Simple Method to Synthesize Na x WO 3 Nanorods and Nanobelts R. Azimirad, †,‡ O. Akhavan, † and A. Z. Moshfegh* ,†,§ Department of Physics, Sharif UniVersity of Technology, P.O. Box 11155-9161, Tehran, Iran, Institute of Physics, Malek-Ashtar UniVersity of Technology, Tehran, Iran, and Institute for Nanoscience and Nanotechnology, Sharif UniVersity of Technology, P.O. Box 14588-89694, Tehran, Iran ReceiVed: March 11, 2009; ReVised Manuscript ReceiVed: May 27, 2009 A simple method for synthesis of Na x WO 3 nanorods and nanobelts on sputtered tungsten films by using sodium in soda lime substrate as the catalyst was reported for the first time. After thermally post annealing thin films in a temperature range of 600-750 °C in N 2 ambient for 80 min, crystalline Na x WO 3 nanorods and nanobelts with [001] direction were formed depending on the annealing temperature. Experimental results reveal that the annealing temperature at 700 °C is the optimum temperature for the growth of sodium-doped tungsten oxide nanorods with maximum density on the surface. According to scanning electron microscopic observations, the synthesized nanorods are ∼50 nm in width and a few micrometers in length at the optimum temperature. It was also observed that increasing annealing temperature facilitates the growth of the Na x WO 3 nanobelts. A solid-liquid-solid mechanism was proposed for describing the growth process of the sodium- doped tungsten oxide nanorods and nanobelts. 1. Introduction Tungsten bronzes are a group of nonstoichiometric com- pounds with the general formula of M x WO 3 , where M is a metal element and x is in the range of 0 < x < 1. Tungsten bronzes have drawn considerable attention in recent decades for their unique properties such as the successive phase transition over a range of temperatures, high electrical conductivity, and some interesting magnetic properties. 1,2 Tungsten bronzes have been applied in many technological applications such as electrochro- mic devices, humidity sensors, solid fuel cells, secondary batteries, ion-sensitive electrodes, etc. Among all kinds of tungsten bronzes known, the sodium tungsten bronzes are the most studied ones ever since their discovery in 1823. 3 It was found their physical properties and structures of the Na x WO 3 are strongly dependent on their compositions. It is known that Na x WO 3 is n-type semiconducting for x < 0.25 but metallic for x > 0.25; meanwhile, their colors range from blue to violet to coppery then to yellow-gold as x changes from 0.4 to 0.98. In addition, the crystalline structure of Na x WO 3 is closely related to x values. 4 There are various literatures that reported different synthetic techniques to prepare sodium tungsten bronzes with powder or thin film structure. 5-7 Recently, we have introduced a new method for growth of Na x WO 3 nanowhiskers 8 and nanobelts with a U-shaped cross section 9 from sputtered W thin films by using the existing sodium, as a catalyst, in soda lime substrates after the heat treatment at 650 °C for different times (15, 80, and 180 min) and at 750 °C for 15 min, respectively. But, the surface density of the Na x WO 3 nanostructures (number of the nanostructures formed per unit area) grown under these experi- mental conditions was low. In this work to determine the optimum annealing temperature for growth of Na x WO 3 nanostructures with high density on the film surface, we have selected the optimum growth time (80 min) 8 and studied the effect of annealing temperature in a range of 550-750 °C on the growth process of the nanostructures. The synthesized samples containing the sodium-doped tungsten oxide nanostructures were characterized and analyzed by scanning electron microscopy (SEM), X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and ultraviolet (UV)-visible spectrophotometry. In addition, a more detailed description for the growth process of the nanostructures has been presented based on the solid-liquid-solid (SLS) mechanism. 2. Experimental Section Initially, thin films of tungsten were deposited on cleaned soda lime substrates by using DC magnetron sputtering tech- nique. The base pressure and Ar sputtering pressure were 2.5 × 10 -6 and 5 × 10 -3 Torr, respectively. Before the deposition process, a presputtering was performed for about 2 min to clean the target surface. The discharge power to grow W thin films was considered about 22 W that resulted in a deposition rate of ∼8.5 nm/min. Thickness of the deposited films was considered to be ∼40 nm monitored in situ by a quartz crystal oscillator and measured by an optical technique. The distance between the target in down and the substrate in up was 40 mm. Some other schematic arrangement of the sputtering system were reported elsewhere. 10 The reaction for the growth of nanorods was carried out in a horizontal quartz tube furnace at various temperatures ranging from 550 to 750 °C. For this step, the deposited samples were placed on an alumina boat located in the furnace. After heating the samples in N 2 environment with a constant flow rate of 400 standard cubic centimeters per minute (sccm) for 80 min (the optimum time for growth of Na x WO 3 1D nanostructure as has been recently reported 8 ), then the furnace was cooled down to room temperature rapidly. * To whom correspondence should be addressed. E-mail: moshfegh@ sharif.edu. Tel: +98-21-6616-4516. Fax: +98-21-6601-2983. † Department of Physics, Sharif University of Technology. ‡ Malek-Ashtar University of Technology. § Institute for Nanoscience and Nanotechnology, Sharif University of Technology. J. Phys. Chem. C 2009, 113, 13098–13102 13098 10.1021/jp902189h CCC: $40.75  2009 American Chemical Society Published on Web 07/01/2009 Downloaded by SHARIF UNIV OF TECH on July 27, 2009 Published on July 1, 2009 on http://pubs.acs.org | doi: 10.1021/jp902189h