| 其他摘要 | Magnetic nanomaterials have been widely applied in the fields of microelectronic devices, sensors, microprobes, high density magnetic recording media and biomedicine because of their special properties. The properties of magnetic nanomaterials are highly dependent on their size distribution, structure, morphology and synthesis method. Recently, synthesis of magnetic nanomaterials with various structures has attracted much attention of researchers. Here, we report a new method for assembling various nanostructures of cobalt and cobalt-nickel, by kinetically controlling the growth of crystals, in the absence of template and external magnetic field. The main results are listed as follows:
(1) Magnetic cobalt chains, self-assembled by microspheres of hexagonal-phase cobalt, have been synthesized via a hydrothermal reduction route in the presence of cobalt chloride, the surfactant sodium dodecylsulfate (SDS) and the complex reagent sodium tartrate. As-synthesized, the chains are 100-300 mm in length and the cobalt microspheres, which consist of nanosheets with an average thickness of about 60 nm, are 5–10 nm in diameter. The morphologies of the microspheres can be controlled by adjusting the concentrations of the surfactant and the complex reagent and also the reaction temperature. The growth mechanism has been proposed, based on time-dependent experiments.
(2) Hierarchical cobalt dendrites are synthesized by a sodium tartrate-assisted hydrothermal route. The route includes the fabrication of the Co dendrites in the solution of cobalt chloride and sodium hydroxide using sodium tartrate as the complex reagent and sodium hypophosphite as the reducing agent. Each Co dendrite is 50–100 μm in length and consists of a main branch with several secondary branches and leaves. The adjustments of the synthetic parameters lead to the formation of the Co products with different morphologies. A possible growth mechanism for the Co dendrites is proposed based on the characterization results of X-ray diffraction and scanning electron microscopy. The results showed that sodium tartrate played two roles in the formation of Co dendrites: it served as complex agent which can form complex with cobalt ions and therefore decreasing the concentration of free cobalt ions in the solution; and it can control the anisotropic growth of crystal by selective absorption - desorption on the special crystal facet.
(3) Cobalt flowerlike architectures composed of hexagonal nanoplatelets have been synthesized by a simple hydrothermal reduction method. The architectures are fabricated by the reaction of CoCl2 with NaOH in the presence of sodium dodecyl benzenesulfonate (SDBS). The diameters of the flowers range from 8 to 10 μm, and the average thickness of the hexagonal sheets is about 100 nm. Higher reaction temperatures and the proper concentration of sodium hydroxide (NaOH) are key requirements for the fabrication of the flowerlike architectures. A growth mechanism for these architectures is proposed on the basis of the characterization by X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The magnetic hysteresis loops at 5 K and 295 K of the cobalt flowerlike architectures show ferromagnetic characteristics.
(4) Cobalt chains with lengths of up to 4-20 μm, self-assembled by flowerlike cobalt submicrospheres, have been synthesized at 200 °C for 4 h by a solvothermal method with the surfactant poly(vinyl pyrrolidone) (PVP). The average diameter of individual flowerlike submicrospheres is 700-900 nm, which are composed of compact nanosheets with an average thickness of about 50 nm. The products were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS). The effects of synthetic conditions, such as reaction temperature and the amount of reducing agent, on the morphology and size of the chains were investigated. The growth mechanism of the chains was proposed, based on the evolution of the structure and the morphology with increasing the reaction time. The magnetic hysteresis loops at 5 and 295 K of the chains show ferromagnetic characteristics. Our work may shed light on the design fabrication of one-dimensional chainlike structures self-assembled by complex three-dimensional architectures of materials.
(5) Magnetic CoNi chains, are synthesized by a surfactant-assisted solvothermal route. The route includes the preparation of CoNi chains in ethylene glycol solution, using cobalt and nickel chloride as precursors and hydrazine monohydrate as reducing agent, with the surfactant poly(vinyl pyrrolidone) (PVP). The typical CoNi chains, with 20－30 μm in lengths, are self-assembled by submicrospheres, and the individual spheres have an averaged diameter of 800nm. The sizes control of individual CoNi spheres can be attained by adjustment the amount of PVP and N2H4•H2O. The magnetic measurement shows that the CoNi chains have a low saturation magnetization and a high coercivity. A possible growth mechanism is proposed, based on the characterization of X-ray diffraction, scanning electron microscopy and transmission electron microscopy.
(6) Uniform 3D hierarchical CuO butterfly-like architectures were fabricated by a surfactant-assisted hydrothermal oriented attachment route. This route included the formation of CuO butterfly-like architectures in a solution of cupric chloride and sodium hydroxide by using sodium dodecyl benzenesulfonate (SDBS) as surfactant. The as-prepared CuO architecture was characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The CuO butterfly-like architectures were assembled from several tens of oriented attachment rhombic nanosheets with a thickness of about 60 nm. A growth mechanism for the formation of the CuO butterfly-like architectures was proposed on the basis of time-dependent experiments. The synthetic parameters such as reaction temperature, the concentration of sodium hydroxide and reaction time all affected the morphology of the CuO architectures. |
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