Surface modification of titania surfaces with dispersed metal oxide
nanoclusters has the potential to enhance photocatalytic activity. These
modifications can induce visible light absorption and suppress charge carrier
recombination which are vital in improving the efficiency. We have studied heterostructures
of Mn<sub>4</sub>O<sub>6</sub> nanoclusters modifying the TiO<sub>2</sub>
rutile (110) and anatase (101) surfaces using density functional theory
corrected for on-site Coulomb interactions (DFT + U). Such studies typically
focus on the pristine surface, free of the point defects and surface hydroxyls
present in real surfaces. In our study we have considered partial hydroxylation
of the rutile and anatase surfaces and the role of cation reduction, via oxygen
vacancy formation, and how this impacts on a variety of properties governing
the photocatalytic performance such as nanocluster adsorption, light
absorption, charge separation and reducibility. Our results indicate that the
modifiers adsorb strongly at the surface and that modification extends light
absorption into the visible range. MnO<sub>x</sub>-modified anatase can show an
off-stoichiometric ground state, through oxygen vacancy formation and cation reduction
spontaneously, and both modified rutile and anatase are highly reducible with
moderate energy costs. Manganese ions are therefore present in a mixture of
oxidation states. Photoexcited electrons and holes localize at cluster metal
and oxygen sites, respectively. The interaction of water at the modified
surfaces depends on the stoichiometry and spontaneous dissociation to surface
bound hydroxyls is favoured in the presence of oxygen vacancies and reduced
metal cations. Comparisons with bare TiO<sub>2</sub> and other TiO<sub>2</sub>-based
photocatalyst materials are presented throughout.
Energy of Oxygen-Vacancy Formation on Oxide Surfaces: Role of the Spatial Distribution
