Competitive Adsorption and Surface Alloying: Underpotential

The initially deposited Sn adatoms displace the sulfate ions from the edge and push themselves toward the interior region of the terrace, maintaining ...
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Competitive Adsorption and Surface Alloying: Underpotential Deposition of Sn on Sulfate-Covered Cu(111) Jia-Wei Yan, Jian-Ming Wu, Qiong Wu, Zhao-Xiong Xie, and Bing-Wei Mao* State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen 361005, China Received March 22, 2003. In Final Form: June 19, 2003 We report an in situ scanning tunneling microscopy study on Sn underpotential deposition (UPD) on sulfate-covered Cu(111) electrode surfaces, which represents a system of anion adsorption in an extreme situation in terms of anion-anion and anion-substrate interactions. Owing to the strong sulfate adsorption, the UPD is initiated with “parasitical” adsorption of Sn adatoms almost exclusively along the periphery of the sulfate adlayer, as manifested in the appearance of brightened Moire´ structure modulation at the terrace edge. A Sn-induced local enhancement of tunneling is offered to explain the brightening of the Moire´ structure. The initially deposited Sn adatoms displace the sulfate ions from the edge and push themselves toward the interior region of the terrace, maintaining the brightened Moire´ pattern at the forefront sites. Consequently, a region free of sulfate is provided for further Sn deposition. In addition to terrace edge reshaping, surface alloying is confirmed by the course-dependent anodic stripping, in which the Sn-covered region of the terrace (excluding the forefront sites) becomes fragmented. The surface alloying is favored, in view of strain relief, by the expanded topmost layer of the reconstructed Cu(111). The overall deposition process may be complete within a time window of several minutes. The present work reveals that when anion-substrate and anion-anion interactions are sufficiently strong and comparable to adatomsubstrate interaction, the formation of metal adlayer is severely restricted and novel features of UPD are displayed.

Introduction Metal underpotential deposition (UPD) results from a strong metal adatom-substrate interaction.1,2 UPD characteristics such as underpotential shift, adlayer structure, and dynamic behavior depend critically on the anion adsorption from electrolyte. A thorough understanding of anion effects on UPD processes is, therefore, highly desirable. Investigations in this field have shown the trend of increasing applications of many sophisticated surface science techniques such as scanning tunneling microscopy (STM),3-8 atomic force microscopy (AFM),9,10 and X-ray scattering methods.11 Information available so far shows three types of anion effects which depend on the strength of interactions for anion-substrate, anion-anion, and anion-adatom pairs. First of all, for systems with strong anion-substrate interactions, anions are adsorbed on the substrate prior to UPD, and additional energy is required for the UPD to proceed via displacement of the adsorbed * To whom correspondence should be addressed. Tel: +86 592 2186979. Fax: +86 592 2183047. E-mail: [email protected]. (1) Kolb, D. M. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, Ch. W. T., Eds.; Wiley: New York, 1978; Vol. 11, p 125. (2) Ju¨ttner, K.; Lorenz, W. J. Z. Phys. Chem. Neue Folge, Bd. 1980, 122, 163. (3) Kolb, D. M. Angew. Chem., Int. Ed. 2001, 40, 1162. (4) Moffat, T. P. Electroanal. Chem. 1999, 21, 211. (5) Itaya, K. Prog. Surf. Sci. 1998, 58, 121. (6) Gewirth, A.; Niece, B. K. Acc. Chem. Rev. 1997, 97, 1129. (7) Weaver, M. J.; Gao, X. P. Annu. Rev. Phys. Chem. 1993, 44, 459. (8) Herrero, E.; Buller, L. J.; Abruna˜, H. D. Chem. Rev. 2001, 101, 1897. (9) Chen, C. H.; Vesecky, S. M.; Gewirth, A. J. Am. Chem. Soc. 1992, 14, 451. (10) Mrozek, P.; Sung, Y. E.; Han, M.; Gamboa-Aldeco, M.; Wieckowski, A.; Chen, C. H.; Gewirth, A. Electrochim. Acta 1995, 40, 17. (11) Ocko, B. M.; Wang, J. In Synchrotron Techniques in Interfacial Electrochemistry; Melendres, C. A., Tadjeddine, A., Eds.; Kluwer: Dordrecht, 1994; p 127.

anions. Second, for systems with strong anion-adatom interactions, anions are induced to coadsorb onto the metal adatoms, in favor of the formation of an ordered metal adlayer. Finally, the above two situations operate sequentially if all interactions are comparable and strong; that is, the preadsorbed anions are displaced by metal adatoms followed by induced coadsorption to form a bilayer. The above-described anion effects can be understood on the basis that the adatom-substrate interaction is stronger than the anion-substrate and anion-anion interactions so that adatoms are able to displace the preadsorbed anions and to form ordered adlayers. There is, however, little information available in cases where the anion-substrate interaction is comparable to or even stronger than the adatom-substrate interaction. Studies on these cases would be equally important not only for a better understanding of anion effects on UPD but also for gaining a clear physical picture of competitive adsorption. Recently, it has been reported that sulfate/(bi)sulfate adsorption-desorption on Cu(111)12-16 involves remarkable charge transfer as is indicated by cyclic voltammograms.12 In situ STM measurements also indicated a remarkable mass transfer and expansion of the topmost layer of the Cu by 12.5%.15 Very likely, the sulfate (this term will be used here to denote either bisulfate or sulfate) covered Cu(111) represents a system of anion adsorption in an extreme situation in terms of anion-substrate and (12) Wilms, M.; Broekmann, P.; Stuhlmann, C.; Wandelt, K. Surf. Sci. 1998, 416, 121. (13) Li, W. H.; Nichols, J. R. J. Electroanal. Chem. 1998, 456, 153. (14) Lennartz, M.; Broekmann, P.; Arenz, M.; Stuhlmann, C.; Wandelt, K. Surf. Sci. 1999, 442, 215. (15) Broekmann, P.; Wilms, M.; Wandelt, K. Surf. Rev. Lett. 1999, 6, 907. (16) Broekmann, P.; Wilms, M; Spaenig, A.; Wandelt, K. Prog. Surf. Sci. 2001, 67, 59.

10.1021/la034500s CCC: $25.00 © 2003 American Chemical Society Published on Web 08/07/2003

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anion-anion interactions, which provides an opportunity to study the anion effect on UPD under such a situation. Because of the low work function of Cu, the number of metals that can be underpotential deposited on Cu is limited. Pb and Cd are the two systems that have been reported using either in situ17-19 or ex situ20,21 surface characterization techniques, in which Pb on Cu is studied in nonspecific perchlorate media17,18 while Cd on Cu(111) shows a bilayer structure in a chloride-containing electrolyte.19 In this paper, we report an in situ STM study on Sn UPD in competition against strong sulfate adsorption at Cu(111) electrode surfaces. The results demonstrate that when anion-substrate and anion-anion interactions are sufficiently strong, the formation of metal adlayer is severely restricted so that the UPD occurs almost exclusively at the periphery of the sulfate adlayer. Surface alloying is favored by the expanded lattice of the reconstructed Cu(111) surface that partially relieves the strain introduced upon incorporation of larger Sn atoms into the Cu surface plane. Experimental Section The Cu(111) single-crystal surface was electrochemically polished in 50% phosphoric acid at 2.1 V (vs a Pt wire) for 30-40 s followed by thorough rinsing with ultra-high-purity water (MiliQ). The surface was then protected by a droplet of water and quickly transferred to an electrochemical or STM cell and immersed under proper potential control. Sn was electrochemically deposited onto Cu(111) single-crystal electrodes from either 0.5 M H2SO4 or 0.5 M HClO4 solution containing SnSO4. The electrochemical measurements were performed on a CHI 631a workstation (CHI, USA) in either an STM cell or a conventional three-compartment cell as indicated. The in situ STM measurements were performed at constant current using a Nanoscope IIIa (Digital Instruments, USA). Electrochemically etched and polyethylene-coated W tips were used. A saturated calomel electrode (SCE) and a platinum wire quasi-reference (∼0.45 V vs SCE) were used as the reference electrodes for the electrochemical and in situ STM measurements, respectively. Potentials quoted in this work are, however, versus SCE. A Pt wire was used as the counter electrode in all cases. All solutions were freshly prepared from AR or higher-grade chemicals using Mili-Q water.

Results The cyclic voltammogram (CV) of Cu(111) in 0.5 M H2SO4 solution given in Figure 1a shows sulfate adsorption features similar to that reported by Wandelt and coworkers.12 On the cathodic sweeping, the broad bump centered ca. -0.43 V is from the reduction of residual oxygen dissolved in the solution of the STM cell. Sulfate desorbs at ca. -0.66 V, which is accompanied by a significant charge withdrawal from the surface and associated with a remarkable cathodic peak (∼120 µQ cm-2) on the CV. Figure 1b is the CV of Cu(111) in a Sn(II)-containing solution. With the equilibrium potential of Sn bulk deposition at -0.52 V, a simple pair of peaks marked as I and I′ is present corresponding to Sn UPD and stripping, respectively. In contrast to the usual reversible adsorption-desorption peaks associated with the formation of ordered adlayers of UPD metals, the deposition and dissolution peaks of the present system are well separated and the corresponding processes (17) Wu, H.-C.; Yau, S.-L. J. Phys. Chem. B 2001, 105, 6965. (18) Moffat, T. P. J. Phys. Chem. B 1998, 102, 10020. (19) Hommrich, J.; Hu¨mann, S.; Wandelt, K. Faraday Discuss. 2002, 121, 129. (20) Brisard, G. M.; Zenati, E.; Gasteiger, H. A.; Markovic, N. M.; Ross, P. N., Jr. Langmuir 1995, 11, 2221. (21) Brisard, G. M.; Zenati, E. Langmuir 1997, 13, 2390.

Figure 1. Cyclic voltammograms of Cu(111) in 0.5 M H2SO4 solution (a) and a solution of 0.5 M H2SO4 and 0.1 mM SnSO4 (b). Also shown are the anodic stripping curves with polarization at -0.5 V for 2, 10, and 30 s, respectively (c). A sweep rate of 50 mV s-1 was used for (a) and (c), but a lower sweep rate of 10 mV s-1 was used for (b) to better follow the deposition and dissolution processes. All measurements were performed in an STM cell.

are kinetically hindered. Assuming a two-electron reduction from Sn(II) to Sn(0), the charge flux associated with UPD peak I is ∼178 µQ cm-2, equivalent to ∼0.4 monolayer (ML) of Sn (assuming the electrosorption valency of 1 for the deposited Sn). If the charge withdrawal associated with sulfate desorption from the Cu surface is considered, the net quantity of UPD metal would be reduced. It is also possible, however, that the sulfate is coadsorbed on Sn with charge donation to Sn. In this case, the estimated Sn coverage would not be influenced significantly by the sulfate desorption and adsorption processes. Using a freshly polished surface for each experiment, the anodic stripping curve afforded almost the same trace after polarization at -0.5 V for 10 s (up to 80 s) as shown in Figure 1c. The independence of the stripping feature on polarization implies that the overall process is completed and the surface reaches a static state shortly even though surface alloying is involved during the UPD, which is proved by in situ STM measurements (vide infra). This is in contrast to Tl UPD on Ag(111) reported by Siegenthaler and co-workers22,23 in which a continuous change of the stripping features was observed with prolonged polarization, and slow structural transformations were encountered. With the immersion potential of -0.25 V, there was a residual current in the potential range up to Sn UPD. There was, however, no indication of any unusual surface change from the in situ STM study at this potential. Nevertheless, to avoid possible surface oxidation, the electrode potential was kept at -0.38 V or more negative before introducing the electrolyte for in situ STM measurements. In view of the possible sulfate influence on Sn UPD, in situ STM measurements on sulfate adsorption and de(22) Siegenthaler, H.; Juttner, K.; Schmidt, E.; Lorenz, W. J. Electrochim. Acta 1978, 23, 1009. (23) Carnai, D.; Oden, P. I.; Mu¨ller, U.; Schmidt, E.; Siegenthaler, H. Electrochim. Acta 1995, 40, 1223.

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Figure 2. In situ STM images of potential-dependent Cu(111) surfaces in a 0.5 M H2SO4 solution at -0.4 V (a,b), -0.65 V (c), and -0.4 V (d); Vb ) -50 mV (tip positive), It ) 0.5 nA. Scan area: 100 nm × 100 nm (a,c,d) and 8 nm × 8 nm (b).

Figure 3. In situ STM images of potential-dependent Cu(111) surfaces in a solution of 0.5 M H2SO4 + 0.1 mM SnSO4. Images were recorded at -0.45 V (a), - 0.47 V (b,c), and -0.37 V (d) at Vb ) - 20 mV and It ) 0.5 nA, with a scan size of 80 nm × 80 nm. Image c was recorded 43 s after image b.

sorption processes were performed first in the supporting electrolyte of H2SO4 solution. Under the potential regime where sulfate is adsorbed, for example, at -0.4 V, the features such as (x7 × x3) structure, the Moire´ pattern and its regular modulation at the terrace edge are clearly observed (see Figure 2a,b). In addition to the Moire´ pattern modulation, the contour of the terrace edge is ragged. As the potential is decreased to -0.65 V or more negative, where sulfate completely desorbs from the surface, the ragged terrace edge smoothes considerably as shown in Figure 2c. A subsequent potential step back to -0.4 V leads to immediate readsorption of sulfate, which is accompanied by the generation of small Cu islands as a result of the expansion of the topmost Cu(111) for reconstruction (see Figure 2d). However, the overall terrace morphology of the Cu(111) surface remained unchanged. These features are consistent with those reported in the literature12,13 and will provide a basis for comparison in an attempt to elucidate anion effects on Sn UPD. Figure 3 gives general pictures of Sn UPD and stripping behavior on a sulfate-covered Cu(111) surface. Prior to

Figure 4. In situ STM images showing structural details of the Sn UPD on the Cu(111) surface at Vb ) -30 mV and It ) 0.5 nA in a solution of 0.1 mM SnSO4 + 0.5 M H2SO4: (panel a and the left-hand image of panel c) -0.38 V; (panel b and the right-hand image of panel c) -0.47 V. Image d is the image 80 s after the recording of image b (the region in the black line square is redisplayed with high brightness and contrast as the inset at the upper left). Scan size: 31 nm × 31 nm (a,b,d) and 10 nm × 6 nm (for both left and right images of image c).

UPD and in the sulfate adsorption region, the characteristic Moire´ pattern is present at the surface (see Figure 3a). Sn UPD takes place after a potential step to -0.47 V by nucleating almost exclusively at the terrace edge while the sulfate adlayer persists over the rest of the surface as shown in Figure 3b. The terrace edge nucleation is confirmed by the obvious protrusions above the terrace edge and the sectional analysis as well (see Figure 4c, right-hand image). Subsequently, UPD rapidly spread over the rest of the surface before the completion of terrace edge decoration. Immediately, the surface loses its integrity, exhibiting severe terrace edge reshaping and complete loss of the sulfate adlayer features as shown in Figure 3c. The fast change of the surface morphology results in tip scratching during imaging. The terrace edge reshaping during deposition cannot be attributed to merely the UPD-driven desorption of sulfate followed by the lifting of the surface reconstruction, which would otherwise lead to smoothing, rather than folding, of the terrace edge contour such as that on a bare copper surface (see Figure 2c). Most likely, the deposition involves surface alloying between Sn and Cu. Subsequent stripping of the UPD Sn further distorted the terrace identity with the generation of small islands as shown in Figure 3d. The terrace

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Figure 5. In situ STM images of the Cu(111) surface before (a) and during (b-e) Sn UPD and after the stripping of the Sn deposit (f) at Vb ) -20 mV and It ) 0.5 nA. Images were recorded at (a) -0.43 V, (b-e) -0.49 V (with a time interval of ∼1 min), and (f) -0.33 V in a solution of 0.1 mM SnSO4 + 0.5 M H2SO4 . Scan area: 71 nm × 71 nm.

reshaping is very different from that observed upon sulfate readsorption (see Figure 2d) and may serve as evidence of surface alloying. The rate of Sn UPD is found to be sensitively dependent on the applied potential as well as on the manner of the potential application. To gain deeper insights into both the structure as well as the dynamics of the UPD process, two sets of experiments are performed further. In the first set, in situ STM measurements were performed at a higher resolution. Since the initial stage of Sn UPD takes place at terrace edges, the area around the step is one of the focuses of the inquiry. On the other hand, it is insensible to monitor the step edge with a scanning area that is too small. There would be a high risk of missing the terrace edge initiation of Sn UPD within the inspection area because of the interference from the rapid second-stage deposition. As a compromise, a scanning area of ∼30 nm was chosen. All the aforementioned sulfate adlayer features, especially the legible Moire´ pattern modulation of the terrace edge contour, are clearly observed prior to UPD as shown in Figure 4a, which provides a basic reference for comparison for the subsequent images after the UPD process. Following stepwise potential decreases by 10 mV down to -0.47 V, Sn nucleates just at the terrace edges as shown by the arrow in Figure 4b, which manifest themselves in the appearance of the brightened Moire´ pattern modulation at the edge. To facilitate comparisons, enlarged images containing sulfate rows near the terrace edges with and without Sn are given in the right- and left-hand pictures of Figure 4c, respectively. A close inspection of the sulfate adlayer structure of these two images reveals that the bright terrace edge on the right is virtually composed of discrete spots that maintain almost exactly the periodicity of the pure sulfate on the left. The corrugation heights are ∼0.03 and ∼0.16 nm at the sulfate- and Sn-covered terrace edges, respectively, a 5-fold difference in contrast. The brightened edge was maintained only for a short time; then UPD proceeded rapidly with Sn quickly spreading toward the center of the terrace and the distortion of the terrace edges. While the surface after this later stage deposition appears cloudy

in general, bright dots can be resolved in various positions such as in the lower right of the top terrace marked by a circle. The bright dots are densely arranged in a nearclose-packed form with an average atomic distance of the two adjacent dots of ∼0.72 nm. This surface appearance is similar to what has been observed by Schmid et al. upon Sn deposition on Cu(111) in ultrahigh vacuum (UHV)24 wherein sparse Sn atoms were incorporated randomly in the Cu(111) surface lattice. In their work, a free-energy-driven surface alloying mechanism is proposed, which lends support to surface alloying of the present system. Based on the measured atomic distance (∼0.72 nm), it is estimated that ∼0.2 ML of Sn is deposited. The coverage is less than the value (0.4 ML) estimated from the CV (see Figure 1b) but is reasonable if taking into account the charge withdrawal due to sulfate desorption. Another piece of information is that the sulfate adlayer features still hold in the regions that are not yet covered by Sn (see the upper right region marked by a square in Figure 4d). They diminish only in the regions that are too small to establish such a coadsorption, as in the bare long narrow belt of ∼2 nm wide in the middle of the top terrace between two domains that contain bright spots. In an effort to observe the dynamics of the Sn UPD, the potential to initiate UPD is carefully controlled by applying a slow potential scan. As shown in Figure 5, an area with several terraces is chosen to include a number of initiation sites and to permit a comparative measurement on the dissolution of the deposited Sn at different coverages in a single experiment. Upon recording the image showing the sulfate adsorption features (see Figure 5a), a slow potential scan was applied and the UPD was initiated at -0.49 V; a time sequence measurement was taken at this potential. At the very beginning, Sn is nucleated in terraces A and B at the respective upper edges as indicated by an arrow (see Figure 5b). Subsequently, other terrace edges such as in terrace C of Figure 5c-e also were nucleated (24) Schmid, A. K.; Bartelt, N. C.; Hwang, R. Q. Science 2000, 290, 1562.

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within a time scale of 1 min. Focusing on the right edge of terrace A in Figure 5c-e, it is interesting that the local Moire´ pattern brightening always appears at the forefront sites of Sn deposition on the terrace (follow the pointed area by the wedge in the square). At the same time, the previous Moire´ pattern brightening disappears followed by new islands on the presumably sulfate-free surface of that region (small white arrow in Figure 5d) and/or growing of the old island (see Figure 5e). Such a Moire´ pattern brightening appearance and disappearance prevails on the entire surface. In this set of experiments, only subtle changes of the terrace edge contour were observed at the early stage of deposition. Course-dependent stripping, which is performed by reversing the potential to -0.31 V before Sn spreads all over a terrace surface, provides additional information important to elucidate the mechanism of the UPD: First, with a very small amount of Sn deposited only at terrace edges, the terrace and its contour remain unchanged (see terrace C in Figure 5f). As Sn extended toward the region slightly away from the terrace edge, the terrace width is reduced to the size enclosed by the forefront of the brightened Moire´ pattern (see terrace A in Figure 5f). Terraces that were largely covered by Sn lost their original characteristics leaving fragmentized copper pieces (see terraces A and B in Figure 5f). The results prove that any Sn deposit that inherits the Moire´ pattern orientation, regardless of the location on a terrace, can be stripped without leaving any noticeable changes on the terrace, and any Sn deposit that remains in the region free of sulfate involves surface alloying with the Cu topmost layer. Discussion The electrochemical and in situ STM results presented above have shown that Sn UPD on the sulfate-covered Cu(111) proceeds by a reversible adsorption at the periphery of the sulfate adlayer along the upper terrace edge at the initial stage. This is followed by rapid spreading of the Sn atoms toward the center accompanied by a surface alloying. The upper terrace edge initiation in Sn UPD signals an important fact that the copper surface is well protected by the presence of the sulfate adlayer and that the energetically most favorable sites for UPD are likely the edge sites. The initial decoration of step edges is observed in a number of previously reported UPD systems owing to a reduced work function at step edges (e.g., UPD of Ag25 and Pd26 on Au(111)). In those systems, the decoration occurs at the lower terrace edge of a step (generally referred to as a “step edge”, see Figure 6a) followed by development toward the lower terrace center. In contrast, our results show a decoration of Sn at the upper terrace edge. The sulfate adlayer, which is adsorbed together with water to form a strong adsorption film, occupies the step edge defined by two copper terraces. Consequently, the arrival of Sn atoms at the step edge would have to proceed either by coadsorption with or by displacing the sulfate and water molecules. A comparative experiment has been carried out in perchloric acid as the supporting electrolyte, in which nucleation is observed randomly on terraces without preference at the step edge (see Figure 4S of the Supporting Information). This further confirms the observed sulfate influence on Sn UPD on Cu(111) surfaces. An interesting aspect is that the bright spots formed at the terrace edge in the initial stage of Sn UPD exhibit a (25) Esplandiu, M. J.; Schneeweiss, M. A.; Kolb, D. M. Phys. Chem. Chem. Phys. 1999, 1, 4847. (26) Kibler, L. A.; Kleinert, M.; Randler, R.; Kolb, D. M. Surf. Sci. 1999, 443, 19.

Figure 6. A conceptual scheme of a surface step edge (a) and proposed models for the Sn adlayer in the initial (b) and subsequent stages of Sn UPD (c). The reconstruction of the topmost Cu layer is omitted for clarity.

detailed structure almost identical to that of the sulfate rows (see Figure 3b). A plausible explanation for the invariance is the direct deposition of Sn on the top of the sulfate layer. However, there is no reason such a process is restricted only to the periphery of the sulfate adlayer. Alternatively, it is probable that only a single row of single Sn atoms are deposited in the form of lateral coadsorption with sulfate at the periphery of the sulfate adlayer and that the bright spots are the results of a Sn-induced local enhancement of tunneling probability that reaches out to several sulfate molecules in the vicinity. The mechanism governing the local enhancement of tunneling probability remains to be understood. Based on the model proposed by Wandelt and co-workers13,14 that sulfate lies on a 2-fold bridge site of the reconstructed topmost layer of the Cu(111) surface, the possible sites for Sn to coadsorb can only be at the interstice between the sulfate adlayer and the topmost Cu layer along the contour of the terrace edge as depicted in Figure 6b. An analogous assignment may be applied to account for the Moire´ pattern brightening

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Table 1. Features of Typical UPD Systems with and without Anion Effect UPD system

anion influence 2-

Au(111)/Sn, SO4 Au(111)/Sb, SO42Au(111)/Ag, SO42Au(111)/Cu, SO42Au(111)/Ag,

I-

Au(111)/Cu, Cl2-

Cu(111)/Sn, SO4 a

negligible negligible repulsive effect on adatomadatom interaction induced adsorption, attractive columbic interaction preadsorbed, attractive columbic interaction preadsorbed, attractive columbic interactions protective strong adsorption film

dominant interactions

metal adlayer structurea

adatom-substrate adatom-substrate adatom-substrate

size-confined cluster Au-Sb surface alloy instable adlattices

adatom-substrate, adatom-anion

adatom-anion, anion-anion

ordered Cu-(bi)sulfate coadsorbed adlattice ordered Ag-I- coadsorbed adlattice Cu/Cl- bilayer

adatom-substrate, anion-substrate, anion-anion

denotative adsorption, Sn-Cu surface alloy

adatom-substrate, adatom-anion

The given adlayer structures correspond to that at low coverage.

of the forefront sites of Sn deposition. We refer to this special type of coadsorption of Sn as “parasitical” adsorption. On the basis of the experimental observations such as the epitaxial structure of Sn at the periphery of the sulfate adlayer, the course-dependent stripping features and other information contained in Figure 2, Figure 4, and Figure 5, we reason that it is the most initially deposited Sn atoms that keep displacing the sulfate (as well as the coadsorbed water molecules) from the edge of the sulfate adlayer film and pushing themselves toward the center of the terrace. As a result, the displacement process continuously provides sites for further Sn deposition on the sulfate-free region of the terrace. Figure 6 depicts the structural pattern and the dynamics of the Sn UPD on the sulfate-covered Cu(111) surface. Assuming that the displacement of sulfate by Sn does not immediately lift the Cu(111) reconstruction, the expanded lattice is expected to be energetically more favorable for surface alloying; strain in accommodating larger Sn atoms in the Cu surface lattice is relieved by the expansion of the surface lattice, leading to fast surface alloying. This contrasts significantly with either the Cu(111)/Sn in UHV or other UPD systems such as Tl on Ag (111) and Ag(100)22,23 and Pb on Au(111).27 The effective size mismatch between Cu and Sn in the present work is reduced by the reconstruction of the Cu(111) surface and is less than those of other systems for comparison. For example, in the case of Tl and Pb on Ag and Au, wherein anion effects are absent, the larger size mismatch prevents the site exchange at the beginning until an ordered layer of Tl is formed and the structural transformation appears as a nonequilibrium phenomenon. In comparison to Cu(111)/Sn in UHV, the faster surface alloying of the present work is likely the result of effects from the electrolyte: the cloud of species from the electrolyte in close contact with the surface hinders diffusion of the Sn islands across the surface, which in turn promotes the site exchange. However, a question as to why the kinetically limited sulfate displacement proceeds efficiently remains to be understood. The above analysis leads us to suggest the following scenario. Owing to the strong sulfate adsorption, the initiation of UPD takes place in the manner of parasitical adsorption along the periphery of the sulfate adlayer close to the upper terrace edge. The presence of such Sn adatoms induces the local enhancement of tunneling that reaches out to several sulfate molecules in the vicinity, leading to the brightening of the Moire´ structure modulation at the terrace edge. The parasitically adsorbed Sn can be stripped off reversibly. Shortly afterward, the initially deposited Sn adatoms begin displacing the sulfate ions from the (27) Green, M. P.; Hansan, K. J. Surf. Sci. Lett. 1991, 259, L743.

edge and advance toward the interior region of the terrace, during which the brightened Moire´ pattern is maintained at the forefront sites. Consequently, a region free of sulfate is provided for further Sn deposition, wherein surface alloying is favored, in view of strain relief, by the expanded topmost layer of the reconstructed Cu(111) surface, leading to terrace edge reshaping and fragmentizing after stripping. The overall deposition process may be completed within a time window of several minutes. Finally, we compare typical UPD systems in view of interactions among substrate, adatoms, and anions; the relative strength among them determines characteristic adlayer structures. Listed in Table 1 are several UPD systems, containing SO42- or halide in the electrolytes, wherein UPD features are outlined in the order from simple adatom-substrate interaction to various multiple interactions. (1) For Sn UPD on Au(111)28 in the presence of sulfate, the sulfate is found to adsorb neither on the Au(111) nor on Sn adatoms in the potential region of interest. The Sn UPD process and adlayer structure are entirely determined by the adatom-substrate interaction. As a result, random size-confined clustering and anisotropic growth behavior on Au(111) (1 × 1) and Au(111) (x3 × 22), respectively, become the characteristics. Similar systems are Pb on Au(111)29 and Sn on Au(100)30 where small islands and clusters are formed, respectively. (2) With an increasing adatom-substrate interaction via directional bonding and electron back-donation, Sb UPD on Au(111) has been observed to induce relaxation of the (x3 × 22) reconstruction to corrugation lines of a trigonal network31 as well as surface alloying.32 (3) Ag on Au(111)25 in sulfate-containing solution starts to show subtle anion influence with a repulsive effect on adatom-adatom interaction within the adatom layer. In this case, various unstable adlayer structures have been observed. (4) However, when attractive adatom-anion interaction becomes significant, the adlayer structure is determined by both the adatom-substrate and adatom-anion interactions. Anions are then induced to incorporate into the adlattice that are stable even at low coverage of adatoms. UPD of Cu33-36 and Hg37 on Au(111) are two typical (28) Mao, B. W.; Tang, J.; Randler, R. Langmuir 2002, 18, 5329. (29) Tao, N. J.; Pan, J.; Li, Y.; Oden, P. I.; DeRose, J. A.; Lindsay, S. M. Surf. Sci. Lett. 1992, 271, L338. (30) Yan, J. W.; Tang, J.; Yang, Y. Y.; Wu, J. M.; Xie, Z. X.; Sun, S. G.; Mao, B. W. Surf. Interface Anal. 2001, 32, 49. (31) Wu, Q.; Shang, W. H.; Yan, J. W.; Xie, Z. X.; Mao, B. W. J. Phys. Chem. B 2003, 107, 4065. (32) Wu, Q.; Shang, W. H.; Yan, J. W.; Mao, B. W. J. Mol. Catal. A: Chem. 2003, 199, 49. (33) Magnussen, O. M.; Hagebo¨ck, J.; Hotlos, J.; Behm, R. J. Faraday Discuss. Chem. Soc. 1992, 94, 329. (34) Manne, S.; Hansma, P. K.; Massie, J.; Elings, V. B.; Gewirth, A. A. Science 1991, 251, 183. (35) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. R. Phys. Rev. Lett. 1995, 75, 4472.

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systems of this case. Even for systems with anion preadsorption such as Ag UPD on Au(111) in iodidecontaining solution,38,39 the follow-up induced anion coadsorption may take place. (5) If the anion-anion interaction is comparable to the adatom-anion interaction, the adlattice can be further restrained and the formation of an adatom/anion bilayer is expected. This is the case for Cu UPD on Au(111)36,40,41 and Cd UPD on Cu(111)19 in the presence of chloride. (6) In the present case of Sn UPD on Cu(111), both the anion-substrate and anion-anion interaction are strongest among the listed systems. This severely restricts the UPD process so that adlayers cannot form in usual way and is possible only at very special sites as an extension of the sulfate adlayer. On the other hand, the persistence of pure sulfate adlayer domains at any stage before the entire surface is deposited with Sn, as well as surface alloying during UPD, clearly indicates that the sulfate-sulfate, sulfate-Cu, and Sn-Cu interactions are comparatively strong. The sulfate-Sn interaction is relatively weak yet effective for electronic coupling between the two species; this leads to the observed tunneling enhancement of sulfate in the vicinity. Conclusions We have shown the unusual behavior of UPD Sn on Cu(111) in the presence of a strongly adsorbed sulfate

Yan et al.

adlayer. The strong sulfate-Cu and sulfate-sulfate interactions hinder the formation of the usual ordered metal adlayer. As a consequence, the initiation of UPD is characterized by the parasitical adsorption of Sn adatoms along the periphery of the sulfate adlayer. The Sn-sulfate interaction is effective so that the presence of a small amount of parasitically adsorbed Sn can induce local enhancement of tunneling that reaches out to several sulfate molecules in the vicinity. The reconstruction of Cu(111) with the expanded topmost layer plays an important role in promoting surface alloying. The extensive information obtained in the present work reveals that as anion-substrate and anion-anion interactions are sufficiently strong and comparable to the adatomsubstrate interaction, the formation of the metal adlayer can be severely restricted and some novel features of UPD are displayed. Acknowledgment. The financial support provided by the Natural Science Foundation of China (NSFC No. 29973040, 29833060, 20021002) is gratefully acknowledged. The authors express heartfelt thanks to Professors Y. L. Chow and C. Malendries for improving the English of the manuscript and Dr. W. Wandlowski for his helpful discussions.

(36) Shi, Z. C.; Wu, S. J.; Lipkowski, J. Electrochim. Acta 1995, 40, 9. (37) Li, J.; Abruna˜, H. D. J. Phys. Chem. B 1997, 101, 244. (38) Sugita, S.; Abe, T.; Itaya, K. J. Phys. Chem. B 1993, 97, 8780. (39) Yamada, T.; Batina, N.; Itaya, K. J. Phys. Chem. B 1995, 99, 8817. (40) Herrero, E.; Glazier, S.; Abruna˜, H. D. J. Phys. Chem. B 1998, 102, 9825. (41) Wu, S.; Lipkowski, J.; Tyliszczak, T.; Hitchcock, A. P. Prog. Surf. Sci. 1995, 50, 227.

Supporting Information Available: Enlarged STM images for better identification of the dynamic behavior described in Figure 5; electrochemical and in situ STM results of Sn UPD in perchloric media. This material is available free of charge via the Internet at http://pubs.acs.org. LA034500S