Structure of a model TiO2 photocatalytic interface

The interaction of water with TiO₂ is crucial to many of its practical applications, including photocatalytic water splitting. Following the first demonstration of this phenomenon 40 years ago there have been numerous studies of the rutile single-crystal TiO₂(110) interface with water. This has provided an atomic-level understanding of the water–TiO₂ interaction. However, nearly all of the previous studies of water/TiO₂ interfaces involve water in the vapour phase. Here, we explore the interfacial structure between liquid water and a rutile TiO₂(110) surface pre-characterized at the atomic level. Scanning tunnelling microscopy and surface X-ray diffraction are used to determine the structure, which is comprised of an ordered array of hydroxyl molecules with molecular water in the second layer. Static and dynamic density functional theory calculations suggest that a possible mechanism for formation of the hydroxyl overlayer involves the mixed adsorption of O₂ and H₂O on a partially defected surface. The quantitative structural properties derived here provide a basis with which to explore the atomistic properties and hence mechanisms involved in TiO₂ photocatalysis.