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Pore Size Engineering in Mesoporous Silicas Using Supercritical CO2 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 chromatography6 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 semiconductor12 and metallic13 nanowires and carbon * To whom correspondence should be addressed. Tel.: +353 (0)21 4903608. Fax: +353 (0)21 4274097. E-mail:
[email protected]. † University College Cork. ‡ University of East Anglia. § Rutherford Appleton Laboratory. | Institut Laue-Langevin. (1) Beck, J. S.; Vartulli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.; Olson, D. H.; Sheppard, E.; McMullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (2) Kresge, C. T.; Leonwicz, M. E.; Roth, W. J.; Vartulli, J. C.; Beck, J. S. Nature 1992, 359, 710. (3) Estermann, M.; McClusker, L. B.; Baerlocher, C.; Merroche, A.; Kessler, H. Nature 1991, 353, 320. (4) Linssen, T.; Cassiers, K.; Cool, P.; Vansant, E. F. Adv. Colloid Interface Sci. 2003, 103, 121. (5) Selvam, P.; Bhatia, S. K.; Sonwane, C. G. Ind. Eng. Chem. Res. 2001, 40, 3237. (6) Gallis, K. W.; Araujo, J. T.; Duff, K. J.; Moore, J. G.; Landry, C. C. Adv. Mater. 1999, 11, 1452. (7) Coleman, N. R. B.; Ryan, K. M.; Spalding, T. R.; Holmes, J. D.; Morris, M. A. Chem. Phys. Lett 2001, 343, 1. (8) Coleman, N. R. B.; Morris, M. A.; Spalding, T. R.; Holmes, J. D. J. Am. Chem. Soc. 2001, 123, 187. (9) Coleman, N. R. B.; O’ Sullivan, N.; Ryan, K. M.; Crowley, T. A.; Morris, M. A.; Spalding, T. R.; Steytler, D. C.; Holmes, J. D. J. Am. Chem. Soc. 2001, 123, 7010. (10) Crowley, T. A.; Ziegler, K. J.; Lyons, D. M.; Erts, D.; Olin, H.; Morris, M. A.; Holmes, J. D. Chem. Mater. 2003, 15, 3518. (11) O’Neil, A. S.; Mokaya, R.; Poliakoff, M. J. Am. Chem. Soc. 2002, 124, 10636.
nanotubes14 allowing the potential creation of multilayered microelectronic device architectures. Amphiphilic block copolymers have emerged as cheap and valuable supramolecular templates for mesostructured 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 surfactants16 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. 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.22 (12) Ryan, K. M.; Erts, D.; Olin, H.; Morris, M. A.; Holmes, J. D. J. Am. Chem. Soc. 2003, 125, 6284. (13) Fukuoka, A.; Araki, H.; Sakamoto, Y.; Sugimoto, N.; Tsukada, H.; Kumai, Y.; Akimoto, Y.; Ichikawa, M. Nano Lett. 2002, 2, 793. (14) Wu, C. G.; Bein, T. Science 1994, 266, 1013. (15) Flodstrom, K.; Alfredsson, Microporous Mesoporous Mater. 2003, 59, 167. (16) Attard, G. S.; Glyde, J. C.; Goltner, C. G. Nature 1995, 378, 366. (17) Huo, Q.; Margolese, D. L.; 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. (18) Huo, Q.; Margolese, D. L.; Stucky, G. D. Chem. Mater. 1996, 8, 1147. (19) Leon, R.; Margolese, D.; Stucky, G. D.; Petroff, P. M. Phys. Rev. B 1995, 52, 4. (20) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (21) Zhao, D.; Sun, J.; Li, Q.; Stucky, G. D. Chem. Mater. 2000, 12, 275.
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Many methods have been reported for controlling the periodic unit size and pore size of mesoporous materials.23-32 The most commonly used technique employed is the introduction of a swelling agent into the structure-directing template. Commonly used swelling agents include large organic hydrocarbons such as dodecane,23 1,3,5-trimethylbenzene,33 triisopropylbenzene,34 tertiary amines,25 and poly(propylene glycol).35 The introduction of these agents can lead to a pore expansion of up to 30%; however, loss of long range order of the mesoporous structure is commonly observed.35 Smarsly et al.36 and Ryan et al.37 recently reported a method of mixing surfactant blends to tailor the pore size of mesoporous silicas, demonstrating angstrom-level control over the mean mesopore diameter. However, the longest block copolymer surfactant chain used in this method governs the largest pore size obtainable. Several researchers38,39 have exploited supercritical carbon dioxide (sc-CO2) in the postsynthesis treatment of mesoporous silicas to extract the surfactant template.39,40 Recently, we reported the controlled expansion of pores within mesoporous silicas using sc-CO2.41 The use of scCO2 as a swelling agent in mesoporous materials was founded on the large amount of literature reported in the polymer sciences where CO2 has found considerable application as an ideal solvent for the impreganation and swelling of polymers.42-50 In particular, Cooper et al.51-53 (22) Meziani, A.; Tourand, D.; Zrabda, A.; Pulvin, S.; Pezron, I.; Clausse, M.; Kunz, W. J. Phys. Chem. B 1997, 101, 3620. (23) Blin, J. L.; Otjacques, C.; Herrier, G.; Bao-Lian, S. Langmuir 2000, 16, 4229. (24) Blin, J. L.; Su, B. L. Langmuir 2002, 18, 5303. (25) Kruk, M.; Jaroniec, M.; Sayari, A. Microporous Mesoporous Mater. 2000, 35, 545. (26) Prouzet, E.; Pinnavaia, T. J. Angew. Chem. 1997, 36, 516. (27) Ulagappan, N.; Rao, C. N. R. J. Chem. Soc., Chem. Commun. 1996, 24, 247. (28) Boissiere, C.; Martines, M. A. U.; Tokumoto, M.; Larbot, A.; Prouzet, E. Chem. Mater. 2003, 15, 509. (29) Yam, V. M.; Li, B.; Zhu, N. Adv. Mater. 2002, 14, 719. (30) Yamanda, T.; Zhou, H.; Asai, K.; Honma, I. Mater. Lett. 2002, 56, 93. (31) Sayari, A. Angew. Chem., Int. Ed. 2000, 39, 2920. (32) Wu, C. N.; Tsai, T. S.; Liao, C. N.; Chao, K. J. Microporous Mater. 1996, 7, 173. (33) Branton, P. J.; Dougherty, J.; White, J. W.; Lockhart, G. Charact. Porous Solids, IV 1997, 668, 60. (34) Kimura, T.; Sugahara, Y.; Kuroda, K. J. J. Chem. Soc., Chem. Commun. 1998, 55, 559. (35) Park, B. G.; Guo, W.; Cui, X.; Park, J.; Ha, C. S. Microporous Mesoporous Mater. 2003, 66, 229. (36) Smarsly, B.; Polarz, S.; Antonietti, M. J. Phys. Chem. B 2001, 105, 10473. (37) Ryan, K. M.; Coleman, N. B.; Lyons, D. M.; Hanrahan, J. P.; Spalding, T. R.; Morris, M. A.; Steytler, D. C.; Heenan, R. K.; Holmes, J. D. Langmuir 2002, 18, 4996. (38) Kawi, S.; Lai, M. W. J. Chem. Soc., Chem. Commun. 1998, 13, 1407. (39) Van Grieken, R.; Stucky, G. D.; Melero, J. A.; Iglesias, J. Langmuir 2002, 6, 10. (40) Lu, X. B.; Zhang, W. H.; He, R.; Li, X. Ind. Eng. Chem. Res. 2003, 42, 653. (41) Hanrahan, J. P.; Copley, M. P.; Ryan, K. M.; Morris, M. A.; Spalding, T. R.; Holmes, J. D. Chem. Mater. 2004, 16, 424. (42) Kazarian, S. G.; Brantly, N. H.; Eckert, C. A. CHEMTECH 1999, 29, 36. (43) Nikitin, L. N.; Gallyamov, M. O.; Vinokur, R. A.; Nicoleac, A. Y.; Said-Gailiyev, E. E.; Khokhlov, A. R.; Jespersen, H. T.; Schaumburg, K. J. Supercrit. Fluids 2003, 26, 263. (44) Royer, J. R.; DeSimone, J. M.; Khan, S. A. Macromolecules 1999, 32, 8965. (45) Sirard, S. M.; Green, P. F.; Johnston, K. P. J. Phys. Chem. B 2001, 105, 766. (46) Wissinger, R. G.; Paulitis, M. E. J. Polym. Sci., Part B: Polym. Phys. 1987, 25, 2497. (47) Zhang, Y.; Gangwani, K. K.; Lemert, R. M. J. Supercrit. Fluids 1997, 11, 115. (48) Kazarian, S. G.; Chan, K. L. Macromolecules 2004, 37, 579. (49) Dong, Z.; Liu, Z.; Han, B.; He, J.; Jiang, T.; Yang, G. J. Mater. Chem. 2002, 12, 3565.
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have shown that sc-CO2 can act as a porogenic solvent in the production of porous poly(methacrylate) monoliths, and Johnston and co-workers45,54 have used CO2 to swell thin films of poly(dimethylsiloxane) on silicon substrates. Indeed, Lee et al.55 used CO2 to swell the interior of CO2in-water microemulsions formed by the fluorinated surfactant potassium carboxylate perfluoropolyether. In this paper we report the mechanism by which scCO2 can be used to control the pore diameters, wall widths, and d spacing in mesoporous silicas while retaining longrange hexagonal ordering. Small angle neutron scattering (SANS) was employed to probe the micellar structures and rod-shaped surfactant complexes of the triblock copolymers in a biphasic water-CO2 system. Experimental Section Preparation and Swelling of Mesoporous Silicas. Hexagonal mesoporous silicas were prepared using a method based on one described by Attard and Goltner et al.,16,56 that is, the hydrolysis of tetramethoxysilane (TMOS) in the presence of PEO-PPO triblock copolymer surfactants P123 (PEO20PPO69PEO20), P85 (PEO26PPO39PEO26), and F127 (PEO106PPO70PEO106) supplied by BASF, U.K. In a typical synthesis P123 (5.0 g) was dissolved in TMOS (9.0 g, 0.0559 mol) to which an aqueous solution of HCl (5.0 g, 0.5 M) was added 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 then divided into two batches. One batch was loaded into a 50-mL stainless steel cell, heated to 40 °C, and pressurized with CO2 to the desired pressure using a 260-mL ISCO syringe pump (Lincoln, NE). The pressure range employed in these experiments varied between 1 and 689 bar. The system was left to stand for 6 days (hereafter termed treated). The second sample was aged at 40 °C for 6 days in air (hereafter termed untreated). Both batches were then subjected to identical thermal treatments, to ensure complete removal of the surfactant template (24 h at 450 °C). Characterization. Powder X-ray diffraction (PXRD) profiles of the mesoporous silicas produced were recorded on a Philips X’Pert diffractometer, equipped with a Cu KR radiation source and accelerator detector. Height and reflected stoller slits of 0.2° θ were used with a programmable divergent slit to maintain a 10-mm footprint at the sample. Sample heights were determined at θ ) 2θ ) 0 at the point when the sample reduced the beam intensity by 50%. The surface areas of the calcined mesoporous silicas synthesized were measured using nitrogen BET isotherms57,58 at 77 K on a Micromeritics Gemini 2375 volumetric analyzer. Each sample was degassed for 12 h at 200 °C prior to a BET measurement. The average pore size distribution of the calcined silicas was calculated using the Barrett-JoynerHalanda (BJH) model58 from a 60-point BET surface area plot. All the mesoporous silicas examined exhibited a type IV adsorption-desorption isotherm typical of mesoporous solids.21,59 In all cases, little hysteresis was observed in the adsorption and desorption isotherms. Desorption isotherms were used to calculate mean pore diameters and distributions. A JEOL 2000 transmission electron microscope (0.5-nm resolution) operating (50) Pai, R. A.; Humayun, R.; Schulberg, M. T.; Sengupta, A.; Sun, J. N.; Watkins, J. J. Science 2004, 303, 507. (51) Cooper, A. I. Green Chem. 1999, 25, 168. (52) Cooper, A. I. J. Mater. Chem. 2000, 10, 207. (53) Cooper, A. I. Adv. Mater. 2003, 13, 1049. (54) Sirard, S. M.; Ziegler, K. J.; Sanchez, I. C.; Green, P. F.; Johnston, K. P. Macromolecules 2002, 35, 1928. (55) Lee, C. T.; Ryoo, W.; Smith, P. G.; Arellano, J.; Mitchell, D. R.; Lagow, R. J.; Webber, S. E.; Johnston, K. P. J. Am. Chem. Soc. 2003, 125, 3181. (56) Goltner, C.; Berton, B.; Kramer, E.; Antonietti, M. Adv. Mater. 1999, 11, 395. (57) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (58) Barrett, E. P.; Joyner, L. G.; Halanda, P. P. J. Am. Chem. Soc. 1951, 73, 373. (59) Donohue, M. D.; Aranovich, G. L. J. Colloid Interface Sci. 1998, 205, 121.
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with a 200-kV accelerating voltage was used to image the mesoporous samples. Samples were prepared for transmission electron microscopy (TEM) by dispersing a small quantity of the mesoporous silica in chloroform and placing a drop of the mixture on a carbon-coated copper grid. Solubility of CO2 in PPO/PEO. To determine the solubility (S) of CO2 within the micelle core with respect to pressure (P) several models were theorized. The lattice models such as the Sanchez-Lacombe46 (S-L) equation of state and Flory-Huggins60 model have been used successfully to describe the thermodynamics of bulk polymer-supercritical mixtures. However, Sato et al. recently showed that Henry’s law could be used to calculate the solubility of CO2 in PPO with high accuracy.61,62 Equation 1 shows the relationship between the solubility of CO2 in PPO with respect to pressure.
S ) KaPNa
(1)
The constants Ka and Na were determined from experimental methods outlined in the following publications.61,63 The solubility of CO2 in the hydrophilic PEO phase of the surfactant was taken to be equal to the solubility of CO2 in water and was calculated on the basis of data previously reported.64,65 SANS Characterization of Mesoporous Silica. SANS experiments were carried out on the LOQ instrument at ISIS at the Rutherford Appleton Laboratory (RAL), U.K., and on D11 at the Institut Laue-Langevin (ILL), Grenoble, France. The absolute scattering cross-section I(Q) (cm-1) was measured as a function of the modulus of momentum transfer Q(A-1) ) (4π/λ) sin(θ/2) where λ is the incident neutron wavelength (2.2-10 Å) and θ is the scattering angle (
