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Carbon nanotubes and diamond-like carbon films produced by cathodic micro-arc discharge in aqueous solutions
Materials Letters 61 (2007) 4916 – 4919 www.elsevier.com/locate/matlet
Carbon nanotubes and diamond-like carbon films produced by cathodic micro-arc discharge in aqueous solutions H.P. Zhao a , Y.D. He a , X.H. Kong a , W. Gao b,⁎ a
Beijing Key Lab for Corrosion, Erosion and Surface Technology, University of Science and Technology Beijing, 100083 Beijing, China b School of Engineering, The University of Auckland, Auckland, New Zealand Received 25 February 2007; accepted 21 March 2007 Available online 30 March 2007
Abstract This letter reports the synthesis of carbon nanotubes and diamond-like carbon films using cathodic micro-arc discharge in aqueous solutions. The conditions and mechanisms for the growth of these structures were briefly discussed based on the experimental observations. © 2007 Elsevier B.V. All rights reserved. Keywords: Nano-materials; Carbon nanotubes; Diamond-like carbon; Cathodic micro-arc discharge
1. Introduction Arc discharge in an inert gas [1] or in a hydrogen atmosphere [2] is a traditional method to synthesize carbon nano-materials, such as fullerene [3], carbon nanotubes (CNTs) [4,5] and diamond-like carbon (DLC) films [6]. Recently, arc discharge in liquid environments has also been developed to synthesize fullerene [7] and CNTs [8–12]. These liquid environments include liquid nitrogen [8,13], deionized water [9,11], aqueous solution NiSO4, CoSO4, FeSO4 and NaCl [10,14]. The device used for arc discharge in liquid environments is quite simple comparing with that in gas atmosphere, because the system does not require vacuum, gases control and heat exchange. However, during the process, the arc discharges take place directly between the cathode and anode. To maintain a steady arc, the gap between the electrodes has to be kept constant. Consequently, a device with precise movement capability is needed to control the gap. Otherwise, the gap would increase gradually as the anode is gradually consumed during this process. Moreover, when the arc discharge takes place in an aqueous solution, water vapor forms around the arc, which is not favorable for the formation of carbon nano-materials. The discharge phenomena associated to electrolysis was discovered more than a century ago [15]. However, the practical ⁎ Corresponding author. Tel.: +64 9 3737599x88175; fax: +64 9 3737463. E-mail address: [email protected] (W. Gao). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.03.081
applications of plasma electrolysis techniques, such as plasma electrolytic oxidation [16], plasma electrolytic nitriding and carburizing [17], were practiced much later. Recently, micro-arc
Fig. 1. Schematic illustration of (a) conventional arc discharge and (b) cathodic micro-arc discharge in aqueous solutions.
H.P. Zhao et al. / Materials Letters 61 (2007) 4916–4919
4917
Fig. 2. TEM and HRTEM images of the products prepared by cathodic micro-arc discharge: (a) a mix of CNFs and CNTs; (b) bamboo-like carbon nanotube; (c) multiwalls of a nanotube; (d) single-grain CNF; and (e) multi-grain CNF.
discharges in aqueous solution have been utilized to ionize media from the solution, so that complex compounds are synthesized on the metal surface through the plasma chemical interactions [18,19]. For examples, He et al. have prepared thick ceramic coatings on alloy surface by cathodic micro-arc discharge (CMAD) in aqueous solutions [20,21] and the phenol in waste water was almost eliminated by anodic micro-arc discharge [22]. This is a relatively new electrochemical treatment process, in which the micro-arc discharge takes place at the surface of cathode or anode, accompanying with
electrochemical reactions. This is different from the conventional arc discharge (Fig. 1). Comparing with the conventional arc discharge, the distance between the cathode and anode does not have significant influence on the micro-arc discharging processes. Therefore, a precise movement device for controlling the distance is not necessary. We found that whether the micro-arc discharge takes place on the surface of cathode or anode is mainly determined by the cathode-to-anode area ratio. For example, in a 0.25 M NaCl aqueous solution, two graphite rods with a diameter of 6 mm
4918
H.P. Zhao et al. / Materials Letters 61 (2007) 4916–4919
Fig. 3. FESEM surface morphologies of the diamond-like carbon (DLC) films produced by micro-arc discharge.
were used as the cathode and anode, the distance between them was 15 mm, and a DC voltage of 130 V was applied (over the triggering value to discharge). We observed a gas envelope formed on the surface of cathode or anode, and the micro-arc discharge occurred in the gas envelope. When the area ratio of cathode-to-anode was 1.7, the micro-arc discharge occurred alternatively between the cathode and anode. When the area ratio was less than 1.7, the micro-arc discharge takes place only on the surface of cathode or anode. This phenomenon is related to the formation of the gas envelope. If the micro-arc discharge requires a critical gas envelope with same amount of gas formed on the surfaces of electrodes, the reduction reaction at the cathode is 2H+ + 2e = H2, and the oxidation reaction at the anode is 4OH− = 2H2O + O2 + 4e, the calculated area ratio should be 2. However, there is another reaction, 2Cl− = Cl2 + 2e, that may occur at the anode, so the real area ratio should be less than 2, which is consistent with the value of 1.7 measured in our experiment. Therefore, CMAD could be easily maintained by using a cathode of a small surface area and an anode with a large area. In this letter, we report our work on carbon nano-materials with various structures synthesized by CMAD in aqueous solutions.
constant and to avoid the solution from evaporation. Most products dropped onto the bottom of the vessel and some deposited on the cathode. The raw products were collected by filtration and washed with deionized water, and then purified by using 4 M HNO3 at 103°C for 1 h. Field-emission SEM, high resolution TEM (JEM-2010) and Raman Spectroscopy (Renishaw, RM2000) were used to characterize these carbon nanotubes and diamond-like carbon films.
2. Experiment Carbon nanotubes (CNTs) and nanofibres (CNFs) were synthesized by CMAD in 0.25 M NaCl solution, in which a graphite cathode doped with 2.5 at.% Ni and 2.5 at.% Co was used. The areas of cathode and anode were 30 and 60 mm2, respectively, and the distance between the two electrodes was 15 mm. A DC voltage of 130 V was applied. In this case, the carbon and catalyst came from the cathode. Diamond-like carbon (DLC) films were synthesized by CMAD in an aqueous solution containing 50 vol.% 0.25 M NaCl aqueous solution and 50 vol.% ethanol. The cathode was a pure Ni rod with a surface area of 30 mm2. The anode is a Pt plate with an area of 60 mm2. The distance between the electrodes was 15 mm. The DC voltage applied to the electrodes was 130 V; the current used in the CMAD process was 1–2 A. The electrolyte vessel was immersed in a big cooling water bath and connected with a condenser to keep the temperature
Fig. 4. Raman spectrum of (a) carbon nanotubes and nanofibres, and (b) diamondlike carbon films.
H.P. Zhao et al. / Materials Letters 61 (2007) 4916–4919
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3. Results and discussion
Acknowledgement
The TEM and HRTEM images show that the products are CNFs and CNTs with 20–30 nm in diameter and 0.5–1 μm in length (Fig. 2a). The morphology and structure of the nano-sized carbon are very different, including bamboo-like tubes (Fig. 2b), multi-wall tubes (Fig. 2c), single-grain CNFs (Fig. 2d) and multi-grain CNFs (Fig. 2e). Fig. 3 shows the morphology of DLC films on a Ni cathode. It can be seen that the DLC coating are quite uniform. However, the coating surface looks rough due to the high impact force and local re-melting effect of micro-arc. Compared with DLC synthesized by unbalanced magnetron sputtering [23], the morphology of DLC synthesized by CMAD is more uniform but rougher. Fig. 4 illustrates the Raman spectrum of the products. Fig. 4a is the spectrum of a mass of CNTs and CNFs measured by a 514.5 nm wavelength laser. A weak D-band (defect band) peak can be seen at 1350.2 cm− 1, indicating that the defective graphite structure is very low [24]. The G-band (graphite band) peak can be clearly seen at 1580.1 cm− 1, which is one of the characteristic Raman peaks of carbon with well-developed graphite structures [24]. Fig. 4b is the Raman spectrum of DLC films. The characteristic peaks at 1371.0 cm− 1 and 1563.7 cm− 1 together with FESEM image confirm the formation of DLC film on the Ni cathode. The critical conditions for synthesizing carbon nano-materials have been established at the surface of cathode using CMAD in aqueous solution. Firstly, the plasma produced in CMAD can supply enough energy for carbon to evaporate. Secondly, the hydrogen gas envelope formed on the surface of cathode can protect the carbon products from oxidation. Thirdly, the fast cooling as a result of pulse micro-arc in aqueous solution provides a condition for the vapor carbon to form nanostructure. Finally, the process can be quite flexible, carbon sources and catalysts for the reaction can be provided either by using graphite cathode or graphite cathode doped with catalyst, or adding them into aqueous solutions, Therefore, CMAD in aqueous solution is a simple and economic way to synthesize nanostructured carbon.
The authors would like to thank the Natural Science Foundation of China for supporting this research under contract no. 20373006.
4. Conclusions In summary, a simple and economic method, cathodic microarc discharge, was successfully developed to prepare carbon nanotubes and diamond-like carbon films in aqueous solutions. An advantage of this process is that the carbon sources and catalysts for the reaction can be provided either by using graphite cathode or graphite cathode doped with catalyst, or by adding them into aqueous solutions.
References [1] D. Ugarte, Chem. Phys. Lett. 198 (1992) 596. [2] X. Zhao, M. Ohkohchi, H. Shimoyama, Y. Ando, J. Cryst. Growth 198/199 (1999) 934. [3] T. Kimura, T. Sugai, H. Shinohara, T. Goto, K. Tohji, I. Matsuoka, Chem. Phys. Lett. 246 (1995) 571. [4] S. Iijima, Nature 354 (1991) 56. [5] C. Journet, W.K. Maser, P. Bernier, A. Loiseau, L.M. Chapelle, S. Lefrant, P. Deniard, R. Lee, J.E. Fischer, Nature 388 (1997) 756. [6] X.L. Peng, Z.H. Barber, T.W. Clyne, Surf. Coat. Technol. 138 (2001) 23. [7] N. Sano, Mater. Chem. Phys. 88 (2004) 235. [8] M. Ishigami, J. Cumings, A. Zettl, S. Chen, Chem. Phys. Lett. 319 (2000) 457. [9] Y.L. Hsin, K.C. Hwang, F.R. Chen, J.J. Kai, Adv. Mater. 13 (2001) 830. [10] H.W. Zhu, X.S. Li, B. Jiang, C.L. Xu, Y.F. Zhu, D.H. Wu, Chem. Phys. Lett. 366 (2002) 664. [11] H. Lange, M. Sioda, A. Huczko, Y.Q. Zhu, H.W. Kroto, D.R.M. Walton, Carbon 41 (2003) 1617. [12] M.V. Antisari, R. Marazzi, R. Krsmanovic, Carbon 41 (2003) 2393. [13] N. Sano, J. Nakano, T. Kanki, Carbon 42 (2004) 686. [14] S.D. Wang, M.H. Chang, M.D. Lan, C.C. Wu, J.J. Cheng, H.K. Chang, Carbon 43 (2005) 1778. [15] N.P. Sluginov, J. Russ, Phys. Chem. Soc. 12 1–2 Phys. (1880) 193 (in Russian). [16] X. Nie, A. Leyland, H.X. Song, A.L. Yerokhin, S.J. Dowey, A. Matthews, Surf. Coat. Technol. 116–119 (1999) 1055. [17] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat. Technol. 122 (1999) 73. [18] T. Pauporté, J. Finne, A. Kahn-Harari, D. Lincot, Surf. Coat. Technol. 199 (2005) 216. [19] T. Paulmier, J.M. Bell, P.M. Fredericks, Development of a novel cathodic plasma/electrolytic deposition technique part 1: production of titanium dioxide coatings, Surf. Coat. Technol. (2006), doi: 10.1016/j.surfcoat.2006.06.049. [20] W. Han, Y. He, D. Wang, R. Xue, W. Gao, Rare Met. 28 (2004) 622. [21] W. Han, Y. He, D. Wang, R. Xue, Trans. Mater. Heat Treatment 26 (2005) 83. [22] W. Han, Y. He, J. Univ. Sci. Technol. Beijing 27 (2005) 662. [23] C. Liu, G. Li, W. Chen, Z. Mu, C. Zhang, L. Wang, The study of doped DLC films by Ti ion implantation, Thin Solid Films 279–282 (2005) 475. [24] M.S. Dresselhaus, G. Dresselhaus, R. Saito, A. Jorio, Raman spectroscopy of carbon nanotubes, Phys. Rep. 47–99 (2005) 409.
Carbon nanotubes and diamond-like carbon films produced by cathodic micro-arc discharge in aqueous solutions H.P. Zhao a , Y.D. He a , X.H. Kong a , W. Gao b,⁎ a
Beijing Key Lab for Corrosion, Erosion and Surface Technology, University of Science and Technology Beijing, 100083 Beijing, China b School of Engineering, The University of Auckland, Auckland, New Zealand Received 25 February 2007; accepted 21 March 2007 Available online 30 March 2007
Abstract This letter reports the synthesis of carbon nanotubes and diamond-like carbon films using cathodic micro-arc discharge in aqueous solutions. The conditions and mechanisms for the growth of these structures were briefly discussed based on the experimental observations. © 2007 Elsevier B.V. All rights reserved. Keywords: Nano-materials; Carbon nanotubes; Diamond-like carbon; Cathodic micro-arc discharge
1. Introduction Arc discharge in an inert gas [1] or in a hydrogen atmosphere [2] is a traditional method to synthesize carbon nano-materials, such as fullerene [3], carbon nanotubes (CNTs) [4,5] and diamond-like carbon (DLC) films [6]. Recently, arc discharge in liquid environments has also been developed to synthesize fullerene [7] and CNTs [8–12]. These liquid environments include liquid nitrogen [8,13], deionized water [9,11], aqueous solution NiSO4, CoSO4, FeSO4 and NaCl [10,14]. The device used for arc discharge in liquid environments is quite simple comparing with that in gas atmosphere, because the system does not require vacuum, gases control and heat exchange. However, during the process, the arc discharges take place directly between the cathode and anode. To maintain a steady arc, the gap between the electrodes has to be kept constant. Consequently, a device with precise movement capability is needed to control the gap. Otherwise, the gap would increase gradually as the anode is gradually consumed during this process. Moreover, when the arc discharge takes place in an aqueous solution, water vapor forms around the arc, which is not favorable for the formation of carbon nano-materials. The discharge phenomena associated to electrolysis was discovered more than a century ago [15]. However, the practical ⁎ Corresponding author. Tel.: +64 9 3737599x88175; fax: +64 9 3737463. E-mail address: [email protected] (W. Gao). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.03.081
applications of plasma electrolysis techniques, such as plasma electrolytic oxidation [16], plasma electrolytic nitriding and carburizing [17], were practiced much later. Recently, micro-arc
Fig. 1. Schematic illustration of (a) conventional arc discharge and (b) cathodic micro-arc discharge in aqueous solutions.
H.P. Zhao et al. / Materials Letters 61 (2007) 4916–4919
4917
Fig. 2. TEM and HRTEM images of the products prepared by cathodic micro-arc discharge: (a) a mix of CNFs and CNTs; (b) bamboo-like carbon nanotube; (c) multiwalls of a nanotube; (d) single-grain CNF; and (e) multi-grain CNF.
discharges in aqueous solution have been utilized to ionize media from the solution, so that complex compounds are synthesized on the metal surface through the plasma chemical interactions [18,19]. For examples, He et al. have prepared thick ceramic coatings on alloy surface by cathodic micro-arc discharge (CMAD) in aqueous solutions [20,21] and the phenol in waste water was almost eliminated by anodic micro-arc discharge [22]. This is a relatively new electrochemical treatment process, in which the micro-arc discharge takes place at the surface of cathode or anode, accompanying with
electrochemical reactions. This is different from the conventional arc discharge (Fig. 1). Comparing with the conventional arc discharge, the distance between the cathode and anode does not have significant influence on the micro-arc discharging processes. Therefore, a precise movement device for controlling the distance is not necessary. We found that whether the micro-arc discharge takes place on the surface of cathode or anode is mainly determined by the cathode-to-anode area ratio. For example, in a 0.25 M NaCl aqueous solution, two graphite rods with a diameter of 6 mm
4918
H.P. Zhao et al. / Materials Letters 61 (2007) 4916–4919
Fig. 3. FESEM surface morphologies of the diamond-like carbon (DLC) films produced by micro-arc discharge.
were used as the cathode and anode, the distance between them was 15 mm, and a DC voltage of 130 V was applied (over the triggering value to discharge). We observed a gas envelope formed on the surface of cathode or anode, and the micro-arc discharge occurred in the gas envelope. When the area ratio of cathode-to-anode was 1.7, the micro-arc discharge occurred alternatively between the cathode and anode. When the area ratio was less than 1.7, the micro-arc discharge takes place only on the surface of cathode or anode. This phenomenon is related to the formation of the gas envelope. If the micro-arc discharge requires a critical gas envelope with same amount of gas formed on the surfaces of electrodes, the reduction reaction at the cathode is 2H+ + 2e = H2, and the oxidation reaction at the anode is 4OH− = 2H2O + O2 + 4e, the calculated area ratio should be 2. However, there is another reaction, 2Cl− = Cl2 + 2e, that may occur at the anode, so the real area ratio should be less than 2, which is consistent with the value of 1.7 measured in our experiment. Therefore, CMAD could be easily maintained by using a cathode of a small surface area and an anode with a large area. In this letter, we report our work on carbon nano-materials with various structures synthesized by CMAD in aqueous solutions.
constant and to avoid the solution from evaporation. Most products dropped onto the bottom of the vessel and some deposited on the cathode. The raw products were collected by filtration and washed with deionized water, and then purified by using 4 M HNO3 at 103°C for 1 h. Field-emission SEM, high resolution TEM (JEM-2010) and Raman Spectroscopy (Renishaw, RM2000) were used to characterize these carbon nanotubes and diamond-like carbon films.
2. Experiment Carbon nanotubes (CNTs) and nanofibres (CNFs) were synthesized by CMAD in 0.25 M NaCl solution, in which a graphite cathode doped with 2.5 at.% Ni and 2.5 at.% Co was used. The areas of cathode and anode were 30 and 60 mm2, respectively, and the distance between the two electrodes was 15 mm. A DC voltage of 130 V was applied. In this case, the carbon and catalyst came from the cathode. Diamond-like carbon (DLC) films were synthesized by CMAD in an aqueous solution containing 50 vol.% 0.25 M NaCl aqueous solution and 50 vol.% ethanol. The cathode was a pure Ni rod with a surface area of 30 mm2. The anode is a Pt plate with an area of 60 mm2. The distance between the electrodes was 15 mm. The DC voltage applied to the electrodes was 130 V; the current used in the CMAD process was 1–2 A. The electrolyte vessel was immersed in a big cooling water bath and connected with a condenser to keep the temperature
Fig. 4. Raman spectrum of (a) carbon nanotubes and nanofibres, and (b) diamondlike carbon films.
H.P. Zhao et al. / Materials Letters 61 (2007) 4916–4919
4919
3. Results and discussion
Acknowledgement
The TEM and HRTEM images show that the products are CNFs and CNTs with 20–30 nm in diameter and 0.5–1 μm in length (Fig. 2a). The morphology and structure of the nano-sized carbon are very different, including bamboo-like tubes (Fig. 2b), multi-wall tubes (Fig. 2c), single-grain CNFs (Fig. 2d) and multi-grain CNFs (Fig. 2e). Fig. 3 shows the morphology of DLC films on a Ni cathode. It can be seen that the DLC coating are quite uniform. However, the coating surface looks rough due to the high impact force and local re-melting effect of micro-arc. Compared with DLC synthesized by unbalanced magnetron sputtering [23], the morphology of DLC synthesized by CMAD is more uniform but rougher. Fig. 4 illustrates the Raman spectrum of the products. Fig. 4a is the spectrum of a mass of CNTs and CNFs measured by a 514.5 nm wavelength laser. A weak D-band (defect band) peak can be seen at 1350.2 cm− 1, indicating that the defective graphite structure is very low [24]. The G-band (graphite band) peak can be clearly seen at 1580.1 cm− 1, which is one of the characteristic Raman peaks of carbon with well-developed graphite structures [24]. Fig. 4b is the Raman spectrum of DLC films. The characteristic peaks at 1371.0 cm− 1 and 1563.7 cm− 1 together with FESEM image confirm the formation of DLC film on the Ni cathode. The critical conditions for synthesizing carbon nano-materials have been established at the surface of cathode using CMAD in aqueous solution. Firstly, the plasma produced in CMAD can supply enough energy for carbon to evaporate. Secondly, the hydrogen gas envelope formed on the surface of cathode can protect the carbon products from oxidation. Thirdly, the fast cooling as a result of pulse micro-arc in aqueous solution provides a condition for the vapor carbon to form nanostructure. Finally, the process can be quite flexible, carbon sources and catalysts for the reaction can be provided either by using graphite cathode or graphite cathode doped with catalyst, or adding them into aqueous solutions, Therefore, CMAD in aqueous solution is a simple and economic way to synthesize nanostructured carbon.
The authors would like to thank the Natural Science Foundation of China for supporting this research under contract no. 20373006.
4. Conclusions In summary, a simple and economic method, cathodic microarc discharge, was successfully developed to prepare carbon nanotubes and diamond-like carbon films in aqueous solutions. An advantage of this process is that the carbon sources and catalysts for the reaction can be provided either by using graphite cathode or graphite cathode doped with catalyst, or by adding them into aqueous solutions.
References [1] D. Ugarte, Chem. Phys. Lett. 198 (1992) 596. [2] X. Zhao, M. Ohkohchi, H. Shimoyama, Y. Ando, J. Cryst. Growth 198/199 (1999) 934. [3] T. Kimura, T. Sugai, H. Shinohara, T. Goto, K. Tohji, I. Matsuoka, Chem. Phys. Lett. 246 (1995) 571. [4] S. Iijima, Nature 354 (1991) 56. [5] C. Journet, W.K. Maser, P. Bernier, A. Loiseau, L.M. Chapelle, S. Lefrant, P. Deniard, R. Lee, J.E. Fischer, Nature 388 (1997) 756. [6] X.L. Peng, Z.H. Barber, T.W. Clyne, Surf. Coat. Technol. 138 (2001) 23. [7] N. Sano, Mater. Chem. Phys. 88 (2004) 235. [8] M. Ishigami, J. Cumings, A. Zettl, S. Chen, Chem. Phys. Lett. 319 (2000) 457. [9] Y.L. Hsin, K.C. Hwang, F.R. Chen, J.J. Kai, Adv. Mater. 13 (2001) 830. [10] H.W. Zhu, X.S. Li, B. Jiang, C.L. Xu, Y.F. Zhu, D.H. Wu, Chem. Phys. Lett. 366 (2002) 664. [11] H. Lange, M. Sioda, A. Huczko, Y.Q. Zhu, H.W. Kroto, D.R.M. Walton, Carbon 41 (2003) 1617. [12] M.V. Antisari, R. Marazzi, R. Krsmanovic, Carbon 41 (2003) 2393. [13] N. Sano, J. Nakano, T. Kanki, Carbon 42 (2004) 686. [14] S.D. Wang, M.H. Chang, M.D. Lan, C.C. Wu, J.J. Cheng, H.K. Chang, Carbon 43 (2005) 1778. [15] N.P. Sluginov, J. Russ, Phys. Chem. Soc. 12 1–2 Phys. (1880) 193 (in Russian). [16] X. Nie, A. Leyland, H.X. Song, A.L. Yerokhin, S.J. Dowey, A. Matthews, Surf. Coat. Technol. 116–119 (1999) 1055. [17] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat. Technol. 122 (1999) 73. [18] T. Pauporté, J. Finne, A. Kahn-Harari, D. Lincot, Surf. Coat. Technol. 199 (2005) 216. [19] T. Paulmier, J.M. Bell, P.M. Fredericks, Development of a novel cathodic plasma/electrolytic deposition technique part 1: production of titanium dioxide coatings, Surf. Coat. Technol. (2006), doi: 10.1016/j.surfcoat.2006.06.049. [20] W. Han, Y. He, D. Wang, R. Xue, W. Gao, Rare Met. 28 (2004) 622. [21] W. Han, Y. He, D. Wang, R. Xue, Trans. Mater. Heat Treatment 26 (2005) 83. [22] W. Han, Y. He, J. Univ. Sci. Technol. Beijing 27 (2005) 662. [23] C. Liu, G. Li, W. Chen, Z. Mu, C. Zhang, L. Wang, The study of doped DLC films by Ti ion implantation, Thin Solid Films 279–282 (2005) 475. [24] M.S. Dresselhaus, G. Dresselhaus, R. Saito, A. Jorio, Raman spectroscopy of carbon nanotubes, Phys. Rep. 47–99 (2005) 409.