Effect of Adsorbed Iodine on the Dissolution and Deposition Reactions

surfaces, the anodic dissolution of Ag occurred exclusively at step-edges in a ... However, whereas jagged monatomic step lines were found on the I-fr...
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J. Phys. Chem. B 1997, 101, 2046-2053

Effect of Adsorbed Iodine on the Dissolution and Deposition Reactions of Ag(100): Studies by In Situ STM Takuya Teshima, Katsuhiko Ogaki, and Kingo Itaya*,† Itaya Electrochemiscopy Project, ERATO, JRDC, 2-1-1 Yagiyama- minami, Taihaku-ku, Sendai 982, Japan ReceiVed: December 3, 1996; In Final Form: January 16, 1997X

In situ scanning tunneling microscopy has been employed to determine the structure of iodine adlayers formed on Ag(100) and to evaluate the role of such adlayers in the dissolution and deposition reactions of the substrate in perchloric acid. In the double-layer potential region, iodine was found to possess a c(2 × 2) structure; this structure was unaffected by anodic dissolution of the Ag substrate. On both bare and I-pretreated Ag(100) surfaces, the anodic dissolution of Ag occurred exclusively at step-edges in a layer-by-layer mechanism. However, whereas jagged monatomic step lines were found on the I-free surface, relatively smooth (straight) step lines existed along the direction of iodine atomic rows ({100}) on the Ag(100)-I surface. Etching rates on Ag(100)-I were dependent on the direction of the step-edges, which suggests that the adlayer near such edges plays a crucial role in the dissolution reaction. The deposition of Ag on Ag(100)-I was also found to occur preferentially at step-edges aligned along a particular direction. A model is proposed to explain the anisotropic dissolution and deposition.

Introduction It is well-known that electrochemical surface reactions are strongly affected by the composition and structure of the substrate-adsorbate-electrolyte interface.1-4 A familiar example is the profound dependence of the underpotential deposition of hydrogen and metal ions not only on the crystallographic orientation of the single-crystal surface but also on the structure of the adsorbed layers.4 The detailed elucidation of the nature of the substrate-adsorbate-electrolyte interface is thus the essential first step if electrochemical reactions are to be understood at the atomic level. In this regard, scanning tunneling microscopy (STM) under electrochemical conditions has become one of the more prominent in situ techniques in the study of interfacial structures with atomic resolution.4,5 The characterization of anodic dissolution and cathodic deposition processes at metal electrodes is of fundamental and practical interest in electrochemical surface science. The siteselective anodic dissolution of Pd in noncorrosive electrolyte catalyzed by a monolayer of iodine was first reported based upon low-energy electron diffraction (LEED).6-8 The dissolution of Pd(111) and Pd(100) surfaces was found to follow a layer-by-layer mechanism. This phenomenon of adsorbatecatalyzed corrosion was recently investigated further by in situ STM. It was found that at low dissolution rates corrosion transpired exclusively at step-edges, not at terraces.9-11 In a case that demonstrated substantial structure sensitivity, the reaction at Pd(100)-c(2 × 2)-I was found to occur anisotropically along the atomic steps aligned in the {100} direction.11 In situ STM studies were also carried out on the anodic dissolution in H2SO4 of Ni from a Ni(100) surface that contained a monolayer of S.12,13 Similar to the Pd(100)-c(2 × 2)-I case, the anodic dissolution of Ni(100)-c(2 × 2)-S13 was found to (i) proceed by a layer-by-layer mechanism, (ii) occur only at stepedges, and (iii) take place anisotropically with the rate higher along [001] than along the [010] step-edges. The unexpected dissolution anisotropy at Pd(100)-I and Ni(100)-S has been † Department of Applied Chemistry, Faculty of Engineering, Tohoku University, Aramaki, Aoba, Sendai 980, Japan. X Abstract published in AdVance ACS Abstracts, February 15, 1997.

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explained in terms of the atomic configurations of I and S atoms at step-edges aligned along different directions. 11,13 It should be noted that, in an in situ STM study of the anodic dissolution of Cu(100) in HCl, anisotropic dissolution along the {100} direction was also found.14 Since the Cl-on-Cu(100) adlayer possesses a c(2 × 2) structure,14 it is possible that a rationale identical with that proposed for Pd(100)-c(2 × 2)-I and Ni(100)-c(2 × 2)-S is also operative in the Cl-(aq)-assisted dissolution of Cu(100). The striking similarities observed for the three different (100)oriented metal substrates (Pd, Ni, Cu) pretreated with different adsorbates (I, S, Cl) but having identical [c(2 × 2)] structures strongly suggest that adlayer structure plays the dominant role in the anodic dissolution of (100)-oriented metal surfaces. The present study, on the structure of chemisorbed iodine and its influence on the dissolution-deposition processes at the Ag(100) electrode surface, was undertaken in order to further assess this role. Experimental Section A commercially grown Ag(100) single crystal (MaTeck Inc., Germany: 10 mm in diameter, 2 mm in thickness) was metallographically polished and sequentially sonicated in acetone, methanol, and pure water. The single crystal was then placed for 2 h in a quartz tube continuously purged with purified H2 and maintained at 1100 K. The sample was cooled to room temperature under H2 steam and subsequently brought into contact with ultrapure water saturated with H2 and then transferred into an electrochemical cell with a droplet of pure water on it to mitigate surface contamination. For the preparation of the iodine monolayer on Ag, the clean surface must be exposed to the KI solution under potential control; immersion at open circuit potential (OCP) leads to the formation of bulk AgI and roughened surfaces.15 It should be mentioned that several recent studies on I-pretreated Ag electrodes using STM16,17 and X-ray photoelectron spectroscopy18 may have been complicated by the presence of AgI multilayers, since the I adlayers in those studies were prepared by immersion at OCP. In this study, the clean Ag(100) electrode © 1997 American Chemical Society

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Figure 1. Cyclic voltammograms of bare Ag(100) (a) and I-Ag(100) (b) electrodes in 0.1 M HClO4 and in 0.1 M HClO4 + 1 mMKI (c). The scan rate was 5 mV/s.

was immersed in a solution that contained 0.1 M HClO4 and 1 mM KI at a specific potential in the double-layer region. After 5 min, the electrode was emersed at the same potential and immediately transferred to the electrochemical cell containing pure HClO4 for the voltammetry and STM experiments. All electrochemical potentials were measured with respect to a reversible hydrogen electrode (RHE) in 0.1 M HClO4. Voltammograms were obtained with a PAR 273-A potentiostat. Experimental details have been described previously.11,15 A Nanoscope III (Digital Instruments, Santa Barbara, CA) with a custom-built electrochemical cell was used for the STM work. All images were acquired in a constant-current mode using a fabricated tip made from a commercial Pt-Ir (80:20) wire. The tip was coated with an insulating enamel to minimize contact of its sides with the electrolyte. The end of the tip was polished by a turning grindstone.11 All tips were previously soaked in the electrolyte solution for several hours to eliminate soluble contanimants before STM measurements were started. Results and Discussion Voltammetry. Figure 1 shows cyclic voltammograms (CV) for the bare Ag(100) (Figure 1a) and I-pretreated Ag(100) (Figure 1b) electrodes in 0.1 M HClO4. Figure 1c shows the CV obtained in 0.1 M HClO4 that also contained 1 mM KI. In the absence of KI, the anodic dissolution of Ag commenced at ca. 0.5 V (vs RHE). The cathodic peaks at 0.55 V observed during the cathodic scan correspond to the electrodeposition of Ag metal from the initially formed Ag+ ions. It is clear that the Ag dissolution-deposition reactions occur reversibly at the same potential regardless of whether I was present or absent at the Ag(100) surface. The case is different, however, for the

Figure 2. Large-scale (150 nm × 150 nm) (a) and atomically resolved (b) STM images of bare Ag(100) surface in 0.1 M HClO4. The Ag(100) and tip electrode potentials were held at 0.1 and 0.6 V, respectively. The tunneling current was 15 nA. Arrows indicate the {110} directions.

hydrogen evolution reaction; this commenced at -0.2 V on the bare Ag(100) electrode but ca. -0.30 V on the Ag(100)-I surface (Figure 1b). A similar adsorbate-induced overpotential for the hydrogen evolution reaction was found on I-pretreated Au and Pt electrodes.19,20 In Figure 1c, the onset potential for the anodic dissolution current was at 0.1 V; the overall reaction in the presence of I- ions involves Ag dissolution to Ag+ ions followed by immediate precipitation of bulk AgI. Figure 1c shows that no bulk AgI phase is formed in the potential range between 0.1 and -0.2 V, the conditions used here in the preparation of the I monolayers. It may be noted that the hydrogen evolution reaction is further shifted to negative potentials when iodide ions are present in solution. It is important to mention that for Ag(111) the CV in the presence of I- ions is characterized by small reversible peaks at potentials slightly negative of bulk AgI formation.15,21 Studies based on LEED and STM showed that the peaks are due to rotational phase transitions in the Ag(111)-I adlayer. No similar

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Figure 3. Large scale (50 nm × 50 nm) (a) and atomically resolved (b) STM images of I-Ag(100) surface in 0.1 M HClO4. The electrode potentials and tunneling current were the same for Figure 2. Arrows in (a) and (b) indicate the [100] directions. A model of c(2 × 2) is depicted in (c).

peaks are found for Ag(100) (Figure 1c). The two cathodic peaks found in Figure 1c are suggestive of an adlayer phase transition that accompanies the reduction of bulk AgI. In Situ STM. (i) I-Free Ag(100). Figure 2a shows a typical large-scale STM image of the bare Ag(100) electrode in 0.1 M HClO4 at 0.1 V. Atomically flat terraces can be seen that extend over a few hundred nanometers without the presence of pits and islands frequently found on Pt and Rh.22,23 A surfaceannealing process may be responsible for the wide terraces on Ag that arise from surface migration, which is faster on Ag than on Pt and Rh.24-26 Steps, mostly monatomic in height, are characterized by arc-shaped (rather than straight) edges; relatively large islands are circular in morphology. A similar surface morphology has been reported for real and quasi-perfect Ag(100) electrodes.25,27 The shape and location of the step lines observed in Figure 2a were not significantly changed in the potential range between 0.4 and -0.1 V, suggesting the absence of potential-induced reconstruction on the bare Ag(100) in contrast to that found on Au(100) electrodes.28,29

Figure 2b shows a high-resolution STM image acquired on the atomically flat terrace. It can be clearly seen that the Ag(100) surface has a square lattice with an interatomic distance of ca. 0.29 nm along the {110} direction; the corrugation amplitude of each Ag atom is about 0.05 nm. The image shown in Figure 2b demonstrates that, under the present conditions, the Ag(100) surface retains the unreconstructed (1 × 1) structure, consistent with previous results.27,30 Images obtained in the potential range between 0.4 and -0.1 V also showed the (1 × 1) structure; the absence of potential-induced reconstruction in this potential region is thus demonstrated. (ii) I-Modified Ag(100). Figure 3 shows typical large-scale (Figure 3a) and high-resolution (Figure 3b) STM images of the Ag(100)-I surface in HClO4 at 0.1 V. It is interesting to note that the monatomic steps now run parallel to the {100} direction, rotated by 45° with respect to the atomic rows of the Ag substrate. The change in the step direction may have occurred upon exposure of the Ag(100) surface to the KI solution. The interatomic I-I distance was found to be 0.4 nm, which

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Figure 4. Time-sequenced STM images showing the etching process of bare Ag(100) in 0.1 M HClO4. The image (a) was acquired at 0.4 V before dissolution was started. The images (b and c) were obtained consecutively at a time interval of 2 min at a potential near the onset of the anodic current. The tunneling current was 10 nA. Arrows indicate the {110} directions. The tip electrode potential was held at 0.6 V.

corresponds to x2 times the lattice constant of Ag(100); in other words, it may be concluded that the I adlayer possesses a c(2 × 2) structure. Previous ultrahigh vacuum-based studies on the chemisorption of iodine from I2 vapor showed a c(2 × 2) LEED pattern on Ag(100).31 Our in situ STM result is thus in good agreement with the ex situ LEED data. A more important observation is that the c(2 × 2) structure was consistently found in the double-layer region (from 0.5 to -0.2 V); at least under the present conditions, no potential-induced structural changes in the iodine adlayer are indicated. It is important to view the potential-independence of the Ag(100)-I adlayer structure in the context of what has been observed for the Ag(111)-I adlayer.32-34 Our recent work on the Ag(111)-I adlayer by means of tandem in situ STM and ex situ LEED15 revealed a continuous compression of the adlayer, from (x3 × x3)R30°, via c(p × x3R-30°), into a rotated hexagonal phase in acidic HI solutions when the I coverage was increased as a result of an increase in the applied potential.15 In general, the (111) planes of Ag and Au exhibit several

incommensurate iodine structures. On the platinum metals, such as Pt(111), Pd(111), and Rh(111), only commensurate structures are found.1,2,20,35-37 The single commensurate structure of I on the Ag (100) surface may be explained by the difference in the coordination numbers of the iodine adatoms on the (100) and (111) surfaces. As shown in the model structure in Figure 3c, each iodine adatom on Ag (100) is on a 4-fold hollow site. Hence, each iodine atom has a coordination number of four. In comparison, an iodine atom on a 3-fold hollow site on Ag(111) has a coordination number of only three. The exceptional stability of I on Ag(100), due to a higher coordination number, may explain the formation of the single commensurate c(2 × 2) structure. Dissolution Process. (i) Bare Ag(100). Figure 4 shows a set of STM images of an I-free Ag(100) electrode during the anodic dissolution in 0.1 M HClO4. Figure 4a, acquired at 0.4 V, reveals atomically flat terraces and monatomic steps similar to what was noted in Figure 2a. Virtually identical images were observed at the same potential for at least 15 min, indicating

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Figure 5. Time-sequenced STM images showing the etching process of I-Ag(100) in 0.1 M HClO4. The images were acquired at the time intervals of 32 s. The tunneling current was 5 nA. The tip electrode potential was held at 0.6 V.

immobilization of the step-lines after attainment of equilibrium. At more positive potentials (parts b and c of Figure 4), the retraction of the monatomic steps is evident. Such a process indicates that at potentials close to the onset of anodic dissolution Ag dissolves only from the step-edges; all terraces remain pitfree. It must be noted, however, that the step-lines have become more irregular (jagged) as the dissolution progresses. Although the step-lines were not always parallel to the Ag atomic rows (the [110] direction), the appearance of jagged step-lines suggests short-range preferential dissolution along the [110]directed step-lines. It is remarkable to note that the jagged lines gradually reverted to smooth lines when the potential was returned to 0.4 V. This potential-induced surface-annealing process is most likely due to surface diffusion of the interfacial Ag atoms. (ii) Ag(100)-I. As was shown in Figure 3a, the monatomic step-lines found on the Ag(100)-I were mostly parallel to the atomic rows of iodine atoms in either the [001] or [010] directions, rotated 45°with respect to the close-packed Ag atomic

rows. Figure 5 shows a set of STM images acquired sequentially after a preselected time interval (32 s) after the onset of (low-current) anodic dissolution. It can be seen that although the step-lines remained nearly parallel to the [001] and [010] directions, they were not always atomically straight. This result indicates the presence of kink sites along the step-lines. For example, whereas the step-line marked by S1 is almost perfectly aligned along the [010] direction, the S2 step is not. The initial jaggedness in the latter (S2) step-lines was rendered smooth as a result of the anodic dissolution; staircase-shaped edges were formed as shown in Figure 5c. The step-edge marked by the arrow in Figure 5a also includes several kink sites of various orientations. This step-line was likewise converted to a staircase-shaped step-edge (Figure 5c). Although it has become clear that the etching of Ag on Ag(100)-I takes place exclusively at the step-edges, similar to that found on the bare Ag(100), one can recognize that the etching rate on Ag(100)-I along the [010] direction is faster than along the [001] direction. As an illustration of this anisotropic etching,

Effect of Adsorbed Iodine

Figure 6. Schematic depictions of anodic dissolution processes of Ag(100) with differently aligned c(2 × 2)-I adlayers on the terraces. The arrows indicate the direction of anodic dissolution reaction.

it can be seen that the step-lines marked by C1 and C2 in Figure 5 remain unchanged while the steps along the [010] direction are progressively retracted. Several other sets of time-dependent STM images were obtained in different areas during the anodic dissolution; invariably, similar anisotropic etching processes were found. The anisotropic dissolution found on Ag(100)-I can be rationalized in terms of the surface structural models shown in Figure 6. These models include three atomic layers with the steps along either [010] or [001] directions; the topmost layer is on the lower right-hand side. In the absence of iodine, the step-lines along [001] and [010] directions are equivalent, but they are nonequivalent in the presence of the c(2 × 2) adlayer. In the proposed model, all the I atoms on the terraces are located at the 4-fold hollow sites, even near the step-edges. As shown in Figure 6a, the arrangement of I near the step-edges is dependent on the direction of the step-edges: (i) the steps along the [010] direction include larger spaces near the edge compared to those along [001]; (ii) the I rows are aligned in a straight line across the three atomic layers along the [010] direction; (iii) in the [001] direction, the I rows are shifted by a halfspace at each step-edge. These differences in the alignment of the atomic rows had been clearly discerned by high-resolution

J. Phys. Chem. B, Vol. 101, No. 11, 1997 2051 STM images of Ni(100)-S.13 The upper-to-lower-terrace diffusion of the first I row is driven by the need to minimize the free energy change during Ag dissolution along the step-edges.13 Because of its open structure, the anodic dissolution along the [010] direction is expected to occur more easily than along the [001] direction. The Ag atoms marked A and B in Figure 6a are thus the first to be dissolved accompanied by the diffusion of the iodine atom from the upper to the lower terrace. After dissolution of A and B, the same process is expected to continue along the [010] direction; this results in the propagation of the anisotropic etching along the direction marked by the arrows in Figure 6. It is important to note that the different alignments of the atomic rows of iodine do not always extend through several atomic layers. The I adlayer on each terrace has an equal probability to be shifted by the half-space with respect to those on lower and upper terraces; such a shift may result in an alternative alignment as shown in Figure 6b. In this case, the anisotropic etching of the topmost layer is expected to proceed along the [001] direction as indicated by the arrow. The etching of the second layer must then propagate along the [010] direction. It should be noted that this mode of anisotropic etching, rotated by 90° with respect to a particular step-terrace configuration, was observed on much higher or much lower terraces (Figure 5). Nevertheless, the overall features for the anodic dissolution of Ag on Ag(100)-I can be explained by the model structures shown in Figure 6. After the image in Figure 5c was acquired, the electrode potential was increased further by 25 mV. Although a faster etching rate occurred, it was still possible to obtain STM images, but the step-lines were now less clear because of enhanced dissolution at the step-edges (Figure 7a). Nevertheless, it was clear that the anodic dissolution still occurs via a layer-by-layer process. Deposition of Ag on Ag(100)-I. After several Ag layers were anodically stripped under the conditions of Figure 7a, the electrode potential was stepped to 0.4 V where dissolved Ag+ ions are expected to be redeposited onto the Ag(100)-I surface. Parts b-d of Figure 7 were sequentially acquired (at 32 s intervals) after the deposition process was initiated. In the first interval (Figure 7b), the surface was found to be rapidly smoothed; in addition, the step-lines were observed to advance to enlarge the atomically flat terraces. It can be seen that the pits found on the terraces marked T1 and T2 have disappeared from parts b and c of Figure 7. The possibility of anisotropic Ag deposition was monitored via the changes in the morphology of the terrace T1. As shown in Figure 7b, the step-edge marked by S1 is jagged and not parallel to the [010] direction. This irregular step-line transformed into a more pronounced staircase shape (Figure 7c). Furthermore, the terrace T1 has expanded toward the [001] direction (Figure 7d). More significantly, it can be recognized that the step-lines along the [010] direction moved more rapidly than the step-lines along the [001] direction. For example, the step-line S1 advanced toward the [001] direction while the step marked by C1 remained unchanged. It is thus clear that (i) the electrochemical deposition of Ag proceeds by a layer-by-layer mode and (ii) the step-lines along the [001] and [010] directions are not equivalent with respect to the deposition reaction. These observations strongly suggest that the growth rate at open-spaced step-lines (Figure 6) would be substantially greater than the growth rate along other directions. In this context, it should be mentioned that similar anisotropic deposition was reported for Cu deposition on Cu-

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Figure 7. STM images (a) acquired in the same area as for Figure 5 under the condition of more rapid anodic dissolution. The images (b, c, and d) were acquired consecutively at time intervals of 32 s after the potential step to 0.4 V, showing the continuous deposition process of Ag+ on I-Ag(100). Arrows indicate the {001} directions.

(100) in HCl.14 Such a process could also be explained by the model proposed here. Acknowledgment. The authors are grateful for financial support from the ERATO Project, JRDC. We thank Professor M. P. Soriaga (Texas A&M University) for his help in the writing of this manuscript. References and Notes (1) Hubbard, A. T. Chem. ReV. 1988, 88, 633. (2) Soriaga, M. P. Prog. Surf. Sci. 1992, 39, 325. (3) Lipkowski, J., Ross, P. N., Eds. Adsorption of Molecules at Metal Electrodes; VCH Publishers: New York, 1992. (4) Trasatti, S., Wandelt, K., Eds. Surface Science and Electrochemistry; Elsevier: Amsterdam, 1995. (5) Siegenthaler, K. In Scanning Tunneling Microscopy II; Wiesendanger, R., Gu¨ntherodt, J., Eds.; Springer-Verlag Press: Berlin, 1992. (6) McBride, J. R. ; Soriaga, M. P. J. Electroanal. Chem. 1991, 303 255. (7) Schimpf, J. A.; McBride, J. R.; Soriaga, M. P. J. Phys. Chem. 1993, 97, 10518.

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