ab initio study of 3d transition metal-doping effects in rutile-TiO2: Role of bandgap tunability in conductivity behaviour

3d transition metal (TM)-doping into 2 × 2 × 2 supercell of rutile-TiO2 has been studied by ab initio band structure calculations based on self-consistent plane-wave method within the first-principle formalism. As a result of doping, 3d states of dopants hybridize with the O 2p and Ti 3d states to provide impurity energy levels, which either modify the valence (conduction) band and/or appear separately in the bandgap of TiO2. We have found that the intermediate impurity energy level shifts towards the valence band (VB) as the atomic number of dopants increases from V to Zn. Band structure calculations reveal that undoped, Sc, Mn, Fe, Co, Ni, Cu, and Zn-doping show the p-type conductivity, whereas doping of V, and Cr in TiO2 lead to the n-type conductivity. On the other hand, for Sc, Cu, and Zn-doping, the Fermi level penetrates into the VB, causing some of the states to appear below the Fermi level which are completely filled with electrons and in turn show inverse Burstein-Moss (BM) effect. As a matter of fact, we have not found BM effect in any of the 3d TM doping case.