Density functional theory study of dopants in polycrystalline TiO2

Abstract

We present a density functional theory (DFT) study of doped rutile and anatase TiO${}_{2}$ in which we investigate the impact of grain boundaries on the physics of atomic defects. The main goal is to obtain information about the positions of the defect levels generated by an oxygen vacancy, a titanium interstitial, cation dopants Nb, Al, and Ga, and an anion dopant N in the electronic band gap having in mind the application of TiO${}_{2}$ as a transparent conducting oxide (TCO) or its use in heterogeneous catalysis. Due to the known deficiency of the local density approximation (LDA) of DFT to yield accurate values for band gap energies for insulators such as TiO${}_{2}$, a self-interaction correction (SIC) to the LDA is employed. The main result of our study is that grain boundaries do affect the defect formation energies as well as the position and shape of the dopant-induced electronic energy levels significantly with respect to the single crystal. According to our study Nb doping may lead to $n$-conducting TiO${}_{2}$ whereas doping with N, Al, or Ga is not promising in order to achieve $p$-conducting TiO${}_{2}$. Furthermore an increase in the photoconductivity of TiO${}_{2}$:N and the colorlessness of TiO${}_{2}$:Al may be explained by our results.