Synthesis And Mechanism Of Highly Active Nanosheet-assembled NiFe₂O₄ Microsphere-based Photocatalys

With the rapid development of modern industry and the accelerated process of urbanization,the problem of water pollution has gradually received global attention.At present,emerging pollutants of global concern mainly include persistent organic pollutants（POPs）,etc.,which are controlled by international conventions.Among them,chlorophenol is a class of pollutants with serious harm to the natural environment and human health,and the efficient degradation is of great significance.Semiconductor photocatalytic technology,which harnesses the green solar energy has been considered as a potential water treatment technology due to its advantages of low cost,mildness and no secondary pollution.Developing highly active photocatalytic nanomaterials and uncovering their photocatalytic mechanisms are key to the development of efficient photocatalysis technology,which is currently a hot research topic.Nickel ferrate（NiFe₂O₄）with a narrow bandgap has attracted much attention because of its wide visible light response range,magnetic separation properties and chemical stability.However,the photocatalytic performance of NiFe₂O₄ is still unsatisfactory in practical applications due to the rapid recombination of photogenerated electron-hole pairs and lack of surface catalytic sites.In this thesis,to address the above key issues,method for controllably preparation of nanosheet-assembled NiFe₂O₄ microsphere,and strategies of introducing an electron modulation platform and constructing a Z-scheme system,have been developed.Based above,three individual works were accomplished and shown below:1.Synthesis of hierarchical NiO/NiFe₂O₄ microsphere photocatalysts.In this work,NiO in-situ introduced nanosheet-assembled hierarchical NiFe₂O₄ microspheres（NiO/NiFe₂O₄）were prepared by ammonium fluoride-modulated solvothermal method and a subsequent calcination treatment.The optimal NiO/NiFe₂O₄ composite can achieve nearly complete degradation of 2,4-dichlorophenol（2,4-DCP）within 3 h under visible light irradiation,which is 1.5-fold improvement compared with NiFe₂O₄ microspheres.The unique micro-nano structure facilitates the separation of photogenerated charges,while the open space between its nanosheets is conducive to the adsorption of 2,4-DCP,and thus the NiFe₂O₄microspheres exhibit good photocatalytic degradation performance.Accordingly,the tightly connected NiO can be employed as a high-level energy electron platform to effectively prolong the lifetime of photogenerated electrons of NiFe₂O₄,which promotes its photogenerated charge transfer and separation,and largely improves its photocatalytic degradation performance.In addition,the NiFe₂O₄ microsphere-based photocatalysts show magnetic separability,which enables rapid recovery of the catalysts in the liquid-phase system.2.Phosphate interface-modulated g-C₃N₄/NiFe₂O₄ Z-scheme photocatalysts.The g-C₃N₄ nanosheets were assembled onto phosphate modified NiFe₂O₄ microsphere by a hydroxyl-induced assembly to form a Z-scheme composite.The 2,4-DCP degradation rate of optimized phosphate-modified g-C3N4/NiFe2O4 composite can reach to about 85%in 2 h,which is 4 times higher than that of individual NiFe2O4 microspheres.This is mainly attributed to the formed Z-scheme heterojunction facilitates charge separation and the introduced phosphate functional molecule bridge solves the problem of restricted interfacial charge transfer due to the lattice mismatch between the two semiconductor components.Meanwhile,the introduced phosphate groups can form an electrostatic negative field on the surface of NiFe₂O₄,which would trap photogenerated holes,synergistically promoting the Z-scheme charge transfer,so as to significantly improve the photocatalytic performance.3.Graphene interface-modulated NiPc/NiFe₂O₄ Z-scheme photocatalysts.NiPc with a central metal was selected to coupling with NiFe₂O₄ to prepare the NiPc/NiFe₂O₄Z-scheme composite photocatalyst via a wet chemistry method.Functionalized graphene（G）was carried out to modulate the interface of the NiPc/NiFe₂O₄ Z-scheme composite.The optimized G-modulated NiPc/NiFe₂O₄ composite exhibits a nearly 100%degradation rate for 2,4-DCP in 1 h,which is 9 times higher than that of individual NiFe₂O₄ microspheres.The improved photocatalytic performance was mainly attributed to the Z-scheme charge transfer between NiPc and NiFe₂O₄,and the accelerated interfacial charge transfer brought by the modified G for increasing the modification loading amount of NiPc.Besides,the abundant Ni-N₄ active sites effectively activate O₂,largely improving the photocatalytic performance.