Synthesis, Modification And Photocatalytic Performance Of Nano-scale TiO₂

Photocatalysis, as one of the most popular technologies for controlling environmental pollution, has received more and more attention. Now, TiO2 has become one of the most common photocatalysts due to its various merits, such as non-toxicity, photo stability, chemical stability, and low-cost. Although it has been widely investigated in the past decade, some problems such as the low photo-quantum efficiency and low photocatalytic activity under visible light still render its practical application. Therefore, modification of TiO2 to extend its absorption edge toward the visible light region and to decrease the recombination of photogenerated electrons and holes has been the hot topic of recent research. In this dissertation, the modified TiO2 nanoparticles have been synthesized by different procedures to improve the utilization efficiency of the visible light.Iron doped TiO2 was prepared by the sol-gel process using MCM-41 as hard template. UV-Vis diffuse reflectance spectroscopy result showed that a red-shift and an enhanced absorption in the visible region were observed in all the prepared samples. TiO2 was not synthesized in the pore of MCM-41 but on its surface. The Ti-O-Si bond was formed between TiO2 and MCM-41. This Ti-O-Si bond fixed the TiO2 on the MCM-41, which restrained the aggregation between TiO2 particles. Fe did not exist as Fe2O3 but substitute Ti to form Ti-O-Fe bond. The particle size and photocatalytic activity of Fe-doped TiO2 prepared using MCM-41 as hard template was smaller and higher than that of TiO2 prepared without MCM-41. The amount of doped iron ion played an important role in affecting its photocatalytic activity.Ultrafine nitrogen-doped TiO2 nanoparticles with narrow particle size distribution and good dispersion were synthesized in the presence of urea and PEG-4000 via a hydrothermal procedure. It was shown that the synthesized TiO2 particles were a mixture of 49.5% anatase and 50.5% rutile with a size of around 5 nm and narrow particle size distribution. The N doping did not narrow the band gap of AR50 but formed an isolated N impurity level above the valence band. The photocatalytic activities were tested in the degradation of an aqueous solution of a dyestuff, methylene blue, under both UV and visible light. The synthesized TiO2 particles showed much higher photocatalytic activity than the commercial TiO2 powders under both UV and visible light irradiations. The high performance may be related to the N doping effect, the reduced particle size, good dispersion, high surface area, and the quantum size effect. Ionic liquid [Bmim]OH was synthesized by ion exchange method using [Bmim]Br as row material, which was synthesized using micro-structured reactor. TiO2 nanocomposites modified with [Bmim]OH were synthesized via a hydrothermal procedure. The TiO2 nanocomposites consisted of pure anatase particles of about 10 nm. The modification of [Bmim]OH on the surface of the TiO2 particles extended the TiO2 absorption edge to the visible light region. The electrochemical redox potentials indicated that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of [Bmim]OH match well with the valence band (VB) and conduction band (CB) of the TiO2 semiconductor. The TiO2 surface is modified by the formation of Ti-O-C bonds rather than by physical adsorption of [Bmim]OH. [Bmim]OH modified TiO2 was much more active than pristine TiO2 under visible light irradiation in photocatalytic degradation of methylene blue in aqueous solution. The improvement of the photocatalytic activity by [Bmim]OH modification may result from the enhanced absorption of visible light, decreased recombination of the photoinduced electrons and holes and enhanced adsorption capacity to MB.A new and facile method for preparing visible light responsive S doped TiO2 has been developed by nonthermal H2S (10%)/H2-plasma treatment. The plasma treatment did not change the catalysts in their phase composition and particle sizes, but successfully doped S in the TiO2 lattice. UV-Vis result indicated that the plasma treatment extended their absorption edges to the visible light region and decreased the band gap energy. The performance in photocatalytic degradation of methylene blue indicated that the plasma treated TiO2 showed much higher activity than the pristine TiO2 under visible light. The increased photocatalytic activity was possible attributed to the enhanced absorption in the visible region and decreased band gap energy which caused by the sulfur doping.