DFT-Based Theoretical Calculations of Nb- and W-Doped Anatase TiO2: Complex Formation between W Dopants and Oxygen Vacancies

The structure and electronic states in Nb-doped Tio(2) (TNO) and W-doped TiO2 (TWO) having the anatase phase were investigated using a first-principles DFT-based band structure method. In addition to the cases where the dopant substituted for a Ti atom, cells containing a dopant (M-Ti, where M = Nb and W) and an oxygen vacancy (V-O) were calculated to clarify the role of oxygen vacancies in the system. Furthermore, cells containing two dopants and an oxygen vacancy (2M(Ti)-V-O) and cells with a dopant and two oxygen vacancies (M-Ti-2V(O)) were calculated. Energetically stable structures were found in the 2W(Ti)-V-O and W-Ti-2V(O) cells sampled, whereas the corresponding structures in TNO did not show any significant energy stabilization. Impurity states were found in the stable 2W(Ti)-V-O and W-Ti-2V(O) structures, and an approach of the two W-Ti atoms was found in 2W(Ti)-V-O. These findings suggest the possible formation of complex structures consisting of W-Ti dopants and oxygen vacancies. Our results are consistent with recent experiments on TWO by Takeuchi et al., and they rationalize the lower electronic conductivity of TWO versus TNO.