Niobium-Doped TiO2: Effect of an Interstitial Oxygen Atom on the Charge State of Niobium.

Abstract

The structural and electronic properties of Nb-doped rutile TiO2 with several doping configurations were investigated by first-principles calculations based on the density functional theory. The calculations show that although the band gap in the NbTi, 2NbTi, and NbTi + Oi systems is small, the intragap states would be the electron-hole recombination center, leading to low photocatalytic efficiency. However, for the 2NbTi + Oi configuration, the impurity states are mainly located at the top of the valence band and the electron-hole recombination would be inhibited, indicating relatively higher photocatalytic efficiency. On the basis of the charge-compensated theory, two electrons on the transition-metal Nb atoms compensate for the same amount of holes on the acceptor level of a nonmetal interstitial O atom, in the model of 2NbTi + Oi. Such donor-acceptor codoping may not only suppress the electron-hole recombination but also maintain a reduced band gap, suggesting that the doping models would exhibit higher photocatalytic activity than pure TiO2. The calculation results show that the interstitial O atoms play an essential role in manipulating the valence state of impurity Nb. The role of the interstitial O atom in Nb-doped rutile TiO2 suggests that it can give rise to beneficial charge-compensation effects.