Hidden Caves in an Aggregate of Single-Wall Carbon Nanohorns

Jan 26, 2009 - We investigated the inner structure of an aggregate made of single-wall carbon nanohorns (SWNHs). This structure has not been clarified...
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J. Phys. Chem. C 2009, 113, 2741–2744

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Hidden Caves in an Aggregate of Single-Wall Carbon Nanohorns Found by Using Gd2O3 Probes Ryota Yuge,*,† Toshinari Ichihashi,† Jin Miyawaki,‡,§ Tsutomu Yoshitake,† Sumio Iijima,†,# and Masako Yudasaka*,†,‡,+ Nano Electronics Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba 305-8501, Japan, SORST, Japan Science and Technology Agency, Yaesu-Dori Building 6F, 3-4-15 Nihonbashi, Chuo-ku, Tokyo 103-0027, Japan, Department of Chemistry, Meijo UniVersity, 1-501 Shiogamaguchi, Nagoya 468-8502, Japan, and Nanotube Research Center, National Institute of AdVanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan ReceiVed: NoVember 18, 2008

We investigated the inner structure of an aggregate made of single-wall carbon nanohorns (SWNHs). This structure has not been clarified because the thousands of SWNHs are densely packed and difficult to observe directly by electron microscopy. We used Gd2O3 as a probe for the electron microscope observation: Gd2O3 was confined inside SWNH via the incorporation of Gd acetate inside SWNHs with holes opened (NHox), followed by heat treatment at 1200 °C in Ar gas. From microscopic study, we found that Gd2O3 formed clumped crystals with sizes of 7-21 nm around the center of the SWNH aggregates, suggesting that there were 10-20-nm caves near the aggregate’s center. Introduction Single-wall carbon nanotubes (SWNTs)1,2 and single-wall carbon nanohorns (SWNHs)3,4 have tubular structures made of single-graphene sheets. SWNHs differ from SWNTs in having long cone-shaped tips with cone angles of about 20° and large tubediameters of about 2-5 nm, and thousands of them form spherical aggregates with diameters of 80-100 nm. The SWNHs have a lot of defects such as pentagonal and heptagonal rings in the hexagonal networks. At these defect sites, holes can be formed easily by oxidation (NHox),5,6 and various kinds of materials, such as C60,7,8 Pt-compounds,9,10 and Gd compounds,11 can be incorporated inside NHox. Potential applications of NHox include gas adsorption,12 capacitors,13 and carriers of drug delivery systems.14-16 New potential applications could emerge from the interesting phenomenon that the holes can be closed by heat treatment in Ar atmosphere at 1200 °C.17,18 The growth mechanism has been investigated,19,20 and large-scale production of 1 kg/day has been achieved.21 Despite these vigorous studies on SWNHs and NHox, the assembly structure of the aggregate has not been clarified previously because of the lack of appropriate experimental procedures. In this study, the inner structure of a spherical aggregate of SWNHs was investigated by using Gd2O3 as a probe material for the electron microscope observation. As a result, we found that many of the aggregates had caves with sizes of 10-20 nm at locations around the centers of the aggregates. Experimental Section SWNH aggregates were prepared by CO2 laser ablation.3 NHox was obtained by heat-treating SWNH at 500 °C for 10 * Correspondingauthors.E-mail:[email protected],[email protected]. Phone: +81-29-861-4818. Fax: +81-29-861-6290. † NEC Corporation. ‡ Japan Science and Technology Agency. § Present address: Institute for Materials Chemistry and Engineering, Kyushu University, 6-1Kasugakoen, Kasuga, Fukuoka, 816-8580, Japan. # Meijo University. + AIST.

min in an oxygen flow (760 Torr, 200 cm3min-1). Gd acetate (50 mg) and NHox (50 mg) were dispersed and stirred in ethanol (20 mL) at room temperature for about 12 h. The mixture was filtered, and black powder was obtained on the filter paper. The black powder was washed with ethanol (20 mL) two times to remove the excess Gd acetate existing outside the NHox and dried for 24 h under vacuum at 80 °C (Gd@NHox). The Gd@NHox was heat-treated for 3 h at 1200 °C in an Ar atmosphere (760 Torr), and HTGd@NHox was obtained. To remove Gd2O3 located outside HTGd@NHox, HTGd@NHox (10 mg) was dispersed in 500 mL of HCl solution, sonicated for 30 min, separated from HCl solution by filtration, and washed with water (500 mL). This rinsing treatment was repeated for five cycles. The obtained HTGd@NHox contained 14% of Gd2O3 according to a thermogravimetric analysis (TGA) (TA2950) performed in the temperature range between room temperature and 1000 °C, at a ramp rate of 10 °C/min, and an atmosphere of O2 100%. TGA was also performed to estimate the quantities of Gd compounds. The structures of the specimens were observed with a transmission electron microscope (TEM) (Topcon 002B) and scanning transmission electron microscope (STEM) (Hitachi HD2300), equipped with electron energy loss spectroscope (EELS) (Gatan Enfina). The TEM and STEM were operated at 120 kV. The states of Gd in the HTGd@NHox were analyzed using EELS. The specific surface area and the pore volume distribution were calculated from N2 adsorption isotherms by theBreunauer-Emmet-Teller(BET)andBarrett-Joyner-Halenda (BJH) methods. N2 gas adsorption measurements were carried out at 77 K using a volumetric apparatus (Shimadzu ASAP2000) after a pretreatment at 150 °C for 12 h under vacuum. Results and Discussion 1. Evidence for the Gd2O3 Encapsulation inside SWNH. The NHox was vacant inside (Figure 1a), but the dark spots of Gd acetate clusters with sizes of 2-4 nm were found inside NHox, as shown in a TEM image of Gd@NHox (Figure 1b).

10.1021/jp810121a CCC: $40.75  2009 American Chemical Society Published on Web 01/26/2009

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Figure 1. TEM image of NHox (a), Gd@NHox (b), HTGd@NHox with type-I Gd compounds (c), and HTGd@NHox with type-I and -II Gd compounds (d). SAED pattern of HTGd@NHox with type-I Gd compounds with arrows indicating weak unidentified diffraction spots (e). SAED pattern of HTGd@NHox with type-II Gd compounds with diffraction indexes of the Gd2O3 crystal (f). Ring-shaped diffractions are from the graphene walls of NHox.

These dark spots disappeared after heat treatment: instead, two types of particles appeared, as seen in the TEM images of HTGd@NHox (Figures 1c and 1d). One of these two types had a horn-like shape (Type I) located at the tips of NHox (Figure 1c). The other was a large particle (Type II) located near the center of the aggregate (Figure 1d). About 70% of the HTGd@NHox aggregates had Gd compounds with the type-II shape, which was evaluated by counting 300 aggregates. Most of the HTGd@NHox aggregates had the type-I Gd compounds. Although the numbers of type-I Gd compounds in one aggregate fluctuated, the trend was for the number of type-I to be small when the aggregate has type-II Gd compounds. We think that both the type-I and type-II Gd compounds were made of Gd2O3 because it is known that Gd acetate is transformed to Gd2O3 by heat treatment above 650 °C in inert gases.22 To confirm that the type-II material was Gd2O3, we performed selected area electron diffraction (SAED) for individual aggregates of HTGd@NHox, such as shown in Figure 1d. As a result, the spots indexed as (222), (400), and (440) diffractions from cubic Gd2O3 were seen in addition to the (100) and (220) rings diffracted from graphitic structures of NHox (Figure 1f). This indicates that the type-II material was Gd2O3 crystal. SAED of type-I material was also observed, but clear diffraction spots ascribable to Gd2O3 were not found; instead,

Figure 2. EELS of type-I Gd compounds of HTGd@NHox marked with a green circle in the inset STEM image. Gd N-edge and C K-edge (a), Gd M-edge (b), and O K-edge (c).

unidentified weak spots were found together with ring-shaped diffraction from NHox graphenes. To identify this type-I Gd compound, we measured EELS, as described below. To elucidate the chemical states of the type-I Gd compounds (Figure 2a, inset, green circle), we performed EELS measurements with a finely focused electron beam with a 0.5-nm diameter and a typical acquisition time of 20 s. The measured spectrum (Figure 2a) showed absorption edges of Gd N(4d) and C K(1s) at 148 and 285 eV, respectively. For the C K-edge, the π* peak at 285 eV and σ* peak at 293 eV correspond to sp2-type bonding in the graphitic walls of SWNHs.23,24 Figure 2b shows the Gd M(3d) absorption obtained for the same particle. The peak positions of the M edge of the lanthanide metals are generally used to determine the valence state. The peak positions of the M5 and M4 were 1184 and 1214 eV,

Aggregate of Single-Wall Carbon Nanohorns

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Figure 3. SEM (upper left), STEM (upper middle), and Z-contrast images (rotation angle: 0-112°) of HTGd@NHox. White circles indicate HTGd@NHox aggregates.

Figure 5. N2 adsorption isotherms (a) and pore volume distributions (b) of SWNHs, NHox, Gd@NHox, and HTGd@NHox. These values were calculated per 1 g of SWNHs or NHox.

Figure 4. Size distribution of type-II Gd2O3 particles in HTGd@NHox obtained from about 300 white spots observed in the Z-contrast image.

respectively: these values agree with a reference spectrum of Gd2O3.25 The observed O K-edge at 530 eV also suggests that the Gd compounds were oxides (Figure 2c). Therefore, we concluded that the type-I Gd compounds were in the trivalent state and were probably Gd2O3. We also measured EELS for type-II Gd compounds, and the results similarly supported them being Gd2O3 (data not shown). Thus, we could confirm that Gd2O3 particles were encapsulated inside SWNH, and there were two types of Gd2O3 particles characterized by morphology and location. To investigate the inner SWNH-aggregate structure, type-II Gd2O3 particles are essential, and their locations were examined precisely, as shown below. 2. Location of Type-II Gd2O3 Particles and Inner Structure of SWNH Aggregate. The type-II Gd2O3 particles seemed to exist around the center of the SWNH aggregate according to the TEM images such as shown in Figure 1d; however, this should be confirmed by rotating the specimen. Figure 3 shows

SEM and STEM images of HTGd@NHox and its Z-contrast images observed by rotating the specimens from 0° up to 112° around the axis indicated in the image for 0°. The SEM image gives an overview of the specimen (Figure 3, upper left). Its STEM images show a large black area, which came from the overlapped HTGd@NHox aggregates (Figure 3, upper middle), indicating that STEM observation is inadequate to find the Gd2O3 location. On the other hand, large type-II Gd2O3 particles were observed clearly in the Z-contrast images as bright spots (Figures 3, 0°) because the image contrast is proportional to the square of the atomic number. In the rotation images (Figure 3, 0-112°), white dashed circles indicate the peripheries of HTGd@NHox aggregates. After the rotation, the white spots of Gd2O3 were present at the same positions in the circles. We concluded that the large type-II Gd2O3 particles were located around the centers of spherical aggregates of SWNHs. We also measured the sizes of type-II Gd2O3 particles in the Z-contrast images. The particle sizes were 7-21 nm, mainly 8-16 nm (Figure 4). This means that there were caves with similar or larger sizes at and around the SWNH aggregate centers. We consider that there were intrinsic caves and Gd compounds gathered in them. This result agrees with the previous model for the SWNH aggregate structure having a cave at the center of the aggregate.3 3. Evidence of Complete Confinement. We have shown that there are empty caves at and around the center of an SWNH aggregate by using Gd2O3 as a probe. We conjectured that the thermal closing of the holes of NHox17,18 was the key to keep

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Figure 6. Schematic illustration of empty NHox (a), Gd@NHox (b), and HTGd@NHox (c). The magenta and blue indicate Gd acetate and Gd2O3, respectively.

Gd2O3 inside NHox. Here, through gas adsorption studies, we show that such a confinement really occurred. Figure 5a shows N2 adsorption isotherms (a) and pore volume distributions (b) of SWNHs, NHox, Gd@NHox, and HTGd@NHox. The BET surface area of the NHox was 1460 m2/g, which is about four times larger than that of the SWNHs (390 m2/g) as previously reported, reflecting that holes were formed.6 The specific surface areas of the Gd@NHox and HTGd@NHox per unit weight of NHox were estimated to be 1100 and 350 m2/g, respectively, which indicates that the holes were closed by the heat treatment in Ar at 1200 °C. The pore size distribution of HTGd@NHox was almost the same as that of SWNHs, and much smaller than those of NHox and Gd@NHox (Figure 5b), which also proves that the holes were closed by the heat treatment. 4. Mechanism of Gd2O3 Particle Formation. Here, we discuss how the Gd2O3 was confined at the tip and/or around the center of a spherical NHox aggregate. Referring to the previous reports,3,26 we note that the SWNH aggregate has a cave near its center, so we draw in Figure 6a a spherical aggregate of NHox with a cave near the aggregate center. In Gd@NHox, the Gd acetate particles existed in the sheaths (Figure, 6b), as presented in the TEM image of Figure 1b. We further consider that, in Gd@NHox, Gd acetate also existed in the caves (Figure 6b). During the heating of Gd@NHox for the purpose of hole-closing, the Gd acetate particles located in the sheaths would move to both ends of the sheath by capillary suction.27,28 As a result, at the tips of the sheath, Gd acetate changed into Gd2O3, forming a cone-shape molded to the tip shapes (Figure 6c). Those moved from the sheath to the center and the Gd acetate already located in the cave could transform into Gd2O3, forming large clumped crystals (Figure 6c). Conclusion In this study, we found that holes of NHox were closed by heat treatment at 1200 °C in Ar even though Gd acetate was incorporated. After this heat treatment, the Gd acetate was found to have changed into Gd2O3, and the Gd2O3 particles were located either at the tips of NHox or around the centers of spherical aggregates. The Gd2O3 particles existing near the center formed clumped crystals with sizes of 7-21 nm. Therefore, we conclude that there are three-dimensional empty spaces, namely caves, around the center of an aggregate of SWNHs. Acknowledgment. This work was in part performed under the management of the Nano Carbon Technology project supported by NEDO.

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