Fabrication Of Nickel-Cobalt Metal-Organic Framework Microflowers By Ultrasonic Spray Method And Their Application In Catalysis

Metal-organic frameworks(MOFs),as a new type of porous materials,have unique properties in porosity,ultrahigh specific surface areas and designable structures,which make them show a great potential in various applications,including gas separation and storage,catalysis,sensing,energy storage and conversion.Micro/nanoflowers are flower-like micro/nanoparticles orderly assembled by two-dimensional nanosheets.Among various hierarchical micro/nanostructures,micro/nanoflowers have been considered as one of the most suitable morphologies.Micro/nanoflower has advantages in large surface area and charge transfer because of the complete exposure of its nanosheets’ surface.Therefore,micro/nanoflowers are promising candidates for the applications in sensing,catalysis and analytical science.Combined metal-organic frameworks with the structure of micro/nanoflowers,MOF micro/nanoflowers exhibit superior performance over MOFs and micro/nanoflowers.Currently,there is hardly any report of MOF micro/nanoflowers,but micro/nanoflowers have mainly consist of inorganic minerals and metal oxides have been prepared and reported.The transformation of nanosheets into micro/nanoflower is a self-assembly process.When the experimental conditions are satisfied for this transformation,it is possible that MOF nanosheets self-assembled into MOF micro/nanoflowers.In this thesis,NiCo-MOF nanosheets were successfully assembled into microflowers by ultrasonic spray method.During the experiment,the metal precursors solution was nebulized by ultrasonic sounds and then sprayed on the surface of the organic ligands solution where the NiCo-MOF crystals are inclined to grow into nanosheets in lateral direction.Finally,the formed nanosheets were aggregated in the zone of the droplets,making them possible for further overlapping.By changing the metal precursor,the concentration and the ratio of two metal precursors,and the concentration of organic ligands,MOF nanosheets were successfully transformed into microflowers under an appropriate experimental condition.The morphology,composition,thermal stability and crystal structure of NiCo-MOF microflowers were respectively characterized and analyzed by SEM,EDS,EDS-Mapping,FT-IR,TG and XRD.The experimental results showed that the microflower is a hierarchical structure,which is orderly assembled by the nanosheets with smooth surfaces.And the microflower having crystal structure of NiCo-MOFs possess regular shape,excellent monodispersity and thermal stability.Subsequently,the synthesized NiCo-MOF microflowers were used as the catalyst in the catalytic reduction of 4-nitrophenol(4-NP)and the reaction was monitored by UV-vis absorption spectroscopy.The result indicated that NiCo-MOF microflowers exhibited excellent catalytic performance in the reaction.In order to compare with the catalytic performance of NiCo-MOF microflowers,bulk NiCo-MOFs were prepared by solvothermal method and were also used as catalyst to reduce 4-NP.By comparing the kinetics data of the reaction of 4-NP catalyzed by NiCo-MOFs with different morphologies,it concluded that microflowers show a better catalytic activity than those in bulk forms,because of nanosheets’ larger surface exposure and smaller thickness.Compared with the traditional methods for preparing MOFs,ultrasonic spray method is more facile and controllable and has been widely used in nowadays industry.Therefore,our investigation indicated that the ultrasonic spray method is an efficient and industrialized means to fabricate MOF microflowers,which lays the great foundation for practical applications in future.