Hydrothermal synthesis of monodisperse WO 3 ·H 2 O square platelet particles Fumiyuki Shiba a, ⁎ , Masakazu Yokoyama a , Yousei Mita b , Tomohiro Yamakawa b , Yusuke Okawa a a Graduate School of Science and Technology, Chiba University, Yayoicho 1-33, Inageku, Chiba 263-8522, Japan b Faculty of Engineering, Chiba University, Yayoicho 1-33, Inageku, Chiba 263-8522, Japan Received 11 July 2006; accepted 22 July 2006 Available online 15 August 2006 Abstract Monodisperse particles of tungsten(VI) oxide monohydrate were prepared in a hydrothermal system at 40 °C, where 20 ml of HCl solution (1.50 mol/l) was added to the same volume of Na 2 WO 4 aqueous solution (0.50 mol/l) with magnetic stirring, followed by standing in an air oven for 168 h. The shape of the particles was square platelet and the mean size was 0.72 μm with 10% of the coefficient of variation. The XRD pattern was in good agreement with the standard JCPDS data for WO 3 ·H 2 O. The effects of counter cation and anion of the starting materials in addition to the preparation temperature and acid concentration were also examined. © 2006 Elsevier B.V. All rights reserved. Keywords: Tungsten(VI) oxide monohydrate; Tungstic acid; Monodisperse particle; Hydrothermal synthesis 1. Introduction Tungsten(VI) oxide and its hydrates, WO 3 ·nH 2 O, are known as electrochromic (EC) substances which alternate their color by electrochemical redox reactions. A deep blue color in the reduced state is based on a mixed valence state of W(V) and W (VI) [1]. Thin films, prepared by vapor evaporation for instance, are common in the WO 3 -based EC devices [2]. However, if particulate materials are applied, availabilities such as simple production processes and hybrid compositions with functional polymers [3,4] are expected. In addition, printing processes are applicable for patterning of the EC materials. Monodisperse particles are particularly expected to give sharp distributions of properties such as response time, precise controls of properties, regular arrangement of particles on electrode, etc. Hydrothermal synthesis from Na 2 WO 4 in a low pH region is one of the typical ways to prepare tungsten oxide hydrates. Freedman [5] investigated influence of acid concentration and temperature on preparation of WO 3 ·H 2 O and WO 3 ·2H 2 O from Na 2 WO 4 solution. Gerand et al. [6] prepar ed WO 3 ·1 / 3H 2 Oin pure water at 120 °C by digesting washed precipitates of WO 3 ·nH 2 O gel or particles, preliminarily prepared from Na 2 WO 4 in acidic conditions. Instead of adding acid solution s, an ion-exchange method was also applied to make Na 2 WO 4 solution acidic to prepare the WO 3 ·2H 2 O particles [7]. How- ever, there seems to be few reports on narrowing the size distribution of these particles. In the present study, the hydro- thermal condition was optimized to prepare monodisperse WO 3 ·H 2 O parti cles. 2. Experimental In the standard condition, 0.50 mol/l of Na 2 WO 4 solution was freshly prepared by dissolving 3.30 g of Na 2 WO 4 ·2H 2 Oin 20 ml of distilled water in a screw-capped bottle. At 40 °C, 20 ml of HCl aqueous solution (1.50 mol/l) was added into the Na 2 WO 4 solution by a transfer pipette under magnetic stirring, which was halted after 1 min from mixing. After 168 h of storage in an air oven at 40 °C, the precipitates were centrifuged at 2000 rpm for 15 min. After removing the supernatant solution, they were re-dispersed in distilled water and centrifuged again. Five sets of the centrifugation process were carried out to wash completely. The effects of the preparation condition were examined by varying either HCl concentration or temperature from the Materials Letters 61 (2007) 1778 – 1780 www.elsevier.com/locate/matlet ⁎ Corresponding author. Fax: +81 43 290 3490. E-mail address: shiba@faculty.chiba-u.jp (F. Shiba). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.129 standard condition. Also, the influence of counter ions of the starting materials was tested. That is, Li 2 WO 4 and K 2 WO 4 were used instead of Na 2 WO 4 . Alternatively, HCl was replaced by HClO 4 , HNO 3 ,orH 2 SO 4 . Because Li 2 WO 4 did not dissolve in distilled water, it was used in a form of dispersion. Particle shape was observed by a scanning electron micro- scope (SEM, Hitachi S-2400). The mean size and coefficient of variation, COV, were obtained with a transmission electron microscope (TEM, JEOL JEM-1200EX). The particles were identified by X-ray diffractometry (XRD) using CuKα radiation with MacScience M18XHF-SRA. The yield of the objective particles was estimated by a gravimetric analysis, in which precipitates fairly separated from soluble salts and gel-like residuals were heated to dehydrate at 750 °C for 30 min and then weighed as WO 3 . All chemicals were purchased from Wako Pure Chemicals except for Li 2 WO 4 which was purchased from Aldrich. They were used as supplied. 3. Results and discussion Particles prepared in the standard condition were in square platelet shape from SEM images, as shown in Fig. 1. Fig. 1 also indicates a histogram of their size distribution obtained from TEM observation. The mean edge length was estimated as 0.72 μm with 10% in COV from more than 200 particles. The average thickness was about 0.2 μm. Fig. 1. SEM image of obtained particles in the standard condition and their size distribution evaluated from TEM observations. Fig. 2. XRD patterns of (a) particles obtained in the standard condition and (b) standard of WO 3 ·H 2 O on the basis of JCPDS data (JCPDS No. 18-1418). Fig. 3. Time evolutions of (a) the mean size of particle and COV and corres- ponding TEM images at (b) 48 h, (c) 56 h, (d) 96 h, and (e) 168 h in the standard condition with the limited repetition of centrifugal washing. 1779F. Shiba et al. / Materials Letters 61 (2007) 1778–1780 The particles were identified as tungsten(VI) oxide monohydrate by a good agreement in the XRD pattern with a JCPDS data for WO 3 ·H 2 O (JCPDS No.18-1418) as shown in Fig. 2. The crystal structure of WO 3 ·H 2 O is anisotropic, where layers consisting of Wand O atoms are stacked via hydrogen bonds [8]. As explained by Livage and Guzman [9], the W atom in the precursor has four coordinated OH groups in an equatorial xy plane with a W_O bond and water molecule in the vertical z-axis. The platelet shape of particles suggests a large dif- ference in the growth rates of the crystal orientations depending on the precursor structure. The change of appearance during the formation process was as follows. At first, a yellow transparent solution was formed by mixing of the Na 2 WO 4 and HCl solutions due to formation of soluble complexes. The mixed solution soon became turbid by the generation of gel-like precipitates and lost fluidity within a few minutes. Reflecting the decrease of complexes, the yellowish color was once reduced until 48 h but gradually increased again by growth of WO 3 ·H 2 O particles, which are also yellow. The apparent volume of precipitate started to decrease at ∼ 72 h and reached to ca. 1 / 3 of the whole volume at 168 h. Fig. 3 shows time evolutions of the mean edge length of objective particles and COV with some corresponding TEM images at 48, 56, 96, and 168 h, where the results were obtained from precipitates discretely prepared for each time. Besides, the repetition of centrifugal washing was limited to two times to retain the gel-like precipitates. Only gel was observed before 48 h but objected particles were formed in the gel- network at least at 56 h. Fig. 3(a) suggests ongoing particle growth even at 168 h. In addition, the gel-like precipitates seemed amorphous in the earlier stage of the generation process and somewhat crystalline at 168 h from dark-field observations with TEM. The standard deviation of particle size was almost constant (ca. 0.07 μm) during the growth period. Thus COV tended to decrease from 22% at 56 h to 10% at 168 h. Due to the gel-like residuals existing at the end of precipitation, as shown in Fig. 3(e), the yield of the objective particles was unfortunately low as 25%. It was raised to 75% by using Li 2 WO 4 instead of Na 2 WO 4 , although particles were polydisperse ones with about 0.4 μm and 25% in mean size and COV, respectively. K 2 WO 4 generated only gel-like precipitates without any objective particles. Moreover, when the concentration of Na 2 WO 4 solution was reduced to 0.1 mol/l, coupled with 0.85 mol/l of HCl, the yield was raised to 80% with a declined monodispersity (12% in COV with 0.46 μm in the mean size). Univalent acids, HClO 4 and HNO 3 , gave almost the same results as that of HCl. On the contrary, H 2 SO 4 formed only gel-like precipitates. Balázsi and Pfeifer [10] observed structural and morphological changes of WO 3 ·2H 2 O particles by a decrease of Na + ion content through a re- peated washing treatment. Therefore, counter ions of tungstates and acids anyway play important roles on the formation process in this system, although the mechanism has not been clear yet. The effect of the concentration of HCl solution was tested in the range of 1.40–1.80 mol/l at 40 °C with Na 2 WO 4 . When increasing the HCl concentration, the mean size was reduced from 0.95 to 0.62 μm and COV tended to decline from 10% to 19%. When temperature was varied in 35–55 °C with 1.50 mol/l of HCl solution, the mean size was decreased from 0.80 to 0.59 μm by the increasing temperature. A minimum COV was shown at 40 °C (20, 10, 13, 12, and 15% in COVat 35, 40, 45, 50, and 55 °C, respectively). Except for 35 °C, the apparent volumes of yellow precipitates at 168 h were almost the same as that of the standard condition (40 °C), suggesting similar yields in these conditions since the apparent volume reflects a degree of transforma- tion to the WO 3 ·H 2 O particles from bulky gels. At 35 °C, the apparent volume of the precipitate was almost the same as that of the whole volume. The larger mean size at 35 °C, even the small conversion to WO 3 ·H 2 O, is due to an inhibited nucleation. Hence this temperature is likely a critical one to form WO 3 ·H 2 O in the system. This observation may be supported by the results of Freedman [5], who has reported that only WO 3 ·2H 2 O was obtained at 25 °C while mixtures of WO 3 ·H 2 O and WO 3 ·2H 2 O were obtained at 50 °C. Generally in monodisperse particle preparations, monodispersity indicated by COV is narrowed by the progress of growth with at least a constant absolute size distribution [11]. Thus longer growth periods relative to nucleation ones are preferred in addition to a clear separation of both steps. In agreement with the mechanisms, COV in the standard condition was decreased to reach the monodisperse particles with the almost constant standard deviation and increasing particle size, as mentioned above. Hence size distribution at the end of the nucleation period has an importance in the final monodispersity. In other words, higher HCl concentration and higher temperature than the standard condition kept a high supersaturation ratio to advance the nucleation period, giving smaller size and declined monodispersity in the present system. 4. Conclusions Monodisperse WO 3 ·H 2 O particles in a square platelet shape were prepared in an optimized hydrothermal system. The narrow size distribution, 10% in COV with 0.72 μm in mean size, was achieved in the standard condition. Acknowledgement We would like to thank Dr. T. Kojima (Department of Applied Chemistry and Biotechnology, Chiba University) for the XRD measurements and Mr. Y. Funabashi (Department of Information and Image Sciences, Chiba University) for the SEM observa- tions. The research was partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Young Scientists (B) and by the Hosokawa Powder Technology Foundation. References [1] M.B. Robin, P. Day, Adv. Inorg. Chem. Radiochem. 10 (1967) 247. 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