Study On The Construction Of MOF And Metal Phosphide High-efficiency Photocatalysts And Their Hydrogen Production Performance

As the degree of industrialization continues to deepen,a series of environmental and energy problems have been triggered.In order to cope with the energy and environmental crisis,it is necessary to develop and utilize clean and renewable hydrogen energy.Therefore,hydrogen evolution through photocatalytic splitting of water is an effective way to alleviate environmental and energy problems.However,the catalytic activity of many photocatalysts is restricted by problems such as weak light trapping ability,severe photo-generated carrier recombination,and low photo-generated carrier utilization rate.In order to construct a photocatalyst with high hydrogen evolution activity,the following work has been done based on a metal organic frameworks（MOFs）:（1）The Co₉S₈ hollow polyhedron was formed after sulfidation and calcination with the metal-organic framework ZIF-67.The hollow polyhedron Co₉S₈ provides abundant support sites for Mn_0.05Cd_0.95S and effectively reduces the agglomeration degree of Mn_0.05Cd_0.95S.The Co₉S₈ hollow polyhedron as the reaction site has a large specific surface area and a mesoporous structure,which is beneficial to the progress of the photocatalytic reaction.A series of tests showed that the introduction of Co₉S₈ hollow polyhedron significantly improved the light-trapping ability and exposed more reaction sites.Co₉S₈ hollow polyhedrons are used as electron capture sites,which can effectively collect electrons and induce the interface charge transfer of Mn_0.05Cd_0.95S to Co₉S₈.Because the Co₉S₈-Mn_0.05Cd_0.95S composite catalyst had a strong light-trapping ability,abundant reaction sites and Co₉S₈-Mn_0.05Cd_0.95S heterojunction accelerate the separation and transfer of charges.（2）Hollow polyhedron materials have the characteristics of low density,high area ratio,strong light absorption and the short distance of mass transport,etc.,which have come to the foreground in recent years.In this work,ZIF-67/Co-Ni double hydroxide（DH）was prepared using ZIF-67 as a precursor and then calcined to form Co₃O₄/NiCo₂O₄hollow double-shell polyhedrons（HDSPs）.This unique self-supporting hollow double-shell catalyst effectively avoids the aggregation of the catalyst and provides a unique reaction platform for the photocatalytic hydrogen production reaction.On the one hand,the thin shell reduces the penetration loss of incident light,and there are multiple refractions and scattering occur inside the shell,thereby effectively improving the light-trapping ability.On the other hand,the Co₃O₄/NiCo₂O₄ HDSPs not only shortens the transport distance of photo-generated carriers but also provides abundant active sites,which effectively inhibiting the recombination of electrons and holes.Density functional theory（DFT）calculations show that the formation of a NiCo₂O₄ shell outside of Co₃O₄ is more conducive to the separation and transfer of electrons.Therefore,the Co₃O₄/NiCo₂O₄HDSPs can facilitate the separation and migration of carriers and effectively increase the hydrogen evolution rate of the catalyst.This work shows the important role of hollow double-shell materials and provides new ideas for the preparation of hollow double-shell catalysts.（3）Three-dimensional self-supporting photocatalytic materials have irreplaceable advantages in terms of structural stability,light trapping,charge utilization,and specific surface area.Under the guidance of density functional theory（DFT）,the p-type Co₃O₄polyhedron and the n-type Mn_0.2Cd_0.8S microrod were successfully combined to build a Mn_0.2Cd_0.8S/Co₃O₄ p-n heterojunction composite catalyst.Co₃O₄ polyhedron provides abundant support sites for Mn_0.2Cd_0.8S microrods,which effectively improves the light-trapping capacity and charge utilization of the catalyst.Under the action of the built-in electric field at the p-n heterojunction interface,the Mn_0.2Cd_0.8S/Co₃O₄ composite material exhibits good carrier separation and migration capabilities.On this basis,we prepared Co₃O₄ with different shell structures（porous polyhedrons,hollow double-shell polyhedrons and hollow single-shell polyhedrons）to optimize the energy/mass transfer process of the catalyst.The experimental results show that our method is successful.Compared with the Co₃O₄ porous polyhedrons,the Co₃O₄ hollow multi-shell polyhedron has a stronger carrier extraction capacity,which effectively promotes the oxidation-reduction reaction.Among them,the Co₃O₄ hollow single-shell polyhedrons has the strongest carrier extraction capacity,which greatly improves the utilization of carriers.Therefore,Mn_0.2Cd_0.8S/Co₃O₄ HSSPs3 has the highest catalytic hydrogen evolution activity,which is 25.3 times that of Mn_0.2Cd_0.8S.This work will provide broad prospects for the preparation of photocatalysts with excellent catalytic performance.