Preparations And Investigations Of Low-dimensional Nanostructured Tungsten Oxide And Tungsten Disulfide

In recent years, one-dimensional (1-D) nanostructured materials have attracted tremendous research interest owing to their unique physical, chemical and optical properties. Among all these kinds of nanomaterials, tungsten oxides are of much importance because of their outstanding electrochromic, gaschromic, thermochromic and optochromic properties as well as their widely applications in electrochromic display, semiconductor gas sensors and photocatalysts. Particularly, 1-D tungsten oxide can be used as precursor for the preparation of tungsten disulfide nanotubes. As one important part of the inorganic fullerene-like materials, tungsten disulfide nanotubes find many potential applications in electron device, catalyst, superior solid lubricant and high performance composite.In this dissertation, systematic research has been focused on the synthetic strategies of low-dimensional tungsten oxide and tungsten disulfide, their formation mechanisms and physical properties. In addition, the morphology, structure and phase transition behaviour of the as-synthesized one-dimensional nanostructured tungsten oxide under thermal processing were demonstrated and possible trasition mechanisms were proposed. Thermal treatment-dependent gas-sensing characteristics were also examined. The as-synthesized products were characterized by using scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM) equipped with energy-dispersive X-ray spectroscopy (EDX), X-ray diffractometer (XRD), transmission electron microscopy (TEM), Fourier infrared spectrophotometer (FTIR), differential thermal analyzer (DTA) and Brunauer-Emmett-Tettler (BET) specific surface area analyser.Bundled W₁₈O₄₉ nanowires were successfully synthesized by a simple solvothermal method with tungsten hexachloride (WCl₆) as precursor and cyclohexanol as solvent. With increasing concentration of WCl₆ in cyclohexanol and increasing reaction time of the hydrothermal process, the bundles became larger and shorter, and finally block-shape product occurred. Ultra-thin W₁₈O₄₉ nanowires with diameter ranging from 2 nm to 15 nm and length of more than 2μm can be finally obtained by modifying the main experimental parameters. TEM, SAED and XRD analysis showed that the ultra-thin W₁₈O₄₉ nanowires exhibit many intrinsic defects such as stacking faults, dislocations and oxygen vacancies. The calculated BET specific surface area and pore volume of the ultra-thin W₁₈O₄₉ nanowires are 151m²/g and 0.51cm³/g.The nanostructured W₁₈O₄₉ bundles underwent a series of morphological evolution with increased annealing temperature, becoming straighter, larger in diameters and smaller in aspect ratio, and eventually becoming irregular particles with size up to 5μm after processing at 1000℃. The calculated specific surface areas dropped to 110m²/g and 66 m²/g after annealing at 400℃and 450℃, equivalent to a moderate decrease of 27.8% and a drastic decrease of 56.2%, respectively. Accordingly, the pore volumes reduced to 0.23cm³/g and 0.13cm³/g. Sensing properties of thin film sensors made from original and thermally processed W₁₈O₄₉ nanowires have been tested with respect to ethanol and a response value as high as 140% can be obtained to 2.7ppm of ethanol even at room temperature. However, the sensitivity of 400℃or 450℃thermally-processed nanowires is not satisfactory, even exposing to high concentration of ethanol. This result is indicative that the original surface character of the nanowires at room temperature is mainly preserved at 400℃, and this temperature can be considered as a top temperature limit in design for high temperature nanodevice and nanotechnology applications where high specific surface areas are important, such as in sensor and fuel cell applications. At 500℃, the monoclinic W₁₈O₄₉ was completely transformed to monoclinic WO₃ phase, which remains stable at high processing temperature.With the original and 400℃thermally-processed W₁₈O₄₉ nanowires as precursor, pure tungsten disulfide can be obtained by means of gas-solid phase reaction. Both of the as-synthesized products are composed of nanotubes, nanorods and nanoparticles. Additionally, intact tungsten disulfide nanotubes with different morphology can be observed when thermally processed nanowires at 400℃were used as precursor, and these nanotubes possess many obvious crystal defects such as layer discontinuities, waving layers, branched layers and dislocations. When thermally processed nanowires at 600℃were used as precursor, irregular nanoparticles mixed with few short nanorods were finally formed. XRD result showed that the resulting product from those thermally processed at 600℃is a mixture of WS₂ and WO₃, indicating the insufficient oxide-to-sulfide conversion. Numerous nano-whiskers have been obtained with ethanol as solvent by using a sonochemical method. The continuous changing supersaturation of tungsten trioxide in the solution may account for the formation of the nano-whiskers. After thermal processing at 500℃, the as-synthesized nano-whiskers transformed to short rods with diameter of about 10nm. Interestingly, the morphological evolution was accompanied by phase transformation, from orthorhombic tungsten trioxide hydrate to monoclinic WO₃. Nano-plates with depth of 60-100nm were formed from ethanol-water mixed solvent, while ultra-thin nano-sheets were synthesized when only water was used as the solvent. Due to a combination of the loss of crystalline water and crystal growth, sizes of both of the nano-plates and nano-sheets were apparently increased. Meanwhile, orthorhombic tungsten trioxide hydrate was transformed to hexagonal WO₃.WO₃ nanoparticles were synthesized by using a high-temperature vapour deposition method with APT·4H₂O as raw material. The morphology of the as-synthesized products was strongly affected by the temperature of the deposition substrate, the reaction temperature and the gas flow rate. At a low gas flow rate, products collected from different areas of the glass tube exhibit various morphologies, from whiskers, rods and quasi-spherical particles to irregular polyhedral particles, depending on the temperature of the deposition substrate. At the same gas flow rate, the product became larger with increased reaction temperature. When the reaction temperature remained unchanged, however, the sizes of the obtained particles decreased with increasing gas flow rate and also the size distribution became more homogenous. Eventually, nanoparticles with diameters ranging from 20nm to 100nm can be obtained when the Ar flow rate reached 6L/min and the reaction temperature was kept at 1350℃.