Synthesis of bundled tungsten oxide nanowires with controllable morphology Shibin Sun a,b, ⁎ , Zengda Zou a , Guanghui Min a a School of Materials Science and Engineering, Shandong University, Jinan 250061, China b Nanotubes Laboratory, Advanced Materials Research Group, School of Mechanical, Materials and Manufacturing Engineering, the University of Nottingham, Nottingham NG7 2RD, UK ARTICLE DATA ABSTRACT Article history: Received 18 September 2008 Received in revised form 12 November 2008 Accepted 12 November 2008 Bundled tungsten oxide nanowires with controllable morphology were synthesized by a simple solvothermal method with tun gsten hexachloride (WCl 6 ) as precursor and cyclohexanol as solvent. The as-synthesized products were systematically characterized by using scanning electron microscopy, X -ray diffraction and transition electron microscopy. Brunauer–Emmett–Teller gas-sorption measurements were also employed. Accompanied by an apparent drop of specific surface area from 151 m 2 g − 1 for the longer nanowires synthesized using a lower concentration of WCl 6 to 106 m 2 g − 1 for the shorter nanowires synthesized using a higher concentration of WCl 6 , a dramatically morphological evolution was also observed. With increasing concentration of tungsten hexachloride (WCl 6 ) in cyclohexanol, the nanostructured bundles became larger, shorter and straighter, and finally a block-shape product occurred. © 2008 Elsevier Inc. All rights reserved. Keywords: Tungsten oxide nanowires Solvothermal Brunauer–Emmett–Teller 1. Introduction One-dimensional (1− D) nanostructured materials, such as nanorods, nanowires and nanotubes have attracted tremen- dous attention owing to their unique physical, chemical and optical properties [1–3]. Among them, tungsten oxides WO x (x=2–3)areofmuchimportanceduetotheirpotential application in electrochromic display, semiconductor gas sensors and photocatalysts [4–6]. Particularly, 1-D W 18 O 49 nanomaterials which exhibit unusual structural defects have received special attention in recent years [7]. Following the first synthesis of W 18 O 49 nanowires by breaking the mircro- trees, a variety of techniques including thermal treatments, vapor phase growth, etc have been employed in the produc- tion of W 18 O 49 nanostructures, and their structure and property characterizations have also been investigated thor- oughly [8–10]. Nonetheless, most of the synthetic methods involved high temperature or complicated procedures, mak- ing them difficult for production in large quantity. Thus, wet chemical methods, which are relative simple and promising in large-scale production, need to be further developed. Very recently, W 18 O 49 nanowires have been successfully synthesized by soft-chemistry methods with WCl 6 or W (CO) 6 as the main precursor in different solvents that are usually alcohol, water and cyclohexanol [11, 12]. Following this facile route, we have reported the successful synthesis of ultra-thin bundled tungsten oxide nanowires via a solvothermal method. In addition, the morphology and phase transforma- tion behaviour of the ultra-thin bundled nanowires under thermal processing were also characterized in detail [13].In this paper, further expanding from our previous work, we demonstrate the synthesis of bundled tungsten oxide nano- wires with various morphologies by simply changing the concentration of tungsten chloride (WCl 6 ) in cyclohexanol. The as-synthesized bundles were systematically character- ized and influences of the morphology on their specific MATERIALS CHARACTERIZATION 60 (2009) 437– 440 ⁎ Corresponding author. School of Materials Science and Engineering, Shandong University, Jinan 250061, China. Tel.: +86 531 88396145; fax: +86 531 82616431. E-mail address: sunshibin1982@yahoo.com.cn (S. Sun). 1044-5803/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2008.11.009 surface areas were also investigated. Possible mechanisms were proposed. 2. Experimental Typically, WCl 6 was dissolved in 2 ml of ethanol in a beaker to obtain a solution. Cyclohexanol was then added to the solution, which was subsequently transferred to and sealed in a PTFE-line 120 ml autoclave. The concentration of WCl 6 in cyclohexanol varied from 0.003M to 0.007M in our experi- ments. The autoclave was heated in a furnace at 200 °C for 6 h, to realize the synthesis. After thorough washing with water, ethanol and acetone several times, the centrifuged products were used for further examination. The structure, morphology and phase composition of the as-synthesized products were then characterized by using a scanning electron microscope (SEM, Philips XL 30, operated at 20 kV) equipped with energy-dispersive X-ray spectroscopy (EDX), a X-ray diffractometer (XRD, Siemens D500, Cu radia- tion) and a transmission electron microscope (TEM, JEOL 2000FX, 200 kV). Selected area electron diffraction (SAED) investigation was also performed during the TEM experiment. Brunauer–Emmett–Teller (BET) gas-sorption measurements were conducted by using an Autosorb-1 sorptometer. The surface area was calculated using the BET method based on adsorption data in the partial pressure (P/P o ) range 0.02–0.22, and total pore volume was determined from the amount of nitrogen adsorbed at P/P o =0.99. 3. Results and discussions Fig. 1 shows SEM images of the as-synthesized products at concentrations ranging from 0.003M to 0.007M. As shown in Fig. 1a, the as-synthesized product at a concentration of 0.003M exhibits ultrathin features with length up to few microns. Further TEM investigation can identify the bundled feature, giving evidence that each 1-D nanostructured bundles consist of nanowires with diameters of about 2-10 nm, as shown in Fig. 2a. This bundled feature was often observed in thin and long 1-D nanostructured materials due to their large surface areas [14]. The SAED pattern (top left inset, Fig. 2b) of the bundles exhibits broadened and strand spots, suggesting that individual nanowires within the bundle all adopted the same growth direction. This is a typical characteristic of bundled 1-D nanostructured materials. Increasing the con- centration to 0.004M, there is no evidence of major changes in morphology except that the bundles become shorter. When the concentration increased to 0.005M, apparent evolution can be observed. As displayed in Fig. 1c, thicker and shorter bundles with uniform size occurred, and they appeared to be composed of small bundles. Fig. 2c is a corresponding TEM image of the bundles synthesized at the concentration of 0.005 M. In comparison with the TEM image of the bundles synthesized at the concentration of 0.003 M (Fig. 2a), major change in the morphology can be seen. The diameter of the bundles increased to about 150 nm and individual nanowires cannot be discriminated. At a high concentration of 0.007 M, the as-synthesized product exhibits block-like structure rather Fig. 1– SEM images of bundled tungsten oxide nanowires synthesized at different concentrations of (a) 0.003 M, (b) 0.004 M, (c) 0.005M and (d) 0.007 M. Inset in Fig. 1d is a high magnification image. 438 MATERIALS CHARACTERIZATION 60 (2009) 437– 440 than bundled feature. The irregular blocks are of about 8 μmin length and 3 μm in diameter. By careful examination (inset in Fig. 1d), it can be found that some bundles are randomly distributed on the surface of these blocks. XRD analysis was carried out to identify the crystalline structure of the as-synthesized products. As shown in Fig. 3a, the main diffraction peaks of the products synthesized at concentrations of 0.003 M and 0.004 M match well with the monoclinic W 18 O 49 phase (JCPDS No.71-2450). It should be noted that the overall intensities of the diffraction spots are weak; whilst the strongest peak intensity of (010) plane indicates that the b010N is the dominant growth direction and that the close- packed plane (010) is roughly perpendicular to the nanowire axis in this monoclinic regime. The HRTEM image (Fig. 2b) indicates that the lattice fringe separations are measured ca.0.38nmand 0.37 nm, respectively, which can be indexed as (010) and (103) of monoclinic W 18 O 49 and in agreement with the election diffrac- tion results (insetin Fig. 2b) [15]. At concentrationsof0.005M and 0.007M, some new peaks can be observed from the patterns of the as-prepared products. We cannot identify the new peaks exactly, buttheyarebelieved to correspondto WO 3 − x because all the as-synthesized products consist only of tungsten and oxygen elements based on EDX results (Fig. 3b). However, t he major peaks can still be assigned to W 18 O 49 with the strongest intensity of (010) plane, indicating that the growth direction of nanowires remained unchanged with increasing concentration. It has been reported that the concentration of the precursors greatly influences the morphology of the hydro- thermal products [16]. The shape of a crystal is determined by the difference in the relative growth rates of the individual crystal planes and the resulting particles are anisotropic in shape under certain supersaturation. In our work, at lower precursor concentration, ultrathin and long bundles com- posed of numbers of nanowires can be obtained, while larger and shorter bundles or even block-shape products were produced with increasing concentration. Here, we believe that low solution concentration contributed to the lower supersaturation of the tungsten source, promoting the growth of tungsten oxide nanowires [11]. At higher concentration, the highly saturated WCl 6 could prohibit the growth of tungsten oxide nanowires along the b010N direction, leading to short nanowires, and finally resulting in shorter and thicker bundles due to agglomeration. As indicated in Fig. 2, the resulting Fig. 3 – (a) XRD patterns of bundled tungsten oxide nanowires synthesized at different concentrations ranging from 0.003 M to 0.007 M. Peaks of monoclinic W 18 O 49 are marked by + and Unassigned peaks are marked by *; (b) EDX profile of bundles synthesized at concentration of 0.007M. Fig. 2 – (a) TEM and (b) HRTEM images of bundled tungsten oxide nanowires synthesized at a concentration of 0.003 M; (c) TEM of bundled tungsten oxide nanowires synthesized at a concentration of 0.005 M. Inset in Fig. 2b is the corresponding SAED patterns of the bundled nanowires in Fig. 2a. 439MATERIALS CHARACTERIZATION 60 (2009) 437– 440 products synthesized at high concentrations are a mixture of different types of WO 3 − x . The formation of the tungsten oxide mixture may be related to the oxygen content in the reaction system. As the precursor concentration increases, the mole fraction of oxygen decreases, which will inevitably lead to oxygen vacancies in the as-synthesized tungsten oxides. In addition, EDX result of the product prepared at concentration of 0.007M indicates that the O/W atomic ratio is about 2.4 (Fig. 3b). Therefore, it is suggested that the as-synthesized products at high concentration should be a mixture of W 18 O 49 (WO 2.72 ) and WO 2 . BET gas-sorption measurements were employed to evalu- ate the specific surface area and porous featur es of the bundled tungsten oxide nanowires. The calculated specific surface area and pore volume of the longer bundles synthe- sized using concentration of 0.003 M are 151 m 2 /g and 0.51 cc/g [12], whilst the shorter bundles synthesized using concentra- tion of 0.005 M are 106 m 2 /g and 0.21 cc/g, respectively. The high specific surface area of the bundled nanowires can be ascribed to a combination of the ultra-thin feature of individual nanowires and the unique packing characteristic of the bundles themselves. The high specific surface area is also associated with the sizes and distributions of the pores, which arise from the solvothermal process at low temperature and inter-nanowire spaces with bundles [12]. The shorter and thicker bundles will consequently lead to decreased pore volumes (e.g. from 0.51 cc/g to 0.21 cc/g) with increasing concentration, which in turn resulted in the decreased specific surface area. 4. Conclusion In summary, bundled tungsten oxide nanowires with con- trollable morphology were prepared by a simple solvothermal method with tungsten hexachloride (WCl 6 ) as precursor and cyclohexanol as solvent. With increasing concentration of tungsten hexachloride (WCl 6 ) in cyclohexanol, dramatically morphological evolution can be observed. The bundles became larger, shorter and straighter, and finally a block- shape product occurred. The resulting longer tungsten oxide bundles exhibit a high specific surface of 151 m 2 g − 1 , which decreased to 106 m 2 g − 1 for shorter tungsten oxide bundles. Acknowledgement We thank the China Scholarship Council (CSC) of the Ministry of Education for sponsoring the study of SBS in the UK, and the EPSRC (UK) for financial support. 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