The Reduction Of Complex Precursor Cobalt Bis(4-pyridine Carboxylate) Tetrahydrate Under External Magnetic Fields And The Synthesis Of Cobalt Micro-/Nano Structures

In recent years,magnetic field has been introduced as a new tool to control chemical reactions and materials synthesis.Anisotropic hexagonal Co nanocrystals are important topics in magnetic nanomaterials research for their high saturation magnetization and magnetic coercivity.The objective of this dissertation is to search the special features of the chemical reactions occurring under magnetic fields and synthesize micro-and nanostructured cobalt magnetic materials.The formation mechanisms and magnetic properties of these materials have also been systematically studied.The main parts of the results are summarized below:1.Cobalt microspheres were prepared through the self-assembly of nanoflakelets formed by reduction of a special metal organic complex cobalt bis(4-pyridine carboxylate) tetrahydrate(CoL₂(H₂O)₄,L=4-pyridine carboxylate) with hydrazine at 160℃for 36 h.The Co fiakelet is ultrathin with the thickness of about 10 nm,whose normal direction([001]) is parallel to the easy magnetization axis.The groups of isonicotinic acid of CoL₂(H₂O)₄ molecule and the agglomerate morphology of the molecules in solution play important roles in the formation of microspheres.Low concentration of CoL₂(H₂O)₄ and high concentration of NaOH in the system are beneficial to the fabrication of the microspheres.The hysteresis loop shows that the assembly exhibits peculiar soft magnetic properties at room temperature,which may result from both the anisotropy and the self-assembly manner of the nanoflakes.The self-assembly manner is isotropic,while the ultrathin nanoflake itself is anisotropic.The sample formed via the same route with an external magnetic field applied irregular.This may be attributed to the fast reaction rate.2.Ordered 3D firtree-like hexagonal cobalt dendritic superstructures have been prepared with a metal complex precursor CoL₂(H₂O)₄ by sodium hypophosphite reduction.The dendrite has a main axis along the[001]direction and the leaves, which grow parallel to the(001) plane,arrange layer by layer along the axis.The complex ligands are proposed to be responsible for the formation of 3D dendrite. The hysteresis loops measured at 300 K reveals a ferromagnetic behaviour with saturation magnetization of 134.0 emu/g and coercivity of 184.9 Oe.The coercivity is lower than that of other cobalt dendritic crystallites,Which may result form the lower total morphology anisotropy of our sample.The products formed via the same route under an external magnetic field are a majority of dendritic microcrystals.This means that the influence of the variation of the morphology of the sample.There are two possible reasons.One is that the growth rate of the dendrites is too fast and the other is the interactions,inducing the growth of dendrites,are much stronger than the magnetic force.These 3D dendritic microcrystals may bring about new opportunities in physical research and have potential applications in micro-/nanodevices.3.The special features of the chemical reactions occurring under magnetic fields are studied.And the details of the effects induced by the combined action of the magnetic field force and these factors in the reaction system are discussed. Particular hexagonal cobalt potato-like microspheres consisting of nanoflakes have been obtained with a metal complex precursor CoL₂(H₂O)₄ by hydrazine reduction. And one-dimensional polycrystalline cobalt chains and wires composed of potato-like particles were formed via the same route with external magnetic fields applied.We systematically investigated the influence of synthetic conditions on the morphology,structures and magnetic properties and it is suggested that the complex molecules tend to agglomerate into spherical clusters that gather along the magnetic lines of force due to magnetic attraction.Then the reduction may occur easily along the lines of magnetic force,as a result the linear chains are formed.The Co nanoflakes,which are the basic units of cobalt spheres,do not assemble into wires or chains directly under a 0.35 T magnetic field.This means that the spherical clusters and the potato-like spheres are stable under the external magnetic field owing to the interaction between complex molecules(or nanoflakes) and that the magnetic field force is weaker than the interactions among complex molecules(or nanoflakes).The hysteresis loops measured at 300 K reveal soft magnetic behaviors of the samples. The coercivity of the AF sample is higher than that of the ZF sample,which may be attributed to the reason that the assembly manner of a potato-like sphere has lower shape anisotropy as a total than that of the one-dimensional chain.