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Fluorination of multi-walled carbon nanotubes (MWNTs) via surface wave microwave (SW-MW) plasma treatment
ARTICLE IN PRESS Physica E 41 (2008) 299–303
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Fluorination of multi-walled carbon nanotubes (MWNTs) via surface wave microwave (SW-MW) plasma treatment Golap Kalita a,, Sudip Adhikari a, Hare Ram Aryal a, Dilip Chandra Ghimre a, Rakesh Afre b, Tetsuo Soga b, Maheshwar Sharon c, Masayoshi Umeno a a
Department of Electrical and Electronics Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai-shi 487-8501, Japan Department of Environmental Technology and Urban Planning, Nagoya Institute of Technology, Nagoya 4668555, Japan c Nanotechnology Research Center, Birla College, Kalyan 421304, India b
a r t i c l e in fo
abstract
Article history: Received 20 December 2007 Accepted 25 July 2008 Available online 8 August 2008
Here, we report fluorination of multi-walled carbon nanotubes (MWNTs) via the surface wave microwave (SW-MW) plasma technique. We have investigated the change in the atomic and structures properties of fluorinated MWNTs using X-ray photoemission electron microscopy (XPS), Raman spectroscopy and transmission electron microscopy (TEM). The XPS study presents a semi-ionic boding character at a low concentration and a covalent bonding character at a high concentration. The Raman spectroscopy study shows an increase in disordered sp3 hybridization in MWNTs with fluorine incorporation. High-resolution TEM study of fluorinated MWNTs shows an induced defect on the sidewall without affecting the inner tubes’ structure. & 2008 Elsevier B.V. All rights reserved.
PACS: 61.46.Fg 81.65.Cf 64.70.Nd Keywords: Carbon nanotubes Surface wave microwave plasma Fluorination Atomic properties Structural properties
1. Introduction Carbon nanotubes (CNTs) as a form of cylindrical graphitic shells [1,2] have attracted the interest of scientists for its application in the field of nanotechnology such as electronic devices, energy storage application, etc. [3–5]. In spite of intensive research on CNTs, practical applications of CNTs in electronic devices are still limited by a number of reasons. Functionalized CNTs are chemically more reactive and can be used for further processing such as thiolization or for integration into a nanocomposite [6–8]. Functionalization of CNTs sidewalls represents a solution in order to improve the interactions between CNTs and the solvent or the polymer matrix and thus increase their dispersion ability [9]. However, the sidewall modification is not easily accessible, since open ends of CNTs are more reactive than the sidewall due to the presence of dangling bonds [10]. There are mainly two different approaches for surface modification of CNTs. One is the noncovalent functionalization, which has been carried out by several processes such as ultrasonication, addition of surfactants, polymer wrapping, etc.
Corresponding author. Tel.: +81 568 511111; fax: +81 568 511478.
E-mail address: [email protected] (G. Kalita). 1386-9477/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2008.07.015
[11–15]. The other approach relies on the covalent grafting of functional groups on the sidewalls of CNTs [16]. Covalent functionalizations of CNTs were achieved by solvent processing, plasma treatment, etc. [17–19]. Among the different techniques, the plasma treatment method is important since it has the advantage of being nonpolluting and due to the possibility of scaling up to produce large quantities necessary for commercial applications. Since fluorination is one of the most effective chemical methods to modify and control physiochemical properties in a wide range, fluorinations of new forms of carbon materials are of great interest [10,20]. There are reports of fluorination of CNTs by plasma treatment and demonstrated sidewall functionalization [21,22]. Fluorination can also enhance the solubility in various solvent and homogeneous composite formations with conducting polymers. Mickelson et al. have also shown in further work that fluorinated single-walled carbon nanotubes (F-SWCNTs) dissolve well in alcohol solvents to yield a long-lasting metastable solution [23]. Fluorination may enhance the wettability of the tubes in a water solvent by inducing a surface dipole layer on the CNTs’ wall [24]. The bonding nature of fluorinated CNTs and the disintegration process upon fluorination are not clearly understood and more study is needed on the interaction of fluorine atoms and CNTs.
ARTICLE IN PRESS G. Kalita et al. / Physica E 41 (2008) 299–303
In this work, we report sidewall functionalization of multiwalled carbon nanotubes (MWNTs) with fluorine atoms via surface wave microwave (SW-MW) plasma using CF4 gas. Fluorination of CNTs was studied with different CF4 gas flow rates to achieve covalent functionalization. Atomic and structure changes of fluorinated MWNTs were studied using X-ray photoemission electron microscopy (XPS), Raman spectroscopy and transmission electron microscopy (TEM). Our result shows that the SW-MW plasma process is an effective tool for CNTs’ functionalization.
Intensity (a.u.)
300
(c) CF4 15 sccm
(b) CF4 10 sccm (a) Not treated
2. Experimental MWNTs were synthesized as described previously [25] using a plant-based precursor turpentine oil by spray pyrolysis at 800 1C. Transition metal particles (Co, Fe) supported on silica gel were used as catalysts for CNTs’ synthesis. Synthesized CNTs were initially heated at 450 1C for 30 min in atmospheric air to remove the amorphous carbon. Pre-heated CNTs were treated with 6 M NaOH and 6 M HCl and washed with water. Purified CNTs were placed in an SW-MW plasma chamber under CF4 flow. CF4 plasma was introduced into the chamber via a surface guide supplied by a 2.45 GHz microwave generator. Gas flow to the chamber was regulated by a mass flow controller; as CNTs were exposed to CF4 plasma with 10 and 15 sccm flow rates. Plasma treatment was performed at a microwave power of 100 W and at a gas pressure of 20 Pa. The time of plasma treatment was kept constant at 10 min for all the experiment. The stage holding MWNTs sample was kept at a 10 cm distance from the plasma showering quartz plate. The MWNT sample was kept at room temperature and exposed to CF4 plasma. The schematic diagram of the SW-MW plasma chamber is shown in Fig. 1. Plasma-treated MWNTs were analyzed by XPS, Raman and HRTEM studies. XPS measurements of as-synthesized CNTs and SW-MW plasma-treated MWNTs were carried out on an SSX-100 photoelectron spectrometer and visible Raman spectroscopy on an NRS-1500 W laser Raman. TEM measurements of MWNTs were carried out by using an FE-TEM, JEOL-2100F.
3. Result and discussion Before SW-MW plasma treatment, purified CNTs were analyzed by the XPS study. XPS measurement of MWNTs shows the presence of carbon and oxygen peaks centered at 284.2 and 534 eV, respectively. Quantitative analysis shows presence of 1.8% (atomic percentage) oxygen atoms along with carbon atoms, which is generally observed in as-synthesized CNTs. XPS study of
Slot plunger
2.45 GHz Microwave
CF4 Surface wave plasma Stage To Vacuum pump Fig. 1. Schematic diagram of the surface wave microwave plasma chamber used for fluorination of MWNTs.
0
200
400 Binding Energy (e.V.)
600
800
Fig. 2. XPS spectra of fluorinated MWNTs at different flow rates of CF4 gas in comparison with pristine MWNTs.
the CF4 plasma-treated MWNTs shows the presence of carbon, oxygen and also fluorine atoms. The XPS spectrum of CF4 plasma-treated MWNTs is given in Fig. 2 in comparison with as-synthesized MWNTs. Quantitative analysis shows an increase in the fluorine content in the MWNTs with increasing CF4 flow rate (Table 1). The F/C ratio varies with the variation of fluorine concentration in MWNTs (Table 1). To acquire more information on the bonding structure of fluorinated MWNTs fitting spectra of C 1s and F 1s were analyzed. Fig. 3(a) shows the deconvoluted XPS C 1s spectra for pristine MWNTs. The spectra show three different peaks representing sp2-hybridized carbon (peak 1, 284.2 eV), sp3 carbon (peak 2, 286.05 eV) and oxygen-related groups (peak 3, 289.8). C 1s spectra of fluorinated MWNTs are presented in Fig. 3b and c. The photoelectron emitted from carbon atoms of fluorinated MWNTs shows a peak centered at 283.48 eV i.e. 1.12 eV lower than the value for the photoelectron emitted from a pure graphite sample. This shift is associated with the weakening of the C–C bonding caused by the redistribution of the electron density with respect to graphene resulting from the curvature of the sheets [26]. After fluorination, the intensity of the sp2 peak became slightly broader and the intensity of the peak decreased. With fluorination of MWNTs, different peaks appear at a higher binding energy as shown in Fig. 3b and c. These peaks at a higher binding energy indicate the existence of carbon species bonded with fluorine. Fitting spectra show the appearance of different peaks in the range of 288–289 and 291–292 eV corresponding to the CFx bonding structure. XPS C 1s peaks at a higher binding energy were ascribed to an sp3-hybridized carbon atom with a covalent C–F bond. This signifies the CFx bond formation in the MWNTs’ sidewalls and successful covalent functionalization. Fig. 4a and b corresponds to XPS F 1s fitting spectra for the fluorinated MWNTs at two different flow rates. The deconvoluted spectra show three different peaks at 688.02, 89.84 and 684.31 eV corresponding to semi-ionic, covalent and CF2QCF2 bonding, respectively. There is an increase in the semi-ionic/covalent ratio with an increase in flow rates. The change in the semi-ionic/ covalent ratio with different fluorine concentrations is given in Table 1. These results show that the bonding configuration can be changed with the variation of fluorine incorporation. Fluorine addition patterns to CNTs have already been analyzed, showing the relative stability of the different addition patterns and the expansion of the fluorine domains. For distinct additional patterns to armchair and zigzag CNTs having a C2F ratio, no significant differences in C–F bond lengths and higher fluorine charges were predicted on addition to a nanotube site. However, longer bond
ARTICLE IN PRESS G. Kalita et al. / Physica E 41 (2008) 299–303
301
Table 1 Presentation of oxygen and fluorine concentration, F/C ratio, type of behavior and semi-ionic/covalent ratio for the C–F bond present in the fluorinated MWNTs with respect to CF4 flow rates ( – not functionalized; J – functionalized) CF4 flow rate (sccm)
Oxygen (at%)
Fluorine (at%)
F/C ratio
Functionalization
Semi-ionic/covalent
Not treated 10 15
1.80 4.12 3.37
0 15.89 23.70
0 0.198 0.377
J
0.097 0.249
100
100
80
80
1
Intensity (Counts)
Intensity (Counts)
J
60 40 20
2
296
60 40 20 4
3
0
1
0
288 284 292 Binding Energy (eV)
298
3
2
5 294 290 286 Binding Energy (eV)
282
Intensity (Counts)
100 80 1 60 40 20 4 5
0 298
3
2
294 290 286 Binding Energy (eV)
282
Fig. 3. XPS C 1s fitting spectra of (a) pristine MWNTs, (b) CF4 plasma-treated MWNTs at 10 sccm flow rate and (c) CF4 plasma-treated MWNTs at 15 sccm flow rate.
3400
2600
Covalent
1800
1600 CF 2=CF 2 1200
C-F
3000
C-F
Semi-ionic C-F
800
Intensity (Counts)
Intensity (Counts)
Covalent
2600 2200
Semi-ionic C-F
1800
CF 2=CF 2
1400 1000
694 692 690 688 686 684 682 Binding Energy (eV)
694
692
690 688 686 684 Binding Energy (eV)
682
Fig. 4. XPS F 1s fitting spectrum of the fluorinated MWNTs at two different CF4 flow rates; presenting covalent, semi-ionic and CF2QCF2 bonding structures.
lengths and higher fluorine charges were predicted on addition to nanotube defect sites [21]. Raman spectroscopy has been extensively used to characterize CNTs because this technique is very sensitive to the structural
disorder of CNTs. Fig. 5 represents the Raman shift of pristine and CF4 plasma-treated MWNTs. The graphite (G) band and disorder (D) band of pristine MWNTs were observed at 1576.7 and 1344.18 cm 1, respectively. For fluorine-incorporated MWNTs,
ARTICLE IN PRESS 302
G. Kalita et al. / Physica E 41 (2008) 299–303
D peak
Intensity (arb. units)
G peak
(c) Induced Defects (b) (a)
1100
1300
1500 Raman Shift (cm-1)
1700
Fig. 5. Raman scattering of pristine MWNTs and fluorinated MWNTs at different CF4 flow rates.
Fig. 7. Presentation of (a) TEM image of purified MWNTs, (b) HRTEM image of pristine MWNTs and (c) HRTEM image of fluorinated MWNTs.
1.1
Intensity ratio of D and G band
1.05 1 0.95 0.9 0.85 0.8 0.75 0.7 0
5
10 Flow rate of CF4 (sccm)
15
20
Fig. 6. Plot of intensity ratio (R) of D and G bands of Raman spectra with respect to the flow rate of CF4.
shifting of the G and D bands was very negligible (G and D bands 1581.2 at 1347.65 cm 1). It was observed that with fluorine incorporation, the D peak became enhanced. This is due to the fact that there are increases in disordered sp3 hybridization in MWNTs with fluorine incorporation. In Fig. 6, the variation of intensity ratio of the D and G bands is presented for pristine MWNTs and fluorinated MWNTs. It is observed that there is an increase in the R value (the intensity of the D band divided by the intensity of the G band) indicating a change in their structural properties in the bulk after fluorination. Fig. 7a shows a TEM image of pristine purified MWNTs. The MWNTs used in these studies had a diameter distribution of 10–40 nm and were several microns in length. Amorphous carbon in the CNT sample was completely absent as observed in the TEM grid. HRTEM measurement of the pristine nanotubes was done, which showed a well-graphitized multi-layer structure as shown in Fig. 7b. Possible structural change of MWNTs with fluorination was measured with an HRTEM study. From HRTEM image 7(c), it is quite clear that the inside wall remains unaffected with some defect on the outermost walls. Mickelson et al. have reported that
with high fluorination of SWCNTs, there is destruction of CNTs’ morphology [23]. As in our case, the fluorine concentration was not as high as there destruction of CNT morphology could occur. In this work, our main objective was attachment of fluorine atoms in the CNT surface for effective covalent functionalization without disturbing the multi-walled structure of CNTs. In the SW-MW plasma system, ion bombardment within the plasma allows sufficient energy for the C–C bonds to be weakened or even broken, hence allowing for the attachment of fluorine in the plasma process. However, the ion bombardment did not result in loss of nanotube structure. Efficient covalent functionalization was achieved without destruction of MWNT samples. In the SW-MW plasma system, plasma can be generated with a high electron density and a low electron temperature, which allows uniform covalent functionalization at a low temperature. The fluorine attachment is seen to remain stable over time, showing that the reported functionalization process is reliable. Although in our experiment a few milligrams of MWNTs were functionalized by fluorination, application of the reported method to large quantities up to several grams can be achieved in one step. Uniform SW-MW plasma can be produced over a large area, which gives the flexibility of efficient functionalization of a large quantity CNTs in a one-step process.
4. Conclusion In conclusion, we successfully attached fluorine atoms to the sidewalls of MWNTs through the SW-MW plasma technique using CF4 at a low temperature. XPS analysis showed fluorine atoms grafted on the surface of the CNTs and can be changed with the flow rate of CF4.From the XPS F 1s spectra, it was observed that with an increase in the flow rate the semi-ionic/covalent bonding ratio increases. Raman studies revealed a structural change in the fluorinated MWNTs. There were increases in disordered sp3 hybridization in MWNTs with fluorine incorporation. HRTEM study shows modification of the outer shell of MWNTs with fluorine atom attachment. In the SW-MW plasma system ion bombardment allows sufficient energy for the C–C bonds to be weakened or even broken, hence allowing for the attachment of fluorine.
ARTICLE IN PRESS G. Kalita et al. / Physica E 41 (2008) 299–303
Acknowledgement The author is grateful to MEXT (Ministry of Education, Culture, Sports, Science and Technology, Japan) for providing a fellowship to carry out this work. References [1] S. Iijima, Nature (London) 354 (1991) 56. [2] P.J.F. Harris, Carbon Nanotubes and Related Structures, Cambridge University Press, Cambridge, London, 1999. [3] X. Gong, J. Liu, S. Badkaran, R.D. Voise, J.S. Young, Chem. Mater. 12 (2000) 1049. [4] E. Kymakis, G.A.J. Amaratunga, Appl. Phys. Lett. 80 (2002) 112. [5] M.W. Rowell, M.A. Topinka, M.D. McGehee, Appl. Phys. Lett. 88 (2006) 233506. [6] Z. Liu, Z. Shen, T. Zhy, S. Hou, L. Ying, Langmuir 16 (2000) 3569. [7] J. Cech, S.A. Curran, D. Ahang, J.L. Dewald, A. Avadhanula, M. Kandadai, S. Roth, Phys. Status Solidi 243 (2006) 3221. [8] R.A. Hatton, N.P. Blanchard, A.J. Miller, S.R.P. Silva, Langmuir 23 (2007) 6424. [9] J. Chen, M.A. Hamon, H. Hu, Y. Chen, A.M. Rao, P.C. Eklund, R.C. Haddon, Science 282 (1998) 95. [10] K.H. An, J.G. Heo, K.G. Jeon, D.J. Bae, C. Jo, C.W. Yang, C.Y. Park, Y.H. Lee, Y.S. Lee, Y.S. Chung, Appl. Phys. Lett. 80 (2002) 4235.
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[11] K.B. Shelimov, R. Esenaliev, A.G. Rinzler, C.B. Huffman, R.E. Smalley, Chem. Lett. 282 (1998) 492. [12] C. Park, Z. Ounaies, K.A. Watson, R.E. Crooks, J. Smith Jr., S.E. Lowther, J.W. Connell, E.J. Siochi, J.S. Harrison, T.L. St. Clair, Chem. Phys. Lett. 364 (2002) 303. [13] A. Hirsch, Angew. Chem. Int. Ed. 41 (2002) 1853. [14] M. Zhang, L. Su, L. Mao, Carbon 44 (2006) 276. [15] M.J. O’Connell, P. Boul, L.M. Ericson, C. Huffman, Y. Wang, E. Haroz, C. Kuper, J. Tour, K.D. Ausman, R.E. Smalley, Chem. Phys. Lett. 342 (2001) 265. [16] E.T. Thostenson, C. Li, T.W. Chou, Compos. Sci. Technol. 65 (2005) 491. [17] A. Gromov, S. Dittmer, J. Svensson, O.A. Nerushev, S.A. Perez-Garcia, L. Licca-Jimenez, R. Rychwalski, E.E.B. Campbell, J. Mater. Chem. 15 (2005) 3334. [18] S. Banerjee, T. Hemraj-Benny, S.S. Wong, Adv. Mater. 17 (2005) 17. [19] N.O.V. Plank, Liudi Jiang, R. Cheung, Appl. Phys. Lett. 83 (2003) 2426. [20] H. Muramatsu, Y.A. Kim, T. Hayashi, M. Endo, A. Yonemoto, H. Arikai, F. Okino, H. Touhara, Chem. Commun. (2005) 2002. [21] A. Felten, C. Bittencourt, J.J. Pireaux, G. Van Lier, J.C. Charlier, J. Appl. Phys. 98 (2005) 074308. [22] Y.S. Lee, T.H. Cho, B.K. Lee, J.S. Rho, K.H. An, Y.H. Lee, J. Fluorine Chem. 120 (2003) 99. [23] E.T. Mickelson, I.W. Chiang, J.L. Zimmerman, P.J. Boul, J. Lozano, J. Liu, R.E. Smalley, R.H. Hauge, J.L. Margrave, J. Phys. Chem. B 103 (1999) 4318. [24] K.A. Park, Y.S. Choi, Y.H. Lee, C. Kim, Phys. Rev. B (2003) 045429. [25] R.A. Afre, T. Soga, T. Jimbo, M. Kumar, Y. Ando, M. Sharon, P. Somani, M. Umeno, Microporous and Mesoporous mater. 96 (2006) 184. [26] A. Felten, J. Ghijsen, J.J. Pireaux, R.L. Johnson, C.M. Whelan, D. Liang, G.V. Tendeloo, C. Bittencourt, J. Phys. D Appl. Phys. 40 (2007) 7379.
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Fluorination of multi-walled carbon nanotubes (MWNTs) via surface wave microwave (SW-MW) plasma treatment Golap Kalita a,, Sudip Adhikari a, Hare Ram Aryal a, Dilip Chandra Ghimre a, Rakesh Afre b, Tetsuo Soga b, Maheshwar Sharon c, Masayoshi Umeno a a
Department of Electrical and Electronics Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai-shi 487-8501, Japan Department of Environmental Technology and Urban Planning, Nagoya Institute of Technology, Nagoya 4668555, Japan c Nanotechnology Research Center, Birla College, Kalyan 421304, India b
a r t i c l e in fo
abstract
Article history: Received 20 December 2007 Accepted 25 July 2008 Available online 8 August 2008
Here, we report fluorination of multi-walled carbon nanotubes (MWNTs) via the surface wave microwave (SW-MW) plasma technique. We have investigated the change in the atomic and structures properties of fluorinated MWNTs using X-ray photoemission electron microscopy (XPS), Raman spectroscopy and transmission electron microscopy (TEM). The XPS study presents a semi-ionic boding character at a low concentration and a covalent bonding character at a high concentration. The Raman spectroscopy study shows an increase in disordered sp3 hybridization in MWNTs with fluorine incorporation. High-resolution TEM study of fluorinated MWNTs shows an induced defect on the sidewall without affecting the inner tubes’ structure. & 2008 Elsevier B.V. All rights reserved.
PACS: 61.46.Fg 81.65.Cf 64.70.Nd Keywords: Carbon nanotubes Surface wave microwave plasma Fluorination Atomic properties Structural properties
1. Introduction Carbon nanotubes (CNTs) as a form of cylindrical graphitic shells [1,2] have attracted the interest of scientists for its application in the field of nanotechnology such as electronic devices, energy storage application, etc. [3–5]. In spite of intensive research on CNTs, practical applications of CNTs in electronic devices are still limited by a number of reasons. Functionalized CNTs are chemically more reactive and can be used for further processing such as thiolization or for integration into a nanocomposite [6–8]. Functionalization of CNTs sidewalls represents a solution in order to improve the interactions between CNTs and the solvent or the polymer matrix and thus increase their dispersion ability [9]. However, the sidewall modification is not easily accessible, since open ends of CNTs are more reactive than the sidewall due to the presence of dangling bonds [10]. There are mainly two different approaches for surface modification of CNTs. One is the noncovalent functionalization, which has been carried out by several processes such as ultrasonication, addition of surfactants, polymer wrapping, etc.
Corresponding author. Tel.: +81 568 511111; fax: +81 568 511478.
E-mail address: [email protected] (G. Kalita). 1386-9477/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2008.07.015
[11–15]. The other approach relies on the covalent grafting of functional groups on the sidewalls of CNTs [16]. Covalent functionalizations of CNTs were achieved by solvent processing, plasma treatment, etc. [17–19]. Among the different techniques, the plasma treatment method is important since it has the advantage of being nonpolluting and due to the possibility of scaling up to produce large quantities necessary for commercial applications. Since fluorination is one of the most effective chemical methods to modify and control physiochemical properties in a wide range, fluorinations of new forms of carbon materials are of great interest [10,20]. There are reports of fluorination of CNTs by plasma treatment and demonstrated sidewall functionalization [21,22]. Fluorination can also enhance the solubility in various solvent and homogeneous composite formations with conducting polymers. Mickelson et al. have also shown in further work that fluorinated single-walled carbon nanotubes (F-SWCNTs) dissolve well in alcohol solvents to yield a long-lasting metastable solution [23]. Fluorination may enhance the wettability of the tubes in a water solvent by inducing a surface dipole layer on the CNTs’ wall [24]. The bonding nature of fluorinated CNTs and the disintegration process upon fluorination are not clearly understood and more study is needed on the interaction of fluorine atoms and CNTs.
ARTICLE IN PRESS G. Kalita et al. / Physica E 41 (2008) 299–303
In this work, we report sidewall functionalization of multiwalled carbon nanotubes (MWNTs) with fluorine atoms via surface wave microwave (SW-MW) plasma using CF4 gas. Fluorination of CNTs was studied with different CF4 gas flow rates to achieve covalent functionalization. Atomic and structure changes of fluorinated MWNTs were studied using X-ray photoemission electron microscopy (XPS), Raman spectroscopy and transmission electron microscopy (TEM). Our result shows that the SW-MW plasma process is an effective tool for CNTs’ functionalization.
Intensity (a.u.)
300
(c) CF4 15 sccm
(b) CF4 10 sccm (a) Not treated
2. Experimental MWNTs were synthesized as described previously [25] using a plant-based precursor turpentine oil by spray pyrolysis at 800 1C. Transition metal particles (Co, Fe) supported on silica gel were used as catalysts for CNTs’ synthesis. Synthesized CNTs were initially heated at 450 1C for 30 min in atmospheric air to remove the amorphous carbon. Pre-heated CNTs were treated with 6 M NaOH and 6 M HCl and washed with water. Purified CNTs were placed in an SW-MW plasma chamber under CF4 flow. CF4 plasma was introduced into the chamber via a surface guide supplied by a 2.45 GHz microwave generator. Gas flow to the chamber was regulated by a mass flow controller; as CNTs were exposed to CF4 plasma with 10 and 15 sccm flow rates. Plasma treatment was performed at a microwave power of 100 W and at a gas pressure of 20 Pa. The time of plasma treatment was kept constant at 10 min for all the experiment. The stage holding MWNTs sample was kept at a 10 cm distance from the plasma showering quartz plate. The MWNT sample was kept at room temperature and exposed to CF4 plasma. The schematic diagram of the SW-MW plasma chamber is shown in Fig. 1. Plasma-treated MWNTs were analyzed by XPS, Raman and HRTEM studies. XPS measurements of as-synthesized CNTs and SW-MW plasma-treated MWNTs were carried out on an SSX-100 photoelectron spectrometer and visible Raman spectroscopy on an NRS-1500 W laser Raman. TEM measurements of MWNTs were carried out by using an FE-TEM, JEOL-2100F.
3. Result and discussion Before SW-MW plasma treatment, purified CNTs were analyzed by the XPS study. XPS measurement of MWNTs shows the presence of carbon and oxygen peaks centered at 284.2 and 534 eV, respectively. Quantitative analysis shows presence of 1.8% (atomic percentage) oxygen atoms along with carbon atoms, which is generally observed in as-synthesized CNTs. XPS study of
Slot plunger
2.45 GHz Microwave
CF4 Surface wave plasma Stage To Vacuum pump Fig. 1. Schematic diagram of the surface wave microwave plasma chamber used for fluorination of MWNTs.
0
200
400 Binding Energy (e.V.)
600
800
Fig. 2. XPS spectra of fluorinated MWNTs at different flow rates of CF4 gas in comparison with pristine MWNTs.
the CF4 plasma-treated MWNTs shows the presence of carbon, oxygen and also fluorine atoms. The XPS spectrum of CF4 plasma-treated MWNTs is given in Fig. 2 in comparison with as-synthesized MWNTs. Quantitative analysis shows an increase in the fluorine content in the MWNTs with increasing CF4 flow rate (Table 1). The F/C ratio varies with the variation of fluorine concentration in MWNTs (Table 1). To acquire more information on the bonding structure of fluorinated MWNTs fitting spectra of C 1s and F 1s were analyzed. Fig. 3(a) shows the deconvoluted XPS C 1s spectra for pristine MWNTs. The spectra show three different peaks representing sp2-hybridized carbon (peak 1, 284.2 eV), sp3 carbon (peak 2, 286.05 eV) and oxygen-related groups (peak 3, 289.8). C 1s spectra of fluorinated MWNTs are presented in Fig. 3b and c. The photoelectron emitted from carbon atoms of fluorinated MWNTs shows a peak centered at 283.48 eV i.e. 1.12 eV lower than the value for the photoelectron emitted from a pure graphite sample. This shift is associated with the weakening of the C–C bonding caused by the redistribution of the electron density with respect to graphene resulting from the curvature of the sheets [26]. After fluorination, the intensity of the sp2 peak became slightly broader and the intensity of the peak decreased. With fluorination of MWNTs, different peaks appear at a higher binding energy as shown in Fig. 3b and c. These peaks at a higher binding energy indicate the existence of carbon species bonded with fluorine. Fitting spectra show the appearance of different peaks in the range of 288–289 and 291–292 eV corresponding to the CFx bonding structure. XPS C 1s peaks at a higher binding energy were ascribed to an sp3-hybridized carbon atom with a covalent C–F bond. This signifies the CFx bond formation in the MWNTs’ sidewalls and successful covalent functionalization. Fig. 4a and b corresponds to XPS F 1s fitting spectra for the fluorinated MWNTs at two different flow rates. The deconvoluted spectra show three different peaks at 688.02, 89.84 and 684.31 eV corresponding to semi-ionic, covalent and CF2QCF2 bonding, respectively. There is an increase in the semi-ionic/covalent ratio with an increase in flow rates. The change in the semi-ionic/ covalent ratio with different fluorine concentrations is given in Table 1. These results show that the bonding configuration can be changed with the variation of fluorine incorporation. Fluorine addition patterns to CNTs have already been analyzed, showing the relative stability of the different addition patterns and the expansion of the fluorine domains. For distinct additional patterns to armchair and zigzag CNTs having a C2F ratio, no significant differences in C–F bond lengths and higher fluorine charges were predicted on addition to a nanotube site. However, longer bond
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301
Table 1 Presentation of oxygen and fluorine concentration, F/C ratio, type of behavior and semi-ionic/covalent ratio for the C–F bond present in the fluorinated MWNTs with respect to CF4 flow rates ( – not functionalized; J – functionalized) CF4 flow rate (sccm)
Oxygen (at%)
Fluorine (at%)
F/C ratio
Functionalization
Semi-ionic/covalent
Not treated 10 15
1.80 4.12 3.37
0 15.89 23.70
0 0.198 0.377
J
0.097 0.249
100
100
80
80
1
Intensity (Counts)
Intensity (Counts)
J
60 40 20
2
296
60 40 20 4
3
0
1
0
288 284 292 Binding Energy (eV)
298
3
2
5 294 290 286 Binding Energy (eV)
282
Intensity (Counts)
100 80 1 60 40 20 4 5
0 298
3
2
294 290 286 Binding Energy (eV)
282
Fig. 3. XPS C 1s fitting spectra of (a) pristine MWNTs, (b) CF4 plasma-treated MWNTs at 10 sccm flow rate and (c) CF4 plasma-treated MWNTs at 15 sccm flow rate.
3400
2600
Covalent
1800
1600 CF 2=CF 2 1200
C-F
3000
C-F
Semi-ionic C-F
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Intensity (Counts)
Intensity (Counts)
Covalent
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Semi-ionic C-F
1800
CF 2=CF 2
1400 1000
694 692 690 688 686 684 682 Binding Energy (eV)
694
692
690 688 686 684 Binding Energy (eV)
682
Fig. 4. XPS F 1s fitting spectrum of the fluorinated MWNTs at two different CF4 flow rates; presenting covalent, semi-ionic and CF2QCF2 bonding structures.
lengths and higher fluorine charges were predicted on addition to nanotube defect sites [21]. Raman spectroscopy has been extensively used to characterize CNTs because this technique is very sensitive to the structural
disorder of CNTs. Fig. 5 represents the Raman shift of pristine and CF4 plasma-treated MWNTs. The graphite (G) band and disorder (D) band of pristine MWNTs were observed at 1576.7 and 1344.18 cm 1, respectively. For fluorine-incorporated MWNTs,
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D peak
Intensity (arb. units)
G peak
(c) Induced Defects (b) (a)
1100
1300
1500 Raman Shift (cm-1)
1700
Fig. 5. Raman scattering of pristine MWNTs and fluorinated MWNTs at different CF4 flow rates.
Fig. 7. Presentation of (a) TEM image of purified MWNTs, (b) HRTEM image of pristine MWNTs and (c) HRTEM image of fluorinated MWNTs.
1.1
Intensity ratio of D and G band
1.05 1 0.95 0.9 0.85 0.8 0.75 0.7 0
5
10 Flow rate of CF4 (sccm)
15
20
Fig. 6. Plot of intensity ratio (R) of D and G bands of Raman spectra with respect to the flow rate of CF4.
shifting of the G and D bands was very negligible (G and D bands 1581.2 at 1347.65 cm 1). It was observed that with fluorine incorporation, the D peak became enhanced. This is due to the fact that there are increases in disordered sp3 hybridization in MWNTs with fluorine incorporation. In Fig. 6, the variation of intensity ratio of the D and G bands is presented for pristine MWNTs and fluorinated MWNTs. It is observed that there is an increase in the R value (the intensity of the D band divided by the intensity of the G band) indicating a change in their structural properties in the bulk after fluorination. Fig. 7a shows a TEM image of pristine purified MWNTs. The MWNTs used in these studies had a diameter distribution of 10–40 nm and were several microns in length. Amorphous carbon in the CNT sample was completely absent as observed in the TEM grid. HRTEM measurement of the pristine nanotubes was done, which showed a well-graphitized multi-layer structure as shown in Fig. 7b. Possible structural change of MWNTs with fluorination was measured with an HRTEM study. From HRTEM image 7(c), it is quite clear that the inside wall remains unaffected with some defect on the outermost walls. Mickelson et al. have reported that
with high fluorination of SWCNTs, there is destruction of CNTs’ morphology [23]. As in our case, the fluorine concentration was not as high as there destruction of CNT morphology could occur. In this work, our main objective was attachment of fluorine atoms in the CNT surface for effective covalent functionalization without disturbing the multi-walled structure of CNTs. In the SW-MW plasma system, ion bombardment within the plasma allows sufficient energy for the C–C bonds to be weakened or even broken, hence allowing for the attachment of fluorine in the plasma process. However, the ion bombardment did not result in loss of nanotube structure. Efficient covalent functionalization was achieved without destruction of MWNT samples. In the SW-MW plasma system, plasma can be generated with a high electron density and a low electron temperature, which allows uniform covalent functionalization at a low temperature. The fluorine attachment is seen to remain stable over time, showing that the reported functionalization process is reliable. Although in our experiment a few milligrams of MWNTs were functionalized by fluorination, application of the reported method to large quantities up to several grams can be achieved in one step. Uniform SW-MW plasma can be produced over a large area, which gives the flexibility of efficient functionalization of a large quantity CNTs in a one-step process.
4. Conclusion In conclusion, we successfully attached fluorine atoms to the sidewalls of MWNTs through the SW-MW plasma technique using CF4 at a low temperature. XPS analysis showed fluorine atoms grafted on the surface of the CNTs and can be changed with the flow rate of CF4.From the XPS F 1s spectra, it was observed that with an increase in the flow rate the semi-ionic/covalent bonding ratio increases. Raman studies revealed a structural change in the fluorinated MWNTs. There were increases in disordered sp3 hybridization in MWNTs with fluorine incorporation. HRTEM study shows modification of the outer shell of MWNTs with fluorine atom attachment. In the SW-MW plasma system ion bombardment allows sufficient energy for the C–C bonds to be weakened or even broken, hence allowing for the attachment of fluorine.
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