Layer-by-layer Assembly And Photocatalytk Performance Of Titanium Dioxide-polyoxometalates Nanocomposite Films

Nanostructured TiO2is widely used for the effective decomposition of various dyes wastewater and organic compounds in air on account of its low cost, innoxiousness, chemical inertness and high photocatalytic performance. However, there are some defects for TiO2photocatalyst, for example, poor quantum efficiency and low solar energy utilization, which seriously limits the wide use in industries. In addition, the formation of a milky dispersion of TiO2nanoparticle results in disadvantages in recycle and separation. In thesis, in order to improve the photocatalytic efficiency and solve the problem of TiO2, polyoxometalates (POMs), an efficient electron acceptor, were added into TiO2multilayer films to enhance separation efficiency of electron-hole pairs. The photocatalytic properties of layer-by-layer self-assembled plate multilayer films were studied in more detail. The outline and contents of this doctoral dissertation are as follows:In chapter1, the crystal structure, principle of the heterogeneous photocatalytic and the research progress of titanium dioxide were introduced firstly. Then, the structure, development history and application in the field of dye wastewater treatment of polyoxometalates were recommended. The influence of several factors in the photocatalytic degradation of dyes was discussed detailedly. Finally, the research status and the recent development of polyoxometalates films, titanium dioxide films, polyoxometalates-titanium dioxide composite films fabricated by layer-by-layer (LbL) self-assembly method were reviewed. The purpose and significance of this thesis are also demonstrated.In chapter2, Photocatalytic multilayer films with different number of bilayers were prepared via an electrostatic layer-by-layer (LbL) self-assembly method. These LbL films were characterized by UV-vis spectroscopy, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Our results indicate that TiO2and H3PW12O40(PW12) were successfully incorporated into the thin films. The as-prepared (Ti02/PW12)10films showed better photocatalytic performance towards methyl orange (MO) solution than (PSS/TiO2)10and (PAH/PW12)10film, which is attributed to the synergistic effect between TiO2and PW12. The effect of experimental parameters including number of bilayers, initial concentration and pH value of dye solution were also studied. The optimum photocatalytic efficiency can be obtained when the concentration of MO is smaller than15mg/L and the acidity of dye solution is2.0. The multilayer films could be easily recovered and reused for several times with little change of degradation, indicating that they were stable under the ultraviolet (UV) irradiation. The detection of active species displays that active holes (h+) play a dominant role for MO photodegradation in the TiO2/PW12system. Taking advantage of immobilization of catalysts on glass slides, the problem of recovery is solved. It is expected that photocatalytic multilayer films have substantial applications in industries.In chapter3, Photocatalytic nanocomposite films (TiO2/SiW12)n(TiO2=titanium dioxide, SiW12=H4SiW12O40) were fabricated via an electrostatic layer-by-layer (LbL) self-assembly method, which is a rapid and simple experimental fabrication process. The results of UV-vis absorption spectra indicated that the LbL assembly of the multilayer films is regular. The XPS and AFM analyses clearly reveal the existence of the TiO2nanoparticles and SiW12polyanions in the composite films, which are in agreement with the results of UV-vis spectra. The removal efficiency increases steadily with an increase of bilayers number and the optimal bilayers number of (TiO2/SiW12)n catalyst is8. In addition, nanocomposite films show excellent photocatalytic performance for the degradation of other six colorful dyes. Photodegradation rate of MO is significantly suppressed by inorganic salts including sodium chloride and sodium sulphate due to the aggregation or association of ionic dye in water. Unlike inorganic salts, nonionic surfactant C12E23plays a positive role in pollutants removal. However, a marked inhibition of anionic sodium dodecylsulfate (SDS) on MO degradation is found at different SDS concentrations.In chapter4, the photocatalytic activities of TiO2/W10O32multilayer thin films prepared by an electrostatic layer-by-layer self-assembly method were tested via the degradation of aqueous methyl orange solution under UV irradiation. The influence of operational parameters including bilayers number of thin films, initial dye concentration and electron acceptor such as hydrogen peroxide were checked to determine the optimum conditions of MO degradation. The experimental results showed that the degradation rate of MO decreases with increasing of initial dye concentration. The (TiO2/W10O32)6/H2O2process proves to promote the decolorization and mineralization of azo dye methyl orange. Kinetics of MO oxidation process has been studied and is found to follow the first-order reaction at different H2O2concentration.