Controllable Synthesis Of Micro/Nano Hierarchical Structure Nickel Hydroxide And Their Electrochemical Properties

The physical and chemical properties of nanomaterials depend on the morphology andsize of the materials. So controlled synthesis of nanomaterials is very important for thedevelopment of nanoscience and nanotechnology. In this study, the preparation of nickelhydroxide micro/nanostructured materials and the effective regulatory factors were studiedsystematically, including hydrothermal synthesis of hierarchical porous β-Ni（OH）₂and NiOnanospheres, mesoporous α-Ni（OH）₂and Ni（OH）₂/graphene composite materials. Theinfluence of reaction conditions on the material structure, morphology and electrochemicalproperty were investigated in depth and the synthesis mechanism was discussed.The main research contents and results are listed as follows.（1） The synthesis of hierarchical porous β-Ni（OH）₂and NiO nanospheres by ahydrothermal method.The uniform flowerlike nanospheres of β-Ni（OH）₂and NiO with hierarchical structureswere synthesized by a facile hydrothermal method. The effect of ammonia and citrate on thestructure and morphology of the products was thoroughly investigated by X-ray powderdiffraction, scanning electron microscopy, transmission electron microscopy and nitrogenadsorption-desorption measurements. Ammonia was found to be crucial for the formation offlowerlike spheres assembled from nanosheets. The addition of citrate could remarkablyreduce the particle sizes and increase the specific surface areas of flowerlike spheres. Apossible formation mechanism based on the experimental results was proposed to understandtheir growing procedures. β-Ni（OH）₂and NiO nanospheres prepared with the addition ofcitrate showed excellent capacitive properties due to their mesoporous structures and largesurface areas, suggesting the importance of controlled synthesis of hierarchical nanostructuresfor their applications.（2） The influence of organic acids on α-Ni（OH）₂structure and its electrochemicalproperties.The nickel nitrate and urea were used as raw material and precipitator respectively, andthe bayberry-like α-Ni（OH）₂nanostucture with high specific surface areas were prepared. Wealso studied the effects of five organic acids on the structure, morphology and electrochemical properties of α-Ni（OH）₂. The five organic acids included maleic acid, amino acid,sulfosalicylic acid, oxalic acid and camphor sulfonic acid. The results showed that the fiveorganic acids have little impact on the morphology of the products at a low concentration, butcould increase the specific surface areas and capacitive properties. With the increase of theamount, these five organic acids have different influence on the specific surface areas, poresize and capacitance performance of the products.（3） The synthesis of Ni（OH）₂/graphene composites.In this study graphene oxide were prepared by a modified hummer’s method.Ni（OH）₂/graphene composites were synthesized by a hydrothermal method with urea asprecipitator and hydrazine hydrate as reducing agent. The structure and morphology of theproducts was thoroughly investigated by transmission electron microscopy, X-ray powderdiffraction, and energy dispersive spectrometer measurements. The results showed that theinterlayer spacing of the grapheme oxide increased to0.86nm, which indicates thesingle-layer sheet structure and the high degree of oxidation. The Ni（OH）₂nanoplates in theNi（OH）₂/graphene composite dispersed onto the reduced grapheme oxide randomly with goodcrystallinity and uniform particle size. Cyclic voltammetry measurement was carried out forNi（OH）₂nanoplates, reduced grapheme oxide and Ni（OH）₂/graphene composite. The resultsshowed that Ni（OH）₂/graphene composite show high specific capacitances, and with theincreasing of the scan rate, the cyclic voltammograms remained unchanged, indicating thatthe composite has a good electrochemical reversibility.