Preparation And Photocatalytic Performance Of ZnO-SnO₂ Nanocomposite Catalysts

In recent years,due to the rapid development of economy and the relative lag of wastewater treatment,the water pollution is serious in our country.Printing and dyeing wastewater is one of the most difficult industrial runoff and it is an urgent problem to be solved.Photocatalytic technology is an economical and feasible method for the degradation of dye wastewater and thus widely used to deal with the organic pollutants.As a new multi-functional semiconductor material,ZnO is considered as the most promising photocatalyst.However,the light absorption range of ZnO is narrow because of its wide band gap energy.Also,the electrons and holes generated by ZnO easily recombine.The above two points inhibit the photocatalytic activity of the catalyst.Therefore,how to improve the photocatalytic property of ZnO has become a new research focus.In this paper,ZnO-SnO₂nanocomposites were prepared by a simple coprecipitation method.The photocatalytic activity of ZnO-SnO₂ was studied using methylene blue?MB?as a simulated organic pollutant.The degradation mechanism of MB was also explored in detail.The main contents of this paper are as follows:?1?ZnO-SnO₂ nanocomposites with different compositions were prepared by a coprecipitation method using ZnCl₂,SnCl₄·5H₂O and NH₄OH as raw materials.The structure,morphology and optical properties of the samples were characterized by XRD,SEM,TEM,HRTEM,EDS,BET,PL and UV-vis spectra.Results showed that the products were generally in granular shape.The particle size decreased obviously and the surface area gradually increased with the increasing content of SnO₂ in the samples.The synthesized ZnO-SnO₂ composites consist of hexagonal wurtzite ZnO and tetragonal rutile SnO₂.Compared with pure Zn O,the fluorescence intensity of the ZnO-SnO₂ samples weakened,indicating that the recombination of photogenerated carriers was inhibited,that is,the separation of photogenerated electrons and holes was promoted.With the increasing content of SnO₂ in the sample,the band gap energy of the samples increased first and then decreased.?2?The prepared ZnO-SnO₂ samples were used as catalysts to study the photocatalytic degradation of MB under different conditions.Results showed that the composite samples exhibited higher photocatalytic activity than pure ZnO.Among the samples,ZS_0.25 showed the best catalytic performance;and the photodegradation efficiency of MB with it reached96.53%after 60 min of UV irradiation,which was nearly 30%higher than that with pure ZnO.After three cycling runs,ZS_0.25 still remained more than 85%catalytic activity towards MB.Thus,ZnO-SnO₂ nanocomposites are a kind of efficient and stable photocatalyst.?3?The photocatalytic reaction mechanism of ZnO-SnO₂ was analyzed.The formation of a heterojunction between ZnO and SnO₂ effectively separated the photogenerated electrons and holes,and thus greatly improved the photocatalytic activity of the composites.During the degradation of MB with the catalyst ZS_0.25,the intermediate products at different stages were tested by UV-Vis,TOC,FT-IR,HPLC and ESI-MS.Ultimately a possible degradation mechanism of MB was proposed.