Mitsuru Wakisaka

Identification and Quantification of Oxygen Species Adsorbed on Pt(111) Single-Crystal and Polycrystalline Pt Electrodes by Photoelectron Spectroscopy

We have positively identified oxygen species on Pt(111) single-crystal and polycrystalline Pt electrodes in N2-purged 0.1 M HF solution by X-ray photoelectron spectroscopy combined with an electrochemical cell. Four oxygen species (Oad, OHad, and two types of water molecules) were distinguished. The binding energies of each species were nearly constant over the whole potential region and independent of the single- or polycrystalline electrodes. The coverages, however, varied considerably and were dependent on the electrode potential. We have for the first time demonstrated clear differences in the surface oxidation processes for Pt(111) and polycrystalline Pt electrodes.

Direct STM elucidation of the effects of atomic-level structure on Pt(111) electrodes for dissolved CO oxidation.

We sought to establish a new standard for direct comparison of electrocatalytic activity with surface structure using in situ scanning tunneling microscopy (STM) by examining the electrooxidation of CO in a CO-saturated solution on Pt(111) electrodes with steps, with combined electrochemical measurements, in situ STM, and density functional theory (DFT). On pristine Pt(111) surfaces with initially disordered (111) steps, CO oxidation commences at least 0.5 V lower than that for the main oxidation peak at ca. 0.8-1.0 V vs the reversible hydrogen electrode in aqueous perchloric acid solution. As the potential was cycled between 0.07 and 0.95 V, the CO oxidation activity gradually decreased until only the main oxidation peak remained. In situ STM showed that the steps became perfectly straight. A plausible reason for the preference for (111) steps in the presence of CO is suggested by DFT calculations. In contrast, on a pristine Pt(111) surface with rather straight (100) steps, the low-potential CO oxidation activity was less than that for the pristine, uncycled (111) steps. As the potential was cycled, the activity also decreased greatly. Interestingly, after cycling, in situ STM showed that (111) microsteps were introduced at the (100) steps. Thus, potential cycling in the presence of dissolved CO highly favors formation of (111) steps. The CO oxidation activity in the low-potential region decreased in the following order: disordered (111) steps > straight (100) steps > (100) steps with local (111) microsteps ≈ straight (111) steps.

Structural effects on the surface oxidation processes at Pt single-crystal electrodes studied by X-ray photoelectron spectroscopy

The oxidation states of Pt(111), Pt(100) and Pt(110) in 0.1 M HF solution were analyzed as a function of the electrode potential by EC-XPS. We have, for the first time, clarified the structural effects on the surface oxidation processes at Pt electrodes on the basis of the coverages of each type of oxygen species.

Osmium nanoislands spontaneously deposited on a Pt(111) electrode: an XPS, STM and GIF-XAS study

Scanning tunneling microscopy (STM) characterized adlayers of spontaneously deposited osmium on a Pt(111) electrode were investigated using ex-situ X-ray photoemission spectroscopy (XPS) and in-situ grazing incidence fluorescence X-ray absorption spectroscopy (GIF-XAS). After a single spontaneous deposition, monoatomic (or nearly monoatomic) nanoislands of osmium are formed. The island diameter varies from 2 to 5 nm depending on the Os coverage, which in turn is adjusted by varying the concentration of the Os precursor salt (OsCl3) in the deposition bath and/or by the deposition time. XPS reveals three oxidation states: a metallic Os (the 4f7 /2 core level binding energy of 50.8 eV), Os(IV) (51.5 eV) and Os(VIII) (52.4 eV). The metallic osmium exists at potentials below 500 mV (vs. RHE) while above 500 mV osmium is oxidized to Os(IV). Electrodissolution of osmium begins above 900 mV and occurs simultaneously with platinum oxidation. At ca. 1200 mV V versus the RHE reference, the oxidation state of some small amounts of osmium that survive dissolution is the Os(VIII). We demonstrate, for the first time, that mixed or odd valencies of osmium exist on the platinum surface at potentials higher that 800 mV. In-situ GIF-XAS measurements of an Os LIII edge also reveal the presence of three Os oxidation states. Namely, below the electrode potential of 400 mV, the X-ray fluorescent energy at maximum absorption is 10.8765 keV, and is characteristic of the metallic Os. In the potential range between 500 and 1000 mV this energy is gradually shifted to higher values, assignable to higher valencies of osmium, like Os(IV). This tendency continues to higher potentials consistent with the third, highly oxidized osmium form present, most likely Os(VIII). The variation of the ‘‘raw edge jump height’’ of Os with the electrode potential, which is equivalent to a drop in osmium surface concentration, demonstrates that the electrochemical stripping of Os begins below 1.0 V versus RHE, as expected from voltammetry. Also, the observed intensity of the white line of Os in the 100/400 mV region is larger than the value reported for metallic bulk Os. This discrepancy may result from the difference in the electronic properties of the metallic Os layers on Pt(111) and the metallic bulk Os: in the potential region between 100 and 400 mV, the 5d electrons in Os and Pt form a mixed electronic band, and the density of electronic states near the Fermi level, the main factor determining the white line intensity, may not be the same as in metallic bulk. The presented results on osmium adlayers are much more comprehensive than those available in our previous work due to the combined STM, GIF-XAS and XPS investigations. A nearly perfect convergence of the in situ and ex situ data is one of the main research outcomes of this project. Finally, platinum XPS spectra taken in the context of Os electrooxidation from the electrode surface are also presented and conclusions are made, that up to 900 mV platinum remain metallic, irrespective of a significant osmium oxidation on its surface. # 2003 Elsevier B.V. All rights reserved.

The structure of a coronene adlayer formed in a benzene solution: Studies by in situ STM and ex situ LEED

Abstract In situ scanning tunneling microscopy (STM) revealed that a highly ordered adlayer of coronene was formed on an Au(111) substrate by immersion in benzene solution containing coronene. High-resolution STM images allowed us to determine the packing arrangement of coronene. The coronene adlayer on Au(111)-(1×1) revealed a commensurate (4×4) symmetry ( θ =0.0625) with a flat-lying orientation at 0.75 V versus the reversible hydrogen electrode (RHE) in aqueous HClO 4 . The adlayer structure of coronene determined by ex situ low-energy electron diffraction (LEED) using an ultrahigh vacuum-electrochemical (UHV-EC) technique was consistent with that determined by in situ STM in aqueous HClO 4 .

In situ STM observation of the CO adlayer on a Pt(110) electrode in 0.1 M HClO4 solution.

We have obtained the first in situ STM molecular image of a CO adlayer on a Pt(110)-(1 x 1) electrode surface in 0.1 M HClO(4) solution. The observed CO adlayer formed an ordered (2 x 1)-2CO structure at saturated coverage. The CO molecules were found to adsorb on top of each Pt surface atom; however, they were tilted with a zigzag configuration along the atomic rows because of the dipole-dipole repulsion of neighboring CO molecules. The high activity of the Pt(110) electrode for surface CO oxidation can be attributed to the low packing density of the adsorbed CO molecules as well as their tilted orientation.

Structures of a CO adlayer on a Pt(100) electrode in HClO4 solution studied by in situ STM

We have obtained the first in situ STM atomic images of a CO adlayer on a Pt(100)-(1 x 1) electrode in 0.1 M HClO(4) solution, exhibiting a phase transition from c(6 x 2)-10CO to c(4 x 2)-6CO at E > 0.3 V vs. RHE.

Structural variations of CO adlayers on a Pt(100) electrode in 0.1 M HClO4 solution: an in situ STM study.

In the present study, we have investigated structures of a CO adlayer on a well-defined Pt(100) electrode surface in 0.1 M HClO4 aqueous solutions saturated with N2, 1% CO/He and 100% CO by using in situ STM. The in situ STM images with molecular resolution demonstrated that highly ordered structures of the CO adlayer, denoted (2 × n) - 2(n - 1)CO with CO coverages of (n - 1)/n, dynamically varied with the electrode potential and the CO partial pressure in solution. As the CO partial pressure increased, more compressed structures of the CO adlayer formed on the electrode surface. In each solution, a phase transition of the CO adlayer on the terrace site was observed to be triggered by increasing the electrode potential, accompanied by a partial desorption of surface CO without charge transfer. A series of in situ STM images revealed transient local structures during the phase transition of the CO adlayer. Specifically, unique structures were found to appear in the vicinity of monoatomic steps in N2- and 1% CO/He-saturated solution, but not in 100% CO-saturated solution.

Adlayer of naphthalene on Rh(111) studied by scanning tunneling microscopy

An adlayer of naphthalene was formed on Rh(111) by vapor deposition. The low-energy electron diffraction analysis showed the adlayer having a (3 ×3) structure. Using a scanning tunneling microscope, individual naphthalene molecules were observed to lie flat on the surface. The C2 axes of the molecules were found to be aligned in the directions of the atomic rows of the Rh substrate.

Analysis of the Surface Oxidation Process on Pt Nanoparticles on a Glassy Carbon Electrode by Angle-Resolved, Grazing-Incidence X-ray Photoelectron Spectroscopy

We have analyzed the surface oxidation process of Pt nanoparticles that were uniformly dispersed on a glassy carbon electrode (Pt/GC), which was adopted as a model of a practical Pt/C catalyst for fuel cells, in N2-purged 0.1 M HF solution by using angle-resolved, grazing-incidence X-ray photoelectron spectroscopy combined with an electrochemical cell (EC-ARGIXPS). Positive shifts in the binding energies of Pt 4f spectra were clearly observed for the surface oxidation of Pt nanoparticles at potentials E > 0.7 V vs RHE, followed by a bulk oxidation of Pt to form Pt(II) at E > 1.1 V. Three types of oxygen species (H2Oad, OHad, and Oad) were identified in the O 1s spectra. It was found for the first time that the surface oxidation process of the Pt/GC electrode at E < ca. 0.8 V (OHad formation) is similar to that of a Pt(111) single-crystal electrode, whereas that in the high potential region (Oad formation) resembles that of a Pt(110) surface or polycrystalline Pt film.