Synthesis And Characterization Of Water-dispersible Titanium Dioxide Nanoparticles With High Photocatalytic Activity

Photocatalytic oxidation can be carried out under ambient conditions using atmospheric oxygen as the oxidant and leads to complete mineralization of pollutants to CO2, water and mineral acids. It is a promising alternative to biological and physical methods because the used nanocrystalline titanium dioxide (TiO2) in photocatalytic oxidation not only possesses high chemical and biological inertness, non-toxicity and relative low cost, but also has other distinctive properties of nanoparticles such as quantum effect and superior catalytic performances, etc. However, TiO2nanoparticles are prone to lose activity and aggregate in photocatalytic process due to its small size, and the difficulty in separating TiO2nanoparticles from the treated water is also ascribed to the fine TiO2nanoparticles. Moreover, anatase TiO2absorbs only around5%ultraviolet part of solar light because of its wide band gap, and artificial light sources are expensive and unstable. All of them are the main barriers to marketing TiO2photocatalytic oxidation.This dissertation focuses on the synthesis, characterization and photocatalytic application of highly water-dispersible TiO2nanoparticles. On one hand, in order to overcome the barriers in TiO2practical application, many efforts were made to exploit new synthetic strategies to obtain TiO2nanoparticles with desirable property. On the other hand, in order to achieve industrial photocatalytic application of TiO2nanoparticles, valuable investigations have been conducted on the relationship among preparation approach, crystal structure, size and photocatalytic performance. The photocatalytic performance of the as-synthesized TiO2nanoparticles were evaluated for the degradation of quinoline as a model pollutant. The photodegradation mechanism of quinoline was also studied. The main findings are as follows:(1) Low-temperature synthesis of water-dispersible anatase TiO2with zeta potential-40mV and average size of9.8±0.6nm. According to the classical theory of Derjaguin, Landau, Verwey and Overbeek (DLVO), water-dispersible anatase TiO2nanoparticles were synthesized at a low temperature (80℃) without any organic surfactants via the mechanism of electrostatic repulsion between nanoparticles. TEM (Transmission Electron Microscope), XRD (X-ray diffractometer), XPS (X-ray photoelectron spectroscopy), FTIR (Fourier Transform Infrared Microscope) and DLS (Dynamic Light Scattering) were used to characterize its morphology, crystallographic structure, surface property and stability etc. By comparing the structure and water-dispersibility of TiO2nanoparticles synthesized by different approaches, it is found that preparation approach would lead to variation in the surface property of TiO2nanoparticles, which finally resulted in the different water-dispersibility of TiO2nanoparticles. This result further proves the advantage of low temperature synthesis of anatase TiO2nanoparticles without any organic surfactants.(2) Controllable synthesis of water-dispersible magnetite nanoparticles with size range from5to30nm. Ligand-exchange approaches and one-step reverse precipitation method were developed to synthesize monodisperse, water-dispersible and carboxylate/amino-functionalized superparamagnetic magnetite nanoparticles. Hereinto, one-step reverse precipitation method uses commercial available, inexpensive, and environmentally acceptable raw reaction materials (water is the solvent), obviously, it provides an economic and green approach for the controlled synthesis of magnetite nanoparticles. The as-synthesized magnetite nanoparticles not only play a vital role in catalyst recycle, but also accelerate its biomedical applications, such as magnetic resonance imaging, drug delivery and bioseparation because of their tunable surface functional group.(3) Low temperature synthesis of magnetically separable Fe3O4/TiO2composite photocatalysts. Based on the coulomb electrostatic forces between Fe3O4and TiO2, magnetically separable Fe3O4/TiO2composite photocatalysts with different molar ratios of Fe3O4to TiO2were synthesized via a modified sol-gel method at low temperature of80℃. This approach avoids the high temperature calcination involved in traditional methods, which usually results in the grain growth and aggregation of the resultant products and therefore the loss of surface area. The morphology, crystal structure, surface property and magnetic performance of the as-synthesized magnetically separable Fe3O4/TiO2composite photocatalysts could be controlled by changing the molar ratios of Fe3O4to TiO2.(4) Low temperature synthesis of visible light responsed TiO2photocatalyst. Graphene oxide (GO) was synthesized via improved Hummers method with the help of ultrasonic treatment. TiO2/GO composite photocatalyst were synthesized based on the synthetic procedure of water-dispersible TiO2nanoparticles without any organic surfactants. GO greatly contributes to its visible-light response. With the increase of GO quantity, the absorption band of TiO2/GO composite presents obvious red shift, correspondingly, the band gap become narrower, and finally the absorption range of visible-light was extended. In TiO2/GO composite photocatalyst, Ti was existed in the form of Ti4+, and there is no formation of Ti-C bond.(5) The photocatalytic application of the as-synthesized TiO2nanoparticles were investigated for the degradation of quinoline as a model pollutant. For the pure water-dispersible TiO2nanoparticles, it is found that different preparation approach could lead to the change in the physical and chemical properties of TiO2nanoparticles, and finally influence the photocatalytic activity of nanoparticles. In this work, we found that the superoxide radical (·O2) played an important role in the photodegradation of quinoline, which is greatly different from most of other publication. For magnetically separable Fe3O4/TiO2composite photocatalysts, it is found that the relative quantity of Fe3O4to TiO2can greatly affect the photocatalytic activity. The Fe3O4/TiO2composites could be easily recovered from the reaction solution by using a permanent magnetic bar and their photocatalytic activity changed little after repetitive uses. For TiO2/GO composite photocatalyst, GO greatly contributes to its visible-light response. GO can be acted as electron-acceptor and inhibit the recombination between photogenerated electron-hole. Its large specific surface area is beneficial to the adsorption of quinoline on the catalyst surface. These factors endow the TiO2/GO composite photocatalyst high and stable photocatalytic activity.