The Construction And Hydrogen Production Performance Of Photocatalytic Materials Based On CdS,MOFs And G-C₃N₄

In view of the three processes of photocatalytic reaction,we have carried out three aspects of research and discussion.（1）Broadening the spectrum absorption range of the semiconductor by introducing the dye molecule photosensitizer;combining the visible light responsive semiconductor material with the MOFs material to enhance the dispersibility of the catalyst and reduce its agglomeration degree,so as to absorb more photons more efficiently and enhance the light capture ability of semiconductor catalyst.（2）By constructing the p-n type heterojunction,the built-in electric field will be formed at their surface contact portion.Under the action of built-in electric field and potential difference,the orientation and efficient separation and transfer of photogenerated electrons and holes can be realized.（3）By introducing transition metal phosphides,sulfides and nickel nanoparticles as non-noble metal co-catalyst to provide the active sites,the electrons can be quickly captured,thereby reducing protons and realizing hydrogen production from water pyrolysis.In this paper,we select the EY eosin molecule,CdS,g-C₃N₄,UiO-66（Zr）,ZIF-67（Co）,WP and NiS_x materials to explore the intrinsic relationship between the composite catalysts,analysis and research on the photo-generated charge transmission mechanism and hydrogen production reaction mechanism of EY dye sensitized photocatalytic system and p-n type heterojunction composite photocatalyst under visible light irradiation.The innovative results achieved are as follows:1.Improve the light absorption capacity of semiconductors and broaden the absorption range of visible light（1）Non-noble metal NiSx modified g-C₃N₄@ZIF-67（Co）heterojunction for high-efficiency photocatalytic H₂ evolution:In this paper,using g-C₃N₄ and ZIF-67 as carriers,EY dye molecules sensitized[g-C₃N₄@ZIF-67/NiSx]composite photocatalyst are successfully prepared by hydrothermal method.Based on SEM and TEM,it is not only known that g-C₃N₄ and ZIF-67 provide space for the loading of NiSx nanoparticles,but also that ZIF-67 as the carrier skeleton can effectively reduce the particle size of NiSx and improve its dispersion.The matching conduction band position between g-C₃N₄ and ZIF-67 provides a feasible thermodynamic path for electron transfer.The NiSx co-catalyst with metallic properties has excellent electron-trapping ability,which can further promote the separation of electrons,thereby improving hydrogen production efficiency.（2）Hydrogen production performance of UiO-66@CdS/WP composites:On the one hand,the octahedral spatial structure of UiO-66 not only provides space for the loading of CdS and WP nanoparticles,but also effectively reduces their particle size and improves their dispersion;on the other hand,the matching energy band position and potential difference between UiO-66 and CdS provides a feasible thermodynamic path for the transfer of photogenerated electrons;the close contact between the three forms a synergistic effect,which together improves the efficiency of photocatalytic H₂ production.Under visible light irradiation,[UiO-66@CdS/WP（10%wt）]photocatalyst produces 395μmol of hydrogen in 5 h,which is 26.33 times that of pure CdS.Further from photoluminescence spectroscopy,photoelectrochemical experiments and Mott-Schottky curves,we reasonably explain the separation and transfer mechanisms of photogenerated electrons and holes.The lower charge recombination rate,the larger electron transfer rate constant(K_ET=2.32×10⁸ s^-1),the higher electron injection efficiency(η_inj=49.1%),the higher photocurrent density and the smaller transfer resistance,which together prove the high charge separation and transfer efficiency.2.Construct the p-n junction to form a built-in electric field to achieve the orientation and efficient separation of photo-generated charges（1）The structure and hydrogen production properties of[CdS/NiWO₄]p-n heterojunction:Firstly,CdS nanorods are prepared by solvothermal method;secondly,NiWO₄ is loaded onto the surface of CdS nanorods by hydrothermal method.The p-type NiWO₄ and n-type CdS form a p-n junction,which forms a built-in electric field at the interface,forming a directional and efficient charge transfer path,effectively slowing down the photocorrosion of CdS.The photocatalytic performance of CdS/NiWO₄ composite catalyst is closely related to the loading of NiWO₄.The[CdS/NiWO₄（30%）]sample has the best H₂ production performance,that is,the maximum H₂ yield in 5 h is 757μmol,which is 7.35 times than that of CdS nanorods.However,excessive NiWO₄ can reduce the light absorption intensity of the CdS nanorods,thereby lowering the hydrogen production activity of the composite catalyst.（2）Study on hydrogen production performance of[Ni-MOF-74/CdS/Co₃O₄]p-n heterojunction:Firstly,spherical Ni-MOF-74 is synthesized,then CdS is introduced,and finally Co₃O₄ is loaded onto the surface of CdS by a simple hydrothermal reaction.Spherical Ni-MOF-74 can be used as a carrier of CdS,thus increasing the specific surface area of the catalyst.At the same time,the built-in electric field at the interface between CdS and Co₃O₄ accelerates the separation and migration of electrons and holes.The photogenerated holes with strong oxidation ability can be quickly transferred to the valence band of Co₃O₄ and further reduced by lactic acid,thus slowing down the corrosion of CdS.（3）H₂ production performance analysis of Ni nanoparticle modified ZIF-67（Co）derived Co₃O₄/CdS p-n heterojunction:Firstly,hollow Co₃O₄ skeleton material is synthesized,then CdS is introduced,and finally Ni nanoparticles are deposited onto the surface of CdS by photoreduction method.The ZIF-67（Co）precursor-derived hollow Co₃O₄ framework with lower density and large volume can be used not only as a hole trapping agent but also as a carrier material for CdS,thereby improving the dispersibility of CdS.Ni nanoparticles act as electron trapping agents and provide reactive sites.Under the action of internal electric field and potential difference,electrons migrate from the CB of Co₃O₄ to the CB of CdS,and further transfer to Ni nanoparticles to reduce H⁺to produce H₂;holes are transferred from the VB of CdS to the VB of Co₃O₄,and further reduced by lactic acid,thus slowing down the photo-corrosion of CdS.The synergistic effect of the above three can improve photocatalytic H₂ production efficiency.3.Introduce the transition metal phosphide co-catalyst to rapidly capture electrons and provide reactive sites（1）High-efficiency photocatalytic H₂ production performance of Orthorhombic WP-coupled UiO-66 composites:Firstly,WP nanoparticles are prepared by temperature programmed solid-state method.Secondly,the WP nanoparticles are loaded onto the surface of the UiO-66 octahedron by ultrasonic-assisted impregnation.In order to explore tthe electron transfer mechanism,PL,UV-vis and Mott-Schottky are characterized and analyzed.We can not only confirm that the modification of WP does improve the electron migration ability,but also that the matching band edge positions between EY dye molecules and UiO-66 provide a feasible thermodynamic path for electron transfer.