Surface orientation dependence of oxide film growth at platinum single crystals

Abstract

At single-crystal surfaces of noble metals, the initial sub-monolayer stages of anodic oxide film formation are distinguishable as two-dimensional surface processes specific to the geometry and orientation of the metal surface plane from which they originate. Below and beyond the monolayer level of oxide film formation, the extension of the film is logarithmic in polarization time for a given controlled electrode potential. The purpose of the present paper is to investigate if and how the continuing logarithmic film growth process may also depend on single-crystal surface orientation, and thus be determined by the geometry of the metal surface underlying the growing oxide film. The underpotential deposition hydrogen profiles, determined following the reduction of small and larger quantities per square centimetre of the oxide film at platinum, show that the geometry of the underlying platinum metal surface on which the oxide film was developed changes detectably with respect to that of the initial oxide-free surface but still retains, to a large extent, its initial two-dimensional structure. Once the Pt(111) or Pt(100) surfaces have been altered by the initial monolayer, they remain essentially unchanged during further oxide formation. The resulting structures are characteristically different from one another and from that of polycrystalline platinum, and remain so (as indicated by underpotential deposition hydrogen profiles following film reduction) even after development of oxide films equivalent to ca. 1000 μC cm −2 in formation charge. Thus, the apparent crystal-face dependence of the oxide film growth kinetics reflects a true specificity with respect to the surfaces from which the films respectively originate.