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Synthesis and characterization of carbon spheres prepared by chemical vapour deposition
Materials Letters 61 (2007) 4854 – 4856 www.elsevier.com/locate/matlet
Synthesis and characterization of carbon spheres prepared by chemical vapour deposition Peng Wang, Jiyong Wei, Baibiao Huang ⁎, Xiaoyan Qin, Shushan Yao, Qi Zhang, Zeyan Wang, Guanghui Xu, Xiangyang Jing State Key Lab of Crystal Material, Shandong University, Jinan 250100, China Received 16 January 2007; accepted 15 March 2007 Available online 20 March 2007
Abstract Carbon spheres, with uniform diameters of about 1 μm, have been achieved via Chemical vapour deposition (CVD). The fabricated materials have been fully characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray analysis (EDX). The results show that the spheres are 95% carbon. The formation mechanism of carbon spheres has also been discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Carbon spheres; Nanomaterials; Chemical vapour deposition; Electron microscopy
1. Introduction Carbon is a versatile element because it can form various structures [1,2]. The successful growth of diamond films [3–5], the fullerene molecule C60 [6] and its family Cn [7,8] and carbon nanotubes has excited many researchers [9,10]. The variety of structures produced by carbon is determined by its unique hybridization sp1, sp2, and sp3 bonding. Carbon spheres as a new material is becoming increasingly important [11–16]. Serp et al. have classified such structures according to their size into fullerenes Cn, carbon onions (2–20 nm, with closed graphite layer) [17], carbon spheres (50 nm to 1 μm) [18,19] and carbon beads (above 1 μm) [20]. Carbon spheres can be fabricated by the methods that are normally used to synthesize carbon nanotubes. Wang et al. developed a mixed-valent oxidecatalytic carbonization (MVOCC) process for the production of carbon spheres. By varying the temperatures, this process produced either carbon spheres or carbon nanotubes when
⁎ Corresponding author. Tel.: +86 531 88364449; fax: +86 531 88366324. E-mail address: [email protected] (B. Huang). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.03.056
methane was used as the carbon source [21]. Qian et al. have also used the CVD method to preparation of carbon spheres with a specific size without catalyst [22]. Carbon spheres can also be prepared at relatively low temperatures
Fig. 1. SEM image of carbon spheres.
P. Wang et al. / Materials Letters 61 (2007) 4854–4856
4855
2. Experimental
Fig. 2. Diameter distribution of carbon spheres.
(650 °C) using transition metal catalysts deposited on a kaolin (hydrated aluminium silicate) support [23]. Jin et al. have reported on the large-scale synthesis and characterization of carbon spheres prepared by the direct pyrolysis of hydrocarbons with two to eight carbon atoms [24]. Chen et al. have reported on the formation of spherical hydrogenated amorphous carbon particulates generated in a radio frequency plasma enhanced chemical vapour deposition (rf-PECVD) system [25]. Recently, Pol et al. have reported on the high yield one-step synthesis of carbon spheres produced by dissociating individual hydrocarbons at their autogenic pressure and low temperatures [26]. These techniques, however, are limited by several factors. Firstly, in a few cases the size distribution can not be easily controlled, Secondly, the catalyst is usually encapsulated in the spheres, which are difficult to separate from the catalyst. In this paper, benzene worked as carbon precursor, aluminium and Fe2O3 powders were first used as the catalyst as far as the authors were aware of. The simple chemical vapor deposition method was used to prepare uniform mono-dispersed carbon spherules without a catalyst encapsulated in the products, and the carbon spheres can be easily separated from the catalyst.
In this study, the pyrolysis of benzene was carried out to synthesize nanosized carbon spheres at atmospheric pressure through a quartz tube reactor in a two-stage furnace system. The quartz tube is 120 cm in length. The temperature of the two furnaces was controlled respectively, and was around 250 °C and 1000 °C. Benzene was injected into a quartz boat, which was put in the first furnace. Aluminium and Fe2O3 powders were mixed together, then were put in the second furnace. The benzene vapour was carried through the system by nitrogen gas (flow rate at 100 cm3/min) and was pyrolyzed at 1000 °C for 10 min. Nitrogen gas flowed through the quartz tube until the temperature of the furnace was decreased to 300 °C. Carbon spheres adhered to the wall of the tube, while the catalyst laid on the inner of tube without affinity. The spheres were collected from wall of the tube by razor, spontaneously separated from the catalyst. The transmission electron microscopy (TEM) was performed with a HITACH H-600 Electron Microscope. The scanning electron microscopy (SEM) images and energy dispersive X-ray analysis (EDX) were taken on a JEOL JSM-6700F scanning electron microscope equipped with INCA400 energy dispersive X-ray detector. The high-resolution transmission electron microscopy (HRTEM) was performed with a Philips Tecnai 20U-TWIN. 3. Results and discussion The SEM and TEM are used to examine the morphology of the carbon spheres. In general, the as-prepared products are dark black powders, soft, and light in weight. The SEM image in Fig. 1 shows the typical morphology of the carbon spheres. The image of the products indicates that the final products consist of a large quantity of carbon spheres in perfect spherical morphology. It can also be seen that some nanorods are on the surface of the spheres, the diameter of carbon spheres is about 1 μm. The size distribution of the particles was determined by SEM examination of 50 particles. The diameters of the particles ranged from 0.8 to 1.1 μm and the diameters of most carbon spheres were 1.0 μm (see from Fig. 2).
Fig. 3. (a) TEM image of carbon spheres; (b) HRTEM image of the regions of coalescence of two carbon spheres shown in the white-circle area in (a).
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P. Wang et al. / Materials Letters 61 (2007) 4854–4856
non-condensed phases of the dissociated products. When the mixture is cooled down, the BSUs congregate, forming the well-shaped carbon spheres (Fig. 2). Some random arrangements of the BSUs are also possible, and nanorods can be clearly seen on the surface of the spheres (Fig. 1).
4. Conclusions In summary, the approximate uniform diameter carbon spheres have been successfully synthesized by the CVD method. The heating conditions have crucial effects on the morphology of the final product. Meanwhile, the growth mechanism of carbon spheres is suggested. The approach described in this paper is a feasible method to fabricate carbon spheres. Acknowledgements This work was supported by a research grant from the National Natural Science Foundation of China (No. 60377041) and a research grant from the State Key Program of China (2004 CB719803). Fig. 4. (a) SEM image of the carbon spheres. (b) EDX of carbon spheres shown in the white-circle area in (a).
Fig. 3a shows the TEM image of the as-prepared carbon spheres. The TEM image indicates the carbon spheres in the sample appear to be of nearly perfect spheres with diameters about 1 μm. Seen from the image, the sample also contains some carbon nanotubes. During the same process, carbon nanotubes, carbon spheres, and other carbon nanostructure were obtained. We give emphasis to the carbon spheres, and other nanostructures will be reported elsewhere. Fig. 3b presents the coalescence region of two spheres. These disordered layers correspond to the small graphene planes of the carbon. The EDX analysis of the spheres (Fig. 4) reveals that the carbon content is 95 wt.%. Further elemental analysis confirms that carbon is the dominant element. They also contain some non-carbon elements, of which the oxygen content is 4.5 wt.%. Oxygen originates from the catalysts, and nitrogen, hydrogen, aluminium, iron could be not detected. A possible mechanism for the formation of carbon spheres is proposed. At 1000 °C, benzene is dissociated to C and H. Upon the dissociation of benzene, C species are obtained and form the basic structural units (BSUs), while methane and hydrogen are also formed. On the other hand, carbon and hydrogen can react with the small amount of oxygen in the catalyst to form water or carbon oxides. The reaction carried out in a nitrogen atmosphere leads to the formation of spherical carbon of 1.0 μm. Inagaki has developed a comprehensive understanding of the growth mechanism of the spherical shape of carbon [27]. He has classified the texture of different graphitic bodies based on the arrangement of the BSUs. The three basic textures are concentric, radial, and random. Inagaki also has claimed that the structure of carbon materials depends strongly on the precursors and heat treatment conditions. Heating conditions (various temperatures, change of atmosphere, various heating rates and various heating times) play an important role in the formation of carbon spheres. According to Inagaki, a solid/liquid interface supports the concentric growth of a sphere. A liquid/liquid interface would lead to a radial growth of the sphere, and solid/gas yields a random texture. The interface in our system is that of fluid/fluid, where one fluid represents the carbonaceous mesophase liquid crystal, and the other represents the
References [1] J. Robertson, Prog. Solid State Chem. 21 (1991) 199. [2] C.N.R. Rao, R. Seshadri, A. Govindaraj, R. Sen, Mater. Sci. Eng., R Rep. 15 (1995) 209. [3] J.C. Angus, C.C. Hayman, Science 241 (1998) 913. [4] W.A. Yarbrough, R. Messier, Science 247 (1990) 688. [5] P. Bachmann, H. Lydtin, Mater. Res. Soc. Symp. Proc. 165 (1990) 181. [6] H.W. Kroto, J.R. Heath, S.C. O'Brien, R.F. Curl, R.E. Smalley, Nature 318 (1985) 162. [7] H.W. Kroto, K. McKay, Nature 331 (1988) 328. [8] L.D. Lamb, D.R. Huffman, R.K. Workman, S. Howells, T. Chen, D. Sarid, R.F. Ziolo, Science 255 (1992) 1413. [9] S. Iijima, Nature 354 (1991) 56. [10] J.W. Mintimire, B.I. Dunlap, C.T. White, Phys. Rev. Lett. 68 (1992) 63. [11] R.L. Jacobsen, M. Mothioux, Nature 385 (1997) 211. [12] Y.N. Xia, B. Gates, Y.D. Yin, Y. Lu, Adv. Mater. 12 (2000) 693. [13] X.Y. Liu, B.C. Huang, N.J. Covolle, Carbon 40 (2002) 2791. [14] M.B. Zheng, J.M. Cao, X. Chang, J. Wang, J.S. Liu, X.J. Ma, Mater. Lett. 60 (2006) 2991. [15] F. Su, X.S. Zhao, Y. Wang, J.Y. Lee, J. Mater. Chem. 16 (2006) 4413. [16] B.Y. Liu, D.C. Jia, Q.C. Meng, J.C. Rao, Carbon 45 (2007) 668. [17] D. Ugarte, Nature 359 (1992) 707. [18] Q. Wang, F.Y. Cao, Q.W. Chen, C. Chen, Mater. Lett. 59 (2005) 3738. [19] Y.Z. Piao, K.J. An, J.Y. Kim, T.Y. Yu, T.W. Hyeon, J. Mater. Chem. 16 (2006) 2984. [20] V.G. Pol, M. Motiei, A. Gedanken, J. Calderon-Moreno, M. Yoshimura, Carbon 42 (2004) 111. [21] Z.L. Wang, Z.C. Kang, J. Phys. Chem. 100 (1996) 17725. [22] H.S. Qian, F.M. Han, B. Zhang, Y.C. Guo, J. Yue, B.X. Peng, Carbon 42 (2004) 761. [23] J.Y. Miao, D.W. Hwang, K.V. Narasimhulu, P.I. Lin, Y.T. Chen, S.H. Lin, L.P. Hwang, Carbon 42 (2004) 813. [24] Y.Z. Jin, C. Gao, W.K. Hsu, Y. Zhu, A. Huczko, M. Bystrzejewski, M. Roe, C.Y. Lee, S. Acquah, H. Kroto, D.R.M. Walton, Carbon 43 (2005) 1944. [25] G.Y. Chen, V. Stolojan, S.R.P. Silva, H. Herman, S. Haq, Carbon 43 (2005) 704. [26] V.G. Pol, S.V. Pol, J.M. Calderon Moreno, A. Gedanken, Carbon 44 (2006) 3285. [27] M. Inagaki, Carbon 35 (1997) 711.
Synthesis and characterization of carbon spheres prepared by chemical vapour deposition Peng Wang, Jiyong Wei, Baibiao Huang ⁎, Xiaoyan Qin, Shushan Yao, Qi Zhang, Zeyan Wang, Guanghui Xu, Xiangyang Jing State Key Lab of Crystal Material, Shandong University, Jinan 250100, China Received 16 January 2007; accepted 15 March 2007 Available online 20 March 2007
Abstract Carbon spheres, with uniform diameters of about 1 μm, have been achieved via Chemical vapour deposition (CVD). The fabricated materials have been fully characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray analysis (EDX). The results show that the spheres are 95% carbon. The formation mechanism of carbon spheres has also been discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Carbon spheres; Nanomaterials; Chemical vapour deposition; Electron microscopy
1. Introduction Carbon is a versatile element because it can form various structures [1,2]. The successful growth of diamond films [3–5], the fullerene molecule C60 [6] and its family Cn [7,8] and carbon nanotubes has excited many researchers [9,10]. The variety of structures produced by carbon is determined by its unique hybridization sp1, sp2, and sp3 bonding. Carbon spheres as a new material is becoming increasingly important [11–16]. Serp et al. have classified such structures according to their size into fullerenes Cn, carbon onions (2–20 nm, with closed graphite layer) [17], carbon spheres (50 nm to 1 μm) [18,19] and carbon beads (above 1 μm) [20]. Carbon spheres can be fabricated by the methods that are normally used to synthesize carbon nanotubes. Wang et al. developed a mixed-valent oxidecatalytic carbonization (MVOCC) process for the production of carbon spheres. By varying the temperatures, this process produced either carbon spheres or carbon nanotubes when
⁎ Corresponding author. Tel.: +86 531 88364449; fax: +86 531 88366324. E-mail address: [email protected] (B. Huang). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.03.056
methane was used as the carbon source [21]. Qian et al. have also used the CVD method to preparation of carbon spheres with a specific size without catalyst [22]. Carbon spheres can also be prepared at relatively low temperatures
Fig. 1. SEM image of carbon spheres.
P. Wang et al. / Materials Letters 61 (2007) 4854–4856
4855
2. Experimental
Fig. 2. Diameter distribution of carbon spheres.
(650 °C) using transition metal catalysts deposited on a kaolin (hydrated aluminium silicate) support [23]. Jin et al. have reported on the large-scale synthesis and characterization of carbon spheres prepared by the direct pyrolysis of hydrocarbons with two to eight carbon atoms [24]. Chen et al. have reported on the formation of spherical hydrogenated amorphous carbon particulates generated in a radio frequency plasma enhanced chemical vapour deposition (rf-PECVD) system [25]. Recently, Pol et al. have reported on the high yield one-step synthesis of carbon spheres produced by dissociating individual hydrocarbons at their autogenic pressure and low temperatures [26]. These techniques, however, are limited by several factors. Firstly, in a few cases the size distribution can not be easily controlled, Secondly, the catalyst is usually encapsulated in the spheres, which are difficult to separate from the catalyst. In this paper, benzene worked as carbon precursor, aluminium and Fe2O3 powders were first used as the catalyst as far as the authors were aware of. The simple chemical vapor deposition method was used to prepare uniform mono-dispersed carbon spherules without a catalyst encapsulated in the products, and the carbon spheres can be easily separated from the catalyst.
In this study, the pyrolysis of benzene was carried out to synthesize nanosized carbon spheres at atmospheric pressure through a quartz tube reactor in a two-stage furnace system. The quartz tube is 120 cm in length. The temperature of the two furnaces was controlled respectively, and was around 250 °C and 1000 °C. Benzene was injected into a quartz boat, which was put in the first furnace. Aluminium and Fe2O3 powders were mixed together, then were put in the second furnace. The benzene vapour was carried through the system by nitrogen gas (flow rate at 100 cm3/min) and was pyrolyzed at 1000 °C for 10 min. Nitrogen gas flowed through the quartz tube until the temperature of the furnace was decreased to 300 °C. Carbon spheres adhered to the wall of the tube, while the catalyst laid on the inner of tube without affinity. The spheres were collected from wall of the tube by razor, spontaneously separated from the catalyst. The transmission electron microscopy (TEM) was performed with a HITACH H-600 Electron Microscope. The scanning electron microscopy (SEM) images and energy dispersive X-ray analysis (EDX) were taken on a JEOL JSM-6700F scanning electron microscope equipped with INCA400 energy dispersive X-ray detector. The high-resolution transmission electron microscopy (HRTEM) was performed with a Philips Tecnai 20U-TWIN. 3. Results and discussion The SEM and TEM are used to examine the morphology of the carbon spheres. In general, the as-prepared products are dark black powders, soft, and light in weight. The SEM image in Fig. 1 shows the typical morphology of the carbon spheres. The image of the products indicates that the final products consist of a large quantity of carbon spheres in perfect spherical morphology. It can also be seen that some nanorods are on the surface of the spheres, the diameter of carbon spheres is about 1 μm. The size distribution of the particles was determined by SEM examination of 50 particles. The diameters of the particles ranged from 0.8 to 1.1 μm and the diameters of most carbon spheres were 1.0 μm (see from Fig. 2).
Fig. 3. (a) TEM image of carbon spheres; (b) HRTEM image of the regions of coalescence of two carbon spheres shown in the white-circle area in (a).
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P. Wang et al. / Materials Letters 61 (2007) 4854–4856
non-condensed phases of the dissociated products. When the mixture is cooled down, the BSUs congregate, forming the well-shaped carbon spheres (Fig. 2). Some random arrangements of the BSUs are also possible, and nanorods can be clearly seen on the surface of the spheres (Fig. 1).
4. Conclusions In summary, the approximate uniform diameter carbon spheres have been successfully synthesized by the CVD method. The heating conditions have crucial effects on the morphology of the final product. Meanwhile, the growth mechanism of carbon spheres is suggested. The approach described in this paper is a feasible method to fabricate carbon spheres. Acknowledgements This work was supported by a research grant from the National Natural Science Foundation of China (No. 60377041) and a research grant from the State Key Program of China (2004 CB719803). Fig. 4. (a) SEM image of the carbon spheres. (b) EDX of carbon spheres shown in the white-circle area in (a).
Fig. 3a shows the TEM image of the as-prepared carbon spheres. The TEM image indicates the carbon spheres in the sample appear to be of nearly perfect spheres with diameters about 1 μm. Seen from the image, the sample also contains some carbon nanotubes. During the same process, carbon nanotubes, carbon spheres, and other carbon nanostructure were obtained. We give emphasis to the carbon spheres, and other nanostructures will be reported elsewhere. Fig. 3b presents the coalescence region of two spheres. These disordered layers correspond to the small graphene planes of the carbon. The EDX analysis of the spheres (Fig. 4) reveals that the carbon content is 95 wt.%. Further elemental analysis confirms that carbon is the dominant element. They also contain some non-carbon elements, of which the oxygen content is 4.5 wt.%. Oxygen originates from the catalysts, and nitrogen, hydrogen, aluminium, iron could be not detected. A possible mechanism for the formation of carbon spheres is proposed. At 1000 °C, benzene is dissociated to C and H. Upon the dissociation of benzene, C species are obtained and form the basic structural units (BSUs), while methane and hydrogen are also formed. On the other hand, carbon and hydrogen can react with the small amount of oxygen in the catalyst to form water or carbon oxides. The reaction carried out in a nitrogen atmosphere leads to the formation of spherical carbon of 1.0 μm. Inagaki has developed a comprehensive understanding of the growth mechanism of the spherical shape of carbon [27]. He has classified the texture of different graphitic bodies based on the arrangement of the BSUs. The three basic textures are concentric, radial, and random. Inagaki also has claimed that the structure of carbon materials depends strongly on the precursors and heat treatment conditions. Heating conditions (various temperatures, change of atmosphere, various heating rates and various heating times) play an important role in the formation of carbon spheres. According to Inagaki, a solid/liquid interface supports the concentric growth of a sphere. A liquid/liquid interface would lead to a radial growth of the sphere, and solid/gas yields a random texture. The interface in our system is that of fluid/fluid, where one fluid represents the carbonaceous mesophase liquid crystal, and the other represents the
References [1] J. Robertson, Prog. Solid State Chem. 21 (1991) 199. [2] C.N.R. Rao, R. Seshadri, A. Govindaraj, R. Sen, Mater. Sci. Eng., R Rep. 15 (1995) 209. [3] J.C. Angus, C.C. Hayman, Science 241 (1998) 913. [4] W.A. Yarbrough, R. Messier, Science 247 (1990) 688. [5] P. Bachmann, H. Lydtin, Mater. Res. Soc. Symp. Proc. 165 (1990) 181. [6] H.W. Kroto, J.R. Heath, S.C. O'Brien, R.F. Curl, R.E. Smalley, Nature 318 (1985) 162. [7] H.W. Kroto, K. McKay, Nature 331 (1988) 328. [8] L.D. Lamb, D.R. Huffman, R.K. Workman, S. Howells, T. Chen, D. Sarid, R.F. Ziolo, Science 255 (1992) 1413. [9] S. Iijima, Nature 354 (1991) 56. [10] J.W. Mintimire, B.I. Dunlap, C.T. White, Phys. Rev. Lett. 68 (1992) 63. [11] R.L. Jacobsen, M. Mothioux, Nature 385 (1997) 211. [12] Y.N. Xia, B. Gates, Y.D. Yin, Y. Lu, Adv. Mater. 12 (2000) 693. [13] X.Y. Liu, B.C. Huang, N.J. Covolle, Carbon 40 (2002) 2791. [14] M.B. Zheng, J.M. Cao, X. Chang, J. Wang, J.S. Liu, X.J. Ma, Mater. Lett. 60 (2006) 2991. [15] F. Su, X.S. Zhao, Y. Wang, J.Y. Lee, J. Mater. Chem. 16 (2006) 4413. [16] B.Y. Liu, D.C. Jia, Q.C. Meng, J.C. Rao, Carbon 45 (2007) 668. [17] D. Ugarte, Nature 359 (1992) 707. [18] Q. Wang, F.Y. Cao, Q.W. Chen, C. Chen, Mater. Lett. 59 (2005) 3738. [19] Y.Z. Piao, K.J. An, J.Y. Kim, T.Y. Yu, T.W. Hyeon, J. Mater. Chem. 16 (2006) 2984. [20] V.G. Pol, M. Motiei, A. Gedanken, J. Calderon-Moreno, M. Yoshimura, Carbon 42 (2004) 111. [21] Z.L. Wang, Z.C. Kang, J. Phys. Chem. 100 (1996) 17725. [22] H.S. Qian, F.M. Han, B. Zhang, Y.C. Guo, J. Yue, B.X. Peng, Carbon 42 (2004) 761. [23] J.Y. Miao, D.W. Hwang, K.V. Narasimhulu, P.I. Lin, Y.T. Chen, S.H. Lin, L.P. Hwang, Carbon 42 (2004) 813. [24] Y.Z. Jin, C. Gao, W.K. Hsu, Y. Zhu, A. Huczko, M. Bystrzejewski, M. Roe, C.Y. Lee, S. Acquah, H. Kroto, D.R.M. Walton, Carbon 43 (2005) 1944. [25] G.Y. Chen, V. Stolojan, S.R.P. Silva, H. Herman, S. Haq, Carbon 43 (2005) 704. [26] V.G. Pol, S.V. Pol, J.M. Calderon Moreno, A. Gedanken, Carbon 44 (2006) 3285. [27] M. Inagaki, Carbon 35 (1997) 711.