Synthesis and characterization of tungsten oxide nanorods from chemical vapor deposition-grown tungsten film by low-temp

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A simple thermal annealing was performed to prepare tungsten oxide nanorods directly from tungsten (W) film. The W film was deposited on Si(100) substrate by chemical vapor deposition (CVD) at 450 °C using W(CO)6. A high density of tungsten oxide nanorods was produced by heating of the W film at 600–700 °C. The morphology, structure, composition, and chemical binding states of the prepared nanorods were characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) analysis. XRD and TEM results showed that the grown nanorods were single-crystalline W18O49. According to XPS analysis, the W18O49 nanorods contained ∼55.69% W6+, ∼32.28% W5+, and ∼12.03% W4+. The growth mechanism based on thermodynamics is discussed for the growth of tungsten oxide nanorods from W film.

I. INTRODUCTION

Recently, one-dimensional (1D) metal-oxide nanostructures such as nanowires, nanorods, nanotubes, and nanobelts have received increasing attention because of their unique applications in nanoelectronics and optoelectronics.1–19 Among them, a significant fraction of the work has been devoted to fabrication of 1D tungsten oxides (WO3−x, 0 艋 x 艋1) due to their many known applications such as in electrochromic windows, optical devices, gas sensors, and photocatalysts.20–23 Tungsten oxide has variant suboxide crystal structures. 24–28 Among these, one of the well-known structures is a monoclinic W18O49, which is highly anisotropic and shows a good mechanical strength.29 Various methods based on thermal treatments, vapor phase growth, oxidation, and wet chemical reaction have been reported for preparing W18O49 nanowires.29–44 Among these synthesis techniques, a simple heating method of the substrate has attracted much attention due to the simplicity of the technique and high crystallinity of the products. Tungsten (W) filament, foil, or film has been used to grow tungsten oxide nanowires by simple annealing. Most of the previous works showed that heating of tungsten foil or film requires high temperatures

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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0158 1320 J. Mater. Res., Vol. 23, No. 5, May 2008 http://journals.cambridge.org Downloaded: 11 Mar 2015

above 1000 °C to produce high crystalline tungsten oxide nanowires.25,31 Otherwise, catalysts or pretreatments were generally required for the growth of tungsten oxide nanowires at low temperatures.30 In this paper, we demonstrate a direct synthesis of W18O49 nanorods using the thermal annealing of chemical vapor deposition (CVD) grown W film. We succeeded in synthesizing W18O49 nanorods at 600–700 °C, which are lower temperatures than any-reported results without pretreatments. The structure, morphology, and composition of the nanorods were characterized by scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), highresolution transmission electron microscopy (HRTEM), and selected area electron diffr