Density functional theory studies of doping in Titania

The structural and electronic properties of rutile and anatase, and the influence of both mono- and co-doping, have been studied using Density Functional Theory. Ge-doped anatase and rutile exhibit different band gap-narrowing mechanisms; in particular, host Ti 3d states move to lower energy regions in anatase and Ge 4s impurities states locate below the conduction band of rutile. For S-doping, S 3p states locate above the top of the valence band and mix with O 2p states, leading to band gap narrowing. For Bi-doping, the energy levels of the Bi 6s states lie below the bottom of the conduction band while the Fermi level EF lies above the gap states, indicating the gap states are fully occupied. For Bi/S–codoping, both S 3p acceptor states and partially occupied Bi 6s donor states hybridised with S 3p appear simultaneously. For N- and W-monodoping, isolated N 2p states above the top of the valence band and W 5d states below the conduction band lead to band gap narrowing. N/W codoping yields significant band gap narrowing. Both studies for Bi/S and N/W codoping rationalise recent experimental data which show that these doped anatase systems exhibit higher visible-light photocatalytic efficiency than respective monodoping.