Energetic and electronic properties of X- (Si, Ge, Sn, Pb) doped TiO2 from first-principles

First-principles density functional theory calculations were performed on the electronic properties of substitutional Si, Ge, Sn, and Pb-doped rutile and anatase TiO2, where Ti was replaced by dopants, to explore the effect of the dopants on the band edges and their possible efficacy for the visible light photocatalysis and solar energy conversion. Based on the optimized structures of X-doped TiO2, the defect formation energies and the density of states were calculated to analyze the energetic properties, band edges, and the band gap states. Our calculated results show that dopants substituing for Ti is energetically unfavorable in the order Si < Sn < Ge < Pb under both O-rich and Ti-rich growth conditions. The electronic structures show that there is a reduction of band gap about 0.1 ∼ 0.55 eV in X-doped rutile TiO2. Si- and Ge-doped anatase TiO2 also decrease by 0.20 and 0.15 eV, but broadening by 0.06 and 0.02 eV occurs in Sn- and Pb-doped systems. The calculated results explain the experimental observations. Furthermore, it can be predicted that Ge-doped TiO2 can be active for overall water splitting under visible light photons whereas Si-, Sn-, and Pb-doped TiO2 are only suitable for photo-oxidation.