Unconventional rate law of water photooxidation at TiO₂ electrodes

Photoelectrochemical oxidation of water over semiconductors is a promising route for the production of sustainable solar fuels. TiO₂ water photooxidation has been intensively studied over the past 50 years, but its rate law and mechanism are still undetermined. The main challenges are that there is no appropriate reaction kinetic model currently, and that both the reaction rate constant and reactant photohole concentration/density are not readily quantified with respect to conventional chemical reactions. Here we report that the rate law and photohole transfer mechanism could be determined by a combination of multiple (photo-) electrochemical techniques. We demonstrate that the kinetics of TiO₂ water oxidation by photogenerated holes can be well-described by a model of surface state mediating charge transfer and recombination. The rate law, in terms of steady-state photocurrent, is the product of the surface hole density exponential dependent rate constant and the surface hole density, with first order for all the surface hole densities studied. This reactant concentration dependent rate constant is conceptually unexpected for an elementary step in conventional chemical reactions. In addition, we find that hydroxyl ions in bulk solutions are involved in the reaction as indicated by observation of the solution pH dependent apparent rate constant. This study may thus lead to key insights both for strategies to evaluate and/or enhance photoelectrochemical performances and for understanding reaction mechanisms.