Hydrogen-Bond-Driven Assembly of Thin Multiwalled Carbon

Phone: +82-55-280-1677., † ... Self-assembly and bundle formation of carbon nanotubes (CNTs) have attracted considerable attentions in the contexts ...
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J. Phys. Chem. C 2008, 112, 15961–15965

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Hydrogen-Bond-Driven Assembly of Thin Multiwalled Carbon Nanotubes Joong Tark Han,† Sun Young Kim,† Jong Seok Woo,† Hee Jin Jeong,† Weontae Oh,‡ and Geon-Woong Lee*,† CNT Electrode Research Group, Korea Electrotechnology Research Institute, Changwon, 641-120, Korea, and Department of Nano Technology, Dong-Eui UniVersity, Busan, 614-714, Korea ReceiVed: May 16, 2008; ReVised Manuscript ReceiVed: August 08, 2008

We report on the formation of the assembled bundle structure of thin multiwalled carbon nanotubes (t-MWNTs) modified with hydroxyl groups, which was assisted by the intertube hydrogen bonding interaction. To achieve this, the t-MWNTs were first functionalized with hydroxide (t-MWNT-OH) by reflux in hydrogen peroxide. For the comparison, the carboxyl-modified t-MWNTs (t-MWNTs-COOH) were prepared by nitric acid treatment as a control sample. After filtration of a functionalized t-MWNT solution, the flexible t-MWNT film was obtained from the t-MWNTs-OH, whereas the t-MWNTs-COOH formed a cracked film. From electron microscope images, it was found that the t-MWNTs-OH can assemble into bundle structures upon densification of the carbon nanotube suspension, while the t-MWNTs-COOH existed in an entangled state within the film. The assembly of the t-MWNTs-OH induced by the intertube hydrogen bonding was demonstrated by observation of the upshift and sharpening of the G band in the Raman spectra and enhanced hydrogen bonds in Fourier transform infrared spectra of nanotube films. 1. Introduction Self-assembly and bundle formation of carbon nanotubes (CNTs) have attracted considerable attentions in the contexts of fabricating a flexible buckypaper (paperlike sheet) and obtaining aligned multiwalled carbon nanotube (MWNT) ribbons and amplification of the micropore surface area of CNT films.1-9 Previous works on disentanglement and alignment of CNTs used a special mechanical trick and the incorporation of foreign materials. Moreover, it is well-known that, for MWNTs, it is difficult to fabricate the buckypaper using a conventional filtration process without severe oxidation and shortening of the nanotubes because of their large diameter and entangled structure of MWNTs. In this point of view, thin MWNTs (tMWNTs)10 have a possibility of forming the assembled structure because of the reduced number of walls (namely, 2-6) and their small diameters (below 6 nm) even after the partial chemical modification. The chemical modification of the CNTs is important to achieve dispersion of the nanotubes in organic solvents and further coating of this solution onto solid substrates.11 Hydrogen peroxide is one of the most powerful oxidizing agents available and is widely used in the functionalization of MWNTs and double-walled CNTs, as well as in the purification and etching of single-walled CNTs (SWNTs).12-15 Moreover, hydroxyl groups can be dominantly attached on the side walls or endcaps of CNTs after oxidation with hydrogen peroxide. The attachment of hydroxyl groups during H2O2 refluxing suggests that both inter- and intramolecular hydrogen bonding between the nanotubes induce CNT assembly during the film formation process (Figure 1). However, little attention has been given to the morphological characteristics of CNT films fabricated with t-MWNTs functionalized with hydroxyl groups. * Towhomcorrespondenceshouldbeaddressed.E-mail:[email protected]. Fax: +82-55-280-1590. Phone: +82-55-280-1677. † Korea Electrotechnology Research Institute. ‡ Dong-Eui University.

In this study, we therefore report on the hydrogen-bond-driven assembly of t-MWNTs treated with hydrogen peroxide, which results in the formation of a flexible MWNT film consisting of assembled t-MWNT bundles after filtration. This phenomenon was illustrated by means of Fourier transform infrared (FT-IR) spectroscopy and Raman spectroscopy measurements performed on the spray-coated t-MWNT films. 2. Experimental Methods The t-MWNTs, with an average diameter of 3-5 nm and a length ranging from hundreds of nanometers to micrometers (obtained from Iljin nanotechnology, Inc.) were used in this study. The t-MWNTs were immersed and refluxed in hydrogen peroxide to attach hydroxyl groups onto the side wall of t-MWNTs for 12, 24, 48, and 65 h at 60 °C (t-MWNT-OH), and for the control experiment, t-MWNTs were refluxed in 30% nitric acid to introduce carboxylic acid groups for 4 h at 90 °C (t-MWNT-COOH). The samples were then extracted several times by vacuum filtration using an alumina filter until the solution reached a pH value of 7. Finally, the filtered t-MWNTs were dispersed in ethanol for 2 h in an ultrasonic bath at a definite concentration. The t-MWNT sheets were then fabricated by means of filtration on an alumina (0.02 µm pore diameter) or polytetrafluoroethylene (PTFE) membrane. The t-MWNT thin films were prepared by spray coater (Fujimori Co., NVD200) with a nozzle of 1.2 mm diameter. The t-MWNT films were then heated under vacuum for 1 h (at 100 °C) in order to remove the remaining chemicals. The corresponding images of the resulting films were obtained by field-emission scanning electron microscopy (FE-SEM, HITACHI S4800). Transmission electron microscopy (TEM) was performed on a Hitachi H-7600 operating at 115 kV. TEM sample was prepared by drop casting of the diluted ethanol solution of the t-MWNTs-OH onto a copper grid (300mesh) coated by carbon (TED PELLA, Inc.). X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 (VG Scientific) spectrometer with monochromatized Al KR X-ray radiation as the X-ray source

10.1021/jp804334f CCC: $40.75  2008 American Chemical Society Published on Web 09/18/2008

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Figure 1. Scheme of the assembling of t-MWNTs functionalized with hydroxyl groups.

for excitation. Raman spectroscopy with the laser excitation line of 633 nm was performed on LabRAM-HR800 Raman spectrometer. For external reflection IR spectroscopy (ERS) analysis, the films on silicon wafers were scanned from 4000 to 400 cm-1 using p-polarized light with an incident angle of 80° and a resolution of 4 cm-1 (FT-IR, Bruker IFS 66v). 3. Results and Discussion Spray-coated samples of the resulting functionalized tMWNTs were studied by XPS and Raman spectroscopy. First, the functional groups on the nanotube surface were confirmed by XPS (Figure S1 in Supporting Information). The main binding-energy peak (at 284.6 eV) was attributed to the C-C 1s, while the other three peaks were assigned to ;C;OH (285.2 eV) and ;CdO (286.5 eV), and ;COOH (289.5 eV). From XPS spectra, it was confirmed that hydroxyl groups were dominantly attached by refluxing in hydrogen peroxide, and the atomic concentration of C-OH on the surface reached a value of 33.5 at % after 65 h (Figure S1f of Supporting Information). Raman spectroscopy was then employed to identify the formation of defects on the t-MWNTs after H2O2 treatment. Figure 2a shows typical G and D bands of the Raman spectra at excitation wavelength of 633 nm for the as-received t-MWNT and t-MWNT-OH films. The ratio of the D-to-G peak intensity (ID/IG) gradually increased with increasing time of the H2O2 treatment. Another quantity of interest is the full width at halfmaximum (fwhm) of the G band. In pregraphitic carbons, ID/IG and fwhm are known to depend on the size of the graphitic crystallite: the smaller these parameters, the higher the degree of crystallinity.16,17 In the present study, however, we found that the fwhm decreased after a 12-h H2O2 treatment, whereas ID/IG gradually increased (Figure 2b). In addition, the fast increase of ID/IG during the 12-h treatment seems to be caused by rapid oxidation of the disordered parts or amorphous carbon. These results indicate that oxidized amorphous carbonaceous materials were removed by the H2O2 treatment. It is well-known that the carboxyl groups after HNO3 treatment lead to a reduction of van der Waals interactions between the CNTs, which is useful for the dispersion of CNTs in aqueous or organic solvents. However, we can assume that the strong intermolecular interaction between functional groups

Figure 2. (A) D and G bands of the Raman spectra (excitation at λ ) 633 nm), (B) fwhm values of the G bands in the Raman spectra of t-MWNTs at different H2O2 treatment times. The inset in part A shows the ratio of the D-to-G-band intensity and the G band shift.

attached on the side walls or endcap of the t-MWNTs can trigger their assembly by enhancing the graphitic layer interaction during film formation (Figure 1). Interestingly, after filtration, we obtained a dark and flexible CNT sheet from the t-MWNTs-OH (Figure 3b), whereas the

Hydrogen-Bond-Driven Assembly of t-MWNT

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Figure 3. Photo images of t-MWNT films prepared with (A) the t-MWNTs-COOH and (B) t-MWNTs-OH (refluxed in H2O2 for 48 h) by filtration. (C, D) FE-SEM images of parts A and B, respectively. (E) FE-SEM image of the t-MWNT-OH (refluxed in H2O2 for 65 h) film. The inset images in parts C, D, and E represent the high magnification image. The arrows in parts C, D, and E indicate the size of the silane sol-coated CNTs and CNT bundles, respectively.

t-MWNTs-COOH formed a cracked film (Figure 3a). The filtered t-MWNT-OH film supported by the PTFE membrane and a free-standing film detached from alumina membrane are bendable as shown in Figure 3b. Parts c and d of Figure 3 show typical FE-SEM images of t-MWNT-COOH and t-MWNT-OH films fabricated by filtration, respectively. The t-MWNT-COOH film consists of the typical randomly entangled nanotubes just with a diameter below 10 nm, whereas the t-MWNTs-OH formed assembled bundle structure with diameter of 20-50nm after filtration. The more tightly assembled bundle structure of t-MWNTs was obtained from the t-MWNTs treated with H2O2 for 65 h (Figure 3e) than for 48 h (Figure 3d). Moreover, by drop-casting the t-MWNT-OH solution on glass, it was possible to fabricate densely packed t-MWNT films at room temperature which is similar to the single-walled CNT film (Figure 4a). However, the t-MWNT-OH bundles formed the loosely packed films at high temperature (100 °C, above the boiling point of ethanol which was used as the dispersing solvent in this study, Figure 4b). Importantly, the t-MWNTs-OH exhibited assembled bundle structures in all cases, even in those in which they were obtained by means of spray coating in which ethanol evaporates very fast (Figure 4c). On the other hand, spraying of the t-MWNTs-COOH gave typical MWNT films composed of

entangled nanotubes (Figure 4d). In order to investigate the detailed morphology of assembled bundle structures of the t-MWNTs-OH, TEM analysis was carried out. The TEM image clearly revealed that the t-MWNTs-OH with a diameter of ca. 3 nm were arranged into bundles (Figure 5b), whereas the t-MWNTs-COOH exist in individual nanotubes (Figure 5a). To illustrate our assumption, the hydrogen bonding of functionalized t-MWNTs was investigated by FT-IR spectroscopy in vacuum with the spray-coated thin t-MWNT films. The characteristic bands of hydrogen bonded hydroxyl groups at ca. 3250 cm-1 appeared clearly from the t-MWNT-OH films, while the weak O-H band was observed from the t-MWNT-COOH film (Figure 6). This fact can provide the evidence that the assembled bundle structure of t-MWNTs-OH may be induced by the intertube hydrogen bonding of the CNTs. If the t-MWNT bundles were formed by assembling through enhanced graphitic interaction by intertube hydrogen bonding, we can expect this bonding to affect the G bands in the Raman spectra of the CNT films. In fact, assembly of the t-MWNTs could be deduced from the Raman spectra of the spray-coated nanotubes. The maximum Raman peak, which is found near 1570 cm-1, the so-called tangential mode (G-mode), related to the graphite E2g symmetric intralayer mode was shifted upward

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Figure 5. TEM image of (A) the t-MWNTs-COOH and (B) the t-MWNT-OH bundles. Figure 4. FE-SEM images of t-MWNT-OH (treated with H2O2 at 60 °C for 48 h) films prepared by drop-casting (A) at room temperature and (B) at 100 °C. (C, D) FE-SEM images of t-MWNT-OH and t-MWNT-COOH films prepared by spray coating at room temperature, respectively.

(by ∼13 cm-1) after H2O2 treatment for 24 h (Figure 2a). Further treatment with hydrogen peroxide did not cause any additional shifts of the G band. Until now, H2O2 treatment had not been reported to induce any large upshift (of ∼10-20 cm-1) in the G-bands of MWNT or SWNT. In the case of nitric acid treatment of the CNTs, large band shifting can occur as a result of charge transfer (NO3-1).15 Up-shifting of the tangential CNT bands is associated with the withdrawal of π electrons. Accordingly, the hydrogen bonding between t-MWNTs-OH can induce the up-shifting of the G-band. If the upshift of the G-band is dominantly dependent on the hydrogen bonding, the G-band should be gradually shifted by increasing the amount of the hydroxyl groups after oxidation with hydrogen peroxide. However, in Figure 2a, there is no shift on the G-band after oxidation for 24 h. Therefore, we believe that the up-shifting of the G-band may be attributed to the increased graphitic layer interaction of the t-MWNTs-OH induced by the intermolecular hydrogen bonding between the hydroxyl groups on the nanotubes. In another point of view, it has been reported that the tangential Raman E2g mode can be shifted to higher wavenumbers upon hydrostatic pressure, which corresponds to a reduction in the bond length and an associated stiffening of the bonds.18 In our case, compressive strain of the C-C bond or a change in the bond angles resulting from hydrogen-bond-induced graphitic layer stacking may lead to an upshift in the tangential mode frequency. Moreover, the sharpening of the G-band of the Raman spectra also gives an evidence of the assembly of the t-MWNTs-OH because fwhm should generally increase after

Figure 6. FT-IR spectra of HNO3-treated and H2O2-treated t-MWNT thin films spray coated on Si wafer.

oxidation.16,17 Kim et al.19 have also reported that the broad G-band of the SWNTs intercalated with NO3- ions becomes sharp by decreasing the interlayer distance of the SWNTs after deintercalation. Previously Li et al.1 have reported that the self-organized ribbons can be formed with MWNTs (diameter, 30 nm), which were treated with a mixture of concentrated nitric acid and sulfuric acid. In that case, however, self-organization was induced by the prolonged heating (at 100 °C) of the acid-treated MWNT solution. Furthermore, Li et al.4 reported the selforganization of MWNTs induced by the capillary flow. In another report, it was suggested that single-walled carbon nanotubes (SWNTs), which are directly modified with multiple hydroxyl groups (SWNTols) can be readily self-assembled into compact, aligned arrays.7 However, the SWNTs were severely oxidized with potassium hydroxide to form the multiple

Hydrogen-Bond-Driven Assembly of t-MWNT hydroxyl groups, in which the relative ratio of the G-to-D modes decreased from 18:1 for the pristine SWNTs to 1.2:1 for the SWNTols. In our study, however, in comparison with the asreceived t-MWNTs, the amount of the hydroxyl groups on the nanotubes slightly increased after H2O2 treatment even for 48 h, as shown in the XPS. This fact indicates that a small increase in the number of hydroxyl groups on the t-MWNTs can induce the assembly of the nanotubes during the film-formation process. Kukovecz et al.20 also suggested that the SWNTs severely functionalized with carboxylic acid groups can form the aligned thick nanotube bundles, promoted by the hydrogen bonding. However, from our study, the assembly of t-MWNTs-COOH induced by the hydrogen bonding between carboxylic acid groups was not observed. Furthermore, they did not provide the direct evidence of intermolecular hydrogen bonding between the CNTs by measuring IR spectra. Importantly, our study provides the direct evidence of the assembly of the nanotubes (which is given by the upshift and sharpening of the G band in the Raman spectrum and hydrogen bonded O-H bands in FTIR spectra of the t-MWNT-OH films). 4. Conclusions We present the assembly of t-MWNTs modified with hydroxyl groups, resulting in formation of a flexible film after filtration. The assembly of t-MWNTs was observed, even in the case of the spraying process in which the volatile solvent evaporates immediately. This assembly process of nanotubes may be caused by hydrogen bonding between the nanotubes, which resulted in the enhanced graphitic layer interaction. FTIR spectra of the t-MWNT-OH films provide the evidence of the intertube hydrogen bonding interaction. The enhanced graphitic layer interaction of the assembled CNT films was demonstrated by the upshift and sharpening of the G-band in Raman spectra. This study shows the possibility of a flexible MWNT buckypaper by introducing the bundled structure into MWNTs. Acknowledgment. We thank Mr. S. G. Lee and Mr. J. H. Park at POSTECH for experimental assistance.

J. Phys. Chem. C, Vol. 112, No. 41, 2008 15965 Supporting Information Available: XPS spectra of asreceived and H2O2-treated t-MWNT films. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Li, Y. -H.; Xu, C.; Wei, B.; Zhang, X.; Zheng, M.; Wu, D.; Ajayan, P. M. Chem. Mater. 2002, 14, 483. (2) Xie, H.; Pikhitsa, P. V.; Kim, Y. J.; Youn, W.; Altman, I. S.; Nam, J. G.; Lee, S. J.; Choi, M. J. Appl. Phys. 2006, 99, 104313. (3) Song, W.; Kinloch, I. A.; Windle, A. H. Science 2003, 302, 1363. (4) Li, Q.; Zhu, Y.; Kinloch, I. A.; Windle, A. H. J. Phys. Chem. B 2006, 110, 13926. (5) Gennett, T.; Dillon, A. C.; Alleman, J. L.; Jones, K. M.; Hasoon, F. S.; Heben, M. J. Chem. Mater. 2000, 12, 599. (6) An, K. H.; Yang, C. -M.; Park, J. S.; Jeong, S. Y.; Lee, Y. H. J. Phys. Chem. B 2005, 109, 10004. (7) Pan, H.; Liu, L.; Guo, Z.-X.; Dai, L.; Zhang, D. Z.; Czerw, R.; Carroll, D. L Nano Lett. 2003, 3, 29. (8) Endo, M.; Muramatsu, H.; Hayashi, T.; Kim, Y. A.; Terrones, M.; Dresselhaus, M. S. Nature 2005, 433, 476. (9) Sreekumar, T. V.; Liu, T.; Kumar, S.; Ericson, L. M.; Hauge, R. H.; Smalley, R. E. Chem. Mater. 2003, 15, 175. (10) Jeong, H. J.; Kim, K. K.; Jeong, S. Y.; Park, M. H.; Yang, C. W.; Lee, Y. H. J. Phys. Chem. B 2004, 108, 17695. (11) Lee, G.-W.; Kumar, S. J. Phys. Chem. B 2005, 109, 17128. (12) Peng, Y.; Liu, H. Ind. Eng. Chem. Res. 2006, 45, 6483. (13) Miyata, Y.; Maniwa, Y.; Kataura, H. J. Phys. Chem. B 2006, 110, 25. (14) Wei, J.; Zhu, H.; Li, Y.; Chen, B.; Jia, Y.; Wang, K.; Wang, Z.; Liu, W.; Luo, J.; Zheng, M.; Wu, D.; Zhu, Y.; Wei, B. AdV. Mater. 2006, 18, 1695. (15) Kim, U. J.; Furtado, C. A.; Liu, X.; Chen, G.; Eklund, P. C. J. Am. Chem. Soc. 2005, 127, 15437. (16) Tuinstra, F.; Koenig, J. L. J. Chem. Phys. 1970, 53, 1126. (17) Nakamura, K.; Fujitsuka, M.; Kitajima, M. Phys. ReV. B 1990, 41, 12260. (18) Sandler, J.; Shaffer, M. S. P.; Windle, A. H.; Halsall, M. P.; MontesMora´n, M. A.; Cooper, C. A.; Young, R. J. Phys. ReV. B 2003, 67, 035417. (19) Kim, H. J.; Jeon, K. K.; An, K. H.; Kim, C.; Heo, J. G.; Lim, S. C.; Bae, D. J.; Lee, Y. H. AdV. Mater. 2003, 15, 1757. (20) Kukovecz, A.; Kramberger, Ch.; Holzinger, M.; Kuzmany, H.; Schalko, J.; Mannsberger, M.; Hirsch, A. J. Phys. Chem. B 2002, 106, 6374.

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