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J. Phys. Chem. C 2008, 112, 1330-1334
Large-Scale Production of Single-Wall Carbon Nanohorns with High Purity Takeshi Azami,†,| Daisuke Kasuya,† Ryota Yuge,† Masako Yudasaka,*,†,‡ Sumio Iijima,†,‡,§ Tsutomu Yoshitake,† and Yoshimi Kubo*,†,⊥ Fundamental Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan, SORST, Japan Science and Technology Agency, c/o NEC Corporation, 34 Miyukigaoka, Tsukuba 305-8501, Japan, and Meijo UniVersity, 1-501 Shiogamaguchi, Nagoya 468-8502, Japan ReceiVed: August 8, 2007; In Final Form: NoVember 5, 2007
To increase the practical daily capacity to produce single-wall carbon nanohorns (SWNHs) with high purity by CO2 laser ablation of graphite, we made a new three-chamber system composed of a target reservoir, a laser ablation chamber, and a collection chamber. The frequency of exchanging targets was reduced by increasing the target size (diameter, 10 cm; length, 50 cm), and the time taken to exchange targets was reduced by adding an automated exchange mechanism. Most of the SWNHs produced in the laser ablation chamber were immediately swept into the collection chamber by an Ar gas carrier. This avoided the problem of fluffy SWNHs blocking the laser beam irradiating the target and enabled the CO2 laser to be operated continuously. The resulting SWNHs had a purity of 92-95% under the optimized conditions of laser power density (1530 kW/cm2) and target rotation speed (1-3 rpm). This increase in purity represents a remarkable improvement on the previous level of 85-90%. A practical production capacity of 1 kg/day was achieved, which is about 100 times greater than the daily capacity attained by the previous single-chamber system.
1. Introduction (SWNH)1-2
The structure of a single-wall carbon nanohorn is similar to that of a single-wall carbon nanotube3-4 but has an irregular shape. The diameter of an individual SWNH ranges from 2 to 4 nm, and the length is 40 to 50 nm. About 2000 of them assemble to form a spherical aggregate with a diameter of about 100 nm. The spherical aggregate is robust and cannot be separated into individual SWNHs. Three types of the spherical aggregates are known. These are called dahlia, bud, and seed aggregates because of their appearances.1-2,5 The dahlia-shaped aggregate has been obtained in gram-order quantities with high purity,1-2 which has enabled a wide range of applied research as well as fundamental research. Representative potential applications are methane storage,6 the separation of hydrogen and deuterium,7 Pt support for fuel cell electrodes,8 supercapacitors,9 and drug delivery.10-11 We think that the daily production capacity must be increased from the gram-order to the kilogram-order to enable further advances in applied research to be made and practical applications to be achieved. In the previous single-chamber SWNH production system using CO2 laser ablation of graphite,1-2 the production capacity was about 10 g/day. This was difficult to increase because of several problems. One was the long time of 1-2 h required to exchange targets; this was inevitable because the chamber had to be opened for target exchange. In addition, the targets had to be exchanged frequently because * To whom correspondence should be addressed. E-mail: yudasaka@ frl.cl.nec.co.jp (M.Y.);
[email protected] (Y.K.). Phone: +81-29-8561940. Fax: +81-29-850-1366. † Nano Electronics Research Laboratories. ‡ SORST. § Meijo University. | Present address: NEC Lamilion Energy, Ltd. 1120 Simokuzawa, Sagamihara, Kanagawa 229-1198, Japan. ⊥ Present address: NEC TOKIN Corporation, 1120 Simokuzawa, Sagamihara, Kanagawa 229-1198, Japan.
the graphite-rod target, which yielded only about 1-2 g of SWNHs, was small. The other problem was the 500-ms onoff laser irradiation mode. The 500-ms off time was necessary to allow many of the SWNHs to be swept toward a trap filter with the ambient Ar carrier gas. Without this 500-ms off phase, fluffy SWNHs permeated the chamber and absorbed the incident CO2 laser light, reducing the intensity of the light that reached the graphite-rod target and stopping SWNH generation. Another problem was that the laser ablation had to be stopped to take out the produced SWNHs. In the present study, we fabricated a new three-chamber system to overcome these problems. This made possible a production capacity of 1 kg/day. 2. Experimental Section A schematic drawing of the SWNH production system is shown in Figure 1. The system is composed of three chambers: a graphite rod target reservoir, a laser ablation chamber, and a SWNH collection chamber. The ambient gas was Ar with a pressure of about 760 Torr. It was introduced from the bottom of the laser ablation chamber and evacuated from the top of the collection chamber. To reduce the target exchange frequency, the target was greatly enlarged to 10 cm in diameter and 50 cm in length, compared with the previous values of 3 and 10 cm, respectively.1-2 To reduce the target exchange time, 10 graphite rod targets were placed in the reservoir chamber, and they were automatically fed into the laser ablation chamber sequentially and returned to the reservoir chamber. This made it possible to exchange targets without breaking the Ar atmosphere. The target exchange took about 15 min, which was much shorter than the previous 1-2 h. The CO2 laser was operated at 3.5 kW in the continuous emission mode instead of the previous 500-ms off-and-on mode. The laser-power density (LPD) was set to 15, 30, or 45 kW/ cm2. These values were set by controlling the laser spot sizes
10.1021/jp076365o CCC: $40.75 © 2008 American Chemical Society Published on Web 01/15/2008
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Figure 1. Schematic illustration of the new three-chamber system for SWNH production.
(about 4.5, 3.2, and 2.6 mm) on the graphite target surface by adjusting the position of a ZnSe lens. During laser ablation, the graphite-rod target was rotated helically about a vertical axis while the laser beam was irradiated horizontally. The laser-beam axis was shifted from the graphite-rod axis so as to irradiate the graphite-rod surface at an angle of 45°. This configuration effectively prevents the laser beam from reheating the carbon plume that formed perpendicular to the rod’s surface. The helical pitch was set at 5 mm, and each rod was rotated a total of 91 times. The target rotation speed (TRS) was varied between 1 and 6 rpm. Most of the fluffy SWNHs were immediately swept by the Ar carrier/ambient gas into the collection chamber. This effectively prevented the SWNHs from permeating the laser ablation chamber. In the collection chamber, high-purity SWNHs were deposited at the bottom of the chamber where there was a gate for attaching an SWNH storage bottle. When the bottle was filled with SWNHs, it was detached without interrupting the continuous production of SWNHs. The impurity-rich SWNHs were sucked away by the vacuum pump and trapped by the filter near the Ar gas outlet at the top of the collection chamber. To evaluate the production capacity of the new system and determine the optimum LPD and TRS, we measured the obtained quantities of high-purity SWNHs. The purities were also investigated by thermogravimetric analysis (TGA) in an oxygen atmosphere with a temperature ramp rate of 10 °C/min. Their structures were studied by transmission electron microscopy (TEM) at an acceleration voltage of 120 kV and with X-ray diffraction (XRD). To estimate the quantities of C60 contained in the SWNHs, we dispersed the SWNHs in toluene and measured the optical absorption spectra of the supernatants. The C60 quantities were estimated from absorption intensity at 330 nm according to the Lambert-Beer law. 3. Results and Discussion Figure 2 shows photographs of the graphite-rod target after CO2 laser ablation. The surface exhibits helical grooves. The laser power density was controlled by adjusting the laser beam’s diameter on the target surface by moving its focal point while keeping the laser source power constant at 3.5 kW. The groove width narrowed, and its depth increased as LPD increased (Figure 3a), but the cross-sectional area of the groove did not depend much on the LPD. The groove depth decreased with TRS in nearly inverse proportion. The ablated target quantities were estimated by measuring the target weight losses, as shown in Figure 3b. These weight losses roughly correspond to the volume losses estimated from the groove cross-section and the density of the graphite target of 1.78 g/cm3. The graphite evaporated in this manner first formed a hightemperature plume containing carbon clusters, leading to the
Figure 2. Photographs of graphite-rod target after laser ablation.
Figure 3. (a) Width and depth of grooves made by laser ablation of the target surface seen in Figure 2. (b) Decrease in the weight of the target caused by laser ablation (solid lines). The weight decrease was estimated from the grooves volume using a target density of 1.78 g/cm3 (broken lines).
SWNH generation.12 The mechanism of SWNH formation is not clear, but one possible mechanism would be that SWNHs are formed by carbon clusters in the plume transforming into carbon droplets and then into SWNHs as the temperature decreased from an initial 10 000 °C to room temperature.13 It is also thought that, soon after the formation of SWNH aggregates, the aggregates collided with each other and physically adhered to form secondary agglomerates with extremely large sizes on the order of millimeters. Secondary agglomerates were carried to the collection chamber by the Ar gas stream and fell to the bottom of the collection chamber. The SWNH quantities collected at the collection chamber were measured. Their deposition rates are shown in Figure 4, together with the target ablation rates obtained from the target weight losses shown in Figure 3b. It is noted that the collected SWNHs accounted for about 40-60% of the total weight loss of the target caused by the laser ablation. The missing portion was found to be trapped by the filter; it contained a lot of nonSWNH carbonaceous materials, as discussed below.
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Figure 4. Target evaporation rates (circles) and SWNH deposition rates at the bottom of the collection chamber (squares). When 15 and 30 kW/cm2, the deposition rates at 6 rpm were not able to be estimated accurately due to the small deposition amounts.
The ablation rate of the graphite target was around 100 g/h, as shown in Figure 4. In consideration of the laser source power of 3.5 kW, this value corresponds to a specific irradiation energy of 126 kJ/g (or 1500 kJ/mol), which is comparable with the reported value of 137 J for 1.5 mg, i.e., 91 kJ/g.14 On the other hand, the energy needed to form a carbon plume, that is, to heat up (to ca. 4000 K) and sublimate graphite, would be about 800 kJ/mol if we assume a specific heat of about 25 J/Kmol and a sublimation energy of 718 kJ/mol. An even higher energy of about 950 kJ/mol would be necessary if the plume temperature were higher (∼10 000 K). In any case, it is estimated that about 60% of the irradiated energy was used for ablation while the rest of it was reflected from the target surface and lost by thermal conduction or radiation. At a high rotation speed of 6 rpm, the ablation rate became very low for lower power densities of 15 and 30 kW/cm2. Under such conditions, since the laser beam moved very quickly on the target surface, the surface temperature could not be raised enough to fully ablate the graphite. The purities of the SWNHs collected from the bottom of the collection chamber were estimated by TGA. A typical TGA result for the SWNHs obtained at a TRS of 2 rpm and an LPD of 30 kW/cm2 is shown in Figure 5a. SWNH combusted at about 610 °C, and graphitic particles combusted at about 730 °C.15 The materials that combusted below 500 °C were non-SWNH carbonaceous impurities such as C6016 and defective parts of the SWNHs where pentagonal and heptagonal rings appeared in the hexagonal graphite networks. The proportion of C60 was estimated to be about 1% from the UV/vis absorption spectrum of the supernatant of the SWNH dispersed in the toluene. Therefore, the chamber-deposited SWNHs obtained at an LPD of 30 kW/cm2 and TRS of 2 rpm included 1% of C60 and 5% of graphitic particles and turned out to have a purity of about 94%. The quantities of graphitic particles contained in the SWNHs generated in other LPD and TRS conditions were similarly estimated by TGA (Figure 5b), and regardless of the LPD and TRS values, C60 was about 1%. Thus, we found that SWNHs with purities between 92 and 95% were produced at a formation rate of 50-70 g/h when LPD and TRS were 15-30 kW/cm2 and 1-3 rpm, respectively. This formation rate can be translated into a production capacity of 1 kg/day, which is about 100 times greater than the previous 10 g/day. Supporting data are presented here to demonstrate the presence of graphitic particle impurities in SWNHs collected
Figure 5. (a) TGA results for SWNHs obtained at 30 kW/cm2 and 2 rpm. (b) Amount of graphitic particles estimated from weight loss in TGA at temperatures from 700 to 800 °C in Figure 5a.
Figure 6. (a) X-ray diffraction analysis of SWNHs obtained at 30 kW/cm2 and 2 rpm. (b) Heights of XRD peaks at about 26°.
on the bottom of the chamber. The XRD of the SWNHs (Figure 6a) shows a peak with a shoulder at 26-27°, which corresponds to the diffractions from graphitic particles.17 The steep gradient seen below 12° may represent the diffractions from the double
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Figure 8. Populations of seed, bud, dahlia, and petal-dahlia aggregates of SWNHs.
4. Conclusion
Figure 7. TEM images of different types of SWNH aggregates.
layers formed by the adjacent SWNH tubules.18 The intensity changes of the peak at 26-27° with LPD and TRS (Figure 6b) corresponds well to the changes in quantity of the graphitic particles estimated from TGA (Figure 5b). Carbonaceous materials were also trapped by the filter at the top of the collection chamber (Figure 1). Their proportions were roughly equal to the differences in proportions of the evaporated targets and the SWNHs collected on the bottom of the collection chamber (cf. Figure 4). They contained more impurities such as graphitic particles (20-30%) and amorphous carbon plus C60 (10-20%) in addition to the SWNHs. We think that these impurities were much finer than the agglomerated SWNH aggregates and thus easily sucked out by the evacuation system along with the Ar carrier gas and trapped by the filter. In this paragraph, we show how the shape of SWNH aggregates depended on LPD and TRS. In addition to the three types of SWNH aggregates having spherical shapes with diameters of about 100 nm (the dahlia, bud, and seed types),1-2 the petal-dahlia aggregate is additionally considered in this report (Figure 7). The dahlia aggregate has long cone-shaped tips sticking through its surface, while the bud aggregate does not. The seed aggregates have lower graphitization than the dahlia and bud aggregates, and their tubules are corrugated. The petal-dahlia aggregates have diameters of about 100 nm and are characterized by having 10 or more petals (arrows in Figure 7). Each petal is a graphitelike plate with a width of about 50 nm and a thickness of 2-3 nm and is made of 5-10 graphene sheets. The four types of aggregates were counted in TEM images, and their populations are shown in Figure 8. It is apparent that the SWNHs produced under the optimum conditions included a lot of dahlia and petal-dahlia aggregates. Their combined population reached 90% for 30 kW/cm2 and 2 rpm.
To increase the practical capacity to produce SWNHs by CO2 laser ablation of graphite, we fabricated a new three-chamber system composed of a target reservoir, a laser ablation chamber, and a collection chamber. We also reduced the target exchange frequency by enlarging the target (diameter, 10 cm; length, 50 cm) and reduced the time required to exchange targets by adding an automated exchange mechanism. Ar carrier gas immediately swept most of the SWNHs produced in the laser ablation chamber into the collection chamber, preventing fluffy SWNHs from blocking the laser beam irradiating the target. Therefore, we could operate the CO2 laser continuously. The optimized conditions for laser power densities and target rotation speeds for SWNH production were found to be 15-30 kW/cm2 and 1-3 rpm, respectively. As a result of these improvements, a practical production capacity of 1 kg/day was achieved, which is about 100 times greater than that attained by the previous single-chamber system. In addition, the purity of SWNHs obtained under the optimum conditions was also greatly improved from the previous 85-90 to 92-95%. It is especially noteworthy that for 30 kW/cm2 and 2 rpm about 90% of the SWNH aggregates were dahlia or petal-dahlia types. Acknowledgment. This work was in part performed under the management of the Nano Carbon Technology Project supported by NEDO. 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) Iijima, S. Physica B 2002, 323, 1. (3) Iijima, S. Nature 1991, 354, 56. (4) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (5) Kasuya, D.; Yudasaka, M.; Takahashi, K.; Kokai, F.; Iijima, S. J. Phys. Chem. B 2002, 106, 4947. (6) Bekyarova, E.; Murata, K.; Yudasaka, M.; Kasuya, D.; Iijima, S.; Tanaka, H.; Kahoh, H.; Kaneko, K. J. Phys. Chem. B 2003, 107, 4681. (7) Tanaka, H.; Kanoh, H.; Yudasaka, M.; Iijima, S.; Kaneko, K. J. Am. Chem. Soc. 2005, 127, 7511. (8) Yoshitake, T.; Shimakawa, Y.; Kuroshima, S.; Kimura, H.; Ichihashi, T.; Kubo, Y.; Kasuya, D.; Takahashi, K.; Kokai, F.; Yudasaka, M.; Iijima, S. Physica B 2002, 323, 124. (9) Yang, C-M.; Kim, Y-J.; Endo, M.; Kanoh, H.; Yudasaka, M.; Iijima, S. J. Am. Chem. Soc. 2007, 129, 20.
1334 J. Phys. Chem. C, Vol. 112, No. 5, 2008 (10) Murakami, T.; Ajima, K.; Miyawaki, J.; Yudasaka, M.; Iijima, S.; Shiba, K. Mol. Pharm. 2004, 1, 399. (11) Ajima, K.; Yudasaka, M.; Murakami, T.; Maigne´, A.; Shiba, K.; Iijima, S. Mol. Pharm. 2005, 2, 475. (12) Kokai, F.; Takahashi, K.; Yudasaka, M.; Iijima, S. J. Phys. Chem. B 1999, 103, 8686. (13) Kasuya, D.; Yudasaka, M. New Diamond 2004, 73, 2. (14) Schaeffer, R.; Pearson, R. K. J. Am. Chem. Soc. 1969, 91, 2153.
Azami et al. (15) Utsumi, S.; Miyawaki, J.; Tanaka, H.; Hattori, Y.; Itoi, T.; Icikuni, N.; Kanoh, H.; Yudasaka, M.; Iijima, S.; Kaneko, K. J. Phys. Chem. B 2005, 109, 14324. (16) Kasuya, D.; Kokai, F.; Takahashi, K.; Yudasaka, M.; Iijima, S. Chem. Phys. Lett. 2001, 337, 25. (17) Fan, J.; Yudasaka, M.; Kasuya, D.; Azami, T.; Yuge, R.; Imai, H.; Kubo, Y.; Iijima, S. J. Phys. Chem. B 2005, 109, 10756. (18) Bandow, S.; Kokai, F.; Takahashi, K.; Yudasaka, M.; Qin, L. C.; Iijima, S. Chem. Phys. Lett. 2000, 321, 514.
