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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We report a novel, scaleable and versatile method for large scale synthesis of tungsten trioxide nanowires and their arrays on a variety of substrates including amorphous quartz, fluorinated tin oxide, etc. The synthesis concept uses the chemical vapor transport of metal oxide vapor phase species onto substrates using air or oxygen flow over hot-filament sources. The hot-filaments were designed to provide uniform heating and gas phase composition over large substrate areas. The results show that the nucleation densities could be varied from 10 6 /cm 2 to 10 10 /cm 2 with decreasing substrate temperature. A thermodynamic model for determining the nucleation density and nanowire diameter is presented. The analysis indicates that the nucleation for nanowires occurs through condensation of sub-oxide, WO 2 species. This is in contrast to WO 3 species condensation into WO 3 solid phase for nanoparticle formation. The dispersion behavior of as-synthesized nanowires in aqueous and organic solvents is also studied.

Inorganic nanowires are expected to play a central role in the re-engineering of products with applications in composites, thin films, nanodispersions, energy conversion devices, sensors, nanoelectronic devices and optics. The synthesis of materials at the nanoscale might also help in the discovery of new phases with interesting properties. However, the synthesis strategies for inorganic nanowires is quite limited and have not reached the level of maturity needed for either bulk manufacturing or for controlling nanowire characteristics such as sub 10 nm diameters and different growth directions. In this regard, we report several synthesis strategies that potentially offer in-situ control over the resulting nanowire characteristics such as size, growth direction and an ability to form two-dimensional networks. The techniques described here could be scaled up easily for bulk production of various nanostructures. Our preliminary results suggest that the nanowires form stable dispersions in both organic and aqueous solvents compared to nanoparticles of the same material. INTRODUCTIONIn the last few years, the synthesis of inorganic nanowires and nanotubes has gained tremendous importance for several reasons 1 (a) the availability of nanowires allows for rapid experimental understanding of the physics and chemistry of materials under reduced dimensions; 2 (b) the synthesis of materials at the nanometer scale could lead to the discovery of new phases through their structural variations, for example -nanotubes; 3 (c) the availability of nanowires and nanotubes in large quantities could allow for bottom-up assembly techniques for several engineering application areas such as composites, electronics, catalysts and optics; 4 and (d) the possibility of growing perfect crystals at the nanoscale. However, the nanowire quantities required for particular applications vary. These requirements range from bulk quantities for composites and thin films to single wires for sensors and electronic devices to large area arrays for field emission based applications. The growth of one-dimensional crystals in the form of micron-scale whiskers has been known for several decades starting in 1940s 5 and was formalized as the Vapor-Liquid-Solid method in 1964. 6 More recently in 1991, a similar strategy was employed to discover the growth of carbon nano-tubular structures. 7The vapor-liquid-solid (VLS) method employs catalyst clusters to template one-dimensional growth. In the VLS technique, a vapor phase solute is selectively dissolved into metal droplets, and it precipitates one-dimensionally due to the metal droplet size confinement. Recognizing the fact that one can reduce the resulting wire size by reducing the metal droplet size, several researchers sought different ways of creating smaller clusters and providing solutes using different means. The notable techniques involved laser ablation of solid targets containing metal catalysts, 8 thermal chemical vapor deposition 9 over catalyst cluster dispersions on supports and gold clusters dis...