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Organic Crystal Templating of Hollow Silica Fibers Fumiaki Miyaji,† Sean A. Davis, Jonathon P. H. Charmant, and Stephen Mann* School of Chemistry, University of Bristol, Bristol, BS8 1TS, United Kingdom Received July 26, 1999 The template-directed sol-gel synthesis of organized inorganic matter offers a new and wide-ranging approach to useful materials with controlled architecture and porosity across a range of length scales.1 Organic templates with extended long-range structure, such as block copolymer lyotropic mesophases,2,3 colloidal crystals,4,5 and bacterial superstructures6 have been used to prepare monolithic forms of porous silica. In contrast, the direct synthesis of discrete inorganic architectures necessitates the use of dispersed organic supramolecular structures with commensurate dimensionality; for example, hollow fibers of amorphous silica have been prepared by template-directed processes using the external surface of self-assembled phospholipid fibers,7 viroid cylinders,8 or organic-gel filaments.9 The use of such specialized molecules, however, has the potential drawback that the costs associated with scale-up are likely to be highly prohibitive. Thus, the recent report by Nakamura and Matsui10 on the formation of silica tubes from ethanol/water/NH4OH/tetraethyl orthosilicate (TEOS) mixtures that contained small amounts of a simple organic acid (for example, racemic dl-tartaric acid), is particularly interesting. The authors speculate that chains of hydrogen-bonded dl-tartaric acid molecules act as a template for the deposition of the silica tube, but this seems highly unlikely considering the submicrometer dimension of the central lumen, and the unusual square-shaped cross section of the channel. For these reasons, we have undertaken further studies to elucidate the template mechanism. Here we show that the incipient crystallization of ammonium dl-tartrate filaments is responsible for patterning the tubular structure through specific interactions involving the {010} and {001} crystal faces. We use a similar mech* Author for correspondence. E-mail:
[email protected]. † Present address: Department of Material Science, Faculty of Science and Engineering, Shimane University, Matsue, Shimane 6908504, Japan. (1) Mann, S.; Burkett, S. L.; Davis, S. A.; Fowler, C. E.; Mendelson, N. H.; Sims, S. D.; Walsh, D.; Whilton, N. T. Chem. Mater. 1997, 9, 2300. (2) Go¨ltner, C. G.; Henke, S.; Weisenberger, M. C.; Antonietti, M. Angew. Chem. Int. Ed. 1998, 37, 613. (3) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (4) Imhof, A.; Pine, D. J. Nature 1997, 389, 948. (5) Antonietti, M.; Berton, B.; Go¨ltner, C. G.; Hentze, H.-P. Adv. Mater. 1998, 10, 154. (6) Davis, S. A.; Burkett, S. L.; Mendelson, N. H.; Mann, S. Nature 1997, 385, 420. (7) Baral, S.; Schoen, P. Chem. Mater. 1993, 5, 145. (8) Shenton, W.; Douglas, T.; Young, M.; Stubbs, G.; Mann, S. Adv. Mater. 1999, 11, 253. (9) Ono, Y.; Nakashima, K.; Sano, M.; Kanekiyo, Y.; Inoue, K.; Hojo, J.; Shinkai, S. Chem. Commun. 1998, 1477. (10) Nakamura, H.; Matsui, Y. J. Am. Chem. Soc. 1995, 117, 2651.
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anism to synthesize hollow silica fibers with triangular or rectangular-shaped channels by coupling TEOS hydrolysis/condensation reactions to the in situ crystallization of ammonium oxalate. Silica tubes were prepared according to previous work10 with several modifications.11 In general, the method involved the addition of concentrated NH4OH to unstirred ethanol-rich solutions of TEOS and dltartaric acid at respective molar ratios of 1:0.038. Washing and filtration of the resulting white precipitate gave a sample that consisted predominantly of flexible, open-ended hollow silica fibers with smooth continuous surface textures (Figure 1). Typically, the tubes were 200-300 µm in length, 0.1-1 µm in width, and contained a single 200-800-nm-wide pseudo-rectangular channel with uniform walls of amorphous silica, 30300 nm in thickness. Larger tubes, up to 5 µm in width, but with similar wall thickness could be produced by adding TEOS after the addition of NH4OH to an ethanol/water solution of dl-tartaric acid (Figure 1c). In many cases, SEM images indicated that the rectangular shape of the central channel was structurally preserved, even in fibers with 50-nm-thick walls and micrometersized channels, suggesting that the mineralized structure was highly uniform and consolidated. This was confirmed by TEM studies which showed the walls to consist of a thin coherent film of amorphous silica with no observable defects (Figure 2). Significantly, a white precipitate was observed when aqueous NH4OH was added to ethanol-rich solutions of dl-tartaric acid in the absence of TEOS. The precipitate formed rapidly in bulk solution and consisted of discrete needlelike crystals that were soluble in water but not ethanol (Figure 3a). The crystals had oblique end faces and cross-sectional shapes that were often identical to the pseudo-rectangular channel morphologies observed in the tubular silica structures (Figure 3b). These observations suggest that the in situ formation of ammonium dl-tartrate crystals is responsible for the silica tubular morphology through a mechanism involving inorganic deposition specifically on the external surface of the organic crystal filaments which dissolve during sample workup to leave hollow fibers, each with a geometrically shaped internal channel. Increases in the crystal width, therefore, should produce larger channels, and indeed decreasing the TEOS/dl-tartaric acid molar ratio to 1:0.1 had no significant effect on the wall thickness but increased the tube width to ∼5 µm. SEM images of the unwashed material, however, showed only a few examples of an internal structure associated (11) Typically, 0.73 g of TEOS was added to 5 mL of absolute ethanol, containing 0.02 g of dl-tartaric acid and 0.06 g of distilled/ deionized water, and the mixture allowed to stand for 30 min (molar ratio, TEOS:H2O:C2H5OH:dl-tartaric acid ) 1:1:24.3:0.038). Subsequently, 2 mL of 28% NH4OH was added, and the solution left to stand for 30 min. Stirring the samples gave lower yields and fractured tubes. The white precipitate was washed with a large amount of water on a brass test sieve (63 µm aperture) to remove colloidal aggregates. Alternatively, the precipitate was collected unwashed on a 0.2-µm nitrocellulose filter. Modifications to the starting compositions, stirring conditions (if any), and time allowed for TEOS hydrolysis prior to addition of NH4OH base, were systematically studied. Similar experiments with d- or l-tartaric acid and oxalic acid additives were undertaken.
10.1021/cm990449v CCC: $18.00 © 1999 American Chemical Society Published on Web 10/15/1999
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Figure 2. TEM image of an individual silica tube showing a continuous thin wall of amorphous silica and central lumen. Scale bar ) 50 nm.
Figure 1. SEM images of (a, top) bundles of flexible silica tubes prepared by addition of aqueous NH4OH to an ethanol/ water mixture of TEOS and dl-tartaric acid, scale bar ) 20 µm; (b, middle) higher magnification image showing smooth surface texture and rhomboidal central channel (arrow), scale bar ) 2 µm; and (c, bottom) silica tube with large geometric channel and thin mineralized walls. The tube is contaminated with secondary aggregates of colloidal silica. Scale bar ) 5 µm.
with the silica tubes (Figure 3c). Instead, most of the unwashed filaments appeared hollow, suggesting that the organic template might be removed by in situ dissolution arising from water molecules generated during the condensation reactions. In further support of this mechanism, powder XRD patterns of both washed and unwashed samples of the silica tubes showed a broad peak centered at 2θ ) 22°, corresponding to amorphous silica, but only the latter showed Bragg peaks at 2θ ) 14.2° and 33.1°. These
reflections were also present as intense peaks in the powder XRD pattern recorded from pure ammonium dltartrate crystals. Similarly, FTIR spectra of unwashed as-synthesized samples showed characteristic absorption bands for tartrate at 1600 and 2360 cm-1 that were absent for the washed material. Thermal analysis showed a distinct weight loss between 200 and 250 °C for the unwashed samples that was also observed for control crystals of ammonium dl-tartrate but not for the washed material (Figure 4). The amount of ammonium dl-tartrate in the as-synthesized silica material was estimated be less than 10 wt %. We propose, therefore, that the addition of aqueous NH4OH plays a dual role in the mechanism of silica tube formation: first as a source of OH- ions for the base catalysis of TEOS condensation, and second as an initiator of ammonium dl-tartrate crystallization, which in turn is responsible for the templated growth of individual filaments. These processes need to be chemically coupled if replication of the organic crystal is to be achieved with good fidelity. Moreover, interactions between the hydrolysis products and the incipient organic crystal are likely to be synergistic. For example, high levels of TEOS could selectively favor crystal growth along the needle axis by blocking sites on the side faces, which in turn produces silica tubes with reduced channel size and high aspect ratio (as shown in Figure 1, parts a and b). The kinetics of these processes is also important: for example, synthesis of the silica tubes was highly dependent on the extent of silica condensation present at the onset of ammonium dl-tartrate crystal growth. The latter occurred immediately on addition of NH4OH, whereas the former was dependent on the H2O/TEOS molar ratio and the amount of time the TEOS solution was allowed to stand prior to base catalysis; optimum conditions for our experiments were in the range of 0-1, and 30-60 min, respectively. In contrast, prolonged TEOS hydrolysis for 1 day, or after 30 min when the H2O/TEOS molar ratio
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Figure 4. TGA profiles for (a) crystalline ammonium dltartrate, (b) unwashed silica tubes, and (c) washed silica tubes. The weight % loss scale for profile a is on the right, and on the left for profiles b and c.
ammonium salts are significantly increased compared with the racemic compounds,12 such that the extent of crystallization on addition of NH4OH is much less. Moreover, our studies on the crystallization of these salts showed that they tended to aggregate rather than remain as discrete needle-shaped particles. In light of these factors, we reduced the period of TEOS aging to 10 min and were successful in synthesizing small amounts of 0.1-0.2-µm-wide hollow silica fibers by template-directed reactions on the surfaces of incipient crystals of enantiomeric ammonium d- or l-tartrate. Single-crystal X-ray diffraction analysis, in association with SEM images, was used to determine the crystallographic axes and faces associated with the ammonium dl-tartrate needlelike particles, and hence the nature of the crystal faces associated with the deposition of the silica tubes. The crystal structure,13 which has not been reported previously, consisted of a monoclinic cell with alternate layers of ammonium and tartrate ions perpendicular to the a axis ([100] direction) (Figure 5). Zone axis analysis of oriented single crystals indicated that the long morphological axis of the needleshaped crystal was coincident with the crystallographic a axis.14 Thus, the preferential crystal growth along this direction can be explained by the net dipole moment generated by alternation of the unicharged layers along the a axis. Presumably, the attachment energies on the (100) surface are sufficiently high that silicate interacFigure 3. SEM images of (a, top) needle-shaped ammonium dl-tartrate crystals formed in the absence of TEOS, scale bar ) 200 µm; (b, middle) high magnification image showing facetted crystal faces, scale bar ) 10 µm; and (c, bottom) assynthesized, unwashed silica tube showing evidence for the presence of an internal organic crystal template (arrow), scale bar ) 2 µm.
was increased to 4, gave only a gel phase of aggregated silica particles several micrometers in size. Likewise, no tubes were formed when the TEOS solution was used immediately after preparation, presumably due to the absence of condensed silicates during crystallization of the organic filaments, which subsequently coalesce into polycrystalline aggregates. Indeed, similar mismatches in the rates of crystallization and silica condensation could be the reason neither d- or l-tartaric acid were reported as being effective for hollow fiber formation.10 The water solubilities of these acids and their respective
(12) Water solubilities (Merck Index, USA, 1996; pp.1551-1552); d-, l-, and dl-tartaric acids, 139, 139, and 20.6 g L-1, respectively. Ammonium d-, l-, and dl-tartrate, 59, 59, and 2.7 g L-1. (13) A single needlelike crystal was mounted on a glass fibre in a Bruker SMART CCD area detector diffractometer with Mo KR radiation (λ ) 0.71073 Å). Intensities were integrated from several series of exposures, each exposure covering 0.3° in ω, and the total data set being a sphere (SMART (diffractometer control) and SAINT (integration) software, Bruker AXS Inc., Madison, WI, 1998. An absorption correction was applied based on multiple and symmetry equivalent measurements (Sheldrick, G. M. SADABS, A program for absorption correction with the Siemens SMART system; University of Go¨ttingen, Germany, 1996). The structure was solved and refined by standard methods (SHELXTL program system version 5.1; Bruker AXS Inc., Madison, WI, 1998). All hydrogen and non-hydrogen atoms were assigned isotropic or anisotropic displacement parameters, respectively. All atoms were refined without positional constraints. Crystal data: (NH4)2C4H4O6; M ) 184.16 g mol-1; monoclinic; a ) 4.8547(6), b ) 16.326(3), c ) 9.696(2) Å; β ) 91.69(2)°; T ) 293 K; space group ) P21/c; Z ) 4; µ ) 0.150 mm-1. A total of 7770 reflections were measured of which 1763 remained after merging symmetry equivalents (Rint ) 0.025), R1 ) 0.028 for 1400 reflections with I > 2σ(I). Full structural data are available from CCDC; reference number CCDC/135442.
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Figure 6. SEM of hollow silica fibers prepared in the presence of incipient ammonium oxalate crystallization, scale bar ) 5 µm.
Figure 5. Crystal structure of ammonium dl-tartrate projected along the c axis. In this view, silica deposition is associated with the slow-growing (010) side faces that lie normal to the b axis (shown as the y axis of the coordinate system), but not the unstable dipolar (100) faces, which are perpendicular to the a axis (x axis) direction of rapid growth. The drawing shows the presence of (010)-surface-adsorbed ammonium ions that could interact with silicate species and induce silica nucleation.
tions are negligible compared with the incorporation of tartrate anions, with the consequence that the face remains free of silica in the presence of TEOS. In contrast, the slow-growing long side faces, which were indexed as (010) and (001) faces from the zone axis diffraction patterns, are electrically neutral with regularly spaced NH4+ ions that can interact with silicate anions and thereby lower the interfacial energy for silica nucleation (Figure 5). Differences in surface roughness between the side and end faces could also account for the preferential growth along the a axis. (14) Zone axis analysis was used to determine the morphological axis of an individual needlelike crystal. The crystal was mounted on a glass fiber with the needle axis approximately coincident with that of the fiber. Zone axis photographs of the hk0 and h0l zones were recorded. Following zone axis photography, face indexing of the longest sides of the crystal was carried out. This showed the thinner edge to be perpendicular to the [010] direction with the largest face normal to the [001] direction, coincident with the b and c axes of the unit cell, respectively. (15) A total of 0.4 g of oxalic acid was dissolved in 5 g of H2O, and 2 mL of 28% NH4OH was added without stirring to give a turbid suspension of small ammonium oxalate needle-shaped crystals. A total of 0.365 g of TEOS was added immediately after the NH4OH solution, and the mixture stirred vigorously for 1 h. The final TEOS:H2O:oxalic acid molar ratio was 1:199.3:2.5. The products were washed with a large amount of water and collected on a brass test sieve.
Finally, we have extended the above studies to the synthesis of hollow silica fibers using needle-shaped organic templates formed from the incipient crystallization of ammonium oxalate hydrate (orthorhombic; P212121; a ) 0.8035 nm, b ) 1.031 nm, c ) 0.3801 nm). Because particulate crystals were formed from ethanol/ water mixtures, we used pure aqueous solutions to obtain a needlelike habit, and modified the sol-gel procedure such that TEOS was added immediately after the mixing of aqueous solutions of NH4OH and oxalic acid.15 The resulting precipitate contained open-ended silica tubes that were 50-100 µm in length, and 1-7 µm in width. Each tube consisted of a single 0.7-6.5µm-wide rectangular- or triangular-shaped channel with a 300-nm-thick wall of amorphous silica (Figure 6). In conclusion, the incipient crystallization of needlelike forms of organic salts, such as ammonium dltartrate or ammonium oxalate, can be used for the surface-specific templating of sol-gel reactions to produce hollow silica fibers and filaments with geometrically shaped channels. By judicious control of both the crystallization and condensation processes, flexible tubes with continuous wall structures can be produced. The development of this method for the synthesis of functionalized silicas, as well as other metal oxide materials is currently in progress. Acknowledgment. We are grateful to the Ministry of Education, Science, Sports and Culture, Japan, for a 1998 Fellowship to F.M., and The Leverhulme Trust for a postdoctoral fellowship to S.A.D. We thank Barry Chapman (University of Bath) for help with powder X-ray diffraction studies. Constructive comments on the manuscript from one of the referees, particularly with regard to the potential influence of TEOS products on crystal morphology, are greatly appreciated. CM990449V
