Systematical investigation on characteristics of a photocatalyst: tantalum oxynitrides.

Tantalum oxynitrides with various nitrogen contents were synthesized from Ta2O5 powder by nitridation under a flow of ammonia at 1123 K for various durations. X-ray powder diffraction, transmission electron microscopy, Ultraviolet-visible spectroscopy, energy-dispersive spectroscopy, elemental analysis and photocatalytic reaction were performed to investigate these samples. Selected-area electron diffraction analysis of the mixed crystalline phases of powder samples revealed that each particle had only one crystalline phase. This indicates that entire particles underwent a rapid structural transformation once their nitrogen content reached a critical value. We discovered a new intermediate crystalline phase of tantalum oxynitride, TaO(a > 1)N(b <1), appeared before the generation of the β-TaON phase. The crystal structure of TaO(a > 1)N(b <1) is suggested to be monoclinic, with unit cell parameters of a = 5.1 Å, b = 35.6 Å, c = 5.4 Å and β = 93.5°. The ratio of nitrogen to oxygen in the samples increased with increasing nitridation duration. The increasing rate is different in the different nitridation stage due to the different structure of the samples. Nitrogen entered the samples quickly during the initial 5 h of nitridation, and a monoclinic β-TaON phase was formed. A mesoporous structure emerged in the nitrided particles during the phase transition, greatly increasing the surface area of the samples. The more the nitrogen entered one sample, the darker the color of it due to the narrower the band gap. H2 and O2 evolved by water splitting from the nitrided samples irradiated with visible light. Change in the evolution rate of H2 and O2 had a relation with the structure of the samples.