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Jiann Shieh National Nano Device Laboratories, Hsinchu, Taiwan, Republic of China

Wei Hao Lai and Min Hsiung Hona) Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China (Received 17 December 2004; accepted 2 June 2004)

The effects of mesoporous structure on grain growth were investigated in this study. The synthesis was accomplished using block copolymer as the organic template and tungsten chloride as the inorganic precursor. Thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy, x-ray diffractometry (XRD), transmission electron microscopy, and N2 adsorption/desorption isotherms were used to characterize the microstructures obtained for different temperatures. TGA and XRD analyses demonstrate that copolymers were expelled at 150–250 °C, and mesoporous structure was stable up to 350 °C. The pore diameter and the surface area evaluated from the Barrett-Joyner-Halenda model and Brunauer–Emmett–Teller method indicated that the average pore diameter is 4.11 nm and specific surface area is 191.5 m2/g for 250 °C calcination. Arrhenius equation used to calculate the activation energy for grain growth demonstrates that the activation energy for grain growth was about 38.1 kJ/mol before mesostructure collapse and 11.3 kJ/mol after collapse. These results show evidence of two different mechanisms governing the process of grain growth. The presence of the pore can be related to the obstacle for grain growth.

I. INTRODUCTION

With the large specific surface area and narrow poresize distribution, mesoporous tungsten oxide acting as a semiconducting ceramic material is currently attracting attention in various applications such as gas sensing, electrochromic, and optical devices.1–4 Generally, mesoporous materials, prepared by self-assembling surfactants as organic templates in sol-gel process,5–8 have poor crystallinity, which is disadvantageous for applications. Most of the mesoporous structure-related work focused on the preparation of silica, but the references concerning the characteristic of mesoporous transition metal oxide are deficient. Until recently, there are a few reported examples of mesoporous WO3, but the relation between the calcined temperature and mesoporous structure

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Address all correspondence to this author. e-mail address: [email protected] DOI: 10.1557/JMR.2004.0362 J. Mater. Res., Vol. 19, No. 9, Sep 2004

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including specific surface area, pore size, and crystallization, especially the activation energy, for grain growth has yet been discussed. The properties of nanostructured materials have been found to depend on the characteristic length associated with them. For example, the sensitivity of semiconducting gas sensor can be improved as the grain size is reduced to the length of space charge layer.9 In the sol-gel process, however, grain growth usually accompanies heat treatment, which is an unavoidable process for dehydroxylation. Grain growth is the result