Mesoporous crystalline SnO 2 of large surface area
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I.A. Rusakova Texas Center for Superconductivity at the University of Houston, Houston, Texas 77204-5932
(Received 17 August 2003; accepted 24 September 2003)
Mesoporous crystalline SnO2 was synthesized by using templating process with cetyltrimethylammonium bromide as the template, combined with a pretreatment process of hexamethyldisilazane vapor prior to thermal crystallization. The combined process resulted in crystalline SnO2 exhibiting large pore volumes and surface areas that cannot be achieved by either of the processes alone, or by the conventional sol-gel process. Fully crystallized SnO2 powder with a pore volume of approximately 0.2 cc/g, a surface area of 220 m2/g, and mesopores mainly of 5 nm in diameter were obtained after heat treatment at 500 °C.
I. INTRODUCTION
Tin oxide, SnO2, is a material of many technological applications, including catalysis, chemical sensing, and electrochemistry and photoelectron chemistry.1–11 Despite different operating mechanisms, most of these applications prefer similar microstructural properties, including large surface area, mesoporosity, and particularly, high crystallinity. Mesoporosity provides large enough pores to facilitate transport of active species during application while simultaneously allowing for a large specific surface area (SSA) and sufficient structural rigidity and strength. The conventional sol-gel process produces mainly micropores with small pore volumes (∼0.1 cc/g).12–21 Pores enlarge on thermal crystallization but only at the expense of tremendous (>50%) loss in surface area due to grain coarsening. High crystallinity is mostly essential for exhibiting required catalytic and electrical properties of the material. Our previous Raman study indicated that a minimum crystallization temperature of 500 °C is required for achieving full crystallization for the sol-gel–derived hydrous SnO2.21 Use of CO2 supercritical drying enabled formation of broadly distributed mesopores near 10 nm only with fragile skeletons.22 The use of supramolecules as a template has lately been shown to be a very powerful tool for directing formation of mesoporous structures of oxides. Although a)
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J. Mater. Res., Vol. 18, No. 12, Dec 2003 Downloaded: 18 Mar 2015
some success based on this methodology has also been reported for SnO2,23–30 there remains much room for further improvement in terms of achieving mesoporosity with both high crystallinity and large SSA. Chen et al.26 for instance, once used a cationic surfactant cetyltrimethyl ammonium bromide (CTAB) as template to obtain a hydrous precipitate containing partially ordered mesoporous structure. Crystallization of the precipitate at 500 °C resulted in a SSA of 140 m2/g. Using other templating reagents, such as a neutral surfactant (tetradecylamine)25 or a block copolymer template,27 or other preparative procedures (e.g., a sonochemical route28) led to smaller SSAs. We previously31 reported a novel crystal
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