Catalytic synthesis of carbon nanotubes and carbon spheres using Kaolin supported catalyst

Materials Science and Engineering B 123 (2005) 102–106

Catalytic synthesis of carbon nanotubes and carbon spheres using Kaolin supported catalyst Zong-Xiang Xu a,∗ , Jing-Dong Lin a , V.A.L. Roy b , Yan Ou a , Dai-Wei Liao a a

State Key Laboratory for Physical Chemistry of the Solid Surfaces, Department of Chemistry, Institute of Physical Chemistry, Xiamen University, Xiamen 361005, China b The Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong Received 16 September 2004; received in revised form 13 June 2005; accepted 2 July 2005

Abstract The substrate for the catalyst is important in chemical vapor deposition for carbon nanostructure synthesis. The importance of substrates in chemical vapor deposition has been explained here by using two different substrates such as Kaolin plate and ceramic plate. Kaolin supported cobalt catalyst was used on these substrates, also without substrate and the catalytic decomposition of C2 H2 was carried out at the optimal temperature (750 ◦ C) in nitrogen atmosphere. Two different kinds of carbon nanostructures, carbon nanotubes and carbon spheres, were obtained and their microstructure was investigated by transmission electron microscopy, high resolution transmission electron microscopy, X-ray diffractometer and laser Raman spectra. The influence of the catalyst substrate and the possible formation mechanism was discussed. © 2005 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Chemical vapor deposition; Transmission electron microscopy; X-ray diffraction; Scanning electron microscopy

1. Introduction With the first discovery in 1991 [1], carbon nanotubes (CNTs) have attracted great interest in the field of synthesis and applications. The synthesis of carbon nanostructures, such as CNTs, graphite spheres and nanoparticles is one of the challenging topics in the field of carbon materials. Surplus research work has been carried out and the synthesis methods have been improved. All these methods can be divided into two parts: one is non-catalytic method, such as arc-evaporation technique, laser vaporization and electrochemical synthesis. The other method is supported catalytic method, which always employs transition metal (Fe, Co, Ni and Cu) as catalysts in chemical vapor decomposition process (CVD). The CVD method is proved to be the most suitable one for the industrial production of carbon

∗

Corresponding author. E-mail addresses: [email protected] (Z.-X. Xu), [email protected] (D.-W. Liao). 0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.07.012

nano- or microstructured materials. In this method, supported template is very important due to interaction with the metal catalyst chemically and also physically [2]. This interaction helps to disperse the metal catalyst, formed in the CVD process, which decides the metal configuration and its chemical character. Various kinds of templates such as MgO, Al2 O3 , SiO2 , glass, zeolite and alumina film have been used, and the CNTs with different shapes and properties were studied, such as helix nanotubes, onion-like carbon spheres, carbon nanorod and carbon nanoparticles [3–6]. In this article, we propose a novel catalyst, Kaolin supported cobalt catalyst which is used to synthesize carbon nanostructures. Different nanoproducts had been obtained when the catalyst was used on different substrates under the same reaction conditions like temperature, time and the gas flow rate. The two main products were CNTs, about 20 ␮m in length with uniform outer diameter of 30–40 nm and the carbon spheres (CS) with 0.5–1.5 ␮m in diameter. The products were characterized to explore their structure by different techniques, and the impact of catalysts was investigated.

Z.-X. Xu et al. / Materials Science and Engineering B 123 (2005) 102–106

2. Experiments The catalyst was prepared by incipient wetness method, Kaolin and cobalt nitrate were mixed in ethanol, the resulting slurry was stirred vigorously [7,8] and it was treated in three different ways to get the catalyst: 1. The obtained mixture was spin-vaporized to dryness at 60 ◦ C, and then dried at 80 ◦ C in the air for overnight to form catalyst A in powder form. 2. Using Kaolin plate as substrate, the obtained slurry was pasted onto a Kaolin plate, and dried in the air at 80 ◦ C for overnight to form catalyst B. 3. Using ceramic plate as substrate, the mixture was pasted on it, and then dried in the air at 80 ◦ C for overnight to form catalyst C. For CVD process, a quartz tube kept inside an electric furnace had been used. A ceramic boat with the catalyst was placed at the isothermal zone where in the middle of the furnace and the synthesis was carried out with all the three catalysts under the same reaction condition. Initially, the tube was heated up to 250 ◦ C from room temperature in a steady flow of nitrogen, 200 ml/min for 40 min, later the temperature was raised up to 750 ◦ C for 80 min, then a controlled flow of acetylene (50 ml/min) was introduced into the quartz tube along with the carrier gas nitrogen (100 ml/min) for about 2 h. After cooling down to ambient temperature with the flow of nitrogen at 50 ml/min, the products in the ceramic boat were collected. Using the same catalyst precursor on different substrate under same reaction conditions, different carbon nanostructures were obtained. When catalyst A was used, CNTs were found mixed with catalyst powder. When catalyst B was used, carbon beads were obtained in the ceramic boat away from the catalyst and the catalyst remained in good flake configuration clinged to Kaolin plate well. While catalyst C curved inside and was separated from ceramic plate after the reaction where no product was found. All samples so far synthesized were sonicated in acetone and dispersed onto a copper grid for transmission electron microscope (JEM II 100CX II), and high-resolution transmission electron microscope (TECNAL F30) to study the microstructure and morphology of the obtained products. The catalysts and the products have also been investigated by scanning electron microscope (LEO 1530) without fur-

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ther treatment. The CNTs and CS were also characterized by Rigaku D/Max-C X-ray diffractometer with Cu K␣ radiation at a scanning rate of 4◦ min−1 from 20◦ to 80◦ . A He–Ne laser beam of wavelength 632.8 nm (Labram I Dilor) was used to perform the Raman spectra of the obtained CNTs and CS.

3. Results and discussions Although we have studied the effect of different reaction conditions, such as reaction temperature, flow rate of N2 and C2 H2 , and also different metal catalyst like Cu, Ni and Fe, we found no difference in the products. So in our present work, all the reactions were carried out under the same optimal conditions. Here, we chose cobalt as metal catalyst, for its greater ability to produce more ordered carbon materials (graphitization ability) [9,10]. 3.1. Transmission electron microscopic studies According to TEM data, the obtained carbon nanostructures can be divided into two categories. Fig. 1a shows the image of the CNTs synthesized by catalyst A where the diameters of these nanotubes are uniform with the outer diameter in the range of 30–40 nm and the inner diameter about 5–10 nm. For the twisted nanotubes, most of them are several micrometers in length. Another type of carbon structure, carbon spheres were shown in Fig. 1b. The CS formed by catalyst B is about 500–1500 nm in diameter. The carbon spheres can also be obtained by heating kerosene under the catalysis of iron particles to 1100 ◦ C [11]. From the above report, in spite of the high reaction temperature, the obtained carbon spheres have no essential difference with ours. Even though no further treatment was done, the obtained spheres were connected together without impurities on them. Fig. 1c shows catalyst C grinded into powder form after reaction, some sphere-like carbon mingles with nanotubes was seen differs from the catalysts A and B, where only carbon nanotubes or carbon spheres were formed. Fig. 2 shows the HRTEM images of CNTs synthesized by catalyst A and CS formed by catalyst B. Fig. 2a revealed that carbon nanotubes grown over catalyst A were multiwalled nanotubes. The observed distances between the unclosed layers were about 0.35–0.36 nm, indicates the similarity of the graphite. The inner diameter is 5–10 nm, which coincides

Fig. 1. TEM images of: (a) carbon nanotubes synthesized by catalyst A; (b) carbon spheres formed by catalyst B; (c) carbon spheres and CNTs mixture formed by catalyst C.

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Fig. 2. HRTEM images of: (a and b) multiwall carbon nanotube formed by catalyst A and (c) multilayer carbon sphere synthesized by catalyst C.

approximately from estimation of TEM image. As shown in Fig. 2b, the nanotube was closed by a cap, and there is a droplet-like metal particle inside the tube near the cap, which proposes the possible mechanism of nanotubes formation. A liquid state is involved from the high surface-to-volume ratio of the nanoparticles for the reduction of their melting point, helps to form carbon nanotubes [12,13]. Fig. 2c shows a single carbon sphere grown over catalyst B, shows its unclosed graphitic layers, similar to nanotube’s multiwalled structure, but more disordered. According to HRTEM data, we also found that there was no metal particle in the carbon sphere, which differs from carbon nanotubes. 3.2. XRD and Raman study of CNTs and CS The XRD pattern of carbon nanotubes and carbon spheres are shown in Fig. 3. A strong and broad peak at about 26◦ belongs to the interlayer spacing (d0 0 2 ) for the multiwalled structure of the CNTs and CS [11]. The cobalt crystallite peaks can be observed at 44.32◦ , 51.72◦ and 76.06◦ , which only present at CNTs, due to the entrapment of cobalt in the middle of these tubes, agrees with HRTEM images in Fig. 2b. This observation suggests that cobalt metal plays different role in the formation process of CNTs and CS. Fig. 4 shows the Raman shift spectrums of CNTs and CS. The strong peak in the region of about 1580 cm−1 occurs on both line (1) and (2) assigned to one of the two Raman active E2g , vibrations of graphite [12], while the band at around 1322 cm−1 almost disappear in CNTs, attributed to non-graphitizable carbon [14]. It indicates that the CNTs are in higher graphitization degree than CS, just agree with the HRTEM result.

Fig. 3. XRD spectra of: (1) CNTs synthesized by catalyst A and (2) carbon spheres synthesized by catalyst B.

3.3. Scanning electron microscopic studies Fig. 5 illustrates the SEM images of the catalysts after reaction. From Fig. 5a and b, the catalyst A was mixed with CNTs, formed bundles and gave cohesion to catalyst A. Fig. 5c shows a cross-sectional view of catalyst C, as shown on its faultage, the CNTs were formed and twisted with the catalyst. Some carbon particles were also found on its front surface. Catalyst C is a combination of catalysts A and B. Not only carbon spheres but also carbon nanotubes were formed on it. Fig. 5d–i represents the SEM images of catalyst B reacted for different time. From the SEM images, the carbon species decomposed on the catalyst followed five different states. At first, when the amorphous carbon decomposed on the surface of the catalyst (Fig. 5d), carbon nanofibers were obtained with 30–40 nm in diameter (Fig. 5e), if the reaction continued, rodlike carbon was obtained with 1–1.5 ␮m in diameter (Fig. 5f), further the continuation of the reaction for more than 1 h, bubble-like carbon layer was formed (Fig. 5g), which led to the formation of carbon spheres (Fig. 5h and i). From these results, the importance of the catalyst substrates was observed. Under normal reaction conditions they interact with the Kaolin template and influence the metal catalyst formed on the surface. For the metal particle, the structure is dependent on the void volume of the support medium, and these particles determine the orientation of the precipitated graphitic lamella [2]. Using different substrate or without substrate, interaction between the metal nanoparticles and

Fig. 4. Raman patterns of: (1) CNTs formed by catalyst A and (2) carbon spheres synthesized by catalyst B.

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Fig. 5. SEM images of: catalyst A after reaction (a and b); catalyst C after reaction (c); catalyst B reacted for different time—(d) 0 min; (e) 15 min; (f) 30 min; (g) 60 min; (h) 90 min; (i) 120 min.

the template was different, this affected the metal catalyst configuration. For catalyst A without substrate, according to HRTEM images, XRD and Raman data, the mechanism of CNTs formation using catalyst A was similar as the tip growth mech-

anism proposed in Refs. [15,16], using other metal oxide as catalyst template. According to SEM images for the catalyst B after different reaction time, the formation mechanism of carbon sphere, as shown in Fig. 6, is proposed also resembles with Ref. [7]. At

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Fig. 6. (a–f) Scheme for carbon spheres formation on catalyst B.

first carbon nanofibers were obtained with the catalyst containing metal particle having size less than 10 nm (Fig. 2b), as like catalyst A. If the reaction continued, under the interaction of Kaolin substrate, the metal particles were sintered, resulting a wider size distribution (Fig. 6d). Here, the diameter of carbon deposition was correlated with the size of catalyst metal particles. With the sintering of the metal particles, the carbon nanofibers became shorter in length and wider in diameter. All these intermediate carbon species contained unstable hexagons or heptagons or reactive dangling bonds, which led to bond switching and migration process. At the end, more stable spherical carbon structures occurred. From the above results, it was shown that under the same reaction condition, the Kaolin substrate caused the sintering of cobalt metal particles. The difference in the size of catalyst makes the difference in the carbon deposition. 4. Conclusion A novel catalyst, Kaolin supported cobalt catalyst was used in the catalytic decomposition of C2 H2 for carbon nanostructure synthesis. It was found that substrate play an important role in the synthesis. Under the same reaction condition, the catalyst was used on different substrates, Kaolin plate and ceramic plate or without substrate and different carbon nanostructures were formed. Carbon nanotubes grown over the catalyst without substrates are about 20 ␮m in length with uniform outer diameter of 30–40 nm. The carbon spheres with 500–1500 nm diameter were formed by the catalyst on Kaolin plate substrate, while using ceramic plate as substrate, a mixture of carbon nanotubes and carbon spheres were obtained. The results of SEM, TEM, HRTEM, XRD and Raman measurements provided the evidence for the formation of CNTs and CS.

Acknowledgements This work was supported by the NSF of China (20273053, 29933040 and 20023001), the 973 Programming (001CB108906) and the NSF of Fujian province (E0310001).

References [1] S. Iijima, Nature 354 (1991) 56–58. [2] R.L. Vander Wal, T.M. Ticich, V.E. Curtis, Carbon 39 (2001) 2277–2289. [3] N. He, Y. Kuang, Q. Dai, Y. Miao, A. Zhang, X. Wang, Mater. Sci. Eng. C 8–9 (1999) 151–157. [4] Y.C. Sui, B.Z. Cui, R. Guardian, D.R. Acosta, L. Mart´ynez, R. Perez, Carbon 40 (2002) 1011–1016. [5] P. Chen, H.B. Zhang, G.D. Lin, Q. Hong, K.R. Tsai, Carbon 35 (1997) 1495–1501. [6] K. Hernadi, A. Fonseca, J.B. Nagy, D. Bernaerts, A.A. Lucas, Carbon 34 (1997) 1249–1257. [7] J.Y. Miao, D. Hwang, C.C. Chang, S.H. Lin, K.V. Narasimhulu, L.P. Hwang, Diamond Relat. Mater. 12 (2003) 1368–1372. [8] J.Y. Miao, D. Hwang, K.V. Narasimhulu, P.I. Lin, Y.T. Chen, S.H. Lin, L.P. Hwang, Carbon 42 (2004) 813–822. [9] Q. Liang, L.Z. Gao, Q. Li, S.H. Tang, Carbon 39 (2001) 897–903. [10] H.S. Kang, H.J. Yoon, C.O. Kim, J.P. Hong, I.T. Han, S.N. Cha, B.K. Song, J.E. Jung, N.S. Lee, J.M. Kim, Chem. Phys. Lett. 349 (2001) 196–200. [11] P. Debabrata, S. Maheshwar, Mater. Sci. Eng. B 96 (2002) 24–28. [12] G. Kamalakar, D. Hwang, L.P. Hwang, J. Mater. Chem. 12 (2002) 1819–1823. [13] A. Weidenkaff, S.G. Ebbighaus, P. Mauron, Mater. Sci. Eng. C 19 (2002) 119–123. [14] I. Mochida, M. Egashira, Y. Korai, K. Yokogawa, Carbon 35 (1997) 1707–1712. [15] S. Amelinckx, X.B. Zhang, D. Bernaerts, X.F. Zhang, Science 265 (1994) 635–637. [16] M. Yudasaka, R. Kikuchi, Y. Ohi, E. Ota, S. Yoshimua, Appl. Phys. Lett. 70 (1997) 1817.