A mechanism for the hole-mediated water photooxidation on TiO2 (1 0 1) surfaces

The mechanism of water photooxidation on TiO2 surfaces is still controversial. Here we report a first-principles density functional study based on a hybrid functional method in which an adsorbed water molecule is found to directly interact with a self-trapped hole at a bridging oxygen site and to transform into an OH• radical species through a concerted proton/hole transfer. This study analyzes both the thermodynamics and kinetics of this step of the reaction, which is generally considered to be the rate determining one. The fate of the OH• radical is then investigated in terms of its reactivity with different surface species, with a second OH• radical, or with a second water molecule coming from the environment. We find that OH• radicals can either acquire a hydrogen from surrounding water molecules or, if they meet, couple to form hydrogen peroxide with highly associated energy gain.