Langmuir 2007, 23, 9155-9157
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Conductive and Mesoporous Single-Wall Carbon Nanohorn/Organic Aerogel Composites Yousheng Tao,*,† Daisuke Noguchi,† Cheol-Min Yang,† Hirofumi Kanoh,† Hideki Tanaka,‡ Masako Yudasaka,§ Sumio Iijima,§,| and Katsumi Kaneko*,† Department of Chemistry, Graduate School of Science, Chiba UniVersity, Chiba 263-8522, Japan, Department of Chemical Engineering, Kyoto UniVersity-Katsura, Kyoto 615-8510, Japan, JST/SORST, c/o NEC Corporation, Tsukuba 305-8501, Japan, and Department of Physics, Meijo UniVersity, Nagoya 468-8502, Japan ReceiVed June 5, 2007. In Final Form: July 12, 2007 Conductive and mesoporous single-wall carbon nanohorn/resorcinol-formaldehyde aerogel composites were fabricated by embedding organic resorcinol-formaldehyde aerogels with single-wall carbon nanohorns. Samples were characterized with transmission electron microscopy, field emission scanning electron microscopy, nitrogen adsorption at 77 K, and direct-current volume electrical conductivity measurement. It was demonstrated that these composites have important properties, such as controllable nanoporosity and high electrical conductivity in the range of 10-4 S m-1, which enables many potential applications.
The single-wall carbon nanohorn (SWCNH), which is similar to the single-wall carbon nanotube (SWCNT), is one of the most attractive new forms of nanocarbons. This nanostructure is a graphitic tube that has a corn-shaped cap with a cone angle of 20° at one end.1 An individual SWCNH has a typical diameter of about 2-3 nm, and SWCNH aggregates (SWCNHs) form a spherical colloid. A laser ablation method is used to produce these nanostructural SWCNHs in bulk quantities under ambient conditions and without the need for a metal catalyst; thus, SWCNHs have high purity. The properties of SWCNHs, such as high surface area, high thermal stability, high yield (>95%) and purity, and conductive graphitic structures, coupled with their unique internal nanopore structures and interstitial nanopore structures make them unique materials with numerous potential applications, such as adsorbents,2-4 molecular sieves and ion sieves,5,6 catalyst support,7 and drug delivery.8-10 Postheat treatment of as-grown SWCNHs in carbon dioxide and thermal oxidization using oxidizing gases or chemical treatments such as using HNO3 or H2O2/H2SO4 provides them with high microporosity.11-14 The oxidized SWCNHs and HNO3-treated SWCNHs could have useful applications as supercritical gas *
[email protected] (Y. Tao);
[email protected] (K. Kaneko). † Chiba University. ‡ Kyoto University. § JST/SORST. | Meijo University. (1) Iijima, S.; Yudasaka, M.; Yamada, R.; Bandow, S.; Suenaga, K.; Kokai, F.; Takahashi, K. Chem. Phys. Lett. 1999, 309, 165. (2) Urita, K.; Seki, S.; Utsumi, S.; Noguchi, D.; Kanoh, H.; Tanaka, H.; Hattori, Y.; Ochiai, Y.; Aoki, N.; Yudasaka, M.; Iijima, S.; Kaneko, K. Nano Lett. 2006, 6, 1325. (3) Bekyarova, E.; Murata, K.; Yudasaka, M.; Kasuya, D.; Iijima, S.; Tanaka, H.; Kanoh, H.; Kaneko, K. J. Phys. Chem. B 2003, 107, 4681. (4) Ajima, K.; Yudasaka, M.; Suenaga, K.; Kasuya, D.; Azami, T.; Iijima, S. AdV. Mater 2004, 16, 397. (5) Murata, K.; Hirahara, K.; Yudasaka, M.; Iijima, S.; Kasuya, D.; Kaneko, K. J. Phys. Chem. 2002, 106, 12668. (6) Yang, C. M.; Kim, Y. J.; Endo, M.; Kanoh, H.; Yudasaka, M.; Iijima, S.; Kaneko, K. J. Am. Chem. Soc. 2007, 129, 20. (7) Bekyarova, E.; Hashimoto, A.; Yudasaka, M.; Hattori, Y.; Murata, K.; Kanoh, H.; Kasuya, D.; Iijima, S.; Kaneko, K. J. Phys. Chem. B 2005, 109, 3711. (8) Murakami, T.; Ajima, K.; Miyawaki, J.; Yudasaka, M.; Iijima, S.; Shiba, K. Mol. Pharm. 2004, 1, 399. (9) Ajima, K.; Yudasaka, M.; Murakami, T.; Maigne´, A.; Shiba, K.; Iijima, S. Mol. Pharm. 2004, 2, 475. (10) Miyawaki, J.; Yudasaka, M.; Imai, H.; Yorimitsu, H.; Isobe, H.; Nakamura, E.; Iijima, S. AdV. Mater 2006, 18, 1010.
storage media.13,14 Bekyavova et al. showed that compressed SWCNHs exhibit excellent adsorptivity of supercritical methane in terms of the American Department of Energy (USA-DOE) target level.3 Additionally, SWCNHs have been shown to absorb liquid ethanol effectively and to act as a catalyst for oxidizing ethanol into acetaldehyde.16 However, their practical applications are limited, because filtration of SWCNHs is difficult due to their colloidal properties. The assembly of SWCNHs in bulk would extend their applicability. We have developed a method for assembling SWCNHs with organic aerogels, namely, single-wall carbon nanohorn/resorcinol-formaldehyde aerogel (SWCNH/RFA) composites prepared by embedding resorcinol-formaldehyde (RF) aerogels with SWCNHs. The RF aerogel was synthesized from the polycondensation of resorcinol (R) with formaldehyde (F) and dried with supercritical CO2 to remove liquid without collapsing or shrinking the gel structure based on a similar procedure described elsewhere.17 RF aerogel is a special type of low-density, opencell foam, which remains the delicate gel structure.17-20 SWCNHs were prepared by laser ablation of graphite in an Ar atmosphere at room temperature.1 The SWCNHs were then dispersed in toluene with a bath-type sonicator for 15 min. The mixture was then transferred to a reaction cell containing RF aerogels, which was evacuated at 383 K and 1 mPa for 2 h prior to use; the SWCNHs was introduced into the larger nanopores of RF aerogels. The elastic property of SWCNHs and structural flexibility of RF aerogels are well-suited for the embedding (11) Bekyarova, E.; Kaneko, K.; Yudasaka, M.; Kasuya, D.; Iijima, S.; Huidobro, A.; Rodriguez-Reinoso, F. J. Phys. Chem. B 2003, 107, 4479. (12) Fan, J.; Yudasaka, M.; Miyawaki, J.; Ajima, K.; Murata, K.; Iijima, S. J. Phys. Chem. B 2006, 110, 1587. (13) Utsumi, S.; Honda, H.; Hattori, Y.; kanoh, H.; Takahashi, K.; Sakai, H.; Abe, M.; Yudasaka, M.; Iiima, S.; Kaneko, K. J. Phys. Chem. C 2007, 111, 5575. (14) Yang, C. M.; Noguchi, H.; Yudasaka, M.; Hashimoto, A.; Iijima, S.; Kaneko, K. AdV. Mater. 2005, 17, 866. (15) Murata, K.; Hashimoto, A.; Yudasaka, M.; Kasuya, D.; Kaneko, K.; Iijima, S. AdV. Mater. 2004, 16, 1520. (16) Nisha, J. A.; Yudasaka, M.; Bandow, S.; Kokai, F.; Takahashi, K.; Iijima, S. Chem. Phys. Lett. 2000, 328, 381. (17) Pekala, R. W.; Alviso, C. T. Mater. Res. Soc. Symp. Proc. 1992, 270, 3. (18) Hanzawa, Y.; Kaneko, K.; Yoshizawa, N.; Pekala, R. W.; Dresselhaus. M. S. Adsorption 1998, 4, 187. (19) Tao, Y.; Hattori, Y.; Matumoto, A.; Kanoh, H.; Kaneko, K. J. Phys. Chem. B 2005, 109, 194. (20) Tao, Y.; Kanoh, H.; Abrams, L.; Kaneko, K. Chem. ReV. 2006, 106, 896.
10.1021/la701660w CCC: $37.00 © 2007 American Chemical Society Published on Web 07/31/2007
9156 Langmuir, Vol. 23, No. 18, 2007
Letters
Figure 1. Field emission scanning electron micrographs (FE-SEMs) and transmission electron micrographs (TEMs) of samples: (A) FE-SEM of RF areogels, (B) FE-SEM and TEM (inset) of as-grown SWCNHs, (C) FE-SEM of SWCNH/RFA-5 composites, and (D) TEM of SWCNH/RFA-5 composites. Arrows indicate interconnected structure of SWCNHs.
process. After separating, washing, and drying, the SWCNH/ RFA composites were obtained. We prepared samples from SWCNHs with concentrations from 0.1 to 5 g L-1, and the samples are named as, e.g., SWCNH/RFA-0.1 with suffix of number referring to the concentration. The samples were studied with a field emission scanning electron microscope (FE-SEM) (JSM6330F, JEOL), a transmission electron microscope (TEM) (JEM400FXII, JEOL), and nitrogen adsorption at 77 K (Autosorb1, Quantachrome). The direct-current (dc) volume electrical conductivities of the composites were measured using a Protek 603 Digital Multimeter at room temperature. Figure 1A shows the FE-SEM image of RF aerogels in which the RF aerogels are composed of interconnected beads and an open-celled structure with continuous porosity. Adsorption and desorption isotherms of nitrogen at 77 K for the RF aerogels are shown in Figure 2a. The adsorption isotherm is an IUPAC type IV curve with a type H1 hysteresis loop, indicating that RF aerogels have large three-dimensional continuous mesopores without necks. RF aerogels have a very pronounced characteristic adsorption hysteresis from P/P0 of 0.8 to ∼1, indicating that they consist of predominantly meso- and macropores with few micropores. In the IUPAC classification, pores having a pore width (w) of 50 nm are called micropores, mesopores, or macropores. Large meso- and macropores are evident in the FE-SEM image shown in Figure 1A. The pore structural characteristics of the RF aerogels were determined from the nitrogen adsorption isotherm at 77 K and are presented in Table 1. Therefore, the large, interconnected meso- and macropores with the greatest volume can be used for nanostructural assembly. Figure 1B shows the morphology of as-grown SWCNHs. A primary SWCNH particle is a tubule with a cone cap, resembling
Figure 2. Adsorption (b) and desorption (O) isotherms of nitrogen at 77 K on (a) RF areogels and (b) SWCNHs. Table 1. Pore Structural Characteristics and Electrical Conductivities of Samplesa sample SWCNH/RFA-5 SWCNH/RFA-2 SWCNH/RFA-0.5 SWCNH/RFA-0.1 SWCNHs RFA
SBET Vtotal Vmicro Vmeso [m2 g-1] [cm3 g-1] [cm3 g-1] [cm3 g-1] 207 382 419 515 340 863
0.23 0.38 0.43 0.53 0.45 4.37
0.06 0.11 0.13 0.17 0.10 0.27
0.17 0.27 0.30 0.36 0.35 4.10
σ [S m-1] 5.0 × 10-4 4.0 × 10-4 SWCNH/ RFA-2 > SWCNH/RFA-5 (Table 1), suggesting that a higher initial concentration of SWCNHs resulted in a more compressed embedding of SWCNHs in RF aerogels. Hence, we can control the porosity of SWCNH/RFA composites. (21) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982; Chapter 3. (22) Barret, P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373.
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Figure 1C,D shows FE-SEM and TEM images of typical SWCNH/RFA composites. These images provide visual evidence of the nanostructures and the nature of the embedded SWCNHs within the RF aerogels. Figure 1C clearly shows that the SWCNHs were distributed throughout the SWCNH/RFA composites; the image is a typical one of a nanoscale pore-filled structure. SWCNHs smaller than the as-grown SWCNHs are evident in the pores of the RF aerogels. The smaller SWCNHs may have resulted from dispersion of the agglomerations of as-grown SWCNHs and from the separation of one colloid of SWCNHs when the as-grown SWCNHs were ultrasonically treated. Relatively small mesopores can be seen in the magnified FESEM image of the monolithic composites; their size (widths
