© Copyright 1999 American Chemical Society
OCTOBER 12, 1999 VOLUME 15, NUMBER 21
Letters Formation of Huge Length Silica Nanotubes by a Templating Mechanism in the Laurylamine/ Tetraethoxysilane System Motonari Adachi,* Toshio Harada, and Makoto Harada Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan Received April 22, 1999. In Final Form: August 12, 1999
Nanosize bundles of huge lengths of silica-nanotubes were synthesized by a surfactant-assisted templating mechanism in a laurylamine hydrochloride/tetraethoxysilane system with the aid of trimethylsilylation treatment. The transmission electron microscopy images confirmed the formation of the bundles of silica nanotubes mentioned above. The trimethylsilylation treatment prevents the condensation reaction of silanol groups between different bundles, provides very long bundles of nanosize tubules, and also removes surfactants from the inside of silica nanotubes without calcination.
1. Introduction Formation of nanotubes offers a prospect of useful nanotechnological applications, e.g., sensor/actuator arrays,1,2 nanowires,3 and optoelectric devices.4,5 Carbon nanotubes play an important role in development of such nanotechnology.6,7 Since researchers at Mobil demonstrated mesoporous silicate and alminosilicate materials (M41S),8,9 a variety of mesoporous materials have been * To whom correspondence may be addressed. Fax: +81-77438-3524. E-mail:
[email protected]. (1) Sakai, H.; Baba, R.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. 1995, 99, 11896-11900. (2) Lindner, E.; Cosofret, V. V.; Ulfer, S.; Buck, R. P. J. Chem. Soc., Faraday Trans. 1993, 89, 361-367. (3) Li, W. Z.; Xie, S. S.; Qian, L. X.; Chang, B. H.; Zou, B. S.; Zhou, W. Y.; Zhao, R. A.; Wang, G. Science 1996, 274, 1701-1703. (4) Trau, M.; Yao, N.; Kim, E.; Xia, Y.; Whitesides, G. M.; Aksay, I. A. Nature 1997, 390, 674-676. (5) Fendler, J. H. Chem. Mater. 1996, 8, 1616-1624. (6) Tans, S. J.; Verschveren, A. R. M.; Dekker, C. Nature 1998, 393, 49-52. (7) Che, G.; Lakshmi, B. B.; Fisher, E. R.; Martin, C. R. Nature 1998, 393, 346-349.
synthesized.10-14 These materials have been obtained as integrated forms of unit structure. Single silica nanotubes or bundles of a few nanotubes, however, have not yet been synthesized. Nakamura and Matsui15 obtained silica gel tubes by the sol-gel method. Lin and Mou16 and Trau et al.4 synthesized tubular structure composed of MCM418,9 (8) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710-712. (9) Beck, J. S.; Vartuli, J. C.; 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, 10834-10843. (10) Ramann, N. K.; Anderson, M. T.; Brinker, C. J. Chem. Mater. 1996, 8, 1682-1701. (11) Tanav, P. T.; Pinnavaia, T. J. Science 1996, 271, 1267-1269. (12) Attard, G. S.; Glyde, J. C.; Goltner, C. G. Nature 1995, 378, 366-368. (13) Lu, Y.; Gabguli, R.; Drewien, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, W.; Guo, Y.; Soyez, H.; Dunn, B.; Hoang, M. H.; Zink, J. I. Nature 1997, 389, 364-368. (14) Lin, H.-P.; Cheng, S.; Mou, C.-Y. Chem. Mater. 1998, 10, 581589. (15) Nakamura, H.; Matsui, Y. J. Am. Chem. Soc. 1995, 117, 26512652. (16) Lin, H.-P.; Mou, C.-Y. Science 1996, 273, 765-768.
10.1021/la9904859 CCC: $18.00 © 1999 American Chemical Society Published on Web 09/16/1999
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Figure 1. Results of SAXS measurements during the reaction. Scattering vector is represented by q. The sample solution was prepared as follows: A 0.1 M LAHC aqueous solution was reacted with TEOS. The mole ratio of TEOS to LAHC was 4. Keys show experimental points, and the solid curves are calculated by assuming the presence of a coaxial double cylinder with outer hydrophilic shell diameter d and inner hydrophobic core diameter di and the length (l) distribution of exponential type, (1/L) exp(-l/L). Variations in d, di, and the mean length L with reaction time t were as follows: (O) t ) 1 h 40 min, d ) 5.0 nm, di ) 3.25 nm, L ) 5.0 nm; (b) t ) 4 h 35 min, d ) 5.0 nm, di ) 3.25 nm, L ) 12.0 nm; (3) t ) 6 h 4 min, d ) 5.0 nm, di ) 3.25 nm, L ) 30.0 nm; (0) t ) 7 h 35 min, d ) 5.0 nm, di ) 3.25 nm, L ) 60.0 nm; (4) t ) 9 h 4 min, d ) 5.0 nm, di ) 3.25 nm, L ) 80.0 nm. The electron densities of the core F1, shell F2 from the geometry, and aqueous solution F3 were calculated as 277, 342, and 332 electrons/nm3, respectively.
by the surfactant templating method. But, their diameters are all micrometer size and much larger than nanosize. It is interesting to examine whether a single silica
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nanotube of real nanosize or a bundle of a few nanotubes can be formed through a surfactant-assisted templating mechanism from the viewpoints of making new materials. There are still no sufficient general models for establishing the mechanistic understanding of the synthesis processes, which is key for rational design of new materials. In most of the reported nanostructure formations by a templating mechanism, either precipitation occurred in the early stage of the formation processes or the materials were synthesized under the conditions of high surfactant concentrations. These circumstances made it difficult to measure the variation in shape and size of produced materials during the microstructure formation, preventing elucidation of the formation processes. In this paper, we present formation of bundles of silica nanotubes in the laurylamine hydrochloride (LAHC)/ tetraethoxysilane (TEOS) system followed by trimethylsilylation treatment. In this system, gelation occurs homogeneously. The shape and size of LAHC/TEOS assemblies can be measured by small-angle X-ray scattering (SAXS) up to the gelation. The formation processes are discussed briefly based on the SAXS results together with transmission electron micrographs and isotherms of nitrogen adsorption. 2. Experimental Section We used a LAHC/TEOS system. TEOS was added to a 0.1 M LAHC aqueous solution (pH 4.5), and the reaction was started in the stirring cell at 313 K. The mole ratio of TEOS to LAHC was adjusted to 4-12. Characterization of the produced materials was made by SAXS, transmission electron microscopy (TEM), scanning electron microscopy (SEM), isotherm of nitrogen adsorption, and 29Si NMR measurements.
3. Results and Discussion The gel formation reaction proceeded as follows. Since TEOS does not dissolve in the aqueous phase, we obtained an emulsified solution in the early stage of reaction. After 2-3 h, the solution became transparent because of the
Figure 2. TEM images of produced materials. All samples shown here were obtained under the following conditions: [LAHC] 0.0918 M and mole ratio [TEOS]/[LAHC] ) 4. All scale bars are 100 nm. (a) Dried gel. Single nanotube (shown by the arrow) and bundle of several nanotubes (upper side) were observed. (b) Calcined gel at 773 K. Bundles of dozens of fine nanotubes were observed in random configuration.
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Figure 3. TEM images of trimethylsilylated dried materials. Scale bar is 100 nm. Sample solution at the reaction time of 9 h was trimethylsilylated. Then the sample was dried at room temperature. Huge length bundles (diameter about 10 nm) composed of several silica nanotubes were observed.
dissolution of TEOS into the aqueous solution due to hydrolysis. After 13-14 h, homogeneous gelation was observed. Figure 1 shows the results of SAXS measurements during the reaction. Before TEOS was added, globular aggregates were observed in dilute aqueous solutions of LAHC (0.1 M). After 1 h and 40 min from the addition of TEOS, the aggregates became short rods with an outer diameter (d) of 5.0 nm, inner hydrophobic core diameter (di) of 3.25 nm, and mean length (L) of 5.0 nm. After 4 h and 35 min, the aggregate shape became rods with d ) 5.0 nm, di ) 3.25 nm, and L ) 12.0 nm. Thereafter, the rods were elongated, keeping the diameter unaltered (d ) 5.0 nm, di ) 3.25 nm). In all experiments of different LAHC concentrations from 0.05 to 0.15 M, the rods with outer diameter 5.0 nm and inner diameter 3.25 nm were always formed in a few hours after adding TEOS and thereafter the rods were elongated, maintaining the same diameter. The TEM image of dried gel indicates the presence of single nanotube (shown by the arrow in Figure 2a). The diameter of the nanotube was around 5 nm. The center of the tube is white and the both edges are black, indicating the formation of single silica nanotube. We can also see bundle of several nanotubes on the upper side of Figure 2a. The TEM image of the materials after calcination (Figure 2b) also shows the presence of long bundles of silica nanotubes in random configuration. The produced silica nanotubes have silanol groups on the surface of the tube and a single tube is converted to their bundles by sticking with each other through silanol condensation between different rods and finally becoming a gel. The silanol condensation also proceeds during the calcination and drying processes. If the silanol groups can
be made inactive in the course of the formation processes, there is a good possibility to make huge lengths of silica nanotubes. We tried trimethylsilylation. The strategy is as follows. First, silica tubes with a nanosize diameter before gelation are prepared. Then silanol groups are made inactive by trimethylsilylation. The procedure of trimethylsilylation is as follows.17 Trimethylchlorosilane (60 mL), isopropyl alcohol (50 mL), and water (50 mL) were mixed well. The sample solution containing silica materials (10 mL) was added to the mixture, and the resultant solution was kept for 2 days at room temperature under stirring. The silica materials were extracted to the alcohol phase. Then the samples were dried at 313 K and analyzed by TEM, nitrogen adsorption isotherm, and 29Si NMR. A TEM image of trimethylsilylated sample is shown in Figure 3. Bundles of huge length silica nanotubes are observed. The diameter is about 10 nm, and the length is beyond a micrometer. Since trimethylsilylation prevents the condensation reaction of silanol groups between different bundles, we can prepare the individual long bundles as shown in Figure 3. Nitrogen adsorption isotherms of the calcined gel at 773 K and the trimethylsilylated samples dried at room temperature were measured (Figure 4). The hysteresis of isotherms confirms the existence of mesoporous structures for both samples. The BET surface area was evaluated as 856 m2/g for calcined sample and 873 m2/g for trimethylsilylated sample, respectively. The pore volumes for both materials were 0.547 and 1.311 mL/g, respectively. The pore size distribution of these samples was obtained by (17) Kamiya, K.; Yoko, T.; Sakka, S. Yogyo Kyokaishi 1984, 95, 242247.
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Figure 5. 29Si NMR spectrum of the trimethylsilylated sample. Q3 and Q4 are defined as the number of 29Si of the type (SiO)3tSi-OH and (SiO)3tSi-O-Sit, respectively. Trimethylsilylated groups (12.0 ppm) were observed.
indicate that the trimethylsilylation treatment removed the surfactants contained in the silica nanotubes without calcination. These peak radii are close to the values measured by SAXS. The 29Si NMR for trimethylsilylation sample is shown in Figure 5. The peak around 12.0 ppm corresponds to trimethylsilylated groups,18 and Q3 (-101 to -102 ppm.) and Q4 (-110 to -111 ppm) represent the number of 29Si of the type (SiO)3tSi-OH and (SiO)3tSi-O-Sit, respectively. The ratio of trimethylsilylated group content against that of (Q3 + Q4) for the sample shown in Figure 5 was 0.43, indicating that silanol groups were changed to trimethylsilane. This value is much higher than that reported for trimethylsilylated Fisher S-157 silica gel 0.089,18 confirming that the obtained sample in our trimethylsilylation experiment had very high surface area in comparison with silica gel. The experimental results mentioned above confirm the formation of long bundles of silica nanotubes due to trimethylsilylation of the tubes in the LAHC/TEOS reaction system.
Figure 4. Nitrogen adsorption isotherm (a) and pore size distribution (b) for the calcined gel at 773 K (b, adsorption; O, desorption) and for the trimethylsilylated dried sample at room temperature (2, adsorption; 4, desorption). The mole ratio of TEOS to LAHC was 4. Hysteresis of isotherms in Figure 4a confirms the existence of mesoporous structures. The peak radius was 1.8 nm for the calcined gel and 1.8-2.2 nm for the trimethylsilylated materials.
the Dollimore and Heal method (Figure 4b). The peak radius of calcined materials was 1.8 nm and that for trimethylsilylated sample was 1.8-2.2 nm. These results
Acknowledgment. The authors gratefully acknowledge Professors T. Kobayashi and S. Isoda for their assistance with TEM experiment and Professor H. Tamon for his assistance with the nitrogen adsorption experiment. We also thank Professor H. Kozuka for helpful discussions. We acknowledge the financial support from a Grant in Aid for Scientific Research (Fundamental Research B), Ministry of Education, Science, Sports and Culture, Japan. LA9904859 (18) Fyfe, C. A.; Gobbi, G. C.; Kennedy, G. J. J. Phys. Chem. 1985, 89, 277-281.
