Electronic Properties of Rutile TiO2 with Nonmetal Dopants from First Principles

The electronic properties of rutile TiO2 with nonmetal dopants are investigated using the first-principles density functional theory. Four types of doping are considered: N doping (substituting N for O), C doping (substituting C for O), N + H codoping (substitutional N and interstitial H), and C + H codoping (substitutional C and interstitial H). The results show that N(C) doping of TiO2 leads to intermediate bands owing to N(C) 2p orbitals and there is no overlapping of these intermediate bands with the valence band to narrow the band gap. These bands are of evident curvatures, and they, acting as stepping stones, are expected to help to relay valence electrons to the conduction band and thus result in a red shift in the optical absorption edges. C + H codoping to TiO2 also gives rise to similar intermediate bands, and N + H codoping induces significant band gap reduction without inducing any bands/states within the forbidden gap. The presence of interstitial H next to a C or N dopant helps to eliminate intermediate states arising from the N(C) 2p orbitals. Also, the C-H and N-H bond lengths in C + H- and N + H-co doped TiO2 are comparable with those in CH4 and NH3 molecules, respectively. It is probable to make use of NH3 and CH4 gases to dope TiO2 for significant optical absorption of visible light for enhanced photovoltaic and photocatalytic functionalities.