Defect engineering toward strong photocatalysis of Nb-doped anatase TiO₂: Computational predictions and experimental verifications

Understanding the roles of point defects in optical transitions is a key to the desirable engineering of photochemical materials. In this study, the origins of the significantly varying optical and photochemical properties of Nb-doped anatase TiO₂were systematically investigated, using density functional theory (DFT) calculations and experimental verifications. We found from DFT calculations that the desirable band gap reduction of anatase TiO₂by ∼0.1 eV reported in many of experimental reports and the resultant improvements of photocatalytic and photovoltaic efficiencies of Nb⁵⁺-doped anatase TiO₂are due to the formation of complex (Nb_Ti-V_Ti)³⁻as the compensator of Nb_Ti⁺. Our experiments demonstrated that the O₂-rich annealing, which is expected to increase the concentration of desirable (Nb_Ti-V_Ti)³⁻complex, narrows band gap of TiO₂and strongly enhances the photocatalytic activity of Nb-doped TiO₂particle. On the contrary, pure TiO₂showed rather worse photocatalytic performances when annealed in O₂-rich atmosphere, which is due to the formation of deep level by O-interstitial defect (O_i). Theoretically obtained charge effective masses could further explain the different photocatalytic activities of undoped and Nb-doped TiO₂.