Electronic Structure of F-Doped Bulk Rutile, Anatase, and Brookite Polymorphs of TiO2

This work investigates the atomic and electronic structure of fluorine-doped (F-doped) rutile, anatase, and brookite polymorphs of TiO2. In a first step, we present a methodological analysis of the performance of several density functional schemes (GGA, GGA+U, and hybrid functionals) in reproducing experimental structures and band gap of rutile and anatase. The PBE+U functional appears to provide the best choice, and it is the one chosen to study suitable supercell models of F-doped bulk rutile, anatase, and brookite. We find that F-doping is thermodynamically stable for all polymorphs. In anatase, the Ti3+ gap states arising from doping are well located in the middle of the band gap which is convenient for photocatalysis purposes. However, in the case of rutile and brookite, F-doping does not lead to a remarkable reduction of the band gap. We present also results concerning F-doping on reduced titania and provide evidence that the presence of oxygen vacancies does not change significantly the electronic properties of F-substituted TiO2., albeit slightly stabilizing the resulting system.