Formation of tungsten oxide nanorods by surfactant-assisted hydrothermal reaction

Tungsten oxide (WO₃) has attracted great attention since it has various applications: electrochromic, photocatalytic, and sensors [1]. Of all of these applications, perhaps electrochromic property is the most interesting as WO₃ film has the ability to undergo optical colouration when applied to it voltage. The colouration of WO₃ film is useful in the formation of various optical devices. WO₃ can change colour from yellowish-transparent to opaque-blue when ions like Li⁺, Na⁺ or H⁺ are inserted or intercalated in its structure. The electrochromism of WO₃ has generated interest in the use of WO₃ in energy-saving smart/dynamic windows, antiglare mirrors, high contrast displays and active camouflage [2]. To allow intercalation of ions, large surface area of WO₃ is desired. High surface area WO₃ can be achieved by the formation of thin WO₃ with nanostructured grains. In this work, WO₃ in the form of nanorods are prepared by seeded growth hydrothermal reaction on the tungsten foils. The foil was thermally oxidised at 400 °C for 30 min as to form a native oxide that will be used as a seed layer. This seed layer will act as a template for the formation of WO₃ nanorods by hydrothermal reaction in reactive solution. The hydrothermal reaction was conducted in an acidic reactive solution with 0.3 M sodium tungstate dihydrate (Na₂WO₄.2H₂O) added to it cetyltrimethylammonium bromide (C₁₉H₄₂BrN; CTAB) or hexamethylamine (C₆H₁₂N₄; HMT). CTAB is a cationic surfactant whereas HMT is nonionic surfactant. In this work, the concentration of the surfactants used was varied from 0.05 to 0.20 M as to investigate the effect of concentration of the surfactant on the growth of the WO₃ nanorods. A surfactant has the ability to hinder gro- th of growing oxide crystals at specific direction. The concentration of surfactant must be optimised as to allow only c-axis growth of WO₃ crystals hence producing only 1 dimensional nanorods structure. X-ray diffraction (XRD) pattern in Fig. 1 shows that after 8 hours of exposure of seeded W foil in reactive solution in the presence of CTAB, monoclinic WO₃ crystallised and formed on the foil. Similarly, in the presence of HMT (Fig. 2), monoclinic WO₃ can be detected (ICDD number: 43-1035). To investigate if indeed nanorods have formed on the surface of the W, field emission scanning electron microscope (FESEM) was used. Fig. 3 (a) and (b) show WO₃ prepared in the presence of 0.05M and 0.10M CTAB, respectively. As seen in the inset in Fig. 3 (a), nanorods have formed in both samples. On the other hand, all the HMT-prepared WO₃ are comprised of compact grains without any noticeable nanostructure as seen in a representative FESEM image shown in Fig. 3 (d). WO₄^2- from the reactive solution react with H⁺ forming WO₃ and the WO₃ grow on the native oxide. The reactions are as follows: WO₄^2- + 2H⁺ + nH₂O → H₂WO₄.nH₂O and H₂WO₄.nH₂O → WO₃ + (n+1)H₂O. Cyclic voltammetric (CV) test was performed to measure the electrochromic properties of the WO₃. For a typical WO₃ nanostructure, the cyclic voltammogram is as shown in Fig. 4.