Synthesis And Properties Of MOFs With 3,6-Carbazole Dicarboxylic Acid And 4-4'-Bipyridine

With the rapid development of society and economy,environmental pollution and energy demand have become serious increasingly.Oxygen evolution reaction(OER)participates in various energy conversion and storage systems,due to slow reaction kinetics,a suitable catalyst is required to catalyze OER.Among the current OER catalysts,there is a lack of constructing the suitable catalyst to improve the performance,with the proton or electron transfer mechanism.Therefore,it is still challenging to develop OER catalysts which are easy to design and construct,and to explain the relationship between the properties and inherent structure at the atomic and molecular scales.In addition,chromium(VI)oxygen anion is related to many human diseases closely,from allergies to genetic mutations,and it is necessary to develop the new materials for the identification and detection by fluorescence method.Metal-organic framework materials(MOFs)is a kind of the new porous materials,by the self-assembly of organic ligands and metal ions.Because of the powerful designability,they can be prepared and studied according to different application.Thus,the project designed and synthesized the new MOFs as the OER electrocatalyst,and to construct a fluorescence sensor for detecting chromium(VI)oxygen anions.The specific content is as follows:Firstly,3,6-carbazole dicarboxylic acid(CDC)and 4,4'-bipyridine(BPY)as ligands,were used to synthesize two isostructural MOFs(FJU-82-Co and FJU-82-Zn)with different metal centers by solvothermal method.Interestingly,their electrocatalytic OER performance is better with the larger proton conductivity.At the current density of 1 m A cm^-2,the overpotential of FJU-82-Co(0.57 V)is much smaller than FJU-82-Zn(1.17 V).At 60°C and 98%humidity,the proton conductivity of FJU-82-Co is 127 times than that of FJU-82-Zn.Coordination of the metal sites in both MOFs are saturated.Although Zn is not the active element of OER,the electrochemical active surface area of FJU-82-Zn is 4.5 times than that of FJU-82-Co.There is no free metal ion detached from the MOFs in the solution after the test,and proton transfer participates in the OER process.We believe that it is not the role of the active cobalt element,but the higher proton conductivity of FJU-82-Co made a major contribution to its better OER performance.For the first time,our work found that the adjustment of the proton conductivity can provide an effective strategy to improve the electrocatalytic OER performance of isostructural crystalline MOF materials.Secondly,FJU-82-Co as a precursor,its pyrolysis derivatives with multi-doped carbon material were obtained,through high-temperature calcination under different atmospheres(Ar/N₂)and the same atmosphere(Ar)at different temperatures(700,800 and 900°C),to study the electrocatalytic OER performance.Among them,the800°C-Ar has the largest specific surface area and the average pore volume of mesopores and micropores,and more graphite N and richer Co-N_x and Co_xN_yin the composition,and the best performance of OER.The experimental results show that FJU-82-Co as a precursor,the doping composition can be adjusted by controlling the pyrolysis conditions to achieve the diversified co-doping for improving the OER performance effectively.Finally,the same ligands and metal ion used as FJU-82-Zn,the crystal growth was controlled by changing the basicity of the solution,to prepare the isomeric MOF(FJU-826)of FJU-82-Zn except solvent molecules.Compared with FJU-82-Zn(triclinic),FJU-826 has upgraded the crystal system(tetragonal),and has improved the fluorescence intensity effectively.Because Cr₂O₇^2-can make the fluorescence of FJU-826 quench and FJU-826 can exist in water stably,a fluorescence sensor was constructed to detect Cr₂O₇^2-in aqueous solution.Fluorescence microscopy,powder diffraction,energy dispersive X-ray spectroscopy(EDS),and ultraviolet spectrum analysis were used to investigate the cause of quenching fluorescence.The results showed that the spectrum of FJU-826 overlaped with that of the analyte,which made fluorescence quench due to electron energy transfer.