Effect of Calcination on Crystallinity for Nanostructured Development of Wormhole-Like Mesoporous Tungsten Oxide Wei Hao Lai, z Lay Gaik Teoh, w,y Yen Hsun Su, z Jiann Shieh, J and Min Hsiung Hon z z Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan y Department of Mechanical Engineering, National Pingtung University of Science and Technology, Neipu, Pingtung 91201, Taiwan z Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan J National Nano Device Laboratories, Hsinchu 30050, Taiwan The effects of calcination on the crystallinity and grain growth model for microstructure development of wormhole-like meso- porous tungsten oxide are investigated in this study. We found that residual mesopores of wormhole-like tungsten oxide can be seen calcined above 5001C, as evidenced by retention of a worm- hole-like mesostructure of tungsten oxide. A model was pro- posed to combine grain growth with wormhole-like mesoporous behavior and to investigate whether the formation of wormhole- like mesoporous tungsten oxide can retard the collapse rate of mesopores during the calcination process. I. Introduction N ANOSTRUCTURED mesoporous materials are widely used for many practical applications due to their large, controllable pore size, and high surface area. 1–3 With its large specific surface area for more reactive sites and narrow pore size distribution, mesoporous tungsten oxide used as a semiconducting ceramic material is now attracting attention in various applications, such as gas sensing, electrochromic apparatus, and optical devices. 4,5 Generally, mesoporous materials of many oxides are prepared using self-assembling surfactants as organic templates in a sol–gel process, 6,7 but this leads to poor crystalli- nity that is disadvantageous for practical applications. It is well known that the calcination temperature strongly affects the crystal structure, particle size, and grain growth, and thus the properties of a mesoporous metal oxide. 8 In order to inhibit grain growth, mechanisms for the reduction of grain boundary mobility and thermodynamic driving force by impu- rity pinning and metastable systems, respectively, have been proposed. 9,10 However, there are few reports that discuss the relationship between the possible model and crystallinity for calcinations of the microstructure development or the strength of the spectrum of wormhole-like mesoporous tungsten oxide. Crystallization of frameworks is strongly affected by the heating rate. Panda 11 demonstrated that increasing the heating can delay crystallization so that a certain glass can sinter to full density. Because faster heating rates retard the nucleation pro- cess, Keddie and Giannelis 12 suggested that samples heated slowly form more crystallites, which corresponds to the authors’ idea in this manuscript. In this work, we have adjusted the calcination temperature with a 11C/min heating rate to investi- gate the effect of a mesoporous structure on the grain growth of a nanocrystallite by microstructure analyses. We also report on the possible model that governs or suppresses the grain growth of wormhole-like mesoporous tungsten oxide, and believe that this attempt is of great significance for the preparation of worm- hole-like nanostructures in the future. II. Experimental Procedure The samples were prepared by sol–gel and reflux processes, using a colloidal solution of tungstic chloride stabilized by the addition of a selected organic poly(alkylene oxide) triblock copolymer. 0.7 g of a poly(alkylene oxide) triblock copolymer of L62 (BASF Pluronic EO 8 PO 30 EO 8 ) was dissolved in 7 g of ethanol solvent (purity: 99.8%). To this solution, 3.5 Â 10 À3 mol of the anhydrous inorganic chloride precursor, WCl 6 (Sigma- Aldrich Co., St. Louis, MO), was slowly added and vigorously stirred in a reflux system at 601C for 48 h. The resulting solu- tions were gelled in an open Petri dish at 601C in air, and then calcined at 2501, 5001, and 7501C for 12 h at a 11C/min heating rate to remove the residual triblock copolymer. The mesostructure of the tungsten oxide obtained was then investigated by X-ray powder diffractometry (Rigaku D/Max-w, Tokyo, Japan), a small-angle X-ray scattering system (Osmic PSAXS-USH-WAXS-002, Troy, MI), transmission electron mi- croscopy (Hitachi Model HF-2000, 200 keV, Tokyo, Japan), and a Raman spectrometer (LabRAM HR, Horiba Jobin Yvon. Inc., Edison, NJ). The nitrogen adsorption and desorption iso- therms at 77 K were measured using a Micrometrics ASAP 2010 system (Norcross, GA) after the samples were vacuum dried at 1501C for 12 h in a N 2 atmosphere. III. Results and Discussion The crystalline structure of as-synthesized tungsten oxide pow- der (Fig. 1(a)) is the monoclinic type of nanocrystals (JCPDS- ICDD 75-2072). The crystal phase of mesoporous tungsten oxide transformed into the cubic phase of nanocrystals (JCPDS-ICDD 41-0905) when it calcined at 2501C (Fig. 1(b)). Furthermore, the crystal phase transformed into orthorhombic crystalline (JCPDS-ICDD 71-0131) when it was calcined above 5001C (Figs. 1(c) and (d)). The mesostructured tungsten oxide was examined by low diffraction angles in the inset of Fig. 1. A well-defined intensity was observed at 2y about 11 for the as- synthesized and calcined samples. There is a broad peak, corre- sponding to the presence of a wormhole-like structure and a lack of long-range order, similar to that reported by others. 13,14 T. Ohji—contributing editor This work was financially supported by the National Science Council of Taiwan, the Republic of China, grants No. NSC 96-2221-E-006-007, NSC 95-2622-E-020-008-CC3, and NSC 96-2221-E-020-013, which are gratefully acknowledged. w Author to whom correspondence should be addressed. e-mail: n5888107@mail.npust. edu.tw Manuscript No. 23223. Received May 17, 2007; approved August 13, 2007. J ournal J. Am. Ceram. Soc., 90 [12] 4073–4075 (2007) DOI: 10.1111/j.1551-2916.2007.02078.x r 2007 The American Ceramic Society 4073