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Langmuir 1997, 13, 2390-2397
Underpotential Deposition of Lead on Cu(100) in the Presence of Chloride: Ex-Situ Low-Energy Electron Diffraction, Auger Electron Spectroscopy, and Electrochemical Studies Gessie M. Brisard* and Entissar Zenati De´ partement de Chimie, Universite´ de Sherbrooke, Sherbrooke, Que´ bec, Canada J1K 2R1
Hubert A. Gasteiger,† Nenad M. Markovic´, and Philip N. Ross, Jr. Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720 Received October 16, 1996. In Final Form: January 21, 1997X Electrochemical and ultrahigh vacuum measurements are presented for the underpotential deposition (UPD) of Pb on Cu(100) in the presence of chloride ions. The chemistry is very similar to that for the (111) surface studied previously by the same methods. UPD Pb forms a compact layer of fully discharged atoms having essentially the same density as that in the (100) plane of bulk Pb. The deposition appears to occur via a single process, the nucleation and growth of Pb islands, but other types of growth modes cannot be excluded. The presence of chloride ion in the electrolyte enhances the kinetics of Pb deposition in both the underpotential and overpotential regions and results in a ≈0.1 V negative shift in the mean potential for UPD versus that in chloride-free perchloric acid. This shift can be accounted for in a simple thermodynamic model for the total Cu/Pb/Cl system. The only real difference in Pb UPD on the two different Cu crystal faces was the absence of an ordered structure for the compact layer on Cu(100), versus the ordered structure observed on Cu(111). This difference appears to be related to a lower mobility of Pb adatoms due to the greater atomic corrugation of the (100) surface.
1. Introduction In contrast to the relatively advanced state of knowledge of the interfacial properties, e.g., capacitance, and the chemistry of electrode processes like metal underpotential deposition (UPD) (refs 1-5 and references therein) and organic molecule adsorption10-13 on Ag and Au electrodes, there have been only a few fundamental studies of these processes on Cu.6-9 Fundamental studies of Cu in solution are much more demanding than the investigations of the more noble group 1B metals, chiefly due to the difficulties in producing oxide free, well-defined Cu surfaces for * To whom correspondence should be addressed: phone, (819)821-7093; fax, (819)-821-8017; e-mail, gessie.brisard@ courrier.usherb.ca. † Current address: Institute of Surface Chemistry and Catalysis, Ulm University, D-89069, Ulm, Germany. X Abstract published in Advance ACS Abstracts, March 15, 1997. (1) Kolb, D. M. Advances in Electrochemistry and Electrochemical Engineering; Gerisher, H., Tobias, C. W., Eds.; Wiley: New York, 1978; Vol. 11, p 125. (2) Hamelin, A. Modern Aspects of Electrochemistry; Conway, B. E., White, R. E., Bockris, J. O.’M., Eds.; Plenum Press: New York, 1985; Vol. 16. (3) Jovic, V. D.; Jovicevic, J. N., Despic, A. R. Electrochim. Acta 1985, 29, 1625. (4) Hamelin, A. J. Electroanal. Chem. 1984, 165, 167. (5) Lipkowski, J.; Stolberg, L. In Adsorption of Molecules at Metal Electrodes; Lipkowski, J., Ross, P. N., Eds.; VCH Publishers: New York, 1992; p 171 ff. (6) Siegenthaler, H.; Ju¨ttner, K. J. Electroanal. Chem. 1984, 163, 327. (7) Vilche, J. R., Ju¨ttner, K. Electrochim. Acta 1987, 32, 1567. (8) Bewick, A.; Jovicevic, J.; Thomas, B. Faraday Symp. Chem. Soc. 1977, 12, 24. (9) Brisard, G. M.; Zenati, E.; Gasteiger, H. A.; Markovic, N. M.; Ross, P. N., Jr. Langmuir 1995, 11, 2221. (10) Hamelin, A.; Morin, S.; Richer, J.; Lipkowski, J. J. Electroanal. Chem. 1991, 304, 195. (11) Ho¨lzle, M. H.; Kolb, D. M. Ber. Busenges. Phys. Chem. 1994, 98, 330. (12) Scharfe, M.; Hamelin, A.; Buess-Herman, C. Electrochim. Acta 1995, 40, 61. (13) Ho¨lzle, M. H.; Krznaric, D.; Kolb, D. M. J. Electroanal. Chem. 1995, 356, 239.
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electrochemical studies. In recent years, with the implementation of specialized ultrahigh vacuum (UHV) chambers equipped with manipulators that can transfer samples from UHV into solution cleanly, these difficulties can be overcome. There have also been significant advances in chemical preparation of Cu surfaces. We reported in our recent study of Cu(111) that by using a specific three-step electropolishing the chemically prepared surface gives identical results for Pb UPD compared to a surface prepared by sputtering/annealing in ultrahigh vacuum,9 and we found that oxygen present at the surface can be limited to very small amounts ( -0.3 V from the independently determined values of ∆θCl ) 0.45 ML, and ∆QCl ) 62 µC/cm2, which might indicate that γCl for the Cu/Cl system could be less than 1. A recent discussion of the electrosorption valency of chloride ion on other metals was presented by Shi and Lipkowski,32 and values of γCl < 1 are typical and in fact most lie within the range of 0.5 ( 0.25. Further insight into the determination of the electrosorption valency of halide on Cu will be obtained from the work under investigation on the Cu/Br system using the flux measurements of bromide on the Pt ring of RRDCu(hkl)E. It is not clear why the Pb deposit does not form an ordered layer at full coverage on Cu(100) as it does on (111). With a short annealing at 0.7 times the bulk melting point of bulk Pb, an ordered (x2×x2)R45° structure formed, probably corresponding to Pb atoms in the 4-fold hollow sites of the Cu(100) surface, which would seem to be a very energetically favorable structure compared with Pb atoms in other sites. The Pb-Pb distance in this structure is 3% larger than that in bulk Pb, which means there is no compressive strain on the Pb atoms. The problem then seems to be one of mobility of the Pb adatoms on the Cu(100) surface, perhaps due to the greater atomic corrugation of this surface relative to the close-packed Cu(111) surface. 4.0. Summary The process of Pb UPD on Cu in the presence of Cl- ion is very similar on both the (111) and (100) faces. In both cases, Pb UPD displaces the Clads layer to form a compact (32) Shi, Z.; Lipkowski, J. J. Electroanal. Chem. 1996, 403, 225.
Deposition of Pb on Cu in Presence of Cl-
layer of fully discharged atoms having essentially the same density as that in the equivalent fcc plane in bulk Pb. The deposition appears to occur via a single process, the nucleation and growth of Pb islands, but other types of growth modes cannot be excluded. The presence of chloride ion in the electrolyte enhances the kinetics of Pb deposition on both surfaces in both the underpotential and overpotential regions and results in a ≈0.1 V negative shift in the mean potential for UPD versus that in (nominally) chloride-free perchloric acid. This shift can be accounted for in a simple thermodynamic model for the total Cu/Pb/Cl system. The only real difference in Pb UPD on the two different Cu crystal faces was the absence of an ordered structure for the compact layer on Cu(100), versus the ordered structure observed on Cu(111). This difference appears to be related to a lower mobility of the Pb adatoms due to the greater atomic corrugation of the (100) surface.
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Acknowledgment. We thank Denis Poulin (Universite´ de Sherbrooke) and Frank Zucca and Lee Johnson (LBNL) for their invaluable help in building many parts and in polishing the single crystals for the experimental setup. E. Zenati acknowledges her fellowship from the Canadian International Development Agency, ACDI Marocco. The financial support from Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. This work was also supported by the Office of Energy Research, Basic Energy Sciences, Materials Science Division of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098.
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