The structure and electronic properties of Nb-doped anatase (TNO) were studied from first principles using the density functional theory based band structure method. Four independent types of unit cells were studied; i.e., pure anatase, anatase with Nb dopant at Ti sites (NbTi), and cells with either interstitial oxygen (Oi) or oxygen vacancies (VO). In addition, a unit cell with a NbTi and Oi, and a cell with NbTi and VO were investigated to clarify the role of nonstoichiometry in TNO. From the calculated results, the importance of the adjacent NbTi- V O and NbTi - O structures was pointed out, and the experimental observation of the relationship between nonstoichiometry and electronic conductivity was rationalized. The shape of the impurity states found in these structures was used to comprehend the experimental observation of carrier concentration and the charge state of Nb dopant. The changes in lattice constants supported the existence of these structures as well. On the contrary, the cell with a simple NbTi did not show significant changes in structure and electronic properties, other than the emission of an electron in the conduction band. A stabilization of the impurity state was observed in the adjacent NbTi-VO structure compared to the VO. The possibility of an essential role of this state in electric conduction was discussed. The formation of the adjacent NbTi - Oi structure by O2 gas annealing was discussed using statistical mechanics. The Gibbs free energies were calculated for Oi atoms in TNO and compared to that of O2 molecules in the gas phase. The analysis was qualitatively consistent with experimental behavior under the assumption of the NbTi - VO structures.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry