Role of the Anion in the Underpotential Deposition of Cadmium on a

These coupled processes resulted in an erroneous coverage of Cd adatoms. ... of Ni anodic dissolution from EIS” [J Solid State Electrochemistry (200...
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J. Phys. Chem. B 2005, 109, 14917-14924

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Role of the Anion in the Underpotential Deposition of Cadmium on a Rh(111) Electrode: Probed by Voltammetry and in Situ Scanning Tunneling Microscopy Liang-Yueh Ou Yang,† Fahd Bensliman,‡ Chia-Haw Shue,§ Yaw-Chia Yang,§ Ze-Haw Zang,† Li Wang,† Shueh-Lin Yau,*,‡,§ Soichiro Yoshimoto,†,‡ and Kingo Itaya*,†,‡ Faculty of Engineering, Tohoku UniVersity, 6-6-04 Aoba, Sendai 980-8579, Japan, CREST, JST, 4-1-8 Kawaguchi, Saitama 332-0012, Japan, and Department of Chemistry, National Central UniVersity, Chungli, Taiwan 320 ReceiVed: March 4, 2005; In Final Form: May 22, 2005

In situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV) were employed to examine the underpotential deposition (UPD) of cadmium on a rhodium(111) electrode in sulfuric and hydrochloric acids. The (bi)sulfate and chloride anions in the electrolytes played a main role in controlling the number and arrangement of Cd adatoms. Deposition of Cd along with hydrogen adsorption occurred near 0.1 V (vs reversible hydrogen electrode) in either 0.05 M H2SO4 or 0.1 M HCl containing 1 mM Cd(ClO4)2. These coupled processes resulted in an erroneous coverage of Cd adatoms. The process of Cd deposition shifted positively to 0.3 V and thus separated from that of hydrogen in 0.05 M H2SO4 containing 0.5 M Cd2+. The amount of charge (80 µC/cm2) for Cd deposition in 0.5 M Cd2+ implied a coverage of 0.17 for the Cd adatoms, which agreed with in situ STM results. Regardless of [Cd2+], in situ STM imaging revealed a highly ordered Rh(111)-(6 × 6)-6Cd + HSO4- or SO42- structure in sulfuric acid,. In hydrochloric acid, in situ STM discerned a (2 × 2)-Cd + Cl structure at potentials where Cd deposition commenced. STM atomic resolution showed roughly one-quarter of a monolayer of Cd adatoms were deposited, ca. 50% more than in sulfuric acid. Dynamic in situ STM imaging showed potential dependent, reversible transformations between the (6 × 6) Cd adlattices and (x3 × x7)-(bi)sulfate structure, and between (2 × 2) and (x7 × x7)R19.1°-Cl structures. The fact that different Cd structures observed in H2SO4 and HCl entailed the involvement of anions in Cd deposition, i.e. (bi)sulfate and chloride anions were codeposited with Cd adatoms on Rh(111).

Introduction Underpotential deposition (UPD) of metal on ordered single crystal electrodes of Au, Pt, Ag, etc. has been one of the most extensively studied subjects in interfacial electrochemistry.1-4 Research employing a wide variety of techniques in the past decade has allowed precise determination of the amount of metal being deposited and the spatial arrangement on the electrode.5,6 Coadsorption of anion, such as (bi)sulfate and halide, appears ubiquitously in UPD. For example, adsorbed (bi)sulfate anions form a (x3 × x3)R30° template on Au(111), under which copper adatoms are deposited into a honeycomb pattern.7-11 In contrast, in situ STM shows a “(5 × 5)”-Cu,Cl structure in the presence of Cl-. The atomic features seen by the STM are thought to be the uppermost HSO4- or Cl- adlayer on top of Cu adatoms.8,12 Although UPD is driven by the attractive interaction between adatom and substrate, an anion that also interacts with the substrate and metal adatom can have a profound influence on this process. For example, while (bi)sulfate and chloride anions assist Cu deposition on Au(111), they suppress Cu deposition on Rh(111).13 Ultimately, the role of the anion depends on how strongly it interacts with the electrode surface. Strongly adsorbed anions such as (bi)sulfate and chloride expectedly impede metal * To whom correspond should be addressed. E-mail: yausl@ atom.che.tohoku.ac.jp. Phone: 81-22-7954177. Fax: 81-22-7954177. † Tohoku University. ‡ CREST, JST. § National Central University.

deposition.14,15 On Rh(111), it is believed that a pseudomorphic Cu adlayer was deposited underneath a well-ordered (x3 × x7)-(bi)sulfate structure.13 More recently, Hoyer et al. examined electrochemical deposition of Pd on Rh(111) in an effort to alter the electrcatalytic property of Rh electrodes by depositing foreign metals.16 In addition, deposition of Cd has important applications in technology; for example, it has been used to produce semiconducting thin films of CdTe and CdSe on Au(111) and Ag(111) electrodes.17,18 Depending on potential, in situ STM reveals a series of structures of Cd adatoms on Au(111) in sulfuric acid.18 A thick CdTe film prepared by electrodeposition is shown to be semiconducting.19 UPD of Cd on the Pt(111) electrode was also reported, showing codeposition of (bi)sulfate and chloride anions with Cd adatoms.20 The present study investigates Cd deposition on Rh(111) in sulfuric and hydrochloric acids. Similarly to the situation of Cu on Rh(111), strongly adsorbed (bi)sulfate and chloride anions caused delays in Cd deposition.13 The fact that Cd adatoms form (6 × 6) in sulfuric acid, but (2 × 2) in hydrochloric acid, indicates that anions were involved in these adlattices. Experimental Section Rhodium(111) single-crystal electrodes were prepared according to a procedure described elsewhere.13,14,21,22 Before each CV or STM imaging experiment, the Rh electrode was annealed with a hydrogen torch and quenched in hydrogen-saturated Millipore water. The as-prepared Rh electrode appeared to be

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hydrophilic. Therefore, upon removal from water it was covered with a thin water film, which protected it from ambient contamination. The rhodium electrode was then quickly transferred to the electrochemical or STM cell for further experiments. Cyclic voltammetry in 0.1 M HClO4 was used to diagnose the as-prepared Rh(111) electrodes. CdSO4 and Cd(ClO4)2, purchased from Aldrich (St. Louis, MO), were ultrapure grade (purity >99.999%). They were used in CV and STM experiments when the electrolyte was sulfuric and hydrochloric acids, respectively. Ultrapure perchloric acid was obtained from Merck Inc. (Darmstadt, Germany). Meanwhile, all the necessary solutions were prepared with Millipore tripledistilled water (resistivity > 18.2 MΩ). The STM used was a Nanoscope-E (Santa Barbara, CA) with a tungsten tip (diameter 0.3 mm) prepared by electrochemical etching in 2 M KOH. After thorough rinsing with water and acetone, nail polish was painted on each tip for insulation. The coated tips were allowed to air-dry for at least 20 min before being used for imaging in solutions. The leakage current of the tip at the open circuit potential was less than 0.05 nA. More than 80% of the as-prepared tips gave good resolution. The electrochemical and STM measurements were carried out with reversible hydrogen electrodes (RHE), and all the potentials herein refer to a RHE scale. Results Cyclic Voltammetry. Figure 1a shows the cyclic voltammograms (CVs) of Rh(111) recorded at 50 mV/s in 0.05 M H2SO4. To elucidate the effect of [Cd2+] on the deposition process of Cd, we obtained CV profiles in 0.05 M H2SO4 solutions containing 1, 10, and 500 mM Cd(ClO4)2, respectively. These results are shown as dotted, broken, and solid lines in Figure 1b, respectively. The potential was scanned at 5 mV/s in 1 and 10 mM Cd2+, whereas it was 50 mV/s in 500 mM Cd2+. Only one pair of redox peaks labeled A1/C1, A/C, A′/C′, and A′′/C′′ were observed for all the CVs in Figure 1 in the potential regions between 0 and 0.8 or 1.0 V. The feature of C1 (A1) observed in the absence of Cd2+ is established to be the coupled processes of hydrogen ((bi)sulfate) adsorption and (bi)sulfate (hydrogen) desorption.13,14 The deposition of Cd in 0.05 M H2SO4 + 1 mM Cd2+ resulted in insignificant characteristics in the CV profile (dotted line in Figure 1b); it merely produced a shoulder to A. This result indicates that the deposition and stripping of Cd overlapped with those of hydrogen. Increasing [Cd2+] to 10 mM did not yield an obvious change in the reduction peak of C′, but the oxidation peak (A′) shifted positively by ca. 100 mV with a clear shoulder, in contrast to the result for Cd/Pt(111), where Cd UPD shifts positively by 30 mV with a 10-fold increase in [Cd2+].20 This difference could reflect the importance of the role of anions, (bi)sulfate, which predominated the Rh(111) surface and thus inhibited Cd deposition. A similar phenomenon was also noted in the deposition of Cu on Rh(111). However, Cd deposition/ stripping characteristics shifted positively to ca. 0.3 V upon increasing [Cd2+] to 0.5 M. Meanwhile, this resulted in separation of the electrode processes of cadmium and hydrogen, and allowed evaluation of the coverage of Cd from the amount of charge contained in C′′ and A′′. Compared to the results observed in 1 and 10 mM Cd2+, where stripping resulted in ca. 200 µC/cm2 charges (A and A′), only about 80 µC/cm2 charge was involved in the stripping of Cd in 500 mM Cd2+. Assuming a stripping process of Cd(ads) f Cd2+ + 2e-, the 80 µC/cm2 charge corresponds to a coverage of 0.17 (Cd/Rh atom). The negative end of the potential window was lowered from 0 to -0.4 V to see how the Cd adlayer affected the reduction

Figure 1. Cyclic voltammograms of Rh(111) recorded in 0.05 M H2SO4 without Cd2+ (a) and with 1 (dotted line), 10 (broken line), and 500 (solid line) mM CdSO4 (b). The potential sweep rate was 5 mV/s, except for the solid line in panel b. The inset in panel b is the CV obtained in 500 mM Cd2+ with the shaded areas denoting the evaluation of charge. The CV in panel c was recorded between -0.45 and 0.8 V at 50 mV/s in 0.05 M H2SO4 + 10 mM CdSO4.

of protons to hydrogen gas. The CV profile shown in Figure 1c was recorded in 0.05 M H2SO4 containing 10 mM CdSO4. The negative potential sweep at 50 mV/s from 0.8 to -0.45 V yielded an UPD peak at 0.1 V, followed by a slow increase of current from 0 to -0.35 V until a sharp, poorly resolved doublet feature emerged at -0.4 V. Evolution of hydrogen gas resulted in the precipitous increase of current density at potentials negative of -0.4 V. The reversal potential sweep from -0.45 to 0.8 V led to two sharp peaks at -0.4 and -0.35 V, followed by a precipitous decrease of current at -0.2 V. The broad peak located between 0 and 0.4 V is attributable to the oxidation of hydrogen, prior to the stripping of Cd at 0.3 V. These characteristics resemble that observed at Pt(111).20 The Cd adlayers on Rh and Pt evidently resulted in higher overpotentials for hydrogen evolution, as the reaction commenced at -0.45 V, rather than at 0.05 V in 0.05 M H2SO4. The doublet feature that emerged near -0.4 V should be associated with Cd deposition and the hydrogen evolution reaction (HER). These two reactions were reversible, producing corresponding features

Underpotential Deposition of Cd on a Rh(111) Electrode

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Figure 2. Cyclic voltammograms of Rh(111) recorded at 50 mV/s in 0.1 M HCl + 1 mM Cd(ClO4)2 with potential scan ranges between 0.05 and 1.0 V (solid line) and -0.4 and 1.0 V (dotted line). The inset shows CV of Rh(111) in 0.1 M HCl.

at the reversal potential sweep. The origin for the precipitous decrease of current starting at -0.2 V in the positive scan is not clear. According to the results reported for Pt(111), it should be associated with HER.20 The reason this feature was pronounced in the positive scan could arise from higher [H+], as protons, not used up in the preceding negative sweep, accumulated to produce a higher current. Continuous potential sweeping between 0.8 and -0.45 V resulted in no change in the CV profile, suggesting a stable Rh substrate without the formation of CdRh alloy. Voltammetry of Cd UPD was also performed in 0.1 M HCl + 1 mM Cd(ClO4)2 to elucidate the role of anions in the UPD of Cd on Rh(111) and the resultant CVs are shown in Figure 2. The solid and dotted traces were acquired with the negative potential windows set at 0.05 and -0.4 V, respectively. The inset shows the steady-state CV for Rh(111) in 0.1 M HCl. Similarly to the results seen in H2SO4, only one pair of peaks appear near 0.1 V, attributable to the deposition/dissolution of hydrogen and Cd adatoms. These redox features are asymmetric with a broader reduction branch, which could support the notion that pre-deposited anions blocked Cd deposition. The charges associated with the deposition and stripping of Cd adatoms were measured to be 120 ( 10 and 170 ( 10 µC/cm2, respectively. The former corresponds to a Cd coverage of 0.23, which agrees with that determined from STM result. The latter appears to be 50 µC/cm2 higher, possibly because of the contribution from hydrogen desorption. In Situ Scanning Tunneling Microscopy of Cadmium Underpotential Deposition on Rh(111) in Sulfuric Acid. Figure 3 shows an STM image obtained on a Rh(111) electrode in 0.05 M H2SO4. This image was acquired at 0.4 V, where the Rh(111) electrode was mostly occupied by (bi)sulfate anions.15,21-23 The (x3 × x7)-(bi)sulfate structure with multiple rotational domains is evident in this image. The inset of Figure 3 reveals a high-resolution scan, showing the internal molecular arrangement within this (x3 × x7) structure. Coadsorption of (bi)sulfate and water is proposed to account for this structure.15,21,22 Switching the potential negatively from 0.4 to 0.1 V yielded STM imaging of a hexagonal array with a nearest neighboring spacing of 0.27 nm, a structure corresponding to the Rh(111) substrate. (x3 × x7) was the only ordered adlattice produced by (bi)sulfate on Rh(111), as reported in refs 13 and 14. Figure 4 reveals two high-resolution STM scans obtained at 0.1 V in 0.05 M H2SO4 containing 1 mM CdSO4. According to the CVs in Figure 1b, UPD of Cd was completed under these experimental conditions. The 100 × 100 nm scan in Figure 4a reveals that UPD of Cd yielded well-ordered arrays on two terraces separated by a monatomic step denoted by a dotted

Figure 3. In situ STM image of Rh(111)-(x3 × x7)-(bi)sulfate at 0.4 V in 0.05 M H2SO4. The numbers denote different rotational domains of the (bi)sulfate structure. The inset shows a high-resolution scan (5 × 5 nm) of this ordered array.

line (∆z ) 0.23 nm). Sketchy dark spots in this image are defects within the adlattice. The higher resolution (11 × 11 nm) scan in Figure 4b reveals the internal atomic arrangement of this ordered array. Cd adatoms appear to be arranged in a nonuniform manner, producing double-walled honeycombs linking with an edge. Hollows in honeycombs are aligned parallel to the close-packed atomic rows of the Rh(111) substrate with two nearest neighboring hollows separated by 1.61 nm, 6 times longer than the Rh-Rh interatomic spacing. This information indicates a structure of (6 × 6) with 6 Cd atoms per unit cell. An identical Cd adlayer was also observed also in sulfuric acid solutions containing 0.5 M Cd2+. It is peculiar to see this nonuniform Cd arrangement, because it left a large portion of the surface (hollows) uncovered. To resolve this issue, we adjusted bias voltage and feedback current to see if hollows in this structure indeed did not contain atomic adsorbates. Shown in Figure 4c is a composite image obtained by changing the imaging conditions from 200 mV and 2 nA to 50 mV and 10 nA when the probe was rastered upward to the middle of this frame (11 × 11 nm). This action led to notable changes in the appearance of the hollows in the STM image. Two STM images in Figure 5a,b acquired by using 200 mV, 2 nA and 50 mV, 10 nA are presented to illustrate further the appearance of STM atomic resolution. The most distinct difference of these two images is the contrast seen at the hollows. Close examination and comparison of these images reveal that weak protrusions 0.05 nm lower than the brightest Cd features appeared at hollows in Figure 5b. Varying imaging conditions made no apparent difference to the other atomic features. If all atomic features including those occupying hollow sites are associated with only one type of chemical species, Cd adatoms or (bi)sulfate anions, it would be difficult to comprehend why only those in the hollows varied with imaging condition. Hence, the most plausible explanation for this STM result is that two different chemical species were coadsorbed on the surface. This contention is in line with the conclusions of most UPD studies reported thus far. In the case of Cd/Rh(111), Cd adatoms competed with (bi)sulfate anions for surface sites and they apparently prevailed at 0.1 V. Hence, the (6 × 6) structure was associated mainly with Cd adatoms, whereas (bi)sulfate anions occupied hollows within this adlattice. Potential-dependent in situ STM images shown in Figure 6 provided further illustration of this issue (vide infra). The

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Figure 4. In situ STM images of Rh(111) recorded at 0.1 V in 0.05 M H2SO4 containing 1 mM CdSO4. The 100 × 100 nm scan in panel a shows the degree of long-range ordering, the density of defects, and a step ledge marked with a dotted line. The high-resolution scan in panel b shows the arrangement of Cd adatoms in this adlattice of (6 × 6) with a double walled honeycomb pattern, as outlined. A composite image acquired after the imaging condition was switched from 200 mV, 2 nA to 50 mV, 10 nA after the tip was rastered upward to the middle of the image in panel c is indicated with a dotted line.

Figure 5. High-resolution in situ STM images of Rh(111)-(6 × 6)6Cd + 1HSO4-. These two images were acquired with 200 mV, 2 nA (a) and 50 mV, 10 nA (b). Weak spots appear in the hollows of the structure in panel b. Scan sizes are 5 × 5 nm.

spacing between two neighboring Cd adatoms varied between 0.54 and 0.65 nm, considerably larger than the van der Waals diameter (0.35 nm) of the Cd atom.24 This large interatomic spacing suggests repulsive interaction among Cd adatoms, possibly because they tended to be positively charged. Panels a-d in Figure 6 show STM images obtained at 0.08, 0.13, 0.15, and 0.4 V, respectively. All images were collected at the same area on the surface 3-5 min after the potential was

altered, when the system reached a stable state. The image in Figure 6a reveals a Rh(111) surface fully covered by Cd adatoms, arranged in the (6 × 6) structure, as seen in Figures 4 and 5. Two minutes after the potential was switched from 0.08 to 0.13 V, in situ STM revealed displacement of the (6 × 6) structure by the (x3 × x7)-(bi)sulfate structure. Shortly after the potential was increased to 0.15 V, the (6 × 6) structure was entirely eliminated, and the (x3 × x7)-(bi)sulfate structure prevailed. These results provide a vivid illustration of dissolution of a metallic adlayer. The unidentified blotches amid the ordered arrays of Cd adatoms in Figure 6a,b are believed to be packing defects within the Cd adlattice. This potential effect on the deposition of Cd was reversible, as the CVs in Figure 1 also reveal. The corrugation heights of atomic features in these two ordered structures is helpful to shed insight into the possible chemical nature of the species made from the (6 × 6) structure. The brightest protrusions in the (6 × 6) structure were 0.06 nm higher than those in the (x3 × x7)-(bi)sulfate structure. If the (6 × 6) structure was made up of (bi)sulfate sitting on a Cd adlayer, it should have appeared 0.35 nm higher than the (x3 × x7)-(bi)sulfate structure. Instead, a corrugation difference of 0.06 nm was observed. This result strongly suggests that the (6 × 6) structure was made up of mainly Cd adatoms with a small amount (θ ) 0.03) of (bi)sulfate anions. In Situ STM of Cd UPD on Rh(111) in HCl Solutions. The involvement of anion in Cd UPD is elucidated further by performing in situ STM imaging with hydrochloric acid. First, the hitherto unreported real-space structures of chloride on Rh(111) is described. Although chloride formed several ordered adlattices on Rh(111), only those observed near the potential region of Cd UPD are described. The STM results in Figure 7a,b were acquired at 0.3 V in 0.1 M HCl. The 30 × 30 nm scan in Figure 7a reveals the degree of ordering of the chloride adlattice, whose internal atomic arrangement is highlighted in Figure 7b. The unfiltered high-resolution STM scan in Figure 7b indicates that chloride is adsorbed in a (x7 × x7)R19.1° structure, as outlined by the rhombus in the image. The edge length of this structure is 0.73 nm and the unit vectors are rotated by 19° from the close-packed atomic rows of Rh(111). The difference in corrugation heights among the protrusions is attributable to different registries of Cl adatoms. This (x7 ×

Underpotential Deposition of Cd on a Rh(111) Electrode

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Figure 6. (a-d) Potential-dependent in situ STM images showing the stripping process of Cd adatoms on Rh(111) in 0.05 M H2SO4 containing 1 mM CdSO4. These images were acquired at 0.08, 0.13, 0.15, and 0.4 V, respectively. It took ca. 2-3 min for the surface to reach a stable state after each potential step. Scan sizes are 50 × 50 nm.

x7)R19.1 structure of Cl was displaced by a Rh(111)-(1 × 1) structure if the potential was switched negatively from 0.3 to 0.1 V. STM imaging of Cd deposition in 0.1 M HCl containing 1 mM Cd(ClO4)2 led to ordered atomic arrays seen in Figure 8a,b. These images were collected at 0.1 V with 50 mV bias voltage and 20 nA feedback current. The ordered array seen in Figure 8b is made of a rhombic unit cell. This unit mesh is determined to be (2 × 2) with one atom per cell. The unit vectors are 0.54 nm in length and aligned in the close-packed atomic rows of the Rh(111) substrate. This structure is apparently different from the (6 × 6) structure found in sulfuric acid solutions, and this difference indicates the important effect of anion in the deposition of Cd on Rh(111). More specifically, anions were likely coadsorbed with Cd adatoms. It is likely that the (2 × 2) structure was associated with chloride anions lying on top of Cd adatoms. Discussion The STM results obtained in this study again illustrate the importance of anion in guiding the UPD process. Figure 9 reveals two structural models to account for the ordered Cd adlayers observed on Rh(111) in sulfuric and hydrochloric acids. These models are proposed on the ground that surface sites with high symmetry, such as hexagonal close pack (hcp) and face center cube (fcc) 3-fold hollow sites, are energetically most favorable on a fcc(111) plane.25 Also, atomic corrugations seen

in STM images reflect the registries of adatoms. Frequently, adatoms give rise to corrugations in the sequence of on-top > 2-fold > hcp 3-fold > fcc 3-fold.26 For example, sulfur adatoms adsorbed at hcp 3-fold hollow sites of Rh(111) are shown to produce corrugation higher than those at fcc 3-fold ones by 0.20.4 Å.27,28 These rules of thumb could hold for Cd adatoms on Rh(111) also, but it is difficult to substantiate this issue because the corrugation pattern seen in the (6 × 6) structure is ill-defined. The dotted and solid circles in the (6 × 6) model in Figure 9a represent coadsorbed (bi)sulfate anions and Cd adatoms, respectively. All ad-species including Cd adatoms and (bi)sulfate anions are assigned to 3-fold hollow sites. This (6 × 6) model results in 6/36 (0.17) and 1/36 (0.028) coverages for Cd and (bi)sulfate, respectively. The coverage of Cd agrees with that determined from coulometry obtained in 0.5 M Cd2+, but is much lower than those observed in 1 and 10 mM Cd2+. This inconsistency is attributable to the interference of concurrent electrode processes involving hydrogen, resulting in an over-estimation of the amount of Cd being deposited. This contention may seem disturbing because the Cd adatom is known to inhibit the reduction of hydrogen, as observed at Pt electrodes.20 On the other hand, it is thought that the degree of blocking can hinge on how many Cd adatoms are deposited on the electrode surface. It is not unlikely that Cd adatoms on Rh(111) were so loosely packed that they exerted a limited restriction on the reduction of proton. In other words, the atomic ratio of 1/6 (Cd/Rh) in the (6 × 6)-Cd structure

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Figure 7. In situ STM images showing the ordered chloride structures of Rh(111)-(x7 × x7)R19.1°-Cl at 0.4 V in 0.1 M HCl. These images were acquired with 50 mV in bias voltage and 10 nA in feedback current. The rhombus in panel b shows the unit cell of the (x7 × x7)R19.1°-Cl structure.

Figure 8. In situ STM atomic images showing the ordered structure of Rh(111)-(2 × 2)-Cd + Cl observed in 0.1 M HCl containing 1 mM Cd(ClO4)2. These images were acquired at 0.1 V with a bias voltage of 10 mV and 5 nA feedback current. The rhombus in panel b denotes the unit cell of (2 × 2).

was too low to block the entire Rh(111) surface from the adsorption of hydrogen adatoms. Quantitatively, the extra charges, roughly 120 µC/cm2, passed in the potential region of the feature of C in Figure 1b, could induce the deposition of half a monolayer of hydrogen adatoms on Rh(111) at 0.05 V. Figure 9b outlines a ball model for the (2 × 2)-Cd + Cl structure observed in HCl + Cd(ClO4)2 solution. This model outlines the codeposition of Cd and Cl on Rh(111) with the chloride adlayer sitting atop the Cd adlayer. As the ball model reveals, each Cl anion resides at the center of three Cd atoms which are adsorbed in equivalent types of sites and arranged in an equilateral triangle. Structurally speaking, both Cd and Cl are arranged in (2 × 2) with one Cd and one Cl per unit cell. Accordingly, the coverage of both Cd and Cl is 0.25. This model can explain the STM results in Figure 8, where all chloride anions residing as the uppermost layer on Rh(111) exhibited identical intensity. The STM atomic resolution of (2 × 2) in Figure 8 revealing spots with equivalent intensity indicates that all chloride anions should coordinate identically with the underlying Cd adadtoms. Similarly, the coulometric results obtained in HCl solution are also inconsistent with that value determined from STM atomic resolution. By the same token, it is proposed that a portion of the charge flew off in the Cd deposition could be associated with the reduction of protons. Close inspection of the ball model in Figure 9b reveals that 1/4 of the 3-fold hollow sites on Rh(111) are not occupied and these

vacant sites could be used to accommodate hydrogen adatoms. It is intriguing to note that this sandwich-like structure of ClCd-Rh bears a strong resemblance to many UPD systems on gold and platinum electrodes in the presence of chloride anions.1-3,30 Ultimately, the amount of adatoms deposited in an UPD process depends mainly on the strength of the adsorbatesubstrate interactions. The stronger this interaction, the more adatoms are deposited. Consequently, it is not surprising to see fewer Cd adatoms being deposited on Rh(111), as compared to that on Pt(111). This issue can be extrapolated from the CV characteristics. In particular, intermetallic interactions of CdRh and Cd-Pt manifest themselves from the potentials at which Cd adatoms are removed from the corresponding electrodes under the same experimental conditions. According to Figure 1b and results reported previously,30 these values are 0.18 and 0.51 V for Rh(111) and Pt(111), respectively. These results imply that the Cd-Rh bond is weaker than the Cd-Pt bond and/or (bi)sulfate anions are more strongly held at Rh(111) than at Pt(111). This 0.33 V difference in potential translates into a 31.8 kJ/mol difference in binding energy in favor of Cd-Pt. Similarly, one can evaluate the adsorptivity of (bi)sulfate anions on Rh(111) and Pt(111) electrodes from the difference in potential of (bi)sulfate adanions desorption. This potential is 0.05 V for Rh(111) and 0.45 V for Pt(111),31 which corresponds to a difference of 38.6 kJ/mol in binding energy in favor of

Underpotential Deposition of Cd on a Rh(111) Electrode

J. Phys. Chem. B, Vol. 109, No. 31, 2005 14923 Finally, we comment on the difference in spatial arrangements of Cd adatoms on Rh(111) and Au(111). Generally speaking, the spatial structure of adsorbate is determined by the adsorbateadsorbate interaction, which is frequently a function of coverage.25 According to results reported for Cd/Au(111), the coverage of Cd varied between 0.4 and 0.66, which is substantially higher than that (θ ) 0.17) of Cd/Rh(111).31,32 This thought could also hold for the coadsorbed anions of (bi)sulfate, whose coverage differ on these two surfaces.31,32 These anionic species which are intermediates in the interactions between Cd adatoms also contributed to the arrangements of Cd deposit. Conclusions

Figure 9. Ball models for the Rh(111)-(6 × 6)-Cd + HSO4- and (2 × 2)-Cd + Cl- structures. The solid and dotted circles in part a represent Cd adatoms and (bi)sulfate anions, respectively. The model in part b contains an uppermost Cl adlayer (in thick lines) and an underlying Cd adlayer (in thin line) on Rh(111).

Rh(111). In addition, the much sharper UPD peak of Cd on Pt(111), as compared to that on Rh(111), implies that the reduction rate of Cd2+ is faster, and attractive interatomic interaction occurs between Cd adatoms. On the other hand, the quality of the electrode surface could also influence the peak shape.23 Again, as found unanimously for all the UPD studied thus far, this study shows the codeposition of (bi)sulfate and chloride anions along with Cd on Rh(111). This coadsorption of anions with metal adatoms is largely attributed to the dipole of the chemical bonding between adatoms and substrate, as they always possess different electron properties. For example, the work functions of Cd, Rh, and Pt are ca. 4.2, 5.1, and 5.6 eV, respectively.24 It is conceivable that the Cd-Rh and Cd-Pt bonds have such a strong polarity that they induce the coadsorption of (bi)sulfate or chloride anions. The strength of the interaction between adatom and anion then determines how many adatoms as well as anions are adsorbed. Given the results that show more chloride was adsorbed than (bi)sulfate, it is reasonable to state that the Cd interacted more strongly with Cl than with HSO4-. Accidentally, it is noted that Cd forms stable complexes with chloride, but not with (bi)sulfate.24 However, this difference in adatom-anion interaction was not great enough to produce a notable change in the CV profiles, as UPD of Cd occurred nearly at the same potentials in sulfuric and hydrochloric acids containing 1 mM Cd2+.

In situ STM and cyclic voltammetry have shed insight on the process of underpotential deposition of Cd on Rh(111) in H2SO4 and HCl solutions. Atomic resolution STM reveals that deposition of Cd produced only one ordered structure, characterized as (6 × 6) and (2 × 2), in H2SO4 and HCl, respectively. This structural difference indicates the involvement of anions in these adlattices. Armed with STM atomic resolution and electrochemical results, we establish that the (6 × 6) structure observed in sulfuric acid mainly consists of Cd adatoms (θ ) 0.17), along with a small amount of (bi)sulfate anion (θ ) 0.028). In contrast, the (2 × 2) structure observed in HCl is likely due to chloride anions (θ ) 0.25) residing on the Cd adlayer. Although in situ STM did not image Cd adatoms in HCl, it is deduced to be arranged in (2 × 2) also. The coverage of Cd can be 0.25 in HCl, 47% higher than that found in H2SO4. In contrast, the coverage of the Cl- anion, 0.25, is nearly 10 times more than that of (bi)sulfate. The weak Cd-Rh interaction results in the commencement of Cd UPD at potentials as negative as 0.1 V, where protons discharge also. Hydrogen adatoms are likely to adsorb at the interstitial sites of the Cd adlattices. Acknowledgment. This work was supported by the National Science Council of the Republic of China under contract No. NSC 93-2119-M-008-002. This work was partially supported by the Ministry of Education, Culture, Sport, Science and Technology, a Grant-in-Aid for the COE project, Giant Molecules and Complex Systems, 2004. References and Notes (1) Itaya, K. Prog. Surf. Sci. 1998, 58, 121. (2) Herrero, E.; Buller, L.; Abruna, H. Chem. ReV. 2001, 101, 1897. (3) Magnussen, O. L. Chem. ReV. 2002, 102, 679. (4) Budevski, E.; Staikov, G.; Lorentz, W. J. Electrochemical Phase Formation and Growth; VCH Publishers: New York, 1996. (5) Abruna, H. D. Electrochemical interfaces: modern techniques for in-situ interface characterization; VCH Publishers: Cambridge, UK; VCH Verlagsgesellschaft: Weinheim, Germany, 1991. (6) Wieckowski, A., Ed. Interfacial Electrochemistry Theory, Experiment, and Applications; Macel Dekker: New York, 2003. (7) Hachiya, T.; Honbo, T.; Itaya, K. J. Electroanal. Chem. 1991, 315, 275. (8) Magnussen, O. M.; Hotlos, J.; Nichols, R. J.; Kolb, D. M.; Behm, R. J. Phys. ReV. Lett. 1990, 64, 2929. (9) Manne, S.; Hansma, P. K.; Massie, J.; Elings, V. B.; Gewirth, A. A. Science 1991, 251, 183. (10) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. l.; Gordon, J. G.; Melroy, O. R. Phys. ReV. Lett. 1995, 75, 4472. (11) Shi, Z.; Lipkowski, J. J. Electroanal. Chem. 1994, 365, 303. (12) Matsumoto, H.; Oda, I.; Inukai, J.; Ito, M. J. Electroanal. Chem. 1993, 356, 275. (13) Wu, Z. L.; Zang, Z. H.; Yau, S. L. Langmuir 2000, 16, 3522. (14) Wan, L. J.; Yau, S. L.; Itaya, K. J. Phys. Chem. 1995, 99, 9507. (15) Krauskopf, E. K.; Wieckowski, A. J. Electroanal. Chem. 1989, 271, 295.

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