The Preparation Of Metal-nonmetal Doped TiO₂ Nanocrystals And Their Photocatalytic Degradation Effect On Formaldehyde

Heterogeneous photocatalytic oxidation technology with semiconductor has beena newly developed in pollution treatment process in recent decades. A wide variety of organic compounds can be completely mineralized to CO₂, H₂O and inorganic constituents as well as non-secondary pollution. Furthermore, such oxidation processes with a cheap oxidizing agent have been operated at ambient temperature and pressure. Therefore, it has become a hot research field both at home and abroad in recent years. Metal oxide semiconductors such as TiO₂, Fe₂O₃, CdS, WO₃, SnO₂, etc., have been largely used as photocatalysts. In particular, TiO₂ was regarded as the most promising green photocatalyst due to its higher photodegradation of the organic contaminants as well as its many advantages (i.e., safety, low cost and no pollution). However, there still exist two major defects for TiO₂ itself as follows:â‘ low photo quantum efficiency due to the higher recombination rate of photogenerated electron-hole pairs;â‘¡only in the small UV fraction of solar light, about 2-4 %, can be utilized by TiO₂ because of its large band gap of 3.0-3.2 eV. How further improve photo-catalytic activity of TiO₂ and increase the utilization ratio to visible light, which has been an important research topic in environmental science.In the paper, our attention was mainly paid on the preparation and modification of nanosized TiO₂ photocatalysts based on the defects of TiO₂ itself. La-doped TiO₂, Ce-doped TiO₂, Zr-doped TiO₂, Sulphur-doped TiO₂, La-S codoped TiO₂, and Ce-S-codoped TiO₂, Zr-S codoped TiO₂ nanocrystallites were synthesized by a sol-gel method. The photocatalytic decomposition of formaldehyde in aqueous solution was used as a probe reaction to evaluate their photocatalytic activities.The experimental results indicate that the 5%La-TiO₂, 5%Ce-TiO₂, 5% Zr -TiO₂ calcined at 500℃achieve the highest photocatalytic activity. The apparent decomposition rate constants of them are 1.80, 1.72 and 1.66 times than that of the best pure TiO₂.Sulphur-doped TizO₂ nanocrystallite photocatalysts were prepared by a simple sol-gel method first using thiourea as sulphur raw material. The experiments of photodegradation of formaldehyde show that the photocatalytic activity of sulphur -doped TiO₂ is appreciably inferior to pure TiO₂ under UV light, but the sulphur-doped TiO₂ samples possess well photocatalytic activity under visible light. Among them, the prepared TiO₂ using 4.0 g thiourea calcined at 400℃achieves the highest photocatalytic activity.Double elements (La-sulphur, and Ce- sulphur, Zr-sulphur -TiO₂) codoped TiO₂ nanocrystallite photocatalysts were prepared by sol-gel method firstly. The research results show that the photocatalytic activities of La-S, Ce-S ,Zr-S codoped TiO₂ calcined at 400℃excelled those of both pure TiO₂ and single sulphur doped TiO₂ under UV light. But the photocatalytic activities of La-S, Ce-S, Zr-S codoped TiO₂ samples compared to the TiO₂ by single sulphur doping could not be enhanced further under visible light.