Fullerenes, Nanotubes and Carbon Nanostructures, 16: 330 334, 2008 Copyright # Taylor & Francis Group, LLC ISSN 1536-383X print/1536-4046 online DOI: 10.1080/15363830802219849 Arc-synthesis of Single-walled Carbon Nanotubes in Nitrogen Atmosphere V. V. Grebenyukov, 1,2 E. D. Obraztsova, 1 A. S. Pozharov, 1 N. R. Arutyunyan, 1 A. A. Romeikov, 1,2 and I. A. Kozyrev 1,2 1 A.M. Prokhorov General Physics Institute, Moscow, Russia 2 Physics Department of M.V. Lomonosov Moscow State University, Moscow, Russia Abstract: Synthesis of single-walled carbon nanotubes by arc-discharge technique has been realized in nitrogen atmosphere with Ni:Y 2 O 3 catalyst. Presence of carbon nanotubes in the synthesized material has been confirmed by a Raman spectroscopy and by a transmission electron microscopy. The synthesis parameters (gas pressure, arc current) have been optimized. Keywords: Single-wall carbon nanotubes, Nitrogen atmosphere, Raman scattering, Gas pressure, Arc electric current INTRODUCTION Single-walled carbon nanotubes (SWNTs) are arguably the most known and studied representatives of the ever-growing nanotube family (1). The arc-discharge method is one of the simplest ways to synthesize them (2, 3). Combined with the ability to produce large amounts of single-walled carbon nanotubes, the arc-discharge method still attracts the attention of scientists due to the possibility to produce new types of carbon nanotubes. One of the advantages of arc-discharge technique is the ability to produce nanotubes in a broad variety of gases. Helium or argon is used as the buffer gas in the majority of works. To the best of our knowledge, there is only one article dedicated to arc-synthesis of SWNTs in nitrogen Address correspondence to V. V. Grebenyukov, A.M. Prokhorov General Physics Institute, 119991, 38 Vavilov Street, Moscow, Russia. E-mail: studentslava @mail.ru 330
Arc-synthesis of Single-walled Carbon Nanotubes in Nitrogen Atmosphere 331 atmosphere (4), whereas this gas can play a key role in synthesis of heterophase C:BN nanotubes (5) or N-doped single-walled carbon nanotubes. The main goal of this work was to find the arc-process parameters providing a stable synthesis of single-walled carbon nanotubes in nitrogen atmosphere, starting from the optimized arc synthesis of SWNTs in He atmosphere (3). EXPERIMENTAL The arc-discharge installation (Figure 1) is a vacuum chamber with the water-cooled walls and cathode. The arc is ignited between two vertically aligned electrodes. The graphite cathode (with diameter of 20 mm) is fixed. The anode level is manually adjusted during the experiment for keeping constant the inter-electrode distance (2 mm). The direct current power supply provides the arc current adjustment in the range 45 195 A with the fixed voltage of several tens of volts. The anode rod (with diameter of 7 mm) is drilled and filled with the catalytic mixture C:Ni:Y 2 O 3 (2:1:1 wt%) consumed during the process. The previous analysis has shown that the optimum parameters for the arc synthesis of SWNTs with our installation in case of helium atmosphere were the following: the gas pressure - 400 500 Torr and the arc current - 75 85 A. Since a thermal conductivity of nitrogen is significantly less than that of helium, it was expected that the arc current Figure 1. A schematic view of the chamber for arc synthesis of SWNTs in nitrogen atmosphere.
332 V.V. Grebenyukov et al. and the ambient pressure should be lower than the values mentioned. The results of work (4) indicated that the most efficient synthesis could be gained by using as low nitrogen pressure as possible. The lower limit appears due to a necessity to keep the arc ignition. The first set of our experiments has been performed under the fixed arc current (65 A) while the nitrogen pressure varied in the range 50 760 Torr. A Raman spectroscopy was used for estimation of the purity and structure of synthesized material and the range of pressure values providing an efficient synthesis. The Raman spectra have been registered with the triple spectrometer S-3000 (Jobin Yvon) with a spectral resolution 2 cm 21. For the Raman scattering excitation, Ar + -ion laser has been used (l5514.5 nm). In the second set of experiments the nitrogen pressure was fixed at 350 Torr, but the arc current was varied from 45 to 115 A. RESULTS AND DISCUSSION The Raman spectra of the soot-like samples synthesized in the first set of our experiments are shown in Figure 2. It is seen that the spectrum shape corresponding to SWNTs (a split band at 1592 cm 21 ) (1) was observed only for some samples. In other cases the spectrum contained two wide bands: D (disorder-induced) at 1340 cm 21 and G (graphite) at 1580 cm 21. Such spectrum shape corresponds to amorphous-like graphite (5). From Figure 2 a range of the nitrogen pressures (100 450 Torr) providing an efficient SWNT synthesis is evident. Usually the Raman band intensity correlates more or less with the material (SWNTs) amount. Images obtained by a high resolution transmission electron microscopy (HRTEM) technique have confirmed this trend and revealed the SWNTs presence in all samples demonstrating the nanotube-like Raman spectra. Also, it cannot be said with confidence that the samples obtained at the pressure beyond the indicated range contain no nanotubes. A special purification of the samples is needed. But since the aim of these experiments was only to find the optimum synthesis parameters, then it may be concluded that under the nitrogen pressures below 100 Torr and above 450 Torr the synthesis process is not enough efficient. This result is different from that published in (4). A possible reason for such a discrepancy is a difference in catalytic mixtures used. In our case Ni catalyst with Y 2 O 3 promoter was used while in (4) only the use of Ni/Co catalyst is mentioned. Such a difference in the catalyst composition usually influences the synthesis efficiency rather than the boundaries of the pressure window corresponding to successful SWNT synthesis. The same research group has performed a laser-ablation synthesis of SWNTs
Arc-synthesis of Single-walled Carbon Nanotubes in Nitrogen Atmosphere 333 Figure 2. The Raman spectra of carbon soot synthesized by arc-discharge technique in nitrogen atmosphere at different nitrogen pressure values (indicated in the Figure). in nitrogen atmosphere (6). Those data are more consistent with our results on arc SWNT synthesis in nitrogen atmosphere. In the second set of experiments the nitrogen pressure was fixed at 350 Torr, and the arc current was varied from 45A to 115A. Besides the sample grown under current 65A a weak signal of SWNTs has been detected in samples synthesized at 55 A and 75 A; therefore, the optimal arc current value of 65 A for the nitrogen pressure of 350 Torr has been established. Since the optimal conditions for SWNT synthesis can be achieved by continuous variation of the pressure-current combinations, the optimum values found should not be considered as the unique ones. The additional study may reveal other combinations. In summary, the possibility of arc-synthesis of SWNTs in nitrogen atmosphere has been shown. By comparison of the Raman spectra obtained for samples synthesized under different conditions the optimum values for the arc current and for the nitrogen pressure have been established. The HRTEM images have confirmed a presence of SWNTs
334 V.V. Grebenyukov et al. in the samples demonstrated the Raman spectra being typical for singlewalled carbon nanotubes. ACKNOWLEDGMENTS The work was supported by FP6 033350, INTAS-05-100008-7871 projects, the young scientist INTAS project 06-6355 for N. Arutyunyan and by RAS Program New materials. REFERENCES 1. Dresselhaus, M.S., Dresselhaus, G., and Avouris, P. (eds.). (2000). In Carbon Nanotubes: Synthesis, Structure, Properties and Applications, Springer-Verlag: Dordrecht, The Netherlands. 2. Journet, C., Maser, W.K., Bernier, P., et al. (1997) Nature, 388: 756 758. 3. Obraztsova, E.D., Bonard, J.-M., and Kuznetsov, V.L. (1999) Nano Structured Materials, 12: 567 572. 4. Makita, Y., Suzuki, S., Kataura, H., and Achiba, Y. (2005) Eur. Phys. J. D, 34: 1434, 287 291. 5. Nistor, L.C., Van Landuyt, J., and Ralchenko, V.G. (1994) Appl. Phys. A, 58: 137 144. 6. Nishide, D., Kataura, H., and Suzuki, S. (2003) Chem. Phys. Lett., 372: 45 50.