Controllable Synthesis And Properties Of Sn₃O₄-based Photocatalytic Materials

At present, energy crisis and environmental issues are two serious issues encountered by mankind. Semiconductor photocatalytic technology is regarded as one of the effective solutions toward the predicaments. However, the conventional photocatalysts, such as TiO2, ZnO, SnO2, etc., possessing wide bandgaps can only absorb UV light, which thus greatly restricts the utilization of solar energy. Meanwhile, this technology meets with high recombination of photoinduced carriers and low photocatalytic quantum yield as well, which limits the wide practical applications. Up to now, to explore the efficient photocatalysts with visible-light response has become one of the frontier scientific explorations in the photocatalysis field.In this thesis, controllable synthesis and properties of heterovalent-tin-based photocatalysts were studied in detail. Solid and hollow heterovalent-tin (Sn3O4) nanostructures were prepared by a facile template-free solvothermal method by modulating the pH and components of the solution. A new type of Sn3O4/TiO2 heterojunctions was designed to achieve high charge separation efficiency. Characterizations were carried out by using XRD, TEM, SEM, N2 adsorption-desorption, XPS, UV-Vis DRS, ESR, and LC-MS to investigate the composition, structures, morphologies, surface states, optical response, surface active species, and generated byproducts in photocatalytic reactions. The photocatalytic activity of the as-prepared catalysts was carried out upon the degradation of azo dyes and benzene, and the selective reduction of p-nitroaniline. Photoelectrochemical measurement and electron spin resonance technique were taken into account to explore the proposed photocatalytic mechanisms.The results are shown as follows:(1) Sn3O4 with hierarchical 3-D nanostructures was synthesized by a facile template-free solvothermal method. The photocatalytic activity of the catalysts was performed by the degradation of methyl orange (MO) and 4-phenylazophenol. It indicates that the Sn3O4 displays an enhancement in the decomposition of MO or 4-phenylazophenol over N-doped TiO2. (2) Self-assembled Sn3O4 hollow spheres were prepared on the conditions that the pH, volume ratio of water/ethanol and reaction time of solvothermal treatment were 3,2:1 and 12 h, respectively. It shows that the photoactivity of selective reduction of p-nitroaniline of the hollow sphere is ca.2 times higher than that of the solid sphere visible light irradiation. (3) Sn3O4/TiO2 heterojunctions were designed and synthesized to obtain the efficient utilization of solar energy and the high photocatalytic quantum yield. The effect of the preparation conditions on the photocatalytic performance of the as-prepared Sn3O4/TiO2 heteroj unctions was discussed in detail. The result shows that the Sn3O4/TiO2 reaches its maximum activity on the conditions of the Sn3O4 content of 5 wt% and the heat-treatment process of 300℃,3 h. Under visible light irradiation, the conversion rate and mineraliztion rate of benzene on the composite is found to be 50% and 55%, respectively, which are correspondingly ca.2 times higher than that of nitrogen-doped TiO2. Notably, Sn3O4/TiO2 also displays higher photocatalytic activity than P25 under simulated solar light irradiation. Under the simulated solar light irradiation, the conversion rate and mineraliztion rate of benzene on the composite the benzene conversion reach 92% and 91%, respectively. More importantly, no distinct deactivation of the heteroj unction could be detected even though the reaction time is prolonged to 24 h, suggesting the high photocatalytic stability of the catalyst.The characteristics and the innovation of this thesis were presented as follows. (1) It is the first time that heterovalent-tin compound has been applied in photocatalysis field while the related photocatalytic mechanisms are also discussed. This work may provide a new insight into the heterovalent-semiconductor photocatalysis and motivate the further exploration of other high efficient heterovalent photocatalysts for practical applications. (2) Hierarchical Sn3O4 hollow spheres have been successfully synthesized via a facile template-free solvothermal process for the first time. The hierarchical Sn3O4 hollow spheres exhibit high photocatalytic activity in the hydrogenation of aromatic nitro compounds under visible light irradiation. (3) Sn3O4/TiO2 heterojunctions has been designed and built up which displays high photocatalytic activity and stability towards the oxidation of gaseous benzene. This work may provide an important strategy to explore and design high efficient visible-light-induced photocatalysts, fully utilizing solar energy and achieving high quantum yield in photocatalytic process.