Periodic hybrid-exchange density functional theory calculations have been used to explore the geometric, electronic and vibrational properties of the first layer of water on a photocatalytically-active surface of TiO2. Although the overall working principle of photoelectrochemical water splitting has been established, remarkably little is known about the underlying reactions that occur at the semiconductor surface, and so currently it is difficult to guide the design of more efficient systems. The purpose of the current work has been to use the rutile TiO2(110) surface as a model system to establish a clear atomistic understanding of water chemistry on TiO2 surfaces and consequently to gain insight into the water splitting reaction mechanisms.

This has led to a number of discoveries regarding: (i) the structure of the oxide-water interface and the water-water and water-surface interactions -- there is an interplay between direct intermolecular interactions and surface-mediated interactions in determining the most favourable adsorption mode; (ii) the oxygen evolution reaction mechanism -- the initial step of the first hole oxidation reaction is found to be energetically undemanding, providing a possible explanation for the high photocatalytic activity of TiO2. Evidence is also provided to suggest that, after this initial step, the photogenerated holes at the surface are more likely to react with adsorbed hydroxyls as opposed to with adsorbed water molecules; and (iii) a novel mechanism in which the interaction between water molecules on a surface can be very strongly affected by surface vibrations which are themselves strongly influenced by surface strain.

The detailed theoretical understanding of the water chemistry and chemical reaction mechanisms developed in this work could be used to predict the structure and properties of other oxide-water interfaces, as well as to design nanostructured semiconducting materials with improved solar-to hydrogen conversion efficiencies.