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Synthesis of carbon nanocoils on surface morphology changed silicon substrates
Materials Letters 60 (2006) 2073 – 2075 www.elsevier.com/locate/matlet
Synthesis of carbon nanocoils on surface morphology changed silicon substrates Z.Y. Huang, X. Chen, J.R. Huang, M.Q. Li, J.H. Liu ⁎ Center for Biomimetic Sensing and Control Research, Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei, Anhui 230031, PR China Received 16 May 2005; accepted 22 December 2005 Available online 17 January 2006
Abstract By the controlled corrosion of HF, silicon substrates appeared to be in a state of concavo-convex morphology. Carbon nanocoils were synthesized on these silicon substrates by the pyrolysis of acetylene with catalytic thermal chemical vapor deposition (CVD). Scanning electron microscopy and transmission electron microscopy were carried out to observe the regular morphology of carbon nanocoils, while Raman spectra was used to characterize the graphitization degree. The growing process of the nanocoils was regarded as the anisotropic activity of catalyst particles resulting from the different thicknesses of FeO layer. © 2005 Elsevier B.V. All rights reserved. Keywords: Carbon nanocoils; Chemical vapor deposition; Silicon substrate
1. Introduction Since the discovery of carbon nanotubes (CNTs) by Iijima [1], great interest has been focused on the physical and electric properties of these novel materials. Besides straight carbon nanotubes, carbon nanocoils (coiled carbon nanofibers and coiled carbon nanotubes) were also predicated and observed experimentally [2]. Because of the intrinsical structure, their properties [3–5] and potential applications as mechanical components such as resonating elements [6,7] and nanosprings are widely researched. Several methods are used for the synthesis of carbon nanocoils. As for carbon nanocoils, catalytic chemical vapor deposition (CCVD) is more effective than other methods. Generally, nanotubes prepared by the CCVD method are in straight or randomly curled morphologies. So far either specific process conditions [8] or a special catalyst [9–12] is available for the synthesis of carbon nanocoils. Little research has been focused on changing the surface morphology of substrate. Bai [13] produced pure coiled carbon structures on the porous aluminum surface of oxide films. In the present
⁎ Corresponding author. Tel.: +86 551 5591 142; fax: +86 551 5592 420. E-mail address: [email protected] (J.H. Liu). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.12.103
work, silicon substrates were corroded by HF firstly, which showed concavo-convex surface morphology. Carbon nanocoils were then synthesized on these silicon substrates by the pyrolysis of acetylene with catalytic thermal chemical vapor deposition. By changing the surface morphology of silicon substrates, regular carbon nanocoils can be obtained while multiwall carbon nanotubes (MWCNTs) were obtained for the non-corroded Si substrate. Scanning probe microscope (AutoProbe CP, Veeco, America) was used for the observation of the surface of corroded silicon substrate. Field emission scanning electron microscopy (FE-SEM, FEI Sirion 200 FEG, America)
Fig. 1. FE-SEM micrographs for the surface morphology of (a) non-corroded and (b) corroded silicon substrate.
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Fig. 2. Scanning probe microscopy image of the corroded silicon substrate.
and transmission electron microscopy (TEM, JEM-200CX, Japan) were conducted to observe the regular morphology of coiled carbon nanotubes. The graphitization degree of the MWCNTs and carbon nanocoils was characterized by Raman spectra apparatus (RAMALOG-6, America). 2. Experimental section Silicon (100) substrates were immersed into 15 wt.% HF for 30 min. A 1.2 g quantity of Fe(NO3)3·9H2O was dissolved in 50 ml ethanol as catalyst, then spin-coated on the surface of the corroded silicon substrates. The spin-coated substrates were positioned in the center of the CVD system, and first heated to 580 °C in a N2 flow (200 sccm), then a H2 flow (20 sccm) was introduced to reduce the catalyst for another 30 min. After the reduction of catalyst, the H2 flow was closed and the temperature was increased to 700 °C with the N2 still on, and C2H2 pyrolyzed under the temperature for 20 min with the flow rate 20 sccm, then slowly cooled down. 3. Results and discussion After the corrosion of HF, the silicon substrates present to be in concavo-convex morphology. Fig. 1(a) and (b) shows FE-SEM micrographs of surface morphology of non-corroded and corroded silicon substrates, respectively. Scanning probe microscopy image of the corroded silicon substrates was shown in Fig. 2. For comparison, the corroded and non-corroded silicon substrates were both used for the synthesis of carbon nanocoils and two different kinds of carbon structure were obtained. Fig. 3 shows the FE-SEM micrographs of (a)
Fig. 4. TEM micrographs of (a) straight carbon nanotubes and (b) carbon nanocoils.
MWCNTs grown on the non-corroded silicon substrate and (b) carbon nanocoils grown on corroded silicon substrate. It can be seen that the MWCNTs grown on the non-corroded flat silicon substrate are straight and with small diameter (those measured CNT diameters are 14 and 17 nm), while the carbon nanocoils grown on the corroded concavoconvex silicon substrate are regularly coiled with measured coil diameter about 480 nm and length about 21 μm. Fig. 4 shows TEM micrograph of the MWCNT and carbon nanocoil, which indicates that the MWCNT is hollow and the carbon nanocoil is also hollow though the inner diameter is small. Whether the nanocoils are a kind of carbon nanotubes need further investigation of high resolution TEM (HRTEM). The TEM diffraction patterns shown in the insets demonstrate that they are amorphous. As shown in Fig. 5, the first-order modes of the Raman spectra of carbon nanocoils show two sharp peaks at 1327.9 cm– 1(D line) and 1603.3 cm– 1(G line), which are alike to the peaks of the MWCNTs. The relative intensity of the D line to G line also does not make any difference between the two structures. Currently the growth of carbon nanocoils or nanotubes is well accepted as the result of anisotropic properties of catalyst particles for carbon deposition [14,15]. From a microscopic point of view, the
Fig. 3. FE-SEM micrographs of (a) straight MWCNTs grown on the non-corroded and (b) carbon nanocoils grown on the corroded silicon substrate.
Z.Y. Huang et al. / Materials Letters 60 (2006) 2073–2075
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through the pyrolysis of acetylene by CVD method. The morphology changed silicon substrates by the corrosion of HF play an important role in the formation of carbon nanocoils. The growing process of the nanocoils was regarded as the anisotropic activity of catalyst particles resulting from the different thicknesses of FeO layer. Acknowledgements
Fig. 5. Raman spectra of the carbon nanocoils and multiwall carbon nanotubes.
structural models for regularly coiled carbon nanotubes are based on the regular incorporation of pentagon–heptagon (P–H) pairs at regular distances in the hexagonal bond network on the wall of a straight carbon nanotube [16]. In this work, using HF etched silicon plates as substrates and Fe as catalyst, regular carbon nanocoils can be reliably obtained through the pyrolysis of acetylene by CVD method. From the results of the experiment, we can draw the conclusion that morphology changed silicon substrates by the corrosion of HF play an important role in the formation of carbon nanocoils. The growth process of the carbon nanocoils underwent as follows: Firstly, the spin-coated Fe catalyst solution was deposited in the concave of the etched silicon substrates prior to the convex, thus a layer of FeO with thicknesses varying in concave and convex of the etched silicon substrate was formed. After reduction the catalyst particles present anisotropy activity at edge or planes of them for the difference in the thicknesses of FeO layer, which has also been found for the CoO layer by Hernadi et al. [17]. In the following process of acetylene pyrolysis, carbon deposited anisotropically on the edge or planes of catalyst particles and P–H pairs was incorporated in the hexagonal bond network at an atomistic scale as bends and coils were formed from a macroscopic point of view. As for the growth process of straight CNTs, Fe solution was deposited on the flat surface of the silicon substrates, no formation of FeO nuclear in selective position was presented, it simply formed on the surface of silicon substrates continuously with the identical thickness and the catalyst particles take effect symmetrically, which result in the straight growth of multiwall carbon nanotubes.
4. Conclusion Using HF corroded silicon plates as substrates and Fe as catalyst, regular carbon nanocoils can be reliably obtained
This project is supported by the National High Technology Research and Development Program of China (863 Program No. 2004AA302030) and National Natural Science Foundation of China (No. 60574094). The testing and analysis are supported by the United Foundation for Testing and Analysis in Hefei, Chinese Academy of Sciences. References [1] S. Iijima, Nature 354 (1991) 56. [2] V. Ivanov, J.B. Nagy, P. Lambin, A. Lucas, X.B. Zhang, X.F. Zhang, D. Bernaerts, G. Vantendeloo, S. Amelinckx, J. Vanlanduyt, Chemical Physics Letters 223 (1994) 329. [3] A. Volodin, M. Ahlskog, E. Seynaeve, C. Van Haesendonck, A. Fonseca, J. B. Nagy, Physical Review Letters 84 (2000) 3342. [4] Xinqi Chen, Sulin Zhang, Dmitriy A. Dikin, Weiqiang Ding, Rodney S. Ruo, Nano Letters 3 (2003) 1299. [5] K. Akagi, R. Tamura, M. Tsukada, S. Itoh, S. Ihara, Physical Review Letters 74 (1995) 2307. [6] D.W. Carr, S. Evoy, L. Sekaric, H.G. Craighead, J.M. Parpia, Applied Physics Letters 75 (1999) 920. [7] A. Volodin, D. Buntinx, M. Ahlskog, A. Fonseca, J.B. Nagy, C. Van Haesendonck, Nano Letters 4 (2004) 1775. [8] M. Lu, H.L. Li, K.T. Lau, Journal of Physical Chemistry B 108 (2004) 6186–6192. [9] Y.K. Wen, Z.M. Shen, Carbon 39 (2001) 2369. [10] J. Xie, K. Mkhopadyay, J. Yadev, V.K. Varadan, Smart Materials and Structures 12 (2003) 744. [11] H.Q. Hou, Z. Jun, F. Weller, A. Greiner, Chemistry of Materials 15 (2003) 3170. [12] L.J. Pan, Z. Mei, N. Yoshikazu, Journal of Applied Physics 91 (2002) 10058–10061. [13] J.B. Bai, Materials Letters 57 (2003) 2629. [14] S. Motojima, I. Hasegawa, S. Kagiya, M. Momiyama, M. Kawaguchi, H. Iwanaga, Applied Physics Letters 62 (1993) 2322. [15] X.Q. Chen, S.M. Yang, S. Motojima, Materials Letters 57 (2002) 48. [16] B.I. Dunlap, Physical Review B 50 (1994) 8134. [17] K. Hernadi, L. Thien-Nga, L. Forro, Journal of Physical Chemistry B 105 (2001) 12464.
Synthesis of carbon nanocoils on surface morphology changed silicon substrates Z.Y. Huang, X. Chen, J.R. Huang, M.Q. Li, J.H. Liu ⁎ Center for Biomimetic Sensing and Control Research, Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei, Anhui 230031, PR China Received 16 May 2005; accepted 22 December 2005 Available online 17 January 2006
Abstract By the controlled corrosion of HF, silicon substrates appeared to be in a state of concavo-convex morphology. Carbon nanocoils were synthesized on these silicon substrates by the pyrolysis of acetylene with catalytic thermal chemical vapor deposition (CVD). Scanning electron microscopy and transmission electron microscopy were carried out to observe the regular morphology of carbon nanocoils, while Raman spectra was used to characterize the graphitization degree. The growing process of the nanocoils was regarded as the anisotropic activity of catalyst particles resulting from the different thicknesses of FeO layer. © 2005 Elsevier B.V. All rights reserved. Keywords: Carbon nanocoils; Chemical vapor deposition; Silicon substrate
1. Introduction Since the discovery of carbon nanotubes (CNTs) by Iijima [1], great interest has been focused on the physical and electric properties of these novel materials. Besides straight carbon nanotubes, carbon nanocoils (coiled carbon nanofibers and coiled carbon nanotubes) were also predicated and observed experimentally [2]. Because of the intrinsical structure, their properties [3–5] and potential applications as mechanical components such as resonating elements [6,7] and nanosprings are widely researched. Several methods are used for the synthesis of carbon nanocoils. As for carbon nanocoils, catalytic chemical vapor deposition (CCVD) is more effective than other methods. Generally, nanotubes prepared by the CCVD method are in straight or randomly curled morphologies. So far either specific process conditions [8] or a special catalyst [9–12] is available for the synthesis of carbon nanocoils. Little research has been focused on changing the surface morphology of substrate. Bai [13] produced pure coiled carbon structures on the porous aluminum surface of oxide films. In the present
⁎ Corresponding author. Tel.: +86 551 5591 142; fax: +86 551 5592 420. E-mail address: [email protected] (J.H. Liu). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.12.103
work, silicon substrates were corroded by HF firstly, which showed concavo-convex surface morphology. Carbon nanocoils were then synthesized on these silicon substrates by the pyrolysis of acetylene with catalytic thermal chemical vapor deposition. By changing the surface morphology of silicon substrates, regular carbon nanocoils can be obtained while multiwall carbon nanotubes (MWCNTs) were obtained for the non-corroded Si substrate. Scanning probe microscope (AutoProbe CP, Veeco, America) was used for the observation of the surface of corroded silicon substrate. Field emission scanning electron microscopy (FE-SEM, FEI Sirion 200 FEG, America)
Fig. 1. FE-SEM micrographs for the surface morphology of (a) non-corroded and (b) corroded silicon substrate.
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Fig. 2. Scanning probe microscopy image of the corroded silicon substrate.
and transmission electron microscopy (TEM, JEM-200CX, Japan) were conducted to observe the regular morphology of coiled carbon nanotubes. The graphitization degree of the MWCNTs and carbon nanocoils was characterized by Raman spectra apparatus (RAMALOG-6, America). 2. Experimental section Silicon (100) substrates were immersed into 15 wt.% HF for 30 min. A 1.2 g quantity of Fe(NO3)3·9H2O was dissolved in 50 ml ethanol as catalyst, then spin-coated on the surface of the corroded silicon substrates. The spin-coated substrates were positioned in the center of the CVD system, and first heated to 580 °C in a N2 flow (200 sccm), then a H2 flow (20 sccm) was introduced to reduce the catalyst for another 30 min. After the reduction of catalyst, the H2 flow was closed and the temperature was increased to 700 °C with the N2 still on, and C2H2 pyrolyzed under the temperature for 20 min with the flow rate 20 sccm, then slowly cooled down. 3. Results and discussion After the corrosion of HF, the silicon substrates present to be in concavo-convex morphology. Fig. 1(a) and (b) shows FE-SEM micrographs of surface morphology of non-corroded and corroded silicon substrates, respectively. Scanning probe microscopy image of the corroded silicon substrates was shown in Fig. 2. For comparison, the corroded and non-corroded silicon substrates were both used for the synthesis of carbon nanocoils and two different kinds of carbon structure were obtained. Fig. 3 shows the FE-SEM micrographs of (a)
Fig. 4. TEM micrographs of (a) straight carbon nanotubes and (b) carbon nanocoils.
MWCNTs grown on the non-corroded silicon substrate and (b) carbon nanocoils grown on corroded silicon substrate. It can be seen that the MWCNTs grown on the non-corroded flat silicon substrate are straight and with small diameter (those measured CNT diameters are 14 and 17 nm), while the carbon nanocoils grown on the corroded concavoconvex silicon substrate are regularly coiled with measured coil diameter about 480 nm and length about 21 μm. Fig. 4 shows TEM micrograph of the MWCNT and carbon nanocoil, which indicates that the MWCNT is hollow and the carbon nanocoil is also hollow though the inner diameter is small. Whether the nanocoils are a kind of carbon nanotubes need further investigation of high resolution TEM (HRTEM). The TEM diffraction patterns shown in the insets demonstrate that they are amorphous. As shown in Fig. 5, the first-order modes of the Raman spectra of carbon nanocoils show two sharp peaks at 1327.9 cm– 1(D line) and 1603.3 cm– 1(G line), which are alike to the peaks of the MWCNTs. The relative intensity of the D line to G line also does not make any difference between the two structures. Currently the growth of carbon nanocoils or nanotubes is well accepted as the result of anisotropic properties of catalyst particles for carbon deposition [14,15]. From a microscopic point of view, the
Fig. 3. FE-SEM micrographs of (a) straight MWCNTs grown on the non-corroded and (b) carbon nanocoils grown on the corroded silicon substrate.
Z.Y. Huang et al. / Materials Letters 60 (2006) 2073–2075
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through the pyrolysis of acetylene by CVD method. The morphology changed silicon substrates by the corrosion of HF play an important role in the formation of carbon nanocoils. The growing process of the nanocoils was regarded as the anisotropic activity of catalyst particles resulting from the different thicknesses of FeO layer. Acknowledgements
Fig. 5. Raman spectra of the carbon nanocoils and multiwall carbon nanotubes.
structural models for regularly coiled carbon nanotubes are based on the regular incorporation of pentagon–heptagon (P–H) pairs at regular distances in the hexagonal bond network on the wall of a straight carbon nanotube [16]. In this work, using HF etched silicon plates as substrates and Fe as catalyst, regular carbon nanocoils can be reliably obtained through the pyrolysis of acetylene by CVD method. From the results of the experiment, we can draw the conclusion that morphology changed silicon substrates by the corrosion of HF play an important role in the formation of carbon nanocoils. The growth process of the carbon nanocoils underwent as follows: Firstly, the spin-coated Fe catalyst solution was deposited in the concave of the etched silicon substrates prior to the convex, thus a layer of FeO with thicknesses varying in concave and convex of the etched silicon substrate was formed. After reduction the catalyst particles present anisotropy activity at edge or planes of them for the difference in the thicknesses of FeO layer, which has also been found for the CoO layer by Hernadi et al. [17]. In the following process of acetylene pyrolysis, carbon deposited anisotropically on the edge or planes of catalyst particles and P–H pairs was incorporated in the hexagonal bond network at an atomistic scale as bends and coils were formed from a macroscopic point of view. As for the growth process of straight CNTs, Fe solution was deposited on the flat surface of the silicon substrates, no formation of FeO nuclear in selective position was presented, it simply formed on the surface of silicon substrates continuously with the identical thickness and the catalyst particles take effect symmetrically, which result in the straight growth of multiwall carbon nanotubes.
4. Conclusion Using HF corroded silicon plates as substrates and Fe as catalyst, regular carbon nanocoils can be reliably obtained
This project is supported by the National High Technology Research and Development Program of China (863 Program No. 2004AA302030) and National Natural Science Foundation of China (No. 60574094). The testing and analysis are supported by the United Foundation for Testing and Analysis in Hefei, Chinese Academy of Sciences. References [1] S. Iijima, Nature 354 (1991) 56. [2] V. Ivanov, J.B. Nagy, P. Lambin, A. Lucas, X.B. Zhang, X.F. Zhang, D. Bernaerts, G. Vantendeloo, S. Amelinckx, J. Vanlanduyt, Chemical Physics Letters 223 (1994) 329. [3] A. Volodin, M. Ahlskog, E. Seynaeve, C. Van Haesendonck, A. Fonseca, J. B. Nagy, Physical Review Letters 84 (2000) 3342. [4] Xinqi Chen, Sulin Zhang, Dmitriy A. Dikin, Weiqiang Ding, Rodney S. Ruo, Nano Letters 3 (2003) 1299. [5] K. Akagi, R. Tamura, M. Tsukada, S. Itoh, S. Ihara, Physical Review Letters 74 (1995) 2307. [6] D.W. Carr, S. Evoy, L. Sekaric, H.G. Craighead, J.M. Parpia, Applied Physics Letters 75 (1999) 920. [7] A. Volodin, D. Buntinx, M. Ahlskog, A. Fonseca, J.B. Nagy, C. Van Haesendonck, Nano Letters 4 (2004) 1775. [8] M. Lu, H.L. Li, K.T. Lau, Journal of Physical Chemistry B 108 (2004) 6186–6192. [9] Y.K. Wen, Z.M. Shen, Carbon 39 (2001) 2369. [10] J. Xie, K. Mkhopadyay, J. Yadev, V.K. Varadan, Smart Materials and Structures 12 (2003) 744. [11] H.Q. Hou, Z. Jun, F. Weller, A. Greiner, Chemistry of Materials 15 (2003) 3170. [12] L.J. Pan, Z. Mei, N. Yoshikazu, Journal of Applied Physics 91 (2002) 10058–10061. [13] J.B. Bai, Materials Letters 57 (2003) 2629. [14] S. Motojima, I. Hasegawa, S. Kagiya, M. Momiyama, M. Kawaguchi, H. Iwanaga, Applied Physics Letters 62 (1993) 2322. [15] X.Q. Chen, S.M. Yang, S. Motojima, Materials Letters 57 (2002) 48. [16] B.I. Dunlap, Physical Review B 50 (1994) 8134. [17] K. Hernadi, L. Thien-Nga, L. Forro, Journal of Physical Chemistry B 105 (2001) 12464.