Preparation And Nitrogen Fixing Performance Of ZnIn₂S₄-based Photocatalyst

Ammonia（NH₃）is currently produced by the Haber Bosch process as a green energy carrier,a potential fuel and an essential chemical for the manufacture of fertilizers,plastics and explosives.Recently,photocatalytic nitrogen fixation reaction has attracted much attention due to its environmental friendliness and cost effectiveness.In order to achieve high photocatalytic nitrogen fixation performance and selectivity,it is necessary to rationally design photocatalysts to optimize light absorption,photo-generated charge separation,N₂ adsorption and activation.ZnIn₂S₄ is a ternary sulfide with properties such as suitable band gap corresponding to visible light absorption and good stability,which is a potential photocatalyst.However,the low photocatalytic activity embodied by pure ZnIn₂S₄ limits its wide application due to the low separation efficiency and migration ability of photoexcited charge carriers.Thus,effective measures to solve these problems are the main research objectives of this thesis.Metal-organic backbone（MOF）is a novel organic-inorganic hybrid material that has attracted much attention in the fields of separation,chemical sensors,gas storage,drug delivery due to its highly ordered pore structure,large specific surface area.Combining MOF with semiconductor materials to form composites can enhance the absorption of light and promote photogenerated carrier separation,which is a feasible way to improve photocatalytic activity and provides a good idea to enhance the photocatalytic performance of semiconductors.In this paper,the photocatalytic activity of ZnIn₂S₄ is enhanced by a new photocatalytic material formed by the composite of ZnIn₂S₄ and MOF.The details of the study and the main conclusions are as follows:（1）Preparation of ZnIn₂S₄ and its optimization.The hydrothermal method was used to synthesize ZnIn₂S₄.The best synthesis method of ZnIn₂S₄ was determined based on the experimental data by comparing different types of zinc sources（ZIF-8,ZIF-L,Zn SO₄ and Zn（CH₃COO）₂）and regulating the hydrothermal time（2 h,3 h,4 h and 5h）.The experimental results showed that the ZnIn₂S₄ synthesized with zinc acetate as the zinc source,hydrothermal temperature of 180℃and hydrothermal heat for 4 h had the best growth morphology,the best photocurrent,the largest absorbable light range,the best nitrogen fixation efficiency and the most outstanding photocatalytic performance.The size of the synthesized ZnIn₂S₄ nanosheets was about 50 nm,the photocurrent density was 0.20 mA·cm^-2,and the nitrogen fixation rate reached 1.78μg·L^-1·h^-1,and after several cycles,the nitrogen fixation performance of ZnIn₂S₄ did not decrease significantly and had good stability of nitrogen fixation.（2）Preparation of ZnIn₂S₄/MIL-125（Ti）composite and its photocatalytic performance.The ZnIn₂S₄/MIL-125（Ti）composites were prepared by hydrothermal method.It was found that the performance of the composites did not show a linear increase with the increase of MIL-125（Ti）loading,and the excessive MIL-125（Ti）particle loading reduced the photocatalytic performance of ZnIn₂S₄.The composite prepared with the addition of 0.8 g MIL-125（Ti）had the best photocatalytic performance,at which the photocurrent density was 0.48 mA·cm^-2 and the nitrogen fixation rate was 3.92μg·L^-1·h^-1.The photocurrent density was more than twice that of pure ZnIn₂S₄,and the nitrogen fixation rate was 2.2 times that of pure ZnIn₂S₄.This excellent photocatalytic performance was attributed to the addition of MIL-125（Ti）which could effectively inhibit the rapid complexation of electron-hole pairs and thus improve the photocatalytic activity of ZnIn₂S₄.Meanwhile,after several cycles of nitrogen fixation,the MIL-125（Ti）particles on the ZnIn₂S₄ nanosheets did not show significant reduction and exhibited good chemical stability.