ELECTROCHEMICAL OXIDE FILM FORMATION AT NOBLE-METALS AS A SURFACE-CHEMICAL PROCESS

The mechanisms of electrochemical oxide film formation at noble metals are described and exemplified by the cases of Pt and Au, especially in the light of recent experimentation by means of cyclic voltammetry, ellipsometry and vacuum surface-science studies using LEED and AES. Unlike the mechanisms of base-metal oxidation, e.g., in corrosion processes, anodic oxide film formation at noble metals proceeds by surface chemical processes involving, initially, sub-monolayer, through monolayer, formation of 2-dimensional OH/O arrays. During such 2-d processes, place-exchange between electrosorbed OH or 0 species on the surface, and Pt or Au atoms within the surface lattice, takes place leading to a quasi-2-d compact film which then grows ultimately to a multilayer hydrous oxide film, probably by continuing injection of ions of the substrate metal and their migration through the growing film under the influence of the field. The initial, sub-monolayer stage of electrosorption of OH involves competitive chemisorption by anions, e.g. HSO4-, ClO4-, Cl-, which inhibits onset of the first stage of surface oxidation. These processes are demonstrable in experiments on single-crystal surfaces. The combination of such anion effects with place-exchange during the extension of the film, leads to a general mechanism of noble metal oxide film formation. The formation of the oxide films can be examined in detail by recording the distinguishable stages in the film's electrochemical reduction in linear-sweep voltammetry which is sensitive down to OH/O fractional coverages as low as 0.5% acid over time-scales down to 50 mu s in experiments on time-evolution and transformation of the states of the oxide films. By means of LEED, AES and STM or AFM experiments, the reconstructions and perturbations (e.g. generation of stepped terraces) which oxide films cause on single-crystal surfaces can be followed.