Os layers spontaneously deposited on the Pt(111) electrode : 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.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC); National Science Foundation (NSF)
- DOE Contract Number:
- DE-AC02-06CH11357
- OSTI ID:
- 961171
- Report Number(s):
- ANL/MSD/JA-45224; TRN: US201010%%990
- Journal Information:
- J. Electronal. Chem., Vol. 554-555, Issue 2003
- Country of Publication:
- United States
- Language:
- ENGLISH
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Related Subjects
ABSORPTION SPECTROSCOPY
BINDING ENERGY
CATALYSIS
CONVERGENCE
DENSITY
DEPOSITION
DISSOLUTION
ELECTROCHEMISTRY
ELECTRODES
ELECTRONS
FERMI LEVEL
FLUORESCENCE
FUEL CELLS
GRAZING
GRAZING INCIDENCE TOMOGRAPHY
HEIGHT
LAYERS
LEVELS
OSMIUM
OXIDATION
PHOTOEMISSION
PLATINUM
PRECURSOR
RANGE
SALTS
SCANNING TUNNELING MICROSCOPY
SPECTRA
SPECTROSCOPY
STRIPPING
SURFACES
VALENCE
VARIATIONS
X-RAY PHOTOELECTRON SPECTROSCOPY