Langmuir 1997, 13, 1853-1855
1853
Preparation of Layered Silica-Dialkyldimethylammonium Bromide Nanocomposites Makoto Ogawa PRESTO, Japan Science and Technology Corporation, and Institute of Earth Science, Waseda University, Nishi-waseda 1-6-1, Shinjuku-ku, Tokyo 169-50, Japan Received September 9, 1996. In Final Form: December 31, 1996
Introduction Self-organization of molecules into highly ordered architecture has attracted increasing attention from a wide range of scientific interests.1 Recently, surfactant mesophases have been used as structure-directing agents for constructing inorganic-organic mesostructured materials including mesoporous silicates.2-16 Much effort has been made to elucidate the mechanism for the formation of mesostructured inorganic-organic nanocomposites. Besides the scientific interest for the formation of the mesostructured materials, these solids may have application as adsorbents, a host for inclusion compounds, catalysis support, and electronic and optical materials. Along these lines, preparations of mesostructured materials with different compositions7-10 and morphology11-17 have been reported so far. I have reported the synthesis of periodic silicaalkyltrimethylammonium bromide nanocomposites by polymerization of tetramethoxysilane in the presence of alkyltrimethylammonium salts.11-13 Since the composites are obtained as transparent films, the synthetic method can be applied to prepare a new class of material. In order to apply the synthetic method, the preparation of inorganic-surfactant mesostructured composites is being investigated. In this paper, the preparation of layered silica-dialkyldimethylammonium bromide nanocomposites is reported as a variation of the silica-surfactant mesostructured composite films. (1) Fendler, J. H. Membrane-Mimetic Approach to Advanced Materials; Springer-Verlag: Berlin, 1994. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710-712. Beck, J. S.; Vartuli, J. C.; Roth, W. L.; Leonowicz, M. E.; Kresge, C. T.; Schmidt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834-10843. (3) 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, 1299-1303. (4) Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. D. Nature 1994, 368, 317-321. Huo, Q.; et al. Chem. Mater. 1994, 6, 1176-1191. (5) Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Nature 1994, 368, 321-323. Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865-867. (6) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988-992. Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc. Chem. Commun. 1993, 680-682. (7) Antonelli, D. M.; Ying, J. Y. Angew. Chem., Int. Ed. Engl. 1995, 34, 2014. Ibid. 1996, 35, 426. (8) Antonelli, D. M.; Nakahira, A.; Ying, J. Y. Inorg.Chem. 1996, 35, 3126-3136. (9) Abe, T.; Taguchi, A.; Iwamoto, M. Chem. Mater. 1995, 7, 1429. (10) Braun, P. V.; Osenar, P.; Stupp, S. I. Nature 1996, 380, 325. (11) Ogawa, M. J. Am. Chem. Soc. 1994, 116, 7941-7942. (12) Ogawa, M. Langmuir 1995, 11, 4639-4641. (13) Ogawa, M. Chem. Commun. 1996, 1149-1150. (14) Ayral, A.; Balzer, C.; Dabadie, T.; Guizard, C.; Julbe, A. Catal Today 1995, 25, 219-224. (15) Yang, H.; Kuperman, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G. A. Nature 1996, 379, 703. (16) Oliver, S.; Kuperman, A.; Coombs, N.; Lough, A.; Ozin, G. A. Nature 1995, 378, 47. (17) Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G. A. Nature 1996, 381, 589.
S0743-7463(96)00877-3 CCC: $14.00
Figure 1. X-ray diffraction pattern of the silica-2C12 composite film (at the TMOS/2C12 ratio of 10:1).
Experimental Section Materials. Tetramethoxysilane (abbreviated as TMOS) and dialkyldimethylammonium bromide ((CnH2n+1)2(CH3)2N+, abbreviated as 2Cn, where n denote the carbon number in the alkyl chain) were obtained from Tokyo Kasei Industries and used without further purification. Sample Preparation. The thin films of silica-surfactant nanocomposites were prepared by the method described in the previous papers11,13 on the preparation of the silica-alkyltrimethylammonium salt nanocomposite films with slight modification. TMOS was partially hydrolyzed by a substoichiometric amount of deionized and distilled water (the molar ratio of TMOS/ H2O was 1:2) under an acidic condition. Then dialkyldimethylammonium bromide was directly dissolved into the prehydrolyzed TMOS and the mixture was allowed to react at room temperature with stirring. The resulting homogeneous solution was spin coated on a Pyrex glass substrate. Thus, transparent thin films formed on the substrate. Gel or glasslike sample was prepared by casting a precursor solution to a Petri dish and air-drying. The products were characterized by X-ray diffraction (XRD), solidstate nuclear magnetic resonance spectra, and elemental analysis.
Results and Discussion By mixture of the prehydrolyzed TMOS and 2C10 powder, a clear viscous solution was obtained. Figure 1a shows the X-ray diffraction pattern of the silica-2C10 composite film (the molar ratio of TMOS/2C10 is 10:1) with the thickness of ca. a few micrometers. A very sharp diffraction peak with the d value of ca. 2.9 nm, which accompanied second-order reflection, was observed in the XRD pattern. Transparent thin films were obtained for the composites with 2C10, 2C12, 2C14, and 2C16. On the other hand, the preparation of silica-2C18 composite was difficult under the experimental condition employed in this study. The mixture of the prehydrolyzed TMOS and 2C18 was inhomogeneous due to the lower solubility of 2C18 to the prehydrolyzed TMOS. Elevated temperature or addition of solvent seems to be necessary for the preparation of silica-2C18 composite. As has previously reported for the synthesis of the silica-alkyltrimethylammonium bromide composites,11 surfactant with short alkyl chain © 1997 American Chemical Society
1854 Langmuir, Vol. 13, No. 6, 1997
Notes
Figure 2. Variation of the d values of the layered silica-2Cn nanocomposites as a function of alkyl chain length: O, silica2Cn; 4, silica-alkyltrimethylammonium bromide composite.9 Figure 4. Solid state 29Si magic angle spinning NMR spectrum of the silica-2C12 composite powder.
Figure 3. X-ray diffraction pattern of the silica-2C12 composite powder (prepared at the TMOS/2C12 ratio of 5:1).
(in the present case, 2C8) does not give a highly ordered mesostructure resulting in the broad X-ray diffraction peak of the product. The relations between the observed d values and the carbon number of the alkyl chain are shown in Figure 2 together with those for the previously reported layered silica-alkyltrimethylammonium bromide nanocomposites.11 The d values changed linearly as a function of the alkyl chain length of the surfactants, suggesting that the composites are composed of surfactant aggregate and thin silica layer in a similar manner proposed for the layered silica-alkyltrimethylammonium bromide nanocomposites. For the following characterization, powder sample was prepared by drying the precursor solution on a Petri dish. Gel or glasslike product formed after the drying, and the product was crushed. The X-ray diffraction pattern of the powder prepared from the TMOS and 2C12 is shown in Figure 3. The d value of the powder sample was determined to be 3.1 nm, which was almost same as that of the corresponding silica-2C12 nanocomposite film. This observation indicate that a mesostructure similar to that of the spin-coated film formed for the powder sample. As mentioned in the previous communication, the diffraction peak observed for the silica-2C12 powder was broad compared with that for the corresponding silica-2C12 nanocomposite film. The solid-state 29Si magic angle spinning NMR spectrum of the silica-2C12 nanocomposite powder is shown in Figure 4. The NMR spectrum showed two broad signals due to Q3 and Q4 (at around -100 and -120 ppm relative to tetramethylsilane) environments of silicon. These results indicate that the tetramethoxysilanes were polymerized to form a siloxane network in the product. Additionally, the broadness of the signals suggests the noncrystalline nature of the silica network. These ob-
Figure 5. A proposed schematic structure of the layered silica2Cn nanocomposite.
servations have been observed for the reported silicasurfactant mesostructured materials.2 In a separate paper on the preparation of the silica-alkyltrimethylammonium salts nanocomposites, the thickness of the thin silica layer can be controlled by simply changing the relative ratios of inorganic to organic components.18 The Q3/Q4 ratios of the 29Si NMR spectra of the silica-alkyltrimethylammonium salt system may be important information on the silica structure in the silica-surfactant mesostructured materials. The composition of the silica-2C12 composite (prepared at the 5:1 molar ratio of TMOS/2C12) was determined to be as follows: C, 38.5; N, 1.7; Br, 10.1; SiO2, 38.2. The Si:2C12 ratio (5.2:1.0) was consistent with the ratio in the starting mixture (5.0:1.0), indicating that the starting mixture was quantitatively converted to the product. The composition did not change significantly upon heating the sample in air at 80 °C for 1 day. (18) Ogawa, M.; et al. Submitted for publication.
Notes
Langmuir, Vol. 13, No. 6, 1997 1855
From these observations, a schematic structure is proposed for the present composites. (The proposed schematic structure is shown in Figure 5.) Although the location of the bromide anions and the configuration of the alkyl chains are unclear at present, the resulting composites are thought to be composed of lamellar surfactant aggregate and thin silica layer. As mentioned in the previous communication on the preparation of layered silica-alkyltrimethylammonium bromide nanocomposites, rapid evaporation of solvent before gelation is essential for the formation of the highly ordered layered composites in the present system. It is thought that 2Cn forms lamellar aggregate and the hydrophilic silica oligomers interact with the hydrophilic head groups of the surfactants during the evaporation of solvents. Being surrounded by the silica layer, the lamellar aggregates solidified upon evaporation of solvents to form the layered structure without crystallization of 2Cn. The polymerization of alkoxysilane in the presence of dialkyldimethylammonium salts19,20 and the adsorption and the polymerization of alkoxysilane into the synthetic bilayer membranes21,22 have previously been reported. In the present system, dialkyldimethylammonium salts were
complexed with alkoxysilane-derived silica to form novel mesostructured composites by the simple reaction. The author has recently investigated the silica-alkyltrimethylammonium salt nanocomposites. The mesostructures of the products have been varied by changing the reaction conditions such as the relative ratios of TMOS to alkyltrimethylammonium salts and the amounts of water in the reaction mixtures.13,18 The variation of the mesostructure included the change between lamellar and hexagonally packed cylindrical surfactant aggregates. Since 2Cn tends to form lamellar aggregates, layered mesostructures predominantly form in the present system.
(19) Dubois, M.; Guik-Krzywicki, Th.; Cabane, B. Langmuir 1993, 9, 673-680. (20) Dubois, M.; Cabane, B. Langmuir 1994, 10, 1615-1617.
(21) Sakata, K.; Kunitake, T. J. Chem. Soc., Chem. Commun. 1990, 504-506. (22) Sakata, K.; Kunitake, T. Chem. Lett. 1989, 2159-2162.
Conclusion Layered silica-dialkyldimethylammonium bromide nanocomposites have been prepared as films and powders by the reaction between tetramethoxysilane and dialkyldimethylammonium bromides under an acidic condition at room temperature. These composites are thought to be composed of lamellar aggregated dialkyldimethylammonium bromides surrounded by a thin silica layer. LA9608775
