7481
2009, 113, 7481–7483 Published on Web 04/13/2009
Transforming Carbon Nanotubes to Few-Layer Graphene with the Assistance of Encapsulated Ferrocene Lunhui Guan* and Jiaxin Li State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, YangQiao West Road 155#, Fuzhou, Fujian 350002, P. R. China ReceiVed: March 9, 2009; ReVised Manuscript ReceiVed: April 3, 2009
Is it possible to form few-layer graphene by direct merging single-walled carbon nanotubes (SWNTs), which are generally regarded as rolled single-layered graphene? In this study, we reported the structure transformation of SWNTs to few-layer graphene assisted by the encapsulated ferrocene molecules. High resolution transmission electron microscopy imaging and Raman spectra revealed that the transformation is quite sensitive to the diameters of the host SWNTs and also the annealing temperature. The encapsulated ferrocene molecules played a key role during the transformation. Graphene-based materials, for their superb properties, have emerged as quite intriguing materials from both perspectives of science and technology.1-5 Single-walled carbon nanotubes (SWNTs), which are regarded as rolled single-layered graphene, have attracted considerable interest of scientists.6 Previous results reported that two or more SWNTs coalesced to form larger SWNTs under electron beam irradiation7 or heat treatments.8 A question comes around, is it possible to form few-layer graphene by direct merging of SWNTs? When considering the coalescence of SWNTs, high energy barriers must have to be overcome.7 Thus, an effective way should be explored to reduce the energy barriers. Filling is a reliable way to modulate the reactivity of SWNTs. Encapsulation of foreign materials into SWNTs’ inner empty cavities may modify the properties of the filling materials, as well as alter the properties of host SWNTs. Previous results indicated that molecules inside SWNTs exhibited totally different structure9 and reactivity10 compared with their bulk materials. However, there is a lack of experimental data to explore the effect on SWNTs resulting from encapsulation. Here, we present the first experimental results on the structural transformation of SWNTs to few-layer graphene assisted by the encapsulated molecules. High resolution transmission electron microscopy (HR-TEM) imaging and Raman spectra revealed that the transformation, which was assisted by the encapsulated ferrocene molecules, is quite sensitive to the diameters of the host SWNTs and the annealing temperature. In this study, Fc molecules were introduced into the high pressure CO conversion (HiPco) SWNTs with a mean diameter of 1.1 nm and the arc-discharged SWNTs with a mean diameter of 1.4 nm by heating the mixtures of Fc and SWNTs in an evacuated glass ampule at 573 K for 48 h. In order to remove ferrocene sticking outside the nanotubes and in between the tube bundles, the SWNT paper was washed with diethyl ether five times until the solvent was colorless. The products, denoted as Fc@ HiPco and Fc@Arc, were annealed in 10-3 Pa for 24 h at * To whom correspondence should be addressed. Phone: 86-59183792835. Fax: 86-591-83792835. E-mail:
[email protected].
10.1021/jp902098b CCC: $40.75
Figure 1. TEM images of (a) the pristine Fc@HiPco, (b) the corresponding HRTEM image, (c) Fc@HiPco annealed at 1473 K, and (d) a bilayer graphene indicated by the arrow in part c.
773, 1123, 1273, and 1473 K, respectively. For comparison, we also annealed intact HiPco SWNTs at 1473 K. The samples were characterized by HR-TEM (JEOL JEM-2010), scanning election microscopy (SEM, JSM 6700F), and Raman spectrometers (Renishaw, 514.5 nm). Our previous result indicated that annealing Fc@CNTs at 1273 K would transfer the encapsulated Fc to inner SWNTs.11 When we increased the annealing temperature to 1473 K, a prominent structure transformation took place for Fc@HiPco. The thermal treatments at high temperature changed the host HiPco SWNTs completely. Figure 1 shows TEM images of the Fc@ HiPco before and after annealing at 1473 K. The low 2009 American Chemical Society
7482
J. Phys. Chem. C, Vol. 113, No. 18, 2009
Letters
Figure 2. Raman spectra of the pristine HiPco SWNTs, with Fc@HiPco and the sample treated at various temperatures. The structural transformation of the SWNTs is diameter-dependent. The smaller tubes collapsed first, and then the larger tubes.
magnification image (Figure 1a) indicated that the pristine HiPco SWNTs existed in the form of bundles and of high purity (g95%). After filling with Fc molecules, the outer surface of SWNTs is still clean. The HR-TEM image (Figure 1b) revealed that some foreign objects are inside the SWNTs. After annealing, we seldom found the typical tubular structure of the host HiPco SWNTs. They have transformed to graphene sheets due to the thermal treatments. Figure 1c shows a typical TEM image of the product after annealing. The product is inhomogeneous, containing mainly few-layer graphene sheets (2-10 layers) and a few graphite particles (see the Supporting Information for more TEM images of the few-layer graphene). The nearly transparent region in Figure 1c can be attributed to the graphene sheets less than three layers. The darker region is ascribed to the overlap of the graphene sheets. We can determine the number of few-layer graphene in a sheet by HR-TEM observations. Indicated by the arrow in Figure 1c is a bilayer sheet, as demonstrated by the HR-TEM image shown in Figure 1d. A SWNT, which was seriously damaged, was lying on the surface of the bilayer sheet. The pristine HiPco SWNTs and Fc@ HiPco before and after annealing at different temperatures were also characterized by Raman spectroscopy, which provides the structural and electronic information of carbon materials. Figure 2 exhibits the evolution of the Raman response for the HiPco SWNTs upon filling and subsequent heat-treating at various temperature. The representative peaks of the pristine SWNTs are the tangential band (G band), the disorder induced band (D band), and the radial breath mode (RBM) that is inversely proportional to the diameters of SWNTs.12 After being encapsulated with ferrocene, the G band and RBM shifted slightly because of the charge transfer between ferrocene and the host SWNTs. The comparison of the Raman spectra was used to investigate the damage of the SWNTs due to the thermal treatments. When the annealing temperature increased, the intensity of the RBM peak decreased, while the intensity of the D band increased, implying that the host SWNTs underwent prominent structure change. It should be noted that the structure transformation of the host SWNTs is diameter-dependent. The reactivity of SWNTs is inversely proportional to their diameters. The thinner tube with a diameter around 0.9 nm (RBM peak located at around 250 cm-1) began to collapse first, and then the tube with a relatively larger diameter around 1.1 nm. As for the sample annealed at 1473 K, the RBM peaks completely disappeared,
Figure 3. Raman spectra of the Fc@Arc-discharged SWNTs and the samples annealed at 1473 K. After the thermal treatment, the outer host SWNTs remain unchanged, while the encapsulated ferrocene molecules decomposed and changed into another shell in SWNT, forming double-walled carbon nanotubes (DWNTs), as demonstrated by the newly appeared RBM peak indicated by the arrow in part a and the inserted HRTEM imaged in part b.
and the line shape of the G band changed significantly, indicating the collapse of the host SWNTs. For comparison, we performed the same heat treatment on the Fc@arc-discharged SWNTs with a mean diameter of 1.4 nm, slightly larger than the mean 1.1 nm of the HiPco SWNTs. Surprisingly, after the thermal treatment, the outer host SWNTs remain unchanged, while the encapsulated ferrocene molecules changed to the inner tube, forming double-walled carbon nanotubes (Figure 3). The formation of few-layer graphene is also demonstrated by the Raman spectra. A single-layer graphene shows a sharp 2D peak below 2700 cm-1 similar to that of SWNTs.13,14 When the number of the sheets increased, the 2D peak became broader and upshifted to positions greater than 2700 cm-1. The Raman spectrum obtained from the sample annealed at 1473 K exhibited a broader and asymmetric 2D peak, indicating that the samples consisted of inhomogeneous few-layer graphene sheets. The encapsulated Fc molecules played a key role in the structure transformation of the host SWNTs during annealing. We performed vacuum heat treatments on the intact HiPco SWNTs and Fc@HiPco at 1473 K, respectively. As expected, the SEM image demonstrated that the intact HiPco SWNTs remained unchanged after the heat treatments (Figure 4a). Previous study also indicated that, even after heating at 2073 K, the HiPco SWNTs still retain a tubular structure.8 On the contrary, after annealing, Fc@HiPco nearly completely changed to few-layer graphene (Figure 4b). The mechanism of the
Letters
J. Phys. Chem. C, Vol. 113, No. 18, 2009 7483 authors thank Mrs. L. Zhou and Mr. F. Bao for helping with the SEM and TEM. We also thank Mr. Y. Li and Prof. R. Chen at Fujian Normal University for helping with the Raman measurement. Supporting Information Available: Description of the sample preparation and a figure showing typical TEM images of a few-layer graphene transformed from Fc@HiPco. This material is available free of charge via the Internet at http:// pubs.acs.org.
Figure 4. SEM images of the samples annealed at 1473 K for 24 h: (a) the intact HiPco SWNTs; (b) Fc@HiPco. The results convincingly demonstrated that the encapsulated Fc molecules played a key role during the structural transformation.
transformation is still unclear; we tentatively speculated that ferrocene decomposed after the heat treatment, forming hydrogen/ hydrocarbon vapor along with the Fe nanoparticles. The hydrogen/hydrocarbon or the Fe nanoparticles with highly reactively etched the host HipCO SWNTs at high temperature, eventually forming few-layer graphene. In summary, the study provided a phenomenon of structure transformation of SWNTs with thinner diameters to few-layer graphene assisted by the encapsulated Fc molecules. The transformation is diameter-dependent and sensitive to the temperature. Our results have the implication for utilizing SWNTs as starting materials to obtain graphene with a reduced number of layers. Acknowledgment. Financial support for this study was provided by Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, and the National Key Project on Basic Research (Grant No. 2009CB939801). The
References and Notes (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666. (2) Li, D.; Kaner, R. B. Science 2008, 320, 1170. (3) Li, X. L.; Wang, X. R.; Zhang, L.; Lee, S. W.; Dai, H. J. Science 2008, 319, 1229. (4) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183. (5) Reina, A.; Jia, X. T.; Ho, J.; Nezich, D.; Son, H. B.; Bulovic, V.; Dresselhaus, M. S.; Kong, J. Nano Lett. 2009, 9, 30. (6) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (7) Terrones, M.; Terrones, H.; Banhart, F.; Charlier, J. C.; Ajayan, P. M. Science 2000, 288, 1226. (8) Yudasaka, M.; Kataura, H.; Ichihashi, T.; Qin, L. C.; Kar, S.; Iijima, S. Nano Lett. 2001, 1, 487. (9) Guan, L. H.; Suenaga, K.; Shi, Z. J.; Gu, Z. N.; Iijima, S. Nano Lett. 2007, 7, 1532. (10) Guan, L. H.; Suenaga, K.; Okubo, S.; Okazaki, T.; Lijima, S. J. Am. Chem. Soc. 2008, 130, 2162. (11) Guan, L. H.; Suenaga, K.; Iijima, S. Nano Lett. 2008, 8, 459. (12) Kasuya, A.; Sasaki, Y.; Saito, Y.; Tohji, K.; Nishina, Y. Phys. ReV. Lett. 1997, 78, 4434. (13) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Phys. ReV. Lett. 2006, 97, 187401. (14) Ferrari, A. C. Solid State Commun. 2007, 143, 47.
JP902098B
