| 英文摘要 |
The acceleration of industrialization promotes the material civilization, which brings serious environmental problems at the same time. The discharge of organic pollutants makes the scarce fresh water resources face severe challenge, which becomes a threat to human health and development. Organic dyes is a widely used industrial materials, which caused water pollution by the improper emissions of organic dyses. Hence, all countries in the world were exploring the effective scientific plans to solve organic dyes in the water. The photocatalytic technology are drawing more and more attention due to the rapidness, efficiency and other advantages. The problems that hinder the rapid development and application of photocatalytic technology are mainly concentrated in the following three aspects: Firstly, most of the catalysts were driving by ultraviolet light, and utilization rate of the sunlight existing is low; Secondly, the existing one-component photocatalysts have some defects, such as: wide bandgap, high recombination rate of electronic (e⁻) and holes (h⁺) and low photocatalytic efficiency; Thirdly, it is difficult to recycle photocatalyst after contact reactions. This is mainly caused by the lack of a reasonable structure and functional integration advantages of photocatalyst materials. Hence, this study aimed at the research of the design, synthesis and application research of novel, highly efficient photocatalysts for degradation of dye with visible light. Based on the hybrid ferrites, this paper designed and constructed the NiFe₂O₄@TiO₂, NiFe₂O₄@TiO₂/RG0, ZnFe₂O₄/RG0 and ZnFe₂O₄/ZnO/Ag photocatalysts with high magnetic responsiveness using reasonable composite method. The effective separation method of e⁻ and h⁺ or reduced the bandgap of photocatalyst was explored. The influence of active substance loading and catalyst efficiency factors were studied systematically to reveal the law of influence. The meaning of the thesis is to prepare highly efficient photocatalyst and provide a method and theotetical foundation for the development of new photocatalyst materials. The main contents of this dissertation are described as follows: The one-step hydrothermal method was used to prepare hybridized M〓Fe〓Fe₂O₄ magnetic nanoparticles by adjusting the proportion of divalent metal cations, the reaction temperature and time. The effects of dosage of raw materials on the particle size and magnetic patterns have also been deeply discussed.The obtained M〓Fe〓Fe₂O₄ magnetic nanoparticles with adjustable range size in 300- 600 nm, have good hydrophilicity, dispersion and high magnetic responsiveness. All of the obtained products have spinel crystalline structure, small grain size and high crystallinity. In addition, the elemental contents were consistent with the target products.Among hybrid ferrites, ZnFe₂O₄ and NiFe₂O₄ have advantages of high yield, perfect morphology, large pore size, high specific surface area and saturated magnetization. Furthermore, ZnFe₂O₄ and NiFe₂O₄ semiconductors can be directly used as photocatalysts due to the narrow bandgap, which could absorb visible light. This study provided a base material of hybrid ferrite materials for modifying the traditional TiO₂ and ZnO photocatalyst. The most effective method to expand the visible light responsive of the photocatalyst is to change the semiconductor electronic structure to adjust the bandgap of the semiconductor. As we all know, compounding with narrow bandgap semiconductor is the easiest implement method. In order to solve low photocatalytic efficiency and recycle rate of the TiO₂ photocatalyst, a new type of composite NiFe₂O₄@TiO₂ photocatalyst has been successfully prepared through a simple hydrothermal and hydrolysis method.The bandgap of NiFe₂O₄@TiO₂ photocatalyst reduced to 1.5 eV due to narrow bandgap of NiFe₂O₄, and the composite photocatalyst extended utilization of sunlight from the ultraviolet to 600 nm. In the process of experiment, when the dosage of TBOT is 0.2 mL, 0.5 mL, TiO₂ can be evenly coated on the surface NiFe₂O₄ forming a modified layer. As TBOT usage increased to 0.7 mL or 1.0 mL, TiO₂ would been crystallized itself. In addition, by adjusting the pH value of preparation system. The higher saturation magnetization intensity of NiFe₂O₄@TiO₂ photocatalyst rods can be acquired in alkaline conditions. Photocatalytic experiment results showed that the NiFe₂O₄@TiO₂ nanorods had higher efficiency for photocatalytic degradation of RhB in visible light than NiFe₂O₄@TiO₂ nanoparticles. The development of NiFe₂O₄@TiO₂ photocatalyst has expanded the TiO₂ utilization of sunlight, but the recombination rate of e⁻ and h⁺ is high, so the photocatalytic efficiency of NiFe₂O₄@TiO₂ photocatalyst is always low. Based on the excellent photoelectric properties of reduced graphene oxide (RGO), this paper used the hydrothermal method, solvent thermal and hydrolysis method to synthesize the NiFe₂O₄@TiO₂/RG0 photocatalyst. Then, the electrochemical properties and its photocatalytic performance of RhB were studied. Compared with NiFe₂O₄@TiO₂, the introduction of graphene effectively regulated the bandgap of TiO₂. The bandgap of NiFe₂O₄@TiO₂/RGO-10% can be reduced to 1.3 eV and e⁻ and h⁺ can be separated effectively. The NiFe₂O₄@TiO₂/RGO photocatalyst had higher utilization rate of sunlight, and the catalytic efficiency increased accordingly.The obtained NiFe₂O₄@TiO₂/RGO-20% photocatalyst showed the strongest adsorption and photocatalytic degradation ability for RhB with k=0.11633 min⁻¹. At the same time, the 2D mesh structure of graphene materials provided a lot of active site, which made the dispersion of NiFe₂O₄@TiO₂ greatly improved. Compared with NiFe₂O₄, ZnFe₂O₄ has higher pore volume, surface area, and stronger dye adsorption ability. Moreover, the bandgap of ZnFe₂O₄ is 1.9 eV, which makes the visible light be absorbed directly. However, ZnFe₂O₄ photocatalyst alone has low catalytic activity and high recombination rate of e⁻ and h⁺, in addition, it is easy to aggregate. Hence, modification is an effective way to broaden the ZnFe₂O₄ application in the field of photocatalysis. In this paper we used the structure characteristics and conductive performance of graphene combined with ZnFe₂O₄ to synthesize composite photocatalyst. In the obtained ZnFe₂O₄/RG0 photocatalysts, zinc element is +2 valence, Fe element is +3 valence, and the structure of ZnFe₂O₄ is cube. The aggregation problem of ZnFe₂O₄ can been solved when GO adding amount is more than 20%. Meanwhile, the adsorption capacity of materials was enhanced. In addition, the development of composite photocatalyst is helpful for the rapid charge migration on the ZnFe₂O₄ surface, and eventually leads to the high photocatalytic efficiency. Photocatalytic degradation of RhB experiments confirmed that after 60 min the decolourization ratio reached to 98% in ZnFe₂O₄/RGO-40% and ZnFe₂O₄/RGO-20% systems with H₂O₂. ZnFe₂O₄/RGO-40% exhibited the highest photocatalytic activity of the degradation of RhB, (k=0.6254 min⁻¹). This suggested that modifying the catalyst ZnFe₂O₄ nanoparticles with graphene could effectively promote the photocatalytic performance. This paper used two semiconductors with special bandgap to construct ZnFe₂O₄/Zn0 heterojunction, impored the photocatalytic activity of ZnFe₂O₄ and broaden absorption wavelength scope of ZnO. The depletion layer between ZnFe₂O₄/ZnO heterojunction can tackle the returns of e⁻ to the VB of ZnFe₂O₄. The migration rate is accelerated of e⁻ and h⁺ due to the plasma synergy effect of elemental Ag, restricted the e⁻ and h⁺ recombination, thus effectively improve the photocatalytic efficiency. In synthesis of ZnFe₂O₄/ZnO/Ag heterostructure, ZnFe₂O₄ is spinel and ZnO is the six-party wurtzite. The lattice fringe and interface are obvious. The best morphology of heterojunction interface is obtained when 10% ZnO has been modified in the surface of ZnFe₂O₄ In the meantime, the saturation magnetization intensity of ZnFe₂O₄/ZnO-10% heterogeneous is highest and degradation is the most favorable. Photocatalytic experiment confirmed that ZnFe₂O₄/ZnO/Ag-10% heterogeneous adsorption capacity of RhB can reach 12.5 ㎎/g. The fitting results of the kinetics of langmuir-Hinshelword( L-H ) equation shows that the value of k is 0.01824 min⁻¹. The recycle experiments confirmed that the photodegradation rate of RhB can reached to 85% after five cycles. Keywords: photocatalyst, heterojunction, nanocomposite, hydrothermal reacation, hybridized ferrite
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