• PDF / 671,321 Bytes
• 6 Pages / 612 x 792 pts (letter) Page_size
• 41 Downloads / 249 Views

DOWNLOAD

REPORT

---

Yu-Zhang Laboratoire de Microscopies et d’Etude de Nanostructures, Université de Reims, 51687 Reims cedex 02, France

A. Gloter Laboratoire de Physique des Solides, Université Paris-Sud, CNRS UMR 8502, 91405 Orsay, France

G.M. Zhang and Z.Q. Xue Department of Electronics, Peking University, Beijing 100871, People’s Republic of China (Received 17 May 2004; accepted 20 September 2004)

Single crystalline nanorods (15–200 nm in diameter and hundreds nanometers in length) have been formed on the carbon-covered W wires by simple electric heating under a vacuum of 5 × 10−4 Pa. The chemical composition and crystalline structure of the nanorods were carefully investigated by various characterization techniques such as scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, energy dispersive x-ray spectroscopy and electron energy loss spectroscopy. After ruling out any possible existence of carbon nanotubes (CNTs), tungsten carbide, W–Fe alloying, and formation of other types of tungsten oxides, monoclinic W18O49 phase has been well identified. The mechanism of nanorod formation of sub-tungsten oxide (∼WO2.7 compared to WO3) will be discussed in relation to the sample preparation conditions.

I. INTRODUCTION

Tungsten oxides (WO3−x, x ⳱ 0 to 1) have many technological applications. They are important in developing smart windows, electrochromic display, and emissivity modulation devices.1–3 The combination of catalytic and n-semiconducting properties of WO3 is very useful in gas sensor fabrication.4,5 Because of their band gaps (2.4–2.8 eV) situated within the solar spectrum range, tungsten oxides are one of the solar energy transforming materials leading to the applications of photocatalytic activity6,7 and photoconductivity.8 On the other hand, some of the sub-tungsten oxides can exhibit charge carrying9 and superconducting properties.10 It should be noticed that the ratio of the surface area to mass of materials plays an important role in the properties exhibition of tungsten oxides.11 Hence, nanostructured tungsten oxides have been increasingly studied in recent years. Solis et al.12 reported that nanocrystalline WO3 films (twodimensional material) greatly enhanced the gas-sensing performance. Li et al.13 showed that WO3 nanoparticles

a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0469 J. Mater. Res., Vol. 19, No. 12, Dec 2004

(zero-dimensional material) improved the photochromic effects. As for one-dimensional (1D) tungsten oxide investigations, Zhu et al.14 first realized tree-like WO3 at micrometer scale by heating a W foil with SiO2 under Ar at 1600 °C, whereas Liu et al.15 synthesized WO2 nanowires by heating W wire partly wrapped with B2O3 power under N2 at the same temperature; Koltypin et al.16 obtained a mixture of WO2–WO3 nanorods by heating amorphous tungsten oxide powder in Ar at 1000 °C, and Hu et al.17 generated hollow fibers of W18O49 by heating WS2 covering W foil in oxygen at 1300–1400 °C. It is known that tungsten oxides are