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Synthesis of Mesoporous Silica Helical Fibers Using a Catanionic-Neutral Ternary Surfactant in a Highly Dilute Silica Solution: Biomimetic Silicification Giung-Ling Lin,† Yi-Hua Tsai,† Hong-Ping Lin,*,† Chih-Yuan Tang,‡ and Ching-Yen Lin‡ Department of Chemistry, National Cheng Kung UniVersity, Tainan, Taiwan 701, and Department of Zoology, National Taiwan UniVersity, Tainan, Taiwan 106 ReceiVed January 18, 2007 Mesoporous silica helical fibers in many different shapes have been synthesized in a highly dilute silicate solution at pH ∼2.0 by using CnTMAB-SDS-P123 (n ) 14-18) ternary surfactant as a template. The mesoporous silica helical fibers possess a well-ordered hexagonal mesostructure, high surface area, and large pore volume. Thus, the microtome sections of the helical fibers demonstrate a concentric mesotructure or two hemiconcentric mesostructures. In addition to triblock copolymer, adding the proper amount of 1-butanol or pentanol can promote the yield of the helical fibers as well. The yield of the surfactant-templated helical fibers is also dependent on the water content, reaction temperature, and pH value of the solution. The mesoporous silica helical fiber can be used as a solid template to prepare mesoporous carbon helical fibers via impregnation of phenol-formaldehyde, pyrolysis, and silica removal.
In nature, numerous helical architectures result from the selfassembly of small building blocks into larger macromolecular structures.1,2 Recently, much effort has been devoted to the synthesis of inorganic materials with helical morphologies in order to explore and apply its power of design and control for the selective preparation of complicated helical chemical architectures.3-7 However, it has been difficult to devise a highyield, controlled synthesis of helical mesoporous materials. In general, the self-assembly of surfactant and inorganic species is a thermodynamically controlled process in which a variety of morphologies can be formed by varying the self-assembly conditions.8-10 As with organic polymers and surfactant micelles, helical organization in the surfactant or polymer-templated mesostructured materials could be readily controlled by adjusting the composition and temperature.11-18 To realize driving forces for the formation of helical morphology, a consideration of shape* Corresponding author. E-mail:
[email protected]. Tel: 886-62757575 ext. 65342. † National Cheng Kung University. ‡ National Taiwan University. (1) Gross, M. TraVels to the Nanoworld: Miniature in Nature and Technology; Plenum: New York, 1999. (2) Sarikaya, M.; Tamerler, C.; K.-Y. Jen, A.; Schulten, K.; Baneyx, F. Nat. Mater. 2003, 2, 577. (3) Zhang, S. Nat. Biotechnol. 2003, 21, 1171. (4) Rowan, A. E.; Nolte, R. J. M. Angew. Chem., Int. Ed. 1998, 37, 63. (5) Seddon, A. M.; Patel, H. M.; Burkett, S. L.; Mann, S. Angew. Chem. 2002, 114, 3114. (6) (a) Yang, Y.; Suzuki, M.; Fukui, H.; Shirai, H.; Hanabusa, K. Chem. Mater. 2006, 18, 1324. (b) Yang, Y.; Suzuki, M.; Owa, S.; Shirai, H.; Hanabusa, K. Chem. Commun. 2005, 4462. (c) Kobayashi, S.; Hamasaki, N.; Suzuki, M.; Kimura, M.; Shirai, H.; Hanabusa, K. J. Am. Chem. Soc. 2002, 124, 6550. (7) (a) Jung, J. H.; Shinkai, S.; Shimizu, T. Chem. Mater. 2003, 15, 2141. (b) Jung, J. H.; Yoshida, K.; Shimizu, T. Langmuir 2002, 18, 8724. (8) (a) Yang, S. M.; Sokolvo, I.; Coombs, N.; Kresge, C. T.; Ozin, G. A. AdV. Mater. 1999, 11, 1427. (b) Yang, H.; Coombs, N.; Ozin, G. A. Nature 1996, 381, 589. (9) (a) Lin, H. P.; Mou, C. Y. Acc. Chem. Res. 2002, 35, 927. (b) Lin, H. P.; Mou, C. Y.; Liu, S. B. AdV. Mater. 2000, 12, 103. (c) Lin, H. P.; Mou, C. Y. Science 1996, 273, 765. (10) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56. (11) (a) Che, S.; Liu, Z.; Ohsuna, T.; Sakamoto, K.; Terasaki, O.; Tatsumi, T. Nature 2004, 429, 281. (b) Ohsuan, T.; Liu, Z.; Che, S.; Terasaki, O. Small 2005, 2, 233. (12) Zhang, Q.; Li, F.; Lu, C.; Wang, Y.; Wan, H. Chem. Lett. 2006, 35, 190. (13) Wang, J.; Wang, W.; Sun, P.; Yuan, Z.; Li, B.; Jin, Q.; Ding, D.; Chen, T. J. Mater. Chem. 2006, 16, 4117. (14) Li, X.; Wu, Y.; Li, Y. Inorg. Chim. Acta 2007, 360, 241.
dependent energies in terms of the bending elasticity of the building blocks could give a more general picture.19,20 In the literature,11-17 it has been shown that the helicity of the resulting mesoporous silica fibers is strongly dependent on the stirring rate, and organic silica sources (i.e., TEOS) have been widely used as the silica source. To mimic silicification in nature, surfactant systems with properties similar to those of phospholipid and inorganic silica precursors are desired.21 It is known that aqueous mixtures of oppositely charged single-tailed cationic and anionic surfactants produce a very rich variety of aggregate microstructures (e.g., spherical and rodlike micelles) at high dilution and can serve as model biological membranes.22,23 In this letter, it is shown that mesoporous silica helical fibers can be synthesized in high yield in a highly dilute silica solution at pH ∼2.0-2.5 by using alkyltrimethylammonium-dodecyl sulfatePluronic 123 ternary surfactant as the template. Here, the catanionic (i.e., cation + anion) surfactant determines the mesostructure, and the Pluronic 123 triblock copolymer was added to reduce the elasticity of the catanionic micelle. Upon the addition of a small amount of triblock copolymer P123 or 1-alkanol, the yield of the helical mesoporous silica fibers was much improved. Because the surfactant-silica assembly is a form of self-assembly with covalent modification,24 the helicity yield is particularly dependent on factors related to reaction kinetics and micellar behavior, such as the reaction temperature, water content, and pH value of the silicas solution. (15) (a) Jin, H.; Liu, Z.; Ohsuna, T.; Terasaki, O.; Inoue, Y.; Sakamoto, K.; Nakanishi, T.; Ariga, K.; Che, S. AdV. Mater. 2006, 18, 593. (b) Wu, X.; Jin, H.; Liu, Z.; Ohsuna, T.; Terasaki, O.; Sakamoto, K.; Che, S. Chem. Mater. 2006, 18, 241. (16) Yang, S.; Zhao, L.; Yu, C.; Zhou, X.; Tang, J.; Yuan, P.; Chen, D.; Zhao, D. J. Am. Chem. Soc. 2006, 128, 10460. (17) Wang, B.; Chi, C.; Shan, W.; Zhang, Y.; Ren, N.; Yang, W.; Tang, Y. Angew. Chem., Int. Ed. 2006, 45, 2088. (18) Kim, W.-J.; Yang, S.-M. Chem. Mater. 2000, 12, 3227. (19) Fuhrhop, J. H.; Helfrich, W. Chem. ReV. 1993, 93, 1565. (20) (a) Zhou, H.; Zhang, Y.; Ou-Yang, Z.-C. Phys. ReV. E 2000, 62, 1045. (b) Ou-Yang, Z. C.; Liu, J. X. Phys. ReV. Lett. 1990, 65, 1679. (21) Mann, S. Biomineralization Principles and Concepts in Bioinorganic Materials Chemistry; Oxford University Press: New York, 2001. (22) Yin, H.; Zhou, Z.; Huang, J.; Zheng, R.; Zhang, Y. Angew. Chem., Int. Ed. 2003, 42, 2188. (23) (a) Shioi, A.; A. Hatton, T. Langmuir 2002, 18, 7341. (b) Xia, Y.; Goldmints, I.; Johnson, P. W. T.; Hatton, A.; Bose, A. Langmuir 2002, 18, 3822. (24) Steed, J. W.; Atwood, J. L. Supramolecular Chemistry; John Wiley & Sons: Chichester, England, 2000.
10.1021/la070154t CCC: $37.00 © 2007 American Chemical Society Published on Web 03/17/2007
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Figure 1. SEM images at different magnifications of various shapes of mesoporous silica helical fibers synthesized with the C18TMAB/ SDS/P123 template (composition: 1.0 g/0.135 g/0.01 g) at pH 2.5: (A) low-magnification image, (B, C) higher-magnification images, (D) single helix, (E) double helix, (F) double helix, (G) double helix, (H) double-helical loop, (I) double-helical ribbon, (J) coupled single-helix strains, (K) triplet helix (one helix winds around a double helix), and (L) multiple strains of helix fibers. Scale bars are 500 nm.
The helical mesoporous silicas were conveniently synthesized by mixing an aqueous cationic-anionic-neutral surfactant solution with an acidified sodium silicate aqueous solution. Then that solution was statically aged for 3 days at 55 °C in a closed vessel. The components of the surfactant mixture are (0.750.70)g of CnTMAB (n ) 14-18), (0.090-0.080)g of SDS, (0.014-0.007)g of P123, and 100.0 g of H2O. The acidified silica solution was prepared by adding a mixture of (2.0-2.35)g of sodium silicate and 10.0 g of H2O to (300-400)g of a (0.0960.072)M H2SO4 aqueous solution and adjusting the pH to 1.04.0. Filtration, washing with water, and drying gave the assynthesized mesoporous silica. After hydrothermal treatment at 60 °C to improve the mesostructure stability and calcination at 580 °C for 6 h in air, the surfactant-free mesoporous silica was obtained. The silica recovery yield was close to 30 wt %. Powder X-ray diffraction (XRD) patterns were obtained on a Sintag X apparatus (λ ) 0.154 nm). The N2 adsorptiondesorption isotherms were obtained at 77 K on a Micromeritics ASAP 2010 apparatus. Before analysis, the sample was outgassed at 200 °C for about 6 h at 10-3 Torr. BET analysis was used to determine the total specific surface area (SBET). The pore size distribution was obtained from an analysis of the adsorption branch using the BJH (Barrett-Joyner-Halenda) method. The scanning electron microscopy (SEM) and transmission electron micrographs (TEM) were taken on an S-800 (Hitachi) operated at an accelerating voltage of 20 keV and an H-7500 (Hitachi) operated at 100 keV, respectively. Figure 1A is a low-magnification SEM image of the mesoporous silica helical fibers obtained from slow silicification of a C18TMAB-SDS-P123 ternary-surfactant mixture in a highly dilute silica solution at pH ∼2.5. It is clear that the yield of the mesoporous silica helical fibers is high (>90%) and that there is little nonhelical mesoporous silica (e.g., gyroids or particulates) in the sample. When observing at higher magnification (Figure
1B,C), we can clearly see that the shapes of the helical mesoporous silicas are different. The length of the helical fibers ranges from a few micrometers to tens of micrometers, and the diameter is on the order of tens of nanometers. Parts D-L of Figure 1 show SEM images of the representative helical fibers. This one-pot synthesis generates single-helix fibers, double-helix fibers, looplike helical fibers, helical ribbons, triple-helix fibers, and multiple strains of helical fibers. In addition to C18TMAB, the mesoporous silica helical fibers in high yield (>80%) can be synthesized by using other alkyltrimethylammonium surfactants (CnTMAB; n ) 14, 16) in an acidified silica solution at pH ∼1.5-2.0 (Supporting Information, Figure S1). It should be mentioned that surfactant compositions that are suitable for the formation of the helical fibers differ slightly. To have more stable mesostructured silica for mesostructural and porosity characterization,9 the dried as-synthesized mesoporous silica was hydrothermally treated in water (pH ∼5.0) at 60 °C for 24 h. Figure 2 demonstrates various HR-TEM micrographs of the mesoporous silica helical fibers. As in many previous studies,11-17 the obvious pitch patterns, well-ordered mesostructure, and nanochannels direction were seen in the mesoporous silica helical fibers. It is clear that the nanochannels are well aligned along the direction of the helical fibers and that the pitch dimension is on the order of tens to a few hundred nanometers. Some images reveal that hollow helical tubes and helicoids were formed as well. Although various helical shapes appear in the ternary surfactant-templated silica, the populations of double-helix fiber and multiple helical nanochannel strains are relatively higher than those of single-helix fibers, triple-helix fibers, multiple stains of helix fibers, and hollow helical tubes. According to the aforementioned EM observations, we provided a simple synthetic method to prepare mesoporous silica helical fibers with different morphologies without using chiral templates. Distinct from the previous reports,11-18 many novel hierarchical
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Figure 2. TEM images of various shapes of mesoporous silica helical fibers after hydrothermal treatment at 60 °C for 1 day: (A) single helix, (B) coupled single helices, (C) multiple helical nanochannels strains, (D) double helix, (E) zip-zap-like fiber, (F) double helix, (G) double helix, (H) double-helical loop, (I) double-helical loop ribbon, (J) multiple strains of helix fibers, (K) triplet helix (one helix winds around a double helix), and (L) double-helix hollow helicoids. Scale bars are 100 nm.
helical structures (i.e., single, double, triple, and multiple helix) are generated in this one-pot synthesis. However, the absence of chirality in the basic building block leads in principle to an equal preference for either left- or right-handed helices.4,11 The mesostructures of the mesoporous silica helical fibers were further examined by microtome TEM and XRD techniques. The XRD patterns at low angles show the three representative peaks of the well-ordered hexagonal mesostructure (Figure 3A). In observations of the microtomed sections of the mesoporous silica helical fibers, we often found a concentric mesostructure or two hemiconcentric mesostructures (Figure 3B,C). Because the thickness of the epoxy film cut by a typical microtome technique ranges from tens to a few hundred nanometers, which is close to the pitch length of the helical fibers, the TEM images of the cross-section of the helical fibers should be a concentric or hemiconcentric mesostructure rather than a honeycomb-like mesostructure of the nonhelical silica fibers. N2 adsorptiondesorption isotherms of the calcined mesoporous silica helical fibers synthesized with the C18TMAB-SDS-P123 template are shown in Figure 3D. The sample has a type IV isotherm with an apparent H1-type hysteresis loop that indicates the channellike mesostructure. The pore size distributions of these samples analyzed by the Barrett-Joyner-Halenda methods are sharp and centered at about 4.19 nm. The BET surface area is around 700-800 m2 g-1. Although the formation mechanism of the mesoporous silica helical fibers is not yet clear,11,16,17 we propose a model based on the theory of shape-dependent energy to explain the formation of the mesoporous silica helical fibers. In the flexible DNA and organometallics supramolecular assemblies,4 the helical structures were produced by optimizing the H-bonding and coordination interactions to achieve a thermodynamically stable (i.e., energyfavorable) structure, whereas in the intertwined fibers and nanochannels (i.e., helicity) the silica-surfactant nanochannels get closer and have a larger contact area than in the straight fibers. Thus, helicity formation can lead to extra covalent SiO-Si bond formation and an enhancement of the H-bond strength of surface Si-OH groups between the contact pitches. Thus, the main driving force for the formation of helical fibers should be the increasing interaction between the surfaces of nanochannels
or submicrometer fibers. It is reasonably supposed that the nanodimension of the mesoporous silica helical fiber has a thermodynamic stable morphology, in contrast to that of the nonhelical fibers. However, twisting and bending penalty energy (i.e., activation energy) is required for the formation of helical morphology. When the elasticity of the surfactant-silica assemblies is small, the twisting and bending penalty energy for the helical structures on the nano- and submicrometer scales is relatively low. To reduce the elasticity of the catanionic surfactant, the addition of triblock copolymer is an effective method.25-27 We found that the yield of the helical mesoporous silica fibers increases with the addition of Pluronic 123 copolymer (Supporting Information, Figure S2). In addition to the triblock copolymer, short-chain alkanols, which usually act as co-surfactant to reduce the rigidity of the surfactant micelles or bilayers, can also be used to improve the helicity yield.28,29 In Figure 4, it is clearly seen that helical fibers in high yield were prepared at a proper CnOH/CnTMAB ratio as well. From the point of view of reaction kinetics, because the covalent linkage of Si-O-Si is irreversible, the formation of mesostructured surfactant-silica assemblies is regarded as a term of “self-assembly with covalent modification”.24 To allow the final product to achieve a thermodynamic minimum structure, a slow reaction rate is required.21 Therefore, a slow silica condensation rate and low concentration of silica species and surfactant can give the slow self-assembly rate necessary to achieve mesoporous silica helical morphologies. According to silica chemistry, the slowest silica condensation rate occurs at pH ∼1.5-2.5.30 When the pH value of the solution is outside this range, the silica condensation rate increases, and the yield of the helical structure would decrease. At pH ∼3.0-4.0, mesoporous silica particulates are dominant (Supporting Information, Figure S3). (25) Yeh, Y. Q.; Chen, B. C.; Lin, H. P.; Tang, C. Y. Langmuir 2006, 22, 7. (26) Bergstrand, N.; Edwards, K. J. Colloid Interface Sci. 2004, 276, 400. (27) Kostarelos, T.; Tadros, F.; Luckham, P. F. Langmuir 1999, 15, 369. (28) Hoffmann, H.; Thungin, C.; Munkert, U.; Meyer, W.; Richer, W. Langmuir 1992, 8, 2629. (29) Valenzuuela, E.; Abuin, E.; Lissi, E. J. Colloid Interface Sci. 1984, 102, 46. (30) Iler, R. K. The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry; John Wiley & Sons: New York, 1979.
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Figure 3. (A) XRD patterns of the calcined mesoporous silica helical fibers. (B) Microtome TEM images of single-strain silica helical fibers. (C) Microtome TEM images of double-strain silica helical fibers. Scale bars are 100 nm. (D) N2 adsorption-desorption isotherm of the calcined mesoporous silica helical fibers.
Figure 4. SEM images of various shapes of the mesoporous silica helical fibers synthesized with the C18TMAB/SDS/CnOH template (composition: 1.0 g/0.135 g/X g) at pH 2.5. (A) C4OH, X ) 0.30; (B) C5OH, X ) 0.15.
In addition to good control for the self-assembly kinetics of the surfactant and silica species, it is also important to maintain the long rodlike micelles in a highly dilute solution as the building block of the mesostructure for the synthesis of helical mesoporous silica fibers. Therefore, we used a catanionic surfactant to produce long helical mesoporous fibers in high yield instead of using the individual cationic or anionic surfactants. On the basis of the surfactant chemistry, the micelle structure of the catanionic surfactant is dependent on the concentration (lyotropic) and temperature (thermotropic).31 For an unsuitable reaction temperature and water content, the yield of the
mesoporous silica fibers decreased, and the shapes transformed to particulates or flower-like morphologies (Supporting Information, Figure S4). Other factors, including the cation/anion ratio, surfactant chain length, silica/surfactant ratio counterion effect, and addition of chiral molecules, will be studied in the future to develop better control of the uniformity and chirality of the helical fibers. Because mesoporous silica helical fibers of large porosity (pore size ≈ 4.5 nm) can be readily prepared in a high yield, they can be used as a solid nanotemplate with which to prepare helical mesostructured materials.32 After impregnating a proper amount of thermosetting phenol-formaldehyde polymer (PF resin/ mesoporous silica helical fibers ) 1.0 in weight) as the carbon source, pyrolysis at 1000 °C under a N2 atmosphere, and HF etching, mesoporous carbon helical fibers were obtained. As the helical mesoporous silica template, the helical morphologies and well-ordered mesostructure are preserved in the resultant carbons (Supporting Information Figure S5). In conclusion, silicification of the CnTMAB/SDS catanionic surfactant in the presence of a triblock copolymer or 1-alkanol in a highly dilute silica solution results in mesoporous silica helical fibers. This knowledge not only will lead to a greater (31) Holmberg, K.; Jonsson, B.; Kronberg, B.; Lindman B. Surfactants and Polymers in Aqueous Solution, 2nd ed.; John Wiley & Sons: Chichester, England, 2003. (32) (a) Lee, J.; Kim, J.; Hyeon, T. AdV. Mater. 2006, 18, 2073. (b) Lin, Y. P.; Lin, H. P.; Chen, D. W.; Liu, H. Y.; Tang, H. T. C. Y. Mater. Chem. Phys. 2005, 90, 339-343.
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understanding of the formation mechanism of helical mesoporous silica but also will enable the design and construction of new functional helical materials. In combination with replicating technology, mesoporous nonsilica material in helical morphologies could be readily obtained. This should open up a new direction for the controlled synthesis and practical use of mesoporous materials for advanced applications such as chiral heterogeneous catalysis, sensors, adsorption, and separation. The ultimate goal is not only to mimic silicification in nature but also to go a step further and be able to preprogram mesoporous materials to produce a specific architecture with defined functionality.
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Acknowledgment. This research was financially supported by the National Science Council of Taiwan (NSC95-2113-M006-011-MY3 and NSC95-2323-B-006-008). Supporting Information Available: SEM images of mesoporous silica helical fibers synthesized with the CnTMAB-SDS-P123 template and TEM images of various shapes of mesoporous carbon helical fibers after impregnation with PF resin, pyrolysis at 1000 °C, and HF etching. This material is available free of charge via the Internet at http://pubs.acs.org. LA070154T
