The Investigation On The Structural Optimization And The Catalytic Performance Of Magnetic Photocatalysis

Semiconductor photocatalyst materials have advantages in the material field. Since magnetic semiconductor photocatalyst not only improves the environmental pollution but also greatly saves the energy from the angle of recycling and utilization energy, they has broad prospects for development. The research in this dissertation is to improve photocatalytic performance of magnetic photocatalyst towards optimizing structure, at the same test conditions, the photocatalytic efficiency of the prepared photocatalyst was the same or higher than P25. The dissertation also disscussed the factors that effected the photocatalytic ability and the relationships between them. It has the vital significance for the optimization of photocatalytic activity of photocatalyst. It mainly includes the following several aspects work:First, a facile approach for the fabrication of magnetic α-Fe2O3@TiO2 nanocomposites has been demonstrated. As a semiconductor which had visible light response, α-Fe2O3 was successfully introduced to the TiO2 photocatalyst.The obtained composites which doped proper amount of hematite expanded the visible light response of the photocatalyst, thus increasing the photocatalytic performance in the UV-vis region. In addition, since the self-built electric field formed in the heterojunction of α-Fe2O3 and TiO2, The formation of heterojunction self-built electric field was the key to improve the photocatalytic efficiency. Due to the formation of heterojunction, photogenerated charge can move quickly to participate in photocatalysis process without being recombination. Simple combination of α-Fe2O3 and TiO2 did not significantly improve the photocatalytic performance. It showed that the optimization for the photocatalytic efficiency was obvious by the formation of self-built electric field. And It was the mechanism of improving the photocatalytic performance after the combination of two kinds of semiconductors. The prepared magnetic α-Fe2O3@TiO2 composites not only had good photocatalytic ability, but also can be magnetic transfer.Second, a simple introduction of sintering TiO2(TiO2-S) to the fabrication of magnetic Fe3O4@TiO2 composites has been demonstrated. And the TiO2-S had high crystallinity which was prepared before hand, improving the photocatalytic efficiency of photocatalyst. The TiO2-S was added in the synthesis of magnetic Fe3O4@TiO2, namely, the hydrolytic TiO2(TiO2-H) which was synthesised by the hydrolysis process together with TiO2-S were compounded on the Fe3O4. In the whole process of photocataly, TiO2-H played a few important roles: first, because TiO2-H was with anatase structure, so it can be involved in the photocatalytic degradation reaction; Second, followed by the isolation role of the magnetite Fe3O4 and TiO2-S, decreasing the recombination rate of electrons and holes; Third, supporting the whole composite structure as a adhesive. Results showed that photocatalyst adding TiO2-S can obtain higher photocatalytic activity than that of without adding TiO2-S samples. It was in contrast to the previously report that the larger specific surface area can obtain higher photocatalytic performance. The crystal structure after adding TiO2-S promoted the effective separation of photogenerated electrons and holes. It has played a more important role than the specific surface area in the process of photocatalytic process. In addition, the prepared composites at room temperature shows excellent magnetic transfer properties.Third, hematite α-Fe2O3 photocatalyst which has visible light response has been prepared by a template method. The photocatalytic degradation for MO was investigated to evaluate the photcatalytic ability of the as-synthesized samples. Brunauer–Emmett–Teller(BET) measurements showed that the influence of the surface areas on the photocatalytic ability of the as-synthesized products was not obvious. The SAED and HRTEM tests demonstrated that sample D and E with twin crystal structure had higher photocatalytic ability. And the other samples inherited polycrystalline characteristics. It indicated that twin crystal structure played a more important role than the specific surface area in the process of photocatalytic process, and then confirming the improvement role that played by twin crystal. The fomation of twin crystal may lead to more active points exposed on the surface of photocatalyst. It was equivalent to provide more active points in the process of photocatalytic, eventually improved the photocatalytic activity of the photocatalyst. In addition, the use of weak magnetization of hematite α-Fe2O3 at room temperature can realize magnetic transfer. Fourth, magnetic photocatalytic Fe3O4@TiO2 composites have been fabricated by changing the concentration of(NH2)2CO in the prepare process. Samples were named low(NH2)2CO concentration group which the(NH2)2CO concentration in the synthesis process was below 2.25 mol/L and high(NH2)2CO concentration group which the(NH2)2CO concentration was above 2.5 mol/L. Photocatalytic degradation experiments of methyl orange showed that the final degradation rates of low(NH2)2CO concentration group samples were higher than that of high(NH2)2CO concentration group, even better than P25 at the same photocatalytic test conditions. It was interesting that samples of low(NH2)2CO concentration group had smaller values of BET surface areas than that of high(NH2)2CO concentration group. It indicated that the improvement of photocatalytic activity which was effected by BET surface areas was not obvious. The reasons that lead to the photocatalytic activity improvement of low(NH2)2CO concentration group samples may be two main factors: first, the diffuse reflection spectrum data showed that the low(NH2)2CO concentration group samples had lower reflectivity, this suggested that low(NH2)2CO concentration group samples structure improved the efficiency of light absorption; Second, The FT-IR test results showed that the samples of the low(NH2)2CO concentration group samples had less NH4+. NH4+ took up less active sites on the surface of the TiO2 particles, leading to the higher photocatalytic activity.