Chemical Liquid Phase Synthesis, Characterization And Properties Of Quasi-one-dimensional Inorganic Nanomaterials

In this paper, based on the hydrothermal and solvothermal methods, we developed several new chemical solution methods to 1D nanomaterials. By using the novel methods, various of nanomaterials have been synthesized, such as SiC nanowires(nanobelts, nanonedles), Te nanotubes (nanowires), Se nanotubes, Te/C nanocables, and tellurides/C nanocables. The formation mechanism of these 1D nanostructures have also been systemically investigated. The main work are described as follows:(1) A simple hydrothermal reduction method, employing sodium tellurate (Na₂TeO₄·2H₂O) as tellurium source and formamide (HCONH₂) as a reductant, was used to prepare and investigate tellurium nanotubes. The diameters of the nanotubes range from 200 to 600 nm and their lengths from 4 to 15μm. Unlike studies has been reported previously,^1.2 a series of electron microscopy characterization results suggests that the growth of tellurium nanotubes under the present experimental conditions is governed by a nucleation-dissolution-recrystallization growth mechanism: sphere-like tellurium nanoparticles initially formed in the hydrothermal system; the sphere-like nanoparticles were gradually dissolved to generate free tellurium atoms in the solution; these tellurium atoms were renewedly transferred onto the surfaces of the sphere-like nanoparticles and evolved into groove-like nanorods; the groove-like nanorods could be grown into tellurium nanotubes eventually.(2) We report a hydrothermal reaction to make t-selenium nanotubes, in the absence of a surfactant or polymer to direct nanoparticle growth, and without externally added forces (such as ultrasonic). A series of electron microscopy characterization results suggest that the growth of t-selenium nanotubes is governed by a nucleation-dissolution-recrystallization growth mechanism. In this mechanism, t-selenium nanoparticles were initially formed in the hydrothermal system, then, the t-selenium nanoparticles started to dissolve into the solution and grow onto large nanoparticles of selenium and sphere-like microparticles were obtained. The sphere-like microparticles then gradually dissolved to generate selenium atoms in the solution; these selenium atoms were renewedly transferred onto the surfaces of the sphere-like microparticles and were recrystallized. Along with the dissolution-recrystallization process, the sphere-like microparticles gradually evolved into a novel groove-like nanostructures. The nanogrooves could grew along the circumferential direction and the tuber axis direction until all sphere-like microparticles had been completely consumed, eventually grew into t-selenium nanotubes. Studies found that this growth mechanism is strongly affected by temperature and concentrations of NaOH. By adjusting temperature and concentrations of NaOH, t-selenium nanotubes; nanowires, microrods, porous microtubes, and polyhedrons can be synthesized, respectively.(3) A green chemical synthetic route was developed to synthesize single-crystalline Te nanowires with an average diameter of 7 nm at low temperature (90°C) by using ascorbic acid as a reducing agent, Na₂TeO₃ as a tellurium source, and surfactant cetyltrimethylammonium bromide (CTAB) as a structure-directing agent. The morphology of Te nanowires in the presence of CTAB is strongly dependent on the reaction conditions such as the concentration of CTAB, reaction time, and reducing agents. A surfactant-assisted solid-solution-solid growth mechanism was identified to explain the formation of ultrathin Te nanowires. The single-crystalline Te nanowires with an average diameter 7 nm display strong luminescent emission in the blue-violet region.(4) We report a general and reproducible route to synthesize coaxial metal tellurides/carbon-rich composite nanocables via an in-situ reaction of metal ions and Te/carbon-rich composite nanocables under hydrothermal conditions at 160°C. These metal tellurides/carbon-rich composite nanocables have hydrophilic, organic-group-loaded surfaces, which indicated their potential applications in biochemistry, biological sensors, and biomaterials.(5) A novel nanostructure, cubic silicon carbide (3C-SiC) nanoparticles encapsulated in branched wave-like carbon nanotubes have been prepared by a reaction of (?). 2-dimenthoxy ethane (CH₃OCH₂CH₂OCH₃), SiCl₄ and Mg in an autoclave at 600°C. According to X-ray powder diffraction, the products are composed of 3C-SiC and carbon. TEM and HRTEM images show that the as-synthesized products are composed of 3C-SiC nanoparticles encapsulated in branched carbon nanotubes with wave-like walls. The diameter of the 3C-SiC cores is approximately 20-40 nm and the thickness of the carbon shells is about 3-5 nm. In Raman scattering Spectroscopy, both TO (Γ) phonon line and LO (Γ) phonon line have red shifts about 6 cm^-1 relative to that for the bulk 3C-SiC. Photoluminescence (PL) spectrum shows that there are two emission peaks: blue light emission (431 nm) and violet light emission (414 nm). A sequential deposition growth process (with cores as the templates for the shells) for the nanostructure was proposed.(6) Silicon carbide nanobelts have been prepared by a reaction of CH3CH2OH, SiCU and Li in an autoclave at 600°C. According to X-ray powder diffraction, the nanobelts crystallize with the structure of 3C-SiC. Electron microscopy investigations have revealed that the nanobelts are usually 50-200 nm wide, 20-60 nm thick, and up to tens of micrometers long. Some unique optical properties are found in the Raman Spectroscopy and photoluminescence emission from the 3C-SiC nanobelts, which are different from previous observations of 3C-SiC materials. A possible lithium-assisted growth mechanism for the nanobelts was proposed.(7) A novel magnesium-catalyzed co-reduction route was developed for the large-scale synthesis of alignedβ-SiC 1D nanostructiires at relative lower temperature (600°C). By carefully controlling the reagent concentrations,β-SiC rod-like and needle-like nanostructures can be synthesized respectively. The possible growth mechanism of the as-synthesizedβ-SiC 1D nanostructures has been investigated. The structure and morphology of the as-synthesizedβ-SiC nanostructiires are characterized using X-ray diffraction, fourier-transform infrared absorption, and scanning and transmission electron microscopes. Raman and photoluminescence properties are also investigated at room temperature. The as-synthesizedβ-SiC nanostructures exhibit strong shape-dependent field-emission properties. Corresponding to their shapes, the as-synthesized nanorods and nanoneedles display the turn-on fields of 12, 8.4, and 1.8 V/μm, respectively.