The Surface Modification of Fluorine‐Doped Tin Oxide by the Underpotential Deposition of Noble Metals

Abstract

The surface modification of fluorine‐doped tin oxide by the underpotential deposition of noble metals (Rh, Ru, Pd, Pt, and Au) strongly influences the rate of sulfide‐polysulfide reduction at the electrode. X‐ray photoelectron spectroscopy (XPS) and scanning electron microscopy are used to characterize as‐prepared and surface modified tin oxide preceding and following electrode operation. The noble metal coverages follow the relative ordering: Au, , , showing an identical trend as the observed enhancement effects. Depth profile analysis shows that rhodium is found more than 100Å into the tin oxide film. This spatial distribution is apparently an artifact of the columnar structure. The XPS atom bonding peaks for the noble metals are shifted from the peak values associated with metallic states by an amount which is dependent on the exposure history of the tin oxide and yet independent of the noble metal species. Tin oxide modified prior to operation as an electrode yields dopant metal energy peaks that are displaced 1.5 eV higher in energy. Postelectrode operation, in contrast, shows 0.5 eV peak shifts. The influence of tin oxide hydration upon the surface oxygen species is discussed and correlated with the binding energy shifts. Finally, the trend of noble metal coverage is analyzed on the basis of interfacial thermodynamics.