| In the past decade, volatile organic compounds (VOCs) are one of the most abundant chemical pollutants in the indoor air. Therefore, development of an efficient technique for removing the aromatic compounds from ambient environment has become a hot topic. Photocatalysis, as advanced oxidation processes, offers a potential method for complete elimination of toxic chemicals. TiO2 is a wide band-gap semiconductor that can produce powerful holes and electrons by absorbing photons. Besides, due to its low cost and good chemical stability, it has been widely studied in photochemical systems, such as TiO2 photocatalytic oxidation for VOCs. However, because of its wide band-gap energy (3.2 eV for anatase form and 3.0 eV for rutile form), TiO2 still suffers from low visible light absorption and high charge recombination rate. Many modification methods have been conceived to increase the hole-electron separation efficiency as well as to extend the absorptive spectral range of TiO2. In our study, several modification methods have been chosen in order to enhance the hole-electron separation efficiency and extend the absorptive spectral range.(1) SiO2/TiO2 composite nanotubes have been synthesized by a facile sol-gel method using ZnO nanowires as template. Compared with the pure TiO2 nanotubes, the UV-Vis absorption edge of SiO2/TiO2 composite nanotubes showed a small blue shift due to the formation of Ti-O-Si group. The photocatalytic activity of the SiO2/TiO2 composite nanotubes in degrading gaseous toluene (63%) was increased by 22% and 12% than Degussa P25 and TiO2 nanotubes, relatively. The improved photocatalytic activity was achieved by quantization of particle size, increase of surface area and hydroxyl content through silica into titania.(2) In order to improve the photocatalytic degradation of toluene, TiO2 nanotube powder was prepared via potentiostatic anodization method and was doped using Ag by an incipient wetness impregnation method. Ag dopant promoted the anatase to rutile phase transformation in low calcination temperature and inhibited effectively the recombination of photoinduced charge carriers. The toluene removal efficiency in photocatalytic oxidation process was 98% in 4 h using the 1 wt.% Ag-doping TiO2 nanotubes. The quantum efficiency of 1 wt.% Ag-doping TiO2 nanotubes was 1.2 times than P25. The main by-product in the gas phase was identified as benzaldehyde or benzoic acid, and then part of them finally could be mineralized into CO2 and H2O. (3) LaVO4/TiO2 nanotubes have been synthesized by sol-gel coupled with hydrothermal method in order to extend the absorptive spectral range of TiO2. It is beneficial for the anatase phase of TiO2 due to the LaVO4 composed to TiO2. The as-prepared samples possess high specific surface area and attractive photovoltage response, which could lead to the enhancement of photocatalytic activity remarkably in degradation of gaseous toluene under visible light. The toluene removal efficiency in photocatalytic oxidation process was about 75% in 6 h over 1 wt.% LaVO4/TiO2 nanotubes under visible light irradiation, which was higher than using TiO2 nanotubes (29%) and Degussa P25 (11%) as photocatalysts. The quantum efficiency of 1 wt.% LaVO4/TiO2 nanotubes was 5 times than P25.Under UV light irradiation, the conversion of toluene was 63% and 98% using SiO2/TiO2 nanotubes and 1 wt.% Ag-doping TiO2 nanotubes as photocatalysts after reaction 4 h, respectively. The 1 wt.% Ag-doping TiO2 nanotubes has higher catalytic activity. Under visible light irradiation, the conversion of toluene was 37% and 75% using 1 wt.% Ag-doping TiO2 nanotubes and 1 wt.% LaVO4/TiO2 nanotubes as as photocatalysts after reaction 6 h, respectively. As the prepared LaVO4/TiO2 nanotubes have a better response to visible light, it has a good prospect in order to improve the utilization of solar energy. |
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