7348
J. Phys. Chem. C 2007, 111, 7348-7351
Plugging and Unplugging Holes of Single-Wall Carbon Nanohorns Ryota Yuge,*,† Masako Yudasaka,*,†,‡ Jin Miyawaki,‡ Yoshimi Kubo,† Hiroyuki Isobe,§,⊥ Hideki Yorimitsu,§,¶ Eiichi Nakamura,§,| and Sumio Iijima†,‡,# Fundamental and EnVironmental Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba 305-8501, Japan, SORST, Japan Science and Technology Agency, c/o NEC Corporation, 34 Miyukigaoka, Tsukuba 305-8501, Japan, Department of Chemistry, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, ERATO, Japan Science and Technology Agency, PRESTO, Japan Science and Technology Agency, and Meijo UniVersity, 1-501 Shiogamaguchi, Nagoya 468-8502 ReceiVed: January 31, 2007; In Final Form: March 13, 2007
We demonstrate that gadolinium acetate could plug holes of single-wall carbon nanohorns (NHox) and that unplugging was also possible. A quantitative evaluation of the plugging and unplugging using C60 as a probe showed that about 50% of NHox were hole-plugged with gadolinium acetates and most of them were unplugged with water. The plugging and unplugging will enhance new uses of NHox and other carbon nanotubes for storing, carrying, and supplying material.
1. Introduction It is known that carbon nanotubes (CNTs) suck various materials such as metals and fullerenes into their inner hollow spaces through holes opened at their tips and/or in their sidewalls.1-3 If the holes could be given plugs that could be attached and removed, the potential applications of CNTs would be expanded even more widely. To show that such plugs are possible, we used single-wall carbon nanohorns (SWNHs),4,5 which is one of the carbon nanomaterials such as single-wall carbon nanotubes.6,7 SWNHs are cylinders made of single graphene sheets with diameters of 2-5 nm and length 40-50 nm. They are of high purity (>90%, no metal), and holes are easily opened by heating (400-600 °C) in an oxygen gas8,9 or air10 as evidenced directly by observations with a transmission electron microscope (TEM)11 and indirectly by the pore-volume increase estimated from nitrogen adsorption.8,9 Even the size and number of holes can be controlled by adjusting the combustion conditions.12,13 The SWNH with open holes is referred to as NHox in this paper. The effects of holes on the incorporation and release of materials, especially C60, were investigated precisely.14 The potential applications of NHox that have been reported include catalyst support,15-17 gas storage media,18,19 molecular sieves,20 and drug carriers.21,22 The plug material used in this study was gadolinium acetate,23 which has the advantages of being removed easily by washing with water and being easily visible with TEM. Its attachment to and removal from the holes of NHox were evidenced by measuring the released quantities of C60 previously incorporated inside the NHox (C60@NHox). * Corresponding authors. Telephone: +81-29-856-1940. Fax: +81-29850-1366. E-mail:
[email protected] (M.Y.);
[email protected] (R.Y.). † Fundamental and Environmental Research Laboratories, NEC Corporation. ‡ SORST, c/o NEC Corporation. § The University of Tokyo. | ERATO. ⊥ PRESTO. # Meijo University. ¶ Present address: Kyoto University, Katsura, Kyoto 615-8510, Japan.
Figure 1. TEM image of SWNHs.
2. Experimental Section SWNHs were obtained by CO2 laser ablation (power 3 kW, laser beam diameter 3.5 mm) of graphite at room temperature (Figure 1).4 To put holes in the graphene walls of SWNHs, we heat-treated the SWNHs at 580 °C for 10 min in oxygen gas.24 The preparation of C60@NHox is reported elsewhere:14 C60 (10 mg) was dissolved in toluene (50 mL), and the C60-toluene solution was mixed with NHox (30 mg) in a Pyrex glass container. The mixture was left in flowing nitrogen gas (760 Torr) at room temperature until the toluene had evaporated. The obtained black powders were C60@NHox 1 (eq 1). The excess C60 remained on the sidewalls of the container. toluene, dry
C60 + NHox 98 C60@NHox 1
(incorporation) (1)
To attach gadolinium acetate plugs to the holes of NHox or to C60@NHox 1, we mixed NHox (10 mg) or 1 with a methanol (5 mL) solution of gadolinium acetate (10 mg) at room temperature for about 12 h (eq 2 or 7). The mixture was filtered, methanol, dry
1 + Gd(OAc)3‚4H2O 98 GdC60@NHox 2 (plugging) (2) toluene
2 98 GdC60@NHox 3
10.1021/jp070831s CCC: $37.00 © 2007 American Chemical Society Published on Web 04/27/2007
(release)
(3)
Plugging and Unplugging Holes of NHox H2O
3 98 C60@NHox 4 toluene
4 98 NHox 5 toluene
1 98 NHox 6
J. Phys. Chem. C, Vol. 111, No. 20, 2007 7349
(unplugging)
(4)
(release)
(5)
(release)
(6)
methanol
Gd(OAc)3‚4H2O + NHox 98 Gd@NHox 7 (plugging) (7) and black powder was obtained on the filter paper. The black powder was washed with methanol (5 mL) two times to remove the excess gadolinium acetate existing outside the NHox and dried (eq 2 or 7), giving GdC60@NHox 2 (17 mg) or Gd@NHox 7. The method of the C60-release experiment (eqs 3, 5, and 6) was the same as described in our previous report:14 the specimens (3.5 mg) were mixed with toluene (300 mL) at room temperature for various periods. The quantity of released C60 in toluene was estimated through the measurement of its optical absorption spectrum. The absorption spectrum of C60 in toluene had a maximum at 336 nm, and the concentration of C60 was calculated from the maximum absorbance according to the Lambert-Beer law. The absorption coefficient of C60 at 336 nm used in the calculation was 54 000 L mol-1 cm-1.14 After the C60 release from GdC60@NHox 2, GdC60@NHox was separated from toluene by filtration and washing with toluene (50 mL, 2 times), and GdC60@NHox 3 was obtained. To remove the gadolinium acetate plug from GdC60@NHox 3 with water (unplugging), GdC60@NHox 3 (5 mg) was dispersed in 50 mL of water, sonicated for 1 min, separated from water by filtration, and washed with water (50 mL) five times. Thus C60@NHox 4 was obtained. The C60 release from C60@NHox 4 was carried out in toluene (eq 5). After the release experiments, the NHox was separated from toluene by filtration and washing with toluene, and NHox 5 was obtained. The C60-release experiment was also carried out for C60@NHox 1 in toluene (eq 6). The specimen obtained after the release experiment was denoted NHox 6. Thermogravimetric analysis (TGA) was performed to estimate quantities of C60 and/or gadolinium acetate. The TGA conditions were a temperature range of 200-900 °C, a ramp rate of 10 °C/min, and an atmosphere of 100% O2. As we previously reported, the quantities of C60 were estimated from the weight decrease measured in a temperature range between 320 and 520 °C.14 The residue after the TGA ended at 900 °C was Gd2O3. The structures of the specimens were observed by TEM using a Topcon 002B with an acceleration voltage of 120 kV. 3. Results and Discussion We show the effect of the plugging and unplugging of gadolinium acetate on the holes of NHox through optical absorption measurements. The optical absorption bands of C60 released from C60@NHox 1 (no plug) in toluene (eq 6) increased in intensity with the immersing period (Figure 2a), and the concentration of C60 in toluene estimated from maximum absorbance was plotted against the immersing period (Figure 2b). The release almost stopped at about 350 min of the immersing period, when the C60 concentration in toluene was almost 0.19 g/g. The TGA results indicated that the C60 quantity against NHox in C60@NHox 1 was 0.22 g/g (Figure 3; see black and red lines). The nonreleased C60 was about 0.03 ()0.22 0.19) g/g, which mostly corresponded to the TGA result of
Figure 2. Optical absorption spectra of C60 released in toluene from C60@NHox 1 (1 f 6) by immersion in toluene for various periods (a). The quantities of C60 released from C60@NHox 1 (1 f 6), plugged GdC60@NHox 2 (2 f 3), and unplugged C60@NHox 4 (4 f 5) in toluene were estimated from the absorption maximum at 336 nm according to the Lambert-Beer law14 and plotted against the immersion period (b). The dotted line indicates the initial C60 quantity in 1, 0.22 g/g, estimated from TGA (Figure 3, red line). The total C60 quantities released from 1, 2, and 4 were 0.19, 0.13, and 0.04 g/g, respectively.
NHox 6 (Figure 3, blue line). The nonreleased C60 was trapped at the tips of NHox or interstitial pores formed among the cylindrical parts of NHox.8 On the other hand, the quantity of C60 released from GdC60@NHox 2 in toluene (eq 3) was small (Figure 2b, red line), which indicated that about 32% {)(0.19 - 0.13)/0.19} of C60 was plug-confined inside NHox in GdC60@NHox 2. Here, we suggest that about 68% ()0.13/0.19) of C60 was adsorbed outside NHox or not plug-confined by the gadolinium acetate plug attachment (eq 2). After the C60-release experiment from GdC60@NHox 2 (eq 3), we collected GdC60@NHox 3 and used it for testing the unplugging. The unplugging was achieved by washing GdC60@NHox 3 with water (eq 4). Here, water dissolved gadolinium acetate but not C60. As a result of this unplugging, we obtained C60@NHox 4. Its TGA-900 °C residue ()Gd2O3) decreased from the initial 19% in GdC60@NHox 2 (Figure 3, green line) to 4% (Figure 3, magenta line), indicating that the gadolinium acetate plug was well removed. Since the plug was removed, the C60 inside NHox in C60@NHox 4 (0.04 g/g (0.04/ 0.19 ) 21% of C60 in 1)) was released again in toluene (eq 5) as shown with a blue line in Figure 2b. Adding a comment about the shortage of the C60 quantity released from 4 (0.04 < 0.19 - 0.13 g/g, Figure 2b), it might be due to the residual gadolinium acetate (Figure 3 magenta line) still confining a little C60. For further confirmations of the gadolinium acetate plug and its unplugging, we focused attention on the NHox sheaths seen
7350 J. Phys. Chem. C, Vol. 111, No. 20, 2007
Figure 3. TG (upper) and DTG profiles (lower) of NHox (black), C60@NHox 1 (red), GdC60@NHox 2 (green), C60@NHox 4 (magenta), NHox 6 (blue), and Gd@NHox 7 (brown). The C60 quantity in 1 was estimated from the weight decrease between 320 and 520 °C.14 The C60-quantity estimations for 2 and 4 were impossible because the combustion-temperature ranges of C60 and NHox overlapped. Their combustions shifted to lower temperatures due to the catalyst action of gadolinium acetate. The residues at 900 °C of 2, 4, or 7 were Gd2O3, and their quantities were about 19% for 2 and 7 and 4% for 4. The graphitic-particle impurities of NHox combusted at temperatures are indicated with an asterisk (/).12
Figure 4. TEM images of C60-encapsulating sheath of NHox (a) and empty sheath (b) seen in C60@NHox 1. TEM images of sheaths encapsulating both C60 and a gadolinium acetate cluster (c) and a gadolinium acetate cluster (d) seen in GdC60@NHox 2. The gadolinium acetate used as the plug might be changed to gadolinium oxide by an electron beam during the TEM observations. Therefore the 2-nm black spots seen in (c) and (d) are likely to be gadolinium oxide particles.
in TEM images. The NHox sheaths were classified into four types, encapsulating only C60 (Figure 4a), empty (Figure 4b), encapsulating both gadolinium acetate and C60 (Figure 4c), and encapsulating only gadolinium acetate (Figure 4d). It should be noted that since gadolinium acetate was easily damaged by the electron beam during our TEM observations, the dark spots located inside NHox in Figure 4c,d were certainly derivatives
Yuge et al.
Figure 5. Schematic table indicating percentages of sheaths encapsulating C60, gadolinium acetate, C60 and gadolinium acetate, and nothing (empty). About 300 sheaths were checked in TEM images for each specimen. A purple circle represents one C60 molecule. A small yellow circle denotes one gadolinium acetate molecule (see Figure 6), and a large yellow circle denotes a cluster of gadolinium acetate molecules.
of gadolinium acetate. It is known that gadolinium acetate is thermally changed to gadolinium oxide at about 700 °C,25 so we consider that the relevant gadolinium derivatives were gadolinium oxide. The NHox numbers were counted and are summarized in Figure 5. The most important counting result for the plug-effect evaluation was that for GdC60@NHox 3 (Figure 5). It indicates that 44% of the sheaths still held C60 inside even after immersion in toluene for more than 3 h (eq 3), though the gadolinium acetate plugs were not obviously seen with TEM. We considered that these NHox could not release the C60 molecules in toluene because the holes were plugged by one or two gadolinium acetate molecules deposited at the hole edges,23 as depicted in Figure 6. Such a small number of gadolinium atoms could not be observed with the TEM equipment used in this study. On the other hand, the plugs of large gadolinium acetate clusters were clearly visible as shown in Figure 4c, which was effective for 7% of the sheath (Figure 5, GdC60@NHox 3). Thus, it can be said that the gadolinium acetate plug was effective in about 50% (∼44% + 7%) of the NHox sheaths. Since about 50% of the sheaths were hole-plugged with the gadolinium acetate (Figure 5), and the quantity of C60 confined in them (GdC60@NHox 3 or C60@NHox 4) was about 32% as explained above using Figure 2, the average number of C60 molecules in each plugged NHox turns out to be about half of that before plugging. The unplugging was also confirmed by counting NHox sheath numbers in TEM images. After the removal of gadolinium acetate plug (3 f 4, eq 4) by washing with water, the gadolinium acetate clusters such as seen in Figure 4c,d were not found (C60@NHox 4 in Figure 5). Since being unplugged,
Plugging and Unplugging Holes of NHox
J. Phys. Chem. C, Vol. 111, No. 20, 2007 7351 Acknowledgment. This work was in part performed under the management of the Nano Carbon Technology Project supported by NEDO. We also thank T. Azami and D. Kasuya for preparing SWNH samples. We also thank Dr. T. Yoshitake for valuable discussions. References and Notes
Figure 6. Schematic images showing that a hole with a diameter of 1.5 nm in the sidewall (a) or one with a diameter of about 0.8 nm at the tip of NHox (b) is plugged by a gadolinium acetate molecule (yellow ball). The gadolinium acetate plug inhibits the release of C60 (purple ball) from inside NHox (b). Here, the diameter of C60 is 0.7 nm3 and that of the gadolinium acetate molecule is about 0.87 nm as estimated from the density of gadolinium acetate crystal (1.61 g/cm3).
most of the C60 in C60@NHox 4 was released in toluene; thus most of NHox 5 became empty (Figure 5). Three interesting phenomena found in Figure 5 are incidentally explained here. The number of C60-encapsulating sheaths in NHox 5 and NHox 6 were small and similar, meaning that the detoured C60-release process (eq 1 f eq 2 f eq 3 f eq 4 f eq 5) is equivalent to the direct process (eq 1 f eq 6). Second, in 5 and 6, C60 were usually found at the tips of NHox (TEM not shown), where the C60 molecules could gain certain stabilization energies through van der Waals interaction due to the narrow NHox diameters at the tips.26 Thus they were not released easily by immersing in toluene. Third, the numbers of sheaths encapsulating gadolinium acetate clusters (Figure 5, large yellow circles) in 2, 3, and 7 were similar, which indicates that the gadolinium acetate plug deposition was not influenced either by the C60 existence inside NHox or by the immersion in toluene. The TGA of 2 and 7 indicated the similar quantities of gadolinium acetate (Figure 3 green and brown). 4. Conclusion We clarified that about 50% of NHox sheaths were holeplugged with gadolinium acetate. The unplugging was possible by removing the gadolinium acetate plugs with water, by which most of the confined C60 became releasable from NHox in toluene. By selecting appropriate plug materials, the effects of plugging and unplugging for the holes of NHox could become useful in applying NHox for storing, carrying, and even supplying materials in various applications.
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