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 impurity 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 E-F lies above the gap states, indicating that the gap states are fully occupied. For Bi/S co-doping, 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 co-doping yields significant band-gap narrowing. Both studies for Bi/S and N/W co-doping rationalise recent experimental data which show that these doped anatase systems exhibit higher visible-light photocatalytic efficiency than respective mono-doping.