Density functional theory calculations of dense TiO2 polymorphs:: Implication for visible-light-responsive photocatalysts

Structural parameters and electronic band gaps of dense TiO2 polymorphs, i.e., alpha-PbO2, baddeleyite, fluorite, and cotunnite types of structures, were calculated using a first-principles density functional method with local-density approximation. The ambient phases, i.e., rutile and anatase, with known theoretical and experimental data were used to ensure the validity of the calculations. The fluorite-type TiO2 turned out to have the narrowest band gap, 1.08 or 2.18 eV after applying a very approximate band gap correction, due to highly symmetrical TiO8 polyhedra with Ti-3d and O-2p orbitals in the most mixed state. Ti with eight coordinated oxygens, as feasible under high pressure or residual stress, may have potential applications as a visible-light-responsive photocatalyst.