Potassium Tungsten Bronze Nanowires: Polarized Micro‐Raman Scattering of Individual Nanowires and Electron Field Emission from Nanowire Films

Abstract

Nanostructures are known to possess large surface-area-tovolume ratios and possible quantum-confinement effects. They have currently been intensely researched because these special properties can give rise to potential applications in nanotechnology, such as the fabrication of nanometer-scale devices. In the family of nanostructures, metal oxide nanostructures have significantly been applied in areas such as chemical/biological sensing, lasering, and displays, and are the most important and widely studied nanostructures. Many methods have been developed for the fabrication of nanostructures, including applying electrochemical techniques, porous aluminum templates, vapor–liquid–solid (VLS) growth and vapor–solid (VS) reactions. Heat-oxide methods have been shown to provide an alternative route for synthesis of metal oxide semiconductor (MOS) nanostructures, with the advantages of mild synthetic conditions, simple manipulation, and large-scale production. The hotplate method, one of the simplest heat-oxide methods in particular, has been successfully applied for the synthesis of various metal oxide (CuO, a-Fe2O3, Co3O4, and ZnO) nanostructures. [7-10] In the past few decades, much attention has been focused on tungsten bronzes owing to their Drude-type optical behavior, photochromic, and superconducting properties. In particular, the hexagonal alkali tungsten bronzes AxWO3 (HTBs, A = K, Rb, Cs, and NH4; 0 < × <1/3) have been the subject of numerous studies. The traditional methods for the preparation of the hexagonal tungsten bronze structure require high temperature reaction of metal tungstate, tungsten oxide, and metal tungsten powder or the electrochemical reduction of tungstate. Here, we demonstrate a simple and convenient process for the synthesis of K0.33W0.944O3 nanowires. Our scanning electron microscopy (SEM) investigations directly confirm that the growth mechanism is that of the tip-growth vapor–solid mechanism. Raman scattering is a well-established technique for elucidating structural properties of nanostructures. More recently, micro-Raman studies on individual nanowires such as SiC, ZnSe, and CuO successfully demonstrate their impressive potential for probing crystal properties of basic building blocks such as single nanowires. However, to the best of our knowledge, there is no polarized micro-Raman scattering study on individual potassium tungsten bronze nanowires to date. For the first time, the electron field emission measurement of potassium tungsten bronze nanowire film was performed in this work. A field-emission scanning electron microscope (FE-SEM) was used to directly observe the morphology of the as-grown samples. After being heated at 450 °C for 10 hours in air, the pretreated tungsten foil was found to be covered with a layer of randomly oriented nanowires with diameters and lengths in the range of approximately 50–200 nm and 5–10 lm (Fig. 1a), respectively. The highest aspect ratio was established to be 100, and the average aspect ratio of these nanowires was around 50, which may give rise to promising field-emission properties. It was noted that the tungsten bronze nanostructures are able to be synthesized into nanorods, nanowires, or nanosheets by changing the heating temperatures. The detailed morphology-controlled synthesis of potassium doped tungsten bronze nanostructures will be discussed in our future work. Figure 1b shows the XRD pattern of the as-prepared samples. All the reflections can be indexed to the hexagonal potassium tungsten bronze K0.33W0.944O3 (Joint Comittee on Powder Diffraction Standards (JCPDS) file No. 81-0005) and no impurities were identified in the XRD pattern. The crystal structural characterization of nanowires was performed by means of high-resolution transmission electron microscopy (HRTEM). The fringe spacing of 0.38 nm (Fig. 2b) concurs well with the interplanar spacing of (0 0 1) plane of hexagonal K0.33W0.944O3 (JCPDS file No. 81-0005), revealing the single crystalloid of the synthesized tungsten bronze nanowires and the growth direction of [0 0 1]. The selected area electron diffraction (SAED) pattern with the zone axis [0 2 0] (inset of Fig. 2a) further confirmed the single crystalline property of nanowires. The ideal hexagonal symmetry structure of AxWO3 (A = K, Rb, and Cs) is schematically shown in the inset of Figure 2c. The WO6 octahedra are formed into a sixmember ring. Consequently, the maximum alkali-metal content is 0.33 if the composition ratio of W is set to be 1. The C O M M U N IC A TI O N