8759
2008, 112, 8759–8762 Published on Web 05/27/2008
Mechanochemically Induced sp3-Bond-Associated Reconstruction of Single-Wall Carbon Nanohorns Koki Urita,† Shinya Seki,† Hideyuki Tsuchiya,† Hiroaki Honda,† Shigenori Utsumi,‡ Chiharu Hayakawa,† Hirofumi Kanoh,† Tomonori Ohba,† Hideki Tanaka,§ Masako Yudasaka,| Sumio Iijima,|,⊥ and Katsumi Kaneko†,* Department of Chemistry, Graduate School of Science, Chiba UniVersity, Chiba 263-8522, Japan, Department of Mechanics and System Design, Faculty of Systems Engineering, Tokyo UniVersity of Science, Suwa, 391-0292, Japan, Department of Chemical Engineering, Kyoto UniVersity, Kyoto, 615-8510, Japan, JST/SORST, c/o NEC Corporation, Miyukigaoka, Tsukuba 305-8501, Japan, and Department of Physics, Meijyo UniVersity, Nagoya 468-8522, Japan ReceiVed: March 16, 2008; ReVised Manuscript ReceiVed: April 3, 2008
Single-wall carbon nanohorns (SWCNHs) and partially oxidized SWCNHs (ox-SWCNHs) were mechanochemically treated for the elucidation of their unique chemical bonding state. The mechanochemical treatment promoted the transformation from sp2 to sp3 bonding from XPS, leading to an increase and decrease in the micropore volume for SWCNHs and ox-SWCNHs, respectively. The changes in the micropore volume and the electrical conductivity responses on CO2 adsorption indicate the occurrence of tubulite-to-tubulite reconstruction in SWCNH through unstable sp3 bonding. Single-wall carbon nanohorns (SWCNH)1–4 reveal quite intriguing potential for novel applications such as adsorbents,5–7 gas sensors,8 catalyst supports,9 and drug-delivery carriers.10 Although there have been many studies on the applications of SWCNHs, their defect-associated nature has not been studied sufficiently. Previous studies using high-resolution electron microscopy suggested the presence of unstable sites such as bending and cap sites that stem from pentagons and heptagons even in the tubular parts of SWCNHs.11 The defective sites in SWCNHs are removed easily, and nanosized holes called nanowindows are introduced on carbon walls by oxidation treatment.12,13 The nanowindows critically cause a change of electrical property of SWCNHs. Our previous study on gas adsorption indicated that SWCNHs and oxidized SWCNHs (oxSWCNHs) exhibits n-type and p-type semiconductivities, respectively.14 The difference in the behavior between SWCNHs and ox-SWCNHs can be ascribed to the different concentrations of pentagons. Hence, it is quite essential to investigate the effect of structural transformations and topological defects, which contribute significantly to their fundamental physical properties. We focused on both SWCNHs and ox-SWCNHs mechanochemically treated by using high-pressure compression and grinding. The pelletized SWCNH and ox-SWCNH obtained with n number of compressions are represented as SWCNH/n and ox-SWCNH/n in this article, respectively. * To whom correspondence should be addressed. E-mail: kaneko@ pchem2.s.chiba-u.ac.jp. † Department of Chemistry, Graduate School of Science, Chiba University. ‡ Department of Mechanics and System Design, Faculty of Systems Engineering, Tokyo University of Science. § Department of Chemical Engineering, Kyoto University. | JST/SORST, c/o NEC Corporation. ⊥ Department of Physics, Meijyo University.
10.1021/jp802276s CCC: $40.75
Figure 1. SEM images of SWCNHs (a-c) and ox-SWCNHs (d-f). Treating numbers (n) are 0, 5, and 9 (from left to right). Scale bar is 100 nm.
We used dahlia-flower-like SWCNH assemblies and oxSWCNH with nanowindows prepared by oxidation at 823 K.13 Each SWCNH was dispersed in ethanol by ultrasonic treatment for 30 min and dried at 343 K.2 We prepared a pelletized SWCNH and ox-SWCNH by repeatedly performing mechanical compression under 500 kg cm-2 in vacuo and grinding SWCNH samples for 10 min under ambient atmosphere. The treated SWCNH samples were characterized thoroughly by using SEM (JSM-6330F), XPS (JPS-9010MX), and Raman spectroscopy (NRS-1000). The nanoporosity changes in the samples were determined by using nitrogen adsorption isotherms at 77 K, with a volumetric apparatus. We also measured their electrical conductivity changes, which should be associated with the bond transformation, by mechanochemical treatment. The samples were pretreated at 423 K and 10 mPa for 2 h prior to the electrical conductivity and adsorption measurements. 2008 American Chemical Society
8760 J. Phys. Chem. C, Vol. 112, No. 24, 2008
Figure 2. Ratio of sp3 to sp2 bonding in compressed SWCNHs (open) and ox-SWCNHs (solid) as a function of their treating numbers (n).
Figure 3. Micropore volume of SWCNHs (a) and ox-SWCNHs (b) as determined by the N2 adsorption isotherm at 77 K as a function of the ratio of sp3 to sp2 bonding.
We characterized the morphological structure of the treated SWCNH and the ox-SWCNH by using SEM. SWCNH/0 (Figure 1a) and ox-SWCNH/0 (Figure 1d) that has not undergone mechanochemical treatment clearly show the spherical forms of the SWCNH assemblies. After the mechanochemical treatment, the morphological structures of both samples change dramatically with the treatment procedures. In particular, the ox-SWCNH/9 (Figure 1f) does not reveal spherical SWCNH assemblies, leading to fused form. On the basis of the SEM images, we can suggest that the mechanochemical treatment causes a rearrangement between the carbon nanohorn aggregates. In particular, ox-SWCNH assemblies are easily transformed into the larger carbon block by the mechanochemical treatment owing to the presence of unstable nanowindows. The C1s peak obtained with X-ray photoelectron spectroscopy (XPS) was examined to elucidate the changes in the C-C
Letters bonding of SWCNHs and ox-SWCNHs that have undergone the mechanochemical treatment. This peak involves five bonding states of carbon: CdC (284.5 eV), C-C (285.5 eV), C-O (286.1 eV), CdO (287.0 eV), and COO (289.0 eV). The detailed deconvolution procedure for the C1s peak can be found in preceding articles.15–19 In this study, we focus on the change in the sp3/sp2 bond ratio with the number of times of mechanochemical treatment, denoted below as “treating number”, as shown in Figure 2. The bond ratio of SWCNHs hardly increases; SWCNH/9 has a bond ratio almost similar to that of oxSWCNH/2. We presume that unstable sites such as bending and cap sites in SWCNHs are removed by the mechanochemical treatment, thereby leading to good agreement between the sp3/ sp2 ratios of SWCNH/9 and ox-SWCNH/2. In contrast, the ratio of ox-SWCNHs varies dramatically with an increase in the mechanochemical treatment. The changes in the bonding state of the both samples clearly correspond to the structural deformation shown in Figure 1. According to a theoretical study,20 the formation of a junction structure in SWCNHs is energetically favorable. We suggest that following intensive mechanochemical treatment the sp3/sp2 bond ratio increases because of formation of junction structures between the defective sites and the sidewalls of carbon nanohorns. A ratio of the peak intensity of the D-band of the disorder structure and the G-band of the graphitic structure (D/G ratio) in Raman spectroscopic analysis can give information on the crystallinity of carbon nanomaterials such as SWCNTs.21,22 In the case of carbon nanohorns, each peak of the disorder structure and graphitic structure is observed around 1345 cm-1 (D-band) and 1590 cm-1 (G-band), respectively. The relationships between the D/G intensity ratio and the sp3/sp2 ratio for SWCNHs and ox-SWCNHs are defined by a smooth curve; the relationship for SWCNHs corresponds to the initial rising part of the total curve, whereas that for ox-SWCNHs forms the upper part (See the Supporting Information, Figure SP1). The mechanochemical treatment opens the SWCNHs as if they were oxidized like the ox-SWCNHs, and thereby the relationship for SWCNHs is similar to that for ox-SWCNHs. The difference in the D/G ratio between the SWCNH and the ox-SWCNH clearly supports our concepts pertaining to structural changes, which were derived from the XPS analyses. In the case of SWCNHs, the mechanochemical treatment breaks the tube and tip structures, producing edge carbons possessing sp3 bonds due to reactions with O2 and/or H2O. ox-SWCNHs are more fragile than SWCNHs owing to the presence of nanowindows; both the D-band intensity and
Figure 4. Electrical conductivity changes in SWCNH/n (a) and ox-SWCNH/n (b) on CO2 adsorption at 303 K. The treating numbers are 2 (circle), 5 (triangle), and 9 (square).
Letters the number of sp3 bonds increase with the treating number, which is similar to the case of SWCNHs. Figure 3 shows the changes in the micropore volume as determined by the N2 adsorption isotherm at 77 K against the sp3/sp2 ratios. We determined the micropore volume by the Rs method,23,24 which is a standard method for determining the nanoporosity of carbon materials accurately. The micropore volume of SWCNHs increases dramatically at low sp3/sp2 ratios and then remains constant, as shown in Figure 3a. In contrast, the micropore volume of ox-SWCNHs decreases with an increase in the sp3/sp2 ratio, as shown in Figure 3b. Because the micropore volume indicates the presence of a pore structure, the increase and decrease in the micropore volume stem from the generation and annihilation of pores, respectively. Hence, Figure 3 shows that the change in the pore volume with the treating number must be associated with a change in the bonding state. In the case of SWCNHs, the mechanochemical treatment produces edge carbons with sp3 bonds, which is accompanied by disorder in the graphene structure, as mentioned above. The mechanochemical treatment produces holes on the walls of the SWCNHs, leading to good accessibility for the molecules in the internal tube spaces. In contrast, the effective pores of ox-SWCNHs must vanish when mechanochemical treatment is performed. Accordingly, the open sites of oxSWCNHs should be closed because of solid-phase reactions between defective SWCNH particles that are highly activated by the mechanochemical treatment. It is well known that mechanochemical treatment can induce a unique solid-phase reaction through the formation of highly concentrated defects.25,26 SWCNHs and ox-SWCNHs consist of only a single atomic wall that attains a highly activated state as a result of the mechanochemical treatment. The differences in the structural changes between the SWCNHs and ox-SWCNHs clearly lead to corresponding electrical conductivity behavior in both SWCNHs (see the Supporting Information, Figure SP2). The electrical conductivity of SWCNHs decreases with an increase of the sp3/sp2 ratio, whereas that of ox-SWCNH remains constant. These different tendencies can be associated with different electrical conductivity responses to gas adsorption. Figure 4 shows the electrical conductivity changes in SWCNH/n and ox-SWCNH/n (n ) 2, 5, 9) resulting from CO2 adsorption at 303 K. A CO2 molecule is a representative electron donor, whose electron affinity value is in the range of -2.1 to -1.1 eV. 27 When an adsorbed CO2 molecule donates an electron to n-type SWCNHs, a new negative charge carrier is created to increase the electrical conductivity of the SWCNHs. In contrast, the electrical conductivity of p-type ox-SWCNHs drops in the low-pressure CO2 region, where the coverage of CO2 molecules on the carbon wall is low, because the electron transfer from CO2 to the ox-SWCNH annihilates hole carriers. The marked difference between SWCNHs and ox-SWCNHs becomes blurred with the progress of the mechanochemical treatment. Mechanochemical treatment performed several times weakens the n-type nature of SWCNHs. Alternatively, after the treatment is performed nine times, the electrical conductivity drops slightly in the low-pressure region for ox-SWCNHs, causing them to lose their p-type nature. These characteristic responses of SWCNHs and ox-SWCNHs should originate from the transformation of the sp2 bond to sp3 bond. In other words, the mechanochemical treatment induces defective structures through the sp3 bond formation in the conjugated π-electron structure of the graphene wall. Therefore, both the representative n-type and p-type nature of SWCNHs and ox-SWCNHs, respectively, tend to disappear with an increase in the treating
J. Phys. Chem. C, Vol. 112, No. 24, 2008 8761 number. Therefore, the electrical conductivity response data for the mechanochemical treatment suggest the formation of a new structure based on the sp3 bond. The present study is the first extensive experimental analysis on the sp2-sp3 structural changes in close correlation with the electrical conductivities of SWCNHs and ox-SWCNHs. The deformation of the tube structure due to the mechanical treatment weakens the n-type and p-type nature in SWCNHs and ox-SWCNHs, respectively. The mechanochemical treatment of SWCNHs reduces the pentagons in the cap and bending sites, which are the highly active sites in a hexagonal network, thereby weakening the n-type nature. In contrast, a reduction of the functional groups causes a decrease in the p-type nature of oxSWCNHs because there are many edge carbons that react with O2 and/or H2O near the nanowindows. In addition, the formation of a new structural junction at the defect sites in ox-SWCNHs was indicated by the transformation of sp2 bonds to sp3 bonds. Acknowledgment. This work on SWCNHs was supported by the Japan Society for the Promotion of Science (JSPS) for Young Scientists, a NEDO nanocarbon project, and a Grantin-Aid for Scientific Research (S) (No. 15101003) from JSPS. Supporting Information Available: (SP1) Intensity ratio of D-band to G-band as a function of the ratio of sp3 to sp2 bonding as determined by XPS analysis. (SP2) Electrical conductivity (σ) of SWCNHs and ox-SWCNHs for the different sp3/sp2 ratios. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Iijima, S.; Yudasaka, M.; Yamada, R.; Bandow, S.; Suenaga, K.; Kokai, F.; Takahashi, K. Chem. Phys. Lett. 1999, 309, 165. (2) Bandow, S.; Kokai, F.; Takahashi, K.; Yudasaka, M.; Qin, L. C.; Iijima, S. Chem. Phys. Lett. 2000, 321, 514. (3) Kasuya, D.; Yudasaka, M.; Takahashi, K.; Kokai, F.; Iijima, S. J. Phys. Chem. B 2002, 106, 4947. (4) Kokai, F.; Takahashi, K.; Kasuya, D.; Yudasaka, M.; Iijima, S. Appl. Surf. Sci. 2002, 197, 650. (5) Bekyarova, E.; Hanzawa, Y.; Kaneko, K.; Albero, J. S.; Ewcribano, A. S.; Reinoso, F. R.; Kasuya, D.; Yudasaka, M.; Iijima, S. Chem. Phys. Lett. 2002, 366, 463. (6) Murata, K.; Kaneko, K.; Kanoh, H.; Kasuya, D.; Takahashi, K.; Koaki, F.; Yudasaka, M.; Iijima, S. J. Phys. Chem. B 2002, 106, 11132. (7) Tanaka, H.; Kanoh, H.; Yudasaka, M.; Iijima, S.; Kaneko, K. J. Am. Chem. Soc. 2005, 127, 7511. (8) Zhu, J.; Kase, D.; Shiba, K.; Kasuya, D.; Yudasaka, M.; Iijima, S. Nano Lett. 2003, 3, 1033. (9) Yuge, R.; Ichihashi, T.; Shimakawa, Y.; Kubo, Y.; Yudasaka, M.; Iijima, S. AdV. Mater. 2004, 16, 1420. (10) Ajima, K.; Yudasaka, M.; Murakami, T.; Maigne, A.; Shiba, K.; Iijima, S. Mol. Pharmaceutics 2005, 2, 475. (11) Ajima, K.; Yudasaka, M.; Suenaga, K.; Kasuya, D.; Azami, T.; Iijima, S.; AdV. Mater. 2004, 16, 397. (12) Murata, K.; Hirahara, K.; Yudasaka, M.; Iijima, S.; Kasuya, D.; Kaneko, K. J. Phys. Chem. B 2002, 106, 12668. (13) 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. (14) 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. (15) Haerle, R.; Riedo, E.; Pasquarello, A.; Baldereschi, A. Phys. ReV. B 2001, 65, 045101–1. (16) Wang, Z. M.; Kanoh, H; Kaneko, K.; Lu, G. Q.; Do, D Carbon 2002, 40, 1231. (17) Roy, S. S.; McCann, R.; Papakonstantinou, P.; Maguire, P.; MacLaughlim, J. A. Thin Solid Films 2005, 482, 145. (18) Ma, P. C.; Kim, J. -K.; Tang, B. -Z Carbon 2006, 44, 3232. (19) Utsumi, S.; Honda, H.; Hattori, Y.; Kanoh, H.; Takahashi, K.; Sakai, H.; Abe, M.; Yudasaka, M.; Iijima, S.; Kaneko, K. J. Phys. Chem. C 2007, 111, 5572. (20) Kawai, T.; Okada, S.; Miyamoto, Y.; Oshiyama, A. Phys. ReV. B 2005, 72, 035428.
8762 J. Phys. Chem. C, Vol. 112, No. 24, 2008 (21) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science of Fullerenes and Carbon Nanotubes; Academic Press: San Diego, CA, 1996. (22) Rao, A. M.; Richter, E.; Bandow, S.; Chase, B.; Eklund, P. C.; Williams, K. A.; Fang, S.; Subbaswamy, K. R.; Menon, M.; Thess, A.; Smalley, R. E.; Dresselhaus, G.; Dresselhaus, M. S. Science 1997, 275, 187. (23) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powders and Porous Solids; Academic Press: London, 1999.
Letters (24) Kaneko, K.; Ishii, C. Colloids Surf. 1992, 67, 203. (25) Suryanarayana, C. Prog. Mater. Sci. 2001, 46, 1. (26) Wang, G.-W.; Komatsu, K.; Murata, Y.; Shiro, M. Nature 1997, 387, 583. (27) Tang, S. Y.; Rothe, E. W.; Reck, G. P. J. Chem. Phys. 1974, 2592, 61.
JP802276S
