STUDIES OF FORMATION OF VERY THIN OXIDE-FILMS ON POLYCRYSTALLINE RHODIUM ELECTRODES - APPLICATION OF THE MOTT-CABRERA THEORY

Anodic polarization of Rh(poly) electrodes in 0.5 M aqueous H2SO4 at potentials between 0.80 and 2.40 V for times up to 10 000 s leads to formation of very thin oxide films and no limit in the extent of surface oxidation is observed. Under such experimental conditions up to 3.25 equivalent monolayers of Rh(OH)3 are formed. It is found that two oxide states (OC1 and OC2) can be distinguished in the linear sweep voltammetry oxide reduction profiles. The plots of the reciprocals of the oxide reduction charges vs the logarithms of the polarization times, 1/q(ox) vs log t(p) are linear and indicate that the oxide formation proceeds according to the inverse-logarithmic oxide formation kinetics. Application of the Mott-Cabrera theory to the growth kinetics allows determination of the potential drop and the electric field across the oxide film; the former is found to depend on the polarization potential, E(p). We propose two possible structures of the surface Rh oxide/hydroxide species from which the distance between the Rh atom and the Rh3+ cation at the inner metal/oxide interface may be estimated as 4.21 angstrom, while the distance between Rh3+ cations in the oxide film is 4.16 angstrom. Consequently, we determine the electric field in the oxide films which is of the order of 10(7) V cm-1.