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In situ synthesis of super-long Cu nanowires inside carbon nanotubes with coal as carbon source
Carbon 44 (2006) 1845–1869 www.elsevier.com/locate/carbon
Letters to the Editor
In situ synthesis of super-long Cu nanowires inside carbon nanotubes with coal as carbon source Zhiyu Wang, Zongbin Zhao, Jieshan Qiu
*
Carbon Research Laboratory, School of Chemical Engineering, Center for Nano Materials and Science, State Key Lab of Fine Chemicals, Dalian University of Technology, 158 Zhongshan Road, P.O. Box 49, Dalian 116012, China Received 23 July 2005; accepted 1 April 2006
Keywords: Carbon nanotubes; Coal; Arc discharge; Electron diffraction; Transmission electron microscopy
Nano-sized copper wires have received considerable attention because of their potential applications in next generation electronic nanodevices in future [1,2]. Up to now, a number of approaches including vapor depositions [1], template synthesis [2,3], solution-phase reactions [4,5] and vapor–solid reaction growth (VSRG) [6] have been developed for making copper nanowires. Despite of the great progresses, some tough problems related to the quality of copper nanowires such as poor oxidation resistance and crystallinity, short length, and nonlinear morphology still need to be addressed [1–7]. Carbon nanotubes (CNTs) are believed to be a good template for making Cu nanowires with perfect 1D morphology and good stability, and this idea has been tested by arc discharge in hydrogen [8] or microwave plasma-assisted CVD [9]. Here, we report a simple arc-discharge method for in situ synthesis of Cu-filled CNTs with coal as carbon precursor. It has been found that super-long crystalline Cu nanowires, of which the average length is over several tens of micrometers and the aspect ratio is ca. 200–360, can be directly fabricated inside the CNTs in large quantity. The preparation experiments were conducted in a traditional DC arc-discharge reactor in argon atmosphere. A high purity graphite tube (10 mm OD, 6 mm ID) filled with
*
Corresponding author. Fax: +86 411 8899 3991. E-mail address: [email protected] (J. Qiu).
0008-6223/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2006.04.001
a mixture of anthracite coal (from Yunnan Province, China) and CuO powders (6150 lm in size) was used as consuming anode while the cathode was a high purity graphite rod (15 mm OD). The weight ratio of CuO to coal powder in the mixture was 1:9. For each run that normally lasted about 15 min, the arc discharge was conducted with DC current of 70 A and voltage of 20 V in argon at 0.08– 0.09 MPa. After the arc discharge was over, the deposits on the cathode were collected and examined using transmission electron microscopy (TEM, JEM-2000EX; HRTEM, Philips Tecnai G2 20). Fig. 1a shows a typical low magnification TEM image of the as-prepared sample, revealing the successful synthesis of CNT-encapsulated nanowires with a length of over several tens of micrometers. It has been found that in some nanowires there are several distorted defects such as kinks and curls, between which the distance varies in a range from hundreds of nanometers to several micrometers. Further high magnification TEM examination (see Fig. 1b) shows that the CNTs are completely filled with continuous nanowires with a diameter ca. 30–80 nm. The aspect ratio of these carbon coated nanowires is 200–360, which is much higher than those reported previously in the literature [8,9]. The repeated experiments indicate that on average, more than 40–50% of the as-prepared CNTs are filled with nanowires, as can be seen in Fig. 2. The selected area electron diffraction (SAED) patterns taken from the nanowires between the distorted section are shown in insets of Figs. 2b and d, showing the diffraction as regular arrays of sharp spots together with the short arc due to the
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Letters to the Editor / Carbon 44 (2006) 1845–1869
Fig. 1. TEM images of Cu-filled CNTs prepared with coal as carbon source: (a) low magnification image showing super-long Cu nanowires in the central cavity of CNTs; (b) high magnification image of three Cu-filled CNTs displayed in (a), indicated by white arrow.
Fig. 3. HRTEM image of one Cu-filled carbon nanotube.
Fig. 2. TEM images and SAED patterns of the Cu-filled CNTs: (a,c) showing most of the CNTs are filled with Cu nanowires. Corresponding magnified images of the section marked by a rectangle in (a) and (c) are shown in (b) and (d), respectively; Insets in (b) and (d) are corresponding SAED patterns of the Cu-filled CNTs.
(0 0 2) diffraction of hexagonal graphite, which gives evidence of the presence of well-developed monocrystalline
structure in the nanowires. These patterns are in good agreement with the typical diffraction pattern of a face-centered cubic (fcc) Cu along the h0 1 1i zone axis, for clarity reasons, some diffraction spots are indexed and labeled in the figures. Further HRTEM examination on a typical 30 nm diameter Cu-filled CNT is shown in Fig. 3, in which monocrystals have been observed in long-range order, as well as the outside coating consisting of well-oriented graphite layers (about 20 layers with a separation of ca. 0.34 nm). The distance between the lattice fringes of encapsulated crystals is measured to be ca. 0.21 nm that is
Letters to the Editor / Carbon 44 (2006) 1845–1869
identical to the d-spacing of (1 1 1) atomic plane of Cu (fcc, d1 1 1 = 0.2087 nm). The results presented above lead one to believe that the nanowires encapsulated inside the CNTs are pure copper nanowires consisting of several long monocrystals. The formation and growth of the Cu-filled CNTs does not follow the well-accepted dissolution–precipitation scheme for graphitization [10] simply because copper does not form stable carbide phase [11] and carbon solubility in bulk copper is rather poor [12]. With the aim of working out and understanding the mechanism involved in the formation process of Cu-filled CNTs, comparison tests with high purity graphite powders as carbon source instead of coal powders were performed under identical conditions, however, few filled CNTs were observed in the final products. This means that coal plays a crucial role in the growth of Cu-filled CNTs in the present work. It is known that coal is a molecular solid consisting of a great number of irregular polymerized polycyclic aromatic hydrocarbon units [13,14], which are joined together by aliphatic benzylic or alkyl ether links or other bridging functional groups [13–15]. It can be easily envisaged that these weak binding linkages can be readily broken once the coal is injected into the high temperature zone of the arc plasma [14–17], releasing a large quantity of reactive fragments of hydrocarbon molecules such as alkyne and aromatic species [8,13,15,16]. These reactive species would serve as building blocks for CNT formation and would enhance the growth rate of CNTs [17,18]. In addition, it has been proposed that the aromatic species could interact with Cu clusters at high temperature to form carbon layers around copper particles because of the dehydrogenation reactions [19]. With these information in mind, it is reasonable for one to assume that various reactive aromatic species generated in situ from the fast pyrolysis of coal in arc plasma may be responsible for the in situ formation of hollow and Cu-filled CNTs. We believe that the approach described here, after being further optimized, will lead to mass production of Cu nanowires with excellent performance, which will offer more opportunities for studying the physical and electronic properties of Cu nanowires.
Acknowledgements This work was partly supported by the National Science Foundation of China (Nos. 20276012 and 20376011), the Natural Science Foundation of Liaoning Province of China (No. 2001101003), and the Program for New
1847
Century Excellent Talents in University by the Education Ministry of China (No. NCET-04-0274). References [1] Liu ZW, Bando Y. A novel method for preparing Cu nanorods and nanowires. Adv Mater 2003;15:303–5. [2] Monson CF, Wolley AT. DNA-templated construction of copper nanowires. Nano Lett 2003;3:359–63. [3] Molares MET, Buschmann V, Dobrev D, Neumann R, Scholz R, Schuchert IU, et al. Single-crystalline copper nanowires produced by electrochemical deposition in polymeric ion track membranes. Adv Mater 2001;13:62–5. [4] Chang Y, Lye LM, Zeng CH. Large-scale synthesis of high-quality ultralong Cu nanowires. Langmuir 2005;21:3746–8. [5] Liu ZP, Yang Y, Liang JB, Hu ZK, Li S, Peng S, et al. Synthesis of copper nanowires via a complex-surfactant-assisted hydrothermal reduction process. J Phys Chem B 2000;107:12658–61. [6] Yen MY, Chiu CW, Hsia CH, Chen FR, Kai JJ, Lee CY, et al. Synthesis of cable-like Cu nanowires. Adv Mater 2003;15:235–7. [7] Liu ZW, Bando Y. Oxidation behaviour of copper nanorods. Chem Phys Lett 2003;378:85–8. [8] Setlur AA, Dai JY, Lauerhaas JM, Chang RPH. Formation of filled carbon nanotubes and nanoparticles using polycyclic aromatic hydrocarbon molecules. Carbon 1998;36:721–3. [9] Zhang GY, Wang EG. Cu-filled carbon nanotubes by simultaneous plasma assisted copper incorporation. Appl Phys Lett 2003;82: 1926–8. [10] Oya A, Otani S. Catalytic graphitization of carbons by various metals. Carbon 1979;17:131–7. [11] Ong TP, Xiong F, Chang RPH, White CW. Nucleation and growth of diamond on carbon–implanted single crystal copper surfaces. J Mater Res 1992;7:2429–39. [12] Ivanov V, Nagy JB, Lambin P, Lucas A, Zhang XB, Zhang XF, et al. Catalytic production and purification of nanotubules having fullerene-scale diameters. Chem Phys Lett 1994;223:329–35. [13] Geldard L, Keegan JT, Young BR, Wilson MA. Pathways of polycyclic hydrocarbon formation during plasma arcing of carbonaceous materials. Fuel 1998;77:15–8. [14] Derbyshire F, Marzec A, Schulten HR, Wilson MA, Davis A, Tekely P, et al. Molecular structure of coals: a debate. Fuel 1989;68: 1091–106. [15] Wilson MA, Moy A, Rose H, Kannangara GSK, Young BR, Mcculloch DG, et al. Fullerene blacks and cathode deposits derived from plasma arcing of graphite with naphthalene. Fuel 2000;79: 47–56. [16] Pang LSK, Wilson MA, Pallasser R, Prochazka L. Isotope effects in plasma arcing experiments with various carbon anodes. Energy Fuels 1995;9:704–6. [17] Qiu JS, Zhou Y, Yang ZG, Wang DK, Guo SC, Tsang SC, et al. Preparation of fullerenes using carbon rods manufactured from Chinese hard coals. Fuel 2000;79:1303–8. [18] Lauerhaas JM, Dai JY, Setlur AA, Chang RPH. The effect of arc parameters on the growth of carbon nanotubes. J Mater Res 1997;12: 1536–44. [19] LaCava AI, Bernardo CA, Trimm DL. Studies of deactivation of metals by carbon deposition. Carbon 1983;20:219–23.
Letters to the Editor
In situ synthesis of super-long Cu nanowires inside carbon nanotubes with coal as carbon source Zhiyu Wang, Zongbin Zhao, Jieshan Qiu
*
Carbon Research Laboratory, School of Chemical Engineering, Center for Nano Materials and Science, State Key Lab of Fine Chemicals, Dalian University of Technology, 158 Zhongshan Road, P.O. Box 49, Dalian 116012, China Received 23 July 2005; accepted 1 April 2006
Keywords: Carbon nanotubes; Coal; Arc discharge; Electron diffraction; Transmission electron microscopy
Nano-sized copper wires have received considerable attention because of their potential applications in next generation electronic nanodevices in future [1,2]. Up to now, a number of approaches including vapor depositions [1], template synthesis [2,3], solution-phase reactions [4,5] and vapor–solid reaction growth (VSRG) [6] have been developed for making copper nanowires. Despite of the great progresses, some tough problems related to the quality of copper nanowires such as poor oxidation resistance and crystallinity, short length, and nonlinear morphology still need to be addressed [1–7]. Carbon nanotubes (CNTs) are believed to be a good template for making Cu nanowires with perfect 1D morphology and good stability, and this idea has been tested by arc discharge in hydrogen [8] or microwave plasma-assisted CVD [9]. Here, we report a simple arc-discharge method for in situ synthesis of Cu-filled CNTs with coal as carbon precursor. It has been found that super-long crystalline Cu nanowires, of which the average length is over several tens of micrometers and the aspect ratio is ca. 200–360, can be directly fabricated inside the CNTs in large quantity. The preparation experiments were conducted in a traditional DC arc-discharge reactor in argon atmosphere. A high purity graphite tube (10 mm OD, 6 mm ID) filled with
*
Corresponding author. Fax: +86 411 8899 3991. E-mail address: [email protected] (J. Qiu).
0008-6223/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2006.04.001
a mixture of anthracite coal (from Yunnan Province, China) and CuO powders (6150 lm in size) was used as consuming anode while the cathode was a high purity graphite rod (15 mm OD). The weight ratio of CuO to coal powder in the mixture was 1:9. For each run that normally lasted about 15 min, the arc discharge was conducted with DC current of 70 A and voltage of 20 V in argon at 0.08– 0.09 MPa. After the arc discharge was over, the deposits on the cathode were collected and examined using transmission electron microscopy (TEM, JEM-2000EX; HRTEM, Philips Tecnai G2 20). Fig. 1a shows a typical low magnification TEM image of the as-prepared sample, revealing the successful synthesis of CNT-encapsulated nanowires with a length of over several tens of micrometers. It has been found that in some nanowires there are several distorted defects such as kinks and curls, between which the distance varies in a range from hundreds of nanometers to several micrometers. Further high magnification TEM examination (see Fig. 1b) shows that the CNTs are completely filled with continuous nanowires with a diameter ca. 30–80 nm. The aspect ratio of these carbon coated nanowires is 200–360, which is much higher than those reported previously in the literature [8,9]. The repeated experiments indicate that on average, more than 40–50% of the as-prepared CNTs are filled with nanowires, as can be seen in Fig. 2. The selected area electron diffraction (SAED) patterns taken from the nanowires between the distorted section are shown in insets of Figs. 2b and d, showing the diffraction as regular arrays of sharp spots together with the short arc due to the
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Letters to the Editor / Carbon 44 (2006) 1845–1869
Fig. 1. TEM images of Cu-filled CNTs prepared with coal as carbon source: (a) low magnification image showing super-long Cu nanowires in the central cavity of CNTs; (b) high magnification image of three Cu-filled CNTs displayed in (a), indicated by white arrow.
Fig. 3. HRTEM image of one Cu-filled carbon nanotube.
Fig. 2. TEM images and SAED patterns of the Cu-filled CNTs: (a,c) showing most of the CNTs are filled with Cu nanowires. Corresponding magnified images of the section marked by a rectangle in (a) and (c) are shown in (b) and (d), respectively; Insets in (b) and (d) are corresponding SAED patterns of the Cu-filled CNTs.
(0 0 2) diffraction of hexagonal graphite, which gives evidence of the presence of well-developed monocrystalline
structure in the nanowires. These patterns are in good agreement with the typical diffraction pattern of a face-centered cubic (fcc) Cu along the h0 1 1i zone axis, for clarity reasons, some diffraction spots are indexed and labeled in the figures. Further HRTEM examination on a typical 30 nm diameter Cu-filled CNT is shown in Fig. 3, in which monocrystals have been observed in long-range order, as well as the outside coating consisting of well-oriented graphite layers (about 20 layers with a separation of ca. 0.34 nm). The distance between the lattice fringes of encapsulated crystals is measured to be ca. 0.21 nm that is
Letters to the Editor / Carbon 44 (2006) 1845–1869
identical to the d-spacing of (1 1 1) atomic plane of Cu (fcc, d1 1 1 = 0.2087 nm). The results presented above lead one to believe that the nanowires encapsulated inside the CNTs are pure copper nanowires consisting of several long monocrystals. The formation and growth of the Cu-filled CNTs does not follow the well-accepted dissolution–precipitation scheme for graphitization [10] simply because copper does not form stable carbide phase [11] and carbon solubility in bulk copper is rather poor [12]. With the aim of working out and understanding the mechanism involved in the formation process of Cu-filled CNTs, comparison tests with high purity graphite powders as carbon source instead of coal powders were performed under identical conditions, however, few filled CNTs were observed in the final products. This means that coal plays a crucial role in the growth of Cu-filled CNTs in the present work. It is known that coal is a molecular solid consisting of a great number of irregular polymerized polycyclic aromatic hydrocarbon units [13,14], which are joined together by aliphatic benzylic or alkyl ether links or other bridging functional groups [13–15]. It can be easily envisaged that these weak binding linkages can be readily broken once the coal is injected into the high temperature zone of the arc plasma [14–17], releasing a large quantity of reactive fragments of hydrocarbon molecules such as alkyne and aromatic species [8,13,15,16]. These reactive species would serve as building blocks for CNT formation and would enhance the growth rate of CNTs [17,18]. In addition, it has been proposed that the aromatic species could interact with Cu clusters at high temperature to form carbon layers around copper particles because of the dehydrogenation reactions [19]. With these information in mind, it is reasonable for one to assume that various reactive aromatic species generated in situ from the fast pyrolysis of coal in arc plasma may be responsible for the in situ formation of hollow and Cu-filled CNTs. We believe that the approach described here, after being further optimized, will lead to mass production of Cu nanowires with excellent performance, which will offer more opportunities for studying the physical and electronic properties of Cu nanowires.
Acknowledgements This work was partly supported by the National Science Foundation of China (Nos. 20276012 and 20376011), the Natural Science Foundation of Liaoning Province of China (No. 2001101003), and the Program for New
1847
Century Excellent Talents in University by the Education Ministry of China (No. NCET-04-0274). References [1] Liu ZW, Bando Y. A novel method for preparing Cu nanorods and nanowires. Adv Mater 2003;15:303–5. [2] Monson CF, Wolley AT. DNA-templated construction of copper nanowires. Nano Lett 2003;3:359–63. [3] Molares MET, Buschmann V, Dobrev D, Neumann R, Scholz R, Schuchert IU, et al. Single-crystalline copper nanowires produced by electrochemical deposition in polymeric ion track membranes. Adv Mater 2001;13:62–5. [4] Chang Y, Lye LM, Zeng CH. Large-scale synthesis of high-quality ultralong Cu nanowires. Langmuir 2005;21:3746–8. [5] Liu ZP, Yang Y, Liang JB, Hu ZK, Li S, Peng S, et al. Synthesis of copper nanowires via a complex-surfactant-assisted hydrothermal reduction process. J Phys Chem B 2000;107:12658–61. [6] Yen MY, Chiu CW, Hsia CH, Chen FR, Kai JJ, Lee CY, et al. Synthesis of cable-like Cu nanowires. Adv Mater 2003;15:235–7. [7] Liu ZW, Bando Y. Oxidation behaviour of copper nanorods. Chem Phys Lett 2003;378:85–8. [8] Setlur AA, Dai JY, Lauerhaas JM, Chang RPH. Formation of filled carbon nanotubes and nanoparticles using polycyclic aromatic hydrocarbon molecules. Carbon 1998;36:721–3. [9] Zhang GY, Wang EG. Cu-filled carbon nanotubes by simultaneous plasma assisted copper incorporation. Appl Phys Lett 2003;82: 1926–8. [10] Oya A, Otani S. Catalytic graphitization of carbons by various metals. Carbon 1979;17:131–7. [11] Ong TP, Xiong F, Chang RPH, White CW. Nucleation and growth of diamond on carbon–implanted single crystal copper surfaces. J Mater Res 1992;7:2429–39. [12] Ivanov V, Nagy JB, Lambin P, Lucas A, Zhang XB, Zhang XF, et al. Catalytic production and purification of nanotubules having fullerene-scale diameters. Chem Phys Lett 1994;223:329–35. [13] Geldard L, Keegan JT, Young BR, Wilson MA. Pathways of polycyclic hydrocarbon formation during plasma arcing of carbonaceous materials. Fuel 1998;77:15–8. [14] Derbyshire F, Marzec A, Schulten HR, Wilson MA, Davis A, Tekely P, et al. Molecular structure of coals: a debate. Fuel 1989;68: 1091–106. [15] Wilson MA, Moy A, Rose H, Kannangara GSK, Young BR, Mcculloch DG, et al. Fullerene blacks and cathode deposits derived from plasma arcing of graphite with naphthalene. Fuel 2000;79: 47–56. [16] Pang LSK, Wilson MA, Pallasser R, Prochazka L. Isotope effects in plasma arcing experiments with various carbon anodes. Energy Fuels 1995;9:704–6. [17] Qiu JS, Zhou Y, Yang ZG, Wang DK, Guo SC, Tsang SC, et al. Preparation of fullerenes using carbon rods manufactured from Chinese hard coals. Fuel 2000;79:1303–8. [18] Lauerhaas JM, Dai JY, Setlur AA, Chang RPH. The effect of arc parameters on the growth of carbon nanotubes. J Mater Res 1997;12: 1536–44. [19] LaCava AI, Bernardo CA, Trimm DL. Studies of deactivation of metals by carbon deposition. Carbon 1983;20:219–23.