A DFT study on the modification mechanism of (Cr, C) co-doping for the electronic and optical properties of anatase TiO2

Abstract

To explore an effective doping configuration for TiO2, thus further improving the photocatalytic performances of anatase TiO2, we systematically study the electronic structures and optical properties for the pure, Cr-doped, C-doped and Cr/C co-doped TiO2, using density functional calculations. We find that three doped systems show higher stability under reduction conditions with relatively lower impurity formation energies. For Cr or C mono-doping, the isolated impurity states appear in the band gap region, thus changing the electronic structures and lowering the excitation energy for electron transition, which would result in response to visible light. The Cr/C co-doped TiO2 shows much bigger red-shift of absorption edge than the mono-doping systems, due to the greater band gap narrowing of the co-doped system. Furthermore, for the co-doped systems, there are more impurity states appearing between the top of the valence band and the bottom of the conduction band. This not only enhances the visible light absorption, but also promotes the separation of photo-excited electron-hole pairs. We also find the doping effect on the electronic structures is localized. The calculated absorption spectra indicate that Cr/C co-doped TiO2 possesses much higher photocatalytic activity under visible light region.