Solvothermal Synthesis Of Nano-or Micro-sized Magnetic Materials

The family of Fe-, Co- and Ni- based nano- or micro-sized magnetic materials has drawn considerable interest because of their unique attributes such as optical, electrical, magnetic, catalytic as well as potential applications in high-density information storage, biological anti-cancer drugs and single-electron devices. Increasing attention is being focused on the synthesis of their nanowires, nanorods, nanotubes with anisotropic structure.Hexagonal-like Co microflakes with an average diameter of 2μm and a thickness of 80 nm have been successfully synthesized by reduction of cobalt nitrate in a polyol/ethylenediamine mixed medium via a solvothermal process in the presence of PVP and dodecanethiol as the structure-directing agents. The products were characterized by XRD, SEM, TEM, HRTEM and SQUID. The results show that Co microflake exhibits well-crystalline hexagonal close-packed (hcp) phase and was self-assembled to chain-like structure and is composed of ultrathin ribbon-like nanostructure with a thickness of 10 nm. This material exhibits a ferromagnetic behavior with a saturation magnetization of 126.6 emu/g, a remnant magnetization of 13.7emu/g, and a coercivity of 163.4 Oe. The influence of synthetic parameters such as reaction time, NaOH concentration and surfactant on crystalline phase and morphology of chain-like assembly of Co nanocrystals was also investigated. The Ostwald ripening growth mechanism was proposed to explain the formation of Chain-like Co assemblies. In the initial stage, the product is mainly composed of Co(OH)₂. Then the precursor was gradually reduced to Co microflake. The effect of NaOH, PVP together with dodecanethiol led to the formation of chain-like assemblies of Co nanocrystals. With a prolonged time, the small crystals dissolve, disappear and then grow on the large crystal, in which the chains-like structure of Co was opened and resulted in the increase of thickness of hexagonal Co microflakes.Cubic Fe₃O₄ hollow microspheres with the diameter of 400 nm has been synthesized in ethylene glycol-ethylenediamine mixed medium via a solvothermal route at 190℃for 24 h in the presence of PVP as the structure directing agent. Ostwald ripening mechanism was also effective to explain the growth of Fe₃O₄ hollow microspheres. Similar solvothermal synthesis route was extended to synthesize Ni and its oxides. The results show that the as-synthesized Ni existed in two phases: cubic and hexagonal. The particle size is about 30 nm. Further work is under way.