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Synthesis of CNTs via chemical vapor deposition of carbon dioxide as a carbon source in the presence of NiMgO
Journal of Alloys and Compounds 647 (2015) 809e814
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Synthesis of CNTs via chemical vapor deposition of carbon dioxide as a carbon source in the presence of NiMgO Ghazaleh Allaedini a, *, Siti Masrinda Tasirin a, Payam Aminayi b a b
Department of Chemical and Process Engineering, Universiti Kebangsaan Malaysia, UKM Bangi, Selangor, Malaysia Chemical and Paper Engineering, Western Michigan University, Kalamazoo, MI, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2015 Received in revised form 30 May 2015 Accepted 1 June 2015 Available online 12 June 2015
Carbon nanotubes were synthesized via the chemical vapor deposition (CVD) method, using Ni/MgO as a catalyst and CO2 as a nontoxic, abundant, and economical carbon source. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM), along with the results from Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy, confirmed the successful formation of CNTs. Energy-dispersive X-ray spectroscopy (EDX) was performed to investigate the weight percentage of the present elements in the synthesized powder, and a significant yield of 27.38% was confirmed. The reaction mechanism was discussed, and the role of the carbon source, catalyst support, and presence of H2 in the reaction environment was elaborated. © 2015 Elsevier B.V. All rights reserved.
Keywords: Carbon dioxide Chemical vapor deposition Carbon nanotubes Ni/MgO
1. Introduction Carbon nanotubes (CNTs), discovered in 1991 [1], are considered a new form of carbon material with unique electrical, mechanical, and chemical properties. CNTs have gained attention because of their potential applications such as additives for high-strength polymer composites, electrode materials for high-capacity batteries, efficient field-emitters as electron sources, and functional components for nanoscale electronic devices [2]. It is clear that future developments in the field of nanotube-based science and technology rely on the highly controlled synthesis of carbon nanotubes. Numerous methods have been employed to synthesize CNTs, including plasma-based arc discharge and laser ablation, in addition to the thermal methods, such as chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), alcohol catalytic CVD (ACCVD), hydrothermal or sono chemical process, and high-pressure CO conversion (HiPCO) [3]. Nevertheless, the CVD method has significant advantages over the other methods. The CVD method is widely used for the carbon nanotube synthesis because of its high production yield and scale-up capability [4]. In the CVD method, the pyrolysis of gas-phase carbon-rich
* Corresponding author. E-mail address: [email protected] (G. Allaedini). http://dx.doi.org/10.1016/j.jallcom.2015.06.012 0925-8388/© 2015 Elsevier B.V. All rights reserved.
molecules (e.g., C2H2, CH4) in the presence of a transition metal catalyst at elevated temperatures (800e1000 C) results in the conversion of the carbon fragments into nanotubes. The use of carbon-containing compounds such as gaseous hydrocarbons (e.g. methane, ethylene, and acetylene) as well as nongaseous sources (e.g. benzene, ethanol, carbon monoxide, and carbon dioxide) have been reported in the literature [5]. Since most of the hydrocarbons used in the previously mentioned methods are hazardous, the focus of this work is to use a non-toxic gas as a direct source of carbon to synthesize CNTs. One approach to accomplish this objective is to utilize CO2 as a cheap, non-toxic, low-energy, and abundant resource. Moreover, when used as a carbon source, the flammability and toxicity of carbon monoxide can be avoided during the operation. CO2 is also an environmentally friendly reagent which is especially useful as a phosgene substitute. CO2 is easily formed by the oxidation of organic molecules during combustion or respiration and can be recovered as a by-product of industrial chemical processes. It can also be acquired from natural reservoirs [6]. As a matter of fact, the increase in the concentration of CO2 in the atmosphere has stimulated significant global research. In order to reduce the effect of CO2 on the global warming, catalytic reduction of CO2 using chemical, biological, and photochemical methods has been investigated [7]. Carbon dioxide has been also reduced for the preparation of organic molecules [8]. However, since CO2 is the most oxidized state of carbon, few industrial processes employ CO2 as
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G. Allaedini et al. / Journal of Alloys and Compounds 647 (2015) 809e814
their raw material. One of the common deterrents of employing CO2 as a raw material is the low energy level of this gas. In other words, a large energy input is required to transform CO2 [9]. Thus, recently, efforts have been made to explore the possibility of thermally splitting CO2 [10]. Few reports are available on the production of carbon nanotubes using carbon dioxide as a source of carbon. Lou et al. [11] used super critical CO2 (scCO2) as a carbon source and the alkali metals (Li or Na) as the reductants to synthesize CNTs under reaction temperatures of 600e750 C [12]. CNTs with different morphologies, depending on the reductants, ranging from double helical, rope-like, porous, and bamboo-like structures were prepared. Tamaura and Tabata reported the complete reduction of CO2 to carbon at 290 C using oxygen deficient ferrites (ODF), represented by the general formula of MxFe3exO4ed, where M is a bivalent metal (Fe, Ni, Co, Cu, Zn, Mg, and Mn) and d is the reduction degree of ferrite [13]. Nano-sized metal particles enable the hydrocarbon to decompose. The most commonly used metals are Fe, Co, and Ni because of their high carbon diffusion capability. In addition, their low equilibrium vapor pressure and high melting point provide compatibility with a wide range of temperature required for a wide range of carbon precursors in the CVD process. It has been reported that these metals have the ability to form low diameter and high curvature CNTs [14]. Catalyst-support interaction is also one of the important factors for selecting the right support for the CVD process, as the substrate material and its textural properties affect the yield and the quality of the obtained CNTs. It is well known that the same catalyst can act differently on a different support [5]. Commonly used supports are quartz, silicon, silicon carbide, silica, alumina, alumino-silicate (zeolite), CaCO3, and magnesium oxide. The morphology of the carbon nanotubes synthesized using magnesium oxide as the support material has a uniform and well-defined structure [15]. In addition, MgO support has the advantage of solubility in acid, and, therefore, the obtained CNTs can be easily purified [16]. Given that CVD is an effective method for CNT production, and that CO2 is an environmental friendly gas, this study investigated the synthesis of CNTs via chemical vapor deposition of carbon dioxide as a carbon source. Ni supported MgO has been employed as the catalyst for the reaction, and the properties of the obtained CNTs have been investigated. To the best of our knowledge, this work is the first to use the combination of CO2 as the carbon source and Ni/MgO as the catalyst for the production of CNTs in the CVD process.
2. Growth mechanism When CO2 is heated beyond its critical point (more than 31 C at 73 atm), the gas and the liquid phases merge into one supercritical phase (scCO2). This phenomenon has drawn attention to CO2 as a carbon source for application in the chemical vapor deposition (CVD) process. Having a combination of both gas and liquid phase characteristics, scCO2 has the advantages of miscibility with other liquids as well as possession of a relatively weaker molecular association as compared with ordinary condensed phases. This results in a higher reactivity of the reactants in the reaction environment [12]. CO2 can be reduced to carbon by active nanometals, among them nickel. The oxygen of CO2 is transferred to the metal surface in the form of CO2 ions. These ions react with metals to form spinel structures. In order to maintain the electrical neutrality of the oxide, active nanometals release electrons which move to the surface and are donated to the carbon in CO2 molecules [13,17]. After the decomposition of the carbon source, carbon diffuses into the metal particles until the solution is saturated. Super-
saturation of the solution results in the precipitation of solid carbon over the metal surface [18]: CO2 þ Ni / C (Graphite) þ NiO2 The catalyst support also plays an important role in maintaining and assisting the catalytic activity [19]. When transition metals and the support meet the CO2 on the catalyst surface, the reduction of CO2 accelerates, and CO2 converts to carbon nanotubes [20]. Magnesium melts at 650 C, and its boiling temperature is equal to 1090 C. The vapor pressure of magnesium at 1000 C is equal to 350 mmHg. At this temperature, CO2 is in its super critical state, and the calculated pressure is equal to 10 kbar (~75 105 mmHg). Therefore, at 1000 C, the super critical CO2, with the assistance of molten MgO support, is decomposed to carbon nanotubes: [17,21]. Mg (l) þ CO2 (g) / MgO (s) þ C (graphite) Various carbon gaseous sources can be used for carbon nanotube production. However, the carbonous or hydrocarbon gases which are meant to be decomposed in the CVD processes are vital to the growth mechanism of CNTs in the CVD process. The gas selection importance is due to its role in the dissociation of carbon precursors into carbon atoms, dissolution and saturation of these atoms over the catalyst metal particles, and further precipitation of these carbon particles until tubular carbon deposits are achieved. Carbon dioxide is more chemically stable than gaseous hydrocarbons at high temperatures. Therefore, it is difficult to activate CO2 in the reaction. The decomposition of carbon dioxide was catalyzed by NieMgO catalyst in this process. The possible reactions formulas are as follows:
CO2 þ e/CO 2 CO 2 /CO ðgÞ þ O
It is well known that at furnace temperatures, two competing reactions of CO hydrogenation: H2 (g) þ CO (g) ⇔ C (s) þ H2O (g) and CO disproportionation: CO (g) þ CO (g) ⇔ C (s) þ CO2 (g) result in carbon liberation. Therefore, the presence of H2 is considered favorable as it provides additional carbon atoms through CO hydrogenation and disproportionation. Consequently, it can be predicted that hydrogen in the reaction environment enhances the CNTs synthesis [20]. 3. Experimental Magnesium Oxide (MgO) M ¼ 40.3 g/mol was purchased from R&M chemicals, Malaysia. Hydrochloric acid (HCl) 37 wt% was purchased from Fisher Chemicals. Nickel(II) nitrate hexahydrate (Ni(NO3)2) 99.99% trace metal basis and oleic acid 99% CH3(CH2)7CH] CH(CH2)7COOH were purchased from Sigma Aldrich, U.S. The employed process is a modification of a study by Budiredla et al. [22]. To synthesize Mg-doped NiO nanoparticles, magnesium nitrate hexahydrate (Mg(NO3)26H2O), nickel nitrate hexahydrate (Ni (NO3)26H2O), and oleic acid (C18H34O2) were used as precursors with ethanol as a solvent. 20 g of magnesium nitrate hexahydrate was added slowly to 100 ml of ethyl alcohol and stirred at 100 C until complete dissolution. Then, 10 g of nickel nitrate hexahydrate was
G. Allaedini et al. / Journal of Alloys and Compounds 647 (2015) 809e814
811
Fig. 1. Schematic picture of the chemical vapor deposition (CVD) instrument.
dissolved in 100 mL of ethyl alcohol, similarly. Both of these solutions were then mixed, and the oleic acid solution was added slowly to the obtained solution with constant stirring to produce a thick white gel. The product was dried at 100 C for 24 h to yield Ni-doped magnesium oxalate dihydrate. The result was then decomposed for 4 h at 600 C. For CNT production, the obtained catalyst was placed in a horizontal stainless steel tubular reactor (Fig. 1) and heated while Ar was being introduced to drive air out. Then the reaction tube was purged with H2, and the temperature was raised to 800 C. Subsequently, the carbon dioxide was introduced at an elevated temperature of 1100 C with a flow rate of 900 cm3 min1, while the flow rate of H2 was raised to 1500 cm3 min1 and kept for 1 h. The as-prepared carbon soot in the tube was in the form of black flakes produced within the catalyst powder in the combustion boat and was collected to be characterized. The obtained carbon soot was dipped in 37 wt% HCl and successively into water to remove the catalyst. The schematic picture of the CVD instrument is presented in Fig. 1.
4. Characterization The Fourier transform infrared spectroscopy (FT-IR) spectra were recorded in transmission mode using a thermo-scientific NICOLET6700 apparatus in the 400e5000 cm1 region to confirm the presence of carbon nanotubes (CNTs). Scanning electron microscopy (SEM) pictures were used to visualize the CNTs and determine their surface morphology using a Zeiss SUPRA55 scanning electron microscope at an operating voltage of 3 kV. Transmission electron microscopy (TEM) was performed using Hitachi477700m10 Kv to investigate the shape and diameter of the obtained CNTs. Energy-dispersive X-ray spectroscopy (EDX) (Zeiss SUPRA55) was used to investigate the weight percentage of the present elements in the synthesized powder. Raman spectrophotometer with a laser of 514 nm wavelength (Horiba Jobin Yvon, LabRam HR800) was used to confirm the formation of CNTs.
Fig. 2. SEM images of (a) NieMgO, (b) carbon grown on NieMgO to form CNTs, (c) CNTs before purification, (d) CNTs after purification.
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G. Allaedini et al. / Journal of Alloys and Compounds 647 (2015) 809e814 Table 1 EDX analysis on the percentages of the present elements before the CVD process and the present elements in the product after the CVD reaction. Element
C O Mg Ni Total
Fig. 3. TEM micrographs of (a) the obtained carbon nanotubes and (b) the tip of a nanotube.
5. Result and discussion The scanning electron microscopy (SEM) images of the products during the different stages of the CVD process are shown in Fig. 2. As can be seen in Fig. 1(a), before the CVD process, the NieMgO particles, which served as the catalyst and support for the reaction,
NiMgO
CNTs
Weight%
Atomic%
Weight%
Atomic%
e 57.91 38.95 3.13 100
e 68.62 30.37 1.01 100
27.38 61.21 10.22 1.19 100
19.34 65.70 14.61 0.35 100
exhibited flower/star like morphology. Fig. 1(b) shows how carbon grew on the surface of NieMgO during the CVD process to form CNTs, and Fig. 1(c) shows the CNTs obtained before purification. Some of the flower-like particles of NieMgO are still observable in this image. Finally, Fig. 1(d) shows the CNTs obtained after purification. The tube-like structure of the CNTs is clearly evident in this image. Transmission electron microscopy (TEM) was performed to confirm the formation of carbon nanotubes. The TEM images of the obtained carbon nanotubes are shown in Fig. 3. The formation of a multi-walled carbon tubular nanostructure can be observed in these pictures. Fig. 3(b) shows the closed tip of a carbon nanotube. The average diameter of the obtained nanotubes was measured to be 8.45 nm. The energy dispersive X-ray analysis (EDX) (Fig. 4) shows the weight percentages of the present elements in the combustion boat before the CVD process begins and its comparison with the present elements in the product after the CVD reaction. The carbon constituted 27.38% of the total weight after the reaction, which represents a considerable amount of yield. The EDX weight and atomic percentages are shown in Table 1 for the NiMgO catalyst and the yield before and after the CVD process. The FTIR spectrum of carbon nanotubes after collection from the stainless steel tube of the CVD furnace is shown in Fig. 5. A characteristic peak at 1480 cm1 is assigned to the C]C bond in the carbon nanotubes. The absorptions at 2850 cm1 correspond to the CeH stretch modes, and the peak observed at ~1640 cm1 is attributed to the CeC stretch band of the carbon nanotubes [23]. The CeC vibrations assigned to the internal defects and the OeH vibration at 3450 cm1 are associated with the amorphous carbon [24]. The intensity of this peak is due to the amorphous carbon, and MgNiO easily forms bonds with the moisture in the environment [25]. The small peak at 671 cm1 is assigned to the vibration mode
Fig. 4. Energy dispersive X-ray analysis (EDX) on the (a) NieMgO nanoparticles before the CVD process and (b) the obtained CNTs after the CVD reaction.
G. Allaedini et al. / Journal of Alloys and Compounds 647 (2015) 809e814
813
Transmittance (%)
From the present study, it can be concluded that Ni as a transition metal, in combination with MgO, could be successfully used as an active catalyst to grow CNTs by employing CO2 as the carbon source in the CVD process. Conflict of interest
C-H C-C
The authors declare that they have no conflict of interest. Mg-O
O-H
Acknowledgment
4000
3400
2800 2200 1600 Wavenumber (cm-1)
1000
400
We would like to acknowledge the support provided by the CRIM, KK-2014-014 and FRGS/2/2013/TK05/UKM/02/3 funds, UKM, Malaysia.
Fig. 5. FTIR spectrum of the carbon nanotubes after collection from the CVD furnace.
References
Intensity ( a.u)
G
0
D G' G"
500
1000 1500 2000 Raman Shift (cm-1)
2500
3000
Fig. 6. Raman spectra of the purified CNTs.
of MgO [26,27]. The presence of the mentioned characteristics in the FTIR spectrum strongly confirms the formation of CNTs. Fig. 6 shows the Raman spectrum of the purified CNT. Graphitelike carbon materials exhibit a Raman band at around 1580 cm1 assigned to the G band, a peak at 1350 cm1 assigned to the D band, and a peak at ~2650 cm1 corresponding to the G0 band [28]. The G and G0 bands represent the two most prominent original graphite features, respectively, whereas the D-band is known to depend on the disorder features of the hexagonal graphitic layers [29e31] as well as the presence of small crystalline grains. Since the G band peak is attributed to the tangential stretching mode of the graphite C]C bond, which is related to the vibration of sp2-bonded carbon atoms in a two-dimensional hexagonal lattice, we can justly conclude the formation of a graphitic nanotube. Moreover, a small peak is observed at 2930 cm1, which is a characteristic of CNTs and is assigned to the G þ D band (or G00 ). This band represents the overtone of the D band [32,33]. 6. Conclusion This study demonstrates that carbon dioxide can be used as a carbon source to synthesize CNTs. The reaction mechanism, which was thoroughly discussed, shows the importance of the carbon source, catalyst support, and presence of H2 in the reaction environment. In this work, NieMgO was used as a catalyst to facilitate the decomposition of CO2 and to produce graphitic carbon nanotubes. The Raman and FTIR spectra were in agreement with the SEM and TEM images and confirmed the formation of carbon nanotubes. The EDX results proved the growth of carbon in the support powder and confirmed an outstanding yield of 27.38%.
[1] P. Ajayan, T. Ebbesen, T. Ichihashi, S. Iijima, K. Tanigaki, H. Hiura, Opening carbon nanotubes with oxygen and implications for filling, Nature 362 (1993) 522e525. [2] A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y.H. Lee, S.G. Kim, A.G. Rinzler, Crystalline ropes of metallic carbon nanotubes, Sci. AAAS-Weekly Pap. Ed. 273 (1996) 483e487. [3] K. Varshney, Carbon Nanotubes: a Review on Synthesis, Properties and Applications. [4] Y.-M. Shyu, F.C.-N. Hong, Low-temperature growth and field emission of aligned carbon nanotubes by chemical vapor deposition, Mater. Chem. Phys. 72 (2001) 223e227. [5] M. Kumar, Y. Ando, Chemical vapor deposition of carbon nanotubes: a review on growth mechanism and mass production, J. Nanosci. Nanotechnol. 10 (2010) 3739e3758. [6] L. Wei, N. Yan, Q. Chen, Converting poly (ethylene terephthalate) waste into carbon microspheres in a supercritical CO2 system, Environ. Sci. Technol. 45 (2010) 534e539. [7] V. Browne, A. Carley, R. Copperthwaite, P. Davies, E. Moser, M. Roberts, Activation of carbon dioxide at bismuth, gold and copper surfaces, Appl. Surf. Sci. 47 (1991) 375e379. [8] E.J. Beckman, Making polymers from carbon dioxide, Science 283 (1999) 946e947. [9] T. Sakakura, J.-C. Choi, H. Yasuda, Transformation of carbon dioxide, Chem. Rev. 107 (2007) 2365e2387. [10] G.S. Simate, K. Moothi, M. Meyyappan, S.E. Iyuke, S. Ndlovu, R. Falcon, M. Heydenrych, Kinetic model of carbon nanotube production from carbon dioxide in a floating catalytic chemical vapour deposition reactor, RSC Adv. 4 (2014) 9564e9572. [11] Z. Lou, C. Chen, Q. Chen, J. Gao, Formation of variously shaped carbon nanotubes in carbon dioxideealkali metal (Li, Na) system, Carbon 43 (2005) 1104e1108. [12] G.S. Simate, S.E. Iyuke, S. Ndlovu, C.S. Yah, L.F. Walubita, The production of carbon nanotubes from carbon dioxide: challenges and opportunities, J. Nat. Gas Chem. 19 (2010) 453e460. [13] Y. Tamaura, M. Tahata, Complete reduction of carbon dioxide to carbon using cation-excess magnetite, Nature 346 (1990) 255e256. n, K. Bolton, The [14] F. Ding, P. Larsson, J.A. Larsson, R. Ahuja, H. Duan, A. Rose importance of strong carbon-metal adhesion for catalytic nucleation of singlewalled carbon nanotubes, Nano Lett. 8 (2008) 463e468. [15] E.D. Dikio, A.J. Kupeta, F.T. Thema, A comparative study of the effect of MgO and CaCO3 as support materials in the synthesis of carbon nanotubes with Fe/ Co as catalyst, J. Chem. 2014 (2014) 6. [16] A. Rashidi, M. Akbarnejad, A. Khodadadi, Y. Mortazavi, A. Ahmadpourd, Singlewall carbon nanotubes synthesized using organic additives to CoeMo catalysts supported on nanoporous MgO, Nanotechnology 18 (2007) 315605. [17] Z. Lou, C. Chen, H. Huang, D. Zhao, Fabrication of Y-junction carbon nanotubes by reduction of carbon dioxide with sodium borohydride, Diam. Relat. Mater. 15 (2006) 1540e1543. [18] A. Moisala, A.G. Nasibulin, E.I. Kauppinen, The role of metal nanoparticles in the catalytic production of single-walled carbon nanotubesda review, J. Phys. Condens. Matter 15 (2003) S3011. [19] A.-C. Dupuis, The catalyst in the CCVD of carbon nanotubesda review, Prog. Mater. Sci. 50 (2005) 929e961. [20] X.-J. Xu, S.-M. Huang, Carbon dioxide as a carbon source for synthesis of carbon nanotubes by chemical vapor deposition, Mater. Lett. 61 (2007) 4235e4237. [21] Y. Zhao, Z. Zhang, X. Qian, Y. Han, Properties of carbon dioxide absorption and reduction by sodium borohydride under atmospheric pressure, Fuel 142 (2015) 1e8. [22] N. Budiredla, A. Kumar, S. Thota, J. Kumar, Synthesis and optical characterization of Mg1-xNixO nanostructures, ISRN Nanomater. 2012 (2012) 8.
814
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[23] M. Bahgat, A.A. Farghali, W.M.A. El Rouby, M.H. Khedr, Synthesis and modification of multi-walled carbon nano-tubes (MWCNTs) for water treatment applications, J. Anal. Appl. Pyrolysis 92 (2011) 307e313. [24] S. Hussain, P. Jha, A. Chouksey, R. Raman, S. Islam, T. Islam, P. Choudhary, Spectroscopic investigation of modified single wall carbon nanotube (SWCNT), J. Mod. Phys. 2 (2011) 538. [25] E. Titus, N. Ali, G. Cabral, J. Gracio, P.R. Babu, M. Jackson, Chemically functionalized carbon nanotubes and their characterization using thermogravimetric analysis, Fourier transform infrared, and Raman spectroscopy, J. Mater. Eng. Perform. 15 (2006) 182e186. [26] P. Tamilselvi, A. Yelilarasi, M. Hema, R. Anbarasan, Synthesis of hierarchical structured MgO by sol-gel method, Nano Bull. 2 (2013) 130106. [27] M. Rezaei, M. Khajenoori, B. Nematollahi, Synthesis of high surface area nanocrystalline MgO by pluronic P123 triblock copolymer surfactant, Powder Technol. 205 (2011) 112e116.
[28] A. Ferrari, J. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. Novoselov, S. Roth, Raman spectrum of graphene and graphene layers, Phys. Rev. Lett. 97 (2006) 187401. [29] F. Tuinstra, J.L. Koenig, Raman spectrum of graphite, J. Chem. Phys. 53 (1970) 1126e1130. [30] J. Robertson, Amorphous carbon, Adv. Phys. 35 (1986) 317e374. [31] W.M. Silva, H. Ribeiro, L.M. Seara, H.D. Calado, A.S. Ferlauto, R.M. Paniago, C.F. Leite, G.G. Silva, Surface properties of oxidized and aminated multi-walled carbon nanotubes, J. Braz. Chem. Soc. 23 (2012) 1078e1086. [32] M.S. Mohamed Saheed, N.M. Mohamed, Z.A. Burhanudin, Effect of different catalyst deposition technique on aligned multiwalled carbon nanotubes grown by thermal chemical vapor deposition, J. Nanomater. 2014 (2014) 11. [33] L. Bokobza, J. Zhang, Raman spectroscopic characterization of multiwall carbon nanotubes and of composites, Express Polym. Lett. 6 (2012) 601e608.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Synthesis of CNTs via chemical vapor deposition of carbon dioxide as a carbon source in the presence of NiMgO Ghazaleh Allaedini a, *, Siti Masrinda Tasirin a, Payam Aminayi b a b
Department of Chemical and Process Engineering, Universiti Kebangsaan Malaysia, UKM Bangi, Selangor, Malaysia Chemical and Paper Engineering, Western Michigan University, Kalamazoo, MI, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2015 Received in revised form 30 May 2015 Accepted 1 June 2015 Available online 12 June 2015
Carbon nanotubes were synthesized via the chemical vapor deposition (CVD) method, using Ni/MgO as a catalyst and CO2 as a nontoxic, abundant, and economical carbon source. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM), along with the results from Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy, confirmed the successful formation of CNTs. Energy-dispersive X-ray spectroscopy (EDX) was performed to investigate the weight percentage of the present elements in the synthesized powder, and a significant yield of 27.38% was confirmed. The reaction mechanism was discussed, and the role of the carbon source, catalyst support, and presence of H2 in the reaction environment was elaborated. © 2015 Elsevier B.V. All rights reserved.
Keywords: Carbon dioxide Chemical vapor deposition Carbon nanotubes Ni/MgO
1. Introduction Carbon nanotubes (CNTs), discovered in 1991 [1], are considered a new form of carbon material with unique electrical, mechanical, and chemical properties. CNTs have gained attention because of their potential applications such as additives for high-strength polymer composites, electrode materials for high-capacity batteries, efficient field-emitters as electron sources, and functional components for nanoscale electronic devices [2]. It is clear that future developments in the field of nanotube-based science and technology rely on the highly controlled synthesis of carbon nanotubes. Numerous methods have been employed to synthesize CNTs, including plasma-based arc discharge and laser ablation, in addition to the thermal methods, such as chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), alcohol catalytic CVD (ACCVD), hydrothermal or sono chemical process, and high-pressure CO conversion (HiPCO) [3]. Nevertheless, the CVD method has significant advantages over the other methods. The CVD method is widely used for the carbon nanotube synthesis because of its high production yield and scale-up capability [4]. In the CVD method, the pyrolysis of gas-phase carbon-rich
* Corresponding author. E-mail address: [email protected] (G. Allaedini). http://dx.doi.org/10.1016/j.jallcom.2015.06.012 0925-8388/© 2015 Elsevier B.V. All rights reserved.
molecules (e.g., C2H2, CH4) in the presence of a transition metal catalyst at elevated temperatures (800e1000 C) results in the conversion of the carbon fragments into nanotubes. The use of carbon-containing compounds such as gaseous hydrocarbons (e.g. methane, ethylene, and acetylene) as well as nongaseous sources (e.g. benzene, ethanol, carbon monoxide, and carbon dioxide) have been reported in the literature [5]. Since most of the hydrocarbons used in the previously mentioned methods are hazardous, the focus of this work is to use a non-toxic gas as a direct source of carbon to synthesize CNTs. One approach to accomplish this objective is to utilize CO2 as a cheap, non-toxic, low-energy, and abundant resource. Moreover, when used as a carbon source, the flammability and toxicity of carbon monoxide can be avoided during the operation. CO2 is also an environmentally friendly reagent which is especially useful as a phosgene substitute. CO2 is easily formed by the oxidation of organic molecules during combustion or respiration and can be recovered as a by-product of industrial chemical processes. It can also be acquired from natural reservoirs [6]. As a matter of fact, the increase in the concentration of CO2 in the atmosphere has stimulated significant global research. In order to reduce the effect of CO2 on the global warming, catalytic reduction of CO2 using chemical, biological, and photochemical methods has been investigated [7]. Carbon dioxide has been also reduced for the preparation of organic molecules [8]. However, since CO2 is the most oxidized state of carbon, few industrial processes employ CO2 as
810
G. Allaedini et al. / Journal of Alloys and Compounds 647 (2015) 809e814
their raw material. One of the common deterrents of employing CO2 as a raw material is the low energy level of this gas. In other words, a large energy input is required to transform CO2 [9]. Thus, recently, efforts have been made to explore the possibility of thermally splitting CO2 [10]. Few reports are available on the production of carbon nanotubes using carbon dioxide as a source of carbon. Lou et al. [11] used super critical CO2 (scCO2) as a carbon source and the alkali metals (Li or Na) as the reductants to synthesize CNTs under reaction temperatures of 600e750 C [12]. CNTs with different morphologies, depending on the reductants, ranging from double helical, rope-like, porous, and bamboo-like structures were prepared. Tamaura and Tabata reported the complete reduction of CO2 to carbon at 290 C using oxygen deficient ferrites (ODF), represented by the general formula of MxFe3exO4ed, where M is a bivalent metal (Fe, Ni, Co, Cu, Zn, Mg, and Mn) and d is the reduction degree of ferrite [13]. Nano-sized metal particles enable the hydrocarbon to decompose. The most commonly used metals are Fe, Co, and Ni because of their high carbon diffusion capability. In addition, their low equilibrium vapor pressure and high melting point provide compatibility with a wide range of temperature required for a wide range of carbon precursors in the CVD process. It has been reported that these metals have the ability to form low diameter and high curvature CNTs [14]. Catalyst-support interaction is also one of the important factors for selecting the right support for the CVD process, as the substrate material and its textural properties affect the yield and the quality of the obtained CNTs. It is well known that the same catalyst can act differently on a different support [5]. Commonly used supports are quartz, silicon, silicon carbide, silica, alumina, alumino-silicate (zeolite), CaCO3, and magnesium oxide. The morphology of the carbon nanotubes synthesized using magnesium oxide as the support material has a uniform and well-defined structure [15]. In addition, MgO support has the advantage of solubility in acid, and, therefore, the obtained CNTs can be easily purified [16]. Given that CVD is an effective method for CNT production, and that CO2 is an environmental friendly gas, this study investigated the synthesis of CNTs via chemical vapor deposition of carbon dioxide as a carbon source. Ni supported MgO has been employed as the catalyst for the reaction, and the properties of the obtained CNTs have been investigated. To the best of our knowledge, this work is the first to use the combination of CO2 as the carbon source and Ni/MgO as the catalyst for the production of CNTs in the CVD process.
2. Growth mechanism When CO2 is heated beyond its critical point (more than 31 C at 73 atm), the gas and the liquid phases merge into one supercritical phase (scCO2). This phenomenon has drawn attention to CO2 as a carbon source for application in the chemical vapor deposition (CVD) process. Having a combination of both gas and liquid phase characteristics, scCO2 has the advantages of miscibility with other liquids as well as possession of a relatively weaker molecular association as compared with ordinary condensed phases. This results in a higher reactivity of the reactants in the reaction environment [12]. CO2 can be reduced to carbon by active nanometals, among them nickel. The oxygen of CO2 is transferred to the metal surface in the form of CO2 ions. These ions react with metals to form spinel structures. In order to maintain the electrical neutrality of the oxide, active nanometals release electrons which move to the surface and are donated to the carbon in CO2 molecules [13,17]. After the decomposition of the carbon source, carbon diffuses into the metal particles until the solution is saturated. Super-
saturation of the solution results in the precipitation of solid carbon over the metal surface [18]: CO2 þ Ni / C (Graphite) þ NiO2 The catalyst support also plays an important role in maintaining and assisting the catalytic activity [19]. When transition metals and the support meet the CO2 on the catalyst surface, the reduction of CO2 accelerates, and CO2 converts to carbon nanotubes [20]. Magnesium melts at 650 C, and its boiling temperature is equal to 1090 C. The vapor pressure of magnesium at 1000 C is equal to 350 mmHg. At this temperature, CO2 is in its super critical state, and the calculated pressure is equal to 10 kbar (~75 105 mmHg). Therefore, at 1000 C, the super critical CO2, with the assistance of molten MgO support, is decomposed to carbon nanotubes: [17,21]. Mg (l) þ CO2 (g) / MgO (s) þ C (graphite) Various carbon gaseous sources can be used for carbon nanotube production. However, the carbonous or hydrocarbon gases which are meant to be decomposed in the CVD processes are vital to the growth mechanism of CNTs in the CVD process. The gas selection importance is due to its role in the dissociation of carbon precursors into carbon atoms, dissolution and saturation of these atoms over the catalyst metal particles, and further precipitation of these carbon particles until tubular carbon deposits are achieved. Carbon dioxide is more chemically stable than gaseous hydrocarbons at high temperatures. Therefore, it is difficult to activate CO2 in the reaction. The decomposition of carbon dioxide was catalyzed by NieMgO catalyst in this process. The possible reactions formulas are as follows:
CO2 þ e/CO 2 CO 2 /CO ðgÞ þ O
It is well known that at furnace temperatures, two competing reactions of CO hydrogenation: H2 (g) þ CO (g) ⇔ C (s) þ H2O (g) and CO disproportionation: CO (g) þ CO (g) ⇔ C (s) þ CO2 (g) result in carbon liberation. Therefore, the presence of H2 is considered favorable as it provides additional carbon atoms through CO hydrogenation and disproportionation. Consequently, it can be predicted that hydrogen in the reaction environment enhances the CNTs synthesis [20]. 3. Experimental Magnesium Oxide (MgO) M ¼ 40.3 g/mol was purchased from R&M chemicals, Malaysia. Hydrochloric acid (HCl) 37 wt% was purchased from Fisher Chemicals. Nickel(II) nitrate hexahydrate (Ni(NO3)2) 99.99% trace metal basis and oleic acid 99% CH3(CH2)7CH] CH(CH2)7COOH were purchased from Sigma Aldrich, U.S. The employed process is a modification of a study by Budiredla et al. [22]. To synthesize Mg-doped NiO nanoparticles, magnesium nitrate hexahydrate (Mg(NO3)26H2O), nickel nitrate hexahydrate (Ni (NO3)26H2O), and oleic acid (C18H34O2) were used as precursors with ethanol as a solvent. 20 g of magnesium nitrate hexahydrate was added slowly to 100 ml of ethyl alcohol and stirred at 100 C until complete dissolution. Then, 10 g of nickel nitrate hexahydrate was
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811
Fig. 1. Schematic picture of the chemical vapor deposition (CVD) instrument.
dissolved in 100 mL of ethyl alcohol, similarly. Both of these solutions were then mixed, and the oleic acid solution was added slowly to the obtained solution with constant stirring to produce a thick white gel. The product was dried at 100 C for 24 h to yield Ni-doped magnesium oxalate dihydrate. The result was then decomposed for 4 h at 600 C. For CNT production, the obtained catalyst was placed in a horizontal stainless steel tubular reactor (Fig. 1) and heated while Ar was being introduced to drive air out. Then the reaction tube was purged with H2, and the temperature was raised to 800 C. Subsequently, the carbon dioxide was introduced at an elevated temperature of 1100 C with a flow rate of 900 cm3 min1, while the flow rate of H2 was raised to 1500 cm3 min1 and kept for 1 h. The as-prepared carbon soot in the tube was in the form of black flakes produced within the catalyst powder in the combustion boat and was collected to be characterized. The obtained carbon soot was dipped in 37 wt% HCl and successively into water to remove the catalyst. The schematic picture of the CVD instrument is presented in Fig. 1.
4. Characterization The Fourier transform infrared spectroscopy (FT-IR) spectra were recorded in transmission mode using a thermo-scientific NICOLET6700 apparatus in the 400e5000 cm1 region to confirm the presence of carbon nanotubes (CNTs). Scanning electron microscopy (SEM) pictures were used to visualize the CNTs and determine their surface morphology using a Zeiss SUPRA55 scanning electron microscope at an operating voltage of 3 kV. Transmission electron microscopy (TEM) was performed using Hitachi477700m10 Kv to investigate the shape and diameter of the obtained CNTs. Energy-dispersive X-ray spectroscopy (EDX) (Zeiss SUPRA55) was used to investigate the weight percentage of the present elements in the synthesized powder. Raman spectrophotometer with a laser of 514 nm wavelength (Horiba Jobin Yvon, LabRam HR800) was used to confirm the formation of CNTs.
Fig. 2. SEM images of (a) NieMgO, (b) carbon grown on NieMgO to form CNTs, (c) CNTs before purification, (d) CNTs after purification.
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G. Allaedini et al. / Journal of Alloys and Compounds 647 (2015) 809e814 Table 1 EDX analysis on the percentages of the present elements before the CVD process and the present elements in the product after the CVD reaction. Element
C O Mg Ni Total
Fig. 3. TEM micrographs of (a) the obtained carbon nanotubes and (b) the tip of a nanotube.
5. Result and discussion The scanning electron microscopy (SEM) images of the products during the different stages of the CVD process are shown in Fig. 2. As can be seen in Fig. 1(a), before the CVD process, the NieMgO particles, which served as the catalyst and support for the reaction,
NiMgO
CNTs
Weight%
Atomic%
Weight%
Atomic%
e 57.91 38.95 3.13 100
e 68.62 30.37 1.01 100
27.38 61.21 10.22 1.19 100
19.34 65.70 14.61 0.35 100
exhibited flower/star like morphology. Fig. 1(b) shows how carbon grew on the surface of NieMgO during the CVD process to form CNTs, and Fig. 1(c) shows the CNTs obtained before purification. Some of the flower-like particles of NieMgO are still observable in this image. Finally, Fig. 1(d) shows the CNTs obtained after purification. The tube-like structure of the CNTs is clearly evident in this image. Transmission electron microscopy (TEM) was performed to confirm the formation of carbon nanotubes. The TEM images of the obtained carbon nanotubes are shown in Fig. 3. The formation of a multi-walled carbon tubular nanostructure can be observed in these pictures. Fig. 3(b) shows the closed tip of a carbon nanotube. The average diameter of the obtained nanotubes was measured to be 8.45 nm. The energy dispersive X-ray analysis (EDX) (Fig. 4) shows the weight percentages of the present elements in the combustion boat before the CVD process begins and its comparison with the present elements in the product after the CVD reaction. The carbon constituted 27.38% of the total weight after the reaction, which represents a considerable amount of yield. The EDX weight and atomic percentages are shown in Table 1 for the NiMgO catalyst and the yield before and after the CVD process. The FTIR spectrum of carbon nanotubes after collection from the stainless steel tube of the CVD furnace is shown in Fig. 5. A characteristic peak at 1480 cm1 is assigned to the C]C bond in the carbon nanotubes. The absorptions at 2850 cm1 correspond to the CeH stretch modes, and the peak observed at ~1640 cm1 is attributed to the CeC stretch band of the carbon nanotubes [23]. The CeC vibrations assigned to the internal defects and the OeH vibration at 3450 cm1 are associated with the amorphous carbon [24]. The intensity of this peak is due to the amorphous carbon, and MgNiO easily forms bonds with the moisture in the environment [25]. The small peak at 671 cm1 is assigned to the vibration mode
Fig. 4. Energy dispersive X-ray analysis (EDX) on the (a) NieMgO nanoparticles before the CVD process and (b) the obtained CNTs after the CVD reaction.
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813
Transmittance (%)
From the present study, it can be concluded that Ni as a transition metal, in combination with MgO, could be successfully used as an active catalyst to grow CNTs by employing CO2 as the carbon source in the CVD process. Conflict of interest
C-H C-C
The authors declare that they have no conflict of interest. Mg-O
O-H
Acknowledgment
4000
3400
2800 2200 1600 Wavenumber (cm-1)
1000
400
We would like to acknowledge the support provided by the CRIM, KK-2014-014 and FRGS/2/2013/TK05/UKM/02/3 funds, UKM, Malaysia.
Fig. 5. FTIR spectrum of the carbon nanotubes after collection from the CVD furnace.
References
Intensity ( a.u)
G
0
D G' G"
500
1000 1500 2000 Raman Shift (cm-1)
2500
3000
Fig. 6. Raman spectra of the purified CNTs.
of MgO [26,27]. The presence of the mentioned characteristics in the FTIR spectrum strongly confirms the formation of CNTs. Fig. 6 shows the Raman spectrum of the purified CNT. Graphitelike carbon materials exhibit a Raman band at around 1580 cm1 assigned to the G band, a peak at 1350 cm1 assigned to the D band, and a peak at ~2650 cm1 corresponding to the G0 band [28]. The G and G0 bands represent the two most prominent original graphite features, respectively, whereas the D-band is known to depend on the disorder features of the hexagonal graphitic layers [29e31] as well as the presence of small crystalline grains. Since the G band peak is attributed to the tangential stretching mode of the graphite C]C bond, which is related to the vibration of sp2-bonded carbon atoms in a two-dimensional hexagonal lattice, we can justly conclude the formation of a graphitic nanotube. Moreover, a small peak is observed at 2930 cm1, which is a characteristic of CNTs and is assigned to the G þ D band (or G00 ). This band represents the overtone of the D band [32,33]. 6. Conclusion This study demonstrates that carbon dioxide can be used as a carbon source to synthesize CNTs. The reaction mechanism, which was thoroughly discussed, shows the importance of the carbon source, catalyst support, and presence of H2 in the reaction environment. In this work, NieMgO was used as a catalyst to facilitate the decomposition of CO2 and to produce graphitic carbon nanotubes. The Raman and FTIR spectra were in agreement with the SEM and TEM images and confirmed the formation of carbon nanotubes. The EDX results proved the growth of carbon in the support powder and confirmed an outstanding yield of 27.38%.
[1] P. Ajayan, T. Ebbesen, T. Ichihashi, S. Iijima, K. Tanigaki, H. Hiura, Opening carbon nanotubes with oxygen and implications for filling, Nature 362 (1993) 522e525. [2] A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y.H. Lee, S.G. Kim, A.G. Rinzler, Crystalline ropes of metallic carbon nanotubes, Sci. AAAS-Weekly Pap. Ed. 273 (1996) 483e487. [3] K. Varshney, Carbon Nanotubes: a Review on Synthesis, Properties and Applications. [4] Y.-M. Shyu, F.C.-N. Hong, Low-temperature growth and field emission of aligned carbon nanotubes by chemical vapor deposition, Mater. Chem. Phys. 72 (2001) 223e227. [5] M. Kumar, Y. Ando, Chemical vapor deposition of carbon nanotubes: a review on growth mechanism and mass production, J. Nanosci. Nanotechnol. 10 (2010) 3739e3758. [6] L. Wei, N. Yan, Q. Chen, Converting poly (ethylene terephthalate) waste into carbon microspheres in a supercritical CO2 system, Environ. Sci. Technol. 45 (2010) 534e539. [7] V. Browne, A. Carley, R. Copperthwaite, P. Davies, E. Moser, M. Roberts, Activation of carbon dioxide at bismuth, gold and copper surfaces, Appl. Surf. Sci. 47 (1991) 375e379. [8] E.J. Beckman, Making polymers from carbon dioxide, Science 283 (1999) 946e947. [9] T. Sakakura, J.-C. Choi, H. Yasuda, Transformation of carbon dioxide, Chem. Rev. 107 (2007) 2365e2387. [10] G.S. Simate, K. Moothi, M. Meyyappan, S.E. Iyuke, S. Ndlovu, R. Falcon, M. Heydenrych, Kinetic model of carbon nanotube production from carbon dioxide in a floating catalytic chemical vapour deposition reactor, RSC Adv. 4 (2014) 9564e9572. [11] Z. Lou, C. Chen, Q. Chen, J. Gao, Formation of variously shaped carbon nanotubes in carbon dioxideealkali metal (Li, Na) system, Carbon 43 (2005) 1104e1108. [12] G.S. Simate, S.E. Iyuke, S. Ndlovu, C.S. Yah, L.F. Walubita, The production of carbon nanotubes from carbon dioxide: challenges and opportunities, J. Nat. Gas Chem. 19 (2010) 453e460. [13] Y. Tamaura, M. Tahata, Complete reduction of carbon dioxide to carbon using cation-excess magnetite, Nature 346 (1990) 255e256. n, K. Bolton, The [14] F. Ding, P. Larsson, J.A. Larsson, R. Ahuja, H. Duan, A. Rose importance of strong carbon-metal adhesion for catalytic nucleation of singlewalled carbon nanotubes, Nano Lett. 8 (2008) 463e468. [15] E.D. Dikio, A.J. Kupeta, F.T. Thema, A comparative study of the effect of MgO and CaCO3 as support materials in the synthesis of carbon nanotubes with Fe/ Co as catalyst, J. Chem. 2014 (2014) 6. [16] A. Rashidi, M. Akbarnejad, A. Khodadadi, Y. Mortazavi, A. Ahmadpourd, Singlewall carbon nanotubes synthesized using organic additives to CoeMo catalysts supported on nanoporous MgO, Nanotechnology 18 (2007) 315605. [17] Z. Lou, C. Chen, H. Huang, D. Zhao, Fabrication of Y-junction carbon nanotubes by reduction of carbon dioxide with sodium borohydride, Diam. Relat. Mater. 15 (2006) 1540e1543. [18] A. Moisala, A.G. Nasibulin, E.I. Kauppinen, The role of metal nanoparticles in the catalytic production of single-walled carbon nanotubesda review, J. Phys. Condens. Matter 15 (2003) S3011. [19] A.-C. Dupuis, The catalyst in the CCVD of carbon nanotubesda review, Prog. Mater. Sci. 50 (2005) 929e961. [20] X.-J. Xu, S.-M. Huang, Carbon dioxide as a carbon source for synthesis of carbon nanotubes by chemical vapor deposition, Mater. Lett. 61 (2007) 4235e4237. [21] Y. Zhao, Z. Zhang, X. Qian, Y. Han, Properties of carbon dioxide absorption and reduction by sodium borohydride under atmospheric pressure, Fuel 142 (2015) 1e8. [22] N. Budiredla, A. Kumar, S. Thota, J. Kumar, Synthesis and optical characterization of Mg1-xNixO nanostructures, ISRN Nanomater. 2012 (2012) 8.
814
G. Allaedini et al. / Journal of Alloys and Compounds 647 (2015) 809e814
[23] M. Bahgat, A.A. Farghali, W.M.A. El Rouby, M.H. Khedr, Synthesis and modification of multi-walled carbon nano-tubes (MWCNTs) for water treatment applications, J. Anal. Appl. Pyrolysis 92 (2011) 307e313. [24] S. Hussain, P. Jha, A. Chouksey, R. Raman, S. Islam, T. Islam, P. Choudhary, Spectroscopic investigation of modified single wall carbon nanotube (SWCNT), J. Mod. Phys. 2 (2011) 538. [25] E. Titus, N. Ali, G. Cabral, J. Gracio, P.R. Babu, M. Jackson, Chemically functionalized carbon nanotubes and their characterization using thermogravimetric analysis, Fourier transform infrared, and Raman spectroscopy, J. Mater. Eng. Perform. 15 (2006) 182e186. [26] P. Tamilselvi, A. Yelilarasi, M. Hema, R. Anbarasan, Synthesis of hierarchical structured MgO by sol-gel method, Nano Bull. 2 (2013) 130106. [27] M. Rezaei, M. Khajenoori, B. Nematollahi, Synthesis of high surface area nanocrystalline MgO by pluronic P123 triblock copolymer surfactant, Powder Technol. 205 (2011) 112e116.
[28] A. Ferrari, J. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. Novoselov, S. Roth, Raman spectrum of graphene and graphene layers, Phys. Rev. Lett. 97 (2006) 187401. [29] F. Tuinstra, J.L. Koenig, Raman spectrum of graphite, J. Chem. Phys. 53 (1970) 1126e1130. [30] J. Robertson, Amorphous carbon, Adv. Phys. 35 (1986) 317e374. [31] W.M. Silva, H. Ribeiro, L.M. Seara, H.D. Calado, A.S. Ferlauto, R.M. Paniago, C.F. Leite, G.G. Silva, Surface properties of oxidized and aminated multi-walled carbon nanotubes, J. Braz. Chem. Soc. 23 (2012) 1078e1086. [32] M.S. Mohamed Saheed, N.M. Mohamed, Z.A. Burhanudin, Effect of different catalyst deposition technique on aligned multiwalled carbon nanotubes grown by thermal chemical vapor deposition, J. Nanomater. 2014 (2014) 11. [33] L. Bokobza, J. Zhang, Raman spectroscopic characterization of multiwall carbon nanotubes and of composites, Express Polym. Lett. 6 (2012) 601e608.