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Single-source precursor route to carbon nanotubes at mild temperature
Carbon 41 (2003) 2101–2104
Single-source precursor route to carbon nanotubes at mild temperature Jianwei Liu a , Mingwang Shao a,b , Qin Xie a , Lingfeng Kong a , Weichao Yu a , Yitai Qian a , * a
Structure Research Laboratory and Department of Chemistry, University of Science and Technology of China, Hefei 230026, People’ s Republic of China b College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, People’ s Republic of China Received 24 January 2003; accepted 10 May 2003
Abstract Carbon nanotubes were synthesized via a single-source precursor route at 500 8C, using iron carbonyl both as carbon source and catalyst. The X-ray power diffraction pattern indicates that the products are hexagonal graphite. Transmission electron microscope (TEM) images of the sample reveal carbon nanotubes with an average inner (outer) diameter of 30 nm (60 nm). High-resolution TEM indicates that fabrication of the carbon nanotube walls was composed of ca. 40 graphene layers. The Raman spectrum shows two strong peaks at 1587 and 1346 cm 21 , corresponding to the typical Raman peaks of graphitized carbon nanotubes. This method avoids the separation of raw material from solvent and simplifies the operation process. At the same time, the research provides a new route to large-scale synthesis of carbon nanotubes. 2003 Elsevier Ltd. All rights reserved. Keywords: A. Carbon nanotubes; B. Catalyst; D. Carbon yield
1. Introduction Carbon nanotubes are interesting materials because of their uses in nanometer-scale electrical devices [1,2], gas storage media [3], nanotweezers [4], and structural composites [5]. Since Iijima discovered carbon nanotubes in 1991 [6], considerable efforts have been made to improve their preparation and study their formation mechanism. In the following years, various methods were demonstrated for the synthesis of carbon nanotubes, such as metal catalyzed chemical vapor deposition (CVD) [7], carbonarc discharge [8], or laser ablation of carbon [9] and solvothermal methods [10,11]. Recently, Rohmund et al. [12] reported that carbon nanotubes were synthesized through thermal chemical vapor deposition from iron carbonyl and acetylene. Subsequently, an interesting method was reported by Gokcen et al. [13], who produced carbon nanotubes from gas-phase reactions of iron carbonyl in carbon monoxide at pressures of 10–100 atm. In *Corresponding author. Tel.: 186-55-1360-1589; fax: 18655-1360-7402. E-mail address: [email protected] (Y. Qian).
the above two methods, iron catalyst particles were obtained from thermal decomposition of iron carbonyl, while acetylene and carbon monoxide served as carbon
Fig. 1. XRD pattern of carbon nanotubes, using Cu Ka X-rays as radiation.
0008-6223 / 03 / $ – see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016 / S0008-6223(03)00207-0
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J. Liu et al. / Carbon 41 (2003) 2101–2104
feedstocks. There are also some reports on the synthesis of carbon nanotubes from CO disproportionation at temperatures of 600–850 8C [14–16]. Here, we report that carbon nanotubes were prepared through a single-source precursor method at 500 8C which employed iron carbonyl both as carbon source and catalyst.
2. Experimental Five ml Fe(CO) 5 were added to a stainless steel
autoclave with a capacity of 60 ml. The autoclave was kept at 500 8C under ca. 4 MPa pressure for 12 h and then cooled to room temperature naturally. The resultant was collected and purified with diluted hydrochloric acid. About 1.88 g products were obtained. The yield of carbon materials was about 85%. The detailed calculations are as follows: 5 ml (Fe(CO) 5 )31.457 g / ml ( r )399.5% (content)57.222 g (total weight); 12.01354195.903 7.22252.214 g (C); 1.8842.214¯85%. XRD patterns were recorded at a scanning rate of 0.028 s 21 in the 2u range from 108 to 708, using Cu Ka radiation
Fig. 2. (a) TEM image of many carbon close-ended nanotubes; (b) TEM image of a single carbon nanotube; (c) an SAED pattern, which was taken at the middle of the nanotube in (b); (d) TEM image of a carbon nanotube with two closed ends, and catalyst in a tip of the carbon nanotube.
J. Liu et al. / Carbon 41 (2003) 2101–2104
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( l50.154178 nm). TEM images and SAED patterns were taken using a Hitachi Model H-800 transmission electron microscope. The Raman spectrum was investigated with a French Labram-HR confocal laser microRaman spectrometer; an argon-ion laser at 514.5 nm was used. Highresolution electron microscopy images were taken on a JEOL-2010 transmission electron microscope.
3. Results and discussion The XRD pattern of the sample is shown in Fig. 1. The intense peaks at ca. 26.38 can be indexed to the (002) diffraction plane of hexagonal graphite. There is no obvious impurity peak. The TEM images, shown in Fig. 2a, indicate that products are nanotubes with an average inner (outer) diameter of 30 nm (60 nm). Fig. 2b displays a single nanotube. The selected area electron diffraction (SAED) pattern (Fig. 2c), which was taken at the middle of the nanotube, exhibits a pair of small but strong (002) arcs which indicate some orientation of the 002 planes in the carbon nanotubes. The nanotubes usually contain catalyst particles encapsulated at the tips of the tubes, as shown in Fig. 2a and d. Fig. 2 shows a typical nanotube with two closed ends. It can be observed that there are catalyst particles at a tip of the nanotube. The EDX spectrum (Fig. 3) shows Fe, Cu, C and O peaks. The Fe signals originated from the catalyst particles, C and Cu from the nanotube and copper grid, respectively, while O signals originated from the surface adsorption of the carbon nanotube. The results indicated that carbon nanotubes were grown from the catalytic particles. A typical HRTEM image of the nanotube is shown in Fig. 4. From the image, the lattice fringes are clearly
Fig. 3. Typical EDX spectrum of catalyst particles encapsulated at the tip of the carbon nanotube.
Fig. 4. HRTEM image of carbon nanotube which shows that the fabrication of the nanotube’s wall is composed of ca. 40 graphene layers.
visible. The fabrication of the walls was composed of ca. 40 graphene layers. Furthermore, the distance between two (002) planes is ca. 0.34 nm typical for the (002) lattice distance in hexagonal carbon. Fig. 5 shows the Raman spectrum of the carbon nanotubes. There exist two strong peaks at 1587 and 1346 cm 21 , corresponding to the typical Raman peaks of graphitized carbon nanotubes. The peak at 1587 cm 21 corresponds to an E 2g mode of graphite and is related to the vibration of sp 2 -bonded carbon atoms in a 2-dimensional hexagonal lattice, such as in a graphene layer [17]. The peak at 1346 cm 21 is associated with vibrations of
Fig. 5. Raman spectrum of carbon nanotubes which shows two graphite peaks at 1346 and 1587 cm 21 .
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carbon atoms with dangling bonds in plane terminations of disordered graphite. This peak is quite high indicating that in the basal plane there exists two-dimensional disorder which is quite common in mild temperature routes [11]. The possible formation mechanism of carbon nanotubes is proposed. Iron carbonyl decomposes at 500 8C to produce carbon and iron. The carbons produced from the above reaction adsorb at the surface of the iron particles. Then they diffuse via the surface of iron particles to form graphite sheets as a cap on the iron particle. While the cap lifts off the iron particle, a closed end with an inside hollow is formed. With constant surface diffusion, hexagonal carbon clusters may grow into nanotubes. The experiment results demonstrate that iron particles produced from the decomposition of iron carbonyl play an important role in the process of nanotube growth. An important implication of our work is the possibility of large-scale solvothermal preparation of bulk carbon nanotubes. As iron carbonyl acted both as catalyst, carbon source and solvent, this method avoids the separation of raw material from solvent and simplifies the operation process. The fact that the iron carbonyl acted as a solvent under this condition helps to accelerate diffusion, adsorption, reaction rate, and crystallization in the formation of carbon nanotubes under solvothermal conditions. All these contribute to a lower reaction temperature compared to other methods using carbon monoxide as the carbon source [14–16]. Another advantage of this experiment is a fuller utilization of the carbon source with the yield of as-prepared products as high as 85%.
4. Conclusion Carbon nanotubes have been successfully synthesized from a single-source precursor at 500 8C and for 12 h via a solvothermal catalytic process. Here, iron carbonyl acted both as catalyst, carbon source and solvent. In particular, the solvothermal method can provide better reaction and crystallization conditions for the growth of carbon nanotubes. Due to surface defects of the carbon nanotubes that may provide channels for gas diffusion, the nanotubes are suitable for gas storage. This work provides a new route to large-scale and low-temperature synthesis of carbon nanotubes.
Acknowledgements This work was supported by the National Natural Science Foundation of China and the 973 National Nanometer Materials Project.
References [1] Frank S, Poncharal P, Wang ZL, de Heer WA. Carbon nanotube quantum resistors. Science 1998;280(5370):1744– 6. [2] Kong J, Zhou C, Morpurgo A, Soh HT, Quate CF, Marcus C et al. Synthesis, integration, and electrical properties of individual single-walled carbon nanotubes. Appl Phys A Mater 1999;69(3):305–8. [3] Liu C, Fan YY, Liu M, Cong HT, Cheng HM, Dresselhaus MS. Hydrogen storage in single-walled carbon nanotubes at room temperature. Science 1999;286(5442):1127–9. [4] Kim P, Lieber CM. Nanotube nanotweezers. Science 1999;286(5447):2148–50. [5] Wu HQ, Wei XW, Shao MW, Gu JS, Qu MZ. Preparation of Fe–Ni alloy nanoparticles inside carbon nanotubes via wet chemistry. J Mater Chem 2002;12(6):1919–21. [6] Iijima S. Helical microtubes of graphitic carbon. Nature 1991;354(6348):56–8. [7] Benito AM, Maniette Y, Munoz E, Martinez MT. Carbon nanotubes production by catalytic pyrolysis of benzene. Carbon 1998;36(5–6):681–3. [8] Zeng H, Zhu L, Hao GM, Sheng RS. Synthesis of various forms of carbon nanotubes by AC arc discharge. Carbon 1998;36(3):259–61. [9] Ma RZ, Wei BQ, Xu CL, Liang J, Wu DH. The morphology changes of carbon nanotubes under laser irradiation. Carbon 2000;38(4):636–8. [10] Jiang Y, Wu Y, Zhang SY, Xu CY, Yu WC, Xie Y et al. A catalytic-assembly solvothermal route to multiwall carbon nanotubes at a moderate temperature. J Am Chem Soc 2000;122(49):12383–4. [11] Shao MW, Li Q, Wu J, Xie B, Zhang SY, Qian YT. Benzenethermal route to carbon nanotubes at a moderate temperature. Carbon 2002;40(15):2961–73. [12] Rohmund F, Falk LKL, Campbell EEB. A simple method for the production of large arrays of aligned carbon nanotubes. Chem Phys Lett 2000;328(4–6):369–73. [13] Gokcen T, Dateo CE, Meyyappan M. Modeling of the HiPco process for carbon nanotube production. II. Reactor-scale analysis. J Nanosci Nanotech 2002;2(5):535–44. [14] Lee JS, Suh JS. Uniform field emission from aligned carbon nanotubes prepared by CO disproportionation. J Appl Phys 2002;92(12):7519–22. [15] Pinheiro JP, Schouler MC, Dooryhee E. In situ X-ray diffraction study of carbon nanotubes and filaments during their formation over Co /Al 2 O 3 catalysts. Solid State Commun 2002;123(3–4):161–6. [16] Herrera JE, Balzano L, Borgna A, Alvarez WE, Resasco DE. Relationship between the structure / composition of Co–Mo catalysts and their ability to produce single-walled carbon nanotubes by CO disproportionation. J Catal 2001;204(1):129–45. [17] Dresselhaus MS, Dresselhaus G, Pimenta MA, Eklund PC. In: Pelletier MJ, editor, Analytical applications of Raman spectroscopy, Oxford: Blackwell Science; 1999, Chapter 9.
Single-source precursor route to carbon nanotubes at mild temperature Jianwei Liu a , Mingwang Shao a,b , Qin Xie a , Lingfeng Kong a , Weichao Yu a , Yitai Qian a , * a
Structure Research Laboratory and Department of Chemistry, University of Science and Technology of China, Hefei 230026, People’ s Republic of China b College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, People’ s Republic of China Received 24 January 2003; accepted 10 May 2003
Abstract Carbon nanotubes were synthesized via a single-source precursor route at 500 8C, using iron carbonyl both as carbon source and catalyst. The X-ray power diffraction pattern indicates that the products are hexagonal graphite. Transmission electron microscope (TEM) images of the sample reveal carbon nanotubes with an average inner (outer) diameter of 30 nm (60 nm). High-resolution TEM indicates that fabrication of the carbon nanotube walls was composed of ca. 40 graphene layers. The Raman spectrum shows two strong peaks at 1587 and 1346 cm 21 , corresponding to the typical Raman peaks of graphitized carbon nanotubes. This method avoids the separation of raw material from solvent and simplifies the operation process. At the same time, the research provides a new route to large-scale synthesis of carbon nanotubes. 2003 Elsevier Ltd. All rights reserved. Keywords: A. Carbon nanotubes; B. Catalyst; D. Carbon yield
1. Introduction Carbon nanotubes are interesting materials because of their uses in nanometer-scale electrical devices [1,2], gas storage media [3], nanotweezers [4], and structural composites [5]. Since Iijima discovered carbon nanotubes in 1991 [6], considerable efforts have been made to improve their preparation and study their formation mechanism. In the following years, various methods were demonstrated for the synthesis of carbon nanotubes, such as metal catalyzed chemical vapor deposition (CVD) [7], carbonarc discharge [8], or laser ablation of carbon [9] and solvothermal methods [10,11]. Recently, Rohmund et al. [12] reported that carbon nanotubes were synthesized through thermal chemical vapor deposition from iron carbonyl and acetylene. Subsequently, an interesting method was reported by Gokcen et al. [13], who produced carbon nanotubes from gas-phase reactions of iron carbonyl in carbon monoxide at pressures of 10–100 atm. In *Corresponding author. Tel.: 186-55-1360-1589; fax: 18655-1360-7402. E-mail address: [email protected] (Y. Qian).
the above two methods, iron catalyst particles were obtained from thermal decomposition of iron carbonyl, while acetylene and carbon monoxide served as carbon
Fig. 1. XRD pattern of carbon nanotubes, using Cu Ka X-rays as radiation.
0008-6223 / 03 / $ – see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016 / S0008-6223(03)00207-0
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J. Liu et al. / Carbon 41 (2003) 2101–2104
feedstocks. There are also some reports on the synthesis of carbon nanotubes from CO disproportionation at temperatures of 600–850 8C [14–16]. Here, we report that carbon nanotubes were prepared through a single-source precursor method at 500 8C which employed iron carbonyl both as carbon source and catalyst.
2. Experimental Five ml Fe(CO) 5 were added to a stainless steel
autoclave with a capacity of 60 ml. The autoclave was kept at 500 8C under ca. 4 MPa pressure for 12 h and then cooled to room temperature naturally. The resultant was collected and purified with diluted hydrochloric acid. About 1.88 g products were obtained. The yield of carbon materials was about 85%. The detailed calculations are as follows: 5 ml (Fe(CO) 5 )31.457 g / ml ( r )399.5% (content)57.222 g (total weight); 12.01354195.903 7.22252.214 g (C); 1.8842.214¯85%. XRD patterns were recorded at a scanning rate of 0.028 s 21 in the 2u range from 108 to 708, using Cu Ka radiation
Fig. 2. (a) TEM image of many carbon close-ended nanotubes; (b) TEM image of a single carbon nanotube; (c) an SAED pattern, which was taken at the middle of the nanotube in (b); (d) TEM image of a carbon nanotube with two closed ends, and catalyst in a tip of the carbon nanotube.
J. Liu et al. / Carbon 41 (2003) 2101–2104
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( l50.154178 nm). TEM images and SAED patterns were taken using a Hitachi Model H-800 transmission electron microscope. The Raman spectrum was investigated with a French Labram-HR confocal laser microRaman spectrometer; an argon-ion laser at 514.5 nm was used. Highresolution electron microscopy images were taken on a JEOL-2010 transmission electron microscope.
3. Results and discussion The XRD pattern of the sample is shown in Fig. 1. The intense peaks at ca. 26.38 can be indexed to the (002) diffraction plane of hexagonal graphite. There is no obvious impurity peak. The TEM images, shown in Fig. 2a, indicate that products are nanotubes with an average inner (outer) diameter of 30 nm (60 nm). Fig. 2b displays a single nanotube. The selected area electron diffraction (SAED) pattern (Fig. 2c), which was taken at the middle of the nanotube, exhibits a pair of small but strong (002) arcs which indicate some orientation of the 002 planes in the carbon nanotubes. The nanotubes usually contain catalyst particles encapsulated at the tips of the tubes, as shown in Fig. 2a and d. Fig. 2 shows a typical nanotube with two closed ends. It can be observed that there are catalyst particles at a tip of the nanotube. The EDX spectrum (Fig. 3) shows Fe, Cu, C and O peaks. The Fe signals originated from the catalyst particles, C and Cu from the nanotube and copper grid, respectively, while O signals originated from the surface adsorption of the carbon nanotube. The results indicated that carbon nanotubes were grown from the catalytic particles. A typical HRTEM image of the nanotube is shown in Fig. 4. From the image, the lattice fringes are clearly
Fig. 3. Typical EDX spectrum of catalyst particles encapsulated at the tip of the carbon nanotube.
Fig. 4. HRTEM image of carbon nanotube which shows that the fabrication of the nanotube’s wall is composed of ca. 40 graphene layers.
visible. The fabrication of the walls was composed of ca. 40 graphene layers. Furthermore, the distance between two (002) planes is ca. 0.34 nm typical for the (002) lattice distance in hexagonal carbon. Fig. 5 shows the Raman spectrum of the carbon nanotubes. There exist two strong peaks at 1587 and 1346 cm 21 , corresponding to the typical Raman peaks of graphitized carbon nanotubes. The peak at 1587 cm 21 corresponds to an E 2g mode of graphite and is related to the vibration of sp 2 -bonded carbon atoms in a 2-dimensional hexagonal lattice, such as in a graphene layer [17]. The peak at 1346 cm 21 is associated with vibrations of
Fig. 5. Raman spectrum of carbon nanotubes which shows two graphite peaks at 1346 and 1587 cm 21 .
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J. Liu et al. / Carbon 41 (2003) 2101–2104
carbon atoms with dangling bonds in plane terminations of disordered graphite. This peak is quite high indicating that in the basal plane there exists two-dimensional disorder which is quite common in mild temperature routes [11]. The possible formation mechanism of carbon nanotubes is proposed. Iron carbonyl decomposes at 500 8C to produce carbon and iron. The carbons produced from the above reaction adsorb at the surface of the iron particles. Then they diffuse via the surface of iron particles to form graphite sheets as a cap on the iron particle. While the cap lifts off the iron particle, a closed end with an inside hollow is formed. With constant surface diffusion, hexagonal carbon clusters may grow into nanotubes. The experiment results demonstrate that iron particles produced from the decomposition of iron carbonyl play an important role in the process of nanotube growth. An important implication of our work is the possibility of large-scale solvothermal preparation of bulk carbon nanotubes. As iron carbonyl acted both as catalyst, carbon source and solvent, this method avoids the separation of raw material from solvent and simplifies the operation process. The fact that the iron carbonyl acted as a solvent under this condition helps to accelerate diffusion, adsorption, reaction rate, and crystallization in the formation of carbon nanotubes under solvothermal conditions. All these contribute to a lower reaction temperature compared to other methods using carbon monoxide as the carbon source [14–16]. Another advantage of this experiment is a fuller utilization of the carbon source with the yield of as-prepared products as high as 85%.
4. Conclusion Carbon nanotubes have been successfully synthesized from a single-source precursor at 500 8C and for 12 h via a solvothermal catalytic process. Here, iron carbonyl acted both as catalyst, carbon source and solvent. In particular, the solvothermal method can provide better reaction and crystallization conditions for the growth of carbon nanotubes. Due to surface defects of the carbon nanotubes that may provide channels for gas diffusion, the nanotubes are suitable for gas storage. This work provides a new route to large-scale and low-temperature synthesis of carbon nanotubes.
Acknowledgements This work was supported by the National Natural Science Foundation of China and the 973 National Nanometer Materials Project.
References [1] Frank S, Poncharal P, Wang ZL, de Heer WA. Carbon nanotube quantum resistors. Science 1998;280(5370):1744– 6. [2] Kong J, Zhou C, Morpurgo A, Soh HT, Quate CF, Marcus C et al. Synthesis, integration, and electrical properties of individual single-walled carbon nanotubes. Appl Phys A Mater 1999;69(3):305–8. [3] Liu C, Fan YY, Liu M, Cong HT, Cheng HM, Dresselhaus MS. Hydrogen storage in single-walled carbon nanotubes at room temperature. Science 1999;286(5442):1127–9. [4] Kim P, Lieber CM. Nanotube nanotweezers. Science 1999;286(5447):2148–50. [5] Wu HQ, Wei XW, Shao MW, Gu JS, Qu MZ. Preparation of Fe–Ni alloy nanoparticles inside carbon nanotubes via wet chemistry. J Mater Chem 2002;12(6):1919–21. [6] Iijima S. Helical microtubes of graphitic carbon. Nature 1991;354(6348):56–8. [7] Benito AM, Maniette Y, Munoz E, Martinez MT. Carbon nanotubes production by catalytic pyrolysis of benzene. Carbon 1998;36(5–6):681–3. [8] Zeng H, Zhu L, Hao GM, Sheng RS. Synthesis of various forms of carbon nanotubes by AC arc discharge. Carbon 1998;36(3):259–61. [9] Ma RZ, Wei BQ, Xu CL, Liang J, Wu DH. The morphology changes of carbon nanotubes under laser irradiation. Carbon 2000;38(4):636–8. [10] Jiang Y, Wu Y, Zhang SY, Xu CY, Yu WC, Xie Y et al. A catalytic-assembly solvothermal route to multiwall carbon nanotubes at a moderate temperature. J Am Chem Soc 2000;122(49):12383–4. [11] Shao MW, Li Q, Wu J, Xie B, Zhang SY, Qian YT. Benzenethermal route to carbon nanotubes at a moderate temperature. Carbon 2002;40(15):2961–73. [12] Rohmund F, Falk LKL, Campbell EEB. A simple method for the production of large arrays of aligned carbon nanotubes. Chem Phys Lett 2000;328(4–6):369–73. [13] Gokcen T, Dateo CE, Meyyappan M. Modeling of the HiPco process for carbon nanotube production. II. Reactor-scale analysis. J Nanosci Nanotech 2002;2(5):535–44. [14] Lee JS, Suh JS. Uniform field emission from aligned carbon nanotubes prepared by CO disproportionation. J Appl Phys 2002;92(12):7519–22. [15] Pinheiro JP, Schouler MC, Dooryhee E. In situ X-ray diffraction study of carbon nanotubes and filaments during their formation over Co /Al 2 O 3 catalysts. Solid State Commun 2002;123(3–4):161–6. [16] Herrera JE, Balzano L, Borgna A, Alvarez WE, Resasco DE. Relationship between the structure / composition of Co–Mo catalysts and their ability to produce single-walled carbon nanotubes by CO disproportionation. J Catal 2001;204(1):129–45. [17] Dresselhaus MS, Dresselhaus G, Pimenta MA, Eklund PC. In: Pelletier MJ, editor, Analytical applications of Raman spectroscopy, Oxford: Blackwell Science; 1999, Chapter 9.