Phase Formation in the Underpotential Deposition of Metals BY ALANBEWICK,JOVAN JOVICEVIC AND BRIANTHOMAS Department of Chemistry Southampton University Southampton SO9 5NH Received 22nd August 1977 The structures of the two-dimensional layers formed in the underpotential deposition of lead onto carefully prepared single crystals of copper are shown to depend upon the substrate orientation for (lll) (100) and (110) surfaces. Nucleative growth processes are observed for first order and for higher order phase transitions. Comparison is made with the behaviour observed on silver sub- strates and the differences are ascribed to the change in atomic radius and in electronegativity. Formation of the underpotential layer is shown to be a necessary precursor to overpotential deposition the mechanism of which is markedly dependent on substrate orientation.Investigations of the deposition of metals onto foreign metal substrates at poten- tials positive to the reversible potential for bulk metal deposition i.e. in the under- potential region have proved to be of considerable interest in their own right as well as being of relevance to studies of the initial stages of bulk metal deposition. Until recently underpotential deposition (UPD) was believed to be caused by the formation of a layer of adsorbed atoms1 Subsequent investigations 2-7 using carefully prepared single crystal substrates with a variety of orientations have revealed a range of more complex processes the detailed nature of which varies with the substrate orientation.Coulometric measurements indicate the formation of epitaxial layers and the con- version of these to close-packed layers which are non-registered but which can be distorted by the structure of the underlying substrate. The formation mechanisms of some of these phases appear to be first order processes while others are of second or of higher order. It has also been dem~nstrated~-~ both for cases involving first order transitions and those involving higher order transitions that the deposition of the phase at constant potential requires the formation and growth of two-dimensional nuclei. These recent developments point to a considerable parallelism between the nature of the phases and the phase transitions observed in underpotential layers and those observed in adsorption onto solid surfaces from the gas phase studied by con- ventional methods and by LEED.Although the electrochemical systems are not amenable to direct structural investigation they have the advantage in readily en- abling the dynamics of the phase formation processes to be investigated. This aspect should be particularly valuable in probing the properties of two-dimensional systems and for example the applicability of new theories of melting in two-dimensional systems.s Our detailed measurements on single crystal surfaces have been restricted to a single substrate material silver. Since we expect the underpotential processes to depend not only on the difference in electronegativity between the substrate and the depositing metal,g but also upon the relative sizes of their constituent atoms a factor which controls epitaxy and registration we have extended the work to other substrates.In this paper deposition of lead onto single crystals of copper will be reported together with preliminary results on the correlation between underpotential deposition and overpotential deposition of lead onto silver.1° ALAN BEWICK JOVAN JOVICEVI~: AND BRIAN THOMAS EXPERIMENTAL It has already been pointed out that quantitative reproducible results depend markedly upon careful chemical polishing of the single crystal surfaces. The technique used for the silver electrodes has been described.* In the case of copper single crystals modified chemical polishing procedures had to be developed for each of the orientations employed (1 1 l) (100) and (110); these will be reported elsewhere.ll Copper is also sensitive to minute traces of oxygen; specially rigorous precautions were necessary to ensure that contamination was below a detectable 1evel.l' Precise potential control was essential for some of the experiments; this was ensured using a HiTek Instruments potentiostat type DT2101 and making use of the internal DVM to calibrate the programming waveform generator.All electrode potentials are quoted with respect to the reversible potential for bulk metal deposition in the solution employed. RESULTS AND DISCUSSION UPD OF LEAD ONTO COPPER (i) LINEAR SWEEP VOLTAMMETRY. Typical linear sweep voltammograms for the three substrate orientations are shown in fig.1 2 and 3 for deposition from perchlorate solutions. No essential -4 N fj -2 4 E N 2 \ .-0 4 0 200 400 €/mV vs Pb FIG.1.-Linear sweep voltammogram at 3 mV s-'; Cu(ll1) electrode; electrolyte mol dmW3 Pb(OAc)2+ 0.5 mol dm-3NaCIO + mol dm-3 HC104. 26 PHASE FORMATION IN UNDERPOTENTIAL DEPOSITION OF METALS I I I I I I 0 200 400 f/mV voPb FIG.2.-Linear sweep voltammogram at 5 mV s-'; Cu(100) electrode; electrolyte mol dm-3 Pb(OAc)2+ 0.5 mol dm-3 NaC104 + rnol dm-3 HC104. differences were observed using acetate or nitrate solutions or using lead concentrations of lov3 mol dm-3 and 10-1 mol dm-3 although the peak positions showed a concen-tration dependence of about 15 mV per decade.'l On the (111) surface deposition occurs in a single sharp peak and there is a single stripping peak which is less sharp.The small shoulder following the sharp deposition peak and the corresponding feature preceding the stripping peak appear to be associated with parts of the electrode surface not possessing the (1 11) orientation. This can be demonstrated by gradually reconstructing the copper surface.l' At very low sweep speeds (-1 mV s-l) the deposition peak is very sharp with a width of about 3 mV at the half peak height and even at this sweep speed the separation of the deposition and stripping peaks is -50 mV. These features suggest a sharp phase for- mation process which is probably first ~rder.~?~ Integration of the deposition and stripping peaks gives a charge value of 229 x C cm-2 table 1.Analogy with the results obtained on ~ilver~.~ would suggest a comparison with the charge required for a close-packed non-registered monolayer which is 310 x C cm-2. The difference is much too large to be accommodated by a change in the extent of anion adsorption. There is very good agreement however with the charge required to form the closest packed epitaxial layer fig. 4 as shown in table 1. A factor of 1.1 to allow for roughness and for a difference in anion adsorption gives perfect agreement. On the (100) surface fig. 2 deposition occurs in two steps with a small shoulder preceding a relatively sharp peak. Corresponding features are observed on the stripping sweep. The total charge involved in the deposition or the stripping processes ALAN BEWICK JOVAN JOVICEVIC:AND BRIAN THOMAS -8.--4- N k e E N I 0 .-\ *-0.-4.- I I I I I I I FIG.3.-Linear sweep voltammogram at 10 mV s-’; Cu(ll0) electrode; electroIyte mol dm-3 Pb(OAc)2+ 0.5 mol dmb3NaC10 + mol dm-3 HC104.is shown in table 1. Again it is too low to correspond to the formation of a close packed plane of lead atoms but there is excellent agreement with the charge required to form the epitaxial layer with the structure shown in fig. 5. It should be noted that this epitaxial structure is very close to that of the (100) face on bulk lead. Comparison with the earlier results obtained on silver leads to the conclusion that the shoulder represents initial random adsorption of lead atoms at favourable coordination sites TABLEMEASURED VALUES OF CHARGE c Cm-’) ASSOCIATED WITH FEATURES ON LINEAR SWEEP VOLTAMMOGRAMS FOR THE UPD OF Pb ON Cu COMPARED WITH CALCULATED CHARGE VALUES FOR VARIOUS STRUCTURES.substrate charge associated charge associated charge for a close orientat ion with the deposition with epitaxial mono- packed layer peak layer of fig. 4 or 5 or 6 310 (100) total charge 273 243 310 prepeak only -50 (110) 1st peak 190 178 310 total charge 322 28 PHASE FORMATION IN UNDERPOTENTIAL DEPOSITION OF METALS FIG.4.-The structure of a complete epitaxial layer of Pb on Cu(ll1). and the sharp peak shows the formation of the epitaxial phase layer the phase transition being of higher order.On the (110) surface the deposition and stripping processes give rise to two over- lapping peaks fig. 3. Each of the peaks is relatively sharp. The charge associated with the first peak is approximately 190 x loe6 C cm-’ compared with a theoretical value of 178 x C cm-2 for the close-packed epitaxial layer shown in fig. 6. Once more there is agreement within a factor of 1.1. It should be noted that the epitaxial structure shown in fig. 6 is very similar to that of the (1 10) face on bulk lead. The total charge encompassed by the two peaks is 322 x C cm-’ which is close to the value required to form a layer of closely packed lead atoms. It appears there- cu (100) aixm 1.28 A FIG.5.-The structure of a complete epitaxial layer of Pb on Cu(100).fore that for this orientation the deposition process follows a very similar pattern to that observed using a silver ~ubstrate.~ The initial process is the formation of an epi- taxial adsorbed layer. Before this layer is fully completed it begins to reorganise into a close-packed structure. Although this layer is likely to be distorted by the substrate as was observed on silve~,~ there is no evidence in the present case for the ALAN BEWICK JOVAN JOVI~EVI~ AND BRIAN THOMAS cu (110) FIG.6.-The structure of a complete epitaxial layer of Pb on Cu(l10). deposition of a second layer in the underpotential region before the onset of bulk deposition. In the preceding discussion possible structures for the surface layers have been inferred by the comparison of observed charge values with those calculated for simple structures related to the arrangement of atoms in an undistorted substrate surface.Although considerable relocation of surface atoms and the reconstruction of surface structures has frequently been observed in other studies [e.g. see ref. (12)] it appears to be unnecessary to introduce this additional complication in the present case. (ii) POTENTIAL STEP EXPERIMENTS Considerable information on the mechanism of phase formation can be obtained by the analysis of the current against time transients observed after initiating the phase formation at constant potential using a potential step. In particular the participation of a nucleation process can be distinguished and the growth geometry and the detailed kinetics of the slow step can be found.This technique was applied to the present system by stepping the potential into the deposition region from a rest potential well positive from this. Fig. 7 shows examples of the transients obtained for the (1 11) surface for a series of growth potentials scanning through and beyond the region of the single sharp peak seen on the voltammograms. For all potentials within or beyond this peak the current against time transients show a well-formed peak which is characteristic of crystal growth from two-dimensional nuclei. The initial part of the rising portion of these transients is linear implying that the layer is formed by the two-dimensional growth of nuclei produced just at the start of the potential step.Further analysis confirms this conclusion. Fig. 10 shows a reasonable linear plot for the data from one of the transients according to the well known equation for this growth mechanism :l3 I= zFnMNok2texp (-nM2Nok2t2/p2). * P 30 PHASE FORMATION IN UNDERPOTENTIAL DEPOSITION OF METALS t I ~o-~s FIG.7.-Current against time transients at a Cu(ll1) electrode in lo-' rnol dm-3 Pb(0Ac)' + 0.5 rnol dm-3 NaClO + mol dm-3 HClO in response to a potential step from +390 mV to (1) +130 mV (2) +120 mV (3) +110 mV (4) + 105 mV (5) + 100 mV. Since the observed nucleation is instantaneous and not progressive it must be con- cluded that the single crystal surfaces contain quite high concentrations of defects. One further property of the peaked transients is of special importance.Integration of any of them leads to the same value of charge and it corresponds to the formation of a complete layer of the close-packed epitaxial structure shown in fig. 4. This is true even for those transients obtained at a potential just at the very start of the deposition peak on the voltammogram. The implication is that the phase formation is a true first order process and the finite width of the peak on the voltammogram is due to kinetic effects even at 1 mV s-l. Details are given elsewherel' of the further analysis of these transients to yield other kinetic data. FIG.8.-Current against time transients at a Cu(100) electrode in mol dm-3 Pb(OAc)* + 0.5 rnol dm-3 NaC10 + rnol dm-3 HClO in response to a potential step from +370 to (1) +70 mV (2) +65 mV (3) +60 mV (4)$55 mV (5) 150 mV.The transients obtained for deposition onto the (100) and (1 10) surfaces are shown in fig. 8 and 9 respectively. In these cases also it is apparent that growth proceeds via a nucleative mechanism but the transients have an additional feature a substantial falling component due to adsorption. This is readily understandable for these two cases since the substrate surface affords particularly favourable sites for adsorption. ALAN BEWICK JOVAN JOVICEVIC AND BRIAN THOMAS I I 0 10 2b 30 4’0 t/ ~o-~s + 0.5 FIG.9.-Current against time transients at a Cu(ll0) electrode in mol dm-3 P~(OAC)~ mol dm-3 NaC104 + mol dm-3 HC104 in response to a potential step from +365 mV to (1) +85 mV (2) $75 mV (3) $65 mV (4) +55 mV (5) +45 mV.I 0 I000 2600 3000 4000 5000 15 10-6s* FIG.10.-Plot of log i/t against t2 for transient (5) in fig. 7. After the application of the potential step nucleative growth develops slowly because of its kinetic limitations whereas adsorption can proceed quite rapidly. Thus ad- sorption can dominate at shorter times even though the final stable form of the deposit is a phase layer. For the (100) surface the adsorption sites and the sites of the lead atoms in the epitaxial layer are the same. However it is reasonable to postu-late that adsorption could proceed by random deposition until approximately half of the sites have been filled and the adatonis are still quite widely separated.Further deposition necessitates bringing adjacent atoms into very close proximity fig. 5 and this requires the development of nuclei to overcome the electrostatic repulsion between separate adatoms connected to the surface by strongly polar bonds. PHASES AND PHASE TRANSITIONS SILVER AND COPPER SUBSTRATES COMPARED The general pattern observed for the UPD of lead5 or thallium2 onto the (1 11) or the (100) or the (1 10)surface of silver was that of adsorption at more positive potentials 32 PHASE FORMATION IN UNDERPOTENTIAL DEPOSITION OF METALS up to a full epitaxial monolayer followed at more negative potentials by a nucleative phase transformation process to form a phase layer which was coulometrically equiva- lent to a close-packed crystal plane.The adatoms in the epitaxial layer were in the most favourable adsorption sites and on the (1 11) surface which does not possess particularly favourable sites the layer was never fully developed. On the other two surfaces the layer could be fully formed but depending upon the concentration the phase transformation could start before completion of the epitaxial layer. A close-packed layer of lead or of thallium would not be registered with the silver substrate but there was evidence that the layer was distorted and partially moulded by the sub- strate structure. On all three surfaces thallium formed a second underpotential layer on top of the first close-packed layer; the details of this deposition were still dependent on the substrate orientation. It was observed that lead formed a second layer ody on the (1 10) substrate.The nucleative phase transformation process appeared to be first order on the (1 11) surface and of higher order on the other two. On changing from silver (atomic radius 0.142 nm) to copper (atomic radius 0.128 nm) there is a significant decrease in size and a significant increase in electronegativity. This decrease in size leads to a larger number of special adsorption sites per unit area on copper ;the sizes of lead and thallium are such (0.175 and 0.170 nm respectively) that the same fraction of these sites can be filled on silver and on copper. As a result the lead or thallium atoms in an epitaxial monolayer are considerably closer together on copper (see fig. 4,5 and 6) than on silver and in the former case they form a layer closer in structure to a further layer of the substrate.Thus it might be expected that the adlayer on copper would more easily participate in a band structure extending over substrate and adlayer thus allowing the development of the properties of a well-defined phase. Further deposition beyond that required for the fully formed epitaxial layer is found only with the (1 10) substrate and the second deposition process always starts at a more positive potential than that required for completion of the first. The epitaxial structure shown in fig. 6 shows the lead atoms arranged in parallel chains with a wide spacing between adjacent chains. It might be expected that further material would be accommodated in a second set of chains situated between and a little above the first set.The measured charge is insufficient for the completion of this process and it must be concluded that as on silver it is energetically more favour- able to form a close-packed structure even though this involves loss of detailed registra- tion. The difference in electronegativities implies stronger bonding to copper than to silver and this will be an additional factor favouring the epitaxial structures which are observed to be more dominant on copper. It is interesting that on both copper and on silver the phase transformation process is first order on the (111) surface and it involves the formation of the phase on an almost bare surface. On the other surfaces the phase transformations are of higher order; they involve phase formation on a surface which already has an appreciable coverage by adsorbed material.There appears to be no correlation between the order of the phase transformation and the degree of registration of the phase; on the (1 11) surface of silver the phase is non registered whereas on copper it is fully registered. This is contrary to the trends which have been reported in other work.14 It is of particular interest that a nucleative growth mechanism is observed for the higher order phase transitions. Electrochemical methods can therefore be used to probe the dynamics of such processes. These kinetic and mechanistic data will be a valuable complement to the structural information obtained by LEED and related methods for phase formation on solid surfaces from the gas phase.For a higher order process proceeding at constant potential it is not clear whether the density of the growth centres will vary with size or to what extent the growth rate constants will vary with ALAN BEWICK JOVAN JOVIE6VI6 AND BRIAN THOMAS the size of the patches. Some indications of variations in these quantities has been obtained.6 OVERPOTENTIAL DEPOSITION OF LEAD ON SILVER In inany cases a very low nucleation overpotential is observed for bulk metal depo- sition in those systems for which UPD is known to occur. However the progression from underpotential to overpotential deposition has not been examined. Harrison et aZ.15 did investigate the overpotential deposition of lead on single crystals of silver but their data for UPD suggest that the surface preparation was not as good as can now be achieved; they observed no effects of substrate orientation in the overpotential region.Similar measurements lousing single crystal electrodes prepared in the same way as for the UPD studies have produced rather different results. In all cases even for deposition at very high overpotentials these studies show that the initiation of bulk deposition by three-dimensional nucleation does not occur until the under- potential monolayer has been laid down; the underpotential layer appears to be an essential precursor to normal bulk deposition. The current against time transients in response to a single potential step into the overpotential region fig. 11 12 and 13 2 4 t/s FIG.11.-Current against time transients at an Ag(l11) electrodein 5 x lo-’ mol dm-3 Pb(OAc)2+ 0.5 mol dmb3 HC104.Potential stepped from +300 to 0 mV for 1 s to preform the underpotential layer then to (1) -13.4 mV (2) -14.5 mV (3) -15.4 mV (4) -15.9 mV (5) -16.8 mV. show interesting variations with substrate orientation. Their major characteristics are (a) rising transients are observed at overpotentials in excess of -13 mV and they are very potential-sensitive as observed by other workers;15 (b)the transients level off to a limiting current at longer times those for higher potentials passing through a maximum due to non-steady state planar diffusion as expected for three-dimensional growth; (c) on the (100) and (1 10) surfaces the rise of current is linear with t2initially indicating the three dimensional growth of nuclei formed instantaneously and with the slow step occurring at the surface of the growing centres; (d)on the (1 11) surface the initial current varies as tY2 indicating that mass transport through hemispherical diffu- sion zones to progressively nucleated three dimensional centres is rate-determining.16 It is clear that the deposition mechanism on the (111) surface differs both in the type of nucleation invoIved and in the nature of the slow step from that on the other two surfaces.This difference will almost certainly be due to the structural reIation- ship between the substrate the underpotential layer and the three-dimensional layer. The underpotential layer is a close-packed plane and the preferred orientation of the 34 PHASE FORMATION IN UNDERPOTENTIAL DEPOSITION OF METALS I t 5 10 t/s FIG.12.-Current against time transients at an Ag(100) electrode in 5 x mol dm-3 Pb(OAc)2 + 0.5 mol dm-3 HC104.Potential stepped from +300 to 0 mV for 1 s to preform the underpotential layer then to (1) -14.2 mV (2) -14.4 mV (3) -14.8 mV (4) -15.2 mV (5) -15.5 mV. -0*4- U 0 2 4 t/s FIG.13.-Current against time transients at an Ag(ll0) electrode in 5 x mol dmF3 Pb(OAc)2 + 0.5 mol drn-3 HC104. Potential stepped from + 300 to 0 rnV for 1 s to preform the underpotential layer then to (1) -15 mV (2) -15.5 mV. three-dimensional layer will be expected to be with a high-density plane parallel to the surface.The closest structural relationships therefore will be met by the (111) system; the other two will possess considerable mismatch. In view of this it is not surprising that the lattice growth rate is considerably higher on the (111) surface and that the nucleative process is progressive. E. Schmidt and H. Gygax J. Electroanalyt. Chem. 1966,12,300; W. J. Lorenz H. D. Hermann N. Wuthrich and F. Hilbert J. Electrochenz. Soc. 1974 121 1167. A. Bewick and B. Thomas J. Electroanalyt. Chem. 1975 65,911. A. Bewick and B. Thomas J. Electroanalyt. Chem. 1976,70 239. A. Bewick and B. Thomas J. Electroanalyt. Chem. in press. A. Bewick and B. Thomas J. Electroanalyt. Chem. in press. A. Bewick and B. Thomas in preparation. ’ D. Dickertmann F. D. Koppitz and J. W.Schultze Electrochim Acta 1976,21,967. * J. M. Kosterlitz and D. T. Thouless J. Phys. C Solid State Phys. 1972 5 124 and 1973 6 1 I81 ; R. L. Elgin and D. L. Goodstein Phys. Reu. 1974 A9,2657. D. M. Kolb M. Przasnyski and H. Gerischer J. Electr.onnalyt.CIiefIi. 1974 54 25. lo B. ‘Thomas PA,D. Thesis (University of Southampton 1976). ALAN BEWICK JOVAN JOVICEVI~:AND BRIAN THOMAS A. Bewick and J. JoviCeviC in preparation. l2 R. Riwan C. Guillot and J. Paigne Surface Sci. 1975 47 183. l3 M. Fleischmann and H. R. Thirsk Advances in Electrochemistry and Electrochemical Engineer- ing ed. P. Delahay (Interscience New York 1963) vol. 3. l4 See for example the review by J. G. Dash Films on Solid Surfaces (Academic Press New York 1975). D. J. Astley J.A. Harrison and H. R. Thirsk J. Electroanalyt. Chetn. 1968 19 325; J. A. Harrison J. Electroanalyt. Chem. 1972,36,71; W. Davison J. A. Harrison and J. Thompson Faraday Disc. Chem. SOC.,1973 56 171. l6 M. Fleischmann J. A. Harrison and H. R. Thirsk Trans. Faraday SOC.,1965,61,2742; D. J. Astley J. A. Harrison and H. R. Thirsk Trans. Faraday Soc. 1968 64,192.