Synthesis of efficient N-containing TiO2 photocatalysts with high anatase thermal stability and the effects of the nitrogen residue on the photoinduced charge separation

Efficient N-containing TiO2 nanoparticles with high anatase thermal stability were synthesized via a hexamethylenetetramine (HMT)-modified sol-hydrothermal process. The results showed that modification with proper amounts of HMT is effective in increasing the onset temperature of the phase transformation of TiO2 from anatase to rutile. The enhancement of the anatase thermal stability of the modified TiO2 was attributed to ammonia produced slowly by hydrolysis of the HMT molecules in the sol-hydrothermal process and, additionally, to the residual nitrogen species after the thermal treatment at high temperatures, as indicated by the XPS examination. Compared with the unmodified TiO2, the modified TiO2 obtained by a thermal treatment at high temperatures exhibited good photocatalytic performance under UV light and was found to even be superior to the commercially available P25-TiO2. It was suggested that the residual N species (Ti-O-N), formed after the thermal treatment at high temperatures, along with the mixed phase composition, large surface area and the increase in the thermal stability, were responsible for the enhanced photocatalytic activity of modified TiO2. It was demonstrated, by means of the surface photovoltage responses of the modified TiO2 in different atmospheres along with the aid of an outer electric field, that the residual N species could effectively capture the photoinduced holes, which was favorable for the effective separation of the photoinduced charges. This work provides a feasible route to fabricate high-performance TiO2-based functional nanomaterials with high anatase thermal stability.