Study On The Synthetic Methods Of Nano-Sized TiO₂ With Enhanced Photocatalytic Activity

In recent years, a great deal of effort has been devoted to developing heterogeneous semiconductor photocatalysts with high photocatalytic activity for their wide environmental applications such as air purication, water disinfection, hazardous waste remediation and water purication. Among various oxide semiconductor photocatalysts, titanium dioxide has been proved to be one of the best photocatalysts due to its high oxidative ability, cost-effectiveness, physical and chemical inertness for widespread environmental applications. However, there still remain some problems in practical applications, such as the preparation of photocatalyst with high photocatalytic activity, the light utilization ratio. The photocatalytic activity of titania must be further enhanced from the point of practical use and commerce. In the present work, aiming at the the existing problems,we try to improve on the fabrication methods and conditions so as to synthesize a serial of TiO₂ photocatalyst with high photocatalytic activity. Lots of studies were done to explain the effect of crystal structure, surface area and other properties on the photocatalytic activity. This can accumulate experience for the development of synthetic methods of high photocatalytic activity. The main results of the present work were as follows:A method was developed to prepare superior photoactive TiO₂ nanopowders under synergistic effect of CTAB and ammonia. The prepared TiO₂ has a characteristic of small crystal size, large surface area and high crystallinity, which can improve the photocatalytic activity. In this method, nano TiO₂ particles were synthesized by a hydrothermal process in the presence of cetyltrimethylammonium bromide (CTAB) and a post-treatment with ammonia. During the hydrothermal process, CTAB can make the agglomeration resulting from the amorphous phase after the hydrolysis dispersed, along with the transformation from amorphous to TiO₂ crystallites, and further induce the assembly of the as-prepared TiO₂ crystallites to form mesoporous titania. The ammonia post treatment can maintain the porous structure and suppress the phase transformation from anatase to rutile, moreover, there is a synergistic effect of CTAB and ammonia. All the effects make it possible to obtain nano-sized TiO₂ simultaneously with fine particle size and high crystallinity. The photocatalytic activity of prepared photocatalysts was obviously higher than that of commercial Degussa P25, and therefore is a promising photocatalyst with high photocatalytic performance.A method was developed to prepare superior photoactive TiO₂ nanopowders under synergistic effect of CTAB and ethylenediamine. The prepared TiO₂ also has a characteristic of small crystal size, large surface area and high crystallinity. It is well known that ammonia is alkalescent, not stable, and apt to decompose during the refluxing treatment; this would bring much discommodity to the work. To overcome the shortcomings of ammonia, ethylenediamine was used as a base during the post treatment. Stable porous TiO₂ photocatalysts, with higher photocatalytic activity than Degussa P25, were also synthesized via a hydrothermal process using CTAB as the template, the post-treatment considerably increased the thermal stability of the porous framework and inhibited the undesirable grain growth and phase transformation during calcinations, and one possible formation mechanism of the stable porous titania was proposed. This work may provide a new insight into preparing high photoactive TiO₂ photocatalysts. A method was developed to prepare a series of TiO₂ samples with stable, controllable crystalline phase content mesoporous TiO₂ by means of the change of solvent polarity. Results demonstrate that crystal phase content of anatase and brookite, particle size can vary with the change of solvent polarity (H2O/EtOH ratios). The effects of crystal structure, composition and surface area on the photocatalytic activity were also studied. A method was developed to prepare nitrogen doped TiO₂ with visible light activity on the basis of TiO₂ with high thermal stability, large surface area and small crystal size. Nitrogen doped TiO₂ powders were prepared by annealing anatase TiO₂ at 800℃in air, subsequently under NH3/N2 flow in a suitable temperature for a certain time. The results revealed that, a proper nitrogen concentration can enhance photocatalytic activity, the catalyst prepared at the best condition showed higher photocatalytic activity than the nitrogen doped Degussa P25 under the same condition.ZrO₂ modified mesoporous S-TiO₂ has been successfully synthesized via the evaporation-induced self-assembly (EISA) in the presence of Thiourea and zirconium oxychloride. Results revealed that Sulphor was doped into the lattice of TiO₂ by the thermal treatment of mixed TiO₂ gels, converting the TiO₂ into a visible-light responsive catalyst. The introduction of ZrO₂ strongly inhibited the undesirable crystallite growth and aggregation, and the crystal phase transformation from anatase-to-rutile structure. Consequently, the ZrO₂ modified S-TiO₂ showed larger BET surface area, higher pore volume, and an improved thermal stability over the corresponding unmodified S-TiO₂ sample. The photocatalytic activity of mesoporous ZrO₂ modified S-TiO₂ composites was superior than that of unmodified S-TiO₂ under visible light irridiation.