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Langmuir 1993,9, 2460-2469
In Situ X-ray Absorption Spectroscopy Studies of Copper Underpotentially Deposited in the Absence and Presence of Chloride on Platinum(11 1) Howell S. Yee and H6ctor D. Abruiia' Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301 Received February 16,1993.I n Final Form: June 23, 199P In situ fluorescence detected X-ray absorption spectroscopy was used to gain insight to the in-plane structure of copper underpotentiallydepositedon a clean and well-ordered Pt(ll1)singlecrystalelectrode in the presence and absence of chloride anions. XANES and EXAF'S data were obtained at a potential of +0.1 V corresponding to a coverage of approximately 0.75 monolayer. The coulometric charge derived from electrochemical measurementa for both systemswas equal in value and, in both cases, less than that required for complete discharge of the electrodepositedcopper. In the absence of chloride, the XANES analysis of the copper ad-layer revealed similarities between metallic copper and the CUZOreference lending support to earlier findings of an incompletely discharged copper ad-layer. The copper-copper bond distance was found to be 2.85 f 0.03 A. In the presence of chloride the bond distance of the copper ad-layer,2.59 f 0.03 A, approached the bulk copper value (2.56 A). In the presence of chlorideno oxygen is present as a backscatterer in the u-polarization. However, oxygen (from either bisulfate or water) is present as a backscatterer in the absence of chloride giving rise to a copper-oxygen bond length of 2.16 f 0.03 A. It appears that the chloride acta as a protective overlayer precluding oxygen (either solvent or electrolyte) adsorption.
Introduction The studies of metal adsorbates and the coadsorption of metals in the submonolayer and monolayer regime on metallic and semiconducting substrates have been very active areas of surface science for the past two decades. As a result of metal adsorption, significant changes in the structural, optical, and electronic properties of substrates have been measured. For instance the adsorbate can cause surface reconstructions, work function changes, charge transfer, change in the dielectric constants, and others.' The coadsorption of metals in the presence of halide ions can profoundly alter not only the properties of the substrate but also the physical and chemical properties of the adlayer as ~ e l l . l - ~ Underpotential deposition (UPD) is a process whereby a metal can be electrodeposited on a foreignmetal substrate a t potentials significantly more positive than that for deposition on the same metal surface. The process of underpotential deposition provides one of the most precise means of controlling coverages from submonolayer to monolayer, and in some cases multilayer coverages, in a quantifiable and reproducible fashion prior to bulk deposition. Since in numerous cases there is a relatively large potential gap that separates monolayer formation from bulk growth, much insight has been obtained in
* Abstract published in Advance ACSAbstracts, August 15,1993.
(1) (a) Zangwill, A. Physics at Surfaces; Cambridge University Press: Cambridge, U.K., 1988. (b) Adamson, A. W. Physical Chemistry of Surfaces; John Wdey: New York, 1982. (c) Somorjai, G. A. Chemistry in Two Dimensions: Surfaces; Cornell University Press: Ithaca, NY, 1981. (d) Hall, R. B., Ellis, A. B., Eds., Chemistry and Structure at Interfaces; VCH Publishers, Inc.: Deerfield Beach, FL, 1986. (2) (a) Kolb,D. M. Aduances inElectrochemistryandElectrochemical Engineering; Gerischer,H., Tobias, C., Eds.;Pergamon: New York, 1978; Vol. 11, p 125. (b) Adzic, R. Aduances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C., Eds.; Pergamon: New York, 1985; Vol. 13, p 159. (3) (a)Xing, X.-K.;Abel, P.; McIntyre, R.; Scherson, D. J. Electroanal. Chem. 1987,216, 261. (b) White, J. H.; Abrufla, H. D. J. E l e c t r o a d . Chem. 1991,300,521. (c) Nichols,R. J.;Beckmann, W.;Meyer,H.;Batina, N.; Kolb, D. M. J. Electroanal. Chem. 1992,330, 381. (d) Batina, N.; Kolb, D. M.; Nichols, R. J. Langmuir 1992,8,2572. (e) Jovicevic, J. N.; Jovic, V. D.; Despic, A. R. Electrochim. Acta 1984,29,1625. (0White, J. H.; Abruda, H. D. J. Phys. Chem. 1990, 94, 894.
0743-7463/93/2409-2460$Q4.00/0
studies of two-dimensional phase formation. Recently, statistically random adsorption of adatoms, different epitaxial superstructures of increasing density up to a compact monolayer, and nonepitaxial 2-D continuous solid phases, have all been found.2 The codepositionof metals from solution in the presence of anions or other surface-active species is important as it is present in numerous processes at the solid/solution interface. Organic additives have been shown to have profound effects on the deposition of metals from both structural and mechanistic v i e ~ p o i n t s . ~With * ~ regards to UPD processes, limited attention has been given to the role of anions in the overall UPD scheme, particularly in the case of single crystal surfaces. It has been recognized that anions can have a strong influence on underpotential deposition, for instance, on the magnitude of the underpotential shifL2s3In addition,the anions stronglyinfluence the structure of the ad-layer, and such structural changes have been observed by several groups. Using a UHV system coupled to an electrochemical cell and a closed sample-transfer, Kolb et al.4a demonstrated that the structure of Cu UPD on Au(ll1) was strongly dependent on the supporting electrolyte. In another ex situ study, ~ a variety of structures present Hubbard et 0 1 . ~reported on the UPD of copper on a Pt(ll1) surface pretreated with iodine. White and Abruiia&have previouslyreported on an in situ surface EXAFS measurement of Cu UPD on Pt(ll1) substrate pretreated with iodine. Most recently, Kolb et al., using LEED and XPS (underUHV conditions) coupled with an electrochemical cell and closed sampletransfer system, reported on the structure of copper UPD on Pt(ll1) in the presence of chloride and bromide.6b (4) (a) Zei, M. S.; Qiao, G.; Lehmpfuhl, G.; Kolb, D. M. Ber. BunsenGes. Phys. Chem. 1987,91,349. (b) Beckmann, H. 0.;Gerisher,H.; Kolb, D. M.; Lehnpfuhl, G. Symp. Faraday SOC.1977,12,51. (c) Stickney, J. L.; h c o , S. D.; Hubbard, A. T. J.Electrochem. SOC.1984,131,260. (d) Hubbard, A. T. Chem. Rev. 1988,88,633, and references therein. (e) hung, L.-W. H.; Gregg, T. W.; Goodman, D. W.Langmuir 1991,7,3205. (5) (a) White, J. H.; Abrufia, H. D. J. Electroanal. Chem. 1989,274, 185. (b) Michaelis, R.; Zei, M. S.;Zhai, R. S.;Kolb, D. M. J. Electroanal. Chem. 1992,339, 299.
0 1993 American Chemical Society
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Studies of Cu UPD on P t ( l l 1 ) Numerous structural tools have been brought to bear on the study of submonolayer and monolayer UPD systems. The earliest studies involved electrochemical and spectroscopic techniques to address questions of the mechanism(s)of monolayer formation as well as structural properties of the UPD layer. Structural characteristics of the UPD layer have been indirectly derived from equilibrium-coverage potential isotherms? Although electrochemical techniques have provided a wealth of information, structuralinferences have been indirect and model dependent. Atomic structural details have been reported using surface sensitive ultrahigh vacuum methods and much data have been obtained with themS4 Various electron spectroscopic techniques have been employed in the structural study of copper UPD on an iodine pretreated Pt(ll1) surface" and most recently in the presence of chloride and bromide.5b It is uncertain what structural changes the UPD adlayer might undergo during the course of transferring the sample from solution to vacuum. Although these ex situ experiments have provided an abundance of information, certain questions remain unanswered and their relation to electrochemicalinterfaces is still a matter of debate. Scanning tunneling (STM) and atomic free (AFM) microscopies have allowed for direct atomic structural measurements in situ on UPD systems. STM and AFM have been employed to study UPD processes of copper on gold and platinum electrodes7a4 and silver and bismuth on gold surface^.^^-^ These studies showed that the UPD process occurred in a well-defined manner and that the ad-layer structures observed were similarto those observed in vacuum. Despite the success and power of STM and AFM, information regarding the solution side of the interface is still lacking. Weakly adsorbed electrolyte species have not been observed (and it is unlikely that they will be with STM or AFM) despite radiotracer studies by Wieckowski and co-workerss that have shown an enhanced adsorption of HSOr during copper UPD on platinum. Recently, with greater access to synchrotron facilities, in situ X-ray spectroscopicand diffractiontechniques have opened a unique window to atomic structural information of UPD systems in a noninvasive manner.+12 Extended X-ray absorption f i e structure (EXAFS),surface EXAFS, and X-ray absorption near edge structure (XANES)have been used to study various UPD systems.1° Information on local structure,atomic environment, and oxidation state of the adsorbed species has been obtained. Such studies have led to the observation of adsorbed oxygen (either from the solvent or the electrolyte) on the UPD adlayer,1° not observed with either STM or AFM, and consistent with partial charge transfer. Specific systems that have been studied include Cu on Au(ll1) and Au(100), Cu on Pt(100), Pt(ll1)-I, and polycrystalline platinum, Ag on Au(lll), P b on Ag(lll), and others.1° In-plane structural measurements of the UPD layer have been carried out ~~~
~
(6) (a) Schultze, J. W.; Dickertmann, D. Surf. Sci. 1976,54,489. (b) Bewick, A.; Thomas,B. J. Electroanal. Chem. 1976, 70,239. (7) (a) Manne, S.; Hansma, P. K.; Maesie, J.; Elinge, V. B.; Gerwirth, A. A. Science 1991,251,183. (b) Sashikata, K.; Furuya, N.; Itaya, K. J. Electroanal. Chem. 1991,316,361. (c) Hachiya, T.; Honbo, H.; Itaya, K. J. Electroanal. Chem. 1991, 315, 275. (d) Nichols, R. J.; Kolb, D. M.; Behm,R. J. J.Electroanal.Chem. 1991,313,109. (e)Chen,C.-H.;Vesecky, S. M.; Gewirth, A. A. J.Am. Chem. Soc. 1992,114,461. (0Chen, C.-H.; Gerwirth, A. A. J. Am. Chem. SOC. 1992,114,5439. (8)Varga, K.; Zelenay, P.; Wieckowski,A. J.Electroanul. Chem. 1992, 330, 453. (9) Abrufia, H. D., Ed.; Electrochemical Interfaces: Modern Techniques for In-Situ Interface Characterization; VCH New York, NY, 1991.
using grazing incident X-ray scattering for specific system8.l' In addition, the use of X-rays in the production of a standing wave field normal to the surface has allowed the electrical double layer to be directly probed.12 In the present paper, we describe our results from an in-plane structural study of the underpotential deposition of copper in the presence and in the absence of chloride onto a clean and well-ordered Pt(ll1) surface. Potentialdependent structural changes were determined in situ by means of XAS in the fluorescence detection mode. The results show marked differences between copper UPD in the presence and absence of chloride with the former giving rise to a more compact copper ad-layer.
Experimental Section Measurementswere conducted at the B-2station of the Cornell HighEnergy Synchrotron Source (CHESS)usingaSi(ll1) double crystal monochromator. The incident beam was collimatedwith a pair of motorized hutch slits and Huber slits juet before the sample. The incident beam intensity was monitored with an ionization chamber with nitrogen as the fill gas. The Cu (Ka) fluorescence was monitored using an energy dispersive Si(Li) detector (Princeton Gamma Tech) in conjunction with a EG&G 673spectroscopyamplifier. A single channel analyzer (Tennelec) served to discriminate against the background and other undesirable contributionsto the signal. A tin lead slit placed directly in front of the detector served to reduce the total counts at the detedor and enhanced the signal to noise ratio. Spectra were collected using the CHESS data acquisition program SPEC. All measurements were taken in-plane (apolarization) at grazing incidence. The angle of the incident beam was approximately 5 mrad. The X-ray absorption spectra were recorded at the Cu K edge, at 8980 eV. The EXAF'S spectra were collected in a 1000-eVwindow from 8800to9800 eV. Within the 1000-eV window three regions were defined, 8800-8900 eV, 8900-9100 eV, and 9100-9800 eV. The step/energy interval for the region 8900-9100 eV was 2 and 4 eV for the others. Data were collected in scans of 20-30 min. Typically 20 scans were collected and averaged to obtain a reasonable signal to noise ratio. The major inflection point in the edge jump was taken to be the position of the edge. Standard analysis of the data consistad of background subtraction, normalization, truncation (10)
(a)Blum,L.;Abrufia,H.D.;White,J.H.;Gordon,J.G.,II;Borges,
G. L.; Samant, M. G.; Melroy, 0. R. J. Chem. Phys. 1986,85,6732. (b) Melroy, 0. R.; Samant, M. G.; Borgee, G. L.; Gordon, J. G., II; Blum, L.; White,J. H.; Albarelli, M. J.; McMillan, M.; A b m , H. D. Langmuir 1988,4,728. (c) White,J. H.; Albarelli, M. J.; Abrufia, H. D.; Blum, L.; Melroy, 0. R.; Samant, M. G.; Borgee, G. L.; Gordon, J. G., I1 J. Phys. Chem. 1988,92,4432. (d) Abrufia, H. D.; White,J. H.; Bommarito, G. M.; Albarelli, M. J.; Acevedo, D.; Bedzyk, M. J. Rev. Sci. Instrum. 1989, 60, 2529. (e) Tourillop, G.; Guay, D.; Tadjeddine, A. J. Electroanul. Chem. 1990,289,263. (0Furtak, T. E.; Wang,L.; Creek,E. A.; Samamta, P.; Hayes, T. M.; Kendall, G.; Li, W.; Liang, G.; Lo, C.-M.Electrochim. Acta 1991, 36, 1869. (g) McBreen, J.; O'Grady, W. E.; Tourillon, G.; Dartyge,E.;Fontaine,A. J.Electroana1. Chem. 1991,307,229. (h)Durand, R.; Faure, €2.; Aberdam, D.; Salem, C.; Tourillon, G.; Guay, D.; Ladoceur, M. Electrochim. Acta 1992, 37, 1977. (i) Tadjeddine, A.; Guay, D.; Ladouceur, M.; Tourillon, G. Phys. Rev. Lett. 1991, 66, 2235. (j) Tadjeddine, A.; Tourillon, G.; Guay, D. Electrochim. Acta 1991,36,1869. (k) Samant, M. G.; Borgee, G. L.; Gordon, J. G., 11; Melroy, 0. R.; Blum, L. J. Am. Chem. SOC. 1987,109, 5970. (11) (a) Toney, M. F.; Melroy, 0. R. Electrochemical Interfaces: Modern Techniques for In-Situ Interface Characterization; Abrufh, H. D., Ed.;VCH, Verlag Chemical: Berlin, 1991. (b) Samant, M. G.; Toney, M. F.; Borgee, G. L.; Blum, L.; Melroy, 0. R. J. Phys. Chem. 1988,92, 220.
(c)Toney,M.F.;Gordon,J.G.;Samant,M.G.;Borges,G.L.;Wieeler,
D. G.; Yee, D.; Sorenaen, L. B. Langmuir 1991,7,796. (d) Toney, M. F.; Gordon, J. G.; Samant, M. G.; Borgee, G. L.; Melroy, 0. R. Phys. Rev. B 1992,45,9362. (e) Chabala, E. D.; Harji, B. H.; Rayment, T.; Archer, M. D. Langmuir 1992,8,2028. (12) (a) Bedzyk, M. J.; Bilderback, D. H.; Bommarito, G. M.; Caffrey, M.; Schildkraut, J. S. Science 1988, 241, 1788. (b) Bedzyk, M. J.; Bommarito,G. M.; Caffrey, M.; Penner, T. L. Science 1990,248,52. (c) Ab-, H. D.; Bommarito, G. M.; Acevedo, D. Science 1990,260,69. (d) Bommarito,G. M.; Acevedo, D.; Rodriguez,J. R.; Abrufia, H. D. X-Rays in Material Analysis II: Novel Applications and Recent Developments; Mills, D. M., Ed.; SPIJZ Proceedings; SPIE: Bellingham, WA, 1991; Vol. 1560, p 166.
Yee and Abruilcr
2462 Langmuir, Vol. 9, No. 9, 1993
of the data set at approximately 20 eV above the edge, conversion of the energy E-scale to wavenumber k-scale, and a k weighing (kl.x(k)) of the data from k = 3 to 13 A-l. Fourier filtering of only the f i t shell contribution to the EXAFS signal was performed. Bond distances were obtained by fitting the oscillatory part of the EXAFS equation to the experimental oscillations with the phase shift for the pair of interest being obtained experimentally from reference compounds. The spectra of Cu foil, CuzO, CuSO4, and CuO, used as reference compounds, were measured in transmission mode. The spectrum of a bulk P t C u alloy was measured in fluorescence mode. The CuCl XAS reference spectrum was generated using a computational code (FEFF) developed by Rehr et al. employing scattering-matrix formulation of the curved-wave multiple-scattering theory.lS Further analysis of the data was carried out with BAN and MFIT (Tolmar Instruments) a commercial EXAFS analysis package. The electrochemical cell, contained inside an aluminum housing, consisted of a cylindrical Teflon body with feedthroughs for electrolytesand electrode connections. A thin layer of solution (of approximately 5 pm) was trapped between the electrode and a 6pm thick polypropylene film,held in place by a Viton O-ring. The film was distended by introducing electrolyte to allow deposition from the bulk solution. Afterward, the thin-layer configuration was reestablished by the removal of electrolyte. Solutions were introduced into the cell with solution bubblers. The aluminum housing was continuouslyflushed with prepurified nitrogen gas, thus ensuring that the effects of oxygen diffusion through the polypropylene would be minimal. The entire cell, with aluminum housing, was mounted on a Huber goniometer stage which allowed for very precise angular motion as well as vertical and horizontal translation of the sample. The cylindrical Teflon body was designed to ensure complete rinsing of the cell and crystal as would be neceseary when coadsorbing anions were deliberately introduced into the cell. However, as a result of such a design, both faces and the edges of the Pt(ll1) crystal were exposed to the electrolyte. Although electrochemical process occurred on the two faces and the edges, the X-ray interacted only with the top face. The electrochemical equipment consisted of a BAS CV-27 potentiostat and voltammograms were recorded on a Soltec X-Y recorder. All potentials were measured against a AgJAgCl reference electrode without regards to the liquid junction. Solutions were prepared using high-purity 18 MQ Millipore Milli-Q water. Aqueous acid solutions were prepared from highpurity (Ultrex) sulfuric acid. Copper ion solutions were prepared by dissolution of CuS04.5HzO (Aldrich Gold Label) in sulfuric acid. The chloride ion solutions were prepared by dissolution of NaCl (Aldrich Gold Label) in sulfuric acid. Prior to introduction into the electrochemical cell, the solutions were degassed with prepurified nitrogen gas which was passed through hydrocarbon and oxygen traps (MG Industries). For the experiments involving chloride, the chemical composition of the electrolytic solution was 0.1 M H804,50 X 10-8 M Cu2+,and 5 X 10-9 M C1-. The low Cu2+ion concentration was necessary'to ensure negligible contribution to the background fluorescence signal. In the thinlayer configuration, we calculated that less than 5% of the absorbing copper atoms are present as ions in the electrolyte. XAS scans were also collected at the rest potential of the system. The Pt(ll1) crystal disk (10 mm diameter and 1.5 mm thickness) used in the XAS experiments was grown from the melt at the Cornell University Materials Preparation Facility. The crystal was oriented by Laue photography and then chemically and metallographically polished. Electrode surface pretreatment involved immersion in hot concentrated nitric acid for 5 h, followed by rinsing with Milli-Q high-purity water. Annealing of the Pt crystal involved placing the Pt disk in a Pt ladle specifically designed for the Pt disk (Micky Roof Designer (13)(a) Rehr, J. J.; Albers, R. C.; Natoli,C. R.;Stern, E.A. Phys. Rev. B 1986,34, 4350. (b) Mustre de Leon, J.; Rehr, J. J.; Fadley, C. S.; Osterwdder, J.; Bullock, E. Physica B 1989, 158,543. (c) Muetre de Leon, J.; Rehr, J. J.; Natoli,C. R.; Fadley, C. S.; Osterwnlder, J. Phys. Rev. B 1989,39,5832. (d) Rehr, J. J.; Albers, R. C. Phys. Reo. B 1990, 41,8139. (e) Mustre, J.; Yacoby, Y.; Stern,E. A.; Rehr, J. J. Phys. Rev. B 1990,42, 10843. (0Rehr, J. J.; Mustre de Leon, J.; Zabinsky, S. I.; Albers, R. C. J. Am.Chem. SOC.1991,113,5135.(g) Mustre de Leon, J.; Rehr, J. J.; Zabinsky, 5. I.; Albers, R. C. Phys. Rev. B 1991,44,4146.
1'olviiIi;il ( V t ~ l l s )
Figure 1. Voltammogram of Pt(ll1) clean and well-ordered surface in XAS cell. The electrolyte was 0.1 M H2SOd. Scan rate, v = 50 mV/s. Reference electrode AgJAgCl. Table I. EXAFS Parametera.
Q (PC/ cm2)
A-B
radius(& NN Au(A2) AE(eV)
chloride
85.81 225.51 311.32' nochloride 309.33
CU-CU
2.59
Cu) Cu-Cu
2.16 2.85
6.7 0.0164 4.2 0.0084 6.2 0.0123
-5.67
15.00 2.26
Bond length, R, effective coordination number NN,DebyeWaller factor Au, and energy variation AE (accuracy: R, 10.03 A; NN,120%). Electrochemicalvalues: Ew = +0.1 V,calculatedcharge Q (&lo% ;1 ML = 420 pC/cm2). The total charge of the two peaks. a
Goldsmith). The Pt crystal was annealed under a CK-02 flame for 6 min followed by cooling under a stream of prepurified nitrogen gas for 2 min before immersion into a degassed solution of 0.1 M HzSO4. This procedure avoided the problem of strains induced by the rapid quenching and produced a clean and wellordered Pt(ll1) surface as evident by the voltammogram in 0.1 M HzSO4 shown in Figure 1. The Pt(ll1) crystal was then transferred to the electrochemicalcell under the protective cover of a 'ladle full" of electrolyte (0.1 M HzSO4). Electrical contact to the disk was made with an embedded Pt wire. The Pt(ll1) crystal was secured by three pressure points in the Teflon body. The experimental setup ensured the integrity of the clean and well-ordered Pt(ll1) surface for extended time periods. Thus, the crystal needed to be annealed approximately once during any24-hperiod. Cyclicvoltammetricscans of thePt(ll1) crystal in 0.1 M H2SO4 were taken every 2 h to verify the presence of a clean, well-ordered Pt(ll1) surface. Underpotential deposition of copper was carried out from the dilute (50 X 10-8 M) aqueous solution of Cu2+(1 X 10-9 M C1when appropriate) in 0.1 M H2SO4. The potential was scanned from the rest potential (+0.65 V) in the negative direction at a rate of 2 mV/s. For XAS measurements the potential was then held at +0.1 V where XAS measurements were made. The copper UPD layer was stripped and redeposited after collecting two scans (1 h period). Before deposition of copper, the cell was rinsed thoroughly with 0.1 M HzSOI. This procedure ensured that time-dependent changes of the adlayer would not be measured and, if present, would be detected electrochemically. The charge corresponding to the stripping of the electrodeposited copper was used in calculating the coverage values which are presented in Table I. In these calculations it was assumed that the copper had an electrosorption valency of two; that is complete discharge of copper. In addition, no correction was made for the difference in the potential of zero charge as this would be a small effect on the calculated charge.
Studies of Cu UPD on P t ( l l 1 )
Results Electrochemistry. UPD refers to the electrodeposition of metal monolayer(s) on a foreign metal substrate at potentials significantly more positive than that for deposition on the same metal surface. This allows for a precise control of the surface coverage prior to the onset of bulk deposition a t which point the deposition is driven by the Nernst equilibrium. Structural studies within the UPD regime can provide a wealth of information on nucleation processes, coverage dependent structure, and electronic properties of a metallic adlayer. In order to establish the cleanliness and order of the Pt(ll1) electrode, a cyclic voltammogram was run in 0.1 M H2SO4. The voltammetry in Figure 1 displays the features that are associated with a clean, well-ordered Pt(111)surface otherwise known as the “butterfly”pattern.14 The voltammetric features at potentials below 0.0 V are attributed to the edges of the crystal. Despite repeated stripping, underpotential deposition of copper, and rinsing with 0.1 M H2S04, the surface of the Pt(ll1) crystal remained clean and well-ordered for extendedtime periods as ascertained from voltammetric measurements. Voltammetries for the UPD of copper at 50 X 10-6 M in the absence and in the presence of chloride at 1X 10-3 M are shown in parts a and b of Figure 2, respectively. Several differences are to be noted. The most notable difference in the voltammetries is the appearance of an additional peak (at about +0.30 V) in the presence of chloride. There is also a shift in the stripping potential of about 30mV to more positive values. In addition, unlike the voltammetry in the absence of chloride where the cathodic features were not very evident, the voltammetry in the presence of chloride gave rise to two somewhat broad peaks. It is clear that the presence of chloride greatly influences the deposition process as demonstrated by the voltammetric responses. Such behavior has been previously documented for chloride, other halides, and molecular adsorbate^.^-^^*^ Most recently, we have also investigated the effect of chloride concentration. At a copper solution Concentration of 50 p M we find that in l.OmM chloride, two well-resolved and stable voltammetric peaks are observed (Figure 3a). These features are similar to those reported previo~hly.~~ At lower chloride concentration (e.g. 50 p M ) the peaks are broadened significantly and the less positive peak (which has been ascribed to interaction with chloride) is greatly suppressed (Figure 3b). In addition the less positive peak is very unstable, so that it is present only on a transient basis. This is depicted in Figure 3c which shows the voltammetric scan for copper UPD on Pt(ll1) when the concentration of both copper and chloride was 50 rM and where the cathodic scan was stopped at +0.1 V for 5 min and then an anodic sweep was initiated. The absence of the second stripping peak is immediately apparent. Throughout this process the Coulombic charge remained constant. This points to kinetic effects in the deposition of copper and that the chloride ions play a key role in the process. Kolb et al. recently reported on a combined electrochemicalUHV study of copper UPD on Pt(ll1) in the presence and absence of chloride and bromide.6b Their findings show that the halides form densely packed incommensurate (4 X 4) and (7 X 7) structures, respectively, on the copper monolayer and that they then form a more open structure at more negative potentials. In (14) (a) Clavilier,J.J.ElectroanaZ.Chem.1980,107,211. (b)Aberdam, D.; Durand, R.; Faure, R.; E l - O m , F. Surf. Sci. 1986, 171, 303. (c) Jaaf-Golze, K. A.; Kolb, D. M.; Scherson, D. J. EZectroanul. Chem. 1986, 200,353.
Langmuir, Vol. 9, No. 9, 1993 2463
-
0.0
0.2
0.4
0.G
I’otciilial (Volls)
0.0
0.2
0.4
0.G
Polenllal Wolls)
Figure 2. Voltammogram at Pt(ll1) electrodes in contact with a 50 pM Cu2++ 0.1 M HzSO4 solution in the preaence and the absence of 1W M C1-. Scan rate,v = 2 mV/s. Reference electrode “1. (a) Cyclic voltammogram of Cu UPD in the absence of chloride. (b)Cyclic voltammogram of Cu UPD in the presence of chloride.
Figure 3. Voltammogram of Cu UPD in the presenceof varying concentrationof chloride: scan rate, v = 2 mV/e; electrolyte,0.1 M HzSO4 and 50 p M Cu2+. Reference electrode Ag/AgCl. (a) 1W M NaCl. (b) 50 pM NaC1. (c) Potential held at + 0.1 V for 5 min in 50 p M NaC1.
addition, the most negative voltammetric feature ie not due to copper deposition (stripping) but instead to halide adsorption/ d e ~ o r p t i o n . ~ ~
2464 Langmuir, Vol. 9, No. 9, 1993 An interesting point is the corresponding charges of the anodic peaks in the absence and presence of chloride. In the presence of chloride (Figure 2b) the s u m of the charges under the two peaks is equal to the charge measured under the singular peak for Cu UPD (Figure 2a, Table I) in the absence of chloride. As noted by several chloride does not adsorb to platinum as strongly as iodide as evident by the ability to rinse the chloride from the platinum surface. Unlike the coadsorption studies with iodide4cJQ2dwhere the platinum surface was pretreated with iodine prior to exposure to the copper electrolyte solution, the underpotential deposition of copper was conducted in the presence of chloride ions in solution. Thus this represents a true competitive adsorption process. It should be noted that the design of the XAS cell allowed for completerinsing of all chloride from the system so that the voltammetry of a clean and well-ordered Pt(ll1) electrode in 0.1 M HzS04 was restored. XANES Information. Useful qualitative information can be extracted from examination of the threshold position and near edge features of the spectrum as they are very sensitive to the electronic state and environment of the absorbing atom. The near edge features of three of the reference compoundsemployed, copper foil, Cu20, and the Pt&u alloy bear a strong similarity to those for the in situ Pt(lll)/Cu XAS data. Figure 4a presents data for the reference compounds and the UPD of copper in the absence of chloride as indicated. For all data sets the threshold position (0 eV) was consistently taken to be the inflection point (prepeak) along the Cu K edge. The edge profile of copper foil (Figure 4a) shows a prepeak feature and two additional peaks at approximately 10and 20eV beyond threshold. For Cu20 (i.e. Cu+)(Figure 4a) the edge profile displays a prepeak which is slightly enhanced, as well as additional peaks at about 10 and 27 eV with the latter being somewhat broad. In the Pt&u alloy the prepeak feature is greatly diminished in comparison to metallic copper and Cu20. The alloy also has strong edge features at 7 and 30 eV with a noticeable shoulder at 12 eV.
ForCuUPDonPt(ll1)intheabsenceofchloride (Figure 4a) there are strong similarities with near edge features of the Pt3Cu alloy. From this observation we can qualitatively state that the underpotentially deposited copper is in an environment similar to that of copper in the PtaCu alloy. It can be surmised that the copper adatoms might be located in either 3-fold hollow or bridge sites (but not atop sites) because of the strong Cu-Pt interaction. However, a definitive assignment of the copper position will not be possible without XAS data in the u-polarization. The X M S spectrumin the absence of chloride also bears a strong resemblance to the CUZO(Cu+) reference compound with the energy separation of the two peaks in the in situ measurement being approximately the same as in CunO. The XANES features for Cu UPD on Pt(ll1) in the presence of chloride (Pt(lll)/Cu/Cl)(Figure4b) bear some similarity to both the metallic copper and the Pt&u alloy references. However similarities to the spectrum of CuzO (i.e. Cu+)are not as evident as in the previous case. The prepeak feature from the in situ measurement is not as pronounced as in either the metallic copper or Cu20 reference but looks quite similar to the Pt&u alloy. In the presence of chloride, a small peak is present at 15 eV which is slightly positive of a similar peak in the Pt&u XANES spectrum and slightly negative of a similar peak found in metallic copper.
Yee and AbruAa P t ( 1 1 l ) / C u 0 1V XANES 201
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Studies of Cu UPD on P t ( l l 1 ) 40, 35
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Figure 5. XANES comparison of 50 &f Cu UPD on a clean and well-ordered Pt(ll1) in the (a) absence and in the (b) presence of 10-9 M chloride. nearly the same. In the absence of chloride, there is significant evidence that suggests that the oxidation state of the copper adlayer is somewhere between 0 and +l. As noted previously, the measured charges are all lower than would be expected for a fully discharged monolayer consistent with a residual charge on the copper ad-layer. Thus, the XANES observations and electrochemical charge data in the absence of chloride are consistent with a copper ad-layer that is not fully discharged;that is there is partial charge transfer. Since the coulometric charge is virtually the same in both cases, a similar statement can also be made in the presence of chloride. It is also clear that the presence of chloride has modified the electronic environment of the copper ad-layer. EXAFS Information. Parts a and b of Figure 6 compare the E W S spectra for the underpotential deposition of copper in the absence and in the presence of chloride, respectively. In the presence of chloride, the near edge prepeak feature is no longer present or it is at best barely discernible. However, the absence of a prepeak in the presence of chloride does not necessarily imply that the deposited copper has no metallic character. Another noticeable difference is the appearance of a double peak a t approximately 8990 eV for Cu UPD in the presence of chloride. In the absence of chloride, a slight shoulder is visible, which becomes more defined in the presence of chloride. More detailed structural information about Cu UPD on Pt(ll1)in the presence and absence of chloride can be obtained by examining the background subtracted data and the Fourier transformed and filtered data of the EXAFS spectra. All of the data sets as well as data for the reference compounds were k = 1weighted. In all of the radial distribution functions (not phase corrected) the predominant peaks appear between 1.5 and 3 A which correspond to actual distance of about 2 to 3.5 A.
8 75
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Energy (keV)
Figure 6. In situ, X-ray absorption spectra of 60 fiM Cu2+UPD on a clean and well-ordered Pt(ll1) surface in the (a) absence and (b) presence of 1FM C1-. Potential held at +0.1 V; scan rate, Y = 2 mV/s; Q T =~ 309 ~ pC/cm2; 8cu = 0.73.
Radial Distribution and k1Weighted Data. Figure 7 shows the radial distribution of the two in situ measurements and the copper foil. Several observations can be made regarding the radial distributions and the k weighted (kl-x(k)) data (over the range of k = 3-13 A-l). The radial distributions of the two in situ measurements show numerous unresolved peaks throughout the entire region. In the region of 1.5-3 A where one would expect to find the first copper-coppernearest neighbor distances, the peak form for the system in the absence of chloride (Figure 7b) is rather complex. The shells are not isolated and they often interfere with one another. Within this particular region there can be several different atomic speciescomprising this particular coordination shell. With chloride present (Figure 7c) a single (though somewhat broad) peak is present at 2.3 A, unlike the doublet present around 2.3 A in the absence of chloride. In the presence of chloride the main peak in radial distribution is very close in position to the main peak in the metallic copper. This would imply that the copper-copper bond distance should be comparable. The presence of chloride has also changed the morphology of the peaks centered around 3 and 4 A, respectively. Recent findings by Abruira et a1.12dcan help in understanding some of the complex features in the radial distribution of the in situ measurement in the absence of chloride. Using X-ray standing waves to study copper UPD on an iodine treated platinum surface (from a Pt/C multilayer),they observed an accumulation of copper near the surface of the platinum electrode. The amount of accumulatedcopper (up to about a monolayer) is potential dependent and in all cases is present in close proximity to the surface (less than 40 A). This layer of copper is likely present in a disordered manner and, in addition, ita interaction with the surface is relatively weak so that it
Yee and Abrufiu
Langmuir, Vol. 9, No. 9, 1993 Ka d i a 1 Di s t i+ib u t i on
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does not survive rinsing with supporting electrolyte. If we assume that a comparable amount of accumulated copper is also present in our system, then it may give rise to a %mearing* effect in the radial distribution because of its disordered nature. This could, in turn, cause a shift in the positions of the peaks in the radial distribution. In addition, this disordered layer could also give rise to unrealistic Debye-Waller factors. From the geometry of the experiment (a-polarization with copper as the absorber atom) and the limited number of species (copper, platinum, oxygen, and chloride) that can serve as backscatterers, it is possible to identify the chemical species that are contributing to the complex features in the radial distribution. For example,for oxygen to be considered as one of the backscatterers would imply that they sit in bridge-sites or 3-fold hollow sites with respect to the copper ad-layer. Another, although less likely, possibility for oxygen backscattering would place it directly in contact with the platinum substrate. Chloride backscattering would place the chloride in environments similar to oxygen. Copper backscatterers would place the coppers either in a 3-Dcluster arrangement or as an adlattice covering the surface. Platinum backscattering would mean that the copper absorber resides in either a bridged-site or a 3-fold-hollow site with respect to the substrate. Keeping these points in mind, we present our findings of the structure of the copper UPD in the presence and in the absence of chloride. Cu Deposited on Pt(ll1) in the Presence of Chloride. We begin with results for the underpotential deposition of copper obtained in the presence of chloride. Assuming complete discharge of the copper ions the measured charge represents a coverage of approximately 0.76 of a monolayer of copper on the platinum surface. Figure 8a presents the magnitude of the Fourier transform obtained from the k weighted (kl.x(k)) data in the range from k = 3 to 11 A-l. In Figure 8b, the open circles
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superimposed on the raw EXAFS (full circles) are the inverse transform of the two peaks in the RDF obtained by using a filtering function (see Figure 8a) to isolate the range between 1.6 and 2.8 A. Both the frequency and amplitude of the filtered transform fit the raw E M S well over the entire spectrum. The shgle peak at approximately2.3 A (not correded for phase shift) can be assigned to a copper-copper absorber-backscatterer pair. The peak is somewhat broader than the correspondingpeak in the metallic copper reference (Figure 7a,c). Though a single peak is present, the width might suggest the presence of other chemical species within this coordination shell. However, fitting with only copper-copper as the absorber-backscatterer proved to be more than adequate; thus, there was no need to perform a multiple component fit. The calculated copper-copper bond length was found to be 2.69 i 0.03 A, which indicates that the copper ad-layer is incommensurate with the platinum substrate. This bond distance is very close to the metallic copper bond length of 2.56 A. In the presence of chloride, it appears that the platinum substrate has little influence in ordering the copper adlattice. It is quite probable that the chloride resides on atop sites, since we observe no chloride backscattering. In addition no oxygen backscattering was observed, which would mean that either there are no oxygen near neighbors or that if present the oxygens also occupy atop sites. Since measurementswere carried out only in the u-polarization, we cannot unambiguously determine the position of the copper with respect to the platinum surface. The fact that scattering by platinum is observed rules out atop sites. However, distinguishing between bridge or 3-foldhollow sites is more difficult. In this case, measurements
Studies of Cu UPD on P t ( l l 1 )
Langmuir, Vol. 9, No. 9, 1993 2467
were obtained. A copper-copper distance of 2.22 A is much shorter than the bulk copper-copper distance of 2.56 A and would only be consistent with the formation of a 3-D cluster such that the 2.22 A represent a projected distance. In such an arrangement, the first copper layer would reside at an angle of approximately 30' with respect to the substrate. 3-D cluster formation of copper underpotentially deposited on electrodispersed platinum has been proposed by Arvia and co-workers15 and Furuya and M ~ t o o . ' ~Measurements in the u-polarization would provide information about formation of 3-D clusters as well as allow the unambiguous assignment of the copper adatom with respect to the platinum substrate. Within the uncertainty of the measurement (i0.03 A) the copper-copper distance (2.84 A) obtained from the fit using copper as the first and second nearest neighbor is essentially the same as that (2.85 A) obtained from the fit using oxygen and copper as backscatterers. We thus conclude that the peak at 2.84 A (second peak) is due to copper near neighbors.
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with a-polarization (electric field vector normal to the surface) would allow an unambiguous determination of the position of the copper ad-layer relative to the platinum surface. Such measurements are currently in progress. CuDepositedonPt(ll1) in the AbsenceofChloride. We next consider data in the absence of chloride. Figure 9a shows the magnitude of the Fourier transform whereas Figure 9b depicts the k weighted (kl.x(k)) data over the range of k = 3 to 12 A-1 (full circles), the back Fourier transform (open circles), and the amplitude function envelope (open triangles). A noticeable difference exists between the radial distributions in the presence and in the absence of chloride. In the presence of chloride there is only one peak around 2 A, whereas in the absence of chloride there is a doublet. This suggest that the chloride might act as a protective overlayer against oxygen (vide infra). Two factors could give rise to the unresolved doublet. One possibility is that several different atomic species constitute a coordination shell or alternatively that there are two coordination shells that are not totally resolved. The unresolved doublet was fitted using copper and oxygen as backscatterers and the bond distances obtained from the fits were 2.16 and 2.85 A for Cu-0 and Cu-Cu, respectively. The presence of oxygen q a backscatterer would indicate that the oxygen occupies sites other than atop sites as has been previously reported for other UPD systems.lOa~ef.i~ The copper-copper bond distance of 2.85 A is significantly larger than that found in the presence of chloride (2.59 A). It was also found that a model with copper as both the f i i t and second nearest neighbors could be employed. For such a model, distances of 2.22 and 2.84A, respectively,
Discussion From the electrochemicalmeasurementsof copper UPD onPt(ll1) inthe presenceof 1X lVMchloride,asplitting effect is seen most clearly in the anodic scan. As expected a similar splitting could not be readily seen in the deposition due, in part, to the low (50 pM) solution concentration of copper. The total charge (sum of the two anodic peaks) measured in the presence of chloride was virtually the same as that obtained in the absence of chloride. We ascribe the less positive peak to copperchloride adsorptionJdesorption interactions, which is in agreement with the recent results of Kolb et aLSb The reason for the similar charges between the two systems (absence vs presence of chloride) is somewhat puzzling. It is quite possible that in the absence of chloride the charge from the anodic peak also includes copper-bisulfate adsorptionJdesorption interactions. In this case, due to a weak interaction, the difference in potentials would be significantly less. For the system in the presence of chloride, the question remains as to whether the presence of chloride has modified the platinum surface or if it influences the copper ad-layer formation. At this time we speculate that the chloride interacts mostly with the copper ad-layer. As we have shown in Figure 3, the splitting of the stripping peaks is strongly dependent on the chloride concentration and, at low chloride concentration, there is a clear time dependenceto the response, suggesting a slow transformation. Iodine as a coadsorbing species in the UPD process has also been studied by a number of group~.~ The ~~ main J ~ difference between these halides is their strength of adsorption to the platinum Substrate. The bond between platinum and iodine is strong enough to resist rinsing with supporting electrolyte whereas chloride is not. The XANES analysis of the copper-chloride UPD data suggest that the electronic state of the copper ad-layer is close to bulk copper and the Pt3Cu alloy. On the other hand there is not much resemblance to the CulO (i.e. Cu+) reference compound. Although the XANES analysis would indicate that in the presence of chloride the copper ad-layer is fully discharged the electrochemical charge measurements indicate otherwise. We base this on the fact that the amount of charge is virtually the same in the (16)Margheritis, D.; Salvarezza, R. C.; Giordano, M. C.; Arvia, A. J. J. Electroaml. Chem. 1987,229, 327. (16)Furuya, N.;M o b , S. J. Electroanul. Chem. 1976, 72, 166.
2468 Langmuir, Vol. 9, No. 9,1993 presence and absence of chloride and in both cases this is less than the charge required for complete discharge of a monolayer of copper. l n the absence of chloride, both the electrochemical and X-ray data suggest that the copper adatoms are not fully discharged (i.e. partial charge transfer). Similar observations have been reported for copper on goldlbJj and copper on platin~m.~~J@J’ It also appears that the copper atoms might be in nonequivalent sites. The shape of the near edge features is likely due to a superpositionof various profiles, giving rise to a general similarity with the Cu+ edge. From the electrochemical data (Table I), the potential at which one monolayer of copper should be present is inconsistent with the measured charge. Goodman and co-workers” have reported a similar discrepancy for the same system in the UHV environment and also concluded that the copper ad-layer is not fully discharged. In the present study, for copper UPD in the absence of chloride we see strong scattering by oxygen and we obtain a copper-oxygen distance of 2.16 A. The fact that scattering by oxygen is observed implies that there is a projection of the Cu-0 bond in the plane, thus excluding atop sites. The oxygens would then occupy either bridge or 3-fold hollow sites. In earlier studies of the underpotential deposition of copper on gold(ll1) it was found that oxygen occupied atop sites.loa Recent findings of Durand et al.loh and Furtak et al.lof on the underpotential deposition of copper on Pt(100) and polycrystalline platinum, respectively, showed strong evidenceof in-plane oxygen backscattering. In the work by Tadjeddineet al.lNJ on copper UPD on Au(lll), a pronounced peak present at approximately 1.9 A was attributed to oxygen backscattering. The data strongly indicate that oxygen is a backscatterer, with the source of oxygen being either the solvent (water) or the electrolyte (0.1 M HzS04). Thus, there is clear precedent for oxygens to occupy positions other than atop sites. Wieckowski and co-workers8J7 and othersl8JS have shown that on a bare single crystal platinum electrode the amount of HS04- adsorbed decreases as the voltammetric scan proceeds further into the negative region (hydrogen adsorption/evolution). However, in recent radiotracer work, Wieckowski et aL8 showed that there was an enhanced adsorption of Hso4- in the presence of copper deposition. At the present time it is not clear whether the excess Hso4- resides on top of the copper adlayer or on the platinum surface. However, the fact that the copper speciesare not fully discharged may be, in part, responsible for this enhanced adsorption. The radial distribution for copper UPD in the presence of chloride showed only a singular peak at about 2.3 A. Fitting the singular peak required only copper-copper as the absorber-backscatterer pair. In the a-polarization, no copper-oxygen nor copper-chloride backscatteringwas seen. However, since we know that chloride is interacting with the surface, the absence of scattering by chloride suggests that they occupy atop sites. In the presence and absence of chloride the coppercopper bond distances were found to be significantly different,with the value of the former being substantially shorter than in the latter, 2.59 vs 2.85 A respectively. In (17) (a)Krauskopf, E. K.; Rice, L. M.; Wieckowski, A. J. Electroanal. Chem. 1988,244,347. (b)Wieckowski, A.; Zelenay, P.;Varga, K. J.Chim. Phys. 1991,88, 1247. (c) Zelenay, P.; Wieckowski, A. J. Electrochem. SOC.1992,139, 2552. (18) Kunimatsu, K.; Samant,M. G.; Seki, H. J. Electroanal. Chem. 1989,258, 163. (19) Faguy, P. W.; Markovic, N.; Adzic, R. R.; Fierro, C. A.; Yeager, E. B. J.Electroanal. Chem. 1990,289, 245.
Yee and Abruitu
both cases the coordination number of the copper is approximately 6 (f20 5% ) and the formation of well-ordered hexagonal structures is observed. Since the electrochemical measurements indicate that there is less than a monolayer of copper present, we can conclude that there is a strong tendency for cluster formation. The recent ex situ work of Kolb et aLsbof Cu UPD in the presence of chloride found that at a monolayer of copper the adatoms were in registry with the platinum substrate. Their finding would sug est a copper-copper distance of approximately the lattice spacing of the platinum surface. 2.77-2.8 However, our in situ measurements indicate a coppelcopper bond length of 2.59 A, which implies that the copper ad-layer is incommensuratewith respect to the platinum surface. However, the measured copper-copper bond distance of 2.59 A is very close to the metallic copper distance of 2.56 A, again suggesting clustering. For the copper deposited in the absence of chloride, the copper-copper bond distance of 2.85 A is larger than the 2.77 A for the platinum substrate. The expansion of the copper adlattice with respect to the platinum substrate (2.85 VEI 2.77 A) would indicate that the platinum surface contributes less to the ad-layer structure. At this coverage the adlayer takes on a more open structure perhaps as a means of minimizing the Coulombicrepulsion arising from the fact that the adlayer is not fully discharged. If bisulfate anions are coadsorbed (as suggested by Wieckowski) in the plane, they might cause a lattice expansion and in so doing stabilize the ad-layer. It would appear that the chloride has a greater influence in giving rise to a more ordered and compact adlattice consistent with its stronger tendencyto adsorbtoplatinum, relative to bisulfate. The chloride ad-layer, which based . ~ the~ topmost, will likely on results by Kolb et ~ 1is likely alter the electronic structure of the interface, and this could account at least in part for the shift in the potential of deposition. From the electrochemical measurements, the amount of charge is the same in the presence and absence of chloride. That being the case, one would expect the repulsive interactions to be the same so that the coppercopper distance would remain relatively unchanged, However, we observe asignificant differencein the coppelcopper distances in the presence and absence of chloride. Horanyi and VerteszOhave shown by radiotracer techniques that during the course of copper deposition there is a simultaneous enhancement in chloride adsorption. Likewise, Wieckowski et aL8 have observed a similar increase in HS04- adsorption during copper UPD. It would appear that chloride is much more effective than the HS04anion in screeningthe charge on the copper. The formation of a CUClad species has been proposed by Horanyi and VertmzOThe formation of such a complex could reduce the overall partial charge of the copper ad-layer thus leading to a more compact adlattice. The presence of chloride also appears to act as a “protective”coating against oxygen adsorption as is evident by the disappearance of the additional peak in the radial distribution, which was earlier ascribed to the presence of oxygen in the plane of scattering. It is interesting to compare our results with those of White and Abruiia.s” In that study, however, the platinum(111)surface was pretreated with iodine. Two interesting points to note from their study was that they found very pronounced copper-copper backscattering at only 0.3
f,
(20) (a) Horanyi, G.; Rzmayer, E. M.; Joo,P. J. Electroanal. Chem. 1983,152,211. (b) Horanyi,G.; Vertes, G. J. Electroanal. Chem. 1973, 4.5, 296.
Studies of Cu UPD on P t ( l l 1 )
monolayer and that the copper-copper bond distance was 2.88 A. No measurement of the copper-copper bond distance was made in the absence of iodine at the same coverage. These numbers suggestthat the iodine overlayer played a significantrole in ordering the copper adlayer at 0.3 monolayer and that clustering was present. In the current study with chloride, we see a contraction in the bond distance from 2.85 A (absence of chloride) to 2.59 A (presence of chloride). As with the iodine, the chloride appearsto be very effectivein orderingthe copper adlattice leading to cluster formation. From the EXAFS analysis it is possible to speculate as to the position of the chloride. Sinceno in-planescattering was observed, it is quite probable that the chloride ions sit on atop sites on the electroadsorbed copper layer. This is also consistent with the recent results of Kolb et a1.6b In their study of Cu UPD on an iodine pretreated Pttlll), White and Abruftasa pointed out that copper played a dominant role as a backacatterer in comparison to the iodine. This would imply that the iodine also resides on top of the copper UPD ad-layer. Based on Auger intensities, Hubbard et al." have suggested that in this system iodine is the topmost layer. In addition XSW studies have conclusively shown this to be the case.21
Conclusions SurfaceEXAFS was used to study the structural changes of the copper UPD ad-layer on a clean and well-ordered Pt(ll1) surface in the presence and in the absence of chloride. In the absence of chloride, the XANES analysis revealed similarities between the copper adlayer and the CuzO reference lending support to earlier findings of a partially discharged copper ad-layer. Strong scattering by oxygen was found and the Cu-O distance was deter-
Langmuir, Vol. 9, No.9, 1993 2469
mined to be 2.16 f 0.03 A. The fact that scattering by oxygen is observed impliesthat there is a projection of the Cu-O bond in the plane, thus excluding atop sites. This would leave bridge and 3-fold hollow sites as possible locationsfor the oxygen. In the u-polarizationwe observed intense copper-copper scattering. In the absence of chloride, the copper-copper bond distance is 2.85 f 0.03 A The coordinationnumber for copperwas approximately 6.2 f 20%. We eee the formationof well-ordered hexagonal surface structures. In the presence of chloride we observe a contraction in the copper adlattice for a comparableamount of deposited copper. The bond distance was found to be 2.59 f 0.03 A in comparison to 2.85 f 0.03 A. The chloride appears to play a significant role in ordering the copper adlayer. The compactness of the adlayer could be attributed to the formation of a CUCLd species whereby the partial charge of the copper ad-layer is greatly reduced, thus leading to a corresponding reduction in the Coulombic repulsion. The presence of chloride also serves as a 'protective" overlayer precluding oxygen adsorption. Current studies are underwayto determinethe exact location of the copper, oxygen, and chloride atom with respect to the substrate surface by probing both the Cu K edge and the Pt L edges in the *-polarization.
Acknowledgment. Our work was generously supported by the National Science Foundation, the Office of Naval Research,and the Army Research Office. We thank Keith Bowers of the LAASP machine shop in aiding in the design and construction of the new Teflon electrochemicalcell. Dr. Kenneth Finkelsteinand Mark J. Keeffe of the Cornell High Energy SynchrotronSource (CHESS) were especially helpful in the early stages of the experi(21) Bedzyk,M.J.;Bilderhack,D.;White,J.;Abrufla,H.D.;Bo"arito, ments. The authors also acknowledgethe assistance of D. L. Taylor, J. Hudson, and S. Chen in collecting the date. G.M.J. Phys. Chem. 1986,90,4926. ~
