Effects of pH values on the physicochemical properties and photocatalytic activities of nitrogen-doped TiO₂

Visible-light-driven nitrogen-doped titanium dioxide (N-TiO₂) was synthesized by the method of coprecipitation, in which titanium tetrachloride was used as Ti-precursor and the mixed solution of ammonia and hydrazine hydrate as nitrogen resource. Effects of pH values on the physico-chemical properties and the activities of the as-prepared N-TiO₂ catalysts were studied. The obtained photocatalysts are found to be of anatase phase, higher specific area, mesoporous structure and visible response by means of XRD, BET and UV-vis spectra analysis. With increasing pH values, the intensity of anatase (101) peaks enhances, and the crystallite size increases, while the BET specific area decreases. The doped impurities are found to be NO_x species, which are confirmed by XPS. Minor band-gap narrowing and the visible-light response are due to nitrogen doping. The experiments of photodegradation of 4-chlorophenol show that the degradation ratio (39.65%) of the catalyst prepared at pH = 3.5 is the highest under visible light irradiation. The photocatalytic activity decreases when pH values increase. Under UV light irradiation, the catalyst prepared at pH = 5.5 has the highest photocatalytic activity, while that derived from pH = 9.5 has the lowest.