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Simple Method to Synthesize NaxWO3 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 NaxWO3 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 N2 ambient for 80 min, crystalline NaxWO3 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 NaxWO3 nanobelts. A solid-liquid-solid mechanism was proposed for describing the growth process of the sodiumdoped tungsten oxide nanorods and nanobelts. 1. Introduction Tungsten bronzes are a group of nonstoichiometric compounds with the general formula of MxWO3, 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 electrochromic 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 NaxWO3 are strongly dependent on their compositions. It is known that NaxWO3 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 NaxWO3 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 NaxWO3 nanowhiskers8 and nanobelts with a U-shaped cross section9 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 NaxWO3 nanostructures (number of the nanostructures formed per unit area) grown under these experimental conditions was low. * 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.
In this work to determine the optimum annealing temperature for growth of NaxWO3 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 technique. 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 N2 environment with a constant flow rate of 400 standard cubic centimeters per minute (sccm) for 80 min (the optimum time for growth of NaxWO3 1D nanostructure as has been recently reported8), then the furnace was cooled down to room temperature rapidly.
10.1021/jp902189h CCC: $40.75 2009 American Chemical Society Published on Web 07/01/2009
Synthetic Method for NaxWO3 Nanorods and Nanobelts
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Figure 2. XRD spectra of the tungsten thin films after annealed at different temperatures: (a) “as deposited”, (b) 600, (c) 700, and (d) 750 °C.
TABLE 1: XRD Peak Position, Lattice Parameter and Calculated Na Content in the NaxWO3(001) Phase Synthesized at Different Annealing Temperatures Figure 1. SEM images of sodium-doped tungsten oxide nanorods and nanobelts after post annealing in N2 atmosphere at different temperatures: (a) 600, (b) 650, (c) 700, and (d) 750 °C (all scale bars are 5 µm).
Surface morphological characteristics of the films were observed by SEM. XRD with a Cu KR radiation source was used to determine phase formation and crystallographic orientation of the samples. XPS with Al KR anode with X-ray incident energy of 1486.6 eV was employed to study the surface atomic composition and chemical state of the samples. All binding energy values were determined by calibrating the C(1s) core level at 285.0 eV. TEM operated at 200 kV was used for detail observation of nanostructures and selected area electron diffraction (SAED) investigation was also performed during the TEM observation. Optical transmission and reflection measurements of the prepared samples were performed in a range of 300-1100 nm wavelength using an UV-visible spectrophotometer with resolution of 1 nm. 3. Results and Discussion The SEM images of the W films after the thermal annealing at different temperatures from 600 to 750 °C have been shown in Figure 1. After annealing the W film up to 550 °C, no any 1D features were observed on the surface. However, by increasing the annealing temperature to 600 °C, low density nanorods with dimensions 400 °C.21 It should be noted that the oxygen can also be originated from residual O2 in the ambient and/or native tungsten oxide on the surface. On the basis of the phase diagram of Na2WO4/WO3,22 the minimum melting point temperature of sodium tungsten bronzes is at ∼600 °C. Therefore, it is reasonable to expect that tiny droplets of low-melting-point liquid containing Na, W, and O form at the growth temperatures higher than 600 °C. These tiny droplets act as the seeds for the growth of nanorods. If more WO3 was dissolved in the droplet to reach the supersaturated state, solid NaxWO3 would precipitate from the droplet in the form of nanorods. Continuous feeding of tungsten and sodium oxides into the liquid droplet sustains the growth of the nanorods. Finally, when the temperature of the system is slowly lowered to room temperature, the growth process was stopped. Figure 7 shows schematically the mechanism for the growth process. In this proposed mechanism, the size of the nanorods is directly related to the size of the initial droplet of the liquid. As observed from Figure 1, the growth of nanorods began at 600 °C and their surface density and width
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Azimirad et al. of the project. The assistance of Mr. M. Goudarzi in the project is greatly acknowledged. References and Notes
Figure 7. Schematic illustration of the mechanism for the NaxWO3 nanorod growth.
increased by increasing the annealing temperature. The increase in width of the nanostructures can be attributed to formation of larger droplets on the surface. However, at 750 °C, due to softening of the glass and diffusion of the film into the substrate (Figure 1d), the density was decreased and the nanorods became nanobelts. This deformation can be as a result of connection and coalescence of the nanorods together due to sintering process. A similar deformation process was also observed by Y. Wu et al. for the growth of hexagonal tungsten trioxide tubes.23 Since this growth process involves SLS phases, it is named as a SLS growth, which is in fact an analogy of the VLS mechanism. It should be noted that there are many reports that used SLS growth mechanism for growth of other materials with 1D nanostructures, such as Si nanowires,24 Ge nanowires,25 and In2O3 nanosheets.26 4. Conclusions In conclusion, we report here a simple and novel method to synthesize sodium-doped tungsten oxide nanorods and nanobelts on soda lime substrates at a relatively low temperature. This new method is very simple and can be easily scaled up to prepare a larger amount of the material. Annealing process at 700 °C was determined as the optimum temperature for growing 1D nanostructure of the NaxWO3 with maximum density on the surface. Furthermore, it is determined that the existing sodium in the soda lime substrate diffuses toward the surface of the annealed samples and this diffusion plays an important role in formation of the 1D tungsten oxide nanostructure. A SLS growth mechanism of the nanorod formation was also proposed to describe formation of NaxWO3 nanorods and nanobelts. Acknowledgment. The authors wish to thank Research Council of Sharif University of Technology for financial support
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