Small diameter carbon nanotubes synthesized in an arc-discharge

Letters to the editor / Carbon 39 (2001) 1421 – 1446

References [1] Dresselhaus MS, Dresselhaus G, Eklund PC. Science of fullerenes and carbon nanotubes, San Diego: Academic Press, 1996. [2] Dresselhaus MS. Future directions in carbon science. Annu Rev Mater Sci 1997;27:1–34. [3] Houriet R, Vacassy R, Hofmann H, Vogel W. Thin film

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growth using ablation of ceramics with a Lina-SparkE atomizer. Mater Res Soc Symp Proc 1998;526:117–22. [4] Houriet R, Vacassy R, Hofmann H. Synthesis of powders and films using a new laser ablation technique. Nanostruct Mater 1999;11(8):1155–63. [5] Tanaka K, Yamabe T, Fukui K, editors, The science and technology of carbon nanotubes, Amsterdam: Elsevier, 1999.

Small diameter carbon nanotubes synthesized in an arc-discharge Yubao Li*, Sishen Xie, Weiya Zhou, Dongsheng Tang, Zhuqin Liu, Xiaoping Zou, Gang Wang Institute of Physics, Chinese Academy of Sciences, PO Box 603, Beijing 100080, China Received 29 January 2001; accepted 26 February 2001 Keywords: A. Carbon nanotubes; B. Arc discharge; C. Transmission electron microscopy

Carbon nanotubes have attracted much attention owing to their special properties and intriguing applications in many fields. It has been demonstrated that they possess excellent mechanical properties and have interesting electric transport behavior. Carbon nanotubes are regarded as an ideal one-dimensional quantum wires and can be either metallic or semiconducting, depending on their diameter and helicity [1]. Therefore, to synthesize carbon nanotubes with well-defined structures is of great importance to their actual applications and the related fundamental research. But, as estimated by Blase et al. [2], small diameter carbon nanotubes tend to be metallic owing to the sufficiently strong s*–p* hybridization effects which dramatically change the band structure. So far, many researchers have tried to synthesize the smallest nanotubes, and the nanotubes with diameters of 0.7 [3], 0.5 [4] and 0.4 nm [5] have been observed. In a previous letter, we reported the synthesis of nanographite ribbons by utilizing a SiC rod as the anode to an arc-discharge in hydrogen [6]. When choosing helium gas as the discharge atmosphere, we found that a similar arc-discharge process led to the creation of small diameter carbon nanotubes in mass. The experiments were carried out in a normal DC arc-discharge apparatus. A SiC rod (7.5 mm in diameter) containing iron impurity acted as the anode and a graphite plate as the cathode. Full details have been published earlier [6]. Before the discharge, 15 kPa pure helium gas was sealed in the chamber and 30 A (35 V) DC electric *Corresponding author. Fax: 186-10-8264-9531. E-mail address: [email protected] (Y. Li).

current was supplied. The discharge process was maintained for 5 min. After the discharge, a column-like deposit had formed on the cathode and the tip of the SiC rod turned black in color. The cathode deposit was directly observed by a scanning electron microscope, and the black powders in its core as well as the black powders left at the tip of anode were dispersed on some microgrids and also checked in a transmission electron microscope. The black core of cathode deposit is composed of lots of long and straight nanofibers and some nanoparticles. Its SEM image (Fig. 1) shows a typical morphology of a cathode deposit formed in a normal carbon-arc process to

Fig. 1. SEM image of the cathode deposit core.

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Letters to the editor / Carbon 39 (2001) 1421 – 1446

˚ multi-walled carbon nanotube. The innermost 4 Fig. 2. HRTEM images of the small diameter carbon nanotubes produced. (a) A typical 4 A ˚ tube is indicated with an open arrow. Some graphite segments deposited on the outermost shell can be seen at site A and a small A ˚ in diameter) is formed at site B. (b) A bamboo-like nanotube with a 3.4 A ˚ single-walled tube confined in the single-walled nanotube (4.7 A ˚ and is open at the ends. innermost core. (c) A short particle-like nanotube. The innermost tube has a diameter of 7 A

produce carbon nanotubes. The diameters of carbon nanotubes range from several to tens of nanometers. In TEM observations, we found that many of the nanotubes formed had a very small inner diameter with a modal value ˚ the smallest diameter being 3.4 A. ˚ of 4 A, Some representative HRTEM images of the nanotubes formed are shown in Fig. 2. Like those observed by other researchers [4,5], the fringes in the core parts of these small diameter nanotubes are shown by the reduced ˚ multi-walled carbon nanotube is contrast. A typical 4 A shown in Fig. 2a. It has 13 shells and the innermost tube ˚ with a 3.4 A ˚ spacing confined in it has a diameter of 4 A, between its neighboring cylindrical shells. Meanwhile, some graphite segments deposited on its outer shell can be seen at site A, which proves a layer-by-layer growth on the outer surface of the growing tubes. It is worth noting that a small single-walled nanotube is formed at site B next to the outermost shell of the multi-walled nanotube in Fig. 2a, ˚ A hemisphere caps its top and its diameter is about 4.7 A. end, while the other end remains open. Such an extremely narrow tube could act as nucleation sites for the growing tubes. According to the calculated values of the unrelaxed nanotube diameters, this single-walled nanotube may possess a (6, 0) structure and is possibly capped by a half-C36 ‘dome’ at its closed end. Fig. 2b shows a bamboo-like nanotube whose inner cylinders are separated into two sections and capped. The innermost tube indicated with an open arrow in Fig. 2b is ˚ in diameter and its one capped end is indicated with 3.4 A a solid arrow. Swada and Hamada (using a Tersoff-like ˚ nanotube is the atomic potential) predicted the 4 A smallest one that can remain energetically stable [7], but using the tight-binding (TB) model, Peng et al. predicted ˚ diameter may be enerthat a (3, 2) tube with a 3.4 A getically stable [8]. According to their analysis, there may be discrepancies of more than 30% between the predicted Tersoff and TB values for the smallest tube diameter due

to the explicit neglect of the p bonds within the Tersoff formation. Our experimental result demonstrates that a ˚ can indeed be stable. nanotube as small as 3.4 A A very short particle-like nanotube is shown in Fig. 2c. ˚ equal to The innermost tube has a diameter of about 7 A, the diameter of a C60 molecule. The innermost tube confined in it remains open at both ends while the outer shells are capped at the ends. This clearly supports the open-ended growth model and reveals that an inward growth process may form the innermost tube confined in some small diameter nanotubes. According to the openended growth model [9], the thickening of the tube structures occurs mainly by outward growth, but the inside surfaces can also thicken to form internal nested structures. Therefore, the growing open-ended nanotube may act as a nanometer-sized cylindrical template for the growth of internal smaller nanotubes. This may account for the growth of some small diameter carbon nanotubes in our experiment. To more clearly comprehend the formation of these small nanotubes, we examined the black residue at the tip of the SiC rod in the TEM and only carbon was seen. A HRTEM image is shown in Fig. 3, from which we can see that some curved graphene layers are formed due to the decomposition of SiC at the tip and the easier evaporation of silicon. Moreover, the front edges of the neighboring graphene sheets tend to be curved and join together to eliminate dangling C–C bonds. The above observation make us propose that the more curved fragments within the arc may favor the formation of small diameter carbon nanotubes in our experiment. According to the energetic growth analysis on nanotubes by Maiti et al. [10], with the further addition of carbon atoms on the edges of growing nanotubes narrower than a critical diameter (about |3 nm), curved pentagon structures may more easily be formed, which leads to the tubes closure. But if a small metal catalyst particle prevents tube

Letters to the editor / Carbon 39 (2001) 1421 – 1446

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carbon nanotubes produced in an arc-discharge process can ˚ be as small as 3.4 A.

References

Fig. 3. HRTEM image of the residue at the tip of the anode after discharge.

closure, very narrow tubes can be grown. In our experiment, the iron impurity contained in the anode was also evaporated by the arc and would be condensed into tiny iron droplets at the cathode. The presence of small iron particles or droplets at the edges of small diameter growing nanotubes might prevent them from closing and could lead to their elongation. In summary, small diameter nanotubes can be effectively synthesized by using a SiC rod as the anode for arc-discharge in helium. Moreover, the diameter of the

[1] Dresselhaus MS, Dresselhaus G, Saito R. Physics of carbon nanotubes. Carbon 1995;33(7):883–91. [2] Blase X, Benedict LX, Shirley EL, Louie SG. Hybridization effects and metallicity in small radius carbon nanotubes. Phys Rev Lett 1994;72(12):1878–81. [3] Ajayan PM, Iijima S. Smallest carbon nanotube. Nature 1992;358:23. [4] Sun LF, Xie SS, Liu W, Zhou WY, Liu ZQ, Tang DS et al. Creating the narrowest carbon nanotubes. Nature 2000;403:384. [5] Qin LC, Zhao XL, Hirahara K, Miyamoto Y, Ando Y, Iijima S. The smallest carbon nanotube. Nature 2000;408:50. [6] Li YB, Xie SS, Zhou WY, Tang DS, Zou XP, Liu ZQ et al. Nanographite ribbons grown from a SiC arc-discharge in a hydrogen atmosphere. Carbon 2001;39(4):626–8. [7] Sawada S, Hamada N. Energetics of carbon nano-tubes. Solid State Commun 1992;83(11):917–9. [8] Peng LM, Zhang ZL, Xue ZQ, Wu QD, Gu ZN, Pettifor DG. Stability of carbon nanotubes: how small can they be? Phys Rev Lett 2000;85(15):3249–52. [9] Iijima S, Ajayan PM, Ichihashi T. Growth model for carbon nanotubes. Phys Rev Lett 1992;69(21):3100–3. [10] Maiti A, Brabec CJ, Roland C, Bernholc J. Theory of carbon nanotube growth. Phys Rev B 1995;52(20):14850–8.

Structure of ultrafine carbon powders prepared using a carbonaceous gel Xuanke Li a , *, Lang Liu b , Shide Shen a b

a Wuhan University of Science and Technology, Wuhan, Hubei 430081, China Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, China

Received 5 January 2001; accepted 6 March 2001 Keywords: C. BET surface area; Transmission electron microscopy; X-ray diffraction

Since Fujii et al. [1] reported that oxidized carbonaceous materials such as mesophase pitch and green coke can be dissolved in alkaline aqueous solution to obtain an acidic insoluble materials called aqua-mesophase, the preparation and application of aqua-mesophase have been studied by many researchers [2–5]. It has been found that aquamesophase can dissolve in alkaline solution or some *Corresponding author. Fax: 186-27-865-35013. E-mail address: [email protected] (X. Li).

organic solvents to form carbonaceous sols and gels [2,3,6–8]. In the present work the surface state, properties and structure of ultrafine carbon powders prepared from carbonaceous gel by supercritical fluid drying method [9] were investigated by TEM, XRD, and nitrogen adsorption. The nitrogen isotherm plot of primary ultrafine carbon powders shows a broad hysteresis loop (Fig. 1a and b) which is unclosed at low relative pressure. It indicates that the mesoporosity and the microporosity exist in primary

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