NANO LETTERS
Coalescence of Double-Walled Carbon Nanotubes: Formation of Novel Carbon Bicables
2004 Vol. 4, No. 8 1451-1454
Morinobu Endo,*,† Takuya Hayashi,† Hiroyuki Muramatsu,† Yoong-Ahm Kim,† Humberto Terrones,‡ Mauricio Terrones,‡ and Mildred S. Dresselhaus§ Faculty of Engineering, Shinshu UniVersity, 4-17-1 Wakasato, Nagano-shi, 380-8553, Japan, AdVanced Materials Department, IPICyT, Camino a la Presa San Jose´ 2055, Lomas 4a. seccio´ n, 78216 San Luis Potosı´, SLP, Me´ xico, and Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307 Received May 19, 2004; Revised Manuscript Received June 9, 2004
ABSTRACT Here, we demonstrate that by coalescing double-walled carbon nanotubes (DWNTs), a novel and stable structure consisting of flattened tubules containing two single-walled tubes (SWNTs) is created. The process occurs due to the coalescence and reconstruction of the outer shells of DWNTs, leaving the inner cylinders almost intact, the latter being encapsulated inside the large diameter coalesced tube (bicable). We propose that the coalescence process is due to: (1) the thermal activity of the outer shells, which is driven by a surface energy minimization process, and (2) a zipping mechanism followed by atom reconstruction, in which two outer shells interact and anneal.
By combining and interconnecting carbon nanotubes and fullerenes, it is possible to generate novel hybrid materials such as peapods,1,2 corrugated tubes,3 nanotube junctions, and networks.4 These materials possess different electronic and mechanical properties when compared to isolated carbon tubules or cages. Recently, carbon nanotube research has focused on the synthesis of DWNTs because they constitute a hybrid between single- and multiwalled tubes and could find applications as field-electron emitters,5 or nanoscale bearings.6 Here, we describe their merging process, which mainly consists of (1) the merging of two outer shells via a zipping mechanism accelerated by the presence of defective sites (e.g., vacancies, interstitials, etc.) within the CVD tubules, and (2) the stabilization of the two inner tubules, remaining inside the wider cylinder. Our experiments were conducted by the thermal annealing of DWNTs produced using chemical vapor deposition. The DWNTs utilized in this study were synthesized using a standard catalytic decomposition of methane over iron metal nanoparticles7-9 and were purified followed by acid treatment and air oxidation at 500 °C. This severe process decreases significantly the amount of SWNTs due to the higher chemical reactivity of SWNTs with oxygen when compared to DWNTs or MWNTs.10 Finally, heat treatment * Corresponding author. E-mail:
[email protected]. † Shinshu University. ‡ IPICyT. § Massachusetts Institute of Technology. 10.1021/nl0492483 CCC: $27.50 Published on Web 07/10/2004
© 2004 American Chemical Society
under an argon atmosphere was carried out at various temperatures from 1500 to 2400 °C for 30 min using a graphite-resistance furnace. TEM observations confirmed that high yields of DWNTs (over 95%) are present in a bundle state (see Figure 1b) before the heat treatment. The TEM used in the present study was a JEOL JEM-2010FEF instrument equipped with an in-column Ω-type energy-filter. The operating voltage of the TEM was 200 kV. The Raman spectra were obtained using a Kaiser HoloLab 5000 system (excitation wavelength: 532 nm, laser power less than 5 mW). Figure 1a depicts the radial breathing mode (RBM) Raman spectra below 400 cm-1 for DWNTs heat treated at various temperatures with a frequency ωRBM, which is inversely related to the tube diameter, dt.11 It is noteworthy that the tube diameters become larger with increasing heat treatment temperature, indicating that tubes are transformed into more stable sp2-like structural forms, that is, larger diameter tubes corresponding to a more gentle curvature of the graphene sheets. The tube diameters are calculated from the RBM peaks (see Table 1) using the equation ωRBM ) 234/dt + 10, where dt is the tube diameter (nm) and ωRBM, the RBM frequency (cm-1).12 There are no significant changes for samples heat-treated at 2100 °C except for the disappearance of the peak located at 312 cm-1 (0.77 nm), suggesting that highly curved and less stable SWNTs are transformed into more stable graphitic structures via the surface reconstruction that is induced thermally.13 From an applications point of
Figure 1. (a) Low-frequency Raman spectra of DWNTs (grown by a catalytic CVD method at around 875 °C) heat treated at various temperatures from 1500 to 2400 °C. Curve fitting with Lorentzian line shapes was carried out in order to get the exact peak position, intensity, and half width at half-maximum (HWHM) intensity. The tendency to downshift the RBM frequency with increasing heat treatment temperature indicates the formation of thermally stable large-diameter tubes. (b) A typical high-resolution transmission electron microscopy (HRTEM) image of DWNTs in a bundle state. (c) HRTEM image of DWNTs at 2100 °C. This image exhibits a sequential reconstruction process of a DWNT: (I) two outer tubes start to merge, through a zipping mechanism, (II) two outer tubes are completely combined into a single large outer tube with an oval shape containing two SWNTs, and (III) two inner SWNTs in a confined space might decompose along the inner wall of an outer shell, to form one inner single shell, like the formation of a DWNT derived from a peapod.16
view, the fabrication of high purity DWNTs using heat treatment processes suggests that they could be used as a 1452
high-current field emitter, owing to their high structural stability and purity (removal of iron metal particles). This is in contrast with SWNTs, which could be easily damaged when the emitter current exceeds a threshold value.14 It is natural that each DWNT within a bundle (Figure 1b) will try to stabilize through a structural transformation when the thermal energy is increased over a critical value, and this transformation occurs from the periphery to the core of a bundle following the temperature gradient. This assumption was confirmed by a detailed HRTEM and Raman scattering study. The abrupt drop in intensity of the RBM peak at 270 cm-1 for samples heat treated at 2200 °C indicates the destruction of DWNTs with inner tubes of 0.9 nm diameter and outer tubes of 1.6 nm diameter. The most intriguing observation was the HRTEM image (Figure 1c) exhibiting a sequential reconstruction behavior of DWNTs at 2100 °C. Here, two adjacent outer shells start to merge (see, (I) in Figure 1c) via a zipping process, similar to the coalescence of SWNTs under electron beam irradiation,15 and a large single outer shell with an oval shape is formed (see, (II) in Figure 1c). To confirm the coalescence of DWNTs, we carried out HRTEM simulations of various types of coalesced nanotubes containing SWNTs (Figure 2). It is clear that these bicables could be stable within a bundle (see Figure 1c). The curvature of the outer shell in the bicable is pronounced for those present in the core of the bundle (see (I) in Figure 1c), whereas the curvature of the outer bicable is more elliptical on the outskirts of the bundle (see (II) in Figure 1c). In this context we propose the possibility of synthesizing a bundle of carbon bicables by coalescing DWNTs at ca. 2100 °C (Figure 3). These carbon bicable bundles may exhibit novel electronic properties. In addition, these novel structures are also by far more stable (thermally) when compared to SWNTs. In this context, it is interesting to note that the two inner SWNTs encapsulated in a wider tubule exhibit a higher thermal stability than that of free-standing SWNTs having the same diameter, and this might be due to the presence of the protective layer (oval-type outer tube). This explanation is strongly supported by the existence of DWNTs with a larger diameter in a sample heat treated at 3000 °C.7 If two inner SWNTs with the same chirality are merged into a single shell through the zipping process, the diameter of the inner shell in a DWNT will be two-fold (e.g., 0.9 × 2 ) 1.8 nm). This means that the interlayer spacing between two shells in a DWNT has to increase exactly two-fold. Alternatively, we suggest a route to explain how two inner SWNTs of different chirality form a single large inner shell of a DWNT (Figure 1c). If the chiralities are different, a complete surface reconstruction of the two inner SWNTs will result in the creation of a graphene nanocluster following the curvature of the outer cylinder, thus preserving the van der Waals interactions.16,17 This tube, exhibiting sometimes a distorted cross-section, will be transformed into a more stable DWNT with a round cross section. As a result, a structurally stable single large-diameter DWNT (1.8 nm inner tube, 3.2 nm outer tube) (see (III) in Figure 1c) is formed once again by coalescing two thinner DWNTs. However, for nanotubes of Nano Lett., Vol. 4, No. 8, 2004
Table 1. Estimated Tube Diameters from the Radial Breathing Mode (RBM) Frequenciesa peak (cm-1) I. D.
92.2
pristine H1500 H2000 H2100 H2200 H2300 H2400
2.29 nm 2.23 nm 2.21 nm 2.20 nm 2.34 nm 2.29 nm 2.17 nm
110.6
1.94 nm 1.93 nm
154.8
186.6
243.1
270.1
313.5
1.61 nm 1.60 nm 1.71 nm 1.70 nm 1.65 nm 1.66 nm 1.67 nm
1.34 nm 1.33 nm 1.33 nm 1.33 nm 1.34 nm 1.33 nm 1.31 nm
0.97 nm 0.97 nm
0.90 nm 0.90 nm 0.91 nm 0.90 nm 0.89 nm 0.89 nm 0.89 nm
0.77 nm 0.78 nm
0.98 nm 0.98 nm
a The tube diameters were calculated using the equation ω -1 12 RBM ) 234/dt + 10, where dt is the tube diameter (nm) and ωRBM, the RBM frequency (cm ). Curve fitting was carried out using Lorentzian line shapes.
Figure 3. HRTEM simulation (using a multi-slice method calculated with a Scherzer defocus) of a crystalline carbon bicable bundle, which could be produced after annealing DWNTs at a fixed temperature. We envisage this type of bundle as novel since their electronic and mechanical properties would be different from a standard SWNT bundle.
Figure 2. (a) Molecular model of two coalesced DWNTs consisting of an outer dumbbell-like tube containing two parallel individual SWNTs, (b) HRTEM image simulation of the model shown in (a) obtained using the multi-slice method at the Scherzer defocus, and (c) HRTEM image of two SWNTs contained inside a dumbbell carbon cylinder, resembling that shown in (a) and (b); the interlayer spacing is ca. 0.34 nm.
the same chirality, this coalescence process may occur faster and would require less thermal energy. This result confirms once again the higher chemical reactivity of the highly curved graphene layers of the thinner tubes. We have produced a new and stable type of coaxial carbon bicable by thermally coalescing DWNTs. We envisage that if the two inner SWNTs of the bicable possess different electronic properties (e.g., semiconductor and a metal), it may be possible to have parallel nanotube transistors within the same single cable. This may have important implications in nanoelectronics. Even though DWNTs exhibit higher thermal Nano Lett., Vol. 4, No. 8, 2004
stability when compared to SWNTs, an excess of thermal energy on DWNTs could transform them into multiwalled tubes. The reconstruction processes using heat treatment could be applicable for controlling the structure and diameter of carbon nanotubes with the desired number of layers and morphology. The nanotube coalescence process using high temperatures might lead to different carbon structures that are not easily produced in conventional sintering. Acknowledgment. This work was supported by the CLUSTER of Ministry of Education, Culture, Sports, Science and Technology (M.E.). We also thank CONACYT-Me´xico grants W-8001-millennium initiative (H.T., M.T.), G-25851-E (H.T., M.T.), 36365-E (H.T.), and 37589-U (M.T.) for financial support. References (1) Smith, B. W.; Monthioux, M.; Luzzi, D. E. Nature 1998, 396, 323. (2) Sloan, J.; Dunin-Borkowski, E.; Hutchison, J. L.; Coleman, K. S.; Williams, V. C.; Claridge, J. B.; York, A. P. E.; Xu, C.; Baily, S. R.; Brown, G.; Friedrichs, S.; Green, M. L. H. Chem. Phys. Lett. 2000, 316, 191. 1453
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NL0492483
Nano Lett., Vol. 4, No. 8, 2004
