Growth of Linear Carbon Chains inside Thin Double-Wall Carbon

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Growth of Linear Carbon Chains inside Thin Double-Wall Carbon Nanotubes C. Zhao, R. Kitaura,* H. Hara, S. Irle, and H. Shinohara* Department of Chemistry and Institute for Advanced Research, Nagoya University, Nagoya 464-8602, Japan

bS Supporting Information ABSTRACT: Fusion reactions of size-selected linear polyyne molecules, C10H2, in the ultrathin one-dimensional (1D) nanospace of double wall carbon nanotubes (DWCNTs) have been observed, leading to the formation of very long linear carbon chains. The formation and growth of long linear carbon chains is investigated by Raman spectroscopy and density-functional tight-binding-based molecular dynamics simulations. The high-resolution transmission electron microscope images of C10H2@DWCNTs show that the encapsulated linear C10H2 molecules have a length of 1.2 1.3 nm. After the high temperature annealing at 1073 1273 K under high vacuum conditions, new Raman bands appear at 1800 1850 cm 1, and the Raman bands arising from encapsulated pristine polyyne molecules at 2000 2200 cm 1 completely disappear. This is clear evidence that a fusion reaction occurs to form long linear carbon chains inside the 1D nanospace of DWCNTs.

1. INTRODUCTION Linear carbon chains (C-chains), which consist of sp-hybridized carbon atoms, have attracted interest in recent years within the context of a wide range of topics such as interstellar molecules1 and carbon nanowires as the so-called fourth allotrope of carbon2,3 after diamond, graphite (graphene), and fullerenes4 together with carbon nanotubes.5 An infinitely long C-chain, sometimes called carbyne, is an ultimate nanowire only one atom wide and therefore is a real one-dimensional (1D) nanomaterial, which is one of the most ideal model systems for exploring chemical and physical nanowire of 1D nanostructure.6,7 Despite the long research history and devoted research efforts, the long linear C-chain, an ultimate carbon wire, is the only carbon allotrope whose real structure and properties are still not clear. This is because the synthesis of such long linear C-chains has been extremely difficult due to their high chemical reactivity and instability; longer C-chains are becoming more and more unstable, and they are easy to react with each other to form polymerized products.8 12 In fact, the maximum length of C-chains achieved by the state-of-the-art organic synthesis technique is limited to up to 24 carbon atoms,13 15 where bulky functional groups at the ends of the carbon chains are required to prevent the known prompt polymerization. To overcome these problems and to obtain long C-chains, we have focused on 1D nanospace of carbon nanotubes (CNTs) as nanosized reaction vessel. The 1D nanospace of CNTs can provide an ideal nanoreaction field to fabricate novel low-dimensional nanomaterials.16 In our previous study, we have developed a nanotemplate reaction to prepare metal nanowires using metallofullerenes nanopeapods as starting materials.17 Here, we report the fabrication of long linear C-chains in an ultrathin 1D nanospace of double-wall carbon r 2011 American Chemical Society

nanotubes (DWCNTs) by employing a high-temperature fusion reaction (nanofusion reaction) of polyyne molecules. A schematic diagram of the fusion reaction is shown in Figure 1. The length of C-chains so-produced has been estimated to consist of more than 50 carbon atoms, and in principle, their length can be extended up to the length of DWCNTs, which is typically several micrometers, corresponding to several thousands of carbon atoms. These long linear C-chains are protected by the graphene wall of DWCNTs, and hence they are stable even at high temperature under ambient conditions. This enables us to carry out detailed structural and spectroscopic characterization using transmission electron microscopy and Raman spectroscopy.

2. EXPERIMENTAL SECTION Sample Preparation. Details of the synthesis of linear-chain polyyne molecules were reported previously.18 Briefly, polyynes of various sizes were generated by laser ablation (532 nm Nd: YAG) of graphite microparticles dispersed in methanol solution. The resulting crude solution contains many different kinds of linear-chain carbon species including C8H2, C10H2, C12H2 together with amorphous carbon materials. In this study, purified C10H2 molecules were selected as a precursor for the nanofusion reaction within DWCNTs. The purification of C10H2 polyynes were carried out by high-performance liquid chromatography (HPLC) using a polymeric octadecyl silica (ODS) packed Received: February 19, 2011 Revised: May 21, 2011 Published: June 07, 2011 13166

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Figure 1. Schematic diagram of nanofusion reaction of C10H2 in inner thin carbon nanotube of DWCNTs. Green, gray, and yellow balls represent carbon atoms in C10H2, carbon atoms in DWCNTs, and hydrogen atoms, respectively.

column (Wakosil 5C18AR, 300 mm 10 mm) with methanol eluent. Soon after the purification, encapsulation of polyynes inside DWCNTs was performed. Prior to the encapsulation of polyyne molecules, DWCNTs were heated at 823 K in dry air for 30 min to remove the end-caps of DWCNTs. For encapsulating polyynes inside DWCNTs, a small piece (ca. 0.2 mg) of such open-ended DWCNTs was dipped in the methanol solution of polyyne, which was kept at 353 K for 24 h. The DWCNTs with polyyne molecules inside were carefully rinsed with methanol and dried in air at 353 K to remove polyyne molecules attached to the outer surface of DWCNTs.19,20 To promote and initiate fusion reactions, such DWCNTs were annealed at 1073 1273 K for 24 h under a high vacuum condition of ∼10 6 Torr in IR heating furnace (IVF298RV, THERMO RIKO). Raman Measurement. Raman spectra were measured using HR-800 (Jovin-Yvon) equipped with Ar+ ion and He Ne lasers operated at 488, 514, and 633 nm, respectively. A notch filter was used to filter out Rayleigh radiation, and Raman signal was detected by charge-coupled device (CCD). A 50, 0.75 NA objective lens was used to focus the laser light onto the samples. All measurements were carried out at room temperature and atmospheric condition. High-Resolution Transmission Electron Microscopy (HRTEM) Observation. Observation was performed using a JEM-2100F (JEOL) operated at acceleration voltage of 80 keV at room temperature under high vacuum (10 8 Torr). For TEM observations, samples were dispersed in 1,2-dichrolobenzene by ultrasonication for 1 h, and the supernatant was dropped onto a copper grid coated with thin carbon film. After a vacuum heat treatment at 473 K for 1 h to remove remaining 1,2-dichrolobenzene, TEM observation was carried out. HRTEM images were recorded by CCD (MSC794 1k x 1k, gatan) with an exposure time of typically 1 3 s. Quantum Chemical MD Simulation. The density functional tight-binding (DFTB) method was employed in direct MD simulation. The unit cell length along the nanotube axis was set to 3.9 nm to allow for the placement of two polyyne molecules with head-to-tail alignment. Ample space was provided between periodic images. Periodic boundary conditions with γ-point approximation were applied; the unit cell length perpendicular to the nanotube axis was chosen as 5 nm to avoid intertube interaction. The MD simulation was performed using a Verlet algorithm and a scaling of velocity thermostat with a 20% overall scaling probability; the MD time integration step and nuclear temperature were set to 0.48 fs and 2700 K, respectively. The high temperature was needed to overcome the barrier to initiate the fusion reaction. Typically, simulations were run until about

Figure 2. UV absorption spectra of crude and isolated C10H2.

10 ps, but we found that initial events were crucial to observe successful fusion reactions.

3. RESULTS AND DISCUSSION Figure 2 shows absorption spectra of crude and isolated C10H2 solutions. As clearly seen in Figure 2, the absorption spectrum of isolated C10H2 shows a distinct peak at 253 nm with several vibronic bands to the blue side, which is identical to the one previously reported.18 This spectrum confirms successful isolation of C10H2 molecules from the crude solution. This highly pure (>99%) C10H2 sample was used for the following experiments. DWCNTs used in this experiment were synthesized by the alcohol chemical vapor deposition method. The typical diameter of inner-tubes of DWCNTs is 0.7 0.8 nm, which corresponds to the inner hollow space size of 0.4 0.5 nm across. This particular size is crucial to form 1D alignment of the polyyne molecules inside the inner space of DWCNTs. As discussed later, a small space size of DWCNTs is critical and necessary to form long linear C-chains by the nanofusion reaction. As shown in Figure 3, the HRTEM image of C10H2@DWCNTs shows linear contrasts aligned inside DWCNTs that have not been observed before encapsulation of C10H2 molecules. These contrasts have also not been observed in the HRTEM image of open-ended empty DWCNTs. The estimated length of such linear contrasts corresponds to 1.2 1.3 nm, which is almost the same as that of C10H2 molecules. These results indicate that the observed linear contrasts are single isolated C10H2 molecules encapsulated and aligned inside the ultrathin nanospace of DWCNTs. The encapsulation of polyyne molecules has also been confirmed by Raman spectroscopy. Parts a, b, and c of Figure 4 show Raman spectra of annealed pristine DWCNTs, C10H2@DWCNTs, and annealed C10H2@DWCNTs, respectively. As shown in Figure 4b, new Raman bands appear in the region of 2000 2200 cm 1 (what we termed as “P-band”19,20) after the encapsulation of C10H2 molecules. The Raman bands have been assigned as stretching vibrations of C10H2 molecules 13167

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Figure 3. (a) HRTEM image of C10H2@DWCNTs, (b) magnified image of (a), (c) a simulated image based on a structure model shown in (d) by multislice method at Scherzer defocus, and (d) the structure model of C10H2@DWCNTs.

Figure 4. Raman spectra measured with excitation energy of 514 nm. (a, b, and c) Raman spectra of pristine DWCNTs, C10H2@DWCNTs, and annealed C10H2@DWCNTs, respectively.

encapsulated in DWCNTs according to the previous report.19,20 In this experiment, an excitation wavelength of 514 nm was used, where intensity of the P-band is expected to be enhanced by resonance effect.21,22 The C10H2@DWCNTs show a red shift of P-band compared to C10H2 molecules in methanol solution. This red shift is caused by strong interaction between C10H2

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molecules and CNTs.19,20 In fact, a recent detailed Raman study shows that the charge transfer from encapsulated polyyne molecules to carbon nanotubes is occurring.21,22 Importantly, after high-temperature annealing, the P-band completely disappears and a new Raman band (hereafter refer to as “L-band”) appeared at 1831 cm 1 (cf. Figure 4c). The L-band has not been observed in the annealed empty DWCNTs. The origin of this L-band should, thus, be attributed to the hightemperature reaction products of C10H2 molecules. Furthermore, the intensity of the L-band did not change at all even after long time heating and ultrasonication in solution, which shows that the products are safely protected by the surrounding graphene wall of DWCNTs. Therefore, the appearance of the L-band in the red side triggered by high-temperature annealing strongly suggests that C10H2 molecules are subjected to thermal fusion reactions inside DWCNTs to form much longer C-chains, which should have lower frequency Raman active modes. It has been reported that the Raman band arising from sphybridized carbon chains shows red shift due to the elongation of C-chain.23 25 Theoretical calculations predict that a steep decrease of Raman frequency should be observed in the short length range (∼20 carbon atoms) followed by convergence to around 1830 1870 cm 1 in the long length range (g50 carbon atoms).23 25 Experimentally, long C-chains produced by arcdischarge also show similar red shifts of the Raman bands.26 On the basis of these studies, the observed red shift can be attributed to the formation of long sp-hybridized C-chain whose length is estimated to be greater than 50 carbon atoms. Our preliminary HRTEM observations of annealed C10H2@DWCNTs also indicate the formation of C-chain in DWCNTs.27 Interestingly, such red-shifted Raman bands have not been observed when CNTs with larger diameters (∼1.4 nm average diameter of the hollow space) are employed as reaction vessels for the nanofusion reaction; L-band was not observed in annealed-C10H2@CNTs (Supporting Information). When the size of CNTs becomes larger, interchain polymerization of C10H2 molecules may easily occur to form two or three-dimensional carbon structures, which are completely different from 1D sp-hybridized C-chains of our interest. In the present experiment, the preparation of atomically thin 1D hollow space of DWCNTs is, therefore, definitely needed to obtain the 1D long carbon chains by fusion reactions. As shown in Figure 4, the width of the observed L-band is narrower than that of P-band (12 and 42 cm 1 for the L- and P-bands, respectively). Previous studies show that P-band broadening occurs whenever environmental effects are important; CNTs of different diameter provides a different environment for encapsulated C10H2 molecules.20 DWCNT samples used in this experiment contain a small amount of SWCNTs (ca. 10 15%) whose diameter ranges in size from 1.2 to 1.7 nm, which leads to spectral broadening arising from the environmental effect. As discussed above, the C-chain formation by fusion reaction in CNTs has not been observed for large-diameter CNTs, and therefore, C-chains produced exist only in thin inner tubes of DWCNTs, which leads to less environmental effect. This is well consistent with observed narrow L-band in annealed-C10H2@SWCNTs samples. As shown above, the long sp-hybridized C-chains spontaneously formed by the simple heat-treatment can be contrasted with those obtained by an elaborated organic synthesis recently reported.13 15 Importantly, as discussed above, the observed Raman bands do not vary after a long time heating and ultrasonication process, which means that the long C-chains formed 13168

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Figure 6. C12H2@(6,6) before and after fusion in the MD simulation. Figure 5. Raman spectra of annealed C10H2@DWCNTs with different excitation energy; red, green, and blue lines corresponds to Raman spectra measured with 633, 514, and 488 nm, respectively.

in this study are isolated and stabilized in DWCNTs; this strong stabilization can lead us to perform detailed spectroscopic characterization as described below. Figure 5 shows Raman spectra of annealed C10H2@DWCNTs at 1473 K measured with three different excitation wavelengths (633, 514, and 488 nm). In all cases, L-bands are clearly observed. As shown in the figures, the frequency of the L-bands changes depending on the laser wavelength. To interpret this, we have to consider the resonance effect in the Raman measurement: Raman intensity of L-band is significantly enhanced when electronic transition energy of C-chains is identical to the excitation energy. The electronic structure of C-chains varies sensitively depending on the difference of the molecular length, which leads to different resonance wavelength in Raman measurements. In addition, different length of C-chains results in different L-band Raman shift. Therefore, the observed Raman results shown in Figure 5 indicate that the products formed in present fusion reaction contains C-chains with various molecular lengths. This is in good agreement with the fact that the newly formed C-chains in this fusion reaction should have a length distribution because the number of precursor molecules (C10H2) encapsulated in each DWCNTs should be different. At present, an attempt to assign the resonance is not successful because the electronic transition energy of C-chain is not fully understood yet. In addition, in the case of C-chain encapsulated in CNTs, the resonance enhancement (which can be assigned as due to dipoleforbidden dark transitions) has been observed due to symmetry breaking arising from the surrounding graphene wall. Moreover, there is a possibility that C-chain ends interact with defects of the inner surface of the DWCNTs, which could affect a peak shape and position of P-bands. These make the assignment even more complicated.21 To further investigate the thermal fusion reaction of polyyne molecules in DWCNTs and to obtain information on the growth mechanism of polyyne fusion reaction, we have performed finitetemperature molecular dynamics (MD) simulation based on the density-functional tight-binding (DFTB) method28,29 using direct calculation of Born Oppenheimer potential energy and gradients. In these simulations, instead of DWCNTs, single-wall CNTs (SWCNTs) with small diameter, typically 0.802 nm (6, 6), were used to simplify the calculations.

Figure 6a shows the initial molecular structures of these simulations. Before the MD simulations, the geometry was fully relaxed and the residual force was found to be 0.015 eV/Å. Figure 6b shows one example of such successful fusion, occurring within a simulation time shorter than 1 ps. As can be clearly seen in the geometry snapshots, two polyyne molecules thermally fuse to form a long carbon chain with 14 carbon atoms. Because of the rather high environmental temperature, the chain frequently breaks in other positions; at lower temperatures, such events should be less likely to occur. Occasionally, hydrogen atoms remained on the fused chain, which are sometimes observed to transfer to the SWCNT sidewall, followed by very fast migration along the tube axis. This result suggests that C C bond formation occurs at the end of polyyne molecules, and then, remaining hydrogen atoms are removed via CNT sidewall to form long C-chain. Interestingly, a formation of long C-chain was not observed when the diameter of SWCNTs was large, consistent with the present experimental results. Thus, the formation of linear C-chain in ultrathin space of CNTs by the nanofusion reaction has been verified also by quantum chemical MD simulations.

4. CONCLUSIONS In conclusion, long sp-hybridized C-chains are formed spontaneously by nanofusion reaction in the atomically thin nanospace of DWCNTs, which results in the formation of a novel 1D carbon nanostructure, C-chain@DWCNTs. In this reaction, the ultrasmall space of DWCNTs plays an essential role to provide an ideal and efficient reaction field. Although the length of 1D linear C-chains formed is estimated to be ∼50 carbon atoms at present, their length, in principle, can be extended up to the length of DWCNTs, which corresponds to several thousand of carbon atoms. This work opens a new field of study on carbyne and materials science in general based on fabrication of 1D nanostructured materials using small CNTs as nanosized reactions. ’ ASSOCIATED CONTENT

bS

Supporting Information. Raman spectra of polyyne@ SWCNTs. Initial and final Cartesian coordinates of successful fusion trajectory of C12H2@(6,6) SWCNTs. This information is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (H.S.); [email protected] (R.K.).

’ ACKNOWLEDGMENT This work has been supported by the Grant-in-Aids for Specific Area Research (No. 19084008) on Carbon Nanotube Nano-Electronics and for Scientific Research A (No. 19205003) of MEXT, Japan, and partly by the Global COE Program in Chemistry, Nagoya University. ’ REFERENCES (1) Kroto, H. W.; Heath, J. R.; Obrien, S. C.; Curl, R. F.; Smalley, R. E. Astrophys. J. 1987, 314, 352. (2) Heath, J. R.; Zhang, Q.; Obrien, S. C.; Curl, R. F.; Kroto, H. W.; Smalley, R. E. J. Am. Chem. Soc. 1987, 109, 359. (3) Duley, W. W.; Hu, A. M. Astrophys. J. 2009, 698, 808. (4) Kratschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354. (5) Iijima, S. Nature 1991, 354, 56. (6) Hu, J. T.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (7) Cui, Y.; Wei, Q. Q.; Park, H. K.; Lieber, C. M. Science 2001, 293, 1289. (8) Springborg, M.; Kavan, L. Synth. Met. 1993, 57, 4405. (9) Ravagnan, L.; Siviero, F.; Lenardi, C.; Piseri, P.; Barborini, E.; Milani, P.; Casari, C. S.; Bassi, A. L.; Bottani, C. E. Phys. Rev. Lett. 2002, 89, 285506. (10) Casari, C. S.; Bassi, A. L.; Ravagnan, L.; Siviero, F.; Lenardi, C.; Piseri, P.; Bongiorno, G.; Bottani, C. E.; Milani, P. Phys. Rev. B 2004, 69, 075422. (11) Cataldo, F. Polym. Degrad. Stab. 2006, 91, 317. (12) Casari, C. S.; Bassi, A. L.; Baserga, A.; Ravagnan, L.; Piseri, P.; Lenardi, C.; Tommasini, M.; Milani, A.; Fazzi, D.; Bottani, C. E.; Milani, P. Phys. Rev. B 2008, 77, 195444. (13) Chalifoux, W. A.; Tykwinski, R. R. C. R. Chim. 2009, 12, 341. (14) Zheng, Q.; Gladysz, J. A. J. Am. Chem. Soc. 2005, 127, 10508. (15) Wesley, A. C.; Robert, M.; Michael, J. F.; Rik, R. T. Angew. Chem., Int. Ed. 2009, 48, 7915. (16) Pichler, T.; Kuzmany, H.; Kataura, H.; Achiba, Y. Phys. Rev. Lett. 2001, 87, 267401. (17) Kitaura, R.; Imazu, N.; Kobayashi, K.; Shinohara, H. Nano Lett. 2008, 8, 693. (18) Tsuji, M.; Tsuji, T.; Kuboyama, S.; Yoon, S.-H.; Korai, Y.; Tsujimoto, T.; Kubo, K.; Mori, A.; Mochida, I. Chem. Phys. Lett. 2002, 355, 101. (19) Nishide, D.; Dohi, H.; Wakabayashi, T.; Nishibori, E.; Aoyagi, S.; Ishida, M.; Kikuchi, S.; Kitaura, R.; Sugai, T.; Sakata, M.; Shinohara, H. Chem. Phys. Lett. 2006, 428, 356. (20) Nishide, D.; Wakabayashi, T.; Sugai, T.; Kitaura, R.; Kataura, H.; Achiba, Y.; Shinohara, H. J. Phys. Chem. C 2007, 111, 5178. (21) Malard, L. M.; Nishide, D.; Dias, L. G.; Capaz, R. B.; Gomes, A. P.; Jorio, A.; Achete, C. A.; Saito, R.; Achiba, Y.; Shinohara, H.; Pimenta, M. A. Phys. Rev. B 2007, 76, 233412. (22) Moura, L. G.; Malard, L. M.; Carneiro, M. A.; Venezuela, P.; Capaz, R. B.; Nishide, D.; Achiba, Y.; Shinohara, H.; Pimenta, M. A. Phys. Rev. B 2009, 80, 161401. (23) Kastner, J.; Kuzmany, H.; Kavan, L.; Dousek, F. P.; Kuerti, J. Macromolecules 1995, 28, 344. (24) Tabata, H.; Fujii, M.; Hayashi, S.; Doi, T.; Wakabayashi, T. Carbon 2006, 44, 3168. (25) Yang, S.; Kertesz, M.; Zolyomi, V.; Kurti, J. J. Phys. Chem. A 2007, 111, 2434.

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(26) Zhao, X.; Ando, Y.; Liu, Y.; Jinno, M.; Suzuki, T. Phys. Rev. Lett. 2003, 90, 187401. (27) It should be mentioned that long linear contrasts are frequently observed inside carbon nanotubes in TEM images even if the interior space of carbon nanotubes is empty. These contrasts (ghost contrasts) arise from an interference of electron waves scattered by carbon nanotubes, and to distinguish the long linear chain from the ghost contrasts is usually not easy. Furthermore, filling ratio of C-chain is fairly low (due to possible escape of C10H2 molecules from CNTs), which makes observation of C-chain by TEM difficult. Therefore, although we have observed linear contrasts inside carbon nanotubes in several TEM images, we have decided not to include the images in the main text due to this ambiguity. In strong contrast, the presence of individual molecules such as polyyne molecules is much easier to recognize and to identify because the limited length of the molecule can provide limited length of the corresponding contrasts. This facilitates to distinguish contrasts of polyyne molecules from the ghost contrasts. (28) Porezag, D.; Frauenheim, T.; Koler, T.; Seifert, G.; Kaschner, R. Phys. Rev. B 1995, 51, 12947. (29) Elstner, M.; Porezag, D.; Jungnickel, G.; Elsner, J.; Haugk, M.; Frauenheim, T.; Suhai, S.; Seifert, G. Phys. Rev. B 1998, 58, 7260.

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