Solvothermal Synthesis Of Co Particles And Magnetic Properties

In modern materials and chemistry science, the precisely architectural manipulation of micro-/nanomaterials with well-defined morphologies and accurately tunable sizes has drawn extensively attention. One of the challenging and urgent tasks in this area is how to rationally control the orientation growth of materials and manipulate their compositions, morphologies, sizes as well as dimensionality, consequently achieving a better understanding of the observed complex phenomena of crystal growth, revealing the underlying fundamental theories and principles and realizing the design of functional materials. Magnetic materials have attracted considerable interest due to their novel properties and many applications in electron, communication, automobile, aerospace, biology and medicine, et al. Cobalt display a wealth of magnetic, electronic, optical, microwave, and catalytic properties. It is believed that the properties of micro-/nanomaterials strongly depend on their size, shape, and dimensionality. Because the multiple crystal structures and morphologies of cobalt have a significant influence on its properties, the synthesis, characterization and magnetic properties of micro- and nano-structured magnetic materials with uniform morphologies have become a hot topic in magnetism research areas and are attracting a growing interest. In this dissertation many efforts have been made to design and synthesize micro-and nanostructured magnetic materials Co with uniform morphologies. The formation mechanisms and magnetic properties of these materials have also been systematically studied. The main contents and major results are given as follows:1. Cobalt micro/nano-crystals with different sizes and morphologies were fabricated via a facile solvothermal method. Marked morphological changes in Co are achieved by varying the reaction conditions, such as dosages of precursors (D), reaction temperatures (T), types of alkali and different solvents during the solvothermal treatment. With remarkably enhanced D, a mass of increase in the Co atoms production rate shifts the balance between structural transition and growth to different extents, and the lack of a globally homogeneous reaction environment for each particle in the solution results in the formation of particles with different morphologies or fluctuation of particle structures in the final product. The different types of alkali kinetically control the nucleation and growth rates and conceivably modulate the size and shape of the final product. And also reduction potential and viscosity rank of the solvents play the important roles in controlling the sizes and morphologies of cobalt. By results of the time-dependent, the formation mechanisms of Co microcrystals in solvothermal systems are conjectured. Moreover, the magnetic measurement indicates that magnetic properties of Co depend on their morphologies.2. Co sub-microchains consist of particles with different morphologies including rare donut shape have been successfully prepared using solvothermal method. Here,1,3-propanediamine (PDA), which serves as both the alkali and template, are introduced the solvothermal reaction. An important phenomenon, which the particle growth can involve attachment characterized by a small misorientation at the interface, is observed from investigation on the morphology and microstructure of Co sub-microchains. Furthermore, Co sub-microchains generate dislocations because of imperfect oriented attachment. By investigation on the growth mechanism and characters of dislocations, we find that synthetic one-dimensional smooth metallic Co chain is very difficult without the aid of any external force such as templates or magnetic field-induced. One thing to be mentioned is that the Co sub-microchains are not oxidized until the environmental temperatures are up to 500℃by the DTA-TG analysis. For magnetic measurement for Co 1D sub-microchains, the values of saturation magnetization (Ms) is 123 emu/g, the values of remnant magnetization (Mr) is 7.1 emu/g, and the values of coercivity (Hc) is 179 Oe.3. Layered Co particals intercalated with dodecyl sulfate (DS) ions were synthesized by precipitation of EG solution of divalent cobalt and sodium dodecyl sulfate (SDS). The hexagonal Co nanosheets with homogeneous thickness can be prepared by exfoliation of layered Co intercalated with DS ions which were mixed with formamide (50 mL) to exfoliate the host layers, and the resulting suspensions were heated for several days at 40℃without agitation. Unusual flower-shaped Co nanostructures had been synthesized using a mild solvothermal treatment with a spot of SDS at 200℃. Based on time-dependent experimental result, we ascribe the oriented attachment mechanism to the growth of the Co nanostructures. The surfactant SDS plays a key role in the formation of the flower-like morphology. The magnetic measurement for samples of hexagonal Co nanosheets, the values of Ms, Mr and He were 148 emu/g,4.9 emu/g and 88 Oe. The values of layered Co were 91 emu/g,9.8 emu/g and 137 Oe.4. Co with different hierarchical structures, such as dendritic particles, flowery particles, flocky spheres, have been successfully prepared using a simple solvothermal process by rationally adjusting the dosages of H2C2O4 and N2H4·H2O in the reaction system. The hierarchical structural Co crystals had hexagonal-close-packed (hcp) structure. The dendritic Co crystal is comprised of several 1-5μm-length dendrites and the hierarchical assemblies radiates from the center. The flowerlike Co submicrocrystals are composed of petal-like crystals with around 1.5μm in diameter and 100-200 nm in thickness. An intergrade from single-crystal to polycrystalline for flowerlike Co submicrocrystals is interestingly observed. The further magnetic measurement for Co crystals with different hierarchical structures, to dendritic particles, flowery particles and flocky spheres, the values of Ms are 124,139, and 131 emu/g; the values of Mr are 3.6,5.4, and 0.9 emu/g; the values of He are 75,105, and 22 Oe. It indicated that the morphology of the Co crystals had an obvious influence on their magnetic properties.