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Control of Pore Morphology in Mesoporous Silicas Synthesized from Triblock Copolymer Templates Kevin M. Ryan,† Nicholas R. B. Coleman,† Daniel M. Lyons,† John. P. Hanrahan,† Trevor R. Spalding, M. A. Morris,† David. C. Steytler,‡ Richard K. Heenan,§ and Justin D. Holmes*,† Department of Chemistry, Materials Section and Supercritical Fluid Centre, University College Cork, Cork, Ireland, School of Chemical Sciences, University of East Anglia, Norwich, NR4 7TJ, U.K., and ISIS-CLRC, Rutherford Appleton Laboratory, Chilton, Oxfordshire, OX11 0QX, U.K. Received February 6, 2002. In Final Form: March 25, 2002 In this paper we describe a novel approach for synthesizing calcined mesoporous silicas with tunable pore sizes, wall thicknesses, and pore spacings using mixtures of triblock copolymer surfactants as hexagonal templating agents. Small angle neutron scattering was used to probe the transition from the liquid crystal phase to the calcined mesoporous silicas formed upon condensation and drying. Powder X-ray diffraction, transmission electron microscopy, and nitrogen adsorption techniques were also used to establish pore diameters, silica wall widths, and the hexagonal packing of the pores within the calcined silicas. By use of a direct templating method, the diameters of mesopores and the spacing between the pores could be tuned with angstro¨m-level precision between the size range of 45-70 and 75-100 Å, respectively. Tailoring of the silica wall widths is also displayed. All the silicas synthesized were highly ordered over distances of at least 2 µm.
Introduction Materials with uniform and tailorable pore dimensions are expected to play a vital role in applications that range from catalysis,1 to molecular separations,2 and even to the fabrication of semiconductor and low dielectric devices.3 Mesoporous materials with variable pore diameters appear to be potentially useful for many of these purposes.4,5 MCM-41 mesoporous materials, with pore sizes ranging from 15 to 100 Å, were first synthesized by Kresge and Beck4,6 using a variety of surfactants, that included cetyltrimethylammonium bromide (CTAB). MCM-41 materials however have found limited use, due in part to the noncommercial availability of the long-chain surfactants used in the preparation and the need for swelling agents that can often lead to the complete extinction of pore ordering.7 Alternatively, amphiphilic block copolymers have recently emerged as cheap and valuable supramolecular templates for mesostructured materials possessing longrange order.8 Liquid crystal templating methods have been developed by a number of research groups to prepare stable * To whom correspondence should be addressed: telephone, +353 (0)21 4903608; fax, +353 (0)21 4274097; e-mail,
[email protected]. † University College Cork. ‡ University of East Anglia. § ISIS-CLRC, Rutherford Appleton Laboratory. (1) Corma, A.; Navaro, M. T.; Pariente, J. P. J. Chem. Soc., Chem. Commun. 1994, 147. (2) Estermann, M.; McCusker, L. B.; Baerlocher, C.; Merroche, A.; Kessler, H. Nature 1991, 352, 320. (3) Brunisma, P. J.; Hess, N. J.; Bontha, J. R.; Liu, J.; Baskaran, S. Mater. Res. Soc. Proc. 1997, 443, 105. (4) Kresage, C. T.; Leonwicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (5) Tanev, P. T.; Pinnavaia, J. Science 1995, 267, 865. (6) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresege, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E.; McMullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (7) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Chmelka, B. F.; Schuth, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176.
mesoporous silicas from short chain ethylene oxide surfactants9 and from triblock copolymer surfactants containing poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) segments.10,11 In particular, Zhao et al.10 demonstrated the synthesis of a family of highly ordered mesoporous silica structures, i.e., SBA-15, with pore dimensions between 20 and 300 Å using commercially available alkyl PEO oligomeric surfactants in acid media without the use of swelling agents. Block copolymer surfactants are ideal as mesoporous templates as they are cheap and readily available due to their use in numerous commercial applications in both the pharmaceutical and cosmetic industries as cleaning, antifoaming, and thickening agents.11-14 Traditionally, tailoring the pore widths of mesoporous silicas prepared from cationic surfactants has been achieved either through synthesizing surfactants of various tail lengths or by adding organic swelling agents, such as decane, or cosurfactants.15 Fine-tuning of pore diameters on nanometer length scales has previously been demonstrated in uncalcined mesoporous materials using CTAB as a template with cetylpyridinium chloride as a cosurfactant.16 This method of mesopore control has however not been extended to calcined silicas, and exact pore size tuning on nanometer and sub-nanometer length scales has remained elusive due to poor sample reproduc(8) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242. (9) Attard, G. S.; Glyde, J. C.; Goltner, C. G. Nature 1995, 378, 366. (10) Zhao, D.; Sun, J.; Li, Q.; Stucky, G. D. Chem. Mater. 2000, 12, 275. (11) Chu, B.; Zhou, Z. Polyoxyalkylene Block Copolymers; Nace, V. M., Ed.; Marcel Dekker: New York, 1996; Vol. 60. (12) Meziani, A.; Tourand, D.; Zradba, A.; Pulvin, S.; Pezron, I.; Clausse, M.; Kunz, W. J. Phys. Chem. 1997, 101, 3620. (13) Sjoblom, J.; Stenius, P.; Danielsson, I. Nonionic Surfactants: Physical Chemistry; Nace, V. M., Ed.; Marcel Dekker: New York, 1987. (14) Schmolka, I. R.; CRC Press: Boston, 1991; p 3620. (15) Ulagappan, N.; Rao, C. N. R. J. Chem. Soc., Chem. Commun. 1996, 2759. (16) Khushalani, D.; Kuperman, A.; Coombes, N.; Ozin, G. A. Chem. Mater. 1996, 8, 2188.
10.1021/la025606a CCC: $22.00 © 2002 American Chemical Society Published on Web 05/10/2002
Pore Morphology in Mesoporous Silicas
ibility. Even though mesoporous silicas templated from triblock copolymers have been prepared with pore diameters ranging from 20 to 300 Å, it has proven difficult to tailor the pore sizes and packing of the mesopores to specific sizes within this broad range.17 Control over mesopore dimensions will be important in a number of applications. We have recently demonstrated that mesoporous silica mesopores can act as templates for forming ordered arrays of semiconductor nanowires,18-20 where the pore size of the silica dictates the diameter of the nanowires produced. Thus, angstro¨m-level control over the mesopore dimensions might subsequently lead to previously unobserved optical and electronic properties of these confined materials. Pore size control on an angstro¨m scale will possibly allow future exploitation of mesoporous materials for molecular sieving, host/guest chemistry, and shape-selective catalysis in a manner similar to the use of zeolites.21-23 In this paper we report angstro¨m-level control over the pore widths and hexagonal spacing between pores in mesoporous silica formed using mixed copolymer surfactants as templating agents. We describe how the diameters of silica mesopores, the thickness of the silica walls, and the center-to-center pore spacings can be controlled on an angstro¨m-scale while retaining long-range hexagonal order on a microscopic scale. Experimental Section Preparation of Mesoporous Silica. Hexagonal mesoporous silica was prepared by a method based on one described by Attard et al.,9,24 i.e., the hydrolysis of tetramethoxysilane (TMOS) in the presence of poly(ethylene oxide) (PEO)-poly(propylene oxide) (PPO) triblock copolymer surfactants, P85 (PEO26PPO39PEO26), P123 (PEO20PPO69PEO20), or P65 (PEO20PPO30PEO20). The surfactants used were supplied by Uniquema, Belgium. In a typical synthesis a mixture of P85 (0.5 g) and P123 (0.5 g) was dissolved in TMOS (1.8 g, 0.0118 mol) and added to an aqueous solution of HCl (1 g, 0.5 M) to give a surfactant concentration of 50 wt %. Methanol generated during the reaction was removed on a rotary film evaporator at 40 °C. The resulting viscous gel was left to condense at 40 °C for 1 week in a sealed flask. Calcination of the condensed gel was carried out in air for 24 h at 450 °C. Any residual surfactant was removed by flowing a 5% ozone stream (Yanco Ozone Generator GE60/MF 5000) over the silica for 30 min. In this paper “calcined mesoporous silicas” refers to silicas that have been both heat treated at 450 °C and ozonized. Characterization of Mesoporous Silica and Related Surfactant Systems. Small angle neutron scattering (SANS) experiments were carried out on the LOQ instrument at ISIS at the Rutherford Appleton Laboratory, U.K. The measurements gave the absolute scattering cross section I(Q) (cm-1) as a function of the modulus of momentum transfer (Q),25 Q (Å-1) ) (4π/λ) sin(φ/2), where λ is the incident neutron wavelength (2.2-10 Å) and φ is the scattering angle (
