Influence of Hole Transport and Thermal Reactions in Photo-Driven Water Oxidation Kinetics on Crystalline TiO₂

The requirement that photogenerated holes accumulate to drive the rate-limiting step is thought to cause slow water oxidation by TiO₂ to form O₂; however, detailed kinetic studies that directly establish the connection between photoabsorption and surface reactions have not been reported. In this work, we use physically realistic kinetic models of photo-driven water oxidation on TiO₂ to evaluate how hole generation, bulk diffusion, surface mobility, and reaction are coupled. The calculations show that hole formation and diffusion in the bulk crystal dominate O₂ formation at low light intensity, resulting in an apparent high-order dependence of the O₂ production rate on holes. As the light intensity increases, the water-splitting reaction becomes nearly independent of hole concentrations because of a buildup of intermediates that can only react thermally. Although it is believed that high hole mobility is a requirement for hole accumulation, a comparison of predicted to observed surface species indicates that immobilized holes dominate the surface reactivity. The primary surface reaction sites are predicted to involve oxygen atoms that bridge two Ti atoms, supplied with OH formed by water dissociation on the Ti sites. Because of the similarity among photocatalytic water oxidation mechanisms on diverse metal oxide semiconductors, which generally have low hole mobilities, the findings from this work may be relevant to them as well. If so, manipulations of hole mobility and acceleration of the rate of thermal steps may provide a general pathway for improving water oxidation efficiency.