Study On The Cu_x-Based Metal–Organic Frameworks For Photocatalytic Water Splitting

Metal–organic frameworks?MOFs?,as new porous materials,have displayed wide application prospects in asymmetric catalysis,gas storage and biomedicine due to their definite structures and diverse functionalities.MOFs are one-dimensional,two-dimensional and three-dimensional space network structures,which are formed by the coordination of multiple metal ions with empty orbitals and organic ligands with rich electrons.At the same time,due to the diversity of metal ions and organic ligands as well as the coordination mode flexibility between metal ions and organic ligands,the structures of MOFs can be well designed and their porous size and topological structures can be optionally controlled.Polyoxometalates?POMs?are a class of nanosized metal-oxygen clusters with rich electrons,thus polyoxoanions as attractive inorganic building units and can show excellent physical and chemical properties such as excellent proton/electron storage and delivery capacity and good stability.Combination of functional polyoxometalates as templates or bridging groups with MOFs can not only increase the structural stability and the porosity of MOFs,but also introduce excellent electromagnetic properties of POMs into the structures of MOFs to obtain functionalized porous POM-based MOFs with novel structures and specific properties.According to the construction strategies of MOFs,we rationally selected photofunctional organic ligands and highly active POMs,constructed photoactive POM-based MOFs with different structures and properties and explored the photocatalytic water splitting for hydrogen and oxygen production of these POM-based MOFs.The specific research contents are as follows:In the first section,the main developments of MOFs for the photolysis of water splitting have been summarized.In the second section,three rigid Cu-Cl-bpy[X=Cl,Br or I]photocatalysts were prepared by using Cu^I ion nodes,4,4?-bipyridine bridging ligands and halogen atoms by the conventional solution method under stirring conditions.These compounds are isomorphic and their 2-D interpenetrating network structures are constructed from cuprous ions as nodes and halogen atoms and 4,4'-bipyridine ligands as bridges.The obvious structural feature of these compounds is that they all contain the dinuclear Cu₂X₂units,which played an important role in the photocatalysis process.Compared with the free ligand bpy,the adsorption bands of UV-Vis spectra of Cu-X-bpy revealed an obvious red-shift.The diffuse reflectance spectra of Cu-X-bpy were also studied.The Tauc-plot fitting results indicated the band-gap energy?Eg?is2.00 eV for Cu-I-bpy,1.90 eV for Cu-Br-bpy and 1.85 eV for Cu-Cl-bpy.These results revealed that Cu-X-bpy could be ideal semiconductive MOFs.The photocatalytic experiments exhibited there compounds behaved as very high efficiency of hydrogen production under UV irradiation in the absence of photosensitizers and cocatalyst.Cu-I-bpy shows the high catalyst stability and could be used repeatedly five times.The recycled powders after the photocatalytic hydrogen evaluation were characterized by PXRD and transmission electron microscope?TEM?.The results revealed that Cu-I-bpy has a good catalytic stability whereas Cu-Cl-bpy and Cu-Br-bpy aren't stable during the course of catalytic processes.TEM pictures of these compounds after the photocatalytic hydrogen evaluation showed that both Cu-Cl-bpy and Cu-Br-bpy had the aggregation of Cu nanoparticles,but this phenomenon was not found in Cu-I-bpy.The results revealed that Cu-I-bpy is a high-efficient catalyst in the photocatalytic water splitting for hydrogen hydrogen generation without additional photosensitizers and cocatalysts,which provides new insights into the design of d¹⁰-based MOFs for long-term solar fuels production in future.In the third section,by the solvothermal method,we used the photosensitive precuror[W₁₀O₃₂]^4–,Cu²⁺ions and the photocatalytic ligand TPB to construct a dual-functionalized POM-based three-dimensional MOF ZZULI–1,in which two types of POM units[W₁₂O₄₀]^4–and[W₆O₁₉]^2–are encapsulated,24-nuclear Cu^I clusters and single Cu^II ions work as mixed nodes and TPB ligands function as linkers.Single-crystal structural analysis reveals that ZZULI–1 crystallizs in a cubic space group Pm-3m and consists of{Cu^I₂₄??₃-Cl?₈??₄-Cl?₆}cages and Cu^II ions as nodes,rigid TPB ligands as linkers and two types of POM units[W₁₂O₄₀]^4–and[W₆O₁₉]^2–.The asymmetry unit comprises two crystallographically independent copper ions.Both Cu^II ions exhibit the five-coordinate square-pyramidal geometry with four nitrogen atoms from four TPB ligands sitting on the basal plane and one oxygen atom of methanol molecule occuppying the vertex position.Compared with the free ligand TPB and POMs,the adsorption band of UV-Vis spectrum of ZZULI–1 reveals a significant red-shift.The photocatalytic activity of ZZULI–1 was studied.This compound can simultaneously complete both the photocatalytic water splitting for hydrogen generation and oxygen evolution.In the hydrogen production process,two Cu nodes can work as the active centers while POM units act as carriers and transporters of protons.In the process of oxygen production,two kinds of POM units play a leading role and can catalyze the splitting of water molecules to produce oxygen in the presence of electron donors and photosensitizers,however,MOF only provides a place for catalysis.This discovery provides more potential and practical ideas and methods for the realization of photocatalytic water overall splitting of hydrogen generation and oxygen evolution.