Solution Reduction Synthesis, Structure And Magnetic Properties Of Transition Metal Particles

Compared with magnetic ferrite particles,ferromagnetic metal and alloy particles have high saturation magnetizations and low coercivities,and are often used as high-density magnetic recording media,magnetic head materials,piezomagnetic transducer,magnetic fluid, MRI contrast agent and etc.For the magnetic properties of ferromagnetic metal and alloy particles are heavily related to their structure,size and shape,so the synthesis of ferromagnetic metal and alloy particles with the controlled structure,size,and shape by simple process is of great scientific and applied significance.The traditional methods for preparing ferromagnetic metal and alloy nanoparticles are by organic reaction or vapor deposition method and so on.There are many disadvantages of these methods,such as low yield,difficulty in post processing etc.Because of the simple operation,mild operating condition,price moderate and large-scale preparation,solution reduction process is widely used in the synthesis of metal and alloy nanoparticles.The base of the solution reduction process is redox reaction.For a redox reaction,the electrode potentials of the oxidization reaction and the reducing reaction determine whether the redox reaction can occur or not,and the speed of the redox reaction.In this work,the reaction conditions of the redox reactions were selected according to the E-pH atlas. According to the E-pH atlas the NaH₂PO₂ was used as the reducing agent to synthesis the cobalt nanoplatelets and cobalt-iron alloy sub-micro particles,respectively.In order to study the shape control mechanism,hydrate hydrazine was used as the reducing agent and the sodium ethoxide as the shape control agent to synthesis Ni nanoparticles in the ethanol solution.The structures and magnetic properties of the as-prepared HCP cobalt particles, Fe₈Co₂ particles and Ni particles were characterized,respectively.1.Cobalt nanoplatelets with hexagonal close-packed crystal structure have been synthesized through the reduction of cobalt chloride by NaH₂PO₂ in a mild aqueous solution. The as-synthesized particles are plate-like in form and have the diameter between 150 and 220 nm and thickness of about 10 nm.X-ray photoelectron spectroscopy analysis implied that the surfaces of the as synthesized samples were oxidized in air and formed a dense oxide layer.A selected-area electron diffraction analysis show that the perpendicular direction of the Co nanoplatelets is a[001]crystallographic direction.2.Iron-cobalt alloy particles with iron content from 10-90%have been synthesized through the reduction of cobalt chloride and ferrous sulfate by NaH₂PO₂ in aqueous solution. As the Fe content of the alloy is 10%,the synthesized particles are single-phase HCP structure.As the Fe content is larger than or equal to 20%,the crystal structure of the alloy is body-centered cubic(BCC).For the as-synthesized Fe₈Co₂ particles,the SEM result show the particles are cubic in shape,the size of the cubes are between 100-300 nm.3.XRD result revealed that the as-synthesized Ni particles are FCC structure.SEM measurement indicated that the particles are spherical in shape and the average diameter of the particles is about 16 nm.The particles are agglomerated in some degree.X-Ray photoelectron spectroscope measurement confirmed the existence of the surface oxidization layer of NiO and Ni₂O₃ on the particles.4.The magnetic properties of the HCP Co nanoplatelets,sub-micron FesCo₂ particles and Ni nanoparticles were investigated at room temperature.The saturation magnetizations of all these three kind of particles are all decreased as compared with the corresponding bulk values.The reasons for the decrease are the oxidation of the surface layer of the particles and the magnetic disorder in the surface or interface of particles.The coercivity of the particles can be influenced by the particle structure,morphology and size.The coercivity of the HCP cobalt nanoplatelets is much larger than the bulk value due to the contribution of the large shape anisotropy.For the particle size is relatively small, and the particles are single domain structure,the coercivity of the Ni nanoparticles is 101.2 Oe.For the sub-micron Fe₈Co₂ particles,it is the increase of the surface anisotropy that makes the coercivity of the sample largen.