Preparation And Adsorption-photocatalytic Performance Of Direct Z-scheme Hierarchical MoS₂/MnO₂

Semiconductor-based heterogeneous photocatalytic oxidation technology is an ideal and promising technology to remove organic pollutants from water.It is great significance to develop photocatalytic materials with high adsorption performance and a wide range of light response.In this paper,molybdenum disulfide（MoS₂）with the hollow tubular structure formed by self-assembly of nanosheets and manganese dioxide（MnO₂）nanosheets were used as precursors,and MoS₂/MnO₂ composite material with the direct Z-scheme hierarchical structure was prepared by freeze-drying without template.A series of test methods were used to characterize the phase composition,micro-morphology,specific surface area,surface chemical states and light absorption performance of the materials.The results show that the presence of O-doping in the defect-forming MoS₂ lattice broadens the photoresponse range and prolongs the lifetime of photogenerated carriers.When the mass ratio of MoS₂/MnO₂ is 3:1,the as-prepared composite material formed a dendritic-like hierarchical structure in which small-sized MnO₂ nanosheets were interwoven on the hollow tubular MoS₂.MoS₂ and MnO₂ were tightly combined and uniformly distributed in the system,and there were electronic interactions and strong heterostructure effects between them.Compared with pristine MoS₂ and MnO₂,the photoresponse range of the composite material was further expanded,indicating that there was a potential photocatalytic performance in the entire spectral region.Taking rhodamine B（RhB）as the target pollutant,the effects of mass ratio of MoS₂/MnO₂and pretreatment method on the adsorption-photocatalytic performance of the material were explored under visible light.The results show that when the mass ratio of MoS₂/MnO₂ is 3:1,the as-prepared composite material exhibited the highest visible light catalytic activity,the removal rate of RhB reached 76.2%within 120 minutes,and the photocatalytic apparent rate constant was 2.03 times and 2.95 times that of pristine MoS₂ and MnO₂,respectively.By improving the pretreatment method,it was found that the adsorption performance of the composite materials prepared by MnO₂ that was not dried after hydrothermal preparation and subjected to 3 h ultrasonic treatment before freeze-drying was greatly improved,the adsorption-photocatalytic removal rate of RhB reached 94.5%in 120 minutes.At the same time,the FT-IR test shows that the residual amount of RhB on the surface of the composite was significantly reduced.When the photocatalytic reaction time was extended to 240 minutes,RhB on the surface of the materials was almost completely degraded.MoS₂/MnO₂ also showed excellent adsorption-photocatalytic performance for RhB under UV irradiation.And the material also showed good adsorption-photocatalytic activity to the colorless pollutant tetracycline（TC）.After three repeated uses under visible light,the removal rate of RhB still reached 91.1%and remained good stability after three times of repeated use.The possible adsorption-photocatalytic mechanism of MoS₂/MnO₂ composite material was studied by PL,EIS and ESR test technology.The unique hierarchical structure of the composite and the radicals that can be generated under dark condition greatly enhanced the adsorption performance.The formation of direct Z-scheme heterojunction between MoS₂ and MnO₂effectively inhibited the recombination of photogenerated e^--h⁺pairs.Organic pollutants were effectively degraded under the collective action of the three active species of·OH,·O₂^-and h⁺.