Electronic Structure and Charge Transfer in the TiO2 Rutile (110)/Graphene Composite Using Hybrid DFT Calculations

摘要

Composite systems of TiO2 with nanocarbon materials, such as graphene, graphene oxide, and carbon nanotubes, have proven to be efficient photocatalyst materials. However, a 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 analyze 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 flat, enabling many possible positions of graphene above the rutile (110) surface. We verify that tensile and compressive strain has a negligible effect on the electronic properties of graphene at low levels of strain. By analyzing 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 significant contribution of Ti orbitals to the two lowest unoccupied bands. We suggest that a transition from graphene-dominated occupied bands to mixed graphene and TiO2-based unoccupied bands is responsible for the experimentally observed photoinduced charge transfer from graphene to TiO2 under visible light irradiation; however, the most stable state for an excess (e.g., photoexcited) electron is localized on the carbon orbitals, which make up the lowest-energy conduction band. This separation of photogenerated electrons and holes makes TiO2-graphene an efficient photocatalyst material.