Seeding-Growth of Helical Mesoporous Silica Nanofibers Templated by Achiral Cationic Surfactant

Transcription

1 pubs.acs.org/langmuir 2009 American Chemical Society Seeding-Growth of Helical Mesoporous Silica Nanofibers Templated by Achiral Cationic Surfactant Longping Zhou, Guosong Hong, Limin Qi,, * and Yunfeng Lu Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry, Peking University, Beijing , China, and Department of Chemical and Biomolecular Engieering, University of California, Los Angeles, California Received March 27, Revised Manuscript Received April 23, 2009 Helical mesoporous silica nanofibers with parallel nanochannels were synthesized in high yield via a novel seedinggrowth method by using the achiral cationic surfactant cetyltrimethylammonium bromide (CTAB) as template without auxiliary additives. A general entropy-driven model taking into account the icelike structure water due to the hydrophobic effect was proposed to explain the formation of helical mesoporous silica nanofibers. It was indicated that helical silica mesostructures could result from a thick layer of highly ordered icelike water around thin silicate seed rods with a proper concentration, which was verified by the effect of various anions and organic additives on the formation of helical mesoporous silica. Introduction Self-organization of small building blocks into highly sophisticated helical architecture has been commonly observed in nature. 1 Inspired by such a fascinating process, extensive research has been conducted to synthesize a large variety of helical materials that are of importance for chiral separation, chiral recognition, and enantioselective catalysis. 2-4 The synthesis of mesoporous silica with helical pore channels is probably one of the most representative examples. For example, Che and coworkers reported the synthesis of helical mesostructured silica with chiral channels by using chiral surfactants as templates, and the formation of chiral structure was initially ascribed to the chiral mesophases consisting of chiral anionic surfactants and silicates. 5-7 However, a recent study by the same group has shown that helical mesostructured silica could also be obtained using achiral cationic surfactants, indicating that the inherent chirality of the template molecule might not be the sole driving force in forming the chiral structure. 8,9 It is worth noting that the synthesis of helical mesoporous silica by using achiral *To whom correspondence should be addressed. liminqi@pku.edu. cn. Fax: (1) Dickerson, R. E.; Drew, H. R.; Conner, B. N.; Wing, R. M.; Fratini, A. V.; Kopka, M. L. Science 1982, 216, 475. (2) Bradshaw, D.; Prior, T. J.; Cussen, E. J.; Claridge, J. B.; Rosseinsky, M. J. J. Am. Chem. Soc. 2004, 126, (3) Rowan, A. E.; Nolte, R. J. M. Angew. Chem., Int. Ed. 1998, 37, 63. (4) Gier, T. E.; Bu, X. H.; Feng, P. Y.; Stucky, G. D. Nature 1998, 395, 154. (5) Che, S.; Liu, Z.; Ohsuna, T.; Sakamoto, K.; Terasaki, O.; Tatsumi, T. Nature 2004, 429, 281. (6) (a) Ohsuna, T.; Liu, Z.; Che, S.; Terasaki, O. Small 2005, 1, 233. (b) Jin, H. Y.; Liu, Z.; Ohsuna, T.; Terasaki, O.; Inoue, Y.; Sakamoto, K.; Nakanishi, T.; Ariga, K.; Che, S. Adv. Mater. 2006, 18, 593. (7) Qiu, H.; Wang, S.; Zhang, W.; Sakamoto, K.; Terasaki, O.; Inoue, Y.; Che, S. J. Phys. Chem. C 2008, 112, (8) Wu, X.; Jin, H.; Liu, Z.; Ohsuna, T.; Terasaki, O.; Sakamoto, K.; Che, S. Chem. Mater. 2006, 18, 241. (9) Qiu, H.; Che, S. J. Phys. Chem. B 2008, 112, (10) Wang, B.; Chi, C.; Shan, W.; Zhang, Y. H.; Ren, N.; Yang, W. L.; Tang, Y. Angew. Chem., Int. Ed. 2006, 45, (11) Wang, J. G.; Wang, W. Q.; Sun, P. C.; Yuan, Z. Y.; Li, B. H.; Jin, Q. H.; Ding, D. T.; Chen, T. H. J. Mater. Chem. 2006, 16, (12) Yang, S.; Zhao, L. Z.; Yu, C. Z.; Zhou, X. F.; Tang, J. W.; Yuan, P.; Chen, D. Y.; Zhao, D. Y. J. Am. Chem. Soc. 2006, 128, (13) Zhang, L.; Qiao, S. Z.; Jin, Y. G.; Cheng, L. N.; Yan, Z. F.; Lu, G. Q. Adv. Funct. Mater. 2008, 18, cationic surfactants with the aid of small molecular additives or using mixed achiral surfactants 14,15 has been reported by some other groups. Nevertheless, it remains a great challenge to realize facile, high-yield, controllable synthesis of helical mesoporous silica with well-defined exterior morphologies and interior channel structures. To date, the mechanism of forming such helical mesostructures using an achiral directing agent remains largely unclear, although a variety of plausible mechanisms have been proposed. It was suggested that tight intermolecular packing between the amphiphilic heads of the planar imidazolium group might cause a staggered wadding of the achiral surfactants, resulting in the twisting of micelles into a helical structure. 16 It was also proposed that the physical confinement of mesostructured silica in porous anodic alumina nanochanels could lead to the formation of helical architectures due to confinement-induced entropy loss. 17 The chiral intermediate species or nuclei 10 and the topological defects existing in the silicate liquid crystal seeds 11 formed in the synthesis process were also presumed to induce the formation of helical mesostructures. Zhao et al. attributed the origin of helical mesostructured materials to a morphological transformation accompanied by a reduction in surface free energy. 12 Che and Qiu supposed that the helical superstructure mainly originated from the asymmetric molecular shape of the surfactant template, which first generated a helical propeller-like micellar packing and then produced the helical mesostructure. 9 Notably, Ying et al. adopted an entropy-driven model, which was previously proposed to understand the helical conformation of long molecular chains in the crowded environment of a cell, 18 to explain the formation of helical mesoporous silica in a highly concentrated ammonia solution where the hydrated ammonia molecules provided a crowded environment for the rodlike surfactant micelles (14) Lin, G. L.; Tsai, Y. H.; Lin, H. P.; Tang, C. Y.; Lin, C. Y. Langmuir 2007, 23, (15) Meng, X. J.; Yokoi, T.; Lu, D. L.; Tatsumi, T. Angew. Chem., Int. Ed. 2007, 46, (16) Trewyn, B. G.; Whitman, C. M.; Lin, V. S. Y. Nano Lett. 2004, 4, (17) Wu, Y.; Cheng, G.; Katsov, K.; Sides, S. W.; Wang, J.; Tang, J.; Fredrickson, G. H.; Moskovits, M.; Stucky, G. D. Nat. Mater. 2004, 3, 816. (18) Snir, Y.; Kamien, R. D. Science 2005, 307, DOI: /la901083u Published on Web 5/8/2009

2 Letter and so the micelles took on a helical conformation to maximize the entropy of the system.19 However, this entropy-driven model cannot be directly used to explain the formation of helical mesostructured silica in many other situations, especially those without a high concentration of ammonia. This result inspired us to propose a more general entropy-driven model for the formation of helical mesoprous silica, which would help us to develop facile synthetic routes for helical mesoporous materials and to explain the formation of helical mesostructured silica under different synthesis conditions. Herein, we report a novel seeding-growth method for the controllable synthesis of single-helix mesoporous silica nanofibers in nitric acid solution by using the normal achiral cationic surfactant cetyltrimethylammonium bromide (CTAB) as template without auxiliary additives. Such a seeding-growth method has been frequently used in the seed-mediated growth of inorganic crystals with controlled size and morphology;20 however, to the best of our knowledge, this method has not yet been used in the synthesis of ordered mesoporous materials. Moreover, a general entropy-driven model taking into account the icelike structure water due to the hydrophobic effect is proposed to explain the current synthesis of helical mesoporous silica nanofibers as well as some previous syntheses of helical mesostructured silica reported in the literature. Figure 1. SEM images of helical mesoporous silica nanofibers. (A-C) SEM images of entangled nanofibers with different magnifications; (D,E) SEM images of single nanofibers showing left-handed (D) and right-handed (E) helices. Experimental Section The synthesis of helical mesoporous silica nanofibers was simply achieved by hydrolyzing tetraethyl orthosilicate (TEOS) in HNO3 solution in the presence of CTAB by a seeding-growth method. In a typical synthesis, a stock aqueous HNO3 solution (2.8 M) was first prepared. Then, 1 ml of aqueous CTAB solution (9.1 mm) was mixed with 0.7 ml of concentrated nitric acid (63 wt %), which was followed by the addition of 0.55 ml of TEOS under vigorous stirring. After 20 s, 100 μl of the resulting seed solution was added to 5 ml of the stock HNO3 solution under vigorous stirring for a while. Then, the growth solution was thermostatted at 23 C for 12 h statically, resulting in the formation of a white silklike product suspended in the solution. The solid product was collected, washed with water, dried at 60 C overnight, and calcined in air at 550 C for 6 h. In the investigation, the volume of the seed solution transferred into the growth solution was varied from 55 to 300 μl. For comparison purposes, the seeding-growth synthesis of mesoporous silica was also carried out in HCl solution instead of HNO3 solution and a series of inorganic salts including Na2SO4, NaCl, NaBr, and NaNO3 were added to this system to examine the effect of anions. The obtained products were characterized by scanning electronic microscopy (SEM, Hitachi S4800, 2 kv), transmission electronic microscopy (TEM, JEOL JEM 200CX, 160 kv), high resolution TEM (HRTEM, Hitachi H9000, 100 kv), powder X-ray diffraction (XRD, Rigaku Dmax-2000, Cu KR), and nitrogen adsorption-desorption measurements (Micromeritics ASAP 2010) with the pore size distribution calculated from the adsorption curve using the Brunauer-Joyner-Halenda (BJH) method. Results and Discussion Figure 1 shows typical SEM images of helical mesoporous silica nanofibers obtained by the hydrolysis of TEOS in aqueous solution of CTAB and HNO3 via a seeding-growth method. In a typical process, TEOS was hydrolyzed in a highly concentrated (19) Han, Y.; Zhao, L.; Ying, J. Y. Adv. Mater. 2007, 19, (20) (a) Murphy, C. J.; Jana, N. R. Adv. Mater. 2002, 14, 80. (b) Gole, A.; Murphy, C. J. Chem. Mater. 2004, 16, Figure 2. (A) XRD pattern and (B) nitrogen adsorption-desorption isotherms and pore size distribution (inset) of helical mesoporous silica nanofibers. HNO3 solution ( 8.2 M) containing 5.4 mm CTAB, and the silicate liquid-crystal seed solution that formed in the first step was quickly transferred into a stock HNO3 solution (2.8 M) without CTAB for the slow seeding-growth of silica mesostructures. The low-magnification SEM image shown in Figure 1A suggests that nanofibers ranging from several micrometers to tens of micrometers in length are the predominant product with a yield higher than 85%. When observing at a higher magnification (Figure 1B), we can see that all the nanofibers show a helical structure, and most of them are unifilar single-helix nanofibers about nm in diameter although they exhibit different shapes and pitches. The high-magnification image shown in Figure 1C shows that the helical nanofibers consist of both left-handed (Figure 1D) and right-handed (Figure 1E) helices. It was found that the ratio between the left-handed and right-handed helices is near 1:1 by counting 500 single-helix nanofibers, indicating that the product is a racemic mixture. The mesostructure of the obtained helical nanofibers was examined by XRD and nitrogen sorption measurements. The XRD pattern shown in Figure 2A clearly reveals a well-ordered two-dimensional (2D) hexagonal mesostructure with a lattice constant of 4.0 nm. The nitrogen sorption isotherms shown in Figure 2B show a steep capillary condensation in the relative pressure (P/P0) range of , indicating uniform mesopores with a BJH pore size of 3.0 nm. The Brunauer-Emmett-Teller (BET) surface area and mesopore volume of the helical nanofibers are 1045 m2 g-1 and 0.95 cm3 g-1, respectively. These results demonstrate that the high-yield helical products are helical mesoporous silica nanofibers. DOI: /la901083u 6041

3 Letter The morphology and mesoporous structure of the helical mesoporous silica nanofibers were also characterized by TEM and HRTEM observations. The TEM image shown in Figure 3A confirms that the helical product mainly consists of single-helix nanofibers. The HRTEM image shown in Figure 3B suggests that these helices have tracklike pore channels that are aligned parallel to the length of the fibers and are hexagonally packed together. The Fast Fourier transform (FFT) images of the helical nanofiber exhibits a three-directional scattering pattern, clearly indicating that the mesopores have a helical orientation, which decomposes into two transversal alignments and a longitudinal one.21 It is known that the nanochannels in helical mesostructured silica usually adopt two alignment forms: twisted hexagonal mesoporous channels5-13,19 and parallel hexagonal mesoporous channels.14,21,22 While a great success has been achieve in the control of the morphology and purity of the helical mesostructured silica with twisted or chiral nanochennels, the control over the morphology and purity of the helical mesostructured silica with parallel nanochennels has turned out to be more difficult.21,22 It is noted that the high-yield synthesis of helical mesoporous silica fibers with parallel nanochannels was recently reported; however, the synthesis was conducted in a rather complicated system involving ternary cationic-anionic-neutral surfactants and the product was a mixture of single-helix fibers, double-helix fibers, triple-helix fibers, helical ribbons, and multiple stains of helical fibers.19 In the present work, the high-yield synthesis of helical mesoporous silica nanofibers with parallel nanochannels and a single-helix structure has been realized by using CTAB as the single template via a novel seeding-growth method. It was found that the seeding-growth process plays a key role in the high-yield synthesis of helical mesoporous silica nanofibers in the conventional synthesis system containing TEOS, CTAB, and HNO3. The acid-catalyzed synthesis of mesoporous silica carried out in similar systems with dilute reactant concentrations could result in hexagonal mesoporous silica with remarkable arrays of curved shapes and surface patterns due to the growth of silicate liquid-crystal embryos or seeds with a hexagonal cross section.23,24 In the current synthesis, the TEOS and HNO3 concentration in the seed solution was very high and irregular silica gels would result from the rapid growth of the initially formed silicate liquid-crystal seeds if these seeds were not transferred quickly into the stock HNO3 solution to form a growth solution with a dilute reactant concentration. Moreover, if TEOS, CTAB, and HNO3 were simply mixed to form a reaction solution with composition concentrations exactly the same as the final growth solution of after seeding-growth, nonhelical fibers and small particles were the main products, which were accompanied by a small proportion of helical fibers (Figure S1, Supporting Information). This result indicates that a separation between the nucleation of silicate liquid-crystal seeds and the subsequent seed growth is favorable for the high-yield formation of helical mesoporous silica nanofibers. The existence of silicate seeds in the initial seed solution was confirmed by the TEM observation (Figure S2, Supporting Information), which showed that some chainlike silicate nanostructures formed in the seed solution after 20 s of aging. Immediately after being transferred into the stock HNO3 solution, these one-dimensional (1D) silicate structures could form rodlike or fiberlike seeds (21) Kim, W. J.; Yang, S. M. Adv. Mater. 2001, 13, (22) Huo, Q. S.; Zhao, D. Y.; Feng, J. L.; Weston, K.; Buratto, S. K.; Stucky, G. D.; Schacht, S.; Schuth, F. Adv. Mater. 1997, 9, 974. (23) Yang, H.; Coombs, N.; Ozin, G. A. Nature 1997, 386, 692. (24) Yang, S. M.; Yang, H.; Coombs, N.; Sokolov, I.; Kresge, C. T.; Ozin, G. A. Adv. Mater. 1999, 11, DOI: /la901083u Figure 3. A) TEM and B) HRTEM images of helical mesoporous silica nanofibers. The FFT image in the inset shows that the mesopores are twisted and well aligned into an hexagonal arrangement. with smoother surfaces because of partial dissolution upon sudden dilution. It was found that the seeding time was important in determining the morphology of the final products and a high yield of the helical structures could be achieved when the aging time of the seed solution was kept between 15 and 40 s. Moreover, the HNO3 concentration in the synthesis system also played a key role in determining the morphology of the final products, and it should be kept between 2 and 7 M in both the seed solution and the growth solution in order to obtain a high yield of the helical structures. Furthermore, it was observed that silica microspheres and irregular particles were the predominant products when a similar concentration of CTAB was added in the growth solution but not in the seed solution. This result indicated that the presence of CTAB in the seed solution was crucial for the formation of the 1D silicate seeds that evolved into the final helical structures in the growth solution. The morphology of the mesoporous silica obtained by the seeding-growth method could be easily adjusted by changing the amount of the silicate liquid-crystal seeds transferred to the growth solution. While the helical mesoporous silica nanofibers shown in Figure 1 were obtained by transferring 100 μl of the seed solution into 5 ml of the stock HNO3 solution, curved thick fibers and gyroids were produced when 300 μl of the seed solution was transferred (Figure S3A, Supporting Information). On the other hand, nonhelical nanofibers about nm in diameter were obtained when 55 μl of the seed solution was transferred (Figure S3B, Supporting Information). The nonhelical fibers obtained at the low seed concentration were somewhat thicker than the helical fibers obtained at the medium seed concentration, which could be rationalized by considering that the gradual addition of the soluble silicate species onto a smaller amount of rodlike silicate seeds would result in the silica fibers with larger diameters. The TEM observation of the nonhelical fibers suggested that these fibers were normal mesoporous silica fibers with parallel channels without any helical characteristics (Figure S4, Supporting Information). The formation mechanism of the helical mesoporous silica nanofibers is not very clear at the present time. However, inspired by the entropy-driven model used by Ying et al. to explain the formation of helical mesoporous silica in a highly concentrated ammonia solution,19 here we tentatively propose a more general entropy-driven model by taking into account the icelike structure water due to the hydrophobic effect to explain the formation of the helical mesoporous silica nanofibers. In the classic iceberg model used to describe the micelle formation, water molecules around surfactant molecules are in a relatively rigid cagelike configuration (termed as iceberg ). When the two hydrophobic chains come together, the gain in entropy as some of this icelike structured water is liberated by the overlapping of solvation shells will compensate for the loss in entropy due to the assembly of the

4 Letter Figure 4. Scheme of entropy-driven formation of helical silica structure based on the overlapping of icelike structure water. (A) Rodlike silicate mesostructure (green) surrounded by icelike structure water (gray). (B) The volume of the icelike structure water is reduced by overlapping itself when the silicate rod takes on a helical conformation, thus increasing the entropy of the system. surfactant molecules. 25,26 Recently, the icelike structure water near a hydrophobic surface has been evidenced Accordingly, an entropy-driven mechanism based on icelike structure water is proposed as follows (Figure 4). During the hydrolysis of TEOS in the CTAB solution, CTAB micelles were gradually incorporated by the partly hydrolyzed silicates to form rodlike mesostructure or liquid-crystal coassembled by silicates and CTAB. The outer layer of the rodlike silicate mesoctructure would be hydrophobic because the hydrophobic C 2 H 5 O- groups carried by the partly hydrolyzed silicates were exposed since these partly hydrolyzed species with a relatively high concentration tended to attach to the head groups of CTAB on the outer surface of the rodlike micelles. These hydrophobic parts are surrounded by the icelike structure water molecules. When the silicate rod takes on a helical conformation, the volume of icelike structure water will be reduced by overlapping itself, thus increasing the entropy of the system. Since the hydrophobic effect could be a long-range interaction extending over distances of up to 100 nm, 30 the overlap volume would be large enough to induce the entropydriven formation of the helical nanofibers. If the gain in entropy due to the overlap volume compensates for the elastic energy due to bending, that is, the depletion interaction dominates the bending stiffness, the silicate rod would bend into a helix spontaneously. In other words, it would be difficult for a silicate rod to bend into a helix if the rod is too thick. Therefore, it may be reasonably deduced that a thicker layer of icelike structure water and thinner silicate fibers would be favorable for the formation of a helical structure. The formation of mesoporous silica with different morphologies by the seeding-growth approach can be explained with the proposed entropy-driven mechanism as schematically illustrated in Figure 5. In the first step, the acid-catalyzed hydrolysis of TEOS occurs very quickly because of the high concentrations of both TEOS and HNO 3, resulting in the rapid nucleation of many silicate mesostructured embryos or liquid-crystal seeds surrounded by icelike water. After being transferred into the growth solution, these liquid-crystal seeds tend to rearrange to (25) Frank, H. S.; Evans, M. W. J. Chem. Phys. 1945, 13, 507. (26) Shinoda, K.; Kobayashi, M.; Yamaguchi, N. J. Phys. Chem. 1987, 91, (27) Chen, X.; Yang, T.; Kataoka, S.; Cremer, P. S. J. Am. Chem. Soc. 2007, 129, (28) Raghuraman, K.; Katti, K. K.; Barbour, L. J.; Pillarsetty, N.; Barnes, C. L.; Katti, K. V. J. Am. Chem. Soc. 2003, 125, (29) Mashl, R. J.; Joseph, S.; Aluru, N. R.; Jakobsson, E. Nano Lett. 2003, 3, 589. (30) Ball, P. Nature 2003, 423, 25. Figure 5. Schematic illustration of the possible growth mechanism of different silica structures in solutions containing different seed concentrations. reduce the volume of icelike water, which can be achieved by three typical ways: (1) to assemble together to form a thicker rod, (2) to form a helix, and (3) to eliminate the icelike water by adsorbing surfactant molecules. When a high concentration of silicate seeds exist, the silicate liquid-crystal seeds are ready to assemble together to form thick fibers or gyroids. At a proper medium seed concentration, the silicate liquid-crystal seeds tend to form helical mesoporous silica nanofibers because of the smaller seed concentration. When a low concentration of silicate seed exists, the reaction proceeds quite slowly and so the CTAB molecules would be easily adsorbed on the surface of the silicate seeds during the slow growth process, considerably eliminating the icelike water. It was further confirmed by the finding that the main product was nonhelical structures when the liquid-crystal seeds were transferred into a HNO 3 solution containing CTAB instead of the stock HNO 3 solution without CTAB (Figure S5, Supporting Information). It could also explain well the observation that helical mesoporous silica fibers usually appeared in highly diluted CTAB solutions. 14 The proposed entropy-driven model indicates that helical silica mesostructures usually result from a thick layer of highly ordered icelike water around thin silicate seed rods with a proper concentration. The factors that could increase the hydrophobicity of the silicate seed rods would increase the thickness and/or ordering of the icelike structure water around the seed rods, thus favoring the formation of helical silica mesostructures. The hydrophobicity of the silicate rods can be significantly influenced by the additives including both inorganic ions and organic molecules. As reported recently, the extent of water orientation near a hydrophobic surface closely follows the Hofmeister series for the anions, for example, nitrate > bromide > chloride > sulfate. 27 As a verification, the effect of anions on the synthesis of mesoporous silica by the seeding-growth method was investigated. As expected, no helical structures were obtained when HCl was used instead of HNO 3 in the synthesis system under otherwise similar conditions; moreover, when a series of sodium salts (i.e., Na 2 SO 4, NaCl, NaBr, and NaNO 3 ) were added to the synthesis system containing TEOS, CTAB, and HCl, helical nanofibers were obtained with a high yield only from the systems with NaBr and NaNO 3 (Figure S6, Supporting Information). This finding suggests that the icelike water with a high orientation or ordering would lead to enough driving force to form a helical structure. Furthermore, the surfactants and organic additives with stronger hydrophobicity would enhance the hydrophobicity of the silicate seed rods, thus favoring the formation of helical silica mesostructures. This deduction is consistent with the DOI: /la901083u 6043

5 Letter findings that helical silica mesostructures usually appeared in the systems involving highly hydrophobic organic additives, such as perfluorooctanoic acid, 12 fluorinated surfactant, 15 and organoalkoxysilane. 13 In conclusion, high-yield synthesis of helical mesoporous silica nanofibers with parallel nanochannels and a single-helix structure has been realized by using the normal achiral cationic surfactant CTAB as template without auxiliary additives via a novel seeding-growth method. A general entropy-driven model taking into account the icelike structure water due to the hydrophobic effect is proposed to explain the current synthesis of helical mesoporous silica nanofibers as well as some previous syntheses of helical mesostructured silica reported in the literature. In combination with replicating strategies, helical nanosilica materials of interest, such as helical mesoporous polystyrene nanofibers (Figure S7, Supporting Information), could be readily obtained. This result may open up a new avenue for the controlled synthesis of helical mesoporous materials, which could find potential applications including chiral catalysis, separation, and sensing. Acknowledgment. This work was supported by NSFC (Grants , , , and ) and MOST (Grant 2007CB936201). Supporting Information Available: Additional SEM and TEM images of the products. This material is available free of charge via the Internet at DOI: /la901083u