Photogenerated Carriers Transport Modulation And Photocatalytic Hydrogen Evolution Performance Of Transition Group Bimetallic Sulfide Semiconductor Materials

The extraction and use of large amounts of non-renewable resources such as fossil energy has caused serious damage and pollution to the earth’s environment,and hydrogen energy is an excellent alternative as a clean energy source with a high calorific value.Solar energy can be converted into hydrogen by splitting water through solid photocatalysts at room temperature and pressure,which is a simple and environmentally friendly way to produce hydrogen.In this paper,Ni Co₂S₄ and Mn_0.2Cd_0.8S,a transition group bimetallic sulphide,were successfully prepared,and composite photocatalysts with excellent hydrogen evolution performance were prepared by different methods.The Ni Co₂S₄/WS₂photocatalysts with core-shell structure were prepared based on Ni Co₂S₄ by in situ hydrothermal method,and the excellent photocatalytic splitting of water for hydrogen evolution was achieved through the simultaneous modulation of surface and photogenerated carriers.The type-II and S-scheme heterostructures were constructed based on Mn_0.2Cd_0.8S semiconductor materials coupled with metal oxides and layered bimetallic hydroxides.By constructing additional photogenerated carrier migration paths,an effective separation of photogenerated carriers at the heterogeneous interface was achieved,and efficient photocatalytic hydrogen evolution performance was realized.（1）Ni Co₂S₄/WS₂ photocatalysts with a core-shell heterogeneous structure were prepared by an in situ hydrothermal method.The synergistic effect of the core-shell enables larger specific surface area of the composite photocatalyst compared to that of a single photocatalyst,providing abundant redox active sites in the photocatalytic hydrogen evolution reaction.The transfer of photogenerated carriers is presumed to be consistent with the transport path of S-scheme heterojunctions by the variation of charge density in the composite photocatalyst.Eosin Y acts as a photosensitizer providing abundant photogenerated electrons to Ni Co₂S₄.The photogenerated electrons enriched in the Ni Co₂S₄ shell layer participate in the hydrogen evolution reaction at the surface of the core-shell structure,improving the photocatalytic performance of the composite photocatalyst.The coupled core-shell photocatalysts showed significantly improved photogenerated carrier separation efficiency and transfer rate,and effectively enhanced photocatalytic hydrogen evolution activity（about 3.35 times that of WS₂）and stability compared to Ni Co₂S₄ and WS₂.（2）Nanorods of Mn_0.2Cd_0.8S and double hexagonal prism Zn O are hydrothermally coupled to form a Mn_0.2Cd_0.8S/Zn O composite photocatalyst with a tight contact interface.The coupling of the composite photocatalysts shows a tight heterojunction contact interface that increases the specific surface area by breaking the agglomeration phenomenon with each other,providing an abundance of redox active sites.the introduction of Mn_0.2Cd_0.8S enhances the photoresponse performance of the photocatalysts to visible light,providing a richer concentration of photogenerated carriers for the hydrogen ion reduction reaction.The migration paths of photogenerated carriers in the photocatalysts were inferred to follow the carrier transport mechanism of Type-II heterojunctions by density flooding theory results,achieving effective separation of photogenerated carriers in space.The composite photocatalyst showed excellent photocatalytic hydrogen evolution performance(about 9.36 times that of Mn_0.2Cd_0.8S)and cycling stability during the evaluation of hydrogen evolution performance.（3）Ni Co-LDH with a layered structure was coupled with Mn_0.2Cd_0.8S nanorods by electrostatic self-assembly to form tightly heterojunctional photocatalysts.The bundle-like structure formed by the stacked nanorods and the tightly packed lamellar structure were electrostatically self-assembled to break the agglomeration state of each other and more redox active sites were exposed.The introduction of Ni Co-LDH broadens the light absorption range of Mn_0.2Cd_0.8S towards the visible wavelength range,effectively enhancing the photoresponse performance of the composite photocatalyst.The experimental results show that the charge density in Ni Co-LDH will be increased after coupling,and the transport direction of photogenerated carriers inside the photocatalyst is in accordance with the migration path of S-scheme heterojunctions.The effective separation of photogenerated carriers in space is achieved,preserving the redox-competent conduction band and valence band potentials.The reduced charge transfer resistance and lower hydrogen evolution overpotential facilitate the hydrogen evolution reaction,and the optimised composite photocatalyst has significantly improved hydrogen evolution performance(approximately 28.17 times that of Mn_0.2Cd_0.8S),and has excellent hydrogen evolution stability.（4）Nanoflower spherical Ni Al-LDH formed from nanosheets by stereocross stacking was prepared using a hydrothermal method,and Mn_0.2Cd_0.8S was anchored on the Ni Al-LDH surface to form a heterojunction composite photocatalyst in close contact.The homogeneous distribution of Mn_0.2Cd_0.8S on the surface of Ni Al-LDH breaks the agglomeration of Mn_0.2Cd_0.8S itself which tends to accumulate in bundles,while Ni Al-LDH as a carrier retains an abundance of redox sites.The introduction of Ni Al-LDH has led to the emergence of new absorption peaks in the visible light range of the photocatalyst,which upon photoexcitation produce a greater abundance of photogenerated carriers to participate in the hydrogen evolution reaction.The photogenerated carriers are effectively separated in space through the Type-II transport path between the heterogeneous interfaces of the composite photocatalyst Mn_0.2Cd_0.8S/Ni Al-LDH,with electrons and holes enriched in Ni Co-LDH and Mn_0.2Cd_0.8S,respectively,suppressing the rapid complexation of the single photocatalyst itself.The decrease in charge transfer resistance and the lowering of the hydrogen evolution overpotential are also important contributors to the increased hydrogen evolution activity of the composite photocatalyst,resulting in a significant enhancement of the composite photocatalyst’s hydrogen evolution performance(approximately 22.06 times that of Mn_0.2Cd_0.8S).