Three‐Dimensional Tungsten Oxide Nanowire Networks

Abstract

Among transition metal oxides, tungsten oxides (WO 3±d )are of great interest and have been investigated extensively owing to their promising physical and chemical properties.[1±6] With outstanding electrochromic, optochromic, and gaschromic properties, tungsten oxides have been used to construct flatpanel displays, photoelectrochromic ªsmartº windows, optical modulation devices, write±read±erase optical devices, gas sensors, humidity and temperature sensors, and so forth.[1±5]Recently, some non-stoichiometric tungsten oxides have attracted considerable attention for their interesting electronic properties, especially superconductivity and charge-carrying abilities. [7] It is well known that nanostructures have unique chemical and physical properties and can be used as elementary units of optoelectronic devices.[8±10] The synthesis of one-dimensional (1D) nanostructures and the assembly of these nanometer-scale building blocks to form ordered superstructures or complex functional architectures offer great opportunities for exploring their novel properties and for the fabrication of nanodevices.[11±13] Thus far, several techniques for the preparation of 1D tungsten oxide nanostructures have been developed. [6,14±17] Although the synthesis of tungsten oxide ªmicro-treesº has been reported, [18] the growth of tungsten oxide nanowire networks remains challenging. In this paper we have successfully synthesized three-dimensional (3D) tungsten oxide nanowire networks using a thermal evaporation approach. Transmission electron microscopy (TEM) investigation indicates that the WO 3±d nanowires have a cubic structure; this is confirmed by X-ray diffraction results (see Supporting Information). Growth along the six equivalent á100ñ directions forms an intersectant 3D network structure. The mechanism that drives such growth is suggested to be the existence of ordered planar oxygen vacancies in the (100) and (001) planes parallel to the [010] growth direction. Field-emission characteristics of these nanowire networks have also been measured. The tungsten oxide nanowire networks were synthesized by the thermal evaporation of W powders in the presence of oxygen. The scanning electron microscopy (SEM) images in Figure 1 show the typical morphology of the as-synthesized products. The high yield of the nanowire networks can be observed from the low-magnification SEM image in Figure 1a. The high-magnification SEM image in Figure 1b clearly demonstrates the shape of the 3D nanowire network. The 3D network is constructed of nanowires with widths ranging from 10 to 180 nm. The WO 3±d nanowires show a polygonal shape and intercross with each other to form the 3D network. The branches are along three perpendicular directions. Notably, there is no obvious stem in the network, making this structure different from previously reported networks and tree-like nanostructures. [11,13,18] Energy-dispersive X-ray spectroscopy (EDS) indicates the exclusive presence of W and O in the sample. Figure 2a shows a typical TEM image of a broken WO 3±d n...

Three dimensional nanowire networks and complex nanostructures of indium oxide

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Formation of self-ordered porous anodized alumina template for growing tungsten trioxide nanowires

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