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2007, 111, 10-12 Published on Web 12/13/2006
Disappearance of Inner Tubes and Generation of Double-wall Carbon Nanotubes from Highly Dense Multiwall Carbon Nanotubes by Heat Treatment Akira Koshio,*,† Masako Yudasaka,‡ and Sumio Iijima‡,§ DiVision of Chemistry for Materials, Graduate School of Engineering, Mie UniVersity, 1577 Kurimamachiya-cho, Tsu, Mie 514-8507, Japan, CNT Project, Solution Oriented Research for Science and Technology (SORST), Japan Science and Technology Agency (JST), c/o NEC Corporation, Fundamental and EnVironmental Research Laboratories, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan, Department of Physics, Meijo UniVersity, Tenpaku-ku, Nagoya 468-8502, Japan, and NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan ReceiVed: NoVember 6, 2006; In Final Form: December 3, 2006
We previously reported that highly dense multiwall carbon nanotubes (hd-MWNTs) with the innermost tube diameters of about 0.4 nm and outside diameters of 5-20 nm were obtained easily with yields higher than 90% by evaporation of graphite with radio frequency (RF) plasma (Koshio et al. Chem. Phys. Lett. 2002, 356, 595). This report shows that vacuum heat-treatment at 2200 °C or higher is useful to preferentially remove single to three inside tubes from hd-MWNTs depending on the heat-treatment temperatures. At the same time, we found that double-wall carbon nanotubes were newly generated, the mechanism of which is unclear.
It is well-known that heating induces carbon nanotubes to change their structures. For example Nikolaev and co-workers reported about diameter doubling of single-wall carbon nanotubes (SWNTs) by heating at the temperature about 1500 °C under argon and hydrogen atmospheres.1 Yudasaka and coworkers reported that the diameters of HiPco SWNTs were increased by heat treatments at 1000-2000 °C.2 Metenier and co-workers reported that the increase of the diameters of arcdischarge SWNTs resulted from coalescence by heating at about 1800 °C under argon flow and that the transformation in multiwall carbon nanotubes (MWNTs) at more than 2200 °C.3 We have previously reported about synthesis and characterization of highly dense multiwall carbon nanotubes (hd-MWNTs).4 The hd-MWNTs were produced by directvaporization of graphite using RF plasma without metal catalysts in a mixture gas of argon and hydrogen. Most of the hd-MWNTs had acute tips with a cone angle of about 20° and had diameters of 5-20 nm. The layers of the hd-MWNTs were packed densely to the center, and the innermost tubes had a diameter of 0.4 nm, the smallest diameter possible for a SWNT. In this paper, we describe structural changes of hd-MWNTs and growth of thin MWNTs, which are including double-wall carbon nanotubes (DWNTs), originating from hd-MWNTs via vacuum heat treatment. The preparation of hd-MWNTs by using radio frequency (RF) plasma was described in a previous paper.4 As-grown hdMWNTs were put into a graphite crucible, and it was set in a vacuum electric furnace. The temperatures were set at 1200* Corresponding author. E-mail:
[email protected]. Fax: +8159-231-5370. † Mie University. ‡ Japan Science and Technology Agency. § Meijo University and NEC.
10.1021/jp0672914 CCC: $37.00
2500 °C. The heating was carried out for 5 h in a vacuum degree better than 10-6 Torr. In the case of more than 2400 °C, the heating was done in Ar gas of 760 Torr in order to prevent vaporizing carbon heaters in a vacuum electric furnace. The obtained samples were characterized by transmission electron microscopy (TEM; Topcon EM-002B). For observation of inner and outer diameter distributions, we measured 100-200 nanotubes directly from more than 20 TEM images for each sample. As-grown hd-MWNTs had inside diameters of about 0.4 nm and outside diameters of about 15 nm (Figure 1a). Their tube walls were a little corrugated; however, they were straightened by heat-treatment at 2200 °C or higher (Figure 1b), and their inner tube diameters exhibited no noticeable enlargements when the heat treatment temperature was 2000 °C or lower (Figure 2a-c). However, when the heat-treatment temperature increased above 2200 °C, more drastic changes started; that is, the inside tubes preferentially disappeared, meaning a part of the tubes had larger inner diameters (Figure 1c). This tendency suddenly became even more critical when the heat-treatment temperature was elevated to 2200 °C or higher (Figure 2d-f). For example, after heat-treatment at 2400 °C, hd-MWNTs changed to the hollow MWNTs with inner diameters of about 3 nm (Figure 1c). However, an infinite enlargement of the inner diameters was not unlikely, and experimental results in Figure 2e,f indicated a preferable inner diameter existing at about less than 3 nm. Assuming the distance between the neighboring walls of hd-MWNTs to be about 0.34 nm referring from the layer-layer distance of graphite, this 3-nm inner diameter corresponded to the disappearance of three or four innermost walls of hdMWNTs. Together with the disappearance of the inner tubes from hdMWNTs, the appearance of DWNTs and thin MWNTs were © 2007 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 111, No. 1, 2007 11
Figure 2. Distributions of inner diameters of (a) as-grown and heattreated hd-MWNTs at (b) 1800, (c) 2000, (d) 2200, (e) 2400, and (f) more than 2500 °C. Distribution of outermost diameters of (a′) asgrown and heat-treated hd-MWNTs at (b′) 1800, (c′) 2000, (d′) 2200, (e′) 2400, and (f′) more than 2500 °C.
Figure 1. TEM images of (a) as-grown hd-MWNTs and (b) hollow MWNTs formed by heat-treatment of hd-MWNTs at the temperature of 2400 °C. Most of them have cone-shaped tips which reflected the tip structure of hd-MWNTs and a typical inner diameter of about 3 nm as shown in (c).
observed after the heat treatments of hd-MWNTs at more than 2200 °C (Figure 3a). Since these DWNTs and thin MWNTs did not coexist with hd-MWNTs before the heat treatments, they were newly produced during the heat treatments. The inner and outside diameters of DWNTs were about 2.8 and 3.8 nm (Figure 3b,c), and those of thin MWNTs were about 1.4 and 4 nm (Figure 3d). (These tubes were not counted in the inner-diameter histograms of the heat-treated hd-MWNTs shown in Figure 2af.) Their length was in order of micrometers and quantity was about 1 piece per 50 pieces of the inner-hollowed hd-MWNTs. Figure 2a′-f′ indicate that outermost tube diameters do not change before and after the heat treatment. The outermost tube
diameters of as-grown and inner-hollowed hd-MWNTs were 5-20 nm. The histograms of outermost tube diameters more than 2200 °C (Figure 2d′,f′) indicate the appearance of thin tubes with diameters of less than 5 nm. This result means that outer tubes were stable during heat treatment and that DWNTs and thin MWNTs were newly produced in the yield of about 2%. It is almost no doubt that the tips of the tubes, most unstable parts of the hd-MWNTs because of the strongest curvatures and pentagons,5,6 were sublimed and opened during heat treatments at high temperatures. These were followed by the sublimation of the inner tubes corresponding to second unstable parts due to the second strongest curvatures. The carbon clusters generated as a result of the sublimation of inner tubes were likely to dissipate outside of the hd-MWNTs through the opened tips, leading to almost no carbon fragment remaining inside the tubes. It is interesting that layer-by-layer removal was possible for hd-MWNTs. This is in contrast with the removal of outsidetubes of MWNTs by combustion in oxygen gas,7,8 when the several layers are combusted at the same time and usually cannot be removed as pre-designed. The layer-by-layer removal became possible in hd-MWNTs perhaps because the thermal stability difference between adjacent tube walls is larger than the thermal energy. Presumably the tips closed when the heat treatment was stopped. Interestingly, the closely layered tip walls seen in the as-grown hd-MWNTs (Figure 1a) could not be reconstructed, but spaces were inserted between the tip walls (Figure 1b). We deduce that the tip structural transformation occurred after heat treatment because the thinner inner tubes were sublimed more quickly than thicker outer tubes. In order to understand the vaporization process of the inner tubes, we briefly estimated the effusion amount of carbon atoms
12 J. Phys. Chem. C, Vol. 111, No. 1, 2007
Figure 3. TEM images of (a) long and thin MWNTs, (b) one of the hollow carbon nanostructures, (c) a double-wall carbon nanotube, and (d) a typical thin MWNT found in the samples after heat-treatment.
and/or clusters from the vaporized inner tubes. From the Young-Laplace equation in the case of a cylindrical structure, the vapor pressure p is given by
p ) γ/r where γ and r are the surface tension and tube radius, respectively. The vapor pressure is inversely proportional to the tube radius. Thus, the vapor pressure of the inner tube increases with a decrease in the tube diameter. That means there is a quicker sublimation of the thinner inner tubes than the thicker outer tubes. For instance, the vapor pressure of the innermost 0.4-nm diameter tube is 50 times as much as that of the outermost 20-nm diameter tube. As the vapor pressure of graphite is about 1.3 × 10-2 Pa at the temperature of 2400 °C, the vapor pressure of the 0.4-nm tube is estimated at 6.5 × 10-1 Pa at least. According to the basis of the Knudsen method, the fundamental relationship of the vapor pressure p and the effusion amount ∆m at the temperature T for the operation time ∆t is given by
p)
(2πRT M )
1/2
∆m A0∆t
Letters where R, M, and A0 are the gas constant, carbon atomic weight, and the area of a pore, respectively. In order to obtain exactly the effusion amount, we must know exactly the values of the tip pore area and the effusion time, but a reasonably good approximation can be made. If a 1-nm2 pore is opened at the tip of the hd-MWNT during heat treatment for 5 h at the temperature of 2400 °C, and the vapor pressure p is 6.5 × 10-1 Pa, we can calculate that the effusion amount is about 3.4 × 10-18 kg corresponding to 1.7 × 108 carbon atoms. This calculated result strongly supports the possibility that thinner inner tubes in the hd-MWNTs could be sublimed more quickly than thicker outer tubes, and the sublimed carbons effuse from the opened tip pore. The reason why the 3-nm diameter was preferred is not obvious, but we can imagine that tube more than 3-nm in diameter needed a higher temperature to degrade thermally. Maiti et al. have reported that the addition of carbon atoms and carbon clusters to tubes thinner than about 3 nm led to nucleation of curved defects during the growth via arc discharge condition.9 The condition of their theoretical calculation are different from that of our experiment; however, both results are essentially the same in that thinner tubes are thermally unstable. A diameter of 3-nm may be the threshold for stable nanotubes formation at high temperature, and the tubes thinner than 3 nm probably were sublimed into carbon fragments in our experiment. A probable mechanism of DWNTs and thin MWNTs growth is a template scheme, where the template is the inside tubes with diameters of about 4 nm, and the carbon feedstock is the sublimating inner tubes. Similar template growth of the carbon nanotubes were seen in the heat treatment of C60 incorporated single-wall carbon nanotubes, namely, C60 molecules coalesce and form the second single-wall carbon nanotubes inside the initial SWNTs.10 In this case, the lengths of the second SWNTs are limited to the number of C60 molecules confined inside the initial tubes; however, in the case of hd-MWNTs, the amount of carbon available from several inner tubes is enough to elongate the template-grown carbon nanotubes that stretch out of the hollowed hd-MWNTs. We first confirmed that thinner inner tubes could be preferentially removed from hd-MWNTs by vacuum heat treatment. After the removal, almost no carbon fragments remained inside the tubes, indicating that thus-formed inner spaces are useful for material storages. References and Notes (1) Nikolaev, P.; Thess, A.; Rinzler, A. G.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1997, 266, 422. (2) Yudasaka, M.; Kataura, H.; Ichihashi, T.; Qin, L.-C.; Kar S.; Iijima, S. Nano Lett. 2001, 1, 487. (3) Me´te´nier, K.; Bonnamy, S.; Be´guin, F.; Journet, C.; Bernier, P.; de La Chapelle, M. L.; Chauvet, O.; Lefrant, S. Carbon 2002, 40, 1765. (4) Koshio, A.; Yudasaka, M.; Iijima, S. Chem. Phys. Lett. 2002, 356, 595. (5) Ebbesen, T. W.; Ajayan, P. M.; Hiura, H.; Tanigaki, K. Nature 1994, 367, 519. (6) Tsang, S. C.; Chen, Y. K.; Harris, P. J. F.; Green, M. L. H. Nature 1994, 372, 159. (7) Tsang, S. C.; Harris, P. J. F.; Green, M. L. H. Nature 1993, 362, 520. (8) Ajayan, P. M.; Ebbesen, T. W.; Ichihashi, T.; Iijima, S.; Tanigaki, K.; Hiura, H. Nature 1993, 362, 522. (9) Maiti, A; Brabec, C. J.; Roland, C. M.; Bernholc, J. Phys. ReV. Lett. 1994, 73, 2468. (10) Bandow, S.; Takizawa, M.; Hirahara, K.; Yudasaka, M.; Iijima, S. Chem. Phys. Lett. 2001, 337, 48.
