N-Doped Titanium Monoxide Nanoparticles with TiO Rock-Salt Structure, Low Energy Band Gap, and Visible Light Activity

Titania nanoparticles are widely studied for photoconversion processes where combining high surface area, charge transport properties, and chemical stability is meaningful. To enhance the conversion efficiency, new compounds with reduced band gap are actively researched to utilize the visible part of the solar spectrum. Some narrowing of the gap can be observed when doping titania with nitrogen, leading to suboxide species. Using laser pyrolysis, we have synthesized for the first time true titanium monoxide TiO nanoparticles with a rock-salt crystallographic structure. The as-formed nanoparticles of black color contain carbon which can be removed by soft annealing under air while maintaining the presence of the TiO phase. These nanoparticles exhibit a very large shift of the light absorption threshold, up to 1.2 eV toward visible range, compared to the anatase form of titania. XPS analysis allows discussion of the role of nitrogen in the formation of this phase and its optical properties. First results indicate efficient photoactivity under UV and visible irradiation.