Crystal Structure,dielectric Relaxation Mechanism And Electrochemical Sensing Behavior Of ZIF-8 And ZIF-8/Nanocarbon-based Composites

Zeolitic Imidazolate Frameworks(ZIFs)have been widely used in gas adsorption,catalysis and other fields due to their unique porous structure,abundant active sites and chemical stability.Recently,ZIFs have also shown great potential in the fields of fuel cells,supercapacitors,electrochemical sensors,micro integrated circuits and other electronic devices.As low dielectric materials,ZIFs exhibit very low intrinsic conductivity.Their electrical properties are usually regulated by adjusting structure,porosity,ligand and compounding with conductive phase.But the influence of synthesis process and crystal structure on their intrinsic electrical properties,and the control mechanism of electrical properties of ZIFs composites have not yet been clarified,need to be further studied.In this paper,based on the control of the crystal structure,micro morphology and guest molecules of ZIF-8 porous materials by different preparation method,we studied the electrical chemistry properties and dielectric relaxation mechanism of ZIF-8 and its composites with nanocarbon materials,and finally designed an electrochemical sensors for detecting dopamine with excellent properties.Firstly,the microstructure and morphology control,intrinsic and interface dielectric response of ZIF-8 and its composites were studied in detail from the aspects of material design,preparation technology,dielectric theory analysis,impedance spectrum simulation.On this basis,the conductivity of ZIF-8/Nanocarbon-based Composites was regulated,and the surface dynamics and mass transfer process of glassy carbon electrode modified by this ZIF-8 composites were analyzed,and the electrochemical sensing behavior were clarified finally.This work provides an important way for understanding the physical and chemical properties of ZIF-8 and its composites with nanocarbon materials,clarifying the influence law of solvent on the intrinsic electrical properties of ZIFs,solving the problems of poor conductivity,and expanding its application in the fields of electrochemical sensors,fuel cells and so on.The main contents are as follows:1.The synthetic methods of ZIFs are various,and can be divided into solid phase method and liquid phase method.In different prepare processes,the existence and types of solvents have great influence on the microstructure and properties of ZIF-8 products.ZIF-8powders were prepared through solid-state synthesis by mixing and calcining.The reaction kinetics and phase evolution were studied.In order to further study the effection of solvent control on ZIF-8 crystal structure and morphology,we prepared ZIF-8 crystals with different morphology and microstructure in three different solvents(methanol,water and ammonia hydroxide).The structure and morphology of ZIF-8s were characterized.The solvents play an important role on grain growth of ZIF-8,furthermore,a small fraction of monoclinic phase can be found in the ZIF-8s prepared in water or in amonium hydroxide.Radio-frequency dielectric and impedance spectra studies of dynamic dielectric response of ZIF-8s indicate two additional dielectric relaxation processes associated with the introduced water molecules in as-prepared samples.The dielectric response can be described by the combination of the dc conductivity,the universal dielectric response,the Cole-Cole relation and intrinsic response.This research revealed the relationship between the crystal structure,morphological characteristics and preparation process of ZIF-8,and clarify the influence of water molecules on its electrical properties.The results could provide technical support and theoretical guidance for the controllable preparation of ZIF-8 and improve its application in electrical devices.2.We prepared ZIF-8/CNT composites by in situ growth of ZIF-8 on carbon nanotubes.The crystal structure,morphology of the composites were investigated.Dielectric permittivity of as-prepared composites is improved more than 10 times,dc and low frequency conductivity increased by 8 orders of magnitude by introduced CNT.Based on the dielectric theoretical model,the dielectric response principle of ZIF-8/CNT composites is deeply analyzed,and the contribution mechanisms of proton long-distance transport,interface effect and rotation of water molecules to the dielectric constants of the composite system in different frequency bands were clarified.The composites have a low percolation threshold(3.2%),where the dc conductivity increases by 4-5 orders of magnitude near the percolation threshold.Furthermore,we find three dielectric relaxation processes in the as-prepared ZIF-8 and ZIF-8/1%CNT composite by electrical modulus.One low-frequency dielectric relaxation associates with long-range proton transport,and the middle-frequency dielectric relaxation is related to Maxwell-Wagner effect between ZIF-8 and CNT,while the high-frequency dielectric relaxation attributes to the short-range hopping process of carries or dipoles orientation.The work provides deep insights on the intrinsic and extrinsic dielectric responses of ZIF-8 based composites and the percolation behavior of hybrid ZIF-8/conducting materials composites.The control technology of conductivity and dielectric of ZIF-8 composites is improved,and ZIFs composites with excellent electrical properties are designed,which can provide theoretical support for expanding their applications in various electrical fields.3.Glassy carbon electrode is the basic electrode commonly used in various electrochemical sensors,and its interface characteristics directly affect the detection performance.We established an equivalent circuit including electrolyte resistance(R_el),charge transport resistance(R_ct),diffusion impedance(R_di,C_di),electrochemical(oxidation/reduction)reaction impedance(R_R,C_R),surface adsorption impedance(R_ads,C_ads),double-layer capacitance(C_DL),and derived the mathematical expression of the equivalent circuit.The impedance spectrum of the electrode system in different states were simulated by changing the five parameters of the mathematical expression.The contribution of different factors to the impedance spectrum of GCE was revealed.The changes of the electrode surface characteristic before and after modification with ZIF-8/IL-RGO nanocomposite were quantitatively compared and analyzed.The mathematical expression provides a good theoretical and methodological support for the modification strategy of glassy carbon electrode and can be used to analyze impedance spectrum accurately.4.Owing to the synergistic effect from large surface area and excellent electrocatalytic activity of ZIF-8 and great electrical conductivity of IL-RGO,a high sensitive sensor for dopamine(DA)can be obtained.A hybrid and hierarchical nanocomposite was successfully prepared by growth of ZIF-8 on the template of ionic liquid(IL,[BMIM][BF₄])functionalized reduced graphene oxide(IL-RGO).The structure and morphology of the ZIF-8/IL-RGO nanocomposite were characterized by X-ray diffraction,scanning electron microscopy,Fourier transform infrared spectrometer and Raman spectroscopy.The results showed that RGO sheets were refrained from restacking by IL,and ZIF-8 nanoparticles grown well on the surface of IL-RGO.ZIF-8/IL-RGO modified electrode exhibits good electrocatalytic activity and electroconductive properties towards DA which were investigated by cyclic voltammetry,differential pulse voltammetry and electrochemical impedance spectroscopy.Compared with bare or IL-RGO modified electrodes,the ZIF-8/IL-RGO modified electrode effectively depressed the oxidation overpotential of DA.The linear response range of DA was from 1.0×10^-7to 1.0×10^-4mol/L with a low detection of limit3.5×10^-8mol/L.In addition,the sensor was shown to provide satisfactory stability for the determination of DA.