J. Phys. Chem. B 2004, 108, 11317-11320
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Dispersing Carbon Nanotubes in Water: A Noncovalent and Nonorganic Way Jin Zhu,*,† Masako Yudasaka,† Minfang Zhang,† and Sumio Iijima†,‡,§ Carbon Nanotube Project, Japan Science and Technology Corporation, c/o NEC, 34 Miyukigaoka, Tsukuba 305-8501, Japan, NEC Corporation, 34 Miyukigaoka, Tsukuba 305-8501, Japan, and Department of Physics, Meijo UniVersity, Tenpaku-ku, Nagoya 468-8502, Japan ReceiVed: February 9, 2004; In Final Form: May 18, 2004
A new noncovalent and nonorganic method has been utilized in dispersing single-walled carbon nanotubes (SWNTs) in water, up to single tube level. SWNTs, otherwise flocculated in aqueous solutions, were stabilized through the addition of highly charged nanoparticles. The dispersed SWNTs could be self-assembled into macroscopic materials in solutions by the application of external stimuli. By anchoring the solution SWNTs onto pyrene-modified Si/SiO2 surfaces, discrete, individual nanotubes have been observed by atomic force microscopy (AFM). This new type of SWNT aqueous dispersions might open up new avenues in the fields of nanostructured materials, biological sensing, and nanotube electronics.
I. Introduction
CHART 1
Single-walled carbon nanotubes (SWNTs) constitute a unique class of one-dimensional functional structures that promise their uses in a variety of areas, including electronics,1 optics,2 and structural materials.3 For many of those applications to be realized, a good dispersion of the materials, preferably up to single nanotube level, is of critical importance. Indeed, efforts have been directed toward such a goal by engineering the nanotube surfaces through either covalent4 or organic approaches.2d,5 However, for covalent functionalizations, the electronic properties of SWNTs are often severely compromised.4 On the other hand, noncovalent organic modifications could sometimes render the pristine nanotube surfaces inaccessible through polymer or surfactant wrapping,5 and as such, predesigned functional structures have to be incorporated into those agents in order to realize their uses. SWNTs, individually dispersed in water and with accessible surfaces for further manipulations, are important for interfacing these materials with biological functionalities and developing biological sensing schemes.6 Here we report a noncovalent and nonorganic strategy for stabilizing aqueous dispersions of SWNTs. The method draws upon a colloid stabilization paradigm recently developed by Lewis et al.7 In their work, negligibly charged, micrometer-sized spheres were stabilized by nanoparticle haloing through the addition of a critical volume fraction of highly charged species. Here we show that such charged particles could be employed in the dispersion of SWNTs in water, up to single nanotube level (Chart 1). It should be pointed out that SWNTs, one-dimensional nanostructures with extremely high aspect ratios and with lateral dimensions on the same order as those of the nanoparticle stabilizers, are fundamentally different from microspheres. As a result, the transition from microspheres to SWNTs is not trivial and might contribute to a further understanding of this new nanoparticle-based stabilization mechanism. * Corresponding author. E-mail:
[email protected]. † Japan Science and Technology Corp. ‡ NEC Corp. § Meijo University.
II. Experimental Section The dispersion experiments were carried out on purified HiPco SWNTs.8 HiPco SWNTs were purchased from Carbon Nanotechnologies Inc. HiPco SWNTs were purified through a two-step procedure: (1) heating in an O2 atmosphere at 300 °C and (2) removal of metal catalysts in HCl at 60 °C for 2 h. Typically, ∼0.4 mg of HiPco was weighed into a small vial. To this container certain volumes of ZrO2 nanoparticle solutions (Nyacol Products, Ashland, MA, 20 wt %, size 5-10 nm, pH ) 3.5, volume from 1 µL to 4 mL) and deionized water (the total volume of ZrO2 solution and water equals 8 mL) were added. The mixtures were then subjected to a sonication for 5 min by an ultrasonic processor (Dr. Hielscher UP400s) equipped with a microtip sonotrode of 3 mm in diameter. For some samples, the sonication process was repeated on a new solution made of half of the sonicated solution and equal volumes of
10.1021/jp0494032 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/09/2004
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Zhu et al.
Figure 1. SWNT dispersion prepared by (A) 800 µL of ZrO2 solution and (B) pure water.
the fresh nanoparticle solution and deionized water. After this procedure, the suspensions were allowed to stand for several hours to several days. The initial gravity-driven sedimentation of unstable, large SWNT bundles was very fast, and after several hours the dark supernatant appeared visually transparent. The supernatant dispersions were then carefully taken out with a pipet and subjected to further experiments if desired. The dispersion of SWNTs were characterized by ultravioletvisible-near-infrared (UV-vis-NIR) spectroscopy (PerkinElmer, Lambda 19), Raman spectroscopy (Jasco, NRS-2000), and atomic force microscopy (AFM) (Digital Instruments, Nanoscope III). III. Results and Discussion The outlook of a typical homogeneous supernatant dispersion is shown in Figure 1A. This initial dispersion (e.g., supernatant taken after 1 day) is stable on the order of hours, which is long enough for a variety of applications. After overnight standing for 15 h, a small amount of precipitates could be found at the bottom of the dispersion. This slow precipitation process might be driven by the gradual formation of large bundles through encountered discrete SWNTs. However, if the dispersion was left standing for an extended period of time (e.g., 7 days), the supernatant SWNTs became extremely stable, plausibly due to the full protection of SWNTs by the nanoparticles and hence negligible aggregation at this stage. Control experiments carried out with only water, at either neutral pH or pH ) 3.5, showed that large agglomerates would quickly settle down at the bottom of the suspension (Figure 1B). This suggests that ZrO2 plays a critical role in the stabilization of SWNT dispersions. In fact, the amount of ZrO2 is the key to the stability of SWNT dispersions. With ZrO2 volume below 1 µL, a large agglomerate of SWNTs settled at the bottom of the vial. With ZrO2 volume above 4 mL, the initially dispersed SWNTs could easily get aggregated in the suspension, and this would eventually lead to the total precipitation of the whole dispersion. Conceivably, the stabilization of SWNT dispersions in water should not be restricted to ZrO2. Other highly charged nanoparticles could likely serve the same purpose and as such should enable these nanoparticle-stabilized SWNT dispersions to be applied under a wide range of experimental conditions and to be biocompatible. Although the exact nanoparticle stabilization mechanism has yet to be further investigated, the utilization of the same type
Figure 2. Tapping mode image (height) of pyrene-modified Si/SiO2 after soaking for 1 day in (A) a SWNT dispersion prepared by 2 mL of ZrO2 solution (some individual SWNTs identified: 1, 1.29 nm; 2, 1.59 nm; 3, 1.70 nm) and (B) the same SWNT dispersion with an additional 15 µL of concentrated HCl (some individual SWNTs identified: 4, 1.43 nm; 5, 1.63 nm; 6, 1.44 nm).
of nanoparticles in the dispersion of nano-/meso-objects indicates that the stabilization is highly likely charge-based. These SWNT dispersions have proven to be easily manipulable due to their quasistable nature. In this regard, the self-assembly behaviors of dispersed SWNTs triggered by external stimuli have been investigated. For example, the addition of 1 mL of ZrO2 solution to a 1 mL dispersion of SWNTs prepared with 800 µL of ZrO2 resulted in a quick aggregation of SWNTs, and macroscopic, precipitated materials settled at the bottom of the container (Supporting Information). The same type of self-assembly event could be initiated by the addition of 50 µL of concentrated HCl solution (Supporting Information).9 The above flocculation processes might operate through a depletion pathway, where upon the addition of the above reagents, bared nanotubes start to interact with each other to form large bundles.10 It could be envisioned that, by introducing biological interconnects into this dispersed aqueous SWNT system, much more controlled and specific self-assembly processes should be possible.11 Still in other applications, nanoparticles could be directly incorporated as one component of the structural or functional materials.12 For example, sol-gel nanotube composites, which might be useful as structural materials or electrochemical sensing devices,13 could be prepared by simple addition of base solution into SWNT/ZrO2 dispersions. The grayish composite was generated through the hydrolysis, condensation of oxide nanoparticles and the rapid formation, precipitation of ZrO2 gels. In addition, hydrogel SWNT/
Dispersing Carbon Nanotubes in Water
J. Phys. Chem. B, Vol. 108, No. 31, 2004 11319 been observed on the surface (Figure 3).18 The dense SWNT matrix might prove useful for the preparation of macroscopic electrodes. IV. Conclusions In summary, aqueous dispersions of SWNTs could be stabilized through the addition of highly charged nanoparticle species. The dispersed SWNTs could be self-assembled in solutions and anchored onto functionalized Si/SiO2 surfaces. This new type of SWNT dispersions might open up new avenues in the fields of nanostructured materials, biological sensing, and nanotube electronics.
Figure 3. Tapping mode image (height) of pyrene-modified Si/SiO2 after soaking for 4 days in the SWNT dispersion prepared by 200 µL of ZrO2 solution.
nanoparticle composites could be generated by heating the dispersions above a critical temperature. The UV-vis-NIR absorption of SWNT dispersions14 revealed typical peak features at the E11 metal (440-645 nm) and E22 semiconductor (600-800 nm) transition areas, while the absorption features at the E11 semiconductor (830-1600 nm) transition area were not clearly observed (Supporting Information). This indicates that the electronic properties of SWNTs were preserved after the dispersion procedure.15 Raman scattering carried out on the nanotubes centrifuged from the supernatant indicates that all the SWNT Raman transition modes basically remain unaffected. The dispersion state of SWNTs in water was further characterized by AFM. Through a previously developed surface modification approach,16 we were able to anchor the solution SWNTs onto pyrene-functionalized Si/SiO2 surfaces and hence study the dispersed SWNTs. Importantly, a fraction of SWNTs on such surfaces, identified by AFM through the height profiling, are individual micrometer-long tubes and well-separated from each other (Figure 2A).17 The existence of individual SWNTs in the original dispersion implies that the SWNTs are protected from aggregating with one another by certain agents, in this case ZrO2 nanoparticles. Considering that ZrO2 nanoparticles are highly charged, the most likely stabilization mechanism is expected to be based on charge repulsion originating from those particles. There is no direct evidence of nanoparticle haloing of ZrO2 around SWNTs at the current stage. We believe that SWNTs are surrounded to a certain extent by those nanoparticles for the charge screening mechanism to work on the basis of the research carried out by Lewis’ group. The surface assembly process and SWNT density could be controlled by changing the accessibility of the solution SWNT surfaces. A more stable dispersion implies less accessibility of SWNT surfaces, since the stability comes from the interactions between SWNTs and charged nanoparticles. With the application of external stimuli, in this case HCl, the SWNT surface became more accessible and the assembly of SWNTs onto pyrene-modified Si/SiO2 surfaces was accelerated accordingly. After the same amount of time, the SWNT density on this surface is higher than that without any external triggers (Figure 2B). Alternatively, the increase in the SWNT density could be achieved through a prolonged soaking of the pyrene-modified Si/SiO2 in SWNT dispersions. For example, after 4 days of incubation, areas with self-assembled, highly inter-networked SWNT structures have
Supporting Information Available: The behavior of nanotube dispersions after the addition of external reagents and the UV-vis-NIR spectra of nanotube dispersions. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Dai, H. Acc. Chem. Res. 2002, 35, 1035. (b) Wind, S. J.; Appelzeller, J.; Martel, R.; Derycke, V.; Avouris, Ph. Appl. Phys. Lett. 2002, 80, 3817. (c) Bachtold, A. P.; Hadley, P.; Nakanishi, T.; Dekker, C. Science 2001, 294, 1317. (2) (a) Misewich, J. A.; Martel, R.; Avouris, Ph.; Tsang, J. C.; Heinze, S.; Tersoff, J. Science 2003, 300, 783. (b) Freitag, M.; Martin, Y.; Misewich, J. A.; Martel, R.; Avouris, Ph. Nano Lett. 2003, 3, 1067. (c) Hartschuh, A.; Pedrosa, H. N.; Novotny, L.; Krauss, T. D. Science 2003, 301, 1354. (d) O′Connell, M. J.; Bachilo, M. J.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C.; Ma, J.; Hauge, R. H.; Weisman, R. B.; Smalley, R. E. Science 2002, 297, 593. (3) (a) Vigolo, B.; Penicaud, A.; Coulon, C.; Sauder, C.; Pailler, R.; Journet, C.; Bernier, P.; Poulin, P. Science 2000, 290, 1331. (b) Ajayan, P. M.; Schadler, L. S.; Giannaris, C.; Rubio, A. AdV. Mater. 2000, 12, 750. (c) Geng, H. Z.; Rosen, R.; Zheng, B.; Shimoda, H.; Fleming, L.; Liu, J.; Zhou, O. AdV. Mater. 2002, 14, 1387. (4) (a) Kovtyukhova, N. I.; Mallouk, T. E.; Pan, L.; Dickey, E. C. J. Am. Chem. Soc. 2003, 125, 9761 and references therein. (b) Banerjee, S.; Kahn, M. G. C.; Wong, S. S. Chem. Euro. J. 2003, 9, 1898. (c) Kahn, M. G. C.; Banerjee, S.; Wong, S. S. Nano Lett. 2002, 2, 1215. (d) Banerjee, S.; Wong, S. S. J. Am. Chem. Soc. 2002, 124, 8940. (e) Georgakilas, V.; Kordatos, K.; Prato, M.; Guldi, D. M.; Holzinger, M.; Hirsch, A. J. Am. Chem. Soc. 2002, 124, 760. (f) Bahr, J. L.; Yang, J.; Kosynkin, D. V.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. J. Am. Chem. Soc. 2001, 123, 6536. (g) Sun, Y.; Wilson, S. R.; Schuster, D. I. J. Am. Chem. Soc. 2001, 123, 5348. (h) Zhao, W.; Song, C.; Pehrsson, P. E. J. Am. Chem. Soc. 2002, 124, 12418. (i) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95. (5) (a) Kang, Y.; Taton, T. A. J. Am. Chem. Soc. 2003, 125, 5650. (b) Star, A.; Stoddart, J. F.; Steuerman, D.; Diehl, M.; Boukai, A.; Wong, E. W.; Yang, X.; Chung, S. W.; Choi, H.; Heath, J. R. Angew. Chem., Int. Ed. Engl. 2001, 40, 1721. (c) Mitchell, C. A.; Bahr, J. L.; Arepalli, S.; Tour, J. M.; Krishnamoorti, R. Macromolecules 2002, 35, 8825. (d) Chen, J.; Liu, H. Y.; Weimer, W. A.; Halls, M. D.; Waldeck, D. H.; Walker, G. C. J. Am. Chem. Soc. 2002, 124, 9034. (e) O’Connell, M. J.; Boul, P.; Ericson, L. M.; Huffman, C.; Wang, Y. H.; Haroz, E.; Kuper, C.; Tour, J.; Ausman, K. D.; Smalley, R. E. Chem. Phys. Lett. 2001, 342, 265. (f) Bandyopadhyaya, R.; Nativ-Roth, E.; Regev, O.; Yerushalmi-Rozen. Nano Lett. 2002, 2, 25. (g) Czerw, R.; Guo, Z.; Ajayan, P. M.; Sun, Y.; Carroll, D. L. Nano Lett. 2001, 1, 423. (h) Moore, V. C.; Strano, M. S.; Haroz, E. H.; Hauge, R. H.; Smalley, R. E.; Schmidt, J.; Talmon, Y. Nano Lett. 2003, ASAP. (6) Star, A.; Steuerman, D. W.; Heath, J. R.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 2002, 41, 2508 and references therein. (7) Tohver, V.; Smay, J. E.; Braem, A.; Braun, P. V.; Lewis, J. A. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 8950. (8) Nikolaev, P.; Bronikowski, M. J.; Bradley, R. K.; Rohmund, F.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. Chem. Phys. Lett. 1999, 313, 91. (9) The solution ZrO2 nanoparticles were not affected under these conditions. In contrast, an addition of 500 µL of saturated NaCl solution to 1 mL of supernatant SWNT dispersion prepared by 200 µL of ZrO2 solution resulted in a slow appearance of iridescent color typical of aggregated SWNT/nanoparticles. (10) (a) Asakuwa, S.; Oosawa, F. J. Polym. Sci. 1958, 33, 183. (b) Vrij, A. Pure Appl. Chem. 1976, 48, 471. (c) Joanny, J. F.; Leibler, L.; de Gennes, P. G. J. Polym. Sci. Polym. Phys. Ed. 1979, 17, 1073. (d) Gast, A. P.; Hall,
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Zhu et al. (15) (a) The loss of E11 semiconductor transition absorption might be due to the acidic environment and/or the presence of bundled SWNTs. (b) Strano, M. S.; Huffman, C. B.; Moore, V. C.; O’Connell, M. J.; Haroz, E. H.; Hubbard, J.; Miller, M.; Rialon, K.; Kittrell, C.; Ramesh, S.; Hauge, R. H.; Smalley, R. E. J. Phys. Chem. B 2003, 107, 6979. (c) Note that SWNTs are better dispersed in sodium dodecyl sulfate (more single tubes) than in ZrO2, which is mainly reflected in the well-resolved NIR features in the 1000-1600 nm region. (16) (a) Zhu, J.; Yudasaka, M.; Zhang, M.; Kasuya, D.; Iijima, S. Nano Lett. 2003, 3, 1239. (b) Extensive rinsing has been used to wash away the nonadsorbed materials on the pyrene-modified SiO2/Si surface. (17) The existence of clean individual SWNTs on Si/SiO2 surfaces suggests that strong adsorption of nanoparticles is not likely under the current experimental conditions. (18) During this surface-anchoring process, SWNT that precipitated out of the dispersion was observed at the bottom of the vial.
