NANO LETTERS
Large Thick Flattened Carbon Nanotubes
2002 Vol. 2, No. 12 1439-1442
Suwen Liu,*,†,‡ Jun Yue,† and Rudolf J. Wehmschulte‡ Department of Materials Science and Engineering, UniVersity of Science & Technology of China, Hefei, Anhui 230026, P.R. China, and Department of Chemistry and Biochemistry, UniVersity of Oklahoma, 620 Parrington OVal, Room 208, Norman, Oklahoma 73019 Received September 9, 2002; Revised Manuscript Received October 17, 2002
ABSTRACT This paper describes large thick flattened multiwalled carbon nanotubes (MWCTs) that have been synthesized using the precursors Co(CO)3NO or Fe(CO)5. TEM and HRTEM clearly show that even thick MWCNTs (more than 30 layers) can be collapsed or partially flattened. The transition from circular to flattened was observed by HRTEM in a partially flattened nanotube. The estimated wall thickness of these flattened nanotubes ranges from 13 to 40 layers, the outer width of the flattened nanotubes (ribbons) lies between 43 and 121 nm. Metal-filled, partially flattened MWCTs were also found in the samples, and TEM and XRD results confirmed that the metals were encapsulated in the carbon shells.
Since the discovery of the buckminsterfullerene molecule (C60),1 a variety of novel structures with trigonally bonded carbon have been discovered: multi- and single-walled carbon nanotubes2,3 (MWCNT and SWCNT), nested spheroidal shells (onions),4 giant fullerene shells,5 interconnected fullerene-like cages,6 cross-linked graphitic cages,7 flattened nanotubes and flattened carbon nanoshells,8,9 and carbon nanoflasks and large carbon nanocontainers.10-12 Ruoff and colleagues13 found that van der Waals forces between adjacent nanotubes can deform them substantially, destroying their cylindrical symmetry. Perhaps the most dramatic deformation so far observed in a carbon nanotube is its occasional complete collapse to a flattened ribbon;8 these collapsed tubes had a wall-thickness of 6 to 9 layers, and the width of the ribbons was around 20 nm. Theoretical modeling demonstrated that a completely collapsed nanotube is favored energetically over the more familiar “inflated” form with a circular cross-section. We present here the results of TEM and HRTEM observation of flattened multiwalled carbon nanotubes (MWCNTs), which clearly show that even thick MWCNTs (more than 30 layers) can be collapsed or partially flattened. The outer widths of the flattened nanotubes (ribbons) are between 43 and 121 nm. HRTEM measurements reveal for the first time the transition from inflated (circular) to flattened tube. Our observations also show some metal-filled nanotubes that have suffered partial collapse along their length. * Corresponding author. Tel (405) 325-2827; Fax (405) 325-6111; E-mail
[email protected]. † University of Science & Technology of China. ‡ University of Oklahoma. 10.1021/nl0257869 CCC: $22.00 Published on Web 11/05/2002
© 2002 American Chemical Society
Carbon nanotube samples were prepared by a chemical method using Co(CO)3NO or Fe(CO)5 as a precursor.11,14 Simply, the syntheses were carried out in a 2 mL closed cell, which was assembled from stainless steel Swagelok parts. A 3/8 in. union part was capped on both sides by a standard plug. For a typical synthesis, 400 mg of magnesium powder and 700 mg of Co(CO)3NO were placed in a cell at room temperature. The vessel was then immediately closed tightly because Co(CO)3NO is an air-sensitive material, and was then heated at 1000 °C for 3 h. The reaction took place at the autogenic pressure of the precursors. The reactions were proposed as follows:11,14 Co(CO)3NO f Co + 3 CO + NO (or Fe(CO)5 f Fe + 5 CO) (1) CO + Mg f MgO + C (nanotubes)
(2)
The products were treated with 50 mL of 8 M HCl at 70 °C for 1 h and then left in the acidic solution overnight at room temperature. During this acid treatment the MgO and the Co metallic particles reacted with HCl and were removed from the outer surface of the CNT. Transmission electron microscopy (TEM) was performed on a JEOL JEM-1220 at 80 kV and high-resolution transmission electron microscopy (HRTEM) on a JEOL JEM-2010 at 120 kV. Specimens for TEM and HRTEM observations were sonicated in ethanol for 10 min and then loaded onto copper grids (200 mesh). X-ray diffraction (XRD) patterns were taken with a Rigaku X-ray diffractometer (model-2028, Cu KR radiation, 20 kV). The two acid-treated samples synthesized from Co(CO)3NO or Fe(CO)5 were characterized by XRD. Figure 1
Figure 1. X-ray diffraction pattern of the acid-treated samples: (a) product synthesized from Co(CO)3NO; (b) product synthesized from Fe(CO)5.
Figure 2. TEM image of partially flattened carbon nanotubes.
shows that for both samples graphite was the dominant feature; however, the samples contain small amounts of Co (Figure 1a) and Fe (Figure 1b) respectively, even after longtime treatment with concentrated HCl. This implies that Co or Fe was encapsulated inside the carbon shells. The majority of the structures imaged by TEM were consistent with previous observations:11 multiwalled cylindrical nanotubes, metal-filled carbon nanoflasks and nanotubes. But we also observed long, completely collapsed or partially collapsed nanotubes in the products. Figure 2 shows a typical TEM image of some flattened nanotubes that were synthesized using the Co(CO)3NO precursor. Completely and partially flattened nanotubes can be easily found in this photo. Based on TEM analysis, the amount of flattened tubes was estimated at over 8%. Such yield makes it easy to study the flattened nanotubes by TEM. Figure 3a shows an example of such a structure identified by arrows. Arrow A points to a bamboo-like node15-17 in this nanotube. The tube wall close to the “compartment”17 structure is over 80 layers thick; however, it becomes thinner and thinner from arrow A to B, and the inner diameter increases. When the number of layers is about 30 and the inner diameter is 43 nm, the tube 1440
Figure 3. (a) TEM image of an individual carbon nanotube. Arrow A shows a “bamboo-like” node. Arrow B points at a flattened region of the tube. (b) Close-up of indicated by arrow B area in (a). Scale bar: 10 nm.
collapses suddenly into a flattened tube. The HRTEM image in Figure 3b reveals the transition region from inflated to flattened tube (arrow B). Chopra et al.8 proposed a simple model to estimate the stability of a flat nanotube based on basic energetic considerations. Upon flattening, all the walls increase their curvature energy, whereas only the inner one gains attractive van der Waals energy. When the latter is larger than the former, the flattened tube will form readily. Obviously, thinner wall and larger inner diameter favor the formation of flattened tubes. As a more general approach, the radius-to-thickness ratio may be applied to predict which nanotubes will be flattened. Assuming that for a given radiusto-thickness ratio (rin/n; rin ) inner radius (nm) of nanotube, n ) number of layers of nanotube), the elastic energy and Nano Lett., Vol. 2, No. 12, 2002
Figure 5. Fe-filled flattened carbon nanotube.
Figure 4. Co-filled and partially flattened nanotubes. (a) Arrows A and B point at the flattened regions of the nanotube; another completely flattened nanotube is marked by arrow C. (b) Another individual partially flattened nanotube. The arrow a points to the flattened parts.
the attractive van der Waals energy both scale approximately with the size of MWNTs, then a critical value for rin/n may be found above which a nanotube will be flattened. In fact, in our sample most radius-to-thickness ratios close to the collapse of the nanotubes are within the 1.02-1.11 range. However, at the transition point in Figure 3b the rin/n is 0.72; that is the smallest ratio we obtained. It probably is the Nano Lett., Vol. 2, No. 12, 2002
critical value for a flattened MWNT forming under this experimental condition. Metal-filled, partially flattened nanotubes were also found in the two samples. Figure 4a shows an interesting individual partially flattened nanotube. Both ends are filled with Co, whereas the middle part is flattened. The upper part is filled with a Co nanorod, the bottom part with a Co particle. The wall thickness of this tube varied from 6 to 35 nm. Arrows A and B point to the transition positions from circular to flattened. At position A there is a sudden transition, whereas at position B the transition is gradual, similar to Figure 3. At position A the number of the layers in the wall is about 35, and the inner diameter is estimated at around 76 nm. At this positon, rin/n is 1.09. In the flattened part, the outer width of the flattened nanotubes is 121 nm. It is also noteworthy that the flattening of this nanotube is not connected to a twist or kink. Twists have been considered the origin of nanotube flattening8 and can also be seen in this sample in the individual completely collapsed tube indicated by arrow C. Another Co-filled nanotube that resembles a cobra is shown in Figure 4b. The “head” is filled with a Co nanoparticle, and the “neck” is flattened. Arrows mark the flattened parts. Contrary to the MWCTs prepared from Co(CO)3NO, only few flattened MWCTs were found in the products that were synthesized from Fe(CO)5. Figure 5 shows such a flattened tube filled with an Fe nanoparticle with an attached Fe nanorod. The shape is similar to the carbon nanoflasks we described before,11 but here the nanotube has completely collapsed shortly beyond the top of the Fe nanorod. There is also an obvious twist in this flattened MWCT. The formation of flattened MWCTs may be explained by the deformation of a conventional hollow cylindrical nanotube by external mechanical forces. On deforming, the inner 1441
tube wall collapses locally and starts a zipper effect, which then flattens the tube down. The van der Waals attraction between opposing and flattened inner walls may act as an adhesive keeping the tube stable in this new position. Chopra et al.8 considered that the twists were the origin of tube collapse, because they found that at least one twist exists in each flattened tube. In our case, we found some flattened MWCTs without any twist (see Figure 4a). It has also been reported that high pressure could flatten carbon nanotubes.18 As we mentioned, the synthesis takes place at the autogenic pressure of the precursors. We suggest that the temporary high pressure at the beginning of reaction may play an important role in the deformation. In summary, thick and large flattened MWCTs were found. The estimated wall thickness of these flattened nanotubes ranges from 13 to 40 layers, and the inner diameters range from 29 to 80 nm. For these flattened nanotubes, the radiusto-thickness ratios are mostly within the 1.02-1.11 range. Metal-filled, partially flattened MWCTs were also found in the samples. Finally, the transition from circular to flattened was observed by HRTEM in a partially flattened nanotube. Acknowledgment. S.L. and J.Y. thank NSFC (No. 50072027) for financial support. R.J.W. gratefully acknowledges the University of Oklahoma for financial support.
1442
References (1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162. (2) Iijima, S. Nature 1991, 354, 56. (3) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (4) Ugarte, D. Nature 1992, 359, 707. (5) Bourgeois, L. N.; Bursill, L. A. Philos. Mag. A 1997, 76, 753. (6) Iijima, S.; Wakabayashi, T.; Achiba, Y. J. Phys. Chem. 1996, 100, 5839. (7) Amaratunga, G. A. J.; Chhowalla, M.; Kiely, C. J.; Alexandrou, I.; Aharonow, R.; Devenish, R. M. Nature 1996, 383, 321. (8) Chopra, N. G.; Benedict, L. X.; Crespi, V. H.; Cohen, M. L.; Louie, S. G.; Zettl, A. Nature 1995, 377, 135. (9) Bourgeois, L. N.; Bursill, L. A. Chem. Phys. Lett. 1997, 277, 571. (10) Liu, S.; Tang, X.; Yin, L.; Koltypin, Y.; Gedanken, A. J. Mater. Chem. 2000, 10, 1271. (11) Liu, S.; Boeshore, S.; Fernandez, A.; Sayagues, M. J.; Fischer, J. E.; Gedanken, A. J. Phys. Chem. B 2001, 105 (32), 7606. (12) Liu, S.; Yue, J.; Gedanken, A. “Large Carbon Nanocontainers”, submitted. (13) Ruoff, R. S.; Tersoff, J.; Lorents, D. C.; Subramoney, S.; Chan, B. Nature 1993, 364, 514. (14) Liu, S.; Tang, X.; Mastai, Y.; Felner, I. Y.; Gedanken, A. J. Mater. Chem. 2000, 10, 2502. (15) Saito, Y.; Yoshikawa, T. Phys. ReV. B 1993, 48, 1907. (16) Kiang, C.-H.; Endo, M.; Ajayan, P. M.; Dresselhaus, G.; Dresselhaus, M. S. Phys. ReV. Lett. 1998, 81, 1869. (17) Lee, C. J.; Park, J. J. Phys. Chem. 2001, 105, 23. (18) Zhang, M.; He, D.-W.; Xu, Y.-F.; Wang, W.-K.; Liang, J.; Wei, B.Q.; Wu, D.-H. Wuli Xuebao 1998, 47, 270.
NL0257869
Nano Lett., Vol. 2, No. 12, 2002
