Composite systems of TiO 2 with nanocarbon materials, such as graphene, graphene oxide and carbon nanotubes, have proven to be e cient photocatalyst materials. However, detailed understanding of their electronic structure and the mechanisms of the charge transfer processes is still lacking. Here, we use hybrid density functional theory calculations to analyse the electronic properties of the ideal rutile (110)-graphene interface, in order to understand experimentally observed trends in photoinduced charge transfer. We show that the potential energy surface of pristine graphene physisorbed above rutile (110) is relatively at, enabling many possible positions of graphene above the rutile (110) surface. We verify that tensile and compressive strain has negligible e ect on the electronic properties of graphene at low levels of strain. By analysing the band structure of this composite material and the composition of the valence and conduction band edges, we show that both the highest occupied states and the lowest unoccupied states of this composite are dominated by graphene, and that there is also a signi cant contribution of Ti orbitals to the two lowest unoccupied bands. We suggest that a transition from graphene-dominated occupied bands to mixed graphene and TiO 2 -based unoccupied bands is responsible for the experimentally observed photoinduced charge transfer from graphene to TiO 2 under visible light irradiation; however, the most stable state for an excess (e.g. photoexcited) electron is localised on the carbon orbitals, which make up the lowest-energy conduction band. This separation of photogenerated electrons and holes makes TiO 2 -graphene an e cient photocatalyst material.

Composites of titanium dioxide (TiO2) and reduced graphene oxide (RGO) have proven to be much more effective photocatalysts than TiO2 alone. However, little attention has been paid so far to the chemical structure of TiO2/RGO interfaces and to the role that the unavoidable residual oxygen functional groups of RGO play in the photocatalytic mechanism. In this work, we develop models of TiO2 rutile (110)/RGO interfaces by including a variety of oxygen functional groups known to be present in RGO. Using hybrid density functional theory calculations, we demonstrate that the presence of oxygen functional groups and the formation of interfacial cross-links (Ti–O–C covalent bonds and strong hydrogen bonds between TiO2 and RGO) have a major effect on the electronic properties of RGO and RGO-based composites. The electronic structure changes from semimetallic to semiconducting with an indirect band gap, with the lowest unoccupied band positioned below the TiO2 conduction band and largely localized on RGO oxygen and carbon orbitals, with some contributions of RGO-bonded Ti atoms. We suggest that this RGO-based lowest unoccupied band acts as a photoelectron trap and the indirect nature of the band gap hinders electron–hole recombination. These results can explain the experimentally observed extended lifetimes of photoexcited charge carriers in TiO2/RGO composites and the enhancement of photocatalytic efficiency of these composites.