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Transformation of singlewalled carbon nanotubes to multiwalled carbon nanotubes and onion-like structures by nitric acid treatment
Synthetic Metals 140 (2004) 1–8
Transformation of singlewalled carbon nanotubes to multiwalled carbon nanotubes and onion-like structures by nitric acid treatment Kay Hyeok Ana,c, Kwan Koo Jeona,c, Jeong-Mi Moona,c, Sung Jin Eumb,c, Cheol Woong Yangb,c, Gyeong-Su Parkd, Chong Yun Parka,c, Young Hee Leea,c,* a
BK21 Physics Division, National Research Laboratory for Carbon Nanotubes, Sungkyunkwan University, Suwon 440-746, South Korea b Department of Advanced Materials Engineering, Sungkyunkwan University, Suwon 440-746, South Korea c Center for Nanotubes and Nanostructured Composites, Sungkyunkwan University, Suwon 440-746, South Korea d Analytical Engineering Laboratory, Samsung Advanced Institute of Technology, Suwon 440-600, South Korea Received 4 September 2002; received in revised form 21 October 2002; accepted 22 October 2002
Abstract We have obtained various phases during the refluxing process of singlewalled carbon nanotubes (SWNTs) in 30% HNO3 acid solution at 100 8C. From the similar treatment in HCl acid solution, we observe no structural phase transformation but enlargement of the bundle size of SWNTs. This strongly suggests that the intercalation of nitric acids into the bundles, which is evidenced by the distribution of smaller diameters of bundles, induces the disintegration of tubes by oxidative etching first and the reformation into multiwalled nanotubes, conecapped phase, and further onion-like structures. The origin of such a transformation mechanism is further discussed. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Carbon nanotubes; Purification; Phase transformation; HNO3 treatment
1. Introduction In order to characterize the carbon nanotubes (CNTs) and improve their performance in the practical devices, the purification of the raw product, which often contains not only the CNTs but also other carbonaceous particles (amorphous carbon particles, fullerenes, nanocrystalline polyaromatic shells) and transition metals, is a necessary step through various oxidation processes. The carbonaceous particles are easily removed during gas phase reaction or thermal annealing in air or oxygen atmosphere, since the etching rate of amorphous carbons is faster than that of CNTs [1–4]. Transition metals are usually introduced during the synthesis as catalysts. The gas phase purification, however, cannot eliminate transition metals. Therefore, the liquid phase reactions in various acids are further necessary in order to melt away the transition metals. It is known that the CNTs are usually cut into small lengths and sometimes broken completely during the liquid phase reaction in * Corresponding author. Present address: Department of Physics, Institute of Basic Science, Sungkyunkwan University, Suwon 440-746, South Korea. Tel.: þ82-31-299-6507; fax: þ82-31-290-5954. E-mail address: [email protected] (Y.H. Lee).
various acids [5,6]. The CNT walls are severely damaged in strong acid solution, although the wall structures could be recovered by the subsequent annealing. Recently, Zhou and co-workers [7] have reported that the bundles of singlewalled carbon nanotubes (SWNTs) become disordered and partially exfoliated, when the SWNTs are immersed in 70% HNO3 solution for a long period of time. They have often observed the isolated SWNTs and large nanoparticles. Monthioux et al. have shown that the acid purification using HNO3 or H2SO4/HNO3 mixture solution cannot avoid an alteration of the SWNT structure. The acidic medium provokes gathering of pre-existing fullerenes into crystallized fullerite. A slight temperature increase has a dramatic effect on SWNT degradation, which can result in a complete amorphisation [5]. Mineral acids containing oxygen atoms are known to help in oxidizing polyaromatic solids like graphite. They are also believed to intercalate and reside inside the bundles of CNTs [7]. In this study, we report the effect of HNO3 treatment to the SWNT structure by means of high-resolution transmission electron microscopy (HRTEM). Comparing the acid treatment in HNO3 solution with that in HCl solution, we learn that nitric acids can be intercalated into SWNT bundles and disintegrate the tube walls into graphitic flakes and then
0379-6779/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(02)01254-7
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reform them into various multi-shell phases such as multiwalled CNTs, cone-capped phases, and onion-like phases, which was never observed from our raw SWNTs. We also discuss the origin and mechanism for the transformation of SWNTs to multiwalled carbon nanotubes (MWNTs) and onion-like structures by nitric acid treatment.
2. Experimental SWNTs were prepared by a conventional catalytic arc discharge [8]. The total amount of catalyst in a graphite powder was fixed to be 5 wt.%, with the ratio of the transition metals (Ni:Co:FeS ¼ 1:1:1), where sulfur was added as a promoter. This increased significantly the yield of CNTs deposited in the whole chamber. The systematic purification procedure is as follows. The cloth-like raw
sample was ground mechanically and transferred to the heating chamber, where the CNT powder was heated at 470 8C for 50 min in air to remove carbonaceous particles. In order to remove the catalysts, the annealed powder was immersed in 6 M hydrochloric acid (HCl) for 24 h. This process was repeated several times until the color of acid was unchanged. The sample was then washed out in deionized water by several times. The detail of synthesis of SWNTs by arc discharge and purification by combining two-step processes of thermal annealing in air and HCl treatment has been described elsewhere [3,8]. The sample obtained at the last step was refluxed in 30% nitric acid (HNO3) at 100 8C for 10 min to 20 h. The obtained suspension was filtered with a polytetrafluoroethylene (PTFE) membrane and washed out in deionized water. After rinsing and drying, a grayish black, thin mat was finally obtained.
Fig. 1. SEM and TEM images of: (a) and (b) the raw SWNT sample; (c) and (d) the thermally treated sample; (e) and (f) HCl-treated sample.
K.H. An et al. / Synthetic Metals 140 (2004) 1–8
The morphology of the sample and the degree of the structural transformation were observed by scanning electron microscope (SEM, Hitachi, S4700), TEM (Hitachi, H9000NA), and Fourier transform (FT) Raman spectroscopy (BRUKER, RFS 100/S) using Nd:YAG laser (1064 nm). Thermogravimetric analysis (TGA) was also carried out using a TA Instruments (TGA 2950) with a linear heating rate of 10 8C/min from 25 to 1000 8C in air flow.
3. Results and discussion 3.1. Structural characterizations of SWNTs treated in nitric acid Fig. 1a shows the SEM image of the ground raw soot collected from the whole chamber. In addition to the SWNT bundles, the white spots, which are the carbonaceous particles, mainly amorphous carbons, are shown in the figure. These structural features might be caused by the variation in the arcing process because of an inhomogeneous distribution of catalysts within the powder in the anode [8]. Furthermore, we can see that the metal particles are mainly embedded in large amorphous carbons, as evidenced in Fig. 1b. The diameter of an individual SWNT is around 1.4 nm, and bundles are composed of a few tenths of SWNTs. This sample was thermally annealed at 470 8C for 50 min in air to remove selectively the amorphous carbons. The SEM image of the oxidized SWNTs is shown in Fig. 1c. The
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figure shows a dramatic decrease of amorphous carbons after oxidation. As a result of oxidation, the weight of the soot is reduced to about 40 wt.% of the original sample. The TEM image of the oxidized SWNTs shown in Fig. 1d suggests that most of the amorphous carbons, which covered the surfaces of SWNT bundles and those surrounding the catalytic metals, were removed. Now the transition metals, as indicated by the black spots in Fig. 1d, are left. The transition metals can be etched away by a HCl treatment. Fig. 1e shows the SEM image of the HCl-treated sample, where the annealed sample was immersed in 6 M hydrochloric acid for 24 h. The corresponding TEM image in Fig. 1f clearly shows that the transition metal particles are well melted away. Yet some carbonaceous particles, which are left during the annealing procedure, still remain in the sample, as illustrated in Fig. 1e. Some transition metals are still left even after the HCl treatment, as can be seen from Fig. 1f. Therefore, a special care should be taken to obtain the purified samples with high quality. We note that the HCl treatment enhances the bundling of SWNTs, as shown in Fig. 1e and f, compared to the annealed samples in Fig. 1c and d, since the entangled SWNTs are released in acid and aggregated themselves by the van der Waals interactions. This acid treatment gives the yield of 70 wt.% of the thermally annealed samples. Thus the total yield of the two steps is about 25–30 wt.%. The HCl-treated SWNT was further refluxed in 30% HNO3 to disperse the SWNT bundles which were aggregated by the HCl treatment. Fig. 2 shows the SEM images of
Fig. 2. SEM images of the HNO3-treated samples as a function of refluxing times: (a) 0.5 h; (b) 1 h; (c) 2 h; (d) 6 h.
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Fig. 3. TEM images of: (a) HCl-treated sample and (b) HNO3-treated sample for 2 h. Bundle size distributions of: (c) HCl-treated sample and (d) HNO3treated sample for 2 h.
the HNO3-treated samples. The SEM images of the HNO3treated samples for 30 min to 1 h (Fig. 2a and b) reveal no significant morphological changes compared to the HCltreated sample. The large bundles of SWNTs enhanced by the HCl treatment are not separated individually yet. At 2 h of HNO3 treatment (Fig. 2c), large portion of the SWNT bundles are dispersed, as evidenced by the opaque backgrounds in the SEM image. This is clearly presented in the TEM image of the SWNT bundles at 2 h of HNO3 treatment, as shown in Fig. 3. The SEM morphology for 6 h treatment is somehow completely different, as shown in Fig. 2d. The SWNTs are broken into small pieces, forming larger diameters. More carbonaceous particles are restructured at this time. These particles and short bundle-like phase are not the same phases as before, which will be discussed later. Fig. 3 shows TEM images and the bundle size distribution of the HCl-treated and HNO3-treated samples for 2 h. The bundle size distributions are estimated from TEM images. One can clearly see that the bundle sizes of the HCl-treated
sample are larger than that of the HNO3-treated one in Fig. 3a and b. While the bundle size of the HCl-treated SWNTs is distributed over the range 10–25 nm, the size of the HNO3treated one is in the range 5–12 nm. We note that the short time treatment of HNO3 is required to disperse the CNT bundles with a minimal damage to the CNT walls. Raman spectroscopy is employed to identify the structural changes of the HNO3-treated samples, as shown in Fig. 4. All the samples have the maximum intensity near 1593 cm1, so called tangential mode, which is related to the graphite E2g symmetric intralayer mode, split due to the curvature of the rolled-up graphene sheet [9]. Two shoulder peaks also appear at 1571 and 1552 cm1, which are not exhibited in the Raman spectra of MWNTs and graphite [3]. A broad band is usually observed around 1280 cm1 in graphite and MWNTs [10–12], which is a characteristic of amorphous carbon or defects, and is also observed in Fig. 4. As the refluxing time in 30% HNO3 increases, the intensity of maximum peak at 1593 cm1 decreases drastically,
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Fig. 4. FT-Raman spectra of the HNO3-treated samples as a function of refluxing times.
whereas the intensity ratio of the 1280–1593 cm1 peak increases, indicating some structural deformation in the tube wall after HNO3 treatment. The radial breathing modes
(RBMs) whose frequencies depend on the tube diameters [13] in the low frequency range 160–200 cm1 are also observed in all of the samples. This is also a characteristic of
Fig. 5. (a) The HNO3-treated samples as a function of refluxing times and MWNT as a reference sample. (b) The maximum temperatures of the differentiated TGAs for the HNO3-treated samples and MWNT. TGA graphs of: (c) the HCl-treated sample and (d) the HNO3-treated sample for 6 h (the solid and dotted lines correspond to the TG and differentiated TG curve, respectively).
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SWNTs, since MWNTs do not show any low frequency mode and the low frequency mode of graphite is at 42 cm1 [3,10]. After the HNO3 treatment, the intensities of the RBM decrease drastically with increasing refluxing time, indicating the structural changes of SWNTs, particularly after 30 min. However, since the two shoulder peaks at 1571 and 1552 cm1 and the RBM in the range 160–200 cm1 are present in the Raman spectra, some SWNTs still remain in the sample during long time refluxing in 30% HNO3. The disappearance of small peaks in the mid-frequency range after 30 min indicates again the structural deformation in the tube wall. Fig. 5 shows the TGA graphs measured in air atmosphere and the peak temperatures of the respective differentiated TGAs (Pdw /dt) of the HNO3-treated SWNTs and MWNTs, which are synthesized by arc discharge method, served as a reference sample. In the TGA curves of the SWNTs treated
by HNO3, The weight loss by burn-off starts from 650 8C, although they are completely burnt-off near 850 8C. The MWNTs start burning-off at higher temperature, compared to that of the SWNTs, although the complete burn-off temperature was rather lower, as shown in Fig. 5a. The TGA curves shift to higher burn-off temperature with increasing refluxing time. The maximum in Pdw /dt increases and saturate at long refluxing time, and yet far below that of the MWNT (820 8C), as shown in Fig. 5b. It may be conjectured that the structural deformation towards the multi-shelled phases, but with more probable defective walls takes place with increasing refluxing time. Fig. 5c and d shows the TGA graphs and the differentiated TGAs of the HCl-treated sample after the thermal treatment in air and those of the nitric acid-treated sample for 6 h after the HCl treatment, respectively. In Fig. 5c, the differentiated TGA peak is narrower, indicating mostly evaporation of
Fig. 6. TEM images of the HNO3-treated samples as a function of refluxing times: (a) 2 h; (b) and (c) 6 h; (d) and (e) 20 h.
K.H. An et al. / Synthetic Metals 140 (2004) 1–8
SWNTs, though a small bump near 550 8C exists, suggesting that carbonaceous particles still remained in the sample. The estimated purity of the sample from the integrated area of the differentiated TGA is about 96 wt.%. However, after the HNO3 treatment (Fig. 5d), the SWNT peak in the differentiated TGA curve is separated, indicating some new phases are created, as will be suggested in the TEM images. 3.2. TEM evidences of transformation of SWNTs into other phases Fig. 6 shows low and high magnification of the TEM images for the SWNTs treated in nitric acid. At the refluxing time of 2 h in HNO3, it is observed that most of the bundle structures of SWNTs are still retained, as shown in Fig. 6a. This is consistent with the Raman spectrum, where the intensity of the RBM and the peak of 1593 cm1 were simply reduced, as shown in Fig. 4. However, some of the SWNT bundles are partially swollen and disordered (dotted circle), which are connected to the complete SWNTs. In fact, this part of the SWNT has a severe curvature. Since the curved part of the tube contains the pentagons and heptagons, this part is more likely to be attacked by the foreign chemicals. Thus it is fair to say that these distorted and swollen bundles are caused by first oxidative etching and then followed possibly by the intercalation of NO3 into the interstitial SWNT bundles. The intercalation by HNO3 treatment has been previously suggested. Monthioux et al. [5] observed that the SWNT bundles are transformed to completely amorphous phase after refluxing in 2–3 M HNO3 for 45 h. They explained this phenomenon in terms of the structural defects of SWNTs which acted as preferred sites for the HNO3 attacks, resulting in complete amorphisation of SWNTs [5]. Zhou and coworkers [7] also found that the SWNTs refluxed in 70% HNO3 for 24 h at room temperature were highly disordered and partially exfoliated. These disordered and exfoliated SWNT bundles were explained by the intercalation of HNO3 and deintercalation by heat-treatment processes. We also note that the presence of oxygen species during the acid treatment plays a crucial role in disintegrating the SWNT walls, as can be evidenced by the two different acid treatments of HCl and HNO3, where the HCl treatment resulted in no structural deformation. Once the tubes start unzipping, the zipping may be prohibited by the saturation of dangling bonds with the protons in the solution [14]. It is interesting to observe some MWNTs formed during the HNO3 treatment for 6 h, as shown in Fig. 6b. The most MWNTs formed by the HNO3 treatment are very defective and typically bamboo-like, as shown in Fig. 6c. Since the length of the MWNTs is short, they are cone-cap shaped. These MWNTs have been never observed from our SWNT samples prepared by arc discharge during HRTEM observations. We believe firmly that SWNTs are transformed to MWNTs during HNO3 treatment. These new phases explain why the differentiated TGA peak becomes broad, as shown
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in Fig. 5d. The origin of formation of such multiwalled and cone-capped phases is not so obvious at this moment. It is reasonable to say that the intercalation of HNO3 into SWNT bundles is first occurred at preferably defective sites and then the unzipping of the tube walls starts by the oxidative etching, where the zipping is prohibited by the presence of the protons, enhancing the stacking of unzipped SWNT bundles. Yet, the driving force to induce curvatures in multishelled phases is not clear. The competition between the edge energy caused by the presence of dangling bonds at the edge of the graphitic flake and the strain energy to induce the curvature to form multiwalled structure will determine which phase to prefer. In HNO3 treatment for long refluxing time of 20 h, most of the SWNT bundles and multiwalled phases formed by HNO3 treatment for 6 h are destroyed and highly altered, as shown in Fig. 5d, although small part of the SWNT bundles still remained, as evidenced by the presence of the RBM peak in the Raman spectra. The HRTEM image reveals the transformation of the entire SWNT bundle and MWNTs to onion-like structures (Fig. 6e) with good crystallinity, in good contrast with the previous reports. At longer reflux time in HNO3, more severe attack to the SWNT bundles takes place and even multiwalled phases would cause to transform into smaller graphitic flakes. This will increase the edge energy, inducing the formation of closed shells such as onion-like phases to minimize the binding energy.
4. Conclusions We have investigated the structural transformation of the SWNTs induced by HNO3 treatment. At shorter refluxing time, the SWNT bundles are partially disordered and exfoliated at preferably defective sites. With increasing refluxing time, the distorted SWNT bundles are transformed into new phases: multiwalled CNTs, cone-capped phases and onionlike phases. Based on our observation, we propose the formation mechanism. Nitric acids first attack the preferably defective sites and intercalate into the SWNT bundles to unzip the tube walls by further oxidative etching. The zipping is prohibited by the presence of protons in the solution which saturate the dangling bonds created by the unzipping. This accelerates the formation of graphitic walls. The formation of multiwalled phases can be determined by the competition between the edge energy generated by the presence of the dangling bonds in the graphitic flakes and the strain energy caused by the formation of curvature in the multiwalled phases.
Acknowledgements This work was supported in part by the MOST through the NRL program and through the CNNC at SKKU and in part by the BK21 program.
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References [1] T.W. Ebbesen, P.M. Ajayan, H. Hiura, K. Tanigaki, Nature 367 (1994) 519. [2] J.L. Zimmerman, R.K. Bradley, C.B. Huffman, R.H. Hauge, J.L. Margrave, Chem. Mater. 12 (2000) 1361. [3] J.-M. Moon, K.H. An, Y.H. Lee, Y.S. Park, D.J. Bae, G.-S. Park, J. Phys. Chem. B 105 (2001) 5677. [4] X.Y. Zhu, S.M. Lee, Y.H. Lee, T. Frauenheim, Phys. Rev. Lett. 85 (2000) 2757. [5] M. Monthioux, B.W. Smith, B. Burteaux, A. Claye, J.E. Fischer, D.E. Luzzi, Carbon 39 (2001) 1251. [6] Z. Jia, Z. Wang, J. Liang, B. Wei, D. Wu, Carbon 37 (1999) 903.
[7] C. Bower, A. Kleinhammea, Y. Wu, O. Zhou, Chem. Phys. Lett. 288 (1998) 481. [8] Y.S. Park, K.S. Kim, H.J. Jeong, W.S. Kim, J.M. Moon, K.H. An, D.J. Bae, Y.S. Lee, G.S. Park, Y.H. Lee, Synth. Met. 126 (2002) 245. [9] P.C. Eklund, J.M. Holden, R.A. Jishi, Carbon 33 (1995) 959. [10] Y.S. Park, Y.C. Choi, K.S. Kim, D.-C. Chung, D.J. Bae, K.H. An, S.C. Lim, X.Y. Zhu, Y.H. Lee, Carbon 39 (2001) 655. [11] H. Hiura, T.W. Ebbesen, K. Tanigaki, H. Takahashi, Chem. Phys. Lett. 202 (1993) 509. [12] W. Li, H. Zhang, C. Wang, L. Xu, K. Zhu, S. Xie, Appl. Phys. Lett. 70 (1997) 2684. [13] A.M. Rao, S. Bandow, E. Richter, P.C. Eklund, Thin Solid Films 331 (1998) 141. [14] S.M. Lee, Y.H. Lee, Unpublished.
Transformation of singlewalled carbon nanotubes to multiwalled carbon nanotubes and onion-like structures by nitric acid treatment Kay Hyeok Ana,c, Kwan Koo Jeona,c, Jeong-Mi Moona,c, Sung Jin Eumb,c, Cheol Woong Yangb,c, Gyeong-Su Parkd, Chong Yun Parka,c, Young Hee Leea,c,* a
BK21 Physics Division, National Research Laboratory for Carbon Nanotubes, Sungkyunkwan University, Suwon 440-746, South Korea b Department of Advanced Materials Engineering, Sungkyunkwan University, Suwon 440-746, South Korea c Center for Nanotubes and Nanostructured Composites, Sungkyunkwan University, Suwon 440-746, South Korea d Analytical Engineering Laboratory, Samsung Advanced Institute of Technology, Suwon 440-600, South Korea Received 4 September 2002; received in revised form 21 October 2002; accepted 22 October 2002
Abstract We have obtained various phases during the refluxing process of singlewalled carbon nanotubes (SWNTs) in 30% HNO3 acid solution at 100 8C. From the similar treatment in HCl acid solution, we observe no structural phase transformation but enlargement of the bundle size of SWNTs. This strongly suggests that the intercalation of nitric acids into the bundles, which is evidenced by the distribution of smaller diameters of bundles, induces the disintegration of tubes by oxidative etching first and the reformation into multiwalled nanotubes, conecapped phase, and further onion-like structures. The origin of such a transformation mechanism is further discussed. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Carbon nanotubes; Purification; Phase transformation; HNO3 treatment
1. Introduction In order to characterize the carbon nanotubes (CNTs) and improve their performance in the practical devices, the purification of the raw product, which often contains not only the CNTs but also other carbonaceous particles (amorphous carbon particles, fullerenes, nanocrystalline polyaromatic shells) and transition metals, is a necessary step through various oxidation processes. The carbonaceous particles are easily removed during gas phase reaction or thermal annealing in air or oxygen atmosphere, since the etching rate of amorphous carbons is faster than that of CNTs [1–4]. Transition metals are usually introduced during the synthesis as catalysts. The gas phase purification, however, cannot eliminate transition metals. Therefore, the liquid phase reactions in various acids are further necessary in order to melt away the transition metals. It is known that the CNTs are usually cut into small lengths and sometimes broken completely during the liquid phase reaction in * Corresponding author. Present address: Department of Physics, Institute of Basic Science, Sungkyunkwan University, Suwon 440-746, South Korea. Tel.: þ82-31-299-6507; fax: þ82-31-290-5954. E-mail address: [email protected] (Y.H. Lee).
various acids [5,6]. The CNT walls are severely damaged in strong acid solution, although the wall structures could be recovered by the subsequent annealing. Recently, Zhou and co-workers [7] have reported that the bundles of singlewalled carbon nanotubes (SWNTs) become disordered and partially exfoliated, when the SWNTs are immersed in 70% HNO3 solution for a long period of time. They have often observed the isolated SWNTs and large nanoparticles. Monthioux et al. have shown that the acid purification using HNO3 or H2SO4/HNO3 mixture solution cannot avoid an alteration of the SWNT structure. The acidic medium provokes gathering of pre-existing fullerenes into crystallized fullerite. A slight temperature increase has a dramatic effect on SWNT degradation, which can result in a complete amorphisation [5]. Mineral acids containing oxygen atoms are known to help in oxidizing polyaromatic solids like graphite. They are also believed to intercalate and reside inside the bundles of CNTs [7]. In this study, we report the effect of HNO3 treatment to the SWNT structure by means of high-resolution transmission electron microscopy (HRTEM). Comparing the acid treatment in HNO3 solution with that in HCl solution, we learn that nitric acids can be intercalated into SWNT bundles and disintegrate the tube walls into graphitic flakes and then
0379-6779/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(02)01254-7
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K.H. An et al. / Synthetic Metals 140 (2004) 1–8
reform them into various multi-shell phases such as multiwalled CNTs, cone-capped phases, and onion-like phases, which was never observed from our raw SWNTs. We also discuss the origin and mechanism for the transformation of SWNTs to multiwalled carbon nanotubes (MWNTs) and onion-like structures by nitric acid treatment.
2. Experimental SWNTs were prepared by a conventional catalytic arc discharge [8]. The total amount of catalyst in a graphite powder was fixed to be 5 wt.%, with the ratio of the transition metals (Ni:Co:FeS ¼ 1:1:1), where sulfur was added as a promoter. This increased significantly the yield of CNTs deposited in the whole chamber. The systematic purification procedure is as follows. The cloth-like raw
sample was ground mechanically and transferred to the heating chamber, where the CNT powder was heated at 470 8C for 50 min in air to remove carbonaceous particles. In order to remove the catalysts, the annealed powder was immersed in 6 M hydrochloric acid (HCl) for 24 h. This process was repeated several times until the color of acid was unchanged. The sample was then washed out in deionized water by several times. The detail of synthesis of SWNTs by arc discharge and purification by combining two-step processes of thermal annealing in air and HCl treatment has been described elsewhere [3,8]. The sample obtained at the last step was refluxed in 30% nitric acid (HNO3) at 100 8C for 10 min to 20 h. The obtained suspension was filtered with a polytetrafluoroethylene (PTFE) membrane and washed out in deionized water. After rinsing and drying, a grayish black, thin mat was finally obtained.
Fig. 1. SEM and TEM images of: (a) and (b) the raw SWNT sample; (c) and (d) the thermally treated sample; (e) and (f) HCl-treated sample.
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The morphology of the sample and the degree of the structural transformation were observed by scanning electron microscope (SEM, Hitachi, S4700), TEM (Hitachi, H9000NA), and Fourier transform (FT) Raman spectroscopy (BRUKER, RFS 100/S) using Nd:YAG laser (1064 nm). Thermogravimetric analysis (TGA) was also carried out using a TA Instruments (TGA 2950) with a linear heating rate of 10 8C/min from 25 to 1000 8C in air flow.
3. Results and discussion 3.1. Structural characterizations of SWNTs treated in nitric acid Fig. 1a shows the SEM image of the ground raw soot collected from the whole chamber. In addition to the SWNT bundles, the white spots, which are the carbonaceous particles, mainly amorphous carbons, are shown in the figure. These structural features might be caused by the variation in the arcing process because of an inhomogeneous distribution of catalysts within the powder in the anode [8]. Furthermore, we can see that the metal particles are mainly embedded in large amorphous carbons, as evidenced in Fig. 1b. The diameter of an individual SWNT is around 1.4 nm, and bundles are composed of a few tenths of SWNTs. This sample was thermally annealed at 470 8C for 50 min in air to remove selectively the amorphous carbons. The SEM image of the oxidized SWNTs is shown in Fig. 1c. The
3
figure shows a dramatic decrease of amorphous carbons after oxidation. As a result of oxidation, the weight of the soot is reduced to about 40 wt.% of the original sample. The TEM image of the oxidized SWNTs shown in Fig. 1d suggests that most of the amorphous carbons, which covered the surfaces of SWNT bundles and those surrounding the catalytic metals, were removed. Now the transition metals, as indicated by the black spots in Fig. 1d, are left. The transition metals can be etched away by a HCl treatment. Fig. 1e shows the SEM image of the HCl-treated sample, where the annealed sample was immersed in 6 M hydrochloric acid for 24 h. The corresponding TEM image in Fig. 1f clearly shows that the transition metal particles are well melted away. Yet some carbonaceous particles, which are left during the annealing procedure, still remain in the sample, as illustrated in Fig. 1e. Some transition metals are still left even after the HCl treatment, as can be seen from Fig. 1f. Therefore, a special care should be taken to obtain the purified samples with high quality. We note that the HCl treatment enhances the bundling of SWNTs, as shown in Fig. 1e and f, compared to the annealed samples in Fig. 1c and d, since the entangled SWNTs are released in acid and aggregated themselves by the van der Waals interactions. This acid treatment gives the yield of 70 wt.% of the thermally annealed samples. Thus the total yield of the two steps is about 25–30 wt.%. The HCl-treated SWNT was further refluxed in 30% HNO3 to disperse the SWNT bundles which were aggregated by the HCl treatment. Fig. 2 shows the SEM images of
Fig. 2. SEM images of the HNO3-treated samples as a function of refluxing times: (a) 0.5 h; (b) 1 h; (c) 2 h; (d) 6 h.
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Fig. 3. TEM images of: (a) HCl-treated sample and (b) HNO3-treated sample for 2 h. Bundle size distributions of: (c) HCl-treated sample and (d) HNO3treated sample for 2 h.
the HNO3-treated samples. The SEM images of the HNO3treated samples for 30 min to 1 h (Fig. 2a and b) reveal no significant morphological changes compared to the HCltreated sample. The large bundles of SWNTs enhanced by the HCl treatment are not separated individually yet. At 2 h of HNO3 treatment (Fig. 2c), large portion of the SWNT bundles are dispersed, as evidenced by the opaque backgrounds in the SEM image. This is clearly presented in the TEM image of the SWNT bundles at 2 h of HNO3 treatment, as shown in Fig. 3. The SEM morphology for 6 h treatment is somehow completely different, as shown in Fig. 2d. The SWNTs are broken into small pieces, forming larger diameters. More carbonaceous particles are restructured at this time. These particles and short bundle-like phase are not the same phases as before, which will be discussed later. Fig. 3 shows TEM images and the bundle size distribution of the HCl-treated and HNO3-treated samples for 2 h. The bundle size distributions are estimated from TEM images. One can clearly see that the bundle sizes of the HCl-treated
sample are larger than that of the HNO3-treated one in Fig. 3a and b. While the bundle size of the HCl-treated SWNTs is distributed over the range 10–25 nm, the size of the HNO3treated one is in the range 5–12 nm. We note that the short time treatment of HNO3 is required to disperse the CNT bundles with a minimal damage to the CNT walls. Raman spectroscopy is employed to identify the structural changes of the HNO3-treated samples, as shown in Fig. 4. All the samples have the maximum intensity near 1593 cm1, so called tangential mode, which is related to the graphite E2g symmetric intralayer mode, split due to the curvature of the rolled-up graphene sheet [9]. Two shoulder peaks also appear at 1571 and 1552 cm1, which are not exhibited in the Raman spectra of MWNTs and graphite [3]. A broad band is usually observed around 1280 cm1 in graphite and MWNTs [10–12], which is a characteristic of amorphous carbon or defects, and is also observed in Fig. 4. As the refluxing time in 30% HNO3 increases, the intensity of maximum peak at 1593 cm1 decreases drastically,
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Fig. 4. FT-Raman spectra of the HNO3-treated samples as a function of refluxing times.
whereas the intensity ratio of the 1280–1593 cm1 peak increases, indicating some structural deformation in the tube wall after HNO3 treatment. The radial breathing modes
(RBMs) whose frequencies depend on the tube diameters [13] in the low frequency range 160–200 cm1 are also observed in all of the samples. This is also a characteristic of
Fig. 5. (a) The HNO3-treated samples as a function of refluxing times and MWNT as a reference sample. (b) The maximum temperatures of the differentiated TGAs for the HNO3-treated samples and MWNT. TGA graphs of: (c) the HCl-treated sample and (d) the HNO3-treated sample for 6 h (the solid and dotted lines correspond to the TG and differentiated TG curve, respectively).
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SWNTs, since MWNTs do not show any low frequency mode and the low frequency mode of graphite is at 42 cm1 [3,10]. After the HNO3 treatment, the intensities of the RBM decrease drastically with increasing refluxing time, indicating the structural changes of SWNTs, particularly after 30 min. However, since the two shoulder peaks at 1571 and 1552 cm1 and the RBM in the range 160–200 cm1 are present in the Raman spectra, some SWNTs still remain in the sample during long time refluxing in 30% HNO3. The disappearance of small peaks in the mid-frequency range after 30 min indicates again the structural deformation in the tube wall. Fig. 5 shows the TGA graphs measured in air atmosphere and the peak temperatures of the respective differentiated TGAs (Pdw /dt) of the HNO3-treated SWNTs and MWNTs, which are synthesized by arc discharge method, served as a reference sample. In the TGA curves of the SWNTs treated
by HNO3, The weight loss by burn-off starts from 650 8C, although they are completely burnt-off near 850 8C. The MWNTs start burning-off at higher temperature, compared to that of the SWNTs, although the complete burn-off temperature was rather lower, as shown in Fig. 5a. The TGA curves shift to higher burn-off temperature with increasing refluxing time. The maximum in Pdw /dt increases and saturate at long refluxing time, and yet far below that of the MWNT (820 8C), as shown in Fig. 5b. It may be conjectured that the structural deformation towards the multi-shelled phases, but with more probable defective walls takes place with increasing refluxing time. Fig. 5c and d shows the TGA graphs and the differentiated TGAs of the HCl-treated sample after the thermal treatment in air and those of the nitric acid-treated sample for 6 h after the HCl treatment, respectively. In Fig. 5c, the differentiated TGA peak is narrower, indicating mostly evaporation of
Fig. 6. TEM images of the HNO3-treated samples as a function of refluxing times: (a) 2 h; (b) and (c) 6 h; (d) and (e) 20 h.
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SWNTs, though a small bump near 550 8C exists, suggesting that carbonaceous particles still remained in the sample. The estimated purity of the sample from the integrated area of the differentiated TGA is about 96 wt.%. However, after the HNO3 treatment (Fig. 5d), the SWNT peak in the differentiated TGA curve is separated, indicating some new phases are created, as will be suggested in the TEM images. 3.2. TEM evidences of transformation of SWNTs into other phases Fig. 6 shows low and high magnification of the TEM images for the SWNTs treated in nitric acid. At the refluxing time of 2 h in HNO3, it is observed that most of the bundle structures of SWNTs are still retained, as shown in Fig. 6a. This is consistent with the Raman spectrum, where the intensity of the RBM and the peak of 1593 cm1 were simply reduced, as shown in Fig. 4. However, some of the SWNT bundles are partially swollen and disordered (dotted circle), which are connected to the complete SWNTs. In fact, this part of the SWNT has a severe curvature. Since the curved part of the tube contains the pentagons and heptagons, this part is more likely to be attacked by the foreign chemicals. Thus it is fair to say that these distorted and swollen bundles are caused by first oxidative etching and then followed possibly by the intercalation of NO3 into the interstitial SWNT bundles. The intercalation by HNO3 treatment has been previously suggested. Monthioux et al. [5] observed that the SWNT bundles are transformed to completely amorphous phase after refluxing in 2–3 M HNO3 for 45 h. They explained this phenomenon in terms of the structural defects of SWNTs which acted as preferred sites for the HNO3 attacks, resulting in complete amorphisation of SWNTs [5]. Zhou and coworkers [7] also found that the SWNTs refluxed in 70% HNO3 for 24 h at room temperature were highly disordered and partially exfoliated. These disordered and exfoliated SWNT bundles were explained by the intercalation of HNO3 and deintercalation by heat-treatment processes. We also note that the presence of oxygen species during the acid treatment plays a crucial role in disintegrating the SWNT walls, as can be evidenced by the two different acid treatments of HCl and HNO3, where the HCl treatment resulted in no structural deformation. Once the tubes start unzipping, the zipping may be prohibited by the saturation of dangling bonds with the protons in the solution [14]. It is interesting to observe some MWNTs formed during the HNO3 treatment for 6 h, as shown in Fig. 6b. The most MWNTs formed by the HNO3 treatment are very defective and typically bamboo-like, as shown in Fig. 6c. Since the length of the MWNTs is short, they are cone-cap shaped. These MWNTs have been never observed from our SWNT samples prepared by arc discharge during HRTEM observations. We believe firmly that SWNTs are transformed to MWNTs during HNO3 treatment. These new phases explain why the differentiated TGA peak becomes broad, as shown
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in Fig. 5d. The origin of formation of such multiwalled and cone-capped phases is not so obvious at this moment. It is reasonable to say that the intercalation of HNO3 into SWNT bundles is first occurred at preferably defective sites and then the unzipping of the tube walls starts by the oxidative etching, where the zipping is prohibited by the presence of the protons, enhancing the stacking of unzipped SWNT bundles. Yet, the driving force to induce curvatures in multishelled phases is not clear. The competition between the edge energy caused by the presence of dangling bonds at the edge of the graphitic flake and the strain energy to induce the curvature to form multiwalled structure will determine which phase to prefer. In HNO3 treatment for long refluxing time of 20 h, most of the SWNT bundles and multiwalled phases formed by HNO3 treatment for 6 h are destroyed and highly altered, as shown in Fig. 5d, although small part of the SWNT bundles still remained, as evidenced by the presence of the RBM peak in the Raman spectra. The HRTEM image reveals the transformation of the entire SWNT bundle and MWNTs to onion-like structures (Fig. 6e) with good crystallinity, in good contrast with the previous reports. At longer reflux time in HNO3, more severe attack to the SWNT bundles takes place and even multiwalled phases would cause to transform into smaller graphitic flakes. This will increase the edge energy, inducing the formation of closed shells such as onion-like phases to minimize the binding energy.
4. Conclusions We have investigated the structural transformation of the SWNTs induced by HNO3 treatment. At shorter refluxing time, the SWNT bundles are partially disordered and exfoliated at preferably defective sites. With increasing refluxing time, the distorted SWNT bundles are transformed into new phases: multiwalled CNTs, cone-capped phases and onionlike phases. Based on our observation, we propose the formation mechanism. Nitric acids first attack the preferably defective sites and intercalate into the SWNT bundles to unzip the tube walls by further oxidative etching. The zipping is prohibited by the presence of protons in the solution which saturate the dangling bonds created by the unzipping. This accelerates the formation of graphitic walls. The formation of multiwalled phases can be determined by the competition between the edge energy generated by the presence of the dangling bonds in the graphitic flakes and the strain energy caused by the formation of curvature in the multiwalled phases.
Acknowledgements This work was supported in part by the MOST through the NRL program and through the CNNC at SKKU and in part by the BK21 program.
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