Direct STM investigation of cinchona alkaloid adsorption on Cu(III)

Scanning tunneling microscopy (STM) combined with cyclic voltammetry has been employed to investigate the adsorption of cinchonine on Cu(111). Similar to cinchonidine, cinchonine forms a long-range ordered adlayer with (4 x 4) symmetry on the substrate. The structural details on molecular adsorption were obtained by high-resolution STM images. On the basis of the previous results and obtained STM images, the quinoline ring is proposed to lie parallel to Cu(111) and serve as an anchoring group. The chiral quinuclidine moiety extends out of the surface to facilitate the interaction with the prochiral reactants.     

In situ scanning tunneling microscopy of 1,6-hexanedithiol, 1,9-nonanedithiol, 1,2-benzenedithiol, and 1,3-benzenedithiol adsorbed on pt(111) electrodes

Cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM) were used to examine four dithiol molecules, including 1,6-hexanedithiol, 1,9-nonanedithiol, 1,2-benzenedithiol, and 1,3-benzenedithiol, adsorbed on well-ordered Pt(111) electrodes in 0.1 M HClO(4). The open-circuit potential (OCP) of Pt(111) electrodes decreased substantially from 0.95 to 0.3 V (versus reversible hydrogen electrode) upon the adsorption of dithol molecules, which indicates that these adsorbates injected electrons into the Pt electrode. For all dithiol molecules, ordered adlattices of p(2 x 2) and (square root 3 x square root 3)R30 degrees were formed when the dosing concentration was lower than 150 microM and the potential of Pt(111) was more negative than 0.5 V. Raising the potential of Pt(111) from 0.1 to 0.4 V or more positive values could transform p(2 x 2) to (square root 3 x square root 3)R30 degrees before it turned disarray. The insensitivity of the structure of dithiol adlayers with their chemical structures was explained by upright molecular orientation with the formation of one Pt-S bond per dithiol molecule. This molecular orientation was independent of the coverage of dithiol molecules, as nucleation seeds produced at the beginning of adsorption were also constructed with p(2 x 2). The triangular-shaped STM molecular resolution suggested 3-fold binding of sulfur headgroup on Pt(111). All dithiols were adsorbed so strongly on Pt(111) electrodes that switching the potential negatively to the onset of hydrogen evolution in 0.1 M HClO(4) or water reduction in 1 M KOH could not displace dithiol admolecules.     

Adsorption and coordination of tartaric acid enantiomers on Cu(111) in aqueous solution

Chiral modifiers have gained much attention because they can induce high enantioselectivity on reactive metal surface in heterogeneous enantioselective catalysis. The high enantioselectivity is attributed to that the chirality of modifiers is bestowed onto the metal surface upon adsorption. Much study on the adsorption of modifiers on metal surface has been performed in an ultrahigh vacuum. In this paper, the adsorption of tartaric acid on Cu(111) has been studied by electrochemical scanning tunneling microscopy (STM) in aqueous solution. It is found that (R,R)-tartaric acid and (S,S)-tartaric acid can form a well-ordered adlayer on the Cu(111) surface with a (4 x 4) symmetry. A dimeric structure is proposed in the temporary model from STM observation.     

Formation of supramolecular cavitands on copper electrode surfaces

The adsorption of 1,1'-dibenzyl-4,4'-bipyridinium molecules (dibenzyl-viologen or DBV(2+) for the sake of simplicity) on chloride precovered Cu(100) has been studied in an electrochemical environment by means of cyclic voltammetry and in situ scanning tunneling microscopy. DBV(2+) spontaneously forms a highly ordered phase on the chloride c(2 x 2) adlayer at potentials close to the onset of the copper dissolution reaction when the pure supporting electrolyte (10 mM HCl/5 mM KCl) is exchanged by one also containing DBV(2+). This ordered phase can be described by a ( radical 53 x radical 53)R15.9 degrees unit cell relating the organic adlayer to the chloride c(2 x 2) structure underneath or alternatively by a ( radical 106 x radical 106)R29.05 degrees unit cell relating the organic layer to the Cu(1 x 1) substrate structure. Thus, the negatively charged chloride layer acts as a template for the adsorption and phase formation of DBV(2+). Compared to the copper-chloride interaction, the DBV(2+)-chloride interaction appears to be weaker since the organic layer can be easily removed from the surface by the tunneling tip when drastic tunneling conditions (low bias voltage, high tunneling current) are applied. A key structural element of the DBV(2+) adlayer is an assembly of four individual DBV(2+) molecules forming square-shaped supramolecular units with pronounced cavities in their center. Characteristically, the supramolecular assemblies reveal a preferential rotational orientation resulting in the appearance of two chiral forms of these assemblies. Furthermore, these two chiral supramolecular assemblies occur in two mirrored domains of the ( radical 53 x radical 53)R15.9 degrees structure. It can be assumed that these viologen-based supramolecular architectures can be used as potential host cavitands for the inclusion of smaller organic molecules.     

Zn adsorption on Pd(111): ZnO and PdZn alloy formation

The adsorption and thermal desorption of Zn and ZnO on Pd(111) was studied in the temperature range between 300 and 1300 K with TDS, LEED, and CO adsorption measurements. At temperatures below 400 K, multilayer growth of Zn metal on the Pd(111) surface takes place. At a coverage of 0.75 ML of Zn, a p(2 x 2)-3Zn LEED structure is observed. Increasing the coverage to 3 ML results in a (1 x 1) LEED pattern arising from an ordered Zn multilayer on Pd(111). Thermal desorption of the Zn multilayer state leads to two distinct Zn desorption peaks: a low-temperature desorption peak (400-650 K) arising from upper Zn layers and a second peak (800-1300 K) originating from the residual 1 ML Zn overlayer, which is more strongly bound to the Pd(111) surface and blocks CO adsorption completely. Above 650 K, this Zn adlayer diffuses into the subsurface region and the surface is depleted in Zn, as can be deduced from an increased amount of CO adsorption sites. Deposition of >3 ML of Zn at 750 K leads to the formation of a well-ordered Pd-Zn alloy exhibiting a (6 x 4 square root 3/3)rect. LEED structure. CO adsorption measurements on this surface alloy indicate a high Pd surface concentration and a strong reduction of the CO adsorption energy. Deposition of Zn at T > 373 K in 10(-6) mbar of O2 leads to the formation of an epitaxial (6 x 6) ZnO overlayer on Pd(111). Dissociative desorption of ZnO from this overlayer occurs quantitatively both with respect to Zn and O2 above 750 K, providing a reliable calibration for both ZnO, Zn, and oxygen coverage.     

On the existence of ordered organic adlayers at the Cu(111)/electrolyte interface

We have reinvestigated the behavior of a Cu(111) electrode in pure and cinchonidine containing aqueous 0.1 M HClO4 solution by cyclic voltammetry (CV) and in situ electrochemical scanning tunneling microscopy (STM). In contrast to previous publications by Wan et al. (Langmuir 2000, 19, 1958-1962 and references cited therein) on Cu(111) in pure 0.1 M HClO4 which claimed an adsorbate-free Cu(111) surface in the entire potential range, we have found a highly ordered hexagonal adsorbate structure with a (4 x 4) unit cell, which is stable in the potential range from hydrogen evolution at -350 to -150 mV (RHE). The adsorbate-free (1 x 1) Cu(111) surface is only visible in a fairly small potential range from -150 to +50 mV. A disordered surface structure is formed at more positive potentials which is interpreted by adsorption of an oxygen-containing species. Furthermore, the formation of a highly ordered cinchonidine adlayer on Cu(111) in 0.1 M HClO4 as reported by Wan et al. (J. Am. Chem. Soc. 2002, 124, 14300-14301) could not be reproduced here. In fact, the similarity of all structures reported by Wan et al. for a great variety of different organic adlayers on Cu(111) in HClO4 solution including cinchonidine with the (4 x 4) superstructure found here already in pure HClO4 solution (i.e., without organic solute) casts serious doubts on the validity of those previous results by Wan et al. in general.     

Structure and bonding of alkanethiols on Cu(111) and Cu(100)

The local structure of the sulfur atom of methanethiolate and ethanethiolate on the Cu(111) and Cu(100) surfaces was investigated from first principles employing the periodic supercell approach in the framework of density functional theory. On the 111 surface, we investigated the (square root 3 x square root 3)R30 degrees and (2 x 2) structures, whereas on the 100 surface, we investigated the p(2 x 2) and c(2 x 2) structures. The landscape of the potential energy surface on each metal surface presents distinctive features that explain the local adsorption structure of thiolates found experimentally. On the Cu(111) surface, the energy difference between the hollow and bridge sites is only 3 kcal/mol, and consequently, adsorption sites ranging from the hollow to the bridge site were observed for increasing surface coverages. On the Cu(100) surface, there is a large energy difference of 12 kcal/mol between the hollow and bridge sites, and therefore, only the 4-fold coordination was observed. The high stabilization of thiolates on the hollow site of Cu(100) may be the driving force for the pseudosquare reconstruction observed experimentally on Cu(111). Density of states analysis and density difference plots were employed to characterize the bonding on different surface sites. Upon interaction with the metal d bands, the pi* orbital of methanethiolate splits into several peaks. The two most prominent peaks are located on either edge of the metal d band. They correspond to bonding and antibonding S-Cu interactions. In the case of ethanethiolate, all the back-bonds are affected by the surface bonding, leading to alternating regions of depletion and accumulation of charge in the successive bonds.     

Nitrate adsorption and reduction on Cu(100) in acidic solution

Nitrate adsorption and reduction on Cu(100) in acidic solution is studied by electrochemical methods, in situ electrochemical scanning tunneling microscopy (EC-STM), surface enhanced Raman spectroscopy (SERS), and density functional theory (DFT) calculations. Electrochemical results show that reduction of nitrate starts at -0.3 V vs Ag/AgCl and reaches maximum value at -0.58 V. Over the entire potential region interrogated adlayers composed of nitrate, nitrite, or other intermediates are observed by using in situ STM. From the open-circuit potential (OCP) to -0.22 V vs Ag|AgCl, the nitrate ion is dominant and forms a (2 x 2) adlattice on the Cu(100) surface while nitrate forms a dominantly c(2 x 2) structure from -0.25 to -0.36 V. The interconversion between the nitrate and nitrite adlattices is observed. DFT calculations indicate that both nitrate and nitrite are twofold coordinated to the Cu(100) surface.     

Molecular adsorption at well-defined electrode surfaces: hydroquinone on Pd(111) studied by EC-STM

The interaction of hydroquinone (H2Q) with well-defined Pd(111) surfaces at preselected potentials in dilute H2SO4 has been studied by molecule-resolved electrochemical scanning tunneling microscopy (EC-STM). H2Q spontaneously undergoes oxidative chemisorption to benzoquinone (Q), which adopts a slightly tilted parallel orientation. Evidently, the surface coordination is through the quinone pi-electron system. At potentials within the double-layer region, a close-packed well-ordered Pd(111)-(3 x 3)-Q adlattice was formed. A potential excursion to 0.7 V, a potential at which the solution-phase Q/H2Q redox reaction takes place, introduced disorder into the organic adlayer; this positive-potential-induced order-to-disorder phase transition is reversible because the ordered (3 x 3)-Q adlattice was regenerated when the potential reverted to 0.4 V. When the potential was poised at 0.2 V, a potential at which hydrogen evolution was initiated, an appreciable fraction of Q was (hydrogenatively) desorbed; the remnant Q molecules were agglomerated in small islands that retained the (3 x 3) symmetry of the full adlayer. Two possible structural models of the Pd(111)-(3 x 3)-Q adlattice are described.     

Adsorption of TTF,TCNQ and TTF-TCNQ on Au(111): An <Emphasis Type="Italic">in situ</Emphasis> ECSTM study

The adsorption and adlayer structures of tetrathiofulvalene (TTF), tetracyanoquinodimethane (TCNQ) and TTF-TCNQ on Au(111) have been systematically investigated by in situ electrochemical scanning tunneling microscopy (ECSTM) and cyclic voltammetry in 0.1 mol L⁻¹ HClO₄. All the three molecules were found to form well-ordered adlayers in the double-layer potential region of Au(111). For TTF and TCNQ adlayers, (6×3) and (4×7) structures have been observed, respectively. A structural transition was observed on TCNQ adlayer at potential negative of 0.08 V vs. the reversible hydrogen electrode (RHE), and induced a new phase with (3 × 12) structure. On the other hand, the charge transfer complex, TTF-TCNQ, self-organized into ordered domains with a lamellar structure different from those of the pure TTF and TCNQ adlayers on Au(111). Its packing arrangement was comparable to surface structures of either single crystal or thin film of TTF-TCNQ. Supported by the National Natural Science Foundation of China (Grant Nos. 20673121, 20733004 & 20821003), the National Key Project for Basic Research (Grant Nos. 2006CB806101 & 2006CB932100) and Chinese Academy of Sciences     

CO和H2分子在Cu(111)面的吸附和溶剂化效应

采用广义梯度近似(GGA)密度泛函理论(DFT)的PW91方法结合周期性模型, 在DNP基组下, 利用Dmol3模块研究了CO和H2在真空和液体石蜡环境下在Cu(111)表面上不同位置的吸附. 计算结果表明, 溶剂化效应对H2和CO的吸附结构参数和吸附能的影响非常显著. 在液体石蜡环境下, H2平行吸附在Cu(111)表面是解离吸附, 而CO 和H2在两种环境下的垂直吸附都是非解离吸附. 相比真空环境吸附, 在液体石蜡环境中, Cu(111)吸附CO时, 溶剂化效应能够提高CO吸附的稳定性, 同时有利于CO的活化. 在真空中, H2只能以垂直方式或接近垂直方式吸附在Cu(111)表面. 当Cu(111)顶位垂直吸附H2, 相比真空环境吸附, 溶剂化效应能够提高H2吸附的稳定性, 但对H2的活化没有明显影响. Cu(111)表面的桥位或三重穴位(hcp和fcc)垂直吸附H2时, 溶剂化效应能明显提高H2的活化程度, 但降低H2的吸附稳定性; 在液体石蜡中, 当H2平行Cu(111)表面吸附时, 溶剂化效应使H—H键断裂, 一个H原子吸附在fcc位, 另一个吸附在hcp位.     

First-principles investigation of transition metal atom M (M = Cu, Ag, Au) adsorption on CeO2(110)

Transition metal atom M (M = Cu, Ag, Au) adsorption on CeO(2)(110), a technologically important catalytic support surface, is investigated with density-functional theory within the DFT+U formalism. A set of model configurations was generated by placing M at three surface sites, viz., on top of an O, an O bridge site, and a Ce bridge site. Prior to DFT optimization, small distortions in selected Ce-O distances were imposed to explore the energetics associated with reduction of Ce(4+) to Ce(3+) due to charge transfer to Ce during M adsorption. Charge redistribution is confirmed with spin density isosurfaces and site projected density of states. We demonstrate that Cu and Au atoms can be oxidized to Cu(2+) and Au(2+), although the adsorption energy, E(ads), of Au(2+) is less favorable and, unlike Cu(2+), it has not been experimentally observed. Oxidation of Ag always results in Ag(+). For M adsorption at an O bridge site, E(ads)(2NN) > E(ads)(3NN) > E(ads)(1NN) where NN denotes the nearest neighbor Ce(3+) site relative to M. Alternatively, for M adsorption at a Ce bridge site, E(ads)(3NN) > E(ads)(2NN) > E(ads)(1NN). The adsorption behavior of M on CeO(2) (110) is compared with M adsorption on CeO(2)(111).     

Self-assembled monolayers of benzylmercaptan and p-cyanobenzylmercaptan on Au(111) surfaces: structural and spectroscopic characterization

The formation of self-assembled monolayers of benzylmercaptan (BM) and p-cyanobenzylmercaptan (pCBM) on Au(111) surfaces is investigated by a combination of X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and scanning tunneling microscopy (STM). The NEXAFS results of pCBM are supported by ab initio calculations. It is found that BM as well as pCBM form well-ordered monolayers with the molecules oriented almost perpendicular to the surface. BM forms a ( radical 3 x radical 3)R30 degrees structure whereas pCBM forms a slightly different c(7 x 7) hexagonal structure. No phase separation is detected for the adsorption of a 1:1 mixture of the two molecules. The implications of the results for the covalent attachment of transition-metal complexes to thiol-functionalized surfaces are discussed.     

Effects of organic additives on zinc electrodeposition at iron electrodes studied by EQCM and in situ STM

Effects of organic additives, such as benzoic acid (BA) and poly(ethylene glycol)s (PEGs), on the initial stage of the zinc electrodeposition have been investigated at iron electrodes using cyclic voltammetry, electrochemical quartz crystal microbalance measurements and in situ electrochemical scanning tunneling microscopy in an acidic zinc chloride solution in efforts to gain a molecular-level understanding of their roles. BA is adsorbed strongly at the sites of more negative potentials on the electrode, although it is randomly adsorbed on the iron surface at around an open circuit potential. Its role seems to control the deposition rate at the dendritic sites by blocking the active surface via adsorption. On the contrary, PEGs are adsorbed more or less evenly with a well-ordered structure on the iron surface and appear to desorb in the underpotential deposition region of zinc ions, which helps inhibit proton reduction by effectively blocking the electrode surface.     

The electrochemical interface between copper(111) and aqueous electrolytes

The electrochemical double layer between Cu(111) electrodes and aqueous electrolytes (F⁻ and SO²⁻₄ at various pH values) was studied by means of linear scan voltammetry and ac impedance measurements. It is found that electrochemisorption of oxygen species proceeds on the Cu(111) surface in the potential regions more negative than the electrodissolution potential of copper. The adsorption-desorption kinetics are analysed; the anodic and cathodic symmetry coefficients are found to be equal (α = β = 0.3), and the standard rate constant is k° = 4 × 10⁻¹⁰cm s⁻¹.     

CO adsorption on copper in alkaline electrolytes: an electrochemical and ellipsometric study

The influence of CO on the passive behavior of copper was analyzed in the potential region near the rest potential in borax solutions (pH 9.2) by cyclic voltammetry, ellipsometry, and surface charge determination techniques. The oxide formation is explained as a sequence of Cu(2)O growth, cation adsorption, Cu(II), and dissolution steps, similarly to previously reported investigations for the metal in free CO solutions. The CO adsorption hinders the cationic defect in the outer oxide layer and accelerates the Cu(2)O growth both at open circuit and in controlled potential experiments. The isoelectric point, iep, obtained at pH 10.8 for both the metal Cu and CuO particles in KCl solutions shifts to pH 10.1 for copper particles in the presence of CO. The iep indicates a CuO coating on Cu metallic particles in the absence of CO.     

Anion—surface interactions: Part I. Perchlorate decomposition and sulphate adsorption hysteresis studied by voltammetry

Cyclic voltammetry has been used to investigate the behaviour of sulphate and perchlorate anions adsorbed on Ir{111} and polycrystalline rhodium. In 0.1 M HClO₄, perchlorate anions decompose on polycrystalline rhodium to give a surface intermediate which inhibits hydrogen adsorption. This is in agreement with a previous study by Wieckowski and coworkers, who showed that the surface intermediate was, in fact, adsorbed chloride. The voltammetry of the same electrode in sulphate containing electrolytes depends critically on the concentration of sulphate and the potential sweep rate. We conclude that hydrogen adsorption can only occur after significant desorption of sulphate. Hysteresis in the adsorption and desorption of sulphate results in an asymmetric hydrogen adsorption region with respect to the potential axis. Ir{111} is shown also to cause decomposition of perchlorate anions, although, contrary to the case of rhodium, the process is very much a chemical one in which electric currents, arising from perchlorate decomposition, were not detected. In sulphuric acid, a strong interaction between anions and the Ir{111} surface is observed, which precludes oxide formation at potentials less than 1 V (Pd---H), behaviour characteristic of a Pt{111} surface in the same electrolyte.     

氧化锌有序结构在Au(111)和Cu(111)上的生长

利用NO₂或O₂作为氧化剂,研究了氧化锌在Au(111)和Cu(111)上的生长和结构。NO₂表现了更好的氧化性能,有利于有序氧化锌纳米结构或薄膜的生长。在Au(111)和Cu(111)这两个表面上,化学计量比氧化锌都形成非极性的平面化ZnO(0001)的表面结构。在Au(111)上,NO₂气氛下室温沉积锌倾向于形成双层氧化锌纳米结构;而在更高的沉积温度下,在NO₂气氛中沉积锌则可同时观测到单层和双层氧化锌纳米结构。O₂作为氧化剂时可导致形成亚化学计量比的ZnO_x结构。由于铜和锌之间的强相互作用会促进锌的体相扩散,并且铜表面可以被氧化形成表面氧化物,整层氧化锌在Cu(111)上的生长相当困难。我们通过使用NO₂作为氧化剂解决了这个问题,生长出了覆盖Cu(111)表面的满层有序氧化锌薄膜。这些有序氧化锌薄膜表面显示出莫尔条纹,表明存在一个ZnO和Cu(111)之间的莫尔超晶格。实验上观察到的超晶格结构与最近理论计算提出的Cu(111)上的氧化锌薄膜结构相符,具有最小应力。我们的研究表明,氧化锌薄膜的表界面结构可能会随氧化程度或氧化剂的不同而变化,而Cu(111)的表面氧化也可能影响氧化锌的生长。当Cu(111)表面被预氧化成铜表面氧化物时,ZnO_x的生长模式会发生变化,锌原子会受到铜氧化物晶格的限域形成单位点锌。我们的研究表明了氧化锌的生长需要抑制锌向金属基底的扩散,并阻止亚化学计量比ZnO_x的形成。因此,使用原子氧源有利于在Au(111)和Cu(111)表面上生长有序氧化锌薄膜。     

Density functional theory study of the adsorption of alkanethiols on Cu(111), Ag(111), and Au(111) in the low and high coverage regimes

The structure, the surface bonding, and the energetics of alkanethiols adsorbed on Cu(111), Ag(111), and Au(111) surfaces were studied under low and high coverages. The potential energy surfaces (PES) for the thiol/metal interaction were investigated in the absence and presence of externally applied electric fields in order to simulate the effect of the electrode potential on the surface bonding. The electric field affects the corrugation of the PES which decreases for negative fields and increases for positive fields. In the structural investigation, we considered the relaxation of the adsorbate and the surface. The highest relaxation in a direction perpendicular to the surface was observed for gold atoms, whereas silver atoms presented the highest relaxation in a plane parallel to the surface. The surface relaxation is more important in the low coverage limit. The surface bonding was investigated by means of the total and projected density of states analysis. The highest ionic character was observed on the copper surface whereas the highest covalent character occurs on gold. This leads to a strong dependence of the PES with the tilt angle of the adsorbate on Au(111) whereas this dependence is less pronounced on the other metals. Thus, the adsorbate-relaxation and the metal-relaxation contributions to the binding energy are more important on gold. The adsorption of thiols on gold was investigated on the 111 surface as well as on a surface with gold adatoms in order to elucidate the effect of thiols on the surface diffusion of gold. The CH(3)CH(2)S radical adsorbs ontop of the gold adatom. The diffusional barrier of the CH(3)CH(2)SAu species is lower than that for a bare gold adatom and is also lower than that for the bare thiol radical. The adsorption of the molecular species CH(3)SH and CH(3)CH(2)SH was also investigated on Au(111). They adsorb via the sulfur atom ontop of a gold atom. On the other hand, the adsorption of the alkanethiol radicals on the perfect 111 surfaces occurs on the face centered cubic (fcc)-bridge site in the low coverage limit for all metals and shifts toward the fcc site at high coverage on copper and silver.     

Adlayers of Sb irreversibly adsorbed on Pt(111): an electrochemical scanning tunneling microscopy study

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 x 3)-3SbO+ structures and one (2 square root(3) x 2 square root(3))R30 degrees-3SbO+ structure. In addition, several unidentifiable transient structures of SbO+ were observed, and their relations to the well-ordered structures of (4 x 3) and (2 square root(3) x 2 square root(3))R30 degrees, 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 (square root(3) x square root(2))-Sb, and domain of bare (1 x 1) Pt(111). During oxidation of elemental Sb at 0.30 V, the Sb domains of the (square root(3) x square root(2)) 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 square root(2a) 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 (square root(2) x square root(2)) structure.