Design And Synthesis Of Two-dimensional Heterojunctions Based On Sulfide And Their Photocatalytic Properties

With the rapid increase in the consumption of fossil energy such as coal,oil and natural gas,the problems of energy shortage and greenhouse effect become increasingly serious.In response to these challenges,scientists have invested considerable efforts in the development of new renewable energy sources and the conversion of greenhouse gases,especially the photocatalytic decomposition of aquatic hydrogen and the reduction of carbon dioxide using solar energy.And they have made many important research advances.In semiconductor materials,transition metal sulfide is an ideal photocatalytic material because of its strong visible light absorption ability and suitable energy band potential.In addition,compared with other semiconductor nanostructures,two-dimensional semiconductor nanosheets have the advantages of large specific surface area,short photogenerated charge migration distance,and multiple surface catalytic active sites,which has become a hot material for photocatalysis research.However,the photocatalytic performance of a single transition metal sulfide semiconductor is significantly reduced due to the high carrier recombination rate and photocorrosion.In order to overcome these defects of transition metal sulfide semiconductors,the construction of semiconductor heterojunction is an effective solution,because the internal electric field in the semiconductor heterojunction can drive the photogenerated electrons and holes to move in the opposite direction to the heterojunction interface,thus significantly improving the charge separation efficiency and stability of the photocatalyst.In addition,recent studies have shown that defect engineering,especially the design of vacancy defects（e.g.,sulfur vacancy and zinc vacancy）,can improve the activity of photocatalysts by facilitating charge separation.Therefore,reasonable design of transition metal sulfide semiconductor nanosheet heterojunction with vacancy defects is expected to achieve efficient photocatalytic reactions.Based on the above research assumptions,this paper takes transition metal sulfide（CdS,CoS_x,ZnIn₂S₄,etc.）semiconductors as the research object,and designs and syntheses a series of porous assembled nanosheets and nano-flower photocatalysts with S,Znvacancies and Z or p-n semiconductor heterojunction.These novel photocatalysts show excellent photocatalytic performance in the reduction reaction of aquatic hydrogen and carbon dioxide.The main findings are summarized as follows:（1）Unique ZnS-CdS-CoS_x porous Reuleaux triangle nanosheets with intimate Z-scheme hetero-interface and S,Znvacancies were produced by using Zn₂Co₃（OH）₁₀·2H₂O as the bimetallic precursor for subsequent sulfurization and Cd²⁺-ex-change reaction.Noticeably,such a novel structure shows desirable features which suggest a promising photocatalyst for visible-light photocatalytic H₂ evolution,including superior charge-separating capability enabled by the Z-scheme charge transfer and synergetic charge-trapping effect of S,Znvacancies,excellent light-harvesting capacity,abundant S₂^2-species as H₂-evolving active sites,a large surface area,and good stability without obvious decline in activity after multiple cycling and long-term tests.The ZnS-CdS-CoS_x heterojunction exhibits an outstanding visible-light-driven H₂-evolving activity as high as 53.43 mmol·g^-1·h^-1,corresponding to a high apparent quantum efficiency of 30.8%at 420 nm,far better than that of Pt-loaded CdS and a great deal of CdS-based photocatalysts reported in the literatures.（2）Unique CdS/ZnS-Ni（OH）₂ nanoflower-structured photocatalyst was prepared by solvothermal method combined with light deposition reaction.Under the irradiation of visible light（λ>400 nm）,CdS/ZnS-Ni（OH）₂ complex shows excellent performance of hydrogen generation by photocatalytic splitting water,and the maximum hydrogen production rate can reach 36.38 mmol·g^-1·h^-1.The apparent quantum efficiency（AQE）of hydrogen production at 400 nm and 420 nm is 36.5%and 31.9%,respectively.The hydrogen production activity of CdS/ZnS-Ni（OH）₂ is significantly higher than that of Pt-loaded CdS and a great deal of CdS-based photocatalysts reported in the literatures.Moreover,CdS/ZnS-Ni（OH）₂ has good photocatalytic stability.The excellent photocatalytic performance of CdS/ZnS-Ni（OH）₂ composites can be attributed to the following three aspects:the nanoflower structure enhances the light capture efficiency through multiple reflection and scattering processes;The larger specific surface area accelerates the kinetics of the surface reaction.The synergistic action of p-n heterojunction and S and Znvacancy results in an efficient photogenic charge separation process.（3）First,ZnIn₂S₄ nanoflowers with rich sulfur vacancy were synthesized by solvothermal method assisted by glycol.Then ZnS nanoparticles with zinc vacancy were grown under solvothermal conditions to obtain ZnIn₂S₄/ZnS nanocomposite photocatalyst.It was found that the loading of ZnS nanoparticles enhanced the light absorption capacity of ZnIn₂S₄.And defects in ZnS and ZnIn₂S₄ provide binding sites for their tight coupling.The photogenerated electrons of ZnIn₂S₄ can be captured by sulfur vacancies,while the photogenerated holes can be transferred to ZnS via zinc vacancies.Therefore,effective spatial separation of photogenerated carrier is achieved.Therefore,under visible light irradiation（λ>400 nm）,ZnIn₂S₄/ZnS composite exhibits good photocatalytic carbon dioxide reduction activity in gas phase.The CO production rate of ZnIn₂S₄/ZnS complex（ZnIn₂S₄/20%ZnS）loaded with 20 wt.%ZnS was up to 20.87μmol·g^-1·h^-1,which was 22.9 times and 4.5 times higher than that of pure ZnS and pure ZnIn₂S₄.