Kenji Sashikata

In situ scanning tunneling microscopy of adsorbed sulfate on well-defined Pd(111) in sulfuric acid solution

In situ electrochemical scanning tunneling microscopy (STM) disclosed that highly-ordered adlayers of adsorbed sulfate/bisulfate with atomic features having a (√3×√7) symmetry formed on a well-defined Pd(111) surface in 10 mM H2SO4 in the double layer potential range. Well-defined Pd(111) surfaces were prepared by a flame-annealing-quenching technique. High resolution STM imaging revealed the structure of water molecules in both the first and the second layers of a water-bilayer. Small reversible peaks at 0.45 V versus a reversible hydrogen electrode were found to be due to a disorder–order phase transition. The Pd(111)-(1×1) and (√3×√7) structures were observed at potentials negative and positive with respect to the small peaks, respectively.

In situ electrochemical scanning tunneling microscopy of single‐crystal surfaces of Pt(111), Rh(111), and Pd(111) in aqueous sulfuric acid solution

In situ electrochemical scanning tunneling microscopy (STM) was applied to single‐crystal platinum(111), rhodium(111), and palladium(111) surfaces in an aqueous sulfuric acid solution. Atomically flat surfaces of Pt(111) are roughened in solutions by the electrochemical oxidation–reduction cycle. It is shown that a single potential cycle causes the formation of many adatoms and very small clusters on the Pt(111) terrace. A steady‐state surface structure can be clearly observed after applying a few potential cycles. The STM image is composed of regularly arrayed islands whose diameter and height are in the ranges of 2–3 and 0.5–0.75 nm, respectively. Atomically flat terrace‐step structures can be also observed on Rh(111) and Pd(111) surfaces. The effect of oxidation–reduction cycle on these surfaces is also discussed.

Effect of underpotential deposition (UPD) of copper on oxygen reduction at Pt(111) surfaces

Abstract The influence of underpotentially deposited Cu adlayers on the electrocatalytic reduction of oxygen at Pt(111) has been studied in 0.05 M H2SO4 solutions using hanging meniscus rotating-disk (HMRD) voltammetry and electrochemical scanning tunneling microscopy (STM). Oxygen reduction at clean bare Pt(111) proceeds by a direct four-electron transfer with the formation of H2O as the primary reaction product. After the formation of the first underpotentially deposited Cu adlayer with (√3 × √3)R30° structure, the oxygen reduction current decreases to a steady-state value which is almost half that observed at bare Pt. This partial inhibition provided by the Cu adatoms can be explained by a change in the oxygen adsorption mechanism from the bridged orientation, favoring a four-electron transfer, to the end on orientation, favoring a two-electron transfer. The effect of coadsorbed halides on underpotential deposition (UPD) as well as the oxygen reduction reaction, has also been examined. Oxygen reduction at Cu-modified Pt(111) in the presence of chloride was completely inhibited after the first UPD process. Further, oxygen reduction in a pure H2SO4 solution on bare Pt(111) was carefully studied using HMRD voltammetry. The oxygen reduction current at 0.02 V was almost half the constant limiting current observed in the potential region between 0.5 and 0.3 V. This result can also be explained by a change in the oxygen adsorption mechanism from the bridged to the end-on orientation.

In situ scanning tunneling microscopy of underpotential deposition of Ag on Pt(111)(7×7)R19.1°−I

Abstract The underpotential deposition (UPD) and bulk deposition of Ag on iodine-coated Pt(111) has been investigated in perchloric acid solutions using electrochemical scanning tunneling microscopy (STM). The UPD of Ag occurred in three steps. A ( 7 × 7 ) R 19.1 ° structure was found at potentials positive with respect to the first UPD peak. A (3 × 3) structure appeared after the formation of the first UPD adlayer. STM atomic images acquired after the second and third UPD peaks corresponded to a ( 3 × 3 ) R 30 ° structure. The same structure was also found for bulk-deposited Ag layers.

Electrochemistry of the I-on-Pd single-crystal interface : studies by UHV-EC and in situ STM

Abstract A single chemisorbed layer of zerovalent I atoms has been found to enhance, at ambient temperatures, the reactivity of Pd electrode surfaces. Three unique reactions (anodic dissolution in non-corrosive electrolyte, regeneration of well-ordered single-crystal surfaces, and “electrochemical digital etching”) have been investigated at Pd(111) and Pd(100) single-crystal electrodes and are described in this paper. Experimental measurements were based upon a combination of electrochemistry (EC), low-energy electron diffraction (LEED), and in-situ scanning tunneling microscopy (STM).

Anodic dissolution and reordering of Pd(110) induced by chemisorbed iodine

Abstract Adsorbate-induced reordering and anodic dissolution, reported previously for Pd(111) and Pd(100) surfaces that contained an ordered iodine adlattice, were examined at an I-pretreated Pd(110) electrode. Experimental measurements were based upon a combination of electrochemistry, electron spectroscopy, and scanning tunneling microscopy; remarkable consistency was observed between the ex situ and in situ results. Similarities and differences exist between Pd(110) and the two other low-index planes. The expected congruencies: (i) well-ordered iodine adlattices (Pd(110)-c(2 × 2)-I and Pd(100)-pseudohexagonal-I) are formed spontaneously upon exposure of a Pd(110) surface to an aqueous solution of iodide, even when the surface was previously disordered by oxidation-reduction cycles; (ii) anodic dissolution of the metal substrate occurs only in the presence of chemisorbed iodine; (iii) the I-catalyzed corrosion does not alter the coverage of the iodine adlayer. The notable disparities: (a) reductive desorption of the chemisorbed iodine does not yield an ordered (1 × 1) surface; neither does the removal of iodine by displacement with and subsequent oxidative desorption of CO; (b) anodic stripping of the metal surface disorders the structure of the iodine adlattice; no reordering takes place upon exposure of such disordered surface to aqueous iodide; (c) the iodine-catalyzed corrosion occurs selectively at stepedges along the {100} and {110} directions; dissolution at the {110}-directed step develops preferentially over that at the {100}-directed edge to form rectangular pits; (d) the propagation of new (smaller) pits at the bottom of the (enlarged) rectangular pits leads to progressive surface roughness.