Controllable Synthesis, Growth Mechanism, And Properties Of One-Dimensional Nanomaterials

One-dimensional (1D) inorganic nanomaterials are ideal systems for investigating the dependence of electrical transport, optical and mechanical properties on size and dimensionality and expected to play important roles as both interconnects and functional components in the fabrication of nanoscale electronic and optoelectronic devices, which are one of the research frontiers and focuses for the nanomaterials field.Referring to the principles of PVD, a novel vacuum vapor deposition (VVD) low temperature route was developed to grow high vapor pressure compounds one-dimensional nanostructures in the thesis. Some typical high vapor pressure compounds (CuCl, Zn, Cd, Se, Te) were chosen to discuss controllable synthesis, growth mechanism of one 1D nanostructures and explore novel function properties due to their sizes, dimensionalities.In the first chapter, the concepts, methods, histories and new developments of nanotechnology are concisely introduced. At the end of this chapter, we pointed out the importance of the search project and summarized the important results obtained in the thesis.In the second chapter, well-aligned CuCl nanorods standing on a Si substrate were prepared via evaporation of CuCl powder in vacuum. Using these CuCl nanorods as a precursor, arrays of CuS nanotubes with diameters in the range 150–250nm and lengths up to several micrometers have been successfully obtained through a gas-solid sulfurization reaction. Scanning electron microscopy indicates that the morphology and orientation of the parent CuCl nanorods are more or less retained after the CuS nanotube growth. The formation of CuS and the consumption of the CuCl precursor occur at the same time as the gas-solid reaction proceeds, and the nanotube growth pathway can be described in terms of Kirkendall effect.In the third chapter, by carefully controlling the experimental parameters, large-scale nanowire networks of group II-B metal (Zn and Cd) on Si substrates are fabricated through vacuum vapor deposition route (VVD). It is shown that the nanowires of networks structure are 20-250 nm in diameter and hundreds of micrometers in length. On the basis of the time-dependent experimental findings, these elementary species condensed at a favorable temperature region to produce one-dimensional nanostructures under the appropriate supersaturation degree. We proposed a phenomenological growth mechanism of the Zn (Cd) nanowire networks that involves vapor-solid mechanism mediated by agglomeration and secondary nucleation phenomenon on the basis of the time-dependent experimental findings. Moreover, the formation process of polycrystalline znic nanowire consisted of about 5 nm nanoparticles was in-situ observed under the irradiation of electron beam during the TEM measurement, showing that the melting point of znic nanowire is significantly lower than that of the bulk. The melting behavior of active metallic nanowire illuminates that the clear lattice fringes in the HRTEM image were difficult to obtain due to the diffusion of the surface atoms.In the fourth chapter, high-quality ultrawide Se and Te nanobelts have been prepared through a simple VVD route at a relatively low temperature. It is found that the morphology of the obtained products is strongly dependent on the evaporation time and temperature. Se urchinlike nanostructures consisting of nanorods and Te open nanorods are formed as intermediates during the nanobelt preparation process. As the evaporation proceeds, these intermediates undergo agglomeration through particle-nanorod collision, and finally Se and Te nanobelts are grown from the roots that are formed by the agglomeration. The growth of t-Se and t-Te nanobelts is believed to follow a vapor-solid mechanism mediated by agglomeration. In addition, it is also demonstrated that thin Te nanobelts transform to helices upon exposure to an electron beam. The successful preparation of Se and Te nanobelts indicates that the simple VVD technique is effective for morphological transformation of materials that have a relatively high vapor pressure at low temperatures.In the fifth chapter, we successfully synthesized two different phases of uranium oxide nanocrystals in high yields from UO₂(OAc)2·2H2O through a facile hydrothermal route using amines as both reducing and structure-directing agents. The sphere-shaped UO₂ particles have a diameter of about 100 nm and each individual particle consists of 15 nm nanocrystal subunits, whereas the U₃O₈ nanorods have a diameter ranging from 80 nm to 100 nm and a length of 500-1500 nm. It is revealed by TEM that the axis of the U₃O₈ nanorods coincides with the [001] crystallographic direction. The sphere-shaped UO₂ particles convert to porous U₃O₈ aggregates by calcination in air at elevated temperatures, and the morphology of the converted U₃O₈ material differ greatly from those of the parent UO₂ spheres and the as-synthesized U₃O₈ nanorods. Catalytic testing demonstrates that the catalytic activity of the porous U₃O₈ sample is distinctly superior to that of the U₃O₈ nanorods for the oxidation of benzyl alcohol to benzaldehyde, although these two materials possess similar BET surface areas. The reason behind this observation is that the former material exposes more {001} crystal planes than the latter. The present results suggest that the catalytic performance of uranium oxides is morphology-dependent.