The Study On The Fabrication Of Transition Metal Selenide/Cadmium Sulfide Based Schottky Heterojunctions And Their High Performance Of Photocatalytic Hydrogen Evolution

In recent years,the global energy crisis and environmental pollution are increasingly seriously hindering the further development of human society.The conversion of solar-to-hydrogen from water splitting with the aid of semiconduction-based photocatalysts is deemed to one of the sustainable and clean techniques to solve the two issues.Despite the remarkable advances since the emergence of photocatalytic technology,the application of the semiconductor-based photocatalysts is siginificantly limited.One of the key challenges for the application of photocatalytic hydrogen（H₂）is the development of efficient photocatalysts,which one should have appropriate band gap and band structure to meet the kinetic and thermodynamic requirements of water splitting for H₂ evolution at the same time.So far,a variety of semiconductor photocatalysts with visible light response have been developed and used in the photocatalytic H₂ evolution.Among them,cadmium sulfide（CdS）is considered to be an attractive photocatalyst because of its narrow band gap and suitable band edge position.However,pure CdS semiconductor photocatalyst is limited by the low separation efficiency of photogenerated electron-hole pairs and serious photocorrosion,leading it is difficult to obtain ideal H₂ evolution efficiency from water decomposition.It is said that loading suitable co-catalyst on the surface of CdS semiconductors is one of the ideal strategies to further improve the H₂ evolution rate.This because it can not only effectively promote the separation of photogenerated charge and increase the carrier transport rate,but also provide a large number of catalytic active sites to accelerate the surface catalytic reaction,so that the catalyst can obtain strong H₂ evolution activity.In this paper,the research status of co-catalysts commonly used to enhance the photocatalytic decomposition of water to H₂ production by CdS-based semiconductors is analyzed systematically.Using elemental selenium（Se）modified CdS nanowires as carrier catalysts,a series of transition metal（Ni、Co、Cu）selenide co-catalysts were designed and synthesized,which significantly improved the light energy conversion efficiency of co-catalysts/CdS_0.95Se_0.05 photocatalytic materials.The internal mechanism of the enhancement of H₂ production performance of heterojunction catalysts was analyzed by density functional theory（DFT）calculations.The specific research content of this thesis is organized as follows:Firstly,Surface-modification of CdS nanowires by doping Se elementsIn this part,CdS nanowires were synthesized by a one-step solvothermal method,and then Se-doped CdS catalysts were successfully prepared by a simple ion exchange method.The changes of phase structure,optical properties and H₂ evolution activity of CdS-based catalysts before and after Se doping were studied systematically.The results show that the ion exchange method does not have a great effect on the phase structure and morphology of CdS nanowires.It can be seen from the steady-state fluorescence（PL）spectra,time-resolved fluorescence lifetime（TRPL）spectra,photocatalytic H₂ production activity analysis and photoelectrochemical studies that proper doping of Se can effectively promote the separation and transfer of photogenerated electron-hole pairs.DFT calculations show that doping Se elements can not only reduce the band gap of CdS_1-xSe_x semiconductors and broaden their response range to visible light,but also reduce the electrostatic potential energy of CdS_1-xSe_x semiconductors and introduce new impurity energy levels to form appropriate electron trapping centers to prolong the lifetime of photogenerated electrons,and finally enhance the H₂ production activity of CdS_1-xSe_x catalysts.In addition,the CdS_0.95Se_0.05 nanowires were chosen as matrix catalysts to further study the effects of different co-catalysts on the activity of CdS-based catalysts for H₂production fron photocatalytic water splitting.Secondly,the performance and mechanism of NiSe₂ promoting water splitting for H₂evolution of CdS_0.95Se_0.05 nanowiresIn this part,NiSe₂/CdS_0.95Se_0.05（Ni CdSSe）Schottky heterojunctions with NiSe₂ as co-catalyst were successfully prepared by solvothermal method.Under the irradiation of visible light（λ≥400 nm）,the best photocatalytic H₂ evolution activity can be obtained when the loading amount of the co-catalyst is set to 10 mol%.The H₂ generation rate can be reached to 798.5μmol·h^-1,and the corresponding apparent quantum efficiency（AQE）is 42.73%,which is 38.4,10.9 and 1.4 times higher than that of pure CdS nanowires,CdS_0.95Se_0.05 catalysts and 1.0 wt%-Pt/CdS_0.95Se_0.05 catalysts,respectively.Comprehensive analysis of photoelectrochemistry and PL resluts,it can be found that the loading of NiSe₂ co-catalyst can significantly improve the separation efficiency of heterojunction photogenerated electron-hole pairs,reduce the resistance in the process of carrier migration and reduce the H₂ evolution overpotential in the process of H⁺reduction.The theoretical calculation results show that the loaded NiSe₂ co-catalyst can form a Schottky heterojunction with CdS_0.95Se_0.05,and form a built-in electric field near the interface between NiSe₂ to CdS_0.95Se_0.05 nanowires.The built-in electric field is beneficial to promote the spatial separation of photogenerated electron-hole pairs,prolong the lifetime of photogenerated carriers,and then improve the H₂ evolution activity of the Ni CdSSe Schottky heterojunctions.Thirdly,the construction,H₂ evolution performance and mechanism of CoSe₂/CdS_0.95Se_0.05 Schottky heterojunctionCoSe₂/CdS_0.95Se_0.05（Co CdSSe）heterojunction catalysts were successfully prepared by a simple solvothermal method.The morphology,structure and optical properties of the heterojunction catalysts were characterized by SEM,TEM,XRD,XPS,DRS and other techniques.The photocatalytic H₂ evolution activity test results show that CoSe₂ as a co-catalyst can significantly improve the H₂ evolution performance of Co CdSSe heterojunction.When the loading of CoSe₂ is 5 mol%,the water decomposition activity of Co CdSSe heterojunction catalyst is the highest under the visible light（λ≥400 nm）irradiation.The highest H₂ generation rate is 1389.5μmol·h^-1（the AQE is 76.10%）,which is 18.9 times and 66.7 times higher than that of pure CdS_0.95Se_0.05 and CdS nanowires,respectively.Photoelectrochemical studies show that the loading of CoSe₂co-catalyst can not only significantly improve the separation efficiency of photogenerated electrons-holes in Co CdSSe heterojunctions,but also reduce the resistance of carrier migration in Co CdSSe heterojunctions and prolong the carrier lifetime.The first-principles calculation results show that the difference of Fermi level between CoSe₂co-catalyst and CdS_0.95Se_0.05 semiconductor results in the formation of Schottky heterojunction after the close contact.The built-in electric field at the heterojunction interface can effectively promote the rapid separation of photogenerated electron-hole pairs.In addition,the metallic CoSe₂ co-catalyst can be used as a temporary storage and transfer device for photogenerated electrons to accelerate the migration of photogenerated electrons to the surface active sites for H⁺reduction,thus improving the performance of heterojunction catalysts for H₂ evolution.Lastly,the preparation of Cu₂Se/CdS_0.95Se_0.05 heterojunction and its photocatalytic H₂ evolution performances studiesCu₂Se/CdS_0.95Se_0.05（Cu CdSSe）composite photocatalysts with Cu₂Se as co-catalyst were prepared by simple solvothermal method.The photoinduced charge behavior of Cu CdSSe heterojunction catalysts was studied by a series of comprehensive characterization methods,such as steady-state fluorescence spectra,transient photocurrent density and surface photovoltage decay curve.First-principles calculations show that the metallic Cu₂Se can significantly affect the electron distribution at the heterojunction interface,so it can be used as a co-catalyst to effectively promote photogenerated electron transport and surface proton reduction in the catalyst.The H₂evolution activity of the optimized Cu CdSSe heterojunction catalyst was 522.9μmol·h^-1,which was 7.1 times and 27.4 times higher than that of pure CdS_0.95Se_0.05 and pure CdS nanowires,respectively.In addition,cycle stability experiments show that the heterojunction catalyst has good stability and durability.