Study On The Electrochemical Synthesis Of Zn, Cu Metal-organic Frameworks And Their Electrocatalytic Performance

Metal-organic frameworks (MOFs) are one kind of inorganic-organichybrid functional material assembled by metal ions and organic ligands throughcoordination bond. Owing to its flexible porosity, large surface area, unsaturatedcoordination bond, its applications range from molecular recognition,magnetism, photoluminescent, drug delivery, gas storage, catalysis and so forth.The most common synthesis methods are hydrothermal/solvothermal, diffusionmethod, microwave synthesis and ultrasonic synthesis. These methods havesome disadvantages to varying degrees, such as high temperature, high pressure,high energy consumption, complex synthesis process, demanding response toequipment and environment and others. In order to solve these problems, thispaper proposes a new electrochemical synthesis method which is clean andcarried out at ambient temperature and pressure. In this paper, the kinetics ofelectrochemical synthesis Zn-based MOFs and Cu-based MOFs in aqueous andionic liquids had been studied. Meanwhile, its application in water splitting tohydrogen and DNA detection had also being investigated. The main contents are as follows.1. Zn (AM-co-AA) metal-organic coordination polymer was successfullysynthesized in an aqueous through electrochemical synthesis method. Thephysical and chemical properties of the product were characterized by atomicabsorption spectrometry, SEM, IR, TG and XRD. The influence of synthesistime and initiator had also been investigated. The mechanism and kinetic rule ofthe electrochemical synthesis process were discussed through cyclicvoltammetry and polarization curve. The results are as follows.(1) When Zn (AM-co-AA) metal-organic coordination polymer wasprepared in the existence of ammonium cerium nitrate initiator at thetemperature25oC, the optimal reaction time of electrochemical synthesis was1.5h. SEM result presented that the product had a distinct loose sphericalstructure; the diameter was about0.5μm. XRD and IR results indicated thatmetal ion bonded to the polymer. The molecular weight was273275calculatedby ubbelohde viscosity, which descripted that polymerization occurred. TGresult showed the stable temperature was267oC-395oC, which stated theelectrochemical synthesis Zn (AM-co-AA) metal-organic coordination polymerhad good thermal stability.(2) Through testing and calculation the electrochemical behavior of thereaction system, the reaction mechanism of the electrode process had beendetermined. The apparent transport coefficient of anode and cathode were3.24and2.33, respectively, the rate controlling step in the electrochemical synthesis process waswhich illustrated free radicals played important role in the electrosynthesisprocess.2. Two kinds of ionic liquid system,1-butyl-3-methyl imidazole chloridesalt and1-butyl-3-methyl imidazole bromine salt, are respectively used assolvents. H2BDC serves as the organic ligand, Zn2+as metal ions, the MOF-5(Cl)and MOF-5(Br) were successfully synthesized by electrochemical method,respectively. The surface morphology, phase, structure characteristics andthermal stability were characterized by SEM, EDS, XRD, IR, TG and othermethods. The kinetics of electrode process was investigated by cyclicvoltammograms, AC impedance and polarization curve. Meanwhile, theelectro-catalytic water splitting to hydrogen performance of the two MOF-5,respectively synthesized by solvothermal method and electrochemical method,had also been compared. The results are as follows.(1) SEM showed that the MOF-5(Cl) synthesized in the ionic liquid1-butyl-3-methyl imidazole chloride salt system presented spherical structure,the diameter was2micrometers. The MOF-5(Br) synthesized in ionic liquid1-butyl-3-methyl imidazole bromine salt system presented uniform flowershaped structure, the diameter was2micrometers. The result demonstrated that the kinds of ionic liquids had great effect on the morphology of MOFs. XRDand IR results illustrated that the synthesized MOF-5(Cl) and MOF-5(Br) wereconsistent with the literatures reported, and further illustrated that the MOF-5material was successfully synthesized by electrochemical method in ionic liquidsystem. TG results demonstrated that the highest thermal stability temperature ofMOF-5(Cl) and MOF-5(Br) were both380oC. Pore structure analysis revealedBET specific surface area of MOF-5(Cl) and MOF-5(Br) were627.3m2g-1and914.7m2g-1, repectively. Which illustrated that electrosynthesis method couldnot affect the porosity of MOFs. The CV curves of reaction system indicatedthat the reaction in1-butyl-3-methyl imidazole ionic liquid chlorine salt systemwas completely irreversible, but the reaction in1-butyl-3-methyl imidazolebromine salt system is reversible.(2) The cathodic reduction mechanism of electrochemical synthesizedMOF-5in the ionic liquid1-butyl-3-methyl imidazole chloride salt system hadbeen determined. CV results indicated that the reaction occurred at the cathodewas irreversible, there had a significant linear relationship between the currentof reduction peak and v1/2, it demonstrated that the reaction was diffusioncontrolled. AC impedance spectra illustrated that the synergistic effect of ionicliquids and organic ligand jointly promoted the electron transfer of reactionelectrode, and further proved that the process was diffusion controlled. Thecathodic reduction mechanism was determined through polarization curves andelectrochemical theory, and ZnCl e ZnClwas the rate controlled step. (3) The MOF-5and MOF-5(Br) respectively synthesized by solvothermalmethod and electrochemical method were used as catalyst for the reaction ofwater splitting to hydrogen, the CV results showed that MOF-5(Br) synthesizedby electrochemical method in ionic liquid system had obvious hydrogenevolution peak and redox peak, it demonstrated that MOF-5(Br) synthesized byelectrochemical method could be used to catalyze water splitting to hydrogen.3. The metal-organic framework material Cu-BTC was successfullysynthesized by electrochemical methond in1-butyl-3-methyl imidazole chloridesalt ionic liquid system, where H3BTC as organic ligand and Cu2+ions as themetal center. The optimum technological parameters of the synthesis reactionwere determined and the product structure, morphology and thermal stabilitywere characterized by XRD, SEM, EDS, IR, TG and pore size analysis. In theend, the synthesis process and electrocatalytic performance of hydrogenproduction from water splitting was systematically discussed by electrochemicalmethods, such as CV, AC impedance, polarization curves. The results are asfollows.(1) The optimum technological parameters of Cu-BTC electrochemicalsynthesis in ionic liquid system were the current density0.025A cm-2, thetemperature45oC, the reaction time2h. SEM image showed that the preparedCu-BTC was uniform octahedral and the diameter was2micrometer. XRD andIR showed Cu-BTC was consistent with the literature reported, which illustratedthat Cu-BTC was successfully synthesized by electrochemical method in ionic liquid system. TG showed the maximum thermal stability temperature ofCu-BTC was310oC. Pore structure analysis revealed BET specific surface areawas753.8m2g-1, pore volume was0.05103cm3g-1and pore size distributedmainly at0.98nm, the results showed Cu-BTC synthesized by electrochemicalmethod in ionic liquid system was microporous material.(2) The Cu-BTC carbon paste electrode was used as cathode forelectrocatalytic water splitting to hydrogen. AC impedance and polarizationcurves showed the Cu-BTC catalyzing water splitting to hydrogen reaction wasa diffusion-controlled reversible reaction. The apparent transport coefficient ofanode and cathode were0.9,1.2, respectively, the number of control step was1.The CV curve displayed the electrodes had significant reduction peak when theCu-BTC/carbon black ratio was2:1, and current density of reduction peak was16times higher than pure carbon black electrode, indicating that Cu-BTC hadgood electrocatalytic activity for hydrogen production from water splitting.4. The spherical MOF-5(Cl) composite, synthesized in1-butyl-3-methylimidazolium chloride ionic liquids by electrochemical method, is used togetherwith gold nanoparticles and DNA molecule to prepare the biological electrode.The morphology of Au nanoparticles and the surface of composite electrodeswere characterized by UV-Vis, SEM, TEM and other ways. The electrochemicalsensor was prepared by the composite electrodes. The detection limit andselectivity of the prepared electrochemical sensors were measured by cyclicvoltammetry, linear sweep voltammetry, and chronoamperometry. The results showed that:(1) SEM images indicated that the chitosan layer did not affect thestructure of MOF-5, the particle size of Au nanoparticles(Au NP) were3-4nm,after the modification of detection DNA, TEM showed better dispersion of AuNP. The CV curves of the different connections of MOF-5, Au NP and detectionDNA illustrated that the current of Au NP modified MOF-5-MGCE (39.92μA)was much bigger than the Au NP-MGCE(1.503μA), it exhibited that the porousstructure increased the possibility of hydrogen adsorbed to the surface of AuNPs, and improved the electro-oxidation ability of Au nanoparticles. The anodicpeak current of the prepared composite electrode is58.03μA, which is muchhigher than these built in other ways. This phenomenon illustrated the framestructure of MOF-5overcome the electron tunnelling caused by the longdistance between the surface of electrode and Au NPs and maximiseelectro-catalytic activity.(2) The CV, linear sweep voltammetry, and chronoamperometry of theelectrochemical DNA sensor at different target DNA concentrationsdemonstrated that the electro-oxidation current gradually increases withincreasing target DNA concentration. The current (12.55μA) at the target DNAconcentration of1pM was significantly higher than the current (8.660μA) atthe target DNA concentration of zero. Target DNA concentrations presented agood linear relationship with current in the range of1pM to100nM. Thecalculated detection limit was0.024pM. These results demonstrated that MOF-5present great potential application in the development ofelectrochemical biosensors for DNA detection. The test with complementaryDNA, base-mismatched DNA and non-complementary DNA illustrated theDNA sensor had good selectivity.