The electronic structure changes and the origin of the enhanced optical properties in N-doped anatase TiO2—A theoretical revisit

Abstract

We have investigated the electronic structure changes and the origin of the enhanced optical properties of N-doped anatase TiO2 using first-principles density-functional theory calculations. To determine the band gap variations induced by N-doping, we developed a new approach to locate the effective valence band maximum (VBM) by characterizing the degree of localization of the N-induced states in the band structures of various N-doped TiO2 systems. Our calculations show that the band gap variations are largely affected by the local bonding structures and doping concentration of the substitutional N atoms. As the N content is up to certain level, some local bonding structures can indeed cause band gap reduction due to the formation of band-like delocalized states above the VBM of TiO2, while other local bonding configurations may simply form localized impurity states in the band gap. Accordingly, the N-induced localized and delocalized electronic states can exist simultaneously to contribute to the enhanced optical properties of anatase TiO2. Our computational approach also provides a new way to investigate the band gap engineering of other wide band gap semiconductor material systems.