Formation of Single-Wall Carbon Nanotube Superbundles

Since their discovery in 1993,1,2 carbon single-wall nanotubes (SWNTs) have been of great interest because of their expected novel electronic,3 mechanical,4 and gas adsorption properties.5 Difficulties arise in measuring these various properties because the SWNTs currently available for analyses have very small bundle sizes (1020 nm) and are typically in random orientations. A vital step for the industrial application of SWNTs is to align the individual tubes into bundles of a size that allows facile measurement of the various physical and chemical properties. This paper is the first to illustrate a set of procedures for the preparation and isolation of aligned SWNT “superbundles”. The synthesis of raw SWNT soot is similar to the pulsed laser vaporization technique described by Guo et al.6 A Nd:YAG laser (1064 nm) was employed to synthesize the carbon nanotubes from 1.2 atom % metaldoped (50:50 Co/Ni) pressed graphite targets.7 The targets were placed in a quartz tube that was heated to a temperature of 1200 °C in a clam-shell furnace. With the laser operating at a frequency of 10 Hz, the laser power and beam size were adjusted to provide ∼20 J/(pulse cm2) at a ∼450-ns pulse width. An argon flow of 100 sccm at 500 Torr was maintained through the reaction vessel for the duration of the synthesis. The raw soot was purified by refluxing in 3 M nitric acid for 16 h, filtering and washing with deionized water on a polytetraflouroethylene (PTFE) filter, and then heating the obtained paper in air for 30 min at 550 °C. This procedure results in tubes of greater than 98 wt % purity when target material is not sputtered and trapped in vaporized soot.7 A representative TEM image of the resultant pure tubes is shown in Figure 1a. The random orientation and small size of the long bundles are apparent. The SWNTs exist as a random tangle because of the conditions under which they are synthesized and purified. The tubes are formed within the high-temperature plasma generated by the laser striking the graphite target, and their formation is rapidly quenched as the tubes diffuse out of the plasma plume. The resulting SWNTs exist in small bundles and are accompanied by other graphitic and amorphous carbon fractions as well as metal nanoparticles. The purification process succeeds in removing the non-nanotube carbon fractions and the metal, but the bundles are still randomly oriented with diameters of only ∼5-20 nm. We have discovered that through the use of ultrasound and polar solvents. it is possible to unwind the intertwined SWNT bundles. Subsequent removal of the ultrasonic perturbation results in realignment and collapse of the SWNTs into the much larger superbundle configuration. A 1.0-mg sample of purified SWNTs was placed in a cylinder containing 10 mL of deionized water or other polar solvents or solvent mixtures. A Heat SystemsUltrasonics Inc. model w-220F Cell Disrupter sonic horn was submersed into the solution and the power was slowly increased to 90 W/cm2. The ultrasonic agitation was continued for a maximum of 120 min. Normally, * Author for correspondence. † On sabbatical from Chemistry Department, Rochester Institute of Technology, 85 Lomb Memorial Drive, Rochester, NY 14623-5604. (1) Bethune, D. S.; Kiang, C.-H.; Vries, M. S. d.; Gorman, G.; Savoy, R.; Vasquez, J.; Beyers, R. Nature 1993, 363, 605. (2) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (3) Saito, R.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Phys. Rev. B 1992, 46, 1804. (4) Yakobson, B. I.; Brabec, C. J.; Bernhole, J. Phys. Rev. Lett. 1996, 384, 2511-2514. (5) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377. (6) Guo, T.; Nikolaev, P.; Thess, A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1995, 243, 49. (7) Dillon, A. C.; Gennett, T.; Jones, K. M.; Alleman, J. L.; Parilla, P. A.; Heben, M. J. Adv. Mater. 1999, 11, 1354. Figure 1. TEM images of (a) purified single-wall nanotubes with small diameter bundles, (b) SWNT superbundles extracted from a 50:50 water/methanol solvent mixture, and (c) a SWNT superbundle at higher magnification. 599 Chem. Mater. 2000, 12, 599-601