Fabrication of N-doped TiO₂ and TiN nanorods by NH₃ treatment: Evolution of morphology, structures, composition and optical properties

We investigate the evolution of morphology, electronic and crystalline structures, elemental composition, and optical properties of TiO2 nanorods (NRs) treated in NH3 in the temperature range of 400-1100 degrees C using both experiments and theoretical calculations. The results show that N-doped TiO2 NRs are obtained in the temperature range of 400-550 degrees C, in which the formation of the N substitutional defects is more energetically favorable than the N interstitial defects. Our theoretical calculations reveal that the formation of N substitutional defects causes a significant bandgap narrowing, which arises from two different mechanisms: the upward shift of the valence band maximum for a low doping concentration and the downward shift of the conduction band minimum for a high doping concentration. Experimentally, a bandgap in the range of 2.22-2.41 eV can be achieved for N-doped TiO2, which enables the visible-light absorption and activates the visible-light photocatalytic activity of the TiO2 NRs. The nitridation starts from 700 degrees C and proceeds rapidly in the temperature range of 800-900 degrees C. However, complete nitridation is achieved when the temperature exceeds 1000 degrees C. The rutile phase is found to be more prone to nitridation than the anatase phase. During nitridation, the TiO2 NRs experience significant changes in morphology, structure, and optical properties. The latter is strongly affected by the remaining O concentration in the materials.