- Email: [email protected]
Synthesis and magnetic properties of aligned carbon nanotubes by microwave-assisted pyrolysis of acetylene
Physica E 54 (2013) 185–190
Contents lists available at SciVerse ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Synthesis and magnetic properties of aligned carbon nanotubes by microwave-assisted pyrolysis of acetylene Dongju Fu a,b,n, Qing Ma a,b, Xierong Zeng c, Jianjun Chen a,b, Weili Zhang a,b, Dongshuang Li a,b a
Research Institute of Tsinghua University in Shenzhen, Shenzhen 518057, China Shenzhen Engineering Laboratory of Active Electrode Material in Lithium Batteries, Shenzhen 518057, China c College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China b
H I G H L I G H T S
Aligned carbon nanotubes were synthesized by microwave-assisted pyrolysis of C2H2. The CNTs with high quality are densely packed and vertically aligned. The reaction temperature strongly influenced the morphologies of the ACNTs. The synthesis temperature of ACNTs by microwave CVD is relatively lower. The Fe3C-filled ACNTs display ferromagnetic properties at room temperature.
art ic l e i nf o
a b s t r a c t
Article history: Received 10 May 2013 Received in revised form 8 June 2013 Accepted 30 June 2013 Available online 6 July 2013
Aligned carbon nanotubes with high quality were synthesized at low temperature by microwave-assisted pyrolysis of acetylene in nitrogen atmosphere. The morphology and structure of the products were characterized by field-emission scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction and Raman spectroscopy. The results indicated that the ACNTs with high crystallinity are densely packed, and some Fe3C nanoparticles are encapsulated in all parts of carbon tubes. In addition, the effect of the reaction temperature on the morphologies of the CNTs was also studied in detail. Magnetic measurements showed that the Fe3C-filled ACNTs display ferromagnetic properties at room temperature, and can be easily manipulated by an external magnetic field. & 2013 Elsevier B.V. All rights reserved.
Keywords: Carbon nanotube Microwave Microstructure Magnetic property
1. Introduction Carbon nanotubes (CNTs) have been received a great attention because of their unusual properties and great potentials in various applications [1–3]. Especially, vertically aligned CNTs (Aligned carbon nanotubes: ACNTs), with the advantages of good orientation, large aspect ratio, and high purity, are attractive because of their wide potential applications, such as field emission devices, magnetic data storage, lithium-ion batteries, and so on [4–6]. The two most common methods used for the production of ACNTs are chemical vapor deposition (CVD) and plasma-enhanced chemical vapor deposition (PECVD) [7–10]. However, the methods need higher growth temperature (above 700 1C) or use various
n Corresponding author at: Research Institute of Tsinghua University in Shenzhen, Shenzhen 518057, China. Tel./fax: +86 755 26551328. E-mail address: [email protected] (D. Fu).
1386-9477/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physe.2013.06.022
complicated facilities, which hindered the preparation of ACNTs in high yield at low cost. Microwave heating technique is widely applied in many areas of chemistry due to its homogeneous, rapid and volumetric heating characteristics [11]. To date there have been some reports on the growth of carbon nanomaterials by the technique using hydrocarbon or solid carbon source [12,13]. In our previous study, we have also reported the direct preparation of carbon nanomaterials with different structures and SiC nanowires in the absence of catalyst by microwave-assisted pyrolysis of hydrocarbon at higher growth temperature (above 1000 1C) [14–16]. To the best of our knowledge, ACNTs prepared at low temperature by microwave pyrolysis chemical vapor deposition is rarely reported. To further lower the growth temperature, in this paper, we report a large-scale synthesis of high-purity ACNTs using ferrocene as catalyst at about 600 1C by microwave-assisted pyrolysis of acetylene in nitrogen atmosphere. The morphology and structure of the ACNTs were investigated by field emission
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Fe
Fig. 1. FESEM images of as-prepared ACNTs (a) Low magnification SEM image of the ACNTs, (b) SEM image of the top part of the ACNTs, (c) and (d) High-magnification SEM image of the ACNTs.
scanning electron microscopy (FESEM), X-ray diffraction(XRD), high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. When all other conditions maintained unchanged, the effect of the reaction temperature on the morphology and structure of ACNTs was studied in detail. The magnetic properties of the ACNTs were investigated at room temperature.
2. Experimental 2.1. Synthesis The experiments were carried out in a “L” quartz tube heated in a self-designed microwave furnace. The experimental apparatus was similar to what was previously described [14]. The microwave power can be adjusted from 0 to 6 kW at a frequency of 2.45 GHz. Before the microwave was switched on, oxygen in the reaction system was eliminated completely by flushing with nitrogen gas. As the temperature was raised to the growth temperature, ferrocene vapor was carried into the reaction zone by N2 flow (carrier gas) and carbon source gas. To investigate the reaction temperature on the morphologies of ACNTs, the reaction was conducted at 550–700 1C with the flow of mixture gas C2H2/N2 (1:4 vol, 40 sccm). After the reaction for 30 min, the furnace was cooled down to room temperature in the atmosphere of N2. A layer of black substance deposited on the wall of the quartz tube was collected for analysis. 2.2. Characterization A field emission scanning electron microscope (FESEM, Hitachi S-4700) was used to study the morphology of the synthesized products. Energy-dispersive X-ray spectrometry (EDS) attached to
FESEM was carried out, which was used for elemental analysis. The X-ray powder diffraction (XRD, D8 ADVANCE, Bruker AXS) with Cu-KaRadiation (λ¼ 0.154060 nm) was used to identify the structure and phase composition. High resolution transmission electron microscopy (HRTEM, JEM-2100F) was used to analyze the nanostructure of the samples. Raman spectroscopy analysis of the samples had been achieved (Renishaw Invia Reflex) using the excitation wavelength as the 514 nm line of the argon ion laser. The magnetic properties of the samples were measured at room temperature using a vibrating sample magnetometer (VSM: Lake Shore 7410) equipped with a maximum applied field up to 24 kOe.
3. Results and discussion Large amounts of CNTs are synthesized by microwave CVD, and the CNTs are deposited on the surface of the quartz tube. Fig. 1 is the typical FESEM image of the ACNTs obtained at 600 1C using C2H2 as carbon source. The CNTs are densely packed and vertically aligned, as shown in Fig. 1a. The average length of ACNTs, which was estimated from Fig. 1a, is approximately about 60 mm. The chemical compositions of the as-prepared ACNTs, are determined using energy-dispersive X-ray analysis of SEM. It was found that Fe existed besides C which was resulted from carbon shells, as shown in the inset of Fig. 1b. Clearly Fe comes from the decomposition of ferrocene, which showing that Fe nanoparticles is in favor of the formation of carbon nanotubes. Fig. 1c and d are high magnification FESEM images showing that the CNTs with high purity, and have a narrow diameter distribution ranging from 50 nm to 80 nm. Moreover, the ACNTs have a smooth surface, and no carbon particles are found. In order to find their nanoscale structure, the sample was investigated using TEM and HRTEM. The TEM image (Fig. 2a) shows that the CNTs with uniform diameter are obtained and no
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Fig. 2. (a) TEM image of the as-synthesized ACNTs, (b) and (c) HRTEM image of the CNTs.
Intensity / a.u.
Raman Intensity(a.u.)
16000
G
12000
8000 D 4000
0
1000 20
30
40
50 2θ / °
60
70
80
Fig. 3. XRD pattern of the as-synthesized ACNTs.
trace of carbon nanoparticles, which indicates the high purity of the CNTs in good agreement with the SEM results. As shown in Fig. 2, Most of the CNTs have a multiwalled structure with a hollow inside and no compartment layer in the tube. Some metal nanoparticles with a liquid-like shape are encapsulated in all parts of the tubes, indicating that Fe catalyst take part in the growth process of CNTs, as indicated by the arrowheads in Fig. 2a. The relatively long iron rods located in the cavity of carbon nanotubes
1200
1400
1600
1800
2000
Wavenumber(cm-1) Fig. 4. Raman spectra of the as-synthesized ACNTs.
are also observed(Fig. 2b). The CNTs can protect the iron fillings from oxidation and mechanical damage. Moreover, it can be seen that the average outdiameter of the CNTs with narrow size distribution is about 50 nm. Fig. 2c is a HRTEM image of Fig. 2a, revealing a relatively high degree of crystallinity. X-ray diffraction was used to access the crystallinity of CNTs and to determine the iron phases present [17]. Fig. 3 shows the XRD patterns of the ACNTs. The highest sharp peak at 2θ¼26.061 can be assigned to graphite (0 0 2), indicating that graphite layers are regularly stacked. The symmetry and narrowness of this peak is indicative of the high
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Fig. 5. FESEM images of the samples grown at different reaction temperatures: (a) 550 1C, (c) 600 1C, (e) 650 1C and (g) 700 1C, High-magnification SEM image of (b), (d), (f), (h) corresponding to (a), (c), (e), (g) images, respectively.
crystallinity present in this sample. Other peaks can be well indexed to orthorhombic phase of Fe3C (JCPDS 77-0255), suggesting that the ACNTs consist of graphite and encapsulated Fe3C. There is no indication of XRD signals assignable to Fe-containing particles, implying that the Fe content in the as-prepared sample is low.
Raman spectroscopy is extremely sensitive to the graphitic character of these structures, and widely employed for the characterization of carbon materials [18]. Fig. 4 shows the Raman scattering spectra of the ACNTs in the range of 1000–2000 cm 1. There exist two strong peaks centered at 1569 cm 1 (G-band) and
D. Fu et al. / Physica E 54 (2013) 185–190
1348 cm 1 (D-band). The G mode is ascribed to E2g mode of graphite lattice and D mode is assigned to A1g mode due to the existence of structural defects of the graphite lattice. The ratio of the intensities of D–G peak is often used to estimate the degree of perfection of graphene planes [19]. The ID/IG value for the ACNTs is 0.23. Low ID/IG ratio indicates a high degree of sp3 bonding in the carbon and that the sample is of high ratio indicates a high quality. The results were consistent with HRTEM observations and XRD analysis shown above. Compared with ordinary CVD (above 700 1C), the microwave chemical vapor deposition can realize the synthesis of ACNTs at lower temperature of 550 1C. So we investigated especially the effect of the reaction temperature on the morphologies of CNTs. At the temperature below 500 1C, catalytic cracking reaction was so difficult that the CNTs product was few. Fig. 5 shows the FESEM images of the deposition products obtained at the growth temperatures of (a) and (b) 550 1C, (c) and (d) 600 1C, (e)and (f) 650 1C, (g) and (h)700 1C. From Fig. 5, it can be seen that the reaction temperature affects the growth process of CNTs greatly. When the temperature was lowered to 550 1C, the nanotubes are densely packed, and this is the result of Van der Waals interactions between the neighboring nanotubes, As shown in Fig. 5a. From SEM images it has been observed that the lengths of the as-grown ACNTs are about 13 mm. With the temperature increasing from
15 10
M (emu/g)
5 0 -5 -10 -15 -20000 -15000 -10000 -5000
0 5000 10000 15000 20000 H (Oe)
Fig. 6. The hysteresis loops of the sample obtained at room temperature.
189
550 1C to 650 1C, the growth rate was enhanced, the lengths of ACNTs is increasing. However, when the temperature was increased to 700 1C, the orientation of CNTs is poor, and so it is difficult to accurately estimate the length of the ACNTs. At a higher magnification (shown in Fig. 5b, d, f, and g), it can be observed that the temperature has also evident influence on the micromorphology of the ACNTs. At 550 1C, many chain-like CNTs with the mean diameter of about 80 nm were obtained(Fig. 5b). When the temperature is raised to 600 1C, the CNTs changed more straight, and the diameter is more uniform. It is interesting that some CNTs with smaller diameter are winding on the surface of the ACNTs when the temperature reached 650 1C (Fig. 5f). Moreover, the diameter is increasing clearly with the increasing reaction temperature. Fig. 5g shows the CNTs with rougher surface obtained at 700 1C, where a small amount of amorphous carbon can be observed. It is might because of the deactivation of the active surface of catalyst particles resulting from higher carbon atom concentration at higher temperature. Based on these findings, the optimum temperature for CNTs growth in the experiment is about 600 1C. The synthesis temperature of ACNTs by microwave CVD is relatively lower compared to common CVD. This may mainly be related to the rapid heating characteristics and the non thermal effect of microwave heating. On the other hand, the Fe nanoparticles from the decomposition of ferrocene act also microwave absorber, and so the temperature of decomposition of C2H2 is achieved quickly. Diamagnetic characteristic was usually discovered for the purified CNTs [20]. The magnetic property of the ACNTs obtained at 600 1C was measured at room temperature. Fig. 6 shows a representative hysteresis loop of the ACNTs. It can be seen that the sample exhibits a typical ferromagnetic behavior and the smooth S-shaped curve is symmetric around H ¼0. The saturation magnetization value (Ms) of the sample is about 14.1 emu/g and very close to that of bulk Fe3C, which is much lower than the Ms of bulk Fe. This can be attributed to the coexistence of some Fe3C nanoparticles and nonmagnetic carbon. Simultaneously, the coercivity (Hc) of the sample was 531.7 Oe, which is much larger than the Hc of bulk Fe. The ferromagnetic character of the materials is characterized by a ratio of remnant to saturation magnetization Mr/Ms of 0.25–0.3 [21]. The Mr/Ms of the ACNTs was 0.27, suggesting the sample showed the ferromagnetic character. As shown in Fig. 7, the magnetic behavior of the ACNTs is macroscopically shown when we approach a permanent magnet to the vial containing the CNT powder, We observe that the sample is totally lifted from the bottom by the magnet by several centimeters, clearly demonstrating its high magnetic moment.
Fig. 7. (a) Photograph of the ACNTs powder in the vial, (b) and (c) Photograph of its response to a permanent magnet.
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The filled ACNTs would be used as absorbers of electromagnetic radiation and nanocontainers for target drug delivery [22,23]. 4. Conclusion In summary, high-purity ACNTs were synthesized at about 600 1C by microwave-assisted pyrolysis of acetylene in nitrogen atmosphere. The CNTs with high crystallinity are densely packed and vertically aligned. XRD and TEM observation show that some Fe3C particles are encapsulated in carbon tubes. The reaction temperature has an important effect on the morphologies and structures of the CNTs. Magnetic measurements reveal that the Fe3C-filled ACNTs exhibit typical ferromagnetic behavior at room temperature, and have potential applications in magnetic recording media, microwave absorber and drug delivery system. Acknowledgment The authors thank the financial support provided by the Science and Technology project of Shenzhen (Grant no.JCYJ20120619140233056, JCYJ20120619140209259 and CXC201104220014A). References [1] S. Iijima, Nature 354 (1991) 56. [2] W.A. deHeer, A. Chatelain, D. Ugarte, Science 270 (1995) 1179. [3] D.W. Kim, K.Y. Rhee, S.J. Park, Journal of Alloys and Compounds 530 (2012) 6.
[4] Q.K. Wang, C.C. Zhu, W.H. Liu, T. Wu, International Journal of Hydrogen Energy 27 (2002) 497. [5] A. Nagata, H. Sato, Y. Matsui, T. Kaneko, Y. Fujiwara, Vacuum 87 (2013) 182. [6] A.S. Raut, C.B. Parker, B.R. Stoner, J.T. Glass, Electrochemistry Communications 19 (2012) 138. [7] M.B. Jakubinek, M.A. White, G. Li, C. Jayasinghe, W. Cho, M.J. Schulz, V. Shanov, Carbon 48 (2010) 3947. [8] S. Wei, W.P. Kang, J.L. Davidson, J.H. Huang, Diamond and Related Materials 15 (2006) 1828. [9] C. Singh, M. Shaffer, I. Kinloch, A. Windle, Physica B: Condensed Matter 323 (2002) 339. [10] X.Z. Wang, Z. Hu, X. Chen, Y. Chen, Scripta Materialia 44 (2001) 1567. [11] X.R. Zeng, F. Deng, J.Z. Zou, Journal of Alloys and Compounds 517 (2012) 176. [12] D.M. Yoon, B.J. Yoon, K.H. Lee, H.S. Kim, C.G. Park, Carbon 44 (2006) 1298. [13] B. Fidalgo, Y. Ferna´ndez, L. Zubizarreta, A. Arenillas, A. Domínguez, J.J. Pis, J.A. Menéndez, Applied Surface Science 254 (2008) 3553. [14] D.J. Fu, X.R. Zeng, J.Z. Zou, H.X. Qian, X.H. Li, X.B. Xiong, Materials Chemistry and Physics 118 (2009) 501. [15] X.R. Zeng, D.J. Fu, H.C. Sheng, S.H. Xie, X.H. Li, Q. Hu, J.Z. Zou, Physica E 42 (2010) 2103. [16] D.J. Fu, X.R. Zeng, J.Z. Zou, L. Li, X.H. Li, F. Deng, Journal of Alloys and Compounds 486 (2009) 406. [17] F.C. Dillon, A. Bajpai, A. Koo's, S. Downes, Z. Aslam, N. Grobert, Carbon 50 (2012) 3674. [18] N. Jeong, Y. Seo, J. Lee, Diamond and Related Materials 16 (2007) 600. [19] S.C. Lyu, B.C. Liu, T.J. Lee, Z.Y. Liu, C.W. Yang, C.Y. Park, C.J. Lee, Chemical Communications (2003) 734. [20] P. Xu, X.J. Han, X.R. Liu, B. Zhang, C. Wang, X.H. Wang, Materials Chemistry and Physics 114 (2009) 556. [21] N. Luo, X.J. Li, X.H. Wang, H.H. Yan, C.J. Zhang, H.T. Wang, Carbon 48 (2010) 3858. [22] A.C. Wright, M.K. Faulkner, R.C. Harris, A. Goddard, A.P. Abbott, Journal of Magnetism and Magnetic Materials 324 (2012) 4170. [23] H.X. Wu, G. Liu, X. Wang, J.M. Zhang, Y. Chen, J.L. Shi, H. Yang, H. Hu, S.P. Yang, Acta Biomaterialia 7 (2011) 3496.
Contents lists available at SciVerse ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Synthesis and magnetic properties of aligned carbon nanotubes by microwave-assisted pyrolysis of acetylene Dongju Fu a,b,n, Qing Ma a,b, Xierong Zeng c, Jianjun Chen a,b, Weili Zhang a,b, Dongshuang Li a,b a
Research Institute of Tsinghua University in Shenzhen, Shenzhen 518057, China Shenzhen Engineering Laboratory of Active Electrode Material in Lithium Batteries, Shenzhen 518057, China c College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China b
H I G H L I G H T S
Aligned carbon nanotubes were synthesized by microwave-assisted pyrolysis of C2H2. The CNTs with high quality are densely packed and vertically aligned. The reaction temperature strongly influenced the morphologies of the ACNTs. The synthesis temperature of ACNTs by microwave CVD is relatively lower. The Fe3C-filled ACNTs display ferromagnetic properties at room temperature.
art ic l e i nf o
a b s t r a c t
Article history: Received 10 May 2013 Received in revised form 8 June 2013 Accepted 30 June 2013 Available online 6 July 2013
Aligned carbon nanotubes with high quality were synthesized at low temperature by microwave-assisted pyrolysis of acetylene in nitrogen atmosphere. The morphology and structure of the products were characterized by field-emission scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction and Raman spectroscopy. The results indicated that the ACNTs with high crystallinity are densely packed, and some Fe3C nanoparticles are encapsulated in all parts of carbon tubes. In addition, the effect of the reaction temperature on the morphologies of the CNTs was also studied in detail. Magnetic measurements showed that the Fe3C-filled ACNTs display ferromagnetic properties at room temperature, and can be easily manipulated by an external magnetic field. & 2013 Elsevier B.V. All rights reserved.
Keywords: Carbon nanotube Microwave Microstructure Magnetic property
1. Introduction Carbon nanotubes (CNTs) have been received a great attention because of their unusual properties and great potentials in various applications [1–3]. Especially, vertically aligned CNTs (Aligned carbon nanotubes: ACNTs), with the advantages of good orientation, large aspect ratio, and high purity, are attractive because of their wide potential applications, such as field emission devices, magnetic data storage, lithium-ion batteries, and so on [4–6]. The two most common methods used for the production of ACNTs are chemical vapor deposition (CVD) and plasma-enhanced chemical vapor deposition (PECVD) [7–10]. However, the methods need higher growth temperature (above 700 1C) or use various
n Corresponding author at: Research Institute of Tsinghua University in Shenzhen, Shenzhen 518057, China. Tel./fax: +86 755 26551328. E-mail address: [email protected] (D. Fu).
1386-9477/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physe.2013.06.022
complicated facilities, which hindered the preparation of ACNTs in high yield at low cost. Microwave heating technique is widely applied in many areas of chemistry due to its homogeneous, rapid and volumetric heating characteristics [11]. To date there have been some reports on the growth of carbon nanomaterials by the technique using hydrocarbon or solid carbon source [12,13]. In our previous study, we have also reported the direct preparation of carbon nanomaterials with different structures and SiC nanowires in the absence of catalyst by microwave-assisted pyrolysis of hydrocarbon at higher growth temperature (above 1000 1C) [14–16]. To the best of our knowledge, ACNTs prepared at low temperature by microwave pyrolysis chemical vapor deposition is rarely reported. To further lower the growth temperature, in this paper, we report a large-scale synthesis of high-purity ACNTs using ferrocene as catalyst at about 600 1C by microwave-assisted pyrolysis of acetylene in nitrogen atmosphere. The morphology and structure of the ACNTs were investigated by field emission
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Fe
Fig. 1. FESEM images of as-prepared ACNTs (a) Low magnification SEM image of the ACNTs, (b) SEM image of the top part of the ACNTs, (c) and (d) High-magnification SEM image of the ACNTs.
scanning electron microscopy (FESEM), X-ray diffraction(XRD), high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. When all other conditions maintained unchanged, the effect of the reaction temperature on the morphology and structure of ACNTs was studied in detail. The magnetic properties of the ACNTs were investigated at room temperature.
2. Experimental 2.1. Synthesis The experiments were carried out in a “L” quartz tube heated in a self-designed microwave furnace. The experimental apparatus was similar to what was previously described [14]. The microwave power can be adjusted from 0 to 6 kW at a frequency of 2.45 GHz. Before the microwave was switched on, oxygen in the reaction system was eliminated completely by flushing with nitrogen gas. As the temperature was raised to the growth temperature, ferrocene vapor was carried into the reaction zone by N2 flow (carrier gas) and carbon source gas. To investigate the reaction temperature on the morphologies of ACNTs, the reaction was conducted at 550–700 1C with the flow of mixture gas C2H2/N2 (1:4 vol, 40 sccm). After the reaction for 30 min, the furnace was cooled down to room temperature in the atmosphere of N2. A layer of black substance deposited on the wall of the quartz tube was collected for analysis. 2.2. Characterization A field emission scanning electron microscope (FESEM, Hitachi S-4700) was used to study the morphology of the synthesized products. Energy-dispersive X-ray spectrometry (EDS) attached to
FESEM was carried out, which was used for elemental analysis. The X-ray powder diffraction (XRD, D8 ADVANCE, Bruker AXS) with Cu-KaRadiation (λ¼ 0.154060 nm) was used to identify the structure and phase composition. High resolution transmission electron microscopy (HRTEM, JEM-2100F) was used to analyze the nanostructure of the samples. Raman spectroscopy analysis of the samples had been achieved (Renishaw Invia Reflex) using the excitation wavelength as the 514 nm line of the argon ion laser. The magnetic properties of the samples were measured at room temperature using a vibrating sample magnetometer (VSM: Lake Shore 7410) equipped with a maximum applied field up to 24 kOe.
3. Results and discussion Large amounts of CNTs are synthesized by microwave CVD, and the CNTs are deposited on the surface of the quartz tube. Fig. 1 is the typical FESEM image of the ACNTs obtained at 600 1C using C2H2 as carbon source. The CNTs are densely packed and vertically aligned, as shown in Fig. 1a. The average length of ACNTs, which was estimated from Fig. 1a, is approximately about 60 mm. The chemical compositions of the as-prepared ACNTs, are determined using energy-dispersive X-ray analysis of SEM. It was found that Fe existed besides C which was resulted from carbon shells, as shown in the inset of Fig. 1b. Clearly Fe comes from the decomposition of ferrocene, which showing that Fe nanoparticles is in favor of the formation of carbon nanotubes. Fig. 1c and d are high magnification FESEM images showing that the CNTs with high purity, and have a narrow diameter distribution ranging from 50 nm to 80 nm. Moreover, the ACNTs have a smooth surface, and no carbon particles are found. In order to find their nanoscale structure, the sample was investigated using TEM and HRTEM. The TEM image (Fig. 2a) shows that the CNTs with uniform diameter are obtained and no
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187
Fig. 2. (a) TEM image of the as-synthesized ACNTs, (b) and (c) HRTEM image of the CNTs.
Intensity / a.u.
Raman Intensity(a.u.)
16000
G
12000
8000 D 4000
0
1000 20
30
40
50 2θ / °
60
70
80
Fig. 3. XRD pattern of the as-synthesized ACNTs.
trace of carbon nanoparticles, which indicates the high purity of the CNTs in good agreement with the SEM results. As shown in Fig. 2, Most of the CNTs have a multiwalled structure with a hollow inside and no compartment layer in the tube. Some metal nanoparticles with a liquid-like shape are encapsulated in all parts of the tubes, indicating that Fe catalyst take part in the growth process of CNTs, as indicated by the arrowheads in Fig. 2a. The relatively long iron rods located in the cavity of carbon nanotubes
1200
1400
1600
1800
2000
Wavenumber(cm-1) Fig. 4. Raman spectra of the as-synthesized ACNTs.
are also observed(Fig. 2b). The CNTs can protect the iron fillings from oxidation and mechanical damage. Moreover, it can be seen that the average outdiameter of the CNTs with narrow size distribution is about 50 nm. Fig. 2c is a HRTEM image of Fig. 2a, revealing a relatively high degree of crystallinity. X-ray diffraction was used to access the crystallinity of CNTs and to determine the iron phases present [17]. Fig. 3 shows the XRD patterns of the ACNTs. The highest sharp peak at 2θ¼26.061 can be assigned to graphite (0 0 2), indicating that graphite layers are regularly stacked. The symmetry and narrowness of this peak is indicative of the high
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Fig. 5. FESEM images of the samples grown at different reaction temperatures: (a) 550 1C, (c) 600 1C, (e) 650 1C and (g) 700 1C, High-magnification SEM image of (b), (d), (f), (h) corresponding to (a), (c), (e), (g) images, respectively.
crystallinity present in this sample. Other peaks can be well indexed to orthorhombic phase of Fe3C (JCPDS 77-0255), suggesting that the ACNTs consist of graphite and encapsulated Fe3C. There is no indication of XRD signals assignable to Fe-containing particles, implying that the Fe content in the as-prepared sample is low.
Raman spectroscopy is extremely sensitive to the graphitic character of these structures, and widely employed for the characterization of carbon materials [18]. Fig. 4 shows the Raman scattering spectra of the ACNTs in the range of 1000–2000 cm 1. There exist two strong peaks centered at 1569 cm 1 (G-band) and
D. Fu et al. / Physica E 54 (2013) 185–190
1348 cm 1 (D-band). The G mode is ascribed to E2g mode of graphite lattice and D mode is assigned to A1g mode due to the existence of structural defects of the graphite lattice. The ratio of the intensities of D–G peak is often used to estimate the degree of perfection of graphene planes [19]. The ID/IG value for the ACNTs is 0.23. Low ID/IG ratio indicates a high degree of sp3 bonding in the carbon and that the sample is of high ratio indicates a high quality. The results were consistent with HRTEM observations and XRD analysis shown above. Compared with ordinary CVD (above 700 1C), the microwave chemical vapor deposition can realize the synthesis of ACNTs at lower temperature of 550 1C. So we investigated especially the effect of the reaction temperature on the morphologies of CNTs. At the temperature below 500 1C, catalytic cracking reaction was so difficult that the CNTs product was few. Fig. 5 shows the FESEM images of the deposition products obtained at the growth temperatures of (a) and (b) 550 1C, (c) and (d) 600 1C, (e)and (f) 650 1C, (g) and (h)700 1C. From Fig. 5, it can be seen that the reaction temperature affects the growth process of CNTs greatly. When the temperature was lowered to 550 1C, the nanotubes are densely packed, and this is the result of Van der Waals interactions between the neighboring nanotubes, As shown in Fig. 5a. From SEM images it has been observed that the lengths of the as-grown ACNTs are about 13 mm. With the temperature increasing from
15 10
M (emu/g)
5 0 -5 -10 -15 -20000 -15000 -10000 -5000
0 5000 10000 15000 20000 H (Oe)
Fig. 6. The hysteresis loops of the sample obtained at room temperature.
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550 1C to 650 1C, the growth rate was enhanced, the lengths of ACNTs is increasing. However, when the temperature was increased to 700 1C, the orientation of CNTs is poor, and so it is difficult to accurately estimate the length of the ACNTs. At a higher magnification (shown in Fig. 5b, d, f, and g), it can be observed that the temperature has also evident influence on the micromorphology of the ACNTs. At 550 1C, many chain-like CNTs with the mean diameter of about 80 nm were obtained(Fig. 5b). When the temperature is raised to 600 1C, the CNTs changed more straight, and the diameter is more uniform. It is interesting that some CNTs with smaller diameter are winding on the surface of the ACNTs when the temperature reached 650 1C (Fig. 5f). Moreover, the diameter is increasing clearly with the increasing reaction temperature. Fig. 5g shows the CNTs with rougher surface obtained at 700 1C, where a small amount of amorphous carbon can be observed. It is might because of the deactivation of the active surface of catalyst particles resulting from higher carbon atom concentration at higher temperature. Based on these findings, the optimum temperature for CNTs growth in the experiment is about 600 1C. The synthesis temperature of ACNTs by microwave CVD is relatively lower compared to common CVD. This may mainly be related to the rapid heating characteristics and the non thermal effect of microwave heating. On the other hand, the Fe nanoparticles from the decomposition of ferrocene act also microwave absorber, and so the temperature of decomposition of C2H2 is achieved quickly. Diamagnetic characteristic was usually discovered for the purified CNTs [20]. The magnetic property of the ACNTs obtained at 600 1C was measured at room temperature. Fig. 6 shows a representative hysteresis loop of the ACNTs. It can be seen that the sample exhibits a typical ferromagnetic behavior and the smooth S-shaped curve is symmetric around H ¼0. The saturation magnetization value (Ms) of the sample is about 14.1 emu/g and very close to that of bulk Fe3C, which is much lower than the Ms of bulk Fe. This can be attributed to the coexistence of some Fe3C nanoparticles and nonmagnetic carbon. Simultaneously, the coercivity (Hc) of the sample was 531.7 Oe, which is much larger than the Hc of bulk Fe. The ferromagnetic character of the materials is characterized by a ratio of remnant to saturation magnetization Mr/Ms of 0.25–0.3 [21]. The Mr/Ms of the ACNTs was 0.27, suggesting the sample showed the ferromagnetic character. As shown in Fig. 7, the magnetic behavior of the ACNTs is macroscopically shown when we approach a permanent magnet to the vial containing the CNT powder, We observe that the sample is totally lifted from the bottom by the magnet by several centimeters, clearly demonstrating its high magnetic moment.
Fig. 7. (a) Photograph of the ACNTs powder in the vial, (b) and (c) Photograph of its response to a permanent magnet.
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The filled ACNTs would be used as absorbers of electromagnetic radiation and nanocontainers for target drug delivery [22,23]. 4. Conclusion In summary, high-purity ACNTs were synthesized at about 600 1C by microwave-assisted pyrolysis of acetylene in nitrogen atmosphere. The CNTs with high crystallinity are densely packed and vertically aligned. XRD and TEM observation show that some Fe3C particles are encapsulated in carbon tubes. The reaction temperature has an important effect on the morphologies and structures of the CNTs. Magnetic measurements reveal that the Fe3C-filled ACNTs exhibit typical ferromagnetic behavior at room temperature, and have potential applications in magnetic recording media, microwave absorber and drug delivery system. Acknowledgment The authors thank the financial support provided by the Science and Technology project of Shenzhen (Grant no.JCYJ20120619140233056, JCYJ20120619140209259 and CXC201104220014A). References [1] S. Iijima, Nature 354 (1991) 56. [2] W.A. deHeer, A. Chatelain, D. Ugarte, Science 270 (1995) 1179. [3] D.W. Kim, K.Y. Rhee, S.J. Park, Journal of Alloys and Compounds 530 (2012) 6.
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