摘要:

AUTHOR INDEX * Barrett M. A. 49 72,98 128,211. Bewick A. 49,114 130. Bockris J. O’M. 85,177 201,208 211. Brusic V. 177,203. Cahan B. D. 36 46 49 50 52 61 85 90 133 Conway B. E. 95 126 135 174. Dignam M. J. 47,208,212 den Engelsen D. 45. Fleischmann M. 130. Genshaw M. A. 87,177,207. Hausen W. N. 27 175. Hayfleld P. C. S. 7. Horkans J. 36,49 52 61. Kolb D. M. 99. Kuhn A. T. 86. McCrackin F L. 192. McIntyre J. D. E. 46 50 55 61 99 126. 203. Mark H. B. Jr. 157 176. Memming R. 145 173 174 175. Meyer F. 17. Miillers F. 145. O’Grady W. E. 211. Paik W. 85 201. Parsons R. 49,72,85,98,128,129,130,173,201 Plieth W. J. 45 89 137 173. Randall E. N. 157,176. Smith L. E. 192. Spamaay M. J. 17 50. Stedman M. 48 64 85 86 88. Stromberg R. R. 192 212. Tadros Th. F. 211. Tuxford A. M. 114 130 132. Vetter K.J. 134 202. Yeager E. 36,49 52 54 61 91 131. 211,212. * The references in heavy type indicate papers submitted for discussion. AUTHOR INDEX * Barrett M. A. 49 72,98 128,211. Bewick A. 49,114 130. Bockris J. O’M. 85,177 201,208 211. Brusic V. 177,203. Cahan B. D. 36 46 49 50 52 61 85 90 133 Conway B. E. 95 126 135 174. Dignam M. J. 47,208,212 den Engelsen D. 45. Fleischmann M. 130. Genshaw M. A. 87,177,207. Hausen W. N. 27 175. Hayfleld P. C. S. 7. Horkans J. 36,49 52 61. Kolb D. M. 99. Kuhn A. T. 86. McCrackin F L. 192. McIntyre J. D. E. 46 50 55 61 99 126. 203. Mark H. B. Jr. 157 176. Memming R. 145 173 174 175. Meyer F. 17. Miillers F. 145. O’Grady W. E. 211. Paik W. 85 201. Parsons R. 49,72,85,98,128,129,130,173,201 Plieth W. J. 45 89 137 173. Randall E. N. 157,176. Smith L. E. 192. Spamaay M. J. 17 50. Stedman M. 48 64 85 86 88. Stromberg R. R. 192 212. Tadros Th. F. 211. Tuxford A. M. 114 130 132. Vetter K. J. 134 202. Yeager E. 36,49 52 54 61 91 131. 211,212. * The references in heavy type indicate papers submitted for discussion.

ISSN:0430-0696    

DOI:10.1039/SF97004FX001

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

General Introduction BY P. C. S. HAYFIELD Imperial Metal Industries Limited P.O. Box 216 Kynoch Works Witton Birmingham B6 7BA Received 7th May 1971 In order to review the present state of optical studies for the detection of and quantitative measurement on adsorbed layers at interfaces it seems appropriate to consider four main topics (i) circumstances leading to the acceptance of optical methods for studying interfaces ; (ii) present-day interests in applying optical methods ; (iii) relative merits of the various optical methods ; (iv) interpretation of optical data. CIRCUMSTANCES LEADING TO THE ACCEPTANCE OF OPTICAL METHODS FOR STUDYING INTERFACES During the 19th century much attention was given by physicists to the charac- teristics of reflection of light from surfaces and during this period and after much debate it became firmly established that interfacial films could exert considerable effects on reflection.There were numerous contributors to this finding,l including such names as Malus Fresnel Brewster and Rayleigh but Drude is perhaps most frequently remembered because in connection with optical studies he derived equations relating to the optical properties and thickness of a surface film on a solid to the characteristics of reflection of polarized light and these equations still carry his name tan $ exp (iA) = FP/FS; 6 = -27& cos ;62dlL where Fi2 FS,, F g 3 and Fi3 are the so-called Fresnel reflection coefficients and d is the thickness of film. These form the basis for interpretation of most present-day ellipsometry and reflectivity data. At this stage optical techniques for evaluating surface films had been formulated but it was after another 40 years to the beginning of the 1930’s before such techniques found appreciable application.While the basic derivation of the Drude equations which involve multiple reflection within a surface film does not seem to have been in much doubt Constable’s explana- tion for the interference colours produced on tarnished copper-his spectrophoto- metric data being published in 19274id not find ready acceptance. However any sceptism existing on the validity of the Drude equations was removed by the experi- mental work reported in the early 1930’s. Using the ellipsometer technique Tronstad and collaborators 4-7 investigated several surface phenomena. Concerning the passivation of stainless steel in sulphuric acid it was shown that growth of film occurred of average thickness 50A and average film refractive index 3.0 and this was presumed responsible for the improved corrosion resistance imparted.In relation to adsorption studies Feachem and Tronstad made ellipsometer studies on films having well-defined properties choosing long-chain fatty acids and alcohols on 7 8 GENERAL INTRODUCTION mercury and their results interpreted on the basis of change in phase A between rp and rs components of reflected elliptically polarized light being linearly proportional to film thickness gave strikingly good agreement between known figures and those calculated as a result of the optical studies (table 1). TABLE 1 .-ELLIPSOMETRIC DATA FOR THE THICKNESS OF ADSORBED LONG CHAIN MOLECULES ON MERCURY.* G(J/ - $0) nj = Cl+ Simplification for thin uniform transparent (A - Ao) sin 2+ (A - Ao) (1 - 1 In%) isotropic films d = C j - film thickness nM in nM ellipsometry data chain length calc.from pelargonic acid 1.2 1.5 lauric acid 1.6 1.7 myristic acid 1.9 2.2 * C. G. P. Feachem and L. Tronstad,Proc. Roy. Sot. A 1934,23,127. Various shortcomings of the method were appreciated such as the fact that interpretation was based upon a perfectly sharp interface between the different media whereas with mercury for example the excursion of surface molecules due to thermal agitation could become a factor. The possibility of error from the roughness of solid surfaces optical anisotropy in films and also changes in absorption due to the increase of free electron concentration induced by applied electric fields were mentioned and it was argued justly by reference to recent work that the magnitude of the errors was smaller than the accuracy of their techniques for measurement of elliptically polarized light.On the theoretical side there was not much incentive for progress because the Drude equations fairly adequately covered the situation. These become difficult to handle in treating optically anisotropic media and Darwin * proposed an alternate theory based upon the film consisting of a three-dimensional array of scattering centres. These equations themselves are complicated and have not been much used but for the special case of an optically isotropic film do simplify to the form of the Drude equations. At the time Tronstad and collaborators were experimenting with the detection of monolayer films Strachan was motivated to put forward a theoretical treatment of reflection from a surface film consisting of a two-dimensional array of scattering centres in the plane of the surface and this like Drude’s equation is of particular interest in connection with present-day considerations of partial monolayer coverage of adsorbed species.PRESENT-DAY INTERESTS I N THE APPLICATION OF OPTICAL STUDIES From the foregoing section it may be seen that the basis for optical studies on interfaces had been both theoretically formulated and experimentally justified in the early 1930’s and all. that remained was for the techniques to be applied and further refined. Development in ellipsometry has mainly come in the precision and speed of recording reflected elliptically polarized light modern equipment incorporating the advances of light detectors and electronic equipment generally but an important consideration also has been the use of computers which largely remove the considerable tedium at one time attached to the computation side, P .C. S. HAYFIELD 9 Archer’s work lo in the early 1960’s on adsorbed films on silicon in the 0-1 nM range is a fine example of the considerable sensitivity and scope for study that can be achieved by ellipsometric methods and it is worth pausing to consider his paper to the 1st International Conference of Ellipsometry as it embodies many of the considerations that will no doubt come up for discussion in the present Symposium. Working at a fixed wavelength and with films of thickness down to one mono- molecular layer he considered that one might reasonably apply the simplified Drude equations where A and $ have the usual connotations used in ellipsometry d is the layer thickness and a and are constants.It seemed to be scarcely credible to suppose that a theory based upon a homogeneous parallel-sided solid film could be extrapolated to predict the change in reflectivity from a surface covered with a partial monolayer. How for example would one define the refractive index of the layer? Perhaps by defining an effective refractive index proportional to coverage e.g. A = Ao-ad; $ = $o-Bd (3) n = 8n2+(1-8) effective film refractive index (4) for a film of refractive index n2 for a complete monolayer and 8 representing fractional coverage. There would perhaps be better justification for using a refractive index based upon molecular refractions nZ,-1= ( IZ’ - I) ( n i - 1) n2,+2 n2,+2 n,+2 4 a + 2 4 b where qa and ab are the fractional volumes of components a and b and proportional to 0.Rearranging and putting qb = 1 -qa this expression simplifies to the perhaps more widely-known Maxwell-Garnet relationship Archer considered it more realistic to think in terms of Strachan’s theory of a two- dimensional array of scattering centres in the plane of the surface and this he showed led to equations of the form a and p are constants oi is the oscillator strength for unit area and assumed pro- portional to coverage by writing oi = Oaf. Hall later commented that the oscillator strengths were more likely to be proportional to the intensity of the scattered wave rather than its amplitude and thus proposed This situation thus set rather a challenge to the experimentalists which has been answered for a number of situations by comparison of the results of ellipsometer investigations with those of other techniques table 2.Archer lo found good corre- spondence for a linear relationship between analysis of isotherms for the thickness of adsorbed molecules-water carbon tetrachloride and acetone on silicon surfaces having relatively smooth surfaces and the results of elIipsometer estimations- 10 GENERAL INTRODUCTION but not for rougher substrates. Tennyson Smith l 2 worked on adsorption on mercury measuring surface tension contact potential and ellipsometry data for coverages from 0.05 to 1.0 and also found that ellipsometer changes were linearly proportional to coverage rather than to the square root of coverage.However it cannot be assumed that a linear relationship exists for all configurations of coverage and it is most valuable to have further evidence such as e.g. reported in the first paper at this Symposium; it involves not only ellipsometry but Auger electron spectroscopy and gas-volumetric analysis. The relationship might be expected to depart from linearity where the sites of adsorption show a preferred regularity such as particular crystallographic sites on a single crystal material. TABLE 2.-uSE OF MULTIPLE TECHNIQUES INCLUDING ELLIPSOMETRY FOR STUDYING ADSORP- adsorption process water vapour etc. on silicon organic molecules on mercury oxygen on tunsten polar organic compounds on various metals various compounds on silicon krypton oxygen etc.on silver oxygen on platinum TION PROCESSES techniques investigator($ ellipsometry B.E.T. Archer ellipsometry surface Tennyson Smith l tension contact potential ellipsometry L.E.E.D. Melmed Layer and field electron emission spectroscopy and Melmed l4 Kruger l 3 and Carrol ellipsometry Miller and Berger l5 radio t racer. ellipsometry B.E.T. Meyer and Bootsma Auger electron spectroscopy Meyer and Spamaay l7 ellipsometry L.E.E.D. Muller S teiger mass spectrometry Somorjai and Morabito ellipsometry Barrett and Parsons l7 reflectivity With the availability of such powerful optical techniques has come a diversity of interests which can arbitrarily be subdivided into three groups. (a) Those interested not so much in mechanism of adsorption as identification of adsorbed ~ p e c i e s l ~ - ~ ~ but information on the geometry of adsorption nature of the attachment forces and strength of the adsorption are also important related features.The work stems largely from trying to apply infra-red absorption spectroscopy to thinner and thinner layers and eventually adsorbed Much use has been made of internal multiple reflection between parallel plate technique^,^ and they have usually been applied with non-polarized light although after a few multiple reflections the beam has become polarized by absorption of the component in the plane of incidence. A new facet to optical studies using reflectivity methods was given in the papers by Koch 27* 28 on the change of reflectivity from surfaces covered with film the film being formed electrochemically.The greater sensitivity to film changes was found in the ultra-violet which is perhaps to be expected since light penetration is limited to a few atom layers only and it would be logical to expect an adsorbed layer to P . C . S . HAYFIELD 11 introduce a proportionately larger effect on the reflection process compared with a situation where the light penetrates more deeply. This result seems to have provided the stimulus for considerable further work on the application of optical methods and in particular modulated reflectivity measurements to electrochemical studies. (b) With the refinement of optical techniques particularly modulation processes giving sensitivity for studying small changes in reflectivity (AR/R N 1 0 3 induced by various perturbing forces has come renewed interest in optical methods of studying the electron structure in the surface layer of semiconductors and metals.S e r a ~ h i n ~ ~ and later Cardonna et have employed this technique with con- siderable success to the fine structure of semiconductors achieving resolutions of peaks and/or dips which may be as narrow as 0.04 eV. Subsequently the technique has been applied to metals the objective being to perturb the surface layer in a manner which can be predicted to affect the free electron concentration. From measurements of change in reflectivity over a range of wavelengths the results are interpreted in terms of dielectric changes with frequency by means of the Kramers-Kronig trans- formations. In turn the dielectric changes are interpreted in terms of free electron concentration.On the basis of the original Drude theory this can be written as E = E - (i4m/m) and E = 5 ; = 1 - [4nNe2/m(w2 + y 2 ) ] CT = Ne2y/m(02 +y2) (9) where E is the complex dielectric constant and CJ is the conductivity; e and m are the charge and mass of the electron N is the concentration of free electrons m the angular frequency and y is a constant describing the viscous damping of the electrons and equals 1 /z where z is a relaxation time. This relationship holds for frequencies less than coo which corresponds to the internal photoelectric effect but from these formulae if the effective number of free electrons changes then accordingly there will be a change in dielectric and hence optical properties. (c) The third branch of optical studies is the extension of optical methods to define aspects of corrosion and electrochemical processes.For example there has been continuing use of ellipsometry to study adsorption processes,16 and others will be discussed in the present Symposium. In connection with basic electrochemical reactions of which adsorption of species forms an integral part there have been both ellipsometric and reflectivity investigations. Particular success has accompanied the modulated reflectivity process which is well exemplified by the papers of McIntyre and Kolb and Bewick and Tuxf0rd.l’ COMPARISON OF REFLECTIVITY AND ELLIPSOMETRIC METHODS The various basic optical parameters that may be measured are itemized below. q number of single reflection multiple reflections R = 3 ( ( r P ) z + r y ( s ) 2 ) Rq reflectivity ( r p ) ( r p ) 2 q ellipsometry tan $ exp (iA) (tan $)q exp (iAq) These include overall reflectivity 3 (rP2 + rs2) rp2 rs2 or ellipsometry A = dp- ds and t,b = tan-l ~ r p / / ~ r s ~ all quantities that can be calculated directly from the Drude equations (1).Some comparisons between reflectivity and ellipsometry changes for the same situation are shown in fig. 1 and other data relating to reflectivity changes alone in fig. 2 and 3. Since the total reflectivity is the sum of rp and rs components and the sign of the reflectivity change with increasing thickness may differ for the two components it is (rSY (rS)2q 12 GENERAL INTRODUCTION better in some respects to measure one or other component individually rather than the resultant of the two. The relative change in AR/R for unfilmed and filmed situations can be improved by multiple reflection between parallel plates.This seemingly is a direct way of improving the sensitivity of both reflectivity and ellipso- metry methods but there are some experimental disadvantages. For example the overall reflectivity level decreases for both methods. With ellipsometry (tan $)* becomes very small so that effectively the component in the plane of incidence vanishes and there remains no basis for a method. Unless the collirnation of the beam is exceptionally good the beam size spreads with consequent loss in both intensity and discrimination. Where however the precision of a single reflection P. C. S . HAYFIELD 13 film thickness in nM FIG. 2.-Comparison of theoretically computed reflectivity curves corresponding to filmed platinum.Single reflection A = 546.1 nM = 75". A. n = 1.39; n2 = 2.0; fi3 = 1.85-3.83'; B. nl = 1.39; W 2 = 2.0-03; j i 3 = 1.85-3.83'; k2 = 0.5; H = 1 . 1 5 ~ lo5 cm-'. technique is adequate for detection of adsorbed layers t h s is clearly the preferred technique and many of the modulated reflectivity methods recently described are of this type. Comparative ellipsometric and reflectivity changes resulting from a particular model of a filmed surface are illustrated in fig. 4. More detailed informa- tion is forthcoming from ellipsometric evaluation but there are instrumentation problems in applying the technique over a wide spectral range. This has now largely been overcome and although not a great deal of ellipsometry has been I I I I 5 0 100 150 2 0 0 film thickness in nM FIG. 3.-Comparison of theoretically computed reflectivity curves for a parallel plate system consisting of filmed platinum surfaces.Three major reflections. A. nl = 1.39; n2 = 2.0; fig = 1.85-3.8i; A = 546.1 nM; = 75"; M.R. = 3. B. nl = 1.39; fi2 = 2.0-0.5j; fi3 = 1.85-3.8i; 1 = 546.1 nM; = 75"; M.R. = 3. 14 GENERAL INTRODUCTION carried out for determining the spectra of molecules adsorbed on metal surface,31 work is in progress in several laboratories. In connection with the studies on metal surfaces per se electro-modulated ellipsometry provides a method of establishing dielectric changes without recourse to the Krainers-Kronig relationships which Buchman and Bashara maintain is advantageou~.~~ film thickness in nM phase change A-A. (degrees) R-Ro RO where RO is the reflectivity at E = +.02V R1- R 0 -3 .O x lo2 '*O 0 t + 1.0 - 1.0 -2 .o I 0 t0.2 I .o 2 .o asterisks corres- pond to phase changes reported in fig.4 p. 39. 0-a-.-a /-@-a-e J I I I I I I I I I I 1 compare with fig. 2 p. 73. compare with fig. 2 p. 103. electrode potential FIG. 4.-Theoretically computed optical changes produced by growth of film on a platinum substrate illuminated with light of wavelength 546.1 nM. nl = 1.39; n2 = 2.0; fi3 = 1.85-3.83; = 75"; M.R. = 1. A further feature of ellipsometry which is worth comment concerns the change in sign of A with change in film thickness and with change in optical properties of the substrate. Thus change due to the substrate can be of either sign dependent upon the direction of the perturbing field or force but for growth of film for oblique angle of incidence at least the sign is always negative see fig.5. INTERPRETATION OF OPTICAL DATA Having determined the optical properties of a surface in terms of an ellipsometry or reflectivity response and changes with change in applied potential temperature or whatever the variant that may be employed the next stage is that of establishing the equivalent optical circuit so that correct interpretation may be applied. In comparison with interpretation applied to growth of thick films and about which most experience is to hand some problems diminish and others become more import- P . C. S. HAYFIELD 15 ant. With thick films the exact determination of film refractive index is very necessary to obtain a film thickness of any accuracy. With thin films for example changing the refractive index from 1.2 to 3.4 was shown by Archer not to lead to more than a maximum error of +34 %.Indeed in some studies on adsorbed layers it has been deemed sufficient to calibrate the ellipsometer (and presumably this would also be applicable to reflectivity methods) by measuring change in response with build-up of barium stearate films these being deposited using the Langmuir trough method. 0 9 0 P - O0 FiG. 5.-Diagram to indicate the change in ellipsometry readings with (a) change in substrate optical constants fij3 and (b) growth of film. However in dealing with ellipsometry changes appreciably smaller than lo or the equivalent AR/R changes such as occur in modulated ellipsometry and reflectivity studies it is necessary to consider changes in reflected optical properties arising from other causes namely in the metal and in the double 1ayer.l'.33 At the present time the response from most modulated optical studies have been interpreted either as arising from changes in the optical properties of the substrate or the film and correction factors for the double-layer contribution have not been applied. Methods of distinguishing between the contribution to optical reflectivity changes introduced by metal film and environment are obviously crucial to correct interpreta- tion and design of experiment to clarify particular circumstances is essential. The paper by Bockris Genshaw and Brusic to the present Symposium for example highlights the effects of surface roughness of the metal/film interface and in a similar manner irregularities at the filmlenvironment interface should also be considered.Some success has attended the treatment of the etched surface of iron as a film layer having an effective refractive index based upon combining molecular refractions in 16 GENERAL INTRODUCTION proportion to the volume of metal and second phase in the interphase region,34 see eqn (5). The gross optical changes which it seems may be associated with non- uniform interfaces between phases and which had been predicted by Fenstennaker and McCra~kin,~~ are likely to affect ellipsometric and reflectivity changes alike. The evidence is that extreme caution should be observed over the interpretation of optical data for systems where roughening or smoothing of an interface may occur such as in the study of active/passive transitions for metals exposed to acid condi- t i o n ~ ~ ~ electropolishing reactions and the lj ke.Similar care seems necessary with those modulated reflectivity and ellipsometry studies where electrode potential changes cover a sufficiently wide range for roughening effects to occur such as those reported by Biegler 37 for platinum. Further clarification over the effects of non- uniform interfacial layers is awaited. Notwithstanding the problems associated with interpretation which may affect the application of optical methods in particular circumstances ellipsometric and reflectivity methods of examination are nevertheless established and very sensitive techniques which can be uniquely applied to in situ measurements of adsorbed layers at interfaces. A. C. Hall Recent Developments in Ellipsometry Symp. Proc.(University of Nebraska) N. M. Bashara A. B. Buckman and A. C. Hall ed. (North Holland Pub. Co.) chap. 1. ' P. Drude Ann. Phys. 1887,32,584 ; 1888,34,489 ; 1889,36,532,865 ; 1890,39,481. F. H. Constable Proc. Roy. SOC. A 1927,115 570; 1928,117 376. L. Tronstad Trans. Faruduy SOC. 1933 29 502. L. Tronstad and T. Hoverstad Trans. Faraday SOC. 1934,30,362. L. Tronstad and T. Hoverstad Tram. Faraduy Soc. 1934,30 11 14. C. G. P. Feachem and L. Tronstad Proc. Roy. SOC. A 1934,23 127. C. G. Darwin Trans. Cambr. Phil. Soc. 1924,23 137. C. Strachan Proc. Cambr. Phil. SOC. 1933,29,116. lo R. J. Archer Ellipsometry in the Measurement of Surfaces aizd Thin Films Symp. Proc. (Nat. Bur. Stand. Misc. Pub. 256) E. Passaglia R. R. Stromberg and J. Kruger ed. ( U S Govern- ment Printing Office Washington D.C.1964) p. 255. l 1 A. C. Hall J. Phys. Chem. 1966,70 1702. '' T. Smith J. Opt. SOC. Amer. 1968 58 1069. l3 A. J. Melmed H. P. Layer and J. Kruger Surface Sci. 1968,9,476. l4 J. J. Carroll and A. J. Melmed Surface Sci. 1969 16,251. l5 J. R. Miller and J. E. Berger J. Phys. Chem. 1966 70 3070. l6 F. Meyer and G. A. Bootsma Surface Sci. 1969 16 221. l7 this Symposium. l 8 R. H. Muller R. E. Steiger G. A. Somorjai and J. M. Morabito Surface Sci. 1969,16,234. l9 H. B. Mark and B. S. Pons Anal. Chem. 1966,38 119. 2o W. N. Hansen T. Kuwana and R. A. Osteryoung Anal. Chern. 1966,38,1811. 21 G. W. Poling J. Electrochem. SOC. 1967 114 1209. 22 G. W. Poling J. Electrochem. SOC. 1969,116 958. 23R. G. Greenler J. Chem. Phys. 1969,50,1963. 24 G. W. Poling Corrosion Sci. 1970,10 359. 25 N.J. Harrick Internul Reflection Spectroscopy (Interscience Publishers 1967). 26 P. T. Kissinger and C. N. Reilley Anal. Chem. 1970,42 12. 27 D. F. A. Koch Nature. 202 3871. 28 D. F. A. Koch and D. E. Wife J. Electrochem. SOC. 1966,113 1302. 29 B. 0. Seraphin Phys. Reu. A 1965,140,1716. 30 M. Cardoma K. L. Shaklee and F. H. Pollak Phys. Rev. 1967,154,696. 31 C. L. McBee and J. Kruger ref. (l) p. 340. 32 A. B. Buckman and N. M. Bashara J. Opt. SOC. Amer. 1968,58,700. 33 M. Stedman Chern. Phys. Letters 1968,2,457. 34 C . J. F. Bottcher Theory of Electric Polarization (Elsevier Publishing Co. Amsterdam 1952). 35 C. A. Fenstermaker and F. L. McCrackin ref. (lo) p. 85. 36 J. O'M. Bockris A. K. N. Reddy and B. Rao J. Electrochem. SOC. 1966,113 1133. 37 T. Biegler J. Electrochem. Soc. 1969 116 1131.

ISSN:0430-0696    

DOI:10.1039/SF9700400007

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Ellipsometric and Gas-Volumetric Investigation of Adsorption Reactions on Clean Silicon and Germanium BY F. MEYER AND M. J. SPARNAAY Philips Research Laboratories N.V. Philips' Gloeilampenfabrieken Eindhoven- Net herlands Received 21st September 1970 The chemical adsorption of a few organic gases on clean silicon and germanium is discussed. The adsorption appears to depend strongly on the presence and number of dangling bonds at the surface. Structures for the adsorption complexes have been proposed based on ellipsometric and gas-volumetric data. There is evidence that the adsorption complexes for CH3Cl and CHJBr on the (1 11) face have a reversible transition at higher temperatures which regenerates dangling bonds. Thermodynamic calculations for this transition give information on the free energy of mutual compensation of these dangling bonds.Chemical adsorption of a number of gases on the clean surfaces of silicon and germanium has been investigated. We have mainly used three different techniques of surface study viz. ellipsometry Auger electron spectroscopy (A.E.S.) and gas- volumetric adsorption measurements on powders. All three methods have a sensitivity of 1-5 % of a monolayer. A combination of the results makes it possible to propose structures for the adsorption complexes. The adsorption appears to be governed by the presence and number of dangling (uncompensated) bonds of the surface atoms and is therefore similar for silicon and germanium surfaces.1 The dangling bonds have some mutual compensation which is probably the reason for the displacements of the surface atoms from their normal positions as observed by low-energy electron diffraction e.g.Si (1 11) - 7 x 7 Ge( 1 1 1) - 2 x 8 etc.2 The degree of compensation on the clean surface is not known but the reactivity of the surface atoms is such that stable molecules such as methyl chloride and methyl bromide adsorb ~hemically.~ There is evidence that the chemical adsorptions are dissociative compensating the dangling bonds with formation of adsorption complexes in which all atoms have their normal va1ency.l. This paper gives a short review of previous work and some new ellipsometric results on the adsorption of organic gases on silicon and germanium; the results are used to obtain information on the degree of mutual compensation of the dangling bonds. EXPERIMENTAL The ellipsometric apparatus has been de~cribed.~ The two angles pertaining to the ellipsometric effects could be determined reproducibly to 0.01 ".The single crystal silicon and germanium samples were cut from high ohmic crystals oriented to within 0.5" of the desired orientation and finely polished. The silicon samples were cleaned by resistive heating to 1200°C for a few minutes and the germanium samples by heating to 850°C for a few hours in a vacuum of The A.E.S. measurements could be performed in combination with elli~sometry.~ An apparatus equipped with a 127" electrostatic analyzer for the measurement of the energy Torr. 17 18 ADSORPTION REACTIONS ON SILICON AND GERMANIUM distribution of the electrons has been used. Temperature measurements on the single crystalline samples were carried out with an Ircon 300C infra-red pyrometer.The gas-volumetric adsorption and desorption experiments were performed on germanium powders only. The powder was obtained by crushing a high-ohmic single crystal in air and cleaned by heating to 650°C in a vacuum of The specific surface area was -0.1 m2/g. Pressure readings of the admitted or desorbed gases were taken with a McLeod manometer and the decomposition products were analyzed by an Atlas M86 mass-spectrometer. Torr for 15 h.'. RESULTS AND DISCUSSION COMBINATION OF ELLIPSOMETRY GAS-VOLUMETRY AND A.E.S. Gas-volumetric adsorption measurements on powders yield the amount of gas adsorbed and therefore the surface coverage since the total surface area can be deter- mined by physical adsorption of krypton at liquid nitrogen temperature interpreting the results with the B.E.T.equation.' Surface cleanliness can be checked by oxygen chemical adsorption at room temperature. The reaction appears to proceed rapidly (exposure - 1 Ton min)' to a coverage of one oxygen atom per germanium or silicon surface atom independent of the crystallographic orientations (1 1 l) (100) and (1 lo).* The further reaction at room temperature is much slower. This seems to be a common feature of the chemical adsorptions on silicon and germanium. The amounts adsorbed in the " fast " reactions seem to correspond to the compensation of the dangling bonds by &ssociation of the adsorbing molecules. The inherent difficulty of the powder measurements is the presence of more than one crystallographic orientation of the surfaces. Calculations based on bond energies predicted a 75 20 5 ratio for (1 11) (100) and (1 lo).' A detailed study of methanol adsorption and subsequent decomposition of the adsorption complex as a function of temperature suggested an 80 20 ratio for (1 11) and Ellipsometry correlates well with these measurements.Ellipsometry is a very accurate optical method whch measures the change in the state of polarization of polarized light upon reflection from a surface. Since the measurements are normally performed with a light spot of 1 mm2 one is able to investigate single crystal surfaces. Comparison with radiotracer techniques O and with gas-volumetric measurements on powders 4* l1 suggested strongly that in many cases the macroscopic theory can be extrapolated to the sub-monolayer region using an effective index of refraction and thickness for the adsorbed layer.It has been demonstrated that in good approxima- tion the Lorentz-Lorenz eqn (1) can be used to relate the macroscopic parameters index of refraction n and thickness d to the microscopic parameters polarizability a surface coverage 8 and the number of surface atoms per cm2 N The value of the atomic or molecular diameter of the adsorbate can be taken for d. It appears that the ellipsometric angle $ for layers in the thickness range 0-50A is related to the absorption coefficient of the layer and the ellipsometric angle A with the real part of the index of refraction and the thickness. The layers discussed in this paper are not strongly optically adsorbing in the visible region and the expected change in @ upon adsorption will therefore be very small.The change in A which contains two unknowns can be interpreted in terms of surface coverage if a value is chosen for the polarizability of the adsorbate. In general literature values for ct have been used. This procedure has been tested for a few physical adsorptions e.g. krypton on clean and oxidized silicon and germanium at liquid-nitrogen tempera- F. MEYER AND M. J . SPARNAAY 19 ture.'" l 2 The coverages 6 were determined independently by gas-volumetry on a powder and a value for 6A (change in A upon adsorption) was calculated using the n and d from eqn (1) in the formulae from the macroscopic theory. Excellent agreement was obtained between calculated and measured values. For chemical adsorptions however anomalous ellipsometric effects both in A and $ have been observed for the first monolayer (a monolayer in chemical adsorption is defined as the coverage where all dangling bonds have just been compensated) whereas further adsorption seemed to give normal effect^.^ There are two possible interpretations.(a) The ellipsometric effects are due to the adsorbate only. This implies that the optical constants of the first monolayer are greatly different from the next layer. Calculations give unrealistic values for the optical constants viz. n < 1 and k> 0.5 in the visible region. Phys. ads. C lean Chem. ads germanium. FIG. I .-Schematic representation of physical and chemical adsorption on clean silicon and (b) The second interpretation of the ellipsometric effects includes a possible change in the substrate optical properties.The effects 6 6 and 6$ could be divided in an adsorbate-dependent part which obeyed the macroscopic theory as for the physical adsorption and an adsorbate-independent part which is due to a change in the substrate upon adsorption. The effect on the substrate which all chemical adsorp- tions have in common is the compensation of the dangling bonds of the surface atoms. This can be described in macroscopic terms as depicted in fig. 1. The top layer of atoms on the clean surface is taken as a layer with optical constants different from the underlying bulk. This layer which we call a transition layer is 0.5 0.4 0.3 3 6Q 0.7 0. I 0.2 01 0.6 0.8 1.0 1-2 1.L t 1.6 8 6h" FIG. 2.-The @A Sl)) curves for chemical adsorption on Si(ll1) and Si(100) measured at an angle of incidence of 70.5" and 69.5" respectively.The wavelength was 0.55 pm. a CH3Cl on Si(ll1) ; A CH3Br on Si(l11) ; 0 CH3C1 on Si(100) ; A CH3Br on Si(100) ; 17 O2 on Si(100). &I represents points which were obtained by adsorbing oxygen on the partially covered surface after the CH3Br adsorption on Si( I 1 1 ). 20 ADSORPTION REACTIONS ON SILICON AND GERMANIUM unaffected by physical adsorption but chemical adsorption which compensates the dangling bonds causes the optical constants to return to their normal bulk values. Ths is optically equivalent to removing the transition layer. The effective optical constants n and k of the transition layer have been calculated from the results for a number of adsorptions where 8 had been determined gas- volumetrically. The differences between the measured ellipsometric effects and those calculated for the adsorbate only gave the values 6A and all/ for the transition layer.A value of 5 has been assumed for the thickness dt. The wave-length dependence of n and k studied in the region 0.34-1.8 pm appeared to be similar to the wave-length dependence of the optical constants of the corresponding amorphous silicon and germarZium.12 This supports this interpretation since it is expected that the amorphous lattice-like material has distorted or dangling bonds throughout its bulk similar to the danghng bonds at a clean single-crystalline surface. It appears that at certain wavelengths (0.55 pm for silicon and 0.80 pm for ger- manium) the adsorbate effect is completely described by 6A whereas the substrate effect is fully reflected in S ~ ." This is a convenient wavelength to measure the number of dangling bonds compensated per molecule adsorbed. The @A plot shows a kink at the " monolayer " coverage as demonstrated in fig. 2. These kinks give valuable information on possible structures of the adsorption complexes if the assumption is made that all atoms in the adsorption complex have their normal valency. Desorption experiments as a function of temperature from a powder surface yield decomposition products which give information on the bonding and the presence of certain units in the complex. It appeared that adsorption complexes on silicon and germanium were the same in nearly all cases. TABLE 1.-" MONOLAYER " COVERAGE ON SILICON AND GERMANIUM CALCULATED FROM ELLIPSOMETRIC DATA adsorbate BonSi(111) BonSi(100) OonGe(111) OonGe(100) CHSSH 0.15 0.55 0.14 0.42 CHsCl 0.13 0.44 0 0.40 CHjBr 0.14* 0.39 0.17 0.44 * The CH3Br adsorption on Si(ll1) does not attain " monolayer " coverage.In this table the extrapolated value is given. STRUCTURE MODELS FOR ADSORPTION COMPLEXES The ellipsometric data for methyl-mercaptide adsorption on silicon and ger- manium as given in table 1 suggested a one-to-six coverage on the (1 11) face and a one-to-two coverage on the (100) face. Gas-volumetric adsorption measurements on a germanium powder yielded an average coverage of 0.23 molecules CH3SH per surface atom in good agreement with ellipsometry if an 80 20 dlstribution for (1 11) and (100) planes was taken. Desorption at higher temperatures gave hydrogen and methane as reaction products. The methane formation depended on the presence of Hz in the system (of at least 0.1 Torr).This same behaviour has been observed for methanol decomposition and both the temperature dependence and the amount desorbed suggested strongly that this reaction takes place on the (100) pIanes. The hydrogen from the (111) planes evolved in two steps; approximately one half desorbed around 200°C and the other half at 450°C. The adsorption complex of CH3SH on a (111) face given in fig. 3 exhibits therefore two binding states; one half of the hydrogen bonded directly to the germanium surface atoms F. MEYER AND M. J . SPARNAAY 21 and one half bonded to carbon. Since hydrogen bonded directly to germanium desorbs at 200°C it is probable that the first desorption step corresponds to those hydrogen atoms in the complex.The ellipsometric data for methyl chloride and methyl bromide adsorption on Si(ll1) and Si( 100) are given in fig. 2. The ellipsometrically-determined coverages given in table 1 suggest a one-to-six coverage on the (1 11) face and a one-to-two coverage on the (100) face. The CH3Br does not attain full coverage on the (111) (-70 %); the remaining dangling bonds can be compensated by subsequent 0 adsorption as indicated in the figure. Ellipsometric and gas-volumetric measure- ments on germanium suggested also one-to-two:coverages on the (100) face and an incomplete (70-90 %) one-to-six coverage for CH,Br on the (111) face. CH3Cl did not adsorb significantly on Ge(ll1) with exposures up to 50 Torr min. CH S H \ / \ / Si Si 7 /"? J\ 'i' CH SH Si Si Si Si Si Si H CH2 H X ,*- I 'i A I I Si Si Si Si Si Si FIG.3.-Structure models of the adsorption complexes of methyl mercaptide and methyl halide (X = C1 or Br) on the (1 11) and (100) faces of silicon or germanium. The adsorption complexes on the (1 1 1) face have to involve a carbon-atom bonded to three surface atoms (using the assumption that all atoms have their normal valency in the complex). Furthermore it is likely that the adsorption complex exhibits a direct germanium or silicon halogen bond since the first adsorption step probably involves this " active " atom (e.g. CH4 does not react chemically). The structures given in fig. 3 seem therefore the most probable. It has been observed in many instances that hydrogen bonded to a germanium surface atom desorbs by heating in vacuum at temperatures of 150-200°C.Examples are H2 adsorption and desorption;13 CH30H and CH3SH decomposition; HCl H2S etc. decomposition.l If the methyl-bromide adsorption complex on germanium powder was heated in vacuum no hydrogen evolution occurred below 350°C. This could be explained by a rearrangement in the structure of the adsorp- tion complex in which the hydrogen is transferred from a germanium atom to the CH-group forming a methylene (CH,) bridge. The driving force for this transition is probably the entropy gain since the CH bridge has a low frequency vibration mode compared to the rigid CH-group. This is treated more quantitatively in the next section. The hydrogen transfer from germanium to carbon regenerates two dangling bonds which can possibly react with a gas molecule. A further adsorption of methyl bromide was indeed observed if the germanium powder was heated in the presence 22 ADSORPTION REACTIONS ON SILICON AND GERMANIUM of CH3Br at temperatures of 150-250°C.This further adsorption did not take place at room temperature after heating the adsorption complex briefly in vacuum to 250"C indicating that the hydrogen transfer is reversible. Heating the germanium powder in CH3Br to temperatures higher than 300°C gave a reaction l4 in which mixed germanes are probably formed i.e. Ge(CH,),Br,-,. The surface proved to be heavily contaminated after such a treatment and could not be cleaned again completely. temp. "C FIG. 4.-The ellipsometric effects 8A and Sl) in deg. representing the " extra " adsorption upon heating a silicon (111) sample (covered by CH3Cl at room temperature and annealed in vacuum at 300°C for 5 min) in CHSC1 to different temperatures.The ellipsometric meaurements were taken at room temperature with an angle of incidence of 71.5" and a wavelength of 0.55 pm. The reaction below 300°C has been investigated by ellipsometry the advantage being that the (111) plane can be studied separately. The ellipsometric data for the adsorption of CH3Cl on Si(ll1) as a function of temperature are given in fig. 4. The procedure was as follows. Methyl chloride was adsorbed at room temperature on the clean silicon surface to approximately " monolayer " adsorption (one-to-six coverage). The crystal with adsorbate was then heated in vacuum to 3Oo0C which gave an irreversible change in A and II/ as measured at room temperature seemingly independent of the adsorbate.This might be correlated with the change in surface structure commonly observed by LEED (compare Si( 1 1 1)-PH3-7 x 7- Si( 1 1 1 )- PH3-l x 1). Room temperature adsorption gave no significant further reaction indicating that no dangling bonds were present after the heating cycle. This was followed by heating in CH3C1 to different temperature for short periods of time (a few minutes) and the ellipsometric effects measured after cooling to room temperature, F . MEYER A N D M. J . SPARNAAY 23 indicated an extra adsorption between 100 and 300°C. The change in $ was small as expected when all dangling bonds stay compensated. The total amount adsorbed after heating in the gas is roughly between a one-to-four and a one-to-two coverage. CH3Br gave similar results.The ellipsometric measurements have also been per- formed at temperatures between 120 and 300°C. Since most data had been obtained however at room temperature we have chosen this temperature as a standard for comparison. THERMODYNAMIC CONSIDERATIONS The adsorption data of CH3CI and CH3Br (or in brief of CH3X) on Ge and Si substrates led us to believe that there is a low-temperature complex (complex (a)) and a high temperature complex ( b ) ; a reversible transition between these two complexes taking place at about 200" (Si) and 150" (Ge). The situation is schemati- cally given in fig. 3. We now consider the transition between complexes (a) and (b) from a statistical point of view and follow roughly the theory given in ref. (16). We limit ourselves to a zeroth approximation because so far we have only rough information as to the complexes (a) and (b).The total number of adsorbed molecules CH3X is assumed to be constant and is denoted as n. The number adsorbed as complex (a) is -$n(l - r ) and the number adsorbed as complex (b) is *n(l+ r ) . The partition function 2 is this system is where the term containing the factorials is the configurational part and where Z, z b are partition functions for individual complexes (a) and (b). Finally f ( r ) denotes a contribution accounting for possible interactions between two neighbouring complexes. In the zeroth approximation it is customary to write for f ( r ) (3) where E,, &ab and Ebb are the interaction energies between two neighbouring complexes (a) (a) and (b) and (b) respectively. The number of neighbours is in this approxima- tion assumed proportional to (1 - r),2 (1 - r)(l + r ) and (1 +r)'.Finally z is a coordination number. j ( r ) = +nz((l- r)2&aa + 2(1- r)(l+ r)Eab -I- (1 + r)2&bb] The procedure is as follows. The Helmholtz free energy P of the system is P = -kTInZ (4) where k is the Boltzmann constant and T the absolute temperature. Equilibrium between the complexes (a) and (b) at temperature Trequires that In the following case eqn (4) and (5) can be written as F = (n/2)kT[( 1 - r ) In (1 - r ) + (1 + r ) In (1 + r)] + (n/2)[( 1 - r)F + (1 + r)&] +f(r) (6) where F = -kTIn 2 and F b = -kTIn z b and where we applied Stirlings' approxi- mation and (aP/ar) = 0. ( 5 ) When the only temperature-dependent term is kT In [( 1 - r)/(l + r)] then a transition takes place at that temperature where r+O provided E + E ~ ~ < ~ E ~ .In that case 24 ADSORPTION REACTIONS O N SILICON AND GERMANIUM there is a high-temperature phase where r = 0 and a low temperature phase where r # 0 and this represents the usual development in the zeroth approximation. However we have a case here where we have a transition from negative to positive r and we ascribe this transition to the temperature dependence of Fa-Fb. The energies E,, Ebb and &,b may serve to " sharpen " or to weaken this transition depend- ing on their sign and magnitude but at this stage we know very little of the physical nature of these energies. In contrast we have at least a crude model of the individual complexes (a) and (b). w e now assume that t&,z&bbE8ab and discuss the difference F,-F'.Inspection of the model depicted in fig. 3 suggests that we can write (8) where is a free energy to be assigned to the mutual compensation of the silicon dangling bonds. The other contributions in eqn (8) pertain to the Si2CH2 the Si3CH and the SiH groups. The important difference between the low-temperature and the high-temperature configurations is the removal of a H-atom from a Si-atom to a C-atom. This removal has as one consequence a loosening of the position of the C-atom because it is now no longer bonded to three Si-atoms (placed in a triangle) but to only two. Consider- ing its vibrations in a plane normal to the plane of the paper (fig. 3) its frequency Vb in the high-temperature SizCHz case will be considerably lower than that in the low-temperature case where we denote this frequency as v vb 4 v a - (9) We apply to these vibrations the free energy expression of a harmonic oscillator with frequency v where h is Planck's constant and assume the classical limit (v<kT/h) for vbn This gives where Fii2C~2 contains contributions F, arising from other frequencies than v b .These contributions are less temperature dependent than the one given by v b . Inserting this equation together with eqn (8) into eqn (7) and ignoring the energies E,, Ebb and &,b one obtains Fa - Fb = FSi 3CH + FSi H - FSi.. .Si - FSi 2CH1 F = $hv + kT In [ 1 - exp (- hv/kT)] FSi2CH2 = F&zCH2 + kT1n lhvblkT (10) (1 1) 1-r kT kT ln- + kT In- = F&2CHZ+FSi...Si - l + r hVb Assuming that the right-hand side of this equation is temperature independent the rate of change of the equilibrium value of r with changing temperature is which is valid near r = 0.The value r = 0 will be defined as the transition point between the low-temperature and the high-temperature configurations. The transi- tion temperature is at about T = 470 K (complex adsorbed on Si) and at about 420 K (Ge). When Vb = 5 x 10" s-1 (a wave number of 25 cm-l) kT-20 hvb (Si case). This means that when dT = 0.1 T a change dr of $- is taking place. This is in qualitative agreement with experiment. Finally we inquire into possible values of the most interesting contribution Issi. . s i . For this purpose we write complex FSi3CHfFSiH-FSi2CH2 = AFH+FSiC Y F . MEYER AND M. J . SPARNAAY 25 where A& is the free energy change involved in the step of the H-atom from a Si-atom to a C-atom and where F i t F p l e x is the free energy connected with the third bond of this C-atom with one of the underlying Si-atoms (fig.3). This is a dangerous procedure but it allows us to make a comparison with the adsorption of CH3SH. Here no " detour " was made by the H-atom. Instead it was disconnected from the surface at about 300°C (Si substrate) whereas at about 500°C other H-atoms probably bonded to the C-atoms were freed. This leads us to believe that AFH cannot be much larger than kT where T-600 K. Therefore since at r = 0 we have Fa-& = 0 (Y = O) Fsi ... si Fsic 3 our estimate is complex This is only a first estimate. However it suggests a working programme. Thus investigations in the far infra-red or microwave region may lead to more information concerning frequencies such as the one denoted here as v,.Such experiments may also lead to a better insight into the nature of the free energy difference AF and of p o m p l e x S i c (and of CONNECTION WITH ELECTRICAL MEASUREMENTS The work described in this paper is a combination of optical and gas volumetric measurements and as such it must be considered as a continuation of previous work consisting of a combination of electrical and gas volumetric measurements. For clean Ge and Si surfaces of samples of intrinsic conductivity type most workers agree l 8 that there is a weakly p-type space charge. This has been found by surface conductance field effect and Hall measurements. For Ge the hole density in the space charge is 3-4 x loll cm-2 and for Si it is even less. The physical origin of the space charge is found in the existence of about 10I5 surface states cm-2.These surface states have both an acceptor and a donor character but the acceptor character slightly dominates and provides for a negative charge in the surface states of 3-4 x 10l1 electrons cm-2 or less (300 K). These electrons are drawn from the bulk of the crystal over a depth beneath the surface of about a Debye length. In this way the positive space charge is created. This weak double layer can probably be ignored in the interpretation of optical data. The electrical effect of the chemisorption of O2 and of the gaseous adsorbates used in this paper is two-fold. First up to a certain coverage which is 10 % of a monolayer for 0 2 1 the space charge density rises (at 300 K) to 6-7 x loll holes cm-2.(Ge) Beyond this coverage (for O2 rising beyond 10 %) there is a decrease of the density to lo1* holes cm-2 or less (Ge) depending on the nature of the chemisorbed molecule. The second effect is a continuous decrease of the surface state density upon an increasing coverage to 10I1 states cm-2 or less (300 K). In the crude " dangling bond " model the second effect is understandable ; chemisorption means a saturation of the dangling bonds and associating the dangling bonds with surface states chemisorption leads to the annihilation of the surface states. The first effect is more difficult to understand in particular the maximum observed in the space charge density. However the first effect and also the second effect lend support to the hypothesis that the change of the ellipsometric angle + observed upon chemi- sorption can be ascribed to a change of the optical constants of a surface layer of the substrate.The chemisorption of widely varying species led in all cases to quali- tatively both the same change of + and the same change of space charge density and 26 ADSORPTION REACTIONS ON SILICON AND GERMANIUM surface state density. It is more obvious to ascribe these changes to changes of properties of the substrate region than to characteristics which the various adsor- bate molecules may have in common. In this respect it would be interesting to investigate possible electrical effects due to the sophisticated chemical reactions discussed in this paper. We thank Mr. E. E. de Kluizenaar for performing most of the ellipsometric measurements . l A.H. Boonstra and J. van Ruler Surface Sci. 1966 4 141 ; A. H. Boonstra Philips Res. Reports suppl. no. 3 (1968). F. Jones I.B.M. J. Research Developt. 1965 9 375. F. Meyer and J. M. Morabito J. Phys. Chem. in press. G. A. Bootsma and F. Meyer Surface Sci. 1969,14 52. J. J. Vrakking and F. Meyer to be published. F. Meyer J. Phys. Chem. 1969,73,3844. S . Brunauer P. H. Emmett and E. Teller J. Amer. Chem. SOC. 1938,60,309. A. Liberman and M. Green J. Phys. Chem. Solids 1962,23,1407. A. J. Rosenberg J. Phys. Chem. Solids 1960,14,175. lo J. R. Miller and J. E. Berger J. Phys. Chem. 1966,70,3070. l1 G. A. Bootsma and F. Meyer Surface Sci. 1969,13,110. l2 F. Meyer E. E. de Kluizenaar and G. A. Bootsma Surface Sci. 1971 27 88. l 3 K. Tamaru J. Phys. Chem. 1957,61,647. l4 E. G. Rochow J. Amer. Chem. SOC. 1947,69,1729. l5 A. J. van Bommel and F. Meyer Surface Sci. 1967 8 381. l6 E. A. Guggenheim Mixtures (The Clarendon Press 1952). l7 A. H. Boonstra J. van Ruler and M. J. Spamaay Proc. Kon. Nederland Akad. Wetens. B 1963,6,64 70. M. J. Sparnaay A. H. Boonstra and J. van Ruler Surface Sci. 1964,2 56. l 8 see textbooks such as A. Many Y. Goldstein and N. B. Grover Semiconductor Surfaces (North-Holland Publ. Co. Amsterdam 1964) and D. R. Frankl EZectrical Properties of Semi- conductor Surfaces (Pergamon Press 1967).

ISSN:0430-0696    

DOI:10.1039/SF9700400017

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Reflection Spectroscopy of Adsorbed Layers BY WILFORD N. HANSEN Utah State University Logan Utah 84321 U.S.A. Received 14th September 1970 General approximate equations are presented for the change in reflectance of a stratified medium due to the presence of an absorbing adsorbed layer. These equations which represent reflection spectra are simple functions of the optical characteristics of the adsorbed species. Experimental examples of monolayer spectra illustrate the sensitivity of reflection methods and the validity of the equations. Band shapes of reflection spectra differ from but are as representative as transmission spectra. The shapes usually depend on polarization but often do not depend on angle of incidence. Intensity is usually linear in absorption coefficient and thickness when the approximate equations are valid.In order to bring the powerful techniques of absorption spectroscopy to full use in the study of surface chemistry and physics it is necessary to obtain spectra of thin layers with ease and accuracy. This paper is concerned with the understanding of reflection spectra from the interfacial region of a three-or-more-phase plane- bounded system. Such a system might represent an organic dye on glass a pro- tective organic film on a metal or some species adsorbed on an electrode immersed in an electrochemical cell. The discussion will be limited to films much thinner than a quarter wavelength typifying adsorbed layers. The idea is to obtain spectra by simple procedures which are characteristic of the molecular species involved (not a fickle function of experimental parameters) and which bear a simple relation- ship to the optical properties especially the absorption coefficient a.Exact equations for reflectance R and reflectance absorbance A = -log, R in a stratified medium can be found in the literature. Here we will use the nomen- clature and exact theory of ref. (1). This theory is extremely useful whenever the real system corresponds close enough to the model assumed. In this case the model is an ideal stratified medium i.e. a stack of parallel plane-bounded layers. The exact equations are far too complicated however to give direct physical insight for multi-phase systems. For this reason we have derived simple approximate equations for reflectance changes caused by very thin films such as absorbing adsorbed layers. The equations are simplest for a thin film on a dielectric or at the surface of a highly reflecting metal.Both internal and external reflection are considered. With some increase in complexity absorbing intermediate and final phases in addition to the one whose spectrum is sought can be accommodated. It will also be seen that reflection methods are sensitive enough to deal with fractions of monolayers. Some aspects of this subject have been discussed in the literature in a different context. Harrick has discussed first-order attenuated total reflection (ATR) theory for thin films on transparent substrates. Francis and Ellison and others 4-6 have discussed reflection from a filmed mirror. Their work was significant in the formulation of the present theory. 27 28 REFLECTION SPECTROSCOPY OF ADSORBED LAYERS THIN FILM O N NON-ABSORBING SUBSTRATE External reflection is considered first.In this case the refractive index of the incident phase n, is less than the index n3 of the final semi-infinite phase. If the exact equations for reflectance in a three-phase system [eqn (3) and (8) of ref. (l)] are expanded in terms of film thickness we obtain to first order and The subscripts refer to the phase phase 2 being the absorbing film phase 3 the sub- strate and phase 1 the incident phase which is usually air. The I and 11 subscripts refer to perpendicular and parallel polarization. For any phase j 5 = A cos ej = (A3-nZ sin2 el)* where A is the complex index of refraction n+ik 6 is the angle of refraction in phasej and k is the extinction coefficient. The angle 8,> is complex if k,> 0 and/or 6 > O, where OC is the critical angle.(This latter condition is possible only for internal reflection where n3 <n,.) The absorption coefficient a which is the quantity directly measured by transmission methods is related to the extinction coefficient by a = 471k/R where R is the wavelength in vacuo h2 is the film thickness in units such that ah is dimensionless; tj+l is complex in general. It is evaluated by taking the complex square root above. The rule in choosing roots is that both Re t,>O and Im t,>O. The &st factor of each equation is the reflectance in the absence of phase 2 and is valid even if phase 3 is absorbing. (Phase 1 is always non-absorbing). In fact eqn (1) and (2) are valid for an absorbing final phase so long as k3 < 1 if Img3 is ignored in the second factor.Experimentally it is most convenient to measure reflection spectra as the change in reflection absorbance (AA = log, (Ro/R)) caused by the presence of the film. (Ro is the reflectance in the absence of the film.) For the present case of external reflection with the first phase air and the final phase transparent we have from (1) and C9 . .- bA = -(-')n2u2h2 4 case In 10 n,2-1 (3) Note that the absorbance will always decrease for I polarization (RI will increase) as the film is added. For 11 polarization it may either increase or decrease. At 8 large but less than Brewster's angle the (n; + k i ) 2 factor which varies greatly through a typical absorption band will determine the behaviour. When this factor is large say 40 its term is small and the next term dominates making AAll negative.When it is much less than unity as it sometimes is AAI will be positive. The reverse is true when 8 is greater than Brewster's angle. To compare spectral shapes of AA with the ones obtained by transmission through thick samples we note that a transmission spectrum in terms of transmission absorbance is a quantity proportional to a and h. So the spectrum of AAl has the same shape as a transmission spectrum multiplied by n2 except for sign ; AA 11 has the same shape as AAL so long as the first term in square brackets can be ignored. When it becomes equal to the second term however the spectrum crosses the no-film axis and becomes positive. WILFORD N. HANSEN 29 For intense bands this will happen on the short wave-length side where n2 becomes low sometimes even less than unity and k2 is also low.Now consider internal reflection where n3<n,. Eqn (1) and (2) still apply. The refractive index of phase 1 is always greater than unity and that of phase 3 is usually so. We therefore write These equations are valid even if phase 3 is absorbing with k3 4 1 and it is noteworthy that they are valid at angles less than and greater than the critical angle. Thus a AA spectrum is continuous in shape as we pass through the critical angle. is obvious. The behaviour of (6) is more complicated. At normal incidence (6) reduces to (5) as it must and AAII is positive. We recall that for n3>n it was negative. As 8 is increased to Brewster's angle O, the denominator inside the parentheses goes to zero making the sensitivity very large but at the same time the energy of the system becomes very small.As 0 passes through Brewster's angle the denominator changes from negative to positive and AA II also changes sign. From OB to OC the radiant power increases to a maximum. At OC the second term in square brackets goes to zero and The behaviour of eqn (5) with and for I polarization AAL = L-(-)n2ct2h2 1 (8 = &). In 10 n cos O1 At angles greater than OC < is negative so that all terms in eqn (6) are positive. Thus in the ATR region the terms in square brackets add and the second term iiicreases in importance as 8 becomes larger. The first term is due to fields normal to the interface while the second term is due to fields parallel to the interface. Eqn (1)-(8) are first order in thickness over wavelength.Nothing is assumed with regard to n2 or k2 so long as the film is thin enough for A A 4 1. (Actually eqn (3)-(8) apply to higher radiant power absorption than eqn (1) and (2).) Thus they are even valid for very thin metal films. The derivation assumes that the thin film is isotropic and homogeneous but these restrictions can be relaxed considerably. For I polarization the electric field vector is always in the plane of the film so the only optical constants involved are those components lying in that plane and per- pendicular to the plane of incidence. For 11 polarization at & the fields are entirely perpendicular to the interfacial plane. Then only those components are effective. When =i= OC both components are effective and different values of n2 and/or a would apply to the two terms of eqn (6).Also eqn (1)-(8) are all valid for an absorbing final phase provided k3 4 1. In fig. 1 are shown four different reflection spectra of a monolayer on a transparent substrate-crystal violet adsorbed on one face of an optical glass prism (n = 1.52). The upper curves were taken per internal reflection near the critical angle. The lower pair were of the same film but per external reflection. Only a single reflection was used in a standard double-beam spectrophotometer. For the upper curves 0 = 43" n = 1.52 n3 = 1. For the lower curves O1 = 45" nl = 1 n3 = 1.52. 30 REFLECTION SPECTROSCOPY OF ADSORBED LAYERS Three of the curves are similar in shape while AAli (internal) is quite different having its maximum at shorter wavelengths. This is as predicted by eqn (6) or (7) and results from the (n2 +k2) factor.A reasonable range for this quantity in the present case is from about 10 at 620 nm to 1 at 490 nm. Also AAII (external) crosses the no-film axis at the left end. This will happen according to eqn (4) whenever the first term in the square brackets dominates over the second. (The denominator in parentheses is negative in the present case.) On the other hand AAII (internal 0,20,) can never cross the no-film axis. A comparison of eqn (3) and (5) shows that the two AAL curves should have the same shape with a ratio AAl (internal)/AAl (external) = - 1.52. This ratio holds for the experimental data at about 580 nm but the ratio is not constant with wavelength. Evidently higher order terms in the original expansion are large enough to be seen.The AAl curves indicate a value of 0.064 for n2k2h2/A at 580 nm. Using this value and guessed values for n2 and k of 1.6 and 1.0 eqn (4) gives -0.57 for AAll (external) compared with the value of -0.62 measured. Such good agreement in absolute intensity will not be found outside the central region of the absorption band. wavelength (nm) FIG. 1.-Reflection spectra of crystal violet monolayer on BK-7 optical glass n~ = 1.52. Upper curves are ATR spectra single reflection at 43". Lower curves are external reflection spectra single reflection at 45". THIN FILM I N A MULTILAYER SYSTEM The rate of absorption of radiant energy at any position in a medium is given by where S is Poynting's vector 0 is the high frequency conductivity and ( E 2 ) is the mean square electric field at the position.This can be made the basis of a powerful method of deriving approximate equations for reflectance in a multilayer system. Values of <E2> can be calculated exactly for any point in a stratified medium.' (rate of energy absorption) = -V . S = a(E2) W/m3 (9) WILFORD N. HANSEN 31 For most cases (E') is affected little by the presence of a very thin film nearby and when the effect is appreciable it can often be accounted for by the decrease in radiant pDwer caused by the absorption in the film. (E2) in the film itself will be continuous for tangential components and (n2 + k2)2 ( E 2 ) will be continuous for normal com- ponents. For simple systems ( E 2 ) values can be predicted as a function of angle of incidence and polarization. For complicated systems curves can be prepared by computer.The general method is to derive approximate equations which give the reflectance in terms of ( E 2 ) which is calculated in the absence of the thin film whose spectrum is sought or with a transparent film in its place to approximate more closely the film-present case. By this scheme eqn (5) and (6) can be derived for the attenuated total reflection (ATR) region where there is no refracted beam. The scheme often fails however when large fractions of the radiant power are lost to absorption or refraction in neighbouring phases. By the above method the fractional change in radiant power in a stratified medium due to absorption in phase j is given by where (E2) is the space-time average field in film j . This equation is exact. For the equation to be useful however we must be able to calculate the (E2) without knowing the a of the film whose spectrum we seek.The R of the equation should also correspond to experimental reality. In some experiments such as the electro- chemical adsorption and removal of a monolayer on a thin metal film electrode the R (most conveniently measured as AA = -log,&) refers to a film-present film- absent situation. In other cases such as an adsorbed film with a strong band in the infra-red but non-absorbing at neighbouring wavelengths AA corresponds to the change in A with wavelength providing absorption by all other phases is relatively constant. We now alter eqn (10) so as to use an ( E 2 ) for vanishingly thin film j or transparent film j where no energy is absorbed. This leads to the following equation njajhj In 10 Ronl cos 6 AAj = (E2> g where Ro is the reflectance when film j does not absorb.This equation is approximate for we have assumed that (E2) M ((l+R,)/2) (E2)g and that AA = 2(1-R,)/ In 10 (1 +R,). Eqn (11) accounts for the absorption in phases other than j but it assumes that this absorption does not depend on the absorption in film j . It is also possible to correct for this latter effect. This leads to the equation njajhj AAj = (E2>q (1 +Ao)Ro In 10 nl cos where A . is the absorbance of the system in the absence of absorption in phase j multiplied by 2.312. This latter factor is usually unimportant and is omitted to sim- plify the eqations. Eqn (12) applies for either polarization. But for I polarization ( E 2 ) is continuous across an interface while for the normal component of 11 polariza- tion it is (n2+k2)2 ( E 2 ) that is continuous.Thus one approach would be to substitute a standard reference film s of similar index and thickness and a = 0 in place of film j for calculation purposes. With reference to this standard state our equations finally become 32 REFLECTION SPECTROSCOPY OF ADSORBED LAYERS and where It and the fields apply to the standard state film. These equations account for absorption in surrounding phases including the final semi-infinite phase. These phases may even be metallic in some cases. While these equations have been found to be valid for a variety of cases the full range of their validity is as yet unknown. They do not take into account changes in phase caused by the presence of film j . Thus eqn ( 5 ) and (6) at 61>6c follow directly from eqn (13) and (14) but not for 6 < OC.Likewise eqn (3) and (4) for external reflection do not follow by the present scheme because phase changes dominate. Eqn (13) and (14) show immediately the nature of a reflection spectrum. Within the range of their validity absorbance is linear in a and h. This implies that a rule analogous to Beer's law is followed and that the spectral intensity is proportional to the amount of absorbing species involved. Thus the absorbances of two thin films are additive and the distribution of absorbing species can be inhomogeneous without affecting the spectrum. The significance of this point becomes clear when we consider that at a molecular level adsorption of molecules on a glass surface might be much like a layer of snow on the Rocky Mountains.In fig. 2 are shown spectra of crystal violet adsorbed on a thin film gold electrode. I I 1 I 4 5 0 5 00 5 5 0 6 0 0 6 5 0 700 wavelength (nm) FIG. 2.-Internal reflection spectra of crystal violet adsorbed on a gold film electrode. Single reflection at slightly greater than critical angle. The first phase is glass which supports a gold film about lOOA thick. The gold film constitutes the second phase. The final phase is aqueous electrolyte containing a small amount of crystal violet. The solution is so dilute that the ATR spectrum of the crystal violet cannot be seen but is sufficiently concentrated to cause adsorption on the gold a condition easily attained. The angIe of incidence was near critical. It is remarkable that the spectra closely resemble ATR spectra even though the optical properties of gold vary widely through this spectral range and the gold is highly absorbing.Another unexpected feature is that the sensitivity is higher for WILFORD N. HANSEN 33 parallel polarization when the film is present than when it is absent. The reason is that ( E 2 ) 11 is greater when the film is present even though the radiation must penetrate through the film! Calculation of ( E 2 ) values for this multiphase system using the equations of ref. (1) confirms this fact. (E2)L is much smaller and the sensitivity is much smaller as indicated by the equtions. THIN FILM ON A METAL Eqn are now derived for the absorbance change due to the presence of a thin film on the specular surface of bulk metal. The most powerful approach is simply to use eqn (12) which is valid even if the metal is not a good reflector and even if it is already covered with a thin oxide layer plus other very thin films.To use the equation effectively we must have knowledge about (E2)g which must be calculated by computer in the general case. For good reflectors however the situation is especially simple. As a general rule all metals are good reflectors in the infra-red and some are good reflectors in the visible. For these metals the mean square fields at the surface relative to those in the incident beam are given by (E2) x 4 ~ 0 s ' 81/k2 (16) where the z direction is normal to the interface and the y direction is normal to the plane of incidence. BB is the pseudo-Brewster Angle. Just how well these equations hold is illustrated by comparison with exact calculations.For the present let n = 3 k = 30 (typical for a metal in the infra-red). At 60" eqn (15) gives 3.00 for (S')~,,. The exact calculation gives 2.95. Eqn (15) gives 4.44 x for (Ez)llx at all angles. The value calculated using exact equations is 4.37 x at = 0" and 4.33 x at = 60". Eqn (16) gives 4 . 4 4 ~ for (E2)1 at 8 = 0" 1.11 x at 60" and 5.40 x at 88". The values from exact calculations are 4.37 x at 0" 1.10 x at 60° and 5.36 x at 88". Substituting the above equation for (Ez)llz into eqn (12) letting Ro = 1 and neglecting ( E ) \Ix we have or The factor in parentheses is the sensitivity and is constant from metal to metal. The sensitivity can be increased rapidly by increasing nl. For perpendicular polarization we can substitute eqn (16) into eqn (12) and similarly obtain The sensitivity is diminished by a factor of l/k2 which is cu.for the metal discussed above. The above equations make clear the physics of the reflection process show what a spectrum will look like relative to an absorption spectrum and show that except for changes in n j absorbance will be proportional to the amount of absorbing S4-2 34 REFLECTION SPECTROSCOPY OF ADSORBED LAYERS material at the interface. This will be true even if the discreteness of the absorbing material is considered. The way to get an intense spectrum is clearly to use parallel polarization alone since any perpendicular component present acts like stray light. With equations written in the form given above the case of multiple reflection is easy to handle. For a single polarization the absorbance at each reflection simply adds to the total.If the sample is uniform and there are Nreflexions A j T = NAj (20) where A, is the overall absorbance. In this case the sensitivity in the above equations is increased by a factor of N. This will not be true however if perpendicular radiation is allowed to come through along with parallel since the ratio of the two will change from point to point. In fact Aoll may be so large compared to AOL that after a number of reflections the light is mostly I which swamps the signal. Polmizers are desirable for quantitative work. A molecular layer of silicone oil was adsorbed on an aluminium mirror and the reflection spectrum of the layer was recorded in the infra-red using a multiple reflection apparatus adjusted to give 100 reflections.The layer formed quickly and reproducibly when a dilute toluene solution of silicone oil was contacted by the mirror. The resulting reflection spectrum is shown in fig. 3 along with a transmission spectrum scaled to give about the same intensity at the 1250 cm-l peak. One curve is shifted vertically relative to the other to aid visual comparison. t I I On30 ' 13bo 1200 I100 1000 9 wavenumber cm- 0 FIG. 3.-Silicone oil adsorbed on aluminium mirror 100 external reflections parallel polarization. Dashed curve is transmission spectrum. First we note the qualitative features. They will be determined by eqn (17). The angle of incidence was 45" and n = 1. Since h is fixed the only remaining factor varying with wavelength is [nj/(n3 + k3)2]aj which determines the shape of WILFORD N.HANSEN 35 the spectrum. From known spectral behaviour and from our determination of the optical constants of silicon oil kj gets very large about unity in the most intense regions. Therefore (Anj)max is also large with low values of n (below unity) to the left of large absorption regions and large to the right. The above equations predict therefore that the reflection spectrum will be more intense to the left of the main absorption bands and less intense to the right. The side bands show that to be just what happens. There are not enough data in the present example to analyze the spectrum in every detail. For example there may be some band distortion in the adsorbed material with consequent spectral changes but this effect is evidently small. Even such a simple spectrum which is easily obtained contains a great deal of information.It is instructive to make an absolute calculation of the thickness of the film from the experimental data. At A = 7.9 pm we take our measured bulk values n = 1.0 kj = 0.27 (giving a = 4200 cm-l) and 8 = 45". From eqn (17) we then have for a single reflection All = 4500 h,. The measured absorbance at 7.9 pn is 0.15 or 0.0015 per reflection for the 100 reflections used. That gives hj = 34 .$ a reasonable answer. The author is greatful to the National Science Foundation for the financial support of Grant no. GP 13767 and to Research Corporation for help in purchasing experi- mental equipment. W. N. Hansen J. Opt. SOC. Amer. 1968,58 380. N. J. Harrick Internal Reflection Spectroscopy (Interscience Publishers 1967). S. A. Francis and A. H. Ellison J. Opt. Soc. Amer. 1959,49,131. R. G. Greenler J. Chem. Phys. 1969,50 1963. G. W. Poling J. Electrochem. SOC. 1969 116,958. Harvey Pobiner Anal. Chem. 1967,39,90.

ISSN:0430-0696    

DOI:10.1039/SF9700400027

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Reflectance Studies of the Gold/Electrolyte Interface BY B. D. C m N JEAN HORKANS AND ERNEST YEAGER Chemistry Dept. Case Western Reserve University Cleveland Ohio 44106 U.S.A. Received 4th September 1970 The gold/electrolyte interface has been examined with both single and multiple reflection tech- niques. Gross errors arising from scattering have been identified with the latter using a large number of reflections. An explanation involving transitions of electrons in surface states (5d-+6s) is proposed to account for the sensitivity of the reflectivity of gold electrodes to potential and to adsorbed ionic and neutral species. Previously proposed explanations involving changes in the Fermi level relative to non-surface electronic states appear untenable. A method is also proposed for examining the refractive index within the double layer if it is lower than that of the bulk solution.A number of papers have recently appeared dealing with the phenomenon of the electro-modulation of light at a reflecting electrode/solution interface. These effects have been detected with single reflection using a phase-lock amplifier,'. ellipso- m e t ~ y ~ ' ~ multiple reflection,6 and with attenuated total reflection.'" The potential dependence of the reflectivity was originally interpreted as a change in the optical constants of the double layer,2 but it was later shown by Hansen and Prostak 7-9 that the changes in the double layer should be too small to cause the observed effect. They re-interpreted the data in terms of a change in the electron concentration in the surface layers of the electrode.Their calculations explain the occurrence of a peak in the AR/R curve for gold at the wavelength of the absorption edge of gold associated with the 5d+6s interband transition. This peak is predicted to be much narrower than that obtained experimentally. It is still not fully clear that the pre- dominant optical effect is attributable to changes in the metal rather than in the double layer. Calculations by Stedman O predict some dependence of reflectivity of the electrode on the changes of the optical constants of the double layer although this effect is smaller than that predicted on the basis of the Hansen-Prostak calculation. Takamura et aL6 reported a large increase in sensitivity by using multiple (- 19) reflections. Their (A,R/R,E) curves were highly sensitive to monolayer and sub- monolayer quantities of oxides adsorbed anions and foreign metal adatoms.Ellipsometry also provides adequate sensitivity for the detection of submonolayer quantities of adsorbates. followed anion adsorption on Hg and Au. Buckman studied the electro-modulation of silver in KCl electrolyte. The purpose of the present paper is to elucidate the mechanism responsible for electro-modulation of the reflectivity. Sirohi and Genshaw 4* EXPERIMENTAL The optical system consisted of a conventional prism monochromator with a tungsten light source and a mirror-cell arrangement for single or multiple reflection measurements. The use of a highly stable light source and photodetector system has afforded sufficient stability to permit the determination of the potential dependence with a single reflection after suitable amplification and without a.c.or square-wave modulation techniques. The 36 B . D. CAHAN J . HORKANS A N D E . YEAGER 37 signal-to-noise and hence the sensitivity to changes in reflectivity with potential were superior to that reported for multiple reflection.6 The electrodes used in these studies were evaporated gold films 5 O00-10 000 A thick. These films were deposited on substrates of flame-polished glass with layers of sputtered tantalum followed by sputtered platinum between the gold and the glass to yield good adhesion.'l The gold film electrodes gave voltammetric and reflectivity data similar to that on mechanically polished gold plates with the added advantage that the light scattered from the surface was very small compared to that obtainable from a polished bulk specimen.Most of the measurements were done in 1 N HC104 to minimize specific adsorption of the anion but measurements were also made in H2S04 and NaC104. The effects of small additions of non-polar materials (e.g. benzene) and adsorbable anions (e.g. the halides) were also studied. RESULTS (Reflectivity potential) curves obtained in the present work with single reflection and with seven reflections are generically similar (fig. 1) to those obtained by Takamura et aL6 with a reported 19 reflections. The magnitude of the reflectivity change per reflection however is larger by a factor of three in the present work. The slopes l/R(aR/aE) of these curves at three potentials in the anodic branch of the sweep have been plotted against wavelength in fig.2 for a single reflection. The analogous curves obtained for seven reflections have the same basic shape and are of the same magnitude after division by a scale factor of seven. Curves obtained in H2S04 showed similar behaviour. There is a shift of the position of the maximum by about 35 mp between 0.2 V and 1.0 V. The cwves also broaden at higher poten- tials. The maxima of the curves occur in a wavelength region similar to that observed by Fejnlieb.2 This is in contrast to the curves obtained by Takamura et aL6 (see inset fig. 2) which exhibit a sharp drop-off in the blue and a maximum displaced to ward longer wavelengths. - 0 0.4 0.8 1:2 1.6 0 0'4 0-8 1-2 1.6 potential against NHE (V) FIG. 1.-Relative reflectivity curve for an evaporated gold film electrode in 1 N HC104 at 540 mp normalized to a reflectivity af 1.0 at 0.0 V.The inset is a similar curve from ref. (6) for multiple reflection at a bulk gold electrode in 0.2 N HC104 at the same wavelength. Relative reflectivity curves (normalized to unit reflectivity at 675 mp) have been obtained for the evaporated gold film electrodes potentiostatted in HC104. These data have been processed digitally and fitted to a ninth-order polynomial. A typical curve and the resultant derivative curve (lIR)(iJR/i?il) against il are shown in fig. 3. 38 REFLECTANCE STUDIES OF GOLD/ELECTROLYTE INTERFACE 16 r 01 I I I I I I I I 400 500 600 700 1 (mt.) FIG. 2.-1/R(3R/3E)~ as a function of wavelength - is at 0.2V -- at 0.6V and - - -at 1.0 V. Slopes are taken from the anodic sweep of curves similar to fig. 1.The insert is the data obtained in ref. (6) using multiple reflection A is at 0.52 V ; B is at 1.03 V. 0'4 0-2 0 mt.) FIG. 3.-The reflectivity (normalized to R = 1.0 at 675 mp) and its derivative 1/R ( ~ R / ~ X ) E for a gold film electrode potentiostatted at 0.0V in 1 N HC104. B . D . CAHAN J . HORKANS AND E. YEAGER Fig. 4 shows the effect of adding 5 x 39 All of the curves described earlier show marked sensitivity to even minute quantities of additives or impurities. M benzene to the solution. At this concentration the benzene should have a surface coverage of only 15 % of a monolayer at the maximum of adsorption (0.5-0.6 V) against NHE),12 yet its effect on the reflectivity is clearly observable. [The noise level was sufficiently low to permit curves with considerable additional amplification to be obtained but they were not included in the present paper because of the difficulty of showing the entire curves within practical space limitations.] Although the curves show unmistakably the optical effects of benzene adsorption the data in its present form are not suitable for the determination of the adsorption isotherm for benzene because of the wide range of potentials used in the scan (> 1.6 V) and the possibility of oxidation products.Further work is planned to study more closely the effects of neutral adsorbed species on the reflectivity. I I I I I I t 1 0 0.4 0-8 I. 2 1.6 potential against NHE (V) FIG. 4.4hange in the relative reflectivity curve of gold (7 reflections) in 1 N HClO at 500 mp upon the addition of 5 x M benzene - in absence of benzene ; -- in presence of benzene.The lower curve has been shifted vertically for purposes of display. DISCUSSION COMPARISONS OF SINGLE AND MULTIPLE REFLECTION TECHNIQUES For a given polarization state of the incident light Zo the intensity Zn after n reflections can be expressed as where R is the reflectivity of the interface. Differentiating and dividing by In gives The implication is that the shape and magnitude of (n/R)(dR/aE)A against R should be independent of the number of reflections. The curves obtained in this study using single reflection and seven reflections exhibit this property within experimental tolerances. The earlier multiple reflection work,6 however deviates significantly (see fig. 2). A consideration of the cumulative effects of 19 reflections shows that at a wavelength where the reflectivity of gold is 0.5 only of the incident light is available for detection.Any stray or scattered light (including extraneous wave- lengths produced by scatter in the monochromator itself) would thus cause a back- ground intensity orders of magnitude greater than the desired signal such that In Zn = RnIO (1) (2) (1 1RXaRlaE)A = (1 /nrn)(aLlaE),i* 40 in eqn (2) is no longer the desired value. The mechanically polished surfaces used in the earlier studies should scatter light down the multiple reflection path giving emergent light equivalent to far fewer reflections than the number calculated on the basis of geometry. Indeed a comparison of the n values obtained in the red (where the reflectivity is relatively high) indicates an effective number of reflections between seven and nine.13 Therefore the displacement toward the red of the peak in the [(l/R)(8R/i?E)4 curve in their work is probably an artifact caused by a burying of the true signal I in the scattered light and the consequent misapplication of the l/n factor in eqn (2).Although quantitatively in error most of the qualitative observations of their work are still valid. REFLECTANCE STUDIES OF GOLD/ELECTROLYTE INTERFACE THE PROBLEM OF THE EDGE SHIFT The theory discussed by Hansen 7-9 ascribes the apparent changes in the optical constants of gold upon electro-modulation to a change in the concentration of free electrons in the surface region of the bulk gold. An increase in the number of free electrons n in the metal will then cause a shift in the plasma frequency of the metal cop = (4me2/rn)+ (3) and the optical properties of the metal will be modulated.Hansen and Prostak * state that the Fermi level will be modulated but that the energy of the bound electrons will be essentially unaffected (Le. the top of the 5d valence band). On this basis a shift of the absorption edge associated with transitions from the 5d to the Fermi level in the middle of the 6s conduction band is to be expected. The idea that the concentration of the electrons within the bulk of a metal film can be varied by an appreciable amount electrochemically is difficult to accept since it violates the principle of electroneutrality within a conductive medium. The excess electrons accumulated within the metal during electrochemical charging of the double layer are statistically localized at the surface by electrostatic interaction with the corresponding ion and dipole charge of opposite charge in the double layer.Stoner et ~11.'~ showed that the resistance of a thin platinum film used as an electrode was unchanged over the entire double layer region of potentials in the presence of a non-adsorbing electrolyte (i.e. HC10J. The concept of a shift in the Fermi level relative to the top of the 5d band in the bulk is contrary to accepted solid-state theory. The energy difference between the Fermi level and the top of the surface electron states however is expected to vary with electrochemical potential. In accounting for their ATR studies of gold film electrodes of - 50 a thickness Hansen and Prostak * assume the change in free electron concentration causes the optical properties of the metal including the optical absorption edge (5d-+6s transition) of gold to shift by 0.0076 eV for a potential sweep of 0.70 V.This shift is estimated on the basis that the double-layer charging leads to a change in the number of free electrons in the 50A gold film and that the (optical constant frequency) curve is then shifted in frequency by the change in cop as predicted by eqn (3). If the change in number of electrons is restricted to 10 % of the film then An/n and hence the frequency shift is 10 fold greater (i.e. 0.076 ev). Hansen and Prostak * found the calculated change in reflectivity with potential to be independent of whether the change in free electrons is in the entire film or only 10 % of it and thus that the reflectivity change is not sensitive to the fraction of the film in which the change in free electron concentration occurs.A similar calculation * performed by the authors shows that this is indeed the * A programme was written in Fortran using the general Fresnel-Drude equations for oblique incidence using a three-layer model. B . D. CAHAN J . HORKANS AND E . YEAGER 41 case for a small A (eV) shift for specular reflection and that this shift of the surface optical constants should lead to an effective wavelength shift of the @,A) curve with potential. The magnitude of this shift is estimated to be 2-3 mp/V at the wavelength of the edge and the shift should be constant (in terms of eV) with wavelength. For a simple linear shift the one point of the curve whose position should be invariant with scale factor is the inflection point.Therefore the fitted polynomial for the single reflection data was analyzed for the location of this point. Computer analysis of these data however showed a negligible simple monotonic trend of the position of the inflection point as the potential was changed. A linear regression analysis of the position of the infection points against potential showed a shift of only 0.12 mp/V with a standard deviation of 0.4mp/V compared to the expected 2-3 mp/V shift. The derivatives (l/R)(dl?/~?A)~ (fig. 3) were calculated from the shape of the (reflectivity wavelength) curves and (1 /R)(dR/dE)A was determined from the (reflectivity potential) curves (fig. 2). The shift of the edge in terms of the energy U (in eV) with respect to potential is then and should according to the edge-shift theory be a constant.From the data in table 1 (aU/aE) is far from constant. This is not unexpected since the peak calculated by Hansen and Prostak * is much narrower than that found experimentally by them by Feinleib,2 or ourselves. While the edge-shift theory can befitted to account for the potential dependence of the peak in the reflectivity at the absorption edge it does not explain the appreciable modulation in either the red or the blue. (3 u/aE)R = - (dU/dA)[(l /R)(3R/aE)A/(l /R)(aR/aA)El (4) TABLE 1 .-WAVELENGTH DEPENDENCE AND POTENTIAL DEPENDENCE OF (8 U / ~ E ) R -(a UIaE)R (eV/V> ( x 103) wavelength A (mi4 0.2 v 0.6 V 1.0 v 450 150 118 98 475 11.4 8.8 7.0 500 4.2 3.4 2.6 525 4.3 4.4 3.5 550 6.1 6.6 6.5 575 8.8 9.6 10.7 600 10.8 13.8 15.4 In view of the lack of a simple shift of the edge itself the displacement of the maximum of (1 /R)(aR/dE) curves (fig.2) with increasing voltage cannot be attributed to a simple linear shift but can be interpreted in terms of a gross distortion of the optical constants in a surface layer of the metal. SURFACE STATE THEORY At a temperature of absolute zero for a perfect crystal if the primary mechanism of light absorption in gold is the 5&6s transition there should be an abrupt step in the (reflectivity photon energy) curve. At higher temperatures the step should become rounded as shown by the Fermi distribution function. This rounding should be symmetric around the inflection point whereas the actual reflectivity curve still shows significant excess absorption,* more than 0.5 eV below the actual absorption * The term excess absorption is here used to denote absorption in the red at wavelengths longer than the absorption edge over and above that expected on the basis of kT (thermal) broadening.42 REFLECTANCE STUDIES OF GOLD/ELECTROLYTE INTERFACE edge while only 2-3 kT (i.e. less than 0.075 eV) is attributable to thermal fluctuations in the electron distribution. Theoretical considerations of the electronic surface states of finite lattices show that the effect of truncation of an infinite lattice generates highly localized surface eigenstates with energies in the band gap i.e. for gold the top of the 5d band is raised and the bottom of the 6s band is lowered at the surface. Fig. 5a is a possible diagrammatic representation of the 5d and 6s bands at a metal-vacuum interface.The cross-hatched areas represent the states induced by introducing a boundary in the crystal. The existence of these states in the forbidden band may account for the shape of the reflectivity curve of gold. The existence of these occupied states makes the (5d)surface +Fermi level transitions possible with photon energies lower than the absorption edge. Conversely it may be possible to calculate the shape of the (5d)surface band from the excess absorption in the red. METAL- VACUUM METAL- ELECTROLYTE FIG. 5.-Schematic representation of energy bands and surface states (cross-hatched area) in gold a metal-vacuum interface ; b metal-solution interface. Fig. 5b illustrates the surface immersed in an electrolyte ; the 5d and 6s bands are distorted because of the potential across the interface.Consequently the (energy distance) relationship of the surface bands is also changed and thus the shape of the excess absorption spectrum can be shifted markedly. If the hypothesis that the round- ing in the red of the reflectivity curve is caused by the excess absorption due to the surface states is correct then we have available sufficiently large energy changes to account for electro-modulation over a wide wavelength range. The modulation of the double layer thus affects a very small region in the metal. Very large effects can be produced by the relatively small charge (33 pC/cm2 for the 0.7 V in the work of Hansen and Prostak 8 associated with the modulation of this double layer. The small shifts in photon energy (0.0076 to 0.076 eV) used by Hansen and Prostaks in their calculations of AR/R against A can be considered as approximating differentials and their resultant curve is equivalent to the derivative of the (reflectivity A) curve.B . D . CAHAN J . HORKANS AND E. YEAGER 43 The half-width of the peak in the (ARIR A) curve derived by Hansen and Prostak is invariant with energy shift as long as the shifts are sufficiently small. Shifts of a size that can no longer be treated as linear do in fact produce broadening and a shift of the peak. With surface states of the type represented in fig. 5 even a few tenths of a volt should be sufficient to produce such non-linear effects. Surface hetero- geneity would also contribute to broadening. EFFECTS I N SOLUTION PHASE The question remains whether there are any appreciable electro-modulation effects specifically due to changes in the refractive index and/or thickness of the double layer.Assuming that these effects do exist it is interesting to consider the best method of observing them independent of the effects within the metal. If we assume that the index of refraction of the double layer is lower than that of the solution for perpendicular polarization there exists a particular angle at high angles of incidence at which the phase difference between the light reflected from the solution/double layer and the double layer/metal interfaces approaches A/2. The resultant interference causes a sharp dip in reflectivity. The assumption of a lower index of refraction in the double layer is not unreasonable at least under some cir- cumstances ; such a layer could exist because of the formation of an ice-like structure the repulsion of anions at negative potentials leaving only surface oriented H30+ ions or the adsorption of a low index of refraction organic material.A three-layer calculation was programmed in Fortran to compute the reflectivity for high angles of incidence varying the index of refraction of the double layer. At each angle of incidence the predicted dip in reflectivity is associated with a unique index of refraction. As the index of refraction approaches that of the bulk solution the required angle of incidence approaches 90". For perpendicular polarization the change of reflectivity can be greater than 20 % for a 0.05 change in index of refrac- tion. The following experimental set-up is being implemented to study this effect.A flat electrode is to be mounted at the centre of rotation of a scanning 8-20 gonio- meter. A small modulation of the fixed average electrode potential will be impressed potentiostatically and the goniometer scanned through small angles. The modula- tion will be detected and divided by the average reflectivity. Any electro-modulation of the index of refraction will show up as a blip at the corresponding angle. From this angle the index of refraction of the double layer can be calculated and hence its dielectric constant. In summary an explanation is offered for the sensitivity of the reflectivity of gold to potential which involves surface electronic states. Specular reflection on gold electrodes is very sensitive to adsorbed species including neutral species and provides a means for studying adsorption phenomena.Further multiple reflection studies can be subject to gross errors particularly associated with scattering phenomena ; this indicates the desirability of performing future work with single or at most a few reflections. Finally a new method has been proposed for measuring the refractive index of the double layer in instances where it is lower than that of the bulk solution. The authors acknowledge the support of this research by the U.S. Office of Naval Research and also the Electrochemical Society through the Edward Weston Fellow- ship to one of the authors (J. H.). J. D. McIntyre 135th National Meeting Electrochem. Sac (New York. May 1969) (extended abstr. p. 578). J. Feinlieb Phys. Reu. Letters 1966 16 1200. A. B. Buckman Surface Sci. 1969 16 193. 44 REFLECTANCE STUDIES OF GOLD/ELECTROLYTE INTERFACE R. S. Sirohi and M A. Genshaw J. Electrochem. SOC. 1969,116,910. M. A. Genshaw private communication. T. Takamura K. Takamura W. Nippe and E. Yeager J. Electrochem. SOC. 1970,117,626. A. Prostak and W. N. Hansen Phys. Rev. 1967,160,600. * W. N. Hansen and A. Prostak Phys. Rev. 1968,174 500. W. N. Hansen Surface Sci. 1969 16,205. lo M. Stedman Chem. Phys. Letters 1968 2,457. 11 B. D. Cahan Ph.D. Thesis (University of Pennsylvania 1968). l2 H. Green and M. Dahms J. Electrochem Soc. 1963,110,1075. l3 T. Takamura K. Takamura and E Yeager unpublished data. l4 J. O'M. Bockris B. D. Cahan and G. E. Stoner Chem. Instr. 1969,1,273. l6 P. Mark in Clean Surfaces G. Goldfinger ed. (Marcel Dekker N.Y. 1970) p. 307. R. J. Archer Ellipsometry (Gaertner Scientific Chicago 1968) p. 11.

ISSN:0430-0696    

DOI:10.1039/SF9700400036

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

GENERAL DISCUSSION Dr. D. den Engelsen (Philips Rex. Lab. Eindhoven) said Langmuir-Blodgett layers have been used before in order to test the validity of the Drude linear approxi- mation of the exact ellipsometric equati0ns.l. We studied Langmuir-Blodgett layers of various compounds of which the unsaturated fatty acid docosenoic acid CH3(CH2),CH=CH(CH2) ,COOH will now be considered. Monolayers of the cis- and the trans-conformation were spread on triply-distilled water (pH = 5.5 temp. 21"C) containing 2 x M CdC12 in some experiments. From known sur- face pressure against area diagrams of spread monolayers of these acids it was concluded that the Langmuir-Blodgett experiment should be done at 20 dyn/cm. Only one monolayer of both compounds could be transferred to the surface of a hydrophylic solid.Experiments to increase the number of deposited monolayers failed e.g. for a hydrophobic surface there was no attachment. For the ellipsometric measurements we deposited the acids on polished etched silicon plates.' The effect of the Si02 layer of about 30A thick was accounted for in the calculations. Since the refractive index could not be determined from one monolayer we assumed the same refractive index of 1.50 for the two conformations. From the measured 6A values a layer thickness of 30.5 A was calculated for the trans- compound and 21.3 A for the cis-compound. A variation of k0.05 in the refractive index changes these values by about 1 A. The cis-acid probably has a lower refractive index than the trans due to the poorer packing which leads to about 22 A for cis whereas 30.5A undoubtedly is the upper limit for trans.From molecular models one deduces a maximum layer thickness of the cis-compound of about 25 A ; therefore these boomerang-shaped molecules are tilted i.e. the line connecting the two extremities is not normal to the surface. Molecular models indicate a layer thickness of about 30 A for the trans ; thus the trans-molecules are not tilted. The surface pressure against area diagrams of the two acids on water at 20dyn/cm indicate cross-sections of the trans and cis of 20 and 30A2/molecule respectively. This leads to a volume of 600 A3/molecule for trans- and a slightly greater volume for cis-docosenoic acid. Dr. W. Plieth (Free University Berlin) said A detailed picture of the adsorption of some compounds is given in fig. 3 of the paper by Meyer and Sparnaay.A dissociation into four particles has been assumed for adsorption of CH3SH and CH3X on a (1 1 1)-face. A dissociation into three particles occur on a (100)-face. Moreover three different radicals CH3- CH2= and CHE are formed on the surface. describes the bond between a surface and an adsorbed molecule as mutual effects between the electron system of the adsorbed molecule and an unknown number of surface atoms. Therefore the compensation The MO-theory of chemisorption Mertens Theroux and Plumb J. Opt. Suc. Amer. 1963,53 788. Hall J. Plzys. Cliem. 1965 69 1654. to be published. Gaines InsolubIe Monolayers at Liquid-gas Interfaces (Interscience 1966) p. 236. Archer J. Opt. SOC. Amer. 1962 52 960. T. B. Grimley Proc. Phys. Soc. 1958 72 103 ; Ado. CataZysis (Academic Press New York) 1960,12 1.45 46 GENERAL DISCUSSION of" dangling bonds " on a surface by undissociated molecules is possible. A dissocia- tion can be combined with the adsorption but it is not necessary for the saturation of the surface bonds. In many cases the bonds in the adsorbed molecule may only become weaker. The experimental results of the authors can be interpreted by the MO-theory. A number of six atoms on a (1 11)-face and a number of two atoms on a (100)-face are occupied in the adsorption bond by one adsorbed molecule. Weakening of intermolecular bonds or perhaps a dissociation into CH3- and SH- or X- can explain the other results. Dr. J. Mchtyre (Bell Telephone Lab. N.J.) said For purposes of correlation eqn (1) and (2) in Hansen's paper represent specialized cases of the general first-order relations for the fractional reflectivity change produced by thin film deposition (cf.eqn (2) of ref. (2). For example on noting that the quantity <t3 - <fl = -n = t3 -cl which in this case is real and also that Im t2 = 2n,k2 = n2a,A/2n the form of eqn (1) is evident by inspection except for a difference in sign. This sign difference results from the use of different definitions for A, 2 and <,. In the general relations of McIntyre and Aspnes,l the Nebraska convention was followed with 2 = nj - ik, etc. so that itj t j and gj are complex conjugates of the analogous quantities in Hansen's paper. These conventions must be adhered to rigorously to obtain the same final numerical result. With regard to the discussion of thin films in multilayer systems the importance of employing the linear-approximation relations should be emphasized in those cases where phase change effects are important since these equations are exact to first order in d/A and take anomalous phase change effects into account.As an example consider the effects produced by deposition of a very thin transparent film on a highly reflecting substrate in a gaseous ambient medium (n = 1.0; n2 = 1.3 k = 0.0; 123 = 3.0 k3 = 30.0; d/A = 1 . 0 ~ In this case a plot of (AR/R),l against 41 exhibits a very sharp peak with a maximum of +2.0 x when cP1 = 89". In contrast an absorbing film with identical refractive index and an extinction coefficient k = 0.1 (characteristic of a moderately strong infra-red absorption band) produces a broader peak of opposite sign with a maximum in (AR/R)II = -2.3 x at 41 = 87".The existence of the anomalous effect produced by the transparent film is not intuitively obvious and is not evident from Hansen's relations for the reflection absorbance All. The effect is clearly predicted however by eqn (2b) of ref. (2). Physically it originates from the rapid variation of the phase change on reflection at the metal substrate 611 near the principal angle (88"). This effect is of importance in studies of the electro-reflectance effect in metals caused by double-layer refractive index modulation. that this is one of the principal sources of the optical modulation effects observed by Walker,5-6 which he attributed to the electrochemical generation of solvated electrons. The relatively large magni- tude of this phase-shift effect near grazing incidence may also be of utility in optical studies of the structure of the double layer at the electrode-solution interface.In fact it has been proposed Dr. B. Cahan (Case Western Reserve University) said Hansen has stated in his earlier papers and in this discussion that he arranged his thin gold films for use in the J. D. E. McIntyre and D. E. Aspzes Surface Sci. 1971 24,417. J. D. E. McIntyre and D. M. Kolb Disc. Faraday Soc. 1970 in press. R. Muller Surface Sci. 1969 16 14. J. D. E. McIntyre to be published. D. C. Walker Can. J. Chern. 1966,44,2226 ; 1967,45 807. D. C. Walker Aital. Chenz. 1967 39 896. GENERAL DISCUSSION 47 ATR electromodulation experiments so that the excess electrons stayed in the metal. I fail to see how this is possible.There is a basic fundamental error in the hypothesis that it is possible to establish an excess of charge inside a metal. The only place excess charge can accumulate macroscopically is at the surface. Any excess charge inside the electrode must immediately flow out through the connecting lead wire since the Fermi levels of the two metals in contact are equal. If this were not the case his equations would lead to the impossible conclusion that the electromodulation effect is dependent on thickness and would go to zero for bulk gold. Prof. M. J. Dignam (University of Toronto) said I report on recent developments that have occurred in the field of reflection spectroscopy at the University of Toronto. My collaborators in this work are Dr. B. Rao Mr. M. Moskovits and Mr. R. Stobie. We have designed and built an automated wavelength scanning ellipsometer which operates in the range 0.5-5 p with a wavelength resolution of about 20A and an angular precision of about 0.01'.To test the capabilities of our technique we have conducted studies on methanol adsorbed on vacuum-deposited gold films using a flow system and hydrogen as the carrier gas. The results from one such study are shown in fig. 1. They were obtained for an angle of incidence of 80" and following four reflections. During measurements on the film covered surface the relative pressure of methanol was maintained at 0.05 to within a few per cent so that no more than a monolayer coverage is expected. v) cd a .- 2 c .- 4 I 3 0.06 0.04 - 0.02 - 0.00 3.2 3.3 3.4 3.5 3.6 wavelength pm FIG. 1 .-Ellipsometric spectrum for methanol adsorbed on silver.It can be shown theoretically that for a good reflecting substrate and not too large angles of incidence the amplitude function In (tan $/tan $),* is to a good approximation proportional to the reflectance absorbance for p-polarized light the proportionality factor being 2.303/2. (A full paper discussing various aspects of i.-r. ellipsometric spectroscopy is in press.') The C-H stretching bands are clearly * 4 and A have their usual meaning with the " bar " referring to bare substrate conditions. M. J. Dignam B. Rao M. Moskovits and R. W. Stobie Can. J Chem. in press. 48 GENERAL DISCUSSION displayed in the amplitude plot of fig. 1 with four of the five peak frequencies in excellent agreement with those obtained by Byholder and Wyatt for methanol adsorbed onto a silica supported dispersion of iron.Since making these measurements instrument sensitivity has been increased about five-fold. It is evident that sufficient sensitivity is available for observing the absorption bands from single reflection measurements and hence that the technique can be used for studying adsorption on single crystal faces. Ellipsometric spectroscopy has a number of advantages over conventional reflec- tion spectroscopy. It is more sensitive to the properties of the adsorbed molecules while at the same time being much less sensitive to absorption of light within the adjacent gas or solution phase. It is also much less sensitive to light intensity and detector drift. That ellipsometric spectroscopy has not been used extensively before now is due undoubtedly to the extreme tediousness of manual ellipsometric measure- ments and data reduction procedures.Our present instrument produces readings in digital form on paper tape at the rate of one per second thus overcoming both problems. Details of the instrument will be published in the near future. Dr. M. Stedman (Nat. Phys. Lab. Teddington) said The proposal of Cahan et ai. for detecting a double layer of refractive index lower than that of the bulk solution is interesting. However any realistic model of a double layer would feature refractive index gradients which would affect computations particularly in critical angle regions. How would the reflectance effect behave for inhomogeneous films? I have attempted to find the reflectance effect reported in the paper of Cahan et al.using my computer programmes which are based on the exact equations and provide output with a numerical resolution of 1 in lolo. The principal cases I have examined SO far are nrnetal = 0.495-2.46i(gOld) Ylfilm = 1.33 nelectrolyte = 1.34 and 1.38 dfilm = lOA A = 5461 A. Output was either in the form of absolute reflec- tance or of the reflectance ratio Rfilm present/I?film absent (= 1 + AI?/R) for p and s polarizations tabulated for ranges of angle of incidence. The only effect discovered was a small deviation from the general trend of reflectance at angles immediately below the critical angle. This consisted of an increase in reflectance which did not exceed lo-* (in terms of AR/R) occurring over a range of about 0.003' ; in addition there appeared to be a sharp drop in reflectance back to its original trend occurring at the critical angle.As the critical angle is approached the Fresnel coefficients approach unity r s 2 3 + 1 rz3+ - 1 where the subscripts 1 2 3 refer to electrolyte film substrate. The total reflection coefficient is defined by Rs = (rsz +r13 exp)/(l +ri2r;3 exp) where exp is an exponential term dependent on film thickness and nearly equal to one in the present case. Both the numerator and denominator become small and numerical resolution is lost at this stage of computation. For example for the system mentioned above with nl = 1.34 we find at = 82.9958' (0 = 82.9960') r ; = 0.9928 and r i 3 and exp are very close to one and numerical resolution in computed Rs drops to about 1 in lo8 (normally 1 in lolo). I conclude that the deviations detected in reflectance near the critical angle did not exceed the uncertainty of computation and hence are probably artefacts.Theoretical considerations indicate that a slight change in the slope of reflectance curves may occur at the critical angle but this was not detected in the computed results within the available accuracy. I would welcome the general com- ments of the authors and would like to know whether their results are related to mine. It would be helpful for the theory of their effect to be clarified to have some G. Byholder and W. V. Wyatt J. Phys. Chem. 1966,70 1745. It is important to consider the accuracy of comptuation. GENERAL DISCUSSION 49 specific quantitative examples of their computed effect and to know the numerical resolution of their computer. Miss M.A. Barrett (University of Bristol) (communicated); I have searched by computation for the effect described in the paper by Cahan et al. of a marked drop in R near the critical angle for films of lower refractive index than the electrolyte. The constants used for the tests included Ytmetal 0.495-2.463 1.3-2.5i and 2-2i; nfilrn from 1.15 to 1.329 with a thickness of 5 A ; neIectrolyte 1.33 and 1.38 ; wavelength 5461 A. The angle of incidence was increased in steps of 0.05" from 75" to 90". R, increased monotonically and almost linearly in this range. Alternatively Ylfilm was increased in steps of 0.001 from 1.31 to 1.329. In both cases there was only a slight irregularity in the general rate of change of R, and R as the critical angle was passed. Under the most favourable conditions the effect did not exceed the sixth decimal place; the accuracy of the computations would be seven significant figures at best.A more detailed comparison of our computations would be valuable. Dr. B. D. Cahan Miss J. Horkans and Prof. E. Yeager (Case Western Reserve University) (conzmunicated) We thank Miss Barrett and Miss Stedman for their communications which prompted us to uncover an unfortunate error in the equations used in the computer programme which predicted the anomalous behaviour at glanc- ing angles of incidence. With a corrected programme the calculation of reflectivity at high angles no longer yields this effect. Experimental data showing peculiarities in the reflectivity of gold in 1N CF,COONa at 89.5" which were originally believed to confirm the predicted behaviour are now unexplained.Dr. R. Parsons (University of Bristol) said I would suggest that the reason that no hydrogen is visible on gold electrodes is that the amount of hydrogen adsorbed on gold is very small. At the reversible potential Breiter Knorr and Volkl estimated that the coverage with adsorbed hydrogen was about 4 %. Dr. A. Bewick (University of Southampton) said In the paper of Cahan et al. it is pointed out that the derivative reflectivity curve for gold fig. 3 is considerably broader than that calculated by Hansen and Prostak. It is suggested that this is due to the neglect of the effect of surface states in the metal. In view of this I wonder if it is wise in this context to use the reflectivity curve obtained at 0.0 V in 1N HClO, when at this potential the surface will possess a certain amount of adsorbed hydrogen.The adsorbed atoms will presumably be associated with surface states or might even induce surface states where none existed in the absence of the chemi- sorbed species as pointed out by Grimley2 and Koutecky.3 In reply to Parsons' comment I would suggest that the effect of 4 % of adsorbed hydrogen on the width of the derivative reflectivity curve might well be measurable. Dr. B. Cahan (Case Western Reserve University) said In reply to Bewick the relative electroreflectivity (1 /R)(aR/aE) in fig. 2 not the derivative reflectivity (l/R)(dR/aA) in fig. 3 is wider than calculated by Hansen and Prostak. Indeed Hansen and Prostak predict that the reflectivity curve should be similar to (l/R) (aR/aL),. The (1 /R)(dR/aE) curves show this broadness at all potentials ; 0.2 0.6 and 1.0 V have been shown in fig.2 as representative. Concerning the effect of Breiter Knorr and Volkl 2. Elektrochem. 1955 59 681. Koutecky Trans. Faraday Soc. 1958 54 1038 ; J. Phys. Chem. Solids 1960,14 233. * Grimley Proc. Phys. SOC. B 1958 72 103 ; J. Phys. Chem. Solids 1960 14 227. 50 GENERAL DISCUSSION hydrogen on the (1 /R)(aR/a& curves if any these curves were practically identical at all potentials between the potentials of visible H2 gas evolution and oxide forma- tion. We emphatically agree with the point about the effect of adsorbed species on surface states. It is specifically because of this interaction that optical methods have such high sensitivity in the study of adsorption. Prof. M. J. Sparnaay (Enschede Netherlands) said Concerning fig.5 in the paper of Cahan et al. what kind of surface states have they in mind? While there will be Tamm states at the metal/vacuum interface is this also the case at the metal/ electrolyte interface ? Do they also have independent evidence for surface states in these systems ? Dr. B. I). Cahan (Case Western Reserve University) said In reply to Sparnaay the surface states we have in mind in fig. 5 are those originating from the truncation of the lattice as well as from specific chemical interactions with liquid-phase species. Independent evidence is available for such surface states within semiconductor electrodes but it is difficult to cite non-controversial independent evidence for metal electrodes. Dr. J. McIntyre (Bell Telephone Lab. N.J.) said A number of models have now been proposed to account for the electroreflectance (ER) effect in metals.For the group Ib metals Cu Ag and Au which exhibit pronounced features in their reflec- tivity spectra the dominant effect in the AR/R spectrum at low angles of incidence is due to a perturbation of the electronic properties of the metal in its surface region. A striking feature of the experimental ER spectra of metals such as Ag and Au,l for external reflection at < 45" is that the signal R-laR/aEproduced by modulation of the electrode potential E is everywhere negative over the photon energy range 1-6 eV. The semi-empirical rigid shift models of Hansen and Prostak,2 however predict an ER spectrum which resembles the derivative of the reflectivity spectrum. Thus for a metal such as Ag which has a deep minimum in R at 3.85 eV peaks of both signs should appear.Similarly if the Fermi level of the metal were modulated by the field together with its plasma frequency the values of the inter-band components of the dielectric constant at the absorption edge corresponding to the excitation of an electron from a d-band to the Fermi surface would undergo changes opposite in sign to those of the free-electron components. Such behaviour should also produce ER peaks of opposite sign. Models based on a simple frequency shift of the optical constants are therefore not generally applicable and experimental deviations from their predictions are not necessarily attributable to a gross distortion of the optical constants of the surface layer of the metal as proposed by Cahan Horkans and Y eager (CHY).In an attempt to resolve these fundamental discrepancies and to provide a theo- retical basis for optical investigations of the surface electronic properties of metals McIntyre and Aspnes (MA) proposed a model based on a first-order approximation for the reflectance change of a two-phase system produced by the generation of a very thin intermediate phase. In this model it is implicitly assumed that the Fermi level is not modulated by the field and that bound electronic states are also unaffected; J. D. E. McIntyre paper presented at the Electrochem. SOC. Meeting (New York 1969) abstr. no. 231. A. Prostak and W. N. Hansen Phys. Rev. 1967,160 600 ; 1968,174 500. J. D. E. McIntyre and D. E. Aspnes Bull. Arner. Phys. Soc. 1970 15 366; in press. J. D. E. McIntyre and D.E. Aspnes Surface Sci. 1971 24 417. GENERAL DISCUSSION 51 the ER effect of metals is attributed to a modulation of the density of the free electron wave-function tails at the metal surface. The mean perturbation of the metallic dielectric constant % in this region is given by where tf is the free-electron component of ern N is the free-electron density of the bulk metal and ANs is the excess surface electronic charge required to shield the applied field. It results therefore that to first order in d/A (cf. eqn (2) of ref. (l)) the ER effect is independent of the transition layer thickness. The MA theory predicts that the ER effect should closely approximate the (positive definite) dielectric loss function Im (Q; l) and quantitatively accounts for the observed features of the ER spectrum of Au including the non-zero response in wavelength regions well-removed from the main peak at 2.6 eV (as noted by CHY) and the uniform sign.The success of this free-electron model in accounting for the features of the experi- mental ER spectra of Ag and Au suggests that surface states of the metal do not contribute significantly to the ER effect. This implies that the orbitals of the 5d core electrons in the Au surface atoms are either " stiff " or are well-shielded from the applied external field by the 6s conduction-band electrons. Experimental deviations from the predictions of the simple free-electron theory however may furnish further information concerning the one-electron wave functions of the surface metal atoms. The shape of the pseudo-plasma edge for Au is determined by the combined contributions of intraband and interband transitions to the complex dielectric con- stant.Examination of analyzed dielectric response function curves for Au 2 p reveals that the edge shape is mainly determined by the real part of thefree electron contribution which varies rapidly and non-linearly with frequency in this energy region. Surface states may be expected to make a relatively small contribution compared to the bulk metal layer of ca. 200A thickness which is sampled by the incident radiation. The dip in R at 640nm in fig. 3 of CHY is not seen in the reflectivity of thin gold films deposited on very smooth substrates. It appears to be analogous to a similar effect observed for silver films which arises from the coupling of the surface plasma oscillation mode to the radiation field through surface roughness.With regard to the distinction between the ER effect produced by (i) a perturbation of the optical properties of a thin surface layer of the metal and (ii) a small change of the real refractive index of the double layer measurements at 41 = 45" are particu- larly useful. Owing to the symmetry between cl and Q3 in the linear-approximation relations for AR/R the dielectric constant of the transition layer can be defined with reference to that of either semi-infinite bounding phase. For case (i) (AR/R), 4 5 ~ z 2(AR/R)s,450 regardless of the actual form of the field-induced shift of the optical constants. However for case (ii) (AR/R)p,450 = 0 whereas (AR/R)s,450 is small but non-zero. In the second case where n 2 z n 1 the Brewster angle (or comple- mentary principal angle) cjhB = tan-l (n2/nl) at which the reflectivity of the interface of two transparent phases vanishes for p-polarized radiation is close to 45".These effects are in fact clearly shown in the computed curves in fig. 1 of Stedman's paper. Experimental ER spectra (AE,) = (&- l)AN,/dN (1) of Ag and Au exhibit close agreement with case (i). J. D. E. McIntyre and D. M. Kolb Symp. Faraday Soc. 1970,4. H. Ehrenreich The OpticuZ Properties of Solids ed. J. Tauc (Academic Press New York 1966) p. 106. J. D. E. McIntyre and D. M. Kolb unpublished results. J. D. E. Mchtyre paper presented at the Electrochem. SOC. Meeting (New York. 1969) abstr. no. 231. 52 GENERAL DISCUSSION According to the MA theory,l the ER effect for a free-electron metal is opposite in sign to that for metals such as Cu Ag Au and Pt for which Re 8,> (cl - 1).Experimental evidence indicates the Drude theory is followed closely by liquid Hg for hcoG3 eV. The ER effect for Hg should provide a sensitive test of the validity of the MA theory which predicts a sign reversal in the experimental ER spectrum near 5.0 eV. Also this theory predicts an ER effect for Hg of opposite sign to that shown by Stedman in her fig. 1 when the refractive index 1.600 - 4.753‘ is used. These optical constants are both considerably higher than those predicted by Drude theory. Dr. B. D. Cahan Miss J. Horkans and Prof. E. Yeager (Case Western Reserve Uniuersity) (communicated) We agree with McIntyre that a model explaining the electroreflectance behaviour of gold in terms of a modulation of the Fermi level of the metal is invalid.We identified this error in concept in the Hansen-Prostak theory in our paper and clearly did not use such in our explanation of the electromodulation. We therefore do not understand McIntyre’s association of our concept of the electro- modulation of surface states with the Hansen-Prostak model. Perhaps a further discussion of the electromodulation phenomenon in gold would be appropriate at this point. The McIntyre-Aspnes (MA) theory seeks to explain this in terms of modulation of the electron density in the exponential tail of the free electron distribution at the surface. The dielectric constant B2 of the surface layer is assumed to differ from that of the bulk metal Eh3 due to the excess surface electronic charge on the electrode.This difference (A&) is postulated to be a result of changes in the free electron contribution & to the dielectric constant which is calculated from the Drude-Zener theory. For an ensemble of free electrons in a conductor this is given by 8 = 1 - w;/(co2- iw/z). Here cop( = (41Te2N/m*)3) is the plasma frequency and z( = oom*/Ne2) is the relaxa- tion time with N the free electron concentration m* the effective mass and oo the d.c. conductivity of the metal. Thus the free electron dielectric constant is approxi- mately linear with the carrier concentration (since the effect of the ico/z term is small in the wavelength region of interest) which leads to McIntyre’s eqn (1). The Drude-Zener equation is based on a classical electron-gas model and considers the motion of individual electrons in the oscillating electric field of the optical waves.The electrons undergo periodic acceleration-deceleration due to this field and decelera- tion resulting from collisions with the ions. The concept of a tail for the free electron distribution extending into the potential energy wall at the limit of the metal phase however is purely quantum mechanical in origin. It is not obvious that the classical mechanical treatment of an oscillator is compatible with the quantum mechanical treatment upon which the existence of the tail in the electron distribution depends. We are forced to ask such questions as the following. What are the scattering ions in the tail of the free electron distribution? What is the physical significance of oo in this region and why should it be invariant with the electron concentration in the tail? Further insight can be gained by consideration of the actual magnitudes of the relevant quantities.The free electron tail occupies a layer ~ 0 . 6 A thick.3 Even if the free electron concentration in the tail were as high as that in the bulk metal J. D. E. McIntyre and D. E. Aspnes Bulf. Amer. Phys. Soc. 1970 15 366; in press. * E. G. Wilson and S. A. Rice Optical Properties and Electronic Structure of Metals and Alloys ed. F. Abelh (North Holland Amsterdam 1966) p. 271. J. D. E. McIntyre private communication. GENERAL DISCUSSION 53 only -2 x 1014 electrons/cm2 of electrode surface could be accommodated in this region. (The actual number should be smaller.) Removal of all these electrons accounts for -30 pC/cm2 of charge.This is less than the charge required to change the potential across the double layer from the e.c.m. to + 1.0 V which is of the order of 50 pC/cm2 (assuming an integral electronic capacitance of 50 pF/cm2). Thus in order to account for the change in the charge on the metal surface one must use more than just the electrons in the free electron tail. It follows that the effects of the applied electric field across the interior then extend into the metal phase beyond just the 0.6A tail of the $$* distribution function for the free electrons. If such is the case this field should be felt by electrons not only in the 6s band but also in the 5d band and the 5d surface states. The fact that the required change in number of electrons for a 1 V potential change is inconsistent with a reasonable value for the number of electrons available in the tail leads to unrealistic values of (At,) in McIntyre’s eqn (1).McIntyre and coworkers do not observe the inconsistency because their modulation (i.e. differential) technique involves a relatively small change in the number of electrons. Whatever model is used however must also be valid for d.c. (i.e. integral) techniques. When a 1 V potential change is considered? AN/N> 1 implying that (8,) is at least of the same magnitude as tf (which is even larger than &,). In view of this any criticism of the size of the optical constant changes suggested in our paper is also applicable to the MA treatment. We do not share however McIntyre’s apparent reservations concerning gross changes in the optical constants at the electrode surface.There are some experimental aspects of the electroreflectance of gold not accounted for by the theory of electromodulation of the free electron tail. We have done a calculation based on the MA theory and find that the calculated effect falls off much more rapidly than the observed AR/R at low energies. In addition there is nothing presently incorporated in the theory to explain the change of the electroreflectance peak as a function of the mean potential of the electrode. It is difficult to understand McIntyre’s interpretation of Ehrenreich (McIntyre’s ref. (6)) as indicating that the shape of the edge in the reflectivity at -2.3 eV is nut due primarily to interband transitions. This reference specifically states on p. 151 that such an edge shape is characteristic of a low-energy interband transition such as “ would typically correspond to Cu or Au near 2 eV ”.It is not the rapid variation in the free electron dielectric constant which determines the shape of the edge but rather the detail in the contribution of the bound electronic states to the dielectric constant. P. 143 of the same reference states that “ It is this peak in [the real part of dielectric constant of the bound electrons 6&p)] which is due to interband transi- tions that is responsible for the characteristic colour of the noble metals.” Further McIntyre’s statement that “ the real part of the free electron contribution . . . varies rapidly in this region ” seems to contradict fig. 9 of the same reference from which it can be seen that the free electron contribution to the real part of the dielectric constant &if) is a smooth function of the energy E of the photon and that deif)/dE near the edge is more than an order of magnitude smaller than it is at 1 eV.It would probably be more accurate to state that &if) has decreased sufficiently to allow the structure in &ib) to dominate in controlling the wavelength dependence of the reflectivity of this region. While surface states are expected to contribute only a few percent to the total absolute reflectivity they can give a contribution to the dielectric constant in the surface layer as great as that for any other bound or free electron in the wavelength regions where transitions involving these energy states are possible. A change in their concentration or energy levels can produce effects at least as large as the observed 54 GENERAL DISCUSSION electromodulation levels.These changes can be caused by interactions with the field at the interface or by a modification of their orbitals due to interactions with adsorbed species. Finally the small dip in R in fig. 3 of our paper is probably due to a shift in baseline over the finite time necessary to obtain the digital data. The dip did not appear in other reflectivity data for similar samples. While it seems that some of the electroreflectance effect can be explained by a modulation of the free electron con- centration at the surface it also appears that this alone cannot account for all of the features seen in the visible. - 2 Prof. E. Yeager (Cleveland) (communicated) Further evidence to support the view that the surface electronic properties are a major consideration in accounting for the change of specular reflectivity with potential is to be found in experiments involving the change of reflectivity with the deposition of monolayers of foreign metal ions on surfaces such as gold.In our laboratory Takamura Takamura and I I have examined monolayers of foreign metal atoms such as lead and cadmium de- posited on gold from perchloric acid solution. The electrodeposition of monolayers of such metals occurs at potentials much less cathodic than those for the bulk metal. I 1 - I) I I I I I I I I I 1 I 0 800 1600 potential (mV) (S.C.E.) FIG. 1.-Relative change in (reflectivity potential) and (current potential) curves for gold in the presence of Pb2+ ions. Electrolyte 5 x lod4 M Pb2++0.2 M HC1O4 ; potential sweep rate 105 mV/s ; wavelength 5200 A.Fig. 1 shows the (reflectivity potential) curves at two different wavelengths and the simultaneously recorded linear sweep voltammetry curve. At 7500 di the reflectance of the gold is decreased by the monolayer of Pb while at 5200A the reflectance is increased. The latter wavelength is close to that for the adsorption edge for gold T. Takamura K. Takamura W. Nippe and E. Yeager J . Electrochem. Soc. 1970 117 626. GENERAL DISCUSSION 55 (= 5500 A) associated with the 5d-6s interband transition. Oxide formation is also evident in these reflectance curves at potentials more anodic than 0.90 V (S.C.E.). These reflectance curves were obtained using a multiple reflectance cell with - 19 reflections. This is far too many reflections for quantitative studies of either the absolute reflectance changes or the wavelength dependence of even the relative changes because of errors introduced by scattering as described in our paper.It is planned to establish the wavelength dependence for monolayers of various metals on gold using a single reflection technique in the near future.* Dr. James McIntyre (Bell Telephone Laboratories) (communicated) The response of Cahan Horkans and Yeager (CHY) to my previous comments concerning their paper indicates a need for some further discussion of physical models for the electro- reflectance effect of metals. The CHY model is certainly different in concept from that of Hansen and Prostak (HP). The intent of my preceding comments was to point out that it is not necessary to postulate a gross distortion of the optical constants of the surface layer to account for the primary features of the observed electro- reflectance (ER) spectra of metals.For small modulating voltages these features seem to be well-explained in terms of the most characteristic feature of metals-the free-electron plasma without invocation of surface-state effects. Secondly the fact that ER spectra of the noble metals are uniquely one-signed even for metals such as Ag which exhibit a pronounced minimum in R cannot be explained in terms of an edge-shift model. We must seek a model which accounts for the features of these spectra over the complete energy range. Both the CHY and HP models have one feature in common-a modulation of the interband component t b of the metallic dielectric constant by the applied electric field.In the HP model Fermi-level modulation produced by anodic polarization of the electrode causes the absorption edge of the 5d-+6s (Fermi surface) interband transition in Au to shift to longer wavelengths. In the edge region the absorption coefficient a and Im &b are raised while Re gb undergoes both positive and negative excursions. Similarly for the free-electron contribution Re tf (large and negative) is shifted positively while Im tf (small and positive) decreases. In the CHY model modulation of the 5d surface state energies by the applied field shifts the components of &b in the same sense as that produced by Fermi-level modulation. In both models the interband effects are reversed for transitions from the Fermi surface to vacant bands at higher energy.Both of the above models differ significantly in this respect from that of McIntyre and Aspnes (MA). The latter model assumes to a fist approximation that &b is not modulated by the applied field. This assumption is based on the facts (cf. Friedel l) that d-states have small orbits compared to sp-valence states of comparable energy are localized and not strongly perturbed by the lattice potential and are ineffective in screening the nuclear charge within the atom. With regard to the use of the Drude relation although this expression was originally derived on a classical basis it also results * This work has been supported by the U.S. Office of Naval Research. J. Friedel Physics ofMetaZs ed. J. M. Ziman (Cambridge University Press Cambridge 1969) chap. 8 p.340. 56 GENERAL DISCUSSION from quantum mechanical treatments of the one-electron model. 1-3 Secondly the concept of a tail on the free-electron distribution at the surface is not uniquely quantum-mechanical. Consider the simple jellium model for a metal in which the positive charge of individual metal ions is replaced by a semi-infinite uniform dis- tribution of positive charge of density No against which the free-electron assembly moves. The steepness with which the free electron density N falls from its uniform interior value No to zero outside the surface is determined by the kinetic energy of the electrons and the potential energy field in which they move. The gradual decrease in electron density at the surface gives rise to a negative surface dipole at the point of zero charge.Electrons exhibit wavelike behaviour and a task of quantum mechanics (cf. Lang and Kohn is to calculate the exact form of the free-electron distribution at the surface in order that surface energies work functions and electro- reflectance effects can be evaluated theoretically. With regard to the electron scatter- ing mechanism the mean free path of an electron on the Fermi surface of a metal such as Au is -3 x 10' A. Electrons in the tail will be scattered by the positive ions of the host lattice with a characteristic relaxation time 2 ~ 2 . 5 x 10-14 s. The classical Lorentz-Sommerfeld relation z = rn*ao/Ne2 is commonly employed to estimate the magnitude of z. In reality z varies slowly with frequency but at visible u.-v. wavelengths w % l/z so that little error results from use of this relation in eqn (1).For the number of electrons in the tail for intuitive purposes we can approximate the electron density distribution in this region by the sum of two terms (applicable inside (z>O) and outside (zG0) of the uniform positive charge distribution respectively) (2) where ZTF is the Thomas-Fermi screening length and I is the screening-length in the solution phase. Significantly ZTF = vF/,/3~, where vF is the velocity of an electron on the Fermi surface and w is the plasma frequency. Then if Is% ITF the number of electrons per unit surface area in the tail region (-2.3 Ztf,<z,<2.3 ZTF) is ca. 2.3 ZTFNO. For a metal such as Au No = 5.9 x e ~ r n - ~ 2.,+0.6 A and ntail= 81 pCcm-2. In their reply to my previous comments CHY discuss a model in which all electrons must be stripped from this tail in order to charge the double layer from the P.Z.C.to + 1.0 V. Such a model seems physically unrealistic however. An applied field will simply cause the tail to shift as a whole relative to the plane z = 0 as electrons flow to or from the external supply. This shift will be accom- panied by a distortion which shortens or lengthens the tail. The exact form of the electron density distribution at a charged metal surface is not known. In the simple MA model however it is not necessary to know the tail shape since for d<;Z it is possible to define an equivalent intermediate phase with sharp boundaries and uniform optical constants which yields the same reflectance properties as the transition layer at the interface.' To first order the quantity AR/R is independent of d.At the point of zero charge therefore the substrate is approximated as a single phase with uniform optical properties. This is analogous to the assumptions commonly made in measurements of the optical constants of metals by ellipsometry. While the views of CHY concerning the electron density distribution at the surface are not in accord with our concepts we concur that the electric field does penetrate C. Kittel Quantum Theory of Solids (Wiley New York 1963) chap 6 p. 99. H. Ehrenreich The Optical Properties of Solids ed. J. Tauc (Academic Press New York 1966) p. 106.. M. Cardona Modulation Spectroscopy (Academic Press New York 1969) chap. 2 p. 9. C. Herring Metal Interfaces (American Society for Metals Cleveland 1952) p. 1. N. D. Lang and W. Kohn Phys.Rev. B 1970,1,4555. N(z) = N o P - 3 exp (- z/4,)3 + (No/2) exp (z/Zs), GENERAL DISCUSSION 57 slightly into the metal surface and is felt by the ion cores of the metal atoms in the first layer. The effective field is strongly attenuated by the conduction electrons however and since d-orbitals in noble metals are not easily deformable (as evidenced by the low compressibility and high cohesive energy of these metals) it is reasonable to assume that the local value of tf in the surface region is modulated much more strongly than &. With regard to the magnitude of (At) provided d<A the incident light beam cannot distinguish whether the perturbation extends only over a charge sheet of infinitesimal thickness (corresponding to a delta-function in the charge density distribution) or whether the variation is more gradual as in the MA tail model.Provided that AR/R is small so that first-order theory is applicable this will always be true. In principle therefore there is no reason why the MA theory cannot be scaled to large potential changes. In fact the integral value of AR/R measured by CHY for a + 1 V potential change at 500 mp (close to the main peak in the ER spectrum) is -6.0 x (The polarization and angle of incidence of the light beam are not specified in their paper). Assuming normal incidence and an integral double layer capacity of 50 pF cm-2 the MA theory yields a value of -5.9 x at this wavelength. Such exact agreement is fortuitous in view of the approximations involved and the neglect of solution double-layer effects,2 but it is evident that the primary change in reflectivity can be attributed with some confidence to free-electron effects.Also when such large potential changes are considered the local value of 2 in the surface region must differ significantly from that in the bulk metal owing to the large excess or deficit of free electronic charge. Consideration of the screening action of the free-electron tail and the insensitivity of the incident light beam to its shape and length indicates that the calculated values of (A&> are realistic. We agree with CHY that under such conditions there are apparent large changes in the optical constants of the surface layer of the metal. This serves to re-emphasize the dangers involved in measuring optical constants of electrode substrate materials immersed in electrolyte solutions.Scaling the MA theory to large potential changes however ignores the pro- nounced variations of double-layer structure with potential. More useful information about the optical and electronic properties of the metal-solution interface can be obtained by using modulation techniques under conditions such that the double- layer structure at a given bias potential is only slightly perturbed. If we can approxi- mate the uncharged metal as a single phase then at normal incidence the differential ER effect at a potential E (measured from the P.z.c.) is given to first order by (""> - R E - - 8nnidIm( A <Afi)E+dE-<Afi)E) 81-23 (3) This result is rigorous and independent of the origin of the optical modulation effect. If we assume as before that at a given bias potential kf is modulated much more strongly by the field than Qb then the differential ER effect is still primarily due to free electrons.However if chemisorbed species alter the magnitude of kb in the interphase region either through a strong interaction with the d orbitals in the surface atom layer of the metal or by introduction of new charge-transfer absorption processes (e.g. in adsorbed 0 or H regions) the effect will be much more complicated. These J. D. E. McIntyre and D. E. Aspnes Surface Sci. 1971,24,417. M. Stedman Chem. Phys. Letters 1968,2,457 ; Symp. Faraday Soc. 1970,4 OOO. J. D. E. McIntyre paper presented at the Electrochemical Society Meeting New York May 1969 (Abstract no. 231). 58 GENERAL DISCUSSION effects will vary non-linearly with the bias potential as will the double-layer capacity itself.It is these second-order effects which we presume to be responsible for the observed shifts in the ER spectra of the noble metals with bias p0tentia1.l'~ The details of these processes are not well understood so that exact relations for their optical effects cannot yet be given. Conceptually however the form of their influence can be seen from eqn (3). Extrinsic chemisorption surface states of the type discussed above are distinctly different in origin from the intrinsic Tamm states (due to lattice termination) discussed by CHY.3 Little or no experimental evidence has yet been found for intrinsic surface My original comment concerning the shape of the reflectivity edge was concerned with the question of whether one can attribute " excess absorp- tion " and assymmetrical broadening of the edge solely to intrinsic surface states as proposed by CHY in their original paper.3 As I noted previously the edge shape a 0 -a 8 -16 -24 -32 I I I 1 I I I I I 0.4 0.2 '0 1.0 2.0 3.0 4.0 5.0 6.0 tw [ev] 1 1 I I I I I I I I 2 3 4 5 6 [evl FIG.1 .-Decomposition of experimental values ',* of the dielectric constant of Au into free and bound . , contributions eexpt = &+b. - &xpt = Eexpt-Z EeXpt; -- &f = &>-k; ; . . . & = 6i-k;. is determined by the combined contributions of and tb to the total dielectric constant of the metal ern = &+ $. (Effects of bound electron surface states are included in tb). The relative importance of the individual terms is shown in fig. 1 which was calculated from the optical constants measured in this laboratory for a vacuum- evaporated Au film on a smooth quartz substrate.Here in the low-energy section J. D. E. McIntyre paper presented at the Electrochemical Society Meeting New York May 1969 (Abstract no. 231). T. Takamura K.Takamura W. Nippe and E. Yeager J. Electrochem. Soc. 1970,117,626. B. Cahan J. Horkans and €3. Yeager Symp. Furduy SOC. 1970,3 36. J. T. Law Semiconductors ed. N. B. Hannay (Reinhold New York 1959) chap. 16 p. 675. D. E. Eastman Phys. Rev. B 1971,3,1769. J. D. E. McIntyre and D. M. Kolb Symp. Furaday SOC. 1970,6,99. GENERAL DISCUSSION 59 of the edge (1.7-2.3 eV) the imaginary components of gf and Eb are small compared to their real parts. The normal incidence reflectivity (in vacuum) thus depends primarily on the real components of 6,.Further in this energy range the variation in Re gb is much smaller than that in Re tf. The net effect of adding the approximately constant term Re tb to the rapidly varying term Re gf is to increase Re k rapidly from a large negative value to a value much closer to unity. As is evident from the above relation the result is a rapid decrease in reflectivity (a pseudo- plasma edge) centred near 5500 A (2.35 eV) which accounts for the characteristic yellow colour of gold when viewed in reflection. Any small structural features in kb at longer wavelengths (e.g. those due to intrinsic surface states in the 5d band) will be strongly masked by the rapid fall in Re Ef towards - 00. This analysis for Au differs in detail from that of Ehrenreich.l The sharpness of the derived peak near 2.5 eV in the plot of Re eb depends slightly on the method of analysis of the optical constant data and the value chosen for a, but more critically on the accuracy of the measurements.The data shown in fig. 1 are in good agreement with recent measurements by Irani et aLY2 and earlier data of Schulz et aL39 However even in the sharper structure in Ehrenreich's plots (cf. fig. 8 of ref. (3)) the variation in Re 6* is smaller by a factor of 5 than that in Re gf for Au in the energy region 1.7-2.5 eV. Our interpretation of the relative roles of Re 6 and Re gf in determining the edge shape of Au thus differs from that of CHY. Finally although Im 6b vanishes outside frequency intervals where absorption occurs Re &b does not since it corresponds to the sum of contributions from all polarizability mechanisms (other than intra-band) over the frequency range O,< cob co.Thus the value of Re gb at the edge is not solely due to the 5&6s interband transition. From the above analysis we conclude as before (i) there is not a priori physical basis for expecting a symmetrical reflectivity edge with rounding solely due to thermal broadening of the Fermi distribution function; (ii) the rounding on the red side of the edge is primarily due to the free-electron behaviour of Au and not to " excess absorption " attributed to intrinsic surface states by CHY. In the experimental ER spectrum of Au a broadening of the low-energy side of the main peak at 2.5 eV is observed as CHY point out. This is illustrated in fig. 2 where the experimental ER spectrum of Au measured in this laboratory using a rapid charge-injection technique is plotted together with the spectrum calculated from the MA theory.While double-layer refractive index modulation can account for some of this broadening its major source may be due to a surface plasma resonance. The conditions for this resonance to occur are that Re ern = E~ (where E~ is the dielectric constant of the transparent bounding phase) and that Im tm be small. Optical constant measurements on electrode substrate metals are generally made in air. Examination of fig. 1 shows that the first condition is satisfied by Au in air near 3.0 eV and again near 4.5 eV. In this energy region however Im & is largely due to inter- band absorption processes and the surface plasma resonance is highly damped. As a result the effects of this resonance do not appear in the measured optical constants of Au.When the bounding medium is an aqueous electrolyte however E~ z 1 8 H. Ehrenreich The OpticulProperties of Solids ed. J. Tauc (Academic Press New York 1966) p. 106. ' G. B. Irani T. Huen and F. Wooten J. Opt. SOC. Amer. 1971,61,128. L. G. Schulz J. Opt. Soc. Amer. 1954,44 357. L. G. Schulz and F. R. Tangherlini J . Opt. SOC. Amer. 1954,44,362. H. E. Bennett J. M. Bennett E. J. Ashley and R. J. Motyka Phys. Reu. 1968,165,755. 60 GENERAL DISCUSSION and the first resonance condition is satisfied near h co= 2.5 eV. Further Im k falls rapidly in this region as the energy is decreased. The conditions for surface plasma oscillations to occur thus appear to be satisfied. The height of the broad surface plasma peak in the spectrum of the dielectric loss function Im 8; l is critically dependent on the roughness of the reflecting surface.In Au this peak will combine with the main loss peak at 2.55 eV and will amplify this peak and broaden it on the low-energy side. For Ag however a small distinctly resolvable peak appears near 4.0 - 3.0 - w w s I 2.0- 1 1.0 - 1.0 2 .o 3.0 4 .O 5.0 6.0 .ha [eYI FIG. 2.-Electroreflectance spectra of Au in 1 M HClO (Ar-saturated) at = 45". EH = 0.4 V. A Qm = + 1.91 pC cm-2 (r.m.s.) at 270 Hz. - (AR/R),,t ; - - - AR/R calc. from McIntyre-Aspnes theory. The dashed sections of the experimental curves indicate wavelength regions where appreciable distortion may occur due to stray light effects. 3.2 eV in the experimental ER spectrum.l Analysis of dielectric constant data for Ag reveals that in air the surface plasma resonance peak in the calculated loss function spectrum would occur near 3.6 eV and tend to be buried in the initial rise of the very large volume plasmon peak.(In the MA theory the ER spectrum of metals is closely related to the loss function spectrum.l 9 Further surface plasma resonance is a phenomenon exhibited by the free-electron plasma and is not associated with bound surface states. The simple MA free-electron model of metallic ER effects can be extended to take this resonance into account Details of this calculation will be given elsewhere. and experiment which is evident in fig. 2 provides convincing evidence of the essential validity of the first- order treatment of McIntyre and Aspnes. Future refinements to include specific J.D. E. McIntyre paper presented at the Electrochemical Society Meeting New York May 1969 (Abstract no. 231). J. D. E. McIntyre and D. E. Aspnes Bull. Amer. Phys. SOC. 1970,15 366. The semi-quantitative agreement between theory GENERAL DISCUSSION 61 effects of the electrical double layer chemisorption surface states and surface plasmons will we hope enhance our ability to investigate in situ the detailed properties of the metal-electrolyte interface by means of electrochemical modulation spectroscopy. Dr. B. D. Cahan Miss J. Horkans and Prof. E. Yeager (Case Western Reserve University) (communicated) In regard to McIntyre’s expansion of his previous com- ments we would re-emphasize our position. It is imperative that any theory of modulation of the reflectivity by potential changes must apply to both integral and differential techniques.Recent measurements in this laboratory in which both techniques were used show good agreement between the two methods. Cahan Horkans and Yeager did not “ discuss a model in which all electrons must be stripped from the tail ” but pointed out that this is a consequence of the MA theory when it is applied to integral changes. We would make a related pointed regarding McIntyre’s eqn (2) which we prefer to write in the following form McIntyre’s choice of limits for the integration of this equation is difficult to understand. The integral was taken over a length of 4.6 ltf (= 2.76 A) which is much larger than the dimensions of the electron tail (= 0.6&. This gives too large a value of ntail because electrons from the bulk metal have also been included.A more meaningful calculation would be the evaluation of the change in the surface charge Antail with potential change across the interface within the basic mathematical framework used to derive the above equation. The value of -6.0 x for AR/R for a 1 V change at 500 mp obtained by McIntyre from our paper was apparently taken from fig. 4. Unfortunately there was a misprint in this figure in the original proof. The fact that the curve was obtained with 7 reflections was omitted and the ordinate was incorrectly labelled. It would seem more appropriate to have taken this value from our fig. 2 which shows R-l(aR/dE)A as a function of wavelength. Finally McIntyre’s discussion of “ second order effects ” such as interactions of liquid phase species with d orbitals involve some of the kinds of effects we originally proposed to be responsible for the observed shifts in electroreflectivity spectra with potential.Dr. J. D. E. McIntyre (Bell Laboratories) (communicated) In reply to CHY it was pointed out explicitly in my previous comments that the MA theory should be equally applicable to both integral and differential techniques provided effects due to double-layer structure and specific adsorption are properly taken into account. Their is one further reservation concerning ER effects in metals which requires mention. For p-polarized radiation (ARIA), is strictly independent of d (to first order) only when (AS) 42%, as can be verified by examination of the linear-approxi- mation relations. Otherwise the thickness of the transition layer must be included.The “ breathing ” action of the electron plasma was discussed in detail in my previous comments. The tail itself is never stripped but only shifted and distorted by the applied field. Since the arguments of CHY against the MA free electron model rest largely on the premise of an insufficiency of electrons in the tail region some further discussion on this point may be helpful. The choice of integration limits for eqn (2) in my previous comment is easily explained. Assuming Zs*ZTF the limits z = -2.3 IT and z = +2.3 ZTF simply correspond to the distances at which N(z) attains values of 0.1 No and 0.9 No, 62 GENERAL DISCUSSION respectively. This choice is analogous to common practice in specifying the rise-time of electronic circuitry. The more restrictive limits z = -ZTF and z = ZTF corre- sponding to the distances at which N(z) = 0.368 No and 0.632 No would lead to ntail = No ZTF.For Au this would yield a value for ntail = 3 . 5 ~ 1014 electrons cm-2 = 57 pC cm-2 which may be compared to the value of 2 x 1014 electrons cm-2 ( 3 0 ~ C c m - ~ ) given by CHY in their earlier comment. Such values under- estimate the number of electrons in the tail region. The cut-off limits for the integra- tion in my previous comments were chosen so as to include most of the electrons in the short-range exponential tail while excluding the long-range Friedel oscillation region. The integration does not include electrons in the bulk metal but only those in the surface region which contribute to the surface dipole. Concerning the dimensions of the tail region it should be emphasized that 0.6A is the value of the Thomas-Fermi screening parameter for Au and not the tail length.For a metal such as Au the Wigner-Seitz radius rs = ($nNo)% is 3.0 a.u. Examina- tion of the recent work of Lang and Kohn reveals that the electron density falls from 0.9 No to 0.1 No over a distance of 0.34 Fermi wavelengths (ca. 1.8 A) for such a metal with an average screening parameter 2=OO.65A. The tail in this case is shorter than that predicted by the approximate eqn (2) in my previous comments due to the Friedel oscillations in the density but the screening lengths are in close agreement. According to eqn (2) displacement of the whole electron density distribution by an amount ZTF relative to the rigid positive charge background without any change in tail shape would give Antailx 1.06 ZTF No in the region -2.3 ZTF<Z< 2.3 ZTF.For Au this corresponds to a surface charge density change of 60 pC cm-2 and provides a rough estimate of the shifts to be expected when large fields are applied to an electrode. A more detailed consideration of the tail shape would require a self-consistent field treatment and hardly seems justified in view of the fact that the MA theory predicts that AR/R is directly proportional to the change in integral surface charge and is insensitive to the precise form of the electron density distribution. These points were also discussed at length in my previous comments. The average value for R-l (AR/AE) at 500 nm reported by CHY in fig. 2 of their paper is ca. - 1 x V-l and is greater by a factor of ca.2 than that predicted by the MA theory. Since we do not have any information concerning the double-layer capacity angle of incidence polarization or electrode optical constants in their experiments a more detailed comparison with theory is not possible. The discrepancy is in the same sense as that illustrated in fig. 2 in my previous comments. One possible cause for this experimental peak height amplification was advanced previously. In my previous communication the data in fig. 4 of CHY was specifically chosen to test the applicability of the MA theory to integral measurements of ARIR. Low-frequency measurements of R-l(AR/AE) are more susceptible to spurious adsorption kinetic effects and changes in double-layer capacity than are the rapid- charge-injection measurements of R-I(AR/AQ) shown in my fig.2. These measure- ments are independent of surface-charge modulation frequency. Frequency-dependent effects are commonly observed however when the potential alone is modulated. AR/R is observed to decrease linearly as logfincreases. Previously reported measure- ments from this laboratory suggested that for potential regions in which no Faradaic reactions occur this frequency dependence is associated with the dispersion of the double-layer capacity of the solid electrodes and the effects of uncompensated electro- lyte resistance in the potentiostatic control circuit. The significant contributions of Yeager and his coworkers have greatly increased our understanding of the optical effects due to specific adsorption and we have GENERAL DISCUSSION 63 endeavoured to acknowledge these specifically in our previous comments. Now that methods for determining the actual absorption spectra of monolayers of chemisorbed species are available rapid progress should be made in elucidating the detailed features of the electronic interactions of these species with the metal surface.

ISSN:0430-0696    

DOI:10.1039/SF9700400045

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Reflectance and Ellipsometry of Metal/Electrolyte Interfaces BY M. STEDMAN Division of Inorganic and Metallic Structure National Physical Laboratory Teddington Middlesex Received 1st October 1970 The improving sensitivity and increasing use of optical reflection techniques in the study of metals and semiconductors in contact with electrolytes makes it timely to review the interpretation of such measurements. The surface charge on the conductor the layer of solvent molecules forming the compact layer the adsorption of ions on the electrode as well as the distribution of ions in the diffuse layer are all features of the electrical double layer which can contribute to optical measurements. Models of these features are described which facilitate computation of their optical effects and speci- men results are presented.The fact that all of these features may make comparable contributions to optical measurements is illustrated for the system mercury+aqueous sodium fluoride and the predicted contributions are compared with experimental results. The measurement of optical change on the reflection of light from a surface ha been used increasingly as a route to the optical parameters of the media at or near the surface. The changes measured may be in intensity (reflectometry) or in the polarization parameters (ellipsometry). Often the optical parameters are not of primary interest but are used as means to probe structure near the surface. Whilst the refinement of instrumentation has taken place rapidly our ability to interpret the measurements remains problematic. Those who make optical measurements on electrodes have to deal with a parti- cularly complicated situation in that the structure of the metal/electrolyte interface has various features with differing dependencies on electrode potential.There has been a tendency in interpreting optical measurements on electrodes to focus attention on to just one feature of double layer structure and to interpret the total optical change in terms of change in that feature only as though all else remained constant. This is often unjustified and it is becoming clear that all aspects of double layer structure in both the metal and the electrolyte phases must be considered. It is timely to survey the range of possible contributions to optical measurements on electrodes and to review the models available for predicting optical effects based on our present ideas of double-layer structure.The discussion that follows will be restricted to interfaces at equilibrium and the effects of Faradaic processes will not be considered. OPTICAL PROPERTIES OF THE DOUBLE LAYER DEFINITIONS The optical parameters used in this paper follow the definitions and sign conven- tions recommended by Muller (Nebraska Symposium 1968). Optical effects are quoted either as changes in the elliptical parameters A and t,b or as relative changes of reflectance AR/R for perpendicular (s) and parallel (p) polarizations. 64 M. STEDMAN 65 BASIC REQUIREMENTS OF A N OPTICAL MODEL The simplest model of a metal/electrolyte interface is one in which the media are completely homogeneous and in which the bulk properties of the metal and electrolyte are applicable up to their interface.This hypothetical model serves as a reference state the optical properties of which can be calculated from the bulk parameters. Structural features of the double layer cause the refractive index to be modified in the region of the interface and deviation from the conceptual reference state can be characterized by the profile of (complex) refractive index with distance from the electrode surface. For the purposes of optical calculation it is convenient to express the profile in stepped form so that the interface region is represented by one or more homogeneous films. The optical properties can then be calculated in the usual way by application of the exact Drude equations. In practice relative rather than absolute optical measurements are made and the electrochemist is mainly interested in the optical changes that occur with electrode potential.Such changes will be classified as those associated with changes occurring in the metal and those in the electrolyte solution. CHANGES I N THE METAL Reflection spectroscopy in which field modulation is applied to the sample through contact with an electrolyte has been widely used during the last decade to study the electronic structure of semiconductors. Generally the magnitude of the effect observed has made it reasonable to neglect the changes in the electrolyte. Recently similar though smaller effects have been observed in metals and have been studied by Hansen and P r o ~ t a k ~ Buckman and others. The work of Hansen and Prostak is of particular interest to electrochemists and they have proposed a simple theory to explain their observations on the potential dependence of the internal reflectance of 50 A gold film electrodes.First the change in the charge on the electrode is expressed as a fractional change in the density of free electrons in a thin surface layer. region is dominated by the free electron contribution the change in electron density can be related to a change in the plasma frequency; this implies a frequency shift of the spectrum and a change in the complex refractive index can be derived. Qualitative support for the theory is strong particularly in the close resemblance of the modulation spectrum to the derivative of the reflectance spectrum of gold. The main quantitative test of the theory that was published concerned the variation of ARJR for angles of incidence between 61 and 76" at 4800A and for a particular potential sweep.Some comments should be made on this test. First the optical constants of gold (viz. 1.80-1.98i) used for computation differ seriously from those referred to in the literature (ca 1.25-1.89 and indeed from those previously reported by Hansen.6 Secondly no computed results were sho,wn for comparison with the observed data for the relative change in reflectance for parallel polarization AR,/R,. When this is done using the same optical data considerable divergence from the observed curve is found. Finally the sign of the AR/R results of Hansen is inconsistent with the direction of the potential step he used and with the changes quoted for the optical constants. One concludes that a more searching test of the theory over a much wider range of conditions is desirable.However there is no reason to doubt the basic soundness of the theory which will provide a most useful guide for predicting the optical behaviour of metal electrodes at least in the free electron regions of their spectra. This is illustrated in fig. 1 which shows the results of applying the theory to mercury and gold ; optical Since the spectrum of gold in the 5000 s4-3 66 ELLIPSOMETRY OF METALIELECTROLYTE INTERFACES effects at 5461 are shown as AA At,,$ and AR/R. Results are restricted to external reflection from the metal. Although internal reflection can be more sensitive for the majority of metals it is unlikely that one can attain the exceedingly thm yet continuous films required by the technique.The full curves show the behaviour predicted for an increase of surface change of 10 pC cm-2 and assume no change in the electrolyte. The broken curves show the behaviour predicted for an increase of solution refractive index of 0.03 in a layer 6A thick and assume no change in the metal. These values were chosen as typical experimental ones but the graphs can be appropriately scaled for any other size of increase. Such graphs provide a useful comparison of the contributions to the total optical effect to be expected from changes in the metal and changes in the electrolyte and can help to select the conditions most appropriate for studying a particular feature of double-layer structure. Absolute reflectances (R and R,) should also be considered particularly if cells with multiple reflections are planned.0 -0 02 . . b b o 90 Angle incidence (deg) -+ 3 a \ \ I -0'04 i j \ I I 0 90 90 4 0 90 01 ' /- A- 0 90 0 90 angle incidence (deg.) FIG. l.-Computed optical effects as a function of angle of incidence. - change only in the metal AqM = + 10 pC cm-2 in 5 A layer. -- change only in the electrolyte An = f0.03 in 6 A layer. Refractive indices electrolyte 1.33 gold 0.495-2.46i mercury 1.600-4.75i. Wavelength 5461 A. CHANGES I N THE ELECTROLYTE First we consider the effects of changes in solvent structure. THE COMPACT LAYER.-The structure of the layer of solvent dipoles comprising the compact or inner double layer differs from that of the solvent in the bulk solution and varies with electrode potential. Evidence for the detailed structure of the inner layer has been scanty and ambiguous and only recently has the use of temperature and pressure as experimental variables in double-layer measurements made possible the derivation of surface excess entropies and volumes.Such measurements reported by Hills Payiie and Hsieh ' 9 * have clarified some aspects of inner layer structure at least for a few simple aqueous electrolytes. The contribution that the inner layer may make to optical measurements on electrodes does not seem to have been discussed in the literature. Three aspects M. STEDMAN 67 of the layer need to be considered viz. orientation of the solvent molecules thickness of the layer and its density or compression. Orientation per se seems unlikely to contribute as a first-order effect. To argue in simple terms the ray reflected from the substrate travels through the film in two directions encountering the dipoles both head first and tail first.The optical effect arising from the interference of this ray with that reflected from the top of the dipole film will thus be similar for the two dipole orientations (see fig. 2). The influence of the orientated dipoles on the sur- rounding media may lead to second-order effects. More important are the inter- related aspects of layer thickness and compression. Hills and Payne have derived ;- FIG. 2. surface excess entropies and volumes as a function of surface charge; they estimate that as the surface charge on mercury in contact with 0.1 N aqueous sodium fluoride increases from -10 to about + 2 0 ~ C c m - ~ a compression of ca. 17 % occurs in the water layer.This is typical of the compressions estimated for a range of aqueous electrolytes onm ercury.8 For a similar compression of bulk water and taking the density as 1.21 g ~ m - ~ the refractive index is 1.405.9 Although the unidirectional compression of the inner layer is not exactly equivalent to the bulk compression of water we may use the bulk refractive index to predict the optical effect. Thus for the system with optical constants Ytelectrolyte 1.334 nfilm 1.405 dfilm 4A n,,,,,(niercury) 1.603-4.729i wavelength 5461 A incidence 70° we compute the following optical changes with respect to the (filmless) reference state AA - 0. 120° A$ 0.013" ARJR 9.07 x These results indicate that the contribution of the inner layer to optical measurements is well within the sensitivity of modern instruments and should not be neglected without careful consideration.ARJR - 3.6 x THE DIFFUSE DOUBLE LAYER.-The excess surface charge on an electrode (metal plus adsorbed ions) is compensated by an ionic atmosphere or diffuse layer composed of ions with concentration excesses or deficiencies with respect to their values in the bulk solution and which are held in equilibrium by the interplay of coulombic and thermal forces ; the profiles of ion concentrations with distance from the electrode surface can be predicted by the Gouy-Chapman theory. The optical effect of the diffuse layer has been discussed.1o The crux of the problem is the conversion of ion concentrations to solution refractive indices and this was done by application of the Lorenz-Lorentz equation using ion refractivities and volumes.Ideally the concentration profiles would then be expressed in a stepped form and the diffuse layer would be approximated as a multi-layered film; in practice treatment as a 68 ELLIPSOMETRY OF METAL/ELECTROLYTE INTERFACES single homogeneous layer suffices. Typically the variation of optical effect with surface charge exhibits asymptotes characteristic of the refractivities and volumes of the dominating ions; maximum curvature occurs near the zero of surface charge. An example in which the optical effect is expressed as AA is shown in fig. 3 curve (b). Since the diffuse layer is a mirror of the charge on the electrode optical methods could provide a route to potentials of zero charge. 4M (@ Cm-*) FIG. 3.-Change of A with surface charge 4M for aqueous sodium fluoride solutions on mercury.Angle of incidence 70" ; wavelength 5461 A. Top Experimental results for a range of concentrations. Bottom Computed results for 0.1 N NaF. Curve a field effect in the metal ; b diffuse double layer ; c inner layer ; d (a + b) ; e (a + b + c). In solutions of sodium and potassium salts it is usually the anion which makes the larger contribution to the refractive index. Typical optical effects of the diffuse layer are as follows for a change in the net surface charge from 0 to +20 pC cm-2 on mercury in contact with 0.1 M aqueous potassium chloride AA - 0.06" A$0.0067" (wavelength 5461 A angle of incidence 70"). ADSORBED IoNs.-The problem of calculating the optical effect of adsorbed ions is similar to that encountered in calculating the effect of ions in the diffuse layer viz.the conversion of surface concentrations of ions to a film refractive index. The method used by the author for the diffuse double layer and referred to above can easily be extended to include adions by simply adding their concentrations to those M. STEDMAN 69 of the ions in the diffuse double layer and forming a single equivalent layer for optical calculation ; detailed equations have been published. O Chiu and Genshaw l1 have adopted a slightly different approach in which they use ion refractivity and volume in the Lorenz-Lorentz equation to derive the " refractive index " of the adsorbed ion. This notional refractive index of the adion is then combined linearly with the refractive index of the solvent in proportion to surface coverage.The combined refractive index is taken to apply to a layer of thickness equal to the length of the adion. However difficulty can arise with ions of low refractivity. For example application of the method to the fluoride ion leads to an imaginary refractive index although the ion is clearly not absorbing. For ions of larger refractivity the two methods appear to lead to similar results. For example the optical effect of a monolayer of bromide ions (reckoned at q1 = - 105 pC cm-') on mercury for optical conditions and data as previously given is computed as follows; (first figure method of the author; second that of Chiu and Genshaw) AA -0.526" -0.634" A$ 0.060" 0.058". More typically for a change in surface charge (qm) from 0 to +20 pC cm-2 the change in bromide ion adsorption (qi) would be about - 26 pC cm-2 and the associated contribution of the bromide adions to the optical effect would be AAN -0.15" A$-0.015" A'RP/RP- 1 x AR,/R,- 1 - x 10-4.It is probable that these models give a useful guide to the optical effect of ions in the double layer. However it is not obvious that the use of ion refractivities derived from measurements on dilute bulk solutions is valid for ions in the double layer environment. Furthermore the process of adsorption not only changes the ion adsorbed but may also significantly modify the substrate e.g. by partial charge transfer. INTERFACE BETWEEN MERCURY AND AQUEOUS SODIUM FLUORIDE This system is a useful one to study and discuss because of the simplifying absence of specific ion adsorption. EXPERIMENTAL The ellipsometer operated at 5461 A and had the usual polarizer/quarter-wave plate/ samplelanalyzer configuration.The circles could be read to less than 0.01" and Faraday cell modulation could be applied on both arms of the instrument. Manual setting was facilitated by inspection of the photomultiplier output trace whilst continuous automatic balancing could be obtained through feedback from phase sensitive detectors. A fused- silica cell with 70" faces contained a mercury pool as the working electrode; a Teflon lid carried nitrogen feeds a platinum counter electrode and a saturated calomel electrode in a probe as a reference. All of the chemicals were freshly distilled or otherwise purified. The cell was driven by a modified 1 A Chemical Electronics potentiostat and the usual procedure was to step from - 1.5 V (sce) to the desired potential and back.Effects of impurity adsorp- tion and of the disturbance of the surface of the mercury pool on stepping the potential were obviated by extrapolation of the optical effect to the start of the step. RESULTS Experimental results for a range of concentrations are presented in fig. 3. Potentials have been converted to surface charges using the data of Russel. Optical effect is shown as change in A relative to the value at zero surface charge. Also shown are the optical effects for a 0.1 N solution as computed from the models 70 ELLIPSOMETRY OF METAL/ELECTROLYTE INTERFACES presented earlier. The inner layer contribution is based on the value previously derived and the shape of the curve follows that given by Hills and Payne for the surface excess volume.The contribution from the metal derives from the data shown in fig. l(d) and is expected to be linear with charge. The diffuse layer contri- bution has been previously computed by the author.1° Combined effects were obtained by addition of the component effects although strictly a multilayer computa- tion should be made for each surface charge. DISCUSSION All of the features of double-layer structure that have been discussed above can contribute to optical measurements in greater magnitude than the sensitivity of modern instruments. Thus any of these features is open to study by optical methods ; equally none should be completely neglected in interpreting results. This has been illustrated by the computations for mercury/aqueous sodium fluoride which predict comparable contributions from the surface charge on the metal and from the inner and diffuse layers in the electrolyte.As would be expected the experimental results presented as a function of surface charge are not very dependent on electrolyte concentration. Comparison of the experimental results with the theoretical ones (fig. 3) shows that the sum of the metal and diffuse layer contributions (curve (d)) reproduces the trend and magnitude of the observations quite well. Inclusion of the inner layer contribution (curve (e)) does not alter the trend but predicts too large an optical effect. Since the bases of the calculations of the metal and diffuse layer contributions are straightforward whilst those of the inner layer effect are speculative it is likely that the latter effect was over-estimated.This is not surprising since compression of water in the compact layer is manifestly different from bulk compression. The observed deviations at high surface charges from the general trend of the results do not correlate with any of the predictions. The complementary observation of A$ as a function of charge would be revealing since the ratio of the metal and electrolyte contributions would be different (fig. l(e)). The present conclusion is that theory and experiment are in reasonable accord but detailed interpretation must await the refinement of both. Much of this paper has been concerned with the conversion of structural data to optical effects; most experimentalists wish to effect the reverse process. A question that arises is whether there are any situations in which double-layer structure may be neglected.In the formation of thick films e.g. in corrosion and passivation studies and generally in any study of extended Faradaic processes changes in A and $ may be many degrees and double-layer effects may well be negligible. However calculation before rejection is always a wise rule. The study of specific ion adsorption is a more difficult case since the change in A may be only a few tenths of a degree. The results of Chiu and Genshaw on the adsorption of the bromide ion on mercury from potassium bromide solutions are of particular interest as excellent agreement with existing adsorption data l 3 was reported without any allowance for the double layer being made. The data of Lawrence Parsons and Payne l 3 show that the net charge (qM+qi) on the electrode is always negative so that the dominant ion in the diffuse layer is always the cation (which has the lower refractivity).This fortuitous situation leads to the expectation that the diffuse layer will have only a small optical effect in this particular case. However a contribution from the metal similar to that already calculated would be expected as well as some contribution from the inner layer. The former would be about 0.05 to 0.1" for the range of potentials used in this work compared with the experimental changes of up to 0.25". Interpretation thus seems less clear than was supposed by Chiu and Genshaw. M. STEDMAN 71 A serious criticsm of all the models used in this paper is that matter in the double layer is treated as though it were the same as bulk matter although in a thin layer.Refinement of these models will require insight into how the properties of substances are modified at interfaces. Unfortunately such information is only likely to be accessible in those experiments for the interpretation of which we need the data. Another dissatisfying feature of the treatment is the division of the double layer into separate components. Such division is neither experimentally possible nor uniquely definable. We may manipulate the double layer but it retains an essential unity. Combining these thoughts with modern views of discreteness in the double layer leads one to seek a microscopic approach to optical calculations. Strachan14 and Sivukhin l 5 have treated molecular films as collections of Hertzian oscillators in their studies of optical reflectance.Perhaps their pioneering work should be taken up again and developed in this context. It is a pleasure to acknowledge many helpful discussions with my colleagues Dr. M. E. Peover and Dr. R. J. King. The work described was carried out at the National Physical Laboratory. Recent Developments in Ellipsometry ; ed. N. M. Bashara A. B. Buckman and A. C. Hall (North- Holland Publishing Co. Amsterdan 1969). R. H. Muller ref. (l) p. 14 and 29. W. N. Hansen ref. (l) p. 205. W. N. Hansen and A. Prostak Phys. Rev. 1968,174,500. A. B. Buckman; ref. (l) p. 193. W. N. Hansen I.S.A. Trans. 1965 4,267. ' G. J. Hills and R. Payne Trans. Faruday SOC. 1965 61 326. S. Hsieh Ph.D. Thesis (University of Southampton 1969). Y. B. Zel'olovith S. B. Kormer M. V. Sinitsyn and K. B. Yushko Soviet Phys. Doklady 1961 6,494. (Note two printing errors in fig. 2 the q- $+I ordinate extends to 0.006" ; in fig. 3 the ordinate extends from 1.332). l o M. Stedman Chern. Phys. Letters 1968 2,457. I 1 Y-C. Chiu and M. A. Genshaw J. Phys. Chem. 1968,72,4325. l 2 C. D. Russel J. Electroanal. Chern. 1963,6,486. l 3 J. Lawrence R. Parsons and R. Payne J. Electroanal. Chern. 1968,16,193. I4 C . Strachan Proc. Cambr. Phil. SOC. 1932 29 116. I s D. V. Sivukhin Soviet Phys. JETP 1956 3 269.

ISSN:0430-0696    

DOI:10.1039/SF9700400064

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Reflectance Studies of Adsorption on a Platinum Electrode BY M. A. BARRETT AND ROGER PARSONS Dept. of Physical Chemistry The University Bristol BS8 1TS Received 1 st September 1970 A simple technique for measuring the intensity of polarized light reflected from an electrode is described. Results for the adsorption on platinum of oxygen hydrogen halides methanol formal- dehyde and formic acid are discussed. The method is particularly sensitive for oxygen and halide5 and the optical properties of the former can be determined within limits. Tronstad demonstrated that adsorbed films of fatty acids on mercury could be detected by the change in the ellipticity of light reflected from the surface. Recently ellipsometry has been used to study adsorbed layers of oxygen 2-5 on platinum elec- trodes as well as the specific adsorption of anions ; and there has been developed a method of studying adsorption on electrodes using the measurement of the intensity of reflected light.7 Although this method necessarily provides less information in a given measurement it has advantages in that the sensitivity can be greatly increased by modulation techniques,'.it is easily adapted for measurements at varying wave- lengths of light and kinetic measurements can be made without the great elaboration required for automatic ellipsometry. The object of the present work is to study the intermediates adsorbed on platinum electrodes during the oxidation of organic molecules. The technique chosen com- bines features from both of those described above. The intensity of the two components of the reflected light is measured.It is also possible to measure the phase difference between the components although this was not usually done. EXPERIMENTAL The scheme of the optical measurements is shown in fig. 1. The light source was tungsten (Osram 48 W) or a deuterium lamp with stabilized power supplies. The light was passed through an Optica plane grating monochromator and then collimated by a quartz lens. In the full arrangement a polarizer could be placed before the cell and a compensator and analyzer after it. The polarizer and analyzer were Glan-Taylor prisms (Lambrecht Crystal Optics) mounted in Bellingham and Stanley circles calibrated to 0.01'. The compensator was a Babinet-Soleil (Gaertner L.135 W). Intensities were measured with a photomultiplier (EMI type 6256B) with a stabilized power supply (Keithley type 245) and recorded on a pen- recorder (Servoscribe RE 511).The cell was a rectangular Suprasil 4.5 x 2.5 x 6 cm or occasionally a glass cell of similar dimensions. The platinum electrode was formed from a bright rolled sheet bent to a U-section to form two parallel sheets about 2 mm apart. The two sheets were slightly offset to allow entry and exit of the light beam. An even number of reflections was used so that the reflected light beam emerged parallel to the incident beam. The number of reflections was dependent on the distance between the sheets and the angle they made with the beam. At first as many as forty reflections were used but it appeared that better results were obtained with the greater intensity remaining after a much smaller number and most of the work described here was done with two reflections.The potential of the electrode was controlled by a potentiostat (Chemical Electronics type TR40/3A) using a 72 M. A . BARRETT AND R. PARSONS 73 platinum auxiliary electrode and a saturated calomel reference electrode. A manual switching unit with potentiometric circuits allowed the electrode potential to be switched from one value to another for example in the cleaning cycle and the kinetic runs. Potentials are quoted with respect to the standard hydrogen electrode. Working Collimating Electrode Calomel Soleil sator \ Photo- / - Silica Cell Electrode multiplier FIG. 1 .-Schematic diagram of the apparatus. METHOD OF OPERATION The apparatus could be used as an ellipsometer. If the analyzer was kept with its axis at 45" to the plane of incidence tan i,b can be determined from the azimuth of the polarizer and A from the retardation of the Babinet-Soleil compensator with its axis permanentIy set at vertical and horizontal positions.In this arrangement the polarizer setting is indepen- dent of the compensator setting which is a decided advantage for rapid measurements. With the components described the apparatus can be used in the u.-v. down to 200nm but the sensitivity is not particularly good in comparison with other ellipsometers. Greater sensitivity of changes in A and continuous recording can be achieved by putting one com- ponent e.g. the compensator out of balance. The intensity then changes with any change in A and the sign of this depends on the direction of the deviation from balance.Any simultaneous differences in absolute reflection intensities will be superimposed on this. 4 FIG. 2.-Recorder trace during cleaning cycle and experiment on a Pt electrode in 0.5 M H2S04 containing 1 M formic acid. The lower part of the figure gives the potential with respect to the normal hydrogen electrode while the upper part shows the relative intensity of the p component of light of wavelength 350 nm at an angle of incidence of 60". (a) 0.5 M HzSOj ; (b) the same with 1 M formic acid. 1s indicates a duration of 60 s. 74 ADSORPTION ON Pt BY REFLECTANCE Thus to interpret the changes it is necessary to separate the two effects. Two methods are available (a) to make equivalent traces with the direction of deviation reversed; (b) the compensator and analyzer are removed and the total intensity recorded which can be sub- tracted from the off-balance curve.Here the polarizer must be left in position of balance in order to give the correct weight to the two polarized components. In either case the sequence of events at the electrode surface must be duplicated accurately. In most of the work however simply the intensity of one or both of the components of the reflected light was recorded. For this the polarizer and the compensator were removed and the analyzer kept with its axis either parallel or perpendicular to the plane of incidence. Reproducibility of the electrode surface was achieved by pre-electrolysis and by carrying out a cleaning cycle of the type recommended by Gilman lo before each measurement. This is shown in fig. 2 together with the corresponding intensity Ip of the parallel component.For the first ten cycles there was an increase in the amplitude of the changes but thereafter they no longer varied with the number of cycles. The oxidizing potential was kept down to 1.5 V to avoid any possibility of oxygen evolution which would greatly affect the intensity of reflected light as the bubbles formed tend to stick to the surface. Besides preparing the electrode surface in a reproducible way this cleaning cycle provides several reference points for the measurement of intensity changes as well as a continual check on the state of the electrode. Intensity changes of 0.01 % were readily detected and the overall reproducibility was about 0.2 %. RESULTS THE REFRACTIVE INDEX OF PLATINUM Published values of the complex refractive index of platinum 4* 6* l-l vary over a wide range as shown in fig.3a and 3b. It therefore seemed worthwhile to determine this quantity which was required for further calculations in this work. This was done using the apparatus as an ellipsometer to measure tan t,9 and A. Measurements were done with the platinum electrode immersed in 0.5 M H2S04 and held at +0.3 V with respect to the hydrogen electrode. Angles of incidence from 60 to 68" were used. The scatter in the results is still disappointingly large and an analysis of the errors involved leads to the conclusion that the main source of error is the non- uniformity of the Babinet-Soleil compensator. Better results could be obtained with a mica retardation plate but this does not transmit below 300nm. Some results were also obtained for the same electrode in air; these are also shown in fig.3a and 3b. However it seems likely that these data are affected by contamination. Since adsorption on platinum at 0.3 V in sulphuric acid is negligible data obtained under these conditions are more likely to be reliable. ADSORPTION OF OXYGEN The variation of intensity of the parallel component (Ip) normal component (In) and of delta after two reflections as a function of potential is shown in fig. 4 for a platinum electrode in 0.5 M H,S04. In agreement with previous work there is a marked lowering of both intensity and A at potentials greater than 0.75-0.8OV corresponding to the adsorption of oxygen. Above 0.80V the intensity drops linearly with potential up to about 1.5 V. These changes d(IJ d(In) and d(A) consequent on the adsorption of the oxygen film were used to narrow down the possible values of the refractive index of the film.The relationships between these changes varies in a complicated way with the refrac- tive index of the adsorbed film (nfilm) so initially computations were made giving a survey of the optical changes to be expected over the entire range of likely values of nfilm for the particular angle of incidence and wavelength employed. These were n I*! I M. A . BARRETT A N D R . PARSONS I I I I X A X o Q 0 OO 0 0 x A +* . X 0 A + + + + 0 + ++ i- 0 0 + 0 a + 0 I I 0 I I I 1 300 4 0 0 500 6 0 0 0 0 0 0 A + + + + O + . + + % + A e x % + I ' 11 I T + I+ 75 3 0 0 4 0 0 5 0 0 60( A/nm (4 FIG. 3.-The complex refractive index of Pt fi = n-ik. (a) n ; (b) k as a function of wavelength 0 in 0.5 M H2S04 at 0.3 V ; 0 in air; 0 Visscher (ref.(4)) ; X Landolt-Bornstein (ref. (1 1)) for solid Pt ; + Landolt-Bornstein for deposited Pt ; A Greef (ref. (1 3)) ; I Rideout and Wemple (ref. (12)). 76 ADSORPTION ON Pt BY REFLECTANCE based on the usual equations for reflection coefficients R of filmed surfaces as given e.g. by Winterbottom l5 (for two reflections I R l4 = I ) . dR, dR and dA were computed using the range of film thickness 0-5A since variation from linearity is negligible in this small thickness range. Many addtional computations were necessary to narrow down the possible values of refractive index. For convenience the complex film index is represented in fig. 5 as a plot in the complex plane with possible values marked out as areas.I*O+*++$ h+ o + *99 I O 0 O 0 o + FIG. EIV (NHT 4.-Intensity of p (0) and n (+) components and A (A) for light reflected twice from electrode in 0.5 M H2S04. Wavelength 350 nm ; angle of incidence 69". a Pt The experimental values of d(I,) d(1,) and d(A) were based on the differences in the values of each quantity between 1.0 and 1.5 V. This eliminates the necessity of an assumption as to the film free value. The choice of 1.5 V could lead to a small error because there is evidence of an inflection in the (intensity potential) curve in this region similar to that found by Biegler and Woods l6 in the coulometric curve. The areas in the complex refractive index plane were narrowed down in the following steps (i) using the experimental values of d(IJ/d(I') together with the fact that both d(1,) and d(1,) are negative; (ii) comparison of dA with d(IJ; this was the most helpful in spite of the poor accuracy of dA ; (iii) comparison of dR at two angles of incidence (the range used was 45-60' with some data up to 68").Fig. 5 shows the resulting areas defined by this procedure at four wavelengths. These determinations are on optical grounds alone and from one end of the segment to the other results in thickness determination ranging from 2 A/V to infinity as the segment approaches the point describing the refractive index of the platinum M . A . BARRETT AND R . PARSONS 77 substrate. Towards this point the various changes become vanishingly small and the computed data are not written out to enough sigruficant places to offer any reliable guide. This is equivalent to extremely large thicknesses in any case so that segments drawn cut off at the point representing 20a/V in fig.5. The experience of the analysis shows that although the accuracy of the experimental data leaves much to be desired improvement of the accuracy would tend to narrow down the segment I 2 3 4 I 2 3 4 k FIG. 5.-Complex refractive index (4 = n-ik) for oxide layer on platinum electrode in 0.5 M H2S04. The shaded area shows values which are compatible with the available optical data. The assumed refractive index of Pt is marked with X . (a) A = 250 nm; (b) A = 290 nm ; (c) A = 350 nm ; (4 h = 400nm. rather than to indicate which portion along its length is the correct one. Resort has to be made to bulk properties to achieve this and following the same assumptions as Visscher about the density and coulometrically determined quantity of PtO or PtOz leading to 6.6A/V and 5.2&V respectively the following results for the refractive index are obtained at 250 nm 2.4- 2i ; for 290 nm 2.7 - 1.6i ; for 350 nm 3.1 - 1.4i (all numbers 1 0.3) and at 400 nm 3.0 - 1.7i (& 0.4) with the latitude in the results covering both types of oxide.These results seem consistent with the value of 3.4- 1.3ikO.5 which is the average value obtained by Visscher at 546 nm. ADSORPTION OF HYDROGEN At potentials more negative than 0.2 V there is again a decrease in intensity but by a much smaller amount than found in the oxygen region. We attribute this to the formation of adsorbed hydrogen which occurs in this region.le At lower angles of incidence there is also evidence of a small maximum in I at about 0.15 V.This maximum occurs also in the normal component I which is greater than its value on a bare surface in the whole of the hydrogen region (fig. 4). In the lower potential region where the changes involve weakly adsorbed hydrogen both d(l,) d(1,) and d(A) are negative for all wavelengths studied in the u.-v. Further- more d(Ip) decreases markedly with decrease in angle of incidence by a factor of 2 in the range 64-45" at 350 nm. 78 ADSORPTION O N Pt BY REFLECTANCE In the region of more positive potentials corresponding to the more strongly bound hydrogen it is not so easy to draw definite conclusions. The optical effects are smaller so that their magnitude and sign are much more dependent on the assump- tions made about the effect of the charge on the metal and ionic adsorption.The least objectionable procedure seems to be to extrapolate the (I,E) plot from the double-layer region. The signs for d(Z,) and d(IJ then depend on wavelength and angle of incidence. Thus the effect of strongly bound hydrogen appears to be different from that of the weakly bound hydrogen. It should be possible in a manner similar to that used for oxygen to find areas corresponding to possible values of nfilm at least for the weakly bound hydrogen. Unfortunately no area has been found that is qualitatively consistent with all the experimental data. It may be that the system cannot be represented as a single film. HALIDES The adsorption of anions on platinum was stuhed in some detail using the ellipsometric method by Ying-Chech Chiu and Genshaw.6 The results obtained for halides differ markedly from those found coulometrically * or usingradiotracers.l8 Since the adsorption of halides provides a straightforward example of optical studies it was decided to check these results by the present method. The results are shown in fig. 6. For the chloride adsorption is detectable in the whole of the double-layer region at M KCl in 0.5 M H2S04. This appears to be more consistent with the coulometric results than with the ellipsometric data. Similarly the independence of the Br- adsorption in the concentration range to molI.-l agrees with E/V (NHE) FIG. 6.-(0) Relative Ip for Pt in 0.5 M H2S04(line) points are for lo-' (61 lowering of Zp due to Br- adsorption deduced from (a) ; (c) the same as (a) but 2.5 x and M KBr added ; M KCl added ; (d) lowering of Zp due to C1- adsorption deduced from (c).Angle of incidence 60". the coulometric results although the latter indicate a constant maximum coverage from 0.2 V to 1.0 V. The present results as far as the potential dependence is concerned agree with those of Genshaw in indicating an increase in coverage with Br- as the potential is increased. However the interpretation that this necessarily indicates an increase in coverage involves the assumption that a given coverage will result in the same optical effect at different potentials without any complicating factor M. A . BARRETT AND R . PARSONS 79 such as a change in the distance of closest approach of the ions or polarization or on the state of the surface of the platinum. An attempt was made to distinguish between these two alternatives by choosing a concentration low enough M) that adjustments to surface coverage would be slow while adjustments of any of the above-mentioned effects would be extremely rapid under any conditions.Potential steps from 0.4 to 0.6 V and from 0.4 to 0.7 V produced adjustments in the reflectivity somewhat faster than those at the same coverage during adsorption from a clean surface but definitely slower than the recorder would be able to indicate. This is taken to indicate that the change is in fact a variation in coverage and the enhanced rate is most likely due to the bulk concentration extending to the surface where there would be a concentration gradient at this coverage when adsorption starts from a clean surface. For adsorption on a clean surface the results plotted as intensity against (time)% give a reasonably straight line evidence that adsorption is diffusion controlled.0.5 I.0 EIV (NHE) FIG. 7. FIG. 8. FIG. 7.-Relative 1' for Pt in 0.5 M H2S04(line) and with 1 M methanol added (X ). Two reflections angle of incidence 68" ; wavelength 400 nm. FIG. 8.-Relative 1 for potential decreasing stepwise 30 s at each potential in 0.5 M H2S04 (0) and in 0.5 M H2S04+1 M CH30H (X). Conditions the same as those for fig. 7. The nature of the reflectivity changes on adsorption of both Cl- and Br- indicate that the simple optical model employed by Genshaw is not strictly correct in assuming no optical absorption in the film. If this were the case Ip would increase with adsorption at all but small angles of incidence and this was definitely not the case either at 550 nm the wavelength used in the ellipsometric studies or at 350 nm used mainly in the present studies.A values are not affected by small amounts of absorp- tion but d(lp) is sensitive to even small values of the absorption coefficient and reverses sign at k N 0.6 under the conditions used. ORGANIC COMPOUNDS Coulometric measurements of the adsorption of the related compounds ; methanol formaldehyde and formic acid are not in good agreement. Thus e.g. Gilman and Breiter l9 reported that the coverage of methanol in a 1 M solution is constant from 0.1 to 0.65 V from anodic sweeps. In contrast Bagotskii and Vasiliev 2o find a marked dependence on potential between 0.05 to 0.4 V. For this reason and in the hope of elucidating the nature of the adsorbed species these compounds have been studied optically.80 ADSORPTION O N Pt BY REFLECTANCE METHANOL.-&. 7 shows the effect of adding methanol to 0.5 M H2S0 to make a 1 M solution. In contrast to the effect of oxygen halides and hydrogen the adsorption of methanol results in an increase of Ip at angle of incidence of 60" and above and wavelengths above 350 nm ; in the range 0.4-0.6 V this amounts to 0.3-0.4 %. At potentials at or below 0.05 V the sensitivity more than doubles because methanol displaces hydrogen. The effect of a monolayer of hydrogen is estimated to be a reduction of Zp by about 0.5 %. However according to Gilman l9 and Bagotskii,20 only about 75 % of the hydrogen is replaceable by methanol. Thus after methanol has been allowed to adsorb at say 0.4 V then on switching to 0.05 V there should be a decrease in intensity of 0.125 %.A decrease of at least this amount is usually observed so that the results are consistent with an approximately constant amount of adsorbed species over the range 0.05-0.6V and tend to support the conclusions of Breiter and Gilman. The sequence on stepwise decreasing the potential starting at 0.8 V is shown in fig. 8 together with the equivalent observations without methanol. The oxide does not appear to be entirely reduced until 0.4 V curve B and the maximum coverage does not appear to have been reached until 0.3 V. This is similar to the results of Gilman and Breiter l 9 on cathodic sweeps except that they find the coverage still increasing at 0.2V. The discrepancy may be due to the rapidity of their sweep which although slow is about 3 times the present rate.There may not have been time for equilibrium to be established. The determination of coverage below 0.6 V by Bagotskii and Vasiliev 2o is analogous to the cathodic sweep of Gilman and Breiter.19 It is claimed that below 0.6V the oxide is completely reduced very rapidly but this leaves the difference between the anodic sweep and cathodic sweep hard to explain. According to the optical results the adsorbed oxygen is not completely gone until 0.4 V in a cathodic stepped sequence and this would suggest that a reduced number of sites are available for methanol adsorption above 0.4 V. Biegler and Koch 21 found that the rapidity with which oxide is reduced is dependent on the length of time the oxidation potential has been held and adopted 15 ms oxidation time in order to obtain a film sufficiently reduced for their technique.From the slope of the intensity recording the rate of methanol adsorption can be determined if we assume the intensity change to be proportional to the amount adsorbed. Every pre-treatment cycle provides a value for the initial rate at 0.5 V. On the basis of the equilibrium coverage being one monolayer the maximum initial rates were found to lie between 0.03 and 0.1 monolayer/s for 1 M methanol and between 0.1 and 0.2 for 2 M methanol. At 0.025 V and 1 M solution the one determination gave 0.03 monolayer/s. No methanol adsorption occurs for the first few seconds at 0.05 V. Data published by Bagotskii 2o indicate a similar delay at 0.2 V the lowest potential for which data is given. The delay is much shorter at higher potentials.Still using the same anodic cleaning sequence but switching to different low potentials several adsorption rates were determined for 0.5 M methanol in 1 M HC104. The maximum rate occurring just after the initial pause but still at virtually zero coverage was considered the relevant quantity to compare with the initial rates of Biegler and Koch.21 The results are collected in a Tafel plot fig. 9 although the present results do not fall on a straight line. The initial rates reported by Biegler and Koch 21 are for 0.2 M methanol but a line corrected to 0.5 M methanol is included in the figure assuming the rate increases in proportion to the concentration. A limit was put on the useful range of potentials treated in this way by the fact that oxide reduction becomes much slower at high potentials.Biegler and Koch21 M. A . BARRETT AND R. PARSONS 81 altered their anodic cleaning procedure to avoid this problem by oxidizing for only 15 ms but such fast switching mechanism was not possible in this work. Although the result at the lowest potential seems consistent with the extrapolation from Biegler's work the slope is different leading to much higher adsorption rates around 0.1 V. His measurements were taken in 1 M H2S04 as opposed to 1 M HC104 of the present work. One other difference between the two methods is that in his work the measure of the rate is really the difference of the anodic current associated with adsorption of methanol and the cathodic current from reducing the oxide if this is not completed by that time.In contrast the change in intensity is in the same direction for both. Thus any error arising from the tail-end of oxide reduction would be in the opposite direction in the two cases. E/V (NHE) FIG. 9.-(u) Rate of adsorption of methanol (0) and formic acid (0) from 0.5 M solutions in 1 M HC104 on Pt. A data of Biegler and Koch (ref. (21)) for 0.2 M methanol in 1 M H2S04; -- recalculated to 0.5 M methanol. (b) Rate of adsorption of formaldehyde on Pt from M HCHO in 0.5 M H2S04. FORMIC AcID.-The formic acid results were identical with those for methanol to within experimental error. This applies to both the optical effect at full coverage and to adsorption rates. FORMALDEHYDE.-Formaldehyde was prepared by refluxing paraformaldehyde in a 0.5 M H2S04 solution with gentle heat for about 5 h.Results were decidedly different from the other two organics. At the same concentration (0.5 M) the In traces were impossible to interpret. In judging the intensity variation due to adsorbed methanol or formic acid two standard intensities were used where the conditions of the surface could reasonably be assumed the same as in the pure electrolyte. These were the hydrogen-covered surface immediately after the oxide was reduced and before there was time for any adsorption of the organic substance and the other was the oxide-covered surface. The results using either of these agreed fairly well. Formaldehyde at the same concentration (0.5 M) gave intensity traces quite different in character from the other two. Formaldehyde adsorption was so much faster that it was impossible to find a point representing full coverage of hydrogen before formaldehyde' had begun to replace hydrogen.Also the shape of the intensity trace during oxidation was 82 ADSORPTION ON Pt BY REFLECTANCE noticeably altered by the presence of formaldehyde. The heavy current needed to oxidize the formaldehyde meant that the potentiostat was unable to bring the potential of the electrode up to 1.5 V in less than 30 s. When this potential was reached there was then no further change in Ip contrary to the normal behaviour during the cleaning cycle. However at low concentrations (0.005 M and 0.05 M) the traces could be interpreted in the same way as for methanol and formic acid and the two possible reference intensities again agree reasonably. Under conditions where a monolayer is expected from coulometric measurements the magnitude of d(Z,) is greater for formaldehyde than for the other two at 350nm though the difference does not show up at 400nm.Adsorption rates were determined for the 0.005 M solution and are plotted in fig. 9b. Even at this concentration adsorption is decidedly more rapid than for 0.5 M methanol or formic acid. Although a few determinations were also made at 0.05 M formaldehyde already the check between the I levels for oxide and hydrogen- covered states was beginning to fail. Loucka and Weber ’’ have likewise reported much more rapid adsorption rates for formaldehyde as compared with the other two organics and also mention a maximum rate at about 0.2 V. If one accepts the intensity at the oxide-covered surface as a standard at 0.5 M the results show a much greater d(1,) than under any other condition and I continues slowly upwards for at least 7 min still showing no signs of ceasing.It seems reason- able to suppose that polymerization is occurring at the surface. The results described so far have been based on the assumption that the observed change in reflected intensity is simply proportional to the amount of adsorbed species. This assumption is only a first approximation and it is necessary to investigate how reliable it is likely to be. When adsorption occurs accompanied by hydrogen displacement a major cause of the increase in I is due to the removal of hydrogen. In the potential range 0.15-0.25 V this would be mainly strongly bound hydrogen while below this potential range it would involve both types of hydrogen with the weakly bound hydrogen giving a proportionately greater effect per atom the proportionality depending on the wavelength and angle of incidence used.Thus the relative influence on I of adsorption on the two types of sites will change with potential. In the potential region above where any adsorbed hydrogen is stable the compli- cation of the optical effect of hydrogen removal is absent. Even here a linear relationship between reflectivity and coverage is dependent on there being only one species or that those present contribute equally to Zp or that the types occur in a constant proportion over the entire range of coverage invariant with time. A further investigation of these factors was made by employing two approaches to coverage determination. This was done with both methanol and formic acid at a concentration of M to give relatively slow adsorption.Between 0.25 and 0.5 V to judge by the simple (reflectivity time) traces made at 60° there was a delay of several seconds before any change occurred followed by a slow rise in intensity fig. 10 ; curves a and 6 check that cathodic pulses do not influence the rate. By the use of cathodic square wave pulses (applied by manual switching) for 1 s or more the hydrogen laid down could be detected by its effect on the intensity. At least 1 s was necessary for the alteration in intensity to be completed and it is not certain to what extent this is due to the recorder. The drop in intensity is a measure of the hydrogen codeposited subject to the same complications of analysis as mentioned above. However regardless of the proportion of the two types of hydrogen co- deposited the definite conclusion from this result is that part of the surface has become blocked to hydrogen codeposition almost immediately on reaching the M .A . BARRETT AND R . PARSONS 83 appropriate potential and sometimes shows overshoot as Smith et aZ.23 found. Thus the species adsorbing initially must either have no optical effect or consist of several species with cancelling optical effects. Further this situation changes after the first few seconds of adsorption since the trend in Ip is upwards for longer times. This argues for the existence of more than one species. It was reasonable to expect that there would be some way of detecting the initial adsorption directly by a trace of Ip. This was in fact found at 250 nm using 45" angle of incidence.The trend was then initially downwards followed as before by a rise. The further conclusions from these observations is that there is a change with time of the nature or proportion of the species. It is possible that the two species involved are the directly adsorbed molecule and some dehydrogenated species like -COH. Oa5 A ] 1.00 A 0.99 -/QQo--o-o C - to I I I i o-o- 1 Q = ~ o - o \ o - ~ 0 c 0 a b 1-00 0 9 9 5 I.0OOb a b 0.995 Bulk methanol has a refractive index close to water and therefore it was expected that even allowing for the effect of dehydrogenation there would be little observable change in the optical parameters. In addition the intensity trace is less stable in the presence of methanol. Although little difference could be detected at 290nm between a methanol covered surface and the film free surface at 400 nm an increase in Ip was observed.As with a positive change in A this positive d(Ip) limits the possible values for the refractive index to two areas but in this case the area inferring 84 ADSORPTION ON Pt BY REFLECTANCE high optical absorption can be considered unlikely. This leads to the conclusion that the refractive index is real and slightly greater than that of the surrounding liquid. As bulk methanol has a slightly lower refractive index this must be due to dehydrogenation producing a more compact film. We are grateful to the Ministry of Defence (Navy) for generously supporting this work and for permission to publish it. We are also indebted to Bhimasena Rao for his valuable preliminary work without which the work described would not have been possible.' L. Tronstad Trans. Faruday SOC. 1935 31 1151. A. K. N. Reddy and J. O'M. Bockris El/@sometry in the Measurement of Surfaces and Thin Flms (Washington 1963) ed. E. Passaglia R. R. Stromberg and J. Kruger N.B.S. Misc. Publ. 256 1964 p. 229. A. K. N. Reddy M. A. Genshaw and J. O'M Bockris J. Chem. Phys. 1968,48 671. W. Visscher Optik. 1967 26 407. R. Greef J. Chem. Phys. 1969,51,3148. ' Ying-Chech Chiu and M. A. Genshaw J. Phys. Chem. 1969,73 3571. ' D. F. A. Koch and D. E. Scaife J. Electrochem. SOC. 1966 113 302. * J. D. E. McIntyre Electrochem. SOC. Meeting. (New York May 1969) Abstract no. 232 ; J. Electrochem. SOC. 1969,116 14OC. A. Bewick and A. M. Tuxford this Symposium. lo S . Gilman Electrochim. Acta. 1964,9 1025.Landolt-Bornstein Zuhlenwerte und Funktionen Band 11 Tie1 8. l2 V. L. Rideout and S. H. Wemple J. Opt. SOC. Amer. 1966 56,749. l 3 R. Greef private communication. l4 A. N. Frumkin Ado. Electrochem. vol. 3 ed. P. Delahay (Interscience. New York 1963). l 5 A. B. Winterbottom Kgl. Norske Videns Skrift. no l. 1955. l6 T. Biegler and R. Woods J. Electroanalyt. Chem. 1969 20 73. V. S. Bagotskii Yu. B. Vassiliev J. Weber and .1. N. Pirtskhalava J. Electroanalyt. Chem. 1970,27 31. N. A. Balashova and V. E. Kazarinov Electroanalyt. Chem. ed. A. J. Bard (Dekker New York) 1969.3 135. l9 S. Gilman and M. W. Breiter J. Electrochem. SOC. 1962 109 622. *O V. S. Bagotskii and Yu. B. Vasiliev Electrochim. Acta. 1966 11 1439. 21 T. Biegler and D. F. A. Koch J. Electrochem. Soc. 1967,114,904. 22 T. Loucka and J. Weber J. Electroanalyt. Chem. 1969,21 329. 23 R. E. Smith H. B. Urbach J. H. Harrison and N. L. Halfield J. Phys. Chern. 1967 71 1250,

ISSN:0430-0696    

DOI:10.1039/SF9700400072

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

GENERAL DISCUSSION Dr. W. Paik and Prof. J. O'M. Bockris (University OfPennsylvania) (communicated) We agree with Stedman that in interpreting ellipsometric data at the metal-solution interface it is necessary to take into account the electroreflectance effect. We doubt at present that there is a significant effect of pressure on the refractive index of the double layer. However Stedmann has assumed following Hansen that as the electron concentration in the metal surface changes due to the potential change there will be a monotonic shift of the NM and KM parameters along the ;1 axis and this assumption clearly does have difficulties near to the optical edge. In work in press,l having aims similar to those of Stedmann we have examined the wave-length dependence of 611 but have used the electron theory of metals to relate the changes in optical properties to frequency (using Br- adsorption on Au thus eliminating the limitation which may exist in her work).In this way we have found a fairly consistent agreement between the observed quantities of those expected on the basis of our calculations of the electroreflectance effect. Dr. R. Parsons (University of Bristol) said Stedman mentioned in her paper the use of ellipsometry to determine points of zero charge. However because there is more often specific adsorption than not this method will not be very useful in general. Perhaps it might be a useful method to determine the point at which the charge on the diffuse layer is zero. I would be interested in her comments and in particular whether she has succeeded in determining points of zero charge this way.Dr. M. Stedman (Nat. Phys. Lab. Teddington) said In reply to Parsons I agree that the presence of specific adsorption would complicate if not preclude the use of optical methods to determine potentials of zero charge. Even in the absence of adsorption it would be difficult to determine with certainty the contribution of the diffuse layer. The remark in my paper was not intended as a practical proposal but was a passing thought which might trigger further ideas. I have not made any attempts to determine potentials of zero charge. Dr. B. Cahan (Case Western Reserve University) said There is some question as to the appropriateness of the use of the Hansen-Prostak model in the calculations used to generate the graphs in fig. 1 of the paper of Stedmann et al.It has been pointed out by several authors (McIntyre Bewick) at this Symposium that this treatment does not give a fit for metals other than Au. We have shown that the magnitude of the effect agrees numerically with the calculated value only at the absorption edge and Stedmann has pointed out that even the sign of the effect is wrong. Since the primary assumption used in treatment is thermodynamically unsound it appears that the agreement at the absorption edge of gold is fortuitous. Therefore there is neither an empirical nor a theoretical basis for the use of this theory for these calculations. The magnitude of the expected changes in reflectivity due to the double layer may be somewhat overstated. While it is true that a AA of -0.120' is well within W. Paik and J. O'M.Bockris to be published. W. Paik M. A. Genshaw and J. O'M. Bockris J. Phys. Chem. 1970,74,4266. 85 86 GENERAL DISCUSSION the resolution obtainable ellipsometrically on a relative basis this is not true for an absolute measurement. With present surface handling techniques it is impossible to prepare two specimens of the same material that agree within this tolerance. The reference state used to determine the above-mentioned 0.120" is a filmless state (i.e, without any double layer) and therefore experimentally unrealizable. A real measure- ment will involve smaller changes than the extremes used here and these effects should be correspondingly lower. This is not to imply that the optical changes in the double layer are unimportant. On the contrary as out recent glancing angle reflection experiments have shown (see discussion on our paper) we are vitally interested in this question but believe that these measurements can and should be made with a system in which the effects of the substrate do not interfere.Dr. M. Stedman (N.P.L. Teddington) said I agree with Cahan that in the light of contributions that have been made to this symposium one would no longer choose the Hansen-Prostak model for predicting the contribution of the metal to the optical behaviour of electrodes. The figure of -0.120' for AA was an estimate of the optical effect of changes in the compression or density of the compact layer for the system aqueous NaF/Hg. It relates to the compression of 17 % derived by Hills and Payne for the surface charge changing from - 10 to +20 pC cm-2. Accordingly AA was computed for a system with compressed film (n = 1.405 d = 4 4 relative to that with an uncompressed film (n = 1.334).The latter is conceptually present though having the same refractive index as the bulk solution one may neglect it for the purposes of computation. It was in this sense that I called the reference state filmless. Therefore the figure for Ad is a true contribution to a real measurement in which the surface charge changes as stated above. Since the magnitude of com- pression is typical of the values derived by Hills and Payne for various electrolytes I conclude that changes in the compact layer may make important contributions to double layer optical effects. The main uncertainty in my calculation lies in estimat- ing the refractive index of the compressed layer.In conclusion most of the optical effects computed in my paper are contributions to the total observable effect of particular features of double layer structure. Individually they are experimentally unrealizable but relate to real systems when summed. Dr. A. T. Kuhn (University of Salford) said Work is in progress under the super- vision of Prof. Orville-Thomas and Dr. A. T. Kuhn at the University of Salford to measure the vibrational frequencies of species adsorbed at platinum electrodes in aqueous solution. Calculations were made with the computer programme prepared at N.P.L. by Dr. M. Stedman to ascertain the required measurement sensitivity to investigate the interaction of infra-red radiation with a partial monolayer of adsorbed species. The results indicated that absorption of radiation by surface species would be low but adequate separation of absorption bands of interest from the background spectrum could be made by observing the reflected radiation by phase-sensitive detection while the electrode potential was modulated by an a.c.wave. The results also gave some indication of the tolerable range of film thicknesses for platinum sputtered on a germanium hemi-cylinder at which penetration of the radiation into the electrolyte would occur while undergoing internal reflection at the platinum/electrolyte boundary. Platinum films have been prepared on germanium substrates with suitable thickness and resistive characteristics. With such films the absorption spectrum of water has been recorded using internal reflection spectro- scopy. GENERAL DISCUSSION 87 Dr.M. A. Genshaw (Elkhart Indiana) said I would with regard to the paper by Stedman suggest a possible method of distinguishing optical changes occurring inside of the metal from those due to changes on the solution side. This is to make kinetic measurements of the optical changes. To stimulate work in this direction I offer the following data which I obtained while at the University of Pennsylvania. A Spectra Physics model 132 He-Ne laser was used as the source in a Rudolph 200-E ellipsometer. The stability of the laser intensity was 3.15 %. The angle of incidence was 65" outside the cell or about 71.5" in the solution. A Motorola MRD-300 phototransistor was used as a photo diode with a operational amplifier with a megohm resistor in the feedback loop to amplify the output (1 V = 1 PA).V (S.C.E.) FIG. 1.-Modulation of A and intensity as a function of potential at 500 Hz and 18" offset. A Hewlett-Packard Wave Analyzer model 302A was used to separate and measure the light modulation. The d.c. amplifier output was 0.15 V/deg. in P (polarizer setting) with the polarizer at 18" from extinction the analyzer at extinction and 0.32 V/deg. at 45". The r.m.s. noise in the output was 5 pV with a d.c. level of 1.3 V whrch is equivalent to 33 pdeg. in P or 66 pdeg. in A. The electrode was modulated with a 100 mV p-p signal and the potential controlled with a Wenking potentiostat. The working electrode was platinum in pH 0.4 1 M H,SO solution. Plots of the potential dependence of the modulation are given in fig. 1 and 2. A strong dependence on potential is noted with a maximum at about 0.5V.The modulation decreases markedly on the oxide-covered surface with the usual hysteresis in oxide reduction being observed. Comparison of fig. 1 and 2 is difficult due to the change in both frequency and offset angle but the apparent increase in the A 88 GENERAL DISCUSSION modulation would indicate that the intensity modulation may be dominant over the A modulation. The frequency dependence is illustrated in fig. 3. Linearity is not observed in linear or full logarithmic plots. This would implicate kinetic control 150 > I- bo 3 120 E C 2 90 -0 4 60 -. 30 120 C .r( 0 V (S.C.E.) FIG. 2.-Modulation of A and intensity as a function of potential at 3000 Hz and 45" offset. of the process causing the modulation. The marked decrease on the oxide-covered surface would suggest that the electron density inside the metal is not the doininant factor in the electromodulation effect as the double-layer capacity and hence the I I00 frequency in Hz FIG.3.-Frequency dependence of electromodulation at 0.6 V and 45" offset. charge introduced into the metal on modulating does not decrease when the oxide phase is formed. I acknowledge the assistance of Mr. 2. Nagy and Mrs. N. El. Nadori in making these measurements. Dr. M. Stedman (N.P.L. Teddington) said I thank McIntyre for pointing out the general relationships between reflectance changes that apply at an angle of GENERAL DISCUSSION 89 incidence of 45”; their simplicity may well make them useful diagnostically. However various considerations enter into the choice of conditions for observing effects due to changes in the metal or in the electrolyte and I believe that computed predictions of the type illustrated in my fig.1 (but based on acceptable models) will still be valuable. The optical constants for Hg used in my calculations were taken from Faber and Smith and appear to be the most reliable available. The refractive index is indeed higher than predicted by Drude theory but the difference is probably The comment that MA theory predicts an ER effect for Hg of sign opposite to that used by myself is interesting. Applied to the theoretical curves in my fig. 3 this would tend to reduce the slope of the total optical effect and improve agreement with experiment. The crucial test would be in the behaviour over a range of wave- length. In reply to Genshaw I agree that time dependence can be a useful diagnostic in analyzing changes due to several concurrent processes.For this reason I used potential stepping in the NaF/Hg experiments to obviate the effects of impurity adsorption. Some double layer processes will be very fast and I doubt if kinetic measurements can provide a complete distinction between changes in the metal and changes on the solution side. Dr. W. J. Plieth (Free University Berlin) said The reflectivity measurements of Br- adsorption shown in fig. 6a and b of the paper of Barrett and Parsons show a steady decrease of reflectivity with increasing potential in the potential range investi- gated. The investigations of Bagotzky Vassiliev Weber and Pirtskhalava and of Balashova and Kazarinov show a constant maximum coverage in the same potential region.The concept of partial charge 5 * has to be considered for the adsorption of Br- ions on platinum. In this case the adsorbed Br- ions are characterized by a partial charge Br”l caused by a partial transfer of electrons to the platinum electrode. The partial charge A- 1 characterizes the probability of finding an electron in the electron system of the adsorbed ion. This partial charge value is combined with an equilibrium distribution function of the solvation energies.’ The partial charge transfer coefficient A increases with the potential. Therefore the properties of the adsorption layer change from the properties of a pure ionic layer to those of a layer of Br atoms. This change should be accompanied by an increase of optical absorption in the adsorption layer.In this case the change in reflectivity in the investigated potential region would not be in contradiction to the otherwise observed constant surface coverage. Barrett and Parsons assume that a change in the properties of the adsorbed film is fast compared with the time constant of the adsorption step. They explain the relative slow change in the reflectivity by the change in the surface coverage. The rearrangement of the solvate molecules on the electrode surface and in the solvation shell of an ion is rate determining for the adsorption of an ion.8 The electronic equilibrium is adjusted during the greater part of the process. The rearrangement of the solvate molecules in the adsorption T. E. Faber and N. V. Smith J. Opt. SOC. Amer. 1968,58,102. N. V. Smith A h .Phys. 1967,16 629. V. S. Bagotzky Yu. B. Vassiliev J. Weber J. N. Pirtskhalava J. Electroanal. Chem. 1970,27 31. N. A. Balashova V. E. Kazarinov Electroanal. Chem. ed. A. J. Bard (Marcel Dekker New York) 1969,3,135. W. Lorenz G. Salit 2. phys. Chem. 1961,218,259; 2. phys. Chem. N.E 1961,29,390. W. J. Plieth K. J. Vetter 2. phys. Chern. N.F. 1968 61 282. K. J. Vetter W. J. Plieth 2. phys. Chem. N.F. 1969 65 181. ti G. Salid W. Lorenz 2. phys. Chern. N.F. 1961,29,408. 90 GENERAL DISCUSSION Iayer is rate determining for a change in the structure of the adsorption layer.' The electronic equilibrium remains adjusted over the whole process. Therefore the rate of a change in the structure and in the charge of the adsorption layer should be of the same order a the rate of the adsorption step.The rate should not be as fast as the authors assume and the decrease in reflectivity between 0 and 1 V is more likely to be due to as change in the structure and in the charge of the adsorption layer than to an increase in surface coverage. It is of interest that optical measurements allow such observations. Dr. B. Cahan (Case Western Reserve University) said Part of the difficulty experienced by Parsons in fitting optical constants to the adsorbed layer of hydrogen on platinum may be resolved by a consideration of some (as yet) unpublished data which I obtained during studies on sputtered Pt-film electrodes. Information on absorbed hydrogen in the metal was obtained by two techniques one involved a bending cantilever beam originally developed far studying the interfacial tension of solid electrodes and the other used conventional voltammetry.The bending beam was a 1 x 10 cm sheet of # 00 cover glass on which was sputtered a thin film of platinum. This substrate is so flexible that even the small forces involved in interfacial changes with potential are sufficient to cause deflections of the free end of the cantilever of 20-30 fringes when measured interferometrically. When FIG. 1.-Two voltammetric sweeps from 0.5 to 0.05 V and back for a thin-film Pt electrode in 1 N HC104. Sweep A starts after a 2-min rest at 0.5 V. Sweep B was the tenth sweep in a con- t inuous series. Sweep speed 25 V/s ; vertical sensitivity 50 ma/cm ; horizontal sensitivity 50 mv/cm. the potential was lowered to the potential at which hydrogen coverage is large the beam was deflected through several thousand fringes.This displacement was rever- sible but 2-3 s were required for complete recovery. Effects of this magnitude could not have been caused by surface forces only and are strong evidence of an absorption into the bulk metal causing severe stresses to be set up in it. The time constant W. J. Plieth Z.pliys. Chem. N.F. 1969 57,:178. GENERAL DISCUSSION 91 associated with the positional recovery agreed well with Bockris’ data for the solubility and diffusion coefficient of hydrogen in platinum. When studying the voltammetry of hydrogen on bulk platinum two current peaks occur before Hz evolution. The position of the more anodic of these two peaks is observed to change as a function of sweep rate and history. With thin films of Pt these peaks are sharpened and it can be seen that shift is only apparent and is caused by a replacement of the first peak with a new one.The figure shows traces from two sweeps from the double-layer region down to hydrogen and back. The only difference between the two is the time the potential was held in the D.L. region. Trace A was run after standing at 0.5 V for 2 min while trace B was the tenth consecutive sweep. While not shown on the figure the crossovers at 350 mV on the cathodic branch and 600mV on the anodic branch are typical isosbestic points which usually implies the gradual replacement of one peak by another too close to it to be completely resolved. The time constant associated with this change- over from A to B is about 1-2 s. Both traces originate from a potential (0.5V) where no hydrogen should be present on the surface.Yet the 50 mV difference between the two first peaks means that the first step in the h.e.r. has been changed by the recent history of the electrode. The implication is that the mechanism has been altered because of a change in the nature of the surface of the Pt. It is unlikely at these potentials that this change has been the result of migration of the platinum surface atoms. It is most likely that the change is induced by the presence of hydrogen below the surface of the metal. Once deposited inside the metal it is then free to diffuse into the bulk but in a thin film rapid saturation occurs. This diffusion of hydrogen into the metal and the resultant stress of the platinum is consistent with the results obtained with the bending beam.Any change in the nature of the platinum surface sufficient to cause a change in the mechanism of the h.e.r. should alter the structure of the surface. This will result in a change of the optical constants of platinum to a depth commensurate with the depth of penetration of the hydrogen. This can explain the inability of Parsons to fit the optical behaviour of the hydrogen layer to any reasonable set of optical constants for a thin layer since the bulk optical constants of the substrate may be changed by the internal hydrogen. The above concept also provides an alternative point of view to that of Bewick regarding the strongly and wealky bound hydrogen. If the H is really inside the surface and can diffuse in or out slowly the slow step may not be the H:dS2H+ but may simply be the slow diffusion of the dissolved hydrogen to the surface.Prof. E. Yeager (Cleveland) said Multiple specular reflection has been used by Dr. T. Takamura Dr. K. Takamura and myself to examine the specific adsorption of halide ions on gold. The experimental arrangement is similar to that used in an earlier paper except that the number of reflections* has been reduced to - 13. The Cl- concentration dependence of the reflectivity is shown in fig. 1 for gold in 0.2 M HC104 in the presence of various concentrations of added C1- at a potential of 0.80V (Ag AgC1) at which voltammetry measurements have shown C1- to be T. Takamura K. Takamura W. Nippe and E. Yeager J. Electrochem. SOC. 1970 117 626. *The use of such a large number of reflections can lead to substantial errors in the absolute value of the reflectivity change and in its wavelenth dependence as pointed out in the earlier paper Cahan Horkans and myself.At a given wavelength however the relative dependence of reflec- follow adsorption and desorption of various species at the electrode 92 10 8 6 - 4 - 2 - 0 - I I I - - A A Y ” - I - I I I Fra. 1 .-ReIat&/e reflectivity change of gold in 0.2 M HC104+ C1- at + 0.8 V against &AgCl at 5600 A. adsorbed. The ordinate represents the relative change in reflectivity between this potential and 0.0 V (Ag AgCl) where specific adsorption of C1- does not occur minus the corresponding change in the absence of Cl- ions in solution. A linear relation- ship exists between the surface concentration of specifically adsorbed halide anion and the reflectivity.Thus the curve fig. 1 corresponds to an adsorption isotherm 12 10 8 $ 6 4 a 4 2 0 20 40 6 0 AQ W/cm2) FIG. 2.-Reflectivity changes against charge for halide adsorption. AQ = Q2- el where Q2 = anodic Q in presence of halide anion and Ql is without halide anion. AR = R2 where Rl R2 are defined analogous to the Q. Ro = highest value on each ( R E ) curve. GENERAL DISCUSSION I I I I I I 93 0 I0 20 30 40 50 time (s) FIG. 3.-Time dependence of reflectivity of AU. Initial potential = -0.3 V; final potential = 3-0.8 V (solid line). Dashed line reverse potential step (time scale shifted +5 s). Electrolyte 0.2 M HC104 ; C1- molarity a 6.3 x ; b 3.1 x lo-’. Wavelength = 560 nm. Insufficient points are presently available at low concentrations to check the mathe- matical form of the isotherm and furthermore the concentrations of Cl- listed for low values may be in some small error because of the possibility of residual C1- in the perchlorate electrolyte.Fig. 2 indicates plots of the reflectivity changes due to the adsorption of the halide ions as a function of their charge as determined from integration of the voltammetry mrves. The concentration of halide anion was held constant as the potential was varied to change the halide adsorption. The linearity of these plots is noted. The 8 6 I 2 0 2 4 6 8 dt s+ FIG. 4.-Reflectivity against dtirne with potential steps. Conditions same as for fig. 3. C1- molarity a 6 . 3 ~ b 3.1 x lo5 ; c 1 . 2 5 ~ 94 GENERAL DISCUSSION increase of slope in going from C1- to I- is in accord with the fact that the I- is more strongly adsorbed and thus interacts more strongly with the surface orbitals e.g.the surface 5d or 6s orbitals. The rate of adsorption of the halide anions has been examined by stepping the potential between values at which halide adsorption does and does not occur. The solid curves in fig. 3 reveal two parts a fast component which is the same as the total change in solutions without C1- ions and a slow component which depends on the C1- concentration with its rate increasing with Cl- concentration. The plot of the slow component of the reflectivity against the square-root of time yields linear plots (fig. 4). When the potential is stepped in the opposite direction resulting in desorption of the halide anion the reflectivity recovers the original value virtually instantaneously within the time resolution of the apparatus (-0.1 s).This indicates that the de- sorption is not diffusion controlled and further that the reflectivity is not caused 6 0 10 r3 (A3) FIG. 5.-Adsorption potential against r3 for anions. by an abnormal concentration of C1- ions in the diffusion layer. Further if the reflectivity were infiuenced to an appreciable extent by refractive index changes in the ionic double layer then the reflectivity should be sufficiently sensitive to detect also the abnormal Cl- concentration in the diffusion layer since the product of the layer thickness times the average refractive index deviation from the bulk value should be comparable for the ionic double layer and the diffusion layer under these conditions. A comparison of the (reflectivity potential) curves with the voltamnictry curves in the presence of halide anions reveals that the change in reflectivity is a maximum GENERAL DISCUSSION 95 at the peak in the voltammetry curve.This is readily understood from the equation for the adsorption current density where R is reflectivity E is potential and q is the charge of the adsorbed ions. Since dR/dq is essentially constant and a linear voltage sweep is used iad is directly pro- portional to dR/dE. The potential of the maximum in the (dR/dE E) curves at high halide concentra- tion with reversible adsorption is some function of the adsorption free energy with the nature of the function dependent on the type of adsorption isotherm. If the anion adsorption depends on the polarizability of the anion then this adsorption potential should be a function of the polarizability and in turn the cube of the ionic radius since the polarizability is proportional to r3.The plot of the adsorption potential against r3 using Pauling radii Since only a small range of radii (1.81-2.16 A) is covered in this plot other functions of Y might prove dso nearly as linear within the precision of the plot. There is relatively little doubt that other factors besides polarizability need to be considered. These results indicate that reflectivity measurements readily lend themselves to the study of halide adsorp- tion on metals such as gold.* is linear (see fig. 5). Prof. B. E. Conway (University of Ottawa) said In relation to the dependence of the optical parameters A and $ on coverage of Pt electrodes by surface oxide referred to in the papers of Parsons Bewick and McIntyre it is of interest to compare the structural details of the form of the electrochemical charging curve for oxide formation examined by Kozlowska and I in very pure solutions with the potential dependence of A.In fig. 1 are shown successive anodic (current (i) potential (V)) profiles at Pt in highly purified 1 M aq. H2S04 taken up to progressively higher anodic termination potentials ; the corresponding cathodic reduction curves are also shown. The curves were obtained potentiodynamically and are hence differential charging curves since the charge for surface oxide formation (or reduction) is given by Q = lidt = ](i/s)dV where s is the rate of potential sweep. In fig. 1 is also shown the integral charge-potential relation expressed in terms of the ratio Q,/& where QH is the charge required to form a monolayer of adsorbed H in the cathodic region (+0.03 to +0.40 V).Several features are of interest (a) the differential (i V ) curves show appreciable structure with two pronounced and one less pronounced peak current being evident. That these are not due to intrinsic heterogeneity of the surface is indicated by the fact that only one peak is observed in cathodic reduction of the surface oxide except when the oxide is formed at high potentials and reduced in a fast sweep.2 However at slower sweeps from less anodic potentials (up to 1.1 V) analysis of the shapes of the reduction curves (fig. 1) indicates some participation of a second species particularly when effects of holding the anodic termination potential at a fixed constant value for various times are examined.Similar structure is observed with single crystal surfaces of Pt. The oxidation behaviour corresponding to the anodic peaks can be represented in terms of successive stages of oxidation corresponding to coverage of Pt sites by * The support of this research by the U.S. Ofice of Naval Research is acknowledged. L. Pauling The Nature ofthe Chemical Bond 3rd ed. (Cornell Univ. Press Ithaca 1960) p. 5 14. D. Gilroy and B. E. Conway Can. J. Chem. 1968,46 875. 96 GENERAL DISCUSSION " OH " then by " 0 " and finally phase oxide formation by place exchange amongst Pt and " 0 " species in the surface beyond ca. 1.1 V. Even at 1.1 V beyond the second main peak (fig. l) it is surprising that a charge of only ca. 1 electron per Pt atom has been passed.The successive stages of Pt oxidation therefore probably involve initially Pt + H,O-+PtOH + Hf + e ; followed by PtOH + P t - d + H+ + e. Since the appearance of peaks normally corresponds to half coverage by the species being electrochemisorbed over the potential range concerned a species (one 0 per 2 Pt sites) competing with OH for coverage of the Pt sites is required to account for the observation of the second peak already at a degree of surface oxidation corres- Pt \Pt ponding to only le per Pt site. *1 1.5 3 0) \ 8 1.0 3.5 0 FIG. 1.-Differential charging curves for oxidation and reduction of Pt electrode surface in cyclic voltammetry up to various anodic termination potentials. Integral oxidation charge curve is also shown in terms of the ratio Qo/QH as a function of anodic potential E (N.H.E.) 1 N H2S04 25°C.(b) The curves of fig. 1 also indicate a progressive irreversibility corresponding to hysteresis between the anodic and cathodic curves. The material initially laid down (first two curves of fig. 1) is evidently reversibly reduced and is probably simply an electrochemisorbed OH radical. However oxidation of the surface up to higher potentials is accompanied by increasing irreversibility. These effects are presumably connected with formation of the surface oxide in growing clumps and the reduction becomes more difficult as the clumps become larger and the fraction of edge sites at N. Sat0 and M. Cohen J. Electrochem. Soc. 1964 111 512. B. E. Conway N. Marincic D. Gilroy and E. J. Rudd J. Electrochem. SOC. 1963 113 1144. GENERAL DISCUSSION 97 which reduction may be more facile becomes smaller (cf.Everett's theory of hysteresis in sorption and desorption at micro-porous substances). The role of edge sites in reduction was suggested as a basis for electrochemical periodicity in reactions at Pt. Fig. 1 shows that reduction from the highest anodic termination potentials is a potential (N.H.E.) FIG. 2.-Changes of ellipsometric parameter A for surface oxidation of Pt referred to horizontal base line (dotted curve) or more correctly sloping bassline (full curve). A is shown as a function of potential (N.H.E.) and surface charge expressed as Qo/QH. 4 = 70.75 ; A = 6328 8 ; sweep rate = 100 mV s-l. relatively slow process and in the cathodic sweeps it is evident that an appreciable excursion to relatively cathodic potentials is required before the current in the cathodic sweep becomes itself cathodic.4 = 70.75' ; A = 6328 8 ; s = 100 mV s-' ; + 0.05 +1.30 V. FIG. 3.-Relation between changes of A in surface oxidation of Pt and differential charging behaviour for surface oxidation from cyclic voltammetry. D. H. Everett Trans. Faruduy SOC. 1954,50 187; 1955,51 1551. J. Wojtowicz N. Marincic and B. E. Conway J. Gem. Phys. 1968,48,4333. s4-4 98 GENERAL DISCUSSION Fig. 2 and 3 shows the relation between changes of A (6A) at Pt and the potential and surface oxidation charge Qo/QH examined by Dr. Laliberte in our laboratory No structure corresponding to fig. 1 is seen in this plot except beyond 1.05-1.1 V (le per Pt) where the (A V ) or (A QJQ,) relation takes a different slope. Also changes of A evidently exactly follow changes of Q determined electrochemically so that there is no basis for distinction of an initial adsorbed “ 0 ” species between 0.8 and 0.95 V from that formed at higher potentials as has been claimed and forcibly argued.2 The only optical distinction and this is a clear one is between the properties of the oxide layer below 1.05 V and above that potential.The state “ 02Pt ” (ix. one 0 per 2 Pt sites) therefore seems to be an electrochemically and physically significant one. These observations and conclusions are consistent with the ARIR results mentioned by Mclntyre and Kolb in this Symposium. Miss M. A. Barrett and Dr. R. Parsons (University of Bristol) (communicated) A study of the coulometric curves in Conway’s discussion remarks and those using both sweeps and stepped potential sequences by Icenhower et aZ.,3 as well as the various optical results suggests to us that the relatively sharp transition at about 1.1 V is typical of sweeps whereas only a slight curvature of the relevant quantities occurs if all oxidation is at a constant potential.For R, the change of slope during potential sweeps is greater than would be predicted by the coulometric curve especially at the longer wavelengths. Thus a plot against coulometrically-determined oxide would show the opposite trend from the delta curve shown by Conway. This confirms a change of optical properties. Another relevant observation i s that oxida- tion at an intermediate potential followed by oxidation at a higher potential yields a smaller dip in R than when the oxidation is carried out entirely at the higher potential. A. K. N. Reddy M. A. Genshaw and J. O’M. Bockris J . Chem. Phys. 1968,48,671. R. Greef J. Chem. Phys. 1969 51 3148 ; cf. M. A. Genshaw and J. O’M. Bockris. J. Cltem. Phys. 1969 51 3149. D. E. Icenhower H. B. Urbach and J. H. Harrison J. Electrochem. SOC. 1970,117,1500.

ISSN:0430-0696    

DOI:10.1039/SF9700400085

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Specular Reflection Spectroscopy of Electrode Surface Films BY J. D. E. MCINTYRE AND D. M. KOLB Bell Telephone Laboratories Incorporated Murray Hill New Jersey 07974 Received 28th September 1970 The optical properties of a series of nonmetallic and metallic surface films of ca monolayer thickness which are adsorbed on metal electrodes have been investigated by means of in situ differential specular reflection spectroscopy over the photon energy range 1.5-5.5 eV. Adsorbed 0 and oxide layers on Pt Au and Ni substrates exhibit intense charge-transfer absorption bands ( a N 105-106 cm-I). The adsorbed H-atom layer on Pt produces an " inverse electroreflectance effects '. The effects of adsorption of monolayer Ag atom films on the oxidation of Pt electrodes were studied optically. The optical properties of very thin Ag films on Pt differ markedly from those of bulk silver and very rapidly with increasing film thickness.Specular reflection spectroscopy is an analytical technique which has been widely employed to investigate the optical properties of strongly absorbing solids such as metals and semiconductors in the energy region above the fundamental absorption edge. It has recently received increased attention as a method of studying in situ the formation and properties of very thin films on surfaces and for detecting inter- mediates and products of heterogeneous chemical and electrochemical reactions. The principles of physical optics on which the method is based are identical to those of the classical ellipsometric method of studying surface films. The two tech- niques differ only in the methods of recording the changes in the polarization state of the light reflected from the surface.In ellipsometric studies the phase change differences A = a,-&, and amplitude attenuation ratios I rp I / I r I = tan $ of light polarized parallel (p) and perpendicular (s) to the plane of incidence are measured in terms of the angular settings of the ellipsometer optical components. Using differential reflection spectroscopy the normalized reflectivity changes (ARIR) and (ARIR), are determined by measurement of the intensity ratios of rays specularly reflected from the filmed and bare surfaces. For surface films whose thickness d is much less than the vacuum wavelength R of the incident radiation the quantity AR/R can be directly related by theory to kinetic parameters of interest such as surface coverage and concentration changes or a perturbation of the dielectric constants of the substrate and/or electrolyte in the double-layer region.Minute intensity changes can be measured rapidly and accurately with modern photodetection systems so that a direct read-out of a pertinent kinetic parameter can be obtained while the experiment is in progress. The method is thus conceptually simpler than ellipsometry and is capable of much faster response than currently available automated ellipsometers. In electrochemical studies for example the electrode potential or current can be rapidly scanned stepped pulsed or modulated while the wavelength is held constant or conversely the wavelength can be scanned coht inuously while the electrode reaction rate is modulated.111 the present communication we present the results of specular reflection studies 011 a series of both non-metallic and metallic layers in the monolayer thickness range which are adsorbed on metal electrode surfaces. 99 100 SPECULAR REFLECTION SPECTROSCOPY THEORY Absolute reflectance measurements are extremely difficult to make when the reflecting surface under study is enclosed in an experimental cell. It is possible to make accurate reflectance ratio measurements however since spectrophotometric errors (cf. Avery,6 Bennett and Bennett 7 and errors due to window reflections electrolyte absorption diffuse scattering and beam defocusing in the cell occur as common factors and cancel. The normalized reflectance change is defined by where R(d) is the reflectance of a three-phase system (ambient film substrate) and R(0) is the reflectance of the corresponding film-free two-phase system.For very thin films (dGA) McIntyre and Aspnes have shown that to first order in d/A the complicated reflectance relations reduce to where is the angle of incidence in the transparent ambient phase (1) with real dielectric constant E~ and refractive index nl and t2 and 8 are the complex dielectric constants of the absorbing thin film and substrate phases respectively. If the surface is fractionally covered it follows that AR/R is linearly proportional to the surface coverage 8 provided that e2 does not vary with 8. It is evident from the form of eqn (2a) and (2b) that the differential reflection spectra of surface films on absorbing substrates are not transmission-li ke but are strongly affected by the optical properties of the substrate and the d/R scaling factor.The film absorption bands themselves are further distorted by anomalous dispersion of the real part of the film refractive index. These effects have been discussed in detail else~here.~~ * The optical properties of thin surface films are characterized by a set of three parameters E; 8; and d where 8; and E; axe the real and imaginary parts of the complex dielectric constant e2 = EJZ-~E;. For very thin surface films the first- order eqn (2u) and (2b) can be solved analytically for E; and .$ if the value of d is known. Since only two quantities (AR/R)s and (AI?/IQ, can be measured spectrophotometrically it is necessary to determine d independently if the film is absorbing.In electrochemical studies d can be estimated from coulometric measure- ments of the charge passed during film formation or removal. EXPERIMENTAL A block diagram of the apparatus employed for single-beam electrochemical- spectroreflectance studies is illustrated schematically in fig. 1. Light from a 150 W xenon arc lamp was double-passed through a grating monochromator (Perkin-Elmer Corp. Norwalk Conn. model E-1). On the second pass the monochromatic radiation was chopped internally at 200 Hz to distinguish it from any d.c. stray radiation which emerged from the exit slit. The exit beam was collimated linearly polarized in a plane perpendicular or parallel to the plane of incidence by means of a Glan-Taylor prism (Karl Lambrecht Corp. Chicago Ill.) and projected through the quartz cell window to strike the test electrode J .D. E . MCINTYRE AND D. M. KOLB 101 surface at an angle of incidence of 45 or 70". The potential of this electrode was controlled relative to that of the reference by a solid-state potentiostat lo and appropriate electronic function generators. The light specularly reflected from the test electrode was monitored by a synchronous phase-sensitive detection system consisting of a photomultiplier (EM1 Electronics Ltd. model 6256B) an FET operational amplifier (Analog Devices Inc. Cambridge Mass. model 149B) connected as a current follower and two lock-in amplifiers (Princeton Applied Research Corp. Princeton N.J. models HR-8 and 122). I 1 AR/R I t REC I x FIG. 1 .-Schematic diagram of electrochemical-spectroreflectance apparatus. L 150 W xenon arc lamp ; M monochromator ; C chopper ; P polarizing prism ; OC optical cell ; W quartz window ; RE reference electrode ; TE test electrode ; CE counter electrode ; S electrolyte solution ; E potentiostat ; DC d.c.bias voltage ; SG voltage sweep generator ; WG waveform generator ; PM photomultiplier tube ; DVC dynode voltage control ; CF current follower ; LIA lock-in amplifier ; RM ratiometer ; REC recorder. In steady-state or low-frequency experiments (e.g. potential steps or slow potential scans at constant wavelength) the photomultiplier dynode voltages were maintained constant by a regdated high-voltage power supply (John Fluke Mfg. Co. Inc. Seattle Wash. model 412B). A single lock-in amplifier (LIA) tuned to the chopping frequency& was used to detect reflectance changes in the range 0-20 %.Zero suppression and a scale expansion of 5X to 20X were employed to obtain maximum detection sensitivity. On occasion an auxiliary beam splitter and photomultiplier were used together with the second LIA (also tuned tofc) to monitor the incident beam intensity. The ratio of the LIA output voltages was monitored electronically to cancel effects of lamp intensity drifts. When electrochemical modulation spectroscopy (EMS) techniques were employed the light reflected from the electrode surface was amplitude-modulated at two frequencies. One LIA was tuned to the frequency fm at which the electrode potential was modulated. The d.c. output voltage from this LIA was proportional to the minute reflectivity changes AR(fm) of the electrode surface. The second LIA was tuned at fc so that its output signal was proportional to the d.c.reflectivity R of the electrode surface. This signal was fed back to a dynode voltage control circuit based on a programmable high-voltage power 102 SPECULAR REFLECTION SPECTROSCOPY supply (Kepco Inc. Flushing N.Y. model ABC 1500M) which maintained the r.m.s. value ipm(fc) of the photomultiplier anode current constant. The ratio AR/R was thus independent of spectral and temporal variations of the source intensity and photodetector response as well as of the optical transfer characteristics of the other system elements. The quantity AR/R could be monitored continuously either at constant A as a function of the bias potential of the test electrode &c or at constant &c as a function of 2. By opti- mizing the signal-to-noise ratio values of AR/R N could be detected using a 1-s time constant.A low-pass filter with 48 db/octave attenuation (Krohn-Hite Corp. Cambridge Mass. model 3202R) was used to reject the large signal at fc from the input of the LIA employed to detect AR(&,). The test electrode consisted of a vacuum-evaporated (Au Ni) or sputtered (Pt) film of metal ca 0.5 pm in thickness deposited on one side of a 3 in. by 1 in. glass or quartz slide. A 200 A undercoating of titanium was employed to provide increased mechanical adherence. The electrode was supported in a vertical plane in a gas-tight 3-in. 0.d. cylindrical Teflon cell and could be rotated horizontally to obtain exact optical alignment. The cell was fitted with interchangeable 22 mm dim. fused quartz windows.Counter and reference electrodes were isolated in Pyrex side-compartments by fritted glass discs and a Teflon stopcock. Stirring was accomplished with a gas bubbler located behind the electrode ; the high-purity gas (Ar or 0,) was pre-saturated by passage through a tower filled with electrolyte. The complete cell assumbly was mounted on an optical rail inside a light-tight housing which was itself mounted on an optical rail of the monochromator. The optical constants of the electrode substrate materials were determined over the photon energy range 0.5-6.2 eV by a Garners-Kronig analysis 11* l2 of normal incidence reflectivity data measured with a recording doubIe-beam spectrophotometer (Cary Instru- ments Monrovia Calif. model 14) equipped with a specular reflectance accessory (Cary Part no.1413). Independent ellipsometric and internal reflection measurements were used to determine the proper extrapolations to be employed for the reflectivity tails outside the accessible energy range. The double-beam spectrophotometer was also used in more recent low-frequency reflectance studies because its freedom from troublesome drift effects and its greater con- venience more than offset its signal-to-noise ratio which was inferior to that of the single- beam system. For oblique incidence (45 and 70") measurements the optical cell was mounted together with two vertical mirrors on a platform in an extended cell compartment (Cary Part no. 1440250) equipped with a Glan-Taylor polarizing prism. Scale expansion and zero suppression were accomplished using a Cary Part no.1490050 transmission slide- wire. A retransmitting potentiometer (Cary Part no. 1090560) was employed for monitor- ing reflectance changes on an external X-Y recorder (Varian ASSOCS. Palo Alto Calif. model F-80). All electrolyte solutions were prepared from reagent-grade chemicals and ultra-high purity water l4 and were used without further purification. RESULTS AND DISCUSSION ADSORBED OXYGEN AND OXIDE LAYERS PLATINUM The reflectance changes of a platinum electrode in both argon- and oxygen- saturated 1.0 M HC104 solutions at 25°C are plotted against electrode potential in fig. 2 for 3000A s- and p-polarized radiation incident at 45"; the potential scan rate was 30 mV s-l. The reflectance of the " bare " platinum surface at 0.4 V was taken as reference. In the adsorbed 0 region (EHZ0.8 V) a hysteresis loop is observed which is similar to that measured by rapid cyclic coulometry.l5- l6 At slower sweep speeds (e.g.3 mV s-l) the formation of the 0 layer i s optically detectable at EH> 0.8 V in agreement with coulometric results the reflectance studies of Koch and the ellipsometric results of Visscher l8 and Greef.lg No sudden onset of a film J . D. E . MCINTYRE AND D. M. KOLB 103 at 0.98 V as reported by Reddy Genshaw and Bockris,20* 21 was observed in this study. As frequently noted during coulometric studies of the 0 layer on platinum,22 the number Qa of coulombs passed during anodic charging exceeded the number Qc consumed during cathodic stripping by as much as 25 %. This effect has variously been ascribed to (i) dissolution of 0 in grain boundaries 23 ; (ii) anodic oxidation of adsorbed impurities 24 ; (iii) parallel cathodic reduction paths yielding H20 and H202 2 5 9 26 ; (iv) cathodic reduction to a " semireduced " state 27; (v) formation of trace amounts of an unidentified soluble species on anodization and a soluble Pt(I1) species on reduction 28 ; (vi) formation of a platinum-oxygen alloy.29 If the surface is initially film-free at EH = 0.4 V before anodization the closure of the (ARIR E ) loop during the reverse scan indicates the surface is returned to its initial state after reduction.Any residual film with a refractive index n2 2 2 would be optically detect- able even if it were a dielectric with extinction coefficient k2 = 0. 2.01 I I I ,' -8.0 I I I I I I I 00 04 08 1.2 1.6 0 EHM FIG. 2.-Variation of normalized differential reflectance with potential for a Pt electrode in 1 .O M HCIOs at 3WA.= 45"; scan rate = 3OmVs-l. - &-saturated solution; 0 a 02- saturated solution. It is also significant that the initial value of the mean square electric field strength of normally incident radiation 30 which penetrates into the platinum metal is ca. 25 % of that of the incident beam. This field is exponentially attenuated by absorp- tion but should be sufficiently large to detect changes of the optical properties of the metal due to oxygen alloy formation in the surface layers. It is clear that the optical properties of such an alloy when formed must be quite different than those of the adsorbed film one monolayer of which can produce a reflectance change as great as 4 % (cf. fig. 3). The curves in fig.2 illustrate that the surface coverage by adsorbed oxygen is independent within the limits of experimental error of whether or not a Faradaic reaction such as O2 reduction occurs on the surface. The ability to monitor adsorbed 104 SPECULAR REFLECTION SPECTROSCOPY 0 or OH coverage rapidly and continuously during the course of an electrochemical reaction should prove of utility in studies of metallic corrosion and electro-organic oxidation reactions. 00 I n o 2.0 3.0 4.0 5.0 6.0 ficl [eYl FIG. 3.-Normal-incidence differential reflection spectra of a Pt electrode in 1.0 M HCIO (Ar- saturated) at a series of electrode potentials. 0 1.2 V ; A 1.4 V; x 1.5 V ; 0 1.8 V. Normal incidence (41 = 0') differential reflection spectra of 0 layers on platinum are plotted in fig.3 as a function of the incident photon energy ku for a series of different film thicknesses. The films were formed by stepping the potential anodically from 0.4V to the indicated levels. Values of AR/R were measured at each wave- length 60-90 s after application of the step as d asymptotically approached its limiting value. Each point represents the average of at least three measurements. In the 2.0-4.0eV energy range the spectra exhibit linear regions. The minima of the reflection spectra shift to lower energies as EH and d increase indicating that a second form of " oxide " is formed at higher potentials. Owing to the influence of the substrate a detailed interpretation of the spectra cannot be made by visual inspection; the results must be analyzed with a computer to obtain the optical constants of the film.The initial linear region is in part due to the d/i2 scaling factor. Such linearity also implies however that the lineshape factor Im[(s - t3)/(q - &)] must remain sensibly constant over this energy range. Differential reflection spectra measured at oblique incidence (45 and 70") exhibit slight curvature in the linear region of the normal incidence spectrum. Such curvature is not predicted by the linear approximation theory (cf. eqn (2a)) and tends to indicate that the optical properties of the film are anisotropic. Fig. 4 illustrates the frequency dependence of computed values of E; and 8; for an 0 layer formed at EH = 1.5 V. Two band maxima in ei corresponding to absorp- tion processes in the thin layer appear at 3.4 and 4.5 eV. A weak band at 2.0 eV is also discernible.The optical constants were calculated on the basis of a two- electron oxidation process and an estimated mean film thickness of 7.5 A. Values of d c 7 A did not yield a continuous set of optical constants over the complete spectral range. For assumed values of d> 7.5 A the computed values of E; and 8; decreased but the energies of the band maxima remained unchanged. Plots of the film absorption coefficient a2 = 47rk2/iZ = 2ne$/n,R exhibit maxima at 3.4kO.l and 4.6 eV. The height of the low-energy peak grows relative to that of the 4.6 eV J . D. E. MCINTYRE AND D. M. KOLB 105 8.0- 6.0 e 4.0 2.0 0.0 peak as the film formation potential is raised thereby causing the shift of the reflection spectra minima to lower energies. The results of previous optical studies of the 0 layer on platinum are sparse and limited to measurements at a single wavelength 5461 (2.27 ev).Visscher l8 has reported 2 = 3.0-1.5i concluding the surface film was Pt(1I) oxide on the basis of comparison with n values for the bulk oxides PtO=H20 (2.6) and RO2*4H20 (1.97). Genshaw 31 concluded the film was probably Pt304 with h2 = 2.8 - 1.4i. The present study yielded fiZ (5461 A) = 2.8-0.7i calculated for PtO. - - - - - - I I I I I The absorption coefficient of the 0 layer on Pt is very high ca. 5 x lo5 cm-’. Such strong absorption indicates the transitions involved cannot be forbidden in first order. Since djdtransitions are not allowed,32 it is probable that the absorption corresponds to the transfer of an electron from an oxygen ion 02- (2p6) to a vacant d level of a nearest-neighbour Pt ion ; no definitive assignment is yet possible however.Further support of the localized nature of the transitions is given by the facts (i) strong absorption occurs at low surface coverages; (ii) AR is linearly proportional to the charge passed during film formation. When C1- is co-adsorbed with 0 on the R surface the magnitude of the reflectance change is decreased particularly in the region of the minima of the ARIA plots and the absorption bands in the near-ultraviolet are shifted to lower energies. GOLD Fig. 5 illustrates the differential reflectance spectrum of the 0 layer formed on gold in Ar-saturated 1.0 M HC104 by stepping the potential anodically from EH = 0.4 to 1.6 V. Distinct minima in the AR/R plots occur at 2.52 3.35 and 4.9 eV. The energy locations of these minima are very close to those of the maxima of the mean-square electromagnetic field strength at the surface of film-free gold <J?Z:)~ = o the values of which were calculated 33 from the measured optical constants of the metal.According to Poynting’s theorem the rate of energy dissipation in the film is proportional to a2(E2) where (E;) is the local field intensity in the film. For very thin films the energy loss is small and ( E i ) N (E&=o. 106 SPECULAR REFLECTION SPECTROSCOPY The frequency dependence oft$ and E; for the 0 layer on gold is shown in fig. 6. The optical constants were calculated for an Au,O film 6.0 A in thickness. Several band maxima in E; are discernible. The fine structure is apparently real and not a result of experimental error.0.0 - 2.0 5 -4.0 a 2 g ti - 6.0 F - 8.0 I I I I I J 1.0 2.0 3-0 4.0 5-0 60 ffo rev] FIG. 5.-Differential reflection spectra = 45") of the 0 layer formed on an Au electrode at EH = 1.6 V in 1 .O M HC104 (Ar-saturated). Yeager and coworkers have proposed that the reflectance change produced by oxide layer formation on gold is due to a shift in the threshold frequency of the 5d-6~ interband transition of the metal. The results of the present work are not consistent with that viewpoint. The optical constants and absorption maxima are FIG. 6.-Frequency dependence of the real and imaginary components of the dielectric constant of the 0 layer on Au in 1.0 M HC104. EH = 1.6 V. characteristic of charge-transfer processes in a semiconducting oxide. In electro- reflectance studies on noble metals McIntyre and Aspnes 34 have shown that the interband component of the dielectric response function of the metal is not affected J .D. E . MCINTYRE AND D. M . KOLB 107 by the high field in the electrical double layer nor is the Fermi level of the metal modulated. The electro-reflectance effect is due rather to a perturbation of the tail of the free electron plasma which tunnels outward from the surface. NICKEL (Current potential) curves measured during cyclic potential scans at 30 mV s-l of a Ni film electrode in Ar-saturated 0.1 M KOH are shown in fig. 7. The lower curve (a) obtained afier a 1 min hold at EH = -0.050 V in the H,-evolution region exhibits a broad peak at 0.17 V during the anodic scan corresponding to the slow formation of a hydrated form of Ni(OH),.The reversible couple at EH = 1.4 V corresponds to the proton-diffusion limited rate 35 of formation and reduction of a higher-valent Ni(II1) oxide nominally NiOOH. The Ni(I1) oxide which is a poor electronic conductor is not removed during the cathodic sweep as shown by the upper curve (b). 1 0.0 0-4 0.8 1.2 1.6 EHM FIG. 7.-Current-potential curves for a Ni electrode in 0.1 M KOH (Ar-saturated). Scan rate = 30 mV s-l. Curve a scanned after 1 min hold at EH = -0.050 V ; curve 6 continuous sweep. The differential reflection spectra = 45") of NiOOH on Ni in 0.1 M KOH illustrated in fig. 8 were obtained by stepping the potential anodically from EH = 0.050V to 1.5 V and scanning the wavelength of the Cary 14 spectrophotometer. Owing to the relatively large reflectance changes adequate sensitivity could be obtained using this technique.The mean film thickness was estimated by coulometry to be 13 A. Application of repetitive potential steps at each wavelength was avoided in this case since it was observed in preliminary experiments that this caused the film thickness to increase continuously due presumably to the effects of surface roughening and the conversion of a-Ni(OH) to P-Ni(OH),. The corresponding values of AR/R measured during reduction of the Ni(OH)2 film to Ni are much smaller (- 1 %) so that the AR/R values measured during oxidation of Ni(OH) closely approximate those which would be measured by oxidation of the bare metal. 108 SPECULAR REFLECTION SPECTROSCOPY Divalent Ni2+ ions are weakly absorbing relative to the trivalent ions Ni3+.36 The spectrum in fig.8 is notable in two respects (i) both negative and positive values of AR/R are observed for this system ; (ii) the reflectance changes are extremely Zarge for a film only 2.3 monolayers thick. 8.0 a n 0.0 2 \ 2 - 8.0 -160 1.0 2.0 3.0 4.0 5.0 6-0 fiw [evl FIG. 8.Differential reflection spectra = 45") of the NiOOH layer formed on a Ni electrode in 0.1 M KOH at EH = 1.5 V. The optical constants of the NiOOH film are shown in fig. 9. Band maxima in 8; occur at 3.0 and 4.1 eV. These bands correspond closely to the charge-transfer absorptions near 3.1 and 4.5 eV observed by McClure 36 and Tippins 37 for Ni3+ ions in corundum. The rising section of the AR/R curve above 5.0 eV is attributable to the tail of the very strong charge-transfer band at 7.0eV observed by Tippins which could not be observed in the present work because of the solvent cut-off.The absolute values of E$ and 8; for NiOOH are extremely large due to the high 40 20 0 P -20 2.0 3-0 4-0 5.0 6.0 h[eV] FIG. 9.-Frequency dependence of the real and imaginary parts of the dielectric constant of NiOOH. l??H = 1.5 v. J . D. E . MCINTYRE AND D. M . KOLB 109 oscillator strengths of the charge-transfer transitions and are in fact greater than those for pure Ni. The negative values of 6; are characteristic of metallic conduction. HYDROGEN ADSORPTION ON PLATINUM In the potential region of H-atom adsorption on platinum (EH = 0.0-0.3 V) there is a small barely discernible reflectance decrease (cf. fig. 2). The resolution attainable in d.c. reflectance experiments (-0.1 %) is too low to permit any details concerning the nature of the adsorbed H film to be obtained.Since the adsorption of H atoms on Pt is fast and reversible the surface converage can be varied rapidly by modulating the electrode potential. EMS techniques can then be employed to enhance the film detection sensitivity by a factor of lo2 to lo3. EH M FIG. lO.-Variation of normalized differential reflectance with potential for a Pt electrode in 1.0 M HC104 (Ar-saturated). A = 3000 A ; +1 = 45" ; p-polarization. Scan rate = 4 mV s-l. The variation of (AR/R)p at 3000 A as a function of potential is shown in fig. 10 for a Pt electrode in Ar-saturated 1.0 M HC104. The electrode potential was slowly scanned at 4mVs-l and simultaneously modulated by a 44mV (pk-pk) square wave at 43 Hz. In the H-adsorption region the signal R-l aR/aE is positive indi- cating that the cathodic deposition of H atoms causes a decrease in the reflectance as observed in the d.c.measurements. The two distinct maxima near EH = 0.1 and 0.18 V are associated with the adsorption of weakly and strongly bonded H atoms. The maximum rate of the Volmer reaction H+ + e +Ha (1) occurs in the range of medium surface coverage where the numbers of occupied and vacant sites for a particular adsorbed form are For this reason the (ARIR EH) plot strongly resembles the variation of the differential capacitance of pt electrodes in the H-adsorption r e g i ~ n . ~ O - ~ ~ 110 SPECULAR REFLECTION SPECTROSCOPY In the double-layer region (EH = 0.4-0.75 V) the signal R-1 dR/dE is negative. Ths is characteristic of the electroreflectance (ER) effect in noble metals 34 for whch Re kb > (q - l) where k, is the interband component of the dielectric response function of the metal.Two mechanisms may be considered to account for the observed positive R-l dR/aE signal in the H-adsorption region (i) a weak charge-transfer absorption process involving excitation of an electron from the species Hd- to a neighbouring Pt'f ion; (ii) a decrease in the free-electron surface charge and conductivity due to the formation of covalent bonds with the adsorbate. The first mechanism seems improbable. The magnitude of the effect and its sign suggests the second mechanism is responsible making it analogous to an " inverse ER effect ". The surface charge on the metal is proportional to the " true " double-layer capacitance measured in the absence of pseudo-capacitance effects.43 Measurements of the optical constants of the adsorbed H-atom layer are currently in progress.SILVER ADSORPTION AND DEPOSITION ON PLATINUM A number of metals (Ag Bi Cd Cu Pb Sn T1) have been reported to form adsorbed layers of ca. monolayer thickness on foreign metal substrates at potentials well positive of the reversible Nernst p ~ t e n t i a l . ~ ~ - ~ ' Fig. 11 illustrates a typical (1,E) curve observed for the deposition of Ag on Pt from a 1.0 M HC104 solution Om2 t -0.q- I I 1 0-4 0.0 I .2 1.6 EH [Vl FIG. 11 .-Current-potential curves for formation and stripping of a one-monolayer Ag deposit on a Pt electrode in 1.0 M HC104. Scan rate = 30 mV s-l ; 1 min hold at 0.4 V. .-- [Agf] = 0; - [Ag+] = 5 . 0 ~ M. 50 pM in AgN03 (EAg+lAg = 0.545 V).On scanning cathodically from 1.5 V at 30 mV s-l the familiar peak corresponding to reduction of the 0 layer is observed at 0.72 V. In the presence of Ag+ ions this peak is broadened due to the " under- potential deposition " sic,51 of Ag atoms. After a 1-2 min hold at 0.4 V a peak is observed in the anodic scan near 0.6 V corresponding to the dissolution of the bulk silver deposit. At 1.15 V a second peak is observed which corresponds to the oxidative desorption of ca. one monolayer of Ag atoms. The inhibiting effect of the adsorbed Ag layer on the reoxidation of the Pt surface is evident from the (ARIR EH) plot in fig. 12 measured at 3200A where bulk silver exhibits a reflectivity minimum. At the starting potential of 1.5 V the surface is completely covered by the 0 layer.On scanning cathodically at 1 mV s-l a depar- J . D. E. MCINTYRE AND D. M. KOLB 111 ture from the normal ([Ag+] = 0) curve occurs near 1.0 V as reduction of the 0 layer commences. As bare Pt sites become vacant Ag atom adsorption commences at a rate jointly controlled by mas transport of Ag+ and the kinetics of PtO red~ction.~' On the reverse scan anodic dissolution of bulk Ag begins at 0.55 V followed by oxidative desorption of the Ag monolayer commencing at 0.9 V. Re-adsorption of 0 on the Pt surface cannot OCCUT until free sites become available. The reflectance rises almost to the value for bare Pt before falling again as the 0 layer reforms. -6.01 I I I I I 0 4 0.8 I -2 1.6 E H [VI FIG. l2.-Variation of normalized differential reflectance with potential for a Pt electrode in 1.0 M HC104 (Ar-saturated).= 0" ; scan rate = 1 mV s-l. -- [Ag+] = 0 ; - [As+] = 5 . o ~ 10-5 M. When bulk silver is plated on a Pt substrate the optical properties of the Ag atomic layers initially deposited differ markedly from those of the bulk metal per se as shown in fig. 13. The reflectance change at 4000& caused by deposition A = 3200A ; drAi 0 2.5 5 7.5 to 125 15 5 4 3 2 a= % I a 1 I I 1 I I FIG. tion 13.-Variation of normalized differential reflectance with cathodic charge passed during deposi- of bulk Ag on a Pt electrode in 1.0 M HC104 (Ar-saturated). EH = 0.4 V; A - 4000 A; = 45". -. experimental - computed. 112 SPECOLAR REFLECTION SPECTROSCOPY of Ag on an initially bare Pt surface at 0.4 V is at first opposite in sign to that pre- dicted by computer calculations made using the optical constants for bulk Ag.52 AR/R exhibits a minimum at a value of Q correspondmg to the deposition of ca.one monolayer of Ag. Values of Q (mC cm-2) were calculated using a roughness factor of 1.3 determined from H-adsorption measurements. The equivalent mean film thickness d shown in the upper scale was computed from the linear approximation theoryY5 using bulk Ag optical constants and adjusting the value of d until the plot of AR/R against d coincided with the linear section of the (ARIR Q) plot. For a Q-value equivalent to one monolayer (ca 0.24 mC cm-2) the corresponding d-value is 1.9 A which is less than the atomic diameter of Ag (2.88 A). This discrepancy may arise in part from experimental error and in part from the choice of bulk optical constants.The values of AR/R for Ag deposition were much less reproducible from one experiment to the next (cf. the different zero-crossing points in fig. 13) than was the case for 0 layer formation. This is to be expected since the optical properties of thin metallic films are due to the collective electronic behaviour of atomic assemblies whereas those of the 0 layer are thought to be due to local electronic transitions (see above). Variations in the initial stages of Ag crystallization may thus account for the observed fluctuations in optical behaviour. The low value of the ratio d/Q suggests the density of the first Ag layers deposited is greater than normal. This may be accounted for by epitaxial deposition on the Pt substrate whose atomic diameters (2.76 A) and crystal structure (b.c.c.) differ from those of Ag (2.88 A f.c.c.).The calculated values of the optical constants (n,k) of these very thin silver films exhibit minima at thicknesses of ca. 4 monolayers. The optical properties of such films differ from those of the bulk metal owing to the differences in polarizability of atoms and submicroscopic particles on a surface,53 the influence of the Maxwell Garnett packing and the decrease of the mean free path of conduction electrons in a thm film These results will be discussed separately. CONCLUSIONS A wealth of new optical techniques is currently appearing to complement the classical ellipsometric method of investigation of interfacial regions. Among these in situ specular reflection spectroscopy offers promise as a method for investigating over a wide spectral range the optical properties of very thin electrode surface films formed in a variety of electrochemical reactions.It is a pleasure to acknowledge helpful discussions with Dr. D. E. Aspnes Dr. M. W. Breiter and Dr. S. Bruckenstein. D. E. Aspnes kindly performed the ellipso- metric measurements. We are also grateful to Mr. W. F. Peck Jr. for technical assistance and Mrs. M. F. Robbins for aid in computer-programming. D. F. A. Koch Nature 1964 202 387. D. F. A. Koch and D. E. Scaife J. Electrochem. Soc. 1966,113,302. J. D. E. McIntyre paper presented at The Electrochemical Society Meeting New York May 1969 (Abstract no. 232). T. Takamura K. Takamura W. Nippe and E. Yeager J. Electrochem. SOC. 1970 117 626. J. D. 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Aspnes Bull. Amer. Phys. SOC. 1970,15,366. 35 D. M. MacArthur J. Electrochem. SOC. 1970 117 422; 1970 117 729. 36 D. S. McClure J. Chem. Phys. 1962 36 2757. 37 H. H. Tippins Phys. Rev. B 1970,1,126. 38 P. D o h and B. Ershler Acta Physicochim. 1940 13,747. 39 A. Eucken and B. Weblus 2. Elektrochem. 1951,55,114. 40 M. Breiter Trans. Symp. Electrode Processes ed. E. Yeager (Wiley New York 19611 p. 307. 41 M. Breiter Trans. Faraday SOC.1966 62,503. 42 M. Breiter J. Electroanal. Chem. 1966 11 157. 43 M. Rosen D. R. Flinn and S. Schuldiner J. Electrochem. Soc. 1969,116,1112. 44 E. Schmidt P. Moser and W. Riesen Helu. chim. Acta 1963,46,2285. 45 E. Schmidt and H. R. Gygax Helv. chim. Acta 1965,48,1584; 1966,49,733 ; 1966,49 1105. 46 E. Schmidt and H. R. Gygax J. Electroanal. Chem. 1966,12,300 ; 1967,13,378 ; 1967,14,126. 47 B. J. Bowles Electrochim. Acta 1965 10 717 731 ; 1970 15 589 737. 48 M. W. Breiter J. Electrochem. SOC. 1967 114 1125. 49 G. W. Tindall and S. Bruckenstein Anal. Chem. 1968,40 1051,1637. 51 G. W. Tindall and S. Bruckenstein Electrochim. Acta. 1971 16,245. 5 2 D. M. Kolb and J. D. E. McIntyre unpublished results. 53 D. W. Berreman J. Opt. Soc. Amer. 1970,60,499. 54 0. S. Heavens Optical Properties of Thin Solids Films (Dover New York 1965) chap. 6 5 5 0. S. Heavens Rep. Prog. Phys. 1960 23 1. 5 6 P. Rouard and P. Bousquet Progress in Optics ed. E Wolf (North-Holland Amsterdan 1965) New York 1969) chap. 6 p. 137. (Pergamon Press New York 1968) chap. 2 p. 4. vol. 2 p. 111. D. C. Johnson D. T. Napp and S . Bruckenstein Electrochim. Acta 1970,15 1493. 1959) vol. 9 p. 399. Muller (Academic Press New York) vol. 9 to be published. G. W. Tindall and S. Bruckenstein J. Electround. Chem. 1969,22,367. p. 155. vol. 4 p. 147.

ISSN:0430-0696    

DOI:10.1039/SF9700400099

出版商:RSC    

年代:1970

数据来源: RSC