Low-temperature hydrothermal synthesis of multiwall carbon nanotubes

Since the discovery of carbon nanotubes (CNTs) by Iijima [1], an endless list of potential applications of CNTs have been proposed. Consequently, a worldwide research interest has been focused on the growth of CNTs to realize their potential applications [2]. Numerous methods for the synthesis of CNTs have been developed [3–11]. Among the variety of methods, chemical vapor deposition (CVD) is the most widely used to synthesize CNTs. Although CVD offers the benefit of significantly lower synthesis temperatures than arc-discharge and laser ablation techniques, it still requires a growth temperature of 600–950 C. Recently, it has been reported that high-temperature hydrothermal process is an alternative route for the synthesis of CNTs [12,13]. Yoshimura and co-workers synthesized CNTs using polyethylene (PE), ethylene glycol (EG) and other sources with and without catalysts Fe/Co/Ni under hydrothermal conditions at 700– 800 C and 60–100 MPa [12,13]. However, the temperature is comparable with the CVD technique. Hence, it is still a challenge to develop a new approach for the synthesis of CNTs at a low temperature. To date, the lowest-reported temperature for the synthesis of multiwall carbon nanotubes (MWCNTs) is 175 C, from decomposition of CCl4 using iron-encapsulated polypropyleneimine dendrimers as a catalyst in supercritical carbon dioxide medium [14]. In this letter, we report the synthesis of MWCNTs by the decomposition of polyethylene glycol (PEG; MW 20,000) in a basic aqueous solution with high concentration of NaOH under hydrothermal conditions at a temperature as low as 160 C, which is the lowest to our knowledge, without the addition of catalyst Fe/Co/Ni. The morphologies and microstructures of the as-prepared MWCNTs were studied with transmission electron microscopy (TEM, JEOL 2010F). In a typical synthesis, 80 mL ethyl alcohol, 10 mL distilled water, 7 g NaOH and 2 g PEG were added to a 250 mL flask. The mixtures were stirred in 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 160 C for 20 h in a furnace, and then cooled down to room temperature. The products were washed with alcohol and distilled water for several times, and then dried in a vacuum oven at 60 C for 10 h. Fig. 1 shows the TEM images of the as-synthesized MWCNTs at different magnifications. It can be seen that the MWCNTs have outer diameters ranging from 9 to 19 nm and inner diameters ranging from 4 to 8 nm. It is worth pointing out that the diameters of the as-prepared MWCNTs by the current low-temperature hydrothermal route are much smaller than those [14] Vesselenyi I, Niesz K, Siska A, Konya Z, Hernadi K, Nagy JB, et al. React Kinet Catal Lett 2001;74:329–36. [15] Willems I, Konya Z, Fonseca A, Nagy JB. Appl Catal A: Gen 2002;229:229–33. [16] Yuan L, Li T, Saito K. Carbon 2003;41:1889–96. [17] Emmenegger C, Bonard JM, Mauron P, Sudan P, Lepora A, Grobety B, et al. Carbon 2003;41:539–47. [18] Zhang M, Yudasaka M, Bandow S, Lijima S. Chem Phys Lett 2003;369:680–3. [19] Motiei M, Hacohen YR, Moreno JC, Gedanken A. J Am Chem Soc 2001;123:8624–5. [20] Dresselhaus MS, Dresselhaus G, Pimenta MA, Eklund PC. In: Pelletier MJ, editor. Analytical applications of Raman spectroscopy. Oxford: Blackwell Science; 1999 [Chapter 9].