Synthesis of Multiwalled Carbon Nanotubes through a Modified Wolff-Kishner Reduction Process Wenzhong Wang,* Bed Poudel, D. Z. Wang, and Z. F. Ren* Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467 Received September 28, 2005; E-mail: renzh@bc.edu; wangwm@bc.edu Since the discovery of carbon nanotubes (CNTs) in 1991 by Iijima, 1 there has been great interest in developing new methods for the synthesis of CNTs due to their potential applications in various technologies. 2-4 Methods developed so far include arc discharge, 5 laser vaporization, 6,7 pyrolysis, 8,9 chemical vapor deposi- tion, 10 high-temperature hydrothermal 11,12 and low- and high- temperature solvothermal. 13,14 Here we report the synthesis of multiwalled CNTs (MWCNTs) by reducing ethyl alcohol with NaBH 4 in a high concentration, strong basic solution without intentionally adding the conventional catalysts of Fe/Co/Ni, fol- lowing a modified Wolff-Kishner reduction process. This technique opens a new route for the synthesis of CNTs. The Wolff-Kishner reduction normally refers to a base-catalyzed process that leads ultimately to the production of alkane from the corresponding aldehydes or ketones, 15-17 which involves first converting aldehydes or ketones to the corresponding hydrazone and then decomposition of this intermediate in the presence of strong basic conditions to yield the reduced alkyl derivative (the corresponding alkane) and nitrogen. 15,16 In this report, we utilize a similar process to directly synthesize MWCNTs through the reduction of ethyl alcohol with NaBH 4 under a strong basic solvent with high NaOH concentration. In a typical synthesis, 80 mL of ethyl alcohol (90%), 4.2 g of NaBH 4 (99.99%), and 15 mL of 10 M NaOH (97%) solution were added to a 250 mL flask. The mixtures were stirred with a magnetic stirrer for 30 min and then transferred to a Parr reactor (model 4750, Parr Company, Moline, IL) with a capacity of 125 mL. The Parr reactor was sealed and then kept at 180 °C for 20 h in a furnace and then cooled to room temperature. The products were washed with alcohol and distilled water several times and then dried in a vacuum oven at 60 °C for 10 h. Panels a and b of Figure 1 show the low- and medium- magnification field emission scanning electron microscopy (FE- SEM) images of the MWCNTs, respectively, demonstrating the diameters of 10-40 nm and lengths of up to several tens of micrometers. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were used to further characterize the microstructure of the as-synthesized MWCNTs. Panels a-d of Figure 2 show the typical TEM and HRTEM images of the as-prepared MWCNTs. The nanotubes exhibit a straight morphology, as shown in a and c of Figure 2. In general, the nanotubes have an outer diameter in the range of 10-40 nm and a length of up to 25 μm. The inner diameters are 3-6 nm. It is clearly seen that some nanotubes have a bamboo-like structure, as shown in b and d of Figure 2. The high- magnification microstructure studies of the tips show that almost all nanotubes have closed ends with different shapes. Figure 2c reveals that some CNTs have a conical tip structure. The HRTEM studies indicate that these conical tips are end-closed and have bamboo-shaped structure (Figure 2d). Figure 3a shows the HRTEM image of an individual MWCNT, indicating the high crystallinity of the walls. The walls are composed of graphite sheets aligned parallel to the tube axis. The inter-wall spacing is about 0.34 nm, consistent with the standard spacing. The selected area electron diffraction (SAED) pattern shown in Figure 3b also exhibits that the nanotube is well-crystallized. During TEM examinations of the as-prepared CNTs, some CNTs with very small inner diameters were detected. In a low-magnifica- tion image, the CNT looks like a solid nanowire (Figure 4a). The high-magnification image demonstrates that the inner diameter is about 2 nm and uniform along its length axis (Figure 4b), and the outer diameter is about 20 nm. The efforts have been made to rationalize the formation process of CNTs. Shi and co-authors 18 used tetraethoxysilane (TEOS) as Figure 1. (a) Low- and (b) medium-magnification SEM images of the as-prepared MWCNTs. Figure 2. TEM and HRTEM images of the as-prepared carbon nanotubes. (a and b) Some carbon nanotubes have a bamboo-like structure. (c and d) Some carbon nanotubes have a conical tip structure. Published on Web 12/01/2005 18018 9 J. AM. CHEM. SOC. 2005, 127, 18018-18019 10.1021/ja056654v CCC: $30.25 © 2005 American Chemical Society