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Elongated Silica Nanoparticles with a Mesh Phase Mesopore Structure by Fluorosurfactant Templating Bing Tan,† Alan Dozier,† Hans-Joachim Lehmler,‡ Barbara L. Knutson,† and Stephen E. Rankin*,† Chemical and Materials Engineering Department, University of Kentucky, 177 Anderson Hall, Lexington, Kentucky 40506-0046, and Department of Occupational and Environmental Health, University of Iowa, 100 Oakdale Campus #124 IREH, Iowa City, Iowa 52242-5000 Received March 1, 2004. In Final Form: June 22, 2004 Mesoporous silica materials with pore structures such as 2D hexagonal close packed, bicontinuous cubic, lamellar, sponge, wormholelike, and rectangular have been made by using surfactant templating sol-gel processes. However, there are still some “intermediate” phases, in particular mesh phases, that are formed by surfactants but which have not been made into analogous silica pore structures. Here, we describe the one-step synthesis of mesoporous silica with a mesh phase pore structure. The cationic fluorinated surfactant 1,1,2,2-tetrahydroperfluorodecylpyridinium chloride (HFDePC) is used as the template. Like many fluorinated surfactants, HFDePC forms intermediate phases in water (including a mesh phase) over a wider range of compositions than do hydrocarbon surfactants. The materials produced by this technique are novel elongated particles in which the layers of the mesh phase are oriented orthogonal to the main axis of the particles.
Several nonionic hydrocarbon surfactants and fluorinated surfactants are known to form mesh phases in aqueous solution.1-3 Mesh phases are “defective” lamellar phases in which nonpolar layers are perforated with channels or pillars of the polar microphase.4 Depending on the arrangement of the pillars, these phases are classified as trigonal, tetragonal, or random mesh phases.1,5 Fluorinated surfactants are prone to form novel “intermediate” phases because the tails of fluorinated surfactants are stiff and favor aggregates with low curvature.2,3,6 In aqueous solution, 1,1,2,2-tetrahydroperfluorodecylpyridinium chloride (HFDePC) has been found to form hexagonal, rectangular, trigonal mesh, and random mesh phases.2 Therefore, this surfactant is a good candidate for beginning to explore whether fluorinated surfactants generate unusual structures when used as pore templates. Despite the unique interfacial properties of fluorinated surfactants, they have only recently begun to be used as templates for porous ceramics.7,8 A large variety of different mesopore architectures have been developed by surfactant templating techniques.9-11 They include 2D hexagonal close packed,12,13 bicontinuous cubic,12,14 lamellar,15 sponge,16 wormhole-like,17 and rectangular18 pore arrangements. However, no silica with a * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: (859)257-9799. † University of Kentucky. ‡ University of Iowa. (1) Burgoyne, J.; Holmes, M. C.; Tiddy, G. J. T. J. Phys. Chem. 1995, 99, 6054. (2) Wang, K.; Ora¨dd, G.; Almgren, M.; Asakawa, T.; Bergensta¨hl, B. Langmuir 2000, 16, 1042. (3) Kekicheff, P.; Tiddy, G. J. T. J. Phys. Chem. 1989, 93, 2520. (4) Tschierske, C. Curr. Opin. Colloid Interface Sci. 2002, 7, 69. (5) Hyde, S. T.; Schro¨der, G. E. Curr. Opin. Colloid Interface Sci. 2003, 8, 5. (6) Puntambekar, S.; Holmes, M. C.; Leaver, M. S. Liq. Cryst. 2000, 27, 743. (7) Rankin, S. E.; Tan, B.; Lehmler, H.-J.; Knutson, B. L. Mater. Res. Soc. Symp. Proc. 2003, 775, 47. (8) Blin, J. L.; Lesieur, P.; Ste´be´, M. J. Langmuir 2004, 20, 491. (9) Ciesla, U.; Schu¨th, F. Microporous Mesoporous Mater. 1999, 27, 131. (10) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56. (11) Edler, K. J.; Roser, S. J. Int. Rev. Phys. Chem. 2001, 20, 387.
mesh phase structure has been reported. In this letter, we report a synthetic route to silica with a random mesh phase pore structure by a liquid phase, one-step synthesis starting from a molecular precursor. Pore structures similar to mesh phase structures have been made by pillaring of clays and microporous solids.19-23 Like random mesh phases, the pillars between layers in these materials are randomly arranged. However, these materials are synthesized by swelling natural or synthetic layered minerals and then introducing metal oxide particles.20 Our one-step synthesis procedure instead produces particles by the direct coassembly of surfactants and silica into an ordered mesophase.24 A silica material, sample UK-2, was synthesized at room temperature by the precipitation of tetraethoxysilane in the presence of HFDePC from an aqueous ammonia solution, as described in the Methods section. The X-ray diffraction (XRD) patterns of the as-synthesized and extracted samples are shown in Figure 1. Because the background is strong in our samples, we show for (12) Beck, J. S.; Vartuli, J. C.; Kenedy, G. J.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, 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. (13) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (14) Kumar, D.; Schumacher, K.; du Fresne von Hohenesche, C.; Gru¨n, M.; Unger, K. K. Colloids Surf., A 2001, 187-188, 109. (15) Tanev, P. T.; Liang, Y.; Pinnavaia, T. J. J. Am. Chem. Soc. 1997, 119, 8616. (16) Behrens, P.; Glaue, A.; Haggenmu¨ller, C.; Schechner, G. Solid State Ionics 1997, 101-103, 255. (17) Zhang, W.; Pauly, T. R.; Pinnavaia, T. J. Chem. Mater. 1997, 9, 2491. (18) Zhao, D.; Huo, Q.; Feng, J.; Kim, J.; Han, Y.; Stucky, G. D. Chem. Mater. 1999, 11, 2668. (19) He, Y. J.; Nivarthy, G. S.; Eder, F.; Seshan, K.; Lercher, J. A. Microporous Mesoporous Mater. 1998, 25, 207. (20) Gil, A.; Gandia, L. M. Catal. Rev.sSci. Eng. 2000, 42, 145. (21) Kwon, O.-Y.; Shin, H.-S.; Choi, S.-W. Chem. Mater. 2000, 12, 1273. (22) Wang, Z.-M.; Yamashita, N.; Kanoh, H. J. Colloid Interface Sci. 2004, 269, 283. (23) Yamanaka, S.; Hattori, M. Stud. Surf. Sci. Catal. 1991, 60, 89. (24) 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.
10.1021/la049474s CCC: $27.50 © 2004 American Chemical Society Published on Web 07/14/2004
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Figure 2. Representative SEM image for UK-2. The arrows indicate some of the particles with openings at one end where the particles appear to have broken.
Figure 1. XRD patterns (a) for amorphous silica and for sample UK-2 (b) as-synthesized, (c) after surfactant extraction, (d) after extraction and heating at 550 °C in air for 4 h, and (e) after extraction and heating in air at 800 °C for 2 h.
comparison (trace a) the XRD pattern of amorphous silica formed by the precipitation of tetraethoxysilane in the presence of sodium dodecyl sulfate, followed by calcination. The pattern for the as-synthesized material (trace b) has low contrast because of the presence of fluorine7 but still indicates a single reflection. After extraction (trace c), the pattern has three distinct reflections. The two reflections at the higher angles are indexed as (001) and (002) reflections of a layered structure with a layer spacing of 3.3 nm. The weak reflection at the lowest angle is consistent with scattering from silica micropillars between the layers, but because of the strong background, there is significant uncertainty in the d spacing. On the basis of the apparent peak position, we can place a lower bound on the average pillar spacing of 4.5 nm. We would not be able to see contributions to the XRD pattern from pillars spaced more than 6.1 nm apart under any circumstances, and contributions from pillars spaced between 6.1 and 4.5 nm apart may be underestimated because of the background. We will use scanning transmission electron microscopy (STEM) (below) to measure the pillar spacing more precisely. In any event, because we do not find a set of reflections that would indicate a periodic 3D arrangement of silica micropillars, the low-angle XRD reflection is interpreted as coming from a random mesh phase (rather than trigonal or tetragonal). In related work with organic polyelectrolyte complexes with fluorinated surfactants, random mesh phase aggregates have been found frequently.25 Another possible interpretation of the XRD pattern is a mixture of a lamellar phase (spacing, 3.3 nm) and a (25) Thu¨nemann, A. F. Prog. Polym. Sci. 2002, 27, 1473.
wormhole-like phase (average pore spacing g 4.5 nm). However, an ordinary lamellar structure would be expected to collapse upon heating, even if it were able to survive surfactant extraction. The thermal stability of this material was checked by heating two separate samples, one at 550 °C in air for 4 h and the other at 800 °C in air for 2 h. Figure 1d shows that this sample was stable at 550 °C, which is consistent with a mesh phase but not with a lamellar phase. The mesh phase begins to collapse only upon heating at 800 °C (Figure 1e). Scanning electron microscopy (SEM) images of sample UK-2 (Figure 2) show that the material is composed of elongated particles that are loosely aggregated into clusters (either in solution or during the process of drying a sample for electron microscopy). Low magnification transmission electron microscopy (TEM) (Figure 3a) confirms the elongated particle morphology. Some of the particles are open at one end, as indicated by the small arrows in Figure 2. These particles are not hollow but instead have shallow depressions at their ends. These appear to result from a “shell” of silica extending beyond the particle core, as a result of either the growth mechanism or fracture. TEM and STEM images at higher magnification (Figure 3b and c) are consistent with a rough shell of amorphous silica surrounding particles with ordered cores. High magnification TEM clearly shows the layered mesopore structure of these particles. Figure 3b shows that each elongated particle has a very well ordered layered pore structure. The region that appears to be amorphous in the lower left corner of Figure 3b is the result of several particles overlapping rather than a mixture of phases. When nonoverlapping particles are visualized, they always have a layered, rather than wormhole-like, structure in TEM. The layered pattern is more clearly shown in the dark field STEM image of a single particle shown in Figure 3c. Remarkably, the slit-shaped pores in these particles are always found to be orthogonal to the main axis of the
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Figure 3. Representative TEM images for UK-2: (a) low magnification image of a particle cluster; (b) intermediate magnification image of particles; (c) dark field STEM image of a single particle; (d) high magnification STEM image (inset, histogram of measured interpillar distances from the STEM image).
particle. Figure 3d shows a high magnification dark field STEM image, providing a close look at the layered structure. Silica pillars can be seen clearly between the layers. The distance between the pillars was estimated by image analysis using all clearly resolved pillar spacings sampled from the entire width of the particle. The histogram of the spacing distribution is shown in the inset of Figure 3d. The distribution is broad, with a mean spacing of 6.3 nm and a standard deviation of 2.1 nm. These spacing estimates could be underestimated because Figure 3d is a 2D projection of a 3D structure, but this effect should be minimal because STEM provides clear contrast only within a shallow depth of field. The measured spacings are consistent with the position of the first XRD peak, within the described uncertainty. The distance between adjacent silica layers (3.1 ( 0.1 nm) is also consistent with the d spacing for the (100) reflection of the mesh phase structure from XRD. As a final confirmation of the mesopores structure, N2 adsorption-desorption isotherms were collected for the extracted sample. The isotherm of UK-2 (Figure 4) is of type IV,26 while the isotherms of microporous, layered materials such as pillared clays are usually of type II and sometimes display H4 hysteresis loops due to a platelet morphology.19,26,27 This confirms that our material is mesoporous (giving rise to the sharp inflection of the type IV isotherm) and does not have a platelet morphology (consistent with Figure 2). The upturn in adsorption at high relative pressure is probably due to nitrogen con(26) Sing, K. S. W.; Everett, D. H.; Hual, R. A. W.; Moscou, L.; Pierottic, R. A.; Rouque´rol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (27) Gil, A.; Gandia, L. M. Chem. Eng. Sci. 2003, 58, 3059.
Figure 4. Nitrogen sorption isotherm and pore size distribution from the modified BJH method28 (inset) for UK-2 after extraction and room-temperature drying.
densation within the clusters of particles illustrated in Figure 3a. This isotherm was converted to an R plot28 to determine the pore texture characteristics relative to macroporous LiChrosphere 1000 silica with an average estimated surface area of 22.7 m2/g.29 We determined a total surface area of St ) 765 m2/g, an external surface area of Sext ) 338 m2/g, and a primary pore volume of Vp ) 0.44 cm3/g. The pore size distribution (PSD) was calculated from the adsorption branch of the isotherm by the Barrett-Joyner-Halenda (BJH) method with a slit pore (28) Sayari, A.; Kruk, M.; Jaroniec, M. Catal. Lett. 1997, 49, 147. (29) Jaroniec, M.; Kruk, M.; Olivier, J. P. Langmuir 1999, 15, 5410.
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geometry and the modified adsorbed film thickness equation of Sayari et al.28 The results, shown in the inset of Figure 4, indicate a slit pore width of 2.0 nm. This estimate is consistent with the slit pore width estimated from the average surface area and pore volume (∼2.1 nm). To conclude, after characterization by XRD, electron microscopy, and nitrogen sorption, the material UK-2 has been found to have a mesh phase mesopore structure with a large surface area and pore volume. It is composed of elongated particles with slit pores oriented orthogonal to their long axis, rather than the platelike morphology that one would expect for a layered material. This suggests that silicate-surfactant aggregates grow by expansion along the [001] direction, rather than in the (001) plane. This can be explained by the tendency of fluorinated surfactants to form bilayer fragments, such as disc-shaped micelles.6,30 We hypothesize that disc micelles may act as precursors to the mesh phase that coassemble with silicates near the tip of growing particles. Strong interactions between the elongated edge of disc micelles and exposed layers of a particle may promote growth in the [001] direction, rather than lateral growth. Our success using HFDePC to make a novel material further suggests that unexplored potential remains for other fluorinated surfactants to be used as pore templates for novel ceramic materials. Methods 1,1,2,2-tetrahydroperfluorodecylpyridinium iodide was prepared as reported previously31 and converted to the chloride (HFDePC) by ion exchange. Its purity was confirmed by 1H NMR (30) Boden, N.; Clements, J.; Jolley, K. W.; Parker, D.; Smith, M. H. J. Chem. Phys. 1990, 93, 9096.
Letters spectroscopy, combustion analysis, and melting point measurements. Full characterization will be provided in a forthcoming detailed report. The synthesis procedure of the materials was based on a room-temperature procedure reported previously.14 A 0.300 g portion of HFDePC was dissolved in 12.44 g of deionized water with 0.681 g of concentrated ammonium hydroxide (29 wt %). After this mixture was stirred for 60 min, 0.968 g of tetraethoxysilane was slowly added. Precipitation began within minutes of adding the silica precursor, but the solution was stirred gently with a magnetic stirrer at room temperature for 24 h. The as-synthesized particulate sample was filtered and dried in air for 2 days before acidic washing. A 150 mL portion of ethanol with 10 mL of concentrated HCl (36 wt %) was used twice to remove the surfactant. Extraction was confirmed by the loss of an isolated pyridinium band at 1490 cm-1 in Fourier transform infrared (FTIR). Powder X-ray diffraction patterns were recorded in the BraggBrentano geometry using a Siemens 5000 diffractometer, with a step size of 0.02° and a scanning speed of 0.008°/min. We utilized 1.540 98 nm Cu KR radiation and a graphite monochromator for the measurement. The scanning electron micrograph was obtained with a Hitachi S-900 instrument. The transmission electron micrographs were taken with a JEOL 2010F instrument. For TEM, the sample was suspended in acetone and deposited by dropping onto a lacey carbon grid. Nitrogen sorption was measured with a Micromeritics Tristar 3000. The sample was degassed under flowing nitrogen at 150 °C for 4 h before the measurement.
Acknowledgment. This material is based upon work partially supported by the National Science Foundation under Grant No. DMR-0210517. B.T. was supported by a Kentucky Research Challenge Trust Fund Fellowship. LA049474S (31) Asakawa, T.; Hisamatsu, H.; Miyagishi, S. Langmuir 1995, 11, 478.
