Zeolitic Imidazolate Frameworks(ZIFs) And Their Composites:Synthesis,Microwave Absorbing And Supercapacitance Applications

Compared with conventional materials,Zeolitic imidazolate structures(ZIFs)delineate a new class of metal organic frameworks contained imidazolate linkers and metal ions,with aluminosilicate zeolites like structures.Over the years,with the gradual expansion of the research on ZIFs,their intrinsic porous characteristics,as well as excellent chemical properties and controllable structural characteristics,led to a wide range of potential applications in the fields of gas adsorption,molecular sensing,energy storage devices,optoelectronics,catalysis,biomedicine,magnetic properties,electromagnetic shielding and stealth technology.Furthermore,ZIF based materials serve as an ideal templates to yield high porous carbons and related nanostructured composites with a large surface area,uniform pores,balanced metal/organics ratio and controllable structure regarding their frameworks.In recent years,the researcher focused on the advance synthesis approach and potential applications in this rapidly developing field.In this dissertation,the preparation of ZIFs,the metal oxides matrices derived from the ZIFs,their nanostructure composites and applications of ZIFs derived metal oxides are mainly studied.The synthesis method,morphology,structural properties of ZIFs are investigated.The magnetic properties,phonon confinements,supercapacitance,electronic polarization,electromagnetic wave absorption and shielding effect are discussed in details.Following are the important aspects of the current research project:(1)Heterostructured TiO2/C/Co nanomaterial has been derived from carbonized zeolitic imidazolate(ZIF-67)porous framework.The obtained uniform nanospheres wrapped with TiO2 have shown the superior microwave absorption properties.Traditionally,the interaction of phonons within a confined system in terms of microwave absorption has not been studied in the literature to date.Here,we demonstrate that the phonon confinement is pivotal in the improvement of impedance matching,along with the multiple interfacial dielectric relaxations at different dielectric media(TiO2/C,C/Co)in the composite.Because of the specially crafted phonon confinement and multiple interface dielectric relaxations,the as-synthesized(TiO2/C/Co)composite manifest a maximum reflection loss(RL)of-42 dB with a broad bandwidth of 5 GHz.This work explains the possible mechanism of electromagnetic shielding in a confined system with multiple interfaces and may lead to design the advanced materials for microwave shielding.(2)Due to the high electromagnetic interference(EMI)pollution,electromagnetic adsorption materials have risen as an important research area in material science and technology.Herein,the prepared carbonized ZIF-67(CZIF)nanocomposites with polypyrrole(PPy)showed significantly enhanced electromagnetic wave absorption performance.The CZIF-PPy nanocomposites were prepared by the solvothermal and in-situ polymerization method.The CZIF-PPy nanocomposites possess a decent reflection loss(RL)value between the 2-18 GHz frequency range.The enhanced surface properties,magnetic-dielectric interfacial polarization and Eddy current loss play an important role to achieve higher reflection loss and broader absorption bandwidth.This work explains the importance of a magnetic-dielectric interface and may lead to design more advanced hybrid electromagnetic shielding systems.(3)Strips like ZIF-67 have been prepared by the wet chemical method and PPy has been deposited on the surface by the in-situ polymerization method and named as SZIF-67@PPy.The structure and morphology of prepared samples were observed by Scanning electron microscope(SEM),Transmission electron microscope(TEM),X-ray diffraction(XRD)and X-ray Photoelectron Spectroscopy(XPS).Further,the prepared samples(PPY,SZIF-67 and SZIF-67@PPy)were used as electrode active materials of supercapacitors.Compared with PPy and SZIF-67,the results show that composite SZIF-67@PPy has enhanced electrochemical performance,high rate capability and good cycle stability.The results indicate that SZIF-67@PPy can become the next generation electrode materials for high-performance flexible supercapacitors.(4)Three-dimensional(3D)ZnO microspheres with the composite of polyaniline(PANI)have been successfully synthesized by a one-pot solvothermal and in situ polymerization method.The obtained microspheres were uniform having a diameter of 4 ?m to 7 ?m.These microspheres,inside cushion of PANI polymer,exhibit excellent microwave absorption properties.Composite of ZnO microspheres with PANI increased the complex permeability and enhanced the dielectric loss.Thus,the microwave absorption properties of the composite have been intensified.Despite the fact that the composite of ZnO with PANI herein dissipates the microwaves by dielectric loss,their performance is admirable compared to most of the PANI-based composites,reported.The morphological,structural and spectral properties have been investigated by using scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD)and the Fourier transform infrared(FT-IR).It is found that the maximum reflection loss value of ZnO@PANI reaches-41 dB at 14 GHz with a thickness of 3.5 mm that is superior to the previously reported composite of PANI with other materials.(5)Heterostructured starfish-like C/CoNiO2 has been prepared by a solvothernal syathesis and was further thermally etched.Owing to the localized electron polar ization and significant magnetic loss,C/CoNiO2 heterostructures with starfish-like morphology exhibited significantly enhanced electromagnetiG attenuation and reflection loss.Such singularities esnable this lightweight C/CoNiO2 to become an outstanding microwave absorber with wide bandwidth and strong absorption.These findings provide new insight into the possible m echanism of microwave absorption due to electron polarization and demonstrate that it is critical to improve microwave absorption performance for future applications.