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J. Phys. Chem. C 2008, 112, 13470–13474
Magnetic-field-controlled Alignment of Carbon Nanotubes from Flames and Its Growth Mechanism Jun Zhang† and Chunxu Pan†,‡,* Department of Physics and Key Laboratory of Acoustic and Photonic Materials and DeVices of Ministry of Education, Wuhan UniVersity, Wuhan, 430072, P R China; Center for Electron Microscopy, Wuhan UniVersity, Wuhan 430072, P R China ReceiVed: April 22, 2008; ReVised Manuscript ReceiVed: June 20, 2008
This paper introduces a novel process for well-aligning growth of carbon nanotubes from flames using a uniform magnetic field generated by a permanent magnet. The results show that the magnetic field also improves the crystallinity of graphite sheets. The theoretical calculation revealed that the magnetic force acting upon the carbon nanotube itself was much larger than that upon the catalyst particle at the tip due to the carbon nanotubes’ diamagnetic property. Different from the electric-field-inducing process, the present work provides a possibility for obtaining well-aligned carbon nanotubes through the “base-growth model”. 1. Introduction It has been well-known that the direction controllable and large-scale growth of well-aligned carbon nanotubes (CNTs) plays a key role for its potential applications in nanoelectrics and nanoscale devices. In the past few years, several process have been developed, such as (1) overcrowding growth. In this case, dense CNTs attract each other through van der Waals forces and lead to a parallel alignment,1 such as in the processes of liquid phase film catalyst assistant chemical vapor deposition (CVD)2 and floating catalyst CVD,3 in which the homogenously distributed catalysts are required and are critical. (2) The wellaligned CNTs grow in a Si porous template4 or anodic aluminum oxide (AAO) nanopores.5 (3) Electric-field-induced growth, which can be processed in a regular CVD,6,7 plasma enhanced chemical vapor deposition (PE-CVD),8-10 and arc discharge11 processes by adding a bias voltage around the substrate intentionally or unconsciously. Generally, the growth mechanism for the alignment in the electric field is due to the torque on the dipole moment along the tube axis, which is induced by highly anisotropic static polarizability, rotates and aligns the CNTs.6 However, in our previous work,12,13 it was found that an additional electrostatic field could not only align the CNTs growth along the direction of the electrostatic force but also improved the diameter uniformity and the crystallinity of graphite sheets. The calculation and modulation revealed that the electrostatic attractive force along the field direction acting upon the catalyst particle at CNT tip was much larger than that on the tube itself, which played a key role for vertical growth of CNTs as a “tip-growth model”.14 In other words, if there were no particles at the tips of the CNTs, then the electrostatic force did not exhibit obvious action upon the well-aligned growth of the CNTs due to the so-called base-grown model. In addition to the above electric field, recently, few papers also focused upon the magnetic field induced growth of CNTs. * Corresponding author. Department of Physics, Wuhan University, Wuhan, 430072, P R China. Tel. +86-27-6236-7023; Fax: +86-27-68752569; E-mail address:
[email protected] (Pan C). † Department of Physics and Key Laboratory of Acoustic and Photonic Materials and Devices of Ministry of Education, Wuhan University. ‡ Center for Electron Microscopy, Wuhan University.
Figure 1. Schematic diagram of the experimental setup.
A. K. Pal et al.15 found that the effect of a magnetic field (100 mT) applied perpendicular to the direction of the electric field during electrodeposition had significant effect on the aligned growth of CNTs. K. H. Lee et al.16 first applied a magnetic bar (140 mT) to orient the iron nanoparticles, then the CNTs preferentially grew from certain facets of the iron catalysts, and at last controlled the growth direction of CNTs in a CVD system. O. Nobuo17 used an external magnetic field (Nd-Fe-B permanent magnet, 500 mT) to induce the shapes of the CNTs bundles by changing the direction of the magnetic field in a PE-CVD apparatus. D. Wei et al.18 indicated that an external magnetic field could promote the coalescence or division of the iron catalyst particles, causing the formation of branched or encapsulated CNTs in a CVD system. From the different route, some researches aligned the CNTs in a magnetic field through the method of modifying CNTs with magnetic particles.19,20 Flames have been demonstrated as a simple and efficient method to obtain one-dimensional (1D) carbon nanomaterials including CNTs and carbon nanofibers (CNFs) with a potentially large mass and large area of production.21-24 In addition, it also helpful to add an electric or magnetic field surrounding the substrate to control the growth of the CNTs, especially wellaligned CNTs in a large area and large quantity. In this article,
10.1021/jp8034852 CCC: $40.75 2008 American Chemical Society Published on Web 08/07/2008
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Figure 2. Macroscopic magnetic field and vector lines around the substrate and the PM.
similar to our previous work,12,13 an external neodymiumiron-boron (Nd-Fe-B) alloy permanent magnet (PM) is applied around the flame. And the effect of the magnetic field on the well-aligned growth of the CNTs based upon the basegrowth model and microstructural changes are examined and calculated. It is expected to introduce a novel process to obtain well-aligned CNTs without catalyst particle at the tip by using a magnetic source. 2. Experimental Section It is well-known that the growth mechanisms of CNTs can be assigned to the “tip-growth model” with a catalyst particle at the tip of the CNT and the base-grown model without particles at the tip.14 However, in most cases, these catalysts are unnecessary and need to be purified. In the present work, in order to reveal the effect of magnetic field on the growth of the CNTs, two processes were used to generate catalysts, that is, coating a Ni(NO3)2 layer for the tip-growth model and electrodepositing a Ni nanocrystalline layer for the base-growth model. The experimental details are as follows: (1) Coating a Ni(NO3)2 layer: the substrate was a cooper sheet 15 × 15 mm in size and pretreated using the following steps. At first, the surface was mechanically polished to a mirror finish; then, the sampling surface was uniformly coated with the Ni(NO3)2 ethanol solution (5 g/20 mL) and dried in air at room temperature; finally, the modified surface was placed face down against the flame when it was inserted into the central core of the flame and maintained for about 40 s.12 (2) Electrodepositing a Ni nanocrystalline layer: the substrate was the same as above. The sampling surface was modified by a Ni nanocrystalline layer using a pulse electrodeposition technique (two pulse numerical controlled electrodeposition power supply, GKDM 30-15, Xin Du, China). The electrolyte and parameters have been described elsewhere.25 The synthesis of the CNTs is the same as above, and the details also described in our previous work.26 Figure 1 shows a schematic diagram of the experimental setup with a magnetic field. A 46 × 46 × 24 mm sized neodymiumiron-boron (Nd-Fe-B) permanent magnet (PM) with a surface magnetic flux of 380 mT was used. It was put above the substrate at a distance of 18 mm. To avoid the magnetic property fading away when the temperature was exceeded to its Courier point during flaming, an aluminum flat 150× 150 mm in size
Figure 3. SEM morphologies of CNTs grown on the plated Ni nanocrystalline layer: (a) without magnetic field, (b and c) with magnetic field.
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Zhang and Pan
Figure 4. TEM micrograph of the well-aligned CNTs grown on the plated Ni nanocrystalline layer: (a) a cluster of aligned CNTs, (b) a single CNT.
Figure 5. HRTEM micrographs of CNTs grown on the plated Ni nanocrystalline layer: (a) without magnetic field, (b) with magnetic field.
was inserted between the PM and the substrate as a barrier layer. Figure 2 illustrates the simulated macroscopic magnetic field and the vector lines around the substrate using a Maxwell 2D simulation program (Maxwell SV, Ansoft Corp). It shows that the magnetic field is parallel to the substrate surface with a uniform field of ∼170 mT. The morphology of the well-aligned CNTs was characterized by using a scanning electron microscope (SIRION SEM, FEI, The Netherlands), a transmission electron microscope (JEM 2010 TEM, JEOL, Japan) and high-resolution transmission electron microscope (JEM 2010FEF HRTEM, JEOL, Japan). The TEM specimens were prepared by scraping the CNTs off the substrate, dispersing them in an ethanol ultrasonic bath for 10 min, and dropping the suspension onto 3 mm diameter copper microgrids. The laser Raman spectra were performed in an England Renishaw-1000 laser Raman spectroscopy instrument in the back-scattering configuration at room temperature. 3. Results and Discussion Generally, the magnetic catalyst nanoparticles have interactions with the external magnetic field, which provides a possibility for improving the alignment of CNTs based upon the tip-growth model. In the present work, lots of experiments using substrate pretreatments, such as coated sampling surface for tip-growth and electrodeposited sampling surface for base-
growth, demonstrated that the magnetic field assistance greatly improved the repeatability, controllability, and stability of the well-aligned growth of CNTs. Figure 3 shows the SEM morphologies of the CNTs that grew without and with a magnetic field respectively. Obviously, comparing to the regular process without a magnetic field, a high density of CNTs with a length of about 1 µm aligned in an orderly fashion and grew perpendicular to the substrate surface when a magnetic field was added. TEM examinations revealed that there is no catalyst particle at the tip of the aligned CNTs, which also exhibited a more uniform diameters about 60 nm, as shown in Figure 4. Further high-resolution transmission electron microcopy (HRTEM) and Laser Raman spectroscopy examination also revealed that the magnetic field, more or less, improves the aligned graphitic degree of order of the CNTs, as shown in Figures 5 and 6, which is similar to the electric-field-induced microstructure transformation.12,27 That is to say, the regular grown of CNTs without a magnetic field have a larger I(D)/ I(G) ) 1.37, indicating lower crystallinity or more amorphous carbon than that with a magnetic field I(D)/I(G) ) 0.80, exhibiting a higher graphitic degree of order. According to our previous work in case of using an electric field assistance,12 the alignment mechanism was concluded from the reason of the electrostatic force acting upon the catalyst
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Figure 6. Raman spectra of CNTs grown on the plated Ni nanocrystalline layer: (a) without magnetic field, (b) with magnetic field.
Figure 7. Local magnetic field around the CNTs: (a) a cluster of aligned CNTs, (b) a single CNT.
Figure 8. The magnetic forces acting upon a catalyst particle and a CNT.
particles at the tips of CNTs which corresponded to the tipgrowth model. In that case, the well-aligned CNTs could not grow upon the electrodeposited sampling surface with a nanocrystalline layer due to the strong combination between Ni nanoparticles and the substrate, corresponding to the basegrowth model. However, the present results indicate that there is not any difference between the two surface pretreatments, which implies that the growth of the CNTs in a magnetic field exhibit a different mechanism. To understand the well-aligned growth mechanism of CNTs in the magnetic field, the magnetic force acting upon a CNT itself and a tip particle was calculated, as illustrated in Figure 7. This calculation was based on a finite element method using
Maxwell SV. In the present model, the catalyst particle and corresponding CNT were assumed to be as a “hemisphere on a hollow-cored post” model.28 It still assumed a Ni catalyst particle on the tip of a CNT and then compared the magnetic forces acting upon the Ni catalyst particle and the CNT. The parameters included the similar CNT diameter 60 nm, wall thickness 20 nm, intertube distance 60 nm, and CNT number 31 (one in the middle, 15 on the left, and 15 on the right) grown in the center of the magnetic field. The calculation showed that the strength of the magnetic field between the tubes was uniformly maintained at 200 mT and only varied around the Ni catalyst particles, as shown in Figure 7. That is, surrounding the catalyst particles, the local magnetic field strength increased to ∼500 mT, and at the top and bottom of the Ni particles it decreased to less than 40 mT. This phenomenon could be explained by the fact that the ferromagnetic property of a Ni catalyst particle was magnetized in the magnetic field to generate an additional magnetic field with the direction parallel to the macroscopic magnetic field. Further calculation for variant length of CNTs, such as 30, 60, 120, 200, 500, 1000, and 2000 nm, is illustrated in Figure 8, which represents the magnetic force variation during the growth of CNTs. The results reveal the following characters: (1) The magnetic forces acting upon both the catalyst particle and the CNT itself were perpendicular to the substrate with similar order of magnitude. The force upon the particle was opposite to the CNT growth direction, whereas the force upon the CNT was along its growth direction. This is because CNT is a kind of diamagnetic substance, and the orientation of carbon
13474 J. Phys. Chem. C, Vol. 112, No. 35, 2008 atomic magnetic torque is opposite to the external macroscopic magnetic field. Therefore, the CNTs are repulsed by the PM. (2) The force acting upon CNT decreased during its growth and reached zero when the length exceeded 2000 nm, and the force acting upon the Ni particles was kept steady during CNT growth. Consequently, the magnetic force acting upon the CNTs enhances the well-aligned growth of the CNTs, whereas the force upon the Ni particles stunts the growth. However, the Ni particle generally lost its ferromagnetism and transformed into paramagnetism in flame when the temperature increased above its Curie point of 358 °C. Therefore, the force acting upon the Ni particles generally could be neglected. That is to say, the magnetic force acting upon the CNTs plays a key role for the well-aligned growth of CNTs in a magnetic field, despite the tip- or base-growth models, which exhibit a different mechanism to an electric field assistance.12,13 The mechanism for well-aligned growth of CNTs in a magnetic field is similar as our previous theory,12 that is, CNT grows along the magnetic field lines. If a CNT deviates at an angle θ with respect to the magnetic field (B), the magnetic force (F) acting on a CNT is divided into a tangential component Ft ) F sin θ and an axial component Fa ) F cos θ. The tangential component Ft is the alignment force drawing the nanotube back to the equilibrium position along the magnetic field lines. The axial component Fa is supposed to generate a tensile stress at the nanotube interface that will accelerate carbon atom precipitation by stress induced diffusion. As a result, a well-aligned CNT array is easily synthesized at the action of a uniformity and parallelity of the magnetic field when using the present process. 4. Conclusions (1) The magnetic field possesses a possibility for growing well-aligned CNTs due to the CNT diamagnetic property, and it also improves the crystallinity of graphite sheets of CNTs. (2) Comparing to growth in an electric field, the growth mechanism in a magnetic field assigns to the base-growth model without catalysts at the tip. It provides a broad and flexible method to obtain well-aligned CNTs with or without a particle at the tip and makes possible for application in nanoelectronic devices. (3) In addition to the present flame process which has advantages such as convenience of applying magnetic field, simple experiment setup, and large area synthesis of well-aligned CNTs, it also is possible to be applied in a regular CVD process.
Zhang and Pan Acknowledgment. We would like to thank Dr. Dongshan Zhao, Mr. Yaoyao Ren, and Mr. Qiang Fu for their excellent technical assistance in HRTEM, TEM, and SEM observations. This work was supported by the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP No. 20070486016.), Ministry of Education, China. References and Notes (1) Lee, C. J.; Kim, D. W.; Lee, T. J.; Choi, Y. C.; Park, Y. S.; Lee, Y. H.; Choi, W. B.; Lee, N. S.; Park, G. S.; Kim, J. M. Chem. Phys. Lett. 1999, 312, 461. (2) Murakami, Y.; Chiashi, S.; Miyauchi, Y.; Hu, M.; Ogura, M.; Okubo, T.; Maruyama, S. Chem. Phys. Lett. 2004, 385, 298. (3) Wei1, B. Q.; Vajtai, R.; Jung, Y.; Ward, J.; Zhang, R.; Ramanath, G.; Ajayan, P. M. Nature 2002, 416, 495. (4) Fan, S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. Science 1999, 283, 512. (5) Che, G.; Lakshmi, B. B.; Fisher, E. R.; Martin, C. R. Nature 1998, 393, 346. (6) Zhang, Y.; Chang, A.; Cao, J.; Wang, Q.; Kim, W.; Li, Y.; Morris, N.; Yenilmez, E.; Kong, J.; Dai, H. Appl. Phys. Lett. 2001, 79, 3155. (7) Avigal, Y.; Kalish, R. Appl. Phys. Lett. 2001, 78, 2291. (8) Bower, C.; Zhu, W.; Jin, S.; Zhou, O. Appl. Phys. Lett. 2000, 77, 830. (9) Chhowalla, M.; Teo, K. B. K.; Ducati, C.; Rupesinghe, N. L.; Amaratunga, G. A. J.; Ferrari, A. C.; Roy, D.; Robertson, J.; Milne, W. I. J. Appl. Phys. 2001, 90, 5308. (10) Law, J. B. K.; Koo, C. K.; Thong, J. T. L. Appl. Phys. Lett. 2007, 91, 243108. (11) Srivastava, A.; Srivastava, A. K.; Srivastava, O. N. Carbon 2001, 39, 201. (12) Bao, Q.; Pan, C. Nanotechnology 2006, 17, 1016. (13) Bao, Q.; Zhang, H.; Pan, C. Comput. Mater. Sci. 2007, 39, 616. (14) Pan, C.; Liu, Y.; Cao, F. Micron 2004, 35, 461. (15) Pal, A. K.; Roy, R. K.; Mandal, S. K.; Gupta, S.; Deb, B. Thin Solid Films 2005, 476, 288. (16) Lee, K. H.; Cho, J. M.; Sigmund, W. Appl. Phys. Lett. 2002, 82, 448. (17) Ohmae, N. Carbon 2008, 46, 544. (18) Wei, D.; Liu, Y.; Cao, L.; Fu, L.; Li, X.; Wang, Yu.; Yu, Gui. J. Am. Chem. Soc. 2007, 129, 7364. (19) Stoffelbach, F.; Aqil, A.; Je´roˆme, C.; Je´roˆme, R.; Detrembleur, C. Chem. Commun. 2005, 36, 4532. (20) Correa-Duarte, M. A.; Grzelczak, M.; Salgueirin˜o-Maceira, V.; Giersig, M.; Liz-Marza´n, L. M.; Farle, M.; Sierazdki, K.; Diaz, R. J. Phys. Chem. B 2005, 109, 19060. (21) Vander Wal, R. L.; Ticich, T. M.; Curtis, V. E. Chem. Phys. Lett. 2000, 323, 217. (22) Yuan, L.; Saito, K.; Pan, C.; Williams, F. A.; Gordon, A. S. Chem. Phys. Lett. 2001, 340, 237. (23) Liu, Y.; Pan, C.; Wang, J. J. Mater. Sci. 2004, 39, 1091. (24) Pan, C.; Liu, Y.; Cao, F. J. Mater. Sci. 2005, 40, 1293. (25) Liu, Y.; Fu, Q.; Pan, C. Carbon 2005, 43, 2264. (26) Pan, C.; Bao, Q. J. Mater. Sci. Lett. 2002, 21, 1927. (27) Bao, Q.; Zhang, H.; Pan, C. Appl. Phys. Lett. 2006, 89, 063124. (28) Forbes, R. G.; Edgcombe, C. J.; Valdre, U. Ultramicroscopy 2003, 95, 57.
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