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Synthesis of single-walled carbon nanotubes from heavy oil residue
Chemical Engineering Journal 211–212 (2012) 255–259
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Synthesis of single-walled carbon nanotubes from heavy oil residue Yongfeng Li a,⇑, Huafeng Wang b, Gang Wang a, Jinsen Gao a a b
State Key Laboratory of Heavy oil Processing, College of Chemical Engineering, China University of Petroleum, Changping, Beijing 102249, China Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
h i g h l i g h t s " The synthesis of single-walled carbon nanotubes (SWNTs) using heavy oil residue as carbon source is realized by a CVD method. " SWNTs can be synthesized from both transition metals catalysts (Fe, Co and Ni) and nonmagnetic catalysts (Au and Pt). " SWNTs synthesized from transition metal catalysts have a narrow diameter distribution.
a r t i c l e
i n f o
Article history: Received 23 May 2012 Received in revised form 27 August 2012 Accepted 4 September 2012 Available online 29 September 2012 Keywords: Heavy oil residue Single-walled carbon nanotube Catalytic method
a b s t r a c t Single-walled carbon nanotubes (SWNTs) were synthesized by a chemical vapor deposition (CVD) method using heavy oil residue as carbon source. Different kinds of metals as catalysts including transition metals (Fe, Co and Ni) and nonmagnetic metals (Au and Pt) are used in the growth of SWNTs. The morphology and structure of the synthesized SWNTs products were characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and atomic force microscopy. It was found that diameters of the as-grown SWNTs strongly depend on the type of catalysts. Compared with the case of nonmagnetic catalysts, SWNTs synthesized from transition metal catalysts have a narrow diameter distribution. Our findings indicate that oil residue is cheaper and suitable industrial carbon source for the SWNT growth with high quality. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The heavy oil residue is a highly aromatic material, as a bottom product after the distillation of petroleum [1], which is used in a furnace or boiler for the generation of heat or ‘‘lighting up’’ facility in many coal-fired power plants. The main drawback of residual oil is its high initial viscosity. It also contains relatively high amounts of pollutants and particularly sulfur which forms sulfur dioxide upon combustion, which makes it very cheap. On the other hand, such byproduct of petroleum industry has great potential as feedstock in making carbon materials [2,3] because it contains a large amount of carbon-rich molecules. Therefore, converting oil residue into high value-added carbon materials will open up new avenues in deep processing and comprehensive utilization of heavy oil residue. During the last two decades, carbon nanotubes (CNTs) including both single-walled CNTs (SWNTs) and multi-walled CNTs (MWNTs) have been a subject of considerable research activities owing to their unique mechanical, thermal, electrical and optical properties [4,5]. There has been a significant amount of work done to ⇑ Corresponding author. Tel./fax: +86 10 89739028. E-mail address: [email protected] (Y. Li). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.09.031
synthesize CNTs, and to explore what might be the key formation reason for these pure carbon materials. Owing to the their enormous advantage, CNTs hold great promise as fundamental building blocks for nanoelectronics, field emitters, drug delivery system, sensors and energy storage devices [6–9]. So much as concerned about the potential applications of CNTs, it is necessary to find a suitable method for synthesis of CNTs in large quantity to meet with the commercial demand in the future. Up to now, much progress has been made on the synthesis of CNTs, among which the chemical vapor deposition (CVD) method has been considered to be practical and controllable in preparing CNTs with high quality [10–16]. In the CVD process, the catalyst and carbon source are the key factors that affect the CNT growth. In previous reports, the synthesis of CNTs from cheap waste cooking oil has been realized [17]. In addition, the synthesis of carbon materials, such as MWNTs [1,2] and carbon microbeads [18], from a byproduct of petroleum deoiled asphalt has been studied. However, the synthesis of SWNTs which is more remarkable compared with MWNTs has not yet been realized from oil residual. Based on this background, we attempt to grow SWNTs from heavy oil residue with different kinds of metals as catalysts including transition metals and nonmagnetic metals. Our results demonstrate that the diameter of as-synthesized SWNTs can be controlled by selecting different catalysts.
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Table 1 The analytical data of heavy oil residue. Element analysis (wt.%) C
H
N
S
86.80
12.12
0.48
0.17
Viscosity (mm2 tS 1)
Aromatics (wt.%)
Carbon residue (wt.%)
Saturates (wt.%)
Resins (wt.%)
Asphaltenes (wt.%)
Metal content (lg g 1)
155.61
28.7
10.36
49.8
19.6
1.90
52.20
Fig. 1. (a) SEM image of the as-grown SWNTs samples over Fe catalyst. (b) SEM image of the as-grown SWNTs samples over Pt catalyst. (c) and (d) TEM images of individual SWNT and bundle SWNTs synthesized over Fe catalyst purified by HCl acid treatment.
2. Materials and methods
2.2. Characterization
2.1. Synthesis of SWNTs
The as-grown SWNTs were characterized using field emission scanning electron microscope (SEM, Quanta 200F), transmission electron microscope (TEM, JEM 2010 microscope, operated at 120 kV), atomic force microscope (AFM, SPM-9600) and Raman spectroscopy (633 nm, He–Ne laser) with a probing spot size of 1 lm2. TEM characterization was prepared by a small amount of purified SWNTs which were dispersed in ethanol by sonication for 30 min to form a homogeneous suspension. Then the solution containing SWNTs was placed onto carbon-coated copper grids for TEM observation. A drop of the supernatant was deposited on a Si substrate for AFM observation in order to estimate the diameter distribution of SWNTs.
The heavy oil residual was supplied from ChangQing in China, and some composition and property data of oil residual characterized by CHNS–O elemental analyzer are shown in Table 1. Catalyst thin films with thickness of 20 nm were prepared by an efficient method of vacuum evaporation on substrates consisting of Si wafers with a 400 nm thick layer of thermal SiO2. Transition metals such as Fe, Co and Ni, and nonmagnetic metals such as Au and Pt, are used as catalysts for synthesizing SWNTs. The oil residue loaded in one quartz boat was placed into the low temperature region of 200–400 °C, near the inlet of quartz tube. The catalysts deposited on Si substrate put in another quartz boat were fixed in the middle of the quartz tube furnace (up to 900 °C). The pyrolysis gases of oil residue were carried into the reaction zone by flowing 10 sccm argon and 5 sccm hydrogen. The total pressure of reactor is about 2.5 kPa. The growth time lasts for 20 min and then the quartz tube was cooled down to room temperature in the flow of Ar gas. The black materials deposited on Si substrate were collected from the quartz tube after the reaction was finished. The purification of as-grown samples was first rinsed in HCl for 12 h to remove the impurities, then washed by deionized water, and dried in air at 100 °C for 2 h to remove absorbed moist.
3. Results and discussion SEM has been employed to study the oil residue-derived SWNTs, and two kinds of morphologies of products are found. The sample obtained from heavy oil residue with Fe as catalyst has film-like morphology. A high magnification SEM image reveals that the film-like sample contains a large amount of fiber-like carbon materials, as shown in Fig. 1a. In contrast, the morphology of samples synthesized over Pt catalyst indicates a different feature,
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Fe as catalyst Intensity (a.u.)
280
0
100
200
300
169
100
200
800
1200 1600 2000
Pt as catalyst
Intensity (a.u.) 0
400
300
400
800
1200
1600
2000
-1
Raman shift (cm ) Fig. 2. (a) Raman spectra of the as-grown SWNTs samples over Fe catalyst. (b) Raman spectra of the as-grown SWNTs samples over Pt catalyst. The insets of (a) and (b) indicate the corresponding AFM images of individual SWNT with diameter of 1.0 nm and 1.4 nm, respectively.
as shown in Fig. 1b, wherein the samples shrink greatly and curl up to form cotton-like materials. The samples synthesized from Fe catalyst are further examined by TEM, and TEM images are shown in Fig. 1c and d which provides direct evidence that the main product consists of SWNTs. Close examination by TEM reveals that the
(a) Intensity (a.u.)
Ni as catalyst 207 274
0
100 200 300 400 0
main components of film-like material are SWNTs which display the morphologies of both individual and bundle consisting of several individual SWNTs, as seen in Fig. 1c and d, respectively, and only a little amount of carbon encapsulated metal particles exist as impurities. According to the TEM observations, it is found that the SWNTs prepared with Fe as catalyst have diameter less than 1 nm. It is necessary to mention that there are many Fe particles with size distribution less than 1 nm on the catalyst thin film, which is confirmed by AFM characterization. Such particle size distribution is desirable for growing SWNTs with diameter in an order of 1 nm. Raman spectroscopy has been used to characterize various kinds of CNTs because it was known as one of the most powerful tools to investigate the vibration properties of CNTs in relation to their structural and electronic properties. Fig. 2a shows the typical Raman scattering spectrum of as-grown SWNTs by chemical vapor deposition of oil residue from Fe catalyst at 900 °C. The Raman spectrum shows a sharp tangential mode (G-band) at 1591 cm 1 and low disorder-induced D-band at 1330 cm 1, which indicates that the carbonaceous products has a high crystallinity. The main radial breathing mode (RBM) in a low frequency range of 280–334 cm 1 verifies that these structures are SWNTs, which is in consistent with TEM observations. Since the peak position of RBM is inversely propositional to the diameter of SWNTs, the diameter distribution of SWNTs can be estimated according to the formula d = 234/(xRBM10) [19], and the estimated diameter of SWNTs is in the range of 0.72–0.87 nm, in agreement with the TEM observations. In contrast, it is found that the SWNTs grown over Pt catalyst have a different diameter distribution, a distinct RBM peak at low frequency of 169 cm 1 is observed, corresponding to SWNTs with diameter of 1.47 nm, as indicated in Fig. 2b. Moreover, the AFM is also used to measure the diameter of individual SWNT, as seen from the insets of Fig. 2, in which the diameter of SWNTs grown over Fe and Pt catalysts is confirmed to be about 1.0 nm and 1.4 nm, respectively, being consistent with the Raman spectroscopy characterization. This finding indicates that the catalyst significantly affects the structure of as-synthesized SWNTs.
Co as catalyst 200
Au as catalyst 192
265
100 200 300 400 0 -1
100 200 300 400
Raman shift (cm )
(b)
Ni catalyst
2 nm
Co catalyst
Au catalyst
2 nm
2 nm
Fig. 3. (a) RBM spectra of three kinds of SWNTs synthesized over Ni, Co and Au catalysts. (b) High-resolution TEM images for the corresponding individual SWNT which grown over Ni, Co and Au catalysts.
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(a)
(b) o
500 C o 600 C o 700 C o 800 C o 900 C
12
IG /ID
Intensity (a.u.)
SWNTs with oil residue as carbon source may not proceed through C2 carbon units although the formation mechanism is not clear. It is well known that the oil residue consists of many kinds of hydrocarbons and their derivatives. During the pyrolysis process, the weak links between the adjacent big aromatic units in oil residue can be destroyed to form small polyaromatic hydrocarbon rings. Furthermore, small hydrocarbon molecules may follow nucleation, growth and termination steps with catalyst, and lead to the growth of SWNTs during the heating process.
16
8 4
4. Conclusions
0
0
500 1000 1500 2000 -1
Raman shift (cm )
500 600 700 800 900 o
Temperature ( C)
Fig. 4. (a) Raman spectra of as-grown SWNTs synthesized over Fe catalyst at different temperatures. (b) IG/ID measured as a function of temperatures.
In order to reveal the critical difference in catalyst activity, we have systematically investigated the formation of SWNTs from other different catalysts including Ni, Co, and Au under the same experimental conditions. Fig. 3a describes the situation in RBM region of Raman spectra of SWNTs grown over the Ni, Co and Au catalysts. The obvious RBM peaks for SWNTs synthesized over Ni and Co are in the range of 207–274 cm 1 and 200–265 cm 1, respectively. The corresponding diameter distribution of SWNTs is in the range of 0.89–1.19 nm, similar to the case of SWNTs grown over Fe catalyst. However, in the case of SWNTs synthesized from Au catalyst, a weak RBM peak at 192 cm 1 is observed, which shows the SWNTs have relative larger diameter about 1.29 nm compared with those synthesized from transition metal catalysts. Furthermore, the diameter of SWNTs grown over different catalysts has been characterized by high-resolution TEM, as shown in Fig. 3b. Obviously, the SWNT synthesized by Au catalyst have relatively large diameter in comparison to that grown by Ni and Co catalysts. Clearly, the above results indicate that the oil residuederived SWNTs grown from transition metal catalysts (Fe, Co and Ni) have a narrow diameter distribution compared with that made from nonmagnetic catalyst (Au and Pt). This phenomenon is possibly ascribed to the lower catalyzing ability and solubility of carbon atoms in liquid phase of nonmagnetic metals to decompose aromatic hydrocarbon molecules. On the other hand, the diameter of SWNTs made from the heavy oil residue can be controlled by selecting different catalysts, i.e. type of the catalyst plays an important role in determining the structure of SWNTs. In addition to catalyst effect, it is found that temperature is another key factor that affects the formation of SWNTs from oil residue. Fig. 4a shows the Raman spectra of SWNTs grown over Fe catalyst at different temperatures of 500–900 °C. When the growth temperature is below 700 °C, the RBM peaks are obviously suppressed, suggesting that there are no SWNTs formed from oil residue. Specially, at 500 °C the D-band intensity is much higher than that of the related G-band. However, the intensity ratio of G- to Dband displays a sharp increase when the temperature increases up to 900 °C, as shown in Fig. 4b. Note that all of Raman spectra are performed under the same experimental conditions, so it can be assumed that the absolute intensity of G-band corresponds with the amount of SWNTs on the substrate. Therefore, it can be concluded that the growth temperature indeed plays an important role in determining the formation of SWNTs. For a comparison, a conventional thermal CVD method with CH4 as carbon source for SWNT growth is normally achieved below 800 °C [20]. This difference may result from different carbon precursors which lead to that SWNTs synthesized from oil residue shows a different mechanism versus the CH4 process. Namely, the formation process of
Our results demonstrated that it is possible to synthesize highquality SWNTs by a CVD method with inexpensive heavy oil residue as the starting material. The diameter distribution of as-grown SWNTs strongly depends on the type of catalysts. It is found that SWNTs grown from transition metals (Fe, Co and Ni) have smaller diameter compared to that of SWNTs synthesized from nonmagnetic catalysts (Au, Pt). This result demonstrates the feasibility of controlling the SWNT diameters by selecting the catalysts. Moreover, it is found that the reaction temperature is the key factor that affects the formation of SWNTs from oil residue. In our case, the growth mechanism of SWNTs is considered to be different from that of SWNTs synthesized from conventional carbon source. Acknowledgements We gratefully thank the National Natural Science Foundation of China (No. 21106184), the Science Foundation Research Funds Provided to New Recruitments of China University of Petroleum, Beijing (No. YJRC-2011-18), and Thousand Talents Program. References [1] J.G. Speight, Handbook of Petroleum Product Analysis, A John Wiley & Sons, Inc., Publication, 2002. [2] X. Liu, Y. Yang, H. Liu, W. Ji, C. Zhang, B. Xu, Carbon nanotubes from catalytic pyrolysis of deoiled asphalt, Mater. Lett. 61 (2007) 3916–3919. [3] X. Liu, Y. Yang, X. Lin, B. Xu, Y. Zhang, Deoiled asphalt as carbon source for preparation of various carbon materials by chemical vapor deposition, Fuel Process Technol. 87 (2006) 919–925. [4] S. Reich, C. Thomsen, J. Maultzsch, Carbon Nanotubes: Basic Concepts and Physical Properties, Wiley-VCH, Germany, 2004. [5] R. Saito, G. Dresselhaus, M.S. Dresselhaus, Physical Properties of Carbon Nanotubes, World Scientific, 1998. [6] D.N. Fubtaba, K. Hata, T. Yamada, T. Hiraoka, Y. Hayamizu, Y. Kakudate, O. Tanaike, H. Hatori, M. Yumura, S. Iijima, Shape-engineerable and highly densely packed single-walled carbon nanotubes and their application as supercapacitor electrodes, Nat. Mater. 5 (2006) 987–994. [7] K. Mizuno, J. Ishii, H. Kishida, Y. Hayamizu, S. Yasuda, D.N. Futaba, M. Yumura, K. Hata, A black body absorber from vertically aligned single-walled carbon nanotubes, Proc. Natl. Acad. Sci. USA 93 (2009) 6044–6047. [8] J. Robertson, G. Zhong, H. Telg, C. Thomsen, J.H. Warner, G. A.D. Briggs, U. Dettlaff-Weglikowska, S. Roth, Growth and characterization of high-density mats of single-walled carbon nanotubes for interconnects, Appl. Phys. Lett. 93 (2008) 163111-1–163111-3. [9] Z.J. Han, B.K. Tay, C.M. Tan, M. Shakerzadeh, K. Ostrikov, Electrowetting control of Cassie-to-Wenzel transitions in superhydrophobic carbon nanotube-based nanocomposites, ACS Nano 3 (2009) 3031–3036. [10] A.M. Cassell, J.A. Raymakers, J. Kong, H.J. Dai, Large scale CVD synthesis of single-walled carbon nanotubes, J. Phys. Chem. B 103 (1999) 6484–6492. [11] D. Venegoni, P. Serp, R. Feurer, Y. Kihn, C. Vahlas, P. Kalck, Parametric study for the growth of carbon nanotubes by catalytic chemical vapor deposition in a fluidized bed reactor, Carbon 40 (2002) 1799–1807. [12] C. Singh, M.S. Shaffer, A.H. Windle, Production of controlled architectures of aligned carbon nanotubes by an injection chemical vapour deposition method, Carbon 41 (2003) 359–368. [13] R.E. Morjan, V. Maltsev, O. Nerushev, Y. Yao, L.K.L. Falk, E.E.B. Campbell, High growth rates and wall decoration of carbon nanotubes grown by plasmaenhanced chemical vapour deposition, Chem. Phys. Lett. 383 (2004) 385–390. [14] H. Ago, S. Imamura, T. Okazaki, T. Saitoj, M. Yumura, M. Tsuji, CVD growth of single-walled carbon nanotubes with narrow diameter distribution over Fe/ MgO catalyst and their fluorescence spectroscopy, J. Phys. Chem. B 109 (2005) 10035–10041.
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[18] Y. Yang, X. Liu, C.Y. Zhang, M. Guo, B. Xu, Controllable synthesis and modification of carbon micro-spheres from deoiled asphalt, J. Phys. Chem. Solids 71 (2010) 235–241. [19] M. Milnera, J. Kurti, M. Hulman, H. Kuzmany, Periodic resonance excitation and intertube interaction from quasi continuous distributed helicities in single-wall carbon nanotubes, Phys. Rev. Lett. 84 (2000) 1324–1327. [20] W.L. Wang, X.D. Bai, Z. Xu, S. Liu, E.G. Wang, Low temperature growth of single-walled carbon nanotubes: small diameter with narrow distribution, Chem. Phys. Lett. 419 (2006) 81–85.
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Synthesis of single-walled carbon nanotubes from heavy oil residue Yongfeng Li a,⇑, Huafeng Wang b, Gang Wang a, Jinsen Gao a a b
State Key Laboratory of Heavy oil Processing, College of Chemical Engineering, China University of Petroleum, Changping, Beijing 102249, China Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
h i g h l i g h t s " The synthesis of single-walled carbon nanotubes (SWNTs) using heavy oil residue as carbon source is realized by a CVD method. " SWNTs can be synthesized from both transition metals catalysts (Fe, Co and Ni) and nonmagnetic catalysts (Au and Pt). " SWNTs synthesized from transition metal catalysts have a narrow diameter distribution.
a r t i c l e
i n f o
Article history: Received 23 May 2012 Received in revised form 27 August 2012 Accepted 4 September 2012 Available online 29 September 2012 Keywords: Heavy oil residue Single-walled carbon nanotube Catalytic method
a b s t r a c t Single-walled carbon nanotubes (SWNTs) were synthesized by a chemical vapor deposition (CVD) method using heavy oil residue as carbon source. Different kinds of metals as catalysts including transition metals (Fe, Co and Ni) and nonmagnetic metals (Au and Pt) are used in the growth of SWNTs. The morphology and structure of the synthesized SWNTs products were characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and atomic force microscopy. It was found that diameters of the as-grown SWNTs strongly depend on the type of catalysts. Compared with the case of nonmagnetic catalysts, SWNTs synthesized from transition metal catalysts have a narrow diameter distribution. Our findings indicate that oil residue is cheaper and suitable industrial carbon source for the SWNT growth with high quality. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The heavy oil residue is a highly aromatic material, as a bottom product after the distillation of petroleum [1], which is used in a furnace or boiler for the generation of heat or ‘‘lighting up’’ facility in many coal-fired power plants. The main drawback of residual oil is its high initial viscosity. It also contains relatively high amounts of pollutants and particularly sulfur which forms sulfur dioxide upon combustion, which makes it very cheap. On the other hand, such byproduct of petroleum industry has great potential as feedstock in making carbon materials [2,3] because it contains a large amount of carbon-rich molecules. Therefore, converting oil residue into high value-added carbon materials will open up new avenues in deep processing and comprehensive utilization of heavy oil residue. During the last two decades, carbon nanotubes (CNTs) including both single-walled CNTs (SWNTs) and multi-walled CNTs (MWNTs) have been a subject of considerable research activities owing to their unique mechanical, thermal, electrical and optical properties [4,5]. There has been a significant amount of work done to ⇑ Corresponding author. Tel./fax: +86 10 89739028. E-mail address: [email protected] (Y. Li). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.09.031
synthesize CNTs, and to explore what might be the key formation reason for these pure carbon materials. Owing to the their enormous advantage, CNTs hold great promise as fundamental building blocks for nanoelectronics, field emitters, drug delivery system, sensors and energy storage devices [6–9]. So much as concerned about the potential applications of CNTs, it is necessary to find a suitable method for synthesis of CNTs in large quantity to meet with the commercial demand in the future. Up to now, much progress has been made on the synthesis of CNTs, among which the chemical vapor deposition (CVD) method has been considered to be practical and controllable in preparing CNTs with high quality [10–16]. In the CVD process, the catalyst and carbon source are the key factors that affect the CNT growth. In previous reports, the synthesis of CNTs from cheap waste cooking oil has been realized [17]. In addition, the synthesis of carbon materials, such as MWNTs [1,2] and carbon microbeads [18], from a byproduct of petroleum deoiled asphalt has been studied. However, the synthesis of SWNTs which is more remarkable compared with MWNTs has not yet been realized from oil residual. Based on this background, we attempt to grow SWNTs from heavy oil residue with different kinds of metals as catalysts including transition metals and nonmagnetic metals. Our results demonstrate that the diameter of as-synthesized SWNTs can be controlled by selecting different catalysts.
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Y. Li et al. / Chemical Engineering Journal 211–212 (2012) 255–259
Table 1 The analytical data of heavy oil residue. Element analysis (wt.%) C
H
N
S
86.80
12.12
0.48
0.17
Viscosity (mm2 tS 1)
Aromatics (wt.%)
Carbon residue (wt.%)
Saturates (wt.%)
Resins (wt.%)
Asphaltenes (wt.%)
Metal content (lg g 1)
155.61
28.7
10.36
49.8
19.6
1.90
52.20
Fig. 1. (a) SEM image of the as-grown SWNTs samples over Fe catalyst. (b) SEM image of the as-grown SWNTs samples over Pt catalyst. (c) and (d) TEM images of individual SWNT and bundle SWNTs synthesized over Fe catalyst purified by HCl acid treatment.
2. Materials and methods
2.2. Characterization
2.1. Synthesis of SWNTs
The as-grown SWNTs were characterized using field emission scanning electron microscope (SEM, Quanta 200F), transmission electron microscope (TEM, JEM 2010 microscope, operated at 120 kV), atomic force microscope (AFM, SPM-9600) and Raman spectroscopy (633 nm, He–Ne laser) with a probing spot size of 1 lm2. TEM characterization was prepared by a small amount of purified SWNTs which were dispersed in ethanol by sonication for 30 min to form a homogeneous suspension. Then the solution containing SWNTs was placed onto carbon-coated copper grids for TEM observation. A drop of the supernatant was deposited on a Si substrate for AFM observation in order to estimate the diameter distribution of SWNTs.
The heavy oil residual was supplied from ChangQing in China, and some composition and property data of oil residual characterized by CHNS–O elemental analyzer are shown in Table 1. Catalyst thin films with thickness of 20 nm were prepared by an efficient method of vacuum evaporation on substrates consisting of Si wafers with a 400 nm thick layer of thermal SiO2. Transition metals such as Fe, Co and Ni, and nonmagnetic metals such as Au and Pt, are used as catalysts for synthesizing SWNTs. The oil residue loaded in one quartz boat was placed into the low temperature region of 200–400 °C, near the inlet of quartz tube. The catalysts deposited on Si substrate put in another quartz boat were fixed in the middle of the quartz tube furnace (up to 900 °C). The pyrolysis gases of oil residue were carried into the reaction zone by flowing 10 sccm argon and 5 sccm hydrogen. The total pressure of reactor is about 2.5 kPa. The growth time lasts for 20 min and then the quartz tube was cooled down to room temperature in the flow of Ar gas. The black materials deposited on Si substrate were collected from the quartz tube after the reaction was finished. The purification of as-grown samples was first rinsed in HCl for 12 h to remove the impurities, then washed by deionized water, and dried in air at 100 °C for 2 h to remove absorbed moist.
3. Results and discussion SEM has been employed to study the oil residue-derived SWNTs, and two kinds of morphologies of products are found. The sample obtained from heavy oil residue with Fe as catalyst has film-like morphology. A high magnification SEM image reveals that the film-like sample contains a large amount of fiber-like carbon materials, as shown in Fig. 1a. In contrast, the morphology of samples synthesized over Pt catalyst indicates a different feature,
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Y. Li et al. / Chemical Engineering Journal 211–212 (2012) 255–259
Fe as catalyst Intensity (a.u.)
280
0
100
200
300
169
100
200
800
1200 1600 2000
Pt as catalyst
Intensity (a.u.) 0
400
300
400
800
1200
1600
2000
-1
Raman shift (cm ) Fig. 2. (a) Raman spectra of the as-grown SWNTs samples over Fe catalyst. (b) Raman spectra of the as-grown SWNTs samples over Pt catalyst. The insets of (a) and (b) indicate the corresponding AFM images of individual SWNT with diameter of 1.0 nm and 1.4 nm, respectively.
as shown in Fig. 1b, wherein the samples shrink greatly and curl up to form cotton-like materials. The samples synthesized from Fe catalyst are further examined by TEM, and TEM images are shown in Fig. 1c and d which provides direct evidence that the main product consists of SWNTs. Close examination by TEM reveals that the
(a) Intensity (a.u.)
Ni as catalyst 207 274
0
100 200 300 400 0
main components of film-like material are SWNTs which display the morphologies of both individual and bundle consisting of several individual SWNTs, as seen in Fig. 1c and d, respectively, and only a little amount of carbon encapsulated metal particles exist as impurities. According to the TEM observations, it is found that the SWNTs prepared with Fe as catalyst have diameter less than 1 nm. It is necessary to mention that there are many Fe particles with size distribution less than 1 nm on the catalyst thin film, which is confirmed by AFM characterization. Such particle size distribution is desirable for growing SWNTs with diameter in an order of 1 nm. Raman spectroscopy has been used to characterize various kinds of CNTs because it was known as one of the most powerful tools to investigate the vibration properties of CNTs in relation to their structural and electronic properties. Fig. 2a shows the typical Raman scattering spectrum of as-grown SWNTs by chemical vapor deposition of oil residue from Fe catalyst at 900 °C. The Raman spectrum shows a sharp tangential mode (G-band) at 1591 cm 1 and low disorder-induced D-band at 1330 cm 1, which indicates that the carbonaceous products has a high crystallinity. The main radial breathing mode (RBM) in a low frequency range of 280–334 cm 1 verifies that these structures are SWNTs, which is in consistent with TEM observations. Since the peak position of RBM is inversely propositional to the diameter of SWNTs, the diameter distribution of SWNTs can be estimated according to the formula d = 234/(xRBM10) [19], and the estimated diameter of SWNTs is in the range of 0.72–0.87 nm, in agreement with the TEM observations. In contrast, it is found that the SWNTs grown over Pt catalyst have a different diameter distribution, a distinct RBM peak at low frequency of 169 cm 1 is observed, corresponding to SWNTs with diameter of 1.47 nm, as indicated in Fig. 2b. Moreover, the AFM is also used to measure the diameter of individual SWNT, as seen from the insets of Fig. 2, in which the diameter of SWNTs grown over Fe and Pt catalysts is confirmed to be about 1.0 nm and 1.4 nm, respectively, being consistent with the Raman spectroscopy characterization. This finding indicates that the catalyst significantly affects the structure of as-synthesized SWNTs.
Co as catalyst 200
Au as catalyst 192
265
100 200 300 400 0 -1
100 200 300 400
Raman shift (cm )
(b)
Ni catalyst
2 nm
Co catalyst
Au catalyst
2 nm
2 nm
Fig. 3. (a) RBM spectra of three kinds of SWNTs synthesized over Ni, Co and Au catalysts. (b) High-resolution TEM images for the corresponding individual SWNT which grown over Ni, Co and Au catalysts.
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(a)
(b) o
500 C o 600 C o 700 C o 800 C o 900 C
12
IG /ID
Intensity (a.u.)
SWNTs with oil residue as carbon source may not proceed through C2 carbon units although the formation mechanism is not clear. It is well known that the oil residue consists of many kinds of hydrocarbons and their derivatives. During the pyrolysis process, the weak links between the adjacent big aromatic units in oil residue can be destroyed to form small polyaromatic hydrocarbon rings. Furthermore, small hydrocarbon molecules may follow nucleation, growth and termination steps with catalyst, and lead to the growth of SWNTs during the heating process.
16
8 4
4. Conclusions
0
0
500 1000 1500 2000 -1
Raman shift (cm )
500 600 700 800 900 o
Temperature ( C)
Fig. 4. (a) Raman spectra of as-grown SWNTs synthesized over Fe catalyst at different temperatures. (b) IG/ID measured as a function of temperatures.
In order to reveal the critical difference in catalyst activity, we have systematically investigated the formation of SWNTs from other different catalysts including Ni, Co, and Au under the same experimental conditions. Fig. 3a describes the situation in RBM region of Raman spectra of SWNTs grown over the Ni, Co and Au catalysts. The obvious RBM peaks for SWNTs synthesized over Ni and Co are in the range of 207–274 cm 1 and 200–265 cm 1, respectively. The corresponding diameter distribution of SWNTs is in the range of 0.89–1.19 nm, similar to the case of SWNTs grown over Fe catalyst. However, in the case of SWNTs synthesized from Au catalyst, a weak RBM peak at 192 cm 1 is observed, which shows the SWNTs have relative larger diameter about 1.29 nm compared with those synthesized from transition metal catalysts. Furthermore, the diameter of SWNTs grown over different catalysts has been characterized by high-resolution TEM, as shown in Fig. 3b. Obviously, the SWNT synthesized by Au catalyst have relatively large diameter in comparison to that grown by Ni and Co catalysts. Clearly, the above results indicate that the oil residuederived SWNTs grown from transition metal catalysts (Fe, Co and Ni) have a narrow diameter distribution compared with that made from nonmagnetic catalyst (Au and Pt). This phenomenon is possibly ascribed to the lower catalyzing ability and solubility of carbon atoms in liquid phase of nonmagnetic metals to decompose aromatic hydrocarbon molecules. On the other hand, the diameter of SWNTs made from the heavy oil residue can be controlled by selecting different catalysts, i.e. type of the catalyst plays an important role in determining the structure of SWNTs. In addition to catalyst effect, it is found that temperature is another key factor that affects the formation of SWNTs from oil residue. Fig. 4a shows the Raman spectra of SWNTs grown over Fe catalyst at different temperatures of 500–900 °C. When the growth temperature is below 700 °C, the RBM peaks are obviously suppressed, suggesting that there are no SWNTs formed from oil residue. Specially, at 500 °C the D-band intensity is much higher than that of the related G-band. However, the intensity ratio of G- to Dband displays a sharp increase when the temperature increases up to 900 °C, as shown in Fig. 4b. Note that all of Raman spectra are performed under the same experimental conditions, so it can be assumed that the absolute intensity of G-band corresponds with the amount of SWNTs on the substrate. Therefore, it can be concluded that the growth temperature indeed plays an important role in determining the formation of SWNTs. For a comparison, a conventional thermal CVD method with CH4 as carbon source for SWNT growth is normally achieved below 800 °C [20]. This difference may result from different carbon precursors which lead to that SWNTs synthesized from oil residue shows a different mechanism versus the CH4 process. Namely, the formation process of
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