Hydrothermal Preparation Of Porous TiO₂ Photocatalytic Materials

In recent years,in order to solve the increasingly serious environmental pollution problems,heterogeneous semiconductor photocatalysis has attracted intense interest for its potential to control aqueous contaminates or air pollutants.Titanium dioxide has been proved to be the most suitable photocatalysts for widespread environmental application because of its biological and chemical inertness,strong oxidizing power, nontoxicity and long-term stability against photo and chemical corrosion.Despite its great potential,the fast recombination rate of photogenerated electron-hole pairs on the surface or in the lattice of TiO₂ hinders the commercialization of photocatalytic oxidation technology.Therefore,the photocatalytic activity of titania must be further enhanced from the point of view of practical use and commerce.To achieve this purpose,well crystalline TiO₂ powders with relative small crystallite size and high specific surface areas are desirable to improve the photocatalytic activity.Among various methods available for the fabrication of TiO₂ nanoparticles,the hydrothermal method has been widely employed due to the low cost equipment required and low temperatures.By changing the hydrothermal reaction conditions,such as reaction temperatures and times,pH values,reactant concentrations and molar ratios,additives and so forth,crystalline products with relative small crystallite size,high specific surface areas,different compositions,structures and morphologies have been obtained.In this dissertation,valuable explorations have been carried out on the synthesis of highly active TiO₂ photocatalysts,TiO₂ hollow structures and composite catalysts by hydrothermal method.The main points could be summarized as follows:In chapter 2,bimodal nanocrystalline mesoporous TiO₂ powders with very high photocatalytic activity could be prepared by hydrothermal method using tetrabutylorthotitanate as titanium source.Hydrothermal treatment enhanced the phase transformation of the TiO₂ powders from amorphous to anatase and crystallization.With increasing hydrothermal temperature or time,the average crystallite size and average pore size increase.In contrast,the BET specific surface areas,pore volumes and porosity steadily decrease.All TiO₂ powders after hydrothermal treatment showed bimodal pore-size distributions in the mesoporous and macroporous regions.An optimal hydrothermal condition(180℃for 10h)was determined.The photocatalytic activity of TiO₂ powders prepared under an optimal hydrothermal condition was nearly 3 times higher than that of Degussa P-25.This may be ascribed to the facts that the sample prepared under an optimal hydrothermal condition has bimodal mesoporous structures,high specific surface area,small crystallite size and good crystallization.In chapter 3,bimodal mesoporous nanocrystalline TiO₂ powders were prepared by hydrolysis of tetrabutylorthotitanate in ethanol-water mixed solutions via a hydrothermal method.The molar ratios of EtOH to H₂O(R)obvious influenced the crystallization,crystallite size,BET specific surface areas and photocatalytic activity of the prepared TiO₂ powders.With decreasing R,the BET specific surface areas, pore volumes and porosity slightly decrease.However,the intensity of diffraction peak from the anatase(101)plane steadily increase.At 150-200℃for 3-24 h,the photocatalytic activity of TiO₂ powders prepared in ethanol-water mixed solutions showed very high photocatalytic activity and greatly exceeded that of Degussa P-25.In chapter 4,bimodal nanocrystalline mesoporous TiO₂ powders with high photocatalytic activity were prepared by a hydrothermal method and then by microwave-assisted drying using tetrabutylorthotitanate as precursor.Compared with the conventional drying,microwave drying can not only shorten drying time but also has an obvious effect on the pore microstructures and photocatalytic activity of TiO₂ powders.Microwave drying promotes the growth of pore and enhances photocatalytic activity.The sample dried in microwave oven for 3 min shows the highest photocatalytic activity.With prolonging the microwave drying time from 6 to 10 min, the photocatalytic activity slightly decreases due to the decrease in the specific surface area and pore volume,but remains higher than that of the sample dried by conventional drying.The TiO₂ powders dried in microwave oven for 3 min displayed an excellent photocatalytic activity probably due to their large surface and pore volume,and their photocatalytic activity was greatly higher than that of Degussa P-25.In chapter 5,novel multiwalled TiO₂ hollow microspheres with bi-phase(anatase and rutile)structures were prepared by a simple one-pot hydrothermal method using (NH₄)₂ TiF₆ as titanium source.Calcination temperatures exhibit a strong effect on photocatalytic activity of the hollow microspheres.At 500℃,TiO₂ hollow microspheres exhibit the highest photocatalytic activity probably due to their bi-phase structure and an optimal mass ratio of rutile to anatase.This novel multiwalled TiO₂ hollow microsphere might find widely potential applications in catalysis,biology, solar cell,separation,purification processes and so on.In chapter 6,novel multiwalled TiO₂/C composite hollow microspheres were prepared by a simple one-pot hydrothermal method using(NH₄)₂TiF₆ as titanium source.Calcination times obvious influenced daylight-induced photocatalytic activity of the as-prepared TiO₂/C composite hollow microspheres.With increasing calcination times,the average crystallite size and average pore size increase.In contrast,carbon content,the BET specific surface areas,pore volumes and porosity steadily decrease.The sample calcined at 450℃for 2 h showed a highest daylight-induced phtotcatalytic activity and greatly exceeded that of Degussa P-25. This may be ascribed to the facts that the sample possesses an optimal carbon content, a relative large surface area and good anatase crystallinity.