with the surface of a stable but wide-band-gap oxide semiconductor; this is followed first by charge transfer from the excited dye into the semiconductor bands, and then by redox reactions of both the charged dye and the injected charge in the semiconductor to yield the redox products and the original starting materials.The problems here are the stability of the dye, the sometimes low quantum efficiency of charge transfer from the excited dye, and the balance between sufficient dye thickness for efficient light absorption and charge transfer inhibition. The subject of dye sensitization is covered in the paper by Prof. Goodenough and co- workers. A final approach to improved electrode stability is to use composite structures. These can involve either electrode materials with a heterogeneous, two-phase composi- t i ~ n , ~ ~ or electrodes with multi-layered structures.The latter case, wherein a thin protective outer layer is deposited over a small-band-gap semiconductor, has not been very successful because layers thick enough to protect the semiconductor inhibit charge transfer, and layers thin enough to permit charge transfer do not provide pro- tection. A composite and multi-layered structure that is more promising is shown in fig. 5. Fer m level I inner n - type 1 outer n-type I 6 c o u nter- I semiconductor I semiconductor electrolyte electrode Vphoto = photovo I t a g e of h e t e r o j u n c tion V = photopotential of semiconductor - e l e c t r o l y t e junction Vnet = V,hoto+ v FIG. 5.-Energy-level diagram for heterojunction electrodes for photoelectrochemical cells.Photo- voltage at solid-solid heterojunction adds in series with photopotential at semiconductor-electroiyte interface. In this case, a heterojunction is formed between two semiconductor layers with dif- ferent band gaps. The larger gap material also forms a protective outer layer, but it is thick enough to absorb virtually all of the light with photon energy above the band gap. The inner and smaller gap semiconductor absorbs the light passed by the first layer. For optimum efficiency equal numbers of photons are separated in each layer. The photovoltage that develops at the heterojunction is in series with the photo-16 PHOTOELECTROCHEMISTRY potential generated at the semiconductor-liquid interface and serves to enhance the total photovoltage of the system; this effect follows from the benefits of multiple, cas- caded absorbers discussed earlier.The maximum quantum efficiency of the system is only 50%, but this is off-set by the enhanced photovoltage. This approach was first demonstrated with an n-CdS/n-GaAs electrode in an electrochemical photovoltaic cell,36 and with n-Si/n-SnO,, and n-GaAs/n-TiO, electrodes in photoelectrolysis cells.37 NOVEL CHEMISTRY The final critical topic in photoelectrochemistry research that will be discussed briefly is the prospect for driving other important chemical reactions besides water splitting. The production of hydrogen gas from water splitting is, of course, of im- mense importance and it remains the major point of focus for research in photo- electrosynthesis.However, many other vital chemical reactions can be driven in photoelectrosynthetic systems. These include N2 red~ction,~~-~O C 0 2 red~ction,4~'~~ decarb~xylation,~~-~~ free radical p~lymerization,~~ metal ion reduction^,^^ and oxida- tion of hydrocarbon^.^^ Some of these reactions are exoergic, so that the light pro- vides the activation energy for the reaction, and others are endoergic, so that light is stored as chemical free energy of the products. In both cases, great opportunities exist for exploring and developing new solar photochemical processes that could have a major impact on how important chemicals are produced in the future. CONCLUSION In this introductory paper I have first tried to show the close and interwoven rela- tionships between the five approaches to solar driven interfacial photoinduced charge separation and redox chemistry that are represented at this Faraday Discussion. Photoelectrochemistry, defined as the field associated with phenomena resulting from the illumination of semiconductor materials in contact with liquid electrolytes, en- compasses the approaches of photoelectrosynthesis and electrochemical photovoltaic cells.The other approaches are photogalvanic cells, microheterogeneous redox systems, and synthetic chloroplasts. Topics in photoelectrochemistry dominate this Discussion, as one would expect from the title, and I have introduced six subjects: conversion efficiency, energetics of semiconductor-electrolyte interfaces, surface modification, catalysis, new electrode materials and novel chemistry.Other important subjects are also addressed in the Discussion that cover new experimental techniques and phenomena in photoelectro- chemistry, organic photoconductors, and organic semiconductors. Together, all of these topics represent most of the latest and most interesting developments in the very active and rapidly growing field of photoelectrochemistry. This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences and the Secretariat of Conservation and Solar Energy, U.S. Dept. of Energy under Contract EG-66-C-01-4042. J. Loferski, NATO Advanced Summer Institute on Photovoltaic and Photo-electrochemical Energy Conversion, Gent, Belgium, 1980 (Plenum Press, New York, in press).R. T. Ross and T. Hsiao, J. Appl. Phys., 1977, 48, 4783. A. F. Haught, Physics Considerations of SoZar Energy Concersion, to be published. J. R. Boton, Science, 1978, 202, 705. P. T. Landsberg, NATO Advanced Summer Institute on Photovoltaic and Photoelectrochemical Energy Conversion, Gent, Belgium, 1980 (Plenum Press, New York, in press).A . J . NOZIK 17 J. R. Bolton, A. F. Haught and R. T. Ross, Proc. Third Int. Conf. Photochemical Conversion and Storage of Solar Energy, 1980, ed. J. C. Connolly, Boulder, Colorado (Academic Press, New York, 1981). R. V. Bilchak, J. S. Connolly and J. R. Bolton, Proc. ASIZSESMeeting, 1980, Phoenix, Arizona, to be published). M. Wolf, Proc. I.R.E., 1960, 48, 1246. D. S. Boudreaux, F. Williams, and A. J. Nozik, J. Appl. Phys., 1980, 51, 2158. (ACS, Washington, D.C., 1980), vol.184. lo A. J. Nozik, D. S. Boudreaux, R. R. Chance and F. Williams, Ado. Chem. Ser., ed. M. Wrighton l1 R. T. Ross, personal communication. l2 A. J. Bard, A. B. Bocarsly, F. F. Fan, E. G. Walton and M. S. Wrighton, J. Amer. Chem. Soc., l3 A. B. Bocarsly, D. C. Bookbinder, R. N. Dominey, N. S. Lewis and M. S. Wrighton, J. Amer. l4 J. A. Turner, J. Manassen and A. J. Nozik, Appl. Phys. Letters, 1980, 37, 489. l5 J. A. Turner, J. Manassen and A. J. Nozik, in Photoeflect at Semiconductor-Electrolyte Inter- l6 W. Kautek, H. Gerischer and H. Tributsch, Ber. Bunsenges. phys. Chem., 1979, 83, 1000. l7 W. Kautek and H. Gerischer, Ber. Bunsenges. phys. Chem., 1980, 84, 645. l8 S. M. Sze, Physics of Semiconductor Devices, (Wiley, New York, 1969), chap.9. l9 M. S. Wrighton, R. G, Austin, A. B. Bocarsly, J. Bolts, 0. Haas, K. D. Legg, L. Nadjo and 2o J. Bolts and M. S. Wrighton, J. Amer. Chem. Soc., 1978, 100, 5257. 21 A. B. Heller, B. Parkinson and B. Miller, Appl. Phys. Letters, 1978, 33, 521. 22 W, D. Johnston, H. J. Leamy, B. A. Parkinson and B. Miller, J. Electrochem. Soc., 1980, 127, 23 A. Heller, in Photoefects at Semiconductor-Electrolyte Interfaces, ACS Symp. Ser., ed. A. J. 24 M. Wrighton, Fourth DOE Solar Photochemistry Conference, 1980, Radiation Laboratory, 25 H. Tributsch, Solar Energy Materials, 1979, 1, 257. 26 J. Gobrecht, H. Gerischer and H. Tributsch, J. Electrochem. Soc., 1978, 125, 2085. 27 H. Tributsch, Ber. Bunsenges. phys. Chem., 1978, 82, 169. 28 H. Tributsch, J.Electrochem. SOC., 1978, 125, 1086. 29 H. J. Lewerenz, A. Heller and F. J. DiSalvo, J. Amer. Chem. Soc., 1980, 102, 1877. 30 H. Gerischer, in Photoefects at Semiconductor-Electrolyte Interfaces, ACS Symp. Ser., ed. 31 R. D. Rauh, J. M. Buzby and S. A. Alkartis, J. Phys. Chem., 1979, 83, 2221. 32 V. Guruswamy and J. Bockris, Solar Energy Materials, 1979, 1, 441. 33 J. B. Goodenough, personal communication. 34 H. S. Jarrett, personal communication. 35 J. G. Mavroides, J. C. Fan and H. J. Zeiger, in Photoefects at Semiconductor-Electrolyte Inter- 36 S. Wagner and J. L. Shay, in Semiconductor Liquid-Junction Solar Cells, ed. A. Heller, (Electro- 37 A. J. Nozik, Second International Conference on the Photochemical Conversion and Storage 1980, 102, 3671. Chem. Soc., 1980,102, 3683. faces, ACS Symp. Ser., ed. A. J. Nozik (ACS, Washington, D.C., 198l), vol. 146. M. Palozzotto, J. Amer. Chem. Soc., 1978, 100, 1602. 90. Nozik (ACS, Washington, D.C., 1981), vol. 146. Univ. of Notre Dame, Indiana. A. J. Nozik (ACS, Washington, D.C., 1981), vol. 146. faces, ACS Symp. Ser., ed. A. J. Nozik, (ACS, Washington, D.C. in press). chem. Soc., Princeton, N.J., 1977). of Solar Energy, 1978, Cambridge, England. C. R. Dickson and A. J. Nozik, J. Amer. Chem. Soc., 1979, 100, 8007. 39 M. Keizurni, H. Yoneyama and H. Tamura, J. Amer. Chem. SOC., 1980, in press. 40 G. N. Schrauzer, and T. D. Guth, J. Amer. Chem. Soc., 1977,99, 7189. 41 M. Halmann, Nature, 1978, 275, 155. 42 T. Inove, A. Fujishirna, S. Konishi and K. Honda, Nature, 1979, 277, 637. 43 B. Aurian-Blajeni, M. Halmann and J. Manassen, Solar Energy, 1981, in press. 44 B. Krauetler and A. J. Bard, J. Amer. Chem. Soc., 1978, 100, 5985; 1978, 100, 2239. 45 B. Krauetler, C. D. Jaeger and A. J. Bard, J. Amer. Chem. SOC., 1978, 100,4903. 46 B. Kraeutler, H. Reiche, 0. J. Bard and R. G. Hocker, J. Polymer Sci., 1979, 17, 535. 47 H. Reiche, W. W. Dunn, A. J. Bard, J. Amer. Chem. Soc., 1979, 83, 2248. 48 A. J. Bard, J. Photochem., 1969, 10, 59.

ISSN:0301-7249    

DOI:10.1039/DC9807000007

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

On the Role of Surface States in Semiconductor Electrode P h o toelec tr ochemical Cells BY ALLEN J. BARD, Fu-REN F. FAN, ALBERTO S. GIODA, G. NAGASUBRAMANIAN AND HENRY S. WHITE Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712, U.S.A. Received 6th May, 1980 Surface states that occur at the semiconductor-liquid interface play an important role in the behaviour of that interface and affect the efficiency of photoelectrochemical solar devices. The nature of such states and evidence for their existence will be briefly reviewed. Their role in dark electron transfer reactions for redox couples with energies within the band-gap region and in mediating surface recombinations will be discussed. The importance of Fermi-level pinning by surface states at moderate densities in GaAs and Si in controlling the open-circuit photovoltage and the observed electrochemical behaviour will be described.The effect of the surface pretreatment on the photo- electrochemical behaviour of p-GaAs and n-WSe2 will be demonstrated. In many ways the development of models for the semiconductor-liquid interface has paralleled that for the metal-semiconductor (M/SC) junction. The earliest models of M/SC junctions (Schottky barriers) by Schottky and Mott proposed a barrier equal to the difference in work functions between the metal and the semi- conductor forming the contact. Subsequent experimental studies, however, showed considerable deviations from the predicted behaviour and the suggestion was put forth3 that surface states at the interface play an important role in determining the barrier height.Many theoretical and experimental studies have subsequently demonstrated the existence of such states and their effects on the junction character- istics. [See, for example ref. (4), (5) and references therein.] In a similar way the barrier at the semiconductor-solution interface has often been taken to be the differ- ence between the flat-band potential of the semiconductor, Vfb, and the redox poten- tial of a solution couple, Vredox, although as early as 1959 the importance of surface states in determining the potential distribution at this interface was indicatedm6 It now appears clear that the behaviour of semiconductor electrodes is critically depen- dent upon the nature of the electrode surface and that the ideal model involving a state- free band gap is rarely applicable.For the purposes of this paper surface states will be taken to mean surface elec- tronic energy levels with energies different from the allowed levels in the bulk semi- conductor. They may be states which arise because the lattice is terminated (" dangl- ing bonds " or " intrinsic surface states "), because of lattice defects, vacancies or differences between the surface composition and that of the bulk, or because of adsorp- tion of electron acceptor or donor species (" impurity states ") which may act as surface states themselves or induce defect states in the semiconductor material. The number, distribution and energies of the surface states may depend not only upon the type of material employed and the crystal face exposed, but also upon the composition of the solution phase and the nature of the surface pretreatment before the junction is formed. Surface states can play an important role in the behaviour of semiconductor elec-20 ROLE OF SURFACE STATES trodes.Dark redox processes of couples with energies located in the gap region may be promoted by such states. Under these circumstances the states may also behave as recombination centres and lead to decreased quantum efficiencies in photelectro- chemical (PEC) cells. A high density of these levels may lead to " Fermi-level pin- ning '' in which the observed photopotential becomes independent of the redox poten- tial of the solution couple and photoprocesses are observed for couples whose energy levels apparently lie well outside the band-gap region (as determined by measurements made in the absence of the redox c o ~ p l e ) .~ - ~ Surface states may also play a role in the catalysis of electron-transfer reactions at the semiconductor electrode surface, in the quenching of spectral sensitization processes by dye layers, and in the photodecomposi- tion reactions of the semiconductors. In this paper we discuss and give several examples of surface-state effects in PEC cells and demonstrate that surface treatments of the semiconductor can be significant factors in the behaviour of these electrodes. EXPERIMENTAL The sources of the semiconductor materials (all single crystals), the method of mounting and producing ohmic contacts, and the apparatus used in the measurements have been pre- viously de~cribed.~-l' The electrodes were illuminated with either a 450 W Xe lamp with suitable filters or a 1.6 mW He-Ne laser.RESULTS AND DISCUSSION FERMI-LEVEL PINNING BY SURFACE STATES PRINCIPLE The qualitative and quantitative aspects of Fermi-level pinning at the semicon- ductor-solution interface have been discussed in some so that only the basic principles will be reviewed here. The model for the surface-state-free-semiconductor function can be represented as shown in scheme l.12913 The flat-band potential, Vfb, measured in a solution in the absence of a redox couple (Vzb), corresponds fairly closely to the energy of the conduction band edge (E,) in an n-type semiconductor or to the valence band edge (&) in a p-type material.At Vfb in the absence of specific adsorption, the potential drop across the Helmholtz layer, ApH, corresponds to that attributable to oriented dipoles at the interface, V,, while the potential drop across the space-charge region of the semiconductor, Aqsc, is zero. (In all cases we assume a n-type I SCHEME 1BARD, FAN, GIODA, NAGASUBRAMANIAN, WHITE 21 reasonably concentrated electrolyte so that the potential drop across the diffuse double layer can be neglected.) When the potential difference between the bulk semiconduc- tor and the solution is varied either by application of an external potential or by intro- duction of an appropriate redox couple (located at Eredox = -eVred0,) in solution, AqH remains largely unchanged while Apse essentially equals that of the applied poten- tial (assuming no specific adsorption of the redox couple and no change in Vd). This is the basis for the usual model in which the band edges are said to remain fixed [i.e., A(AqH) x 01 and in which the open-circuit photopotential (AV) under high illumina- tion intensities is For couples with energies outside the band gap, no photoresponse is expected.Eqn (1) is the basis for the selection of redox couples with potentials corresponding to energies near the valence band edge (for n-type) or near the conduction band edge (for p-type) to maximize the output photovoltage of a PEC cell. The flat-band potential will be shifted by specific adsorption of ions, since the potential drop across the Helmholtz layer in the presence of surface charge, qs, is given by AV = I y& - yredox 1- (1) where C, is the Helmholtz layer capacitance, E is the dielectric constant, co the per- mittivity of free space, and d the thickness of the Helmholtz layer.Thus adsorption of anions (qs < 0) will cause a negative shift in Vfb, while cation adsorption (qs > 0) causes a positive shift. If surface states are present, they may be filled or emptied either by interaction with solution redox couples, or by photoprocesses or by charge redistribution within the semiconductor. The overall effect of such filling or emptying of the surface states is to produce surface charge which is a function of the number of surface states and their occupancy. As suggested by eqn (2) the flat-band potential will shift, producing a shift in the relative location of the conduction and valence band edges with respect to a solution redox-couple energy level. If the surface state density is sufficiently large, a potential change between semiconductor and solution will result in an almost equal change in AqH (with Aqsc remaining almost constant).Under these conditions the Fermi level is pinned to the energy level of the surface states (Ess) and the photopoten- tial is independent of vredox (scheme 2): n-type P-tY Pe ‘redox SCHEME A V w e(E, - Ess) A V z e(E,, - EJ (n-tYP4 (P-tYPe) *22 ROLE OF SURFACE STATES When Fermi-level pinning exists, photoeffects may be observed for couples whose redox potentials are such that they are apparently located outside of the gap region (based on the VOfb value).We have previously suggested7 that the high density of surface states can be viewed as a metal overlayer on the semiconductor. This over- layer forms a Schottky barrier with the semiconductor with a height which is indepen- dent of the solution redox couple. The overlayer will come into electronic equili- brium with the solution redox couple. The main point of the argument is that surface charge can cause shifts in the flat- band potential and under some conditions lead to pinning. While we have described these effects in terms of surface states (rather broadly defined), electron injection into the conduction band or hole injection into the valence band (inversion layer forma- tion,14 or the modification of the electrode surface by the attachment of donor or acceptor rnolecule~~~ can also produce such effects. We and Wrighton and co-workers 7-9 previously gave examples of the Fermi-level I 1 f ( Me C N *) Fc( MeCN) I I I I Lc- F- - ’ i I ’ > s 0.9- 0.4.i f ’ 9 M”2’’: /*$I; 0 0 0 PA” ’ b I I I I I 0 -0.4 -0.8 -1.2 -1.6 -2.0 Vredox/V US. SCE FIG. 1.-Photovoltage as a function of the standard potentials of redox couples. Photovoltage is taken to be the difference in the standard potential of the redox couple and the peak of the photo- cathodic wave in a cyclic voltammetry scan of the illuminated semiconductor under conditions where photocurrent is limited by diffusion of the redox reagent in the quiet solution (0 for n-GaAs/MeCN, 0.1 mol dm-3 TBAP); A V in p-GaAs/H,O (symbolized by A) is the difference between the standard potential of the redox couple and the onset photopotential of the photocathodic wave.Abbrevia- tions : TMPD = NNN’N’-tetramethyl-p-phenylene-diamine; Ox-1 = oxazine-1 ; BQ = benzo- quinone; HV = NNr-dihepty1-4,4’-bipyridium; MV = NN’-dimethyl-4,4’-bipyridium; AQ = anthraquinone; bpy = 2,2’-bipyridine; TPTZ = 2,4,6-tripyridyl-s-triazine; DPA = diphenylanthra- cene: EDTA = ethylenediamine-tetra-acetic acid. pinning effects at n- and p-GaAs and p-Si in acetonitrile (MeCN) and aqueous solutions. Typical results for n-GaAs in MeCNI6 and p-GaAs in water9 are shown in plots of A V vs. Vredox in fig. 1. The deviation of the results from eqn (1) and the near independence of VA for couples spanning a potential range wider than Eg for GaAs is consistent with the pinning model.Liquid ammonia cells.-A particularly striking result of Fermi-level pinning is observed in electrochemical studies of p-GaAs and p-Si in liquid a m m ~ n i a . ~ ~ * ’ ~BARD, FAN, GIODA, NAGASUBRAMANIAN, WHITE 23 Typical results are shown in fig. 2. The cyclic voltammetric scan in NH3/0. l mol dm-3 KI at a Pt electrode [curve (a)] shows that the cathodic current attributable to solvated electron injection starts at -2.4 V [us. Ag/Ag+(NH,)], with the collection of electrons occurring on scan reversa1.l' At p-GaAs in the same solution negligible currents are observed in the dark in this potential range. However, under steady or chopped illumination with light of energy greater than Eg, a cathodic photocurrent commencing + C 0 C i C u l i g h t I 2 0 0 p A cm d a r k Ib I I / I I I I I I potential/V us.Ag/AgNO,(O. 1 mol dm-j) FIG. 2 . 4 ~ 7 ) Cyclic voltammetric background current for a Pt disc electrode in liquid ammonia. Scan rate 200 mV s-l. (6) A similar scan for p-GaAs and p-Si in the dark. (c) p-GaAs under con- stant (sun-lamp) illumination. Scan rate 100 mV s-'. (d) p-Si under constant (sunlamp) illumina- tion. Scan rate 100 mV s-l. at ca - 1.5 V is observed [curve (c)]. The blue coloration near the electrode surface during the cathodic pulses clearly indicates that the process is the photoinjection of electrons which occurs at potentials considerably less negative than the reversible value for this process. The VV",,-value for n-GaAs determined by capacitance measurements in this solvent (Schottky-Mott plot) is ca - 1.2 V.Very similar effects are found for p-Si in liquid ammonia [curve (d)] where V& for n-Si in the absence of excess electrons is ca -0.8 V.I8 For both semiconductors, PEC cells can be constructed. The cell p-GaAs/e;, KI, NH,/Pt shows an open-circuit photovoltage (V,,) of 0.7 V, while the24 ROLE OF SURFACE STATES cell p-Si/e;, KI, NH3/Pt yields Yo, = 0.57 V. Details of the behaviour of these and several other semiconductors with several redox couples in liquid NH3 will be pub- lished elsewhere.I8 p-WSe2-N I TR 0 BENZENE SYSTEMS With p-WSe, in MeCN photocurrents are also observed for couples located at redox potentials more negative than the conduction band edge [as determined from V;l, of p- and n-WSe, in 0.1 mol dm-3 tetra-n-butyl ammonium perchlorate (TBAP)/MeCN].The band structure and relevant energy levels are shown in fig. 3. Consider the volt- ammetric behaviour of nitrobenzene (PhN02) shown in fig. 4. The photoelectro- FIG. 3,--Schematic representation of the energetic situation at p-WSe2/electrolyte interface. chemical behaviour of PhN0, is of interest because highly concentrated solutions of it and its radical anion can be prepared in MeCN, thus minimizing the mass transfer limitations often observed in non-aqueous PEC cells.20 The reduction of PhNO, at Pt occurs at - 1.33 V us. SCE [curve (a)]. Under illumination with the chopped beam of a He-Ne laser (1.6 mW) no photocurrent is observed at p-WSe, even when the potential is swept to - 1 .O V [curve (b)] for a solution of 0.2 mol dmP3 PhNO, + 0.1 mol dm-3 TBAP/MeCN.However, if a small amount (ca. 0.095 mmol d1l1-~) of PhN02: is electrogenerated in the solution at a Pt electrode, a photocurrent is observed at p-WSe, beginning at ca. -0.4 V [curve (c)]. Similar effects are observed with anth- racene and anthraquinone at p-WSe, in MeCN. No cathodic photocurrents areBARD, FAN, GIODA, NAGASUBRAMANIAN, WHITE 25 observed unless radical anion is first generated in the solution. We propose that the electrogenerated radical anions inject charge into the p-WSe, surface. This negative charge causes a shift in V,, to more negative values until pinning at the potential of the PhNO,/PhNO,: is obtained. At this point the conduction band is located at an energy where photogenerated electrons can be injected to produce more PhNO,:.! i v - _./--. 4 j20$ ___ .'.-L-rL <A- 1 - 0.6 - 0.8 -1 .o /bJ 0 -0.2 potential/V us. SCE FIG. 4.-(u) Cyclic voltammogram on Pt of the reduction of nitrobenzene. (b) Dark voltammetric curve on p-WSe, in acetonitrile solution containing 195 mmol dm-3 nitrobenzene. 0.1 mol dm-3 TBAP was used as the supporting electrolyte. (c) Current-potential charac- teristic under chopped light on p-WSe2 in acetonitrile solution contaning 0.2 mol dm-j nitrobenzene and 0.095 mmol dm-3 nitrobenzene radical anion. Scan rate 20 mV s-l. EFFECTS OF SURFACE PRETREATMENTS There have been many studies demonstrating the importance of semiconductor surface treatment or modification in the electrochemical behavio~r.'~*~'-~' For ex- ample the surface treatment of n-GaAs with RuCl, produced higher efficiencies in PEC cells based on the Se:-/Se2- redox system.21*22 Surface modification by the attachment of electroactive sites can suppress the photoanodic decomposition of the semiconductor 1atti~e.l~ Surface treatments have long been used in the fabrication of solid-state devices.We discuss here two examples of the application of surface treatments to suppress recombination and improve photoresponse. EFFECT OF Cl- ON n-WSe,/MeCN SYSTEM Layered-type transition metal chalcogenides such as MoSe, and WSe2 have been investigated rat her extensively . 26-34 The behaviour of such materials is critically de- pendent upon the character of the surface. Thus single crystals which show smooth and defect-free van der Waals surfaces (l.c axis) (referred to here as " Type S ") pro- duce low dark currents and high efficiencies in PEC cells. However, the presence of defects, discontinuities or exposed edges on the van der Waals surface produce elec- trodes (referred to here as " Type E ") which show appreciable dark currents and26 ROLE OF SURFACE STATES poorer photoresponse presumably because these edges provide sites for dark oxida- tion (at the n-type material) and r e ~ o r n b i n a t i o n . ~ ~ . ~ ~ We report here experiments on n-WSe, in MeCN and the effect of surface treatment on the behaviour of Type E electrodes. Typical behaviour of a Type E n-WSe, electrode in MeCN/0.2 mol drn-3 TBAP containing thianthrene (TH) is shown in fig. 5. A quasireversible oxidation 1.6 1.2 0.8 0.4 0.0 -0.4 potentiallv us.SCE FIG. 5.-Effect of bulk halide on dark currents at Type E n-WSe2 electrode. (a) 5 mmol dm-3 ihianthrene (TH); (b) 5 mmol dm-3 TH and 15 mmol dm--3 TBAI; (c) 5 mmol dm-3 TH and 10. mmol dm-3 TBABr; ( d ) 5 mmol dm-3 TH and 10 mmol dm-3 TEACl. Scan rate 100 mV s-' 0.2 mol dm-3 TBAP as the supporting electrolyte. wave is shown in the dark [curve (a)]; the charge transfer leading to the oxidation has been attributed to conduction along the edges of the van der Waals planes to surface di~continuities.~~ Addition of 5-10 mmol dm-3 of I-, Br- or C1- to the solution causes significant decreases in the dark current [curves (b), (c) and (41. Addition of Cl- also affects the photo-oxidation of TH at n-WSe, (fig. 6); the anodic photo- current for TH oxidation starts at potentials ca.0.23 V less positive in solutions containing C1-. Neither Br- nor I - show this effect on the TH photocurrent in MeCN. To demonstrate that this effect results from the interaction of C1- with the surface discontinuities, the experiment illustrated by fig. 7 was undertaken.BARD, FAN, GIODA, NAGASUBRAMANIAN, WHITE 27 A fresh Type E electrode was prepared and the dark- and photo-oxidation of TH was observed [curves (a) and (b)]. The electrode was then removed and dipped into a MeCN solution of 7.0 mmol dm-3 TBACl in the dark without any external electrical connection. After 30 s, the electrode was removed, rinsed thoroughly with MeCN and placed back into the original TH-containing solution. The resulting cyclic voltammograms [curves (c) and ( 4 1 showed an immediate decrease in the dark current and a negative shift (ca.180 mV) of the onset potential for photocurrent. The maxi- mum photocurrent for TH oxidation increased by ca. 25% following this surface mA mi2 potential/V us. SCE FIG. 6.-Effect of chloride on photo-oxidation of thianthrene; 5 mmol dm-3 TH, 10 mmol dm-3 TBACl and 0.2 mol dm-3 TBAP in acetonitrile solution. Solid lines indicate dark current; broken lines indicate photocurrent; (a) TH at platinum, (b) TH only, (c) 10 mmol dm-3 TEACl, ( d ) 5 mmol dm-3 TH and 10 mmol dm-3 TEACl. treatment. This improved photocurrent-potential curve remained unchanged for at least 30 min of continuous cycling. Wher, a similar experiment was carried out with a Type S electrode, no changes in the dark oxidation current (which was negligible) or the photocurrent was found by a C1- pretreatment.Note that the decrease in dark current for the Type E electrode upon treatment with Cl- takes place without any possibility of photo-oxidation occurring during the exposure of the electrode to C1- so that the formation of a light-induced complex between the electrode and C1- is28 ROLE OF SURFACE STATES unlikely. The observed effect can be ascribed to interactions of the C1- with surface discontinuities leading to modification or passivation of these sites. Similar shifts in photopotential of redox couples in aqueous media have been found by Tributsch and c o - w ~ r k e r s ~ ~ ~ ~ ~ by the addition of the 1-/12 couple. 1.2 0.8 0.4 0.0 -0.4 potential/V 03.SCE FIG. 7.-Effect of dipping Type E n-WSe2 electrode into 7 mmol dm-3 TEACl solution. (a) Dark oxidation of 5 mmol dm-3 TH on untreated electrode. (b) Photo-oxidation of 5 mmol dm-3 TH on untreated electrode. (c) Dark oxidation of TH after C1- treatment. (d) Photocurrent after C1- treatment. HNO, ETCHING OF p-GaAs Surface pretreatment also affected the dark and photoresponses of a p-GaAs electrode. The behaviour of an HCI-etched p-GaAs single-crystal electrode in aque- ous solution of NN’-dimethyl-4,4’-bipyridinium (or methyl viologen, MV2 +) has been reported.’O When a p-GaAs crystal (100 face) was polished first with Sic paper and then with 0.5 pm alumina powder on felt until a mirror-like surface is obtained and then used as an electrode in the MV2+ medium, the dark i-Vcurves were practically the same as those observed at a metal electrode and no photoresponse was obtained (fig.8) [curve (a)]. A scanning electron micrograph of this surface is shown in fig. 9 (a). X-ray fluorescence measurements of this electrode surface show K-series peaks forFIG. 9.-Scanning electron micrographs for the electrode surface with different treatments : (a) poli- shed with alumina; (6) etched in 7.9 mol dmP3 HNO, for 15 s ; (c) etched in 7.9 mol dm-3 for 30 s ; (d) 1 min etching; ( e ) 2 min etching; (f) 5 min etching; ( g ) 10 min etching. [To face page 29BARD, FAN, GIODA, NAGASUBRAMANIAN, WHITE 29 r 0 c u T 1 - 0 u 0 c 0 5 i - 0 I. U 0 c CI T 1 0.1 . . : I 1 1 I I I 0 -400 -800 E/mV us. SCE FIG.S.-Cyclic voltammograms of methyl viologen (MV2+) in 0.5 mol dm-3 Na2S0, aqueous solu- tion on p-GaAs electrode. Scan rate 50 mV s-l. (a) On alumina-polished electrode: (-) in the dark; (- - -) under illumination. (6) On electrodes etched with 7.9 mol dm-3 HN03 acid: (-) in the dark; under illumination: (- - -) 15 s etching, (. * a ) 30 s etching, (-.-.-) 1 rnin etching, (---) 2 rnin etching, (A++) 5 rnin etching, and (- - -) 10 min etching. Ga (9.25 keV) and As (10.54 keV) with the Ga peaks larger than the ones for As. The effect of etching the electrode with 7.9 mol dm-3 HNO, for various lengths of time is illustrated in fig. 8 and 9. Even a very brief immersion (ca. 15 s) causes a signi- ficant decrease in the dark current and improved photoresponse [curve (b)].Con- tinued etching for times up to 10 rnin showed constant improvement in the photo- response and changes in the electrode surface. X-ray fluorescence measurements show a continual increase in the ratio of As/Ga peak heights with the electrode which has been etched for 10 rnin yielding almost no Ga peak. The high As levels appear to be associated with the crystallites which are formed on the surface during etching, since X-ray fluorescence measurements on the exposed flat surfaces of the electrodes in fig. 9 (b), (c) and (d) continue to show a As/Ga peak height ratio close to that for the polished electrode. Etching with 0.9 mol dm-3 HN03 for periods of up to 80 rnin was ineffective in improving the dark current or photoresponse of the p-GaAs elec- trode and an electrode so treated showed behaviour similar to that in fig.8(a).30 ROLE OF SURFACE STATES These results can be interpreted in terms of the existence of a very disturbed surface with a high population of states formed by the polishing. Grinding or polishing of semiconductors is known to induce states in the gap.35 The material then shows a metallic behaviour. Etching of the surface removes the damaged layer and may also cause passivation of surface states. Recent experiments have suggested that deposi- tion of small amounts of metal on the electrode surface can produce similar effects. CONCLUSIONS The results here, as well as numerous past studies of junctions to semiconductors, demonstrate the importance of surface states in the interpretation of photoelectro- chemical behaviour.For semiconductors where pinning is observed these effects allow photoprocesses to occur which would not be predicted from V;,, measurements. This has been demonstrated here for p-type materials where the photoreduction of couples with very negative redox potentials is found. In principle a similar effect should be possible at n-type semiconductors and couples with very positive redox levels. However, in this case hole injection into the valence band with decomposition of the semiconductor may occur. Surface states may also play an important role in the catalysis of surface chemical reactions in a manner similar to that seen in electrochemical reactions at metal elec- trodes. For example the photoproduction of hydrogen probably involves as a first step the formation of a hydrogen atom (Ha).Since the potential for this reaction in bulk aqueous solution is very negative (ca. -2.1 V us. NHE), production of H2 will probably require the presence of surface sites which will adsorb H* quite strongly and also promote their combination. Similarly adsorption of hydroxyl radicals is re- quired for O2 production. An understanding of the chemical and physical nature of the surface states on a molecular level is clearly needed. Such an understanding is only now beginning to emerge in studies of semiconductor surfaces in a high vacuum environment. For example the pinning of GaSb, GaAs and InP surface at the same level by submono- layer coverage of metals and oxygen has recently been reported and ascribed to the induced formation of defect levels by the adatorn~.~ Induced defect levels and im- purity levels may similarly be formed at the semiconductor-liquid interfaces (i.e., the surfaces may change in the act of forming the junction), but in situ molecular or micro- scopic characterization of these will be difficult.A general observation from all of these results is that the character of the semiconductor-liquid junction is very specific for the particular conditions existing at that junction and the method of pretreatment of the surface. It will be very difficult therefore to provide a general theoretical model which will allow prediction of the interface properties simply from the characteristics of semiconductor and solution phases. Similar opinions have been expressed con- cerning the metal-semiconductor Schottky barrier.36 The support of this research by the National Science Foundation, the Solar Energy Research Institute and the Office of Naval Research is gratefully acknowledged. We are indebted to Dr.Richard Malpas for the liquid ammonia measurements. We also acknowledge a fellowship to A. S. G. by the Consejo Nacional de Investigaciones Cienti- ficas y Tkcnicas de la Republica Argentina. W. Schottky, Naturwiss., 1938, 26, 843. N. F. Mott, Proc. Cambridge Phil. Soc., 1938, 34, 568. J. Bardeen, Phys. Rev., 1947,71, 717.B A R D , F A N , GIODA, NAGASUBRAMANIAN, WHITE 31 W. E. Spicer, I. Lindau, P. Skeath, C. Y . Su and P. Chye, Phys. Rev. Letters, 1980, 44, 420. J. 0. McCaldin, T. C. McGill and C. A.Mead, Phys. Rev. Letters, 1976, 36, 56. M. Green, J . Chein. Phys., 1959, 31, 200. ’ A. J. Bard, A. B. Bocarsly, F. R. Fan, E. G. Walton and M. S. Wrighton, J . Amer. Chem. SOC., 1980, 102,3671. A. B. Bocarsly, D. C. Bookbinder, R. N. Dominey, N. S. Lewis and M. S. Wrighton, J . Amer. Chem. SOC., 1980,102,3677. F-R. Fan and A. J. Bard, J. Amer. Chem. SOC., 1981, in press. lo F-R. F. Fan, B. Reichman and A. J. Bard, J . Amer. Chem. SOC., 1980, 102, 1488. l1 F-R. F. Fan, H. S. White, B. Wheeler and A. J. Bard, J . Electrochem. SOC., 1980, 127, 518. l2 R. Memming, in ElectroanaZyticaZ Chemistry, ed. A. J. Bard (Dekker, New York, 1979), pp. l3 H. Gerischer, in Physical Chemistry-An Advanced Treatise, ed. H. Eyring, D. Henderson l4 W. Kautek and H. Gerischer, personal communication. l5 J. M. Bolts, A. B. Bocarsly, M. C. Palazzotto, E. G. Walton, N. S. Lewis and M. S. Wrighton, 1-84. and W. Jost (Academic Press, New York, 1970), pp. 463-542. J . AFner. Chem. SOC., 1979, 101, 1378 and references therein. P. A. Kohl and A. J. Bard, J . Electrochem. SOC., 1979, 126, 59. R. E. Malpas and A. J. Bard, manuscript in preparation. l7 R. E. Malpas, K. Itaya and A. J. Bard, J . Anzer. Cheni. SOC. 1979, 101, 2535. l9 T. Teherani, K. Itaya and A. J. Bard, Noun J . Chein., 1978, 2,481. 2 o P. A. Kohl and A. J. Bard, J . Electrochein. Soc., 1979, 126, 603. 21 A. Heller, B. A. Parkinson and B. Miller, Appl. Fhys. Letters, 1978, 33, 521. 22 B. A. Parkinson, A. Heller and B. Miller, J . Electrochem. Soc., 1979, 126, 954. 23 W. H. Brattain and P. J. Boddy, Surface Sci., 1966, 4, 18. 24 H. Gerischer, N.B.S. Spec. Pub., 1975, 455, 1 . 25 S. R. Morrison, The Chemical Physics of Surfaces (Plenum Press, New York, 1977). 26 H. Tributsch, J . Electrochem. SOC., 1978, 125, 1086. 27 H. Tributsch and J. C. Bannett, J. Electroanalyt. Chem., 1977, 81, 91. 28 H. Tributsch, Ber. Bunsenges. phys. Chem., 1977, 81, 361. 2 9 H. Tributsch, Ber. Bunsenges. phjs. Cheiii., 1978, 82, 169. 30 H. Tributsch, H. Gerischer, C. Clemen and E. Bucher, Ber. Bunseiiges. phys. Chem., 1979, 83, 31 L. F. Schneemeyer and M. S. Wrighton, J . Amer. Cheni. SOC., 1979, 101, 6496. 32 F-R. Fan, H. S. White, B. Wheeler, and A. J. Bard, J . Anter. Cheni. SOC., 1980, 102, 5142; J. 33 H. J. Lewerenz, A. Heller and F. J. Disalvo, J . Anter. Chem. Soc., 1980, 102, 1877. 34 W. Kautek, H. Gerischer and H. Tributsch, Ber. Bunsenges. phys. Chem., 1979, 83, 1000. 35 A. Aresti, P. Manca and A. Spiga, Chem. Phys. Letters, 1979, 63, 139 and references therein. 36 G. Margaritondo, J. E. Rowe and S. B. Christman, Phys. Rev., 1976, 14B, 5396. 655. Electrochem. SOC.. 1980, 127, 518.

ISSN:0301-7249    

DOI:10.1039/DC9807000019

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

Pulsed Laser-induced Photoelectrochemistry at Polycrystalline and Single-crystal Semiconductor Electrodes BY SUZANNE B. DEUTSCHER AND JEFFERY H. RICHARDSON Chemistry Department, Lawrence Livermore Laboratory, Box 808 L-325, Livermore, California 94550, U.S.A. AND SAM P . PERONE, JEFF ROSENTHAL AND JAMES ZIEMER Department of Chemistry, Purdue University, W. Lafayette, Indiana 47907, U.S.A. Receiced 6th May, 1980 Our studies have focused on the production and detection of transient phenomena related to u.v.-visible photoelectrolysis at various types of semiconductor electrodes. For this work we have utilized xenon flash-lamp and pulsed dye-laser sources. We are reporting on two significant aspects of this work : a theoretical/experimental description of the time dependence of pulse photocurrents and experimental studies directed at the detection of transient intermediates in photoelectrolysis pro- cesses.Two experimental measurement approaches have been investigated. One of these involves controlled-potential chronoamperometry, utilizing potential steps synchronized with pulsed irradia- tion for detection of transient photoproducts. The other approach involves photocoulostatic poten- tiometry, where open-circuit photovoltage excursions are studied with sub-microsecond time resolu- tion. The former approach allows selective monitoring of various potential regions for qualitative identification of transient photoproducts, whereas the latter approach allows much faster time resolu- tion of photo-induced events. These studies were conducted with single-crystal n-TiO,, p-Gap, p-GaAs and polycrystalline n-Ti0,.Because of the much lower ohmic resistance, the polycrystalline electrodes allowed the best time resolution for controlled-potential experiments. A discussion of the limitations of time-resolved measurements, as well as the nature and significance of transient photo- induced processes is presented. Most photoelectrochemical studies of semiconductor electrode processes have employed continuous or chopped light sources resulting in relatively long time experi- ment~.’-’~ As a prerequisite to a complete understanding of the processes taking place, studies of the transient effects should be conducted. It is the purpose here to demonstrate that flash irradiation can provide a new perspective on semiconductor photoeffects which complements observations made with continuous sources. We have reported previously on the limited time-resolution of potentiostatic photocurrents measured with pulsed At best one can achieve resolu- tion of ca. 500 ns, with ca.50 ps more typical if correction for induced charging cur- rents is made.15 The results reported here for potentiostatic measurements fall in this range. Barker et a1.16 proposed coulostatic-flash measurements to obtain micro- second time-resolution in photoemission studies with mercury electrodes. We recently achieved 200 ns time resolution in coulostatic-flash mercury photoemission studies.” The present work demonstrates that we can observe photopotential tran- sients at semiconductor electrodes with at least 15 ns time resolution.We have analysed the electrical and electronic characteristics of photoelectrochemical cell and measurements circuitry to ensure the validity of short-time measurements; and we have suggested possible interpretations for the observed behaviour.34 PULSED LASER-INDUCED PHOTOELECTROCHEMISTRY Various types of electrodes were studied, single-crystal, polycrystalline, n-type and p-type. One objective was t o develop an understanding of the response of an elec- trode/solution interface to potential step and potentiostatic-flash irradiation perturba- tions. Another objective was to examine the transient sub-microsecond photopoten- tial excursions obtained with coulostatic-flash experiments. EXPERIMENTAL POTENTIOSTATIC-FLASH INSTRUMENTATION The excitation source used was a S321 xenon flash lamp manufactured by Xenon Corp.The total energy per flash was 200 J, with a pulse width of 10 ps. Transient behaviour was monitored with a potentiostat constructed from selected components [see ref. (1 8)J; unity- gain bandwidth was ca. 5 MHz. Current measurement was made with a Burr-Brown 36225 differential-input instrumentation amplifier, for which the 3 db roll-off point is 2 MHz at a gain of 100. Current-time curves were obtained from oscilloscope traces or from a mini- computer data acquisition system, for which the maximum sampling rate for a 10-bit word was 167 kHz. Coulomb measurements were obtained by numerical integration of the data acquired by the minicomputer system. Currents which decayed too rapidly to be monitored accurately by the computer were recorded by oscilloscope, and the areas under the traces were measured by planimetry.RC bridge experiments and cyclic voltammetry were con- ducted using a PAR 173 potentiostat. COULOSTATIC-FLASH INSTRUMENTATION The pulsed laser source was a Molectron model UV 1000 nitrogen laser (337 nm) and Molectron model DL 200 dye laser with 10 ns pulse width. The coulostatic-flash instrumen- tation has been described," but some modifications to the measurement approach have been made for this study. They are as follows: ( a ) for photopotential transients in a time domain from ns to 10 p s , the working electrode and Pt quasi-reference electrode were directly con- nected (and a.c.-coupled) to a Tektronix type 7A13 (105 MHz bandwidth) differential ampli- fier plug-in which was mounted in a Tektronix model 7623A oscilloscope.All control/ monitoring electronics were located inside the Faraday cage enclosing the cell. The y-axis output from the scope back panel was connected externally to a type 7A19 (225 MHz band- width) plug-in amplifier (50sZ input, a.c.-coupled), mounted in a Tektronix model 7844 oscilloscope. (b) For photopotential transients in the time domain from 1 ps to seconds, the Pt quasi-reference electrode was capacitively coupled (0.47 pF) to the saturated calomel reference electrode (SCE), the SCE and working electrode potentials were input to a wide- band pass (5 MHz) high-input-impedance (10" n) differential amplifier (4.6 x), with d.c. offset bias so that the d.c.level of transients could be conveniently measured. Its output was in turn connected to a Tektronix type 7A16-A amplifier plug-in, with 1 Mi2 input impedance, and 225 MHz bandwidth. Traces were recorded photographically. The philosophy behind the above measurement approaches is documented The important point is that the Pt quasi-reference electrode reflects accurately any transient changes in the cell potential in a time domain of <10 ps. Capacitive coupling of the two reference electrodes allows measurements of photopotential transients referenced t o the SCE for times > I ps. However, when only the magnitude of photopotential transients was required the quasi- reference electrode could be monitored directly, without coupling to the SCE. CELL AND ELECTRODES The photoelectrochemical cell design is basically as described previously.20 Four different single crystal n-Ti02 electrodes were used for coulostatic-flash studies, representing a rangeDEUTSCHER, RICHARDSON, PERONE, ROSENTHAL AND ZIEMER 35 TABLE 1 .--SUMMARY OF ELECTRODE CHARACTERISTICS (n-TiOz) ~~ electrode A E,,(MS)IV us.SCE - 0.4 ET,(C.W./U.V./d.C.)/v US. SCE -0.8 ETH(pulse iJ/V us. SCE -0.55 ETH(C.f*)/V ?IS* SCE -1.3 ETH(c.w./u.v./a.c.)/v US. SCE - 1.1 N,( MS)/cm - C,(E = + 1 .O V)/pF R,(E = +1.0 vyn 1.6 x 10" 0.14 3 30 area/cm2 0.1 1 B -0.75 -1.1 -1.3 3.0 x lo2' 2.2 29 0.14 C -0.53 -0.88 -0.80 - 0.50 -0.90 1.4 x 10'' 1.8 38 0.20 D -0.80 -0.93 -0.70 -0.50 -1.1 1.5 x lozo 1.8 N 86 0.21 ~~ All data obtained with 1.0 mol dm-3 KN03 electrolyte. EFB = flat-band potential; ETH = thres- hold potential; ND = donor density; MS = Mott-Schottky plot; c.w./u.v.refers to photocurrent against E experiments with C.W. U.V. laser source; a.c. refers to lock-in detection; d.c. refers to direct current measurement; c.f. refers to coulostatic-flash experiments; R,, C, refer to impedance bridge measurements of cell resistance and capacitance at 1 kHz. TABLE 2.-cHARACTERISTICS OF SEMICONDUCTOR ELECTRODES USED IN POTENTIOSTATIC- FLASH STUDIES material used dopant and type dopant le~el/cm-~ orientation flat-band potential (at pH 7)/V us. SCE mean capacitancea / p F cm-2 contact stable potential range /V us. SCE manufacturer preparation pretreat men t GaAs (111) 0.20 2 silver epoxy +0.2, - 1 .o General Diode 1 4 Ti02 Ti(n) <OOl) -0.74 4 x 1019 5 indium f1.0,-0.5 NL Industries 2 5 pol y-Ti02 Ti(n) 6 x 10l6 -0.50 - 0.01 silver epoxy + 1 .O, -0.8 Alfa Products 3 none pol y-Fe,O Feb) 1 x lo2' - 0.40 20 silver epoxy + 1 .o, -0.1 Alfa Products 3 none GaAs was used as received from General Diode, already doped.A pure crystal of Ti02 was doped in a reducing atmosphere(H,) for 30 min at 650 "C. The oxides were formed on a piece of metal foil (either Ti or Fe) by heating in a dry oxygen atmosphere to 1000 "C and allowed to anneal in the furnace as it cooled to room temperature. GaAs was etched in H,SO, (95%)/H202 (30%)/- H 2 0 (3: 1 : 1) for 90 s" before a set of runs. Ti02 crystals were rinsed in methanol before use. (1 Mean capacitance is defined as the average capacitance of the electrode over the potential range studied.of resistivities and donor densities. characteristics are summarized in table 1 . tiostatic-flash studies are described in table 2. These were designated as electrodes A-D, and their The semiconductor electrodes used in the poten- PHOTOELECTROCHEMICAL PROCEDURES The coulostatic-flash experimental procedure has been described in detail e1~ewhere.l~ It involves first adjusting the cell voltage to the desired initial potential, E,, for the semi- conductor working electrode using the polarograph. A solid state electronic switch which opens the cell circuit is triggered at 5 s intervals. After ca. 30 ps time delay the pulsed laser is triggered and the open-circuit potential difference between the working and reference36 PULSED LASER-INDUCED PHOTOELECTROCHEMISTRY electrodes is monitored as described above. After a 2 s interval the cell circuit is closed again and the intial potential reimposed by the polarograph.Pulse photocurrent measurements were made as described previously.20 RESULTS AND DISCUSSION POTENTIOSTATIC-FLASH EXPERIMENTS Since potential steps can be used to identify intermediates and products of photo- redox reactions,21 an understanding of the response of a semiconductor/electrolyte interface to a potential step must be formulated. In doing this, first consider the equivalent circuit for a semiconductor-electrolyte interface given in fig 1 . 2 2 3 2 3 The Rsc FIG. 1 .-Analogue representation of an electrochemical cell. Rsol : represents the cell impedance Cdf the double-layer capacitance, C,, the space-charge capacitance, R,, and C,, the surface-state impedance and capacitance, respectively, R,, and R, the impedance of the space-charge region and semiconductor bulk, respectively.Rr represents the faradaic impedance. The total current passed through the cell is denoted by isol which is equal to the sum of the currents passed through the space- charge region (isc) and surface states (i)SS. impedance to ion transport through the cell is represented by the double-layer capacitance (or Helmholtz capacitance), c d , ; the capacitance of the space-charge region of the semiconductor electrode, Csc ; the impedance to charge transport through the space-charge region and the bulk of the semiconductor electrode are represented by R,, and R,, respectively. The product R,,C,, is the charging time-constant of sur- face states at the interface.The faradaic impedance is represented by Rf, which is set to infinity for the potential step theory developed here. When a potential step is applied across this equivalent circuit the changing potential across the space-charge region will cause the space charge capacitance to change in a manner expressed by the Mott-Schottky relationship lo, 11*24 where E is the permittivity of free space, E , is the dielectric constant of the semicon- ductor, e is the charge of an electron, N is the number of donors or acceptors in the semiconductor, VSc is the potential drop across the space-charge region and 9 = kT/e. To determine the importance of this effect some simplifications in the equivalent circuit of fig.1 will help. First, assume that the effects of surface states can be neg- lected, eliminating R,, and C,, from the circuit. This leaves the space-charge capaci- tance and double-layer capacitance in series so that the total capacitance, CT = c , , c d l / ( c , c + c d l ) . In general we can impose the constraint that C,, <C,,, leaving the capacitance of the semiconductor-electrolyte interface to be representedDEUTSCHER, RICHARDSON, PERONE, ROSENTHAL AND ZIEMER 37 by only Csc. Finally, the impedances of the space-charge region, the bulk of the semiconductor and of the solution can be combined into one net impedance. The simplified circuit is shown in fig. 2 and consists of only two elements, the interfacial R FIG.2.-Simplified analogue equivalent to an electrochemical cell with a semiconductor working electrode. R represents the combined cell and semiconductor impedance and C the interfacial capacitance. capacitance (C) and the net solution-semiconductor impedance (R). Since faradaic current is not considered in understanding the potential step response, Rf is allowed to go to infinity and is not included in fig. 2. The interfacial capacitance is shown as a variable capacitor since it is voltage dependent. For a potential step applied to the simplified circuit of fig. 2 the following boundary values are defined, t < to; V = V,, i = 0 and V,, = V, Vf - vo t = to; V = Vf, i = i,,, = ~ R (2) and Vsc = Vo (3) Vf - R t > to, v = vf, i = -‘Sc Substitution of the relationship and eqn (1) into yields _- - dt. -~ di 2 idvf-q- Ri Integration of eqn (8) results in 4 < I = - - R (‘f - 9) m z where dvo - q l - d V f - y‘vo - q F d G t = exp( - t/RC,) and (4)38 PULSED LASER-INDUCED PHOTOELECTROCHEMISTRY A plot of In i against t is curved, with an intial slope of l/RCo and a final slope of l/RCf.But, for small potential steps or steps through a potential region for which the space-charge capacitance changes very little, a In i against t plot is nearly linear. With this in mind we can now proceed to the more complicated and accurate equivalent circuit and study its response to a potential step. If we assume that Csc changes very little for the potential step to be applied to the cell, the following set of equations re- sult from the application of Kirchhoff's laws to the circuit in fig.1, assuming Rf is infinite. 1 1 %c - = 0 c s c j s s R s s + 4ss - - css All resistances and capacitances retain the definitions given previously. The ion flux, represented as current, is denoted by isol ; current through the space-charge capacitance, isc; current through the surface states, iss. The magnitude of the poten- tial step is Vf - Vo, the difference of the final and initial potentials. The charge on each of the capacitors in the circuit is denoted by q with the appropriate suffix. [Recall that i = dq/dt, so that differentiation of eqn (12) and (13) will replace the q terms with i. J This system of equations is solvable by differentiation and separation into a system of linear o p e r a t o r ~ .~ ~ The result is as follows, where Rr2 + l / C d l Rr2 + 1/c , 6 2 = - r l = - +(a - d m , r2 = - + (a + da2 - 4b), R = Rsol + R,, + R, and 1 1 c=-+-. C d , c s c The important observation here is that the total cell current decays as a function of the sum of two exponential functions. Furthermore, if C,, and Cs, < c d , and Rss 9 R (i.e., the surface states possess a long time constant and the double-layer capacitance is large compared with the surface-state and space-charge capacitances,DEUTSCHER, RICHARDSON, PERONE, ROSENTHAL AND ZIEMER 39 which are not unreasonable hold : the following two simplifications (16) (17) 1 - = (R + Rss)Css and r1 1 -= RC,,. r2 Although it is difficult to test eqn (15) rigorously, experimental observations do demonstrate two different regions of exponential decay.An example of this beha- viour is shown in fig. 3. Table 3 lists the initial and final slopes (l/rl and 1/r2) from the semilog plots of fig. 3, where several different potential steps were applied to a - 6 -7 7 -8 E n 3 -9 U C - -10 -11 I 1 I I I I 1 0 2 4 6 a 10 12 14 tlms FIG. 3.-Semilog plot of the current response to a potential step for GaAs. Potential steps are from -1,000 to -0.800 V (a), -0.600 V (b), -0.400 V (c), -0.200 V ( d ) and 0.000 V (e). UU-U wwrmng cicurvuc. fiiong wirn rnese values are iisrea rne KL proaucts or me cell (an RC value is the product of the cell impedance and capacitance as determined by the bridge technique). Notice that the initial and final decay constants do not Y - ____ - - tions which lead to eqn (16) and (17) is not maintained.Nevertheless, the type of behaviour described by eqn (1 5) is at least qualitatively followed. Moreover, there is TABLE 3.-cOMPARISON OF THE INITIAL AND FINAL DECAY CONSTANTS FROM EQN (15) TO THE RC OF THE CELL DETERMINED BY THE RC BRIDGE METHOD AT VARIOUS POTENTIALS. THE WORKING ELECTRODE IS GaAs IN A pH 7 BUFFER SOLUTION. 1 .ooo 0.800 0.50 12 27 1 .ooo 0.600 1.60 14 29 1 .ooo 0.400 2.30 17 30 1 .ooo 0.200 3.60 24 33 1 .ooo 0.000 3.80 29 3540 PULSED LASER-INDUCED PHOTOELECTROCHEMISTRY reasonably close agreement between l/r, and the RC value obtained from the bridge technique. Bear in mind, however, that the above derivation and discussion provides us with no information about the energetic location of surface states at the interface (relative to the valence and conduction bands); nor is there any information available to evaluate the density of surface states or relative numbers of surface states with different time constants.We can only demonstrate the existence of surface states which are slow in comparison with the space-charge time constant (which is deter- mined by RC,,). Also Slow surface states are evidenced by the large values of l/rl. K a .,E . -- 1 0 1 0 U 0 10 20 30 40 50 FIG. 4.-Current response of the cell to a potential step (ca. 100 mV) for each type of semiconductor electrode, (a) poly-TiO,, (6) GaAs, (c) sc-Ti0, and (d) Fe203. All solutions are in pH 7 buffer. The potential of each electrode is + 1 .O, - 1 .O, + 1 .O and + 1 .O V vs. SCE, respectively. note that no information regarding Cdl is obtained from the above analysis of current- time behaviour.A graphic summary of surface contributions to the charging current profiles for each type of electrode studied is presented in fig. 4. Notice that for the polycrystal- line TiO, electrode the charging time is very short. This suggests that either the cell and electrode resistance (R) is very small and/or that the surface states are very fast (i.e., R&, is small). Semilog plots of current against time do not show two distinct slopes. At the other extreme, GaAs, single-crystal TiO, and polycrystalline Fe,O, demonstrate a considerable contribution from slow surface states (relative to the space- charge decay constant). Semilog plots for these electrodes show two distinct decay constants (see fig.3). The dependence of photocurrent on potential for flash irradiation is fundamentallyDEUTSCHER, RICHARDSON, PERONE, ROSENTHAL AND ZIEMER 41 the same as that for continuous irradiation, as shown in fig. 5. Procedurally, the process of flash irradiation can be broken down as follows. Before illumination a potential is applied to the cell so that a certain amount of band-bending is present in the space-charge region of the semiconductor. This causes a majority carrier defi- ciency at the surface relative to the bulk. At the onset of irradiation electron-hole pairs are formed and, since a potential gradient exists across the space-charge region, the newly formed electrons and holes migrate in opposite d i r e ~ t i o n s ~ ~ s ~ ~ until a steady- state concentration gradient is formed.This happens very quickly compared with the duration of the flash and ion migration in the solution.28 In an n-type semicon- ductor the electrons promoted to the conduction band are received in the bulk of the //O GaAs / 0.5 # iJmA cm-2 FIG. 5.-Photocurrent-voltage profile for each type of electrode taken at the current maximum after the flash. A11 solutions are in pH 7 buffer. The potential of each electrode is the same as that in fig. 4. semiconductor, while the holes in the valence band migrate to the surface. If the energetics are correct water will be oxidized to give oxygen. The electrons of a p-type semiconductor, however, are accumulated at the surface in the conduction band and may reduce the solution, while the holes in the valence band migrate into the bulk.As a result of this process two sources of perturbation on the original potential field exist. First, the steady-state redistribution of electrons and holes in the space-charge region creates a new potential distribution which has already been referred to as a p h o t o ~ o l t a g e . ~ ~ * ~ ~ Secondly, transport of charge across the double layer in the form of faradaic current discharges the double-layer capacitance, changing the potential drop across it. After termination of the irradiation the remaining electron-hole pairs recombine on a time-scale fast relative to the flash d ~ r a t i o n . ~ ~ ~ ~ ~ Hence the electrode production of electron-hole pairs ceases and no further chemical reaction occurs at the interface.All that remains at non-equilibrium is the potential drop across the double-layer and space-charge capacitances, which will be recharged at a rate go- verned by eqn (1 5). Quantitative evidence for this is lacking for the semiconductor- electrolyte interface. Notice, however, that the same trends are observed for the response of the electrode to a potential step or flash irradiation. An electrode which demonstrates a short decay-constant for a potential step will re-establish equilibrium very rapidly after a flash, and an electrode with a long potential step decay-constant will require a relatively long time to establish equilibrium after a flash (cf: fig. 4 and 6).42 PULSED LASER-INDUCED PHOTOELECTROCHEMISTRY One set of experiments involved flash irradiation with a delayed potential step.For these experiments an electrode is held at a potential favourable for a photo- current to flow upon flash irradiation. After the flash is over (25 p s delay), a potential step of a predefined magnitude is applied to the cell so that at the new potential any products or intermediates of the photoreaction might be detected electrochemically. The timing of the experiment for an n-type electrode is clarified in fig. 7. At time to I 0 1 0 0 50 100 tips FIG. 6.-Typical photocurrent response for poly-Ti02 (b), GaAs (c), Sc-Ti02 ( d ) and Fe203 (e) com- pared to the Xe flash lamp profile (a). The photocurrent responses are from the output of the current measurement amp of the potentiostat. The flash lamp profile was measured with a Motorola MRDSOO photodiode (rise-time = 1 ns).All solutions are pH 7 buffer solution, and the electrode potentials are the same as in fig. 4. the flash is initiated with a positive potential applied, and the current response of the cell is monitored. At time t l , after a delay of 7, a negative potential step is applied sufficient to reduce photoproducts formed by the flash. If z is large enough, the cur- rent flow through the cell induced by the delayed potential step will be identical to the current induced without the preceding flash. That is, only step-charging current is observed. As z decreases, however, fewer of the products produced as a result of the flash will have had a chance to diffuse away from the electrode, so that there will be a farradaic contribution to the step-induced current.The temporal behaviour of theDEUTSCHER, RICHARDSON, PERONE, ROSENTHAL AND ZIEMER 43 faradaic current can be obtained by subtraction of the response due to the potential step alone from the response observed with the flash and step. In other words, the faradaic and charging-current contributions are additive [this is not precisely true at times which are small compared with the RC of the cell; see ref. (21) and (34) for more on this subject}. The faradaic contribution should vary inversely as the square FIG. 7.-Timing diagram for a flash-potential step experiment. The initiation of the flash is at to. After a delay time (z) the potential step is applied at tl. root of z, by analogy to heat-transfer as the electrochemically active product diffuses away from the electrode.Another consideration coupled to a decrease in z is the amount of current still flowing as a result of the initial flash. When z is so small that a potential step is ini- tiated while a significant amount of current is still present, as a result of the flash, the potential drop across the semiconductor/electrolyte interface will be unknown. 0 ‘flash ‘f ‘0 FIG. 8.-Relationship of charge passed during a step, flash and Aash-step. The total charge passed through the cell by the potentiostat as a result of a flash at a potential Vo is (Ql + Qz); for a step from Yo to V,, -Qz; for a flash at V, followed by a step to Vr, Ql. Consequently the time dependence of the charging current is unknown and a point by point subtraction, retaining temporal integrity, is not valid.By reference to fig. 8 it can be shown, however, that the faradaic and charging current contributions are separable by subtraction of the total charge passed. In fig. 8 the total interfacial capacitance is plotted as a function of the potential drop across it. The area under the curve between two potentials is the number of coulombs accumulated on the capacitor when the potential is changed from one to the other. As the capacitance44 PULSED LASER-INDUCED PHOTOELECTROCHEMISTRY against voltage curve is plotted in fig. 8 for a potential step from V, to V,, the amount of capacitative charge passed will be, qs = - Q2. The amount of charge passed as the result of a flash can be represented by a potential step from Yo to Vflash (assuming an instantaneous flash generating an initial photovoltage of Yflash - V,) where the total number of coulombs passed will be, q F = Ql + Q2.(Note, however, that the mea- sured current is opposite in sign to qs, because it results from the potentiostat attempt- ing to change the potential from Yflash to Yo after the flash.) For a flash coupled with a step the final potential will be Vf and the charge passed, qFS = el, if no faradaic processes are initiated as a result of the potential step. Thus, subtraction of the charge passed during a step alone from the charge passed during a flash with delayed step should be equal to the charge passed for a flash alone. This can be summarized by defining qNET such that qNET = qFS - q S - qF.If no faradaic process is initiated by the delayed potential step, qNET = 0. However, if the products of the flash are reducible or oxidizable at the new potential, qNET # 0. Due to the assumption of an instantaneous flash however, eqn (18) is only precisely true for electrode/electrolyte systems with a long time-constant. In other words, the longer it takes a system to respond to a flash perturbation the more instantaneous the flash will appear. Fig. 9 summarizes the dependence of qNET on the potential stepped to (V,) for all four electrodes. The assumptions which lead to the calculation of qNET via eqn (18) are strictly followed for only the slower responding electrode, Fe,O, (see fig. 6). How- ever, the trends observed for iron oxide and the other electrodes are the same.Notice (18) FIG. 9.-Plot of the qNET against potential for each type of electrode, sc-Ti02 (a), Fe203 (b), poly- Ti02 (c) and GaAs (d). The residual q is obtained by subtracting qs from qFS and then subtracting the amount of charge passed for a flash at the initial potential from the result. that qNET is non-zero at all potentials, except the initial potential, for which the poten- tial step is zero. To test the presumption that the non-zero qNET is the result of faradaic current a set of experiments were conducted with a constant potential step, but variable delay time, z. By doing this, the initial difference between the current due to a potential step and a potential step applied after a flash could be measured over a wide range of delay times.Polycrystalline TiOz was chosen for this study since it responds most quickly to a flash, leaving little charging current to interfere with theDEUTSCHER, RICHARDSON, PERONE, ROSENTHAL AND ZIEMER 45 delayed potential step (see fig. 6). The result follows typical Cottrell behaviour (fig. 10). That is, the net current detected by a potential step after the flash decays as l / d z . This result is consistent with the detection of a stable photoproduct (probably oxygen) which is diffusing away from the electrode. The lifetime of this photo- product is > 15 ms. Because there are no sharp steps in the plots of qNET against potential (fig. 9), as in a polarogram, it appears that significant back reaction of product occurs at the elec- trode, even at potentials favourable for photo-breakdown of the solvent.(It should 600 i I 1 0 50 100 z-*/s-f FIG. 10.-Cottrell plot for poly-Ti02. The potential of the electrode before and during the flash is + 1 .000 V us. SCE. The potential stepped to is -0.400 V us. SCE. The current (i) is measured at the initiation of the step. be pointed out that the fact that qNET goes to zero at the initial potential is inherent in the design of the experiment and does not imply that the product is not being con- sumed by the electrode.) The evidence here suggests that at an n-type semiconductor electrode some of the photo-oxidation product is reduced. The driving force for the backreaction of the photoproduct at the electrode can be broken down into two contributions. First the charge transfer which takes place during irradiation drives the potential across the space-charge region toward flat-band.30*31 This results in a cathodic shift for an n-type electrode which is more favourable for reduction.Likewise for a p-type electrode, the shift in potential across the space-charge region is anodic. For the electrodes used, the amount of charge passed as a result of a flash experiment is about the same as a potential step of 400 to 800 mV. Secondly, the use of surface states for electron transfer between the product and the electrode is suggested here since suitable overlap between the product and the conduction band is unlikely. When the valence-band energy and the energy of the donor orbital of OH-46 PULSED LASER-INDUCED PHOTOELECTROCHEMISTRY are in close proximity the energy level of the oxidized form of OH- (0,) will be too high to interact favourably with the valence band.29p32 For example, a cyclic voltam- mogram of oxygen saturated electrolyte with a polycrystalline TiO, electrode shows no distinct O2 wave, only a gentle rise that begins at ca.0.3 V vs. SCE (also true for other n-type electrodes used here). It has already been reported in the literature that sur- face states can furnish a significant amount of ~ u r r e n t . ~ ~ * ' ~ * * ~ Also, it has been re- ported that etching can reduce surface-state contributions 23 and that etching increases the efficiency of the semiconductor electrode's ability to convert radiant to electrical en erg^.^ Thus, from the observations in the literature and this study it appears reasonable that backreaction uia surface states of the product of the photo-induced decomposition of water can be significant.COULOSTATIC-FLASH STUDIES In contrast to pulse photocurrent measurements, coulostatic (open-circuit) photo- potential (E,) measurements made with a pulsed laser source are not limited in response time by the cell time c o n ~ t a n t . ' ~ * ~ ~ The response is limited by the RC time-constant defined by the cell impedance (R = R, + R,,) and the combined value of the inter- electrode stray capacitance, the input capacitance of the measurement electronics, and cable capacitance. This combined value, C,, is typically less than 100 pF. Thus, for electrodes A to D (table l), respectively, RC, <4 <3 <4 and <5 ns, where R is the value measured at high frequency with a vector impedance meter in 1.0 mol dm'3 KN03.These response time constants are comparable to or less than the pulse- width of the laser source (10 ns), thus allowing time-resolved studies in a time domain of 210 ns as shown below. Fig. 11 and 12 show typical photopotential (E,) behaviour for coulostatic flash experiments in 1.0 mol dm-3 KN03, using the nitrogen laser (337 nm). Irradia- tion with the nitrogen pumped dye laser (520 nm) resulted in photopotential excur- sions at least four orders of magnitude lower (using comparable laser intensities). Several features are worth noting. First of all, it is clear that the rise-time (tg5%) is ca. 12 ns, which suggests that it is limited by the laser pulse-width and/or the bandwidth of the measurement electronics.The potential changes sharply in a more negative direction, but then decays back towards the initial potential, Ei. When Ei is very positive of EFB, the final photopotential, (Ep)F, is reached after ca. 1-20 ms for the electrodes studied here. When Ei is near EFB, not only is there a much smaller initial excursion of the photopotential, but (Ep)F + Ei ; also, (E,JF is reached in < ca. 1 ms. The net magnitude of photopotential excursion achieved after the decay, (AEp)F, is taken to be indicative of the net quantity of charge transferred, or " photocharge " (Q,). Qp can be calculated from the relationship, Qp = C,(AE,)F where C, is the combined capacitance of C,,, C,, and Cdl. An indication of the efficiency of the photoelectrolysis process can be obtained from the ratio Q,/A, where A is the electrode area.The data in table 4 clearly show that electrode B is the most efficient and elec- trode A is the least efficient. By comparison with table 1, the trend in efficiency cor- relates well with the donor density. (However, because the laser pulse intensity varied &25% due to optical alignment, the efficiency data can only be interpreted semi-quantitatively .) The times, tF, required for the photopotential to decay to (AEJF were only esti- mated from visual inspection of photographic traces, but do indicate a qualitative in- verse correlation between tF and the donor density. Finally, the quantum efficiency Table 4 summarizes the time-dependent behaviour of four electrodes.DEUTSCHER, RICHARDSONy PERONE, ROSENTHAL AND ZIEMER 47 P EP a P J P _i 1 1 1 1 1 1 - / a / l 5 mV I time time L _I time FIG.11 .-Coulostatic-flash photopotential transients at positive potential. Electrode C; E, = + 1 .O V us. SCE; 1.0 mol dm-3 KNOB; light intensity cu. 10 k W cm-z.48 PULSED LASER-INDUCED PHOTOELECTROCHEMISTRY time 1 time time FIG. 12.-Coulostatic-flash photopotential transients near EFB. Electrode C; Ei = -0.5 V us. SCE; 1.0 mol dm-3 KNOJ; light intensity EZ 10 k W cm-2.DEUTSCHER, RICHARDSON, PERONE, ROSENTHAL AND ZIEMER 49 (Q,/total photons incident) can be estimated to be ca. This value is rather low, but certainly includes the uncertainties in measuring the total charge transferred and the number of photons absorbed in the depletion layer during each 10 ns laser pulse.Several different factors may be involved. The first factor is that the initial photopotential is caused by electron- hole separation, with the subsequent decay caused by electron-hole recombination. The rates of such processes in the bulk for highly doped semiconductor materials are typically found to be in the sub-microsecond time domain.36 However, longer The nature of the Ep decay is certainly of interest. TABLE 4.-sUMMARY OF TIME-DEPENDENT BEHAVIOUR OF SEVERAL n-Ti02 ELECTRODES WITH PULSED-LASER COULOSTATIC-FLASH EXPERIMENTS IN 1 .O mol dm- KNO, (A) For E1 = +1.5 V us. SCE A B C D z"/ns 12 10 15 12 tF/ms N 20 - 2.2 - 1 - 1 (C\E,)F (AEp)ma x (bQp/A)/pC cm-f 0.024 0.56 0.10 0.12 0.15 0.76 0.59 0.41 CS/@ 0.12 1.96 1.56 1.46 (B) For Ei near EFB for each electrode A B C D z"/ns 12 10 12 12 t F b -lo00 -50 -175 -150 0.10 0.18 0.13 0.20 (AEPlF (AEp)max ('Qe,/A)/pC 0.017 0.067 0.044 0.032 EilV US.SCE -0.7 -0.6 -0.5 -0.8 cs IPF 0.53 3.5 3.1 3.4 "z = tgs% = rise time of photopotential in ns, based on a smoothed fit to the rising portion of the = (AE,)FC,; A = working electrode area in cm'. (The C, value used was taken from curve. MS plots or was taken as the maximum measured value from the MS plot when Ei < EFB.) lifetimes might occur in the space-charge region at a semiconductor-electrolyte inter- face (band bending at the interface facilitates the separation of electrons and holes). The second factor is oxidation by holes migrating to the surface (of either the sol- vent or species at the electrode surface).Note that this process would tend to increase AEp with time. An opposite effect would be the dark reduction of photoproducts; i.e., the photopotential decay might represent the dark back reaction of photo-oxidation products produced by the flash. This would be consistent with the observations in the potentiostatic-flash studies that such back reactions may occur, probably via sur- face states, even when the electrode potential is quite positive of EFB. The rate (or efficiency) of the back reaction appears to increase as the potential is made less posi- tive, and this is consistent with the observation here that the decay time constant decreases as Ei approaches E F B (table 4). The fourth possible explanation is that the very high rate of charge injection50 PULSED LASER-INDUCED PHOTOELECTROCHEMISTRY effected by the intense 10 ns laser pulse causes a transient non-equilibrium expansion of the space charge region in the semiconductor, followed by a relaxation back to its equilibrium dimensions, with associated overshoot and decay of the photopotential.(This decay may or may not involve hole-electron pair recombination, depending on the survival of excess surface holes.) Such a relaxation effect has been observed with coulostatic electrical charge injection at a mercuryldilute-electrolyte i n t e r f a ~ e ~ ~ n ~ ~ and has been described in terms of a transient over-expansion of the Helmholtz layer (interfacial double layer) followed by a relaxation back to an equili- brium thickness, with corresponding changes in the measured double-layer voltage analogous in nature and time dependence to those observed here.The nature of steady-state photopotentials generated under conditions where charge transfer to solution is blocked has been described previously; 42-44 and experi- mentally observed instantaneous photovoltaic response has been d e ~ c r i b e d . ~ ~ ’ ~ ~ G e r i ~ c h e r ~ ~ suggested that short light pulses should be used for these measurements to obtain a better characterization of the space-charge region free from charge-transfer and surface effects. However, no reliable measurements have been reported on a time-scale comparable with our studies. The possibility that a transient photopoten- tial overshoot and decay might be observed has been suggested p r e v i o ~ s l y .~ ~ ~ ~ ~ Laser and Bard28 also pointed out that the response of the semiconductor space-charge layer to coulostatic electrical charge injection is analogous to that predicted by Feldberg41 for diffuse double-layer relaxation, except that it should occur in a much shorter time-domain. Unfortunately, the pulsed laser-induced photopotentiometry experi- ments performed here do not correspond exactly to the electrical coulostatic charge injection arrangement described by Laser and Bard.28 Nor do the conditions agree with those used to simulate steady-state photo potential^^^ or to simulate the relaxation of photogenerated free carriers.47 However, ref. (47) does demonstrate that distinct transient minima and maxima in the concentration-distance profiles of minority and majority charge carriers can be generated, perhaps leading to the kind of relaxation processes observed here.The time-scale of the transient behaviour predicted 47 is several orders of magnitude shorter than observed here. However, the conditions chosen for the simulations correspond to light fluxes and donor densities several orders of magnitude lower than those used here. To investigate further the merits of the fourth explanation for the photopotential decay, the time dependence was compared with that predicted theoretically for double- layer r e l a ~ a t i o n . ~ ~ In that case, assuming equal mobilities for positive and negative charge carriers, theory predicts that the observed double-layer potential should follow a t - 2 / 3 dependence during the latter part of the decay when the potential is near EpZC (potential of zero charge).Essentially the same theoretical concepts can be imposed on the response of the semiconductor space-charge region to an instantaneous light- induced charge injection. Holes and electrons are considered to replace cations and anions, with their associated mobilities and concentrations; the thickness of the deple- tion layer replaces the Helmholtz-layer thickness, and EFB replaces Epzc. The suggestion that Helmholtz-layer relaxation could also contribute to the time dependence observed here is of course appropriate. However, the effects have not been observed at mercury electrodes with concentrated electrolyte( > -0.05 mol d r n 7 and relatively small charge injection.37 Thus, we should not expect to see Helmholtz- layer relaxation effects here, with 1.0 mol dm-3 KN03 electrolyte and Q,/A < 1 pC cm-2.The value of Ei for the experimental data in fig. 13 was near EFB. The data plotted are for only the latter part of the decay, where a linear dependence on t‘2/3 was observed. This A typical plot of E, against t -2/3 is shown in fig. 13 for electrode D.DEUTSCHER, RICHARDSON, PERONE, ROSENTHAL AND ZIEMER 51 50 40 30 20 10 I I I I I 1 0 0.1 0.2 0.3 0.4 0.5 0.6 t-213pS-Z13 FIG. 13.-Transient photopotential us. t-213 for El near E F B . 1.0 mol dm-3 KN03, electrode D, -0.5 V us. SCE, gain = 4.6. behaviour is analogous to double-layer relaxation observation~,~~ where linear depen- dence of cell potential on t -2/3 is predicted after coulostatic charge injection when Ei is near Epzc.41 The behaviour shown in fig.13 was typical of all the electrodes used in these studies. Non-linear dependence on t - 2 / 3 was observed at short times, as TABLE 5.-sUMMARY OF t-213 ANALYSIS OF PHOTOPOTENTIAL DECAY FOR & NEAR EFB A electrode B C D EJV US. SCE 0.7 -0.6 - 0.7 -0.5 - 0.8 "S/V ps-2I3 0.089 0.012 0.0025 0.027 0.01 3 "SIPS N 30 -4 -4 N 15 N 15 dqp/pC cm-2 0.01 7 0.067 0.021 0.044 0.032 "CZ/lO4 ~ r n - ~ 0.027 5.0 5.0 2.33 2.5 '( AEpMmV 3.6 2.7 0.78 2.8 2.0 *105y 0.22 1.21 1.74 1.75 1.59 S is the slope of a AEp against t - 2 / 3 plot. begins. ' density = C, (AEp)F/A (see tables 1 and 2 for C,, A). ( =6 x ts is the time at which linear dependence on t - 2 / 3 is the intercept of the AE, against t - ' I 3 plot as t + 03.'q, is the photocharge Cz = ND/L, where L is Avogadro's number 7 = SCz3/2/qp5'3. indicated in table 5. Non-linear dependence on t - 2 / 3 was observed at all times when Ei was very positive of EFB (e.g., at + 1.0 V us. SCE), for all electrodes; the slope of a linear least-squares fit on a log-log plot of Ep us. t becomes increasingly more positive as Ei becomes more positive of EFB. CONCLUSIONS These studies demonstrate for the first time that photoinduced electrode processes can be observed reliably in the nanosecond to millisecond time domains. The ob- served limitations on time resolution are consistent with those predicted. The time dependence of transient potentiostatic photocurrents is clearly related to the cell time52 PULSED LASER-INDUCED PHOTOELECTROCHEMISTRY constant (microseconds) ; whereas the time dependence of transient coulostatic photo- potentials can be studies with ca.10 ns time resolution. The fact that the rise-time is ca. 12 ns in 1.0 mol dm-3 electrolyte may indicate that photoinduced transfer of charge to the electrolyte solution is essentially instantaneous with a 10 ns laser pulse. The observed subsequent photopotential decay may be related to a phenomenon of space-charge relaxation which exhibits a decay time varying from ca. 1 to ca. 20 ms as the potential becomes more positive of EFB. A significantly shorter and less dramatic decay is observed with increased doping density. Alternative explanations suggested here include photoproduct back reaction or hole-electron recombination. Because these other processes are known to occur to some extent, their possible con- tributions to the initial photopotential transient behaviour should be investigated fur the r .These studies have provided a foundation on which to base further investigations of transient behaviour with other semiconductor photoelectrodes and other photo- electrolysis processes. It should be possible to study the dynamics of redox stabilized photoelectrodes as well as dye-sensitized photoelectrolysis on a previously inacces- sible time-scale. Modification of the local environment (e.g., non-aqueous solvents) may be necessary to fully exploit this technique for studying chemical processes as opposed to processes inherent to the semiconductor electrode.The authors thank Jackson Harrar and Lloyd Steinmetz for their contributions to this study. This work was supported by the U.S.D.O.E. contracts no. W-7405- ENG48(LLL) and DE-AC02-77ER04263. A002 (Purdue). L. Hollan, J. C. Tranchart and R. Memming, J. Electrochem. SOC., 1979, 126, 855. J. O’M. Bockris and K. Uosaki, J. Electrochem. Soc., 1977, 124, 1348. K. Nakatani and H. Tsusomura, Bull. Chem. SOC. Japan, 1977,50,783. A. B. Bocarsly, J. M. Bolts, P. G. Cumins and M. S. Wrighton, Appl. Phys. Letters, 1977,31, 568. R. H. Wilson, L. A. Harris and M. E. Gerstner, J. Electrochem. Soc., 1979, 126, 844. L-S. R. Yeh and N. Hackerman, J. Electrochem. Soc., 1977, 124, 833. K. L. Hardee and A. J. Bard, J . Electrochem.SOC., 1976, 123, 1024. J. H. Kennedy and K. W. Frese Jr, J. Electrochem. SOC., 1978, 125, 709. 419. a J. S. Curran and W. Gissler, J. Electrochem. SOC., 1979, 126, 56. lo S. M. Wilhelm, K. S. Yun, L. W. Ballenger and N. Hackerman, J. Electrochem. Soc., 1979,126, l1 L. A. Harris and R. H. Wilson, Ann. Rev. Muter. Sci., 1978, 8, 99. l2 R. L. VanMeirhaeghe and F. Cardon, Ber. Bunsenges. phys. Chem., 1979, 83,236. l3 D. Laser and S. Gottesfeld, J. Electrochem. SOC., 1979, 126, 475. l4 S. S. Fratoni and S. P. Perone, Analyt. Chem., 1976, 48,287. Is K. F. Dahnke, S. S. Fratoni and S. P. Perone, Analyt. Chem., 1976, 48,296. l6 G. C. Barker, A. W. Gardner and G. Bottura, J. Electroanalyt. Chem., 1973, 45, 21. 17J. H. Richardson, S. B. Deutscher, A. S. Maddux, J. E. Harrar, D.C. Johnson, W. L. Schmel- zinger and S. P. Perone, J. Electroanalyt. Chem., 1980, 109, 95. S. S. Fratoni, Ph.D. Thesis (Purdue University, W. Lafayette, IN, 1976). l9 C. C. Herrmann, G. G. Perrault and A. A. Pilla, Analyt. Chem., 1968, 40, 1173. 2o S. P. Perone, J. H. Richardson, B. S. Shepard, J. Rosenthal, J. E. Harrar and S. M. George, in New Applications of Lasers to Chemistry (A.C.S. Symp. Ser., Washington, D. C., 1978), vol. 85, pp. 126-170. 21 B. S. Hall, K. F. Dahnke, S. S. Fratoni Jr, and S. P. Perone, J. Phys. Chem., 1977, 81, 866. 22 S. Toshima, Progress in Surface Membrane Science, ed. J. F. Daniel], M. D. Rosenberg and z3 M. Tomkiewicz, Semiconductor Liquid-Junction Solar Cells, ed. A. Heller (The Electrochemical 24 F. M. Delnick and N. Hackerman, J. Electrochem. SOC., 1979, 126, 732. 25 Any number of good books on differential equations will demonstrate the technique used to D. A. Cadenhead (Academic Press, New York, 1974), vol. 4, pp. 231-297. SOC., Princeton, N. J., 1977), proceedings vol. 77-3.DEUTSCHER, RICHARDSON, PERONE, ROSENTHAL A N D ZIEMER 53 derive eqn (15). An example is Introduction to Diflerential Equations by Shepley L. Ross (Xerox, Lexington, MA, 1974). 26 M. Nishida, Nature, 1979, 277, 202. 27 W. Schmickler, Ber. Bunsenges. phys. Chem., 1978, 82, 477. 28 D. Laser and A. J. Bard, J. Electrochem. Soc., 1976, 123, 1828. 29 H. Gerischer, Physical Chemistry: An Advanced Treatise, ed. H. Eyring (Academic Press, New 30 H, Reiss, J. Electrochem. Soc., 1978, 125, 937. 31 H. Gerischer, J. Electrochem. SOC., 1966, 113, 1174. 32 F. Williams and A. J. Nozik, Nature, 1978, 271, 137. 33 R. J. Nelson and R. G. Sobers, J. Appl. Phys., 1978, 49, 6103. 34 S. S. Fratoni, Jr., and S. P. Perone, J. Electrochem. SOC., 1976, 123, 1672. 35 H. S. Carslaw and J. C. Jaeger Conduction of Heat in Solids (Oxford University Press, 2nd edn, 36 S. M. Sze, Physics of Semiconductor Devices (Wiley-Interscience, New York, 1969). 37 F. C. Anson, R. F. Martin and C. Yarnitzky, J. Phys. Chem., 1969, 73, 1835. 38 C. Yarnitzky and F. C. Anson, J. Phys. Chem., 1970, 74, 3123. 39 J. Newman, J. Phys. Chem., 1969, 73, 1849. 40 R. P. Buck, J. Electroanalyt. Chem., 1969, 23, 219. 41 S. W. Feldberg, J. Phys. Chem., 1970, 74, 87. 42 C. G. B. Garrett and W. H. Brattain, Phys. Rev., 1955, 99, 376. 43 E. 0. Johnson, Phys. Reu., 1958, 111, 153. 44 D. Laser and A. J. Bard, J. Electrochem. Soc., 1976, 123, 1833. 45 P. J. Boddy and W. H. Brattain, J. Electrochem. SOC., 1963, 110, 570. 46 P. J. Boddy and W. H. Brattain, Ann. N. Y. Acad. Sci., 1963, 101, 683. 47 D. Laser and A. J. Bard, J. Electrochem. Soc., 1976, 123, 1837. 48 R. K. Rhodes and R. P. Buck, J. Electroanalyt. Chem., 1979, 103, 19. York, 1970), vol. 9A. 1959).

ISSN:0301-7249    

DOI:10.1039/DC9807000033

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

Evaluation of the Flat-band Potentials by Measurements of Anodic/Cathodic Photocurrent Transitions BY HANS R. SPRUNKEN, ROLF SCHUMACHER AND RALPH N. SCHINDLER Institut fur Physikalische Chemie, Universitat Kiel, Olshausenstrasse 40-60, 2300 Gel, West Germany Received 2nd May, 1980 The photoprocesses on semiconducting n-Ti02 and n-SrTiO, electrodes in the presence of reducible surface species are described. These species are generated by pre-illumination with the band-gap light of the electrodes being operated under reverse bias. The amount formed in the course of this pre- illumination can be re!ated to the negative charge required to reduce them. This charge correlates proportionally with V,,, the voltage at which transition from anodic to cathodic photocurrent occurs. The influence on Vt, of the parameters surface coverage as function of pre-illumination time, wave- length, photon flux and pH of the electrolyte is investigated. The occurrence of the cathodic photoprocess is discussed in terms of an electron-tunnelling mechanism through the space-charge barrier to the solution and a joint model proposed by Bard and Gerischer.The influences of the cathodic photoeffect on the onset potential of the usual anodic photoprocess may affect flat-band potential determinations from curves of iph against V. Experimental evidence is provided that the most reliable values of Vfb from this method can be obtained by scanning from cathodic to anodic potentials. Also irradiation with long-wavelength light, close to the band-gap energy, and high light intensity are recommended.The increased interest in fundamental processes occurring on semiconductor electrodes is well documented in a number of publications which have appeared over the last decade. Further insight into the process is required because of its significance in the field of energy research. One important feature, the determination and evaluation of flat-band potentials Vfb, has attracted particular,research as the knowledge of this parameter is of essential importance for the energy calibration of the semi- conductor-liquid junction.lV2 Various techniques have been suggested for measuring VfbeS7 The most common method utilizes the capacitance shift as a function of the potential drop inside the semiconductor interface. Another method suggested by Butler connects photocur- rent against voltage measurements with the theory of Gartner.g In this case the flat- band position is defined as the potential where the photocurrent starts to flow.The determination of Vfb can be then obtained from plots (iP,J2 against V where iph denotes the photocurrent. The energy position of the flat-band potential has been shown in many cases to be dependent on the nature of the electrolytic solution, e.g., the pH, and on additives such as chalcogenides.l0V1l Because of variation of the pH and the sulphide ion concentration in both cases a shift of 60 mV per concentration decade of [H+] and [S2-] has been found. It is generally agreed that these shifts are caused by surface adsorption of these species. In this paper we report on complications which one may face using the method suggested by Butler to determine Vfb.It will be shown that an additional photo- process occurs when n-TiO, electrodes are operated in the reverse-bias mode and in the presence of reducible species. This cathodic photoprocess may give rise to an apparent onset potential V,, for the appearance of the anodic photoprocess. It will56 FLAT-BAND ANODIC/CATHODIC PHOTOCURRENTS be shown that the cathodic photocurrent is affected by experimental parameters such as light intensity, wavelength, electrolyte pH and the amount of the reducible species present. Influences on V,, of this cathodic process can be minimised by scanning from negative to positive potentials and using long-wavelength light, close to the band-gap energy, of high intensity. The occurrence of the cathodic photo- process is discussed in terms of an electron tunnelling mechanism through the space- charge barrier to the solution and a joint model proposed by Bard and G e r i ~ c h e r .~ * ~ ~ Finally, it should be noted that cathodic photoeffects on n-semiconductors have been reported p r e v i ~ u s l y . ' ~ ~ ~ EXPERIMENTAL Polycrystalline Ti02 electrodes were prepared by oxidation of Ti-foils (Metallwerk Plansee) in an air stream at temperatures between 700 and 800 "C. Before oxidation the specimens were cleaned in acetone, concentrated HzS04 and in HF (5%). Finally they were washed in doubly-distilled water. Different doping densities ND were made by vacuum annealing at temperatures ranging from 600 to 800 "C.TiO, single crystals, purchased from Hrand Djevahirdjian S.A., Monthey were vacuum treated in the same way. Polycrystalline SrTiO, doped with Nb was provided by Philips Research Laboratories, Aachen. Some of the Ti02 and SrTiO, single crystals were kindly supplied by Sandia Laboratories, Albuquerque. The oxides were made semiconducting by hydrogen reduction. Ohmic contacts were obtained with Ga/In. The rear sides of the single-crystal electrodes were insulated by epoxy cement. The electrochemical experiments were performed in a closed-cell arrangement. In order to avoid perturbation of the processes occurring at the working electrode by species generated at the counter electrode (platinized Pt) the compartments were separated with the aid of porous fritted glass discs.IA drops were made negligible by placing the reference electrode close to the working electrode. Flushing of the working electrode compartment with various gases could be arranged through a glass valve placed at the bottom of the cell. The measurements were carried out in 0.05 or 0.5 mol dm-3 H2S04 and 1 mol dm-3 NaOH, respectively, prepared with doubly-distilled water. Measurements at around pH 7 were carried out in 0.5 mol dm-3 Na,S04 solutions which were adjusted by adding H2S04 or NaOH. All chemicals used in this study were reagent grade. The potential of the semiconductor electrode was adjusted and scanned by a Wenking potentioscan POS 73 (Bank Elektronik). The usual scan rate was 2 mV s-l. The illumination of the exposed sample area (0.2 cm2) was performed by a 450 W xenon light source (Osram).The light could be separated into its components using a grating mono- chromator (M 25, Jobin Yvon) and was focused by quartz lenses onto the semiconducting electrodes. For most of the experiments the light was chopped with frequencies ranging from 0.05 to 2 Hz. Photocurrents were amplified with an Ithaco model 391 A lock-in system. Absolute light intensities were measured with a photodiode model OSD 50-1 (Laser Optronic) calibrated to a standard of the U.S. National Bureau of Standards. RESULTS Unexpected changes in the photoresponse of n-TiO, and n-SrTiO, electrodes were observed when the potential of the electrodes was swept from positive values into the cathodic direction (referred to as sweep C ) compared with scans from negative to positive potentials (sweep A).The scan directions are indicated by arrows. The sweeps were recorded with the lock-in amplifier system. Using the calibrated photodiode the output signals were converted into quantum efficiencies, q. As can be seen from sweep C in fig. 1 the direction fo the recorded photocurrents reverses at potentials more negative than -0.1 and -0.5 V This behaviour is illustrated in fig. 1.H . R . SPRUNKEN, R . SCHUMACHER AND R . N . SCHINDLER 57 m F l " " " ' " " " ' 1 I * 0.04- P 0.02 1 , , , , 1 l , , , 1 , . , - 0.5 0 0.5 potential/V 0s. SCE FIG. 1 .-Typical anodic qa and cathodic q' quantum-efficiency-potential curves taken for poly- crystalline n-Ti02 and n-SrTi03 electrodes in H,SO, solution of pH 3 and pH 1, respectively.C and A denote the scan direction. (i) n-Ti02: ND N 2 x 1017 cm-', h = 340 nm. (ii) n-SrTi03: ND N 5 x 10'' crne3, h = 350 nm. During both experiments N, was bubbled through the electro- for Ti02 and SrTiO, electrodes, respectively. We refer to this reductive photo- current as cathodic photoeffect iEh. This photoresponse was observed only for the sweeps C. The decrease of i,"h at potentials more negative than -0.2 and -0.5 V recorded on TiOz and SrTiO, electrodes, respectively, indicates the depletion of species adsorbed on the electrode surface. The reverse anodic scans A taken immediately after sweeps C show no reductive photocurrent iih. These results strongly suggest that photo-oxidized products like oxygen play an important role in initiating this cathodic photoeffect.lyte. - 1 .o -0.5 potential/V us. SCE 0 FIG. 2.-Typical current-voltage diagrams of n-Ti02 (ND 21 5 x 1017 ~ r n - ~ ) in 0.1 mol dm-3 NaOH. Prior to scanning nitrogen was bubbled through the solution. Dashed curve taken in the dark. Solid curve taken under illumination with h = 380 nm. For completeness in fig. 2 current against potential curves are given recorded in the dark and under band-gap illumination. Comparing the results of fig. 1 and 2 differences in the photocurrent curves were found around potentials where reductive dark currents appear. In this context we refer to the literature16-20 on forward cur- rents recorded in the dark under reverse bias.58 FLAT-BAND ANODIC/CATHODIC PHOTOCURRENTS According to a number of author^^'-^^ values for flat-band potentials v f b can be evaluated from photocurrent against voltage (iph against V ) curves.The photo- current onset potential Yo, for the photoprocess is considered to be the true flat-band potential Vfb. However, our results reveal that two different V,, values are found when tracking sweeps A or C. Thus, the determination of v f b from iph measurements seems to be uncertain due to contributions from possible cathodic photoprocesses. For convenience we define the potential where the anodic photoprocess changes over to the cathodic one as the transition potential V,,. In the following we report on experimental parameters which influence Vtr. In order to elucidate the electrode processes leading to the cathodic photoeffect we will also present more experimental evidence on that subject.The growth of the reductive photoprocess is influenced by the time of illumination under depletion conditions. This observation is represented in fig. 3(a) where the potential/V us. SCE 4 illumination timelmin FIG. 3.-Growth of the reductive photoresponse of polycrystalline n-TiO, pretreated at 0.5 V in 0.1 mol dm-3 NaOH as function of illumination time. (a) Behaviour of the anodic i;h and cathodic i&, photocurrents. The illumination times before the scans (2)-(8) were 1,2,3,5, 10, 15 and 40 min. During the experiments N2 was bubbled through the electrolyte. ( b ) Plots of the peak area and the transition potential Iftr as function of the time of The scan directions are indicated by arrows. illumination. photoresponse of an n-Ti02 electrode in the potential range - 1 .O to-0.6 V for various illumination times is plotted.The peak areas obtained in 7 C sweeps are taken as a measure of the value of the reductive photocurrent and plotted as curve (1) in fig. 3(b). A shift of Vt, as a function of illumination time was found only after short-time irradiations The diagram shows rapid initial growth with subsequent saturation.H. R . SPRUNKEN, R. SCHUMACHER AND R . N . SCHINDLER 59 [curves (2) and (3) in fig. 3(a)]. All other sweeps yield identical Vtr values. The change in Vtr is plotted in fig. 3(b) curve (2) as well. The exponential behaviour of curve (1) suggests that the species generated under these conditions are adsorbed on the TiOz surface. From the saturation limit we can obtain some idea of the electrode coverage.Assuming a two-electron process and a particle diameter of 4 A, 0.10 - 0.05- I I I l l I I I I I 1 I , 300 350 400 1 Inm I I I I I 3 00 350 40 0 i /nm FIG. 4.-Normalized spectral responses q" and qc obtained under anodic and cathodic bias in an electrolyte of pH 13. The scan directions are indicated. (a) Ti02: ND FZ 5 x loi7 ~ m - ~ , curves (1) and (2) taken at potentials of 0.5 V, curve (3) at a potential of -0.9 V. (b) SrTi03: ND z 2 x 1017 ~ m - ~ , curve (1) taken at a potential of A0 V, curve (2) obtained at a potential of -1.1 V.60 FLAT-BAND ANODIC/CATHODIC PHOTOCURRENTS an estimate of the number of adsorbed species per unit area yields 8 x lOI3 ~ m - ~ , which corresponds to a coverage of 0.5 monolayer. The valuations are based on coulometric data.Although the description of iih as function of coverage is ade- quately represented by measurements of the peak areas we preferred V,, as appropriate alternative. Fig. 3(6) indicates a reasonable correlation between peak area and Vtr. The stable nature of the species attached to the electrode surface could be proved. Even after a long delay at rest potential between light-off and the beginning of the 0.02 0.01 7 .+ E $ 0 .1. 0.01 c- 0.02 I I I l l l 1 I l l , : I 300 3 50 400 ;i/nm FIG. 5.-Normalized anodic qa and cathodic q" spectral response for polycrystalline n-Ti02 (ND z 5 x 10'' C M - ~ ) recorded at a potential of -0.85 V in 1 mol dm-3 NaOH. The light intensities were increased from time to time [curves (3) to (I)] by a factor of 4.scan into the negative direction the reductive photocurrent remained constant. Also vigorous stirring of the solution close to the electrode before the run did not affect the reduction peak intensity. The wavelength dependence of the photoprocess for polycrystalline n-Ti02 and SrTiO, at potentials positive and negative to Vt, is given in figs. 4(a) and (6). The photocurrents obtained were converted into quantum efficiencies q" and q" and plotted on the ordinate. Pre-treatment of the TiO, electrode for 15 min at - 1.5 V yielded curve (l), fig. 4(a). Electrodes not pre-treated this way showed the behaviour given in curve (2). This behaviour did not change in repetitive scans, whereas the influence of the pre-treated electrodes was of transient nature only. Curve (3) in fig.4(a) describes the cathodic photoresponse at -0.9 V obtained in the same wavelength region as curve (1) and (2). Reproducible cathodic quantum efficiencies were obtained for both scan directions if oxygen was bubbled through the electrolyte. From thisH . R . SPRUNKEN, R . SCHUMACHER AND R . N . SCHINDLER 61 we conclude that the cathodic photoeffect can be stabilized by dissolved oxygen which is supplied to the working electrode. Comparing curves (1) and (2) it is interesting to note that the presence of oxygen on the electrode generated by the anodic photo- process leads to a lowering of qa at short wavelengths whereas qc in the same wave- length region remained considerably higher. As in the case of TiOz the wavelength characteristics of polycrystalline n-SrTi03 were taken when oxygen was bubbled through the electrolyte. The figure shows that SrTiO, behaves qualitatively in the same way as TiO,. However, polarization at -1.8 V did not alter the anodic quantum efficiency at all.Fig. 4(a) and (b) reveal that separation of the photo-oxidation processes can be established by record- ing the spectral response at potentials considerably more cathodic or anodic, respectively, than the transition potential Vtr. Using an We next consider the influence of wavelength and photon flux on V,,. 1’ 1 I 1 - 0.90 - 0 . 8 5 - 0.80 potential/V us. SCE FIG. 6.-Transition potential Vt, of polycrystalline n-Ti02 (ND M 5 x 10’’ ~ m - ~ ) as function of the light intensity at the wavelengths: h(1) = 330, h(2) = 350, h(3) = 370, h(4) = 390 nm.iPh against Vscan obtained at J. = 345 nm the potential V,, was identified as -850 mV. Polarization of the electrode with that potential and illumination with J. = 380 nm resulted in an anodic photoeffect. Fig. 5 demonstrates the photoresponse at V,, = -850 mV recorded with the lock-in amplifier system in the spectral range 280-420 nm. The figure also reveals the influence of light intensity on the photoresponse. As shown the anodic process rises proportionally with the light intensity whereas for the cathodic process saturation was found. More detailed information on the dependence of Vt, on wavelength, photon flux62 FLAT-BAND ANODIC/CATHODIC PHOTOCURRENTS and pH is provided in fig. 6 and 7.Each curve in fig. 6 was obtained for one wave- length but for various intensities. The wavelengths of curves (1)-(4) are given in the figure captions. All measurements were carried out with the lock-in amplifier system. I I I I I I -1 .o 0 2 4 6 0 10 12 14 PH FIG. 7.-Dependence of Von and V,, on pH. Yon represents the onset potential of the anodic photo- current whereas V,, denotes the transition potential obtained on polycrystalline n-Ti02. The differ- ence between Yo, and Vtr depends on experimental conditions. DISCUSSION An evaluation of the flat-band potentials V,, taken from the onset potential of the commencing anodic photocurrent on semiconductor electrodes encounters unexpected difficulties because of an extra photoprocess which can occur under reverse bias.The following discussion is divided into two sections. In the first the experimental results will be discussed in order to elucidate the processes which are responsible for the occurrence of the cathodic photoeffect. The influences of that cathodic photo- process on the determination of the flat-band potential as suggested by Butler* are then discussed in the second section. As most of the results are obtained on poly- crystalline n-TiO, the discussion is centred on that material. However, the results show that the phenomena described can also be observed on polycrystalline n-SrTi03 and on single crystals of both materials as well. CATHODIC PHOTOPROCESSES UNDER REVERSE BIAS EFFECT OF SURFACE AREA As mentioned above, the phenomenon of the cathodic photoeffect observed on polycrystalline n-TiO, and n-SrTiO, also appears on both single-crystal materials.H.R . SPRUNKEN, R . SCHUMACHER AND R . N. SCHINDLER 63 We assume that the less pronounced effects on single crystals compared with the poly- crystalline materials arise from the smaller surface areas. This is supported by the results of fig. 3(a) and (b). Assuming a planar n-TiO, surface which is covered with one monolayer, the calculation for the number of adsorbed particles for the exposed area yields 1.6 x Coulometric determination of the generated species on the same area yields 8 x 1013 particles corresponding to a coverage of 50%. As reported in the 1iteratu1-e~~’~’ coverages in the range 0.5-20% are observed with the solid-gas interface. This discrepancy with our result of 8 x 1013 particles suggests a consider ably higher surface area of the polycrystalline materials used.EFFECT OF REDUCIBLE ADSORBATES DISCUSSED FOR OXYGEN To elucidate the effect of oxygen on the semiconductor-liquid interface we turn first to results found on semiconductor-gas interfaces. Electron transfer from the conduction band of a semiconductor such as TiO, to adsorbed electronegative species has been studied extensively. It seems generally agreed that after oxygen adsorption 0, molecules are f~rmed.~’-~O This species and further reduction products such as OH., H02* were identified in e.s.r. investigation^.^^^^^ On hydrated surfaces in addi- tion OH- and HO, were The formation of negative ions on the surface reduces the mobile electron density [el which affects the space-charge region of the semiconductor.It was demonstrated from conductivity measurements that charge transfer from TiO, to adsorbates, e.g. oxygen, results in a surface p-type c h a r a ~ t e r . ~ ~ ~ ~ ~ As a consequence the work function increases, resulting in an upward band bending. At the same time the electric field at the surface E, strengthen^.^^ This p-type be- haviour could be avoided by additional surface doping using electron donors such as vanadium and niobium.37 The photogenerated forward current in the electrochemical system was only observed when reducible species resulting from either anodic photoproduction or from diffusion of dissolved electrolyte oxygen were adsorbed on the electrode surface. We therefore conclude that in electrochemical systems also electrons are trans- ferred from the conduction band of n-TiO, to reducible adsorbates as discussed for the solid-gas interface.For electrochemical systems further oxygen reduction products and intermediates, especially H202, are discussed in the l i t e r a t ~ r e . ~ ~ * ~ ~ ~ TRANSITION POTENTIAL (Vtr) SHIFTS DUE TO CHANGES I N ELECTRON DENSITY [el AND THE ELECTRIC FIELD E , AT THE SURFACE In this section we explain the observed shifts of V,, in both potential directions by means of changes in [el and E, on the electrode surface. We recall that at the transi- tion potential, V,,, by definition no photoprocesses are observed because all the photogenerated carriers recombine. The concentration of mobile electrons within the depletion region of the elec- trodes can be changed by the following three methods: (i) different adsorbate con- centrations, (ii) illumination with various light intensities using monoenergetic photons and (iii) by varying the wavelength of the incident radiation.All three treatments resulted in a shift of the transition potential V,, (see fig. 3 and 6). We first deal with case (i) where the initial [el is lowered by increasing the adsorbate concentrations at the surface. As mentioned above this successive lowering of [el causes at the same time a rise in E,. It is obvious from the adsorption experiments where E, is increased that the cathodic photoprocess is favoured. This results in64 FLAT-BAND ANODIC/CATHODIC PHOTOCURRENTS a Yt,-shift to more positive potentials, as indeed was observed.According to the results of fig. 3(a) taken from curve (2) the potential V,, (2) = -780 mV was found for the lowest electrode coverage. For the next higher coverage at this potential the electrode showed a cathodic photoprocess [see curve (3)]. This results in a more positive potential V,,(3) = -705 mV. To understand the independence of V,, for higher coverages [curves (4)-(8)] we refer to the Results section. The observations found for case (i) are supported by experiments where [el was changed by illumination, case (ii) (see fig. 6). In contrast to the coverage experiments where [el was successively lowered, the experiments with increased photon fluxes lead to an increase in [el and thus to a reduction in E,. Hence, the potential V,, should move in the opposite direction as compared with case (i), as is supported by the results.The wavelength and intensity dependence of V,, as demonstrated in fig. 6 reveals that V,, is more strongly affected by wavelength than by photon flux. At short wavelengths [A(l) = 330 nm], [el at the surface can be changed considerably by varying the light intensity because all electrons are produced within the penetration depth ( 1 / ~ ) of 30 nm. Increasing the photon flux per unit area from1 x 1013 to 12 x lOI3 hv s-l shifts the potentials Vt, by 65 mV. On the other hand at A(4) = 390 nm this shift yields only 20 mV. At this wavelength the electrons are produced within an ex- tended penetration depth of l/rc = 3000 nm which alters the electron density [el close to the surface only slightly.It then follows that V,, shifts to more negative values for long wavelengths as compared with shorter wavelengths. Of course this is only valid for comparable photon fluxes. It seems to be conclusive that electron generation in deeper regions of the semiconductor does not show up as forward current, because of the reduced probability with which electrons reach the surface. The effect of extra surface doping by niobium and vanadium, 37 as well as by hydro- gen, 41 confirms this tendency. The doping procedures were similar to those noted in the literature. Surface doping caused by in-diffusion of hydrogen as reported re- ~ e n t l y ~ ' . ~ ~ influences qc in the same way as do the electron donors V5+ and Nb5+. Hydrogen can also leave the electrode by diffusion.Accordingly, reversible behaviour of the cathodic photoeffect should be restored, as indeed was observed. These addi- tional dopants increase the electron concentration at the surface and thus diminish the cathodic photoprocess and intensify the anodic one. Our conclusion is that surface doping may also result in a reduction of the E, initially caused by adsorption case (i). This result is unexpected in that an increase in E, which is connected to a higher potential drop at the semiconductor interface favours a forward current and diminishes the anodic photoresponse. One may suggest an electron tunnelling mechanism through the space-charge barrier as being responsible for this cathodic photoeffect. TRANSITION POTENTIAL (Vtr) SHIFTS OWING TO SURFACE STATES The observed shifts of Ytr given in fig.6 can be explained alternatively by models proposed by Bard and Gerischer and used by ~ t h e r ~ . ~ ~ ~ ~ * ~ ~ * ~ ~ In the model of Bard intermediate levels and/or surface states are included to mediate charge transfer from semiconductor to solution states. At V,, the number of electrons which flow from the conduction band via surface states to solution species is equal to the sum of holes which are lost by recombination or in oxidation processes. If the intensity of the light is increased [see case (ii) above] according to Gerischer the quasi-Fermi level of holes is much more affected than isH . R . SPRUNKEN, R . SCHUMACHER AND R . N. SCHINDLER 65 the quasi-Fermi level of electrons.As a consequence the anodic photocurrent is favoured, which results in the observed V,, shift to more negative potentials. To explain the wavelength dependence [case (iii)] using this model we assume that the population probability of the surface states is increased due to electrons photogenerated close to the surface. Electrons generated in deeper regions of the semiconductor (i.e., by low-energy photons) must first diffuse to the surface to be trapped there by the empty surface states. In our case surface states were produced either by anodic photogeneration or by adsorption of dissolved oxygen. The density of these states alters also the cathodic photoeffect (see fig. 3) and simultaneously the qa in the short- wavelength range [see fig. 4(a)]. EVALUATION OF THE FLAT-BAND POTENTIAL As shown in the previous sections the determination of flat-band potentials V f b taken from (iph,V) measurements as proposed by Butler' may lead to significant errors in the present system.In this method the onset potential Yon of the anodic photocurrent determines Vfb. It seems to be established that only in the absence of reducible species produced by illumination under strong depletion conditions does this method yield reliable values of Von and thus of Vfb. Sweeping from positive to negative potentials under illumination leads to an apparent Yon as demonstrated by fig. 1. We referred to this apparent Von as the transition potential Vtr. This V,, was found to depend very strongly on the experimental conditions such as surface coverage with reducible adsorbates, e.g., oxygen, illumination intensity, wavelength and pH-value of the electrolyte.As shown in fig. 7 for Y,, and Yon potentials a shift of 60 mV per pH unit was found, which coincides with the well-known pH shift of Vfb. The difference between Yon and V,, depends on the conditions during the measurement. It seems to be indicated that the most reliable values of V f b can be determined in the absence of reducible adsorbates and using high light intensities and long wavelengths. The former can be verified for n-type material when sweeping from negative to positive potentials. Utilizing photocurrent against voltage measurements according to the methodof Butler 'the flat-band potential is determined by the potential where (iph)2 is zero. This evaluation is restricted to wavelengths much longer than the space-charge barrier and the diffusion length.The above restrictions eliminate the influence on Vfb determinations initiated by the wavelength dependence of the cathodic photoprocess. Additionally, capacity measurements carried out to deter- mine Vf, yielded values close to Van. Note that the plots of 1/C2 against Vwere found not to be linear in most cases. Non-linear behaviour of Mott-Schottky plots has often been described and discussed in the l i t e r a t ~ r e . l ' ~ ~ ~ - ~ ~ Finally note that flat- band potentials for TiO, given in a recent paper of T o r n k i e ~ i c z ~ ~ were found to be ca. 200 mV more negative than the results taken from i:, against Y measurements. The work is supported in part by the Bundesminister fur Forschung und Tech- nologie (BMFT) Bonn, within project ET 4274 A.H. Gerischer, Topics Appl. Pltys., 1979, 31, 1 1 5. R. Memming in ElectroanaIyticaI Chemistry, ed. A. J. Bard (Marcel Dekker, New York, 19791, vol. 1 1 , p. 1-84. V. A. Myamlin and Y. V. Pleskov, Electrochemistry of Semiconductors (Plenum Press, New York, 1967), p. 1 1 1. H. Gerischer in Physical Chemistry, an Advanced Treatise, ed. H. Eyring, D. Henderson and W. Jost (Academic Press, New York, 1970), vol. IX A, p. 463. P. A. Kohl and A . J. Bard, J. Amer. Chem SOC., 1977,99,7531.66 FLAT-BAND ANODIC/CATHODIC PHOTOCURRENTS M. Tomkiewicz, J. Hectrochem. SOC., 1979, 126, 2220. L. J. Handley and A. J Bard, J. Electrochem. SOC., 1980,127, 338. M. A. Butler, J. Appl.Phys., 1977, 48, 1914. W. W. Gartner, Phys. Rev., 1959, 116, 84. D. S. Ginley and M. A. Butler, J. Electrochem. SOC., 1978, 125, 1968. chap. 4, p. 77. 126,419. lo T. Inoue, T. Watanabe, A. Fujishima and K. Honda, Bull. Chem. SOC. Japan, 1979,52,1243. l2 H. Gerischer in Solar Power and Fuels, ed. J. R. Bolton (Academic Press, New York, 1977), l3 S. M. Wilhelm, K. S. Yun, L. W. Ballenger and N. Hackerman, J. Electrochem. 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ISSN:0301-7249    

DOI:10.1039/DC9807000055

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

Pho tocurrent Spectroscopy of Anodic Oxide Films on Titanium BY JEROME F. MCALEER AND LAURENCE M. PETER Department of Chemistry, The University, Southampton SO9 5NH Received 2nd May, 1980 Photocurrent spectroscopy has been used to examine the early stages of anodic film growth on titanium. Analysis of the photocurrent conversion efficiency as a function of wavelength for films formed at different potentials has shown that the film is essentially pure TiO, above ca. 1.5 V us. SCE (at pH 0). Photocurrent spectroscopy has also been used to follow changes in the structure and thickness of the anodic film during breakdown at higher voltages. The recent upsurge of interest in the photoelectrochemistry of semiconductors owes its origin to the present search for alternative forms of energy conversion, and our understanding of the properties of the semiconductor-electrolyte interface is now advancing rapidly as the potential usefulness of a variety of semiconducting materials is investigated.Although the great majority of recent experimental work has been oriented towards solar energy conversion, at least as a distant goal, it is clear that the concepts and methodology of photoelectrochemistry are more widely applicable. Photocurrent spectroscopy is a powerful in situ method which can be used to study the solid-state and dielectric properties of anodic films on metal~.l-~ These anodic films are important; they can play a role in corrosion and catalysis for instance, and if they are semiconductors they can be characterised by photoelectrochemical methods.In a detailed investigation of the formation of anodic films on titani~rn,~~’ we have combined photocurrent spectroscopy with more conventional electrochemical and optical techniques in an attempt to characterise the passivation process and the properties of the protective oxide layers. The results of these measurements have led to a clearer understanding of the solid-state properties of the anodic films formed on titanium under different electrochemical conditions, and at the same time have helped to clarify the mechanisms of film breakd~wn.~.’ It is appropriate to consider only the photoelectrochemical measurements here; further details of other electro- chemical and optical experiments are presented el~ewhere.~.~ EXPERIMENTAL Titanium disc electrodes, 0.5 cm2 in geometric area, were machined from commercial purity metal (IMI 130) provided by IMI Titanium.The discs were embedded into “ Kel-F ” holders with epoxy resin and mounted on a Teflon rod with an O-ring seal. The electrodes were ground flat and then polished on successively finer grades of diamond. A fresh surface was generated for each experiment by polishing the electrode on 1 pm diamond, followed by 0.05 pm alumina. For the measurements at potentials above 6 V us. SCE, the electrodes were rotated at a speed sufficient to prevent bubble formation during oxygen evolution. Measure-68 PHOTOCURRENT SPECTROSCOPY OF ANODIC FILMS ments were carried out in 1 mol dmV3 H2S04 and 1 mol dm-3 H3P04 solutions prepared from AnalaR grade chemicals and triply distilled water.Photocurrents were excited by chopped monochromatic light and detected by a lock-in amplifier. Absolute conversion efficiencies (electrons per incident photon) were measured using the photocurrent response recorded directly at a chopping rate t l Hz. The power of the incident radiation, which was measured with a calibrated RCA 935 photodiode, was in the range 2-50 pW in the region 300-400 nm. The electrochemical instrumentation was con- ventional, except that a 100 V/1 A potentiostat was modified as a programmable voltage source for measurements in the range 6-60 V. RESULTS GROWTH OF THIN OXIDE FILMS It is convenient to define "thin" oxide films in the present context as those formed on titanium at potentials < 6 V us. SCE (at pH 0).These films are < 30 nm thick, and they behave more simply than films formed at higher potentials. Titanium can be classified as a valve metal6 and in many ways it resembles in its behaviour metals like aluminium and tantalum. Although oxide films on titanium are generally less stable than their counterparts on the other valve metals, there is no doubt that the growth of insulating oxide takes place by the field-assisted migration of ions or vacancies. Titanium differs from other valve metals in the way in which the oxide film is apparently able to recrystallise at low over voltage^,^ and the breakdown of the film is important in applications to corrosion protection and electrocatalysis. The current efficiency for film formation can be less than unity, and this may go some way to explain the wide range of the characteristic growth parameters which have been reported in the l i t e r a t ~ r e .~ Our own measurement^^*^ have shown that za, the product of ion-charge and half-jump distance, is 0.9 nm for films grown in the range 0-2 V us. SCE in 1 mol dmW3 H2S04. The exchange current, i,,, for the injection of ions at the metal-oxide interface was found from galvanostatic and potentiodynamic measurements in the same potential range to be 6 x lo-" A cm-2, in reasonable agreement with the results of Ahrens et aL8 The interpretation of electrochemical measurements of oxide film growth is not easy. Calculation of the film thickness requires knowledge of the film stoichiometry, current efficiency, the density of the deposit and surface roughness.Similarly, evaluation of capacitance data is only possible if both the relative permittivity and surface roughness are known. For this reason, direct optical methods for the deter- mination of films thickness are valuable. Interferometry has been used for thick oxide films on t i t a n i ~ m , ~ * ~ whereas ellipsometry1°-12 can be used even for very thin films. Careful analysis of photocurrent data provides an alternative way in which to determine the thickness of the oxide layer, while at the same time providing informa- tion about solid-state properties. Ti02 is an insulating or semiconducting material, which occurs widely ip nature and can crystallise in three non-cubic forms, anatase, brookite and rutile. The photoelectrochemistry of reduced specimens of the crystal form rutile has been widely studied.The material is an n-type semiconductor with a band-gap of 3.06 eV, and it shows a characteristic anodic photocurrent at potentials more positive than Efb, the flat-band potential. Anodic films on titanium are also p h o t o ~ e n s i t i v e , ~ ~ ~ ' ~ ~ ~ although no detailed quantitative study of their photoelectrochemistry appears to have been made. Fig. 1 shows a cyclic voltammogram and the corresponding photocurrent measured during the growth of the oxide film on titanium at 100 mV s-l in H,P04. The photocurrent rises linearly with potential beyond 0.5 V vs.J . F . MCALEER A N D L . M. PETER 69 SCE, and on the reverse sweep the photocurrent falls again almost linearly with potential, although a tail can now be seen which extends as far as -0.3 V us.SCE. The observation that the photocurrent increased linearly with potential immediately suggested that the film thickens at a uniform rate, as would be expected for growth by the high field migration of ions or vacancies. On the other hand, the almost linear decrease of photocurrent with potential on the reverse scan is less readily understood, since an approximately square-root dependence of photocurrent on potential might be expected for a highly doped semiconductor. We have established that the properties of the anodic oxide film on titanium depend greatly on the rate at which the film is grown. The dielectric behaviour and donor distribution of films grown at sweep I -i -0 i 2 3 i 5 E/Vvs.SCE FIG. 1 .-Cyclic voltammogram (left-hand scale) and corresponding photocurrent iph for a titanium electrode in 1 mol dm-3 H3P04. Excitation wavelength 335 nm. Sweep rate 100 mV s-I. rates above 5 mV s-l are anornalo~s,~*~ and for the present we shall therefore focus attention on the properties of films grown at lower sweep rates (1 mV s-I). The photocurrent response observed during the slow potentiodynamic formation of the anodic film is shown in fig. 2. Two points are immediately striking. First, the increase of photocurrent with potential on the forward scan is divided into two linear regions, with a change of slope at ca. 1.5 V us. SCE. Secondly, the photocurrent on the reverse potential scan has the more familiar shape observed with single crystal semiconductor electrodes.We consider first the photocurrent during growth. At the high fields which are needed to overcome the potential energy barriers for ion migration (>lo6 V cm-I), the efficiency of charge-carrier separation must be essen- tially unity since the characteristic thickness of the space-charge region, the Schottky length, L,, is greater than the film thickness itself. Even if space-charge is present, the field at the metal-oxide contact must still be large enough to move ions across the70 PHOTOCURRENT SPECTROSCOPY OF ANODIC FILMS interface. Under these conditions, the whole of the anodic film is active in the photo- electrochemical conversion process, and if reflection at the metal-oxide contact is taken into account, it follows that the conversion efficiency, @, is given simply by @A = 1 - (exp - 2ccALf), (1) where t c ~ is the absorption coefficient of the oxide at the wavelength 1 and Lf is the film thickness.For sufficiently thin films, the exponential term may be linearised, and a linear dependence of @ on Lf is then predicted. In order to use the observed variation of @ with potential during growth of the film to derive the thickness potential relationship, the appropriate value of CCA is needed. -1 0 1 2 3 4 5 E/V us. SCE FIG. 2.-Photocurrent measured for a titanium electrode in 1 mol dmW3 H3PO4 during a cyclic sweep at 1 mV s-l. Excitation wavelength 335 nm. The reliability of this approach to the determination of film thickness was tested in the following way. First of all, a series of photocurrent excitation spectra were measured under the " steady-state " conditions obtained by holding the electrode at a given potential for the 10 min.Fig. 3 shows a set of photocurrent spectra converted to give the wavelength dependence of the conversion efficiency. These spectra all show a smooth increase of @ with photon energy, and the absence of any reduction of efficiency at short wavelengths suggests that surface recombination is negligible under these conditions. These conversion efficiencies were then used to construct the plots of --In (1 - 0) against electrode potential shown in fig. 4. Provided that is independent of film thickness, these plots should provide a direct measure of Lf as a function of potential. The absorption spectrum of the anodic oxide film is unknown, but Mollers et aZ.13 have reported the absorption spectrum of CVD films of TiOz, and we have used their values of aA to convert the data in fig.4 into a plot of Lf against electrode potential. The success of the method is evident in fig. 5 , which shows that the data obtained at different wavelengths fall onto a common straight-lineJ . F. MCALEER AND L. M. PETER 71 hlnm FIG. 3.-Spectral dependence of the photocurrent conversion efficiency, Q,, at a titanium electrode in 1 mol dmW3 H2SOo. The electrode potentials were (vs. SCE): (1) 1.0, (2) 1.5, (3) 2, (4) 3, ( 5 ) 6 V. Slit resolution 9 nm. plot which exhibits the same change of slope seen in the linear sweep measurement of photocurrent (fig. 2). The relationship between film thickness and voltage is often referred to as the " anodising ratio ".The concept arises from the inverse logarithmic law for high- field oxide growth,6 since it can be shown that the rate of thickening at constant poten- tial becomes negligible in a fairly short time. Reported anodising ratios for titanium lie in the range 1-5 nm V-l; the rather large scatter probably reflects the different techniques and assumptions used. The analysis of the photocurrent data in fig. 5 O.1 0 2 4 6 FIG. E/V us. SCE 4.-Treatment of the photocurrent spectra in fig. 3 according to eqn( 1). The wavelengths chosen for analysis are (a) 320, (b) 330, (c) 340 and (d) 350 nm.72 PHOTOCURRENT SPECTROSCOPY OF ANODIC FILMS gives an anodising ratio of 6 nm V’I up to 2 V and 3.6 nm V-l above 2 V. These results may be contrasted with the values calculated from fig.2, viz. 6.4 nm V-I below 1.5 V and 3.3 nm V-I from 1.5 to 5.4 V. We have also measured “ steady-state ” anodising ratios by ellipsometry using McCrackin’s program.14 For 10 V films, an anodising ratio of 3 nm V-l was found. Dynamic anodising ratios under linear scan conditions were also calculated from the za and io values obtained for the growth of the oxide in the potential region from 0 to 2 V us. SCE. At 1 mV s-’, the calculated a E/V us. SCE FIG. 5.-Apparent film thickness as a function of electrode potential. The plot was constructed from the data shown in fig. 4, using the absorption coefficients for TiOz given in ref. (13). Eqn (1) shows that the axis parameter is equivalent to the film thickness, Lr.Wavelengths as as follows: 0, 330; +, 340; A, 350 nm. anodising ratio was 3.5 nm V-l, very close to the value obtained from the photo- current measurements at potentials above ca. 2 V us. SCE. Rather lower values around 2.5 nm V-’ have been measured by reflectance and ellip~ometry,’-~~ but this is not surprising since the data were obtained at higher sweep rates than those used here. Since we have established 4*5 that the electron-donor distribution is non- uniform, it is not easy to relate measurements made at different sweep rates. A change in the properties of the film was also seen when the geometric capacity of the film was measured during growth at 1 mV s-l, although in this case the change in shape occurred at a lower potential (fig. 6). A problem arises immediately when fig.5 and 6 are compared. Whereas fig. 5 suggests that the film thickness is zero at 0.5 V vs. SCE, it is clear from fig. 6 that the inverse capacitance relationship is obeyed down to at least 0 V; clear evidence that the film is already growing at this potential. It is difficult to establish exactly where film growth commences, since a natural oxide film is already present at the beginning of the electrochemical experiment. We can conclude from the comparison of fig. 5 and 6 that the first few monolayers of oxide on titanium do not give rise to a photocurrent. There are two possible reasons for this. First, if the oxide is very thin, photo-excited carriers will be quenched by electronJ . F . MCALEER AND L . M. PETER 73 exchange with the metal.This mechanism is probably restricted to the first one or two monolayers of oxide. Secondly, the film formed at low overpotentials may not be TiO, at all. X.P.S. and Auger of the anodic film on titanium have sug- gested that the oxygen/titanium ratio in films formed below 2 V is only 1.6-1.7. The oxide film formed in air appears to be close in stoichiometry to TiO, whereas at low potentials Ti203 appears to be formed.” The photocurrent results suggest that a photo-inactive oxide grows on the titanium electrode at potentials less than 0.5 V us. SCE. This oxide appears to behave as a normal dielectric, since the plots of inverse capacity are linear. The unusually high value of the anodising ratio obtained from the analysis of the photocurrent below 2 V can be explained if it is assumed that growth is taking place at the surface between the lower oxide and the Ti02 film -1 0 1 2 3 4 5 E/V us.SCE FIG. 6.--Inverse capacitance of a titanium electrode measured during growth of the oxide film at 1 mV s-’ in 1 mol dm-j H3P04. Frequency 110 Hz. as well as at the TiO,/electrolyte interface. The TiO, film could then grow at the expense of the underlying photoinactive oxide. Alternatively, TiO, may be nucleated as islands in the lower oxide phase.” Whatever the mechanism below 1.5 V, it seems reasonable to conclude that the film is essentially TiO, beyond 2 V, and that further growth is restricted to the oxide/solution interface, ions being transported across the film by the electrical field. Armstrong and Quinn lS have observed an interesting peak at 1.7 V in the linear sweep voltammogram obtained with 20 nm evaporated Ti films in 1 mol dm-3 HC104. This peak, which is also associated with an abrupt change in surface conductance, may be related to a restructuring of the film which occurs when it is entirely converted to TiO, from the lower oxides.It is striking that this peak occurs close to the potential at which the photocurrent-voltage relationship changes its slope (fig. 2). The capacitance data were also used to estimate the dative permittivity of the oxide at potentials above 2 V us. SCE. The surface roughness of the electrodes, although remarkably reproducible, was unknown, so that an arbitrary surface rough- ness factor ,%’ = 1.5 was chosen. The relative permittivity, Ef, of films grown at 1 mV s-’ was then calculated to be 60. The cf values found for films grown at higher sweep rates was reproducibly lower, and changes in film structure are reflected in an increase of geometric capacitance of films with The changes are slow; films grown at 1 V s-’ and then left overnight at the formation potential were indis- tinguishable from films grown at 1 mV s-’.74 PHOTOCURRENT SPECTROSCOPY OF ANODIC FILMS PHOTOCURRENT-VOLTAGE CHARACTERISTICS OF THIN FILMS The photocurrent recorded during the reverse scan in fig.2 represents the current against voltage relationship for an oxide film of fixed thickness. In the case of the slowly grown film, an analysis of the curve was made taking reflection at the metal- oxide interface into account.In the absence of surface recombination, the photo- current conversion efficiency is given by l6*I7 exp ( - 2 4 ) 1 + 2aL, ' @ 2 ! 1 - where L,, the Schottky length, is related to the potential drop, Ay, in the semiconductor by and Lp is the diffusion length for holes. A plot of -ln(l - @) against A@ should therefore be a straight line with slope proportional to Ng3 and intercept -ln(l + 2aLp). The donor density can also be obtained from the well-known Mott-Schottky relationship and fig. 7 is the corresponding experimental plot for a film grown at 1 mV s-'. The linear relationship between C-2 and E observed experimentally indicates that the *1 6 N I L44 1 -1 0 1 2 3 4 5 EIV us. SCE FIG. 7.-Mott-Schottky plot for the oxide film grown at 1 mV s-I to 5.4 V us. SCE in 1 mol dm-3 H3P04 (see fig.6). The slope corresponds to a donor density of ca. 1 x lozo ~ r n - ~ (for Ef = 60, 92 = 1.5). distribution of electron donor states is spatially homogeneous. The intercept does not give the flat-band potential directly; the potential drop in the Helmholtz layer is not negligible since the field strength at the surface is appreciable at such a high donor density (lo2* ~ m - ~ ) . Correction for the potential drop in the Helmholtz layer can beJ . F. MCALEER A N D L . M. PETER 75 made if the capacity of the electrical double-layer is known,l* but in the present case the flat-band potential in 1 mol dm-3 H3P04 was estimated from the photocurrent onset to be -0.3 V us. SCE. The double-layer capacity was then calculated from the Mott-Schottky plot to be ca.40 pF cm-2. The Fig. 8 illustrates the treatment of the photocurrent according to eqn (2). 0.2 n 8 I I H W - 0.1. 0 / / /’ 0 ./. / / / I ’ 1 I I 1 2 3 Ap+/V+ FIG. 8.-Treatment of the photocurrent-voltage curve for the oxide film grown at 1 mV s-’ in 1 mol dm-3 H3P04 [see eqn (2)]. (-.--.--.) Theoretical line for L, = 0. (- - - -) Theoretical line for L, = 20 nm. experimental data correspond to an L, value of ca. 2 nm, although this estimate may not be reliable since surface recombination causes the current to decrease as the elec- trode potential is lowered. Similar low values of L, have been obtained in this way by Kennedy and Fresel’ for cc-Fe203, but values for thermally grown TiOa films are generally higher.” An interesting feature of these results is that they show that the Schottky length coincides with the film thickness at the formation voltage.This is clear in fig. 8, where the theoretical lines were drawn for the experimentally determined values of E and Nd. This result suggests that the film grows by high-field migration until the field at the metal-oxide interface is reduced to zero by the space charge in the film, i.e., until L, = Lf. The growth limit of anodic films at constant potential is therefore determined by the donor density, and a true steady-state thickness can be defined. The importance of surface recombination in the determination of 4~ is clear from fig. 8. The photocurrent begins to fall rapidly when the electrode potential is re- duced below 1 v. We therefore examined the photocurrent response to check that a true steady-state photocurrent was measured by the lock-in amplifier.The photo- current transients in fig. 9 show that at potentials above ca. 0.5 V, the photocurrent response is sufficiently ideal to justify quantitative evaluation. At lower potentials,76 PHOTOCURRENT SPECTROSCOPY OF ANODIC FILMS the photocurrents show some overshoot and decay, probably associated with trap- ping and recombination. Interestingly, very similar photocurrent transients were obtained with the oxide film grown at higher sweep rates, although in this case the photocurrent against voltage relationship indicates a very high density of trapping and recombination centres. Evidently the characteristic time constants for these bulk processes is very much shorter than the time-scale of the experiment (seconds).The very low value of L, (two to three orders of magnitude lower than for single crystal rutile) 2o shows that the minority-carrier lifetime in slowly grown anodic films 1 v 0.75 V 0.5 V 0.5 V 0.3 V 0.2 v 0.1 v H 1 s FIG. 9.-Photocurrent response to chopped illumination for an oxide film grown in 1 mol dm-3 H3P04 at 1 mV s-I to 5.4 V us. SCE. Excitation wavelength 335 nm. is small. It is usual to assume that the time taken for a minority carrier to cross the space charge region is very much smaller than q,, the hole lifetime. While this as- sumption is still justified in the case of the slowly grown films, it is no longer valid for films grown at sweep rates above 10 mV s-l. The effective mobility of charge car- riers appears to be reduced sufficiently that only a certain fraction of carriers generated inside the space-charge layer escape recombination or trapping.The almost linear dependence of the photocurrent on potential suggests that carrier transport in the space-charge region is dominated by field emission from traps.4 A quantitative treatment of the problem of photocurrents in amorphous anodic films is not available at present, although Williams and Wright,21 for instance, have shown that pulsed photopotential measurements may offer a useful experimental approach. THICK OXIDE FILMS ON TITANIUM The breakdown of oxide films on valve metals generally takes place at rather high voltages, and it appears to be accompanied by recrystallisation.Titanium behavesJ . F. MCALEER AND L . M. PETER 77 differently; oxygen evolution is already appreciable at potentials around 2 V us. SCE (at pH 0), and changes in film structure are evident at 10 V. Leach and Sidgewick7 have reported recently that the oxide film formed on titanium at low current densities recrystallises, and they have identified crystallites of brookite embedded in an amor- phous matrix. On the other hand, di Quorto et aZ.22 have shown that a minimum field strength is required to stabilise thick oxide films on titanium; a reduction of the current density below a critical value was found to result in a catastrophic breakdown of the film. Under these circumstances it is not possible to distinguish between re- crystallisation as cause or effect of the breakdown.The clearest evidence of film breakdown can be seen in Iinear sweep experiments, and fig. 11 shows that breakdown phenomena are evident on the forward and reverse 0.8 0.6 QP 0.4 0.2 0 280 300 3 20 340 360 380 h/nm FIG. 10.-Time dependence of the spectral response of a titanium electrode held at 30 V in 1 mol dm-3 H2S04; (1) immediately after film formation, (2) after 10 min and (3) after 15 min. sweeps, We have characterised both types of breakdown by a number of tech- n i q u e ~ , ~ . ~ including interferometry, impedance analysis and photocurrent spectro- scopy; a complete discussion is outside the scope of this paper which considers only the relevant results obtained by photocurrent spectroscopy. The " forward breakdown ", which occurs as the potential is increased in the linear sweep, appears to be a process of continued microfracture and repair of the film, whereas the " reverse breakdown ", which takes place when the potential is relaxed, appears to be a catastrophic process which is initiated by the nucleation and propagation of fractures in the film; the whole film appears to " explode ", and a burst of oxygen bubbles can be seen to form on the electrode.We first became aware that the oxide film on titanium was unstable during studies of the excitation spectra for films formed at constant potential. Above ca. 10 V, the magnitude of the conversion efficiency and its spectral distribution were found to be time dependent. The effect became more obvious as the formation voltage was increased, and fig.10 shows as an example how the spectral response of a 30 V film changed when the electrode was held at the formation voltage for 15 min. Clearly, the film has more than doubled in thickness in this time, and also the response at short wavelengths has changed considerably, so that the conversion efficiency now passes78 PHOTOCURRENT SPECTROSCOPY OF ANODIC FILMS 0 10 20 30 40 50 cell voltagelv FIG. 1 1 .-Cyclic voltammogram for titanium in 1 mol dm -3 H2S04, showing the " forward break- down " (1) and " reverse breakdown " (2). Sweep rate 1 V s-l. through a maximum. This behaviour is characteristic of surface recombination ; photons of higher energy are absorbed close to the surface where the effects of surface recombination have a predominant influence on the photocurrent.Surface recom- bination centres can be generated, for example, by the mechanical abrasion of single- crystal surfaces,23 and in the present case it seems reasonable to conclude that film breakdown gives rise to areas which are mechanically damaged. We have shown above (see, for example, fig. 2) that the photocurrent increases smoothly during film growth as more light is absorbed. If the oxide films were stable, the conversion efficiency should approach unity when virtually all incident light is absorbed in the film. In fact, this behaviour was not observed experimentally. At potentials above ca. 7 V, the photocurrent began to decrease and then levelled 0.2, a) 0.1 0 0 10 20 30 40 50 cell voltage/V FIG. 12.-Photocurrent conversion efficiency during a cyclic voltage scan (see fig.11). The electrode was rotated at 20 Hz during the experiment. Sweep rate 1 V s-l.J . F . MCALEER AND L. M. PETER 79 out at a constant but lower value. This effect was not simply due to the scattering of the incident light during oxygen evolution since the electrode was rotated in order to prevent bubble formation. Fig. 12 shows how the photocurrent changed during a linear sweep experiment. On the forward sweep, the photocurrent decreased to a roughly constant value, and on the reverse sweep it increased abruptly. A comparison of fig. 11 and 12 makes it clear that the " forward breakdown " is responsible for the initial reduction of the photocurrent, and we attribute this to the increased rate of recombination and trapping in the stressed and damaged oxide.The reverse scan is particularly interesting. The photocurrent increases suddenly when " reverse break- down " takes place, and impedance measurements show that the capacity of the film also increases abruptly at the same point. Apparently the oxide films breaks down completely at this point, and a new barrier film of about half the original thickness is formed underneath the damaged original layer. This type of film breakdown is particularly easy to miss in an experimental study. Ex situ studies of the oxide film necessarily involve the inadvertent formation of a film which has suffered the reverse FIG. 13.-Photocurrent conversion efficiency for a titanium electrode held at 50 V in 1 mol dmW3 HzS04; (1) before the reverse breakdown and (2) after the reverse breakdown.breakdown. The film appears to be stabilised in situ by the electrical field until a certain critical minimum value is reached when the film collapses. The sudden in- crease in photocurrent at this point probably shows that the total film thickness increases as a fresh barrier film grows under the fractured original. The effect of the reverse breakdown is also evident on subsequent potential cycles. It quite clearly results in the formation of a thicker film with a high density of recom- bination centres, and this is best seen in the corresponding changes in the photocurrent conversion efficiency and its dependence on wavelength. Fig. 13 illustrates the effect80 PHOTOCURRENT SPECTROSCOPY OF ANODIC FILMS of reforming a film which has undergone reverse breakdown.When the electrode is brought back to the original formation potential, the photocurrent spectrum changes considerably. First, it is clear from the response at longer wavelengths that the film thickness has increased by at least a factor of two. Secondly the fall in response at short wavelengths shows that breakdown and reformation gives rise to a high density of surface states, and electron microscopy has confirmed that blistering and cracking of the film do occur. These results show that photocurrent spectroscopy is a convenient in situ technique which is sensitive to changes in the thickness and structure of the oxide film on titanium. The need for such a technique is pressing when ex situ methods are excluded by the irreversible changes which can occur when the control of electrode potential is interrupted. Sensibly combined with other optical and electrochemical techniques, photocurrent spectroscopy has shown great potential as a method for the study of the growth and properties of semiconducting anodic films on metals.This study has also shown why anodised titanium films are unsuitable for photoelectrolysis ; thin films, although perfect in other respects, absorb too little light, whereas thicker films are mechanically damaged and are characterised by a high density of trapping and recombination centres. We thank IMI Titanium for advice and the supply of the titanium electrodes. J. M. also thanks the S.R.C. for financial support. L. M. Peter, Electrochim. Acta, 1978, 23, 1073. L. M. Peter, J. Electroanalyt. Chem., 1979, 98, 49. L. M. Peter, Surface Sci., in press. J. F. McAleer, Ph.D. Thesis (University of Southampton, 1980). J. F. McAleer and L. M. Peter, to be published. L. Young, Anodic Oxide Films (Academic Press, London and New York, 1961). ' J. S. L. Leach and D. Sidgewick, 5e Journe'es d'Etude du Titane et de Ses Alliages, Nantes (1978). M. Ahrens, K. D. Allard and K. E. Heusler, Werkt. und Korros., 1975, 26, 694. L. Arsov, M. Froehlicher, M. Froment and A. Hugot-Le-Goff, Compt. rend. C, 1974, 279, 485. lo C. K. Dyer and J. S. L. Leach, J. Electrochem. SOC., 1978,125, 1032. l1 D. Laser, M. Yaniv and S. Gottesfeld, J. Electrochem. SOC., 1978, 125, 358. l2 S. Gottesfeld, S. Srinivasan, M. Yaniv and D. Laser, J. Physique, 1977, C5, 145. l3 F. Mollers, H. J. Tolle and R. Memming, J. Electrochem. SOC., 1974, 121, 1160. l4 F. L. McCrackin, Nut. Bur. Stand. Tech. Note, 479, 1969. l6 V. A. Myamlin and Y. V. Pleskov, Electrochemistry of Semiconductors (transl.) (Plenum, New l7 Z. A. Rotenberg, T. V. Dzhavrishvili, Yu. V. Pleskov and A. L. Asatiani, Sou. Electrochem., la B. Pettinger, H. R. Schoppel,! T. Yokoyama and H. Gerischer, Ber. Bunsenges. phys. Chem., l9 J. H. Kennedy and K. W. Frese, J. Electrochem. SOC., 1978, 125, 709. 2o R. H. Wilson, Semiconductor Liquid-Junction Solar Cells, in Proc. Airlie Con$, 1977 (Electro- 21 D. K. Williams and G. A. Wright, Electrochim. Acta, 1979, 24, 1179. 22 F. di Quarto, H. Gerischer and K. Doblhofer, Electrochim. Acta, 1978, 23, 195. 23 H. Gerischer, F. Hein, M. Lubke, E. Meyer, B. Pettinger and H-R. Schoppel, Ber. Bunsenges. N. R. Armstrong and R. K. Quinn, Surface Sci., 1971, 67,451. York, 1967). 1977. 13, 1803. 1974,78,1024. chemical SOC., Princeton, N.J.). phys. Chem., 1973,77,284.

ISSN:0301-7249    

DOI:10.1039/DC9807000067

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

Correlation between the Non-stoichiometry of Titanium Dioxide and its Photoelectrochemical Behaviour BY JAQUES GAUTRON Laboratoire de Physique des MatCriaux Solides, Universitk de Tours, 37200-Tours, France AND PHILIPPE LEMASSON Laboratoire d’Electrochimie Interfaciale-C.N.R.S., 1 place Aristide-Briand, 92 190-Meudon Bellevue, France AND JEAN-FRANCIS MARUCCO Laboratoire des Composks non Stoeichiomktriques, E.R.A. 680 du C.N.R.S., UniversitC de Paris-Sud, 9 1405-Orsay Cedex, France Received 12th May, 1980 TiOz (rutile) has been reduced in well-defined conditions to give n-type TiOzhx semiconductors in which both the nature and concentration of the defects attributed to reduction are known. The influence of the value of x in TiOz-x on the efficiency of such a material when used as a photoanode is very marked and, in particular, there exists a value of x for which this efficiency is a maximum.The influence of x on the electrochemical behaviour of Ti02-, is only noticeable for electrodes whose active surface has been previously mechanically polished. Finally the broadening of the absorption spec- trum towards the visible as x increases can be attributed to the existence of deep acceptor levels which are excited by the light, giving free holes that can be involved in the OH- oxidation process. The use of electrolyte/semiconductor junctions for the conversion of solar energy was first demonstrated in the work on TiO, by Fujishima et al.’ Rather large effi- ciencies, ranging between 8 and 12%, have recently been obtained with such devices in which the semiconductor was either a 11-VI compound such as CdSe2 or a 111-V compound such as G ~ A s .~ With such semiconductors, the main problem is poor stability, i.e., the existence of decomposition mechanisms due to the influence of the electrolyte and/or illumination. On the other hand, some oxides like Ti02 are suit- ably stable but their high energy gap (ca. 3 eV) permits the transformation of only a small part of the solar spectrum. A broadening of the absorption spectrum of Ti02 towards the visible is possible by doping and, so far has led to a small increase being obtained in the visible range.4 More generally, the photoelectrochemical properties of a semiconductor are strongly connected with its photoelectronic characteristics, in particular the number and nature of intrinsic and/or extrinsic defects, the diffusion length of the minority carriers and the light absorption coefficient.Titanium dioxide TiO, can be used only after a reducing treatment at high tempera- ture which makes it n-type semiconducting at room temperature where it exists in the Ti0,- form, x being the oxygen deficiency. Its photoelectronic properties appear to be mainly connected with its post-reduction state. However, for this oxide, one of the most studied, paradoxically no serious correlation between its photoelectrochemical82 PHOTO E L E CTR o c HEMI s TR Y o F Ti02-x behaviour and its post-reduction state has hitherto been established to our knowledge. This undoubtedly arises from the fact that reduction has usually been achieved either in vacuum or in a hydrogen atmosphere.These treatments are carried out with ease, but, on the other hand, the defect nature is then very difficult to check. However, work by Subbarao et aZ.5 in which x is experimentally determined, albeit after hydro- gen reduction, should be mentioned. In the present work, we try to establish the correlation between the thermodynamic and photoelectrochemical properties of Ti02-x. Unlike the work cited above reduc- tion is achieved here by a thermodynamic equilibrium in such a manner that it is pos- sible to determine the number and nature of defects in TiOz-x at high temperature. By well-controlled quenching, the reduced material is cooled down to room tempera- ture. The electrochemical study is then made on the materials obtained using classical methods.EXPERIMENTAL SAMPLE PREPARATION The materials used were either single crystals [OOl] oriented (Djevahirdjian Co.) or pellets sintered from high purity Johnson-Matthey oxide powder (density ca. 0.9). BULK The results are presented in fig. 1. Reduction was achieved at 1100 “C by means of CO + CO, reducing mixtures in order to control the oxygen partial pressure as well as pos- sible. At thermodynamic equilibrium, both the electrical conductivity 0 and the departure from stoichiometry x (by thermogravimetry) is measured (sensitivity 5 x lo-’ for x). The resulting data have been reported previo~sly.~ Before quenching, the CO 3. C02 mixture is removed and changed for purified argon. During this operation, it is verified that both 0 and x remain unchanged. The equilibrium phase is thus fixed and the defect distribution in the material bulk becomes more homo- geneous.The quenching is then achieved by removal from the furnace, the sample cooling rapidly down to room temperature. As the non-stoichiometry does not change experimentally, this implies that the defect concentration originating in the reduction process remains constant. On the other hand, the diminution of the electrical conductivity demonstrates that the ioniza- tion state of these defects diminishes, this fact being emphasized by the increase in the drift mobility. Using the classical formalism developed by Kroger and Vink, the defect state of TiOz-, can be described schematically as follows: at large reductions (large x), the donors are Ti:*; these donors are changed for V> at smaller reductions.In the latter case, the existence of acceptors must also be taken into account: Vk;” for high-purity Ti02 and Meki when tri- valent impurities of the Me type (Cr, Al, Fe . . .) exist in the original material *.’ SURFACE The electrical properties of the samples indicate that they are homogeneous, at least in the bulk. During quenching, a surface reoxidation (resulting in a “ crust ”) is difficult to avoid and, as long as the electrochemical properties of the material are mainly dependent on the surface quality, it is necessary to remove this crust. In order to demonstrate this phenomenon, two different surface preparations were used * In fact, this type of impurity always exists in TiOz.J .GAUTRON, P . LEMASSON AND J . F . MARUCCO 83 0 -1 b E: M I -2 -3 -15 -10 - 5 log10 Po, -2 -3 -4 7 0 FIG. 1 .-Typical plots of log,, cr against log,, por at 1100 (a) and 20 "C (b) after quenching, and of log x against loglo por at 1100 "C for TiOz (c). for the single crystals: the first one, which allows electrochemical measurements as a function of x, combines mechanical polishing and chemical etching; the second one is only an etching. In both cases, the etching mixture is that proposed by Tamura et al: (NH4)$304 + HzS04 (1 : 1) at 250 "C for 1 h. As demonstrated by our measurements, this type of etching does not produce a noticeable surface reoxidation. APPARATUS A N D MATERIALS ELECTRODES Ohmic contacts at the electrodes were of indium attached by means of ultrasonic solder in the laboratory atmosphere. A gold wire was then soldered to these contacts.Back and sides of the electrode were insulated from the electrolyte with the aid of epoxy resin (Scotch- cast, 3M Co.). No further treatment, either mechanical or chemical, was applied to these electrodes as single crystals. CELL The cell was of the classical three-electrode type with a quartz window for light passage and a gold counter-electrode. The reference electrode used was a mercury-mercurous-84 P HOT0 E L E C TROC H E M I STR Y 0 F Ti02-x sulphate one immersed in a saturated potassium sulphate aqueous solution. by e.s.m. (0 V us. e.s.m. = +0.65 V us. NHE). We denote it ELECTROLYTES Solutions were prepared with Millipore purified water and chemicals were Merck Suprapur grade.Three different media have been used: molar H2S04 and KCl and decimolar KOH. Unless otherwise specified, results are reported only for the last case. The electrolytes were in contact with the laboratory atmosphere. APPARATUS Cyclic voltammetry was performed with the aid of a potentiostat (Wenking 66TS1) con- trolled by a voltage scan generator (Wenking VSG72). Impedance measurements were made at 100 kHz: the electrical perturbation was applied as a low-amplitude current; the response of the electrochemical cell was measured with the aid of a double phase-sensitive detector (Brookdeal 411) and analysed as a series circuit. In the photoelectrochemical measurements, the illumination was provided by a 150 W xenon arc lamp and a monochromator (Jobin-Yvon HRS2).If necessary, light was chopped at a frequency of 140 Hz and, in this case, the currents were analysed by means of a single phase-sensitive detector (PAR 121). RESULTS ELECTROCHEMICAL CHARACTERIZATION IN THE DARK CYCLIC VOLTAMMETRY As already demonstrated by many authors, the TiOz-x oxides are stable as electro- chemical electrodes. In the voltammetric measurements we performed, the principal aim was to find the potential range to use in further electrochemical measurements and, incidentally, to demonstrate that the electrodes we used showed no anomalous behaviour in comparison with those previously reported in the literature. In 0.1 mol dmW3 KOH, the potential range was determined to be - 1.8 to +2.5 V us. e.s.m. IMPEDANCE MEASUREMENTS In the range defined above, the impedance measurements were interpreted with the aid of a classical Schottky model for the semiconductor-electrolyte j u n ~ t i o n .~ Following the usual simplifying hypothesis, we assumed that the whole potential drop took place in the space-charge layer of the semiconductor and that the space- charge capacity was equal to the measured capacity C . Although results for this type of measurements differ from one author to another on these materials, they are important here for two main reasons: (i) they give an indication on the free carrier density in the space-charge region; (ii) the flat-band potential determination enables the semiconductor energy levels to be located with respect to the reference electrode. It is important to note that the Schottky plots for the three types of electrodes are straight lines in a potential range of ca.2 V. The results obtained in the particular case of mechanically polished and chemically etched single crystals are presented in (1) Polishing + etching: the free carrier densities given by the C-’ against Y fig. 2. slopes are in good agreement with those deduced from the relation u = npeJ . GAUTRON, P. LEMASSON AND J . F . MARUCCO 85 where 0 is the electrical conductivity, e the electron charge, n the electron concentration and p the drift mobility of the electrons. The results obtained are shown in table 1 where a value p = 0.3 cm2 V-' s-l determined by us6 has been used. (2) Etching withoutpolishing: For - 12.5 < log poz < -8.4, the values found for ND from the capacity measurements range between 4 x 10l8 and 6 x 1OI8 ~ m - ~ .10i1F-2 cm4 -2 -1 0 1 ViV us. e.s.m. Fr~.2.--Schottky plots for the interface Ti02-,/0.1 mol dm-3 KOH for various x values: (a) 4 x lot6, (6) 9 x lo", (c) 2.1 x lot8, ( d ) 7 x lot8, ( e ) 9 x 1Ol8, (f) 2 x loL9. The experimental points are reported only for (f), x = 47 x (ND = 2 x lOI9 crn-9, indicating a slight deviation from the straight line at low and large potential values. The deviations are identical for the other x values. The approximate constancy of ND implies that, during the quenching, the crystal sur- faces are re-oxidized to about the same degree and that they do not follow the stoichio- metry of the crystal bulk. This observation confirms the importance of surface treat- ment in electrochemistry.(3) Sintered electrodes: The electrode surface is removed by a rough polishing. As the true surface areas are difficult to define directly, impedance measurements provide an opportunity to obtain them by assuming that the ND value which appears in the expression of the Schottky slope equals the n value thermodynamically determined (in this particular case, we used a value p = 0.5).6 This leads to an effective roughness factor of ca. 3.86 P H o TO EL E c TR o c HEM IS TR Y OF Ti02+ TABLE CO COMPARISON OF ELECTRON DENSITIES IN THE VARIOUS CRYSTALS REDUCED AT DIFFERENT Po2 VALUES CALCULATED FROM THE 0 MEASUREMENTS AND THE c-2 SLOPES log10 Po2 X n / ~ r n - ~ N , / c ~ - ~ (0 measurements) (C-2 slope) -14.5 47 x 1 0 - 4 3.2 x 1019 2 x 1019 -12.5 18 x 10-4 1.1 x 1019 0.9 x 1019 - 10.5 7 x 10-4 0.5 x 1019 0.7 x 1019 - 9.4 4.5 x 10-4 3 x 1Ol8 2.1 x 1Ol8 -8.4 3.3 x 10-4 1.3 x 10l8 0.9 x 1Ol8 -8.0 2.85 x a 2 x 1017 4 x 10l6 The large discrepancy between the two values is easily understood from the shape of the Q against po2 plot at large log po2 values (fig.1). For the three different types of electrodes, for a given electrolytic medium, the flat-band potential values are similar. and in molar HzSOJ leading to a pH effect of ca. 60 mV per unit, in good agreement with the literature. In 0.1 mol dm-j KOH, we obtain Vfb = -2.3 V vs. e.s.m. V,, = -1.6 V us. e.s.m. PHOTOELECTROCHEMISTRY (a) INFLUENCE OF x Single-crystal electrodes polished + etched as previously indicated were used as photoanodes.For these experiments, the photocell characteristics have been deter- mined for a wavelength of 335 nm: for wavelengths < 340 nm, the photoresponse of the electrodes for different x values is constant and a maximum [fig. 6(c)]. The results obtained are collected in table 2 for xvalues ranging between 2.4 x and 47 x loe4. The photocell efficiency as a function of the donor concentration n is plotted in fig. 3. For n 21 5 x lo1* ~ r n - ~ , the efficiency is a maximum. This result is in agree- ment with that of Tamura et aL8 obtained with samples reduced in argon atmosphere and for an incident light wavelength of 400 nm. TABLE 2.-PHOTOCELL MEASUREMENTS AT DIFFERENT X VALUES. vsc IS THE SHORT-CIRCUIT VOLTAGE, Js, THE SHORT-CIRCUIT CURRENT, vOc THE OPEN-CIRCUIT VOLTAGE AND ?j' THE EFFICIENCY. -7.6 - 8.0 - 8.4 -9.4 - 10.5 - 12.5 - 14.5 X n - vsc JSC - vo, 2.4 x 2 x 10l6 0.513 0.85 0.964 2.85 x 2 x lW7 0.527 1.3 0.960 3.34 x 1.3 x lo1* 0.535 1.67 0.959 4.6 x 3 x 10'' 0.532 2.37 0.973 6.98 x 5 x 10l8 0.540 2.88 0.994 17.9 x 1.1 x lOI9 0.521 2.13 0.967 47 x 10-4 3.2 x 1019 0.507 1.17 0.970 / ~ r n - ~ /V vs.e.s.m. /PA cm-2 /Vus. e.s.m. rl (%) 4.4 6.5 10.2 16.0 21 .o 11.3 7.4J . GAUTRON, P . LEMASSON AND J . F . MARUCCO 87 FIG. 3.-Efficiency of photocells having Ti02-x anodes as a function of the Nd value. For a given wavelength, A, the penetration depth I of light into the semiconductor is given by where a. is the absorption coefficient value of the semiconductor at 1. The space charge-region in the semiconductor (i.e., the semiconductor region where an electric field exists that can separate the light-created charges) has a thickness W expressed by where E is the static dielectric constant of the semiconductor and V, is the surface potential.In the particular case of our photocell efficiency measurements, V, is constant and has the value 1.4 V with an accuracy of 50 mV. The dielectric constant value used is that measured in the direction parallel to the c crystallographic axis and its value is 173. For large x values, the space-charge region is thin and the electron-hole pairs created therein are efficiently separated by the large electric field F ( cc Na) existing in this region. On the other hand, pairs created out of this region have a small lifetime as a result of the small diffusion length of minority carriers (Lp N 0.2 and do not contribute to the photocurrent.For small x values, the space-charge layer is thicker and all the incident light is absorbed in it but the electric field is too weak and not effectively used in the whole region where it exists. Therefore, it seems logical to imagine that there exists a value (ND),-, for ND which corresponds, for a given 1, to a maximum efficiency. This condition can be accounted for by the relation Taking a. = lo5 cm-l,ll together with the E and V, values previously indicated, we obtain (ND)o = 3 x lo1' cm-3 in good agreement with our experimental results. I = l / a , u/ = (2&&0/d?~)* v$ w = l/cc,.88 P H o TOEL E c TR o c H E MI s TR Y OF Ti02+ (b) SURFACE INFLUENCE As often mentioned, surface preparation plays an important role in the shape of the photoresponse.12 On the other hand, as we demonstrated previously, it is impor- tant to know the surface non-stoichiometry. In fig.4 are shown the photocurrent-potential curves obtained with three different electrodes at constant x. The single-crystal electrodes when only etched give a steeper photocurrent rise than those which are both polished and etched. This observation 3 2 1 0 -1 V/V us. e.s.m. FIG. 4.-Dhotocurrent plotted against applied potential for various electrode types : (1) single-crystal polished + etched, (2) single crystal etched, ( 3 ) sintered electrode. can be interpreted as corresponding to a smaller recombination process in the former than in the latter case.Note that the photocurrent obtained with a sintered electrode presents a plateau at a much lower current value than do the single crystals, the current rise, however, being steep: in this case, the L, value is presumably lower than in a single crystal. In fig. 5 are shown the measured photocell characteristics for two single-crystal electrodes of identical x, one being only etched the other both polished and etched. This comparison reinforces the previous observation on the influence of surface recom- bination centres mainly due here to the great amount of dislocations produced by the polishing and difficult to eliminate by a chemical etching. The efficiency obtained is twice as big with an etched electrode as with a polished and etched one. From the observations just presented, it follows that there exists an incompati- bility between obtaining a well-defined product (e.g., characterised by the x value) and that of a product with interesting practical features.(C) INFLUENCE OF THE NATURE OF THE DEFECTS The results presented above show the influence of only one parameter (x) connected to the concentration of defects produced by reduction but not to their nature. In fig. 6(a), where we report the spectral responses of two different sintered elec- trodes (one highly reduced, x = 47 x the influence of the nature of the defects is clearly demonstrated : in the latter case, a spec- tral broadening towards the visible range appears with, however, much lower current values than at the absorption maximum. and the other weakly, x = 1.8 xJ .GAUTRON, P . LEMASSON AND J . F . MARUCCO 89 This result should be correlated with those reported by Ghosh and Maruska4 on Ti02 doped with various transition elements : they attributed the observed broadening to the dopants. In order that a more accurate comparison can be achieved, we prepared, in the same reducing conditions, Cr doped electrodes ([Cr]/[Ti] = 5 x lo-'). The results obtained are reported in fig. 6(b) and are closely similar to those in fig. 6(a). On first analysis, reduction seems to play a prominent role in comparison with the introduced impurities. This seems quite normal as the doping by an impurity of type 3 is chemi- cally equivalent to a red~cti0n.l~ In order to demonstrate the influence of the specific 5 4 3 N I 5 d ;;;' 2 1 0 -0.5 -1 VlV us.e.s.m. FIG. 5.-Compared efficiencies of two different photocells having the same non-stoichiometry with two' different surface preparations: (- - - - - -) etching alone q = 43%; (--*-*- ) etching + polishing q = 21%. The electrolyte is 0.1 mol dm-3 KOH. absorption of the impurities, a study with variable concentrations would be necessary. In both cases, the broadening can be attributed, following the defect equilibrium associated with the reduction and/or the existence of dopants, to acceptor levels located at the bottom of the forbidden gap.* These levels may be due to titanium vacancies (V&'') and/or chromium in titanium sites (Crki). An energy diagram can then be established that enables an interpretation of the observed photocurrents.Generally speaking, the semiconductor is more or * These acceptors have a large ionization energy of ca. 1 eV.P HO TOE L E CTROC HE MI STR Y OF Ti02-x 390 440 490 540 590 390 440 490 540 590 waveIengt h/nm wavelength/nm U C h E .2 36% p ( C ) -D a- . 290 340 390 440 wavelength/nm FIG. 6.-(a) Photocurrent (arb. units) as a function of light wavelength for sintered TiOZwx electrodes (6) Photocurrent plotted against light wavelength for TiO2-,-Cr sintered electrodes of large and small degrees of reduction: (* * * * .) x = 15 x (c) Typical photocurrent against light wavelength plot for TiOz-x electrodes. of large and small degrees of reduction: (. . . . .) x = 47 x (- x = 1.8 x 10-4. (- ) x = 2 x EC - - H?- FD I I I I 1 I I I I I I I I high x Low x semiconductor TiO, - x eV 2 H+/H 1 -1 electrolyte KOH FIG.7.-Schematical representation of the energetic transitions in Ti02-x excited by light. At large x values, an electron is excited from the acceptor state to the conduction band and its corresponding hole in the acceptor level EA recombines with a conduction band electron. At low x values, after the excitation, the hole remains available for the OH- oxidation process. In the figure are also reported the donor levels ED and EA corresponding to two different x values.J . GAUTRON, P . LEMASSON AND J . F . MARUCCO 91 less compensated, this compensation depending on the relative proportions of the donor and acceptor concentrations. During the quenching, the acceptors are ionized, the electrons coming more probably from the donors than from the valence band.The excitation of an electron from the acceptor level to the conduction band creates a hole in the former; if the material is weakly reduced, the recombination probability of this hole will be rather small, and therefore it will be available for the OH- oxida- tion process; on the other hand, if the reduction is large, the recombination probabi- lity of the hole is large and its efficiency i n the oxidation process is zero. In this way, the experimentally observed progressive broadening of the absorption spectrum can easily be accounted for as x continuously diminishes. A. Fujishima, K. Honda and S . Kukuchi, Kogyo Kogaku Zasschi, 1969, 72, 108. A. Heller, K. C. Chang and B. Miller, J. Electrochem. SOC., 1977, 124, 697. B. A. Parkinson, A. Heller and B. Miller, Appl. Phys. Letters, 1978, 33, 521. A. K. Ghosh and H. P. Maruska, J . Electrochem. SOC., 1977, 124, 1516. S. N. Subbarao, Y. H. Yun, R. Kershaw, K. Dwight and A. Wold, Mater. Res. Bull., 1978, 13, 1416. J. F. Marucco, J. Gautron and P. Lemasson, J. Phys. Chem. Solids, in press, ’ J. F. Marucco, J. Gautron, P. Lemasson and J. P. Loup, Compt. rend. C, 1979,289, 117. a H. Tamura, H. Yoneyama, C. Iwakura and H. Sakamoto, J . Electroanalyt. Chem. Interfacial Electrochem., 1977, 80, 357. V. A. Myamlin and Yu. V. Pleskov, Electrochemistry of Semiconductors (Plenum Press, New York, 1967). D. M. Eagles, J. Phys. Chem. Solids, 1964, 25, 1243. lo H. P. Maruska and A. K. Ghosh, Solar Energy Mater., 1979, 1,237. l2 R. H. Wilson, L. A. Harris and M. E. Gerstner, J . Electrochem. Soc., 1979, 126, 844. l3 P. Kofstad, Non-Stoichiometry, Difusion and Electrical Conductivity in Binary Metal Oxides l4 D. N. Mirlin, I. I. Reshina and L. S. Sochava, Sov. Phys. Solid State, 1970, 11, 1995. (Wiley, New York, 1972).

ISSN:0301-7249    

DOI:10.1039/DC9807000081

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

GENERAL DISCUSSION Mr. M. P. Dare-Edwards (Oxford) said : The evidence put forward by Prof. Bard for “ Fermi-level pinning ” on p-type GaAs comes only from measurements and correla- tion of the photovoltage behaviour of the electrode in the various electrolytes. No correlation is demonstrated between the dark-current behaviour and the standard potentials of the redox couples. Indeed, similar work performed here in Oxford has shown that despite the similarity in standard redox potentials of the 13-/1- and Fe3+/ Fe2 + couples, there is totally different dark-current behaviour of electrodes immersed in the two media. The time-dependent responses of the electrodes towards illumina- tion is also markedly different for these two couples. Much of Prof. Bard’s work was carried out using chopped illumination which, unfortunately, masks much of the time- dependent behaviour.There are also no reported measurements of flat-band poten- tials of p-GaAs in these media which may provide important additional evidence for Prof. Bard’s theory. From the observed results, it would seem more important to correlate behaviour with the detailed chemical interactions at the electrode/electrolyte interface than to use a generalized “ Fermi-level pinning ” model. From our own work, there is an obvious chemical interaction between p-GaAs and the I:/I- medium, seen by the appearance of a clear surface film on the electrode even following short periods of use. Another more general point is that the minimum concentration of surface states required to achieve “ Fermi-level pinning ’’ will be highly dependent on the nature and quantity of compensating ionic charge adsorbed within the inner Helmholtz layer.Prof. Bard alludes to the behaviour of “ Fermi-level pinned ” electrodes as if coated with a monolayer of metal. A.c. cyclic voltammetry of p-GaAs following long periods of use at cathodic potentials in the dark or after illumination reveals the presence of surface species which may be considerably enhanced by deliberate addition of Ga3+ to the electrolyte. The beha- viour correlates well with literature results on pure gallium metal. Use of p-GaAs for periods of up to 24 h can produce bulk Ga metal at the surface, sufficient to be seen with the much less sensitive d.c. cyclic voltammetry. As a final point, in the reduction of PhN02 by p-WSe,- it should be made more clear what Prof.Bard means by the 0.095 mol dmY3 PhNOi- produced at the Pt electrode being a “ small amount ”. At such levels quoted in the paper, specific ion adsorption of the PhN0;- could produce a negative flat-band shift sufficient to explain the observed results. This allusion may, in fact, be a reality. Prof. A. J. Bard (Texas) said: The “ Fermi-level pinning ” model at GaAs is drawn from a number of different experiments, involving redox couples which probably do not involve specific adsorption, in aqueous and non-aqueous (e.g., MeCN and liquid NH3) solutions. For p-GaAs in aqueous solution, we find that the onset photopoten- tial, 0.75 V us. SCE, in the presence of the I-/I, couple, is far more positive than the reported flat-band potential, Vfb, in the absence of redox couples 0.1-0.75 V us.SCE, at pH 0.1-3 This positive shift of the flat-band potential cannot be explained by the specific adsorption of the negatively charged ions (I- or I;). As indicated above, the reported values of Vfb of p-GaAs in aqueous solutions show a good deal of scatter,94 GENERAL DISCUSSION probably because of surface effects. We have used impedance techniques to measure V,, of p-GaAs in supporting electrolyte with or without redox couples. The results are complicated, especially in the presence of redox couples, which show interferences because of Faradaic processes. Results on the dark currents for p-GaAs with these couples have been previously reported (see, e.g., fig. 1, 3 and 4 in Fan and Bard).4 Continuous illumination experiments (for up to 8 h in an I-/I< solution) are also re- ported in this paper.Of course a more detailed picture of the interface would be desirable and clearly kinetic factors and the double-layer structure play a role in the behaviour of redox couples at semiconductors. However, the Fermi-level pinning model appears to explain quite well a number of observations at G ~ A S ~ B ~ and Si,6 especially when compared with the ideal model usually employed. - O A - 06 I 4 8 4 4 \ 4 0 36 energy /eV FIG. 1 .-X-ray photoelectron spectra of two p-GaAs electrodes. Electrode A was maintained at 0 V. Electrode B was scanned to -2.1 V and illuminated in liquid NHs containing 0.1 mol dm-3 KI. (From unpublished work by R.Malpas, K. Itaya and A. J. Bard.) With respect to the question of formation of bulk Ga metal at the surface, small amounts of Ga formation can probably occur under some circumstances. For example Kohl has found production of less than monolayer amounts of Ga on photo- reduction of p-GaAs in aqueous solutions. It is unlikely, however, that bulk Ga metalGENERAL DISCUSSION 95 deposition can explain the behaviour in most of the experiments reported for GaAs. For example, we considered this possibility in our original work on the GaAs/CH,CN interface5 and rejected it based on X-ray (X.P.S.) and Auger electron spectroscopic analysis of the electrode following use. No evidence of an increase in the relative amount of Ga was observed in this case, nor in the case of p-GaAs used to photo- generate solvated electrons in liquid ammonia, which represents rather drastic reduc- ing conditions.8 In the latter case two identical p-GaAs electrodes were placed in a liquid NH3 + 0.1 mol dm-3 KI solution with electrode A maintained at 0.0 V and electrode B scanned under illumination to potentials where solvated electrons are generated.After washing the electrodes and dismantling the cell in an inert-atmo- sphere glove box, the electrodes were transferred to an X.P.S. spectrometer. The As 3d peaks in the X-ray photoelectron spectrum for these electrodes are shown in fig. 1. The peak at 41 eV on both spectra has been previously assigned to As in GaAs, with the shift in binding energy between this peak and that for pure As metal (-0.7 eV) caused by the increased negative charge on the As.The significant difference between the two spectra is the change in the ratio of this peak to that centred around 44 eV. The latter peak has been previously assigned to an arsenic-oxygen species (probably AS,^^).^ Thus at 0 V, where the As/As,O, peak ratio is small, it appears that a sur- face oxide film exists on the electrode, whereas at -2.1 V, where the largest photo- effects are observed, the peak ratio is large suggesting reduction of this oxide film, thus producing a cleaner GaAs surface. The removal of the oxide films from anod- ized GaAs, by etching with HC1 or aqueous NH, solutions, has previously been shown to yield a surface rich in elemental AS.^ The Ga peaks in the X.P.S. spectra of the two GaAs samples were essentially the same.Finally Ga metal would be thermodynamically unstable in the acidic aqueous solu- tions and in the presence of oxidants such as 1, or Fe3+. The concentration of electrogenerated PhN0;- in the p-WSe, experiment was 0.095 mmol dm-3. Thus the negative shift of the onset photopotential V,, of p-WSe, due to specific ion adsorption of the PhNOS- seems unlikely. An experiment has been performed by dipping a p-WSe, electrode in a PhNOi- solution for a few minutes. After rinsing the electrode with solvent, a photovoltammetric experiment was per- formed with this electrode in a PhNO, solution. No shift of Yo, was observed com- pared with an electrode which had not been pretreated with PhNOI-. L. J. Handley and A. J. Bard, J. Electrochem.Soc., 1980, 127, 338. W. H. Laflere, R. L. Van Meirhaeghe, F. Cardon and W. P. Gomes, Swface Sci., 1976,59,401. R. Memming in Semiconductor Liquid-Junction Solar Cells, ed. A. Heller, Proceedings of a Con- ference on the Electrochemistry and Physics of Semiconductor Liquid Interfaces under Illumi- nation held at Airlie, Virginia, 1977, p. 38. F. R. F. Fan and A. J. Bard, J. Amer. Chem. Soc., 1980, 102, 3677. P. A. Kohl and A. J. Bard, J. Electrochem. Soc., 1979, 126, 59. A. B. Bocarsly, D. C. Bookbinder, R. N. Dominey, N. S. Lewis and M. S . Wrighton, J . Amer Chem. SOC., 1980, 102, 3683. ’ P. A. Kohl and F. W. Ostermayer Jr, 158th Meeting, Electrochem. SOC., Sept. 1980, Abstr. 289 J. Electrochem. SOC., 1980, 127, 371C. * R. E. Malpas, K. Itaya and A. J. Bard, to be published.C. C. Chang, P. H. Citrin and B. Schwartz, J. Vac. Sci. Technol., 1977, 14, 943. Prof. L. R. Faulkner (Urbana) said : There has been discussion among investigators interested in chemically modified semiconductor electrodes about the possibility that electroactive groups chemically attached to the surface could serve as agents for pin- ning the Fermi level of the electrode. The idea is that these groups act essentially as a set of surface states. However, it is important to note a difference between that96 GENERAL DISCUSSION situation and the usual idea of a surface state as part of or at least a rigid termination of the lattice. The distinction is that most chemical modification schemes involve flexible connection of electroactive species to a surface, whereas surface states are generally considered as rigid.In order for a collection of surface states to pin the Fermi level, they must offer a significant capacity for accepting net charge into a surface layer, so that the voltage drop across the Helmholtz layer changes as the electrode potential. Conventional surface states have this property. On the other hand, electrodes modified with electro- active agents usually do not because their flexible connections allow charge compen- sating ions to mingle among the electroactive sites. Thus charge transfer into the layer does not necessarily result in either a net charge in the layer or a change in the potential drop across the Helmholtz layer. In order for electroactive species at a chemically modified electrode to act as agents for Fermi-level pinning the attachment scheme must not allow penetration of their plane by counter ions.Prof. A. J. Bard (Texas) said: Prof. Faulkner’s point is certainly well-taken. If the attached electroactive group is predominantly in the diffuse double layer, it will essentially behave as a solution reactant and not cause pinning. However, some modi- fied electrodes (e.g., those involving “ adsorptive attachment ”) probably have a signi- cant amount of the species at the surface which can exchange charge with the electrode. These would behave as surface states. Prof. J. O’M. Bockris and Dr. S. U. M. Khan (Texas) and Dr. K. Uosaki (Sapporo) said: We wish to comment on the roots of the idea of double-layer pinning, suggested by Bard et al.We consider this concept to have been contained in the work of Mino Green, who published the first treatment of the structure of the double layer at a semiconductor/ solution interface in 1959. Green’s version of the situation concerning the movement of potential within and without the semiconductor, during change of electrode potential, was, however, qual- itative.’ His paper shows a figure portraying the double layer at a semiconductor/ solution interface in the absence and presence of surface states. The double-layer pinning model which Bard et al. present is tantamount to that of Green in the presence of surface states. Green published numerical calculations of the degree to which a change of the electrode potential would be partitioned over the potential drop inside the semiconductor and over the Helmholtz double layer.Typically, at low surface state concentrations, and with an electrode potential change of several hundred mV, the change over the Helmholtz layer was a few mV. However, at higher surface states, e.g. (1/10 ionized), the p.d. in the double layer for a 1 V change of the electrode potential approached 1 V, with the corresponding change in the interior being reduced to a few mV. The small change of the p.d. within the semiconductor implies, for a doped semiconductor, a limitingly small change in the position of the Fermi level. One may call the stationary Fermi level pinned. However, we should like to suggest that the term “ double-layer pinning ” for these happenings does not indicate what is happening as well as the descriptions “ inner p.d.change dominant ” and “ Helmholtz change dominant ” (respectively, for the case for which the potential is pinned at the surface of the electrode, and for which the Fermi level is pinned). The former descriptions help understand the relevant model, particularly in respect to the electrode kinetics, which each limiting condition would imply. In nearly all the papers published in photoelectrochemistry to date, the first caseGENERAL DISCUSSION 97 treated by Green (internal potential difference dominant) has been gratuitously as- sumed to be the relevant one, and the possibility that there will be sufficient surface to make the second case relevant has been neglected.2 Which is the more general case ? Although the requisite experiments have not yet been done, it seems likely that the high surface state and Helmholtz p.d. dominant is more general than the case usually treated.The evidence is circumstantial, and suffers under our poor knowledge of the nature and concentration of surface states. These are assumed to arise from localized bonds on the surface. However, it seems likely that they could arise from bonds which arise from the solution side. For example, in hydrogen or oxygen evolution, and if the steady state of an intermediate adsorbed chemical radical is high, each bond of such a radical to the surface would be associated with a surface state. As the con- centration of these states would vary with potential, they would affect the Helmholtz p.d. during a variation of the electrode potential.Correspondingly, the specific adsorption of ions on the surface of semiconductors seems likely to occur, and could cause surface states to occur in the presence of redox systems. Specific adsorption has been measured, for example by Genshaw, Paik and Bockris, on the surface of so-called iron and chromium which was likely to consist of oxide films, and found to be far higher than the corresponding adsorption with which we are used to dealing on rner~ury.~ The attainment of full coverage was reached for small concentrations of ions in the solution. This suggests that specific adsorption occurs with a large degree of charge transfer so that there is little repulsion between the adsorbed anions and such a picture may apply to the semiconductor/solu- tion interfaces, implying surface states at more than 0.1 of full coverage.As specific adsorption is potential dependent, there may be a correspondence between the surface states implied and changes in the Helmholtz double layer. If these thoughts are applicable, then diagrams which are drawn in photoelectro- chemical models would have to be modified to take account of the change of Helmholtz potential with electrode potential. It would become more likely that the photocur- rent/potential relation at a semiconductor/solution interface would be rate controlled, at low biases, by transfer of holes and electrons through the double layer (the quan- tum efficiency being controlled by the ratio of the rate constant for this passage, com- pared with the rate constant for the surface recombination reaction). This was the position taken by us in interpretation of the (ip, u) relations for hydro- gen evolution at low biases in the photoelectrochemical behaviour of p-type electrode^.^ We calculated the magnitude of the potential difference in the Helmholtz layer from the slope of the Mott-Schottky plot and derived the magnitude of the potentials in the Helmholtz layer.Thus, the phrase such as “ double-layer pinning ” is perhaps quantitative, since each semiconductor/solution interface has to be evaluated (for a given change of electrode potential) in respect to the degree of change of potential within and without each solid for itself, and in respect to the solution it contacts. Double-layer pinning would refer to the extreme case where the changes of p.d.would be entirely in the Helmholtz plane (“ metallization ”). We have to wait until we get a better measure of the concentration of surface states to know how often we come to this situation. At this time, “ Helmholtz-layer dominant,” or “ inner-layer dominant ” seems more appropriate terms to describe these situations. M. Green, J. Chem. Phys., 1959, 31, 200. J. O’M. Bockris and K. Uosaki, J. Electrochem. Soc., 1977, 124, 1. W. Paik, M. A. Genshaw and J. O’M. Bockris, J. Phys. Chern., 1970, 74, 4266. J. O’M. Bockris and K. Uosaki, J. Electrochem. Soc., 1978, 125, 2.98 GENERAL DISCUSSION Prof. A. J. Bard (Texas) said: As we point out in our paper the roots of the con- cept of Fermi-level pinning are contained in considerations of the semiconductor/ metal interface dating back to Bardeen’s work in 1947 [ref.(3) of my paper]. How- ever, Green’s contribution in 1959 to the theory of the semiconductor/electrolyte interface was certainly of importance and we have referenced this work both in this paper [ref. (6)] and our original publication on Fermi-level pinning [ref. (7)]. At the time of Green’s publication, as he points out, no actual experimental examples of this effect were available, and the importance of such effects have generally not been recog- nized in more recent studies. An especially important aspect of the Fermi-level pin- ning phenomenon, not dealt with in the original work of Green, is the shifting of the level to energies corresponding to couples well outside the semiconductor conduction and valence band edges (as determined from measurements in the absence of redox couple).The term “ Fermi-level pinning ” has its origins in the semiconductor/metal litera- ture.’ Terms such as “ double-layer pinning ” or “ band-edge unpinning ” are less‘ descriptive. In terms of the nature of surface states, I prefer the distinction drawn by Gerischer [ref. (1 3), p. 4751 where those that contribute to Fermi-level pinning involve electron exchange rather than simple specific adsorption of ions. See, for example, S. M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1969). Prof. H. Geriscber (Berlin) said: Fermi-level pinning has been found on semi- conductor surfaces in vacuo if surface states are present to large enough concentrations. This is caused there by the presence of dangling bonds.The same effect has often been observed at semiconductor heterojunctions owing to the presence of interfacial electronic levels within the band-gap generated by structural imperfections or chemical impurities at such interfaces. It is not surprising that similar problems will arise at semiconductor/electrolyte contacts. One particular reason is that the chemical com- position of the semiconductor surface deviates from the bulk, as is the case with ger- manium or silicon or many compound semiconductors which form either an oxidic monolayer or even a thin oxide layer at such a contact. However, there was some hope that the reactivity of the surface could also prevent the generation of electronic states within the energy range of the band gap, since very stable bonds formed on the surface should have energy levels within the valence band, and their antibonding counterparts may be located within the conduction band.Such a type of surface state would not influence the distribution of the electric charge at the contact. The enormous dependence of the electronic properties of these contacts on surface pretreatment and etching procedures indicate that there is an enormous variability of the surface properties. I believe, however, that not all the shifts of the band edges observed in electrochemical systems can be attributed to the presence of surface states. One should consider that semiconductors with not too wide a band-gap can easily form an inversion layer if minority carriers are generated by light absorption or can directly be injected into the surface from the electrolyte owing to the presence of a suitable redox system therein. If the energy position of surface states is very close to the band edges, as might often be the case, it will be very difficult to distinguish be- tween Fermi-level pinning owing to surface states and control of the space-charge layer by inversion.Both situations lead to an up- or down-shifting of the band edges at the interface, if the applied potential is varied. To demonstrate the similarity between both situations I would like to show in fig. 2 the situation where a p-type semiconductor is polarized cathodically to such an extent that the band-gap would be exceeded by the Schottky barrier height if a deple-GENERAL DISCUSSION 99 acceptor Ec sur t ace states Redox system Flat band potential no surface states with surface states energy terms in the absence of Red FIG.2.-Formation of an inversion layer or Fermi-level pinning by surface states. tion layer were formed with no minority carriers available on the surface. In contact with a very cathodic redox system, electrons can be injected into the surface and in the absence of surface states an inversion layer will be formed causing some upwards shift of the band edges owing to the resulting potential drop in the Helmholtz double layer. In the presence of surface states as shown on the right-hand side of the picture the Fermi level will be pinned to the surface states in equilibrium with the redox system.Assuming that these surface states have acceptor character and can pick up negative charge, the potential drop in the Helmholtz double layer will now be controlled by this charge and the band edges at the interface will be shifted upwards even further. If the semiconductor is illuminated, photocurrents will be found in both cases without much difference. This is shown in fig. 3 on the left-hand side for a semiconductor without surface states, and on the right-hand side for a semiconductor with surface states located somewhere in the Larger differences can be expected if the polarization is varied. I bias I anodic (forwardl 1 bias with inversion layer anodic (forwardl 1 bias with surface state control of lhe Fermi level FIG. 3.-Semiconductor-redox-electrolyte junction at cathodic and anodic bias.100 GENERAL DISCUSSION band-gap.Most conclusive should be the behaviour in the dark, which is indicated in this figure by the course of the Fermi levels of electrons and holes. Under cathodic bias, i.e. in the reverse direction, the electrode will show a blocking be- haviour in both cases. The current will be controlled by the rate of electron-hole pair generation in the depleted region of the inversion layer or, in the case of the presence of surface states, directly underneath the surface, where ,,EF and deviate from each other. In the forward direction, however, a larger current would be expected if an inversion layer is formed, in accordance with the normal behaviour of p-n junc- tions. This is indicated in fig.3 by the large deviation of the Fermi levels of electrons and holes in the transition range between the n-type surface and the p-type bulk. In the presence of surface states the forward current would be controlled by the injection of electrons from surface states into the conduction band, which can only reach a high level if these states are very close to the band edge. My conclusion is that a distinc- tion between Fermi-level pinning by surface states and the formation of an inversion layer will be possible if the surface states are not too close to the band edges. Other- wise one can hardly distinguish whether the shift of the band edges at the interface is caused by the presence of surface states or by the accumulation of minority carriers at the surface.I have the impression that the results shown in fig. 4 of Prof. Bard's paper can easily be interpreted by the formation of an inversion layer. The difference between the onset of the photocurrent and the standard potential of this redox system at the plati- num electrode is ca. 0.7 V. This is just the photovoltage one would expect for a semi- conductor with a band gap of 1.15 eV, if an inversion layer is formed in contact with a redox system.l Another reason for this interpretation is that one can hardly see why surface states of such high concentration should be formed on the surface of layered materials, where in the absence of large concentrations of dislocations or steps the surface should be perfect and have no dangling bonds. W. Kautek and H. Gerischer, Ber.Bunsenges. phys. Chem., 1980, 84, 645. Prof. A. J. Bard (Texas) said: In general, we agree with the comments of Prof. Gerischer on the difficulty of distinguishing between a shift of the band edges at the interface being caused by the presence of surface states or by inversion, when the surface states are close to the band edges. In addition to the differences pointed out by Prof. Gerischer in the forward-bias voltammetric curves, the reverse-bias voltam- metric curves can be very different for these two models if the density of surface states is high and they are also close to the conduction band edge (for n-type) or the valence band edge (for p-type) semiconductors. Majority charge-carrier injection from the redox species is possible based on the Fermi-level pinning model but not on the inver- sion model.The results with p-WSe, in non-aqueous solution can apparently be interpreted by the formation of an inversion layer. However, Fermi-level pinning still cannot be completely excluded, since capacitance measurements (fig. 4) taken in aqueous solu- tion show a frequency dispersion which cannot be interpreted simply by pure inversion. There are at least two ways of explaining this frequency dispersion, one involving sur- face states and the other inversion along with a Faradaic process. Further work is required to give a definitive answer. The variation of Vredox- V,,, with T/redox for p- and n-WSe, for electrodes with smooth and edge-free surfaces is shown in fig. 5 . ' q 2 Note that while p-WSe, shows a levelling of the photovoltage at Vredox values corres- ponding to the conduction band edge, for n-WSe, pinning occurs at potentials signi- ficantly negative of the valence band edge.The surface states that cause this pinning also lead to recombination effects in the p-WSe, and small photovoltages for couplesGENERAL DISCUSSION 101 V/V US. SCE 0.8 0.4 10 0.4 I I I I I I FIG. 4.-Plots of capacitance against potential on “ smooth ” p-WSe2 electrode at various frequencies in 0.5 mol dm-j Na2S04 aqueous solution (pH % 5 ) . Curve 1 : 10 kHz; curve 2: 5 kHz; curve 3: 4 kHz; curve 4: 3 kHz; curve 5: 2 kHz; curve 6: 1 kHz. 1 c, 00 4 6 0 3 5 0 7 2 / I / I -2.0 -1.2 P= +0.4 @yfbi+; . 2 ’f b(n1 f, - E , - f, Vr/dox/V US. SCE FIG. 5.-Plot of Yredox - Yo, against Vredox for various redox couples at n- and p-type WSez in CH3CN solution.1, anthracene (O/ - 1); 2, phthalonitrile (O/ - 1); 3, nitrobenzene (0/ - 1); 4, ruthenium 2, 2’-bipyridine (+2/ + 1) ; 5 , azobenzene (O/ - 1) ; 6, anthraquinone (O/ - 1) ; 7, benzoquinone (O/-1); 8, methyl viologen (+2/+1); 9 = tetracyanoquinone dimethane (+l/O); 10, NNN”’- tetramethyl-p-phenylene diamine (+ 1 /O) ; 11, NNN”’-tetraphenyl-p-phenylene diamine ( + 1/0) ; 12, idodide/iodine; 13, bromide/bromine; 14, chloride/chlorine; 15, thianthrene (+ l/O); 16, ruthenium 2,2’-bipyridine (+3/+2).102 GENERAL DISCUSSION located at these energies. Similarly for n-WSe, recombination effects appear for couples located very near the conduction band edge. Thus pinning in p-WSe, could occur by either inversion or surface states close to the conduction band edge.The results suggest that even on apparently smooth layer-type compound electrodes, surface states can exist. H. S. White, F. R. Fan, and A. J. Bard, J. Electrochem. SOC., in press. G. Nagasubramanian and A. J. Bard, J. Electrochem. SOC., in press. Dr. A. J. Nozik (Colorado) said: I would like to point out that an alternative mechanism to Fermi-level pinning can be invoked in certain cases to explain the photoelectrochemical behaviour described by Prof. Bard. This mechanism is based on the effects of inversion, whereby the charge density and capacitance in the semi- conductor space-charge region can become greater than that of the Helmholtz layer. Inversion occurs when the band bending is sufficiently large that the Fermi level at the semiconductor surface lies closer to the minority carrier band than to the majority carrier band.' The resulting large increase in charge density and capacitance near the semiconductor surface causes additional changes in the electrode potential (in the direction of increased reverse bias) to drop across the Helmholtz layer rather than the semiconductor space-charge layer. This situation destroys the constancy of the posi- tions of the semiconductor band edges with respect to the electrolyte redox potentials, and allows the semiconductor band edges to move with applied potential (in the inver- sion region) with respect to the electrolyte redox levels.We call this effect " band- edge unpinning ". Recent experiments 2-4 with p-Si in non-aqueous electrolytes show that the dependence of the capacitance as function of electrode potential, light inten- sity and surface-oxide thickness is totally consistent with the behaviour expected for p-Si in inversion.This mechanism for band-edge unpinning works best for small- band-gap semiconductors, where inversion can be readily achieved. It also predicts, of course, that the band edges only become unpinned in the potential regions where inversion exists; it does not depend upon the presence of surface states. S. M. Sze, Physics of Semiconductor Devices (Wiley-Interscience, New York, 1969), chap. 9. J. A. Turner, J. Manassen and A. J. Nozik, Appl. Phys. Letters, 1980, 37, 489. J. A. Turner, J. Manassen and A. J. Nozik, Photoefects at Semiconductor-Electrolyte Interfaces, ACS Symp.Ser (ACS, Washington D.C., in press). M. Klausner, J. A. Turner and A. J. Nozik, to be published. Dr. R. Schumacher (Kiel) said: We have recently found changes in the overall capacitance c,, of n-Ti02 single-crystal electrodes illuminated with band-gap light. These changes are given in fig. 6 as the difference between the illuminated and un- illuminated sample. The most interesting feature of this illustration is that in the anodic potential mode c,, is diminished, whereas at negative potentials c,, increases. At the transition potential V,, no photocapacitance is observed. These changes, how- ever, are only detectable on n-TiO, samples which have been operated for a certain time period in the anodic potential mode and simultaneously illuminated with band- gap light.For illustration the increase of the photocapacitance as function of the pretreatment time is given in fig. 7. Samples which had not been treated in that way do not show any detectable photocapacitance. We assume that this photoelectrochemical pretreatment procedure covers the ex- posed electrode area with states which can be reversibly filled with or emptied of elec- trons. This, of course, can be carried out either by varying the cell potential and/or by illumination. Assuming that the capacities of the Helmholtz layer cH and that of the semiconductor c,, are shunted in series the potential drop across the semiconduc-GENERAL DISCUSSION 103 tor-liquid junction controls the recorded overall capacitance cov. That means at positive potentials c,, determines c,, whereas at potentials close to Vfb the C, is the controlling factor.A rough estimation on the contributions of the photoinduced capacitance revealed that changes of ca. 50% with respect to the dark behaviour were I I * I .5 potential / V us SCE. FIG. 6.-The course of the anodic (c&) cathodic (cpch) photocapacitance recorded with a lock-in ampli- fier svstem operated with chopped light in 0.1 mol dm-j NaOH as function of the potential. observed. The changes of c,, in both directions are explainable assuming light- induced variations of the charge distribution within the surface states. More details on the charge-transfer mechanism involved are given elsewhere.’ We suppose these significant changes of c,, due to illumination to be a strong indi- cation that the existence of surface states in which electric charge can be trapped timelmin was operated for certain time periods under band-gap illumination in 0.1 mol dm-j NaOH.FIG. 7.--Increase in the anodic photocapacitance due to the pre-treatment procedure. The sample favours a situation where a possible shift of the bands relative to electrolyte states seems conclusive. This result is in agreement with the conclusions presented in Prof. Bard’s paper. H. R. Sprunken, R. Schumacher and R. N. Schindler, Ber. Bunsenges. Phys. Chem., 1980, 10,1040. Dr. A. J. Nozik (Colorado) said: In response to Prof. Bard’s informally expressed criticism of the term “ band-edge unpinning ” I believe it is a perfectly valid descrip- tion of the effect we are discussing.In the previous models of the semiconductor- electrolyte interface it was generally acceped that the positions of the semiconductor band edges were fixed (indeed pinned) with respect to the redox levels in the electrolyte,104 GENERAL DISCUSSION and independent of electrode potential. In our current understanding, we realize that under certain conditions (e.g. surface states, inversion) the positions of the band edges may become unfixed, or unpinned, and move with applied potential. These special conditions, therefore, lead to band-edge unpinning. Dr. S . U. M. Khan and Prof. J. O'M. Bockris (Texas) said: It will be of interest to have proof from the theoretical point of view of the experimental observation that : 1*2 We make an attempt to prove relation (1) by considering charge transfer at the semi- conductor-solution interface as the rate-determining step, and the potential drop mainly in the Helmholtz layer for a semiconductor having surface state^.^ We also assume that the acceptor and donor electronic states in redox ions in solution are Boltzmannian.We assume that the onset potential, Yonset, in photoelectrochemical kinetics of semiconductors arises when the cathodic photocurrent, e.g., of p-type semiconductors, equals the anodic dark current across the conduction band. Following the experi- mental observation of Bockris and Uosaki that the Yonset is independent of photon energy, and hence considering the photoexcited electron transfer from the bottom of the conduction band to the acceptor ion, e.g., M3+(aq) ion, in solution, we can ex- press the cathodic photocurrent as : VOnse, - YZedox 2: constant 2: 0.4 V.(1) C A UL CT O"t + l i b pTc i,,,,(cathodic) = - e, I, (1 - RA) x exP - [A&(3) + peoAP]/kT = A exp - [AE0(3) + j?eoAp]/kT (2) where e, is the electronic charge; C, is the acceptor concentration, M3+ (aq), in the reaction plane; CT is the concentration of total number of sites in reaction plane; Z, is the intensity of light; RL is the reflection coefficient of light; aA is the adsorption coefficient of light; LD is the diffusion length of electrons in the semiconductor; P$ is the WKB tunnelling probability across the interfacial barrier for a cathodic process ; /? is the symmetry factor; Ap is the potential drop at the semiconductor-solution interface and AE0(3) is the amount of energy needed to excite M3+ (as) from the ground state to the activated state, so that radiationless photoelectron transfer is possible from the bottom of the conduction band to activated M3+ (as) ion in solution.AE,(3) is obtained using a cycle for the process M3+(aq) + p-Sc(e) AE(3)t M:"+(aq) and becomes: where AFo is the free energy of the reaction and AFt(2) is the free energy of activation of the reverse reaction, AEo(3) = AF" + AFj(2) M3+(aq) + p-Sc(e) -+ M2+(aq), M2+(aq) + M3+(aq) + p-Sc(e). (3) The dark anodic current corresponding to electron transfer to the conduction band can be expressed as: kT h i,,,,(anodic) = eo - CD 6 P$e - [ A W ) - & W / k *GENERAL DISCUSSION 105 where k is Boltzmann's constant; h is Planck's constant ; CD is the concentration of the donor, M2+(aq), ion; 6 is the width of the double layer at the p-Sc-solution inter- face; Pa is the WKB tunnelling probability across the interfacial barrier for an anodic process and AE(2) is the amount of energy needed to excite M2+(aq) from the ground state to the activated state, so that the radiationless electron transfer is possible from the excited M2+(aq)* to the vacant levels of the bottom of the conduction band of p-Sc electrode, and is obtained from a cycle for the process M2+(aq) ____+ M3+(aq)* + p-Sc(e) AE(3) and becomes: AEo(2) = -AF" + AFt(3) = AF(2) where AFl(3) is the free energy of activation for the reaction M3+(aq) + p-Sc(e) -+ M2+(aq).At Yonset we use the equality of current densities from eqn (2) and (4), i.e., Ae- + Be&vl/kT = Be - [AE0(2) - Be0Avl/kT or, for /3 = 1/2, eoAq = kT In AIB - AE0(3) + AE0(2).Using eqn (3) and (5) for AE0(3) and AE0(2), respectively, we get (Aq)onset = 5 1 , AIB - AF"/eo e0 or ( 5 ) in potential scale, or (7) - kT Yonset - V'redox = - In A/B = constant. e0 Hence, relation (1) is proved in eqn (7). Putting the value of A and B from eqn (2) and (4) in eqn (7) and considering (1 - R,) 21 1, and P: 'v P;, Zo = 2.97 x 1017 photon cm-2 s-l for an intensity of 1 sun, having hv = 2.1 eV. C,/C, = 0.5 for 0.5 rnol dm-3 solution; cq = lo4 cm-'; LD-l = lo3 cm'l; kT/h = 1012-5; C,, = 0.5 mol dm-3 = 3.0 x lo2' molecule ~ m - ~ ; 6 = 5 A. We get: - _ - - kT In (9 x 10-9-5) e0 = 0.5 V which is comparable with the experimental value.'106 GENERAL DISCUSSION F.F. Fan and A. J. Bard, J. Amer. Chem. Soc., 1980, 102, 3677. V. Guruswamy and J. O’M. Bockris, Energy Research, 1979, 3, 389. M. Green, J. Chem. Phys., 1959, 31, 200. J. O’M. Bockris and K. Uosaki, J. Electrochem. Soc., 1977, 124, 1348, Prof. J. O’M. Bockris (Texas) said : The reaction of photoelectrochemical processes to pulses is a main path to the determination of the rate-determining step in the steady state. Hence, I advocate detailed examination of the (current, time) line under potentiostatic conditions, not only at very low (nanosecond) or intermediate (milli- second) ranges but also up to the range of seconds where slow surface states (lifetime ca. seconds) can still be coming into play, and until steady state is reached.The rate- determining step may change as the occupancy of the surface with surface radicals changes and from the point of view of device orientation, it is the rate-determining step in the steady state which is important. An initial reduction of a surface oxide may be important at p-type semiconductors and affect the course of the (current, time) transient. Prof. A. B. Ellis (Wisconsin) said: In their potentiostatic flash experiments Dr. Richardson and Prof. Perone suggest that oxygen is the detected photo-oxidation pro- duct based on a lifetime which exceeds 15 ms. Is there any other evidence that the product is O2 and not, perhaps, a species such as H202? Mr. Z. Harzion (Tel Auiv) said: I would like to describe results of some fast photo- current transient measurements performed on the CdSe/polysulphide system in which a slightly different approach was adopted. We measured at both closed and Rseries Rlood FIG.8.-Equivalent circuit for photoelectrochemical cell. open circuit conditions and analysed the results by plotting the decay time z against the resistance of the external load in a two-electrode cell. Small potential perturb- ations were employed so that constant values of the interfacial elements could be safely assumed. A linear dependence is found in the small-R, region, while at higher values of RL the slope falls gradually. This general behaviour could be simulated by a simple equivalent circuit shown in fig. 8. In this circuit the semiconductor-electrolyte inter- face is represented by a capacitor C, with a shunt resistor, Ri, across it.The rest of the cell and circuit is represented by a resistor Rout, where Rout = Rseries + Rload, Rseries being the total series resistance of the photoelectrochemical cell. A current source represents the source of illumination and is connected in parallel with C. The measured response of this circuit to a short light pulse can be simulated by its re-GENERAL DISCUSSION 107 sponse to a delta function of injected current, and is calculated by Laplace transform- ation as: AQ iout = R C exP(-t/z) out where z is given by z = (Ri,JIROut)C. T = (Rseries + Rload)C and a plot of z against Rlcad should be linear with slope C and intercept Rseries C. In the high-Riload region the decay will occur primarily via the internal route, Rin, and in terms of this equivalent circuit z = RinC.For example, measurements taken before and after surface etching revealed that etching resulted in higher photocurrent peaks, larger charge per pulse, shorter decay time and a lowered Rseries. This might might be due to the removal of a damaged layer rich in traps. (Note that even at small Rload values, " closed circuit," the resolution rise-time is limited by pulse width as in the case of '' open circuit ".) One question which remains open at this point is the possible contribution of the Faradaic impedance for photohole transfer to the measured decay time: In the CdSe/ polysulphide system, the measured Rseries for an etched electrode was found to be simply the sum of bulk SC and solution ohmic resistances. More experiments with different hole acceptors are required to clarify if RF can have any direct effect on such photocurrent decay curves, or if the larger Helmholtz capacitor acts as a short during fast transient measurements, Z .Harzion, N. Croitoru and S. Gottesfeld, (a) Bull. Israel Phys. Suc., April 1980, (b) extended abstract of the 3rd Int. Conf. on Photochemical conversion and storage of Solar Energy, Boulder, Colorado, August 1980. When Rout < Xi,, Prof. W. J. Albery (London) said: Although in this work Perone et al. have sim- plified the equivalent circuit in fig. 1 of their paper to that given in fig. 2, I would like to ask if these new techniques will be able to measure the different elements of the more complicated circuit in fig. 1. Results from a.c.impedance techniques are often difficult to interpret and it would be helpful if your more direct technique could be used to obtain values of C,, and C,, for instance. Prof. H. Gerischer (Berlin) said: The coulostatic technique is certainly the most powerful technique for studying fast electrode reactions at metals. In the case of semiconductors, however, I believe that a serious limitation arises from the presence of a Helmholtz double layer and a space-charge layer which act as capacitors in series with respect to the potential distribution. The potential changes which are observed directly during the light pulse must be attributed to the charge separation occurring in the space-charge layer of the semiconductor. Electron-hole pairs generated therein will be separated immediately and generate a counter-voltage which can be observed externally.If minority carriers drift from the charge-free bulk to the space-charge layer, there might consecutively be a further potential change in the same direction due to a further increase of the surface charge. However, I cannot see how one can distinguish whether the electric charge stays on the surface of a semiconductor or moves further through the Helmholtz double layer in connection with an electro- chemical reaction. Electrostatically, this means that the electric charge which acts as counter-charge to that one accumulated in the bulk of the semiconductor is moving only a few Angstrom further apart. If the space charge layer is extended over 100 and often more A, this latter step can only cause a negligible variation of the overall potential drop which can be measured externally.Therefore I believe that no con- clusion can be drawn from these experiments on the charge-transfer reaction occur- ring at the interface.108 GENERAL DISCUSSION Prof. R. S. Davidson (London) said: We have observed transient overshoots of photocurrent with photosensitive electrodes made by depositing powdered titanium dioxide on platinum meshes.' The magnitude of the overshoot was dependent upon the electrolyte. Has Prof. Perone found the magnitude of his transient phenomena to be affected by the type of electrolyte used? H. H. Chambers, R. S. Davidson, R. R. Meek and R. M. Slater, J.C.S. Faraday I, 1979, 75, 25 17. Prof. A. J. Bard (Texas) said: To amplify on the comments of Prof.Davidson on the effect of electrolytes on transients observed at Ti02, let me add the following. Such transients occur on a much longer time-scale than those discussed in the paper by Perone et al. They are more significant at polycrystalline Ti02 and largely arise from the effects of recombination processes caused by the reduction of photogenerated oxidants which are reduced via surface states at the Ti02. They are much less promi- nent when electrolytes which do not produce oxygen (e.g., oxalate, acetate) are em- ployed. Your work demon- strates that very short photocurrent pulses can be observed when the cell time-constant is very small. Your work also shows that the time-dependence of the photocurrent pulse is dictated by the cell internal RC time-constant if no longer-lived current- producing processes occur. Thus, your observations are completely consistent with our studies which state that time-resolved potentiostatic photocurrent measurements are limited (assuming infinitesimally short light pulses) by the cell internal time-con- stant, specifically by RUG',, when C,, < Cdl.In your work I believe the values for R, and C,, were ca. 10 SZ and 10-20 nF, respectively, and your observed photocurrent pulses exhibited half-widths of ca. 100 ns, which is totally consistent with our statement regarding limitations on potentiostatic photocurrent measurements. On the other hand, open-circuit measurements of photopotential transients (which we call '' coulo- static-flash " experiments) are not subject to the same limitations.If the cell is treated as a voltage source with internal resistance Ri, the limiting rise-time is defined by the time-constant, RiCM, where CM is the total capacitance in parallel with the measurement probe. CM includes stray interelectrode capacitance (typically 20 pF), cable capaci- tance and measurement amplifier input capacitance. In our studies to date, we have found CM to be ca. 50-100 pF, and have used cells with Ri < 100 R. Thus, we have conducted experiments where the measurement rise-time is < 10 ns and the time resolution is limited by the pulse width of the nitrogen laser source (ca. 10 ns). Secondly in response to Prof. Albery I comment that we did provide a derivation for the response of the more complicated circuit of fig.1 to a potential step or to the equivalent step induced by a pulse photocharge. The goal was not to provide a tool to obtain better measurements of C,, or C,,, but rather to define the cell time-con- stant limitations to potentiostatic photocurrent measurements with a pulsed-laser source. An additional fringe benefit of this work is pointed out by your comment. That is, we can estimate RC,, and (R + R,,)C,, from the shape of potential-step current-time curves. From our experience, the A.c.-bridge techniques provide much more precise measurements of cell component characteristics. However, the poten- tial-step measurements at least demonstrate the time domains over which the cell electrical characteristics remain constant and where the transitions occur.Such measurements provide a complementary perspective on cell characteristics and em- phasize the need for variable-frequency a.c.-bridge measurements. They are also directly meaningful for pulsed photocurrent measurements. Prof. S. P. Perone (Indiana) said: I turn first to Mr. Harzion.GENERAL DISCUSSION 1 09 Thirdly in reply to comments by Prof. Gerischer I would remark that: (A) With regard to possible problems related to high laser intensities, we have been guided by the following facts: (1) Even though peak power can be high for our 10 nS pulse (up to 1 mW cm-2), the equivalent C.W. power is kept low ( < 1 mW) with a repetition rate of ca. 1 Hz. (2) We confine our studies to regions of laser intensity low enough to avoid saturation or other non-linear effects.(For these studies the upper limit was 10 kW cm-2). (3) The potential dependence of the " photoeffect '' is the same with pulsed laser irradiation as for low level C.W. irradiation. (4) We do not observe any long-term deterioration in the electrochemical behaviour or physical appearance of the Ti02 electrodes. Thus, we conclude that, at least for these electrodes under the conditions employed here, no perceptible physical damage occurred due to pulsed laser irradiation. Also, we have avoided irradiation conditions in this work where saturation or non-linear effects might distort the results. (B) With regard to the possibility that the time dependence of observed photo- potential transients might reflect primarily hole-electron recombination, I believe that this certainly must contribute some part of the observed behaviour.However, several other processes may also contribute.' These might include: charge transfer, space- charge relaxation, and long-term re-equilibration of electrode/solution Fermi levels. (C) With regard to coulostatic-flash experiments we agree that the change in the open-circuit cell potential with a light pulse does not in itself indicate charge transfer to the solution. Indeed, an initial negative excursion would occur with a 10 ns light pulse due to the production of electron-hole pairs and their separation. Simultan- eously, the double layer at the electrode-solution interface would become re-oriented to reflect the new potential field. However, if charge transfer to solution occurs sub- sequently, additional negative charge will enter the space-charge region of the semi- conductor, causing a further negative excursion of the electrode potential.Because we do not observe this subsequent step with Ti02 electrodes, one might conclude that charge transfer to solution is simultaneous with the light pulse, giving only a single step. We have conducted coulostatic-flash experiments with Ti02, where current- doubling reducing agent (tiron) was added to the solution. We observed concentra- tion-dependent enhancement of the initial photopotential excursions (up to double the response), with essentially the same rise-time, equivalent to the rise-time of the laser pulse (< 500 ns). Because response enhancement could only result from charge transfer to solution, these experiments demonstrated that we could measure the effect as a further negative excursion of the photopotential. One cannot infer that the charge-transfer rate is large if the recombination rate is fast, because charge transfer might only be allowed to occur during the light pulse.Thus, the efficiency of charge transfer would be directly related to the rate constant(s), but no characteristic time dependence would be observed. Our results to date with CdS electrodes suggest that charge-transfer processes after a 10 ns pulse can cause a slower negative photopotential excursion beyond that related to initial light-induced charge separation. This conclu- sion is based on observed correlations between recombination luminescence decay and the time dependence of the coulostatic-flash photopotential.Dr. J. H. Richardson (California) said: Prof. Gerischer has a good point in asking if the laser coulostatic technique is only monitoring physical processes occurring within the semiconductor and not measuring the rate of charge transfer across the interface. So far we have interpreted our results largely within a framework of physical processes occurring within the semiconductor and at the surface [electron-hole generation, separation and recombination, hole drift and diffusion, surface reactions (" extrinsic holes ")I. However, with regard to his specific comment concerning the capacitance110 GENERAL DISCUSSION differences between the space-charge region and the double layer, we saw only one potential step (< 10 ns) even with highly doped TiOt electrodes where the space-charge capacitance approached that of the double layer (to less than an order of magnitude difference).This would imply that the rate constant for charge transfer is also <lo ns. A different approach to measuring charge-transfer rates would be by spectroscopic techniques. While not a direct measurement of the charge-transfer rate, our time- resolved luminescence measurements may be thought to monitor Faradaic processes which must kinetically compete with radiative and non-radiative recombination. The observed longer luminescence lifetimes with CdS electrodes at negative potentials where hydrogen is being evolved may reflect a slower rate of chemical reaction at the interface. If so, that result is also consistent with a rate of charge transfer being of the order of 10 ns.Prof. H. Gerischer (BerZin) said: I address my remarks to Dr. Spriinken and his co-authors. It is surprising that such a large cathodic photoeffect has been observed in these experiments. I suppose that in this cathodic potential range a considerable dark cur- rent was already flowing through the interface. My question is: Have you measured the transient during a light pulse and seen that there is no relaxation in it which could cause misleading interpretations of the effect seen with lock-in techniques. Have you a satisfying explanation for the occurrence of these cathodic photoeffects ? Dr. M. D. Archer (Cambridge) said: Taking up the same theme, I notice that you mention in your paper that you have observed cathodic photocurrents resulting from the diffusion of dissolved oxygen to the electrode, as well as those resulting from the presence at the electrode surface of adsorbed oxygen produced by prior photoanodic generation.We have also observed cathodic photocurrentswith a well-defined poten- tial-independent plateau that apparently result from reduction of dissolved oxygen at a titanium-doped n-cobalt ferrite electrode in oxygen-saturated 1 mol dm-3 KOH, and will shortly publish the results. A preliminary investigation of the mass-transport characteristics of this photocurrent, using a rotating-disc electrode and oxygen or ferri- cyanide in solution, shows that the photocurrent declines considerably as the rotation speed is increased from 0 to 2000 r.p.m., but it becomes virtually independent of rota- tion speed above 2000 r.p.m.Independence of photocurrent and rotation speed is predicted both by your hypothesis that oxygen adsorption on the electrode determines the photocurrent, and by the hypothesis of Vandermolen et al. [your ref. (17)], that the saturation photocurrent is determined by rate-limiting capture of electrons from the conduction band by electronic surface states. However, neither model accounts for a decline in photocurrent between 0 and 2000 r.p.m. The photothermal hypothesis of Decker et al.' does not fit the form of our transient data either. I wonder if you have any feeling as to the role of mass transport in your observa- tions ? F. Decker, J. F. Julioa and M. Abramovich, Appl.Phys. Letters, 1979, 35, 397. Dr. W. Gissler (Zspra) said: Questions have been asked concerning the origin of the " anomalous " cathodic photocurrent which was observed in n-type Ti02. Such anomalous photocurrents have also been observed with other semiconductors and were recently discussed for p-type trigonal selenium.' Anomalous photocurrents occur together with relatively large dark currents which are caused in n-type semi- conductors by an electron transfer from the conduction band to the interface uia tunnelling through the space-charge layer. The dark current is increased if the tunnel-GENERAL DISCUSSION 111 ling probability is increased by narrowing the space-charge layer. This occurs in potentiostatic experiments by illumination due to an increased space-charge carrier density and it might be caused also by a Fermi-level pinning effect.In both cases the resulting current increase is no real photocurrent. A better description might be " light-induced dark-current enhancement." W. Gissler, J. Electrochem. SOC., 1980, 127, 1713. Dr. P . Pichat (Villeurbanne) said: I would like to comment on the oxygen species occuring at the U.V. illuminated Ti02 surface in 02. From photoconductivity measurements as a function of oxygen pressure, we infer- red the existence of 0- adsorbed species in addition to that of 02- species.' More- over, these 0- species are also formed on other U.V. illuminated n-type semiconductors, such as Zr02, ZnO, Sb204 and CeO,. Support for the formation of these dissociated oxygen species and their importance in photocatalytic oxidation reactions was recently gained from a study of NO interaction with illuminated TiO,.' Nitrogen oxide, which captures surface free electrons, decomposes yielding N, and N20.The resulting oxygen atoms were evidenced by the photocatalytic oxidation of alcohols over Ti02 in the presence of NO. Furthermore, we found3 that the oxidation of I- and Br- ions in illuminated aqueous suspensions of Ti02 requires the presence of dissolved O2 and a mechanism involving 0- species was tentatively suggested. In conclusion, con- cerning the effects of oxygen on illuminated Ti02 we agree with the authors, but we think that 0- species must be considered. ' J-M. Herrmann, J. Disdier and P. Pichat, Proc. 7th Znt. Vac. Congr. and 3rd Znt.Conf. Solid Surfaces ed. R. Dobrozemsky et al. (F. Berger und Sohn, A-3580 Horn, Austria, 1977), vol. 11, P. Pichat, H. Courbon, J. Disdier, M-N. Mozzanega and J-M. Herrmann, 7th Int. Congr. Catal., Tokyo (Japan), 1980, preprint F 7. J-M. Herrmann and P. Pichat, J.C.S. Farday Z, 1980, 76, 1138. p. 951. Dr. A. J . Nozik (Colorado) said: With regard to the problem of determining the flat-band potential from Mott-Schottky (MS) data, I would like to report on some recent work done at SERI that shows one must be very careful in interpreting these types of plots. Mott-Schottky data obtained from TiO, crystals that were prepared by the usual method reported in the literature (polished, etched, reduced or just polished and reduced) invariably showed non-linear MS plots and dispersion effects.However, if these crystals were then re-ground with 600 grit Sic paper and etched in concentrated H2S04 at 150-160 "C, the MS plots became perfectly linear and showed no dispersion. The measured flat-band potentials were consistent with literature values, and showed the expected pH dependence. These ideal MS plots were found to be dependent on the electrolyte concentration. At concentrations of 0.10 mol dm-3, the MS plots showed large dispersion effects; the ideal behaviour was not ob- tained until the electrolyte concentration exceeded 1 .O mol dm-3. However, it was also possible to obtain linear Mott-Schottky plots that yielded very negative x-axis intercepts; this was achieved by just grinding the crystals after reduction, without the H2S04 etch.These types of plots always showed dispersion effects, but the intercepts were still always anomalously negative, These very negative flat-band potential values (up to 1 V more negative than expected) were found to have no signficance with respect to photocurrent-voltage curves. The onset potential for anodic photocurrent was found to be the same for crystals showing ideal MS plots and for those showing anomolous MS plots. The lesson to be learned here is that the flat-band potential from MS plots can only112 GENERAL DISCUSSION be considered valid if the plots are perfectly linear, show no significant dispersion effects, and also show a reasonable pH dependence. MS plots that are simply linear at one frequency cannot be used to define the flat-band potential.With regard to the pH dependence of the flat-band potential, it was found that for several types of boules (Al-free, prism grade, optical grade) it varied from 57 mV/pH unit to 67 mV/pH depending on the boule type. The scatter of flat-band potential data in the literature is believed to be related to this generally uncontrolled or un- reported variable. Prof. J. 0’ 111. Bockris (Texas) said: Does not the Butler and Ginley approach to the determination of the fiat-band potential (iL, V ) relation essentially imply that the surface states are sufficiently low so that one is the inner p.d. dominant state?’ This and other assumptions are tacitly behind the Butler and Ginley deduction of their approach to ffat-band determination. However, there is an increasing amount of evidence that surface states in high concentrations do frequently appear on certain crystal planes of the semiconductor-solution interface.Thus, I would like to draw attention to some unpublished work of my former graduate student, Dr. J. F. McCann, who found that for numerous cases of photo- electrochemical reactions involving oxides (e.g. , In,O,), the difference between the Mott-Schottky flat band potential and that indicated by the intersection of the iph with the potential axis differed by up to 900 mV.2 Nevertheless, one must be cautious of attributing the difference always to surface states. As McCann showed, it may be in- terpreted also in terms of the effect of surface recombination. This would be another reason for not accepting the results of the Butler and Ginley approach to flat-band potential determination until much prior knowledge had been gathered about the interface concerned. This situation draws attention to the poverty of our present possibilities for the measurement of flat-band potential.The Mott-Schottky plot (when it does not vary with frequency) offers no smooth path. Is it affected by fast surface states? Is the Helmholtz capacitance correctly allowed for? (This may be increasingly difficult with a higher degree of surface states. What of the true area?) One of our principal lacks is methods of determining the flat-band potential. There are some dozen methods of determining the potential of zero charge at solid- solution interface^.^ Some of these could be turned into flat-band potential deter- minations.Thus, Bard and Handley have shown that Gockhshtein’s piezoelectric method can be used for this p u r p o ~ e . ~ ~ ~ Would the friction method be applicable?6 But note that determination of the flat-band potential without a knowledge of the surface states is like a determination of the potential of zero charge without knowledge of specific adsorption. M. A. Butler, J. Appl. Phys., 1977, 48, 1914; D. S. Ginley and M. A. Butler, J. Electrochem. SOC., 1978, 125, 1968. J. F. McCann, Ph.D. Thesis (Flinders University of South Australia, 1980). J. O’M. Bockris and S. U. M. Khan, Quantum Electrochemistry (Plenum Press, New York, 1979). A. Yu. Gokhshtein, Elektrochim., 1966, 2, 1318; Electrochim. Acta., 1970, 15, 219; Doklad. Akad. Nauk S.S.R., 1971, 200, 20.J. O’M. Bockris and R. K. Sen, Surface Sci., 1972,30, 237. Dr. D. S . Ginley (New Mexico) said: As Prof. Bockris has mentioned, the measure- ment of flat-band potentials can be fraught with difficulty. This is true not only because of the difficulties of the measurement techniques themselves, but because even the most stable materials we seek to measure may in fact be time variant. This is * A. J. Bard and L. Handey, J. Electrochem. SOC., 1980, 127, 338.GENERAL DISCUSSION 113 due not only to changes in the species adsorbed on the surface but to actual changes in the near-surface stoichiometry. We have found that the electromigration of ions across the semiconductor-elec- trolyte interface can play a large role in determining electrode properties. The field in the depletion region is only ca.1 V but this potential is dropped over a distance <1 pm. This gives rise to electric fields of lo4 V cm-l and larger. These fields are large enough to promote the movement of ions across the interface. Both materials have Fig. 9 illustrates the crystal structures for TiOz and Gap. FIG. 9.-Crystal structures for (a) rutile [OOl] and (b) zincblende [110]. 0 0 0 0 0 0 I , O f 0 0 . . I 0 0.1 0.2 0.3 0.4 0.5 0.6 depth/pm Change in donor profile in the near surface region after ageing. FIG. 10.-Doping profile of TiOz wafer anodically aged (+5 V, 30 h).114 GENERAL DISCUSSION TABLE DEPENDENCE OF Vfb ON AGEING CONDITIONS FOR SrTi03 bias illumination conditions" V&' US. SCE anodic anodic cathodic cathodic virgin -1.11 on +5 V us.SCE -1.15 PCb 21 h PC 4 days on +1 V us. SCE - 1.15 virgin -1.17 Off 0.2 mA-CC" 25 h - 0.95 Off 0.2 mA-CC 21 h - 0.96 a The anodic and cathodic ageing experiments were done consecutively, PC, potential control; tromigration will be of consequence predominately for positive ions since the radii of all negative ions are too large to allow entrance into the channels. Fig. 10 illustrates the doping profile of an anodically aged TiO, wafer. When run anodically, the field in the depletion region is such so as to force the removal of Ti3+ interstitials. This ageing phenomenon gradually reduces the doping level in the near- surface region, widening the depletion region allowing for the collection of electron- hole pairs from photons with a longer wavelength and deeper penetration depth.This in effect red-shifts the photoresponse. The donor distribution can be restored by thermal annealing of the sample. Since the number of atoms removed from the depletion layer is small, no change in electron affinity and consequently flat-band poten- tial is associated with this form of ageing. Substitutional doping is the only obvious way to avoid this problem. CC, current control. I 300 350 wavelength Inm 400 FIG. 11 .-Changes in the photoresponse associated with the cathodic ageing of the electrode: 1 , virgin Sr-Ti03; 2, cathodically aged (2 mA-CC for 25 h); 3, 0.0 V us. SCE for 27 h.GENERAL DISCUSSION 115 Table 1 illustrates that under cathodic ageing conditions the situation is substanti- ally different.Here the potential is such as to drive small positive ions into the surface region. This is most pronounced for hydrogen, deuterium and lithium. The top half of the table substantiates that there are no changes in V f b associated with anodic ageing. In the lower half of table 1 for cathodic ageing there is a considerable change in Vfb upon the electroinjection of hydrogen. The initial ageing period is sufficient to saturate the surface so subsequent ageing does not alter Vfb. These re- sults clarify the problem some investigators have had in measuring reproducible flat- band potentials after cathodically cycling their electrodes. Fig. 11 illustrates the changes in the photoresponse associated with the cathodic ageing of the electrode, The overall quantum efficiency is significantly reduced, which suggests that the injected H + significantly increases recombination centre den- sities.Interestingly, anodically cycling the electrode removes a substantial portion of the hydrogen, restoring the photoresponse; thus a significant portion remains, al- though capable of altering Vfb but not the photoresponse. Clearly the effects shown here can significantly alter electrode properties and must be taken into account when measurements are made, especially if they are to take place over any appreciable time span. The forced electromigration of ions may as well be a potentially useful tech- nique for electrode modification. There is a question on the talk presented as well. Is it possible that the observed p-type photoeffect is due to an accumulation effect? Dr.H. S. Jarrett (Delaware) said: We found in our laboratory that the flat-band potential Vfb for pure ( 4 0 ppm impurity) Ti02 crystals as determined from Mott- Schottky plots is in acceptable agreement (<O. 1 eV, but always more cathodic) with the potential V,, where photocurrent begins to flow. These data are reproducible in the same electrolyte over several days providing the electrolyte is kept clean. We define V,, as the potential at which the photocurrent exceeds the dark background current by 0.1 pA for a light flux 2 10'' photons The current characteristics were obtained point by point allowing a few minutes at each potential to attain steady-state conditions. No V,, is seen by this method, although current characteris- tics of the type reported here were seen for scan rates as low as 5 mV s-l.For 0.1 % vanadium-doped crystals, however, the agreement between v f b and V,, is poor. For various samples of n-V:Ti02 cut from the same single-crystal boule, (Vfb - VOcI x0.5 5 0.1 eV with the flat-band potential being more cathodic (see fig. 12). From the analysis of surface capacitance for semiconductors with deep im- purities,' one expects that C-2 is not linear, but has a maximum2 at a potential below flat-band corresponding to the energy of the filled acceptor state. The figure indicates such behaviour, although a clear maximum is not found but only an inflection and change of slope. In addition, the theory predicts that the two straight line segments of C2 above and below the maximum should both extrapolate to v f b as they do in this figure, which is for a fresh electrolyte sample.However, we also find that this be- haviour is not always obeyed and that over a period of several hours the more shallow segment of C - 2 tends to curve and become even more shallow so that the potential ob- tained by extrapolation to Cm2 = 0 of this more shallow segment moves to more cathodic potentials. Extrapolation of the steeper segment remains within 0.1 eV of v f b of the fresh sample. Such behaviour seems also to have been observed by Kennedy and Frese3 for m-Fe203, which should also have deep levels caused by the various valence states of iron. Despite these variations in capacitance, VOc remains constant and the entire cur- In their case, I V,, - VOcI is of the order of 0.2 eV.116 GENERAL DISCUSSION rent characteristic remains unchanged.Note, however, that vanadium doping seems to increase electron-hole recombination, and the photocurrent quantum effi- ciency is reduced by about two orders of magnitude from pure TiO,. H. S. Jarrett, A. W. Sleight, H. H. Kung and J. L. Gillson, J. Appl. Phys., 1980, 51, 3916. J. F. Dewald, Bell Systems Tech. J., 1960, 39, 615. J. H. Kennedy and K. W. Frese Jr, J. Electrochem. SOC., 1978, 125, 723. FIG. 12.-Plot of C against V for 0.1 % vanadium-doped crystals. N = 1.2 x 1020, pH = 2.6. Dr. M. A. Butler (New Mexico) said: Several people have raised the question about the usefulness of photocurrent onset potential Yo, as a measure of the flat-band poten- tial Vfb. I would like to point out some of the limitations of using V,, and some of the advantages.After all we are interested in obtaining the maximum amount of information about a specific semiconductor-electrolyte interface rather than having a blind tool for measuring flat-band potentials. The use of an (I2, Y ) plot as previously proposed’ has built into it a number of assumptions: (1) monochromatic light, (2) optical absorption depth c(-l much larger than depletion layer thickness W and (3) a uniform donor distribution. This first assumption is important as white light results in a spectrally weighted photoresponse which will not necessarily satisfy condition (2) and should not be ex-GENERAL DISCUSSION 117 pected to yield a linear (Z', Y ) plot. The second condition allows expansion of the full expression for the photocurrent : leC(1 - GL} and thus fulfils the conditions necessary for a linear (Z2, V ) plot.It also restricts the light to energies just above the bandgap. The third condition has become more important since ion migration has been re- cognized as a mechanism which generates non-uniform donor profiles. Finally re- combination at midgap states in the near surface region of the semiconductor can also result in disagreements between Yo, and Vfb. The important point is to consider disagreement between V,, and v f b not as an insoluble problem but as new information which will provide better insight into the properties of the semiconductor-electrolyte interface. Here we assume that V,, is Perhaps this can be best illustrated by fig.13. I 2 / vo " vfb V FIG. 13.-Plot of I 2 against V : (A) surface region is depleted of donors; (B) near-surface recombina- tion processes. determined by a linear extrapolation of an (Z', V ) plot and that Y f b is determined by some other means such as Mott-Schottky plot. In (A) we show the kind of behaviour that is observed when the near-surface region is depleted of donors. While the actual photocurrent onset occurs at v f b , extrapolation of the linear part of ( I 2 , V ) gives an intercept at more negative (positive) potentials for N(P)-type semiconductors. In (B) we illustrate the kind of behaviour expected with near-surface recombination pro- cesses. Here Vf, and V,, determined from a linear extrapolation agree but the actual photocurrent onset is at more positive (negative) potentials for N(P)-type semiconduc- tors.Thus these kinds of comparisons can provide insight into the properties of the interfacial region. It should be pointed out that the breaks in the curves are not al- ways as clear-cut as illustrated. Care must be taken to make measurements over a large enough potential region so that one is not deceived as to what is the true linear portion of the curve. M. A. Butler, J. Appl. Phys., 1977, 48, 2019. M. A. Butler, J. Electrochem. SOC., 1979, 126, 338.118 GENERAL DISCUSSION Prof. N. Armstrong (Arizona) said: I address my remarks to Drs McAleer and Peter. It should be made more clear that the titanium surface, as these authors have pre- pared it, probably consists of a gradient of oxide phases, from Ti02 at the very surface, to sub-oxide phases beneath, and finally the pure metal itself.We initially pointed this out as the authors indicated in ref. (15) of their paper, but enlarged the discussion in J. Electrochem. SOC., 1978,125, 1790. The (current, voltage) behaviour in fig. 1 of their paper can therefore be interpreted as the oxidation of a sub-stoichiometric oxide region extending to a thickness of up to ca. 50-60 A, followed by the formation of a true oxide-metal interface. Our initial experiments indicated that this transition occurs at ca. 1.5-1.7 V us. SCE which is notable when one considers the change in photocurrent response that they observe in this same potential region. I think that this fact should lead to a certain degree of caution in the interpretation of film thicknesses, since these are based upon assumptions of film stoichiometry which are not always well-under- stood.Prof. W. P. Gomes (Gent) said: In fig. 7 of their paper, the authors find the inter- section of the Mott-Schottky plot with the Y-axis ( Yo) to be ca. 1 V more negative than the photocurrent onset: and attribute this discrepancy to the contribution of the Helm- holtz layer to the (capacitance, voltage) behaviour. Now, assuming that the Helm- holtz capacitance C, is voltage-independent, the relationship between Vo and the flat- band potential Vfb can be expressed by: kT &o&eNd Yo = Vf, + 7 - ___ 2 cg * Using the data from the paper, i.e., & = 60, Nd = lo2' C M - ~ and c, = 40 pF cm-2, one finds a value of ca.-25 mV only for the last term in the foregoing expression. Hence, if one wants to interpret the difference between Yo and the photocurrent onset in this manner, one would have to assume a much lower value for C, (2: 6 pF cm-2). Dr. L. M. Peter (Southampton) said: Prof. Gomes has pointed out that our calcu- lation of the double-layer capacity is in error; a value of 6 pF cmm2 is indeed obtained from fig, 7 of our paper if E f b is assumed to be -0.3 V us. SCE. However, the flat- band potential cannot be obtained from the extrapolation shown in the figure, even when the small shift due to the potential drop in the Helmholtz layer is allowed for. We neglected to point out that the plot changes slope close to the flat-band potential, and to illustrate this we have plotted in fig.14 more data obtained at potentials below 1.5 V us. SCE in the measurement. E f b is seen to be close to -0.3 v us. SCE. The change in the slope of the Mott-Schottky plot, which has also been noted by Potter' in this discussion, can be interpreted as evidence for a two-level donor system or alter- natively in terms of a spatially inhomogeneous distribution of donors. Both cases have been treated in detail elsewhere.2 R. Potter, Firudiy Disc. Chern. Soc., 1980, 70, 124. J. F. McAleer, Ph.D. Thesis (University of Southampton, 1980). Dr. D. S. Ginley (New Mexico) said: Two questions come to mind concerning the (1) Is it really possible to measure the film thickness by this technique if the doping Clearly there is a validity of the techniques employed to measure the film thickness.level and composition of the film change as a function of depth?GENERAL DISCUSSION 119 0 -0.5 0 0.5 1 .o V/V us. SCE FIG. 14.-Mott-Schottky plot for the oxide film grown at 1 mV s-l to 5.4 V us. SCE in 1 mol dm-3 H3P04. The plot includes additional data points not shown in fig. 7 of our paper, and shows clearly a change in slope close to Efb. whole region of suboxides at the growth interface. As well, one expects that Ti3+ interstitials will age out of the interface under mild conditions. Is there any observed increase in the long-wavelength response associated with depletion-layer widening ? (2) Many people have observed the ph,otostimulated instability of Ti02 films in acidic media at potentials <6 V. Have the authors evaluated their films for stability and how does this affect their other measurements? Dr.L. M. Peter (Southampton) said: The determination of the film thickness by the method outlined in our paper should be insensitive to the doping level in the oxide. In fact, we have shown that the doping distribution is essentially homogeneous for slowly grown films. The point is that the photocurrent is measured during film growth, i.e., under conditions where the depletion region extends throughout the en- tire film (an excess charge is necessary on the metal to bring about film growth in the first place). The donor density and distribution should not, therefore, influence the photocurrents directly. We agree that lower oxides are probably present adjacent to the metal, and this point is discussed in our paper (see fig.5). We have also studied the photostimulated breakdown and growth of TiO, films on titanium, but we do not believe that the effect is important at the low intensities used in our work. The intact nature of the films below 2 V us. SCE is adequately demonstrated by the ideal behaviour of the (l/C, E ) relationship. At higher potentials, the breakdown phenomena discussed in our paper are not sensitive to illumination at the low intensities used for photo- current spectroscopy. Dr. M. Froelicher (Paris) said: We are in fair agreement with the authors on the occurrence of some modification in the oxide at ca. 1-1.5 V, at least in the electrolytic medium with which we are working, i.e. 0.5 mol dm-3 sulphuric acid.We observe that the optical indices of thicker anodic films have been measured,' which could lead to a better choice for al. However, the main point under discussion is not this choice of at, but some discrepancies in the hypothesis and conclusion of the argu- ment. The authors assume that the film has the optical properties of Ti02, then that120 GENERAL DISCUSSION it is Ti02, and they conclude that it “ may not be TiO, at all ”. In this case, what can be the significance of the chosen a2. In fact, in sulphuric acid, we proved from electroreflectance data that the film keeps this TiO, nature down to the corrosion poten- tial, and probably, it is the growth law which is modified, maybe in relation to incom- plete disappearance of native oxide, the rutile modification of Ti02, which can then present a different electrochemical behaviour than does anodic anatase.G. B. Blondeau, M. Froelicher, M. Froment and A. Hugot-Le Goff, Thin Solid Films, 1977,42, 147. Dr. L. M. Peter (Southampton) said: We cannot agree with Dr. Froelicher that our argument is inconsistent. We have made it abundantly clear in our paper we set out to test whether the thickness could be determined using the a2 values for Ti02. The success of this approach is evident in fig. 5 of our paper which shows how the thickness data obtained by analysis at different wavelengths fall onto a common plot. There can be little doubt that we are dealing with Ti02 here. We introduced the hypothesis that a lower oxide may be present at lower potentials in order to explain the late onset of the photocurrent during the growth scan; apparently the first 2 nm or so of the film are photoinactive.It is this film which “ may not be TiO, at all ”. The change in growth law suggested by Froelicher as the origin of the break in slope of the (apparent thickness, potential) plot (fig. 5) should also give rise to a change of slope in the ( l / C , E ) plot at the samepotential. We have not found this to be the case, and prefer a model in which the “ TiOz film could . . . grow at the expense of the underlying photoinactive oxide ”. Dr. M. A. Malati (Chatham) (communicated): The results presented in some figures of Dr. Peter’s paper (e.g., fig. 6 might) have been affected by the presence of &Pod. In a continuation of our study of the photoinduced oxidation of normal primary alcohols by anatase,’ we have noticed that when a Degussa anatase P25 was pretreated with H3P04, the yield of Ti3+ ions produced in n-butanol dropped from 1.5 x loe3 mol to ca.mol in 60 min as a result of the pretreatment2 This has been ascribed to the adsorption of phosphate at the Ti4+ surface sites., The specific adsorption of phosphate and of sulphate by P25 has been previously rep~rted.~ The photoenhanced decomposition of KMn04 solution in presence of Ti02 (anatase or r ~ t i l e ) ~ has been also in~estigated.~ A Tioxide sample of rutile, CLDD/ 1124,6 was shaken with H3P04 (1 mol dmm3) and then washed and irradiated with Hanovia chromatolite lamp as a suspension in KMn04 solution (1.99 x mol dm-3). The rate of photodecomposition dropped from 2.2 to 0.86 pmol min-’ as a result of the pretreatment with H3P04.’ This may be also ascribed to the adsorption of phosphate on the rutile surface.A. D. Buss, M. A. Malati and R. Atkinson, J. Oil Colour Chem. ASSOC., 1976, 59, 369. M. A. Malati and N. J. Seager, J. Oil Colour Chem. Assoc., in press. R. Flaig-Baumann, M. Herrmann and H. P. Boehm, Z. Anorg. allg. Chem., 1970,372,296. V. V. Sviridov and L. V. Potanina, Dokl. Akad. Nauk Beloruss. SSR, 1968, 12, 813. M. W. Rophael and M. A. Malati, Chemie f i r Labor und Betrieb, 1981, in press. M. A. Malati and A. E. Smith, Powder Technol., 1979, 22, 279. ’ R. Cobb, unpublished results. Dr. L. M. Peter (Southampton) (communicated) ; Although the effects of pH changes on the flat-band potential of Ti02 have been widely investigated, less is known about the influence of anion absorption.The systems described by Dr. Malati are notGENERAL DISCUSSION 121 \ - Gaertner "<approximation - \ strictly comparable with ours since we are usually concerned with the saturation photo- current obtained when surface recombination processes are unimportant. However, we have observed that the stability of the films is sensitive to the anion present in acid and it is clear that further study of anion adsorption on oxides is needed. J. F. McAleer, Ph.D. Thesis (University of Southampton, 1980). * J. F McAleer and L. M. Peter, to be published. Dr. H S. Jarrett (Delaware) said: These comments are addressed to Dr. Gautron and his colleagues. I have obtained a theoretical expression for the photocurrent density at the surface of an illuminated Schottky barrier by first solving the diffusion equation for photo- injected minority carrier density with finite lifetime, 7,.The solutions are subject to the boundary conditions that at the surface the minority carrier current is proportional to the minority surface-charge density and that the carrier density and its gradient are continuous at the barrier/bulk interface. For sufficiently high potential, the photo- current efficiency is always unity, and the behaviour observed by Gautron et al. would not be observed. However, such potentials can never be attained because inversion or tunnelling would occur first. Therefore, one must consider the photocurrent effi- ciency at some practical potential below the onset of inversion, say, 1 V beyond fiat- band potential for a semiconductor such as Ti02. At such potentials, various intrinsic and extrinsic parameters defining the semi- conductor now play a role in the photocurrent efficiency.One important parameter that emerges from this analysis is a characteristic time T,, the transit time across the rl 0.4 0.3 0.2 0.1 0 lo-' 1 10 lo2 lo3 Ni IN1 FIG. 15.-Plot of q against Ni/Nl: aLp = 0.1, V = 0.1 eV. Schottky barrier, which is inversely proportional to the impurity concentration, Ni. As Ni increases, z,, which may exceed z, at low impurity concentration, can become shorter than z, at the higher concentrations. For Z,/z, B 1, photocurrent efficiency is low because minority carriers are lost by recombination within the barrier during the long time these carriers take to traverse the wide barrier.For z,/ z, - 1 , the barrier narrows, and the minority carriers now traverse the bar- rier in a time comparable with or shorter than the time to recombine. Thus, the photocurrent efficiency increases with increasing impurity concentration. However, this trend cannot continue without limit. For zt/z, < 1, my expression for the photocurrent reduces to that originally122 GENERAL DISCUSSION obtained by Gaertner.' At such high impurity concentrations, the barrier is so narrow that although all carriers generated within the barrier traverse it before recombining, most of the minority carriers are generated in the bulk of the semiconductor because the penetration depth of light into the semiconductor greatly exceeds the barrier width.The bulk is field-free, and these carriers do not contribute to the photocurrent but remain in the bulk where they are lost by recombination. Thus, the photocurrent efficiency becomes low again after having passed through a maximum at intermediate concentrations. This behaviour, which is demonstrated in fig. 15, is qualitatively similar to that found by Gautron et al. The efficiency is determined at 1 V into the depletion region. NJN, is the impurity concentration normalized to N,, the impurity concentration at which z,/ z, is unity. All intrinsic parameters, z,, carrier mobility, diffusion constant, optical absorptivity, and surface transfer coefficient are assumed to be independent of impurity concentration.' W. W. Gaertner, Phys. Rev., 1959, 116, 84. Dr. M. A. Butler (New Mexico) said: I would like to comment on the nature of the sub-band-gap response shown in fig. 6 of the paper by Lemasson et al. Recently1 we have considered this photoresponse in Ti02 and SrTi03 in some detail with respect to the mechanism responsible for it and its spatial origin. It exhibits the following characteristics : (1) a linear dependence on light intensity, (2) a square-root dependence on the potential across the semiconductor and (3) the quantum efficiency to the two- thirds power is linear in photon energy. The potential dependence has been inter- preted as indicating a bulk origin from states uniformly distributed throughout the depletion layer. This follows directly from the dependence of depletion-layer thick- ness on potential,' and supports the identification of these states with defect states.The linear dependence on light intensity, even under laser illumination at > 200 mW cm-2, suggests that a two-step photoexcitation process is likely since it is doubtful that impurity-band conduction through the deep levels would be large enough to sup- port the kind of photocurrents observed. Such a two-step process can give a linear dependence for real intermediate states with appropriate lifetimes. This simple idea has been further developed using density of states arguments with simple parabolic bands to explain the observed spectral dependence. M. A. Butler, M. Abramovich, F. Decker and J. F. Juliao, J . Electrochem. Soc., 1981, 128, 200.* M. A, Butler, J . Appl. Phys., 1977, 48, 1914. Dr. P. Lemasson (Meudon) said: The existence of a sub-band-gap photoresponse with a diminishing degree of reduction is correlated to a lower donor concentration together with a more important role of the acceptors. The latter influence the sub- band-gap photoresponse strongly. As a consequence of the method of sample pre- paration we used, these acceptors are certainly distributed uniformly in the bulk; this uniformity exists right up to the surface. From the above, it follows that our explanation of the observed sub-band-gap photoresponse by a tunnelling of holes inside the energy band (level) of acceptors can be considered as possible. However, as suggested by Butler, a two-step absorption mechanism is equally pos- sible.In all the cases the problem of positioning the level(s) concerned within the for- bidden gap remains.GENERAL DISCUSSION 123 Dr. S. Ginley (New Mexico) said: (1) Has an examination been made of the de- pendence of quantum efficiency on wavelength ? The longer wavelengths penetrate more deeply and should be a better probe of the depletion layer than the short wave- lengths employed in the study. (2) Is the added Cr in your Ti02 electrodes acting simply to increase the density of impurity levels? How is the above-band-gap photo- response for the TiO, affected by Cr addition? Dr. P. Lemasson (Meudon) said: The photocell efficiency has been measured for wavelengths up to 410 nm. However, at 335 nm, the wavelength at which the results presented above have been obtained, the penetration depth of the light is always suffi- cient in comparison with that of the space-charge region.for a band- bending V, = 1.8 V, corresponding to the measured V,, (this gives the minimum value of V, and consequently of W in our measurements). For all values ND > (ND)o, the penetration depth I is larger than W. Following our experimental results, doping by chromium does not seem to influence the sub-band-gap photoresponse. An interpretation of the above may be that incorporation of chromium in Ti02 is equivalent to a reduction of the solid: it leads to the creation of autocompensating oxygen vacancies and not to acceptors (titanium vacancies). The consequence of the preceding remark is that during the incorporation of chromium the density of the impurity levels concerned remains unchanged.The above-band-gap photoresponse has not been studied by us in detail and in the present state of our experiments we cannot give reliable information concerning the influence of doping by chromium. The usually assumed relation lo = l/cco leads to (ND)o = 3 X loL8 Prof. A. J. Bard (Texas) said: The reported flat-band potentials (and hence the location of the energy levels for Ti02) appear to be quite different from those given by most authors. For example, in 1 mol dme3 H2S04, the reported value of V,, in this paper is -1.6 V us. saturated mercury sulphate electrode which corresponds to ca. -0.95 V 21s. NHE. In most studies' the position of Vfb in 1 mol dm-3 acid has been located at ca.0 V us. NHE. This finding is important since production of hydrogen at counter-electrodes in n-TiO, cells does not occur very readily and the production of oxygen appears to occur via processes in the valence band involving the formation of hydroxyl radical. Based on the potentials and energy-level diagram given here, hydro- gen formation should occur quite easily but hydroxyl formation would not be possible. See, for example, W. P. Gomes and F. Cardon, in Semiconductor Liquid-Junction Solar Cells, ed. A. Heller (Electrochem. SOC. Proc., Princeton, N.J.), vol. 77-3, p. 120. Dr. P. Lemasson (Meudon) said : The flat-band potential value we assume and that usually assumed are both indicated in the energy diagram presented in fig. 16. The short-circuit and open-circuit potential values experimentally determined have been used in the determination both of the band bendings and of the position of the metal Fermi level.It then appears that the Fermi level in solution lies between the levels corresponding to the H+/H2 and OH-/02 couples. With both flat-band potential values, the working of such a photocell may similarly be explained by a photogalvanic pr0cess.l J. G . Mavroides, D. I. Tchernev, J. A. Kafalas and D. F. Kolesar, Mater. Res. Bull., 1975, 10, 1023.124 GENERAL DISCUSSION 2p++H20-1/20,+ 2H+ T i 0 2 - x e l e c t r o l y t e metal pH =14 FIG. 16.-Energy diagram of a photocell with TiOz anode with two different band bendings corres- ponding to V,, and Vs, and with two possible flat-band potential values: Vfb (our measurements) and V'fb (Gomes measurement).The latter corresponds approximately to the energy level of couple H+/H2. Reactions occurring both at the anode and at the cathode are mentioned as in ref. (l), previous page. Mr. R. Potter (Southampton) said: The use of Mott-Schottky plots to determine flat-band potentials is not without danger. In particular it is necessary to measure capacitance data over a range of frequencies and over a sensible range of potential. Fig. 17 illustrates a set of Mott-Schottky plots which we have obtained for a hydrogen- reduced rutile specimen which has been prepared by the methods described by Dare- Edwards and Hamnett.' The change in slope which is evident in the Mott- Schottky plots could well have remained undetected if data had not been taken at potentials below this point.An extrapolation of the upper part of the plots would clearly lead to an erroneous value of the flat-band potential. We attribute the change in slope of the Mott-Schottky plot to a two-level donor state; the ratio of the slopes is close to 2 : 1, suggesting that a two-stage ionisation of titanium interstitials or oxygen vacancies may be involved. We cannot completely exclude the possibility that the results may be due to a spatial inhomogeneity in the donor distribution, but the ratio of the Mott-Schottky slopes and the sharpness of the discontinuity make this unlikely. The quality of the crystal surface is demonstrated by the exact correspondence of the flat-band potential with the onset of the photocurrent (fig.18), and this proves that the flat-band potential of the rutile sample in 1 mol dm-3 H2S04 was +0.2 V vs. NHE. The unusually negative Efb values found by Gautron et al. may therefore result from the way in which the Mott-Schottky plots in fig. 2 of their papers are extrapolated. M. P. Dare-Edwards and A. Hamnett, J. Electoanalyt. Chern., 1979, 105, 283. Dr. A. Hamnett (Oxford) (partly communicated): I would like to make several comments on the paper by Gautron et al. The authors have reported studies on the effects of doping level on the photoelectrochemical properties of rutile in the sub- stoichiometric (TiOz.J region. Implicit in much of their interpretation is the assump-GENERAL DISCUSSION 3-01 125 0 0 o n I 2.5 2 .o N 1 . 5 z 2 . N rj 1 . O 0 .5 0 0.0 0.5 1 .o 1.5 2 .o V/V us. SHE FIG. 17.-Mott-Schottky plot for single-crystal rutile. Preparation: Etched 1 : 1 (NH4)2S04 + HzS04, reduced in H,at 650 "C. Electrolyte 1 mol dmd3 H2S04. Frequencies: 0,110; 0, 1 kHz. tion that the nature of the defect centre does not alter as x increases. However, there is little evidence for this assumption. Indeed, recently James' has reviewed at length the defect equilibria in TiOz and his conclusions are that in the very slightly sub- stoichiometric region (x lo-^) Schottky disorder dominates, the effective point defect being an oxygen-ion vacancy. At higher values of x , the dominant defects are (132) shear planes that order for x 2 0.02. It seems likely that such shear planes are in kinetic equilibrium with isolated Ti3+ interstitials, but that such ions are in rela- tively small concentration. The first point to be made is therefore that the multitude of equilibria possible in TiOzPx makes it unlikely that any conceptually simple model will suffice to explain all the data.The second point concerns the erroneous flat-band potential predicted from fig. 2 of the paper. In materials with complex defect structure, such as Ti02-x, the flat-band potential cannot be obtained by extrapolation of the Mott-Schottky plot from a poten- tial region remote from flat-band. In the presence of deep traps that may ionise in126 GENERAL DISCUSSION the surface region in the presence of an anodic bias, it can be shown2v3 that a kink in the Mott-Schottky plot is to be expected. The potential at which the change in slope occurs provides a rough estimate of the trap energy, and from the data of Gautron et al.an energy ca. 1 eV below the conduction band is found for the lightly reduced samples, reasonably 100- 90 - 8 0 - 70- 6 0 - 9 5 0 - e W 40 - 30 - 2 0 - 1 0 - close to that expected for a singly occupied oxygen vacancy. 0 0 0 0 0 0 0 0 0 0 1 I I I V/V us. SHE I 0 .o 0.5 1.0 1.5 2.0 FIG. 18.-Photocurrent conversion efficiency, a, as a function of voltage for the same crystal in 1 mol dm-3 H2S04. Wavelength 330 nm. Additional evidence for the presence of deep traps is provided by the authors’ own con- ductivity data which shows a drop of ca. 2 units as the temperature falls from 1100 to 300 K. The third point I wish to make concerns the efficiency curve plotted in fig.3 of the paper. The authors state that the optimal efficiency corresponds to a value of the depletion-layer width equal to the reciprocal absorption coefficient. This is greatly oversimplified. Transport of minority carriers depends on the interplay of mass and field-assisted mechanisms and the relative importance of these depends in turn on the minority-carrier diffusion length L, and the electrochemical rate constant ko at the surface. Recently we have treated this problem theoretically allowing for recom- bination in the depletion layer, and it has emerged that only for very restricted ranges of these parameters should a maximum in efficiency with ND be seen. The authors’ own data of fig. 3 cannot be fitted by this formula. Instead we have fitted the efficiencyGENERAL DISCUSSION 127 for ND values below ca.2 x 10l8 cm-3 and have obtained values of ca. 7 x cm for L, and 0.04 cm s-' for ko assuming Jarrett's value of cm2 s-' for the diffusion coefficient for holes. These values are quite reasonable, but they do not predict a maximum efficiency with ND. There are two possible explanations for the sharp fall- off. First, the maximum in efficiency corresponds quite well to the first unambiguous appearance of shear planes; in other words, the character of the defects is changing, and such a change will be reflected in the band structure and in such parameters as recombination rates that must be put into the model. This must become significant once the shear planes order. The second possibility is that majority-carrier tunnelling may become significant for large values of ND, The oxidation of water is not a one- electron process and easily-reduced high-energy intermediates are likely to be in- volved.The back reduction of these species by electrons tunnelling through the de- pletion layer from the bulk becomes increasingly likely as the depletion layer contracts with increasing donor density. The fourth point is that the effects of mechanical polishing can only be removed by high-temperature (700 "C) reduction and annealing followed by extensive etching using a strong etchant such as boiling concentrated sulphuric acid.' Finally, we have also observed small sub-bandgap photocurrents in lightly doped SrTiO,. Recent p.e.s. data from this laboratory on the spinel system Lil+x Ti2--x04 have revealed that the formation of oxygen vacancies gives rise to electronic states ca.1 eV above the 0 2p band. It is quite possible that holes created in such levels in TiOz and SrTiO, can tunnel to the surface more rapidly than electron recapture can occur. Above this doping level, the photocurrent vanishes in SrTi0, as well as Ti02, and since the dominant defect in SrTiO, remains an oxygen vacancy, this sug- gests that the authors are correct in assuming that recombination becomes too fast. We have seen much larger sub-bandgap photocurrents in Cr-doped SrTiO,, the initial optical process here being undoubtedly a Cr3 + -Ti4+ charge-transfer.6 However the structural chemistry of Cr doped into SrTi0, is quite different from Cr doped Ti02.In the latter case shear structures form even at very low concentrations of Cr. R. James, Disorder and Non-stoichiometry in Rutile and Corundum Structured Metal Oxides AERE Harwell, Technical Publication 814 (1979). V. A. Myamlin and Yu. V. Pleskov, Electrochemistry of Semiconductors (Plenum Press, New York, 1967). J. H. Kennedy and K. W. Frese, J. Electrochem. SOC., 1978, 125, 723. W. J. Albery, P. N. Bartlett, M. P. Dare-Edwards and A. Hamnett, J. Electrochem. SOC., to be submitted. M. P. Dare-Edwards and A. Hamnett, J. Electroanalyt. Chem., 1979, 105, 283. G. Campet, M. P. Dare-Edwards, A. Hamnett and J. B. Goodenough Nouv. J. Chim., 1980, 4, 501. Dr. P. Lemasson (Meudon) said: I thank Drs. Potter and Hamnett for their re- marks, and deal with the various points in sequence.A complete set of experimental results in KOH medium is presented in fig. 19. The following remarks can be made : (1) In the (i, Y ) characteristic, a cathodic current rises at potentials lower than - 1 .O V/e.s.m. As oxygen is present in solution, the corresponding plateau may be attributed to oxygen reduction.' (2) Hydrogen evolu- tion begins at potential values lower than - 1.8 V/e.s.m. ( 3 ) A change in slope appears below - 1 .O V/e.s.m. in the Mott-Schottky plot (capacities being measured at 100 kHz). An extrapolation of this portion of curve, which is indeed not a straight line, leads to a flat-band potential Vfb = - 1.6 V/e.s.rn.128 GENERAL DISCUSSION (4) In the (iph, V ) characteristic, the Yo, value is V,, = -1.5 V/e.s.m. From these observations, the following conclusions can be drawn : (a) The change in slope in the Schottky plot seems to be connected to the rise of a cathodic current.In such conditions, the main question is the validity of a Schottky model because of the perturbation of charge equilibrium. Furthermore, this model assumes the existence of a fully depleted layer and is not applicable for band bendings lower than 0.3 VS3 (arb. units) I - - - - r - - 1 1 * V1e.s.m. FIG. 19.-Dark current (idark) photocurrent (ipb) and Schottky (C2) plots (in arbitrary units) as func- tions of the applied voltage (0. These two reasons explain why we have not chosen to introduce two donor levels in our paper ; such changes in slope exist for many semiconductor-electrolyte junction Schottky plots even in cases where physical measurements indicate clearly the existence of only one donor level.(b) It seems possible to explain the difference that exists between the Yo, and the Vfb values with the help of recent theories concerning the current voltage characteris- tics of semiconductor-electrolyte junctions under ill~mination.~ Besides, in our particular case, suggestions concerning a possible influence of an inhomogeneity of our samples seem ruled out more easily than when samples are reduced in a hydrogen atmosphere. This particular point is developed in that part of our answer concerning the preparation of our samples. (c) If the flat-band potential is - 1.5 V/e.s.m. as usually assumed, it seems difficult to explain how hydrogen evolution takes place only at potentials more negative than - 1.8 V/e.s.m. The quality of the surface preparation for surface recombination rate is closely connected to the etching used after polishing.The etching procedure proposed by Dr. Hamnett, while leading to low surface recombination rates [as proved by the shape of the (photocurrent, potential) charac- teristics] is nevertheless inconvenient for us as it is oxidizing.GENERAL DISCUSSION 1 29 Because we needed an electrode surface with the same stoichiometry as the bulk, we used a poorly oxidizing etchant which was not sufficient to remove completely the mechanically damaged layer. As mentioned in our paper, there seems to be an incompatibility between obtaining a surface presenting the property we needed and obtaining a good efficiency, i.e., a low surface recombination rate.In this range, the shape of conductivity and thermogravimetry isotherms may be ex- plained, as usual, by means of a point defect m0de1.~ For x > 0.01, large discon- tinuities occur, both in conductivity and thermogravimetry isotherms, indicating the existence of new phases (Magneli From fig. 1 in our paper, it is clearly pointed out that we are not in this region. After the quenching of our samples, they were investigated with the electron microscope and it was shown that there are no shear planes in them.8 At room temperature, it is quite evident that shear planes exist only when passage from high to low temperature is sufficiently slow that a point defect reorganization is possible. It then seems logical to assume the existence at room temperature of point defects (donors + acceptors), in an ionization state different from that at high tem- perature and with relative amounts depending on the degree of reduction.Conse- quently, the samples of Ti02-x we have studied here are all included in the mono- phasic domain of rutile. Finally, the way in which our samples were prepared, as explained in our paper and completed by the previous explanation concerning the nature of defects, indicates that they have a very good spatial homogeneity during the high-temperature equili- brium. This is not true for samples reduced in vacuum or in a hydrogen atmosphere which are usually used in electrochemical experiments. At high temperature, the non-stoichiometry range lies between < x < ' P.Clkhet, C. Martelet, J. R. Martin and R. Olier, Electrochim. Acta, 1979, 24, 457. ' P. Lemasson, A. M. Baticle and P. Vennereau, Surface Sci., 1976, 59, 177. V. A. Myamlin and Yu. V. Pleskov, Electrochemistry of Semi-conductors (Plenum Press, New York, 1967). J. Reichman, Appl. Phys. Letters, 1980, 36, 574. P. Kofstad, Non-stoichiometry, Diflusion and Electrical Conductivity in Binary Metal Oxides (Wiley, New York, 1972). J. F. Baumard, D. Panis and A. M. Anthony, J. Solid State Chem., 1977, 43, 20. J. S. Anderson and A. B. Khan, J. Less Common Metals, 1970,22, 219. * M. G. Blanchin, P. Faisant, C. Picard, M. Ezzo and G. Fontaine Phys. Stat. Solidi (a), 1980,60, 357. Dr. H . R. Sprunken (Kiel) said: There appears to be an inconsistency concerning the relation between light penetration depth, " best " doping density (N,) and photo- efficiency.As shown in fig. 6 (c) of Gautron's paper the influence of the doping den- sity of photocurrent will be small for a wavelength A < 340 nm. In contrast a strong influence is shown in fig. 3 with a maximum efficiency around ND = 5 x 10'' cm-j. Similar dependences on doping densities have been observed previously by Tamura for ;1 = 400 nm irradiation' and by us using white light.2 On the other hand if the results shown in fig. 3 were obtained with A = 355 nm3 corresponding to the light penetration depth l/m0 = lom5 cm the authors used for their (ND)o calculation the observed efficiency should include a 30% contribution from diffusion provided Lp 21 2 x cm is assumed.This contribution would be negligible only for photon energies close to the band gap, i.e., around 400 nm. I wonder whether Gautron et al. have applied corrections for this diffusion. Do they agree that their calculated value of (ND)o = 3 x 10l8 cm-j is not the " best '' doping density for other photon energies, i.e., other penetration depths l/ao?130 GENERAL DISCUSSION If one calculates the " best " doping density ND for irradiation with light close to the band gap, a density of ND 2: 2 x 1014 is obtained'*2 in contrast to the experi- mental findings. Do the authors consider in their discussion in addition to weak electric fields in the space-charge layer also processes via surface states to be responsible for the small efficiency observed for the low doped materials? Reactions mediated by these surface states would become more pronounced the smaller the doping density of the semiconducting electrode and thus the electric field of the depletion layer.' H. Tamura, H. Yoneyama, C. Iwakura, H. Sakamoto and S. Murakami, J. Electrounulyt. Chem. Interfacial Electrochem., 1977, 80, 357. H. R. Sprunken, R. Schumacher and R. N. Schindler 2nd. Int. Conf. Photochem. Conversion and Storage of Solar Energy, Cambridge, (1978), extended abstracts. D. M. Eagles, J. Phys. Chem. Solids, 1964,25, 1243. Dr. P. Lemasson (Meudon) said: In the case of the work cited as ref. (2) in the above question, it seems that anodes are not constituted by well-defined oxides. Indeed, the electrodes are prepared by thermal oxidation of Ti foils in air and some comments can be made : (1) The electrode surface, essential to determine the Mott-Schottky slope, is not easy to determine with good precision.(2) The thickness of the layers is not precise but it does not seem impossible for a 400 nm wavelength, a value at which penetration depth of light is approximately 10 pm, that the metal background will be reached by light, thus leading to multiple reflexions in the oxide layer. (3) Homo- geneity for such samples is quite questionable. In our case, the materials 'are massive in form and, as previously indicated, the assumption of homogeneity seems realistic in regard to the preparation used (see the answer on this subject to Dr. Hamnett). During measurement at fixed potential value of the (photocurrent, wavelength) characteristic, no noticeable modification of the shape in relation to the Ti02-x substoichiometry has been observed: in each case, a smooth maximum is found around 340 nm.In these conditions, the arguments developed on this question regarding the ND value for the maximum efficiency are ruled out. So far as the influence of minority-carrier diffusion on the photocell efficiencies is concerned, we are not certain that it will be important in comparison with space- charge region generation. A model which includes bulk recombination has recently been developed1 leading to diffusion being less important. An influence of the wavelength value of incident light (corresponding to energies higher than the forbidden gap) on the position of the maximum may be expected but our purpose here was to locate this maximum, if it existed, given that for each N,, value the photocurrent is a maximum at a given potential value.Although some discrepancies exist between the different results, it is interesting that in three different works on Ti02 electrodes (this one and the two cited above) a maximum efficiency has been observed at relatively similar N,, values. The dif- ferences between the three sets of results may certainly be accounted for by the dif- ferent types of electrode preparation used by the respective authors. ' J. Reichman, Appl. Phys. Letters, 1980, 36, 574. Prof. W. J. Albery (London) said: I would like to comment on the explanation offered by Gautron et al. for the maximum shown in their fig. 3. In collaboration with Dr. Hamnett, we have recently solved the differential equation describing the transport and kinetics of photogenerated minority carriers, including recombinationGENERAL DISCUSSION 131 in the space-charge layer.’V2 A rigorous solution can be obtained in terms of confluent hypergeometric functions, but we have shown that a good approximate solution is:- where, in addition to the symbols in Gautron’s paper 8, = eV,/kT, k = D/Lp,2 and k’ (in cm s-l) describes the reaction of the holes at the surface of the semiconductor. In the denominator the term in L, describes loss of holes from the surface by recombi- nation in the field-free region. Because of the large value of 8, this term can be ig- nored. The term in W describes the loss of the holes in the space-charge region. For the reasons stated by Gautron et al., q can show a maximum as W is varied. However, the condition R,W = 1 suggested by them is too simple, since the position 20 g E- 10 0 0 0 I I I 1 0 ’ ~ 1 OI6 10” NDlCrn - FIG. 20.-Variation of q with ND. The points are taken from fig. 3 of the paper of Gautron et a/. The curve is calculated from eqn (1) using the values of the parameters in the text. of the maximum must also depend on k’, k, 8, and L,. It is also true that the maxima predicted by eqn (1) are not nearly as pronounced as are the experimental results in fig. 3 of their paper. Fig. 20 shows the best fit that I can obtain to the points in fig. 3 of the paper, where I have used the following parameters:- ROW = 0.50 UOL, = 0.04 and k’/cco D = 1.1. In this case the condition for R,W is similar to that suggested by the authors, L, is smaller and using the value of D suggested by Jarrett3 we obtain a plausible value of k’ of 10 cm s-l.132 GENERAL DISCUSSION W. J. Albery, P. N. Bartlett, M. P. Dare Edwards and A. Hamnett, J. Electrochem. SOC., sub- mitted for publication. H. Jarrett, Faruduy Disc. Chern. Soc., 1980, 70, 121. ’ W. J. Albery, Chem. SOC. Rev., to be published. Dr. P. Lemasson (Meudun) said: I now respond to comments by both Prof. Albery and Dr. Hamnett. For the reasons mentioned in the answer to Dr. Ginley on the penetration depth of light into the semiconductor, the whole space-charge region is penetrated by light in our photocell measurements. Of course, it is difficult to discuss the physical model proposed in the questions for the reason it is not yet published and not available to us. However, some further remarks can be made: (1) The experimental points plotted in fig. 3 of our paper are reliable and are not in question. (2) The observed maximum is less pronounced when efficiency is measursd with sintered photoanodes. This case corresponds to a very low L, value. (3) The L, value given by the fitting proposed by Prof. Albery which presents a maximum, seems unrealistic (L, = 4 x lo-’ cm) whereas the rate constant (ko = 10 cm s-’) seems probable. (4) In the calculation proposed by Dr. Hamnett, only a very small number of our experimental points is involved and if the L, value (7 x cm) is acceptable, the rate constant (ko = 0.04 cm sdl) seems too low. (5) The shape of the photocell characteristic reported in fig. 5 of our paper can be explained by means of the model proposed by Reichmanl in the case where recombinations in the space-charge region are negligible. The de- crease in efficiency with increasing ND values can then be accounted for by the increase of bulk recombinations as light penetrates more deeply than the space-charge region: this is the case for ND Concerning specifically the &, values, we measured them for the three different types of electrodes we used, following the method proposed by TyagaL2 The results are 3 x lo1’ ~ r n - ~ . single crystals etched: L, fi 2 x cm single crystals etched + polished: L, E 2 x cm sintered electrodes : Lp 21 8 x cm These results seems to indicate that even the L, value used by Dr. Hamnett in his fitting is inadequate as long as our efficiencies in fig. 3 correspond to the second type of electrodes. l J. Reichman, Appl. Phys. Letters, 1980, 36, 574. V. A. Tyagai, Fiz. Tverd. Telu, 1964, 6, 1602.

ISSN:0301-7249    

DOI:10.1039/DC9807000093

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

GENERAL DISCUSSION 133 ADDITIONAL REMARKS The following remarks were received on 1 lth December, 1980, after typesetting of the General Discussion sections had commenced. They have not been read in proof by the authors. Prof. R. N. Schindler (Kiel) said : In response to Prof. Gerischer (p. 110) I wish to remark that the presence of reducible species at the semiconductor/electrolyte inter- face is a prerequirement for the observation of a cathodic photoeffect using our n-type materials. The photocurrent is superimposed on a reductive dark current and can be observed also without a lock-in system (see fig. 2 of our paper). It is clearly not of the transient type as described in the literature. To avoid the observation of transient phenomena we used a chopping frequency of 0.1 Hz and a scanning rate of 2 mV s-l.In addition, the existence of behaviour as shown in fig. 3 of our paper excludes a transient character of the observed effect. We correlate the occurrence of a large cathodic photoeffect to the high surface area of our polycrystalline material and to the low electron density of our samples (ND < 1017 ~ m - ~ ) . In such systems the presence of suitable electron acceptors in the interface will effectively influence the position of the Fermi level for electrons at the surface and eventually lead to an inversion of the majority-carrier concentration. Cathodic photoeffects were also obtained with single-crystal material, although much less pronounced. Prof. R. N. Schindler (Kiel) said: We agree with Dr. Pichat (p. 11 1) that adsorbed electron scavengers like oxygen reduce the immediate electron-hole recombination by trapping photogenerated surface electrons.If the scavenger is an oxygen-containing species photocatalytic oxidation processes over Ti02 will occur. In addition to electron capture, which represents a heterogeneous electron transfer, we have also presented evidence' for a reduction of electron mobility by adsorbed electron scavengers not containing oxygen, e.g. SF6 and halogenomethanes. In these cases the " trapped " conduction band electrons become visible by e.s.r. We antici- pate that this reduced mobility should also become apparent in Dr. Pichat's conducti- vity measurements. Similarly, in the photoelectrochemical cell the cathodic photo- effect can only be observed in the presence of an electron-accepting species.Electron donors reduce the cathodic photoeffect. Prof. R. N. Schindler (Kiel) said: We thank Dr. Ginley for his observation (p. 112). First we would like to answer his direct question. We do not believe that the p-type photoeffect on TiOz and SrTi03 is due to accumulation of electrons at the sur- face. The experiments reported in our paper revealed just the opposite, namely a depletion of electrons at the surface. In this context we refer to the following points: It can be seen in fig. 2 of our paper that a sufficient cathodic dark current flows in addition to a relatively small photocurrent, so that accumulation of photoproduced electrons would appear to be negligible. This suggests that photo- and dark electrons might be transported to the electrolyte via the same channel.The latter point is con- sistent with the charge-transfer model of Bard.l In agreement with this explanation is the result that cathodic quantum efficiency increases with the concentration of redu- cible electrolyte species.2 Finally we would like to mention that an accumulation of photoelectrons at the surface should also depend on the electron density of the sample which, of course, is determined by the donor concentration ND. Our experiments revealed, however, that cathodic photocurrents decrease with increasing doping den- sity. E. Serwicka, M. W. Schlierkamp and R. N. Schindler 2. Naturforsch., in press.134 GENERAL DISCUSSION Furthermore we would like to respond on Dr. Ginley’s cathodic ageing experi- ments, because we observed changes of the spectral response after cathodic biasing of Ti02 as well.Our experiments, however [see fig. 4(a) curves 1 and 2 of our paper] showed an increased photoresponse only in the short-wavelength range, which may be due either to additional protons introduced close to the surface and/or to the removal of adsorbed oxygen. The changes in the spectrum mentioned by Dr. Ginley and shown in fig. 11 of the General Discussion section might be interpreted as follows: Prolonged treatment under cathodic conditions leads to very high donor densities which probably extend to depths beyond the charge barrier. This reflects the de- creased wavelength response (curve 2) observed in the whole spectrum range. An influence due to adsorbed oxygen can be excluded under these experimental conditions.In our experiments, however, the observed change supports milder loading conditions. Similar to others we did not observe changes in the spectral response of SrTi0,3 which was loaded electrochemically. We take this observation as an additional evidence for Dr. Ginley’s strong reduction conditions. Finally attention is drawn to the information given in the table concerning vfb changes in the positive direction after cathodic ageing of SrTi03. The vfb values given are probably obtained by extrapolation of iph2 to zero similar to ref. (4). In fig. 20 is shown a cyclic photocurrent-potential diagram for a highly doped Ti02 electrode. t I I I I 1 -2 - 1 0 + 1 + 2 potential/V us. SCE FIG. 20.-Cyclic photocurrent-potential curve of a hydrogen-reduced Ti02 single crystal in 0.1 mol dmd3 NaOH under illumination with 13.= 350 nm. The scan rate was 3mV s-’. pH = 13. The scan starts at -2 V. The V,, shift observed resembles the Yfb shifts quoted in table 1. The decrease in photocurrent around +0.7 V is related to protons being forced out of the cry~tal.~ Care should be exercised to deduce Vfb values from elec- trodes loaded with hydrogen since shifts in V,, could result from a number of unspeci- fic changes of the surface character of the material. For example, as already pointed out by Prof. Bockris, the presence of recombination centres excludes the validity of the (iph2, Y ) relation to determine reliable vfb values. P. A. Kohl and A. J. Bard, J. Amer. Chem. SOC., 1977, 99, 7531.H. R. Sprunken, R. Schumacher and R. N. Schindler, Ber. Bunsenges. phys. Chem., 1980, 84, 1040. L. A. Harris, M. E. Gerstner and R. H. Wilson, J. Electrochem. SOC. 1979, 126,850. D. S. Ginley and M. L. Knotek, J. Electrochem. SOC., 1979, 126, 2165. R. Schumacher, Ber. Bunsenges. phys. Chem., 1980, 84, 125.GENERAL DISCUSSION 135 Dr. R. Schumacher ( K i d ) said: I am grateful to Dr. Garrett for the description of his experiments (p. 115). As far as his results on pure Ti02 electrodes are con- cerned it is not surprising that no V,, was obtained in the point-to-point measure- ments since all reducible species are removed this way. Also, his observation on vanadium-doped TiO, crystals clearly demonstrates the absence of cathodic photo- processes.This result confirms our observation on vanadium, niobium- and hydro- gen-doped Ti02 electrodes. In all these cases no V,, was apparent. The course of the Mott-Schottky plot (MSP) in Dr. Garrett’s diagram which shows a shallow maxi- mum around -0.4 V(SCE) may be taken as further evidence for the absence of charge-transfer processes involving acceptor states on the electrode surface. Accord- ing to his suggestion this maximum is due to the filling of acceptor states inside the crystal. On the other hand acceptor states on the surface of the electrode should cause a minimum in the MSP close to Vfb. All this seems to indicate that doping of Ti02 with electron donors such as hydro- gen, niobium and vanadium prevents the photoreduction of reducible adsorbates at the semiconducting electrode before v f b is reached.This observation is in accord- ance with an e.s.r. investigation on powdered n-Ti02 samples which demonstrated that surface adsorbtion of electron acceptors like O2 leads to the formation of 0, ions only on samples which do not contain dopants such as vanadium, niobium and hydro- gen, whereas for samples which do contain those additives no 0; species was observed spectroscopically. p 2 Hence, the difference of ca. 0.5 V between Vfb and V,, from your experiments cannot be traced back to contributions due to the filling of electron-acceptor states located on the electrode surface. We therefore suggest small photo responses below the defined V,, should also be taken into consideration so that Voc shifts towards Finally, we would like to mention that the observed relatively negative position of Vfb compared with samples without containing niobium are in agreement with results reported by other^.^,^ This behaviour may be caused by modification of Ti02 electrodes with electron donors as mentioned above. One might speculate that this treatment leads to an upward shift of the bands at the surface which results in a Vf, shift to more negative potential^.^ Vfb. E. Serwicka and R. N. Schindler, in preparation. E. Serwicka, R. N. Schindler and R. Schumacher, to be published. J. Vandermolen, W. P. Gomes and F. Cardon, J. Electrochem. SOC., 1980,127, 324. P. Salvador, Solar Energy Materials, 1980, 2, 413. H. R. Sprunken, R. Schumacher and R. N. Schindler, Ber. Bunsenges. phys. Chem., 1980, 10, 1040.

ISSN:0301-7249    

DOI:10.1039/DC9807000133

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

Photodecomposition of Semiconductors Thermodynamics, Kinetics and Application to Solar Cells BY HEINZ GERISCHER Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D 1000 Berlin 33, West Germany Received 28th May, 1980 Photodecomposition of semiconductors is caused by reactions of electrons or holes at the surface. Thermodynamic criteria for such processes are derived. The thermodynamics and kinetics depend on the energy position of the band edges and the concentration of electrons and holes at the surface which can both vary under illumination. The role of competing redox reactions and the influence of surface states on the decomposition reactions are discussed. Consequences for photoelectro- chemical solar cells are outlined. Photodecomposition is a phenomenon common to all semiconducting electrodes in photoelectrochemical cells. It is the prominent obstacle to their application in liquid junction solar cells and it is therefore very important in understanding the conditions which lead to photodecomposition.This problem has already been discussed from thermodynamic and kinetic points of ~iew.l-~ Some earlier conclusions ' n 3 will be modified in this paper. 1. THERMODYNAMIC ASPECTS 1 . 1 . DECOMPOSITION POTENTIALS OF SEMICONDUCTORS Semiconductors in contact with an electrolyte can decompose in electrochemical reactions either by oxidation or reduction. It was found in the earliest stages of semi- conductor electrochemistry that oxidative decomposition is caused by reactions with hole^^-^ and reductive decomposition by reactions with electron~.'9~ In some cases, electrons and holes are involved in different steps of the overall reactions.In order to provide a general formulation of this process we shall consider the decomposition of a binary-compound semiconductor MA and describe this by the following reactions : MA + (z - x)h+ + soh $ Mz+ solv + A + xe- MA + (z - y)e- + solv + M + A': solv + yh+. (1) (2) It turns out that x and y in these equations are practically zero if the semiconductors have band-gaps > - 1 eV.' A combination of both processes can describe the electro- chemical mechanism of a neutral decomposition reaction. The above reactions have been formulated as reversible processes although they are in most cases irreversible, proceeding only in the direction from left to right.In this paragraph, however, we shall deal with the thermodynamics of these reactions and consider them as reversible. Combining reactions (1) or (2) with the electrode reaction of the reference system, e.g., the standard hydrogen electrode,138 PHOTODECOMPOSITION OF SEMICONDUCTORS one can formulate chemical reactions for which the free-energy difference may be available in thermodynamic tables. This gives for the oxidative process, MA + zH+ solv + M": solv + A +$H2 and for the reductive reaction, (4) Knowing the AGO values of reactions (4) and (5), one ultimately obtains the standard decomposition potentials co in the electrochemical scale, with reaction (3) as the refer- ence zero, from -- AGO - cOdecomp d 3 F where the plus sign is used for the anodic decomposition [reaction (l)] and the minus sign for the cathodic one [reaction (2)].The free energy of the electrons and the holes depends on the electrode potential and is therefore variable. The thermodynamic data of all the other reactants of eqn (1) and (2) are invariable for a given composition of the system. Knowing the decom- position potentials for a semiconductor one can predict its stability from the free energy, i.e., the redox potential of electrons and holes in the electrode. If the redox potential of holes is more positive than the anodic decomposition potential, p&decomp, decomposition can proceed; otherwise not. If the redox potential of electrons is more negative than the cathodic decomposition potential, ncdecomp, decomposition is possible.These thermodynamic conditions for decomposition, however, do not imply that decomposition really occurs if these limits are exceeded. Kinetics modify the real behaviour very much as we shall discuss later. 1.2. FREE ENERGY OF ELECTRONS AND HOLES AND THE ELECTROCHEMICAL SCALE OF REDOX POTENTIALS The free energy of electrons in a solid is described by the position of the Fermi level, EF, because electrons obey Fermi statistics.' The reference state for the elec- tron is either the vacuum level or, in the case of a semiconductor, one of the band edges which form the energy gap. The concentration of electrons in the conduction band, n, is related to the energy of the bottom of this band, E,, to the effective density of states, N,, in this band and the Fermi energy EF by EF - Ec n N T - exp (- -) kT * (7) This is an approximation in which the electrons are treated as if they were all species For referred to a potential-energy datum at the bottom of the conduction band.n << N,, a simpler approximation is Analogous equations describe the concentration of holes in the valence band within the same approximations :H. GERISCHER 139 i f p < Nv: EF = Ev - kTln __ (9 where p is the hole concentration in ~ m - ~ and Nv is the effective density of states in the valence band in cm3. At equilibrium, the concentrations of electrons and holes are coupled by the rela- tion n x p = (Nc - n)(NV - p ) exp (1 1) where Ec - Ev = AEgag, the width of the band gap. For n < Nc and p < N,, relation (1 1) reads n x p = N c N v e x p ( - E&EV). Eqn (7)-(10) demonstrate that the free energy of electrons and holes is strictly corre- lated to the energy position of the band edges, Ec and Ev.It is therefore very impor- tant to know these energies. They are related to the work functions (Ev) and to the electron affinity (Ec) of a semiconductor. However, since in electrochemistry elec- tron energies are measured as redox potentials uersus a standard electrode as the refer- ence, it is necessary to know the positions of Ec and Ev on such a scale. One obtains the correlation between the electrochemical scale and the vacuum level if one knows the Fermi energy of the reference system in relation to the vacuum level. For the standard hydrogen electrode, this would be the Fermi level of the electrons in the metal on which the electrode reaction (3) is in equilibrium.This value is not known exactly, but is in the range -4.5 to &0.2 eV.lO*ll The exact value is not important from the conceptual point of view. We shall use the correlation EF = -eoc + const (13) where the constant is ca. -4.5 eV. There are several ways of measuring the position of the band edges on the electro- chemical scale. The most reliable comes from capacity measurements in the potential range where a depletion layer is formed at the contact to the electr~lyte.l~-~~ Another convenient method is by observing the onset of a photoc~rrent.~~ The position of the band edges can vary if the electric charge on the surface varies with the potential applied. This can be caused by an excess of electric charge either in electronic surface states, or in form of adsorbed ions or by a very high concentration of electrons or holes at the surface (approaching degeneracy).The position of Ec and Ev relative to the reference electrode in the electrolyte is then altered by a varying potential drop in the Helmholtz double layer at the interface. Such a variation of the position of the band edges can also be a consequence of illumination as we shall discuss later. 1.3. QUASI-FERMI ENERGIES OF ELECTRONS AND HOLES UNDER ILLUMINATION In the illuminated region of a semiconductor, where light is absorbed and electron- hole pairs are generated, equilibrium between electrons and holes is no longer pre- served. Both charge carriers are in an excess to such an extent that, in the steady state, recombination and transport locally compensate the generation rate.However, since the dissipation of excess energy within the electron bands occurs very rapidly, one can still assume that the distribution of electrons and holes over the quantum140 PHOTODECOMPOSITION OF SEMICONDUCTORS states of their respective bands remains in thermal equilibrium. The increase in free energy can then be described by the excess concentrations, and the free energy of electrons and holes can locally be represented by their respective quasi-Fermi levels,16 .E: and pEz Denoting the excess concentrations by An* and Ap* we can write n* = n +An* p* = p 4- Ap* (16) where n and p may be the equilibrium concentrations according to eqn (12).One sees that drastic changes in free energy under illumination can only be expected for carriers with a low equilibrium concentration such that Ac* > c. These are the minor- ity carriers of a doped semiconductor. In an intrinsic or very low doped material, the deviation in free energy can be large for both types of electronic carriers. 1.4. STABILITY AGAINST PHOTODECOMPOSITION We have stated in section 1.1 that the decomposition in the anodic direction re- quires the redox potential of holes to be more anodic than the oxidative decomposi- tion potential. For decomposition in the cathodic direction it is necessary that the redox potential of the electrons is more cathodic than the reductive decomposition potential. The redox potentials of holes and electrons can then be replaced by their Fermi or quasi-Fermi energies and the decomposition potential by the equivalent Fermi energies for decomposition according to eqn (1 3), A decisive factor for the reactivity is the local free energy of electrons and holes at the surface.Consequently, stability against photodecomposition means where the index s means " surface ". In order to predict stability or instability we must get some idea of the E$,s values which will considerably differ from the bulk values if light is falling onto the electrode. 2. KINETIC ASPECTS 2 . 1 . RATE OF DECOMPOSITION REACTIONS It has been stated in section 1.1 that anodic decomposition of semiconductors with a wider-band gap occurs with holes as reactants and cathodic decomposition with elec- trons as reactants.It appears that the rate-determining step in most cases is the first or second one-electron transfer step in a series of consecutive oxidation or reduc- tion steps which are needed for the net reaction. The real process is much more com- plex since structural influences like surface orientation, structural defects, efc., have to be taken into a c c o ~ n t . l ~ - ~ ~ We shall, however, use here a very simplified model for the description of the reac- tion kinetics which is represented by fig. 1. This picture shows the bond breaking inH. GERISCHER 141 two steps. The first is induced by a hole and results in the formation of a new bond between a nucleophilic ligand and one of the previously connected surface atoms. The second step completes the bond-breaking by a second hole with the formation of a new bond between the other surface atom and a second ligand.We assume that g: X x x -.sol v g X + h++ X- - + s o l v @ X +h'+ X- R + h++ X - - f-- I - P FIG. 1.-Model for the two first steps of anodic decomposition of a semiconductor. one of these steps is rate-determining and the following reaction steps completing the removal of the kink site atom from the crystal lattice are fast. The kinetics of the two reaction steps of this scheme can be described by the fol- lowing equation: ps is the surface concentration of holes, [I] the concentration of the intermediate and AqH the potential drop in the Helmholtz double layer, a1 and cc2 are the charge-trans- fer coefficients for the attachment of the ligand X- to the surface atoms in step 1 and 2 where the charged ligand has to pass the Helmholtz double layer.* Dl = 1 - al.Applying the steady-state condition, v1 = v2, one gets for the rate of decomposi- tion, The two limiting cases are as follows: First step rate-determining, second step rate determining, 3 * If the ligands were uncharged, but lose an ion during their attachment, like HzO forming an OH- bond, the same type of equation would be valid.2142 PHOTODECOMPOSITION OF SEMICONDUCTORS The rate equations resulting from this simplified model are typical of all electrolytic decomposition reactions, although they will usually be even more complex. They show that it is not only the concentration of holes (or electrons in cathodic decompo- sition) which controls the rate but also the position of the band edges at the surface which varies with the voltage drop ApH in the Helmholtz double layer.Whether and how a change in ApH can be induced by illumination will be discussed in the next sec- ion. We see in these rate equations that p s and ApH are the important parameters influencing the rate of anodic decomposition. For cathodic decomposition this role is played by n, and AyH, To establish the connection with the thermodynamic treat- ment, we shall use eqn (7)-(lo), (14) and (15) in order to describe the surface concen- trations. 2.2, SEMICONDUCTOR/ELECTROLYTE CONTACT UNDER ILLUMINATION The electronic charge carriers which take part in photoreactions are overwhelmingly generated in the bulk. Transport from the bulk to the surface occurs either by dif- fusion or by migration in an electric field.The mean diffusion length of excess car- riers depends on their lifetime and can vary to a large extent. Electric migration is important in the space-charge layer, which has a considerable extension only under conditions where a depletion layer is formed. The combination of electron-hole pair generation by light absorption with diffusion, recombination and electric migration in an electric field, which itself is dependent on the distribution of the mobile charge carriers, makes the theoretical calculation of the real charge distribution in the sta- tionary state of illumination extremely complicated. Some solutions have been out- lined based on simplifying a s s ~ m p t i o n s . ~ ~ - ~ ~ In order to understand the principal features, we need not go into such detail.A qualitative representation of the situa- tion found in an illuminated boundary layer at a semiconductor electrolyte contact will be sufficient for this purpose. We consider first a blocking semiconductor electrolyte contact in order to show how much, and in what way, illumination can change the free energy of electrons and holes at the surface. We compare the situation in the dark and under illumination for different positions of the Fermi level in the bulk, i.e., for different voltages applied. Fig. 2 shows the course of the band edge positions and of the quasi-Fermi levels in the boundary layer of the semiconductor for three characteristic situations. Fig. 2 is constructed for an n-type semiconductor with the penetration depth of the light being larger than the extension of the space-charge layer under illumination.For a p-type semiconductor with equivalent properties, the figures would have to be inverted with the band edges and Fermi levels exchanged in order to represent the analogous situa- tions. The energy scale in fig. 2 is related to the flat-band energy, Efb, of the particular semiconductor. The illuminated case represents the steady state where all generated electron-hole pairs disappear by recombination. The Fermi level in the bulk is as- sumed to be held constant by an external voltage source. Fig. 2(a) shows the situation at the flat-band potential. The bands remain prac- tically flat under illumination (Dember voltages are neglected).The Fermi level of the majorities (electrons) is nearly unchanged, but the quasi-Fermi level of the minorities (holes) deviates in the illuminated layer largely from the equilibrium value and reaches its maximum deviation at the surface. At very high intensity, pEZ can approach Ev. Fig. 2(b) represents the depletion layer with a positive excess charge on the n-type semiconductor. The large band-bending in the dark indicates the presence of an elec-H . GERISCHER 143 tric field in the space-charge layer which, under illumination, separates electron- hole pairs if they are generated therein or reach this region by diffusion. This charge separation acts in opposition to the electric field present in the dark, and the band bend- ing decreases. With a fixed Fermi level in the bulk, the energy difference between the bulk and the electrolyte remains constant.This is only possible if the voltage drop lost in the space-charge layer is compensated by an equal voltage gain in the Helm- holtz double layer. In order to obtain this compensation, surface charge is needed. dark iII uminated E lbJ FIG. 2.-Energy diagrams of an n-type semiconductor electrode at three different electrode potentials in the absence of any electrode reaction. If surface states with donor character are available in the band gap at sufficient den- sity, they can perform this function. Otherwise, an accumulation of the minority carriers at the surface is needed, and an inversion layer is obtained. This is assumed in fig. 2(6). For completeness, fig.2(c) shows the situation where an accumulation layer is formed in the dark. This has the consequence of an increased recombination rate at the surface, as indicated in fig. 2(c) by the upward bending of &$ at the interface. With regard to stability we have to correlate the quasi-Fermi levels at the surface with the semiconductor decomposition potentials. This means that a current can pass the interface. We shall first discuss the case where decomposition reactions are the only144 PHOTODECOMPOSITION OF SEMICONDUCTORS possible redox reactions. Although this case is rather unrealistic, since redox reactions of the solvent are unavoidable, it provides some instructive information. Fig. 3 com- pares the situation of an n-type semiconductor in the dark and under illumination for three different positions of the decomposition potentials relative to the band edges.As in the dark situation, the flat-band potential is used. The three cases differ in the position of the decomposition potentials and in their distance from each other. dark iilu minat ed 1 la) \ E I - n E decornp - [fb -p 'decornp - € f b -n Edecom p p fdecomp - EV E - E l f c , s n decornp \ \ I b €",S 9- p ldecornp n Ed ecom p p ldecomp - p fdecom p FIG. 3.-n-Type semiconductor at the flat-band potential showing 3 different positions of the de- composition Fermi levels. In the absence of other redox reactions which could compensate the charge con- sumption of an electrochemical decomposition reaction, unidirectional photodecom- position cannot proceed in a steady state.Recombination controls the steady-state situation and may be catalysed by the intermediates of the decomposition process as indicated in fig. 3(a) and (b) by the dashed arrows. In case (a), the accumulation of electrons leads to an upward shift of the band edges until anodic decomposition is prevented by recombination. In case (b), cathodic decomposition is avoided by a depletion of electrons. Case (c) shows a situation where both decomposition reactions have Fermi ener- gies within the gap and can both proceed under illumination. How fast these occur is a question of reaction rates. This scheme represents the electrochemical mechanism of photodecomposition such as seems to occur with Cu,O, which disproportionates under illumination to Cu and C U O .~ ~H. GERISCHER 145 2.3. COMPETITION BETWEEN DECOMPOSITION AND REDOX REACTIONS We now proceed to the more realistic assumption that, besides decomposition reactions, redox reactions with species of the electrolyte are possible. Again, we shall discuss typical and interesting cases in terms of the position of the characteristic energy levels in the dark and under illumination. The cases to be discussed here cor- respond to the four types derived previously1g3 from a comparison between the posi- tion of the band edges and the decomposition potentials. Fig. 4 shows an n-type semiconductor which is stable against cathodic decomposi- i II urn ina ted dark - nE decomp - fredox - p Ed ec o rn p FIG. 4.-n-Type semiconductor in contact with a slightly oxidising redox system: (a) dark; (b) slow; (c) fast redox reaction.tion because ,,Edecomp>EC. The redox system has a redox Fermi level located in the band gap and electron exchange may be fast enough to establish equilibrium such that a depletion layer is formed [fig. 4(a)]. Two possible situations under illumination are shown in fig. 4(b) and (c). If the redox reaction is slow, as for example the oxidation of water due to the slow reaction steps involved, can pass pEdecomp as shown in fig. 4(b). It then depends on the kinetics of decomposition (cf. section 2.1) whether it occurs at a noticeable rate or not. If the redox reaction is very fast, the quasi-Fermi levels of both charge carriers will be held at the surface close to the redox Fermi level. A great deal of the recom- bination between electrons and holes occurs then via the anodic and cathodic current of the redox reaction which compensate each other.The decomposition Fermi level cannot be reached by the holes. This is depicted in fig. 4(c). The situation of fig. 5 is much less favourable. The redox system is so strongly dark i I lu m ina t e d fF - - - ---- - EV fr::: - redox - p [decamp ICl f ' - [redox - p Ed eco rn p FIG. 5.-n-Type semiconductor in contact with a strongly oxidising redox system: (a) dark, (b) slow, (c) fast redox reaction.146 PHOTODECOMPOSITION OF SEMICONDUCTORS oxidising that it can even inject holes in the dark, with the result that an inversion layer is formed at the contact. If the redox reaction is sluggish, ,,E$ will be shifted downwards at illumination, partly by a further increase inp, but mainly by a downward shift of Ev (increase in ApH) as shown in fig.5(b). This can accelerate decomposition according to section 2.1 to a considerable extent, as is indicated in this figure. If, however, the hole-exchange rate is high, the quasi-Fermi level of holes will remain closely pinned to the redox Fermi level in solution, and the decomposition rate will remain negligible under illumination as indicated in fig. 5(c). Fig. 6 shows the equivalent situations for a p-type semiconductor with respect to d a r k i l l u r n i n a t ed la/ Ibl /C/ P fF FIG. 6.-p-Type semiconductor in contact with a redox system forming a depletion layer: (a) dark, (6) slow, (c) fast redox reaction. cathodic decomposition.Now, the quasi-Fermi level of electrons plays the decisive role in decomposition. This is indicated in fig. 6(b) and (c), where (b) depicts the unfavourable situation with a slow redox reaction where an upward shift of the band edges occurs by an accumulation of electrons at the surface. The kinetic model discussed in section 2.1 has shown that the rate of a decomposi- tion reaction can be of higher order with respect to the surface concentration of holes. A competing one-electron transfer redox reaction will in such cases become less pro- tective with increasing illumination intensity. If the competing redox reaction is it- self complex, containing several steps, and the rate-determining step is not the first step to occur then its rate will also follow a higher order inp,.In such cases, increas- ing illumination intensity will not in every case favour decomposition in relative terms. However, each increase of p , or AyH will accelerate anodic decomposition in absolute terms, each increase of n, and - A q H will accelerate cathodic decomposition if these reactions are possible. Therefore, increasing light intensity will in all cases increase the risk of photodecomposition. A systematic investigation of the competi- tion between redox reactions and the anodic decomposition process has recently been conducted by go me^.'^ 2.4. ROLE OF SURFACE STATES Surface states can catalyse the rate of recombination and of redox reactions at a semiconductor s ~ r f a c e . ~ ~ ~ ~ ~ - * ~ They usually can exchange electrons rapidly with redox couples in solution if their energy levels are in the same range as those of the redox system.Electron exchange with the bulk of the semiconductor will be fast if the surface states have energy levels close to one of the band edges. Surface states too far away from the band edges will only pick up charge in the downhill directionH . GERISCHER 147 with respect to energy. That is to say, donor states can trap holes from the valence band but cannot inject electrons into the conduction band; acceptor states will pick up electrons from the conduction band but cannot inject holes into the valence band. Only with very highly doped semiconductors are the reverse processes possible via t~nnelling.~~ -31 A consequence of this picture is that surface states with energies in the band gap will have a different state of occupation under illumination.If they are present in considerable concentrations, this can contribute to the charge in the Helmholtz double layer and thus cause a corresponding shift of the band edge positions at the interface with all its consequences for the kinetics (cf. section 2.1). On the other hand, surface states catalyse recombination and can pin the quasi-Fermi level to this energy range either by fast recombination or, in the absence of recombination, by a rapid electron transfer to a redox system in solution. Fig. 7 shows these two functions of surface states with donor character at an n-type dark ill u rn inat e d FIG. 7.--n-Type semiconductor with surface states of donor character in contact with a redox elec- trolyte.semiconductor. They are neutral in the occupied state and positively charged if they are vacant. The left-hand side of fig. 7 represents a situation at equilibrium where the surface charge has shifted the band edges downwards compared with their position at the flat-band potential, where all surface states would be occupied. This is indicated by the arrow representing the energy drop in the Helmholtz double layer Under illumination, the positive charge will increase by hole capture from the valence band. This causes a further downward shift of the band edges at the interface, while in the bulk the band edges shift upwards because of the decreased space charge. This is depicted at the right-hand side of fig. 7. The amount of this shift depends on the illumination intensity, on the concentration of surface states and, most important, on the electron and hole capture rates by the surface states.If both rates are very fast, the shift will be small. If hole capture is the faster process, the downward shift will be large and pE: may pass the Fermi level for decomposition as assumed in fig. 7. If the hole capture is slow, pinning of pEg to the range of the surface-state energy will not occur. In this case will easily pass Edecomp and the surface-states have little effect on stability, or even worsen the situation by accumulating positive charge and shifting the band edges further downwards. Fig. 8 shows the analogous situation for an n-type semiconductor with surface states of acceptor character, which are negatively charged if occupied, and neutral if vacant.In this case, accumulation of charge in surface states can only cause an up- (-eoA%)-148 PHOTODECOMPOSITION OF SEMICONDUCTORS lifting of the band edges at the interface. This improves the stability against anodic photodecomposition. It may, however, cause cathodic decomposition if the uplift goes too far (somewhat farther than shown on the illuminated case of fig. 8). For p-type semiconductors, the situation is fully analogous, as one can easily derive in the same way. The general conclusion is that surface states can help prevent photo- decomposition by fast catalysis of surface recombination. They can, however, also I acceptor ‘dew rn p d a r k illurninat ed FIG. 8.-n-Type semiconductor with surface states of acceptor character in contact with a redox elec- trolyte. increase the tendency to photodecomposition if they accumulate charge of the same sign as that of the minority carriers and shift the band edges at the surface by a varia- tion of ApH in the direction of the minority band.3 . APPLICATION TO SOLAR CELLS Electrochemical solar cells are either of the regenerative or storage type.32-34 Both types are based on the formation of a Schottky barrier at the semiconductor-electrolyte interface. Optimal efficiency can be reached if the barrier height in the dark becomes identical to the band gap. This corresponds to a slight modification of fig. 4 and 5 with Eredox close to the valence band edge or to fig. 6 with &dox close to J!&.Under illumination, we have again very similar situations as in fig. 4-6 with the only difference being that the anodic current through the interface is not compensated by a cathodic current. The current of the majority carriers flows now into the bulk of the semi- ductor and from there via an external loop with resistance Rext to the counter-elec- trode. The result is a smaller upward or downward shift of the Fermi level in the bulk and, at the surface, a larger deviation of the quasi-Fermi level of the minority carriers from the Fermi level of the redox system. Fig. 9 shows this situation for an n-type semiconductor. With decreasing Rext one approaches the situations of fig. 2 with a fixed Fermi level in the bulk, while increasing Rext leads to the situations of fig.4-6. In the first case, the risk of photodecomposition is largest since the greatest deviation of E$ is then obtained for the minority carrier. With respect to the competition between a redox reaction and the decomposition process, the same situation is found as pointed out in sections 2.3 and 2.4. The case of a storage cell will be discussed for a redox battery with two different redox couples in the two compartments of the cell, separated by a membrane withH . GERISCHER 149 selective permeability. If one of the electrodes is a metal and the other a semiconduc- tor, the electrodes must be disconnected in the dark in order to prevent discharge. Direct contact between both electrodes (ReXt + 0) will pin the Fermi level in the bulk of the semiconductor electrode close to the Fermi level of the metallic counter-elec- trode, because the redox reaction there will have a much larger rate constant and pro- se mi condu ctor e I e c t ro l y t e m e t a I semiconductor electrolyte m e t a l d a r k i l l u m i n a t e d FIG.9.-Energy diagram of a regenerative photoelectrolytic cell with a slow redox reaction at the semiconductor electrode. ceed at a smaller overvoltage. In order to store energy, the quasi-Fermi level of the minority carriers must differ at the surface by more than the open-cell voltage of the battery system from the Fermi level in the bulk of the semiconductor. In the dark we therefore arrive at a situation similar to that shown in fig. 4(a), while the illuminated case corresponds to fig. 5(b) or (c).This is depicted in fig. 10. I I / i.F-t--- I I \ I I '\I,\ 1 \ \ r e d o x I r e d o x semiconductor electrolytelelectrolyte m e t a l s e m i - 1 redox I redox I conduct or electrolyte electrolyte metal FIG. 10.-Energy diagram of a photoelectrochemical storage cell with a slow (a) or fast (b) redox reaction at the semiconductor electrode. With respect to photodecomposition, the statements made in section 2.3 and 2.4 are valid. If the decomposition Fermi levels are not outside the band gap, it is a question of competing kinetics whether, and how fast, decomposition occurs. In contrast to the previous case of a regenerative cell, the danger that a shift of the band150 PHOTODECOMPOSITION OF SEMICONDUCTORS edges at the surface by an accumulation of surface charge increases the free energy of the minority carriers at the semiconductor surface too far is diminished in a storage cell.The reason is that the decrease of the free energy of the majority carriers in the bulk which would be connected with such a shift would cause the rate of the redox reaction at the counter electrode to decrease and finally fully to cancel the charging current. In summarising, the discussion in this paper has shown that in order to analyse the tendency to photodecomposition one has to take into account the possibility of a variation of the voltage drop in the Helmholtz double layer caused by the contact with the redox electrolyte or by illumination. This will often have an unfavourable in- fluence since it is more likely that this shift makes the minority carriers at the surface more reactive (downwards shift for holes or upwards shift for electrons).However, this effect can also lead to an improvement of stability if the added surface charge has the opposite sign from the minority carriers. To find such a system should be a chal- lenge for future research. For the electrochemical solar cells the previously derived criteria of ~tabilityl-~ can still be used if one relates the decomposition Fermi energies to the correct position of the band edges at the surface of the semiconductor under working conditions instead of to their position at the extrapolated flat-band potential in the dark. How fast decomposition really occurs even provided it is thermodyna- mically possible, cannot be predicted from general principles.This must be checked experimentally from case to case. All light absorbed would then be lost by recombination. H. Gerischer, J . Electroanalyt. Chem., 1977, 82, 133. A. J. Bard and M. S. Wrighton, J. Electrochem. SOC., 1977, 124, 1706. H. Gerischer, J . Vac. Sci. Technol., 1978, 15, 1422. W. H. Brattain and C. G. B. Garrett, BellSyst. Tech. J., 1955, 32, 1. F. Beck and H. Gerischer, Ber. Bunsenges. phys. Chem., 1959, 63, 500. R. Williams, J. Chem. Phys., 1960, 32, 1505. H. R. Schoeppel and H. Gerischer, Ber. Bunsenges. phys. Chem., 1971,75, 1237. W. Shockley, Electrons and Holes in Semiconductors (Van Nostrand, Princeton, 1950). S. Trasatti, Advances in Electrochemistry and Electrochemical Engineering, ed. H. Gerischer and C. W. Tobias (Wiley, New York, 1977), vol. 10, p. 213. New York, 1967). ' H. Gerischer and W. Mindt, Electrochim. Acta, 1968, 13, 1329. lo F. Lohmann, 2. Naturforsch., 1967, 22A, 813. l2 J. F. Dewald, Bell. Syst. Tech. J., 1960, 39, 615. l 3 cJ V. A. Myamlin and Yu. V. Pleskov, Electrochemistry o j Semiconductors (Plenum Press, l4 R. A. L. Van den Berghe, F. Cardon and W. P. Gomes, Surface Sci., 1973, 39, 368. l5 M. A. Butler, J . Appl. Phys., 1977, 48, 1914. l6 W. Shockley, Electrons and Holes in Semiconductors (Van Nostrand, Princeton, 1950), p. 302. e.g., H. Gerischer, F. Hein, M. Luebke, E. Meyer, B. Pettinger and B. Schoeppel, Ber. Bunsen- ges. phys. Chem., 1973, 77, 284. R. L. Meierhaage, F. Cardon and W. P. Gomes, Ber. Bunsenges. phys. Chem., 1979, 83,236. l 9 W. Kautek, H. Gerischer and H. Tributsch, Ber. Bunsenges. phys. Chenz., 1979, 83, 1000. 'OW. W. Gaertner, Phys. Rev., 1959, 116, 84. R. H. Wilson, J. Appl. Phys., 1977, 48, 4292. 22 H. Reiss, J. Electrochem. Sac., 1978, 125, 937. 23 J. Reichman, Appl. Phys. Letters, 1980, 36, 574. 24 K. Hauffe and K. Reinhold, Ber. Bunsenges. phys. Chem., 1972, 76, 616. 25 W. P. Gomes, lecture at the ACS-meeting in Houston, March 1980, in course of publication. 26 H. Gerischer, Surface Sci., 1969, 13, 265. 27 K. H. Beckmann and R. Memming, J . Electrochem. SOC., 1969, 116, 368. 29 P. J. Boddy, J. Electrochem. SOC., 1968, 115, 199. 30 F. Moellers and R. Memming, Ber. Bunsenges. phys. Chem., 1972, 76, 469, 475. 31 B. Pettinger, H. R. Schoeppel and H. Gerischer, Ber. Bunsenges. phys. Chem., 1974, 78, 450, S . N. Frank and A. J. Bard, J . Amer. Chem. SOC., 1975, 97, 7427. 1024.H . GERISCHER 151 32 cf. H. Gerischer in Solar Power and Fuels, Proc. 1st Int. Conf. Photochemical Conversion of 33 cf. Semiconductor Liquid-Junction Solar Cells, ed. A. Heller, Proceedings Vol. 77-3 (The Elec- 34 cf. H. Gerischer in Topics Appl. Phys., 1979, 31, 115. Solar Energy 1976, ed. J. R. Bolton (Academic Press, New York, 1977), p. 77. trochem. Society, Princeton, 1977).

ISSN:0301-7249    

DOI:10.1039/DC9807000137

出版商:RSC    

年代:1980

数据来源: RSC