Preparation Of Novel TiO₂ Complex And Its Properties For Photocatalytic Degradation Of Methylene Blue

Titanium dioxide（TiO₂）, for its non-toxicity, low cost, chemical stability and highly photoactivity, becomes an ideal environment-friendly semiconductor material. It has got more and more attention in solving the energy crisis and environmental pollution. But in practice, there still exists the low utilization rate of photogenerated carrier recombination rate and other issues, which greatly limits the practical application of a wide range of TiO₂. Therefore, it’s important to use appropriate methods to expand the spectral response range of TiO₂ and suppress the light-generated electrons and holes compound. In this thesis, the solid phase method, base on following raw materials,melamine（C3H6N6）, bismuth nitrate（Bi（NO3）3·5H2O）, lanthanum nitrate, industrial TiO₂, surrounded nanometer TiO₂ photocatalyst, used doping and compound, prepared three kinds of high efficiency visible light catalytic activity nanocomposite photocatalyst: Bi₄Ti₃O₁₂/Bi₂O₃/TiO₂, Bi₂O₃/g-C3N4/TiO₂, La-doped TiO₂/g-C3N4, and adopted a series of characterization ways of complex sample preparation such as X-ray diffraction（XRD）, X-ray photoelectron spectroscopy（XPS）, Fourier transform infrared spectroscopy（FT-IR）, ultraviolet-visible absorption spectroscopy（UV-Vis）, fluorescence spectroscopy（PL）, transmission electron microscopy（TEM） and N2 adsorptio-desorption analysis to characterize. Under the 12 W LED light irradiation, I evaluated visible light catalytic properties of the composite sample by using methylene blue as targets. The main studies of this paper are as follows:（1） Bi₄Ti₃O₁₂/Bi₂O₃/TiO₂ composite photocatalyst were successfully synthesized via a facile solid-phase method of directly heating bismuth nitrate and commercial TiO₂ mixtures. This paper examined the different Bi（NO3）3·5H2O and TiO₂ ratio, different calcination temperature and calcination time on the effect of composite photocatalyst with visible light photocatalytic degradation of methylene blue to determine the bestconditions for sample preparation: Bi（NO3）3·5H2O/TiO₂ ratio was 2.5:1; Calcination temperature is 600℃; Calcining time was 5h. In this condition, after 180 min 12 W LED light irradiation, the degradation rate of composite photocatalyst(Bi₄Ti₃O₁₂/Bi₂O₃/TiO₂)to methylene blue reached 96.8%, which was higher 40% than the pure Bi₂O₃ catalyst,and about 50% than the pure TiO₂. Through a series of characterization analysis, the results showed the presence of anatase TiO₂, α-Bi₂O₃ and perovskite Bi₄Ti₃O₁₂ in the Bi₄Ti₃O₁₂/Bi₂O₃/TiO₂ catalysts. Compared with TiO₂, composite sample fluorescence intensity obviously decreased. The band gap width is lowered to 2.6 e Vand. the absorption band edge red shifted.Through the formation of heterojunction, The complex samples of Bi₄Ti₃O₁₂, Bi₂O₃ and TiO₂ phase, effectively inhibited the compound of photoinduced electron and hole, improved the utilization of composite samples of visible light, and enhanced the visible light photocatalytic activity.（2） Bi₂O₃/g-C3N4/TiO₂ composite photocatalyst were successfully synthesized via a facile solid-phase method of directly heating bismuth nitrate,g-C3N4 and commercial TiO₂ mixtures. This paper examines the effect of Bi content, different C3H6N6 and TiO₂ ratio, different calcination temperature and calcination time on the effect of composite photocatalyst with visible light photocatalytic degradation of methylene blue to determine the best conditions for sample preparation: bismuth content was 0.35%;industrial TiO₂ with C3H6N6 quality ratio was 1:2.5; calcination temperature was 520℃;calcining time was 5h. In this conditions, after 180 min 12 W LED light irradiation, the degradation rate of composite photocatalyst to methylene blue reached 98.1%, but degradation rate of TiO₂, g-C3N4 and g-C3N4/TiO₂ to methylene blue were 19.9%,49.1% and 66.1%, respectively. Through a series of characterization analysis, the results show that the Bi₂O₃/g-C3N4/TiO₂ composite photocatalyst has g-C3N4, γ-Bi₂O₃ and anatase TiO₂.With respect to TiO₂, composite sample fluorescence intensity obviously decreased. The band gap width is reduced to 1.8 e V and the absorption band edge red shifted. Mixed sample stacked visible light response of g-C3N4 and Bi₂O₃, making visible light absorption enhancement. Elongated tubular γ-Bi₂O₃ and the flake of g-C3N4,were situated on the surface of the TiO₂ particles,which formed solid heterojunction. Electrons and holes through the transfer between the interfaces of Bi₂O₃,g-C3N4 and TiO₂ can effectively prolong the photocarrier lifetime, and significantlyimprove the photocatalytic activity.（3） In La N3O9·6H2O, the raw materials commercial TiO₂ and C3H6N6 were first prepared by calcining muffle La-doped samples with different content of TiO₂ and g-C3N4, and then a certain percentage of methanol mixed sample La-doped TiO₂ and g-C3N4.This paper examines the different La doping amount, different g-C3N4 and4%La/TiO₂（Lanthanum content accounted for the TiO₂’ mass fraction is about 4%）, the effect of composite photocatalyst with visible light photocatalytic degradation of methylene blue to determine the best conditions for sample preparation: La doping amount for the proportion was 4%; g-C3N4 and 4%La/TiO₂ were 2:1. In this conditions,after 180 min 12W LED lights irradiation, removal rate of La-doped TiO₂/g-C3N4 composite light catalyst to methylene blue reached 97%; and removal rate of TiO₂,g-C3N4 and g-C3N4/TiO₂ samples to methylene blue were 19.7%, 48.9%, 66.4%respectively. Through a series of characterization analysis results demonstrated that most of the La existed in TiO₂ crystal gap, and formed Ti-O-La bond. Doped lanthanum increased oxygen vacancies on the surface of TiO₂, and formed the capture well the active capture center in TiO₂. Trapping hole, it effectively inhibited the recombination of photogenerated electron hole; and TiO₂ introduced impurity level, narrowed the band gap of TiO₂, expanded the scope of its absorption, and improved the photocatalytic activity. In addition, the formed heterojunction structure of g-C3N4 and La doped TiO₂ composite, promoted effectively the photogenerated electron hole separation and improved the photocatalytic activity of the samples.