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Improved carbon nanotube growth inside an anodic aluminum oxide template using microwave radiation
Journal of Physics and Chemistry of Solids 116 (2018) 203–208
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Improved carbon nanotube growth inside an anodic aluminum oxide template using microwave radiation Sedigheh Dadras *, Maryam Faraji Department of Physics, Alzahra University, Tehran, 1993893973, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: Anodic aluminum oxide template Carbon nanotube Ferrocene Graphite Microwave
In this study, we achieved superfast growth of carbon nanotubes (CNTs) in an anodic aluminum oxide (AAO) template by applying microwave (MW) radiation. This is a simple and direct approach for growing CNTs using a MW oven. The CNTs were synthesized using MW radiation at a frequency of 2.45 GHz and power was applied at various levels of 900, 600, and 450 W. We used graphite and ferrocene in equal portions as precursors. The optimum conditions for the growth of CNTs inside a MW oven were a time period of 5 s and power of 450 W. In order to grow uniform CNTs, an AAO template was applied with the CNTs synthesized under optimum conditions. The morphology of the synthesized CNTs was investigated by scanning electron microscopy analysis. The average diameters of the CNTs obtained without the template were 22–27 nm, whereas the diameters of the CNTs prepared inside the AAO template were about 4–6 nm.
1. Introduction Carbon nanotubes (CNTs) have unique structures and properties, such as well-defined hollow interiors, inert surface properties, and resistance to acid and base environments, which make them excellent candidates for use as supports in catalytic reactions [1–7], energy storage, and conversion media [8–11]. The special structural characteristics of CNTs greatly influence the performance of the supported particles [12–15]. Properties such as the inner diameter, wall thickness, length, crystallinity, electronic structure, size of the CNT hollow core, and electron density distinguish these materials from other species of carbonaceous materials [14,16]. In addition, CNTs have outstanding mechanical, chemical, and electronic properties, which make them among the most fascinating materials in the field of nanotechnology. The use of CNTs as fillers in composites for improving the electrical conductivity or mechanical properties requires a facile and direct approach for producing bulk quantities [6]. Extensive studies have investigated various CNT synthesis techniques, but arc discharge [17], laser ablation [18], and chemical vapor deposition [19] are the main methods employed. These widely used CNT synthesis methods have been successful despite the high production costs. These methods require feed stock gases, vacuum, high temperature chamber, inert atmosphere, and a long process time [6]. Microwaves (MW) are electromagnetic waves in the frequency range
of 0.3–300 GHz [20]. It is important to understand the MW absorption properties of raw materials. MW-assisted heating is a simple, time-saving, and low-cost method that consumes less energy than the other techniques mentioned above, where this method has been used for organic [21] and inorganic synthesis [22], the preparation of catalysts [23], and in mineral treatment processes [24]. Applying direct heating via MW radiation is an early step in CNT synthesis processes. Recently, CNTs were synthesized with MW heating for 3–5 min using conducting polymers as precursors [20]. Liu et al. [25] reported a “pop-tube” technique for CNT growth using a MW oven at room temperature for 15–30 s. Nie et al. [26] synthesized CNTs by applying radiation to ferrocene for 15–20 s with single C-fibers in a MW oven. Takagaki et al. [27] used a specially designed MW oven in order to grow CNTs, where they used complicated multi-step processes before applying MW radiation to the precursor materials. MW radiation was applied in a nitrogen gas atmosphere for 5 min. Some other studies also used as MW oven as a heat source for CNT growth but they were conducted in a vacuum or inert atmosphere and the heating duration was tens of minutes [20,27–30]. The template carbonization technique is a suitable method for synthesizing precisely controlled CNTs with the desired diameter and open caps. The template serves as a structural framework within which the carbon material can be generated by carbonization from precursors. The template carbonization method involves the carbonization of an organic gas or polymer in the nano-space of an inorganic template and releasing
* Corresponding author. E-mail address: [email protected] (S. Dadras). https://doi.org/10.1016/j.jpcs.2018.01.039 Received 29 July 2017; Received in revised form 20 January 2018; Accepted 22 January 2018 0022-3697/© 2018 Elsevier Ltd. All rights reserved.
S. Dadras, M. Faraji
Journal of Physics and Chemistry of Solids 116 (2018) 203–208
without the porous AAO template, and using ironic catalysts (ferrocene and/or iron nitrate). The novel feature of our method is the synthesis of CNTs in a MW oven with an AAO template, which facilitates the well-organized growth of CNTs in a short time period. 2. Experiments 2.1. Preparation of the porous AAO template An aluminum plate (99.5%) was degreased in acetone and rinsed in ethanol solution, before drying in the air atmosphere. In order to obtain a mirror finish, the aluminum sheet was electropolished in a mixture of perchloric acid/ethanol solution [35], where we used the two-step anodization method. In the first anodization step, a voltage of 42 V was applied to a 0.3 M oxalic acid solution at 17 C for 3 h. Subsequently, the AAO film was chemically etched in a mixture of phosphoric acid and chromic acid solution at 60 C for 2 h. The second anodization step was performed under the same conditions as the first, which resulted in the formation of a highly ordered AAO template with a pore diameter and depth of 58 nm and 1 μm, respectively. a scanning electron microscopy (SEM) image of the AAO template is shown in Fig. 1. 2.2. CNT synthesis using two methods 2.2.1. Synthesis of CNTs without a template in a MW oven Precursor materials comprising graphite rods extracted from common pencil (HB2 grade), and ferrocene (batch S6170678-Merck) were mixed in equal ratios (50:50 mg, 1:1) [20]. The mixed powder was then placed in an isolated alumina crucible and subjected to thermal MW radiation at a frequency of 2.45 GHz and various power levels (900, 600, and 450 W) for different time periods. Finally, the optimum conditions were determined. The conditions employed for the synthesis of two samples are described below. Sample1. Equal ratios of 50 mg graphite and ferrocene were mixed well. The mixture was then placed in a closed-lid crucible and irradiated with MW radiation in three steps at power levels of 900, 600, and 600 W for 10, 5, and 5 s, respectively. We determined the most suitable time and power level for the synthesis of the CNTs by trial and error. Fig. 2a shows an SEM image of the CNTs synthesized in this process. Sample 2. The same precursors were irradiated in one step at 450 W in an open-lid crucible for 5 s and the CNTs obtained in this process are shown in Fig. 2b. We found that the optimum conditions for the synthesis
Fig. 1. SEM image of the AAO template prepared in this study.
the deposited carbon from the template [31]. Two-step anodization can be employed to obtain an anodic aluminum oxide (AAO) film from aluminum metal in an acidic medium. The uniform AAO film has straight nano-size channels with tailored length and diameter [32,33]. The porous AAO template has parallel and straight channels and a highly uniform distribution of cylindrical pores arranged in a hexagonal array. Porous AAO films are ideal for templating in CNT synthesis. The other advantages of AAO films are their mechanical and thermal stability, even at a high carbonization temperature of 900 C in an inert gas atmosphere [34]. In this study, we synthesized CNTs inside a MW oven at room temperature and an air atmosphere as an inexpensive method. In order to grow uniform CNTs, a porous AAO template was applied to the optimally prepared CNTs. We used two alternative methods for synthesis with and
Fig. 2. SEM images of the synthesized CNTs in: (a) sample 1 and (b) sample 2.
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Journal of Physics and Chemistry of Solids 116 (2018) 203–208
Fig. 3. SEM images of sample 3 (a) over and (b) inside the AAO template.
Fig. 4. SEM images of sample 4 (a) over and (b) inside the AAO template.
of CNTs were power of 450 W for 5 0.2 s in an open-lid crucible.
placed in the mixed powder. Three-step MW radiation at a power of 450 W for 5 s was applied to the sample in an open-lid crucible. Fig. 4a and b shows SEM images of the CNTs obtained from sample 4 using the AAO template and iron nitrate solution. We used SEM (VEGA3 TESCAN) to investigate the morphology of the synthesized CNTs and measurement software was employed to determine the average diameters of the CNTs. X-ray diffraction (XRD) analysis of the samples was conducted using a X'Pert MPD model (Philips, Holland) X-ray diffractometer with Co-Kα radiation (λ ¼ 1.79 Å), which operated at 40 kV and 40 mA. The results were analyzed using X'Pert high score software. We employed transmission electron microscopy (TEM; Zeiss EM10C, 80 kV) with an ultrasonic device (Misonix S3000) to investigate the morphology of the synthesized CNTs. Raman analysis of the samples was performed using an Almega Thermo Nicolet dispersive Raman spectrometer in the spectral range of 100–4200 cm1 and at a second harmonic 532 nm with an Nd:YLF laser at a resolution of 4 cm1.
2.2.2. CNT synthesis using a porous AAO template and MW radiation An AAO template was applied to obtain ordered and homogeneous CNTs. Two samples were prepared from two different solutions: ferrocene dissolved in ethanol and iron nitrate dissolved in deionized (DI) water. Sample 3. First, 0.1 g of ferrocene (Batch S6170678, Merck) was dissolved in 1 cm3 of pure ethanol and applied to the AAO template, before ultrasonication at a power of 60 W for 10 min. The AAO template was taken from the solution and dried. Next, graphite and ferrocene were mixed in an equal ratio (50:50 mg) and the template was placed inside the mixed powder in the alumina crucible. We applied MW radiation at 450 W to the prepared sample for 5 s and then repeated this process three times at the same power and duration. Fig. 3a and b shows SEM images of the CNTs obtained from sample 3 with a AAO template. Fig. 3a shows images over the AAO template and Fig. 3b illustrates the CNTs inside the AAO template. Sample 4. First, 1 g of iron nitrate (Art. 3883 and Batch 8564068) was dissolved in 1 cm3 of pure DI water and applied to the AAO template, before ultrasonication at a power of 60 W for 10 min. Next, graphite and ferrocene were mixed in an equal ratio (50:50 mg) and the template was
3. Results and discussion The average diameters of the CNTs were measured as 27 nm and 22 nm in samples 1 and 2, respectively. The CNTs in sample 2 had smaller diameters and better uniformity than those in sample 1. Thus, we
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Journal of Physics and Chemistry of Solids 116 (2018) 203–208
Fig. 5. SEM images of the synthesized CNTs in: (a) sample 3 and (b) sample 4 over the AAO templates.
Fig. 6. EDX spectrum of the synthesized CNTs in (a) sample 3 and (b) sample 4.
magnifications. The average diameter of the synthesized CNTs in sample 3 was about 6 nm. Fig. 4a and b shows SEM images of sample 4 at two different magnifications. The average diameter of the CNTs in sample 4 was about 4 nm. SEM images of samples 3 and 4 are shown in Fig. 5a and b, respectively. The average diameter of the CNTs grown in sample 3 outside the template and in ferrocene solution was about 12 nm, with perfect growth of the CNTs (Fig. 5a). Fig. 5b shows the CNTs grown in sample 4 outside the template and in iron nitrate solution, with imperfect growth of the CNTs. Energy dispersive X-ray (EDX) spectroscopy was conducted for the CNTs synthesized with the template in samples 3 and 4, and the results are shown in Fig. 6a and b, respectively. EDX analysis detected the common elements of both the templates and the CNTs. The highest peak was attributed to carbon in both samples. The XRD patterns were used to determine the structural phases of the samples. The XRD spectrum obtained for the CNTs prepared under optimum conditions (sample 2) is shown in Fig. 7, where 2θ varied from 10 to 100 with steps of 0.04 . The most prominent reflection was at the diffraction profile of 2θ ¼ 30.87 , which was indexed as C (002). The sharpening of the (002) C-peak may be attributable to the combined effect of the CNT and some graphitic flakes that could not be removed. The peak at 2θ ¼ 35.24 can be attributed to Fe2C3 (JCPDS file No. 391346) and the peak at 2θ ¼ 41.63 indicates the formation of Fe2O3 (JCPDS No. 391346). Fe2C3 is also considered to be an active phase of Fe for CNT growth [25,28]. The XRD peaks at 50.9 , 52.35 , and 64.4 were assigned to the (103), (018), and (004) lattice planes of the CNTs,
Fig. 7. XRD patterns of the synthesized CNTs (sample 2).
conclude that it is possible to synthesize CNTs with smaller diameters by using MW radiation at 450 W. Fig. 3a and b shows SEM images of sample 3 at two different
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Fig. 8. TEM images of the synthesized CNTs (sample 2).
catalytic solutions were used comprising ferrocene and iron nitrate in ethanol and DI water, respectively. The obvious advantages of AAO-CNT in this MW-assisted synthesis method are the excellent uniformity of the CNTs obtained and the short process time compared with the conventional CNT synthesis methods. Acknowledgment The authors acknowledge financial support by Al Zahra University. References [1] R.M.M. Abbaslou, A. Tavasoli, A.K. Dalai, Effect of pre-treatment on physicochemical properties and stability of carbon nanotubes supported iron FischerTropsch catalysts, Appl. Catal. Gen. 335 (2009) 33–41. [2] A. Tavasoli, R.M.M. Abbaslou, M. Trepanier, et al., Fischer-Tropsch synthesis over cobalt catalyst supported on carbon nanotubes in a slurry reactor, Appl. Catal. Gen. 345 (2008) 134–142. [3] W. Chen, X.L. Pan, X.H. Bao, Tuning of redox properties of iron and iron oxides via encapsulation within carbon nanotubes, J. Am. Chem. Soc. 129 (2007) 7421–7426. [4] H.B. Zhang, X.L. Pan, J.Y. Liu, et al., Enhanced catalytic activity of sub-nanometer titania clusters confined inside double-wall carbon nanotubes, ChemSusChem 4 (2011) 975–980. [5] M.C. Bahome, L.L. Jewell, K.D. Padayachy, et al., Fe-Ru small particle bimetallic catalysts supported on carbon nanotubes for use in Fischer-Tropsch synthesis, Appl. Catal. Gen. 328 (2007) 243–251. [6] W. Ma, E.L. Kugler, J. Wright, et al., Mo-Fe catalysts supported on activated carbon for synthesis of liquid fuels by the Fischer-Tropsch process: effect of Mo addition on reducibility, activity, and hydrocarbon selectivity, Energy Fuels 20 (2006) 2299–2307. [7] P. Serp, M. Corrias, P. Kalck, Carbon nanotubes and nanofibers in catalysis, Appl. Catal. Gen. 253 (2003) 337–358. [8] C.Z. Meng, C.H. Liu, L.Z. Chen, et al., Highly flexible and all-solid state paper like polymer supercapacitors, Nano Lett. 10 (2010) 4031–4205. [9] A. Izadi-Najafabadi, S. Yasuda, K. Kobashi, et al., Extracting the full potential of single-walled carbon nanotubes as durable supercapacitor electrodes operable at 4 V with high power and energy density, Adv. Mater. 22 (2010) E235–E241. [10] P. Serp, E. Castillejos, Catalysis in carbon nanotubes, ChemCatChem 2 (2010) 41–47. [11] W. Chen, Z.L. Fan, L. Gu, et al., Enhanced capacitance of manganese oxide via confinement inside carbon nanotubes, Chem Commun. 46 (2010) 3905–3907. [12] X.L. Pan, Z.L. Fan, W. Chen, et al., Enhanced ethanol production inside carbonnanotube reactors containing catalytic particles, Nat. Mater. 6 (2007) 507–511. [13] J. Zhang, J.O. Muller, W.Q. Zheng, et al., Individual Fe-Co alloy nanoparticles on carbon nanotubes: structural and catalytic properties, Nano Lett. 8 (2008) 2738–2743. [14] S.J. Guo, X.L. Pan, H.L. Gao, et al., Probing the electronic effect of carbon nanotube in catalysis: NH3 synthesis over Ru nanoparticles, Chem. Eur J. 16 (2010) 5379–5384. [15] R.M.M. Abbaslou, J. Soltan, A.K. Dalai, Effects of nanotubes pore size on the catalytic performances of iron catalysts supported on carbon nanotubes for FischerTropsch synthesis, Appl. Catal. Gen. 379 (2010) 129–134. [16] J.P. Tessonnier, O. Ersen, G. Weinberg, et al., Selective deposition of metal nanoparticles inside or outside multiwalled carbon nanotubes, ACS Nano 3 (2009) 2081–2089. [17] C. Liu, H.M. Cheng, H.T. Cong, et al., Synthesis of macroscopically long ropes of well-aligned single-walled carbon nanotubes, Adv Mater. 12 (2000) 1190–1192. [18] J.E. Fischer, H. Dai, A. Thess, et al., Metallic resistivity in crystalline ropes of singlewall carbon nanotubes, Phys. Rev. B 55 (1997) R4921–R4924. [19] H.M. Cheng, F. Li, X. Sun, et al., Bulk morphology and diameter distribution of single-walled carbon nanotubes synthesized by catalytic decomposition of hydrocarbons, Chem. Phys. Lett. 289 (1998) 602–610. [20] R. Bajpai, H.D. Wagner, Fast growth of carbon nanotubes using a microwave oven, Carbon 82 (2015) 327–336.
Fig. 9. Raman spectroscopy image of synthesized CNTs (sample 2).
respectively (JCPDS file No. 411487). The peaks in the XRD spectra demonstrate the structural integrity of the CNTs produced by MW heating. TEM images obtained for the synthesized CNTs in sample 2 are shown in Fig. 8. TEM analysis demonstrated that the final CNTs comprised a mixture of single and multiwall CNTs. Fig. 9 shows the Raman spectroscopy results obtained for the CNTs synthesized in sample 2. Two peaks were observed in the high frequency Raman spectroscopy range and they were characteristic of CNTs. One of the peaks in the frequency range of 1500–1600 cm1 was due to the interaction between phonons and the hexagonal graphite structure (G band), while the other was related to the disorder and structural defects (D band) in the frequency range of 1350 cm1. The D band was caused by the presence of amorphous carbon and crystalline particles in the samples. The peak in the frequency range of 2690 cm1 was related to the summit of the second order. The peak intensity ratio of the G band relative to the D band indicated the purity and quality of the CNTs, i.e., IG/ID ¼ 0.63 for this sample [36]. 4. Conclusion In this study, we developed a simple and direct technique for producing CNTs in only 5–20 s with the assistance of MW oven heating. The precursor materials comprising graphite and ferrocene were mixed together and heated in a MW oven. Ferrocene was used to generate catalyst iron nanoparticles and to supply carbon atoms. The use of graphite resulted in the uniform absorption of MW radiation as well as the rapid spread of heat. We used a novel AAO template method, which we demonstrated was very useful for the preparation of CNTs with specific and controllable shape, diameter, and length. Furthermore, two
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Journal of Physics and Chemistry of Solids 116 (2018) 203–208 [30] K. Ohta, T. Nishizawa, T. Nishiguchi, R. Shimizu, Y. Hattori, S. Inoue, et al., Synthesis of carbon nanotubes by microwave heating: influence of diameter of catalytic Ni nanoparticles on diameter of CNTs, J. Mater. Chem. 2 (2014) 2773–2780. [31] T. Kyotani, Synthesis of various types of nano carbons using the template technique, Bull. Chem. Soc. Jpn. 79 (2006) 2–1337. [32] H. Masuda, F. Hasegwa, S. Ono, Self-ordering of cell arrangement of anodic porous alumina formed in sulfuric acid solution, J. Electrochem. Soc. 144 (1997) L127–L130. [33] C.R. Martin, Membrane-based synthesis of nanomaterials, Chem. Mater. 8 (1996) 1739–1746. [34] P.X. Hou, C. Liu, C. Shi, H.M. Cheng, Carbon nanotubes prepared by anodic aluminum oxide template method, Chin. Sci. Bull. 57 (2012) 2–3. [35] S. Dadras, E. Awani, R.S. Dariani, Simple fabrication of highly ordered anodic aluminum oxide (AAO) films, Int. J. Mater. Res. 106 (2015) 902–908. [36] A.A. Mamedov, N.A. Kotov, M. Prato, D.M. Guldi, J.P. Wicksted, A. Hirsch, Molecular design of strong single-wall carbon nanotube/polyelectrolyte multilayer composites, Nat. Mater. 1 (2002) 190.
[21] P. Lidstr€ om, J. Tierney, B. Wathey, J. Westman, Microwave assisted organic synthesis - a review, Tetrahedron 57 (2001) 9225–9283. [22] K.J. Rao, B. Vaidhyanathan, M. Ganguli, P.A. Ramakrishnan, Synthesis of inorganic solids using microwaves, Chem. Mater. 11 (1999) 882–895. [23] X. Zhang, D.O. Hayward, D.M.P. Mingos, Microwave dielectric heating behavior of supported MoS2 and Pt catalysts, Ind. Eng. Chem. Res. 40 (2001) 2810–2817. [24] K.E. Haque, Microwave energy for mineral treatment processes - a brief review, Int. J. Miner. Process. 57 (1999) 1–24. [25] Z. Liu, J. Wang, V. Kushvaha, S. Poyraz, H. Tippur, S. Park, et al., Poptube approach for ultrafast carbon nanotube growth, Chem Commun. 47 (2011) 9912–9914. [26] H. Nie, M. Cui, T.P. Russell, A route to rapid carbon nanotube growth, Chem Commun. 49 (2013) 5159–5161. [27] Y. Takagaki, H.D.N. Tran, K. Ohta, Development of facile synthetic methods of carbon nanotubes using a domestic microwave oven, Bull. Chem. Soc. Jpn. 83 (2010) 1100–1106. [28] E.H. Hong, K.H. Lee, S.H. Oh, C.G. Park, Synthesis of carbon nanotubes using microwave radiation, Adv. Funct. Mater. 13 (2003) 961–966. [29] U.O. Mendez, O.V. Kharissova, M. Rodriguez, Synthesis and morphology of nanostructures via microwave heating, Rev. Adv. Mater. Sci. 5 (2003) 398–402.
208
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Improved carbon nanotube growth inside an anodic aluminum oxide template using microwave radiation Sedigheh Dadras *, Maryam Faraji Department of Physics, Alzahra University, Tehran, 1993893973, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: Anodic aluminum oxide template Carbon nanotube Ferrocene Graphite Microwave
In this study, we achieved superfast growth of carbon nanotubes (CNTs) in an anodic aluminum oxide (AAO) template by applying microwave (MW) radiation. This is a simple and direct approach for growing CNTs using a MW oven. The CNTs were synthesized using MW radiation at a frequency of 2.45 GHz and power was applied at various levels of 900, 600, and 450 W. We used graphite and ferrocene in equal portions as precursors. The optimum conditions for the growth of CNTs inside a MW oven were a time period of 5 s and power of 450 W. In order to grow uniform CNTs, an AAO template was applied with the CNTs synthesized under optimum conditions. The morphology of the synthesized CNTs was investigated by scanning electron microscopy analysis. The average diameters of the CNTs obtained without the template were 22–27 nm, whereas the diameters of the CNTs prepared inside the AAO template were about 4–6 nm.
1. Introduction Carbon nanotubes (CNTs) have unique structures and properties, such as well-defined hollow interiors, inert surface properties, and resistance to acid and base environments, which make them excellent candidates for use as supports in catalytic reactions [1–7], energy storage, and conversion media [8–11]. The special structural characteristics of CNTs greatly influence the performance of the supported particles [12–15]. Properties such as the inner diameter, wall thickness, length, crystallinity, electronic structure, size of the CNT hollow core, and electron density distinguish these materials from other species of carbonaceous materials [14,16]. In addition, CNTs have outstanding mechanical, chemical, and electronic properties, which make them among the most fascinating materials in the field of nanotechnology. The use of CNTs as fillers in composites for improving the electrical conductivity or mechanical properties requires a facile and direct approach for producing bulk quantities [6]. Extensive studies have investigated various CNT synthesis techniques, but arc discharge [17], laser ablation [18], and chemical vapor deposition [19] are the main methods employed. These widely used CNT synthesis methods have been successful despite the high production costs. These methods require feed stock gases, vacuum, high temperature chamber, inert atmosphere, and a long process time [6]. Microwaves (MW) are electromagnetic waves in the frequency range
of 0.3–300 GHz [20]. It is important to understand the MW absorption properties of raw materials. MW-assisted heating is a simple, time-saving, and low-cost method that consumes less energy than the other techniques mentioned above, where this method has been used for organic [21] and inorganic synthesis [22], the preparation of catalysts [23], and in mineral treatment processes [24]. Applying direct heating via MW radiation is an early step in CNT synthesis processes. Recently, CNTs were synthesized with MW heating for 3–5 min using conducting polymers as precursors [20]. Liu et al. [25] reported a “pop-tube” technique for CNT growth using a MW oven at room temperature for 15–30 s. Nie et al. [26] synthesized CNTs by applying radiation to ferrocene for 15–20 s with single C-fibers in a MW oven. Takagaki et al. [27] used a specially designed MW oven in order to grow CNTs, where they used complicated multi-step processes before applying MW radiation to the precursor materials. MW radiation was applied in a nitrogen gas atmosphere for 5 min. Some other studies also used as MW oven as a heat source for CNT growth but they were conducted in a vacuum or inert atmosphere and the heating duration was tens of minutes [20,27–30]. The template carbonization technique is a suitable method for synthesizing precisely controlled CNTs with the desired diameter and open caps. The template serves as a structural framework within which the carbon material can be generated by carbonization from precursors. The template carbonization method involves the carbonization of an organic gas or polymer in the nano-space of an inorganic template and releasing
* Corresponding author. E-mail address: [email protected] (S. Dadras). https://doi.org/10.1016/j.jpcs.2018.01.039 Received 29 July 2017; Received in revised form 20 January 2018; Accepted 22 January 2018 0022-3697/© 2018 Elsevier Ltd. All rights reserved.
S. Dadras, M. Faraji
Journal of Physics and Chemistry of Solids 116 (2018) 203–208
without the porous AAO template, and using ironic catalysts (ferrocene and/or iron nitrate). The novel feature of our method is the synthesis of CNTs in a MW oven with an AAO template, which facilitates the well-organized growth of CNTs in a short time period. 2. Experiments 2.1. Preparation of the porous AAO template An aluminum plate (99.5%) was degreased in acetone and rinsed in ethanol solution, before drying in the air atmosphere. In order to obtain a mirror finish, the aluminum sheet was electropolished in a mixture of perchloric acid/ethanol solution [35], where we used the two-step anodization method. In the first anodization step, a voltage of 42 V was applied to a 0.3 M oxalic acid solution at 17 C for 3 h. Subsequently, the AAO film was chemically etched in a mixture of phosphoric acid and chromic acid solution at 60 C for 2 h. The second anodization step was performed under the same conditions as the first, which resulted in the formation of a highly ordered AAO template with a pore diameter and depth of 58 nm and 1 μm, respectively. a scanning electron microscopy (SEM) image of the AAO template is shown in Fig. 1. 2.2. CNT synthesis using two methods 2.2.1. Synthesis of CNTs without a template in a MW oven Precursor materials comprising graphite rods extracted from common pencil (HB2 grade), and ferrocene (batch S6170678-Merck) were mixed in equal ratios (50:50 mg, 1:1) [20]. The mixed powder was then placed in an isolated alumina crucible and subjected to thermal MW radiation at a frequency of 2.45 GHz and various power levels (900, 600, and 450 W) for different time periods. Finally, the optimum conditions were determined. The conditions employed for the synthesis of two samples are described below. Sample1. Equal ratios of 50 mg graphite and ferrocene were mixed well. The mixture was then placed in a closed-lid crucible and irradiated with MW radiation in three steps at power levels of 900, 600, and 600 W for 10, 5, and 5 s, respectively. We determined the most suitable time and power level for the synthesis of the CNTs by trial and error. Fig. 2a shows an SEM image of the CNTs synthesized in this process. Sample 2. The same precursors were irradiated in one step at 450 W in an open-lid crucible for 5 s and the CNTs obtained in this process are shown in Fig. 2b. We found that the optimum conditions for the synthesis
Fig. 1. SEM image of the AAO template prepared in this study.
the deposited carbon from the template [31]. Two-step anodization can be employed to obtain an anodic aluminum oxide (AAO) film from aluminum metal in an acidic medium. The uniform AAO film has straight nano-size channels with tailored length and diameter [32,33]. The porous AAO template has parallel and straight channels and a highly uniform distribution of cylindrical pores arranged in a hexagonal array. Porous AAO films are ideal for templating in CNT synthesis. The other advantages of AAO films are their mechanical and thermal stability, even at a high carbonization temperature of 900 C in an inert gas atmosphere [34]. In this study, we synthesized CNTs inside a MW oven at room temperature and an air atmosphere as an inexpensive method. In order to grow uniform CNTs, a porous AAO template was applied to the optimally prepared CNTs. We used two alternative methods for synthesis with and
Fig. 2. SEM images of the synthesized CNTs in: (a) sample 1 and (b) sample 2.
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S. Dadras, M. Faraji
Journal of Physics and Chemistry of Solids 116 (2018) 203–208
Fig. 3. SEM images of sample 3 (a) over and (b) inside the AAO template.
Fig. 4. SEM images of sample 4 (a) over and (b) inside the AAO template.
of CNTs were power of 450 W for 5 0.2 s in an open-lid crucible.
placed in the mixed powder. Three-step MW radiation at a power of 450 W for 5 s was applied to the sample in an open-lid crucible. Fig. 4a and b shows SEM images of the CNTs obtained from sample 4 using the AAO template and iron nitrate solution. We used SEM (VEGA3 TESCAN) to investigate the morphology of the synthesized CNTs and measurement software was employed to determine the average diameters of the CNTs. X-ray diffraction (XRD) analysis of the samples was conducted using a X'Pert MPD model (Philips, Holland) X-ray diffractometer with Co-Kα radiation (λ ¼ 1.79 Å), which operated at 40 kV and 40 mA. The results were analyzed using X'Pert high score software. We employed transmission electron microscopy (TEM; Zeiss EM10C, 80 kV) with an ultrasonic device (Misonix S3000) to investigate the morphology of the synthesized CNTs. Raman analysis of the samples was performed using an Almega Thermo Nicolet dispersive Raman spectrometer in the spectral range of 100–4200 cm1 and at a second harmonic 532 nm with an Nd:YLF laser at a resolution of 4 cm1.
2.2.2. CNT synthesis using a porous AAO template and MW radiation An AAO template was applied to obtain ordered and homogeneous CNTs. Two samples were prepared from two different solutions: ferrocene dissolved in ethanol and iron nitrate dissolved in deionized (DI) water. Sample 3. First, 0.1 g of ferrocene (Batch S6170678, Merck) was dissolved in 1 cm3 of pure ethanol and applied to the AAO template, before ultrasonication at a power of 60 W for 10 min. The AAO template was taken from the solution and dried. Next, graphite and ferrocene were mixed in an equal ratio (50:50 mg) and the template was placed inside the mixed powder in the alumina crucible. We applied MW radiation at 450 W to the prepared sample for 5 s and then repeated this process three times at the same power and duration. Fig. 3a and b shows SEM images of the CNTs obtained from sample 3 with a AAO template. Fig. 3a shows images over the AAO template and Fig. 3b illustrates the CNTs inside the AAO template. Sample 4. First, 1 g of iron nitrate (Art. 3883 and Batch 8564068) was dissolved in 1 cm3 of pure DI water and applied to the AAO template, before ultrasonication at a power of 60 W for 10 min. Next, graphite and ferrocene were mixed in an equal ratio (50:50 mg) and the template was
3. Results and discussion The average diameters of the CNTs were measured as 27 nm and 22 nm in samples 1 and 2, respectively. The CNTs in sample 2 had smaller diameters and better uniformity than those in sample 1. Thus, we
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Fig. 5. SEM images of the synthesized CNTs in: (a) sample 3 and (b) sample 4 over the AAO templates.
Fig. 6. EDX spectrum of the synthesized CNTs in (a) sample 3 and (b) sample 4.
magnifications. The average diameter of the synthesized CNTs in sample 3 was about 6 nm. Fig. 4a and b shows SEM images of sample 4 at two different magnifications. The average diameter of the CNTs in sample 4 was about 4 nm. SEM images of samples 3 and 4 are shown in Fig. 5a and b, respectively. The average diameter of the CNTs grown in sample 3 outside the template and in ferrocene solution was about 12 nm, with perfect growth of the CNTs (Fig. 5a). Fig. 5b shows the CNTs grown in sample 4 outside the template and in iron nitrate solution, with imperfect growth of the CNTs. Energy dispersive X-ray (EDX) spectroscopy was conducted for the CNTs synthesized with the template in samples 3 and 4, and the results are shown in Fig. 6a and b, respectively. EDX analysis detected the common elements of both the templates and the CNTs. The highest peak was attributed to carbon in both samples. The XRD patterns were used to determine the structural phases of the samples. The XRD spectrum obtained for the CNTs prepared under optimum conditions (sample 2) is shown in Fig. 7, where 2θ varied from 10 to 100 with steps of 0.04 . The most prominent reflection was at the diffraction profile of 2θ ¼ 30.87 , which was indexed as C (002). The sharpening of the (002) C-peak may be attributable to the combined effect of the CNT and some graphitic flakes that could not be removed. The peak at 2θ ¼ 35.24 can be attributed to Fe2C3 (JCPDS file No. 391346) and the peak at 2θ ¼ 41.63 indicates the formation of Fe2O3 (JCPDS No. 391346). Fe2C3 is also considered to be an active phase of Fe for CNT growth [25,28]. The XRD peaks at 50.9 , 52.35 , and 64.4 were assigned to the (103), (018), and (004) lattice planes of the CNTs,
Fig. 7. XRD patterns of the synthesized CNTs (sample 2).
conclude that it is possible to synthesize CNTs with smaller diameters by using MW radiation at 450 W. Fig. 3a and b shows SEM images of sample 3 at two different
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Fig. 8. TEM images of the synthesized CNTs (sample 2).
catalytic solutions were used comprising ferrocene and iron nitrate in ethanol and DI water, respectively. The obvious advantages of AAO-CNT in this MW-assisted synthesis method are the excellent uniformity of the CNTs obtained and the short process time compared with the conventional CNT synthesis methods. Acknowledgment The authors acknowledge financial support by Al Zahra University. References [1] R.M.M. Abbaslou, A. Tavasoli, A.K. Dalai, Effect of pre-treatment on physicochemical properties and stability of carbon nanotubes supported iron FischerTropsch catalysts, Appl. Catal. Gen. 335 (2009) 33–41. [2] A. Tavasoli, R.M.M. Abbaslou, M. Trepanier, et al., Fischer-Tropsch synthesis over cobalt catalyst supported on carbon nanotubes in a slurry reactor, Appl. Catal. Gen. 345 (2008) 134–142. [3] W. Chen, X.L. Pan, X.H. Bao, Tuning of redox properties of iron and iron oxides via encapsulation within carbon nanotubes, J. Am. Chem. Soc. 129 (2007) 7421–7426. [4] H.B. Zhang, X.L. Pan, J.Y. Liu, et al., Enhanced catalytic activity of sub-nanometer titania clusters confined inside double-wall carbon nanotubes, ChemSusChem 4 (2011) 975–980. [5] M.C. Bahome, L.L. Jewell, K.D. Padayachy, et al., Fe-Ru small particle bimetallic catalysts supported on carbon nanotubes for use in Fischer-Tropsch synthesis, Appl. Catal. Gen. 328 (2007) 243–251. [6] W. Ma, E.L. Kugler, J. Wright, et al., Mo-Fe catalysts supported on activated carbon for synthesis of liquid fuels by the Fischer-Tropsch process: effect of Mo addition on reducibility, activity, and hydrocarbon selectivity, Energy Fuels 20 (2006) 2299–2307. [7] P. Serp, M. Corrias, P. Kalck, Carbon nanotubes and nanofibers in catalysis, Appl. Catal. Gen. 253 (2003) 337–358. [8] C.Z. Meng, C.H. Liu, L.Z. Chen, et al., Highly flexible and all-solid state paper like polymer supercapacitors, Nano Lett. 10 (2010) 4031–4205. [9] A. Izadi-Najafabadi, S. Yasuda, K. Kobashi, et al., Extracting the full potential of single-walled carbon nanotubes as durable supercapacitor electrodes operable at 4 V with high power and energy density, Adv. Mater. 22 (2010) E235–E241. [10] P. Serp, E. Castillejos, Catalysis in carbon nanotubes, ChemCatChem 2 (2010) 41–47. [11] W. Chen, Z.L. Fan, L. Gu, et al., Enhanced capacitance of manganese oxide via confinement inside carbon nanotubes, Chem Commun. 46 (2010) 3905–3907. [12] X.L. Pan, Z.L. Fan, W. Chen, et al., Enhanced ethanol production inside carbonnanotube reactors containing catalytic particles, Nat. Mater. 6 (2007) 507–511. [13] J. Zhang, J.O. Muller, W.Q. Zheng, et al., Individual Fe-Co alloy nanoparticles on carbon nanotubes: structural and catalytic properties, Nano Lett. 8 (2008) 2738–2743. [14] S.J. Guo, X.L. Pan, H.L. Gao, et al., Probing the electronic effect of carbon nanotube in catalysis: NH3 synthesis over Ru nanoparticles, Chem. Eur J. 16 (2010) 5379–5384. [15] R.M.M. Abbaslou, J. Soltan, A.K. Dalai, Effects of nanotubes pore size on the catalytic performances of iron catalysts supported on carbon nanotubes for FischerTropsch synthesis, Appl. Catal. Gen. 379 (2010) 129–134. [16] J.P. Tessonnier, O. Ersen, G. Weinberg, et al., Selective deposition of metal nanoparticles inside or outside multiwalled carbon nanotubes, ACS Nano 3 (2009) 2081–2089. [17] C. Liu, H.M. Cheng, H.T. Cong, et al., Synthesis of macroscopically long ropes of well-aligned single-walled carbon nanotubes, Adv Mater. 12 (2000) 1190–1192. [18] J.E. Fischer, H. Dai, A. Thess, et al., Metallic resistivity in crystalline ropes of singlewall carbon nanotubes, Phys. Rev. B 55 (1997) R4921–R4924. [19] H.M. Cheng, F. Li, X. Sun, et al., Bulk morphology and diameter distribution of single-walled carbon nanotubes synthesized by catalytic decomposition of hydrocarbons, Chem. Phys. Lett. 289 (1998) 602–610. [20] R. Bajpai, H.D. Wagner, Fast growth of carbon nanotubes using a microwave oven, Carbon 82 (2015) 327–336.
Fig. 9. Raman spectroscopy image of synthesized CNTs (sample 2).
respectively (JCPDS file No. 411487). The peaks in the XRD spectra demonstrate the structural integrity of the CNTs produced by MW heating. TEM images obtained for the synthesized CNTs in sample 2 are shown in Fig. 8. TEM analysis demonstrated that the final CNTs comprised a mixture of single and multiwall CNTs. Fig. 9 shows the Raman spectroscopy results obtained for the CNTs synthesized in sample 2. Two peaks were observed in the high frequency Raman spectroscopy range and they were characteristic of CNTs. One of the peaks in the frequency range of 1500–1600 cm1 was due to the interaction between phonons and the hexagonal graphite structure (G band), while the other was related to the disorder and structural defects (D band) in the frequency range of 1350 cm1. The D band was caused by the presence of amorphous carbon and crystalline particles in the samples. The peak in the frequency range of 2690 cm1 was related to the summit of the second order. The peak intensity ratio of the G band relative to the D band indicated the purity and quality of the CNTs, i.e., IG/ID ¼ 0.63 for this sample [36]. 4. Conclusion In this study, we developed a simple and direct technique for producing CNTs in only 5–20 s with the assistance of MW oven heating. The precursor materials comprising graphite and ferrocene were mixed together and heated in a MW oven. Ferrocene was used to generate catalyst iron nanoparticles and to supply carbon atoms. The use of graphite resulted in the uniform absorption of MW radiation as well as the rapid spread of heat. We used a novel AAO template method, which we demonstrated was very useful for the preparation of CNTs with specific and controllable shape, diameter, and length. Furthermore, two
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