First-principles study of the influences of oxygen defects upon the electronic properties of Nb-doped TiO₂ by GGA + U methods

The influence of oxygen defects upon the electronic properties of Nb-doped TiO₂ has been studied by using the general gradient approximation (GGA)+U method. Four independent models (i.e., an undoped anatase cell, an anatase cell with a Nb dopant at Ti site (Nb_Ti), an anatase cell with a Nb-dopant and an oxygen vacancy (Nb_Ti+V_O), and an anatase cell with a Nb-dopant and an interstitial oxygen (Nb_Ti+O_i)) were considered. The density of states, effective mass, Bader charge, charge density, and electron localization function were calculated. The results show that in the Nb_Ti+V_O cell both e_g and t_2g levels of Ti 3d orbits make contributions to the electronic conductivity, and the oxygen vacancies (V_O) collaborate with Nb-dopants to favor the high electrical conductivity by inducing the Nb-dopants to release more excess charges. In Nb_Ti+O_i, an unoccupied impurity level appears in the band gap, which served as an acceptor level and suppressed the electronic conductivity. The results qualitatively coincide with experimental results and possibly provide insights into the preparation of TCOs with desirable conductivity.