Bandgap engineering of oxygen-rich TiO_2+x for photocatalyst with enhanced visible-light photocatalytic ability

TiO₂, as a photocatalyst, has attracted substantial attention since the discovery of water splitting property on the TiO₂ electrodes. However, its efficiency of water splitting is limited by its wide bandgap (~3.0 eV). Here, we predict its bandgap can be efficiently reduced by incorporating excess oxygen atoms on the basis of first-principles calculations. We show that the excess oxygen is more stable to bond to Ti atom and to form ordered structure. The narrowing of bandgap in oxygen-rich TiO₂ originates from the ordered excess oxygen because the coupling between them shifts the conduction band bottom down. The bandgap of TiO_2+x decreases with the increase of the density of excess oxygen (x). A bandgap of 1.3 eV can be achieved at x = 0.5. The oxygen-rich TiO₂ shows intrinsic semiconducting characteristic without localized states within the bandgap, indicating lower trapping centers. The enhancement of visible-light absorption due to the narrowed bandgap and the intrinsic semiconductor characteristic result in the improvement of the photocatalytic performance in oxygen-rich TiO_2+x .