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Formation of Whiskers of Silicate Mesostructures Hiroaki Imai,* Noriko Takahashi, Ryo Tamura, and Hiroshi Hirashima Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan Received May 31, 2000. In Final Form: September 14, 2000 Whiskers of mesostructured silicate having a sharp top and hexagonal habits were formed on a flat substrate by surfactant-templated synthesis in an acidic solution system. In the initial stage of whisker formation, truncated cones of hexagonal mesosilicate covered the surface through heterogeneous nucleation. Whisker structures were constructed on the truncated cones by surfactant micelles accompanied with silicate through a hexagonal plan. The formation of sharp whiskers is ascribed to instability of the surface of the mesostructured silicate-surfactant composite in a diffusion field.
Introduction Recently, mesoscopically ordered silicate materials have been synthesized by the organization of surfactant molecules and silica-precursor species.1,2 Various kinds of mesophases such as hexagonal, cubic, and lamellar, similar to a liquid-crystal array, have been observed in silica composites.2 Because a homogeneous array of mesopores is easily prepared by removing surfactants from the composites, the resulting silicate mesostructures are applicable for molecular selective catalysts and membranes. Morphological studies have revealed that macroscopic precipitates consisting of hexagonally ordered silicates showed noncrystallographic symmetry including gyroids, helicoils, and curved tubules.3-5 The morphological control of mesostructures is extremely important for the production of designed mesophases which are useful for various applications. Mesostructured silica films were prepared at the mica-water, graphite-water, and airwater interfaces.6-8 Microscopic patterning of mesostructures was successfully achieved by infiltrating a reaction fluid into microcapillaries 9 and by using self-assembled monolayers.10 Spheres,11,12 fibers,13,14 and hollow tubules15 * To whom correspondence should be addressed. E-mail: hiroaki@ applc.keio.ac.jp. (1) Kresge, C. T.; Leonowicz, M.; Roth, W. J.; Vartuli, J. C.; Beck, J. C. Nature 1992, 359, 710. (2) Monnier, A.; Schu¨th, F.; Huo, Q.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299. (3) Yang, H.; Coombs, N.; Ozin, G. A. Nature 1997, 386, 692. (4) Yang, H.; Kuperman, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G. A. Nature 1996, 379, 703. (5) Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G. A. Nature 1996, 381, 589. (6) Sokolov, I.; Yang, H.; Ozin, G. A.; Kresge, C. T. Adv. Mater. 1999, 11, 636. (7) Yang, S. M.; Sokolov, I.; Coombs, N.; Kresge, C. T.; Ozin, G. A. Adv. Mater. 1999, 11, 1427. (8) 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. (9) Trau, M.; Yao, N.; Kim, E.; Xia, Y.; Whitesides, G. M.; Aksay, I. A. Nature 1997, 390, 674. (10) Yang, H.; Coombs, N.; Ozin, G. A. Adv. Mater. 1998, 9, 811. (11) Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Schu¨th, F. Science 1996, 273, 768. (12) Huo, Q.; Feng, J.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1997, 9, 14. (13) Huo, Q.; Zhao, Z.; Feng, J.; Weston, K.; Buratto, S. K.; Stucky, G. D.; Schacht, S.; Schu¨th, F. Adv. Mater. 1997, 9, 974. (14) Schmidt-Winkel, P.; Yang, P.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Adv. Mater. 1999, 11, 303. (15) Lin, H.; Cheng, S.; Mou, C. Chem. Mater. 1998, 10, 581.
consisting of hexagonal mesosilicates were also produced using silicate-precursor and surfactant systems. The morphogenesis and morphokinetics of mesostructured silicate were discussed based on the relationship between the synthesis conditions and the morphology.16-18 The investigation of the formation mechanism of noncrystallographic morphology is essential for understanding the self-assembly of an inorganic-surfactant complex. This paper reports a novel morphology of surfactantsilicate hexagonal mesostructures formed as a result of a diffusion field. Specific whiskers consisting of mesostructured silicates were constructed on a flat surface in a precursor solution in restricted ranges of pH, silicate concentration, and reaction time. The formation mechanism is discussed from the points of view of the assembly of precursor micelles with a hexagonal plan and the surface instability of the silicates. Experimental Section Mesostructured silicates were prepared in an acidic system containing tetraethoxysilane (TEOS) (95%, Kanto Chemicals) and cetyltrimethylammonium chloride (CTAC) (Kanto). Typical molecular ratios for the precursor solutions were 2.7 TEOS/1.3 CTAC/3.4 HCl/280 water. The mixed solutions were vigorously stirred at room temperature for 10 min in a bottle. Glass slides were immersed in the mixed solutions and then kept at 25 °C in an incubator. The surface of the substrates for deposition of silicate whiskers was arranged downward to prevent the accumulation of the precipitates. Silicate-surfactant composites precipitated in a bottle and deposited on a substrate were dried at 60 °C in air for 1 h and then characterized by X-ray diffraction (XRD) (Rigaku RAD-C) using Cu R radiation, optical microscopy, and scanning electron microscopy (SEM) (Hitachi S-2150).
Results and Discussion After an induction period of a couple of hours, white particles started to precipitate in the solutions containing chemicals with typical molar ratios. The precipitated particles showed complex shapes including gyroids, which had already been reported by Yang et al.3,17,18 A typical XRD pattern (Figure 1) revealed that the particles consisted of a hexagonal unit cell with d100 ∼ 4.4 nm. The well-defined facets of the precipitates showing hexagonal basal faces are attributable to hexagonally aligned silicate-surfactant micelles. (16) Yang, H.; Ozin, G. A.; Kresge, C. T. Adv. Mater. 1998, 10, 883. (17) Yang, H.; Vovk, G.; Coombs, N.; Sokolov, I.; Ozin, G. A. J. Mater. Chem. 1998, 8, 743. (18) Yang, S. M.; Yang, H.; Coombs, N.; Sokolov, I.; Kresge, C. T.; Ozin, G. A. Adv. Mater. 1999, 11, 52.
10.1021/la000758w CCC: $20.00 © 2001 American Chemical Society Published on Web 12/09/2000
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Figure 1. Typical XRD patterns of precipitated particles and deposition (truncated cones) on a substrate in the solutions containing chemicals with typical molar ratios (2.7 TEOS/1.3 CTAC/3.4 HCl/280 water).
Figure 2. A typical scanning electron micrograph of a truncated conelike morphology formed on a substrate. This image was observed on a substrate kept for 3 h at 25 °C in the precursor solution with typical molar ratios.
Figure 3. Typical scanning electron micrographs of whiskers having a smooth surface formed on truncated cones: (a) small cones with a sharp top and (b) tall whiskers. These morphologies were occasionally observed on a substrate kept longer than 8 h.
A truncated conelike morphology (Figure 2) was formed on a downward surface of glass substrates in the precursor solutions at the same time as the particles precipitated. The number of cones formed on the substrate decreased upon increasing the amounts of TEOS and HCl from the typical molar ratios. An increase in the proton concentration in the solution promotes the hydrolysis of TEOS molecules. Thus, these changes in the synthesis conditions increase the production rate of silicate-surfactant composite species in the solution. A high production rate dominantly induces precipitation through homogeneous nucleation, suppressing the number of truncated cones formed through heterogeneous nucleation on the surface. The facet of the truncated body indicates that the cones are based on a hexagonal plane. Because the top face of the truncated cones is parallel to the surface, a hexagonal array is aligned parallel to the surface of the substrate. In this case, the XRD pattern of the mesostructures is similar to that of a layered structure because of the disappearance of the (110) peak (Figure 1). The d100 spacing (∼4.1 nm) of the truncated cones on the substrate is smaller
than that of the precipitates, suggesting that the hexagonal array is compressed in the direction normal to the substrate as reported by Aksay et al.8 The precipitation in the precursor solutions containing chemicals with typical molar ratios almost finished within 6 h. After precipitating, the surface was entirely covered with the truncated cones, and mesosilicate whiskers having a sharp top (Figure 3) then started to grow on the basal cones. It can be inferred that small cones with a sharp top (Figure 3a) were in an earlier stage of the growth than tall whiskers (Figure 3b). The formation of the whiskers was markedly activated after keeping for 8-9 h, and many tall whiskers having hexagonal habits (Figure 4) were finally formed on the substrates. The number of tall whiskers increased with the reaction time after an induction period (Figure 5). Some of the truncated cones suddenly changed into whiskers, but gradual growth of all the truncated cones covering the surface was not observed. Thus, small whiskers with a smooth surface and faceted tall ones were observed simultaneously on
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Figure 5. Variations of the SiO2 concentration (b) in the solutions containing chemicals with typical molar ratios and the number of whiskers on a substrate (0). The concentration was estimated by weighing the residue of the solutions after evaporation of water and firing of organic compounds.
Figure 4. Typical scanning electron micrographs of whiskers having hexagonal facets: (a) an image of a whole whisker and (b) a magnification of the facets. These faceted morphologies were commonly observed on a substrate kept longer than 10 h. Cracks of the whiskers in the direction normal to the substrate were formed with shrinkage during drying.
the same substrate. The direction of growth of the whiskers was almost perpendicular to the surface. The hexagonal facets exhibited by the whiskers were parallel to one another (Figure 4b). These results show that the whiskers were directly constructed on the basal truncated cones with the same hexagonal plan. Thus, sharp whiskers are obviously different from previously reported gyroid and helicoil morphologies.5-7 Small truncated cones with random orientation were occasionally observed on the basal cones. In this case, the formation of silicate whiskers was prevented by the small cones. The whisker formation was observed only in a restricted range of pH ∼ 0.1 and
[TEOS]/[CTAC] ∼ 2. At a low pH below 0, most silicate precursors were consumed for the formation of precipitates through homogeneous nucleation as mentioned above. On the other hand, neither precipitation nor formation of the cones occurred in several days at a high pH above 0.5 because silicate precursors are relatively stable in that condition. Therefore, a moderate production rate is important for the formation of mesosilicate whiskers. The [TEOS]/[CTAC] ratio is also essential for the construction of whisker morphologies. It can be deduced that the surfactant micelles covered with hydrolyzed silicates are assembled in the aqueous solution. Precursors having a specific molar ratio of [TEOS]/[CTAC] ∼ 2 would be required for the ordered construction of tall whisker structures. Surfactant micelle rods are relatively flexible because bending of a micelle rod slightly affects the total energy.19 In this case, the curved surface morphology is easily formed with disclinations and dislocations in liquid crystals.16,20 Because round shapes are advantageous to decrease the surface energy, gyroids of liquid crystals accompanied with silicate are commonly prepared through homogeneous nucleation in the precursor solution as illustrated in Figure 6.18 However, hexagonal arrays of the tubular micelles have been reported to be aligned along the lattice of freshly cleaved mica5 and graphite surfaces8 because the adsorption of the micelles was regulated by the crystalline surface. On the other hand, chaotic morphologies were commonly observed at the air-water interface.9 The micelle arrays were freely assembled on noncrystalline surfaces. It can be inferred that the conelike morphology is formed through heterogeneous nucleation, as shown in Figure 6, because gyroids are also preferred on amorphous solid surfaces. Because the surface energy is relatively high at a convex surface having a great curvature, the radius of the top of the conelike morphology would be restricted below a critical value. Thus, the deposited cones show truncated shapes having almost the same radius. The top face of the cones was parallel to the surface of the substrates, indicating that the tubular micelles covered with silicates were aligned parallel to the surface. (19) Helfrich, W. Z. Naturforch. 1973, 28C, 693. (20) Feng, J.; Huo, Q.; Petroff, P. M.; Stucky, G. D. Appl. Phys. Lett. 1997, 71, 620.
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Figure 7. A schematic model of instability of a surface in a diffusion field. Fluctuation of the surface increases with the difference of the slope of the concentration u in the z direction du/dz ((1) < (2) < (3)).
V ) D du/dz
Figure 6. A schematic formation model of hexagonal facets for precipitates and truncated cones on a substrate: (a) a hexagonal rod of hexagonal silicate mesostructures, (b) winding a hexagonal rod, and (c) formation of a gyroid through homogeneous nucleation and a truncated cone through heterogeneous nucleation.
However, this assumption is contradictory to the formation of whiskers with a relatively sharp top. Another mechanism is required to explain the production of whisker morphologies. Figure 5 shows time-dependent variations of the SiO2 concentration in the solution containing chemicals with typical molar ratios and the number of whiskers taller than 0.1 mm. The silicate precursors were remarkably consumed by the formation of precipitates in the initial stage. Then, precipitation through homogeneous nucleation was reduced with decreasing the concentration. After the precipitation was suppressed, whiskers started to be constructed on the basal cones by the assembly of the residual silicate precursors. In this stage, the silicate precursors were consumed only at the surface of the mesosilicates already formed on the substrate because the homogeneous nucleation was finished. Thus, a concentration profile such as that illustrated in Figure 7 is achieved near the surface of a substrate. The concentration of the precursors at the surface is lower than that in the bulk of the solution because of the continuous growth of the silicate-surfactant composite on the substrate. The growth velocity V in a diffusion field is expressed as
(1)
with the diffusion constant D and the slope of the concentration in the z direction du/dz. In this diffusion field, the growth behavior at the surface becomes unstable. As shown in Figure 7, a relatively convex part of the surface can grow faster than the other parts because of the greater concentration slope at the top of the convex part. Finally, the morphology of whiskers having a relatively sharp top is suddenly induced by some fluctuation at the surface because of the instability. Relatively small whiskers having a smooth surface were occasionally found on the samples, but most of the whiskers showed hexagonal facets. Thus, it is suggested that the facets are constructed by the assembly of tubular micelles accompanied with silicates through a hexagonal plan on the smooth surface of the initial whiskers formed by the surface instability. It was proposed that the hexagonal mesophase resulted from a transformation of a lamellar mesophase of the surfactant-inorganic systems.15,21 Yang et al.7 described that helicoids of mesostructured silica were formed by the folding of hexagonal silicate film. In this case, however, the hexagonal array is directly assembled on the basal surface of the substrate with the precursor micelle rods, forming the whisker morphologies. In summary, whiskers of silicate mesostructures showing hexagonal habits were found to be constructed on the flat surface of a substrate in a restricted range of reaction conditions. The unique morphology of the silicatesurfactant composite is attributed to the assembling with a hexagonal plan of precursor micelles accompanied with silicate transported in a diffusion field. The mesosilicate whiskers provide a large amount of ordered mesopores on a substrate. Therefore, this novel morphology of mesostructured materials would be applicable for a new type of adsorbents and supports of catalysts. LA000758W (21) Yada, M.; Hiyoshi, H.; Ohe, K.; Machida, M.; Kijima, T. Inorg. Chem. 1997, 36, 5565.
