Studies on TiNxOyFz as a visible-light-responsive photocatalyst

A titanium fluorooxynitride (TiNxOyFz) is studied as a visible-light-responsive photocatalyst. Nitridation of NH4TiOF3 at 773 K for 1 h under NH3 flow yields fine particulate TiNxOyFz(particle size, 100-300 nm) with an anatase crystal structure and high crystallinity. X-ray diffraction and absorption measurements confirm that the TiNxOyFz material exhibits similar structural characteristics to anatase TiO2. The energy gap of TiNxOyFz is estimated to be 2.34 eV, substantially smaller than the band gap energy of TiO2 with the same anatase crystal structure (3.26 eV). Density functional theory calculations indicate that the bottom of the conduction band of TiNxOyFz is composed of empty Ti3d orbitals and that the doping level is formed by N2p orbitals in the forbidden band above the valence band consisting of O2p orbitals. TiNxOyFz. is demonstrated to function as a visible-light-driven photocatalyst for water oxidation into molecular oxygen at wavelengths as long as 540 nm through photoexcitation from the N2p doping level to the conduction band formed by empty Ti3d orbitals.