Electron-transfer reactions on pure and modified oxide films

Abstract

The applicability of the band structure model to anodic oxide films is discussed, and data for special systems are given. Various ETR mechanisms described in Fig. 1 of this paper can be distinguished by determination of the transfer coefficients. Large activation energies are correlated with the distance between the Fermi level Ef and the energy at which the maximum current flows. Maximum rate constants, km ≈ 105 cm/s, correspond to adiabatic transitions. At highly doped and thin films, smaller values indicate tunneling processes. The film thickness exerts a strong influence owing to changes of the Debye length and changes of the band structure. The electronic influence of the oxide and the redox system can be rationalized using normalized exchange current densities, and the energetic difference between Ef and the next band of the oxide. Modification of oxide films can be caused by an increase of the density of states at the oxide surface (e.g. diodes), by an increase of the number of donors, or by the generation of an inner surface below an outer semipermeable film. Anodic oxygen evolution is extensively discussed as an example of the continuos variation of oxide properties achieved by oxide mixing, metal incorporation, ion implantation or even inhibition.