| Metallic tungsten and tungsten-based materials have unique chemical, physical, electrical and mechanical properties, which make them very useful not only in the field of structural materials, but also in the fields of optics, catalysis, gas sensing and other functional materials. However, there are some limitations in the methods for preparing these materials. For example, these processes are complicated and/or difficult to control the morphologies of products. To overcome above-mentioned limitations, this thesis developed facile routes to synthesize tungsten-based nanocrystalline materials on the basis of soft chemistry and topotactic transformation processes. The as-obtained products were characterized, and the possible formation mechanisms were also investigated. Major results were described as the following:(1) Purchased tungstic acid and n-octylamine are used as the starting materials to synthesize tungstate-based inorganic-organic hybrids using n-heptane as the reaction solvent. Tungstic acid reacts with n-octylamine in a reverse-micelle-like media to form highly ordered tungstate-based inorganic-organic hybrid nanobelts with lamellar mesostructures, which is stacked alternately by single-octahedral W-O layers and bilayer-arranged n-alkyl chain arrays with a large tilt angle via a "dissolution-reorganization" process. The mole ratios of n-alkylamine to tungstic acid are about 2-5. The lengths of the hybrid nanobelts are about 5-10μm. It's found that water can speed up the reaction for the formation of hybrids, and too much water reduces the length-to-diameter ratios of the hybrid products. When the mass ratio of water to tungstic acid is up to 1:2, the hybrid product shows a platelike morphology. (2) H2WO4 nanosheets have been prepared by selectively removing the organic species from the tungstate-based inorganic-organic hybrids, and WO3 nanosheets with lengths of 200-300 nm and widths of 100-200 nm have been obtained by dehydrating H2WO4 nanosheets at 300-500℃. WO3 nanoparticles have been obtained by pyrolyzing the tungstate-based inorganic-organic hybrids at 500-600℃in air, and the obtained product took on a loose-aggregate-like morphology.(3) Tungsten carbide micro-/nanocrystals have been synthesized by pyrolyzing a single-source precursor of tungstate-based inorganic-organic hybrid nanobelts in a sealed quartz tube. A mixture of W2C and WC are obtained at 850℃. When the temperature reaches 1050℃, the product is a pure hexagonal WC phase. Meanwhile, a number of WC nanocrystals with sizes of about 100 nm are organized together to form porous WC microspheres that have a narrow particle size distribution. The result shows that the pressure condition has a vital effect on the formation of tungsten carbide nanocrystals, and there are no tungsten carbides obtained when the hybrid nanobelts are pyrolyzed under an Ar atmosphere. The sizes of tungsten carbides are about 1-2μm when the precursors are the mixture of WO3 nanosheets obtained from the hybrid nanobelts and activated carbon.(4) Tungsten nitride nanocrystals are prepared by a temperature-programmed reaction under NH3 using tungstate-based inorganic-organic hybrid nanobelts as the precursor at 650-800℃for 2 h. The as-obtained tungsten nitrides nanocrystals are 100 nm x200 nm in size, which retain a one-dimensional morphology of the precursor.(5) Cubic tungsten nanocrystals with a wide range of particle-size distribution (from 100 nm to 2μm) can be obtained by pyrolyzing tungstate-based inorganic-organic hybrids under an Ar condition at 975-1300℃for 2-5 h. The mechanism has been discussed.
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