Facile Synthesis And Magnetic Properties Of Hierarchical Micro/Nanocomposites Of Co And Co_xC(x=2,3)

Three-dimensional?3D?hierarchical micro/nanocomposites constructed from lowdimensional nanostructured units?zero,one and two-dimensional nanomaterials?not only have the excellent properties of the low-dimensional nanostructure unit,but also the multistage 3D structure synergistic nature.Therefore,this novel multistage micro/nanocomposite has attracted much attention because of the application prospect in biomedical field such as electron,magnetic sensor and targeted drug carrier.Most of the conventional methods for achieving 3D hierarchical architectures require the use of surfactants,catalysts,and various soft or hard templates which are expensive,cumbersome and not environmentally friendly.The disadvantages limits their potential for further application.In this paper,by selecting different solvents?castor oil,ethylene glycol,tetraethylene glycol,etc.?,we tried to use simple,efficient chemical reduction method and improved dropping method to prepared the sphere,spherical chain,corn rod and flower-like 3D hierarchical micro/nano-materials.The influence of the experiment parameters on the morphology,structure and magnetic properties was studied.And the main results are as follows: 1.Preparation and magnetic properties of sphere and spherical chain cobalt micro-structureThe hierarchical microspheres were prepared by the method of modified dropping method,and the one-dimensional spherical cobalt chain was prepared by the addition of auxiliary magnetic field.We found that a mesophase complex was obtained when the basic ratio([NaOH]/[Co²⁺])b = 2.The hcp-Co can be obtained only as the basic ratio b ? 5.At a high reduce ratio([N₂H₄·H₂O]]/[Co²⁺]),r = 28,the self-assembled flower-like cobalt microspheres were abtained which have an average diameter of about 3 ?m and consist of cobalt nanosheets with a thickness of about 50 nm.The magnetic properties of the samples are closely related to the reduction ratio.Increasing the concentration of cobalt salt or decreasing the hydrazine hydrate content,the saturated magnetization Ms of the obtained cobalt powder decreases,but the coercivity Hc is greatly improved.While decreasing the concentration of cobalt salt or increasing the hydrazine hydrate content is the opposite trend.With a auxiliary magnetic field at 80 mT,the obtained microspheres would be arranged in order to form a one-dimensional spherical chain.2.Corn-like hierarchical cobalt microstructure prepared with dropping methodThe microstructure of corn-like hierarchical cobalt microstructure was successfully prepared by dropping method with TEG as solvent and hydrazine hydrate as reducing agent.The solute concentration,reaction temperature and the NaOH concentration have important influence on the morphology of the hierarchical architectures.Solute concentration affect the nuclear formation and growth.The reaction temperature plays an important role in the growth of dendrites in the hierarchical structure.The suitable conditions for the formation of monodisperse corn-like hierarchical architectures is at a solute concentration of 0.025 M and 80 100 ? reaction temperature.The magnetic properties is apparently depend on the reaction temperature.The optimum saturation magnetization and coercivity of the metal cobalt can be obtained under the reaction conditions at 100?(Ms = 162.1 A·m2·kg^-1,Hc = 21.87 kA·m^-1).3.Preparation and magnetic properties of Co and Co_xC?x = 2,3?microstructures by polyol methodThe Co and Co_xC?x = 2,3?permanent magnet nanocrystalline materials were prepared by polyol method using TEG as solvent and PVP as surfactant.It was found that the composition and structure of the reaction product were closely related to the basic ratio([NaOH] / [Co²⁺]).When b is less than 2,it tends to form metal cobalt with a hexagonal structure.When b is greater than or equal to 2,the reaction tends to form a Co_xC alloy phase structure.The variation of solute concentration and NaOH concentration had a great influence on the morphology of the prepared samples.With higher concentrations of NaOH?0.3 M and 0.4 M?,cobalt particles show a cylindrical and spherical morphology with a porous net structure on the surface.This particular morphology brings an improved adsorption performance.The prepared Co_xC alloy phase shows a much better coercivity(up to 74.07 kA·m^-1)than the hcp structural Co.while compared to the hcp Co,a lower saturation magnetization(Ms 46.15 A·m2·kg^-1)was founded.4.Preparation and magnetic characterization of carbon-coated cobalt nanoparticles with a facie green methodCarbon-coated cobalt nanoparticles were prepared by a simple two-step method without any other surfactant and catalyst but only a natural polyol castor oil?CSO?as solvent.The reaction mechanism was studied and the optimum technological conditions were explored.The reduction process of cobalt is happened in two stages: liquid phase reaction and wet chemical reduction reaction process,resulting in a stable metal Co nano-crystalline material.The optimum conditions for the preparation of nano-scale cobalt by two-step method are as follows: the optimum concentration of cobalt salt is at 4-6 M,the chemical reaction is carried out at the temperature of liquid phase? 120 ??for 1-2 h,and then annealing at 350 ? for 2 h,an amorphous carbon-coated cobalt nanocrystalline phase was obtained.The reduced metal Co nanocrystals are easily oxidized to form a Co3O4 phase while the process parameters are exceeded.The coercivity of cobalt nanocrystals is up to Hc 16.1 kA·m^-1?203 Oe?,which is higher than bulk cobalt.