Design Of ZnIn₂S₄-based Photocatalytic Materials And Study On The Degradation Performance Of Organic Pollutants

With the rapid development of science and technology,it has achieved steady economic growth,while inevitably causing serious environmental problems.The organic wastewater in various industries（including dyes,antibiotics,etc.）is directly discharged into rivers and lakes,causing serious water pollution problems.The increasing water pollution causes great damage to the living environment of animals and plants.It is urgent to explore clean and efficient wastewater treatment methods to provide a green living environment for human beings.As an advanced oxidation technology,solar-driven semiconductor photocatalysis technology is considered as one of the most economical and effective ways to treat organic pollutants due to its high efficiency,environmental protection and low energy consumption.Among many photocatalysts,indium zinc sulfide（Zn In₂S₄）is considered as an ideal photocatalyst due to its suitable band gap and good light absorption performance.However,like most other photocatalysts,Zn In₂S₄（ZIS）still has some deficiencies,such as high recombination rate of photogenerated carriers,low sunlight utilization rate and weak redox capacity of materials.In view of the current shortcomings of Zn In₂S₄,the strategies of heterostructure construction,carbonized polymer dots（CPDs）composite and defect regulation are adopted to enhance the optical absorption performance and photogenerated carrier transfer and separation efficiency of materials to effectively improve the photocatalytic degradation performance of materials in this paper.The effects of modification strategies on the composition,morphology,surface structure,photoelectric properties and photocatalytic properties of materials were studied through a series of characterization,and the mechanism of photocatalytic degradation pollutants was clarified.The specific research contents are as follows:Firstly,the direct Z-scheme Zn In₂S₄/Bi VO₄ heterojunction is constructed via in situ growth of 2D ultrathin Zn In₂S₄ nanosheets on 3D Bi VO₄（BVO）surface.Synchronous illumination X-ray photoelectron spectroscopy and theoretical calculation confirm that the transmission mode of electrons follows the direct Z-scheme heterostructure.The construction of the direct Z-scheme heterojunction not only promotes the separation of photogenerated carriers,but also retains the strong redox ability of the material.All the Zn In₂S₄/Bi VO₄ composites present enhanced photocatalytic degradation performance of Tetracycline（TC）.Among them,ZIS/BVO-0.10 can degrade 60%TC after 180 min light radiation and the degradation rate is 1.8times and 4.4 times than that of Zn In₂S₄ and Bi VO₄,respectively.The free radical trapping experiment and electron spin resonance（ESR）spectroscopy prove that the active species produced in the degradation process were superoxide radicals（·O₂^-）,hydroxy radicals（·OH）and holes（h⁺）.In addition,the mechanism of photocatalytic degradation is analyzed in detail according to the energy band position of the material.Secondly,3D ultra-thin Zn In₂S₄ composites modified with 0D CPDs（CPDs/Zn In₂S₄）are constructed.Based on the previous work,this work simplifies the synthesis step,further raises the separation efficiency of carriers,and the photocatalytic performance of the materials is greatly improved.The electron microscope images,X-ray photoelectron spectroscopy（XPS）,Raman spectroscopy and Fourier transform infrared spectroscopy（FT-IR）of CPDs/Zn In₂S₄ materials confirm that CPDs are successfully introduced into the surface of Zn In₂S₄ nanosheets.The photoelectrochemical tests show that CPDs,as transmission medium and electron acceptor,can effectively enrich electrons and further strengthen the separation of photogenerated electrons and holes.The degradation experiment shows that all the composites exhibit enhanced photocatalytic degradation activity,in which the optimized 7 wt%CPDs/ZIS composites can degrade 97%Rhodamine B（Rh B）after180 min light radiation and the degradation rate is about 2.1 times than that of Zn In₂S₄monomer.At the same time,the removal rate of 2-Mercaptobenzothiazole（MBT）by 7wt%CPDs/ZIS reach 74%and its degradation rate was 2.3 times than that of Zn In₂S₄monomer.The intermediate products in the process of degradation are determined by liquid chromatography-mass spectrometry（LC-MS）and the possible degradation pathway is inferred.The active species produced during the reaction are determined by ESR spectrum and free radical trapping experiment and the mechanism of degradation is given.Finally,2D ultrathin Zn In₂S₄ nanosheet materials with different surface S-vacancy concentrations are prepared by low-temperature heat treatment.This work further simplifies the synthesis raw materials and synthesis steps,and greatly enhance the photocatalytic degradation activity of the material.Electron paramagnetic resonance（EPR）spectra prove that there are abundant S vacancies on the surface of ultra-thin Zn In₂S₄ nanosheets.As electron traps,the S vacancy on the surface of the material can quickly capture electrons,effectively inhibiting the recombination of photogenerated electrons and holes.The results of degradation experiments show that the ZIS-100 can remove 99%Rh B after 30 min light radiation and 97%MBT after 180 minutes light irradiation.The experimental results show that constructing surface defects can effectively strengthen the photogenerated carrier separation and increase the surface active site,thus greatly improving the efficiency of photocatalytic degradation of pollutants.High-resolution liquid chromatography-mass spectrometry（HPLC-MS）is used to detect the small molecules produced in the degradation process in order to speculate the possible degradation pathway.The results of ESR and free radical trapping experiments show that·O₂^-,h⁺and singlet oxygen（¹O₂）play important roles in the degradation process.Finnally,the mechanism of photocatalytic degradation of ultrathin Zn In₂S₄ materials with abundant S vacancy is described in detail.