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Oxygen Functionalization of Multiwall Carbon Nanotubes by Microwave-Excited Surface-Wave Plasma Treatment Changlun Chen,†,‡ Bo Liang,† Akihisa Ogino,† Xiangke Wang,*,‡ and Masaaki Nagatsu*,† Graduate School of Science and Technology, Shizuoka UniVersity, 3-5-1, Johoka-ku, Hamamatsu 432-8561, Japan, and Key Laboratory of NoVel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, 230031 Hefei, P. R. China ReceiVed: February 9, 2009; ReVised Manuscript ReceiVed: March 9, 2009
In this work, oxygen-containing groups were introduced onto multiwall carbon nanotubes (MWCNTs) by using microwave-excited Ar/O2 surface-wave plasma (SWP) treatment. The changes of the atomic contents and structure properties of MWCNTs as a function of gas flow rate, treatment time, and plasma power were analyzed using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. A mechanism of MWCNT oxidation was proposed, based on XPS analysis, which showed how oxygen-containing groups, such as C-O, CdO, and O-CdO, were generated on the surfaces of MWCNTs. The surface morphology of MWCNTs was observed by field emission scanning electron microscopy (FE-SEM). The results indicated that Ar/O2 plasma treatment greatly enhanced the content of oxygen, modified structure properties, induced more surface defects, and improved the dispersion of MWCNTs in aqueous solution. The integrity of the nanotube patterns was not damaged. I. Introduction Carbon nanotubes (CNTs) are promising new material for a variety of potential applications, due to their unique physical and chemical properties. However, CNTs usually tend to agglomerate in bundles or other aggregates resulting in poor solubility in most solvents and poor chemical and biological compatibility, which greatly hinders the applications of CNTs in real work.1-4 To improve their solubility, CNTs are functionalized by different methods. The easiest way to covalently attach chemical groups (e.g., carboxylic groups) is by oxidation, such as nitric (HNO3) and sulfuric (H2SO4) acid oxidation,5,6 air oxygen,7,8 ozone oxidation,9,10 and plasma oxidation,11-16 resulting in the formation of carboxyl and carbonyl groups on the surfaces of the nanotubes. These groups can also be used as sites for further functionalization by other molecules.6,17,18 The tips of CNTs were shown to be more reactive than their sidewalls,19 and the treatment of CNTs with certain acids (e.g., refluxing in HNO3 and/or H2SO4) was proved to open the nanotube tips and to introduce oxygen-containing groups at the opened ends.20-24 However, due to the rather harsh conditions involved, most oxidation reactions result in the opening of the nanotube tips, detrimental damage of their sidewalls, or both. A particularly attractive option is the chemical modification of CNTs while largely retaining their structural integrity. It is important that these treatments are surface specific so that the bulk properties are preserved. Plasma treatment is an efficient method in the field of surface modifications. The excited species, radicals, electrons, ions, and UV light within plasma strongly interact with the surfaces of CNTs breaking the CdC bonds and creating active sites for binding of functional groups, and, as a result, chemical and physical modifications occur on the surfaces. Compared to other * Corresponding authors. Tel.: +81-53-478-1081. E-mail: tmnagat@ ipc.shizuoka.ac.jp (M.N.);
[email protected] (X.W.). † Shizuoka University. ‡ Chinese Academy of Sciences.
chemical modification methods, the plasma treatment method has the advantages of shorter reaction time, nonpolluting process, and providing a wide range of different functional groups depending on plasma parameters such as power, used gases, treatment time, and pressure. Thus, this method offers the possibility of scaling up to produce large quantities necessary for commercial use. In the microwave-excited surface-wave plasma (MW-SWP) system, plasma can be generated with high electron density and a low electron temperature, which allows uniform covalent functionalization of nanotubes at low temperature. Ruelle et al.25 reported that atomic nitrogen chemical groups could be grafted onto the surfaces of CNTs using microwave N2, and that the surfaces of CNTs were not significantly damaged. Khare et al.26 reported that single-wall CNTs were successfully functionalized through a microwave discharge of ammonia. To the best of our knowledge, the introduction of oxygen-containing groups on CNTs by using MW-SWP plasma treatment has not been reported so far. In this work, the oxygen-containing groups were introduced onto the surfaces of multiwall carbon nanotubes (MWCNTs) by using Ar/O2 MW-SWP treatment. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were used to estimate the influence of Ar/O2 mixture gas flow rate, treatment time, and plasma power on the generation of oxygen-containing groups onto the surfaces of MWCNTs. The surface morphology of MWCNTs before and after plasma treatment was observed by field emission scanning electron microscopy (FE-SEM), and a mechanism of MWCNT oxidation was proposed based on the analysis of XPS results, which showed that oxygencontaining groups, such as C-O, CdO, and O-CdO, were generated on the surfaces of MWCNTs. II. Experimental Section The MWCNTs used in this work were synthesized by using chemical vapor deposition (CVD) of acetylene in hydrogen flow at 760 °C using Ni-Fe nanoparticles as catalysts.24 The plasma
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TABLE 1: Plasma Treatment Parameters Used for MWCNT Oxidation and XPS Analysis as a Function of Gas Flow Rate, Plasma Power, and Treatment Time MWCNTs untreated MWCNTs O2 40 sccm, 700 W, 15 min O2 70 sccm, 700 W, 15 min O2 110 sccm, 700 W, 15 min Ar/O2 70/40 sccm, 700 W, 15 min Ar/O2 70/40 sccm, 700 W, 10 min Ar/O2 70/40 sccm, 700 W, 5 min Ar/O2 70/40 sccm, 500 W, 5 min
peak fwhm element (eV) (eV) atom % C 1s O 1s C 1s O 1s C 1s O 1s C 1s O 1s C 1s O 1s C 1s O 1s C 1s O 1s C 1s O 1s
284.1 532.7 284.1 533.5 283.8 532.8 283.9 533.3 284.0 533.6 284.1 533.3 284.1 533.3 283.5 532.5
1.61 2.28 2.16 2.08 2.50 2.00 2.56 2.21 2.69 1.97 2.24 1.82 2.25 1.84 1.76 1.91
88.68 11.32 80.01 19.99 73.86 26.14 66.23 33.77 58.77 41.23 61.72 38.28 64.10 35.90 78.54 21.46
device for MWCNT modification consisted of a cylindrical stainless vacuum chamber with a diameter of 40 cm and a height of 40 cm. There was a 2.45 GHz microwave generator in the upper part of the chamber. The microwaves were guided through rectangular waveguide and introduced into the chamber through slot antennas.27 The reflected microwave power was minimized by adjusting the E-H tuner. During the surface-wave plasma discharge operation, microwaves damped exponentially below the quartz window where the electron density (more than 3.6 × 1011 cm-3) generally exceeded the cutoff density of 7.4 × 1010 cm-3.27 A high-density plasma with uniform density was formed in the vacuum chamber and broadened in the downstream region owing to particle diffusion. MWCNT powder was placed in a glass beaker. The stage supporting the glass beaker
Figure 1. Emission spectra of Ar/O2 MW-SWP.
was kept at a 22.5 cm distance from the plasma-showering quartz plate. O2/N2 mixture gases of 99.99% purity were introduced through mass flow meters. Microwave plasma powers were 500 and 700 W, and the treatment times were 5, 10, and 15 min, respectively (listed in Table 1). XPS measurements of MWCNTs before and after plasma treatment were carried out on an ESCA-3400 spectrometer (Shimadzu, Japan). Raman spectroscopy was carried out on an NR-1800 laser Raman spectrometer (JASCO, Japan). FE-SEM measurements of MWCNTs were carried out by using a JSM-6320F FE-SEM (JEOL). III. Results and Discussion Optical Emission Spectroscopy. Optical emission spectroscopy (OES) is often used for diagnostics of reactive plasmas. The main advantage of this method is the noninvasive character of measurements. Several wavelengths corresponding to atomic transitions in argon and oxygen are used to analyze the plasma emission spectra. The most significant oxygen line in our experimental conditions is the 777.2 nm line (seen in Figures 1A, C, and D), which corresponds to the deexcitation of the oxygen atoms from the state 5P(O*), produced by the following ways: e + O2fe + O* + O and e + O2fe + O*, i.e., dissociative excitation or direct impact excitation of the oxygen atoms, respectively.28 In the presence of Ar, the content of active oxygen is enhanced due to the production of oxygen atoms by the quenching reaction ArM + O2fAr + O* + O*, where ArM is a metastable atom. The content of active oxygen also enhances with increasing of plasma power (Figure 1D). The active oxygen can be created in the bulk of plasma or on the surfaces of MWCNTs. The active oxygen species are highly reactive and interact with the surfaces of MWCNTs. MWCNT oxidation may be attributed to the existence of the active oxygen species. XPS Analysis. XPS analysis is performed to determine the chemical modification of MWCNTs before and after plasma
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Figure 2. Influence of plasma treatment time on the relative contents of carbon and oxygen at an Ar/O2 mixture gas flow rate of 70/40 sccm, plasma power of 700 W, and pressure of 0.1 torr.
treatment. Figure 2 shows the influence of plasma treatment time on the contents of carbon and oxygen on MWCNT surfaces at anAr/O2 mixture gas flow rate of 70/40 sccm, plasma power of 700 W, and pressure of 0.1 torr. For MWCNTs before plasma treatment, oxygen atoms come from air contamination oxidation. From Figure 2, it can be seen that the relative content of oxygen increases mainly during the first 5 min, and thereafter slightly increases. Thus, the oxidation of MWCNTs by plasma can be achieved very rapidly. This result is crucial because the treatment time is one of the most important parameters for the industrial application of MWCNT treatment. Table 1 shows the XPS results of plasma-treated MWCNTs with the relative contents of carbon and oxygen expressed as atom %, as a function of gas flow rate, treatment time, and plasma power. The additional oxygen-binded configurations contribute to the slight increase in the full width at half-maximum (fwhm) (∼0.15-1.08 eV) of C 1s. The relative content of oxygen increases with the gas flow rate, treatment time, and plasma power, while the relative content of carbon decreases, indicating a plasma oxidation effect. Ar/O2 mixture gas plasma treatment enhances the relative content of oxygen on the surfaces of MWCNTs, due to the enhancement of the content of active oxygen content in the presence of Ar. After plasma treatment for 15 min at an Ar/O2 mixture gas flow rate of 70/40 sccm, plasma power of 700 W, and pressure of 0.1 torr, the relative content of oxygen on the surfaces of MWCNTs can reach up to 41.23 atom %. Considering that XPS is a surface sensitive technology, probing about 5 nm deep into the material, one can estimate that about all the aromatic sites on the surfaces of MWCNTs are oxidized, and the surfaces of MWCNTs are coated with the oxygen-containing groups. XPS can also be used to identify groups attached to the surfaces of MWCNTs. Figure 3 shows the high-resolution XPS C 1s spectra of MWCNTs before and after plasma treatment for 15 min at an Ar/O2 mixture gas flow rate of 70/40 sccm, plasma power of 700 W, and pressure of 0.1 torr. In order to further explain the process of plasma oxidation, the C 1s peak is deconvolved into five component Gaussian peaks.13,29 The main peak (1) at 284.1 ( 0.2 eV corresponds to the sp2hybridized graphite-like carbon atoms (CdC). Peak 2 centered at 285.1 ( 0.2 eV is attributed to the sp3-hybridized carbon atoms (C-C). Peaks 3 at 286.2 ( 0.2 eV, 4 at 287.2 ( 0.2 eV, and 5 at 288.9 ( 0.2 eV are considered to originate in carbon atoms binded to one and two oxygen atoms, respectively, because electronegative oxygen atoms induce a positive charge
Figure 3. XPS analysis of MWCNTs before (A) and after (B) plasma treatment for 15 min at an Ar/O2 mixture gas flow rate of 70/40 sccm, plasma power of 700 W, and pressure of 0.1 torr.
on a carbon atom. Hence, they correspond to C-O (e.g., alcohol, ether), CdO (e.g., ketone, aldehyde), and O-CdO (e.g., carboxylic, ester) species, respectively. The quantitative analysis (Figure 3B compared to Figure 3A) indicates that the sp2 CdC fraction decreases after plasma treatment, whereas the C-O, CdO, and O-CdO fractions increase. These changes suggest that the CdC bonds are oxidized and new C-Ox groups are generated on the surfaces of MWCNTs by plasma treatment. It is believed that various oxidative reactions occur during plasma treatment. Using oxygen plasma treatment, free radicals can be created on the treated surfaces, which can then couple with active species from the oxygen plasma environment.30-32 Relative contents fitted for the components forming the C 1s XPS spectra as a function of gas flow rate, plasma power, and treatment time are tabulated in Table 2. From Table 2, in the case of pure O2 plasma treatment, the C-O fraction first increases and then decreases with increasing pure O2 flow rate, and the O-CdO fraction increases with the pure O2 flow rate. The possible reaction mechanisms that occur during the plasma treatment include the generation of the C-O, CdO, and O-CdO bonds, as shown in Figure 4. Since the π bonds in CdC are active and are the most susceptible to plasma attack,11,30 it is believed that radicals are first generated on the dissociated π bonds in CdC, which then further react with active oxygen atoms. This explains the decrease in the CdC fraction
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TABLE 2: Curve Fitting Results of XPS C 1s Spectra MWCNTs untreated MWCNTs O2 40 sccm, 700 W, 15 min O2 70 sccm, 700 W, 15 min O2 110 sccm, 700 W, 15 min Ar/O2 70/40 sccm, 700 W, 15 min Ar/O2 70/40 sccm, 700 W, 10 min Ar/O2 70/40 sccm, 700 W, 5 min Ar/O2 70/40 sccm, 500 W, 5 min
sp2 sp3 C-O CdO O-CdO (%) (%) (%) (%) (%) 53.5 44.2 44.0 42.8 35.6 35.8 41.4 47.9
38.6 4.7 39.3 8.0 41.0 1.9 41.6 1.4 36.4 13.0 36.8 12.6 37.3 9.7 39.6 6.4
1.8 2.4 3.0 1.6 0.8 0.4 1.9 2.5
1.4 6.1 12.1 12.7 14.4 14.0 9.7 3.6
after plasma treatment. This process may produce C-O bonds, and then the C-OH bonds are formed through stabilization by hydrogen atom transfer from the same or a neighboring chain. The hydrogen atoms can be introduced during the synthesis phase of MWCNTs or via atmosphere absorption. Oxygen radicals are also generated on the surfaces of MWCNTs. The new CdO bonds are believed to be formed from these oxygen radicals through intramolecular reorganization on the C-C bonds, as shown in Figure 4b. The formation of O-CdO bonds is believed to be due to the CdO bonds through the combination of the plasma-generated radical on the CdO bonds with the active oxygen atoms. After stabilization with proton transfer,
HO-CdO can be formed, as shown in Figure 4c. Compared to pure O2 plasma treatment, Ar/O2 plasma treatment enhances the C-O and O-CdO fractions, and the C-O and O-CdO fractions increase with increasing plasma power and the treatment time. The efficiency of Ar/O2 mixture gas plasma treatment is higher than that of pure O2 plasma treatment, since the content of active oxygen in Ar/O2 mixture gas plasma is higher than that in pure O2 plasma (as shown in Figure 1). Ar atoms and/or ions present in the plasma can also interact with the surfaces of MWCNTs creating active sites for further oxygen functionalization. Table 2 shows that when the plasma treatment time was 15 min compared with MWCNTs plasma-treated for 10 min at an Ar/O2 mixture gas flow rate of 70/40 sccm, plasma power of 700 W, and pressure of 0.1 torr, there are almost no changes in the fractions of C-O, CdO, and O-CdO groups, with only a slight decrease in the CdC fraction. This suggests that only a small amount of CdC bonds have been oxidized. The further oxidation effect of the plasma is quite weak in this period, only a few new oxygen groups being generated on the surfaces. This phenomenon suggests that there is a saturation state for surface oxidation, for which the surface cannot accept new oxygen species from the plasma environment due to the oxidation level and molecular steric hindrance. At a treatment time of 10 min,
Figure 4. Possible mechanism of MWCNT oxidation by Ar/O2 MW-SWP: (A) generation of C-O bonds; (B) generation of CdO bonds; (C) generation of O-CdO bonds; (D) transfer between carboxyl and lactone.
Oxygen Functionalization of MWCNTs
Figure 5. Influence of O2 gas flow rate on Raman shift of MWCNTs for treatment time 15 min at a plasma power of 700 W.
it is believed that the level of the oxidation on the surface approaches or reaches this saturation state. Therefore, further increase of the treatment time cannot obviously alter the surface functionality. This is also consistent with the results shown in Figure 2, where the content of oxygen for 10 and 15 min treated samples does not show any significant difference. In addition to the above reactions shown in parts A-C of Figure 4, plasma can also induce inner chemical reactions on the surfaces of MWCNTs, e.g., a HO-CdO bond and a neighboring CdO bond can form lactone groups by plasma treatment as shown in Figure 4D. Except for the above-mentioned reaction mechanism in Figure 4, other reaction mechanisms may have been involved during the plasma treatment because the active plasma particles are able to induce radicals on different chemical bonds on the surfaces of MWCNTs. However, it is believed that most of the chemical reactions occur between the plasma-generated radicals and the active species in the plasma atmosphere. Herein, we take the radicals generated on the CdC bonds as an example and illustrate the possible reactions on the surfaces of MWCNTs by plasma treatment. Raman Analysis. Raman Spectroscopy has been extensively used to characterize various carbon materials, including nanotubes, this technique showing a high sensitivity to the disorder on the surfaces based on the optical skin depth.33 Figures 5 and 6 show the Raman spectra of MWCNTs before and after Ar/O2 plasma treatment as a function of gas flow rate, treatment time, and plasma power. For the untreated MWCNTs, the G peak at 1572.8 cm-1 is the E2g2 model corresponding to the movement in the opposite direction of two neighboring carbon atoms in a graphitic sheet, and it indicates the presence of crystalline graphitic carbon in MWCNTs. The D peak at approximately 1345.9 cm-1 is an A1g breathing mode. This mode is generally attributed to the defects in the curved graphite sheet, sp3 carbon, or other impurities. The R ) ID/IG ratio, where Icorresponds to the peak area of the Lorentzian functions, allows us to estimate the relative extent of structural defects. This ratio is 1.23 for the untreated MWCNTs. The enhancement of ID/ IG for the plasma-treated MWCNTs can be interpreted as being attributed to the microstructure of carbon sheets in tubes and the increasing of oxygen content. Compared to untreated MWCNTs, blue shift of each peak position of the treated MWCNTs takes place, which can be caused by the increased disorder and defect density in the treated MWCNTs.34,35 From
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Figure 6. Influence of treatment time and plasma power on Raman shift of MWCNTs at an Ar/O2 mixture gas flow rate of 70/40 sccm.
TABLE 3: Raman Feature of MWCNTs Before and After Ar/O2 MW-SWP Treatment as a Function of Gas Flow Rate, Plasma Power, and Treatment Time MWCNTs untreated MWCNTs O2 40 sccm, 700 W, 15 min O2 70 sccm, 700 W, 15 min O2 110 sccm, 700 W, 15 min Ar/O2 70/40 sccm, 700 W, 15 min Ar/O2 70/40 sccm, 700 W, 10 min Ar/O2 70/40 sccm, 700 W, 5 min Ar/O2 70/40 sccm, 500 W, 5 min
peak (cm-1)
intensity (au)
(D) 1345.9 (G) 1572.8 (D) 1351.5 (G) 1586.6 (D) 1355.9 (G) 1587.2 (D) 1359.2 (G) 1587.7 (D) 1355.3 (G) 1586.1 (D) 1353.1 (G) 1584.1 (D) 1355.4 (G) 1586.6 (D) 1345.4 (G) 1574.9
5.889 4.784 1.755 1.264 1.646 1.203 1.446 1.134 1.446 1.069 1.472 1.151 1.823 1.399 4.881 3.873
ID/IG 1.23 1.39 1.37 1.28 1.36 1.28 1.30 1.26
Table 3, with pure O2 plasma treatment, R values slightly increase due to the enhancement of surface defects and embedment of oxygen atoms. However, with increasing O2 gas flow rate, R value appears to decrease. This might be the result of the removal of some amorphous carbon layers, sp3 carbon, and other impurities. Lee et al.34 investigated CNT purification using atmospheric pressure plasma and reported that the impurities in CNTs (amorphous carbon, dangling bonds, and defects) are more reactive than the well-structured sp2 carbon
Figure 7. Dispersion properties of MWCNTs (0.5 g/L in deionized water) before (A) and after (B) plasma treatment for 15 min at an Ar/ O2 mixture gas flow of 70/40 sccm, plasma power of 700 W, and pressure of 0.1 torr, settling for 2 h (left) and 20 days (right).
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Figure 8. SEM images of MWCNTs before (A) and after (B) plasma treatment for 15 min at an Ar/O2 gas flow of 70/40 sccm, plasma power of 700 W, and pressure of 0.1 torr.
networks and can be easily removed by plasma treatment. From XPS analysis, Ar/O2 mixture plasma treatment enhances the oxygen fraction on the surfaces of MWCNTs. However, no remarkable difference of R values between the plasma treatment of pure O2 and that of Ar/O2 is observed. Slight increase of R values and blue shift of peak positions of the treated MWCNTs indicate the change of the surface structure of MWCNTs and the introduction of oxygen atoms, but plasma conditions applied in the experiments do not destroy the integrity of MWCNT structure. Dispersion Properties. Compared to the untreated MWCNTs, the MWCNTs after Ar/O2 MW-SWP treatment have a very good dispersion in aqueous solution. As can be seen in Figure 7, MWCNTs after plasma treatment disperse well in deionized water and do not form aggregation after a long settling time, whereas the untreated MWCNTs form aggregation at the bottom of the bottle in a short period of settling time. After 2 h of settling time, most of the untreated MWCNTs form aggregation at the bottom of the bottle (see bottle A in the left part of Figure 7). After 20 days of settling time, all untreated MWCNTs form aggregation at the bottom of bottle (see bottle A in the right part of Figure 7). However, no aggregation forms in the suspension of MWCNTs after plasma treatment even after 20 days of settling time (see bottle B in the right part of Figure 7). The oxygen-containing groups introduced on the surfaces of MWCNTs by plasma treatment are hydrophilic. The dispersion of plasma-treated MWCNTs is therefore improved. The high dispersion of plasma-treated MWCNTs is crucial for the application of MWCNTs as sorbents in the removal of pollutions from wastewaters.24,36 SEM Images. The typical SEM images of MWCNTs before and after Ar/O2 plasma treatment are shown in Figure 8. The differences of the surface morphology between untreated and plasma-treated samples are observed. The surfaces of MWCNTs before plasma treatment are smooth and tidy. However, after plasma treatment, the smooth MWCNT surfaces become rough, and the surface defects of treated MWCNTs are enhanced, but the integrity of the nanotube patterns is not damaged, which is consistent with the Raman analysis. The measurements of MWCNT mass before and after Ar/O2 plasma treatment indicate that the loss of MWCNT mass is less than 8%. There is not a large loss of mass. IV. Conclusions Oxygen-containing groups are introduced on MWCNT surfaces by using Ar/O2 MW-SWP treatment. The analysis results of XPS and Raman spectroscopy indicate that mixture gas flow rate, treatment time, and plasma power affect the atomic contents and structure properties of MWCNTs. SEM images of MWCNTs
suggest that after plasma treatment, the smooth MWCNT surfaces become rough, and the surface defects of plasma-treated MWCNTs are enhanced, but the integrity of the MWCNT patterns is not damaged. The dispersion of MWCNTs in deionized water is improved by using Ar/O2 MW-SWP treatment. Ar/O2 MW-SWP treatment is an efficient method for oxygen functionalization of MWCNTs. Acknowledgment. This work has been supported in part by the Grants-in-Aid for Scientific Research and performed under Research and Education Funding for Research Promotion, MEXT, Japan. Partial support from National Natural Science Foundation of China (20677058) and 973 project (2007CB936602) from MOST of China is also acknowledged. References and Notes (1) Qin, S.; Qin, D.; Ford, W. T.; Resasco, D. E.; Herrera, J. E. J. Am. Chem. Soc. 2004, 126, 170. (2) Coleman, J. N.; Khan, U.; Ryan, K.; Blau, W. J.; Gun’ko, Y. K. Carbon 2006, 44, 1624. (3) Cadek, M.; Coleman, J. N.; Ryan, K. P.; Nicolosi, V.; Bister, G.; Fonseca, A.; Nagy, J. B.; Szostak, K.; Beguin, F.; Blau, W. J. Nano Lett. 2004, 4, 353. (4) Tseng, C. H.; Wang, C. C.; Chen, C. Y. Chem. Mater. 2007, 19, 308. (5) Wiltshire, J. G.; Khlobystov, A. N.; Li, L. J.; Lyapin, S. G.; Briggs, G. A. D.; Nicholas, R. J. Chem. Phys. Lett. 2004, 386, 239. (6) Hamon, M. A.; Chen, J.; Hu, H.; Chen, Y. S.; Itkis, M. E.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. AdV. Mater. 1999, 11, 834. (7) Ajayan, P. M.; Ebbesen, T. W.; Ichihashi, T.; Iijima, S.; Tanigaki, K.; Hiura, H. Nature (London) 1993, 362, 522. (8) Kuznetsova, A.; Popova, I.; Yates, J. T.; Bronikowski, M. J.; Huffman, C. B.; Liu, J.; Smalley, R. E.; Hwu, H. H.; Chen, J. G. J. Am. Chem. Soc. 2001, 123, 10699. (9) Simmons, J. M.; Nichols, B. M.; Baker, S. E.; Marcus, M. S.; Castellini, O. M.; Lee, C. S.; Hamers, R. J.; Eriksson, M. A. J. Phys. Chem. B 2006, 110, 7113. (10) Mawhinney, D. B.; Naumenko, V.; Kuznetsova, A.; Yates, J. T.; Liu, J., Jr.; Smalley, R. E. J. Am. Chem. Soc. 2000, 122, 2383. (11) Ago, H.; Kugler, T.; Cacialli, F.; Salaneck, W. R.; Shaffer, M. S. P.; Windle, A. H.; Friend, R. H. J. Phys. Chem. B 1999, 103, 8116. (12) Lee, Y. J.; Kim, H. H.; Hatori, H. Carbon 2004, 42, 1053. (13) Okpalugo, T. I. T.; Papakonstantinou, P.; Murphy, H.; Mclaughlin, J.; Brown, N. M. D. Carbon 2005, 43, 2951. (14) Hou, Z.; Cai, B.; Liu, H.; Xu, D. Carbon 2008, 46, 405. (15) Imsaka, K.; Kato, Y.; Suehiro, J. Nanotechnology 2007, 18, 335602. (16) Felten, A.; Bittencourt, C.; Pireaux, J. J. J. Appl. Phys. 2005, 98, 074308. (17) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95. (18) Esumi, K.; Ishigami, M.; Nakajima, A.; Sawada, K.; Honda, H. Carbon 1996, 34, 279. (19) Dai, L. Polym. AdV. Technol. 1999, 10, 357. (20) Liu, J.; Rinzler, A. G.; Dai, H.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; Rodriguez-Macias, F.; Shon, Y. S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253.
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