| In recent years, environmental pollution becomes more and more serious all over the world, environmental issues have attracted considerable attention by more and more people. Among these the dyestuff industrial effluents is one of the industrial wastewater which is difficult to deal with because of its high chroma, large quantity, complex composition. These dyestuff not only pollutes the ecological environment, but also does great harm to the human beings. Photocatalytic technology has emerged as one of the most promising technologies in energy and environment area, it represents an easy way to utilize the inexhaustible solar energy to eliminate the most organic matter to non-toxic CO2, H2O and other small molecule. Among those photocatalysts, ZnO and TiO2 are widely studied due to its high catalytic activity, security, non-toxic, long life and other advantages. Unfortunately, ZnO (Eg=3.3 eV) and TiO2 (Eg=3.2 eV) have low utilization of the total solar energy impinging on the surface of the earth due to its wide bandgap. In addition, the recombination of photo-electrons and holes on the surface of ZnO and TiO2 semiconductor are greatly reduced the photocatalytic activity of the catalyst. So in this thesis, we have modified the structure and morphology of ZnO semiconductor, synthesized the composites base on ZnO or TiO2 in order to improve the photocatalytic activity. The main contents and results are summarized as follows:ZnO nanorods with various ratio of length to diameter have been fabricated by using Zn(CH3COO)2·2H2O, LiOHH2O, Zn(NO3)2-6H2O, HMT as reactant and combining the facile sol-gel and hydrothermal methods. The obtained ZnO nanorods were investigated by using electron microscopy, X-ray diffraction spectroscopy, electron absorption spectrum and photoluminescence spectrum. The results show that the spectral properties of the ZnO nanorods can be adjusted by controlling the concentration of reactant. The photodegradation of methylene blue in the presence of ZnO has also been investigated in detail. It is found that the ZnO nanorods prepared in 12.0 mM reactant condition exhibit the optimum activity which is six times as large as the prepared ZnO seeds, and that the activity of the optimum ZnO can reserve above 80% still after 5 times repeated use.The quantum-sized zinc oxide-graphene oxide (ZnO-GO) hybrid has been prepared by using GO dispersed in the ethanol as carrier, zinc acetate dihydrate (Zn(CH3COO)2·2H2O) and lithium hydroxide monohydrate (LiOH-H2O) as reactant. Nano-scale hybrid material of ZnO-graphene (ZnO-rGO) is then obtained by thermally treating ZnO-GO at different temperatures. The results reveal that both sides of the graphene sheets are coated with ZnO particles, and that the lattice constants and the band gap energy of ZnO have been changed compared with the pure quantum-sized ZnO particles. The photodegradation of methylene blue has been measured detailedly in the presence of hybrid materials; and it is found that ZnO-rGO prepared at 200℃ containing 2.5% graphene exhibits the highest activity which is three times as large as that of pure ZnO. The experiments indicate that after the catalyst has been used 5 times repeatedly, the degradation rate reserves above 80% still compared with the first time although some ZnO particles have peeled off the graphene sheets.In order to exclude the influence of dye adsorption on photocatalytic efficiency, the nano-sized ZnO-graphene hybrid has been prepared through combining the facile sol-gel process and hydrothermal methods by using Zn(NO3)2·6H2O and hexamethylenetetramine (HMT) as growing reactants in the presence of ZnO-graphene oxide (ZnO-GO) seeds. The obtained products have been characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy and UV-vis absorption spectroscopy. The results show that the GO has been converted to reduced-graphene oxide during the hydrothermal process due to the released reductant by HMT. The photo-degradation of methylene blue in the presence of ZnO-graphene (excluding the influence of the dye adsorption on the catalyst) has also been investigated in detail. It is found that the preparation conditions have significant effects on photo-catalytic properties of the composites, and that ZnO-graphene prepared in the optimal conditions exhibits the optimum activity. This facile and low-cost method will make the composite a perfect candidate in applications of photo-catalysis and other areas.In order to improve the utilization of visible light, the composites of g-C3N4 and TiO2 (g-C3N4/TiO2) have been synthesized by directly heating melamine containing different amount of TiO2 in semi-closed stainless steel container. The obtained samples have been characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-visible diffuse reflectance spectroscopy (DRS), photoluminescence spectrum (PLS), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results reveal that the microstructures, DRS and PLS of the composites vary with the changes of the components. The XPS peaks of Ti3d in the composites have shifted to low binding energy compared with pure TiO2. The photodegradation properties of the composites have been evaluated in detail by using methylene blue (MB) as target under visible light irradiation (400-600 nm). The results indicate that the optimum composite of g-C3N4/TiO2 (the content of TiO2 in the composites is 26.4 and 80.0 wt.%) has a remarkable enhancement for MB photodegradation compared with pure g-C3N4 and TiO2. Moreover, the activity of the optimum composite can retain 91% of initial activity after four times of cyclic use. |
