NANO LETTERS
Ex-MWNTs: Graphene Sheets and Ribbons Produced by Lithium Intercalation and Exfoliation of Carbon Nanotubes
2009 Vol. 9, No. 4 1527-1533
Abraham G. Cano-Ma´rquez,† Fernando J. Rodríguez-Macı´as,† Jessica Campos-Delgado,† Claudia G. Espinosa-Gonza´lez,† Ferdinando Trista´n-Lo´pez,† Daniel Ramírez-Gonza´lez,† David A. Cullen,‡ David J. Smith,‡ Mauricio Terrones,† and Yadira I. Vega-Cantu´*,† AdVanced Materials Department and Laboratory for Nanoscience and Nanotechnology Research (LINAN), IPICYT, Camino a la Presa San Jose´ 2055, Col. Lomas 4a, San Luis Potosí, SLP, 78216, Me´xico, and Department of Physics and School of Materials, Arizona State UniVersity, Tempe, Arizona, 85287-1704 Received November 25, 2008; Revised Manuscript Received February 16, 2009
ABSTRACT We found that multiwalled carbon nanotubes (MWNTs) can be opened longitudinally by intercalation of lithium and ammonia followed by exfoliation. Intercalation of open-ended tubes and exfoliation with acid treatment and abrupt heating provided the best results. The resulting material consists of: (i) multilayered flat graphitic structures (nanoribbons), (ii) partially open MWNTs, and (iii) graphene flakes. We called the completely unwrapped nanotubes ex-MWNTs, and their large number of edge atoms makes them attractive for many applications.
Carbon nanotubes are usually described as rolled graphene sheets; however, the generation of graphene by unrolling carbon nanotubes has yet to be explored in detail. In this Letter we report an efficient method in which open-ended multiwalled carbon nanotubes (MWNTs) are intercalated with ionic Li in liquid NH3 thus unfolding concentric tubules into exfoliated graphitic ribbons and stacked sheets (exMWNTs). These unwrapped tubes exhibit a large number of exposed edges that could be functionalized and are succeptible of further chemical modification, thus making this material attractive for different applications. Ex-MWNTs could also be easily functionalized and dispersed, using methods developed for carbon nanotubes (CNTs),1-3 facilitating potential applications. Single-walled carbon nanotubes (SWNTs) dispersed in liquid NH3 with Li form “nanotube salts”,4 in which SWNTs absorb electrons released by Li, and then react with alkyl or aryl halides.5 This Birch reduction method has also been applied to functionalize MWNTs6 and as * Corresponding author: e-mail,
[email protected]; fax, (+) 52 444 834-2010; telephone, (+) 52 444 834-2000 ext 7236. † Advanced Materials Department and Laboratory for Nanoscience and Nanotechnology Research (LINAN), IPICYT. ‡ Department of Physics and School of Materials, Arizona State University. 10.1021/nl803585s CCC: $40.75 Published on Web 03/04/2009
2009 American Chemical Society
an approach for in situ polymerization reactions to produce SWNT composites.7 While using the nanotube salts method to produce MWNT composites by in situ polymerization of aniline, acrylates, and other monomers, we found by transmission electron microscopy (TEM; Figure 1) several damaged nanotubes and some appearing partially unwrapped and exfoliated. We considered that this effect was a consequence of Li intercalation between the concentric graphene cylinders followed by exfoliation (unwrapping) of the MWNTs. This exfoliation would have taken place when the material was treated with a 10% (v/v) HCl solution, used to remove residual Li in the final step of the nanotube salts method described above. These initial results motivated us to study this phenomenon in more detail, as described below. Intercalation of alkali metals and other chemical species in graphite has been known since the 19th century.8 The reaction of MWNTs with Li at high pressures and temperatures has been reported to produce Li2C intercalation compounds, or lithium carbide, depending on conditions.9 Electrochemical intercalation of Li in MWNT has been reported to be reversible, causing little damage to the nanotubes.10 Other studies have found that, for MWNT produced by chemical vapor deposition (CVD) of acety-
Figure 1. TEM images showing damaged and open nanotubes after treatment with Li in liquid NH3 followed by treatment with HCl solution.
lene, no intercalation occurs; Li deposits only over the outside wall, due to surface functional groups.11 Exfoliation has been reported in electrochemical intercalation when propylene carbonate was used as an electrolyte; after deintercalation, nanotubes were observed to have diameters enlarged by a factor of 2 over the original value indicating that the walls might be ruptured, but unwrapping was not reported12 and the graphene cylinders remained in a concentric arrangement. It is possible that exfoliation of nanotubes has not been observed before in the intercalation of Li since the increase in the interlayer distance of graphite from 3.35 to 3.71 Å8 after Li intercalation might not be enough to break MWNT walls. However, the propylene carbonate reports indicate that exfoliation can occur when a cointercalant is used. The simultaneous intercalation of Li and NH3 has been reported for graphite13 under similar conditions to those used in this work, increasing interlayer distances to ca. 6.62 Å. Such a lattice expansion is likely to induce extreme stress within the concentric cylinders, enough to break nanotube walls, allowing exfoliation and unwrapping. In addition, these expanded interlayer spacings could be accessed by water molecules, thus producing strong exothermic reactions between Li and H2O that result in the unwrapping of the tubes. To the best of our knowledge, the simultaneous intercalation of Li and NH3 in MWNTs has not been reported hitherto. Hydrogenation of arc-grown MWNTs by Birch reduction14 caused some damage of the walls, but no opening was observed, possibly because these nanotubes are highly crystalline, with closed end, thus preventing intercalation of Li or a Li-NH3 complex. In this study we used nanotubes grown by CVD of ferrocene-toluene mixtures at 825 °C,15 which may exhibit some open ends caused by the purification treatment used, a modification of a hydrothermal procedure.16 Acid purification techniques have been shown to open the tips of nanotubes,17 which would facilitate intercalation. The original procedure consisted of dispersing MWNTs (∼25 mg) by sonication in dry tetrahydrofuran (THF, ACS grade, Tedia, dried by reflux and distillation with LiAlH4, ∼50 mL used), adding them to liquid NH3 (99.95%, Praxair, condensed by a mixture of acetone and liquid N2, ∼200 mL 1528
used). The reaction flask was kept cold with an acetone-dry ice bath (ca. -77 °C), and Li was then added, in a 10:1 ratio Li/C (w/w). For better handling, lithium was used in the form of granules (99% Li, ∼0.5% Na, “high sodium”, reagent grade, Aldrich). The intercalation was allowed to proceed for a few hours. After evaporation of NH3 a 10% HCl solution was stirred into the flask containing intercalated MWNTs (slowly and preferably under inert atmosphere, since this exfoliation treatment is highly exothermic) and then recovered with a microfiltration membrane (Millipore, PTFE, 0.45 µm pores). This only produced small amounts of ex-MWNTs (∼0-5%), resulting mostly in partially exfoliated and damaged nanotubes. Scanning transmission electron microscopy (STEM) images, taken in an FEI XL-30 SFEG, were used to compare the efficiency of different intercalation-exfoliation treatments. The treated nanotube materials were dispersed by sonication in ethanol for 1-2 min, preparing very dilute solutions, and deposited over lacey carbon TEM grids (2-3 drops), where they were observed thoroughly to evaluate the extent of damage and/or exfoliation. Individual nanotubes were classified by their appearance as intact, damaged (only outer layers were exfoliated), partially exfoliated (open on one or both ends, damaged walls on unopened part), and ex-MWNT (nanotubes completely unwrapped along their axes). These four stages are illustrated in Figure 2. At least 40 nanotubes were classified and counted to estimate efficiency of each treatment. Where more exfoliation was apparent, about 100 nanotubes or more from each batch were classified and counted to determine the percentage of exMWNTs more accurately. Image Pro Plus (Media Cybernetics Inc.) was used to perform the measurements of lengths and diameters on the scanning electron microscopy (SEM) images, using the scale bar in the images to calibrate. The damaged appearance of nanotubes was in some cases due to oxidative cutting, as explained below and as shown in Figure 2E-G. In all subsequent experiments and in order to find a better intercalation-exfoliation treatment, we used a 10:1 ratio Li/C (w/w), to ensure maximum Li intercalation and thus enhance exfoliation and opening of MWNTs. Preliminary results indicated that with lower amounts of Li the extent of Nano Lett., Vol. 9, No. 4, 2009
Figure 2. Representative STEM images of MWNTs in four different states after the exfoliation treatments: (A) intact; (B) damaged; (C) a long tube partially exfoliated, along with shorter damaged tubes; (D) exfoliated MWNTs. SEM observation of MWNTs cut in acid reveals surface damage, as seen in (E); however the oxidative cutting does not unwrap the nanotubes as shown by (F) and (G) (SEM and STEM, respectively).
exfoliation was lower. Other experiments also showed that the volume of NH3 could be an important factor, ∼10 mL of NH3 per mg of MWNTs was used based on those observations. We found that refluxing for 2 h in 10% HCl was more effective for exfoliating MWNTs than room temperature stirring, as shown by increases in the percentage of exMWNTs on STEM images. Sonication in a bath (at ∼60 °C) for 2 h in 10% HCl was equally effective. Refluxing after sonication was only moderately more effective. Using HNO3 instead of HCl did not produce more unwrapped tubes, only more damaged tubes, most likely due to oxidation by nitric acid. Using purified MWNTs (50-100 µm long) as in the experiments above, half of the tubes were partially exfoliated (unwrapping running up to 2-3 µm from the tip), ∼20% were intact, and almost no ex-MWNT appeared. Figure 3 shows a clear example of the partial unwrapping we observed. On the basis of the report by Liu et al.18 we then opened and cut nanotubes by sonicating for 28 h in 3:1 concentrated HNO3:H2SO4, prior to Li-NH3 intercalation. The cutting process ensures that we have open tips and it can also introduce damage on the nanotube walls as shown in Figure 2E; however we did not observe opening Nano Lett., Vol. 9, No. 4, 2009
and unwrapping of MWNTs with the oxyacid treatment alone. After the cutting process, most nanotubes show lengths
