Electronic and optical properties of Zr, N mono-doped and co-doped anatase TiO2: first-principles calculations

Abstract

The growth conditions, crystal structural, electronic, optical and photocatalytic properties of undoped, N-, Zr mono-doped TiO2 and four kinds of Zr/N co-doped TiO2 (Zr–N1, Zr–N2, Zr–N3 and Zr–N4) with different Zr and N co-doped positions were investigated by first-principles calculations. It is found that N substituted preferentially for O under Ti-rich conditions. While Zr substituted preferentially for Ti under O-rich conditions, and has the lowest formation energies (−4.56 eV) due to Zr having the same valence electron distribution as Ti. And, co-doped samples prepared in the experiment may contain all co-doped modes; owing to the formation energies of them being very close under Ti-rich or O-rich conditions. On the other hand, the optical absorption threshold and the optical absorption intensity of Zr–N1, Zr–N2 and Zr–N4 co-doped TiO2 are more enlarged than the N mono-doped and Zr–N3 co-doped TiO2, due to the smaller carrier effective mass and the curved and wide impurity level (IL) appearing in the band gap, which not only increase the carrier mobility and reduce the electron–hole recombination, but also reduces the electronic transition energies. Thus, Zr–N1, Zr–N2 and Zr–N4 co-doped configurations can explain the mechanism of visible light absorption and photocatalytic activity enhancement for co-doped TiO2 and give a good explanation of the previous experimental observation.