Electronic properties of anatase-TiO2 codoped by cation-pairs from hybrid density functional theory calculations

We have attempted to modulate the band gap of anatase-titania using hybrid density functional theory (DFT) calculations to examine cation codoping at two Ti sites by Al with either Nb or Ta, and by Cu with either Mo or W to achieve significant gap narrowing via creation of shallow gap states, without creating recombination centres. Cu- and Mo/W-doped systems exhibit larger gap-narrowing than those of Al- and Nb/Ta-codoped TiO2 owing to strong d–d repulsion in the former two cases. At the same time, codoping serves to decrease transparent conducting behavior relative to monodoping. Upon increasing the doping concentration, the gap-narrowing mechanisms and transparent conducting characteristics do not change.