characterization of h - wo3 nanorods synthesized by hydrothermal process

Characterization of h-WO 3 nanorods synthesized by hydrothermal process S. Salmaoui a , F. Sediri a,b, * , N. Gharbi a a Laboratoire de Chimie de la Matière Condensée, IPEIT, Université de Tunis, 2 rue Jawaher Lel Nehru 1008, B.P. 229 Montfleury, Tunis, Tunisia b Faculté des Sciences de Tunis, Université Tunis-Elmanar, 2092 Elmanar, Tunis, Tunisia article info Article history: Received 10 November 2009 Accepted 15 February 2010 Available online 19 February 2010 Keywords: Tungsten oxide Aniline Hydrothermal synthesis Nanorods abstract Hexagonal tungsten oxide nanorods have been synthesized by hydrothermal strategy using Na 2 WO 4 Á2H 2 O as tungsten source, aniline and sulfate sodium as structure-directing templates. Tech- niques X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectros- copy (FTIR) and Raman spectroscopy have been used to characterize the structure, morphology and com- position of the nanorods. The h-WO 3 nanorods are up to 5 l m in length, and 50–70 nm in diameter. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction One-dimensional (1D) nanostructure materials, such as nano- tubes, nanowires, nanorods and nanobelts have attracted consider- able attention due to their remarkable physicochemical properties and their great potential for nanodevices [1–6]. The outstanding structural versatility of the metal oxides compounds such as tung- sten oxides, and their derivatives has been receiving significant attention especially with respect to applications in electrochromic or photochromic devices, secondary batteries, gas sensors and photocatalysts [7–9]. In these fields, recent studies showed that WO 3 nanostructured have superior sensitivity compared with those of bulk materials [10,11]. For such an application, character- istics of the material such as size, shape and crystallographic struc- ture are very important and strongly depend on the growth method. Until now various methods for the synthesis of nanostruc- tured tungsten oxides with various morphologies have been re- ported such as electrospinning [12], chemical vapour deposition [13], pulsed laser irradiation [14], electro-deposition [15], va- pour–solid growth [16], gas deposition [17], precipitation [18], sol–gel [19] and hydrothermal methods [20–24]. Moreover, WO 3 can crystallize according to several structures. All the WO 3 structures can be described as deformations of the ReO 3 cubic perfect model. Such a perfect structure is composed of a three-dimensional network of WO 6 octahedral linked by their oxygen corner. Among various crystal structures of WO 3 , hexago- nal form is of great interest owing to its well-known tunnel struc- ture in which WO 6 octahedrons share their corners with each other forming hexagonal tunnels along c-axis [25]. Hexagonal tungsten oxide (h-WO 3 ) has been widely investigated, especially as an inter- calation host for obtaining hexagonal tungsten bronzes MxWO 3 (M = K + ,Li + , etc.) and a promising material for negative electrodes of rechargeable lithium batteries [26–28]. Although many methods have been developed to elaborate nanostructured tungsten oxides, to the best of our knowledge, it is the first time to report the optimization of reaction conditions of the synthesis of hexagonal tungsten oxide nanorods using ani- line with sodium sulfate as structure-directing templates by hydrothermal self-assembling process. 2. Experimental 2.1. Hydrothermal synthesis All of the chemical reagents were analytical grade. They were purchased from across and used without further purification. Na 2 - WO 4 Á2H 2 O was used as tungsten source. The aniline and sodium sulfate together have been used as structure-directing template for the first time. Hexagonal WO 3 nanorods were hydrothermally synthesised from a mixture of Na 2 WO 4 Á2H 2 O, H 2 O, C 6 H 5 –NH 2 , HCl and Na 2 SO 4 in the molar ratio 1:258:1:2.8:5. Reactants were introduced in this order and stirred a few minutes before introducing the solution in a Teflon-lined steel autoclave and the temperature set at 180 °C for 3 days under autogenous pression. The pH of the solution remains close to pH % 1 during the whole synthesis. The obtained powder was washed with acetone to remove organics residues and then dried at 80 °C. 0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2010.02.025 * Corresponding author. Address: Faculté des Sciences de Tunis, Université Tunis- Elmanar, 2092 Elmanar, Tunis, Tunisia. Tel.: +216 71872600; fax: +216 71871666. E-mail addresses: faouzi.sediri@ipeit.rnu.tn (F. Sediri), neji.gharbi@ipeit.rnu.tn (N. Gharbi). Polyhedron 29 (2010) 1771–1775 Contents lists available at ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly In order to control the morphology of h-WO 3 a set of experi- ments was performed by varying the molar ratio Na 2 SO 4 :W from 0.15:1 to 5:1. 2.2. Characterization techniques X-ray diffraction (XRD) patterns were obtained on a X‘Pert Pro Panalytical diffractometer with Co K a radiation (k = 1.78901 Å). The XRD measurements were carried out by a step scanning meth- od (2h range from 3° to 60°), the scanning rate is 0.03°/s and the step time is 3 s. Scanning electron microscopy (SEM) images were obtained with a Cambridge Instruments Stereoscan 120. Transmission electron microscopy (TEM) studies were recorded on JEOL 100 CX II electron microscope operated at 200 KV. One droplet of the powder dispersed in CH 3 CH 2 OH was deposited onto a carbon-coated copper grid and left to dry in air. Fourier transform infrared spectra (FTIR) were recorded with a Nicolet 380 Spectrometer. Raman spectroscopy was performed using a Jobin Yvon T 64000 Spectrometer. 3. Results and discussion 3.1. X-ray diffraction By controlling the appropriate molar ratio of the starting mate- rials (Na 2 SO 4 :W), the nanorods can be prepared conveniently with high purity. Fig. 1 shows the XRD patterns of the as-obtained prod- ucts at different molar ratio Na 2 SO 4 :W = (a) 5:1, (b) 2.5:1, (c) 1.25:1, (d) 0.62:1 (e) 0.30:1 and (f) 0.15:1, respectively. All the dif- fraction peaks can be perfectly indexed to hexagonal tungsten oxide crystalline phase (h-WO 3 ) with lattice constants of a = 7.298 Å and c = 3.899 Å (JCPDS # 33-1387). The peak intensities in the reported spectra were not exactly the same as the reference spectrum because of differences in the molar ratio. No peaks of any other phases or impurities were observed from the XRD patterns, indicating that h-WO 3 crystalline phase with high purity could be obtained using the present synthetic process. 3.2. Scanning and transmission electronic microscopy To understand the formation of the rod-like morphology, we conducted the experiment in the presence of different amount of Na 2 SO 4 while keeping other conditions unchanged. The morphol- ogy of the products was investigated by SEM and TEM images. It is found that the amount of Na 2 SO 4 has clear effects on the forma- tion of nanorods. Comparative experiments have shown that when the molar ratio Na 2 SO 4 :W = 0.62:1, only irregular particles of nano- rods and nanowires were formed (Fig. 2a). With increasing the mo- lar ratio to 1.25:1, nanorods were generated with an increase in the diameter (Fig. 2b). When the molar ratio = 2.5:1 the h-WO 3 rod- like nanostructures became predominant products (Fig. 2c). SEM and TEM images, shown in Fig. 2d and e, indicate that with aNa 2 SO 4 :W ratio of 5:1, a rod-like structure with diameters of about 50–70 nm and length of about 5 l m were obtained. Close observation shown in Fig. 2f revealed that these rod-shape prod- ucts are formed by the oriented attachment of large, numerous, highly aligned, and closely packed nanoneedles. According to the literature [29], sodium sulfate tends to induce the formation of the 1D nanostructures of h-WO 3 . Thus, the size and the yield of the rod-like nanostructures increase with increasing the amount of the salt. As a result, the presence of an appropriate amount of Na 2 SO 4 plays a key role in the formation of the h-WO 3 nanorods evolved from the oriented attachment of h-WO 3 nanoneedles. Controlled experiments have shown that the presence of aniline without Na 2 SO 4 , only irregular particles coexist with a small frac- tion of nanoneedles of orthorhombic WO 3 Á1/3H 2 O (JCPDS # 35- 0270) (Figs. 3a and 4a) were obtained. Furthermore, when the preparations are made without aniline and sodium sulfate only regular plate-like of monoclinic WO 3 (JCPDS # 72-1465) are formed (Figs. 3b and 4b). Our results show that the addition of so- dium sulfate without aniline leads to the formation of irregular nanoneedles of the h-WO 3 (Fig. 5). As reported in the literature, organic molecules were found to play an important role in the controlling the morphologies. Thus, it appears that aniline, due to its anisotropic character, is used as capping agent which induce and enhance the directing role of Na 2 SO 4 in the formation of 1D h-WO 3 nanostructures [24,30,31]. In view of these results, we can conclude that aniline and so- dium sulfate play important roles on the types of structure, mor- phology and size of particles constituting the product. The details of the effect of sulfate on the formation of WO 3 nanocrystal are not clear up to date. However, it is known that the anisotropic growth of the particles can be explained by the spe- cific adsorption of ions to particular crystal surface, therefore, inhibiting the growth of these faces by lowering their surface en- ergy [32,33]. This study allowed us to determine the optimum conditions of synthesis of h-WO 3 nanoneedles. Indeed, the observation by SEM (Fig. 2d) of the obtained product with the molar ratio Na 2 SO 4 :Na 2 - WO 4 = 5:1 shows that the synthesized material is made of homog- enous phase with uniform particles which display nanorods mor- phology sizing about 5 l m in length. TEM image (Fig. 2e) shows that the as-synthesized product exhibits the same rod-like mor- phology. These nanorods are straight and uniform in diameter in the range of 50–70 nm. The high-resolution HRTEM image in Fig. 6, clearly, reveals that the inter-plane distance is 0.383 nm. This could be indexed as [001] of the h-WO 3 crystal, according to JCPDS # 33-1387. This re- sult is good agreement with X-ray diffraction. 3.3. Infrared spectroscopy The structure information was further provided by FTIR spec- troscopy. Fig. 7 displays the FTIR spectrum of as-synthesized WO 3 Fig. 1. XRD patterns of h-WO 3 synthesized at different molar ratio Na 2 SO 4 :W (a) 5:1, (b) 2.5:1, (c) 1.25:1, (d) 0.62:1 (e) 0.30:1 and (f) 0.15:1. 1772 S. Salmaoui et al. /Polyhedron 29 (2010) 1771–1775 nanorods. The broad band located between 949 and 870 cm À1 ob- served for the WO 3 is present in many tungsten oxide compounds. It is attributed to the stretching of short W@O bonds that are also present in h-WO 3 , while the bands located at 817 and 728 cm À1 are assigned to O–W–O stretching modes [34]. The vibrational bands centered at 659 and 590 cm À1 are attributed to W–O–W stretching modes [35]. 3.4. Raman spectroscopy Raman spectroscopy was used to characterize this material since this technique is suitable to obtain details of the WO 3 chem- ical structure (Fig. 8). Well defined peaks centered at 236, 265, 328, 612, 702 and 807 cm À1 , can be observed. According to the litera- ture [35,36], these bands can be assigned to the fundamental modes of crystalline h-WO 3 . The bands at 807 and 702 cm À1 are attributed to the symmetric and asymmetric vibrations of W 6+ –O bonds (O–W–O stretching modes), while the bands at 328, 265 and 236 cm À1 can be attributed to the W–O–W bending mode of the bridging oxygen. The band at 460 cm À1 can be attributed to the characteristic band of crystalline WO 3 [37–39]. FTIR and Ra- man results confirm those obtained by X-ray diffraction. This study suggests to propose a mechanism for the formation processes of h-WO 3 nanorods under hydrothermal conditions: Fig. 2. SEM images for the products prepared at different molar ratio Na 2 SO 4 :W (a) 0.62:1, (b) 1.25:1, (c) 2.5:1 and (d) 5:1. TEM images for the products prepared at molar ratio Na 2 SO 4 :W (e) and (f) 5:1. S. Salmaoui et al. /Polyhedron 29 (2010) 1771–1775 1773 Fig. 3. SEM images of the as-obtained materials: (a) with aniline and without sodium sulfate (b) without sodium sulfate and aniline. Fig. 4. XRD patterns of the as-obtained materials: (a) without sodium sulfate (b) without sodium sulfate and aniline. Fig. 5. SEM image of the as-obtained material in presence of Na 2 SO 4 only. Fig. 6. HRTEM image of the individual WO 3 nanorod. Fig. 7. IR spectrum of h-WO 3 nanorods. 1774 S. Salmaoui et al. /Polyhedron 29 (2010) 1771–1775 Na 2 WO 4 þ 2HCl þ nH 2 O ! H 2 WO 4 ÁnH 2 O þ 2NaCl ð1Þ H 2 WO 4 ÁnH 2 O ! Hydrothermal treatment C 6 H 5 —NH 2 þNa 2 SO 4 WO 3 þðn þ 1ÞH 2 O ð2Þ In this treatment, with sufficient energy provided by the hydro- thermal system, the WO 3 nuclei were formed. In the presence of aniline and Na 2 SO 4 , these nuclei serve as seeds and following grow along the c-axis direction of h-WO 3 unit cell due to the aniline and sulfate preferentially absorb on the faces parallel to the c-axis of the WO 3 , resulting in the formation of the nanorods. More in-depth studies are necessary to further understand their growth process, which can provide important information for structure design and morphology controlled synthesis of oxides. 4. Conclusion In summary, we have successfully synthesized h-WO 3 nanorods through a hydrothermal process. The h-WO 3 nanorods are up to some l m in length, and 50–70 nm in diameter. The pure hexagonal phase crystalline WO 3 nanostructure was confirmed by XRD, SEM, TEM, HRTEM, IR and Raman analyses. 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