Fe（In）-based Metal-Organic Frameworks Photocatalysts:Design,Derivation And Photocatalytic CO₂ Reduction Properties

Metal-organic frameworks（MOF）with a high surface area,regular pore structure,and open channels,have a wide range of applications,particularly in catalysis.In catalytic systems,because two-dimensional MOFs have more sufficient exposed catalytic active sites compared to their three-dimensional counterparts,they are regarded as more efficient photocatalyst.Fe（In）-based MOF photocatalysts have regular structures and can be easily prepared,making them very promising photocatalysts for photocatalytic CO₂ reduction.However,Fe（In）-based MOF photocatalysts yet exhibit several big weaknesses:they have a weak visible light response and low absorption coefficient,which limit their full use of sunlight;on the other hand,their Lowest Unoccupied Molecular Orbital（LUMO）position does not match with the reduction potentials of some energy-related reactions.To overcome the weaknesses of Fe（In）-based MOF materials,we have tried to address above issues in this thesis from the following aspects:First,in order to solve the problem of limited response of Fe-MOF photocatalyst to visible light,we propose a dye-sensitized photocatalytic system for photocatalytic CO₂reduction in which[Ru（bpy）]₃²⁺was used as a sensitizer and two-dimensional Fe-MOF nanosheet（Fe MNS）was employed as a catalysts.In this system,combining the good visible light absorption of[Ru（bpy）]₃²⁺and the large specific surface area and sufficiently exposed active sites of two-dimensional MOF,the electrons from the dye molecules can be more effectively injected into Fe MNS for photocatalytic CO₂ reduction.It has been found that the injection of dye electrons can significantly increase the Fermi level of Fe-MOF by dye sensitization and thus raise its reduction potential so as toperform the catalytic reaction of CO₂ reduction to CO.In order to further improve the photocatalytic performance of Fe-MOF,we optimized the electronic structure of these Fe-MNSs by doping Co ions,and then regulated their photocatalytic activities by adjusting the ratios of Fe/Co ions.Second,using the mechanism of hot electron injection of surface plasma,we loaded plasmonic metal Au nanorods（NRs）on the surface of Fe MNS and constructed AuNRs/Fe MNS composite nanostructures,which were then successfully used for photocatalytic CO₂reduction.On the one hand,the plasmonic AuNRs can be used as a light absorber,which can effectively absorb visible light and make up for the lack of light absorption of Fe-MOFs.On the other hand,the hot electrons injected by AuNRs can also significantly increase the Fermi levels of Fe-MOFs,thereby realizing the reduction from CO₂ to CO.Furthermore,the formed interface of AuNRs/Fe MNS metal-semiconductor heterojunction facilitates the transfer of hot electrons generated by plasmonic resonance to Fe-based MOF nanosheets.The results show that Fe-based MOF nanosheets exhibit good photocatalytic activity with the assistance of plasmonicmetal nanostructures.The roles of different components in the system have been explored in detail and corresponding working mechanism is proposed.Third,considering the weak visible light response of the In-based MOF and the mismatch between the LUMO position and the reduction potential of a specific catalytic reaction.Herein,we present a convenient method to obtain a hybrid photocatalyst consisting of MnS and In₂S₃ nanosheets with assembled hierarchical structures using Mn²⁺-loaded MIL-68（In）submicro-rods as templates.Due to the dispersive Mn²⁺and In³⁺ions in templates,numerous small p-n heterojunctions of MnS/In₂S₃ could be simultaneously produced in each hierarchical particle.The p-type MnS and n-type In₂S₃with an original type II band alignment can create a stronger built-in electric field after the formation of p-n heterojunctions,which is favorable for charge separation and migration to catalyst surface.The prepared MnS/In₂S₃ heterojunctions show 4-fold higher photocatalytic activity toward CO₂ reduction than pristine MnS and In₂S₃.The MnS/In₂S₃ hierarchical structures were well characterized and their working mechanism was explored.