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.
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