Mechanism of the doping species controlling the UV photocatalytic activity of nitrogen doped TiO₂

Nitrogen-doped and pure TiO₂ photocatalysts were synthesized by precipitation method using TiCl₄ as titanium precursor. Both X-ray diffraction (XRD) and N₂ adsorption-desorption isotherms show that the as-prepared samples consis of anatase phase except for trace brookite phase with mesoporous structure. X-ray photoelectron spectroscopy (XPS) confirms that the incorporated nitrogen element exists as the chemical state of NO_x. The UV-Vis diffuse reflection absorption spectra (UV-Vis) reveal that the nitrogen-doped TiO₂ shows a new absorption region at 400-550 nm. The photocatalytic activities of the nitrogen-doped TiO₂ utilized for the photodegradation of 4-chlorophenol (4-CP) are higher than those of the pure TiO₂ under UV and visible light irradiation. The improved photocatalytic activities under UV light irradiation of the nitrogen-doped TiO₂ are attributed to the presence of NO_x. With no influence on the energy gap of TiO₂, NO_x can extend the optical response of TiO₂, excite more photoinduced electrons-holes for photocatalysis and reduce the recombination probabilities of carriers.