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Deoiled asphalt as carbon source for preparation of various carbon materials by chemical vapor deposition
Fuel Processing Technology 87 (2006) 919 – 925 www.elsevier.com/locate/fuproc
Deoiled asphalt as carbon source for preparation of various carbon materials by chemical vapor deposition Xuguang Liu a,b , Yongzhen Yang a,c , Xian Lin a,b , Bingshe Xu a,c,⁎, Yan Zhang a,c a
Key Laboratory of Interface Science and Engineering in Advanced Materials of Taiyuan University of Technology, Ministry of Education, Taiyuan 030024, China b College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China c College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China Received 21 April 2006; received in revised form 24 June 2006; accepted 29 June 2006
Abstract Various carbon materials, including vapor grown carbon fibers (VGCFs) and carbon trees, were synthesized by chemical vapor deposition in argon atmosphere, using deoiled asphalt as carbon source and ferrocene as catalyst. Pure carbon microbeads (CMBs) were also obtained by this method in the absence of ferrocene. The influence of different growth parameters, such as ferrocene content, reaction temperature, retention time and argon flow rate, was investigated, with respect to morphology and product yield. The products were characterized by field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and Raman spectroscopy. © 2006 Elsevier B.V. All rights reserved. Keywords: Deoiled asphalt; Chemical vapor deposition; Carbon materials
1. Introduction Deoiled asphalt (DOA) with resins, asphaltenes, a small quantity of saturates and aromatics, is a carbon-rich by-product of petroleum processing. With carbon dominating in its elemental composition (C, H, S, O and N), DOA will release CH4, CO, H2, N2, CO2, H2S and low molecular hydrocarbons upon heat-treatment. Converting DOA into carbon materials with high added value is an economically competent route. Chemical vapor deposition (CVD) results in carbon materials with different conformations and microstructures, including vapor grown carbon fibers (VGCFs) [1–11], carbon nanotubes (CNTs) [12–15], carbon microbeads (CMBs) [16–18], carbon trees [19] and so on, which have attracted much attention due to their special atomic configurations and important applications in engineering, such as hydrogen storage, field emission devices and electrode materials. So far, various carbon materials have been grown by the
decomposition of hydrocarbons, such as benzene, toluene, methane, acetylene, ethane, cyclohexane, hexane and so on [1–19]. For the preparation of carbon materials by CVD method with DOA as the carbon source, the formation of products depends on the thermal cracking gases from DOA. The species of thermal cracking gases might take part in the formation process of products through a kind of synergic effects [12,20]. Recently, our group has reported the preparation of VGCFs from DOA [20]. The objective of this paper was to give a systematic description of the effect of synthesis parameters on carbon materials by CVD. Ferrocene content, reaction temperature, growth time and argon flow rate were varied in order to study their effect on the morphologies of the products and to find their relationships with product yield. The morphology and structure of the products were investigated by field emission scanning electron microscopy (FE-SEM), high resolution transmission electron Table 1 The analytical data of deoiled asphalt (DOA)
⁎ Corresponding author. College of Materials Science and Engineering, Taiyuan University of Technology, 79, West Yingze Street, Taiyuan 030024, China. Tel./fax: +86 351 6010311. E-mail address: [email protected] (B. Xu). 0378-3820/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2006.06.007
Elemental analysis (wt.%) C
H
N
S
88.11
9.05
1.35
0.42
H/C (mol mol− 1)
Softening point (°C)
Carbon residue (wt.%)
1.23
199
41.12
920
X. Liu et al. / Fuel Processing Technology 87 (2006) 919–925
Table 2 Effect of ferrocene content on form of the products at 900 °C for 30 min in argon atmosphere of 150 ml/min Sample The percent of Mass of products (g) Products' ferrocene in morphologies in Reaction Outlet Residue raw material (%) reaction zone a zone of tube in boat 1 2 3 4 5 6
0 5 10 20 30 40
0.06 0.13 0.19 0.45 0.65 0.80
0.06 0.13 0.11 0.06 0.08 0.01
0.73 0.72 0.70 0.58 0.57 0.48
CMB VGCF VGCF + few AC CNP + CNTs + VGCF CNP + CNT CNP + CNT
outside the furnace. After the reaction zone was heated in argon atmosphere to growth temperature, the reaction was proceeded by moving the quartz tube to make the boat into the reaction zone. The products were collected from the inner wall of the quartz tube after reaction was finished. The process parameters were varied in the following range: ferrocene ratio in the total mass of ferrocene and DOA from 0 to 40%, reaction temperature from 700 to 1200 °C, growth time from 20 to 50 min, Ar flow rate from 40 to 480 ml/min. 2.3. Characterizations of VGCFs
a
CMB represents carbon microbead; VGCF represents vapor grown carbon fiber; AC represents amorphous carbon; ACNP represents agglomerated carbon nanoparticles; CNT represents carbon nanotube. (Observed through FE-SEM).
microscopy (HRTEM), Raman spectroscopy and X-ray diffraction (XRD) techniques.
JSM-6700F field emission scanning electron microscopy (FESEM) was used to characterize and analyze the morphology of the products, D/max-3C automatic X-ray diffraction (XRD, CuKα) was used to analyze the phases. Laser Raman spectrum was performed in Raman spectroscopy instrument (JY-T64000, 488 nm Ar ion laser).
2. Experimental
3. Results and discussion
2.1. Raw materials
3.1. Effect of ferrocene content
Deoiled asphalt was supplied by China University of Petroleum-Beijing. Ferrocene (A.R.) was chosen as catalyst. Some analytical data of DOA are shown in Table 1.
The change of the morphology and yield of the products with ferrocene content is shown in Table 2 and Fig. 1. Product yield increased with the increase of ferrocene content from 0 to 40 wt.%. Fig. 1 shows the FE-SEM images of carbon materials obtained at different ferrocene contents. CMBs with diameters of 400–600 nm were produced in the absence of ferrocene (Fig. 1a), but the yield was very low (Table 2). As the content of ferrocene increased up to 5%, VGCFs were obtained with relatively uniform diameters of 150–200 nm, smooth surface and high purity, as shown in Fig. 1b. Fig. 1c is similar to Fig. 1b, but VGCFs were shorter and a small quantity of amorphous carbon (AC) attached to VGCFs when
2.2. Synthesis The synthesis of products was conducted in a cylindrical furnace composed of a horizontal quartz glass tube with an inner diameter of 32 mm and a length of 1000 mm. 2.0 g of mixed powder of DOA and ferrocene was placed in a small quartz boat, which was located at first in the cool zone of the quartz tube
Fig. 1. FE-SEM images of the products from DOA-ferrocene mixtures with the content of ferrocene (a) 0, (b) 5%, (c) 10%, (d) 20%, (e) 30%, and (f) 40% in the reaction temperature zone.
X. Liu et al. / Fuel Processing Technology 87 (2006) 919–925
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Table 3 Effect of the reaction temperature on the products (reaction time: 30 min, Ar: 150 ml/min, 1.8 g DOA + 0.2 g ferrocene) Sample Reaction Mass of products (g) temperature Reaction Outlet of (°C) zone tube
Products' morphologies in Residue in reaction zone a boat
1 2 3 4 5
0.68 0.70 0.63 0.66 0.65
800 900 1000 1100 1200 a
0.04 0.19 0.35 0.35 0.68
0.01 0.11 0.09 0.07 0.04
ACNP VGCF + fewer AC VGCF + fewer AC VGCF + fewer AC VGCF
See footnote in Table 2.
ferrocene content changed from 5 wt.% to 10 wt.%. When ferrocene content reached 20 wt.%, agglomerated carbon nanoparticles (ACNPs), nanotubes (CNTs) and bulky carbon fibers were coexistent (Fig. 1d). After that, further increasing ferrocene induced larger quantity of agglomerated CNPs and a few of short CNTs (Fig. 1e and f). It is known that among various factors, which determine the growth of the VGCFs, catalyst seems to be one of the key factors [11]. Early studies indicated that the size of metal particles is one of the important criteria for VGCFs growth and carbon fibers grown on smaller diameter catalyst particles have higher growth rate than those grown on larger particles [13]. Carbon atoms from thermal cracking gases of DOA only deposited on the inner wall of quartz tube to form CMBs in the absence of catalyst, which is in agreement with Ref. [18]. As catalyst content increased, the amount and sizes of Fe particles from ferrocene pyrolysis became appropriate to form VGCFs. When the content of ferrocene was over 20%, Fe particles formed in very rapid rate and easily reunited to form less active agglomerates while the amount of active iron particles became less, so a part of carbon atoms deposited on iron agglomerates to
Fig. 3. XRD patterns of VGCFs at different temperatures.
form a few of short and thick VGCFs and a lot of ACNPs. Simultaneously, the rest of carbon atoms deposited on the surface of highly active iron nanoparticles to form CNTs. 3.2. Effect of reaction temperature From Table 3, it can be seen the total yield of products increased with temperature increasing from 800 to 1200 °C. Fig. 2 shows the effect of different reaction temperatures on the morphology of products in reaction zone. In Fig. 2a, ACNPs were formed at 800 °C almost exclusively but with very low yield. Typical FE-SEM images of VGCFs with varying diameters can be observed in Figs. 1c and 2b–f. VGCFs with central hollow cores were relatively uniform for different temperatures (Figs. 1c and 2b–c). They did not change obviously in diameters from 900 to 1100 °C but their purity became lower with the increasing temperature. The length of VGCFs at 1000 °C was longer than at other temperatures. When the temperature was further increased
Fig. 2. FE-SEM images of the products from DOA at different temperatures (a) 800 °C, (b) 1000 °C, (c) 1100 °C, and (d) 1200 °C. (e) Oriented VGCFs grown on Fe film at 1200 °C, the pinhole of Fe as an arrow shows. (f) The cross-section of a broken fiber in (e).
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Table 4 Effect of argon flow rate on the products (temperature: 900 °C, reaction time: 30 min, 1.8 g DOA + 0.2 g ferrocene)
Table 5 Effect of the reaction time on the products (temperature: 900 °C, Ar: 150 ml/ min, 1.8 g DOA + 0.2 g ferrocene)
Sample Argon (ml/min)
Products' morphologies Reaction zone Outlet of Residue in in reaction zone a tube boat
Sample
1 2 3 4 5
0.26 0.23 0.19 0.13 0.08
1 2 3 4
40 80 150 300 480 a
Mass of products (g)
0.09 0.11 0.11 0.09 0.05
0.65 0.68 0.70 0.69 0.71
VGCF + AC VGCF+AC VGCF + fewer AC VGCF + fewer AC VGCF + AC
Reaction time (min) 20 30 40 50
a
Mass of products (g) Reaction zone
Outlet of tube
Residue in boat
0.17 0.19 0.26 0.25
0.08 0.11 0.09 0.06
0.73 0.70 0.65 0.65
Products' morphologies in reaction zone a VGCF + fewer AC VGCF + fewer AC Nanofiber Carbon microtree
See footnote in Table 2.
See footnote in Table 2.
to 1200 °C, there were, at least, two types of VGCFs that had quite different lengths but similar diameter of about 1 μm. The shorter VGCFs with a few micrometres in length are shown in Fig. 2d while the longer oriented VGCFs, with circular cross-section and the length of about 2 cm, are shown in Fig. 2e–f. Fig. 2e is a low-magnification micrograph showing that the oriented fibers, with high purity and very narrow diameter distribution around 1 um, were grown on porous Fe film formed by Fe atoms from ferrocene pyrolysis. This indicates that the peripheries of the pinholes of underlying Fe film possibly confined the growth of oriented fibers. This can be further verified by the fact that fibers grew perpendicularly from pinholes. This may suggest a similar mechanism as oriented micro-sized carbon fibers on a nickel-coated silicon substrate by CVD [9]. In Fig. 2f, the crosssection of a broken fiber displays concentric, layered structure of deposited carbon layer. In this work, it is not clear whether fibers are completely hollow or not, because it is difficult to characterize fibers via HRTEM due to relatively large dimension of fibers. Thus, further investigation is needed to clarify the detailed microstructure of aligned VGCFs.
Moreover, XRD result also reveals the structural feature of VGCFs. The higher sharp peak of (002) reflections centered at ∼26° indicated that graphene layers were regularly stacked and the degree of graphitization varied indistinctly with the increase of reaction temperature (Fig. 3). These results suggest that reaction temperature also had an important effect on the nucleation and growth process of VGCFs. The effect can be catalyst particles and the concentration of pyrolytic gases generated from the precursors. At low temperatures, catalytic cracking reaction was so difficult that the yield was very low. Not only was the deposited rate of carbon atom lower than the formation rate of tubular structure because of low concentration of pyrolytic gases, but the catalyst was also unable to play its role at temperatures lower than 800 °C, so carbon fibers couldn't be formed. When the temperature reached 900 °C, iron particles for VGCFs growth began to exhibit catalytic activity. Small quantities of long and homogeneous VGCFs were synthesized. Furthermore, increased reaction temperature resulted in improved yield of VGCFs following enhanced growth rate. At higher reaction temperatures, the concentration of pyrolytic carbon became
Fig. 4. FE-SEM images of the products from DOA at different argon flow rates (ml/min) (a) 40, (b) 80, (c) 300, and (d) 480.
X. Liu et al. / Fuel Processing Technology 87 (2006) 919–925
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Fig. 5. FE-SEM images of the products from DOA at different times (a) 20 min and (b) 40 min. (c) The enlarged image in (b). (d) 50 min. (e) The enlarged image in (d).
supersaturated and led to the deactivation of the active surface of catalyst particles. On the other hand, the overhigh temperature could induce the agglomeration of catalyst particles, thus reducing the activity of catalyst and inducing the formation of VGCFs with low aspect ratio. 3.3. Effect of argon flow rate The flow rate of carrier gas, argon, directly controls the duration time of cracking gases and growth of catalyst particles in the reaction zone. Therefore, the influence of the flow rate of argon on the yield and morphologies of products was investigated through changing argon flow rate and fixing other factors (shown in Table 4 and Fig. 4). Table 4 shows that the yield of products decreased with increasing gas flow. A group of FE-SEM images are shown in Fig. 4. It can be found that VGCFs of 500–600 nm in diameter were curled and dumpy at 40 ml/min argon flow rate (as shown in Fig. 4a). Straight
VGCFs with diameters of 300 nm were obtained and amorphous carbon particles were also found at 80 ml/min argon flow rate (as shown in Fig. 4b). The average diameters of obtained VGCFs with high purity decreased to about 180 nm when the flow rate was increased to 150 ml/min (as shown in Fig. 1c). As shown in Fig. 4c and d, though the diameter of VGCFs became smaller, they had relatively low purity. When appropriate argon flow rate was used, the residence time of the pyrolysis gases in the reaction zone was suitable to enhance collision probability between Fe atoms/clusters and carbon clusters, thus forming a large quantity of uniform Fe particles, which provided the sites of nucleation for the inner core of carbon fibers. It can be deduced that a suitable level of carrier gas can provide more space for mass growth of VGCFs with good quality in the reaction zone. However, when the argon flow rate was very small, the excessive carbon atoms, which were formed by decomposing pyrolysis gases with high density, deposited on the surface of catalyst particles to form amorphous
Fig. 6. HRTEM images of VGCFs from DOA (10% ferrocene, 1000 °C, 150 ml/min Ar, 30 min). (a) HRTEM images of VGCFs. (b) The enlarged images of the walls of the VGCFs in (a).
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Fig. 7. Raman spectrum of VGCFs from DOA (10% ferrocene, 1000 °C, 150 ml/min Ar, 30 min).
carbon layer, resulting in short and thick VGCFs. If the argon flow rate was high enough to carry the pyrolysis gas out of the reaction zone too quickly, the catalyst Fe particles do not have enough time to catalyze the growth of VGCFs despite that the high flow rate prohibited the aggregation of catalyst particles. 3.4. Effect of deposition time The experiments were carried out for 20 min, 30 min, 40 min and 50 min, respectively, and the other conditions were kept the same. The relationship between products and deposition time is shown in Table 5. It can be seen that the total yield of products increased with deposition time prolonged from 20 min to 40 min and changed vaguely when the deposition time was over 40 min. There was no sufficient growth period when the experiment went on for 20 min, thus resulting in lower yield of products. Although the absolute yield of products increased with increasing deposition time, the concentration of pyrolysis gases became lower. Therefore, the yield no longer increased when deposition time reached a certain degree. The products at various deposition times were observed by FESEM (Fig. 5). Fig. 5 shows that the morphologies of products varied with increasing deposition time. VGCFs with an average diameter of 180 nm were obtained and there was not much difference between the shapes for the two samples obtained with deposition time of 20 min and 30 min, respectively, as shown in Figs. 5a and 1c. With the increase of the deposition time, the appropriate ratio of pyrolysis gases to Ar made smaller Fe nanoparticles appear. Many curled and wrapped carbon nanofibers (VGCNFs) of ∼45 nm in diameter are found in Fig. 5b–c. As shown in Fig. 5d–e, carbon microtrees were obtained when the experiment was performed for 50 min. The root and branch of the trees were composed of many carbon nanobeads, as shown in Fig. 5e. On the other hand, the elucidation of the growth model of carbon microtrees needs further research. 3.5. HRTEM and Raman analysis of VGCFs Fig. 6 shows HRTEM images of VGCFs using 10% ferrocene in 150 ml/min argon atmosphere at 1000 °C for 30 min. It can be
found that the walls of VGCFs with hollow cores (Fig. 6a) can be divided into two different structures (Fig. 6b), that is, the highgraphitized inner layer shells and low-graphitized outer layer shells. It can be also observed that shuttle-like catalyst particles are encapsulated in the tips of VGCFs (as an arrow denoted in Fig. 6a). This result agrees with the general structures of VGCFs and obeys the vapor–liquid–solid growth mechanism [11]. The structure of the samples was also verified by Raman spectrum, which is shown in Fig. 7. Raman spectrum has two peaks centered at 1354 cm− 1 (D-band) and 1583 cm− 1 (G-band). The G-band is assigned to one of the two E2g modes corresponding to stretching vibrations in the basal-plane of graphite. The D-band may be attributed to the disorder and imperfection of the carbon crystallites. The high relative intensity of G- to D-band (IG/ID = 1.08) implies that the obtained VGCFs are mainly composed of microcrystalline graphite full of defects, in agreement with HRTEM observation. 4. Conclusion In summary, four experimental parameters evidently influenced the growth of carbon materials, including ferrocene content, reaction temperature, deposition time and argon flow rate. An effective mass production of pure VGCFs could be achieved in argon atmosphere by CVD method using DOA as carbon source and ferrocene as catalyst under optimized preparation parameters, that is, 10 wt.% of ferrocene, reaction temperature of 1000 °C, deposition time of 30 min, and argon flow rate of 150 ml/min. The obtained VGCFs with about 180 nm in diameter were pure, uniform and relatively straight. Pure CMBs were formed in the absence of ferrocene. VGCNFs and carbon trees were also produced by this method. Acknowledgements This research work was supported by National Basic Research Program of China (Grant No. 2004CB217808), National Natural Scientific Foundation of China (Grant No. 20471041, 90306014), Natural Science Foundation of Shanxi Province (Grant No. 20051018) and Shanxi Research Fund for Returned Scholars (Grant No. 200428). References [1] J.M. Ting, N.Z. Huang, Carbon 39 (2001) 835. [2] Y.Y. Fan, H.M. Cheng, Y.L. Wei, G. Su, Z.H. Shen, Carbon 38 (2000) 789. [3] M. Endo, Y.A. Kim, T. Hayashi, K. Nishimura, T. Matusita, K. Miyashita, M.S. Dresselhaus, Carbon 39 (2001) 1287. [4] S.M. Yang, X.Q. Chen, S.J. Motojima, M. Ichihara, Carbon 43 (2005) 827. [5] X.Q. Chen, S.M. Yang, S.J. Motojima, M. Ichihara, Mater. Lett. 59 (2005) 854. [6] B.Q. Wei, R. Vajtai, P.M. Ajayan, Carbon 41 (2003) 179. [7] J.N. Xie, P.K. Sharma, V.V. Varadan, V.K. Varadan, B.K. Pradhan, S. Eser, Mater. Chem. Phys. 76 (2002) 217. [8] Y.Y. Li, S.D. Bae, A. Sakoda, M. Suzuki, Carbon 39 (2001) 91. [9] Y.J. Li, S.P. Lau, B.K. Tay, G.Y. Chen, Z. Sun, J.S. Chen, G.F. You, D. Sheeja, Diam. Relat. Mater. 10 (2001) 878. [10] S.R. Mukai, T. Masuda, K. Hashimoto, H. Iwanaga, Carbon 38 (2000) 475. [11] S. Bai, F. Li, Q.H. Yang, H.M. Cheng, J.B. Bai, Chem. Phys. Lett. 376 (2003) 83.
X. Liu et al. / Fuel Processing Technology 87 (2006) 919–925 [12] J.S. Qiu, Y.L. An, Z.B. Zhao, Y.F. Li, Y. Zhou, Fuel Process. Technol. 85 (2004) 913. [13] G. Giuseppe, V. Ricardo, A. Julien, N. Patrick, J.L. Marc, G. Signorino, G. Cabriele, P.H. Cuong, Appl. Catal., A Gen. 279 (2005) 89. [14] Z.J. Zhang, C. Dewan, S. Kothari, S. Mitra, D. Teeters, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 116 (2005) 363. [15] E. Flahaut, C. Laurent, A. Peigney, Carbon 43 (2005) 375. [16] 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.
925
[17] P. Serp, R. Feurer, P. Kalck, Y. Kihn, J.L. Faria, J.L. Figueiredo, Carbon 39 (2001) 621. [18] Y.Z. Jin, C. Gao, W.K. Hsu, Y.Q. Zhu, A. Huczko, M. Bystrzejewski, M. Roe, C.Y. Lee, S. Acquah, H. Kroto, D.R.M. Walton, Carbon 43 (2005) 1944. [19] X.Y. Guo, Carbon 43 (2005) 1098. [20] Y.Z. Yang, X.G. Liu, B.S. Xu, T.B. Li, Carbon 44 (2006) 1661.
Deoiled asphalt as carbon source for preparation of various carbon materials by chemical vapor deposition Xuguang Liu a,b , Yongzhen Yang a,c , Xian Lin a,b , Bingshe Xu a,c,⁎, Yan Zhang a,c a
Key Laboratory of Interface Science and Engineering in Advanced Materials of Taiyuan University of Technology, Ministry of Education, Taiyuan 030024, China b College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China c College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China Received 21 April 2006; received in revised form 24 June 2006; accepted 29 June 2006
Abstract Various carbon materials, including vapor grown carbon fibers (VGCFs) and carbon trees, were synthesized by chemical vapor deposition in argon atmosphere, using deoiled asphalt as carbon source and ferrocene as catalyst. Pure carbon microbeads (CMBs) were also obtained by this method in the absence of ferrocene. The influence of different growth parameters, such as ferrocene content, reaction temperature, retention time and argon flow rate, was investigated, with respect to morphology and product yield. The products were characterized by field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and Raman spectroscopy. © 2006 Elsevier B.V. All rights reserved. Keywords: Deoiled asphalt; Chemical vapor deposition; Carbon materials
1. Introduction Deoiled asphalt (DOA) with resins, asphaltenes, a small quantity of saturates and aromatics, is a carbon-rich by-product of petroleum processing. With carbon dominating in its elemental composition (C, H, S, O and N), DOA will release CH4, CO, H2, N2, CO2, H2S and low molecular hydrocarbons upon heat-treatment. Converting DOA into carbon materials with high added value is an economically competent route. Chemical vapor deposition (CVD) results in carbon materials with different conformations and microstructures, including vapor grown carbon fibers (VGCFs) [1–11], carbon nanotubes (CNTs) [12–15], carbon microbeads (CMBs) [16–18], carbon trees [19] and so on, which have attracted much attention due to their special atomic configurations and important applications in engineering, such as hydrogen storage, field emission devices and electrode materials. So far, various carbon materials have been grown by the
decomposition of hydrocarbons, such as benzene, toluene, methane, acetylene, ethane, cyclohexane, hexane and so on [1–19]. For the preparation of carbon materials by CVD method with DOA as the carbon source, the formation of products depends on the thermal cracking gases from DOA. The species of thermal cracking gases might take part in the formation process of products through a kind of synergic effects [12,20]. Recently, our group has reported the preparation of VGCFs from DOA [20]. The objective of this paper was to give a systematic description of the effect of synthesis parameters on carbon materials by CVD. Ferrocene content, reaction temperature, growth time and argon flow rate were varied in order to study their effect on the morphologies of the products and to find their relationships with product yield. The morphology and structure of the products were investigated by field emission scanning electron microscopy (FE-SEM), high resolution transmission electron Table 1 The analytical data of deoiled asphalt (DOA)
⁎ Corresponding author. College of Materials Science and Engineering, Taiyuan University of Technology, 79, West Yingze Street, Taiyuan 030024, China. Tel./fax: +86 351 6010311. E-mail address: [email protected] (B. Xu). 0378-3820/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2006.06.007
Elemental analysis (wt.%) C
H
N
S
88.11
9.05
1.35
0.42
H/C (mol mol− 1)
Softening point (°C)
Carbon residue (wt.%)
1.23
199
41.12
920
X. Liu et al. / Fuel Processing Technology 87 (2006) 919–925
Table 2 Effect of ferrocene content on form of the products at 900 °C for 30 min in argon atmosphere of 150 ml/min Sample The percent of Mass of products (g) Products' ferrocene in morphologies in Reaction Outlet Residue raw material (%) reaction zone a zone of tube in boat 1 2 3 4 5 6
0 5 10 20 30 40
0.06 0.13 0.19 0.45 0.65 0.80
0.06 0.13 0.11 0.06 0.08 0.01
0.73 0.72 0.70 0.58 0.57 0.48
CMB VGCF VGCF + few AC CNP + CNTs + VGCF CNP + CNT CNP + CNT
outside the furnace. After the reaction zone was heated in argon atmosphere to growth temperature, the reaction was proceeded by moving the quartz tube to make the boat into the reaction zone. The products were collected from the inner wall of the quartz tube after reaction was finished. The process parameters were varied in the following range: ferrocene ratio in the total mass of ferrocene and DOA from 0 to 40%, reaction temperature from 700 to 1200 °C, growth time from 20 to 50 min, Ar flow rate from 40 to 480 ml/min. 2.3. Characterizations of VGCFs
a
CMB represents carbon microbead; VGCF represents vapor grown carbon fiber; AC represents amorphous carbon; ACNP represents agglomerated carbon nanoparticles; CNT represents carbon nanotube. (Observed through FE-SEM).
microscopy (HRTEM), Raman spectroscopy and X-ray diffraction (XRD) techniques.
JSM-6700F field emission scanning electron microscopy (FESEM) was used to characterize and analyze the morphology of the products, D/max-3C automatic X-ray diffraction (XRD, CuKα) was used to analyze the phases. Laser Raman spectrum was performed in Raman spectroscopy instrument (JY-T64000, 488 nm Ar ion laser).
2. Experimental
3. Results and discussion
2.1. Raw materials
3.1. Effect of ferrocene content
Deoiled asphalt was supplied by China University of Petroleum-Beijing. Ferrocene (A.R.) was chosen as catalyst. Some analytical data of DOA are shown in Table 1.
The change of the morphology and yield of the products with ferrocene content is shown in Table 2 and Fig. 1. Product yield increased with the increase of ferrocene content from 0 to 40 wt.%. Fig. 1 shows the FE-SEM images of carbon materials obtained at different ferrocene contents. CMBs with diameters of 400–600 nm were produced in the absence of ferrocene (Fig. 1a), but the yield was very low (Table 2). As the content of ferrocene increased up to 5%, VGCFs were obtained with relatively uniform diameters of 150–200 nm, smooth surface and high purity, as shown in Fig. 1b. Fig. 1c is similar to Fig. 1b, but VGCFs were shorter and a small quantity of amorphous carbon (AC) attached to VGCFs when
2.2. Synthesis The synthesis of products was conducted in a cylindrical furnace composed of a horizontal quartz glass tube with an inner diameter of 32 mm and a length of 1000 mm. 2.0 g of mixed powder of DOA and ferrocene was placed in a small quartz boat, which was located at first in the cool zone of the quartz tube
Fig. 1. FE-SEM images of the products from DOA-ferrocene mixtures with the content of ferrocene (a) 0, (b) 5%, (c) 10%, (d) 20%, (e) 30%, and (f) 40% in the reaction temperature zone.
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Table 3 Effect of the reaction temperature on the products (reaction time: 30 min, Ar: 150 ml/min, 1.8 g DOA + 0.2 g ferrocene) Sample Reaction Mass of products (g) temperature Reaction Outlet of (°C) zone tube
Products' morphologies in Residue in reaction zone a boat
1 2 3 4 5
0.68 0.70 0.63 0.66 0.65
800 900 1000 1100 1200 a
0.04 0.19 0.35 0.35 0.68
0.01 0.11 0.09 0.07 0.04
ACNP VGCF + fewer AC VGCF + fewer AC VGCF + fewer AC VGCF
See footnote in Table 2.
ferrocene content changed from 5 wt.% to 10 wt.%. When ferrocene content reached 20 wt.%, agglomerated carbon nanoparticles (ACNPs), nanotubes (CNTs) and bulky carbon fibers were coexistent (Fig. 1d). After that, further increasing ferrocene induced larger quantity of agglomerated CNPs and a few of short CNTs (Fig. 1e and f). It is known that among various factors, which determine the growth of the VGCFs, catalyst seems to be one of the key factors [11]. Early studies indicated that the size of metal particles is one of the important criteria for VGCFs growth and carbon fibers grown on smaller diameter catalyst particles have higher growth rate than those grown on larger particles [13]. Carbon atoms from thermal cracking gases of DOA only deposited on the inner wall of quartz tube to form CMBs in the absence of catalyst, which is in agreement with Ref. [18]. As catalyst content increased, the amount and sizes of Fe particles from ferrocene pyrolysis became appropriate to form VGCFs. When the content of ferrocene was over 20%, Fe particles formed in very rapid rate and easily reunited to form less active agglomerates while the amount of active iron particles became less, so a part of carbon atoms deposited on iron agglomerates to
Fig. 3. XRD patterns of VGCFs at different temperatures.
form a few of short and thick VGCFs and a lot of ACNPs. Simultaneously, the rest of carbon atoms deposited on the surface of highly active iron nanoparticles to form CNTs. 3.2. Effect of reaction temperature From Table 3, it can be seen the total yield of products increased with temperature increasing from 800 to 1200 °C. Fig. 2 shows the effect of different reaction temperatures on the morphology of products in reaction zone. In Fig. 2a, ACNPs were formed at 800 °C almost exclusively but with very low yield. Typical FE-SEM images of VGCFs with varying diameters can be observed in Figs. 1c and 2b–f. VGCFs with central hollow cores were relatively uniform for different temperatures (Figs. 1c and 2b–c). They did not change obviously in diameters from 900 to 1100 °C but their purity became lower with the increasing temperature. The length of VGCFs at 1000 °C was longer than at other temperatures. When the temperature was further increased
Fig. 2. FE-SEM images of the products from DOA at different temperatures (a) 800 °C, (b) 1000 °C, (c) 1100 °C, and (d) 1200 °C. (e) Oriented VGCFs grown on Fe film at 1200 °C, the pinhole of Fe as an arrow shows. (f) The cross-section of a broken fiber in (e).
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Table 4 Effect of argon flow rate on the products (temperature: 900 °C, reaction time: 30 min, 1.8 g DOA + 0.2 g ferrocene)
Table 5 Effect of the reaction time on the products (temperature: 900 °C, Ar: 150 ml/ min, 1.8 g DOA + 0.2 g ferrocene)
Sample Argon (ml/min)
Products' morphologies Reaction zone Outlet of Residue in in reaction zone a tube boat
Sample
1 2 3 4 5
0.26 0.23 0.19 0.13 0.08
1 2 3 4
40 80 150 300 480 a
Mass of products (g)
0.09 0.11 0.11 0.09 0.05
0.65 0.68 0.70 0.69 0.71
VGCF + AC VGCF+AC VGCF + fewer AC VGCF + fewer AC VGCF + AC
Reaction time (min) 20 30 40 50
a
Mass of products (g) Reaction zone
Outlet of tube
Residue in boat
0.17 0.19 0.26 0.25
0.08 0.11 0.09 0.06
0.73 0.70 0.65 0.65
Products' morphologies in reaction zone a VGCF + fewer AC VGCF + fewer AC Nanofiber Carbon microtree
See footnote in Table 2.
See footnote in Table 2.
to 1200 °C, there were, at least, two types of VGCFs that had quite different lengths but similar diameter of about 1 μm. The shorter VGCFs with a few micrometres in length are shown in Fig. 2d while the longer oriented VGCFs, with circular cross-section and the length of about 2 cm, are shown in Fig. 2e–f. Fig. 2e is a low-magnification micrograph showing that the oriented fibers, with high purity and very narrow diameter distribution around 1 um, were grown on porous Fe film formed by Fe atoms from ferrocene pyrolysis. This indicates that the peripheries of the pinholes of underlying Fe film possibly confined the growth of oriented fibers. This can be further verified by the fact that fibers grew perpendicularly from pinholes. This may suggest a similar mechanism as oriented micro-sized carbon fibers on a nickel-coated silicon substrate by CVD [9]. In Fig. 2f, the crosssection of a broken fiber displays concentric, layered structure of deposited carbon layer. In this work, it is not clear whether fibers are completely hollow or not, because it is difficult to characterize fibers via HRTEM due to relatively large dimension of fibers. Thus, further investigation is needed to clarify the detailed microstructure of aligned VGCFs.
Moreover, XRD result also reveals the structural feature of VGCFs. The higher sharp peak of (002) reflections centered at ∼26° indicated that graphene layers were regularly stacked and the degree of graphitization varied indistinctly with the increase of reaction temperature (Fig. 3). These results suggest that reaction temperature also had an important effect on the nucleation and growth process of VGCFs. The effect can be catalyst particles and the concentration of pyrolytic gases generated from the precursors. At low temperatures, catalytic cracking reaction was so difficult that the yield was very low. Not only was the deposited rate of carbon atom lower than the formation rate of tubular structure because of low concentration of pyrolytic gases, but the catalyst was also unable to play its role at temperatures lower than 800 °C, so carbon fibers couldn't be formed. When the temperature reached 900 °C, iron particles for VGCFs growth began to exhibit catalytic activity. Small quantities of long and homogeneous VGCFs were synthesized. Furthermore, increased reaction temperature resulted in improved yield of VGCFs following enhanced growth rate. At higher reaction temperatures, the concentration of pyrolytic carbon became
Fig. 4. FE-SEM images of the products from DOA at different argon flow rates (ml/min) (a) 40, (b) 80, (c) 300, and (d) 480.
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Fig. 5. FE-SEM images of the products from DOA at different times (a) 20 min and (b) 40 min. (c) The enlarged image in (b). (d) 50 min. (e) The enlarged image in (d).
supersaturated and led to the deactivation of the active surface of catalyst particles. On the other hand, the overhigh temperature could induce the agglomeration of catalyst particles, thus reducing the activity of catalyst and inducing the formation of VGCFs with low aspect ratio. 3.3. Effect of argon flow rate The flow rate of carrier gas, argon, directly controls the duration time of cracking gases and growth of catalyst particles in the reaction zone. Therefore, the influence of the flow rate of argon on the yield and morphologies of products was investigated through changing argon flow rate and fixing other factors (shown in Table 4 and Fig. 4). Table 4 shows that the yield of products decreased with increasing gas flow. A group of FE-SEM images are shown in Fig. 4. It can be found that VGCFs of 500–600 nm in diameter were curled and dumpy at 40 ml/min argon flow rate (as shown in Fig. 4a). Straight
VGCFs with diameters of 300 nm were obtained and amorphous carbon particles were also found at 80 ml/min argon flow rate (as shown in Fig. 4b). The average diameters of obtained VGCFs with high purity decreased to about 180 nm when the flow rate was increased to 150 ml/min (as shown in Fig. 1c). As shown in Fig. 4c and d, though the diameter of VGCFs became smaller, they had relatively low purity. When appropriate argon flow rate was used, the residence time of the pyrolysis gases in the reaction zone was suitable to enhance collision probability between Fe atoms/clusters and carbon clusters, thus forming a large quantity of uniform Fe particles, which provided the sites of nucleation for the inner core of carbon fibers. It can be deduced that a suitable level of carrier gas can provide more space for mass growth of VGCFs with good quality in the reaction zone. However, when the argon flow rate was very small, the excessive carbon atoms, which were formed by decomposing pyrolysis gases with high density, deposited on the surface of catalyst particles to form amorphous
Fig. 6. HRTEM images of VGCFs from DOA (10% ferrocene, 1000 °C, 150 ml/min Ar, 30 min). (a) HRTEM images of VGCFs. (b) The enlarged images of the walls of the VGCFs in (a).
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Fig. 7. Raman spectrum of VGCFs from DOA (10% ferrocene, 1000 °C, 150 ml/min Ar, 30 min).
carbon layer, resulting in short and thick VGCFs. If the argon flow rate was high enough to carry the pyrolysis gas out of the reaction zone too quickly, the catalyst Fe particles do not have enough time to catalyze the growth of VGCFs despite that the high flow rate prohibited the aggregation of catalyst particles. 3.4. Effect of deposition time The experiments were carried out for 20 min, 30 min, 40 min and 50 min, respectively, and the other conditions were kept the same. The relationship between products and deposition time is shown in Table 5. It can be seen that the total yield of products increased with deposition time prolonged from 20 min to 40 min and changed vaguely when the deposition time was over 40 min. There was no sufficient growth period when the experiment went on for 20 min, thus resulting in lower yield of products. Although the absolute yield of products increased with increasing deposition time, the concentration of pyrolysis gases became lower. Therefore, the yield no longer increased when deposition time reached a certain degree. The products at various deposition times were observed by FESEM (Fig. 5). Fig. 5 shows that the morphologies of products varied with increasing deposition time. VGCFs with an average diameter of 180 nm were obtained and there was not much difference between the shapes for the two samples obtained with deposition time of 20 min and 30 min, respectively, as shown in Figs. 5a and 1c. With the increase of the deposition time, the appropriate ratio of pyrolysis gases to Ar made smaller Fe nanoparticles appear. Many curled and wrapped carbon nanofibers (VGCNFs) of ∼45 nm in diameter are found in Fig. 5b–c. As shown in Fig. 5d–e, carbon microtrees were obtained when the experiment was performed for 50 min. The root and branch of the trees were composed of many carbon nanobeads, as shown in Fig. 5e. On the other hand, the elucidation of the growth model of carbon microtrees needs further research. 3.5. HRTEM and Raman analysis of VGCFs Fig. 6 shows HRTEM images of VGCFs using 10% ferrocene in 150 ml/min argon atmosphere at 1000 °C for 30 min. It can be
found that the walls of VGCFs with hollow cores (Fig. 6a) can be divided into two different structures (Fig. 6b), that is, the highgraphitized inner layer shells and low-graphitized outer layer shells. It can be also observed that shuttle-like catalyst particles are encapsulated in the tips of VGCFs (as an arrow denoted in Fig. 6a). This result agrees with the general structures of VGCFs and obeys the vapor–liquid–solid growth mechanism [11]. The structure of the samples was also verified by Raman spectrum, which is shown in Fig. 7. Raman spectrum has two peaks centered at 1354 cm− 1 (D-band) and 1583 cm− 1 (G-band). The G-band is assigned to one of the two E2g modes corresponding to stretching vibrations in the basal-plane of graphite. The D-band may be attributed to the disorder and imperfection of the carbon crystallites. The high relative intensity of G- to D-band (IG/ID = 1.08) implies that the obtained VGCFs are mainly composed of microcrystalline graphite full of defects, in agreement with HRTEM observation. 4. Conclusion In summary, four experimental parameters evidently influenced the growth of carbon materials, including ferrocene content, reaction temperature, deposition time and argon flow rate. An effective mass production of pure VGCFs could be achieved in argon atmosphere by CVD method using DOA as carbon source and ferrocene as catalyst under optimized preparation parameters, that is, 10 wt.% of ferrocene, reaction temperature of 1000 °C, deposition time of 30 min, and argon flow rate of 150 ml/min. The obtained VGCFs with about 180 nm in diameter were pure, uniform and relatively straight. Pure CMBs were formed in the absence of ferrocene. VGCNFs and carbon trees were also produced by this method. Acknowledgements This research work was supported by National Basic Research Program of China (Grant No. 2004CB217808), National Natural Scientific Foundation of China (Grant No. 20471041, 90306014), Natural Science Foundation of Shanxi Province (Grant No. 20051018) and Shanxi Research Fund for Returned Scholars (Grant No. 200428). References [1] J.M. Ting, N.Z. Huang, Carbon 39 (2001) 835. [2] Y.Y. Fan, H.M. Cheng, Y.L. Wei, G. Su, Z.H. Shen, Carbon 38 (2000) 789. [3] M. Endo, Y.A. Kim, T. Hayashi, K. Nishimura, T. Matusita, K. Miyashita, M.S. Dresselhaus, Carbon 39 (2001) 1287. [4] S.M. Yang, X.Q. Chen, S.J. Motojima, M. Ichihara, Carbon 43 (2005) 827. [5] X.Q. Chen, S.M. Yang, S.J. Motojima, M. Ichihara, Mater. Lett. 59 (2005) 854. [6] B.Q. Wei, R. Vajtai, P.M. Ajayan, Carbon 41 (2003) 179. [7] J.N. Xie, P.K. Sharma, V.V. Varadan, V.K. Varadan, B.K. Pradhan, S. Eser, Mater. Chem. Phys. 76 (2002) 217. [8] Y.Y. Li, S.D. Bae, A. Sakoda, M. Suzuki, Carbon 39 (2001) 91. [9] Y.J. Li, S.P. Lau, B.K. Tay, G.Y. Chen, Z. Sun, J.S. Chen, G.F. You, D. Sheeja, Diam. Relat. Mater. 10 (2001) 878. [10] S.R. Mukai, T. Masuda, K. Hashimoto, H. Iwanaga, Carbon 38 (2000) 475. [11] S. Bai, F. Li, Q.H. Yang, H.M. Cheng, J.B. Bai, Chem. Phys. Lett. 376 (2003) 83.
X. Liu et al. / Fuel Processing Technology 87 (2006) 919–925 [12] J.S. Qiu, Y.L. An, Z.B. Zhao, Y.F. Li, Y. Zhou, Fuel Process. Technol. 85 (2004) 913. [13] G. Giuseppe, V. Ricardo, A. Julien, N. Patrick, J.L. Marc, G. Signorino, G. Cabriele, P.H. Cuong, Appl. Catal., A Gen. 279 (2005) 89. [14] Z.J. Zhang, C. Dewan, S. Kothari, S. Mitra, D. Teeters, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 116 (2005) 363. [15] E. Flahaut, C. Laurent, A. Peigney, Carbon 43 (2005) 375. [16] 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.
925
[17] P. Serp, R. Feurer, P. Kalck, Y. Kihn, J.L. Faria, J.L. Figueiredo, Carbon 39 (2001) 621. [18] Y.Z. Jin, C. Gao, W.K. Hsu, Y.Q. Zhu, A. Huczko, M. Bystrzejewski, M. Roe, C.Y. Lee, S. Acquah, H. Kroto, D.R.M. Walton, Carbon 43 (2005) 1944. [19] X.Y. Guo, Carbon 43 (2005) 1098. [20] Y.Z. Yang, X.G. Liu, B.S. Xu, T.B. Li, Carbon 44 (2006) 1661.