Density Functional Characterization of the Band Edges, the Band Gap States, and the Preferred Doping Sites of Halogen-Doped TiO2

First-principles density functional theory (DFT) electronic structure calculations were carried out for model halogen-doped anatase TiO2 structures to evaluate the effect of halogen doping on the band edges and the photocatalytic activity of TiO2. The model structures of X-doped TiO2 were constructed by using the 48-atom 2 × 2 × 1 supercell of anatase TiO2 with one O or one Ti atom replaced with X (=F, Cl, Br, I). The unit cell parameters and the atom positions of the resulting X-doped TiO2 with X at an O site (X@O) and that with X at a Ti site (X@Ti) were optimized by performing first-principles DFT calculations. On the basis of the optimized structures of X-doped TiO2 with X@O and X@Ti, the defect formation energies and the plots of the density of states were calculated to analyze the band edges, the band gap states, and the preferred doping sites. Our work shows that the doping becomes more difficult in the order F < Cl < Br < I for X-doped TiO2 with X@O, while the doping becomes less difficult in the order F < Cl < Br < I for X-doped TiO2 with X@Ti. Under O-rich growth condition, it is energetically more favorable to substitute Br and I for a Ti site than for an O site, while it is energetically more favorable to substitute F and Cl for an O site than for a Ti site. Under Ti-rich condition, it is energetically more favorable to substitute all X (=F, Cl, Br, I) for an O site than for a Ti site. The I atoms in I-doped TiO2 with I@Ti are present as I5+ (s2) ions, while in X-doped TiO2 with X@Ti (X = F, Cl, Br) the F, Cl, and Br are present as F3+ (s2p2), Cl4+ (s2p1), and Br4+ (s2p1) ions, respectively. I-doped TiO2 either with I@Ti or with I@O has a doubly filled band gap state, which significantly reduces the optical band gap with respect to that of undoped TiO2.