Pore Size Engineering in Mesoporous Silicas Using Supercritical CO 2 John P. Hanrahan, † Mark P. Copley, † Kirk J. Ziegler, † Trevor R. Spalding, † Michael A. Morris, † David C. Steytler, ‡ Richard K. Heenan, § Ralf Schweins, | and Justin D. Holmes* ,† Department of Chemistry, Material Section and Supercritical Fluid Centre, University College Cork, Cork, Ireland, School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich NR4 7TJ, United Kingdom, ISIS-CLRC, Rutherford Appleton Laboratory, Chilton, Oxfordshire OX110QX, United Kingdom, and Institut Laue-Langevin, 6 rue Jules Horowitz BP 156-38042 Grenoble, CEDEX 9, France Received November 30, 2004. In Final Form: February 10, 2005 In this paper we investigate the use of supercritical carbon dioxide (sc-CO2) for synthesizing calcined mesoporous silicas with tunable pore sizes, wall thickness, and d spacings. Small angle neutron scattering was used to probe the controlled swelling of the triblock copolymer surfactant templating agents, P123 (PEO20PPO69PEO20), P85 (PEO26PPO39PEO26), and F127 (PEO106PPO70PEO106), as a function of CO2 pressure. The transition from the liquid crystal phase to the calcined mesoporous silicas, formed upon condensation and drying, was also studied in detail. Powder X-ray diffraction, transmission electron microscopy, and nitrogen adsorption techniques were used to establish pore diameters, silica wall widths, and the hexagonal packing of the pores within the calcined silicas. Using a direct templating method, the diameters of mesopores and the spacing between the pores could be tuned with a high level of precision. The swelling process was observed to have no detrimental effects on the quality of silica formed, a distinct advantage over conventional swelling techniques, and all of the silicas synthesized in this study were highly ordered over distances of at least 2000 Å. Introduction Mesoporous materials, 1,2 with uniform and tailorable pore dimensions and high surface areas, are currently being employed in a number of applications that include molecular protein separations, 3 catalysis, 4,5 and chroma- tography 6 and as templates for controlling the aspect ratio of quantum-confined nanoparticles and nanowires. 7-11 In particular, mesoporous thin films have recently been utilized as templates for creating high-density arrays of semiconductor 12 and metallic 13 nanowires and carbon nanotubes 14 allowing the potential creation of multilayered microelectronic device architectures. Amphiphilic block copolymers have emerged as cheap and valuable supramolecular templates for mesostruc- tured materials possessing long-range order. 15 Liquid crystal templating methods have been developed by a number of research groups to prepare stable mesoporous silicas from short chain ethylene oxide surfactants 16 and from triblock copolymer surfactants containing poly- (ethylene oxide) (PEO) and poly(propylene oxide) (PPO) segments. 17-20 In particular, Zhao et al. 20,21 demonstrated the synthesis of a family of highly ordered mesoporous silica structures, that is, SBA-15, with pore dimensions ranging between 20 and 300 Å using commercially available alkyl PEO oligomeric surfactants in acid media. 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