Preparation And Characterization Of TiO₂-Based Nanomaterials Used As Oxygen Sensor And Photocatalyst

The properties of TiO₂ materials are dependent on it's crystal phase. In this respect, the conductivity of TiO₂ with rutile phase is sensitized to oxygen gas, while TiO₂ with anatase phase has high photocatalytic activity. Up to now, only tetravalent titanium (Ti4+) has been used as the source materials prepared for rutile phase applied as oxygen sensor. In the case that tetravalent titanium was used, the rutile phase was obtained through a higher sintering temperature (≥700℃). This leads to that the properties of oxygen sensor made by the materials would attenuated due to it's higher operating temperature (≥250℃), and with bad stability and selectivity. On another application, it is known that the method of the metallophthylocyanine (or metalloporphyrin) sensitized TiO₂ is an efficiency process for advancing photocatalytic activity. And now, dye sensitization of wide-band gap nanocrystalline semiconductors is attracting owing to the two dyes show strong absorption at visible light section and stability of physical and chemical properties. The synthesis processes for TiO₂ sensitized by the dyes were all chosen by dip-coating with TiO₂ nanoparticles or spin-coating on TiO₂ film. However, the interaction between the dyes and TiO₂ is infirmness and the efficiency of these sensitized TiO₂ is limited In this thesis, the source materials with trivalent titanium (Ti3+) is firstly used at the synthetic procedures for rutile TiO₂. The rutile doped with pentavalent or hexavalent impurity ions was obtained at lower sintering temperature (400℃) during sol-gel process. It is hoped that by using this process the substantial increases in oxygen sensing properties by doping with the impurity will be obtained. On the other hand, three photocatalysts were obtained by using the general sol-gel process and an in-situ, self-assemblying process, respectively. The catalytic activities of modified TiO₂ were examined by using rhodamine B and methylene orange solutions as organic substrates under visible– light excitation. The characterization of structure and spectra for modified TiO₂ was made. And a new mechanism of enhanced activity was elucidated. 1. Nanoscale TiO₂ powders doping with V⁵⁺, Nb⁵⁺,Ta⁵⁺, Cr⁶⁺, Mo⁶⁺, W⁶⁺ ions were prepared using TiCl3 as a source matter. The ruitle with doped TiO₂ was obtained at lower sintering temperature (400℃,1h). It has been found that the 18mol% V5+ incorporated TiO₂ shows the maximum sensitivity (S=16), lowest operating temperature (80℃), shorter response time, good selectivity and stability. A mechanism of the anatase-rutile transformation in doped TiO₂ is suggested. 2. The Zeta potential and susceptibility with temperature were first carried out in the gas sensing materials, and used to analyze the relationship among the charge behavior on the surface, structure and gas properties. The mechanism, i.e., enhancement in sensitivity and selectivity for TiO₂ doped by pentavalent and hexavalent impurity ions, has been proposed. 3. Using TiOPc with an axial ligation as sensitizer, TiOPc/TiO₂ compound was obtained by a general sol-gel process. The non-aggregated TiOPc molecules are self-organized on the TiO₂ particle surface only at TiOPc: TiO₂=1mol%. The 54% of rhodamine B was degraded by this photocatalyst under visible light (15W, 2h). 4. Cobalt phthalocyanine (CoPc)/TiO₂ and Zinc tetraphenylporphyrin (ZnTPP) / TiO₂ were synthesized by using a novel method, i.e., in-situ process. The monolayer of dye molecules is self-organized on the TiO₂ particle surface only at suitable molar ratio of dye and TiO₂. The IR and fluorescence spectra showed distinctive interaction between sensitizer molecule and TiO₂ at the axial direction of dyes. The charges excited by visible light are rapidly separated, and at the same time the energy is transfer efficiently from dye to semiconductor. The enhanced activity originates from the cooperative functions of the two components of the photocatalyst, sensitizer and TiO₂. The degradation of rhodamine B by ZnTPP/TiO₂ photocatalyst is up to 90% under same visible light (15W, 2h).