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

•Using ab initio calculations, we find that narrowed band gap of Nb-doped TiO2 originates from the complex form of defect, (NbTi-VTi)3−.•To increase the concentration of desirable defect, (NbTi-VTi)3−, we suggest O-rich annealing on Nb-doped TiO2 photocatalyst.•We experimentally verify that O-rich annealing on Nb-doped TiO2 narrows its band gap and notably improves the photocatalysis.•From carrier effective mass calculations, we could explain the enhanced photocatalysis of Nb-doped TiO2 crystal.

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 TiO2 were systematically investigated, using density functional theory (DFT) calculations and experimental verifications. We found from DFT calculations that the desirable band gap reduction of anatase TiO2 by ∼0.1 eV reported in many of experimental reports and the resultant improvements of photocatalytic and photovoltaic efficiencies of Nb5+-doped anatase TiO2 are due to the formation of complex (NbTi-VTi)3− as the compensator of NbTi+. Our experiments demonstrated that the O2-rich annealing, which is expected to increase the concentration of desirable (NbTi-VTi)3− complex, narrows band gap of TiO2 and strongly enhances the photocatalytic activity of Nb-doped TiO2 particle. On the contrary, pure TiO2 showed rather worse photocatalytic performances when annealed in O2-rich atmosphere, which is due to the formation of deep level by O-interstitial defect (Oi). Theoretically obtained charge effective masses could further explain the different photocatalytic activities of undoped and Nb-doped TiO2.

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