- Email: [email protected]
High-purity single-wall carbon nanotubes synthesized from coal by arc discharge
2170
Letters to the Editor / Carbon 41 (2003) 2159 – 2179
[10] Matsuo Y, Sugie Y. Electrochemical intercalation of lithium into pyrolytic carbon from graphite oxide. Denki Kagaku (Electrochemistry) 1998;66:1288–90.
[11] Matsuo Y, Sugie Y. Electrochemical lithiation of carbon prepared from pyrolysis of graphite oxide. J Electrochem Soc 1999;146:2011–4.
High-purity single-wall carbon nanotubes synthesized from coal by arc discharge Jieshan Qiu a , *, Yongfeng Li a , Yunpeng Wang a , Tonghua Wang a , Zongbin Zhao a , Ying Zhou a , Feng Li b , Huiming Cheng b a
Carbon Research Laboratory, Center for Nano Materials and Science, Department of Materials Science and Chemical Engineering, Dalian University of Technology, 158 Zhongshan Road, P.O. Box 49, Dalian 116012, China b Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China Received 15 November 2002; accepted 22 May 2003
Keywords: A. Carbon nanotubes, Coal; B. Arc discharge; C. Raman spectroscopy, Transmission electron microscopy
Single-wall carbon nanotubes (SWNTs) exhibiting many unique and useful physical and chemical properties [1] have drawn great attention around the world. Extensive experimental and theoretical efforts are being pursued to understand their structures, mechanism of formation, as well as their electronic, vibrational, mechanical, chemical and hydrogen storage properties [2–5]. These research activities have led to many potential applications of SWNTs, and at the same time, also led to great demands for this exciting new material. SWNTs were first prepared by metal-catalyzed DC arcing of graphite rods in helium [6,7]. The graphite anode was filled with metal powders (Fe, Co, Ni) and the cathode was of pure graphite. SWNTs prepared in this way generally deposited behind the cathode in the form of web-like materials. In the past decade various alternate synthesis strategies and methods have been explored and developed in the hope of mass-producing cheap and highpurity SWNTs [5,7–17]. Compared with other methods, the arc discharge method is simple, cheap and easy to implement, and has been widely used because of its potential merits to make a massive production of SWNTs. It was reported that Journet et al. [8] had successfully obtained SWNTs from graphite by arc discharge with a mixture of 1% yttrium and 4.2% nickel (by number) as catalyst and the content of SWNTs in the collected deposits was about 80%. However, in the conventional arc discharge process, a graphite electrode is normally used, which consequently results in the high cost of SWNTs in practical production because the graphite electrode is *Corresponding author. Tel.: 186-411-370-5939; fax: 186411-363-3080. E-mail address: [email protected] (J. Qiu).
already a kind of value-added carbon. One of the feasible options for low cost SWNTs is to replace the value-added graphite electrodes with cheap ones obtained from other cheap carbonaceous materials. For this purpose coal appears to be a better candidate because it is one of the cheap and abundant carbon sources in nature. Several groups have investigated the possibility of preparing carbon nanotubes and other carbon nanomaterials from coal [18–24]. Over the past 7–8 years our group has been studying the features and feasibility of producing fullerenes and carbon nanotubes from Chinese coals [21– 24]. It has been found that the higher the carbon content in raw coals is, the higher the yield of fullerenes and carbon nanotubes is [22,23]. Here we report the preparation of high-purity SWNTs from an anthracite coal by arc discharge with iron as catalyst. The preparation of SWNTs was conducted in an arc discharge reactor inside which a cage made of iron wires was put around the electrodes, here the wire cage acted as the substrate for collecting SWNTs. With the wire cage inside, as schematically shown in Fig. 1, the SWNTs in web-like or film-like forms deposited on the cage surface instead of depositing on the chamber walls of the arc reactor. It was found that this simple innovation greatly reduced the amorphous carbon contaminants in SWNTs. In addition to the SWNTs, very small amount of hard deposits was found on the front face of the cathode and some soot containing higher fullerenes (C 84 , C 104 , C 106 , etc.), that was confirmed by LD–TOF– MS, was found on the inner walls of the reactor. In this work, one typical Chinese anthracite coal was used. Its proximate and ultimate analysis data is shown in Table 1. The coal sample without any pretreatment was crushed and sieved to 150 mm, and fully dried before use. The dried coal powder was mixed with coal tar, and the
0008-6223 / 03 / $ – see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016 / S0008-6223(03)00242-2
Letters to the Editor / Carbon 41 (2003) 2159 – 2179
Fig. 1. Schematic diagram of the arc discharge reactor, showing that a wire cage is placed inside for collecting SWNTs. (1) Reactor wall; (2) iron wire cage; (3) coal-derived carbon anode containing iron catalyst; (4) graphite cathode. Table 1 Analysis data of coal sample Proximate analysis (%)
Ultimate analysis (daf %)
Mad
Ad
Vdaf
C
H
N
S
O*
2.78
3.92
15.18
87.86
3.66
0.73
0.36
7.39
*By difference.
weight ratio of tar to coal was 20–80%. The mixture was subsequently pressed at about 0–20 MPa to make hollow coal rods (10 mm O.D., 5 mm I.D., ¯120 mm length). The hollow coal rods were put into a tube furnace and heated to 1173 K at a rate of 10 K min 21 and were heated at the final temperature for 4 h before cooling down to room temperature, and finally the coal-based hollow carbon rods (designated as HCRs) were obtained. The heat treatment was conducted in flowing nitrogen to avoid the oxidation of the hollow tubes. The HCRs were filled with a mixture of iron powder (|117 mm) and carbon powder in a ratio of 2:1 by weight, and both ends of the HCRs were sealed with graphite plugs (5 mm in diameter, 3 mm in length). The carbon powder used in the mixture for filling the tubes was obtained by crushing some HCRs obtained as de-
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scribed above. In the arc discharge experiments, the filled HCRs were used as anode and a high-purity graphite electrode (16 mm in diameter, 30 mm in length) was used as cathode. These two electrodes were surrounded by a cage made of iron wires, and in the arcing process the formed SWNTs deposited on the cage surface, as schematically shown in Fig. 1. The arc discharge was conducted with a current of 50–70 A and a voltage of 30–50 V in a helium atmosphere at 0.065 MPa. The distance between two electrodes was kept constant at about 1–2 mm by manually feeding the coal-based anode, i.e. the filled HCRs. The arc discharge experiments normally lasted about 20 min. After the arc discharge was finished, the wire cage was taken out of the arc reactor, and the web-like or film-like deposits on the cage surface were directly peeled off and examined by scanning electron microscopy (SEM, JSM-5600LV), transmission electron microscopy (TEM, JEM-2000EX) and Raman spectroscopy (JY LabRam HR800, 632.8 nm He–Ne laser). It should be noted that once the SWNTs-containing web-like deposits were peeled off the wire cage, they tended to shrink and / or curl up immediately to form bundles that look like somewhat of the ash left behind after a piece of paper is burned. SEM has been employed to study as-formed film-like deposits that were peeled off the wire cage and to check the alignment of nanotubes. The typical SEM images of SWNTs-containing deposits peeled off the wire cage are shown in Fig. 2. The image shown in Fig. 2a (scale bar 100 mm) shows that the coal-derived film-like deposits peeled off the wire cage shrink greatly and curl up to form cotton-like threads or bundles. The image in Fig. 2b (scale bar 10 mm) shows a section as circled in Fig. 2a, wherein nanotubes with a high packing density can be seen, and the carbon nanotubes seem to be aligned quite well along their axis. The coal-derived SWNTs were further examined by TEM and the typical TEM images are shown in Fig. 3. The
Fig. 2. SEM images of coal-derived film-like SWNTs peeled off the wire cage. (a) A typical SEM image of a curled SWNTs rope or bundle that looks like cotton threads; Scale bar5100 mm; (b) an image of the circled section as shown in Fig. 1a, showing numerous high-purity SWNTs. Scale bar510 mm.
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Letters to the Editor / Carbon 41 (2003) 2159 – 2179
Fig. 3. TEM images of coal-derived film-like SWNTs. (a) A low magnification image of SWNTs bundles consisting of a large amount of entangled carbon nanotubes with some contaminants such as carbon-coated nanoparticles; Scale bar5100 nm; (b) a high magnification image showing seven SWNTs bundles. Scale bar520 nm.
TEM observation reveals that a large number of SWNTs bundles exist and entangle together in the film-like deposits. Other impurities such as carbon-coated nanoparticles can also be seen, but much less than in the case of traditional arc discharge. The improvement in purity of SWNTs can be attributed to the innovation in the collection method adopted in our work. Fig. 3b shows a high magnification image of coal-derived SWNTs, wherein about seven SWNTs bundles and several nanoparticles (10–20 nm in diameter) can be seen clearly, indicating that the purity of coal-derived SWNTs is quite high. Raman spectroscopy is one of powerful techniques for identifying and studying SWNTs, and has provided important insights into the structure of nanotubes. Fig. 4 shows the typical Raman spectra of the as-prepared filmlike SWNTs deposits peeled off the wire cage, which were measured with a laser light in a wavelength of 632 nm. A broad peak called the D-band can be clearly seen at 1340 cm 21 , which is related to the disordered or defective graphitic structure. The main peak called the G-band appears at 1590 cm 21 , which is one of the characteristic Raman peaks of carbons with well-developed graphitic structures [25]. It is known that the ratio of the peak intensities at 1590 to 1340 cm 21 (called the G / D value) is a good index of the SWNTs content, and the G / D value increases with increasing SWNTs in the sample. The average G / D value of the coal-based SWNTs was calculated to be around 22, which is quite high, indicating that the film-like deposits synthesized from coal contain a high
content of SWNTs. This is consistent with the SEM and TEM observation discussed above. Other characteristics of SWNTs are the Raman peaks in the low frequency region, which are strongly diameterdependent. The inset in Fig. 4 shows the Raman spectra of coal-derived SWNTs material in the wave number region below 250 cm 21 , which are attributed to the radial breathing modes (RBM) of SWNTs. As shown in the inset, for the coal-derived SWNTs in this work, several strong
Fig. 4. Raman spectra of SWNTs obtained from coal in the frequency region of 0–2000 cm 21 , the inset showing Raman spectra of SWNTs in the lower frequency region below 250 cm 21 .
Letters to the Editor / Carbon 41 (2003) 2159 – 2179
peaks appear at 115, 138, and 195 cm 21 , respectively. It has been well established that the frequency of the RBM is in inverse proportion to the diameter of the SWNTs [26], and this can be correlated by the following equation, vr 5 238 /d 0.93 , where vr is the RBM frequency in cm 21 and d is the SWNTs diameter in nm [27]. According to this equation, the RBM frequencies of 115, 138, and 195 cm 21 correspond to 2.19, 1.79, and 1.24 nm, respectively. This means that the diameters of the coal-derived SWNTs are in a range between 1.24 and 2.19 nm, which is a bit larger than the diameter distribution of SWNTs produced from graphite by arc discharge. This large dispersion could be due to both the catalyst used (iron) and the coal-derived carbon. Work is now in progress with a hope of clarifying the reason why the diameter distribution of coal-based SWNTs with iron as catalyst by arc discharge is large. In summary, we have demonstrated that it is possible to produce high-quality SWNTs from inexpensive coal-based carbons by arc discharge with iron as catalyst. The purity of SWNTs bundles is remarkably improved by simply putting a wire cage around the carbon electrodes for collecting SWNTs, which helps to greatly reduce the contaminants such as soot and carbon coated nanoparticles in the SWNTs deposits. It should be noted that the conclusions about the quality of SWNTs samples in this letter are drawn on the basis of examining more than 10 samples both in TEM and Raman. The present work is a step toward mass-production of cheap SWNTs. We believe that further exploration will provide an economical means of producing reasonably priced SWNTs from coal in quantity.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 29976006), the Natural Science Foundation of Liaoning Province (No. 9810300701 and No. 2001101003), and the Education Ministry of China. We also would like to thank the reviewers for helpful comments.
References [1] Yakobson BI, Smalley RE. Am Sci 1997;85:324. [2] Dresselhaus MS, Dresselhaus GF, Eklund PC. Science of fullerenes and carbon nanotubes. New York: Academic Press; 1996. [3] Saitto R, Fujita M, Dresselhaus GF, Dresselhaus MS. Appl Phys Lett 1992;60:2204.
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[4] Dresselhaus MS, Dresselhaus G, Eklund PC, Saito R. Phys World 1998;11:33. [5] Harris PJF. Carbon nanotubes and related structures. Cambridge, UK: Cambridge University Press; 1999. [6] Iijima S, Ichihashi T. Nature 1993;363:603. [7] Bethune DS, Kiang CH, de Vries MS, Gorman G, Savoy R, Vazquez J, Beyers R. Nature 1993;363:605. [8] Journet C, Maser WK, Bernier P, Loiseau A, Lamy de la Chapelle A, Lefrant S, Deniard P, Lee R, Fisher JE. Nature 1997;388:756. [9] Dai H, Rinzler AG, Nikolaev P, Thess A, Colbert DH, Smalley RE. Chem Phys Lett 1996;260:471. [10] Cheng HM, Li F, Su G, Pan HY, He LL, Sun X, Dresselhaus MS. Appl Phys Lett 1998;72:3282. [11] Laplaze D, Bernier P, Maser W, Flamant G, Guillard T, Loiseau A. Carbon 1998;36:685. [12] Nikolaev P, Bronikowski M, Bradley RK, Rohmund F, Colbert DT, Smith KA, Smalley RE. Chem Phys Lett 1999;313:91. [13] Kitiyanan B, Alvarez WE, Harwell JH, Resasco DE. Chem Phys Lett 2000;317:497. [14] Colomer JF, Stefan C, Lefrant S, van Tendeloo G, Willems I, Konya Z, Fonseca A, Laurent C, Nagy JB. Chem Phys Lett 2000;317:83. [15] Thess A, Lee R, Nikolaev P, Dai HJ, Petit P, Robert J, Xu CH, Lee YH, Kim SG, Rinzler AG, Colbert DT, Scuseria GE, Tomanek D, Fischer JE, Smalley RE. Science 1996;273:483. [16] Bronikoowski MJ, Willis PA, Colbert DT, Smith KA, Smalley RE. J Vac Sci Technol A 2001;19(4):1800. [17] Falhaut F, Govindaraj A, Peigney A, Laurent C, Sousset A, Rao CNR. Chem Phys Lett 1999;300:236. [18] Pang LSK, Wilson MA. Energy Fuels 1993;7:436. [19] Williams KA, Tachibana M, Allen JL, Grigorian L, Cheng SC, Fang SL, Sumanasekera GU, Loper AL, Williams JH, Eklund PC. Chem Phys Lett 1999;310:31. [20] Wilson MA, Patney HK, Kalman J. Fuel 2002;81(1):5. [21] Qiu JS, Zhou Y, Wang LN, Tsang SC. Carbon 1998;36(4):465. [22] Qiu JS, Zhou Y, Yang ZG, Wang DK, Guo SC, Tsang SC, Harris PJF. Fuel 2000;79:1303. [23] Qiu JS, Zhang F, Zhou Y, Han HM, Hu DS, Tsang SC, Harris PJF. Fuel 2002;81:1509. [24] Li YF, Qiu JS, Zhao ZB, Wang TH, Wang YP, Li W. Chem Phys Lett 2002;366(5 / 6):544. [25] Kasuya A, Sakaki Y, Saito Y, Tohji Y, Nishina Y. Phys Rev Lett 1997;78:4434. [26] Saito R, Dresselhaus G, Dresselhaus MS. Physical properties of carbon nanotubes. London: Imperial College Press; 1998, chapter 10. [27] Rols S, Righi A, Alvares L, Anglaret E, Almairac R, Journet C, Bernier P, Sauvajol JL. Eur Phys J B 2002;18:201.
Letters to the Editor / Carbon 41 (2003) 2159 – 2179
[10] Matsuo Y, Sugie Y. Electrochemical intercalation of lithium into pyrolytic carbon from graphite oxide. Denki Kagaku (Electrochemistry) 1998;66:1288–90.
[11] Matsuo Y, Sugie Y. Electrochemical lithiation of carbon prepared from pyrolysis of graphite oxide. J Electrochem Soc 1999;146:2011–4.
High-purity single-wall carbon nanotubes synthesized from coal by arc discharge Jieshan Qiu a , *, Yongfeng Li a , Yunpeng Wang a , Tonghua Wang a , Zongbin Zhao a , Ying Zhou a , Feng Li b , Huiming Cheng b a
Carbon Research Laboratory, Center for Nano Materials and Science, Department of Materials Science and Chemical Engineering, Dalian University of Technology, 158 Zhongshan Road, P.O. Box 49, Dalian 116012, China b Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China Received 15 November 2002; accepted 22 May 2003
Keywords: A. Carbon nanotubes, Coal; B. Arc discharge; C. Raman spectroscopy, Transmission electron microscopy
Single-wall carbon nanotubes (SWNTs) exhibiting many unique and useful physical and chemical properties [1] have drawn great attention around the world. Extensive experimental and theoretical efforts are being pursued to understand their structures, mechanism of formation, as well as their electronic, vibrational, mechanical, chemical and hydrogen storage properties [2–5]. These research activities have led to many potential applications of SWNTs, and at the same time, also led to great demands for this exciting new material. SWNTs were first prepared by metal-catalyzed DC arcing of graphite rods in helium [6,7]. The graphite anode was filled with metal powders (Fe, Co, Ni) and the cathode was of pure graphite. SWNTs prepared in this way generally deposited behind the cathode in the form of web-like materials. In the past decade various alternate synthesis strategies and methods have been explored and developed in the hope of mass-producing cheap and highpurity SWNTs [5,7–17]. Compared with other methods, the arc discharge method is simple, cheap and easy to implement, and has been widely used because of its potential merits to make a massive production of SWNTs. It was reported that Journet et al. [8] had successfully obtained SWNTs from graphite by arc discharge with a mixture of 1% yttrium and 4.2% nickel (by number) as catalyst and the content of SWNTs in the collected deposits was about 80%. However, in the conventional arc discharge process, a graphite electrode is normally used, which consequently results in the high cost of SWNTs in practical production because the graphite electrode is *Corresponding author. Tel.: 186-411-370-5939; fax: 186411-363-3080. E-mail address: [email protected] (J. Qiu).
already a kind of value-added carbon. One of the feasible options for low cost SWNTs is to replace the value-added graphite electrodes with cheap ones obtained from other cheap carbonaceous materials. For this purpose coal appears to be a better candidate because it is one of the cheap and abundant carbon sources in nature. Several groups have investigated the possibility of preparing carbon nanotubes and other carbon nanomaterials from coal [18–24]. Over the past 7–8 years our group has been studying the features and feasibility of producing fullerenes and carbon nanotubes from Chinese coals [21– 24]. It has been found that the higher the carbon content in raw coals is, the higher the yield of fullerenes and carbon nanotubes is [22,23]. Here we report the preparation of high-purity SWNTs from an anthracite coal by arc discharge with iron as catalyst. The preparation of SWNTs was conducted in an arc discharge reactor inside which a cage made of iron wires was put around the electrodes, here the wire cage acted as the substrate for collecting SWNTs. With the wire cage inside, as schematically shown in Fig. 1, the SWNTs in web-like or film-like forms deposited on the cage surface instead of depositing on the chamber walls of the arc reactor. It was found that this simple innovation greatly reduced the amorphous carbon contaminants in SWNTs. In addition to the SWNTs, very small amount of hard deposits was found on the front face of the cathode and some soot containing higher fullerenes (C 84 , C 104 , C 106 , etc.), that was confirmed by LD–TOF– MS, was found on the inner walls of the reactor. In this work, one typical Chinese anthracite coal was used. Its proximate and ultimate analysis data is shown in Table 1. The coal sample without any pretreatment was crushed and sieved to 150 mm, and fully dried before use. The dried coal powder was mixed with coal tar, and the
0008-6223 / 03 / $ – see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016 / S0008-6223(03)00242-2
Letters to the Editor / Carbon 41 (2003) 2159 – 2179
Fig. 1. Schematic diagram of the arc discharge reactor, showing that a wire cage is placed inside for collecting SWNTs. (1) Reactor wall; (2) iron wire cage; (3) coal-derived carbon anode containing iron catalyst; (4) graphite cathode. Table 1 Analysis data of coal sample Proximate analysis (%)
Ultimate analysis (daf %)
Mad
Ad
Vdaf
C
H
N
S
O*
2.78
3.92
15.18
87.86
3.66
0.73
0.36
7.39
*By difference.
weight ratio of tar to coal was 20–80%. The mixture was subsequently pressed at about 0–20 MPa to make hollow coal rods (10 mm O.D., 5 mm I.D., ¯120 mm length). The hollow coal rods were put into a tube furnace and heated to 1173 K at a rate of 10 K min 21 and were heated at the final temperature for 4 h before cooling down to room temperature, and finally the coal-based hollow carbon rods (designated as HCRs) were obtained. The heat treatment was conducted in flowing nitrogen to avoid the oxidation of the hollow tubes. The HCRs were filled with a mixture of iron powder (|117 mm) and carbon powder in a ratio of 2:1 by weight, and both ends of the HCRs were sealed with graphite plugs (5 mm in diameter, 3 mm in length). The carbon powder used in the mixture for filling the tubes was obtained by crushing some HCRs obtained as de-
2171
scribed above. In the arc discharge experiments, the filled HCRs were used as anode and a high-purity graphite electrode (16 mm in diameter, 30 mm in length) was used as cathode. These two electrodes were surrounded by a cage made of iron wires, and in the arcing process the formed SWNTs deposited on the cage surface, as schematically shown in Fig. 1. The arc discharge was conducted with a current of 50–70 A and a voltage of 30–50 V in a helium atmosphere at 0.065 MPa. The distance between two electrodes was kept constant at about 1–2 mm by manually feeding the coal-based anode, i.e. the filled HCRs. The arc discharge experiments normally lasted about 20 min. After the arc discharge was finished, the wire cage was taken out of the arc reactor, and the web-like or film-like deposits on the cage surface were directly peeled off and examined by scanning electron microscopy (SEM, JSM-5600LV), transmission electron microscopy (TEM, JEM-2000EX) and Raman spectroscopy (JY LabRam HR800, 632.8 nm He–Ne laser). It should be noted that once the SWNTs-containing web-like deposits were peeled off the wire cage, they tended to shrink and / or curl up immediately to form bundles that look like somewhat of the ash left behind after a piece of paper is burned. SEM has been employed to study as-formed film-like deposits that were peeled off the wire cage and to check the alignment of nanotubes. The typical SEM images of SWNTs-containing deposits peeled off the wire cage are shown in Fig. 2. The image shown in Fig. 2a (scale bar 100 mm) shows that the coal-derived film-like deposits peeled off the wire cage shrink greatly and curl up to form cotton-like threads or bundles. The image in Fig. 2b (scale bar 10 mm) shows a section as circled in Fig. 2a, wherein nanotubes with a high packing density can be seen, and the carbon nanotubes seem to be aligned quite well along their axis. The coal-derived SWNTs were further examined by TEM and the typical TEM images are shown in Fig. 3. The
Fig. 2. SEM images of coal-derived film-like SWNTs peeled off the wire cage. (a) A typical SEM image of a curled SWNTs rope or bundle that looks like cotton threads; Scale bar5100 mm; (b) an image of the circled section as shown in Fig. 1a, showing numerous high-purity SWNTs. Scale bar510 mm.
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Letters to the Editor / Carbon 41 (2003) 2159 – 2179
Fig. 3. TEM images of coal-derived film-like SWNTs. (a) A low magnification image of SWNTs bundles consisting of a large amount of entangled carbon nanotubes with some contaminants such as carbon-coated nanoparticles; Scale bar5100 nm; (b) a high magnification image showing seven SWNTs bundles. Scale bar520 nm.
TEM observation reveals that a large number of SWNTs bundles exist and entangle together in the film-like deposits. Other impurities such as carbon-coated nanoparticles can also be seen, but much less than in the case of traditional arc discharge. The improvement in purity of SWNTs can be attributed to the innovation in the collection method adopted in our work. Fig. 3b shows a high magnification image of coal-derived SWNTs, wherein about seven SWNTs bundles and several nanoparticles (10–20 nm in diameter) can be seen clearly, indicating that the purity of coal-derived SWNTs is quite high. Raman spectroscopy is one of powerful techniques for identifying and studying SWNTs, and has provided important insights into the structure of nanotubes. Fig. 4 shows the typical Raman spectra of the as-prepared filmlike SWNTs deposits peeled off the wire cage, which were measured with a laser light in a wavelength of 632 nm. A broad peak called the D-band can be clearly seen at 1340 cm 21 , which is related to the disordered or defective graphitic structure. The main peak called the G-band appears at 1590 cm 21 , which is one of the characteristic Raman peaks of carbons with well-developed graphitic structures [25]. It is known that the ratio of the peak intensities at 1590 to 1340 cm 21 (called the G / D value) is a good index of the SWNTs content, and the G / D value increases with increasing SWNTs in the sample. The average G / D value of the coal-based SWNTs was calculated to be around 22, which is quite high, indicating that the film-like deposits synthesized from coal contain a high
content of SWNTs. This is consistent with the SEM and TEM observation discussed above. Other characteristics of SWNTs are the Raman peaks in the low frequency region, which are strongly diameterdependent. The inset in Fig. 4 shows the Raman spectra of coal-derived SWNTs material in the wave number region below 250 cm 21 , which are attributed to the radial breathing modes (RBM) of SWNTs. As shown in the inset, for the coal-derived SWNTs in this work, several strong
Fig. 4. Raman spectra of SWNTs obtained from coal in the frequency region of 0–2000 cm 21 , the inset showing Raman spectra of SWNTs in the lower frequency region below 250 cm 21 .
Letters to the Editor / Carbon 41 (2003) 2159 – 2179
peaks appear at 115, 138, and 195 cm 21 , respectively. It has been well established that the frequency of the RBM is in inverse proportion to the diameter of the SWNTs [26], and this can be correlated by the following equation, vr 5 238 /d 0.93 , where vr is the RBM frequency in cm 21 and d is the SWNTs diameter in nm [27]. According to this equation, the RBM frequencies of 115, 138, and 195 cm 21 correspond to 2.19, 1.79, and 1.24 nm, respectively. This means that the diameters of the coal-derived SWNTs are in a range between 1.24 and 2.19 nm, which is a bit larger than the diameter distribution of SWNTs produced from graphite by arc discharge. This large dispersion could be due to both the catalyst used (iron) and the coal-derived carbon. Work is now in progress with a hope of clarifying the reason why the diameter distribution of coal-based SWNTs with iron as catalyst by arc discharge is large. In summary, we have demonstrated that it is possible to produce high-quality SWNTs from inexpensive coal-based carbons by arc discharge with iron as catalyst. The purity of SWNTs bundles is remarkably improved by simply putting a wire cage around the carbon electrodes for collecting SWNTs, which helps to greatly reduce the contaminants such as soot and carbon coated nanoparticles in the SWNTs deposits. It should be noted that the conclusions about the quality of SWNTs samples in this letter are drawn on the basis of examining more than 10 samples both in TEM and Raman. The present work is a step toward mass-production of cheap SWNTs. We believe that further exploration will provide an economical means of producing reasonably priced SWNTs from coal in quantity.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 29976006), the Natural Science Foundation of Liaoning Province (No. 9810300701 and No. 2001101003), and the Education Ministry of China. We also would like to thank the reviewers for helpful comments.
References [1] Yakobson BI, Smalley RE. Am Sci 1997;85:324. [2] Dresselhaus MS, Dresselhaus GF, Eklund PC. Science of fullerenes and carbon nanotubes. New York: Academic Press; 1996. [3] Saitto R, Fujita M, Dresselhaus GF, Dresselhaus MS. Appl Phys Lett 1992;60:2204.
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