Construction Of Layered Double Hydroxides Photocatalyst And Its Hydrogen Production Performance

Photocatalytic technology can directly convert solar energy into clean and storable hydrogen energy,which provides a new ideal way for energy utilization and pollution control.As is known to all,when a single semiconductor material is photoexcited,photogenerated electrons and photogenerated holes are easy to recombine,resulting in low hydrogen production efficiency,which seriously hinders the development of photocatalytic technology.In addition,there are still some problems in the photocatalyst,such as insufficient active sites on the surface and low redox capacity,which are not conducive to the photocatalytic reaction.In order to solve these problems,the efficient hydrogen evolution photocatalyst based on layered double hydroxides（LDHs）was synthesized by means of heterojunction construction and morphology control.The specific research contents are as follows:（1）Based on the appropriate conduction band and valence band positions of CdS and CoAl LDH,the S-scheme heterojunction CdS@CoAl LDH composite photocatalyst was successfully constructed by electrostatic self-assembly of CdS and CoAl LDH,and the photocatalytic hydrogen evolution performance of CdS@CoAl LDH composite photocatalyst was studied under visible light irradiation.The results showed that the hydrogen production of the optimal proportion of CdS@CoAl LDH（CCA-2）composite catalyst with 10 mg reached 248μmol(4960μmol h^-1g^-1)after visible light irradiation for5 h,which was significantly higher than that of single material CdS(1384μmol h^-1g^-1)and CoAl LDH(96μmol h^-1g^-1).The test results show that the construction of S-scheme heterojunction can effectively separate photogenerated electrons and holes,so that a large number of photogenerated electrons can participate in the water decomposition reaction,at the same time,the photogenerated holes are consumed by sacrificial reagents.Therefore,the CdS@CoAl LDH composite photocatalyst can efficiently utilize electrons and show excellent photocatalytic hydrogen production activity.（2）A novel Mn_0.2Cd_0.8S@CoAl LDH S-scheme heterojunction photocatalyst was successfully prepared by coupling Mn_0.2Cd_0.8S nanorods with CoAl LDH nanosheets by physical mixing method,and the performance of Mn_0.2Cd_0.8S@CoAl LDH photocatalytic hydrogen evolution under visible light irradiation was studied.The results showed that the hydrogen production of 10 mg Mn_0.2Cd_0.8S@CoAl LDH（MCCA-3）with optimal CoAl LDH load is 1178μmol(23560μmol h^-1g^-1)within 5 h,which was 23 times that of Mn_0.2Cd_0.8S(1020μmol h^-1g^-1)and 512 times that of CoAl LDH(46μmol h^-1g^-1).The results show that an S-scheme electron transfer channel is formed at the contact interface between Mn_0.2Cd_0.8S and CoAl LDH,which promotes the separation of photogenerated carriers and improves the hydrogen evolution performance of the photocatalyst.（3）The ZIF-67@Ni-Fe LDH composite photocatalyst with S-scheme heterostructure was prepared by in situ driven growth and applied to photocatalytic hydrogen evolution.In the dye sensitization system,the optimal ratio of composite catalyst ZIF-67@Ni-Fe LDH（ZNF-3）shows strong hydrogen evolution activity under visible light,the hydrogen evolution activity of 10 mg ZNF-3 can reach 105μmol(2100μmol h^-1g^-1),3 and 13 times higher than ZIF-67(626μmol h^-1g^-1)and Ni-Fe LDH(160μmol h^-1g^-1),respectively.The unique dodecahedron structure of ZIF-67 provides sufficient space for the dispersion of Ni-Fe LDH,which effectively prevents the agglomeration of Ni-Fe LDH.In addition,the formation of S-scheme heterostructure in ZIF-67@Ni-Fe LDH improves the utilization of photogenerated carriers,which is conducive to hydrogen evolution reaction.（4）Through a simple physical mixing method,Ni Al LDH was coupled on the surface of ZIF-67 to obtain ZIF-67@Ni Al LDH S-scheme heterojunction photocatalyst,furthermore,the P-ZIF-67@Ni Al LDH composite catalyst was obtained by introducing phosphorus into ZIF-67@Ni Al LDH through low temperature phosphating.The results showed that 10 mg ZIF-67@Ni Al LDH（ZNA-3）composite showed 80μmol(1600μmol h^-1g^-1)photocatalytic hydrogen evolution activity after 5 h of visible light irradiation.Under the same test conditions,the hydrogen evolution activity of P-ZIF-67@Ni Al LDH reached 150μmol(3000μmol h^-1g^-1),which was 6 times that of ZIF-67(526μmol h^-1g^-1)and 32 times that of Ni Al LDH(94μmol h^-1g^-1).The results show that the construction of S-scheme heterojunction can effectively improve the separation efficiency of photogenerated electron-hole and prolong the lifetime of photogenerated carriers,so as to improve the performance of hydrogen production of photocatalyst.In addition,the introduction of phosphorus forms a special electron transfer channel on the surface of the catalyst,which accelerates the transfer of photogenerated charge carriers,and also facilitates the improvement of hydrogen evolution performance.