Synthesis and Optical Properties of Colloidal Tungsten Oxide Nanorods Kwangyeol Lee, Won Seok Seo, and Joon T. Park* National Research Laboratory, Department of Chemistry and School of Molecular Science (BK 21), Korea AdVanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea Received January 2, 2003 ; E-mail: jtpark@mail.kaist.ac.kr Nanostructured materials are expected to play a crucial role in the future technological advance in electronics, 1 optoelectronics, 2 and memory devices. 3 One-dimensional nanostructures in particular offer fundamental opportunities for investigating the effect of size and dimensionality on their collective optical, magnetic, and electronic properties. Various 1-D nanostructured metal oxides have been obtained via several different synthetic approaches, including solvothermal methods, 4 template-directed syntheses, 5 sonochemis- try, 6 thermal evaporation, 7 and gas-phase catalytic growth. 8 Control over the dimension of the prepared nanocrystals, however, is rarely accomplished due to the required harsh reaction conditions. Controlled colloidal nanocrystal growth under mild conditions in the presence of structure-directing surfactants has attracted much attention due to flexible processing chemistry in terms of solubility and nanocrystal dimension and has been successfully applied for a number of metals 9 and metal chalcogenides. 10 However, its ap- plication to the growth of 1-D metal oxide is extremely rare. 11 Among various metal oxides, WO 3-x has found useful applica- tions in electrochromic devices, 12 semiconductor gas sensors, 13 and photocatalyses. 14 Sodium-doped WO 3 is also reported to be a high- temperature superconductor with T c ≈ 90 K. 15 In addition, one- dimensional nanostructured tungsten oxide has been used as a structure-directing precursor for WS 2 nanotube, 16 a useful material in tribological applications and catalyses; the dimension of oxide nanorod is directly transferred to the resulting WS 2 nanotube after reaction with H 2 /H 2 S. Thus far, preparation of single-crystalline, 1-D nanostructured tungsten oxide in mass quantity has been accomplished by heating a tungsten foil, covered by SiO 2 plate, in an argon atmosphere at 1600 °C 17 or recently by electrochemically etching a tungsten tip, followed by heating at 700 °C under argon. 18 The employed harsh conditions, contamination by platelets, and uncontrolled size hamper systematic investigations on size-depend- ent properties of the oxide nanorod itself as well as of inorganic derivatives prepared from the oxide. Herein we report a simple large-scale preparation of soluble and highly crystalline tungsten oxide nanorods of varying sizes by a mild, solution-based colloidal approach. A stirred slurry of 0.70 g of W(CO) 6 (Strem, 99%), 1.33 g of Me 3 NO‚2H 2 O (6 equiv, Aldrich, 98%), and 8.5 g of oleylamine (16 equiv, Aldrich, 70% (technical grade)) in a 100-mL Schlenk tube, connected to a gas bubbler, was slowly heated in an oil bath from room temperature to 270 °C over 2 h. Over the course of the reaction, a vigorous frothing was observed, accompanied by a series of color changes from brown, bluish green, pink, to white. Gas evolution subsided at the bath temperature of 250 °C, and the reaction mixture became a clear, deep-green solution. The reaction mixture became a viscous, deep-blue-colored oil at the bath temperature of 270 °C, and was further aged at the same temperature for 24 h. The cooled viscous blue oil was diluted with toluene (20 mL), and to the resulting blue solution was added ethanol (50 mL) to form a blue precipitate. Centrifugation, redissolution in toluene, and precipitation by ethanol gave a blue powder, which can be easily redispersed in various solvents such as dichloromethane, toluene, and chlorobenzene. 19 The structure of the product was examined with transmission electron microscopy (Omega EM912 operated at 120 kV) and high- resolution transmission electron microscopy (HRTEM; Philips F20Tecnai operated at 200 kV). 20 A rodlike morphology with average diameter of 4 ( 1 nm and average length of 75 ( 20 nm (aspect ratio ≈ 20) is observed as shown in Figure 1a. The diameter of nanorods is uniform throughout their length. The selected area electron diffraction (SAED) as shown in Figure 1b exhibits two intense rings corresponding to lattice spacings of 3.78 Å (inner ring) and 1.89 Å (outer ring), suggesting the preferential rod growth in one direction. The unidirectional growth of the nanorods is clearly shown in the HRTEM image (Figure 1c), and the lattice spacing along the direction of rod growth is found to be 3.78 Å, consistent with the SAED pattern. The X-ray powder diffraction (XRD, Rigaku D/MAX-RC (12 kW) diffractometer using graphite-monochromatized Cu-K radia- tion at 40 kV and 45 mA) pattern as shown in Figure 2 gives information about the possible stoichiometry of the prepared tungsten oxide nanorods, and it matches best the W 18 O 49 reflections (JCPDS card No: 05-0392) among various tungsten oxide systems. Figure 1. (a) a TEM micrograph of 75 ( 20 nm tungsten oxide nanorods, (b) a selected area electron diffraction pattern (SAED), and (c) a high- resolution TEM image. Figure 2. XRD pattern of 75 ( 20 nm tungsten oxide nanorods. Published on Web 02/26/2003 3408 9 J. AM. CHEM. SOC. 2003, 125, 3408-3409 10.1021/ja034011e CCC: $25.00 © 2003 American Chemical Society