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Fabrication of ordered single-walled carbon nanotube preforms
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Letters to the Editor / Carbon 43 (2005) 2215–2234
Fabrication of ordered single-walled carbon nanotube preforms Hao Yu, Weiping Zhou, Guoqing Ning, Qunfeng Zhang, Guohua Luo, Fei Wei
*
Key Laboratory of Green Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University, Beijing 100084, PR China Received 24 November 2004; accepted 26 March 2005 Available online 24 May 2005
Keywords: Carbon nanotubes; Chemical vapor deposition; Electron microscopy; Microstructure
The most exciting properties of carbon nanotubes (CNTs) are in their 1-dimensional nanostructure, e.g. tensile strength [1], transport properties along the axis [2–4], and the possibility of fabricating nano electronic elements [5,6]. In order to harness the properties on the nanometer scale for the commercial application of CNTs, the alignment of nanotubes in ordered arrays is of great interest. Only in a few recent reports [7–9], do workers claim to synthesize efficiently oriented SWNTs. Therefore, researches on post-treatments to align disordered and entangled SWNTs are significant for future applications of SWNTs. Some possibilities include the use of magnetic fields [10], chemical bonding [11], shear forces [12], superacids [13], Langmuir–Blogett membranes [14], and flowing gases [15]. However, only 1-D [12,16] aligned or 2-D [11,15] arrays have been obtained up to now. It will be difficult or time-consuming to prepare SWNT 3-D bulk preforms by these methods, although they are obviously important for nanocomposites. In the present work, a simple and scalable method is proposed for aligning disordered and entangled SWNTs to form bulk ordered material. SWNTs were synthesized by chemical vapor deposition of methane on Fe/MgO catalyst. The catalyst was synthesized by the dipping method. The weight ratio of the salt (Fe(NO3)3 Æ 9H2O) in solution to magnesium oxide was 1:10. The SWNTs were produced in a quartz fluidized bed reactor 50 mm in diameter. The reactant gas was composed of argon, hydrogen and methane with a composition ratio of 10:1:1 which was introduced into the reactor at 1173 K for 30 min. The as-grown products containing abundant SWNTs were ultra-sonicated in 6 M HCl for 2 h to remove the MgO support and iron particles. The purified products were composed of clean
*
Corresponding author. Tel.: +86 10 62785464; fax: +86 10 62772051. E-mail address: weifei@flotu.org (F. Wei). 0008-6223/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.03.033
SWNTs and disordered and entangled bundles of SWNTs (SEM, JSM 6700F, as shown in Fig. 2A). They were ground into fine particles and dispersed in either distilled water or chloroform, by ultrasonication for 2 h. The orientation of the SWNTs was conducted by shear flow on a plane sheet as shown in Fig. 1. The resultant suspension was slowly fed from the wall of a rectangular vacuum filter. The seeping holes were linearly aligned so the bulk flow moved along a direction perpendicular to the seeping hole alignment. The feeding speed adopted ensured a constant level of liquid on the filter so that a steady bulk laminar flow can be achieved on the sheet except for corner turbulence. A piece of filter paper was put in the filter to retain the sediment. Finally, the deposits on the paper were washed several times with solvents and dried at 383 K to give a solid composed of oriented SWNTs (Fig. 2B). SEM observation of the sediment in the laminar flow region show that the microstructure of the SWNT aggregates differed significantly from the original (Fig. 2C–E). The deposit along the flow direction is made up of large assemblies of SWNTs. Fig. 2C shows the morphology of the surface of the deposit. All of the SWNTs deposited on the surface are oriented along the flow direction vertical to the filter holes. The orientation of SWNTs inside the deposit can also be observed at the edge. Fig. 2D and its insert E show a SEM of the edge of the deposit with thickness of about 1 lm, almost of the SWNTs at the edge were aligned along the flow direction. It should be noted that the SWNTs are still in the form of bundles as shown in Fig. 2E. In other words, the solvents can orient individual SWNTs, smaller than 2 nm in diameter, or their bundles, about 10 nm in diameter, despite the aggregation and splitting of the SWNT bundles in Fig. 2E. In the methods reported in the literature, debundling and surface-functionalization are often required to make the oriented SWNT material. In our method, however, a well-aligned deposit was obtained without surfactants or
Letters to the Editor / Carbon 43 (2005) 2215–2234
lk Bu w flo
Vacuum
SW NT deposits
Filter paper
Bulk flow
Turbulence
Seeping hole
Fig. 1. Schematic diagram of the method for aligning SWNTs with a shear flow on a sheet. The lower part shows a cutaway view of filter, in which the boundary layer is shown.
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pretreatment. This can be attributed to the steady shear force resulting from the designed bulk flow on the sheet. When the floating disordered SWNTs or their bundles sank freely under laminar flow and through the boundary layer, they were sheared and oriented due to the velocity gradient, as shown in Fig. 1. It should also be noted that the orientation only took place in the laminar flow region and the sediment near the corner did not contain ordered arrays due to the turbulence. Such shear induced orientation of SWNTs was also reported by Vigolo [12]. In the present work, the shear flow can hierarchically act with SWNTs, so the ordered SWNTs can accumulate layer by layer. When the process is uninterrupted, it is relatively easy to prepare bulk deposits of SWNTs, which is difficult with the other known methods. In summary, a novel and simple approach to align disordered and entangled SWNTs to form macroscopic bulk arrays on a large scale is proposed. The method is based on a well-designed, steady laminar flow on a sheet in which the SWNTs are aligned by shear forces in the flowing fluid. The present method can be effectively used with both SWNTs and their bundles, and will significantly simplify the process of purification and dispersion to prepare composites based on SWNTs. Since the direct synthesis of dense and ordered SWNTs is difficult, this
Fig. 2. Optical and SEM images of the deposits: (A) a SEM image of purified disordered SWNTs, (B) an ordered deposit with size on the centimeter scale, (C) the surface of the aligned deposit, (D) the edge of a membrane of SWNT sediment, with insert (E) highlighted in (D) at high magnification. Scale bar in (E) 100 nm.
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Letters to the Editor / Carbon 43 (2005) 2215–2234
method can be a useful technical option to make nanocomposites and anisotropic materials. Acknowledgements This work was supported by a grant from the national 863 program (No. 2003AA302630) and NSFC key program (No. 20236020).
References [1] Treacy MMJ, Ebbesen TW, Gibson JM. Exceptionally high Youngs modulus observed for individual carbon nanotubes. Nature 1996;381:678–80. [2] Frank S, Poncharal P, Wang ZL, de Heer WA. Carbon nanotube quantum resistors. Science 1998;280:1744–6. [3] Odom TW, Huang JL, Kim P, Lieber CM. Atomic structure and electronic properties of single-walled carbon nanotubes. Nature 1998;391:62–4. [4] Ebbesen TW, Lezec HJ, Hiura H, Bennett JW, Ghaemi HF, Thio T. Electrical conductivity of individual carbon nanotubes. Nature 1996;382:54–6. [5] Deheer WA, Chatelain A, Ugarte D. A carbon nanotube fieldemission electron source. Science 1995;270:1179–80. [6] Tans SJ, Verschueren ARM, Dekker C. Room-temperature transistor based on a single carbon nanotube. Nature 1998;393: 49–52.
[7] Zhu HW, Xu CL, Wu DH, Wei BQ, Vajtai R, Ajayan PM. Direct synthesis of long single-walled carbon nanotube strands. Science 2002;296:884–6. [8] Murakami Y, Chiashi S, Miyauchi Y, Hu MH, Ogura M, Okubo T, et al. Growth of vertically aligned single-walled carbon nanotube on quartz substrates and their optical anisotropy. Chem Phys Lett 2004;385:298–303. [9] Hata K, Futaba DN, Mizuno K, Namai T, Yumura M, Iijima S. Water-assisted highly efficient synthesis of impurityfree single-waited carbon nanotubes. Science 2004;306: 1362–4. [10] Walters DA, Casavant MJ, Qin XC, Huffman CB, Boul PJ, Ericson LM, et al. In-plane-aligned membranes of carbon nanotubes. Chem Phys Lett 2001;338:14–20. [11] Rao SG, Huang L, Setyawan W, Hong SH. Large-scale assembly of carbon nanotubes. Nature 2003;425:36–7. [12] Vigolo B, Penicaud A, Coulon C, Sauder C, Pailler R, Journet C, et al. Macroscopic fibers and ribbons of oriented carbon nanotubes. Science 2000;290:1331–4. [13] Davis VA, Ericson LM, Parra-Vasquez ANG, Fan H, Wang Y, Prieto V, et al. Phase behavior and rheology of SWNTs in superacids. Macromolecules 2004;37:154–60. [14] Whang D, Jin S, Wu Y, Lieber CM. Large-scale hierarchical organization of nanowire arrays for integrated nanosystems. Nano Letters 2003;3:1255–9. [15] Huang SM, Maynor B, Cai XY, Liu J. Ultralong, well-aligned single-walled carbon nanotube architectures on surfaces. Adv Mater 2003;15:1651–5. [16] Ericson LM, Fan H, Peng H, Davis VA, Zhou W, Sulpizio J, et al. Macroscopic, neat, single-walled carbon nanotube fibers. Science 2004;305:1447–50.
Letters to the Editor / Carbon 43 (2005) 2215–2234
Fabrication of ordered single-walled carbon nanotube preforms Hao Yu, Weiping Zhou, Guoqing Ning, Qunfeng Zhang, Guohua Luo, Fei Wei
*
Key Laboratory of Green Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University, Beijing 100084, PR China Received 24 November 2004; accepted 26 March 2005 Available online 24 May 2005
Keywords: Carbon nanotubes; Chemical vapor deposition; Electron microscopy; Microstructure
The most exciting properties of carbon nanotubes (CNTs) are in their 1-dimensional nanostructure, e.g. tensile strength [1], transport properties along the axis [2–4], and the possibility of fabricating nano electronic elements [5,6]. In order to harness the properties on the nanometer scale for the commercial application of CNTs, the alignment of nanotubes in ordered arrays is of great interest. Only in a few recent reports [7–9], do workers claim to synthesize efficiently oriented SWNTs. Therefore, researches on post-treatments to align disordered and entangled SWNTs are significant for future applications of SWNTs. Some possibilities include the use of magnetic fields [10], chemical bonding [11], shear forces [12], superacids [13], Langmuir–Blogett membranes [14], and flowing gases [15]. However, only 1-D [12,16] aligned or 2-D [11,15] arrays have been obtained up to now. It will be difficult or time-consuming to prepare SWNT 3-D bulk preforms by these methods, although they are obviously important for nanocomposites. In the present work, a simple and scalable method is proposed for aligning disordered and entangled SWNTs to form bulk ordered material. SWNTs were synthesized by chemical vapor deposition of methane on Fe/MgO catalyst. The catalyst was synthesized by the dipping method. The weight ratio of the salt (Fe(NO3)3 Æ 9H2O) in solution to magnesium oxide was 1:10. The SWNTs were produced in a quartz fluidized bed reactor 50 mm in diameter. The reactant gas was composed of argon, hydrogen and methane with a composition ratio of 10:1:1 which was introduced into the reactor at 1173 K for 30 min. The as-grown products containing abundant SWNTs were ultra-sonicated in 6 M HCl for 2 h to remove the MgO support and iron particles. The purified products were composed of clean
*
Corresponding author. Tel.: +86 10 62785464; fax: +86 10 62772051. E-mail address: weifei@flotu.org (F. Wei). 0008-6223/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.03.033
SWNTs and disordered and entangled bundles of SWNTs (SEM, JSM 6700F, as shown in Fig. 2A). They were ground into fine particles and dispersed in either distilled water or chloroform, by ultrasonication for 2 h. The orientation of the SWNTs was conducted by shear flow on a plane sheet as shown in Fig. 1. The resultant suspension was slowly fed from the wall of a rectangular vacuum filter. The seeping holes were linearly aligned so the bulk flow moved along a direction perpendicular to the seeping hole alignment. The feeding speed adopted ensured a constant level of liquid on the filter so that a steady bulk laminar flow can be achieved on the sheet except for corner turbulence. A piece of filter paper was put in the filter to retain the sediment. Finally, the deposits on the paper were washed several times with solvents and dried at 383 K to give a solid composed of oriented SWNTs (Fig. 2B). SEM observation of the sediment in the laminar flow region show that the microstructure of the SWNT aggregates differed significantly from the original (Fig. 2C–E). The deposit along the flow direction is made up of large assemblies of SWNTs. Fig. 2C shows the morphology of the surface of the deposit. All of the SWNTs deposited on the surface are oriented along the flow direction vertical to the filter holes. The orientation of SWNTs inside the deposit can also be observed at the edge. Fig. 2D and its insert E show a SEM of the edge of the deposit with thickness of about 1 lm, almost of the SWNTs at the edge were aligned along the flow direction. It should be noted that the SWNTs are still in the form of bundles as shown in Fig. 2E. In other words, the solvents can orient individual SWNTs, smaller than 2 nm in diameter, or their bundles, about 10 nm in diameter, despite the aggregation and splitting of the SWNT bundles in Fig. 2E. In the methods reported in the literature, debundling and surface-functionalization are often required to make the oriented SWNT material. In our method, however, a well-aligned deposit was obtained without surfactants or
Letters to the Editor / Carbon 43 (2005) 2215–2234
lk Bu w flo
Vacuum
SW NT deposits
Filter paper
Bulk flow
Turbulence
Seeping hole
Fig. 1. Schematic diagram of the method for aligning SWNTs with a shear flow on a sheet. The lower part shows a cutaway view of filter, in which the boundary layer is shown.
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pretreatment. This can be attributed to the steady shear force resulting from the designed bulk flow on the sheet. When the floating disordered SWNTs or their bundles sank freely under laminar flow and through the boundary layer, they were sheared and oriented due to the velocity gradient, as shown in Fig. 1. It should also be noted that the orientation only took place in the laminar flow region and the sediment near the corner did not contain ordered arrays due to the turbulence. Such shear induced orientation of SWNTs was also reported by Vigolo [12]. In the present work, the shear flow can hierarchically act with SWNTs, so the ordered SWNTs can accumulate layer by layer. When the process is uninterrupted, it is relatively easy to prepare bulk deposits of SWNTs, which is difficult with the other known methods. In summary, a novel and simple approach to align disordered and entangled SWNTs to form macroscopic bulk arrays on a large scale is proposed. The method is based on a well-designed, steady laminar flow on a sheet in which the SWNTs are aligned by shear forces in the flowing fluid. The present method can be effectively used with both SWNTs and their bundles, and will significantly simplify the process of purification and dispersion to prepare composites based on SWNTs. Since the direct synthesis of dense and ordered SWNTs is difficult, this
Fig. 2. Optical and SEM images of the deposits: (A) a SEM image of purified disordered SWNTs, (B) an ordered deposit with size on the centimeter scale, (C) the surface of the aligned deposit, (D) the edge of a membrane of SWNT sediment, with insert (E) highlighted in (D) at high magnification. Scale bar in (E) 100 nm.
2234
Letters to the Editor / Carbon 43 (2005) 2215–2234
method can be a useful technical option to make nanocomposites and anisotropic materials. Acknowledgements This work was supported by a grant from the national 863 program (No. 2003AA302630) and NSFC key program (No. 20236020).
References [1] Treacy MMJ, Ebbesen TW, Gibson JM. Exceptionally high Youngs modulus observed for individual carbon nanotubes. Nature 1996;381:678–80. [2] Frank S, Poncharal P, Wang ZL, de Heer WA. Carbon nanotube quantum resistors. Science 1998;280:1744–6. [3] Odom TW, Huang JL, Kim P, Lieber CM. Atomic structure and electronic properties of single-walled carbon nanotubes. Nature 1998;391:62–4. [4] Ebbesen TW, Lezec HJ, Hiura H, Bennett JW, Ghaemi HF, Thio T. Electrical conductivity of individual carbon nanotubes. Nature 1996;382:54–6. [5] Deheer WA, Chatelain A, Ugarte D. A carbon nanotube fieldemission electron source. Science 1995;270:1179–80. [6] Tans SJ, Verschueren ARM, Dekker C. Room-temperature transistor based on a single carbon nanotube. Nature 1998;393: 49–52.
[7] Zhu HW, Xu CL, Wu DH, Wei BQ, Vajtai R, Ajayan PM. Direct synthesis of long single-walled carbon nanotube strands. Science 2002;296:884–6. [8] Murakami Y, Chiashi S, Miyauchi Y, Hu MH, Ogura M, Okubo T, et al. Growth of vertically aligned single-walled carbon nanotube on quartz substrates and their optical anisotropy. Chem Phys Lett 2004;385:298–303. [9] Hata K, Futaba DN, Mizuno K, Namai T, Yumura M, Iijima S. Water-assisted highly efficient synthesis of impurityfree single-waited carbon nanotubes. Science 2004;306: 1362–4. [10] Walters DA, Casavant MJ, Qin XC, Huffman CB, Boul PJ, Ericson LM, et al. In-plane-aligned membranes of carbon nanotubes. Chem Phys Lett 2001;338:14–20. [11] Rao SG, Huang L, Setyawan W, Hong SH. Large-scale assembly of carbon nanotubes. Nature 2003;425:36–7. [12] Vigolo B, Penicaud A, Coulon C, Sauder C, Pailler R, Journet C, et al. Macroscopic fibers and ribbons of oriented carbon nanotubes. Science 2000;290:1331–4. [13] Davis VA, Ericson LM, Parra-Vasquez ANG, Fan H, Wang Y, Prieto V, et al. Phase behavior and rheology of SWNTs in superacids. Macromolecules 2004;37:154–60. [14] Whang D, Jin S, Wu Y, Lieber CM. Large-scale hierarchical organization of nanowire arrays for integrated nanosystems. Nano Letters 2003;3:1255–9. [15] Huang SM, Maynor B, Cai XY, Liu J. Ultralong, well-aligned single-walled carbon nanotube architectures on surfaces. Adv Mater 2003;15:1651–5. [16] Ericson LM, Fan H, Peng H, Davis VA, Zhou W, Sulpizio J, et al. Macroscopic, neat, single-walled carbon nanotube fibers. Science 2004;305:1447–50.