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Surface modification of multi-walled carbon nanotubes by O2 plasma
Applied Surface Science 253 (2007) 8945–8951 www.elsevier.com/locate/apsusc
Surface modification of multi-walled carbon nanotubes by O2 plasma Tao Xu a,b, Jinghui Yang a, Jiwei Liu b, Qiang Fu a,* a
Department of Polymer Materials, Sichuan University, State Key Laboratory of Polymer Materials Engineering, Chengdu 610065, China b Institute of Chemical Materials, Academy of Engineering Physics of China, Mianyang 621900, China Received 25 October 2006; received in revised form 8 May 2007; accepted 8 May 2007 Available online 18 May 2007
Abstract The surface modification of multi-walled carbon nanotubes (MWCNTs) by O2 plasma was carried out in this study. In order to achieve a relatively homogeneous treatment of MWCNTs powder, a rotating barrel fixed between the two discharge electrodes was used. The effect of plasma treatment parameters, such as power, time, and positions of samples (inside and outside the barrel), on the morphology and structure of MWCNTs surface was systematically analyzed by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The results showed that the direct discharge (outside the barrel) could result in not only a quick grafting of polar functional groups but also an easy damage of MWCNTs after longer time, particularly under intensive power. It was found that the surface of MWCNTs powder might be changed in three steps—expansion (loosed structure formed), peel off and oxidization with increasing of treatment time during the irradiation. In this way, a complete purification of MWCNTs powder could be finished within 30 min via plasma treatment. Our work suggested that plasma treatment could be a simple and nonpolluting method for a large scale purification of MWCNTs. # 2007 Elsevier B.V. All rights reserved. Keywords: Multi-walled carbon nanotubes; Plasma; Purification
1. Introduction Carbon nanotube (CNT), since it was discovered by Iijima [1] in 1991, has excited worldwide interest among the material researchers. Due to the unique electrical, mechanical and thermal properties [2] caused by the one-dimensional nanostructure, carbon nanotubes have become the most promising materials in many scientific and technological fields [3–6], particularly, as advanced filler materials in polymer composites. However, the formation of a homogeneous composite is difficult. The inactive surface (containing a layer of amorphous carbon) of CNT makes weak interfacial bonding between polymer matrix and CNT. The agglomeration of CNTs into bundles results in poor CNT dispersion in polymer matrix [7,8]. To overcome these problems, a modification of the carbon nanotubes by changing their surface chemical composition has proven to be efficient.
* Corresponding author. Tel.: +86 28 85460953. E-mail address: [email protected] (Q. Fu). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.05.028
At present, the main approach for modifying the carbon nanotubes is chemical convalent attachment of functional groups. Acid oxidation, which is the most commonplace, is in this category. The CNTs are opened at the end and the terminal carbons are converted into carbarrelylic acids by oxidization in concentrated sulfuric or nitric or mixed acid [9]. Besides, a lot of work has been done on the amination [10,11], fluoration [12,13], and long alkyl chain grafting [14] of the carbon nanotubes through the chemical reactions. Several other methods, such as polymer wrapping [15], electrochemical [16], mechanic-chemical [17] treatment, and plasma treatment, have been reported for the functionalization of the CNTs. Compared with other chemical modifications methods, the plasma treatment method is of great importance because of its nonpolluting property, which is not negligible for an industrial fabrication, and shorter reaction time. Thus, this method provides the possibility of scaling up to produce large quantities necessary for commercial use. It has been widely used for surface activation of various materials, such as organic polymer [18], ceramic [19] and metals [20]. The plasma is usually obtained by glow discharge when the gas is exposed to an electromagnetic field of radio-frequency (rf) at low pressure. So
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the different type of gas permits the fixation of different chemical groups on the surface of CNTs for various applications [21–23]. Valentini [10] has reported the fluorinated single-wall carbon nanotube by plasma, and made CNTs more active for a further chemical modification. The article by Valentine [24] has demonstrated that taking advantage of the plasma polymerization technology, they deposited a thin polymer coating containing the corresponding functional groups on the aligned CNTs for the electrode ally units. Felten et al. [25] made the rf plasma efforts on the nanotube powder attached to a scotch tape in order to avoid dispersion. The chemical components produced by various types of gas were analyzed by XPS. However, the interactions between nanostructures of MWCNTs and plasma particle beams are not clear until now, meanwhile, the industry production of purifying a large scale homogeneous of MWCNTs is not available. In this work, the oxygen plasma technology was used to modify the raw-MWNTs powder in a large scale through our homemade equipment. A PMMA rotating barrel was fixed between the two electrodes. The powder could be rotating with the barrel when being treated by plasma, in order to strengthen the dispersion and obtain a relatively homogeneous modified MWCNTs powder. The effect of variations of plasma treatment parameters, such as power, time, and positions of samples inside and outside the barrel, on the MWCNTs surface morphology and structure was systematically analyzed by Xray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). XPS was used to detect the subtle chemical change of CNTs surface, and SEM was used to observe the morphology change caused by plasma. It was demonstrated that the MWCNTs surface morphology strongly depended on the plasma treatment conditions, and the transformation course was presented. 2. Experiment MWCNTs prepared by chemical vapor deposition (CVD) were purchased from the Organic Chemical Limited Company, Chengdu, China. The lengths were about 50 mm, and the outer diameters ranged from 60 to 80 nm. After drying, one part of
MWCNT was directly put into the rotating barrel (referred to as MWCNT powder), and the other part was attached to a scotch tape as a film (MWCNT film) before they put into the plasma chamber. The inductive coupled plasma was generated in rf-600 (Southwest Academy of Nuclear Physics) with a rotating barrel fixed between the two discharge electrodes, as shown in Fig. 1. The reflective frequency was 13.56 MHz. The diameter of radio-frequency (rf) plate electrode was approximately 350 mm, and the spacing of the electrode and samples was 150 mm. The oxygen plasma treatment conditions for MWCNT film were as follows: oxygen flow rate of 80 sccm, operating pressure of 10 Pa, a bias of 200 V, the power between 100 and 250 W, process duration of 1–5 min, and the average temperature of samples of about 100 8C during the O2 plasma treatment. Taking the carbon nanotubes films as a guide for powder treatment, Carbon nanotube powder was put into the rotating barrel, so the glow discharges made the indirectly effect on the carbon nanotube. It was demonstrated that the treatment result was directly relative to the excited species density controlled by power and the treatment space, so-called power density. When the barrel was added, the space of treatment became smaller, and the power was turned to 800 W, the treatment time ranged from 1 to 30 min. In order to determine the chemical changes during the treatments, XPS measurements were used to analyze with a system equipped with a hemispherical electron energy analyzer (ESCALAB250, England). A mono-chromatized Al Ka line (hn = 1486.6 eV) was used as the photon source, and photoelectrons were collected at an angle of 558 relative to the sample surface normal. The energy resolution of system was 0.9 eV. In the spectrum analysis, the background signal was subtracted by Shirley’s method. The samples were prepared by attaching carbon nanotubes to a scotch conductive tape in a form of thin film, in order to avoid dispersion during the pumping. And the software Avantage 2.52 was used to do peak fitting and quantitative analysis. The samples were prepared by dispersing in water through sanitation and observed under an acceleration voltage of 20 kV with a JEOL JSM-5900LV for SEM experiment.
Fig. 1. Schematic diagram of the rf discharge plasma equipment. (1) Inox vacuum chamber, (2) upper electrode, (3) observation window, (4) down electrode, (5) geers, (6) rotating barrel and (7) gas inlet.
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Fig. 2. XPS spectrum of raw multi-walled carbon nanotubes. The C 1s curve fitting suggests the existence of three species: (1) sp2 carbon atoms of original carbon nanotube; (2) sp3 carbon atoms of amorphous carbon layer; (3) oxygen from air contamination.
3. Results and discussion As reference spectra, the C 1s peaks of the as-grown MWCNT are recorded. There are three peaks indicating three different chemical states of C 1s shown in Fig. 2, which can be used to identify functional groups attached to the CNT. The main peak (1) at 283.8 eV corresponds to the sp2 carbon atoms of original carbon nanotube, similar to the graphite; (2) 284.7 eV matches with the sp3 carbon atoms, indicates the amorphous carbon layer at the surface of CNTs; (3) 289.0 eV is attributed to the oxygen–carbon from air contamination. This is consistent with the fact that nonpurified nanotube powder also contains amorphous carbon and that defects exist at the surface of the nanotubes [26]. Fig. 3 shows the XPS C 1s spectrum of CNTs film treated by oxygen plasma. It can be found that there is one more chemical states compared with Fig. 2, and the new-formed function is attributed to oxidation carbon at 286.4 eV of C O. Table 1 summarizes the different chemical states of C 1s and its relative percentage along with the oxygen atom percentage after XPS peak fitting and quantitative analysis. At first, it can be known that the CNT main structure survived the irradiation as the chemical state of sp2 carbon at 283.9 eV remains the same and the relative percentage also keeps at about 62%. Secondly, the content of sp3 carbon decreases from 23.8 to 5.0% after plasma
Fig. 3. XPS C 1s spectrum of CNT film treated by oxygen plasma under the condition: power 100 W, treatment time: 5 min. The curve fitting suggests the existence of three species: (1) sp2 carbon atoms of original carbon nanotube; (2) sp3 carbon atoms of amorphous carbon layer; (3) C O; (4) air contamination.
irradiation for 5 min, while that of oxygen-contained functional groups increases from zero to 30.9%. This means that the oxygen-contained functional group can be easily grafted to sp3 carbon due to its low energy and relatively high reactivity. Meanwhile, the content of C–O–O is slightly decreased because of the etching effect of O2 plasma. At last, the relative oxygen atom percentage calculated from XPS oxygen narrow spectra (not shown here) increases immediately from 1.6 to 9.1% after 1 min exposure, then gradually increases to 16.6% in 5 min. The results of O 1s and C 1s are consistent, and they both indicate that the oxygen-contained carbon groups have grafted on the CNT after O2 plasma treatment. Furthermore, the effect of long treatment time and high power of plasma has been studied. The power was changed from 100 to 150, 200 and 250 W at an elevated time of 5 min. Fig. 4 is the XPS survey spectra with different process duration and different power. Nickel is the catalyst for the CVD synthesis of carbon nanotubes. Nickel particles are usually encapsulated in the nanotubes as there are not any signals of nickel in the XPS spectrum of pristine carbon nanotubes (seen in Fig. 2). Only after prolonging the exposure time to 10 min or
Fig. 4. XPS survey spectra of carbon nanotubes film treated by oxygen plasma under the following conditions: (a) 100 W, 10 min; (b) 200 W, 5 min; (c) 250 W, 5 min. Inset: XPS Ni 2p spectrum of catalyst of carbon nanotubes obtained by oxygen plasma treatment under the same conditions.
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Table 1 Relative percentage of the four XPS components of carbon atoms and the calculated ratio of carboxyl groups and oxidized carbons
MWCNT–pristine MWCNT film: 1 min MWCNT film: 2 min MWCNT film: 3 min MWCNT film: 4 min MWCNT film: 5 min
sp2 (eV, %) graphite
sp3 (eV, %) contamination
C O (eV, %)
COO (eV, %)
[O], %
283.9, 283.9, 283.9, 283.9, 283.9, 283.9,
284.7, 284.8, 284.7, 284.6, 284.8, 284.7,
286.3, 286.2, 286.1, 286.1, 286.1,
289.0, 289.9, 290.5, 290.1, 289.9, 289.7,
1.6 9.1 11.3 11.5 13.5 16.6
68.5 62.6 61.9 62.4 63.2 59.9
23.8 13.9 9.9 6.8 4.5 5.0
16.7 21.8 25.4 27.8 30.9
7.6 6.8 6.2 5.2 4.3 4.1
The samples were treated in the form of film.
much longer, or raising the power to 200 W or more and keeping the exposure time at 5 min, do the signals of nickel appear (seen in Fig. 4). The signals of transition metals (Ni) reveal that the CNTs are etched and destroyed by the oxygen plasma. This phenomenon is matched with the results obtained by Felten [25]. It is concluded that the increase of the power (over 200 W) can increase the amounts of the reactive particles, leading to the more chances to react with the CNTs. The long treatment time (over 10 min at 100 W) may lead to destroy the carbon nanotubes, too. Fig. 5 gives the C 1s spectrum of MWCNT powder after plasma treatment. The peak at 286 eV (C O) can be clearly seen when the treatment time is less than 5 min. This result is well coordinated with that (shown in Fig. 3) obtained by the
direct discharge. It is clearly demonstrated that MWCNT powder can be modified in the PMMA barrel, and the intensity of O 1s spectrum increases sharply with plasma treatment time increasing. With the further increase of treatment time, more than 10 min (including 10 min), however, the intensity of C O peak is gradually eliminated and the curve in the spectrum is very similar to the curve of raw-MWCNTs (seen in Fig. 5B). Fig. 6 shows the relative oxygen atom percent with different process duration time of MWCNT powder. The grafting ratio increases sharply and reaches a maximum value (6.2%) within a very short duration time, about 5 min. And then the grafting ratio decreases. When the time increases to 30 min, the oxygen atom percentage is only 0.9%, even less than the rawMWCNTs, and Ni element is not detected at any time, which is different from the direct discharge. 3.1. SEM For a further discussion, SEM is used to observe the morphologies of the carbon nanotube shown in Fig. 7. The tubes of raw-MWCNTs can be clearly seen, where the amorphous carbon layer is deposited on the surface of CNTs, connecting with each other. During a short period of time, such as 3–5 min, there is no obvious difference with the rawMWCNTs. With the treatment time increasing, the morphology
Fig. 5. XPS of C 1s spectra of CNTs powder treated by oxygen plasma at power 800 W, with the different treatment time. (A) a: 5 min, b: 4 min, c: 3 min, d: 2 min, e: 1 min. (B) a: 30 min, b: 25 min, c: 20 min, d: 15 min, e: 10 min.
Fig. 6. The relative oxygen atom percent with different process duration time of MWCNT powder.
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Fig. 7. SEM images of CNT powder treated by oxygen plasma at power of 800 W. (a) Pristine, (b) 3 min, (c) 5 min, (d) 10 min, (e) 20 min, (f) 25 min and (g) 30 min. The scale bar is same (1 mm) for all images.
of CNTs is evidently changed. When treated for 10 min, the tubes are expanded. Twenty and 25 min later, seen in Fig. 7e and f, respectively, carbon nanotubes become ambiguous and one can not even divide a single carbon nanotube from the others. To our surprise, the carbon nanotubes become clearly seen after plasma treatment for 30 min and the amorphous carbon domains are eliminated and the impurities are removed. The software of SMILE-VIEW was applied to detect the diameter of CNTs. It is found that the diameter of CNTs after
treated for 10 min was 85 nm in average, 71 nm for 30 min treatment whereas 81 nm for the raw-MWCNTs. Combined with XPS results and SEM figures, the two different irradiation methods lead to different phenomena. The MWCNTs irradiated directly can be grafted with polar functional groups quickly, but tends to be bent or destroyed due to the ion bombardment under higher power and longer time. The irradiation effect of MWCNTs in the barrel is different, the polar functional groups are much less, and the
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diameter of the MWCNTs increases first, about 85 nm in 10 min and then reduces to 71 nm in 30 min. The possible reason is discussed as follows. During the experiment in the rotating magnetron-sputtering apparatus, the ions of the O2 plasma have high energy but lower effect compared with that treated directly in the chamber, so that the process of transformation of MWCNTs is delayed. As far as the transformation of MWCNTs is concerned, it might be three steps during the process of irradiation. At first, ion bombardment causes the creation of vacancies and interstitials in MWCNTs by knock-on collisions if the carbon nuclei of MWCNTs receive the energy from the ion. The superficial structure including defects and amorphous carbon nanowires is loosed because of ion sweeping, which leads to the diameters expanded. From SEM figures, the morphology of MWCNTs treated under 10 mins can reveal such phenomena, and the results are the same with the previous paper as well [22,26,27]. Secondly, the ion beams continue to react with the amorphous carbon until they are peeled totally from the carbon nanotubes. The surface morphology of MWCNTs after being treated for
20 min becomes rough and be coated with amorphous carbon, which makes the nanotube’ s framework structure unclear. Thirdly, the amorphous carbon is oxidized under O2 plasma irradiation due to the sp3 bonding energy is quite low to sp2 structure, and the diameter of MWCNTs becomes small which is observed in Fig. 7 g. Meanwhile, the disappearance of C O in XPS spectra also reveals the phenomenon. The functional groups along with the defects and amorphous carbon are eliminated due to oxygen plasma will react with those more easily than with carbon nanotubes. In summary, the MWCNTs in the rotating barrel has experienced three steps—expansion (loosed structure formed), peel off and oxidization. Fig. 8 shows the proposed model for the transformation of MWCNTs under O2 plasma treatment. CNTs are considered to actually undergo a purification process, while carbon atoms sputtering from MWCNTs will react with oxygen ion to form carbon dioxide at the surface of original carbon nanotube framework. Compared with traditional oxidation by acid (24 h), the plasma treatment time is shortened and the whole process is nonpolluting. Meanwhile, the amount
Fig. 8. The proposed model for the transformation process of the MWCNTs in the rotating barrel under O2 plasma treatment.
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of treatment can be adjusted with the volume of barrel; and the maximum can reach up to 10 g in our study. 4. Conclusion Oxygen plasma treatment is an effective way for modifying the carbon nanotube with proper parameters. However, the direct discharge can graft polar function more quickly but longer exposure time or higher energy can destroy the carbon nanotube. The interaction mechanism of MWCNTs with O2 plasma in the rotating barrel is different. XPS and SEM results show that the carbon nanotubes in the rotating barrel are totally purified through 30 min plasma treatment. There are more advantages than chemical modification, such as nonpolluting, shorter treatment time. It is concluded that the carbon nanotubes treated by plasma is available. As a new method for purifying a large scale of the MWCNT powder, plasma treatment is valuable for industry production. Acknowledgements The authors thank Drs. Zhong Wei and Ai Lu for their guide and instructions. This work was financially supported by the Special Foundation of CAEP (No. 62601080320). Reference [1] S. Iijima, Nature 56 (1991) 354. [2] M. Dresselhaus, P. Avouris, Carbon Nanotubes: Synthesis, Properties and Application, Springer Verlag, Berlin, Heidelberg, 2001. [3] J.P. Lu, J. Phys. Chem. Solids 58 (1997) 1649. [4] S.S. Xie, W.Z. Li, Z.W. Pan, B.H. Chang, L.F. Sun, J. Phys. Chem. Solids 61 (2000) 1153. [5] Q.M. Gong, Z. Li, X.D. Bai, D. Li, Y. Zhao, J. Liang, Mater. Sci. Eng. 384 (2004) 209. [6] H.L. Zeng, C. Gao, D.Y. Yan, Adv. Funct. Mater. 16 (2006) 812.
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Surface modification of multi-walled carbon nanotubes by O2 plasma Tao Xu a,b, Jinghui Yang a, Jiwei Liu b, Qiang Fu a,* a
Department of Polymer Materials, Sichuan University, State Key Laboratory of Polymer Materials Engineering, Chengdu 610065, China b Institute of Chemical Materials, Academy of Engineering Physics of China, Mianyang 621900, China Received 25 October 2006; received in revised form 8 May 2007; accepted 8 May 2007 Available online 18 May 2007
Abstract The surface modification of multi-walled carbon nanotubes (MWCNTs) by O2 plasma was carried out in this study. In order to achieve a relatively homogeneous treatment of MWCNTs powder, a rotating barrel fixed between the two discharge electrodes was used. The effect of plasma treatment parameters, such as power, time, and positions of samples (inside and outside the barrel), on the morphology and structure of MWCNTs surface was systematically analyzed by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The results showed that the direct discharge (outside the barrel) could result in not only a quick grafting of polar functional groups but also an easy damage of MWCNTs after longer time, particularly under intensive power. It was found that the surface of MWCNTs powder might be changed in three steps—expansion (loosed structure formed), peel off and oxidization with increasing of treatment time during the irradiation. In this way, a complete purification of MWCNTs powder could be finished within 30 min via plasma treatment. Our work suggested that plasma treatment could be a simple and nonpolluting method for a large scale purification of MWCNTs. # 2007 Elsevier B.V. All rights reserved. Keywords: Multi-walled carbon nanotubes; Plasma; Purification
1. Introduction Carbon nanotube (CNT), since it was discovered by Iijima [1] in 1991, has excited worldwide interest among the material researchers. Due to the unique electrical, mechanical and thermal properties [2] caused by the one-dimensional nanostructure, carbon nanotubes have become the most promising materials in many scientific and technological fields [3–6], particularly, as advanced filler materials in polymer composites. However, the formation of a homogeneous composite is difficult. The inactive surface (containing a layer of amorphous carbon) of CNT makes weak interfacial bonding between polymer matrix and CNT. The agglomeration of CNTs into bundles results in poor CNT dispersion in polymer matrix [7,8]. To overcome these problems, a modification of the carbon nanotubes by changing their surface chemical composition has proven to be efficient.
* Corresponding author. Tel.: +86 28 85460953. E-mail address: [email protected] (Q. Fu). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.05.028
At present, the main approach for modifying the carbon nanotubes is chemical convalent attachment of functional groups. Acid oxidation, which is the most commonplace, is in this category. The CNTs are opened at the end and the terminal carbons are converted into carbarrelylic acids by oxidization in concentrated sulfuric or nitric or mixed acid [9]. Besides, a lot of work has been done on the amination [10,11], fluoration [12,13], and long alkyl chain grafting [14] of the carbon nanotubes through the chemical reactions. Several other methods, such as polymer wrapping [15], electrochemical [16], mechanic-chemical [17] treatment, and plasma treatment, have been reported for the functionalization of the CNTs. Compared with other chemical modifications methods, the plasma treatment method is of great importance because of its nonpolluting property, which is not negligible for an industrial fabrication, and shorter reaction time. Thus, this method provides the possibility of scaling up to produce large quantities necessary for commercial use. It has been widely used for surface activation of various materials, such as organic polymer [18], ceramic [19] and metals [20]. The plasma is usually obtained by glow discharge when the gas is exposed to an electromagnetic field of radio-frequency (rf) at low pressure. So
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the different type of gas permits the fixation of different chemical groups on the surface of CNTs for various applications [21–23]. Valentini [10] has reported the fluorinated single-wall carbon nanotube by plasma, and made CNTs more active for a further chemical modification. The article by Valentine [24] has demonstrated that taking advantage of the plasma polymerization technology, they deposited a thin polymer coating containing the corresponding functional groups on the aligned CNTs for the electrode ally units. Felten et al. [25] made the rf plasma efforts on the nanotube powder attached to a scotch tape in order to avoid dispersion. The chemical components produced by various types of gas were analyzed by XPS. However, the interactions between nanostructures of MWCNTs and plasma particle beams are not clear until now, meanwhile, the industry production of purifying a large scale homogeneous of MWCNTs is not available. In this work, the oxygen plasma technology was used to modify the raw-MWNTs powder in a large scale through our homemade equipment. A PMMA rotating barrel was fixed between the two electrodes. The powder could be rotating with the barrel when being treated by plasma, in order to strengthen the dispersion and obtain a relatively homogeneous modified MWCNTs powder. The effect of variations of plasma treatment parameters, such as power, time, and positions of samples inside and outside the barrel, on the MWCNTs surface morphology and structure was systematically analyzed by Xray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). XPS was used to detect the subtle chemical change of CNTs surface, and SEM was used to observe the morphology change caused by plasma. It was demonstrated that the MWCNTs surface morphology strongly depended on the plasma treatment conditions, and the transformation course was presented. 2. Experiment MWCNTs prepared by chemical vapor deposition (CVD) were purchased from the Organic Chemical Limited Company, Chengdu, China. The lengths were about 50 mm, and the outer diameters ranged from 60 to 80 nm. After drying, one part of
MWCNT was directly put into the rotating barrel (referred to as MWCNT powder), and the other part was attached to a scotch tape as a film (MWCNT film) before they put into the plasma chamber. The inductive coupled plasma was generated in rf-600 (Southwest Academy of Nuclear Physics) with a rotating barrel fixed between the two discharge electrodes, as shown in Fig. 1. The reflective frequency was 13.56 MHz. The diameter of radio-frequency (rf) plate electrode was approximately 350 mm, and the spacing of the electrode and samples was 150 mm. The oxygen plasma treatment conditions for MWCNT film were as follows: oxygen flow rate of 80 sccm, operating pressure of 10 Pa, a bias of 200 V, the power between 100 and 250 W, process duration of 1–5 min, and the average temperature of samples of about 100 8C during the O2 plasma treatment. Taking the carbon nanotubes films as a guide for powder treatment, Carbon nanotube powder was put into the rotating barrel, so the glow discharges made the indirectly effect on the carbon nanotube. It was demonstrated that the treatment result was directly relative to the excited species density controlled by power and the treatment space, so-called power density. When the barrel was added, the space of treatment became smaller, and the power was turned to 800 W, the treatment time ranged from 1 to 30 min. In order to determine the chemical changes during the treatments, XPS measurements were used to analyze with a system equipped with a hemispherical electron energy analyzer (ESCALAB250, England). A mono-chromatized Al Ka line (hn = 1486.6 eV) was used as the photon source, and photoelectrons were collected at an angle of 558 relative to the sample surface normal. The energy resolution of system was 0.9 eV. In the spectrum analysis, the background signal was subtracted by Shirley’s method. The samples were prepared by attaching carbon nanotubes to a scotch conductive tape in a form of thin film, in order to avoid dispersion during the pumping. And the software Avantage 2.52 was used to do peak fitting and quantitative analysis. The samples were prepared by dispersing in water through sanitation and observed under an acceleration voltage of 20 kV with a JEOL JSM-5900LV for SEM experiment.
Fig. 1. Schematic diagram of the rf discharge plasma equipment. (1) Inox vacuum chamber, (2) upper electrode, (3) observation window, (4) down electrode, (5) geers, (6) rotating barrel and (7) gas inlet.
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Fig. 2. XPS spectrum of raw multi-walled carbon nanotubes. The C 1s curve fitting suggests the existence of three species: (1) sp2 carbon atoms of original carbon nanotube; (2) sp3 carbon atoms of amorphous carbon layer; (3) oxygen from air contamination.
3. Results and discussion As reference spectra, the C 1s peaks of the as-grown MWCNT are recorded. There are three peaks indicating three different chemical states of C 1s shown in Fig. 2, which can be used to identify functional groups attached to the CNT. The main peak (1) at 283.8 eV corresponds to the sp2 carbon atoms of original carbon nanotube, similar to the graphite; (2) 284.7 eV matches with the sp3 carbon atoms, indicates the amorphous carbon layer at the surface of CNTs; (3) 289.0 eV is attributed to the oxygen–carbon from air contamination. This is consistent with the fact that nonpurified nanotube powder also contains amorphous carbon and that defects exist at the surface of the nanotubes [26]. Fig. 3 shows the XPS C 1s spectrum of CNTs film treated by oxygen plasma. It can be found that there is one more chemical states compared with Fig. 2, and the new-formed function is attributed to oxidation carbon at 286.4 eV of C O. Table 1 summarizes the different chemical states of C 1s and its relative percentage along with the oxygen atom percentage after XPS peak fitting and quantitative analysis. At first, it can be known that the CNT main structure survived the irradiation as the chemical state of sp2 carbon at 283.9 eV remains the same and the relative percentage also keeps at about 62%. Secondly, the content of sp3 carbon decreases from 23.8 to 5.0% after plasma
Fig. 3. XPS C 1s spectrum of CNT film treated by oxygen plasma under the condition: power 100 W, treatment time: 5 min. The curve fitting suggests the existence of three species: (1) sp2 carbon atoms of original carbon nanotube; (2) sp3 carbon atoms of amorphous carbon layer; (3) C O; (4) air contamination.
irradiation for 5 min, while that of oxygen-contained functional groups increases from zero to 30.9%. This means that the oxygen-contained functional group can be easily grafted to sp3 carbon due to its low energy and relatively high reactivity. Meanwhile, the content of C–O–O is slightly decreased because of the etching effect of O2 plasma. At last, the relative oxygen atom percentage calculated from XPS oxygen narrow spectra (not shown here) increases immediately from 1.6 to 9.1% after 1 min exposure, then gradually increases to 16.6% in 5 min. The results of O 1s and C 1s are consistent, and they both indicate that the oxygen-contained carbon groups have grafted on the CNT after O2 plasma treatment. Furthermore, the effect of long treatment time and high power of plasma has been studied. The power was changed from 100 to 150, 200 and 250 W at an elevated time of 5 min. Fig. 4 is the XPS survey spectra with different process duration and different power. Nickel is the catalyst for the CVD synthesis of carbon nanotubes. Nickel particles are usually encapsulated in the nanotubes as there are not any signals of nickel in the XPS spectrum of pristine carbon nanotubes (seen in Fig. 2). Only after prolonging the exposure time to 10 min or
Fig. 4. XPS survey spectra of carbon nanotubes film treated by oxygen plasma under the following conditions: (a) 100 W, 10 min; (b) 200 W, 5 min; (c) 250 W, 5 min. Inset: XPS Ni 2p spectrum of catalyst of carbon nanotubes obtained by oxygen plasma treatment under the same conditions.
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Table 1 Relative percentage of the four XPS components of carbon atoms and the calculated ratio of carboxyl groups and oxidized carbons
MWCNT–pristine MWCNT film: 1 min MWCNT film: 2 min MWCNT film: 3 min MWCNT film: 4 min MWCNT film: 5 min
sp2 (eV, %) graphite
sp3 (eV, %) contamination
C O (eV, %)
COO (eV, %)
[O], %
283.9, 283.9, 283.9, 283.9, 283.9, 283.9,
284.7, 284.8, 284.7, 284.6, 284.8, 284.7,
286.3, 286.2, 286.1, 286.1, 286.1,
289.0, 289.9, 290.5, 290.1, 289.9, 289.7,
1.6 9.1 11.3 11.5 13.5 16.6
68.5 62.6 61.9 62.4 63.2 59.9
23.8 13.9 9.9 6.8 4.5 5.0
16.7 21.8 25.4 27.8 30.9
7.6 6.8 6.2 5.2 4.3 4.1
The samples were treated in the form of film.
much longer, or raising the power to 200 W or more and keeping the exposure time at 5 min, do the signals of nickel appear (seen in Fig. 4). The signals of transition metals (Ni) reveal that the CNTs are etched and destroyed by the oxygen plasma. This phenomenon is matched with the results obtained by Felten [25]. It is concluded that the increase of the power (over 200 W) can increase the amounts of the reactive particles, leading to the more chances to react with the CNTs. The long treatment time (over 10 min at 100 W) may lead to destroy the carbon nanotubes, too. Fig. 5 gives the C 1s spectrum of MWCNT powder after plasma treatment. The peak at 286 eV (C O) can be clearly seen when the treatment time is less than 5 min. This result is well coordinated with that (shown in Fig. 3) obtained by the
direct discharge. It is clearly demonstrated that MWCNT powder can be modified in the PMMA barrel, and the intensity of O 1s spectrum increases sharply with plasma treatment time increasing. With the further increase of treatment time, more than 10 min (including 10 min), however, the intensity of C O peak is gradually eliminated and the curve in the spectrum is very similar to the curve of raw-MWCNTs (seen in Fig. 5B). Fig. 6 shows the relative oxygen atom percent with different process duration time of MWCNT powder. The grafting ratio increases sharply and reaches a maximum value (6.2%) within a very short duration time, about 5 min. And then the grafting ratio decreases. When the time increases to 30 min, the oxygen atom percentage is only 0.9%, even less than the rawMWCNTs, and Ni element is not detected at any time, which is different from the direct discharge. 3.1. SEM For a further discussion, SEM is used to observe the morphologies of the carbon nanotube shown in Fig. 7. The tubes of raw-MWCNTs can be clearly seen, where the amorphous carbon layer is deposited on the surface of CNTs, connecting with each other. During a short period of time, such as 3–5 min, there is no obvious difference with the rawMWCNTs. With the treatment time increasing, the morphology
Fig. 5. XPS of C 1s spectra of CNTs powder treated by oxygen plasma at power 800 W, with the different treatment time. (A) a: 5 min, b: 4 min, c: 3 min, d: 2 min, e: 1 min. (B) a: 30 min, b: 25 min, c: 20 min, d: 15 min, e: 10 min.
Fig. 6. The relative oxygen atom percent with different process duration time of MWCNT powder.
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Fig. 7. SEM images of CNT powder treated by oxygen plasma at power of 800 W. (a) Pristine, (b) 3 min, (c) 5 min, (d) 10 min, (e) 20 min, (f) 25 min and (g) 30 min. The scale bar is same (1 mm) for all images.
of CNTs is evidently changed. When treated for 10 min, the tubes are expanded. Twenty and 25 min later, seen in Fig. 7e and f, respectively, carbon nanotubes become ambiguous and one can not even divide a single carbon nanotube from the others. To our surprise, the carbon nanotubes become clearly seen after plasma treatment for 30 min and the amorphous carbon domains are eliminated and the impurities are removed. The software of SMILE-VIEW was applied to detect the diameter of CNTs. It is found that the diameter of CNTs after
treated for 10 min was 85 nm in average, 71 nm for 30 min treatment whereas 81 nm for the raw-MWCNTs. Combined with XPS results and SEM figures, the two different irradiation methods lead to different phenomena. The MWCNTs irradiated directly can be grafted with polar functional groups quickly, but tends to be bent or destroyed due to the ion bombardment under higher power and longer time. The irradiation effect of MWCNTs in the barrel is different, the polar functional groups are much less, and the
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diameter of the MWCNTs increases first, about 85 nm in 10 min and then reduces to 71 nm in 30 min. The possible reason is discussed as follows. During the experiment in the rotating magnetron-sputtering apparatus, the ions of the O2 plasma have high energy but lower effect compared with that treated directly in the chamber, so that the process of transformation of MWCNTs is delayed. As far as the transformation of MWCNTs is concerned, it might be three steps during the process of irradiation. At first, ion bombardment causes the creation of vacancies and interstitials in MWCNTs by knock-on collisions if the carbon nuclei of MWCNTs receive the energy from the ion. The superficial structure including defects and amorphous carbon nanowires is loosed because of ion sweeping, which leads to the diameters expanded. From SEM figures, the morphology of MWCNTs treated under 10 mins can reveal such phenomena, and the results are the same with the previous paper as well [22,26,27]. Secondly, the ion beams continue to react with the amorphous carbon until they are peeled totally from the carbon nanotubes. The surface morphology of MWCNTs after being treated for
20 min becomes rough and be coated with amorphous carbon, which makes the nanotube’ s framework structure unclear. Thirdly, the amorphous carbon is oxidized under O2 plasma irradiation due to the sp3 bonding energy is quite low to sp2 structure, and the diameter of MWCNTs becomes small which is observed in Fig. 7 g. Meanwhile, the disappearance of C O in XPS spectra also reveals the phenomenon. The functional groups along with the defects and amorphous carbon are eliminated due to oxygen plasma will react with those more easily than with carbon nanotubes. In summary, the MWCNTs in the rotating barrel has experienced three steps—expansion (loosed structure formed), peel off and oxidization. Fig. 8 shows the proposed model for the transformation of MWCNTs under O2 plasma treatment. CNTs are considered to actually undergo a purification process, while carbon atoms sputtering from MWCNTs will react with oxygen ion to form carbon dioxide at the surface of original carbon nanotube framework. Compared with traditional oxidation by acid (24 h), the plasma treatment time is shortened and the whole process is nonpolluting. Meanwhile, the amount
Fig. 8. The proposed model for the transformation process of the MWCNTs in the rotating barrel under O2 plasma treatment.
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of treatment can be adjusted with the volume of barrel; and the maximum can reach up to 10 g in our study. 4. Conclusion Oxygen plasma treatment is an effective way for modifying the carbon nanotube with proper parameters. However, the direct discharge can graft polar function more quickly but longer exposure time or higher energy can destroy the carbon nanotube. The interaction mechanism of MWCNTs with O2 plasma in the rotating barrel is different. XPS and SEM results show that the carbon nanotubes in the rotating barrel are totally purified through 30 min plasma treatment. There are more advantages than chemical modification, such as nonpolluting, shorter treatment time. It is concluded that the carbon nanotubes treated by plasma is available. As a new method for purifying a large scale of the MWCNT powder, plasma treatment is valuable for industry production. Acknowledgements The authors thank Drs. Zhong Wei and Ai Lu for their guide and instructions. This work was financially supported by the Special Foundation of CAEP (No. 62601080320). Reference [1] S. Iijima, Nature 56 (1991) 354. [2] M. Dresselhaus, P. Avouris, Carbon Nanotubes: Synthesis, Properties and Application, Springer Verlag, Berlin, Heidelberg, 2001. [3] J.P. Lu, J. Phys. Chem. Solids 58 (1997) 1649. [4] S.S. Xie, W.Z. Li, Z.W. Pan, B.H. Chang, L.F. Sun, J. Phys. Chem. Solids 61 (2000) 1153. [5] Q.M. Gong, Z. Li, X.D. Bai, D. Li, Y. Zhao, J. Liang, Mater. Sci. Eng. 384 (2004) 209. [6] H.L. Zeng, C. Gao, D.Y. Yan, Adv. Funct. Mater. 16 (2006) 812.
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