Synthesis And Photocatalytic Activity Of Mesoporous TiO₂

Quasi-nanospheres mesoporous TiO₂ particles(m-TiO₂) with anatase structures have been prepared directly by using tetrabutyl(ortho) titanate(TBOT) as a precursor and surfactants as templates in ethanol-water co-solvent,in which the hydrolisis speed of TBOT can be slowed down due to tha adding of ethanol.The influence of hydrothermal reaction time, hydrothermal temperature,the volume ratio of ethanol to water,the pH value of the medium, the type of template and its calcination temperature have been investigated.A complete characterization of the synthesized products has been performed by the X-ray diffraction,the low temperature nitrogen adsorption,the scanning electron microscope and the transmission electron microscope.The results indicate that with the increasing of the hydrothermal temperature,the crystallization of the products can be enhanced and the average particle sizes are also widened,whereas the ordered mesoporous structure and the surface areas are declined.When the hydrothermal reaction time is prolonged,the crystallization are enhanced a little which indicates that the hydrothermal time has few affects on the synthesis process. The other important factor on both crystallization and mesoporous structure of the products is the volume ratio of ethanol to water.In addition,the pH value of the reaction medium can influence mesoporous structure when P123 was adopted as template in the synthesis of m-TiO₂.The suitable synthesis condition will be the neutrality or the acidic environment.For the remove of the template,it is found that through two steps of calcination the template can be easily removed and the products possess good crystallinity.But with a further increasing of calcining temperature,such as 450℃,the thermal stability of the products decreased and the mesoporous structure was destroyed completely.On the basis of the above experiment,the optimal synthesis parameters for preparing m-TiO₂ have been determined,which are as follows:in the neutral medium,taking three block copolymer P123 as template,the co-solvent with the volume ratio of ethanol to water 1.5:1,the hydrothermal reaction were carried out at 80℃for 2 h,afterwards the samples was firstly calcined at 200℃for 1 h and then calcined at 350℃for 2 h to remove template.By the observation of TEM,it can be seen that the morphology of the as-prepared m-TiO₂ is quasi-nanospheres with the average diameter about 200 nm,which are made up of a large quantity of nanoparticles with the size approximately 10 nm.The N₂ adsorption isotherm shows the surface area is up to 182.7 m^·g^-1 and the pore diameter is about 6.85 nm through the BJH method to calculate.The photocatalytic avtivity and recycle life of the as-prepared m-TiO₂ has been valuated by taking the methyl orange(MO) solution as simulative pollutant under UV irradiation.The results indicate that the m-TiO₂ is a good photocatalyst for the degradation of MO(97.7%) after a first 50-Min photocatalytic treatment,and it also owns long circulatory life than commercial photocatalyst P25 because it still show high photocatalytie activity of 90%for MO degradation after a five 50-min photocatalytic process, which is much better than that of P25,the latter is only 56%for MO degradation after a five 50-min recycle.The mesoporous TiO₂ is further modified by silver nanopartieles through physical impregnation and chemical deposition method,respectively.The silver nanoparticles which were used in physical impregnation have been pre-synthesized in large quantities by reducing silver nitrate(AgNO₃) with ethanol and/or water in the presence of poly(vinylpyrrolidone) (PVP).The effects of the proportion of ethanol to water,the concentration of AgNO₃ and the reaction time on the shapes and sizes of silver nanoparticles have been investigated.At last, the quasi-nanospherical silver nanoparticles were chosen to impregnate on the surface of the as-prepared m-TiO₂ by the assistance of ultrasonic.The chemical deposition of silver nonpartieles loading on m-TiO₂ was by silver mirror reaction.The results of the photodegradation of MO show that all the silver modified m-TiO₂ exhibit higher photocatalytic activity than the pure m-TiO₂ and P25.With the increasing of silver conent, the photocatalytic activity was elevated first and then decreased,and the sample of Ag-2(wt)%/m-TiO₂ possesses the best property.Under the same conditions,there is little difference on photocatalytic activity between the Ag/m-TiO₂ samples derived from two prepared methods.However,the comparison of recycle efficiency of Ag-2(wt)%/m-TiO₂ samples shows that the product prepared by chemical deposition is better than that of by physical impregnation.After a five 20-min photocatalytic process,it still exhibits high photoeatalytic degradation of MO about 88%.