Adlayers of Sb Irreversibly Adsorbed on Pt(111): An Electrochemical

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J. Phys. Chem. B 2006, 110, 10814-10821

Adlayers of Sb Irreversibly Adsorbed on Pt(111): An Electrochemical Scanning Tunneling Microscopy Study Jisheng Zhao, Changhoon Jung, and Choong Kyun Rhee* Department of Chemistry, Chungnam National UniVersity, Daejeon, 305-764, Korea ReceiVed: December 21, 2005; In Final Form: April 11, 2006

This work presents an electrochemical scanning tunneling microscopy study of Sb irreversibly adsorbed on Pt(111) at various potentials. At an open circuit potential (0.46 V vs a Ag/AgCl electrode), well-ordered structures of SbO+ were found: four (4 × 3)-3SbO+ structures and one (2x3 × 2x3)R30°-3SbO+ structure. In addition, several unidentifiable transient structures of SbO+ were observed, and their relations to the wellordered structures of (4 × 3) and (2x3 × 2x3)R30°, regarding structural evolution, were proposed. At a reducing potential (0 V), the Pt(111) surface was covered with irreversibly adsorbed Sb which consisted of three different domains: protruded domain, domain of uniaxially incommensurate (x3 × x2)-Sb, and domain of bare (1 × 1) Pt(111). During oxidation of elemental Sb at 0.30 V, the Sb domains of the (x3 × x2) structure were oxidized, while the protruded domains were not oxidized. After underpotential deposition of additional Sb onto the Pt(111) covered with irreversibly adsorbed Sb, the whole surface was filled with the Sb domains where each Sb atoms were separated by the x2a distance (a ) one Pt-Pt distance, 0.277 nm). The observed electrochemical inactivity below 0.3 V was discussed in terms of the protruded domain of a presumable incommensurate (x2 × x2) structure.

Introduction Electrochemical modification of solid electrode surfaces with foreign metals has been recognized as an important research field in both fundamental and technological areas. Particularly, underpotential deposition, recently reviewed by Abruna,1 has enjoyed tremendous attention regarding monolayer formation of metal adatoms. In underpotential deposition, a metallic monolayer of an element is electrochemically deposited on an electrode surface of different elements at potentials more positive than that of the electrode of the same element. Because of the reversibility of underpotential deposition, it is easy to control the coverage of foreign metal precisely and reproducibly to tune the surface properties of electrodes. On the other hand, irreversible adsorption, known as the immersion method2,3 also, operates in a way different from underpotential deposition. In irreversible adsorption, foreign metal ions, generally oxygenated ions, adsorb onto an electrode surface of different elements. The adsorbed metal ions then undergo a redox process as they remain adsorbed. The dissolution of the irreversibly adsorbed species takes place oxidatively at potentials more positive than the potential where the surface redox process occurs. Therefore, such a wide potential range, in which irreversibly adsorbed metal ions remain adsorbed, would be one of the advantages the irreversible adsorption offers in various applications such as fuel cells.4,5 On Pt single crystals, various metal ions, such as Te,6-12 Sb,11,13-15 and As,16-19 have been known to adsorb irreversibly. Recently, the irreversible adsorption of Te on Pt(111) has been investigated by means of electrochemical scanning tunneling microscopy (EC-STM)9,12 and X-ray photoelectric spectroscopy (XPS).10 In the EC-STM study, it was revealed that the pristine layer of irreversibly adsorbed Te on Pt(111) was a two* To whom correspondence may be addressed. Tel: 82-42-821-5483. Fax: 82-42-821-8896. E-mail: [email protected].

dimensional monolayer of TeO2+ and that after reduction, the elemental Te atoms were mobile to form a compressed layer of (8 × 11). The XPS results confirmed the coexistence of Te and O in the pristine layer and the absence of O in the reduced layer. Furthermore, underpotential deposition of Te onto Pt(111) covered with irreversibly adsorbed Te induced a dramatic change in the voltammogram: the complete vanishing of the redox charge related to the adsorbed Te. Our EC-STM study demonstrated that during the underpotential deposition of Te, the existing layer of Te was compressed ultimately to a rectangular c(2 × x3), whose Te coverage was 0.5. Obviously, the disappearance of the Te charge was relevant to the compressed superlattices of Te. On Au single-crystal electrodes, the irreversible adsorption of metal ions, e.g., Sb20-25 and Pt,26 took place as well as on Pt electrodes. The pristine layer of Sb on Au(111) electrode was a uniform layer of SbO+ in a (x3 × x3)R30° arrangement. It was clearly verified that part of the reduced Sb adatoms were mobile enough to form a surface alloy with the substrate Au atoms. When the modified surface was oxidized, the alloyed Sb species were oxidatively stripped, while the unalloyed Sb species was oxidized to a slowly desorbing adspecies. On Au(100), on the other hand, the pristine layer of irreversibly adsorbed Sb showed a quasi-(2 × 2) structure of oxygenated Sb(III) species, and the underpotential deposition of additional Sb developed two distinctive arrangements, one of which was proposed to be an array of an Sb2 dimer.24 This work presents EC-STM results to extend our previous voltammetric work of Sb on Pt(111).15 Various atomic structures of the pristine layer of irreversibly adsorbed Sb were observed. Also, real time images of Sb-covered Pt(111) were obtained during electrochemical treatments, e.g., reduction and oxidation. The changes in the voltammograms of irreversibly adsorbed Sb on Pt(111) are fully described in conjunction with the EC-STM results.

10.1021/jp057422l CCC: $33.50 © 2006 American Chemical Society Published on Web 05/12/2006

Sb Adsorbed on Pt(111)

Figure 1. Cyclic voltammograms of Pt(111) covered with irreversibly adsorbed Sb: (a) in 0.05 M H2SO4 and (b) in Sb2O3-saturated 0.05 M H2SO4. The solid lines and the dashed lines were obtained during the initial cycle and the ninth cycle, respectively.

Experimental Section To obtain the Pt(111) single crystals used in this work, a Pt wire (0.5 mm diameter, Aldrich, 99.99%) was melt to a singlecrystal bead in a hydrogen-oxygen flame. For voltammetric experiments, a (111) facet of the bead crystal was polished to a mirrorlike finish, while for EC-STM (Nanoscope III, Digital Instruments, U.S.A.) work, a (111) facet was used without polishing. Annealing in a hydrogen flame, followed by quenching in hydrogen-saturated water, normally produced well-ordered and clean Pt(111) electrode surfaces in both experiments. A homemade EC-STM cell was employed, and W tips (0.25 mm diameter, Aldrich), made by electrochemical etching in 1 M KOH solution with 15 V ac and coated with a nail polish or a melted polyethylene, were exclusively used. The Sb solution was obtained by saturating 0.05 M H2SO4 (Merck, Suprapur) with Sb2O3 (Aldrich, 99.99%), and its concentration of Sb was approximately 0.05 mM. The potentials of Pt(111) electrodes in EC-STM and voltammetric experiments were controlled against a Ag/AgCl reference electrode with an 1.0 M solution of Cl-, and the potentials in this work were reported as measured. The crystallographic orientation of the Pt(111) surface was confirmed with the atomic image of I on Pt(111).27,28 Results and Discussion Electrochemical Behavior of Sb on Pt(111). Figure 1 shows the cyclic voltammograms of Sb irreversibly adsorbed on Pt(111). The electrochemical behavior of Sb irreversibly adsorbed on Pt(111) was fully documented in ref 15. Briefly, the saturation coverage of irreversibly adsorbed Sb was measured to be 0.33-0.38 from the initial reductive scan (the solid line in Figure 1a), based on the redox process of Sb(III, ad) + 3e a Sb(0, ad). As the cyclic voltammetric scan was continued in 0.05 M H2SO4 solution, however, the apparent charge related to the electrochemical process of Sb decreased slowly as shown with the dashed line in Figure 1a. Such a decrease of the Sb

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Figure 2. Micrographs of the pristine Sb layers on Pt(111) in (4 × 3) arrangements. The micrographs were obtained without potential control. Scan size: 10 nm × 10 nm. Tip bias: 0.878 mV. Set point: 1.7 nA.

charge was proposed to be due to the formation of an Sb species electrochemically inactive below 0.3 V. Furthermore, the redox charge of the Sb adlayer disappeared completely as illustrated with the dashed line in Figure 1b, when the voltammetric scan was undertaken in the Sb2O3-saturated 0.05 M H2SO4 solution. Under this specific condition, additional Sb (corresponding roughly to the coverage of 0.1) was deposited underpotentially onto the irreversibly adsorbed Sb layer existing on the Pt(111) surface, so that the whole Sb layer became electrochemically inactive. Therefore, the two conceptually different processes, i.e., the irreversible adsorption and the underpotential deposition, were cooperative to form a full monolayer of reduced Sb, whose coverage was ∼0.45. The electrochemically inactive Sb species was oxidatively stripped at potentials higher than 0.35 V. The Adlayers of Sb at Open Circuit Potential. In Figure 2, there are four atomic images of Sb irreversibly adsorbed on Pt(111) in 0.05 M H2SO4 solution without potential control, i.e., at open circuit potential (∼0.46 V). The lattice parameters of the unit cells in the images were measured to be 1.11 ( 0.03 nm in the [11h0] direction and 0.84 ( 0.01 nm in the [1h01] direction, respectively, which correspond to a (4 × 3) structure taking one Pt-Pt distance (a ) 0.277 nm) into account. Although the unit cells of the four images were identical, the details in the arrangement of the observed spots within each unit cell were different from each other. Specifically, the (4 × 3) structure of Figure 2a (designated as (4 × 3)-I) was an array of small parallelograms (0.56 ( 0.01 nm in the [11h0] direction and 0.37 ( 0.02 nm in the [1h01] direction), while that of Figure 2b (denoted as (4 × 3)-II) showed eight bright spots along the sides of the unit cell. The (4 × 3) structure of Figure 2c (designated as (4 × 3)-III), on the other hand, consisted of the wide rows of spots in the [1h01] direction and the isolated bright spots between them. In Figure 2d (denoted as (4 × 3)-IV), the aggregates of seven spots were arranged in the (4 × 3) pattern. A close examination of the EC-STM images disclosed that within the (4 × 3) unit cells, there were seven atomic spots:

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Figure 3. Schematics of the (4 × 3) structures of the pristine Sb layer: (a) (4 × 3)-I, (b) (4 × 3)-II, (c) (4 × 3)-III, (d) (4 × 3)-IV. The open circles represent Sb(III) ions, and the filled circles stand for O2- ions. The gray open circles of Sb(III) in (b), (c), and (d) denote the unobservable Sb(III) ions, while the ones in (a) designate the dim spots at the corners of the parallelograms. See the details in the text.

three bright spots and four dim spots. One exception was the (4 × 3)-I structure, whose unit cell consisted of three bright spots and five dim spots. Probably, the dim spots at one of the corners of the parallelograms in Figure 2a would not be observable in the images in parts b-d of Figure 2 because of an extremely low tunneling efficiency of those particular atoms. Such an absence of STM spots despite existence would be exemplified with the atomic structures of I on Pt(111).27,28 Then, the number of the spots, including the unobservable spot, in one (4 × 3) unit cell was eight, which indicates that the spot coverage, regardless of their brightness, was 0.66. Because the electrochemically measured coverage of Sb was 0.33, half of the eight spots must be Sb atoms and the other half must be O atoms. In other words, the coverages of Sb and O were 0.33, respectively; thus, the chemical identity of the irreversibly adsorbed Sb was a two-dimensional oxide of SbO+, as proposed on Au(111)20,21,23,25 and Au(100).20,22 The positive charge of the oxide monolayer might be compensated by the adsorption of HSO4- ion as proposed for the monolayer of irreversibly adsorbed TeO2+ on Pt(111).9 The schematic models in Figure 3 are proposed for the (4 × 3) structures observed in Figure 2. Here, the bright and dim

Zhao et al.

Figure 4. (a) A micrograph of the pristine Sb layer on Pt(111) in a (2x3 × 2x3)R30° arrangement and (b) its corresponding schematics. The open circles represent Sb(III) ions, and the filled circles stand for O2- ions. Scan size: 10 nm × 10 nm. Tip bias: 0.878 mV. Set point: 1.7 nA.

spots were arbitrarily assigned to Sb and O atoms, respectively. In the models for the (4 × 3)-I, (4 × 3)-II, and (4 × 3)-III structures, the Sb atoms (open circles) are arranged in a c(4 × 3) pattern in parts a-c of Figure 3. Phenomenologically, however, the Sb atoms at the center of the c(4 × 3) unit cell (gray open circles) were not observable with STM, except in the (4 × 3)-I structure. It should be noted that the Sb atoms at the center of the unit cell in the (4 × 3)-IV structure, not observable with STM, are presumably located close to the corners of the (4 × 3) unit cell to account for the local stoichiometry of SbO+. On the other hand, the respective locations of the O atoms (filled circles) were determined based on the dim spots in Figure 2. Figure 4 shows another atomic image of Sb irreversibly adsorbed on Pt(111) and a proposed model. The respective lattice parameters of the unit cell in Figure 4a were 0.96 ( 0.03 nm in the [12h1] direction and 0.95 ( 0.05 nm in the [2h11] direction, indicating that the unit cell in Figure 4 was (2x3 × 2x3)R30°. In the unit cell, there were three bright spots and four dim spots, so that according to the same argument as in the (4 × 3) structures, the coverages of Sb and O were 0.33, respectively. In Figure 4b, the Sb atoms are arrayed in a c(2x3 × 2x3)R30° pattern, equivalent to a (x3 × x3)R30° pattern, with the central Sb atom of low electron tunneling efficiency, and the O atoms are located at appropriate positions according to the dim spots in Figure 4a. The sequential images in Figure 5 demonstrate the mobility of the Sb species adsorbed irreversibly on Pt(111). The shape

Sb Adsorbed on Pt(111)

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Figure 5. Sequential micrographs of the pristine Sb layer on Pt(111). Scan size, 12 nm × 12 nm. Tip bias, 0.878 mV. Set point, 1.7 nA.

and location of the defect site, indicated with arrows in Figure 5, changed as the time elapsed, and the change was accompanied with jittering lines to denote the movement of the atoms during imaging. The structure of the domains inside the circle and rectangle, on the other hand, transformed from (4 × 3)-I (Figure 5a) to (4 × 3)-II (Figure 5d). In addition, a straight domain boundary at the lower part of Figure 5a changed to the bent one in Figure 5d. On the pristine layers of Sb, several distinctive but unidentifiable structures were found. The images in Figure 6 were mosaics of domains different in structure. In Figure 6a, three new structures were observed adjacent to the (4 × 3)-I and (4 × 3)-III domains: domains of (1 × 1) in square symmetry (rectangular (1 × 1)) as surrounded with dashed lines, domains inside the rectangles (R phase), and a domain in the triangle (β phase). Because the Pt(111) substrate has a hexagonal symmetry, the rectangular (1 × 1) area was an incommensurate part covered with irreversibly adsorbed Sb, not a part of bare Pt(111). In particular, the rectangular (1 × 1) domain at the left bottom of Figure 6a transformed to the (4 × 3)-I domain as seen inside the oval in Figure 6b. Near the rectangular (1 × 1) and (4 × 3)-I structures, the R phase with discernible bright lines was always found. On the other hand, the β phase, apparently similar to a not-well-ordered (4 × 3)-I structure, was related to the growth of the (4 × 3)-III structure (compare the domains in the circles of parts a and b of Figure 6). In addition, an array of features such as ladder (γ phase), enclosed with the solid lines in Figure 6c, was observed adjacent to the (4 × 3)II structure. In Figure 6d, two domains were found: a (1 × 1) domain in hexagonal symmetry (hexagonal (1 × 1)), enclosed with the dashed lines, and a domain surrounded with solid lines (δ phase). Since the defects in the hexagonal (1 × 1) domains were not observed normally on a bare Pt(111) surface, the domains were surely covered with irreversibly adsorbed Sb. Furthermore, the δ phase would be a transient structure to the (2x3 × 2x3)R30° structure because of the jittered lines between the δ phase and (2x3 × 2x3)R30° domains. Figure 7 suggests a possible phase transition in the pristine layer of Sb irreversibly adsorbed on Pt(111). When the

Figure 6. Micrographs of mosaics of structurally different domains of Sb. Image (b) is subsequent to the image (a). Scan size: (a) 15 nm × 15 nm, (b) 15 nm × 15 nm, (c) 12 nm × 12 nm, and (d) 20 nm × 20 nm. Tip bias: 0.878 mV. Set point: 1.7 nA.

Figure 7. Proposed structural evolution in the pristine layer of SbO+ on Pt(111) at open circuit potential.

irreversible adsorption of Sb on Pt(111) is initiated, the (1 × 1) structures of square and hexagonal symmetries might be formed. The incommensurate rectangular (1 × 1) structure may evolve to the (4 × 3) structures: (i) to the (4 × 3)-I structure via the R phase, (ii) to the (4 × 3)-II structure via the γ phase, and (iii) to the (4 × 3)-III structure via the β phase. In addition, the (4 × 3)-I structure may change further to the (4 × 3)-II and (4 × 3)-III structures by arranging the O atoms. Furthermore, it is more likely that the (4 × 3)-I and (4 × 3)-II structures would change to the (4 × 3)-IV structure, because the (4 × 3)-IV structure was generally found near the (4 × 3)-I and (4 × 3)-II structures (for example, refer to the top parts of panels a and b of Figure 2). However, the transition from the (4 × 3)-III structure to the (4 × 3)-IV structure is questionable, due to the absence of an experimental observation of the close relationship between them. On the other hand, the hexagonal (1 × 1) domains would transform to the (2x3 × 2x3)R30° structure via the δ phase. The Adlayers of Sb at Reductive and Oxidative Potentials. In Figure 8, shown are the sequential micrographs of Sb on Pt(111) observed in 0.05 M H2SO4 solution during reduction/ oxidation cycles. After reduction of Sb on Pt(111) at 0 V in

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Figure 8. Sequential micrographs of Sb on Pt(111) observed along reduction-oxidation cycles in 0.05 M H2SO4 solution. Scan size: 100 nm × 100 nm. Tip bias: -451 mV. Set point: 4.0 nA.

0.05 M H2SO4 solution, Figure 8a was obtained. The reduced Sb layer on Pt(111) was fairly uniform with small void spaces of irregular shape. When the electrode potential was moved from 0 to 0.30 V to oxidize the Sb layer, the electrode surface became filled with wormlike protruded features (Figure 8b). Once the oxidized surface was reduced again, the Sb layer, as in Figure 8c, became similar to that in Figure 8a. It is noteworthy, however, that the void spaces in Figure 8c were not same as those in Figure 8a. This observation indicates that during the reduction/oxidation cycle, there was a movement of the Sb atoms. In the following voltammetric cycles, such morphological changes were persistent as shown in panels d-f of Figure 8. A close examination of panels b, d, and f of Figure 8 also clearly revealed that the density of the wormlike protrusions increased and converged to a certain ultimate value. On the Pt(111) surface covered with the oxidized Sb layer, there were two kinds of Sb domain as in Figure 9. In Figure 9a, three domains different in brightness were clearly discernible: bright, dim, and dark ones. The height difference (0.28 nm) of the bright domain from the dark one was close to the diameter of elemental Sb (0.286 nm29), while that of the dim part (0.15 nm) was similar to the diameter of Sb(III) ion (0.180 nm29) (Figure 9b). This observation obviously supports that the bright and dim parts were the domains of Sb(0) and Sb(III), respectively. (The dark part was the surface of the Pt(111) electrode. See below.) In other words, the wormlike protrusions were the domains of Sb(0), surrounded with the domains of Sb(III). This clear evidence indicates that at 0.30 V one part of the reduced Sb underwent oxidation, while the other part remained unoxidized, i.e., electrochemically inactive. The assignment of the wormlike feature to an electrochemically inactive species is coherent with the electrochemical results: as the cyclic voltammetric scan was continued, the charge related to the adsorbed Sb was decreased (Figure 1a), while the density of the wormlike feature increased (Figure 8). Figure 10 shows sequential images of the Sb layers on Pt(111) during a cycle of reduction and oxidation. The Pt(111) surface with adsorbed Sb at 0.30 V was covered with the wormlike features. As the electrode potential moved down to 0.25 V, the domains of the electrochemically inactive species became larger, especially in the vicinity of the existing protrusions. This particular growth of the bright rims around the protrusions during the cathodic potential shift is more evidence

Figure 9. (a) A micrograph of Pt(111) covered with irreversibly adsorbed Sb on Pt(111) at 0.30 V, and (b) the cross-sectional profile along the line in (a).

that the wormlike features were the domains of Sb(0). In other words, as the Sb(III) was reduced, the height difference between the two domains disappeared. With further cathodic shift to 0 V, the bright rims of Sb(0) continuously grew to leave void spaces, which would be bare Pt(111) domains uncovered with Sb (see below). When the direction of the potential shift was reversed, the morphological change was reversed also, and the wormlike features eventually emerged. A comparison of the images before reduction with those after reoxidation, however, revealed that the shapes and sizes of the protrusions were changed. Figure 11 shows the images of a Pt(111) surface covered with reduced Sb as the number of voltammetric cycles increased in Sb2O3-saturated 0.05 M H2SO4 solution. As shown in Figure 1b, such a voltammetric treatment increased the coverage of

Sb Adsorbed on Pt(111)

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Figure 12. Atomically resolved micrographs of Sb observed at 0 V. The Pt(111) surface was covered with (a) irreversibly adsorbed Sb only and (c) with irreversibly adsorbed Sb and underpotentially deposited Sb. The cross-sectional profiles along the lines in (a) are in (b). See the details in the text. Scan size: (a) 20 nm × 20 nm and (c) 20 nm × 20 nm. Tip bias: (a) 64.09 mV and (c) 40.28 mV. Set point: (a) 1.4 nA and (c) 1.7 nA. Figure 10. Sequential micrographs of irreversibly adsorbed Sb on Pt(111) observed during a cycle of reduction and oxidation in 0.05 M H2SO4 solution. Scan size: 50 nm × 50 nm. Tip bias: -414 mV. Set point: 1.0 nA.

Sb and turned the whole Sb-covered surface electrochemically inactive below 0.30 V. Upon saturation of a Pt(111) electrode with irreversibly adsorbed Sb only, the first image in Figure 11 was obtained at 0 V. The surface was partially covered with electrochemically inactive protrusions. One more cycle of oxidation-reduction (the second image) in the Sb-containing solution noticeably increased the density of the protrusions, and

the continuing voltammetric scans kept increasing gradually the coverage of the electrochemically inactive Sb(0). After the eighth cycle, the Pt(111) surface became fully covered with the electrochemically inactive Sb(0). Because the surface, such as the last image in Figure 11, was electrochemically inactive as confirmed with the dashed voltammogram in Figure 1b, the layer of Sb(0) should not be oxidized below 0.30 V. Indeed, there was no change in STM image (not shown here) after shifting the electrode potential from 0 to 0.30 V. Figure 12 shows the atomic arrangements of Sb atoms on Pt(111) at 0 V. In Figure 12a, three distinctive domains were

Figure 11. Sequential micrographs observed at 0 V during cyclic underpotential deposition of Sb in Sb2O3-saturated 0.05 M H2SO4 solution onto the Pt(111) electrode covered with irreversibly adsorbed Sb. Scan size: 100 nm × 100 nm. Tip bias: -298 mV. Set point: 0.8 nA.

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Figure 13. Schematics of the overall processes of Sb on Pt(111).

observed on the Pt(111) covered with irreversibly adsorbed Sb only: protruded domains (solid oval), domains of atomic rows running parallel to each other (dashed circle), and small domains of dense atomic spots (dotted circle). The atomic profiles of line 1 and line 2 in the region of atomic rows running parallel to each other in Figure 12a are shown in Figure 12b. The distance between the spots along line 1 was 0.39 ( 0.01 nm, indicating that the atomic row has the periodicity of x2a (a ) 0.277 nm, one Pt-Pt atomic distance). These x2a atomic rows were positioned at the period of x3a, as observed to be 0.48 ( 0.01 nm in the profile of line 2 (Figure 12b). Thus, the surface was partially covered with the domains of a uniaxially incommensurate (x3 × x2) superlattice. On the other hand, the protrusions were the aggregates of a number of spots, normally less than 10. Although the lateral arrangements of the spots were difficult to detail statistically due to the small sizes of the protrusions, the closest distance between the spots was approximately x2a. This observation obviously means that the local coverage of the protrusions was higher even than that of the (x3 × x2) structure. The existence of such dense domains of reduced Sb led us to a conclusion that during the reduction, the Sb domains of coverage 0.33 were compressed to the (x3 × x2) adlattices and protrusions. As a result of the compression, bare Pt(111) sites of (1 × 1) were observed as in the dashed circle in Figure 12a. The period of the (1 × 1) domain was 0.28 ( 0.02 nm as shown in Figure 12b. Figure 12c is an atomic image of the Pt(111) surface fully covered with totally electrochemically inactive Sb after under-

Zhao et al. potential deposition of Sb onto the irreversibly adsorbed Sb layers (refer to Figure 1 and Figure 11). The surface was a mosaic of irregularly shaped domains separated by trenches. Although the details inside the domains were hard to figure out, the closest spot-spot distance inside the domains was x2a again. The particular value implies that the atomic structure in the domains would be an incommensurate (x2 × x2). To achieve a full monolayer of Sb, as mentioned previously, the irreversible adsorption and underpotential deposition should be cooperative. The electrochemically determined coverage of each process is 0.33-0.38 and 0.1, respectively, the sum of which is reasonably consistent with the value estimated from the incommensurate (x2 × x2) structure. The Overall Processes of Sb on Pt(111). Figure 13 is an illustration of the electrochemical processes of Sb irreversibly adsorbed on Pt(111). During the irreversible adsorption of Sb without potential control, a monolayer of SbO+ is formed in the (4 × 3) and (2x3 × 2x3)R30° arrangements via several transient superlattices. As the two-dimensional SbO+ layer is reduced, the O atoms are stripped off from the surface, and the remaining elemental Sb atoms are reorganized to the electrochemically active domains and the electrochemically inactive domains. The electrochemical inactivity may result from the narrow space between the Sb atoms in the incommensurate x2a rows. For the reduced Sb atoms to be oxidized to an adsorbed two-dimensional oxide, room between the adsorbed Sb(III) ions would be needed for the O atoms from the water molecules. Considering that the sum of the diameters of Sb(III) (0.180 nm) and O2- (0.248 nm) ion is 0.428 nm, then, the space between Sb atoms separated by x2a (0.39 nm) would not be enough to accommodate the incoming O atoms during oxidation, while the room provided the Sb atoms separated by x3a (0.48 nm) would be spacious enough. If so, the Sb atoms, not separated in an enough distance, would remain unoxidized; such Sb atoms should need a higher overpotential to be oxidized. Indeed, the electrochemically inactive Sb, such as protrusions, was oxidatively stripped at potentials higher than 0.35 V. On the other hand, the compression of the reduced Sb domains leaves vacant (1 × 1) Pt(111) sites among the Sb domains. When the underpotential deposition of Sb is allowed, an additional amount of Sb atoms is deposited onto such empty sites, so that further compression of the Sb domains takes place to turn the entire Sb-covered surface electrochemically inactive. Summary This work presents the results of EC-STM studies on Sb irreversibly adsorbed on Pt(111) related to its electrochemical behavior. The pristine layer of Sb was a two-dimensional oxide monolayer of SbO+ with the coverage of 0.33 for each element, which showed a class of (4 × 3) structure and a (2x3 × 2x3)R30° structure. Furthermore, a few discernible but unidentifiable structures were observed and proposed to be transient structures to the well-ordered ones. Upon electrochemical reduction, the O atoms in the oxide layer were stripped to leave a layer of Sb(0), a part of which was rearranged to compact electrochemically inactive clusters below 0.3 V. When an additional amount of Sb was underpotentially deposited onto the Pt(111) covered with irreversibly adsorbed Sb, a full monolayer of the incommensurate x2a rows, presumably incommensurate (x2 × x2), was formed, so that the whole surface remained electrochemically inactive. The reason for the presence of electrochemically inactive Sb(0) was concluded to be the narrow rooms between the Sb(0) atoms in the x2a row not to accommodate the incoming O atoms during oxidation.

Sb Adsorbed on Pt(111) Acknowledgment. The authors are grateful to the Central Research Facility, Chungnam National University, Korea, for permission to use the STM instrument. This work was supported by a Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (R05-2004-000-10247-0). References and Notes (1) Herrero, E.; Buller, L. J.; Abruna, H. D. Chem. ReV. 2001, 101, 1897-1930. (2) Janssen, M. M. P.; Moolhuysen, J. Electrochim. Acta 1976, 21, 869-878. (3) Janssen, M. M. P.; Moolhuysen, J. Electrochim. Acta 1976, 21, 861-868. (4) Ross, P. N. The Science of Electrocatalysis on Bimetallic Surfaces. In Electrocatalysis; Ross, P. N., Lipkowski, J., Eds.; Wiley-VCH: New York, 1998; pp 43-74. (5) Waszczuk, P.; Crown, A.; Mitrovski, S.; Wieckowski, A. Methanol and formic acid oxidation on ad-metal modified electrodes. In Handbook of Fuel Cells: Fundamentals, Technology and Applications; Vielstich, W., Lamm, A., Gasteiger, H. A., Eds.; John Wiley & Sons Ltd.: New York, 2003; Vol. 2; pp 635-651. (6) Herrero, E.; Llorca, M. J.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1995, 383, 145. (7) Herrero, E.; Llorca, M. J.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1995, 394, 161. (8) Herrero, E.; Rodes, A.; Perez, J. M.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1996, 412, 165. (9) Rhee, C. K.; Kim, D. J. Electroanal. Chem. 2001, 506, 149-154. (10) Zhou, W. P.; Kibler, L. A.; Kolb, D. M. Electrochim. Acta 2002, 47, 4501-4510. (11) Ku, B.; Kim, T.; Jung, C.; Zhao, J.; Rhee, C. K. Bull. Korean Chem. Soc. 2005, 26, 735-739.

J. Phys. Chem. B, Vol. 110, No. 22, 2006 10821 (12) Rhee, C. K.; Jung, C.; Ku, B. J. Solid State Electrochem. 2005, 9, 247-253. (13) J. M. Feliu; Fernandez-Vega, A.; A. Aldaz, J. C. J. Electroanal. Chem. 1988, 256, 149. (14) Gomez, R.; Fernandez-Vega, A.; Feliu, J. M.; Aldaz, A. J. Phys. Chem. 1993, 97, 4769-4776. (15) Jung, G.; Park, H.; Rhee, C. K. J. Electroanal. Chem. 1998, 453, 243-247. (16) Herrero, E.; Rodes, A.; Perez, J. M.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1995, 393, 87-96. (17) Orts, J. M.; Rodes, A.; Feliu, J. M. J. Electroanal. Chem. 1997, 434, 121-127. (18) Blais, S.; Jerkiewicz, G.; Herrero, E.; Feliu, J. M. Langmuir 2001, 17, 3030. (19) Zhou, W. P.; Kibler, L. A.; Kolb, D. M. Electrochim. Acta 2004, 49, 5007-5012. (20) Jung, G.; Rhee, C. K. J. Electroanal. Chem. 1997, 436, 277-280. (21) Wu, Q.; Shang, W.-H.; Yan, J.-W.; Mao, B.-W. J. Mol. Catal. A: Chem. 2003, 199, 49-56. (22) Hara, M.; Inukai, J.; Yoshimoto, S.; Itaya, K. J. Phys. Chem. B 2004, 108, 17441-17447. (23) Jung, C.; Rhee, C. K. J. Electroanal. Chem. 2004, 566, 1-5. (24) Yan, J. W.; Wu, Q.; Shang, W. H.; Mao, B. W. Electrochem. Commun. 2004, 6, 843-848. (25) Jung, C.; Rhee, C. K. J. Phys. Chem. B 2005, 109, 8961-8966. (26) Nagahara, Y.; Hara, M.; Yoshimoto, S.; Inukai, J.; Yau, S.-L.; Itaya, K. J. Phys. Chem. B 2004, 108, 3224-3230. (27) Schardt, B. C.; Yau, S.-L.; Rinaldi, F. Science 1989, 243, 1050. (28) Yau, S.-L.; Vitus, C. M.; Schardt, B. C. J. Am. Chem. Soc. 1990, 112, 3677-3679. (29) Huheey, J. E. Inorganic Chemistry: Principles of Structure and ReactiVity, 3rd ed.; Murray Printing Co.: New York, 1983.