Langmuir 1999, 15, 2227-2229
Preparation of Blow-Molded Macroscopic Bubbles of Mesoporous Silica by a Supramolecular Templating Approach Makoto Ogawa*,†,‡ and Naoki Yamamoto‡ PRESTO, Japan Science and Technology Corporation, and Department of Earth Sciences, Waseda University, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan Received July 28, 1998. In Final Form: December 14, 1998
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and hollow spheres of mesoporous silica have successfully been prepared by means of the solvent evaporation method.26 They have potential applicability as chromatographic stationary phases and optical waveguides. Since the reaction conditions are very simple, the solvent evaporation method is a promising synthetic way to prepare silica-surfactant mesostructured materials in a controlled morphology. In this paper, we report the preparation of hollow spheres of centimeter size of the silica-surfactant mesostructured materials as a novel macroscopically structured material.
Introduction
Experimental Section
The preparation of inorganic-organic mesostructured materials by using supramolecular assemblies of surfactants to template the reactions of inorganic species has attracted increasing interest as a biomimetic approach to the fabrication of an inorganic/surfactant biphase array.1 The preparation of silica-based mesoporous materials by using surfactant aggregates as structure-directing agents is a successful example of this synthetic strategy.2-6 Although these solids have potential applicability as adsorbents,7 catalysts,8 and hosts for inclusion compounds,9 the mechanism for the formation and the detailed structure of the resulting composites are still controversial and the applications are still hindered. For the applications of mesoporous silica, it is very important to control the structures not only at the nanometer scale of the pore size but also at the macroscopic scale.10 The morphology in the wide size range is also worth investigating from basic scientific viewpoints, since the controlled morphology can be related to biominerals. Accordingly, the preparation of thin films, hollow and hard spheres, and fibers of the silica-surfactant mesostructured materials has been reported so far. The first successful example of controlling the macroscopic morphology of the silica-surfactant mesostructured materials was the rapid solvent evaporation method for the preparation of transparent thin films coated on solid substrates.11-18 The resulting films are possible candidates for such application as separation and optoelectronics due to their transparency and homogeniety. Recently, fibers
The silica-surfactant mesostructured materials were prepared as follows: tetramethoxysilane (abbreviated as TMOS) was partially hydrolyzed by a substoichiometric amount of deionized and distilled water (the molar ratio of TMOS/H2O was 1:2) under acidic conditions for 1 h at room temperature. Initially the mixture was an emulsion, but it became homogeneous as the hydrolysis proceeded. Then hexadecyltrimethylammonium chloride (abbreviated as CTAC) was added and the mixture was allowed to react at room temperature for 5 min. A glass pipet with 2 and 3 mm as the internal and external diameters, respectively, was dipped into the precursor solution; then it was blown like a soap bubble. Thus, transparent bubbles with diameters on the centimeter scale were obtained. The resulting bubble was gently place on a glass plate and calcined in air at 600 °C for 1 h to remove surfactant to obtain a porous silica bubble.
* Corresponding author. Address: Institute of Earth Science, Waseda University, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 16950, Japan. † PRESTO, Japan Science and Technology Corporation. ‡ Department of Earth Sciences, Waseda University. (1) Fendler, J. H. Membrane-Mimetic Approach to Advanced Materials; Springer-Verlag: Berlin, 1994. (2) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988. (3) Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc., Chem. Commun. 1993, 680. (4) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (5) Monnier, A.; Schu¨th, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299. (6) Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Nature 1994, 368, 321. (7) Mercier, L.; Pinnavaia, T. J. Adv. Mater. 1997, 9, 500. (8) Maschmeyer, T.; Rey, F.; Sankar, G.; Thomas, J. M. Nature 1995, 378, 159. (9) Wu, C.-G.; Bein, T. Science 1994, 264, 1757. (10) Mann, S.; Ozin, G. Nature 1996, 382, 313. (11) Ogawa, M. J. Am. Chem. Soc. 1994, 116, 7941. (12) Ogawa, M. Langmuir 1995, 11, 4639. Ogawa, M.; Igarashi, T.; Kuroda, K. Chem. Mater. 1998, 10, 1382. (13) Ogawa, M. Chem. Commun. 1996, 1149. Ogawa, M.; Ishikawa, H.; Kikuchi, T. J. Mater. Chem. 1998, 8, 1783.
Results and Discussion The photograph of the product prepared at the TMOS/ CTAC ratio of 4 is shown in Figure 1a as a typical example. Scanning electron microscopy revealed that the sphere is continuous. When the relative TMOS/CTAC ratios varied from 3 to 15, bubbles with similar sizes were obtained. It is possible to prepare many bubbles during one batch experiment. The hollow spheres of the silica-surfactant mesostructured materials have been synthesized previously by utilizing emulsion chemistry.27,28 The solvent evaporation method by spray drying also yielded hollow spheres.26 (14) Ogawa, M. Langmuir 1997, 13, 1853. (15) Ogawa, M.; Igarashi, T.; Kuroda, K. Bull. Chem. Soc. Jpn. 1997, 70, 2833. (16) Lu, Y.; Gangull, R.; Drewien, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, W.; Guo, Y.; Soyez, H.; Dunn, B.; Huang, M. H.; Zink, J. I. Nature 1997, 389, 364. (17) Anderson, M. T.; Martin, J. E.; Odinek, J. G.; Newcomer, P. P.; Wilcoxon, J. P. Microporous Mater. 1997, 10, 13. (18) Martin, J. E.; Anderson, M. T.; Odinek, J. G.; Newcomer, P. P. Langmuir 1997, 13, 4133. (19) Yang, H.; Kuperman, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G. A. Nature 1996, 379, 703. (20) Hillhouse, H. W.; Okubo, T.; van Egmond, J. W.; Tsapatsis, M. Chem. Mater. 1997, 9, 1505. (21) Tolbert, S. H.; Scha¨ffer, T. E.; Feng, J.; Hansma, P. K.; Stucky, G. D. Chem. Mater. 1997, 9, 1962. (22) Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273, 892. (23) Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G. A. Nature 1996, 381, 589. (24) Huo, Q.; Feng, J.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1997, 9, 14. (25) Huo, Q.; Zhao, D.; Feng, J.; Weston, K.; Buratto, S. K.; Stucky, G. D.; Schu¨th, F. Adv. Mater. 1997, 9, 974. (26) Bruishman, P. J.; Kim, A. Y.; Liu, J.; Baskaran, S. Chem. Mater. 1997, 9, 2507. (27) Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Schu¨th, F. Science 1996, 273, 768. (28) Singh, P. S.; Kosuge, K. Chem. Lett. 1998, 101.
10.1021/la980943y CCC: $18.00 © 1999 American Chemical Society Published on Web 02/26/1999
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Notes
Figure 3. X-ray diffraction patterns of the blow-molded macroscopic bubbles of the calcined silica-CTAC prepared at the TMOS/CTAC ratio of (a) 3, (b) 4, (c) 7, (d) 9, and (e) 15.
Figure 1. Photographs of the blow-molded macroscopic bubbles of the silica-CTAC mesostructured material prepared at the TMOS/CTAC ratio of 4 (a) before and (b) after the calcination in air at 600 °C.
Figure 4. Variation of the d values of the silica-CTAC mesostructured materials as a function of the TMOS/CTAC ratio before (open circles) and after (closed circles) the calcination.
Figure 2. X-ray diffraction patterns of the blow-molded macroscopic bubbles of the silica-CTAC hollow spheres prepared at the TMOS/CTAC ratio of (a) 3, (b) 4, (c) 7, (d) 9, and (e) 15.
However, the size of the spheres has been in the range of micrometers. The large hollow spheres with diameters of centimeters obtained in the present study are novel macroscopically structured materials. The powder X-ray diffraction patterns of the products prepared at the TMOS/CTAC ratios 3, 4, 5, 7, and 15 are shown in Figure 2. In the X-ray diffraction patterns, a single diffraction peak appeared in the low 2θ region, showing the formation of periodic mesostructures in the products. As observed for the spin-coated films of the silica-CTAC mesostructured materials,15 the diffraction peak became broad with the increase in the TMOS/CTAC ratios. When the precursor solution was allowed to react until gelation, the resulting product was amorphous.11 Accordingly, the mesostructures of the bubbles were thought to form by solvent evaporation, as reported for the preparation of the thin films and fibers.11-16,26
The samples have been calcined in air at 600 °C for 1 h to remove surfactant. The macroscopic morphology was retained during the calcination, as shown by the photograph (Figure 1b) of the calcined hollow sphere of the silica-CTAC mesostructured material (TMOS/CTAC ) 4). However, the mechanical strengths of both as-prepared and calcined bubbles are not satisfactory and should be improved before their utilization. The removal of surfactant was confirmed by the infrared spectra. The infrared absorption bands ascribable to CTAC, such as the CH stretching vibration at around 2900 cm-1, disappeared due to calcination. The X-ray diffraction patterns of the calcined samples are shown in Figure 3. Although the diffraction peaks shifted to a higher 2θ region due to calcination, the single diffraction peaks remained even after the removal of surfactant. Thus, the calcined hollow spheres possess a periodic mesostructure even after the removal of surfactant. The change in the d values after the calcination is shown in Figure 4. As reported previously for the preparation of the thin films of silica-surfactant mesostructured materials, the d values decreased upon calcination.13 The decrease in the d values has also been observed for
Notes
Figure 5. Transmission electron micrograph of the calcined silica-CTAC mesostructured material prepared at the TMOS/ CTAC ratio of 4. Scale bar ) 50 nm.
the silica-surfactant mesostructured materials synthesized under acidic conditions.29 The condensation of the silica walls during the heating may be the cause of the shrinkage. A transmission electron micrograph of the calcined product (prepared at the TMOS/CTAC ratio of 4) is shown in Figure 5. A honeycomb-like pore arrangement was seen in the TEM image of the calcined product. From the nitrogen adsorption/desorption isotherms, the BET surface area of the calcined product was determined to be
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approximately 700 m2/g, which is comparable to those typical for MCM-41 and FSM-16. The average pore diameter was determined to be approximately 1.9 nm. The pore size observed for the TEM image (1.9 nm) was consistent with that determined from the nitrogen adsorption/desorption isotherms. Thus, it was shown that the calcined silica bubbles possess a periodic porous microstructure. The preparation of self-standing films of mesoporous silica has been reported for possible applications such as separation membranes and spectroscopic investigation.21,23,24 The hollow sphere of porous silica is regarded as a self-standing film for such applications. As shown in the X-ray diffraction patterns, the products prepared at higher TMOS/CTAC ratio gave less ordered microstructures. TEM images indicated that the products prepared at the TMOS/CTAC ratios of 9 and 15 possess disordered microstructures. Thus, the TMOS/CTAC ratios of the precursor solutions must be lower for the preparation of porous silica with periodic porous structures. In summary, blow-molded bubbles with centimeter size range of the silica-alkyltrimethylammonium chloride mesostructured materials have been successfully synthesized by blowing the precursor solution containing tetramethoxysilane and alkyltrimethylammonium salts under acidic conditions. The blow-molded bubbles have been converted to porous silica while retaining the macroscopic morphology. LA980943Y (29) Huo, Q.; Margolese, D.; Feng, P.; Gier, T.; Siegr, P.; Leon, R.; Petroff, P.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176.
