Large-scale hydrothermal synthesis of WO 3 nanowires in the presence of K 2 SO 4 Xu Chun Song a, ⁎ , Yi Fan Zheng b , E. Yang a , Yun Wang a a Department of Chemistry, Fujian Normal University, Fuzhou 350007, P.R. China b Coll Chem Engn & Mat Sci, Zhejiang University Technol, Hangzhou, Zhejiang 310014, P.R. China Received 7 July 2006; accepted 21 December 2006 Available online 30 December 2006 Abstract WO 3 nanowires were fabricated by a hydrothermal method in the presence of K 2 SO 4 . The nanowires exhibit a well crystallized one- dimensional structure with 10 nm in diameter and several microns in length. Effects of other alkali salts (KNO 3 , NaNO 3 and Na 2 SO 4 ) on the morphologies of WO 3 nanocrystals were also investigated. The important role of K 2 SO 4 salt in the WO 3 nanowires synthesis has been demonstrated. © 2006 Elsevier B.V. All rights reserved. Keywords: Hydrothermal; WO 3 ; Nanowires; K 2 SO 4 1. Introduction Over the past few years, much effort has been devoted to the synthesis of semiconductor nanowires, nanorods, and nanobelts, because of the importance of understanding the dimensionality confined transport phenomena and fabricating nanodevices and nanosensors [1–4]. Many attempts have been made to synthesize one-dimensional nanostructured materials [5–14]. Of the meth- ods used in 1D nanostructure synthesis, hydrothermal processes have emerged as powerful tools for the fabrication of anisotropic nanomaterials with some significant advantages, such as controllable particle size and low-temperature, cost-effective, and less-complicated techniques. Under hydrothermal conditions, many starting materials can undergo quite unexpected reactions, which are often accompanied by the formation of nanoscopic morphologies that are not accessible by classical routes. Among various metal oxides, WO 3 is a versatile wide band- gap semiconductor for many valuable applications. WO 3 has found useful applications in semiconductor gas devices [15], electrochromic devices [16], and photocatalyses [17].Thusfar, preparation of single-crystalline, 1D 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 [18] or recently by electrochemically etching a tungsten tip, followed by heating at 700 °C under argon [19].The employed harsh conditions, contamination by platelets, and uncontrolled size hamper systematic investigations on size- dependent properties of 1D nanostructured tungsten oxide itself as well as of inorganic derivatives prepared from the oxide. Recently, the hydrothermal synthesis of ultralong and single- crystalline Cd(OH) 2 nanowires using alkali salts as mineralizers was reported by Tang et al. [20]. The 1D nanostructure synthesis using inorganic salt instead of surfactant and water-soluble high molecule has strong points in non-pollution, low-cost, easy- cleanout and recovery. Herein, we describe a facile inorganic route for synthesis of uniform WO 3 nanowires in aqueous solution. This novel method is based on treating freshly prepared H 2 WO 4 in the presence of K 2 SO 4 salt under hydrothermal conditions at 180 °C for 12 h. 2. Experime ntal 2.1. Synthesis of WO3 Na 2 WO 4 (1 g) was dissolved in 30 ml deionized water to form a transparent solution. A (3 mol l − 1 ) HCl solution was added dropwise into the above solution under continuous Materials Letters 61 (2007) 3904– 3908 www.elsevier.com/locate/matlet ⁎ Corresponding author. Tel.: +86 591 87441126; fax: +86 591 83465376. E-mail address: songxuchunfj@163.com (X.C. Song). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.12.055 stirring until tungstenic acid was precipitated thoroughly. Next, the centrifuged precipitate was dissolved in 30 ml deionized water, 40 g K 2 SO 4 was added to the system and agitated to form starchiness, and then transferred into Teflon-lined autoclave with a capacity of 50 ml. Hydrothermal treatments were carried out at 180 °C for 12 h. After that, the autoclave was allowed to cool down naturally. The finally products were collected and washed with deionized water and ethanol several times and dried in air at 80 °C. The WO 3 nanowires were finally obtained. 2.2. Characterization The morphologies were characterized using scanning electron microscopy (SEM, Hitachi S-4700 II, 25 kV) and transmission electron microscopy (T EM, JEM200CX, 120 kV). The composition of the product was analyzed by energy dispersive X-ray detector (EDX, Thermo Noran VANTAG-ESI, 120 kV). The X-ray diffraction (XRD, Thermo ARL SCINTAG X'TRA with CuKα irradiation, λ =0.154056 nm) was used to analyze the crystallinity. 3. Results and discussion The morphologies of the final products were demonstrated in Fig. 1a–c. On the basis of the SEM check, the proportion of the nanowire morphology was estimated to be about 100% (Fig. 1a). As shown in the SEM images, the average diameter of these uniform nanowires was about 10 nm and the length was up to several microns (Fig. 1b). Therefore, the nanowires reached a high aspect ratio of more than 500. A TEM image of a single nanowire with diameter of about 10 nm was shown in Fig. 1c. The selected area electron diffraction (SAED) taken from different parts of nanowires indicated that the sample was single crystalline with a preferential growth in the (001) direction. The energy dispersive spectrometry (EDS) analysis was employed to determine the composition of the tungsten oxide nanowires. As shown in Fig. 2, only oxygen and tungsten elements existed in the nanowires with molar ratio of about 3 (O/W). A representative XRD pattern for our synthesized tungsten oxide nanowire was displayed in Fig. 3. All the main peaks can be indexed undisputedly to hexagonal WO 3 (JCPDS card 35-1001), which are consistent with general features of nanomaterials. Diffraction peaks of Fig. 1. (a) SEM, (b) SEM, (c)TEM images of WO 3 nanowires synthesized at 180 °C for 12 h with 40 g K 2 SO 4 . Fig. 2. EDS patterns of WO 3 nanowires. Fig. 3. XRD patterns of WO 3 nanowires. 3905X.C. Song et al. / Materials Letters 61 (2007) 3904–3908 (001) are stronger compared with the rest, indicating that the [001] is the major growth direction. This agrees well with the SAED results. The morphologies of synthesized WO 3 with different amounts of K 2 SO 4 were shown in Fig. 4a–c. Fig. 4a is the TEM image of the sample obtained without K 2 SO 4 , which has a lamellar structure with diameter of about 100 nm. With the addition of 15 g K 2 SO 4 , the morphologies of synthesized WO 3 shown in Fig. 4b are the mixture of nanorods and nanoparticles. Nanorods and nanowires were obtained for the products synthesized with 30 g K 2 SO 4 (Fig. 4c). As the content of K 2 SO 4 reaches 40 g, Fig. 4d shows the TEM image of the solid sample, where nanowires with lengths around several micrometers and diameter of about 10 nm are the major product. From the results, it can be speculated that the content of the assisted K 2 SO 4 plays an important role in the specific morphologies of WO 3 . No nanowires of WO 3 could be obtained without K 2 SO 4 , and the purity of the nanowires only depends strongly on the content of K 2 SO 4 . We have also carried out synthesis with 40 g K 2 SO 4 at 160 and 200 °C individually. It could be found that short WO 3 nanorods but generally mixed with nanoparticles were produced at 160 °C. However, the nanowires were obtained at 200 °C, and the morphology was similar with that obtained at 180 °C. In addition, we have carried out analogous experiments with different inorganic salts for comparison. Fig. 5 displayed the TEM image of the obtained WO 3 with the addition of NaNO 3 , KNO 3 and Na 2 SO 4 . It can be seen that no WO 3 nanowires could be obtained with any content of KNO 3 . With an increase in the content of KNO 3 , the dimension of nanoparticles only became smaller. Similarly, the nanorods and nanoparticles were obtained in the products with the addition of NaNO 3 or Na 2 SO 4 . The nanowires of WO 3 were not obtained at last with any content of KNO 3 and Na 2 SO 4 . It could be concluded from the results mentioned above that K 2 SO 4 plays an important role in the synthesis of WO 3 nanowires. The XRD patterns for WO 3 synthesized with different kinds of inorganic salts were compared in Fig. 6. It is obvious that the crystalline phases for WO 3 nanocrystals are discriminatory at different conditions. The hexagonal phase of WO 3 (JCPDS card 33-1387) was obtained without salts or in the presence of KNO 3 , NaNO 3 and NaSO 4 (see Fig. 6a–d). Among this, the intensity for the diffraction peaks grew weaker as follows: no salt N KNO 3 N NaNO 3 N NaSO 4 . On the other hand, Fig. 6e exhibited the hexagonal reflections (JCPDS card 35-1001) in the presence of K 2 SO 4 . Combined with the TEM results, it could be concluded that the inorganic salts had a significant effect on the crystalline phase and the corresponding morphology of WO 3 . The morphologies and dimensions of synthesized nanocrystals were controlled not only by the inner structure, but also affected by the Fig. 4. TEM images of WO 3 synthesized at 180 °C for 12 h with different amounts of K 2 SO 4 : (a) 0 g, (b) 15 g, (c) 30 g, and (d) 40 g. 3906 X.C. Song et al. / Materials Letters 61 (2007) 3904–3908 surrounding conditions such as temperature, pressure, and composition of the solution [21]. The formations of nanowires first need the anisotropy during the growth process for the nanoparticles. In our experiments, the presence of the salts is also an important factor influencing the crystallization process and the growth of the WO 3 nanowires. With the addition of different inorganic salts, the compositions and the corresponding properties of the solution are different in t he hydrothermal conditions. The changes in the surrounding conditions would affect the crystalline phase, and further affect the morphologies and dimensions of the WO 3 nanocrystals. As the reaction mechanism and hydrothermal conditions are complicated, the exact reason for the 1D nanostructure synthesized in the presence of inorganic salts needs further investigations. 4. Conclusion In summary, tungsten o xide nanowires with relatively uniform diameters ranging from 10 to 20 nm and lengths up to several micrometers were synthesized on a large scale. With the distinctive and promising properties of tungsten oxide, the as-synthesized nanowires may serve as functional materials in the fabrication of nanosized sensors and flat panel display systems. The important role of K 2 SO 4 salt in the synthesis has been demonstrated. This aqueous route should be feasible for large-scale p roduction of low-dimensional nanostructured tungsten oxide. Acknowledgment We wish to acknowledge the financial support from the Natural Science Foundation of Fujian Province (no: 2006J0153). References [1] D. Wu, J. Liu, X.N. Zhao, A.D. Li, Y.F. Chen, N.B. Ming, Chem. Mater. 18 (2006) 547. [2] Hao X. Mai, L.D. Sun, Y.W. Zhang, Y. Si, W. Feng, H.P. Zhang, H.C. Liu, C.H. Yan, J. Phys. Chem., B 109 (2005) 24380. [3] J.G. Yu, J.C. Yu, W.H. Ho, L. Wu, X.C. Wang, J. Am. Chem. 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Large-scale hydrothermal synthesis of WO 3 nanowires in the presence of K 2 SO 4 Xu Chun Song a, ⁎ , Yi Fan Zheng b ,. from the oxide. Recently, the hydrothermal synthesis of ultralong and single- crystalline Cd(OH) 2 nanowires using alkali salts as mineralizers was reported