Electronic properties of oxygen-deficient and aluminum-doped rutile TiO2 from first principles

The electronic properties of stoichiometric, defective, and aluminum-doped rutile TiO2 have been investigated theoretically with periodic quantum-chemical calculations. Theoretical results obtained with the Perdew-Wang density functional method [Phys. Rev. B 45, 13244 (1992)] and with a density functional-Hartree-Fock hybrid method are compared. Occupied defect states are observed in the band gap of rutile due to the presence of oxygen vacancies, which is in accord with previous theoretical studies and the experimentally observed coloring. For the investigation of aluminum doping, three different situations have been considered: substitution of a single Ti atom by an Al atom, cosubstitution of Ti by Al and O by Cl, and substitution of two Ti by two Al combined with the formation of an O vacancy. In the last two cases, aluminum doping does not introduce band gap states, and the band gap is even increased compared to undoped rutile. We conclude that stoichiometric Al doping reduces pigment coloring induced by oxygen vacancies in rutile and also suppresses the photocatalytic activity of titania pigments.