Fullerene Pipes
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E. W. Wong, P. E. Sheehan, C. M. Lieber, Science 277, 1971 (1997); Yakobson B. I., Smalley R. E., Am. Sci. 85, 324 (1997).
A. G. Rinzler et al. Appl. Phys. A in press.
Y. J. Chen, et al., Adv. Mater. 8, 1012 (1996); Ebbesen T. W., Ajayan P. M., Hiura H., Tanigaki K., Nature 367, 519 (1994).
In a typical procedure a raw sample of nanotubes (8.5 g) was first refluxed in 1.2 liters of 2.6 M nitric acid for 45 hours. Upon cooling the solution was transferred to polytetrafluoroethylene centrifuge tubes and spun at 2400 g for 2 hours. The supernatant acid was decanted off replaced by deionized water and vigorously shaken to resuspend the solids followed by a second centrifuge-decant cycle. The solids were resuspended in 1.8 liters of water with 20 ml of Triton X-100 surfactant (Aldrich) and adjusted to pH 10 with sodium hydroxide. The suspension was then transferred to the reservoir of a tangential flow filtration system (MiniKros Lab System; Spectrum Laguna Hills CA). The filter cartridge used (M22M 600 01N; Spectrum) had mixed cellulose ester hollow fibers of 0.6-mm diameter 200-nm pores and a total surface area of 5600 cm 2 . The buffer consisted of 44 liters of 0.2 volume % Triton X-100 in water of which the first 34 liters were made basic (pH 10) with sodium hydroxide and the final 10 liters were at pH 7. The cartridge inlet pressure was maintained at 6 psi. A control valve was added to the exit so that the outflow rate was restricted to 70 ml min −1 .
These cut tubes were prepared in a two-step process: cutting and polishing. In a typical example 10 mg of the purified SWNT “bucky paper” (Fig. 1 B) was suspended in 40 ml of a 3:1 mixture of concentrated H 2 SO 4 /HNO 3 in a 100-ml test tube and sonicated in a water bath [Cole Palmer (Verona Hills IL) model B3-R 55 kHz] for 24 hours at 35° to 40°C. The resultant suspension was then diluted with 200 ml of water and the larger cut SWNTs were collected on a 100-nm pore filter membrane (type VCTP; Millipore) and washed with 10 mM NaOH solution. The cut tubes were then further polished by suspension in a 4:1 mixture of concentrated H 2 SO 4 :30% aqueous H 2 O 2 and stirring at 70°C for 30 min. After filtering and washing again on a 100-nm pore filter the cut nanotubes were suspended at a density of 0.1 mg/ml in water with the aid of 0.5 weight % Triton X-100 surfactant.
T. J. Mason and J. P. Lorimer Sonochemistry: Theory Applications and Uses of Ultrasound in Chemistry (Halsted New York 1988).
Taking into account AFM tip broadening the possibility remains that some of these apparent individual tubes are actually “rafts” of several parallel tubes in van der Waals contact lying on the HOPG surface.
A 20-μl sample of cut nanotube suspension (0.07 mg/ml) in 0.5% aqueous Triton X-100 was injected into an FFF instrument (Model F-1000-FO FFFractionation; LLC Salt Lake City UT) operating with 0.007% Triton X-100 in water mobile phase at 2 ml min −1 and a cross-flow rate of 0.5 ml min −1 .
Electrodeposition was performed by placing 20 μl of the nanotube suspension on the surface of a freshly cleaved HOPG substrate (Advanced Ceramics Cleveland OH) confining the droplet within a viton O-ring (4-mm outer diameter 1.7 mm thick) capping the trapped suspension with a stainless steel electrode on top of the O-ring and applying a steady voltage of 1.1 V for 6 min. When suspended in water the nanotubes are negatively charged and are therefore driven by the electric field onto the HOPG surface. After deposition the HOPG-nanotube surface was washed with methanol on a spin coater to remove the water and Triton X-100 surfactant.
Bubke K., Gnewuch H., Hempstead M., Hammer J., Green M. L. H., Appl. Phys. Lett.71, 1906 (1997); A. G. Rinzler and R. E. Smalley, unpublished data.
K. Kinoshita Carbon Electrochemical and Physicochemical Properties (Wiley New York 1988) pp. 199–201.
We thank K. Smith and V. Colvin for helpful discussions. Supported by the NSF the Office of Naval Research the Advanced Technology Program of Texas and the Robert A. Welch Foundation.