NANO LETTERS
Supermolecular Self-Assembly of Graphene Sheets: Formation of Tube-in-Tube Nanostructures
2004 Vol. 4, No. 11 2255-2259
Zhenping Zhu,*,†,‡ Dangsheng Su,† Gisela Weinberg,† and Robert Schlo1 gl† Department of Inorganic Chemistry, Fritz-Haber-Institute of the MPG, Faradayweg 4-6, Berlin D-14195, Germany, and State Key Laboratory of Coal ConVersion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taoyuan South Road 27, Taiyuan 030001, P. R. China Received July 27, 2004; Revised Manuscript Received September 7, 2004
ABSTRACT Graphitic impurity nanoparticles were reorganized outside and inside of carbon nanotubes to produce novel tube-in-tube nanostructures. The graphitic nanoparticles were disintegrated into small graphene sheets by an intercalation−exfoliation process with nitric acid, during which the graphene sheets were simultaneously modified with carboxyl and hydroxyl groups at their edges. The modified graphene sheets were self-organized outside and inside of pristine carbon nanotubes in an acid-catalyzed esterification process, leading to an assembly of wellconstructed tube-in-tube nanostructures.
Carbon nanotubes are basically constituted by sp2 C-C covalent bonds as in graphite planes. Their syntheses have been highly successful following various routes, such as laser evaporation or arc-discharge of graphite, catalytic chemical vapor deposition, and decomposition of organic explosives.1-4 These methods are based on a common key process: the assembly of small carbon species (Cn) generated at high temperatures. The studies on the structures of carbon nanotubes have shown that the practically obtained nanotubes are highly defective and have a local structure similar to that of turbostratic graphite.5,6 The presence of discontinued defects in the tube structures means that an individual tube could be actually viewed as an assembly of small graphene sheets and that they could be directly synthesized from the graphene sheets under mild conditions if proper organization technology is available. Because of the anisotropic lamellar structure of graphite, single or thinly stacked graphene sheets can be easily obtained by an intercalation-exfoliation process against bulk graphite with inorganic acids such as nitric, suphuric, and perchloric acids.7-10 This process has been developed industrially over 15 years to produce flexible graphite for the application of sealing gaskets. In the synthesized carbon nanotube samples, graphitic impurity nanoparticles are always present. They seriously hamper the accurate characterization of the bulk properties of nanotubes and affect their practical applications. To remove these impurities, various purification methods have * Corresponding author. E-mail:
[email protected]. † Fritz-Haber-Institute. ‡ Institute of Coal Chemistry. 10.1021/nl048794t CCC: $27.50 Published on Web 09/23/2004
© 2004 American Chemical Society
been developed.11-17 Although the graphitic nanoparticles intrinsically contain richer sub-stable nonhexagonal rings and thus are more reactive than carbon nanotubes,11 the presence of defects in the tube structures renders the purification difficult. Furthermore, carbonaceous impurities are also frequently present in the inner voids of tubes.5 These internal impurities are more resistant and survive even under purification-purposed deep oxidation that causes severe damage to the tubes.18,19 How to use these impurity graphitic nanoparticles as a valuable carbon sources is of great interest but, to our knowledge, has not yet been achieved. Here we report a simple two-step process to reorganize in-situ the graphitic impurity nanoparticles outside and inside of the coexisting carbon nanotubes and to assemble novel tube-in-tube nanostructures, in which the complete separation of the graphitic particles from nanotubes is not necessary. The graphitic impurity nanoparticles are first disintegrated into small graphene sheets by the intercalation-exfoliation process with nitric acid, in which carboxyl and hydroxyl functional groups are modified at the edges of the exfoliated graphene sheets. The subsequent esterification treatment makes the graphene sheets self-assemble outside and inside of carbon nanotubes and dramatically produces carbon tubein-tube nanostructures. Both double- and triple-channel tubes are formed in this way. The employed starting material was obtained commercially (Applied Science Ltd., OH). It was produced by ironcatalyzed hydrocarbon decomposition and mainly consists of carbon nanotubes together with carbon nanoparticle impurities. The nanotubes have lengths up to tens of
Figure 1. (a) TEM image of pristine carbon nanotubes. (b) Highresolution TEM image of graphitic impurity nanoparticles. The black objects filled inside of the graphitic particles are iron particles.
micrometers and exhibit a diameter distribution of 50-170 nm, peaked at 70 nm. They show multiwalled structures and have opened ends (Figure 1a), by which guest materials could easily get into the tube channels. The carbon impurity nanoparticles are present not only external to the tubes but also inside the tube channels (Figure 1a). Figure 1b shows a high-resolution transmission electron microscopic (TEM) image of the external nanoparticles, which exhibit a graphitic structure, similar to the structure of the tube walls. The encapsulated black objects are iron particles, initially used to catalyze tube nucleation and growth. To exfoliate the graphitic nanoparticles, 3.7 g of raw material was introduced into 200 mL concentrated HNO3 and refluxed for 10 h, with weak magnetic stirring. The resulting suspension was filtrated, and the remaining solid was washed sequentially with deionized water (20 mL) and ethanol (20 mL) for three times (pH ) 6.2 for the last filtrate of ethanol) and dried at 110 °C in air for 12 h. The acid treatment plays multiple roles of (1) intercalating NO3- ions into the interlayer spaces through defect-involved “doors” 2256
and exfoliating the large particles into small and thin graphene sheets; (2) modifying the edges of the graphene sheets with carboxyl and hydroxyl groups through an oxidation reaction at the edge carbon atoms; (3) removing the amorphous carbon nanoparticles (more reactive than graphitic particles) and the iron catalyst particles.12-14 During this treatment, although some defect-rich carbon nanotubes are also undesirably cut into shorter pieces,13 most of nanotubes remain unchanged in morphology and structure. Since the graphitic nanoparticles intrinsically contain rich pentagonal rings, they are more reactive than nanotubes11 and are somewhat selectively exfoliated and modified. The resulting small graphene sheets with carboxyl and hydroxyl groups at their edge carbon atoms can be viewed as special supermolecules and desirable for further structural organization as described below. To assemble the graphene supermolecules into defined structures, 50 mg of the exfoliated sample was dispersed in 50 mL tetrahydrofuran (THF) by magnetic stirring for 40 min at room temperature. Concentrated H2SO4 (0.2 mL) was then added to the resulting suspension and refluxed for 10 h. After the reaction, the suspension was filtered, washed four times with 10 mL ethanol, and dried at 110 °C overnight. During this reaction, the graphene supermolecules are reintegrated via an esterification linkage between the carboxyl and hydroxyl groups at their edges, in which H2SO4 molecules serve as catalysts. Figure 2a shows a TEM image of the obtained materials, indicating dramatic changes in composition and morphology. Carbonaceous impurities are significantly reduced for both the materials external to the tubes and the materials in the cavities of the tubes. Large amounts (about 70% as observed by TEM) of the pristine single-channel tubes have been transformed into multichannel tube-in-tube nanostructures. Undoubtedly, the newly formed tube walls are the consequence of the assembly of the functionalized graphene sheets. As shown by Figure 2b, the tube-in-tube nanostructures are well constructed, with uniform wall thickness along full tube and large interval spaces between the outer and inner tubes. The tube ends are normally open, which facilitates further intuitional observations of the perfect structures. Figure 2c demonstrates the scanning electron microscopic (SEM) image of an individual tube-in-tube assembly at its open end. Both the outer and inner tube moieties are clearly observable, confirming the encased tubular structures. The high-yield formation of tube-in-tube structures other than re-integrated carbon particles indicates that the functionalized graphene sheets have a preference to assemble in the direction of the pristine tubes. Combined with the uniformity of the wall thickness of the newly formed tube moieties, it also suggests that the graphene sheets have very strong self-managing and self-tailoring abilities, even in the used mild wet chemical environment. For the formation of the tube-in-tube nanostructures, there are two possible assembly modes, occurring outside or inside of the pristine tubes. The observations of the assembly morphologies suggest that both of the modes occur. The assemblies shown in Figure 3a represent two typical different Nano Lett., Vol. 4, No. 11, 2004
Figure 2. (a) TEM image of the materials after the HNO3-based exfoliation and subsequent esterification reactions, showing the transformation of a large fraction of the original single-channel nanotubes to tube-in-tube structures. (b) An individual tube-in-tube assembly with open end. (c) SEM image of the end of a tube-intube assembly.
morphologies. The upper one exhibits relatively narrow and uniform interval spaces along the length, and the wall of outer tube is rather similar to the wall of the inner tube in shape and winding. The bottom one exhibits wide and irregular interval spaces, and the wall of outer tube is not matched with the wall of the inner tube. The former situation, also displayed by the assemblies shown in Figure 3b, likely associated with an exterior assembling mechanism, in which the pristine inner tubes may serve as templates to direct the formation of the new tube walls. This is strongly supported by the special morphology of the assembly at the underside of Figure 3b. The bamboo-shaped inner tube is markedly the pristine tube. Similar shaped nanotubes can be produced from the high-temperature nanotube growth process20,21 and are frequently observed in the present sample before the assembling. The latter situation may be connected with an interior assembly mechanism because the inner tube moieties in this type of assemblies are often rather thinner than the pristine tubes as shown by the tube-in-tube assembly in Figure 3c, in which the inner tube has a diameter of 15 nm only. In this case, the diameter and wall thickness of the inner tubes is believed to be dependent on the available amount of carbon impurities in the channels of the pristine tubes. The continuity of filling the graphene sheets inside the tubes would lead to the formation of a fully constructed tube-in-tube structure along the entire length. Local lack of the building blocks would leave behind discontinued inner Nano Lett., Vol. 4, No. 11, 2004
Figure 3. (a) TEM images of two morphologically distinct tubein-tube assemblies. They are likely produced by different mechanisms: exterior (upper image) and interior (bottom image) assembly (see text). (b) Two tubes produced by exterior assembly. The inner tube moiety of the bottom image shows a bamboo-like shape, markedly the pristine tube moiety. (c) A tube produced by interior assembly, with very wide and irregular interval spaces. The inner tube moiety is very thin (15 nm) and shows two breaking parts (the arrows).
tubes as shown in Figure 3c. In addition, the irregular and wide interval spaces between the outer and inner tubes seem to mean that the new inner tubes assemble freely, although they are confined in the channels of the pristine outer tubes. When the exterior and interior assembly mechanisms work simultaneously, triple-channel tube-in-tube structures could be obtained. Figure 4a shows such an assembly, in which three tubes with different diameters are encased together. The presence of the triple tube-in-tube structures can be considered as another evidence of the exterior and interior assembling mechanisms. The left assembly in Figure 4b displays a situation where the outer tube is constructed along the entire length of the middle pristine tube, while the inner tube is only in short segments, possibly as the consequence of limited carbon building blocks inside the pristine tubes. This observation suggests that the carbon building blocks for the assembly of the inner tubes come from the initially filled carbon nanoparticles rather than from the external sources. We estimate that transporting enough external 2257
Figure 4. (a) TEM image of a triple-channel tube-in-tube structure, formed from simultaneous exterior and interior assembly of the graphitic sheets. (b) A special triple-channel assembly (left image), in which the narrow inner tube is constructed partially and is discontinued.
carbons into a long nanotube for the construction of a new tube is extremely diffusion-controlled and thus is difficult. Figure 5a presents the high-resolution TEM image of the walls of a tube-in-tube assembly. The wall of the inner tube shows a fishbone-like graphitic structure with interlayer distances of 0.34 nm, which is similar to the structure of pristine tubes (Figure 5b). The outer tube is clearly the newly assembled moiety. It exhibits pre-graphitic short-rangeordered structure, with larger interlayer distances of about 0.35 nm and many discontinued and dislocated defects. Such a structure is a reflection of the soft chemical characteristic of the assembling process and the non-carbon-atom-involved linkage mode of the small graphene segments. Tailoring the structures into well-ordered graphitic structures is possible by annealing treatments at high temperatures,22 which could clip off the involved oxygen-containing groups and weld the small graphene sheets together by forming C-C bonds. Fabrication of tube-in-tube nanostructures has been accomplished before. Martin et al.23 pre-filled iron catalyst nanoparticles into carbon nanotubes and used the conventional chemical vapor deposition technology to grow the inner tube moieties within the pristine tubes under the iron catalysis. This method is imaginative and intelligent, but it is difficult to selectively bring the desired number of catalyst particles into the tubes, which is necessary to assemble wellconstructed tube-in-tube structures. The present method is based on a soft chemical technology and it should be easier to rationally control the tubular structures and to produce them. The multisurface and multichannel characteristics of the tube-in-tube nanostructures should be greatly beneficial for the improvement or tuning of nanotube properties and for wide potential applications in catalysis, gas storage and sensing, electrode materials, and so on, like the conventional single-channel nanotubes.23-31 2258
Figure 5. (a) High-resolution TEM image of the two walls of a double-channel tube-in-tube structure. The outer tube shows a disordered pre-graphitic structure and the inner tube shows a fishbone-like graphitic structure. (b) The structure of a representative pristine nanotube.
In summary, we use the graphitic impurity nanoparticles as valuable carbon source and assembly the graphene sheets exfoliated from them into novel tube-in-tube nanostructures by soft chemical technologies. This strategy may lead to a new horizon in dealing with the impurity carbons and to a general soft chemical route toward structural nanomaterials. The exterior-interior assembly strategy should be applicable to cast non-carbon tube-in-tube nanostructures. Acknowledgment. This work was performed under the auspices of ELCASS and supported by Chinese Academy Sciences and Institute of Coal Chemistry following the “Bairen” program. Z.Z. thanks the Alexander von Humboldt Foundation for the fellowship support. References (1) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science of Fullerenes and Carbon Nanotubes; Academic Press: San Diego, 1996. (2) Ajayan, P. M. Chem. ReV. 1999, 99, 1787. (3) Dai, H. J. Top. Appl. Phys. 2001, 80, 29.
Nano Lett., Vol. 4, No. 11, 2004
(4) Lu, Y.; Zhu, Z. P.; Wu, W. Z.; Liu, Z. Y. Chem. Commun. 2002, 21, 2740. (5) Zhou, O.; Fleming, R. M.; Murphy, D. W.; Chen, C. H.; Haddon, R. C.; Ramirez, A. P.; Glarum, S. H. Science 1994, 263, 1744. (6) Harris, P. J. F.; Green, M. L. H.; Tsang, S. C. J. Chem. Soc., Faraday Trans. 1993, 89, 1189. (7) Furdin, G. Fuel 1998, 77, 479. (8) Chung, D. D. L. J. Mater. Sci. 1987, 22, 4190. (9) Inagaki, M.; Tashiro, R.; Washino, Y.; Toyoda, M. J. Phys. Chem. Solids 2004, 65, 133. (10) Kang, F.; Zheng, Y. P.; Wang, H. N.; Nishi, Y.; Inagaki, M. Carbon 2002, 40, 1575. (11) Ebbesen, T. W.; Ajayan, P. M.; Hiura, H.; Tanigaki, K. Nature 1994, 367, 519. (12) Hiura, H.; Ebbesen, T. W.; Tanigaki, K. AdV. Mater. 1995, 7, 275. (13) Liu, J.; Rinzler, A. G.; Dai, H. J.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; Rodriguez-Macias, F.; Shon, Y. S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253. (14) Dillon, A. C.; Gennett, T.; Jones, K. M.; Alleman, J. L.; Parilla, P. A.; Heben, M. J. AdV. Mater. 1999, 11, 1354. (15) Tohji, K.; Goto, T.; Takahashi, H.; Shinoda, Y.; Shimizu, N.; Jeyadevan, B.; Matsuoka, I.; Saito, Y.; Kasuya, A.; Ohsuna, T.; Hiraga, K.; Nishina, Y. Nature 1996, 383, 679. (16) Harutyunyan, A. R.; Pradhan, B. K.; Chang, J.; Chen, G.; Eklund, P. C. J. Phys. Chem. B 2002, 106, 8671. (17) Hou, P. X.; Bai, S.; Yang, Q. H.; Liu, C.; Cheng, H. M. Carbon 2002, 40, 81.
Nano Lett., Vol. 4, No. 11, 2004
(18) Tsang, S. C.; Harris, P. J. F.; Green, M. L. H. Nature 1993, 362, 520. (19) Ajayan, P. M.; Ebbesen, T. W.; Ichihashi, T.; Iijima, S.; Tanigaki, K.; Hiura, H. Nature 1993, 362, 522. (20) Lee, C. J.; Park, J. J. Phys. Chem. B 2001, 105, 2365. (21) Li, Y.; Chen, J.; Ma, Y.; Zhao, J.; Qin, Y.; Chang, L. Chem. Commun. 1999, 12, 1141. (22) Andrews, R.; Jacques, D.; Qian, D.; Dickey, E. C. Carbon 2001, 39, 1681. (23) Che, G.; Lakshmi, B. B.; Fisher, E. R.; Martin, C. R. Nature 1998, 393, 346. (24) Mestal, G.; Maksimova, N. I.; Keller, N.; Roddatis, V. V.; Schlo¨gl, R. Angew. Chem., Int. Ed. 2001, 40, 2066. (25) Planeix, J. M.; Coustel, N.; Coq, B.; Brotons, V.; Kumbhar, P. S.; Dutartre, R.; Geneste, P.; Bernier, P.; Ajayan, P. M. J. Am. Chem. Soc. 1994, 116, 7935. (26) Modi, A.; Koratkar, N.; Lass, E.; Wei, B.; Ajayan, P. M. Nature 2003, 424, 171. (27) Collins, P.; Bradley, K.; Ishigami, M.; Zettl, A. Science 2000, 287, 1801. (28) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622. (29) Terrones, M. Annu. ReV. Mater. Res. 2003, 33, 419. (30) Dillon, A. C.; Heben, M. J. Appl. Phys. A 2001, 78, 2128. (31) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787.
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