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Large-scale purification of single-wall carbon nanotubes prepared by electric arc discharge
Diamond & Related Materials 15 (2006) 1098 – 1102 www.elsevier.com/locate/diamond
Large-scale purification of single-wall carbon nanotubes prepared by electric arc discharge Xinluo Zhao, Masato Ohkohchi, Sakae Inoue, Tomoko Suzuki, Takenori Kadoya, Yoshinori Ando * 21st COE Program ‘‘Nano Factory’’, Department of Materials Science and Engineering, Meijo University, Shiogamaguchi 1-501, Tenpaku-ku, Nagoya 468-8502, Japan Available online 9 January 2006
Abstract High-yield single-wall carbon nanotubes (SWNTs) have been mass-produced by dc arc discharge evaporation of a carbon electrode including 1 at.% Fe catalyst in hydrogen mixed gas [i.e., H2 – inert gas (Ne, Ar, Kr, Xe), or H2 – N2]. The as-grown SWNTs have high-crystallinity due to the high temperature of arc plasma, and the coexisting Fe catalyst nanoparticles are embedded in very thin amorphous carbon because of the in situ etching effects of hydrogen. A macroscale purification technique, which is a whole liquid-phase purification process, first reflux treatment in H2O2 solution and then rinsing with hydrochloric acid, has been developed to eliminate the coexisting Fe catalyst nanoparticles and obtain SWNTs with purity higher than 90 at.%. D 2005 Elsevier B.V. All rights reserved. Keywords: Nanotubes; Electric arc discharge; Etching; Vibrational properties characterization
1. Introduction Single-wall carbon nanotubes (SWNTs) exhibit many interesting mechanical and electrical properties due to their unique one-dimensional structure, and have attracted considerable attention because of their potential applications [1,2]. A variety of synthesis and purification methods have been developed for attaining high-yield SWNTs [3 –11], especially, the impurity-free SWNTs have been successfully synthesized by water-assisted catalytic chemical vapor deposition (CCVD) [12]. However, even now, a challenging problem is still to develop a method for mass-production of high-crystallinity SWNTs with controlled diameters at high yield and low cost. Meanwhile, it is also important that the metal catalyst nanoparticles coexisting with the as-grown SWNTs can be easily eliminated. Three methods, laser ablation [3], arc discharge [4], and CCVD [5 –7,12], are generally used to synthesize high-yield SWNTs. The crystallinity of SWNTs prepared by laser ablation or arc discharge is usually higher than that of SWNTs grown by CCVD, although the latter is of great advantage to mass-produce SWNTs.
* Corresponding author. Tel.: +81 52 838 2409; fax: +81 52 832 1170. E-mail address: [email protected] (Y. Ando). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.11.002
Comparing with laser ablation, electric arc discharge technique is cheaper and easier to implement, but usually has led to low yield of SWNTs when the evaporation experiments are carried out in inert gas [13,14]. By using H2 – Ar or H2 – N2 mixed gas as atmospheric gas, huge webs of SWNTs with the yield higher than 70 at.% have been prepared by dc arc discharge evaporation of a carbon electrode including 1 at.% Fe catalyst [15,16]. Moreover, the coexisting Fe catalyst nanoparticles can be removed by a simple two-step purification process (heating in air at 693 K and hydrochloric acid treatment). Here, we report the mass-production of high-yield SWNTs by H2 – Ne, H2 – Kr, and H2 – Xe arc discharge. It will be also shown that these SWNTs obtained by electric arc discharge in H2 –inert gas (Ne, Ar, Kr, Xe), or H2 –N2 gas mixture can be easily purified by a liquid-phase and macroscale purification process, reflux treatment in H2O2 solution and rinsing with hydrochloric acid, to obtain SWNTs with purity higher than 90 at.%. 2. Experimental The apparatus for preparing SWNTs by dc arc discharge evaporation, in which two electrodes were installed vertically, was same as that previously used [15,16]. The lower carbon electrode (anode: 6 mm diameter, 100 mm long, ¨ 4 g mass)
X. Zhao et al. / Diamond & Related Materials 15 (2006) 1098 – 1102
containing 1.0 at.% Fe catalyst was fixed at the center of a vacuum chamber. The upper, pure carbon electrode (cathode: 10 mm diameter, 100 mm long) was equipped with a computercontrolled step motor for translation along the vertical axis. The vacuum chamber was linked to both H2 and inert gas (Ne, Ar, Kr, Xe), or N2 gas lines through two mass flow controllers as well as a rotary pump to allow continuous flow of atmospheric gas of various mixing ratios at 50 –500 Torr. A dc arc discharge was generated by applying 50 A, and typical synthesis time was 3 –20 min. During the arcing, the lower Fe –C anode was evaporated and consumed, a carbon deposit was formed on the upper cathode surface, and a huge SWNT web was simultaneously grown surrounding the upper cathode. The precise electrode gap adjustment of 2 mm was achieved by controlling the arc voltage through an automatic feedback circuit, which senses the arc voltage variation between cathode and anode, accordingly adjusts them [17]. In order to keep constant gas pressure, gas flow capacity, and the mixing ratio of H2 – inert gas (or H2 – N2), two mass flow controllers were used. A liquid-phase purification process was developed to purify the as-grown SWNTs. First, 100 mg of as-grown SWNT webs in 100 ml of distilled water was pulverized by cooking mixer, and refluxed in 15% H2O2 solution at 383 K for 2 h. Then, the residual material was soaked in 36% hydrochloric acid (100 ml) for 12 h and centrifuged, leaving black sediment and a yellowgreen supernatant liquid, which was decanted off. For removing the substantially trapped acid contained in the sediment, we repeatedly re-suspended the sediment in distilled water (twice) or ethanol (once) by shaking ultrasonically, followed by centrifuging and decanting of the supernatant liquid. After the ethanol was evaporated using a hot plate at 473 K, the weight of purified SWNTs was measured to determine the mass yield (m purified/m as-grown), which was found to be typically 20 – 30%. As-grown and purified SWNTs were evaluated by scanning electron microscopy (SEM, Topcon ABT-150F) equipped with an energy-dispersive X-ray analysis system (EDX, Horiba EMAX-5770W), transmission electron microscopy (TEM, Hitachi H-7000), and high-resolution transmission electron microscopy (HRTEM, JEOL JEM2010F). Raman spectra were recorded using a Raman spectrometer (Jobin Yvon, RAMANOR T64000) with 514.5 nm excitation (Ar+-laser).
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3. Results and discussion Fig. 1a shows a characteristic SEM image of SWNT web prepared by dc arc discharge evaporation (arc current, 50 A; evaporation time, 3 min) of carbon electrode containing 1.0 at.% Fe in 50% H2 –50% Kr gas mixture under a pressure of 200 Torr. Long entangled SWNT bundles can be observed in abundance, and the bright points attaching on their surface are Fe catalyst nanoparticles. When the preparation experiments were carried out in 50% H2 – 50% Ne gas mixture, even though other conditions were same with that of 50% H2 –50% Kr, the SWNT bundles became thinner (see Fig. 1b). On the contrary, the evaporation of C – Fe anode in 50% H2 –50% Xe results in the formation of thicker SWNT bundles. It should be pointed out that these preparation experiments have been done without the flow of atmospheric gas since Ne, Kr and Xe gases are very expensive. Obviously, under the same pressure of atmospheric gas, the bigger the atomic mass, the thicker the SWNT bundles. Fe element is an effective catalyst for forming SWNTs [6,15,16], but the high solubility of Fe in graphite and the high reactivity of Fe with oxygen cause self-deactivation, which hampers the growth of SWNTs. When using H2 gas as a reducing agent, the atomic hydrogen in arc discharge not only keep the Fe catalyst active, but also selectively etching the amorphous carbon attached to the surface of Fe catalyst nanoparticles or SWNTs [15,16]. However, the high reactivity and thermal conductivity of H2 gas make the arc plasma unstable. In the present experiments, Ne, Kr or Xe gas has been added into H2 gas to stabilize the arc plasma. It has been found that the yield of SWNTs and the thickness of SWNT bundles have relations with the atomic mass, the thermal conductivity and pressure of atmospheric gas, because they have an effect on the temperature of arc plasma. It is well known that Ar (atomic weight: 18) and N2 (14) gas have the thermal conductivities of 38.8 10 6 and 54.5 10 6 cal cm 1 s 1 deg 1, respectively. Ne (atomic weight: 10) gas has much higher thermal conductivity (108.5 10 6 cal cm 1 s 1 deg 1) than those (21.2 10 6, 12.4 10 6) of Kr (36) and Xe (54) gas. Usually, the gas with higher thermal conductivity can result in the arc plasma with higher temperature. The electric current of dc arc discharge also has an effect on the temperature of arc plasma. In our previous experiments, we have applied 30 –70 A to generate a dc arc discharge plasma in
Fig. 1. SEM images of as-grown SWNTs: (a) prepared in 50% H2 – 50% Kr, (b) prepared in 50% H2 – 50% Ne.
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H2 – Ar gas mixture, and found that 50 A is the best current for preparing high-yield SWNTs [15]. As the current was too low, the C –Fe anode could not be evaporated uniformly. When the current was higher than 50 A, the C – Fe anode was evaporated heavily, and the carbon smoke took place, resulting in the decrease of yield of SWNTs. In the case of 50 A, the temperatures of arc plasma during SWNTs growth under the various H2 – inert gas mixtures are 3600 – 4900 K based on the spectroscopic study [18]. It has been found that the C2 concentration (column densities) vary from 1 1015 to 3.8 1015 cm 1, and the C2/Fe, Fe+/Fe and C/C2 ratios by spectra intensities are in the ranges 1.6– 16, 0.25– 1.0, and 0.17 –3.2, respectively. It should be pointed out that the balance of these constituent species is also important in highyield SWNT production. Fig. 2a –d show the Raman spectra of SWNTs prepared in 50% H2 – 50% N2, 50% H2 – 50% Xe, 50% H2 – 50% Kr, and 50% H2 –50% Ne gas mixture. The multiple splitting of Gbands (1550 – 1650 cm 1) can be clearly seen in a high frequency region, 1200 – 1700 cm 1. A low intensity of Dband at 1345 cm 1 is observed, which indicates the high purity and high crystallinity of as-grown SWNTs. Radial breathing modes (RBMs) are observed in the insets, 100 –300 cm 1, and four RBM peaks appear at 185, 245, 260, and 267 cm 1. Using the correlation [19] between diameter d (nm) and RBM frequency x (cm 1): d = 224 / (x 14), the RBM peak at 267 cm 1 should originate from the SWNTs with 0.9 nm in diameter, and the RBM peaks at 185, 245 and 260 cm 1 can be
assigned to the SWNTs with diameters of 1.31, 0.97 and 0.91 nm. It can be seen that as using Xe, Kr, or Ne instead of N2, the peak at 267 cm 1 become relatively stronger to the modes at 185 cm 1. This means that the diameter distribution of SWNTs can be changed by varying the kind of inert gas. The SWNTs prepared by dc arc discharge evaporation in inert gas have a narrow diameter distribution, and the diameters are around 1.4 nm [4,14]. As adding H2 gas into inert gas, very thin SWNTs with 0.9 nm in diameter have been grown in dc arc discharge plasma, resulting in that the diameter distribution of SWNTs becomes broader (0.9 – 1.3 nm). It can be speculated that the atomic hydrogen in arc plasma play an important role in the creation and growth of thin SWNTs. Once the thin SWNTs with 0.9 nm in diameter are created, the atomic hydrogen can terminate the dangling carbon bonds at the growing edge of thin SWNTs, keeping them open to form long SWNTs. H2 gas selectively etches the amorphous carbon attached to the surface of Fe catalyst nanoparticles or SWNTs by forming gaseous hydrocarbons (for example, CH4, C2H2, and so on). This serves as in situ purification process and improves the purity of SWNTs. However, this also results in that the highest yield of SWNT production, i.e., the ratio of obtained SWNTs mass to the evaporated anode mass, is only ¨ 10% [16]. If mass-production of SWNTs was carried out in a sealing chamber filled with H2 –inert gas mixture, the composition of atmospheric gas in the chamber would change with the arc discharge evaporation of anode. Therefore, we mass-produced SWNTs under H2 –Ar or H2 – N2 mixed gas flow, even though
Fig. 2. Raman spectra of SWNTs prepared by dc arc discharge in: (a) 50% H2 – 50% N2, (b) 50% H2 – 50% Xe, (c) 50% H2 – 50% Kr, and (d) 50% H2 – 50% Ne gas mixture at 200 Torr. The inset shows the low-frequency region, 100 – 300 cm 1.
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Fig. 3. SEM images (a) and low-magnification TEM image (c) of as-grown SWNTs prepared in 50% H2 – 50% Ar. SEM image (b) and low-magnification TEM image (d) of purified SWNTs.
the preparation experiments in H2 – Ne, H2 – Kr or H2 –Xe gas mixture were carried out in a sealing chamber. With the help of the computer-controlled feeder, we can make the as-grown SWNTs to have the same quality. It has been found that the production rates of SWNTs in H2 – Ar and H2 – N2 gas mixture can reach to 7.7 and 13 mg/min, respectively. As a whole C –Fe anode was evaporated, a huge SWNT web with the mass of more than 200 mg could be obtained. At present, it is capable of generating ¨ 10 g/day in our laboratory. EDX analyses show that the as-grown SWNTs contain ¨ 10 at.% of Fe element. HRTEM investigations show that the
coexisting Fe catalyst nanoparticles are embedded in very thin amorphous carbon, and are nearly spherical, with diameter of a few nm. These Fe catalyst nanoparticles can be removed by a simple two-step purification process (heating in air at 693 K and mild hydrochloric acid treatment) [15,16], but this purification method is not suitable to purify SWNTs on a large scale. Therefore, a new liquid-phase purification process, first reflux treatment in H2O2 solution and then rinsing with hydrochloric acid, has been developed for large-scale purification of SWNTs. Fig. 3a and c show a typical SEM image and a lowmagnification TEM image of as-grown SWNTs produced in
Fig. 4. EDX spectra of as-grown SWNTs prepared in 50% H2 – 50% Ar (a), and purified SWNTs (b). The percentage of Fe decreases from 11.39 to 0.06 at.% by the liquid-phase purification process, and that of C increases from 82.19 to 94.96 at.%.
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H2 – Ar gas mixture, respectively. Some Fe catalyst nanoparticles attached to the surface of SWNT bundles can be observed as dark dots in Fig. 3c. After the liquid-phase purification, the SWNT bundles become thicker, as shown in Fig. 3b, and the morphology of purified SWNTs is very similar to that of ‘‘buckypaper’’ [8]. Only few Fe catalyst nanoparticle can be found in the low-magnification TEM image of purified SWNTs (see Fig. 3d). EDX analyses indicate that the percentage of Fe decreases from 11.39 to 0.06 at.% by purification, and that of C increases from 82.19 to 94.96 at.%. Remaining impurities are O (3.92 at.%), Cl (1.06 at.%), and a few other forms of carbon (carbon nanoparticles, pieces of graphitic carbon). Fig. 4a and b show the EDX spectra taken from as-grown SWNTs and purified SWNTs, respectively. It can be seen that after the liquid-phase purification process, the peak of C element become very strong, and the peak of Fe become very weak. On the basis of TEM, HRTEM, and SEM observations, as well as EDX analyses, it has been estimated that the purity of SWNTs is higher than 90 at.%. The high reactivity of Fe catalyst nanoparticles and presence of very thin amorphous carbon layer attached to their surface are crucial in the present liquid-phase purification process. During the reflux in H2O2 solution, Fe nanoparticles with diameter of a few nm act as a catalyst for resolving H2O2 into H2O and O2. This chemical process will result in the occurrence of active oxygen, which can remove the amorphous carbon and also transforms Fe into iron oxide. We have used mild hydrochloric acid to wash out iron oxide because it does not damage the SWNTs. It should be pointed out that this liquid-phase purification process is not effective for purifying the SWNTs prepared by dc arc discharge evaporation of Ni 4% – Y 1%-doped carbon electrode in He gas [4,14]. We have tried to reflux these SWNTs, which usually coexist with Ni catalyst nanoparticles embedded in thick amorphous carbon, in 20% H2O2 solution at 383 K for 24 h, but we have failed to remove the amorphous carbon. It has been found that adding Fe ultrafine particles into H2O2 solution is helpful to purify this kind of SWNTs, and the detail will be published elsewhere. 4. Conclusions Gram quantities of high-yield SWNTs have been produced by dc arc discharge evaporation of a carbon electrode including 1 at.% Fe catalyst in hydrogen mixed gas [i.e., H2 –inert gas (Ne, Ar, Kr, Xe), or H2 –N2]. The as-grown SWNTs have highcrystallinity SWNTs, and the coexisting Fe catalyst nanoparticles are embedded in very thin amorphous carbon. By
exploiting the catalyst effects of Fe nanoparticles resolving H2O2 into H2O and O2, a liquid-phase purification process has been developed to eliminate the coexisting Fe catalyst nanoparticles and obtain the SWNTs with purity higher than 90 at.%. This macroscale purification technique can be scaled up to industrial levels, and will accelerate the studies of physical properties and practical applications of bulky SWNTs. Acknowledgements We thank Dr. M. Hiramatsu of Meijo University for allowing the use of the Raman spectrometer. This work was supported by Grant-in-aid for Intellectual Cluster Project by the Ministry of Education, Culture, Sports, Science and Technology, Japan, Aichi Prefecture and the Aichi Science and Technology Foundation. References [1] M.S. Dresselhaus, G. Dresselhaus, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, New York, 1996. [2] R. Saito, G. Dresselhaus, M.S. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London, 1998. [3] A. Thess, R. Lee, P. Nikolaev, et al., Science 273 (1996) 483. [4] C. Journet, W.K. Maser, P. Bernier, et al., Nature 388 (1997) 756. [5] H.M. Cheng, F. Li, G. Su, H.Y. Pan, L.L. He, X. Sun, M.S. Dresselhaus, Appl. Phys. Lett. 72 (1998) 3282. [6] P. Nikolaev, M.J. Bronikowski, R.K. Bradley, et al., Chem. Phys. Lett. 313 (1999) 91. [7] S. Maruyama, R. Kojima, Y. Miyauchi, S. Chiashi, M. Kohno, Chem. Phys. Lett. 360 (2002) 229. [8] A.G. Rinzler, J. Liu, H. Dai, et al., Appl. Phys., A 67 (1998) 29. [9] E. Dujardin, C. Meny, P. Panissod, J.-P. Kintzinger, N. Yao, T.W. Ebbesen, Solid State Commun. 114 (2000) 543. [10] D. Chattopadhyay, I. Galeska, F. Papadimitrakopoulos, Carbon 40 (2002) 985. [11] A.R. Harutyunyan, B.K. Pradhan, J. Chang, G. Chen, P.C. Eklund, J. Phys. Chem., B 106 (2002) 8671. [12] K. Hata, D.N. Futaba, K. Mizuno, T. Namai, M. Yumura, S. Iijima, Science 306 (2004) 1362. [13] D.S. Bethune, C.H. Kiang, M.S. de Vries, et al., Nature 363 (1993) 605. [14] Y. Ando, X. Zhao, K. Hirahara, K. Suenaga, S. Bandow, S. Iijima, Chem. Phys. Lett. 323 (2000) 580. [15] X. Zhao, S. Inoue, M. Jinno, T. Suzuki, Y. Ando, Chem. Phys. Lett. 373 (2003) 266. [16] Y. Ando, X. Zhao, S. Inoue, T. Suzuki, T. Kadoya, Diamond Relat. Mater. 14 (2005) 729. [17] T. Suzuki, Y. Guo, S. Inoue, et al., J. Nanoparticle Research, in press. [18] T. Okazaki, H. Lange, N. Murata, et al., Diamond Relat. Mater. (private communication). [19] A.M. Rao, J. Chen, E. Richter, et al., Phys. Rev. Lett. 86 (2001) 3895.
Large-scale purification of single-wall carbon nanotubes prepared by electric arc discharge Xinluo Zhao, Masato Ohkohchi, Sakae Inoue, Tomoko Suzuki, Takenori Kadoya, Yoshinori Ando * 21st COE Program ‘‘Nano Factory’’, Department of Materials Science and Engineering, Meijo University, Shiogamaguchi 1-501, Tenpaku-ku, Nagoya 468-8502, Japan Available online 9 January 2006
Abstract High-yield single-wall carbon nanotubes (SWNTs) have been mass-produced by dc arc discharge evaporation of a carbon electrode including 1 at.% Fe catalyst in hydrogen mixed gas [i.e., H2 – inert gas (Ne, Ar, Kr, Xe), or H2 – N2]. The as-grown SWNTs have high-crystallinity due to the high temperature of arc plasma, and the coexisting Fe catalyst nanoparticles are embedded in very thin amorphous carbon because of the in situ etching effects of hydrogen. A macroscale purification technique, which is a whole liquid-phase purification process, first reflux treatment in H2O2 solution and then rinsing with hydrochloric acid, has been developed to eliminate the coexisting Fe catalyst nanoparticles and obtain SWNTs with purity higher than 90 at.%. D 2005 Elsevier B.V. All rights reserved. Keywords: Nanotubes; Electric arc discharge; Etching; Vibrational properties characterization
1. Introduction Single-wall carbon nanotubes (SWNTs) exhibit many interesting mechanical and electrical properties due to their unique one-dimensional structure, and have attracted considerable attention because of their potential applications [1,2]. A variety of synthesis and purification methods have been developed for attaining high-yield SWNTs [3 –11], especially, the impurity-free SWNTs have been successfully synthesized by water-assisted catalytic chemical vapor deposition (CCVD) [12]. However, even now, a challenging problem is still to develop a method for mass-production of high-crystallinity SWNTs with controlled diameters at high yield and low cost. Meanwhile, it is also important that the metal catalyst nanoparticles coexisting with the as-grown SWNTs can be easily eliminated. Three methods, laser ablation [3], arc discharge [4], and CCVD [5 –7,12], are generally used to synthesize high-yield SWNTs. The crystallinity of SWNTs prepared by laser ablation or arc discharge is usually higher than that of SWNTs grown by CCVD, although the latter is of great advantage to mass-produce SWNTs.
* Corresponding author. Tel.: +81 52 838 2409; fax: +81 52 832 1170. E-mail address: [email protected] (Y. Ando). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.11.002
Comparing with laser ablation, electric arc discharge technique is cheaper and easier to implement, but usually has led to low yield of SWNTs when the evaporation experiments are carried out in inert gas [13,14]. By using H2 – Ar or H2 – N2 mixed gas as atmospheric gas, huge webs of SWNTs with the yield higher than 70 at.% have been prepared by dc arc discharge evaporation of a carbon electrode including 1 at.% Fe catalyst [15,16]. Moreover, the coexisting Fe catalyst nanoparticles can be removed by a simple two-step purification process (heating in air at 693 K and hydrochloric acid treatment). Here, we report the mass-production of high-yield SWNTs by H2 – Ne, H2 – Kr, and H2 – Xe arc discharge. It will be also shown that these SWNTs obtained by electric arc discharge in H2 –inert gas (Ne, Ar, Kr, Xe), or H2 –N2 gas mixture can be easily purified by a liquid-phase and macroscale purification process, reflux treatment in H2O2 solution and rinsing with hydrochloric acid, to obtain SWNTs with purity higher than 90 at.%. 2. Experimental The apparatus for preparing SWNTs by dc arc discharge evaporation, in which two electrodes were installed vertically, was same as that previously used [15,16]. The lower carbon electrode (anode: 6 mm diameter, 100 mm long, ¨ 4 g mass)
X. Zhao et al. / Diamond & Related Materials 15 (2006) 1098 – 1102
containing 1.0 at.% Fe catalyst was fixed at the center of a vacuum chamber. The upper, pure carbon electrode (cathode: 10 mm diameter, 100 mm long) was equipped with a computercontrolled step motor for translation along the vertical axis. The vacuum chamber was linked to both H2 and inert gas (Ne, Ar, Kr, Xe), or N2 gas lines through two mass flow controllers as well as a rotary pump to allow continuous flow of atmospheric gas of various mixing ratios at 50 –500 Torr. A dc arc discharge was generated by applying 50 A, and typical synthesis time was 3 –20 min. During the arcing, the lower Fe –C anode was evaporated and consumed, a carbon deposit was formed on the upper cathode surface, and a huge SWNT web was simultaneously grown surrounding the upper cathode. The precise electrode gap adjustment of 2 mm was achieved by controlling the arc voltage through an automatic feedback circuit, which senses the arc voltage variation between cathode and anode, accordingly adjusts them [17]. In order to keep constant gas pressure, gas flow capacity, and the mixing ratio of H2 – inert gas (or H2 – N2), two mass flow controllers were used. A liquid-phase purification process was developed to purify the as-grown SWNTs. First, 100 mg of as-grown SWNT webs in 100 ml of distilled water was pulverized by cooking mixer, and refluxed in 15% H2O2 solution at 383 K for 2 h. Then, the residual material was soaked in 36% hydrochloric acid (100 ml) for 12 h and centrifuged, leaving black sediment and a yellowgreen supernatant liquid, which was decanted off. For removing the substantially trapped acid contained in the sediment, we repeatedly re-suspended the sediment in distilled water (twice) or ethanol (once) by shaking ultrasonically, followed by centrifuging and decanting of the supernatant liquid. After the ethanol was evaporated using a hot plate at 473 K, the weight of purified SWNTs was measured to determine the mass yield (m purified/m as-grown), which was found to be typically 20 – 30%. As-grown and purified SWNTs were evaluated by scanning electron microscopy (SEM, Topcon ABT-150F) equipped with an energy-dispersive X-ray analysis system (EDX, Horiba EMAX-5770W), transmission electron microscopy (TEM, Hitachi H-7000), and high-resolution transmission electron microscopy (HRTEM, JEOL JEM2010F). Raman spectra were recorded using a Raman spectrometer (Jobin Yvon, RAMANOR T64000) with 514.5 nm excitation (Ar+-laser).
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3. Results and discussion Fig. 1a shows a characteristic SEM image of SWNT web prepared by dc arc discharge evaporation (arc current, 50 A; evaporation time, 3 min) of carbon electrode containing 1.0 at.% Fe in 50% H2 –50% Kr gas mixture under a pressure of 200 Torr. Long entangled SWNT bundles can be observed in abundance, and the bright points attaching on their surface are Fe catalyst nanoparticles. When the preparation experiments were carried out in 50% H2 – 50% Ne gas mixture, even though other conditions were same with that of 50% H2 –50% Kr, the SWNT bundles became thinner (see Fig. 1b). On the contrary, the evaporation of C – Fe anode in 50% H2 –50% Xe results in the formation of thicker SWNT bundles. It should be pointed out that these preparation experiments have been done without the flow of atmospheric gas since Ne, Kr and Xe gases are very expensive. Obviously, under the same pressure of atmospheric gas, the bigger the atomic mass, the thicker the SWNT bundles. Fe element is an effective catalyst for forming SWNTs [6,15,16], but the high solubility of Fe in graphite and the high reactivity of Fe with oxygen cause self-deactivation, which hampers the growth of SWNTs. When using H2 gas as a reducing agent, the atomic hydrogen in arc discharge not only keep the Fe catalyst active, but also selectively etching the amorphous carbon attached to the surface of Fe catalyst nanoparticles or SWNTs [15,16]. However, the high reactivity and thermal conductivity of H2 gas make the arc plasma unstable. In the present experiments, Ne, Kr or Xe gas has been added into H2 gas to stabilize the arc plasma. It has been found that the yield of SWNTs and the thickness of SWNT bundles have relations with the atomic mass, the thermal conductivity and pressure of atmospheric gas, because they have an effect on the temperature of arc plasma. It is well known that Ar (atomic weight: 18) and N2 (14) gas have the thermal conductivities of 38.8 10 6 and 54.5 10 6 cal cm 1 s 1 deg 1, respectively. Ne (atomic weight: 10) gas has much higher thermal conductivity (108.5 10 6 cal cm 1 s 1 deg 1) than those (21.2 10 6, 12.4 10 6) of Kr (36) and Xe (54) gas. Usually, the gas with higher thermal conductivity can result in the arc plasma with higher temperature. The electric current of dc arc discharge also has an effect on the temperature of arc plasma. In our previous experiments, we have applied 30 –70 A to generate a dc arc discharge plasma in
Fig. 1. SEM images of as-grown SWNTs: (a) prepared in 50% H2 – 50% Kr, (b) prepared in 50% H2 – 50% Ne.
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H2 – Ar gas mixture, and found that 50 A is the best current for preparing high-yield SWNTs [15]. As the current was too low, the C –Fe anode could not be evaporated uniformly. When the current was higher than 50 A, the C – Fe anode was evaporated heavily, and the carbon smoke took place, resulting in the decrease of yield of SWNTs. In the case of 50 A, the temperatures of arc plasma during SWNTs growth under the various H2 – inert gas mixtures are 3600 – 4900 K based on the spectroscopic study [18]. It has been found that the C2 concentration (column densities) vary from 1 1015 to 3.8 1015 cm 1, and the C2/Fe, Fe+/Fe and C/C2 ratios by spectra intensities are in the ranges 1.6– 16, 0.25– 1.0, and 0.17 –3.2, respectively. It should be pointed out that the balance of these constituent species is also important in highyield SWNT production. Fig. 2a –d show the Raman spectra of SWNTs prepared in 50% H2 – 50% N2, 50% H2 – 50% Xe, 50% H2 – 50% Kr, and 50% H2 –50% Ne gas mixture. The multiple splitting of Gbands (1550 – 1650 cm 1) can be clearly seen in a high frequency region, 1200 – 1700 cm 1. A low intensity of Dband at 1345 cm 1 is observed, which indicates the high purity and high crystallinity of as-grown SWNTs. Radial breathing modes (RBMs) are observed in the insets, 100 –300 cm 1, and four RBM peaks appear at 185, 245, 260, and 267 cm 1. Using the correlation [19] between diameter d (nm) and RBM frequency x (cm 1): d = 224 / (x 14), the RBM peak at 267 cm 1 should originate from the SWNTs with 0.9 nm in diameter, and the RBM peaks at 185, 245 and 260 cm 1 can be
assigned to the SWNTs with diameters of 1.31, 0.97 and 0.91 nm. It can be seen that as using Xe, Kr, or Ne instead of N2, the peak at 267 cm 1 become relatively stronger to the modes at 185 cm 1. This means that the diameter distribution of SWNTs can be changed by varying the kind of inert gas. The SWNTs prepared by dc arc discharge evaporation in inert gas have a narrow diameter distribution, and the diameters are around 1.4 nm [4,14]. As adding H2 gas into inert gas, very thin SWNTs with 0.9 nm in diameter have been grown in dc arc discharge plasma, resulting in that the diameter distribution of SWNTs becomes broader (0.9 – 1.3 nm). It can be speculated that the atomic hydrogen in arc plasma play an important role in the creation and growth of thin SWNTs. Once the thin SWNTs with 0.9 nm in diameter are created, the atomic hydrogen can terminate the dangling carbon bonds at the growing edge of thin SWNTs, keeping them open to form long SWNTs. H2 gas selectively etches the amorphous carbon attached to the surface of Fe catalyst nanoparticles or SWNTs by forming gaseous hydrocarbons (for example, CH4, C2H2, and so on). This serves as in situ purification process and improves the purity of SWNTs. However, this also results in that the highest yield of SWNT production, i.e., the ratio of obtained SWNTs mass to the evaporated anode mass, is only ¨ 10% [16]. If mass-production of SWNTs was carried out in a sealing chamber filled with H2 –inert gas mixture, the composition of atmospheric gas in the chamber would change with the arc discharge evaporation of anode. Therefore, we mass-produced SWNTs under H2 –Ar or H2 – N2 mixed gas flow, even though
Fig. 2. Raman spectra of SWNTs prepared by dc arc discharge in: (a) 50% H2 – 50% N2, (b) 50% H2 – 50% Xe, (c) 50% H2 – 50% Kr, and (d) 50% H2 – 50% Ne gas mixture at 200 Torr. The inset shows the low-frequency region, 100 – 300 cm 1.
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Fig. 3. SEM images (a) and low-magnification TEM image (c) of as-grown SWNTs prepared in 50% H2 – 50% Ar. SEM image (b) and low-magnification TEM image (d) of purified SWNTs.
the preparation experiments in H2 – Ne, H2 – Kr or H2 –Xe gas mixture were carried out in a sealing chamber. With the help of the computer-controlled feeder, we can make the as-grown SWNTs to have the same quality. It has been found that the production rates of SWNTs in H2 – Ar and H2 – N2 gas mixture can reach to 7.7 and 13 mg/min, respectively. As a whole C –Fe anode was evaporated, a huge SWNT web with the mass of more than 200 mg could be obtained. At present, it is capable of generating ¨ 10 g/day in our laboratory. EDX analyses show that the as-grown SWNTs contain ¨ 10 at.% of Fe element. HRTEM investigations show that the
coexisting Fe catalyst nanoparticles are embedded in very thin amorphous carbon, and are nearly spherical, with diameter of a few nm. These Fe catalyst nanoparticles can be removed by a simple two-step purification process (heating in air at 693 K and mild hydrochloric acid treatment) [15,16], but this purification method is not suitable to purify SWNTs on a large scale. Therefore, a new liquid-phase purification process, first reflux treatment in H2O2 solution and then rinsing with hydrochloric acid, has been developed for large-scale purification of SWNTs. Fig. 3a and c show a typical SEM image and a lowmagnification TEM image of as-grown SWNTs produced in
Fig. 4. EDX spectra of as-grown SWNTs prepared in 50% H2 – 50% Ar (a), and purified SWNTs (b). The percentage of Fe decreases from 11.39 to 0.06 at.% by the liquid-phase purification process, and that of C increases from 82.19 to 94.96 at.%.
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H2 – Ar gas mixture, respectively. Some Fe catalyst nanoparticles attached to the surface of SWNT bundles can be observed as dark dots in Fig. 3c. After the liquid-phase purification, the SWNT bundles become thicker, as shown in Fig. 3b, and the morphology of purified SWNTs is very similar to that of ‘‘buckypaper’’ [8]. Only few Fe catalyst nanoparticle can be found in the low-magnification TEM image of purified SWNTs (see Fig. 3d). EDX analyses indicate that the percentage of Fe decreases from 11.39 to 0.06 at.% by purification, and that of C increases from 82.19 to 94.96 at.%. Remaining impurities are O (3.92 at.%), Cl (1.06 at.%), and a few other forms of carbon (carbon nanoparticles, pieces of graphitic carbon). Fig. 4a and b show the EDX spectra taken from as-grown SWNTs and purified SWNTs, respectively. It can be seen that after the liquid-phase purification process, the peak of C element become very strong, and the peak of Fe become very weak. On the basis of TEM, HRTEM, and SEM observations, as well as EDX analyses, it has been estimated that the purity of SWNTs is higher than 90 at.%. The high reactivity of Fe catalyst nanoparticles and presence of very thin amorphous carbon layer attached to their surface are crucial in the present liquid-phase purification process. During the reflux in H2O2 solution, Fe nanoparticles with diameter of a few nm act as a catalyst for resolving H2O2 into H2O and O2. This chemical process will result in the occurrence of active oxygen, which can remove the amorphous carbon and also transforms Fe into iron oxide. We have used mild hydrochloric acid to wash out iron oxide because it does not damage the SWNTs. It should be pointed out that this liquid-phase purification process is not effective for purifying the SWNTs prepared by dc arc discharge evaporation of Ni 4% – Y 1%-doped carbon electrode in He gas [4,14]. We have tried to reflux these SWNTs, which usually coexist with Ni catalyst nanoparticles embedded in thick amorphous carbon, in 20% H2O2 solution at 383 K for 24 h, but we have failed to remove the amorphous carbon. It has been found that adding Fe ultrafine particles into H2O2 solution is helpful to purify this kind of SWNTs, and the detail will be published elsewhere. 4. Conclusions Gram quantities of high-yield SWNTs have been produced by dc arc discharge evaporation of a carbon electrode including 1 at.% Fe catalyst in hydrogen mixed gas [i.e., H2 –inert gas (Ne, Ar, Kr, Xe), or H2 –N2]. The as-grown SWNTs have highcrystallinity SWNTs, and the coexisting Fe catalyst nanoparticles are embedded in very thin amorphous carbon. By
exploiting the catalyst effects of Fe nanoparticles resolving H2O2 into H2O and O2, a liquid-phase purification process has been developed to eliminate the coexisting Fe catalyst nanoparticles and obtain the SWNTs with purity higher than 90 at.%. This macroscale purification technique can be scaled up to industrial levels, and will accelerate the studies of physical properties and practical applications of bulky SWNTs. Acknowledgements We thank Dr. M. Hiramatsu of Meijo University for allowing the use of the Raman spectrometer. This work was supported by Grant-in-aid for Intellectual Cluster Project by the Ministry of Education, Culture, Sports, Science and Technology, Japan, Aichi Prefecture and the Aichi Science and Technology Foundation. References [1] M.S. Dresselhaus, G. Dresselhaus, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, New York, 1996. [2] R. Saito, G. Dresselhaus, M.S. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London, 1998. [3] A. Thess, R. Lee, P. Nikolaev, et al., Science 273 (1996) 483. [4] C. Journet, W.K. Maser, P. Bernier, et al., Nature 388 (1997) 756. [5] H.M. Cheng, F. Li, G. Su, H.Y. Pan, L.L. He, X. Sun, M.S. Dresselhaus, Appl. Phys. Lett. 72 (1998) 3282. [6] P. Nikolaev, M.J. Bronikowski, R.K. Bradley, et al., Chem. Phys. Lett. 313 (1999) 91. [7] S. Maruyama, R. Kojima, Y. Miyauchi, S. Chiashi, M. Kohno, Chem. Phys. Lett. 360 (2002) 229. [8] A.G. Rinzler, J. Liu, H. Dai, et al., Appl. Phys., A 67 (1998) 29. [9] E. Dujardin, C. Meny, P. Panissod, J.-P. Kintzinger, N. Yao, T.W. Ebbesen, Solid State Commun. 114 (2000) 543. [10] D. Chattopadhyay, I. Galeska, F. Papadimitrakopoulos, Carbon 40 (2002) 985. [11] A.R. Harutyunyan, B.K. Pradhan, J. Chang, G. Chen, P.C. Eklund, J. Phys. Chem., B 106 (2002) 8671. [12] K. Hata, D.N. Futaba, K. Mizuno, T. Namai, M. Yumura, S. Iijima, Science 306 (2004) 1362. [13] D.S. Bethune, C.H. Kiang, M.S. de Vries, et al., Nature 363 (1993) 605. [14] Y. Ando, X. Zhao, K. Hirahara, K. Suenaga, S. Bandow, S. Iijima, Chem. Phys. Lett. 323 (2000) 580. [15] X. Zhao, S. Inoue, M. Jinno, T. Suzuki, Y. Ando, Chem. Phys. Lett. 373 (2003) 266. [16] Y. Ando, X. Zhao, S. Inoue, T. Suzuki, T. Kadoya, Diamond Relat. Mater. 14 (2005) 729. [17] T. Suzuki, Y. Guo, S. Inoue, et al., J. Nanoparticle Research, in press. [18] T. Okazaki, H. Lange, N. Murata, et al., Diamond Relat. Mater. (private communication). [19] A.M. Rao, J. Chen, E. Richter, et al., Phys. Rev. Lett. 86 (2001) 3895.