Organic-Vapor-Induced Repeatable Entrance and Exit of C60 into

J. Phys. Chem. C 2007, 111, 9719-9722

9719

Organic-Vapor-Induced Repeatable Entrance and Exit of C60 into/from Single-Wall Carbon Nanohorns at Room Temperature Jin Miyawaki,*,† Masako Yudasaka,*,†,‡ Ryota Yuge,‡ and Sumio Iijima†,‡,§ JST/SORST, c/o NEC, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, NEC, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, and Meijo UniVersity, 1-501 Shiogamaguchi, Tenpaku-ku, Nagoya 468-8502, Japan

Downloaded by UNIV OF NEBRASKA-LINCOLN on August 31, 2015 | http://pubs.acs.org Publication Date (Web): June 19, 2007 | doi: 10.1021/jp069002h

ReceiVed: December 28, 2006; In Final Form: May 14, 2007

We found that ethanol vapor forced C60 encapsulated inside single-wall carbon nanohorns (SWNHs) to exit from SWNHs at room temperature, and in reverse, toluene vapor forced the ejected C60 to re-enter. Interestingly, these entrance and exit behaviors were repeatable. Thermogravimetric analyses suggested C60-toluene pseudosolvate formation in internal hollow nanospaces of SWNHs. We consider the mechanism of the backand-forth transportations; the coincorporated toluene lured C60 from inside to outside SWNHs through adsorbed layers of ethanol formed on the SWNH surfaces. In the re-entrance, the toluene-adsorbed layer mediated transportation of C60 from outside to the most stable sites, that is, inside SWNHs.

Introduction The entrance and exit of organic and inorganic materials into and from internal hollow nanospaces of carbon nanotubes (CNTs)1 have been investigated since the discovery of CNTs, and their unique characteristics and potential applications have been identified.2-5 However, the entrance and exit mechanisms have not been well investigated except for theoretical calculations due to the lack of the appropriate experimental procedures. To investigate the incorporation and exit phenomena, we adopted C60 as a probe molecule in this study. The CNTs used here were single-wall carbon nanohorns (SWNHs)6 because they are available with high purity and the following characteristics are suitable for the relevant studies. Individual SWNHs consist of tubule (2-4 nm in diameter, 40-50 nm in length) and closedtip parts. Thousands of SWNHs are assembled to form a spherical aggregate with a diameter of about 100 nm. Controlled heating in oxidative gases of as-grown SWNHs having closed structure affords hole openings at the tips and topological defects in the sidewalls,7-11 enabling various materials to be incorporated in the internal nanospaces.12-17 We found that C60 molecules repeatedly enter and exit into/ from SWNHs at room temperature by exposure to ethanol or toluene vapor. The layers of adsorbed organic vapors mediate the back-and-forth movements. Since the variations of organic vapors and materials to be encapsulated are enormous, we believe that various types of environment-responsive hybrid nanometer-scale systems accompanied with the material transportations could be realized. Experimental Section SWNHs were produced by CO2 laser ablation (power 3 kW, beam diameter 3.5 mm) of graphite (99.999%) in Ar (760 Torr) * To whom correspondence should be addressed. E-mail: jin_m@ frl.cl.nec.co.jp (J.M.); [email protected] (M.Y.). Phone: +81-29856-1940. Fax: +81-29-850-1366. † JST/SORST. ‡ NEC. § Meijo University.

at room temperature.6 The graphite did not contain any metal catalysts. The purity was about 95%, containing 5% impurities of micrometer-sized graphitic particles.18 To create holes in the SWNH walls, we heated them in a dry air flow at a ramping rate of 1 °C/min to 550 °C, followed by natural cooling to room temperature.11 The hole-opened SWNHs are referred to as “NHox” here. The starting material was C60-incorporated NHox prepared using the nanoprecipitation method.17 Briefly, NHox immersed in C60-toluene solution was left in a N2 flow at 760 Torr until the toluene evaporated. The C60-incorporated NHox (C60/tolu@NHox), a black powder, remained at the bottom of the preparation container. The C60 quantity was 0.13 g/g of NHox, as determined by thermogravimetric analysis (TGA) carried out in O2. A certain amount of residual toluene, which plays an important role in the C60 ejection as described later, was coincorporated with the C60. To remove the coincorporated toluene in C60/tolu@NHox, a portion of C60/tolu@NHox was heated at 150 °C for 30 min in vacuum below 1 × 10-5 Torr (C60@NHox(HTHV)). Ejection of C60 from C60/tolu@NHox or C60@NHox(HTHV) was demonstrated by exposure to ethanol-saturated air in a closed container for various periods. The specimens after the exposure to ethanol vapor are designated as having “(EtOH)” as a suffix. The period of the ethanol vapor exposure was also shown on demand, for example, C60/tolu@NHox(EtOH,24h). To confirm that the ejected C60 was again incorporated inside NHox, the specimens were exposed to toluene-saturated air at room temperature in a closed container for various periods. Two specimens were also prepared as references; NHox without incorporated C60 was exposed to ethanol or toluene vapor at room temperature for 5 h (EtOH@NHox, Toluene@ NHox). The structures of the obtained specimens were observed with transmission electron microscopy (TEM; 120 kV acceleration voltage) and X-ray diffraction analysis (XRD; Cu KR radiation at 45 kV and 40 mA). The quantities of toluene and ethanol were estimated from TGA carried out in He after the specimens were placed in the TGA apparatus for 30 min under a He flow (100 cm3/min).

10.1021/jp069002h CCC: $37.00 © 2007 American Chemical Society Published on Web 06/19/2007

9720 J. Phys. Chem. C, Vol. 111, No. 27, 2007

Miyawaki et al.

Figure 1. XRD patterns of NHox (A), C60/tolu@NHox (B), and C60/tolu@NHox after exposure to ethanol-vapor-saturated air for 24 h (C60/tolu@NHox(EtOH,24h)) (C). Peaks at around 26° correspond to graphite impurities of NHox.18 The broad peak at around 20° for C60/tolu@NHox is characteristic of C60 incorporated in NHox.17 Peaks with an asterisk for C60/tolu@NHox(EtOH,24h) agree with the (111), (220), (311), etc. diffraction peaks of the cubic C60 crystal.19


Results and Discussion The incorporated C60 exhibited a broad XRD peak at around 20° (Figure 1), reflecting the disordered arrangement of the C60 molecules inside the NHox,9,17 which was observed in the TEM image (Figure 2b). Although there is a possibility that a part of the C60 molecules adsorb in spaces between individual SWNHs, that is, interstitial sites, most of the C60 molecules were considered to be incorporated inside hollow nanospaces of the NHox, because we have found that C60 tends to crystallize on external surface of as-grown (closed) SWNH aggregates, exhibiting sharp XRD peaks.17 Exposure of C60/tolu@NHox to ethanol-saturated air for 24 h at room temperature resulted in C60 ejection, as evidenced by newly appearing XRD peaks corresponding to C60 nanocrystals (Figure 1) and TEM observation (Figure 2d). An image of C60 molecules at the edges of C60 polycrystalline particles is shown in the inset of Figure 2d. The TEM image showed the lattice patterns of the C60 crystals; the spacing was about 0.8 nm, which corresponds to the (111) layer spacing of the cubic C60 crystal.19 Naturally, the inside of the NHox became almost empty, and only a few C60 molecules remained at the narrow tips of the NHox (Figure 2c). The degree of C60 ejection depended on the exposure period (Figure 3); the characteristic XRD peaks of C60 suddenly became obvious after 3 h of exposure, and the crystals grew slowly afterward. Their sizes, estimated from the (220) or (311) diffraction peak width using the Scherrer formula,20 were about 40 nm at 3 and 5 h and about 60 nm at 24 h of exposure. Before considering the ejection mechanism, we investigated the role of the residual toluene in this ejection phenomenon. We found that the C60 and toluene interacted and stabilized each other, as evidenced by the slower desorption rate of toluene from C60/tolu@NHox than from toluene-adsorbed NHox without C60 (Toluene@NHox) (Figure 4). Such an interaction suggests the formation of a pseudosolvate of C60 and toluene inside the NHox. The difference in the residual toluene quantity between C60/tolu@NHox and Toluene@NHox after 120 h of evacuation was about 0.01 g/g of sample (Figure 4), so the molar ratio of toluene to C60 was about 1:1. This ratio is close to that reported for toluene-C60 solvates, 2:1.21 As the solvates are stable only below room temperature in the bulk phase,21 the pseudosolvates inside the NHox should be stabilized by the confinement effect inside the nanometer-scaled space. Similar stabilization effects caused by encapsulation inside the NHox have been reported.22,23 The interaction of C60 with toluene induced their comovement from inside the NHox during the C60 ejection process. This is inferred from the finding that toluene in C60/tolu@NHox was forced to eject together with C60 by the ethanol vapor; the

Figure 2. TEM images of NHox (a), C60/tolu@NHox (b), and C60/tolu@NHox(EtOH,24h) (c, d). The inset shows a magnified image of C60 nanocrystals of C60/tolu@NHox(EtOH) showing the lattice pattern with 0.8 nm spacing corresponding to the (111) layer spacing of the cubic C60 crystal and individual C60 molecules at crystal edges.

toluene quantity in the C60/tolu@NHox decreased from about 4% to 1% when C60 was ejected by the exposure to ethanol vapor for 5 h (Figure 5).24 Although ethanol strongly affects the ejection of toluene and C60 from inside the NHox, ethanol itself adsorbed on NHox weakly; its residual quantity was about 0.3%, as seen in Figure 5. On the basis of the results described above, we propose the following mechanism for ejection of C60 from C60/tolu@NHox by exposure to ethanol vapor: (1) Ethanol adsorbs outside and inside the NHox. Inside the NHox, it mixes with the toluene and C60. (2) The toluene quantity differences in the ethanol layers inside and outside the NHox induce toluene diffusion from the inside to the outside. In this movement, C60 also diffuses outside the NHox due to the strong interaction between C60 and toluene, resulting in the formation of a C60-toluene-

Entrance and Exit of C60 into/from SWNHs

J. Phys. Chem. C, Vol. 111, No. 27, 2007 9721


Figure 6. XRD patterns of C60@NHox(HTHV) before (A) and after (B) exposure to ethanol vapor for 24 h. XRD peaks corresponding to C60 nanocrystals ejected by ethanol for C60@NHox(HTHV) were not observed, indicating that coincorporated toluene assisted C60 movement from inside to outside NHox.

Figure 3. (a) XRD patterns of C60/tolu@NHox after ethanol vapor exposure for various periods. (b) Change in the C60 (222) peak area with exposure period for C60/tolu@NHox exposed to ethanol vapor (b) and for C60/tolu@NHox(EtOH,24h) exposed to toluene vapor (0). A 2 h incubation period was observed for C60 ejection by ethanol vapor, while a rapid drop in peak area with toluene vapor exposure indicates fast re-entrance of C60.

Figure 4. Changes in residual toluene quantity for C60/tolu@NHox (b) and Toluene@NHox (0) with evacuation in high vacuum (below 1 × 10-5 Torr) at room temperature for various periods. The delayed toluene desorption for C60/tolu@NHox indicates interactions between C60 and toluene inside NHox. Toluene quantities were estimated from TGA weight loss between 110 and 250 °C carried out in He.

Figure 5. TGA curves measured in He after the specimens were left in air for 10 min: (A) C60/tolu@NHox; (B) C60/tolu@NHox after exposure to ethanol vapor for 5 h; (C) EtOH@NHox obtained by exposing NHox to ethanol vapor for 5 h at room temperature. Ethanol vapor induced a decrease in toluene quantity in C60/tolu@NHox, but the adsorption quantity of ethanol itself into NHox was negligibly small.

ethanol layer on the outside surface. (3) Since the surrounding atmosphere is saturated with ethanol vapor, only toluene evaporates from the C60-toluene-ethanol layer on the outside surface, leading to C60 crystallization because of the low

Figure 7. Change in XRD patterns of C60/tolu@NHox due to repeated exposure to ethanol or toluene vapor at room temperature: (A) original C60/tolu@NHox; (B) after 5 h of exposure of the sample in (A) to ethanol vapor; (C) after exposure of the sample in (B) to toluene vapor for 20 h; (D) after exposure of the sample in (C) to ethanol vapor for 20 h; (E) after exposure of the sample in (D) to toluene vapor for 24 h. The appearance and disappearance of characteristic C60 diffraction peaks indicate repeated ejection and entrance of C60 from/into NHox by ethanol or toluene vapor exposure.

solubility of C60 in ethanol.25 (4) Steps 2 and 3 are repeated until most of the toluene has evaporated. (5) Finally, once the surrounding atmosphere of ethanol-vapor-saturated air is replaced by ethanol-absent air, the ethanol has desorbed and an almost empty NHox with C60 crystals outside is left. In this mechanism, we assume that the driving force of the C60 ejection is the toluene quantity differences inside and outside the NHox (steps 2 and 3). In other words, the incorporated toluene (small molecule) lured C60 (large molecule) from inside to outside the NHox. Indeed, when no toluene was coincorporated with C60 inside the NHox (C60@NHox(HTHV)),26 C60 ejection by exposure to ethanol did not occur (Figure 6). Furthermore, it should be noted that no characteristic XRD peaks of C60 were observed after the removal of the coincorporated toluene by heating in vacuum (C60@NHox(HTHV)). This indicates that the appearance of the XRD peaks of C60 nanocrystals after the ethanol exposure of C60/tolu@NHox (see Figures 1 and 2) actually stems from the crystallization of the ejected C60 from inside the NHox, but not from that of amorphous C60 particles deposited outside the NHox from the beginning. Reversely, the ejected C60 re-entered the NHox by exposure to toluene-vapor-saturated air in a closed container at room temperature. The XRD data revealed that the characteristic diffraction peaks of the C60 polycrystalline particles disappeared after the toluene vapor exposure (curves B and C in Figure 7). The rate of this re-entrance was much higher than the exit rate, and the re-entrance was mostly finished within 20 min (Figure 3b). Interestingly, the C60 ejection by exposure to ethanol vapor

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and the re-entrance by exposure to toluene vapor were repeatable (curves D and E in Figure 7). The re-entrance mechanism is explained as follows. The C60 nanocrystals outside the NHox dissolve in the toluene-adsorbed layers formed outside and inside the NHox, and the dissolved C60 moves to the most stable sites, namely, inside the NHox. Once the specimen was removed from the closed container saturated with toluene vapor, most of the adsorbed toluene evaporated, and the C60-toluene pseudosolvate remained inside the NHox. We previously reported a liquid-phase method for C60 incorporation, which we called “nanocondensation”, in which the importance of thin adsorbed toluene layers formed outside CNTs to C60 incorporation was highlighted.27 We ascribe the slow start of the C60 ejection (Figure 3b) to steps 1 and 2 above. On the other hand, the rapid C60 re-entrance may reflect high rates of toluene-adsorbed layer formation and C60 dissolution. Conclusions We demonstrated that C60 encapsulated inside the NHox was ejected from the NHox and formed C60 nanocrystals on the outside of the NHox by exposure to ethanol vapor. The ejected C60 re-entered the NHox when it was exposed to toluene vapor. These processes took place at room temperature and were repeatable. The strong interaction of C60 with toluene inside the NHox suggests the pseudosolvate formation of toluene and C60 inside the nanospaces of the NHox. We conclude that the C60 ejection from the NHox in the ethanol-saturated vapor was mediated by the adsorbed layers of ethanol inside and outside the NHox. By exposure to ethanol vapor, the ethanol-adsorbed layers formed inside and outside the NHox, in which the C60toluene pseudosolvates dissolved, followed by diffusion to outside the NHox. Because the pseudosolvates were not stable outside the NHox, toluene evaporated to the surrounding atmosphere, so the C60 accumulated outside the NHox. In the re-entrance process of C60, the toluene-adsorbed layers transported the dissolved C60 to inside the NHox, giving rise to the re-formation of the C60-toluene pseudosolvate inside the NHox. Acknowledgment. We thank Dr. Takeshi Azami and Dr. Daisuke Kasuya for preparing the SWNHs and Dr. Hideki Tanaka for fruitful discussions. Supporting Information Available: Figures showing the TGA curves in He for Toluene@NHox before and after ethanol vapor exposure and for C60/tolu@NHox and C60@NHox(HTHV) and UV/vis spectra of C60 released in toluene from C60@NHox(HTHV). This material is available free of charge via the Internet at http://pubs.acs.org.

Miyawaki et al. References and Notes (1) Iijima, S. Nature 1991, 354, 56. (2) Ajayan, P. M.; Iijima, S. Nature 1993, 361, 333. (3) Smith, B. W.; Monthioux, M.; Luzzi, D. E. Nature 1998, 396, 323. (4) Takenobu, T.; Takano, T.; Shiraishi, M.; Murakami, Y.; Ata, M.; Kataura, H.; Achiba, Y.; Iwasa, Y. Nat. Mater. 2003, 2, 683. (5) Yanagi, K.; Miyata, Y.; Kataura, H. AdV. Mater. 2006, 18, 437. (6) Iijima, S.; Yudasaka, M.; Yamada, R.; Bandow, S.; Suenaga, K.; Kokai, F.; Takanashi, K. Chem. Phys. Lett. 1999, 309, 165. (7) Murata, K.; Kaneko, K.; Steele, W. A.; Kokai, F.; Takahashi, K.; Kasuya, D.; Hirahara, K.; Yudasaka, M.; Iijima, S. J. Phys. Chem. B 2001, 105, 10210. (8) Bekyarova, E.; Kaneko, K.; Yudasaka, M.; Kasuya, D.; Iijima, S.; Huidobro, A.; Rodriguez-Reinoso, F. J. Phys. Chem. B 2003, 107, 4479. (9) Ajima, K.; Yudasaka, M.; Suenaga, K.; Kasuya, D.; Azami, T.; Iijima, S. AdV. Mater. 2004, 16, 397. (10) Utsumi, S.; Miyawaki, J.; Tanaka, H.; Hattori, Y.; Itoi, T.; Ichikuni, N.; Kanoh, H.; Yudasaka, M.; Iijima, S.; Kaneko, K. J. Phys. Chem. B 2005, 109, 14319. (11) Fan, J.; Yudasaka, M.; Miyawaki, J.; Ajima, K.; Murata, K.; Iijima, S. J. Phys. Chem. B 2006, 110, 1587. (12) Murata, K.; Kaneko, K.; Kanoh, H.; Kasuya, D.; Takahashi, K.; Kokai, F.; Yudasaka, M.; Iijima, S. J. Phys. Chem. B 2002, 106, 11132. (13) Murata, K.; Hirahara, K.; Yudasaka, M.; Iijima, S.; Kasuya, D.; Kaneko, K. J. Phys. Chem. B 2002, 106, 12668. (14) Hashimoto, A.; Yorimitsu, H.; Ajima, K.; Suenaga, K.; Isobe, H.; Miyawaki, J.; Yudasaka, M.; Iijima, S. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 8527. (15) Murakami, T.; Ajima, K.; Miyawaki, J.; Yudasaka, M.; Iijima, S.; Shiba, K. Mol. Pharm. 2004, 1, 399. (16) Ajima, K.; Yudasaka, M.; Murakami, T.; Maigne´, A.; Shiba, K.; Iijima, S. Mol. Pharm. 2005, 2, 475. (17) Yuge, R.; Yudasaka, M.; Miyawaki, J.; Kubo, Y.; Ichihashi, T.; Imai, H.; Nakamura, E.; Isobe, H.; Yorimitsu, H.; Iijima, S. J. Phys. Chem. B 2005, 109, 17861. (18) Fan, J.; Yudasaka, M.; Kasuya, D.; Azami, T.; Yuge, R.; Imai, H.; Kubo, Y.; Iijima, S. J. Phys. Chem. B 2005, 109, 10756. (19) Powder diffraction file no. 81-2220. (20) Scherrer, P. Go¨tt. Nachr. 1918, 2, 98. (21) Korobov, M. V.; Mirakyan, A. L.; Avramenko, N. V.; Olofsson, G.; Smith, A. L.; Ruoff, R. S. J. Phys. Chem. B 1999, 103, 1339. (22) Yudasaka, M.; Fan, J.; Miyawaki, J.; Iijima, S. J. Phys. Chem. B 2005, 109, 8909. (23) Miyawaki, J.; Yudasaka, M.; Imai, H.; Yorimitsu, H.; Isobe, H.; Nakamura, E.; Iijima, S. J. Phys. Chem. B 2006, 110, 5179. (24) We also found that toluene was ejected by exposure to ethanol vapor in the absence of the incorporated C60; TGA results showed a remarkable decrease of the incorporated toluene quantity for Toluene@NHox after ethanol vapor exposure for 5 h (Supporting Information, Figure S1). (25) Ruoff, R. S.; Tse, D. S.; Malhotra, R.; Lorents, D. C. J. Phys. Chem. 1993, 97, 3379. (26) C60@NHox(HTHV) was obtained by heating C60/tolu@NHox at 150 °C in vacuum (about 1 × 10-5 Torr) for 30 min, by which the amount of coincorporated toluene was reduced from about 4% to almost zero (Supporting Information, Figure S1). No thermal degradation of the C60 occurred. About 0.13 g of C60/g of NHox was released from C60@NHox(HTHV) in liquid toluene (Supporting Information, Figure S2), which agrees with the initial C60 quantity (0.13 g/g of NHox) of C60/tolu@NHox. (27) Yudasaka, M.; Ajima, K.; Suenaga, K.; Ichihashi, T.; Hashimoto, A.; Iijima, S. Chem. Phys. Lett. 2003, 380, 42.

Organic-Vapor-Induced Repeatable Entrance and Exit of C60 into

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