Synthesis and Characterization of Mesostructured Tin Oxide with Crystalline Walls

Shortly after the original synthesis of silica-based mesoporous molecular materials (MCM-41) with uniformly sized pores,1,2 the supramolecular templating approach was extended to the synthesis of non-siliceous mesoporousmaterials based onboth transition andmaingroup metal oxides because of their potential application as solid electrolyte devices, high surface area catalysts, and sorbents, as well as in host-guest chemistry.3,4 The mesostructured materials of the oxides of W, Sb, and Pb are among the first reported examples of hexagonal mesoporousphasesbasedonmetal oxides.5 Subsequently, three-dimensional mesostructures based on a variety of metal oxides such as the oxides of Ti,6,7 Zr,8-10 Hf,11 V,12,13 Nb,14 Ta,15 and Al16,17 were synthesized via various templating routes. The obtainedmesoporousmetal oxide materials usually consist of amorphous walls, although structurally ordered walls can form in certain cases such as the case of ZrO2. It has been pointed out that since manyapplications require crystallinematerials of specific crystalline structure, it would be advantageous to synthesize mesoporous materials with desired crystalline frameworks.18,19 Recently, hexagonal mesoporous manganese oxide with walls composed of microcrystallites of dense phases of Mn2O3 and Mn3O4 has been synthesized and characterized as mixed-valent semiconducting catalysts.20 It is worthwhile to explore the synthesis of mesostructuredmaterialswith crystallinewalls based on other metal oxides by means of supramolecular templating. Tin oxide is a kind of wide energy gap semiconductor and finds many technological applications such as catalysts for oxidation of organics, solid-state gas sensors, and optical electronic devices.21-23 The success in many of these applications relies on crystalline SnO2 with a uniform nanosize pore structure. Thus, synthesis of mesoporous SnO2 with crystalline walls would be of industrial interest. Recently, a mesoporous phase based on SnO2 has been successfully synthesized by using an anionic surfactant aerosol OT (AOT).7 However, the obtainedmesoporous SnO2 has not been shown to consist of crystalline walls. Here, we report on the synthesis of mesostructured tin oxide with walls composed of nanocrystallineSnO2with the tetragonal cassiterite structure. High-resolution transmission electron microscopy (HRTEM) was used to directly examine themicrostructure of the mesophase with crystalline walls for the first time.