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Anisotropic Dissolution of an Au(111) Electrode in Perchloric Acid Solution Containing Chloride Anion Investigated by in Situ STMsThe Important Role of Adsorbed Chloride Anion Shen Ye, Chikara Ishibashi, and Kohei Uosaki* Physical Chemistry Laboratory, Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060, Japan Received July 6, 1998. In Final Form: November 3, 1998 The anodic dissolution process of Au(111) in a 0.1 M perchloric acid (HClO4) solution containing chloride anion (Cl-) was investigated using an in situ scanning tunneling microscope. The initial dissolution of gold was observed at the step sites when the electrode potentials became more positive than +1.0 V. The rate of the anodic dissolution increased as the potential became more positive. When the potential became more positive than +1.35 V, dissolution on the terraces was also observed. The dissolution of Au(111) anisotropically proceeds in a layer-by-layer mode. Step lines along the [110] direction, which were found in the doublelayer region, disappeared, and ones along the [211] direction were newly formed during the dissolution process. The gold surface became rougher when the electrode potential became more positive than +1.45 V, where anodic dissolution and oxide formation simultaneously took place. The gold surface was completely passivated at +1.7 V. The mechanism for the anisotropic dissolution is discussed in relation to the structure of the chloride adlayer on the Au(111) electrode surface.
1. Introduction Elucidating the dissolution mechanism of noble metals is important for both industrial applications and fundamental science. Gold is known to have an excellent chemical stability and is widely used in industry and as an accessory material.1,2 It becomes unstable and dissolves in the positive potential region, especially in solutions containing Cl-.1-10 Recently, we investigated in detail the anodic dissolution process of Au(111) in HClO4 solution containing various concentrations of Cl- based on electrochemical quartz crystal microbalance (EQCM) measurements11 and clearly showed that gold dissolves through a 3e- mechanism in the positive potential region. A number of issues, especially the surface states of the gold electrode during the anodic dissolution process, are, however, still unclear. STM is a powerful method to investigate the atomic and electronic structures at the electrode/electrolyte interface and has been widely used for surface charac* Corresponding author. Telephone: +81-11-706-3812. Fax: +81-11-706-3440. E-mail:
[email protected]. (1) Schmid, G. M.; Curley-Fiorino, M. E. In Encyclopedia of Electrochemistry of the Elements; Bard, A. J., Ed.; Marcel Dekker: New York, 1975; Vol. IV. (2) Puddephatt, R. J. The Chemistry of Gold; Elsevier Scientific Publishing Company: Amsterdam, 1978. (3) Kirk, D. W.; Foulkes, F. R.; Graydon, W. F. J. Electrochem. Soc. 1980, 127, 1069. (4) Kelsall, G. H.; Welham, N. J.; Diaz, M. A. J. Electroanal. Chem. 1993, 361, 13. (5) Heumann, T.; Panesar, H. S. Z. Phys. Chem. 1965, 229, 84. (6) Gaur, J. N.; Schumit, G. M. J. Electroanal. Chem. 1970, 24, 279. (7) Cadle, S. H.; Bruckenstein, S. J. Electroanal. Chem. 1973, 48, 325. (8) Frankenthal, R. P.; Thompson, D. E. J. Electrochem. Soc. 1976, 123, 799; 1982, 129, 1192. (9) Horikoshi, T.; Yoshimura, S.; Kokubo, N.; Sato, E. Nippon Kaga Kukaishi 1983, 1118. (10) Diaz, M. A.; Kelsall, G. H.; Welham, N. J. J. Electroanal. Chem. 1993, 361, 25. (11) Ye, S.; Ishibashi, C.; Uosaki, K. J. Electrochem. Soc. 1998, 145, 1614.
terization during metal dissolution and deposition processes.12,13 Although gold single-crystal electrodes are widely used as substrates in STM studies, STM observations of the gold surface during the dissolution process in a Cl--containing solution are limited compared with those of other metals such as Cu,14-18 Pd,19 Ag,20 and Ni.21 Trevor et al. found that the terraces of an Au(111) surface in a HClO4 solution containing a trace amount of Cl- were smoother than those in a Cl--free solution after the formation and reduction of gold oxide, suggesting that Cl- increased the mobility of gold atoms on the gold electrode surface.22 Honbo et al. reported that pits were formed after the reduction of gold oxide on an Au(111) surface in a Cl--free solution but not in a Cl--containing solution.23 Haiss and Sass investigated surface species during dissolution of Au(111) in 0.1 M HClO4 containing 5 mM CsCl by in situ STM.24 An ordered adlayer was observed at +0.95 V (vs SCE) and was assigned to a dimer of the Au-Cl complex formed as a product of the dissolution. Although an ordered chloride phase on a gold surface has not yet been observed by STM, grazing incident angle X-ray diffraction measurements showed that an (12) Nanoscale Probes of the Solid/Liquid Interface; Gewirth, A. A., Siegenthaler, H., Eds.; Kluwer Academic Publishers: Dordrecht, 1995. (13) Gewirth, A. A.; Niece, B. K. Chem. Rev. 1997, 97, 1117. (14) Suggs, D. W.; Bard, A. J. J. Am. Chem. Soc. 1994, 116, 10725. (15) Suggs, D. W.; Bard, A. J. J. Phys. Chem. 1995, 99, 8349. (16) Moffat, T. P. In Electrochemical Nanotechnology; Lorenz, W. L., Plieth, W., Eds.; Wiley-VCH: Weinheim, 1998; p 171. (17) Vogt, M. R.; Mo¨ller, F. A.; Schilz, C. M.; Magnussen, O. M.; Behm, R. J. Surf. Sci. 1996, 367, L33. (18) Vogt, M. R.; Lachenwitzer, A.; Magnussen, O. M.; Behm, R. J. Surf. Sci. 1998, 399, 49. (19) Sashikata, K.; Matsui, Y.; Itaya, K.; Soriaga, M. P. J. Phys. Chem. 1996, 100, 20027. (20) Teshima, T.; Ogaki, K.; Itaya, K. J. Phys. Chem. B 1997, 101, 2046. (21) Ando, S.; Suzuki, T.; Itaya, K. J. Electroanal. Chem. 1996, 412, 139. (22) Trevor, D. J.; Chidsey, C. E. D.; Loiacono, D. N. Phys. Rev. Lett. 1989, 62, 929. (23) Honbo, H.; Sugawara, S.; Itaya, K. Anal. Chem. 1990, 62, 2424. (24) Haiss, W.; Sass, J. K. J. Electroanal. Chem. 1997, 431, 15.
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ordered adlayer of chloride on Au(111) was formed at +0.68 V (vs Ag/AgCl) in 0.1 M Cl- solution and was uniformly compressed with increasing potential.25 In the present study, anodic dissolution of Au(111) in a 0.1 M HClO4 solution containing Cl- was investigated by in situ STM. Gold dissolution took place when the potential became more positive than +1.0 V. It is found that gold anisotropically dissolves in a layer-by-layer mode. Anisotropic dissolution on Au(111) is attributed to the lower etching rate on gold atomic rows along the [211] direction stabilized by the chloride adatom. 2. Experimental Section STM measurements were carried out on atomically flat (111) facets formed on a gold single-crystal surface prepared by the Clavilier method.26 Electrochemical characterization was performed on an Au(111) electrode surface, which was cut and mechanically polished, followed by annealing with a gas/O2 flame and quenching with ultrapure water. Electrolyte solutions were prepared using Suprapure grade HClO4 (Wako Pure Chemicals), NaCl (Wako Pure Chemicals), and Milli-Q water. Solutions of 0.1 M HClO4 with various concentrations of chloride (0.1-100 mM) were prepared. They were deaerated with purified Ar gas for at least 20 min. In situ STM measurements were carried out using a NanoScope E instrument (Digital Instruments) in the ECSTM mode with a homemade electrochemical cell. STM images were obtained in constant-current mode unless otherwise stated. STM tips were simply cut Pt/Ir wire (80:20, φ ) 0.25 mm) insulated with nail polish to minimize the faradic current. A small quasi-reversible hydrogen electrode and a Pt wire were used as the reference and counter electrodes, respectively. Electrochemical measurements were performed using a potentiostat (Hokuto Denko, HA-151) and a function generator (Hokuto Denko, HB-111). A quasi-reversible hydrogen electrode and a Pt wire were used as the reference and counter electrodes, respectively. All potentials are reported with respect to the reversible hydrogen electrode (RHE).
3. Results and Discussion 3.1. Effect of Cl- Adsorption on the Electrochemical Behavior of Au(111). Figure 1 shows cyclic voltammograms (CVs) of an Au(111) electrode in a 0.1 M HClO4 solution with (a) 0 mM and (b) 1 mM Cl- in the potential region between +0.2 and +1.7 V. The CV obtained in the 0.1 M HClO4 solution was consistent with that reported in the literature27,28 and exhibited two anodic peaks at +1.37 and 1.56 V in the positive-going potential sweep and a large cathodic peak at +1.18 V in the negativegoing potential sweep (Figure 1a). The anodic and cathodic peaks are attributed to the formation (reaction I, forward) and reduction (reaction I, backward), respectively, of oxide on the Au(111) surface.
Au + H2O f AuO + 2H+ + 2e-
(I)
The inset of Figure 1a shows an enlarged CV in the potential region between +0.2 and +1.2 V. A small anodic peak (denoted as peak I) was found around +0.75 V. Peak I is an intrinsic characteristic of the Au(111) electrode surface which has been attributed to the surface recon(25) Magnussen, O. M.; Ocko, B. M.; Adzic, R. R.; Wang, J. X. Phys. Rev. B 1995, 51, 5510. (26) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205. (27) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. Electrochim. Acta 1986, 31, 1051. (28) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. J. Electroanal. Chem. 1987, 228, 429.
Figure 1. CVs of an Au(111) electrode obtained in a 0.1 M HClO4 solution containing Cl- at a concentration of (a) 0 and (b) 1 mM in the potential region between +0.2 and +1.7 V. The insets show enlarged CVs in the double-layer region. The sweep rate is 20 mV/s.
struction from the (x3 × 22) structure to the (1 × 1) structure arising from a large difference in the potential of zero charge (pzc) for the two structures.29-33 The CV in a Cl--containing solution (Figure 1b) is quite different from the one in the Cl-- free solution (Figure 1a). A significant increase in the anodic current started at +1.25 V. Two anodic peaks were found at +1.40 and +1.51 V. When the potential became more positive than +1.55 V, the anodic current quickly decreased. As discussed in our previous EQCM study,11 the pronounced anodic current in the positive-going potential sweep was attributed to the 3e- oxidative dissolution of gold (reaction II, forward).
Au + 4Cl- f AuCl-4 + 3e-
(II)
In the more positive potential region, the dissolution reaction competed with oxide formation (reaction I, forward) on the gold surface and passivation was completed around +1.7 V. The cathodic peaks observed at +1.21 and +1.10 V in the negative-going potential sweep were related to the simultaneous oxide reduction (reaction I, backward)/gold dissolution (reaction II, forward) and the redeposition of the dissolved gold species (reaction II, backward), respectively, as already confirmed by EQCM measurements.11 An enlarged CV obtained in the potential region between +0.2 and +1.2 V (inset of Figure 1b) shows an anodic peak at 0.5 V (peak I) and a pair of redox spikes at +1.04 V (denoted as peak II). The position of peak I (29) Scherson, D. A.; Kolb, D. M. J. Electroanal. Chem. 1984, 76, 353. (30) Kolb, D. M.; Schneider, J. Surf. Sci. 1985, 162, 764. (31) Kolb, D. M. Prog. Surf. Sci. 1996, 51, 109. (32) Wang, J.; Ocko, B. M.; Davenport, A. J.; Isaacs, H. S. Phys. Rev. B 1992, 46, 10322. (33) Shi, Z.; Lipkowski, J. J. Electroanal. Chem. 1996, 403, 225.
Anisotropic Dissolution of an Au(111) Electrode
Figure 2. Sequence of in situ STM images (500 × 500 nm2) of an Au(111) surface in a 0.1 M HClO4 solution at (a) +0.8 V, (b) +1.7 V immediately after sweeping from +0.8 V (2 mV/s), and (c) +0.8 V immediately after sweeping from +1.7 V (2 mV/ s). Etip ) +1.2 V. iT ) 5 nA. The STM images are shown by the surface plots with a Z-range of 2.5 nm.
shifted to a more negative potential compared to the one in Cl--free solution (Figure 1a) as a result of chlorideinduced lifting of the surface reconstruction.29-33 Peak II is not observed at all in the Cl--free solution and has been attributed to the formation of an ordered chloride adlayer on the Au(111) surface.25,31,33 3.2. STM Observation of an Au(111) Surface in Cl-Free HClO4 Solution. Figure 2 shows a sequence of in situ STM images (500 × 500 nm2) of an Au(111) surface in 0.1 M HClO4 solution during its oxide formation and
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reduction processes. Wide atomically flat terraces and straight step lines of monatomic height (ca. 0.22 nm) were observed in the double-layer region (Figure 2a). Most step lines were straight and crossed each other at 60° or 120°. The step lines were parallel to the [110] direction, that is, the nearest neighbor direction of the Au(111) substrate. An atomic image of hexagonal symmetry with a nearest neighbor distance of 0.29 nm, which is in agreement with the atomic distance of Au(111), was observed in a highresolution measurement, showing that a well-defined Au(111)-(1 × 1) phase was exposed at this potential. Small islands with monatomic height on the terraces were formed by the lifting of the surface reconstruction from (x3 × 22) to (1 × 1), since the former has approximately 4% more surface gold atoms than the latter.33 Figure 2b was obtained at +1.7 V, where a full monolayer of oxide is formed (Figure 1a), immediately after the potential was slowly swept from +0.8 to +1.7 V (2 mV/s). Although no change was found in the shape of the step and terrace after the oxide formation, an increase in the roughness of the terraces was observed (Figure 2b). The oxide-induced surface roughening has been reported in the literature.22,23,34,35 STM observations after the potential was swept back to +0.8 V, at which the gold oxide is completely reduced (Figure 1a), showed that the morphology of the Au(111) surface irreversibly changed (Figure 2c). Compared to the cases of its initial states at the same potential (Figure 2a), the surface roughness increased. Some small pits with the size of several nanometers were observed on the terraces in the high-resolution STM image. Pit formation has been previously observed and partly attributed to the irreversible gold-oxygen place exchange during the oxide reduction process.23,34 The shape of the pits, however, seemed to be different from those reported by Honbo et al.23 and Gao et al.34 This difference should be due to the fact that they used a more positive potential limit (1.85 and 1.90 V vs RHE for the former23 and latter,34 respectively). 3.3. STM Observation of an Au(111) Surface in a Cl--Containing HClO4 Solution. Figure 3 shows a sequence of in situ STM images (1 × 1 µm2) of an Au(111) surface obtained in a 0.1 M HClO4 solution containing 1 mM Cl-. Large terraces and monatomic step lines were observed at +0.8 V (Figure 3a), as was the case in Cl--free solution (Figure 2a). The step lines, which were parallel to the [110] direction, crossed each other at 60° or 120°. No change was observed in the images in a continuous STM observation at the same potential for at least 10 min, showing the gold surface was stable at this potential even in the solution containing Cl-. Although efforts were made to obtain an STM image of atomic resolution, no clear atomic image has been obtained yet. When the potential was slowly swept (2 mV/s) in the positive direction, drastic changes in the shape of the step lines were observed. Figure 3b shows an STM image obtained during the potential sweep from +0.99 V (top) to +1.09 V (bottom) while rastering the tip downward. It should be noted that the potential region for Figure 3b is close to that of peak II shown in Figure 1b. Although no change was found on the terraces, a number of kinks were formed in the originally straight step lines. No change in the terrace structure shows that the kink formation is not due to the migration of gold atoms. Thus, the gold surface really started to dissolve from the step sites at this (34) Gao, X.; Weaver, M. J. J. Electroanal. Chem. 1994, 367, 259. (35) Vitus, C. M.; Davenport, A. J. J. Electrochem. Soc. 1994, 141, 1291.
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Figure 3. Sequence of in situ STM images (1 × 1 µm2) of an Au(111) surface in a 0.1 M HClO4 solution containing 1 mM Cl-: (a) at +0.8 V; (b) at +0.99 to +1.09 V during a potential sweep (2 mV/s) with the rastering tip downward; (c) 0 min, (d) 6 min, and (e) 40 min, respectively, after the potential was swept from +1.09 to +1.27 V; (f) potential was stepped from +1.27 to +1.36 V at the moment shown by the arrow with the rastering tip downward; (g) next scan after part f with the rastering tip upward; (h) 3 min after part f with the rastering tip downward. Etip ) +1.2 V. iT ) 5 nA. The Z-range of the STM images was 5 nm. A compass was drawn to indicate the orientation of the surface. 50 s is necessary to capture one STM image. See text for details.
potential. The change in the outline of the step edge became more obvious on the bottom part than that on the top part
in the image, demonstrating that the gold dissolution rate increased as the electrode potential became more positive.
Anisotropic Dissolution of an Au(111) Electrode
About 4% of surface gold atoms were dissolved between the STM images of Figure 3a and b. According to the EQCM results, however, gold started to dissolve from approximately 1.25 V,11 that is, more positive than the present potential onset. The dissolution of 4% surface gold atoms corresponds to the weight decrease of approximately 20 ng/cm2, that is, an ∼1 Hz frequency increase. This mass change should be easily detected, since our EQCM system can usually detect a mass change of less than 2 ng/cm2 (0.1 Hz). This discrepancy should be explained by considering the weight increase due to the simultaneous chloride adsorption in the potential region of peak II. A surface X-ray diffraction measurement showed that the chloride coverage (θCl) increased with a slope of approximately 1.5%/V after the ordered chloride layer was constructed.25 Thus, potential-induced “compression” of the chloride adlayer and initial gold dissolution seemed to take place in the potential region around peak II, resulting in a net mass change too small to be detected by the EQCM measurements. Figure 3c shows an STM image at +1.27 V immediately after the potential was swept to this potential. The dissolution mainly proceeded at the step sites and the originally straight step lines were etched like the teeth of a saw. The dissolution also proceeded on the terrace (Figure 3c) but only from spots where small pits or defects were found at more negative potentials (Figure 3a and b), as indicated by the black circles. Parts d and e of Figure 3 show STM images at +1.27 V after 6 and 40 min, respectively, after Figure 3c was captured. The Au(111) surface was mainly etched from its step edge. At the beginning of the dissolution, only steps along the [110] direction were observed (Figure 3b and c). As shown by the arrows in Figure 3d, some of the step lines were rotated approximately 30° and new step lines running along the [211] direction, that is, the next nearest neighbor direction of the Au(111) substrate, were observed. The step edges along the [110] direction were still dominant in Figure 3d. The step edges along the [211] direction, however, became dominant after the electrode was kept at +1.27 V for a long time (Figure 3e). These results suggest that anodic dissolution of the Au(111) surface is an anisotropic etching process in which step edges preferentially retreated along the [211] direction. Dissolution behavior drastically changed at more positive potentials, as shown in Figures 3f-h. The potential was stepped to a more positive potential (+1.36 V), where gold still dissolves without oxide formation, at the moment shown by an arrow in Figure 3f while rastering the tip downward. Acceleration of the dissolution rate, especially on the terraces, was observed. Many small pits were formed on the terraces. In the bottom part of the image, rectangular-shaped pits were clearly observed. In the next STM scan (Figure 3g), rectangular pits were clearly observed on all terraces. It is interesting to note that the rectangular pits were parallel to each other on the same domain. Furthermore, the direction of the long axis of the rectangular pits was parallel to the [211] direction of the substrate, that is, the new step direction after initial dissolution (Figure 3e). The pits grew bigger and then merged together. Figure 3h shows an STM image captured approximately 2 min after Figure 3g. The top layer of the Au(111) was almost etched away, and the next layer of gold started to dissolve. Nearly all of the step lines were orientated only along the [211] direction, and the step line along the [110] direction was absent. These results demonstrated that gold was dissolved layer-by-layer in an anisotropic way in this potential region.
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Figure 4. Schematic model illustrating the chloride adlayer on an Au(111) surface based on the surface X-ray diffraction measurement. See text for details.
STM observations were also carried out in solutions of various Cl- concentrations. It was found that the onset potential of the dissolution depended on the Cl- concentration. The higher the Cl- concentration, the more negative the onset potential. This potential dependence is consistent with that of peak II. In solutions containing Cl- of less than 0.1 mM, the dissolution rate was very low and both the [211] and [110] step lines were observed after the electrode was kept at +1.36 V for 20 min. In a solution of higher Cl- concentration, that is, more than 10 mM, the anisotropic characteristics were hard to observe as a result of the very high dissolution rate. Preferential etchings from step sites of Au(111) were also observed in solutions containing CN- or F- 36,37 as well as an organic molecule, tetramethylethiourea,38,39 although no clear anisotropic dissolution feature was reported in these studies. Anisotropic dissolution studies on copper single crystals in aqueous chloride solution by Suggs and Bard have shown that the step edges always retreat along the [211] direction on Cu(111)14 and along the [100] direction on Cu(100).15 The higher reactivity on the [100] step edges, which have a high density of kink sites relative to that of the [110] step edges on the Cu(100) surface, was considered as the reason for the anisotropic dissolution behaviors of Cu(100).15 On the other hand, Vogt et al. proposed that the preferential formation of the [100] step edge resulted from its higher stability since the [100] step edges on the Cu(100) covered by a (2 × 2) chloride adlayer were more stable than the [110] step edges on the Cu(100)-(1 × 1) surface by 3-4 orders of magnitude.17,18 The same idea can be applied to the present case. Figure 4 illustrates a real-space model of the ordered chloride adlayer and underlying Au(111) at the potential of peak II, based on the surface X-ray diffraction measurements.25 (36) Li, Y.-Q.; Chailapakul, O.; Crooks, R. M. J. Vac. Sci. Technol., B 1995, 13, 1300. (37) Zamborini, F. P.; Crooks, R. M. Langmuir 1997, 13, 122. (38) Bunge, E.; Port, S. N.; Roelfs, B.; Meyer, H.; Baumga¨rtel, H.; Schiffrin, D. J.; Nichols, R. J. Langmuir 1997, 13, 85. (39) Bunge, E.; Nichols, R. J.; Roelfs, B.; Meyer, H.; Baumga¨rtel, H. Langmuir 1996, 12, 3060.
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The chloride adlayer with a close-packed hexagonal structure was not commensurate with the gold substrate, and the interatomic distance was close to the van der Waals diameter of chloride. The highest density of the chloride adatom was found in the step lines along the [211] direction, and the adlayer was compressed more with increasing potential.25 As demonstrated in Figure 3, the original step lines along the [110] direction disappeared and new [211] step lines were observed on the surface after anodic dissolution in 0.1 M HClO4 solution containing 1 mM Cl-. The lateral interactions between the adsorbed chloride and gold were considered to stabilize atom rows along the [211] direction more than those in the other directions because of its highest chloride adatom density. In other words, a much lower etching rate on the [211] step lines was expected, and as a result, the anisotropic dissolution of Au(111) in the Cl--containing solution was observed. When the potential became more positive than 1.45 V, the gold surface became rougher. This was attributed to simultaneous oxide formation at this potential.11 Anodic dissolution appeared to take place only on the oxide-free sites and stopped when the surface was totally covered with oxide. No morphology change was found when the potential was more positive than +1.7 V, as the gold surface was completely passivated (Figure 1b).
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4. Conclusion The anodic dissolution process of Au(111) in Cl-containing HClO4 solutions was studied by in situ STM. Gold dissolution occurred at step edges as the potential became more positive than +1.0 V, at which formation of an ordered chloride adlayer has been previously observed by surface X-ray diffraction measurements. New step lines running along the [211] direction were formed during the dissolution process. For potentials more positive than +1.35 V, the etching of gold took place at both the step and terrace sites. The present study demonstrates that gold anisotropically dissolves in a layer-by-layer mode. The lower etching rate was found for gold atomic rows along the [211] direction, in which the highest chloride adatom density array has been previously observed. Therefore, the anisotropic dissolution on Au(111) observed can be attributed to stabilization of the [211] step lines by the preferred adsorption of the chloride adatom. Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Area of “Electrochemistry of Ordered Interfaces” (No. 09237101) from the Ministry of Education, Science, Sports and Culture, Japan. LA980812X
