Mechanisms of charge transfer at the semiconductor-electrolyte interface: Oxygen electroreduction at naked and platinized n-TiO2 electrodes

The electroreduction of oxygen at a n-TiO2 single crystal was studied by the rotating ring-disc electrode technique. The overpotential of the reaction was found to be higher at acidic than at basic pH, this pH effect being more important at low bandbending than at high bandbending. At potentials positive of flat band the cathodic current was not found to be proportional to the O2 concentration of the electrolyte, both at acidic and basic pH. The amount of H2O2 generated as final product of the reaction was found to be higher in the low current region (high bandbending) than in the high current region (small bandbending). — All these experimental features was explained on the basis of a two-step kinetic model involving bandgap surface states probably associated with chemisorbed OH− groups. The first, limiting step of the reaction was considered to be the formation of the superoxide ion. The generation of H2O2 as final product was determined by the facility of these species to be chemisorbed on the TiO2 surface. — The main effect of Pt deposition is to reduce the overpotential for O2 electroreduction, the same mechanism as for the naked electrode being apparently valid.