Synthesis Of Highly Visible-Light-Responsive TiO₂ Photocatalyst Using Low Temperature Solvothermal Method

The photocatalysis is considered as one of promising technology to address the global energy supply issue and environmental problems. In the last decade, considerable efforts have been devoted to the development of photocatalytic materials, with the purpose of the effective utilization solar energy. As a well-known photocatalyst, Titanium dioxide （TiO₂） has attracted increasing attention over the past dacades due to its non-toxicity, stability, low cost, high chemical and biologic resistance. However, the main drawback is that TiO₂ has poor absorption visible light, low activity and high recombination rate of photogenerated electron/hole pairs, which hinder its further application in industry. In order to improve the photocatalytic activity and extend the photoresponse of TiO₂ into the visible region of the solar spectrum, a lot of efforts have been directed towards the doping of TiO₂ lattice to modify its electronic band gap and shift its absorption edge to the visible light region. One way this may be achieved is to dope TiO₂ with either nonmetal （or anion） or metal （or cations）. In this thesis, different series of doped TiO₂ materials have been prepared by low-temperature solvothermal method. The SnO2/ZnO/TiO₂ composite photocatalysts were also synthesized by sol-gel and solid-state methods.A series of N-doped TiO₂ samples have been achieved using solvothermal method in an organic amine/ethanol-water reaction system. The effects of different starting N:Ti molar ratio on the catalyst structure, surface property and catalytic activity have been investigated. Characterization results show that N dopant has a significant effect on the crystallite size and optical absorption of TiO₂. It was found that the N-doped TiO₂ catalysts have enhanced absorption in the visible light region, and exhibit higher activity for photocatalytic degradation of model dyes （e.g. MB and MO）. The catalyst with the highest performance was the one prepared using N:Ti molar ratio of 1.0. Electron paramagnetic resonance （EPR） measurement suggests the materials contain Ti3+ ions, with both the degree of N doping and oxygen vacancies make contributions to the visible light absorption of TiO₂. Visible light driven S doped and F doped anatase TiO₂ samples have been achieved via solvothermal method using different F and S containing compounds as dopant. The S and F doping level have a remarkable effect on the crystal phase, optical absorption, surface composition and catalytic performance of doped TiO₂ samples. The activity of obtained photocatalysts under visible light illumination can be enhanced significantly for photocatalytic degradation of methylene blue. There are two distinct S species existed in the S doped TiO₂ system, one is the S atom substitutes for Ti4+ and incorporates into the TiO₂ lattice to form Ti-O-S linkage, the next one is the SO2 absorbed on the surface of TiO₂. In the F doped TiO₂ system, the substitute F atoms that occupied oxygen sites in the TiO₂ lattice and lead to the formation of Ti-F. The creation of the oxygen vacancies during the S and F doping process contributes to the enhanced visible light photocatalytic activity.Fluorine-sulfur （F-S） co-doped TiO₂ materials have been prepared under solvothermal condition. Thiourea and ammonia fluoride were used as starting precursor. XPS analysis shows that the F and S elements successfully doped into TiO₂ lattice and combines with Ti4+ cations to form Ti-S and Ti-F in the TiO₂. The crystal phase analysis shows that F-S co-doping restrains the crystal growth and phase transformation from anatase to rutile. The obtained samples present pure anatase after annealing at 700°C for 3h, indicating the catalysts have high thermal stability. The F-S co-doped TiO₂ has a higher photocatalytic activity than that of mono-doped F- and S-doped samples under visible light irradiation. It is believed that the co-doping gives rise to a localized state in the band gap of the oxide and creates active surface oxygen vacancies, both which are responsible for visible light absorption and the promotion of electrons from the localized states to the conduction band. Characterization by Electron Paramagnetic Resonance revealed the presence of a superoxide radical （O2·–） which may be mainly responsible for photodegradation of Methylene Blue under visible light.To control the morphology of TiO₂, a template-solvothermal methodology was developed to synthesize antase mesoporous TiO₂ microsphere at low temperature （140°C）. Meanwhile, using a co-doping approach red-shift the threshold of TiO₂ adsorption into visible light region and improve its photocatalytic efficiency for degradation of MO. The obtained mesoporous TiO₂ microsphere possesses higher photocatalytic stability and stronger durability in the successive photoreaction process. In addition, low-temperature solvothermal approach was also extended into the synthesis of alkaline-doped TiO₂ catalysts. The effects of the doping level of alkali metal cations on the crystal phase, morphology, chemical composition, surface property and activity have been studied. The characterization results show that the optimum alkaline doping level is 3wt%, which give a smaller average particle size and highest photocatalytic degradation rate of the dye.The preparation of SnO2/ZnO/TiO₂ composite semiconductor photocatalysts from different Sn and Zn precursor by solid-state and non-water sol-gel methods was reported in this thesis. It was found that both of the synthesis methods are very efficient, which could introduce Sn and Zn ions into TiO₂ lattice and lead to the deformation of catalyst lattice. XRD analysis shows that the crystalline phase of composite semiconductor samples prepared by solid state method is pure rutile. The samples prepared by non-water sol-gel methods are anatase phase at the low doping level, and main rutile phase at high doping level, indicating SnO2 and ZnO doping could improve the phase transformation of the composite semiconductor. The phtocatalytic activity increased significantly for the non-water sol-gel synthesized SnO2/ZnO/TiO₂ composite semiconductor. This is due to the coupling different band gap of semiconductor could promote the separation of photogenerated electron/hole pairs, resulting in the enhanced photocatlytic performance of catalyst.