Study The Doping And Modification Of Nano TiO₂ And Its Photocatalytic Activity

Nano titanium dioxide, used as a photocatalyst, can degrade the trace organic compounds to non-toxic substances and can kill bacteria and virus, therefore, it is of great value in the practical applications. However, it suffered from the disadvantages that the nano titanium dioxide cannot show the excellent photocatalytic activity until irradiated with UV light and the utilization of the solar rays is too limited and ineffective, which hinders its widespread applications to a great extent. In order to improve the photocatalytic activity and make full use of the solar rays, the nano titanium dioxide must be modificated. With these in mind, the main purpose of the paper is to prepare nano TiO₂ doped with several non-metal elements and rare-earth elements and improve the photocatalytic activity in visible light by means of controling the particle size of nano titanium dioxide and the phase transformation process, changing the crytal structure of titanium dioxide, along with suppressing the recombination of photogenerated electrons and holes. The nano titanium dioxide powders were synthesized by sol-gel method with the use of hydrochloric acid as the catalyst. The hydroxypropyl cellulose (HPC) was utilized as the surfactant for the first time. According to the experimental results, the optimum molar rate of the materials was shown as follows: Ti(OC4H9)4: C4H9OH: H2O: HCl = 1:27:2.8:0.24. HPC, used as the surfactant, provided strong steric stabilization in the sol-gel process, which can decrease the collision frequency between particles, baffle the growing of nano TiO₂ particles and effectively undermine the further conglomeration of grains in the heat treated process. We studied the influence of heat treatment in different atmospheres (air and nitrogen) on the phases of titanium dioxide, the size of the particles as well as the photocatalytic activity. The results demonstrated that the transformation of titanium dioxide from anatase to rutile calcined in nitrogen took place at the temperature of 400 ℃,which is lower than that treated in air(650 ℃). The results of X-ray diffractometer (XRD),X-ray photoemission spectrum (XPS) and diffuse reflectance spectra (DRS) revealed that during the heat-treated process in nitrogen, the oxygen atoms of TiO₂ were replaced by nitrogen atoms to form the compounds of TiO₂-xNx, which caused the formation of oxygen defects. The defects can easily become the centers of photoactivity in TiO₂, resulting in the increasing of the quantity of light absorbed, and the bandgap broadening. Therefore, the N doping caused the absorption edge of TiO2 to shift into the visible light region. Further photocatalytic experiments proved that TiO2 treated at lower temperature in nitrogen possessed higher photocatalytic activity than that treated in air. The activiation energy of nano-TiO2 calcined in air and nitrogen for crystal growth are 15.99 kJ·mol-1 and 8.84 kJ·mol-1, respectively. The growing process of TiO2 crystals is controlled by diffusion. S-doped uniform TiO2 photocatalysts were prepared using thiourea as the doping agent for the first time in China. The influence of sulfur doping on the crystal structure of nano TiO2, the energy level state and the surface state were investigated. The doping mechanism was also discussed. The results of photocatalytic degradation methylene blue demonstrated that the doped TiO2 exhibited the highest photocatalytic activity when the mole ratio of thiourea and tetrabutyltitanate[Ti (OC4H9) 4] is 3.5 and the doped TiO2 was calcined at 500℃ for 2 h. The results of crystal structure analysis showed that the growth of nano TiO2 crystal particles was suppressed effectively and the crystal grains were refined after sulfur doping. The transformation of TiO2 from anatase phases to rutile was also restrained and the temperature of transformaton is higher than 650℃. The results of X-ray photoelectron spectroscopy (XPS) showed that the trace of sulfur ions (S4+) substituted partially for the lattice titanium ions (Ti4+), which resulted in the localized crystal deformation of TiO2 and the bandgap between valence band and conduction band narrowed. The strong absorption of the S-doped TiO2 for visible light was observed from the results of diffusion reflectance spectra (DRS). The doping technique and the investigation on the mechanism proposed here were described for the first time in China. The TiO2 photocatalyst modified with carbon black was prepared. The results based on the X-ray diffraction (XRD) indicated that the transformation temperature from anatase to rutile was reduced to 400-500℃ . Carbon black modification suppressed the growth of nano titanium dioxide crystals. The size of TiO2 crytals was about 7.3 nm when calcined at 400℃ in air, resulting in the quantum size effect enhanced. Moreover, nano carbon black used as a photosensitizer can sensilize titanium dioxide to the much broader visible portion of the spectrum and can further improve the catalytic activity of TiO2 in visible light. The photocatalytic activity mechanism was proposed as follows: *2 22 2 2S ?h?γ → S ?T?iO ?→ TiO g? + S + ?O?→ S + + TiO +Og? Activated carbon, used as an excellent carrier, possesses strong adsorption ability. The organic molecules can be absorbed to the surrounding of catalysts and form acircumstance with relatively high concentration of organic molecules. The organic molecules further diffused to the surface of nano-titanium dioxide and were quickly absorbed on the surface of TiO2, which provided a good organic circumstance for improving the photocatalytic activity of TiO2. The research on the photocatalytic activity of activated carbon loaded nano TiO2 prepared in different atmospheres (air and nitrogen) was carried out. The results indicated that the phase transformation of TiO2 from anatase to rutile calicined in nitrogen is lower than that treated in air. The results of photocatalytic degradation of methylene blue brought out that the highest removal rate of the catalyst was obtained when calcined at 400℃ in N2 which is higher than that treated in air at the same temperature. However, the highest removal rate of the catalyst in air was obtained when calcined at the temperature of 550℃. The improvement in the photocatalytic activity of the catalysts derived from the synergistic mechanism between activated carbon with the high adsorption ability and nano TiO2 itself with the high catalytic activity. Three kinds of catalysts including Dy2O3-TiO2, CeO2-TiO2 and Gd2O3-TiO2 were prepared using tetrabutyltitanate[Ti(OC4H9)4] as the starting material. The photocatalytic activity of the catalysts was found to be improved which may be associated with the fact that the structure deformation of TiO2 took place and the crystal size of the TiO2 particles was restrained after the rare earth elements doped into the crystal lattice. Additionally, some rare earth dioxide was converd on the surface of nano-TiO2 particles, which resulted in the separation of electrons and holes. We found that the degradation rate on methylene blue is highest with the use of 1.25 wt % CeO2-TiO2, and that the degradation rate on methyl orange, however, is highest by using 1.25 wt % Gd2O3-TiO2. Hence, we concluded the photocatalyst doped with different rare earth elements showed a selective photocatalytic activity to different degradation agents.