Construction And Photocatalytic Hydrogen Evolution Performance Of Metal Sulfide-based Heterojunction Catalysts

With the rapid development of China’s economy and society,environmental pollution and energy shortage have become increasingly prominent.The establishment of clean and efficient energy system and the realization of green and sustainable development society have become hot spots in scientific research.Hydrogen（H₂）energy is a clean energy source with non-pollution.Accelerating the development of H₂ energy industry is expected to fundamentally achieve sustainable energy development.There are many ways to produce H₂,from the perspective of green chemistry,photocatalytic H₂ evolution is an ideal method to produce H₂.In order to promote the development of photocatalytic technology,the design and construction of efficient photocatalysts are very important.Among these photocatalysts,metal sulfides are favored because of their suitable bandgap,excellent visible light response,and adjustable photoelectrochemical properties.However,the photocatalytic activity of single metal sulfides is limited due to low separation efficiency of photogenerated carriers,few exposed active sites,and weak redox ability.Therefore,metal sulfide-based heterojunction photocatalysts are selected for research in this thesis including internal electric field to drive the efficient separation of photogenerated carriers in Cd In₂S₄-based heterojunction photocatalysts,construction of ZnIn₂S₄-based heterojunction photocatalysts with different morphologies to increase the effective catalytic active sites,synergistic coupling of metal-organic frameworks（MOFs）and Zn_0.5Cd_0.5S to construct heterojunction system and further to give photocatalyst dual advantages and enhanced redox ability.Through the above three modification strategies,the photocatalytic H₂ evolution activity of metal sulfide-based heterojunction photocatalysts is evidently improved.The main research contents of this thesis are shown as follows:（1）Construction and photocatalytic H₂ evolution performance of Cd In2S4-based heterojunction catalysts driven by internal electric fielda.The single internal electric fields driven Co₉S₈/Cd In₂S₄ heterojunction photocatalyst is constructed by hydrothermal method and in-situ synthesis method.The internal electric field as the driving force can effectively regulate the migration behavior of photogenerated carriers in the photocatalytic reaction system.Under visible light,the photocatalytic H₂ evolution activity of 5%Co₉S₈/Cd In₂S₄ can reach1083.6μmol h^-1 g^-1,which is 6.4 times that of pure Cd In₂S₄(170.5μmol h^-1 g^-1).The enhanced photocatalytic activity can be mainly attributed to the presence of the internal electric field,which accelerates the separation efficiency of photogenerated carriers.Moreover,the introduction of Co₉S₈ improves the light response ability and reduces the hydrogen evolution overpotential.b.The double internal electric fields driven NH₂-UiO-66/CoFe₂O₄/Cd In₂S₄（NU6/CFO/CIS）heterojunction photocatalyst is constructed by hydrothermal method,high temperature calcination method,and in-situ synthesis method.Compared with heterojunction photocatalyst with single internal electric field,double internal electric fields driven photocatalyst has dual carrier transport channels,faster carrier separation efficiency,and stronger redox capacity.When using 100 mg of photocatalyst,25%NU6/2%CFO/CIS shows excellent photocatalytic H₂ evolution activity(283.26μmol h^-1),and its activity is 13.5 times and 2.5 times higher than that of CIS(21.01μmol h^-1)and 2%CFO/CIS single p-n junction(113.11μmol h^-1).X-ray photoelectron spectroscopy,theory calculations and Pt ion probe method fully demonstrate the“double internal electric fields”mechanism,that is,under the drive of visible light and double internal electric fields,the electrons on CFO will transfer to CIS and NU6,and then reduce H⁺to H₂.（2）Construction and photocatalytic H₂ evolution performance of ZnIn₂S₄-based heterojunction catalysts with different morphologiesa.The 3D flower-shaped CoFe₂O₄/ZnIn₂S₄（CFO/ZIS）p-n junction photocatalyst is constructed by hydrothermal method and in-situ synthesis method.The 1%CFO/ZIS p-n junction photocatalyst exhibits excellent photocatalytic H₂ evolution activity(800.00μmol h^-1 g^-1),which is about 3.7 times that of pure ZIS(217.12μmol h^-1 g^-1).The enhancement of photocatalytic activity can be mainly attributed to the effective dispersion of CFO nanoparticles on 3D flower-shaped ZIS,and the construction of p-n junction can enhance the visible light response ability and accelerate the separation efficiency of photogenerated carriers.b.The 2D/2D Sn Nb₂O₆/Ni-ZnIn₂S₄（SNO/Ni-ZIS）S-scheme heterojunction photocatalyst is constructed by hydrothermal method and in-situ synthesis method.Among them,the 25%SNO/Ni_0.4-ZIS exhibits excellent photocatalytic H₂ evolution activity(2807μmol g^-1 h^-1),which is about 4.49 times and 2.00 times that of ZIS and Ni_0.4-ZIS.2D nanosheet can fully expose active sites and reduce mass transfer resistance,thereby improving the contact and reaction possibilities of reactants and active sites.Meanwhile,the transmission distance of electrons can be reduced,and the recombination of photogenerated carriers can be also suppressed.The improvement of photocatalytic activity is attributed to the 2D nanosheets that can give the photocatalytic system more effective active sites.In addition,benefitting from the synergistic modification of the interior and the surface,Ni doping and the construction of S-scheme heterojunction improve photogenerated carrier separation efficiency and redox capacity.（3）Construction and photocatalytic H₂ evolution performance of MOFs/Zn_0.5Cd_0.5S heterojunction catalystsa.The 2D Ni-MOFs/Cu-Zn_0.5Cd_0.5S-x（NMCZ-x）type-II heterostructure photocatalyst is constructed by hydrothermal method and mechanical grinding method.The photocatalytic H₂ evolution activity of NMCZ-10 is as high as 5771.2μmol g^-1 h^-1,which is 6.26 times and 2.68 times higher than that of Zn_0.5Cd_0.5S(922.6μmol g^-1 h^-1)and Cu_0.1-Zn_0.5Cd_0.5S(2155.4μmol g^-1 h^-1).The enhancement of photocatalytic H₂ evolution activity can be mainly attributed to the inherent porous structure of MOFs,which promotes the direct contact between catalyst and reactant molecules.In addition,the energy band bending at the interface of heterojunction photocatalyst provides a driving force for the separation and migration of photogenerated carriers.b.The NH₂-MIL-125（Ti）/C₆₀/Zn_0.5Cd_0.5S（NMTCZ）S-scheme heterojunction is constructed by hydrothermal method and in-situ synthesis method.Under visible light irradiation,the NMTCZ-45 shows comparable photocatalytic H₂ evolution activity,and its photocatalytic H₂ evolution activity reaches 7825.20μmol h^-1 g^-1,which is93.05 times,6.38 times and 2.65 times higher than that of 2%C₆₀/NH₂-MIL-125（Ti）(84.10μmol g^-1 h^-1),Zn_0.5Cd_0.5S(1226.24μmol g^-1 h^-1),and 45%NH₂-MIL-125（Ti）/Zn_0.5Cd_0.5S(2953.28μmol g^-1 h^-1).The enhancement of photocatalytic activity can be mainly attributed to the construction of S-scheme heterojunction,which not only accelerates the separation and migration of photogenerated carriers,but also endows the photocatalytic system with higher redox capacity,thus speeding up the photocatalytic H₂ evolution reaction.