| The refractory metal tungsten and molybdenum have excellent physical and chemical properties, such as high strength, low work function, high electron emissivity, etc. Nanostructural material has important research significance and practical value due to its differences of characteristics from the macroscopic material. Because of their special structural and superior properties (mechanics and field emission), tungsten and molybdenum nanomaterials are widely used in national defense industry, biological medicine and so on.In this paper, with tungsten and molybdenum trioxide as raw materials, W nanowhiskers, nanosheets/nanobelts and Mo nanostructural materials synthesized by a simple chemical vapor deposition were characterized systematically by the X-ray diffraction(XRD), scanning electron microscope(SEM), transmission electron microscope(TEM) and atomic force microscope(AFM). Also, the growth mechanism of W nanowhiskers, nanosheets/nanobelts and Mo nanocones were explored. Moreover, the mechanical properties of W nanowhiskers and nanoshees were measured by nanoindentation instrument.Main conclusions show as follows:1) The main as-synthesized products were W nanosheets/nanobelts with a length of10to60μm and a thickness of100～300nm when the deposition time increased to24h. W nanowhiskers with a diameter of50-500nm and a length of10to100microns were also synthesized.2) A growth model for the as-synthesized W nanosheets/nanobelts was proposed. The thermodynamics and kinetics of crystal growth were found to be the decisive roles on the well-defined geometrical shape and two-dimensional growth of the W nanosheets/nanobelts.3) The mechanical properties of W nanostructural materials on the single crystal Si substrate were measured by nanoindentation instrument. The W nanowhiskers exhibited a hardness of6.2GPa and an elastic modulus of225GPa while the nanosheets/nanobelts exhibited a hardness of6.3GPa and an elastic modulus of176GPa.4) The Mo nanocones with a uniform height of～2μm and a good shape consistence were synthesized through reducing the molybdenum trioxide by hydrogen. By changing the deposition temperature, the MoO3nanowhiskers, MoO2nanosheets and Mo nanoparticls were also synthesized.5) According to surface energy minimization and competitive growth theory, the synthesis mechanism of Mo nanocones was explored based on the Wulff theorem. |
