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

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 Nb_Ti, 2Nb_Ti, and Nb_Ti + O_i systems is small, the intragap states would be the electron-hole recombination center, leading to low photocatalytic efficiency. However, for the 2Nb_Ti + O_i 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 2Nb_Ti + O_i. 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.