Density functional theory based first-principle calculation of Nb-dopedanatase TiO_2and its interactions with oxygen vacancies and interstitialoxygen

摘要

The structure and electronic properties of Nb-doped anatase (TNO) were studied from firstprinciples using the density functional theory based band structure method. Four independent typesof unit cells were studied; i.e., pure anatase, anatase with Nb dopant at Ti sites (Nb_(Ti)), and cells witheither interstitial oxygen (O_i) or oxygen vacancies (V_O). In addition, a unit cell with a NbT, andand a cell with Nb_(Ti) and V_o were investigated to clarify the role of nonstoichiometry in TNO. Fromthe calculated results, the importance of the adjacent Nb_(Ti)-V_o and Nb_(Ti)-O_istructures was pointedout, and the experimental observation of the relationship between nonstoichiometry and electronicconductivity was rationalized. The shape of the impurity states found in these structures was usedto comprehend the experimental observation of carrier concentration and the charge state of Nbdopant. The changes in lattice constants supported the existence of these structures as well. On thecontrary, the cell with a simple Nb_(Ti)did not show significant changes in structure and electronicproperties, other than the emission of an electron in the conduction band. A stabilization of theimpurity state was observed in the adjacent Nb_(Ti)-V_o structure compared to the o. The possibilityof an essential role of this state in electric conduction was discussed. The formation of the adjacentNb_(Ti)-O_i structure by 2gas annealing was discussed using statistical mechanics. The Gibbs freeenergies were calculated for O_iatoms in TNO and compared to that of O_2molecules the gasphase. The analysis was qualitatively consistent with experimental behavior under the assumption ofthe Nb_(Ti)–V_o structures.