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A scalable and versatile method for the large-scale synthesis of tungsten trioxide nanowires and their arrays on a variety of substrates, including amorphous quartz and fluorinated tin oxide, is reported. The synthesis involves the chemical-vapor transport of metal oxide vapor-phase species using air or oxygen flow over hot filaments onto substrates kept at a distance. The results show that the density of the nanowires can be varied from 10(6)-10(10) cm(-2) by varying the substrate temperature. The diameter of the nanowires ranges from 100-20 nm. The results also show that variations in oxygen flow and substrate temperature affect the nanowire morphology from straight to bundled to branched nanowires. A thermodynamic model is proposed to show that the condensation of WO(2) species primarily accounts for the nucleation and subsequent growth of the nanowires, which supports the hypothesis that the nucleation of nanowires occurs through condensation of suboxide WO(2) vapor-phase species. This is in contrast to the expected WO(3) vapor-phase species condensation into WO(3) solid phase for nanoparticle formation. The as-synthesized nanowires are shown to form stable dispersions compared to nanoparticles in various organic and inorganic solvents.

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Here, we report that the postsynthesis nitridation of tungsten oxide nanowires can result in single crystal nitride nanowires when the initial diameters of the nanowires are less than 10 nm. For nanowires with diameters greater than 10 nm, the nitridation of nanowires resulted in polycrystalline but highly oriented tungsten nitride domains in nanowires. Partially nitrided nanowires show an epitaxial relationship between the nitride nuclei and the oxide nanowire, being oriented with respect to the axial direction of the nanowires. The stress resulting from the strain due to lattice mismatch between oxide and nitride phases seems to control the critical size of the nitride nuclei within oxide nanowires. The photoluminescence data show that the resulting bandgap of the tungsten nitride nanowires was downshifted to 2.95 eV from 3.54 eV for tungsten oxide nanowires.