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Boron-catalyzed multi-walled carbon nanotube growth with the reduced number of layers by laser ablation
30 June 2000
Chemical Physics Letters 324 Ž2000. 224–230 www.elsevier.nlrlocatercplett
Boron-catalyzed multi-walled carbon nanotube growth with the reduced number of layers by laser ablation K. Hirahara a , K. Suenaga a
a,)
, S. Bandow a , S. Iijima
a,b,c
‘Nanotubulites’ Project, Japan Science and Technology Corporation, c r o Department of Physics, Meijo UniÕersity, 1-501 Shiogamaguchi, Tenpaku-ku, Nagoya 468-8502, Japan b NEC, 34 Miyukigaoka, Tukuba 305-8501, Japan c Department of Physics, Meijo UniÕersity, Nagoya 468-8502, Japan Received 6 April 2000; in final form 27 April 2000
Abstract Multi-walled carbon nanotubes with the reduced number of layers Ž2 or 3 layers. are dominantly produced by the laser ablation method using a carbon target mixed with boron. Chemical analysis with sub-nanometer resolution has revealed that the obtained nanotubes are composed of pure carbon layers with no boron concentration, while the boron carbide particle is found to be encapsulated at the nanotube tip. It is considered that the boron Žin the form of boron carbide. acts as a catalyst during the nanotube formation. A critical particle size of the boron carbide for the tubular growth is found to be ; 5 nm. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Carbon nanotube w1x is finding numerous applications. Especially it shows great potential as a storage medium since it may accommodate other atoms or molecules in its interlayer spaces andror central hollow. Therefore controlling the number of layers and the inner diameter is a key to this application. Recently diameter-controlled nanotube comprising single graphene layer Žsingle-walled carbon nanotubes. can be obtained in a certain amount by selecting the proper catalysts and changing the growth temperature w2–4x. However, controlling the number of layers of multi-walled carbon nanotubes has not ) Corresponding author: Fax q81-52-834-4001; e-mail: [email protected]
been achieved yet, except a case for the doublewalled boron nitride nanotube w5x. Like nickel, cobalt and iron that are the most famous catalysts for the carbon graphitization, boron is also known as one of the graphitization catalysts w6,7x. Adding boron has been shown to reduce the graphitization temperature w7x. The boron-catalyzed graphite is considered as a promising candidate for anodes in lithium battery w8x and attracts huge interests in the application viewpoint. However, the microstructure of the boron-catalyzed graphite has not yet been well understood, and neither has the catalytic behavior of the boron for the nanotube growth been reported so far. Here we demonstrate the production of the multiwalled carbon nanotubes by a laser ablation technique using boron as a catalyst. Distributions of the
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 5 6 7 - 4
K. Hirahara et al.r Chemical Physics Letters 324 (2000) 224–230
number of layers and the inner diameter derived from a statistic analysis performed on the produced nanotubes are different from those of the other metal-catalyzed carbon nanotubes. The carbon nanotubes with double- and triple-walled are selectively formed with the boron catalyst, while the other metal catalysts generally lead to the dominant production of the single-walled carbon nanotubes under the laser ablation condition. In order to reveal how the boron behaves as a catalyst during the nanotube growth, a detailed chemical analysis to trace the boron concentration in the produced nanotubes is made. The result may give new reflections upon the previous works reporting the boron-doped carbon nanotubes w9–13x in which the doping rate of substitutional boron into graphite layer have been claimed more than 5 or 10 at%.
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centimeter per minutes.. The soot deposited at down stream side of the furnace was collected and dispersed in ethanol by sonication. The suspension prepared in this way was dropped onto an electron microgrid for transmission electron microscopy ŽTEM. and then dried. Products were observed by a high-resolution TEM ŽJEOL JEM-2010F. operated at 120 kV. Elemental mapping and bonding state analysis were owing to dedicated scanning transmission electron microscopes ŽSTEM, VG-HB501 and HB601UX. equipped with parallel electron energy loss spectrometer ŽGatan PEELS 666 or DIGIPEELS., which provide a 0.5 nm probe with a high current flux. Elemental profiles are calculated from the absorption edge weights normalized for each element Žboron, carbon and nitrogen. that are involved in the EELS spectra sequentially recorded by scanning the probe at the area of interest Žsee details in Ref. w14x..
2. Experimental Samples were produced by a laser ablation method with a secondary harmonic Ž l s 532 nm. of a pulsed Nd:YAG laser w3x with a repetition rate of 10 Hz and a power density of ; 20 mJrpulse on the surface of the target. Targets for ablation were prepared by mixing a carbon powder Žkoujyundo-chemical, 99.9% with ; 5 mm in grain size. with either boron ŽB, rare-metallic, 99%., boron nitride ŽBN, nilaco, 99%. or boron carbide ŽB 4 C, koujyundo-chemical, 99% with ; 10 mm in grain size. 1 by ball milling for 15 h, and compressed at ; 3500 kgrcm2 to make 10 mm in diameter and ; 5 mm thick pellet. Wide range of boron atomic ratio Ž0.5–50 at% for boron. against C was employed for targets. Laser ablations were carried in the environment temperature range between 5008C and 12008C at a 500 Torr of Ar with a constant flow rate of 100 sccm Žstandard cubic
1 Use of any kind of these powders for the source of boron additives did not affect so much on the final products. Even in the case the BN was used for the starting material, the involved nitrogen was released during the ablation and no trace of the nitrogen was detected within the nanotubes or nanocapsules. However in using a B 4 C mixed target the amount of soot itself was comparatively lower than cases of B or BN. B 4 C has a high melting point, 24508C; therefore, in this case, it is considered that it was harder to vaporize while a laser irradiation.
Fig. 1. Low-magnification TEM images. Ža. Multi-walled carbon nanotubes found in the web-like deposit in the laser ablation experiment. Žb. Spherical nanocapsules involving BC x particles.
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3. Results Nanotubes are found in the web-like deposit together with nanocapsules when the mixed pellets are prepared with the boron atomic ratio to carbon between 0.5 and 30%. No nanotubes are found without the boron catalyst. The yield of nanotubes is maximal when the furnace temperature is set to 6008C. No web-like deposit is formed with the furnace temperatures below 5008C, neither above 9008C. There is no clear dependence of the yield, the diame-
ter and the length of the nanotubes on the furnace temperature between 5008C and 9008C. Fig. 1 shows low magnification TEM images for the typical products of the experiments described above. A region containing a number of nanotubes is shown in Fig. 1a. Besides the nanotubes, nanocapsules in roundly shape are also found and actually more abundant in the deposit as shown in Fig. 1b. These nanocapsules are made of several graphite layers and each of them involves a particle of boron carbide ŽBC x . in a wide range of carbon content
Fig. 2. Ža. A TEM image of a three-layer MWNT with a BC x particle at its tip. Žb. Chemical profiles across the tip Žalong the line x 1 –x 2 . indicate that the encapsulated nano-particle at the tip consists of the boron carbide. Žc. Chemical profiles across the graphitic layers of nanotube Žthe line y1 –y 2 . prove that the layers consist of the pure carbon and no boron concentration can be detected in the graphitic layers. Žd. An illustration of the proposed structure for the boron-catalyzed carbon nanotube.
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Fig. 3. Ža. A TEM image of a nanocapsule involving a BC x particle with a larger diameter. Žb. EELS chemical profiles across the nanoparticle suggest that the encapsulated boron carbide ŽBC x . within the graphite layers. Žc. The boron and carbon K near-edge structures recorded at the indicated points at a and b . Žd. Reference spectra for the pure amorphous boron, the stoicheometric B 4 C and the hexagonal BN.
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Ž0 - x - 0.25, from the pure boron upto the B 4 C. Ždiscussed later.. Fig. 2 shows results of EELS chemical analysis performed on one of the nanotubes present in the soot. A typical structure is shown in the Fig. 2a. It consists of three layers in the trunk, while the tip region is made of four layers. A nanometer-scale particle about 5 nm or below is encapsulated at the nanotube tip. Chemical profiles of boron and carbon across the tip of the nanotube are recorded along the indicated line x 1 –x 2 Žshown in Fig. 2b.. The carbon profile exhibits two peaks at the outside of the nanotube tip and a hump at the center, while the boron profile locates exclusively inside the tip. These profiles unambiguously prove that the outer layers of the tip are made of carbon and that the involved particle at the tip consists of boron and carbon Žboron carbide, BC x .. From numbers of analysis on the similar tip structures we conclude that the composition of the BC x particle is not uniform and ranges widely from the pure boron Ž x s 0. to the B 4 C Žthe carbon-saturated boron carbide, x s 0.25.. On the other hand, the body of the nanotube consists of pure carbon as shown in the line-profiles ŽFig. 2c. recorded along the indicated line y 1 –y 2 in the Fig. 2a. After numerous investigations of the obtained nanotubes, it is concluded that the boron can be found particularly at the tip and never detected in the body of nanotubes. Since the detection limit for the boron concentration of the present technique can be at most 0.5 ; 1.0 at%, the result suggests that the incorporation of boron into the graphite layer hardly occurs at least under the present experimental condition and therefore indicates that the estimated doping rate of the boron into the nanotube is nearly zero Ždiscussed later.. Fig. 2d illustrates a scheme for the typical structure of the boron-catalyzed nanotube derived from the analysis described above. Fig. 3a shows a TEM image of a nanocapsule which is the dominant product of the experiment. This nano-capsule is composed of about 10 graphite layers and encapsulates a crystalline particle, whose diameter is about 20 nm. Chemical analysis by means of EELS ŽFig. 3b. also indicates that the layers are made of pure carbon and that the crystalline particle is the boron carbide ŽBC x .. The EELS near-edge structure ŽFig. 3c. confirms that the encapsulated
particle is the boron carbide while the outer layers of the particle are made of graphite. The boron and carbon K-edges obtained from the encapsulated particle resemble to those for the B 4 C reference spectrum and are not likely to that for the pure amorphous boron ŽFig. 3d., suggesting that the encapsulated BC x particle is almost saturated with carbon, namely, the x is close to 0.25.
Fig. 4. Ža. Distribution of the number of walls for the produced carbon nanotubes by the present experiment. It shows that multiwalled carbon nanotubes consisting of 2 or 3 layers are selectively synthesized. Žb. Distribution of the inner diameter Ž2.1 nm in average., which is relatively large compared with the typical one of nanotubes by laser ablation Ž0.9–1.5 nm.. Žc. The size distribution of the boron carbide particle encapsulated at the nanotube tip ŽI. or in the spherical nano-particle ŽII.. Smaller boron carbide particle is indispensable for the nanotube formation. This critical size of the boron carbide particle is around 5 nm.
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Fig. 4 presents results of numerical analyses made on the produced nanotubes and nanocapsules. Statistics for the number of layers and the inner diameter of the nanotubes are given in Fig. 4a,b, respectively. The former indicates that the nanotubes with the reduced number of the layers are exclusively produced by the present method and the double- and triple-walled nanotubes are dominant by more than 75% frequency. The inner diameter of the nanotubes is 2.1 nm in average and obviously larger than the previously reported ones Ž0.9–1.5 nm for the typical metal catalyzed single-walled nanotubes. w2–4x. More intriguing is a distinction in the size distribution of the boron carbide ŽBC x . particle at the tip of the nanotubes and in the nanocapsules shown in Fig. 4c. While the mean size of the BC x particle encapsulated in the nanocapsules is around 5–11 nm, the typical size of the BC x particle at the nanotube tip is 2–5 nm. This clear difference in the size distribution confirms that the particle size of catalyst is an important factor for the nanotube growth.
4. Discussion The optimal furnace temperature for the nanotube growth with the boron catalyst during the laser ablation is around 6008C. It is considerably lower than that for the other metal catalyst Ž800–15008C for the typical metal-catalyzed laser ablation experiments.. So far, in such a low temperature range, multi-walled carbon nanotubes have not been produced by laser ablation method. This result seems to be consistent with the previous work reporting the substantially lower graphitization temperature when the boroncatalyst was used for the carbon graphitization w7x. The BC x particle is frequently found at the nanotube tip and the composition of the BC x particle widely ranges from the pure boron to the B 4 C Ž0 - x - 0.25.. This suggests that boron acts as a catalyst through the formation of the boron carbide during the nanotube growth. Therefore we infer the catalytic behavior of the boron as follows. At first, the target surface is heated to the high temperature momentarily by the laser shot and the boron carbide droplet is formed. Then it dissociates upon cooling and the pure carbon graphite layers segregate at the surface of the particle. Since the amount of carbon initially
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involved in a B 4 C particle can afford a nanotube of only a few nanometer length, the carbon supply should be continuously fed to the catalytic particles during the nanotube growth. The similar situation has been reported for the BNrC nanotube growth in which the HfB x metallic particle acts as a catalyst and the chemical composition widely ranges from the pure hafnium to the HfB 2 w15x. The wide-range solubility of the constituent within the catalytic particle might be essential for this kind of nanotube growth. Judging from the Fig. 4c, smaller boron carbide particle in diameter seems to be indispensable for the nanotube formation. A critical size of the particle leading to the nanotube growth is estimated around 5 nm. The boron carbide particles with a larger diameter than 5 nm lead to the formation of the nanocapsule in round shape. The smaller particles have larger curvatures on their surfaces. Therefore the graphite layers covering them have often hexagon network discontinuity, involving the bent and distorted regions. Such unstable places could act as growth nuclei for the one-dimensional growth of the graphite sheets. This could be the reason why the initial nanotube is formed at the surface of the small particle. If the boron particle size in the vapor phase can be well controlled at a few nanometer scale, it might be possible to obtain multi-walled carbon nanotubes with a uniform diameter in higher yield. The statistic analysis of the number of layers and the inner diameter of produced nanotubes presented in the Fig. 4a,b clearly demonstrates the distinct features of the boron-catalyzed carbon nanotubes. The reduced number of layers Ždominantly 2–3 layers. and the larger inner diameter ; 2.1 nm are unique to the boron-catalyzed nanotubes and have not been reported so far for the other laser ablated nanotubes with the metal catalysts. The specific features found for the boron-catalyzed nanotubes may be commonly expected in the boron-catalyzed graphite. The fact might be a clue to explain the reason why the boron-catalyzed graphite shows the good performance for the lithium battery anode w8x, i.e., this crude quality of the graphitization, the reduced number of the stacking layers and the large pores, can be beneficial for this application use. In the present work, no boron concentration can be detected within the graphitic layers of the nan-
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otubes and the boron Žin the form of carbide. is found to be segregated at the nanotube tip. This result contradicts the previous works reporting the boron-doped carbon nanotubes within the layers w9– 13x. The authors of these papers have claimed that the maximum doping rate of boron into the nanotubes are found to be 5 at% w11x or 10 at% w12,13x, or even 25 at% w9x, despite the solubility of boron into the graphite is limited to 2.35 at% w16x. From our viewpoint, their chemical compositions derived from single EELS spectrum are not sufficient to conclude the entire doping rate because the boron can be segregated at nanometer scale and may not be homogeneously distributed in the graphite layers w17–19x, which is more likely in the view of the present results. We do not deny the possibility of synthesizing the boron-doped carbon nanotubes, however, more careful analysis is indeed required to confirm the high doping rate of boron into the graphite layer and should be demonstrated more clearly.
5. Conclusion Multi-walled carbon nanotubes with the reduced number of layers are produced by laser ablation at a relatively lower growth temperature Žaround 6008C.. The results of the chemical analysis suggest that the boron additives act as a catalyst through the formation of boron carbide at the tip during the nanotube growth and that the doping the graphite layers with boron can never be realized under the present condition.
Acknowledgements This work is supported by an international cooperative research project ŽICORP. established be-
tween the Japan Science and Technology Corporation ŽJST. and Centre National de la Recherche Scientifique ŽCNRS.. K.S. thanks C. Colliex and M. Tence for their assistance in the STEM instrumentation. The authors are indebted to N. Fukushi for bringing the Ref. w8x to their attention. References w1x S. Iijima, Nature 354 Ž1991. 56. w2x S. Bandow, S. Asaka, Y. Saito, A.M. Rao, L. Grigorian, E. Richter, P.C. Eklund, Phys. Rev. Lett. 80 Ž1998. 3779. w3x O. Jost, A.A. Gorbunov, T. Pichler, R. Friedlein, M. Knupfer, M. Reibold, H.D. Bauer, L. Dunsch, M.S. Golden, J. Fink, Appl. Phys. Lett. 75 Ž1999. 2217. w4x H. Kataura, Y. Kumazawa, Y. Maniwa, Y. Ohtsuka, R. Sen, S. Suzuki, Y. Achiba, Carbon Ž2000. in press. w5x J. Cumings, A. Zettle, Chem. Phys. Lett. 316 Ž2000. 211. w6x H.N. Murty, D.L. Biederman, E.A. Heintz, Fuel 56 Ž1977. 305. w7x A. Oya, R. Yamashita, S. Otani, Fuel 58 Ž1979. 495. w8x M. Endo, C. Kim, K. Nishimura, T. Fujino, K. Miyashita, Carbon 38 Ž2000. 183. w9x Z. Weng-Sieh, K. Cherrey, N.G. Chopra, X. Blase, Y. Miyamoto, A. Rubio, M.L. Cohen, S.G. Louie, A. Zettle, R. Gronsky, Phys. Rev. B 51 Ž1995. 11229. w10x Ph. Redlich, J. Loeffler, P.M. Ajayan, J. Bill, F. Aldinger, M. Ruhle, Chem. Phys. Lett. 260 Ž1996. 465. w11x D.L. Carroll, Ph. Redlich, X. Blase, J.-C. Charlier, S. Curran, P.M. Ajayan, S. Roth, M. Ruhle, Phys. Rev. Lett. 81 Ž1998. 2332. w12x D. Golberg, Y. Bando, K. Kurashima, T. Sasaki, Appl. Phys. Lett. 72 Ž1998. 2108. w13x W. Han, Y. Bando, K. Kurashima, T. Sato, Chem. Phys. Lett. 299 Ž1999. 368. w14x K. Suenaga, C. Colliex, N. Demoncy, A. Loiseau, H. Pascard, F. Willaime, Science 278 Ž1997. 653. w15x K. Suenaga, F. Willaime, A. Loiseau, C. Colliex, Appl. Phys. A 68 Ž1999. 301. w16x C.E. Lowell, J. Am. Ceramic Soc. 50 Ž1967. 142. w17x O. Stephan, P.M. Ajayan, C. Colliex, Ph. Redlich, J.M. Lambert, P. Bernier, P. Lefin, Science 266 Ž1994. 1683. w18x Y. Zhang, H. Gu, K. Suenaga, S. Iijima, Chem. Phys. Lett. 279 Ž1997. 264. w19x V. Serin, R. Brydson, A. Scott, Y. Kihn, O. Abidate, B. Maquin, A. Derre, Carbon 38 Ž2000. 547.
Chemical Physics Letters 324 Ž2000. 224–230 www.elsevier.nlrlocatercplett
Boron-catalyzed multi-walled carbon nanotube growth with the reduced number of layers by laser ablation K. Hirahara a , K. Suenaga a
a,)
, S. Bandow a , S. Iijima
a,b,c
‘Nanotubulites’ Project, Japan Science and Technology Corporation, c r o Department of Physics, Meijo UniÕersity, 1-501 Shiogamaguchi, Tenpaku-ku, Nagoya 468-8502, Japan b NEC, 34 Miyukigaoka, Tukuba 305-8501, Japan c Department of Physics, Meijo UniÕersity, Nagoya 468-8502, Japan Received 6 April 2000; in final form 27 April 2000
Abstract Multi-walled carbon nanotubes with the reduced number of layers Ž2 or 3 layers. are dominantly produced by the laser ablation method using a carbon target mixed with boron. Chemical analysis with sub-nanometer resolution has revealed that the obtained nanotubes are composed of pure carbon layers with no boron concentration, while the boron carbide particle is found to be encapsulated at the nanotube tip. It is considered that the boron Žin the form of boron carbide. acts as a catalyst during the nanotube formation. A critical particle size of the boron carbide for the tubular growth is found to be ; 5 nm. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Carbon nanotube w1x is finding numerous applications. Especially it shows great potential as a storage medium since it may accommodate other atoms or molecules in its interlayer spaces andror central hollow. Therefore controlling the number of layers and the inner diameter is a key to this application. Recently diameter-controlled nanotube comprising single graphene layer Žsingle-walled carbon nanotubes. can be obtained in a certain amount by selecting the proper catalysts and changing the growth temperature w2–4x. However, controlling the number of layers of multi-walled carbon nanotubes has not ) Corresponding author: Fax q81-52-834-4001; e-mail: [email protected]
been achieved yet, except a case for the doublewalled boron nitride nanotube w5x. Like nickel, cobalt and iron that are the most famous catalysts for the carbon graphitization, boron is also known as one of the graphitization catalysts w6,7x. Adding boron has been shown to reduce the graphitization temperature w7x. The boron-catalyzed graphite is considered as a promising candidate for anodes in lithium battery w8x and attracts huge interests in the application viewpoint. However, the microstructure of the boron-catalyzed graphite has not yet been well understood, and neither has the catalytic behavior of the boron for the nanotube growth been reported so far. Here we demonstrate the production of the multiwalled carbon nanotubes by a laser ablation technique using boron as a catalyst. Distributions of the
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 5 6 7 - 4
K. Hirahara et al.r Chemical Physics Letters 324 (2000) 224–230
number of layers and the inner diameter derived from a statistic analysis performed on the produced nanotubes are different from those of the other metal-catalyzed carbon nanotubes. The carbon nanotubes with double- and triple-walled are selectively formed with the boron catalyst, while the other metal catalysts generally lead to the dominant production of the single-walled carbon nanotubes under the laser ablation condition. In order to reveal how the boron behaves as a catalyst during the nanotube growth, a detailed chemical analysis to trace the boron concentration in the produced nanotubes is made. The result may give new reflections upon the previous works reporting the boron-doped carbon nanotubes w9–13x in which the doping rate of substitutional boron into graphite layer have been claimed more than 5 or 10 at%.
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centimeter per minutes.. The soot deposited at down stream side of the furnace was collected and dispersed in ethanol by sonication. The suspension prepared in this way was dropped onto an electron microgrid for transmission electron microscopy ŽTEM. and then dried. Products were observed by a high-resolution TEM ŽJEOL JEM-2010F. operated at 120 kV. Elemental mapping and bonding state analysis were owing to dedicated scanning transmission electron microscopes ŽSTEM, VG-HB501 and HB601UX. equipped with parallel electron energy loss spectrometer ŽGatan PEELS 666 or DIGIPEELS., which provide a 0.5 nm probe with a high current flux. Elemental profiles are calculated from the absorption edge weights normalized for each element Žboron, carbon and nitrogen. that are involved in the EELS spectra sequentially recorded by scanning the probe at the area of interest Žsee details in Ref. w14x..
2. Experimental Samples were produced by a laser ablation method with a secondary harmonic Ž l s 532 nm. of a pulsed Nd:YAG laser w3x with a repetition rate of 10 Hz and a power density of ; 20 mJrpulse on the surface of the target. Targets for ablation were prepared by mixing a carbon powder Žkoujyundo-chemical, 99.9% with ; 5 mm in grain size. with either boron ŽB, rare-metallic, 99%., boron nitride ŽBN, nilaco, 99%. or boron carbide ŽB 4 C, koujyundo-chemical, 99% with ; 10 mm in grain size. 1 by ball milling for 15 h, and compressed at ; 3500 kgrcm2 to make 10 mm in diameter and ; 5 mm thick pellet. Wide range of boron atomic ratio Ž0.5–50 at% for boron. against C was employed for targets. Laser ablations were carried in the environment temperature range between 5008C and 12008C at a 500 Torr of Ar with a constant flow rate of 100 sccm Žstandard cubic
1 Use of any kind of these powders for the source of boron additives did not affect so much on the final products. Even in the case the BN was used for the starting material, the involved nitrogen was released during the ablation and no trace of the nitrogen was detected within the nanotubes or nanocapsules. However in using a B 4 C mixed target the amount of soot itself was comparatively lower than cases of B or BN. B 4 C has a high melting point, 24508C; therefore, in this case, it is considered that it was harder to vaporize while a laser irradiation.
Fig. 1. Low-magnification TEM images. Ža. Multi-walled carbon nanotubes found in the web-like deposit in the laser ablation experiment. Žb. Spherical nanocapsules involving BC x particles.
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3. Results Nanotubes are found in the web-like deposit together with nanocapsules when the mixed pellets are prepared with the boron atomic ratio to carbon between 0.5 and 30%. No nanotubes are found without the boron catalyst. The yield of nanotubes is maximal when the furnace temperature is set to 6008C. No web-like deposit is formed with the furnace temperatures below 5008C, neither above 9008C. There is no clear dependence of the yield, the diame-
ter and the length of the nanotubes on the furnace temperature between 5008C and 9008C. Fig. 1 shows low magnification TEM images for the typical products of the experiments described above. A region containing a number of nanotubes is shown in Fig. 1a. Besides the nanotubes, nanocapsules in roundly shape are also found and actually more abundant in the deposit as shown in Fig. 1b. These nanocapsules are made of several graphite layers and each of them involves a particle of boron carbide ŽBC x . in a wide range of carbon content
Fig. 2. Ža. A TEM image of a three-layer MWNT with a BC x particle at its tip. Žb. Chemical profiles across the tip Žalong the line x 1 –x 2 . indicate that the encapsulated nano-particle at the tip consists of the boron carbide. Žc. Chemical profiles across the graphitic layers of nanotube Žthe line y1 –y 2 . prove that the layers consist of the pure carbon and no boron concentration can be detected in the graphitic layers. Žd. An illustration of the proposed structure for the boron-catalyzed carbon nanotube.
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Fig. 3. Ža. A TEM image of a nanocapsule involving a BC x particle with a larger diameter. Žb. EELS chemical profiles across the nanoparticle suggest that the encapsulated boron carbide ŽBC x . within the graphite layers. Žc. The boron and carbon K near-edge structures recorded at the indicated points at a and b . Žd. Reference spectra for the pure amorphous boron, the stoicheometric B 4 C and the hexagonal BN.
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Ž0 - x - 0.25, from the pure boron upto the B 4 C. Ždiscussed later.. Fig. 2 shows results of EELS chemical analysis performed on one of the nanotubes present in the soot. A typical structure is shown in the Fig. 2a. It consists of three layers in the trunk, while the tip region is made of four layers. A nanometer-scale particle about 5 nm or below is encapsulated at the nanotube tip. Chemical profiles of boron and carbon across the tip of the nanotube are recorded along the indicated line x 1 –x 2 Žshown in Fig. 2b.. The carbon profile exhibits two peaks at the outside of the nanotube tip and a hump at the center, while the boron profile locates exclusively inside the tip. These profiles unambiguously prove that the outer layers of the tip are made of carbon and that the involved particle at the tip consists of boron and carbon Žboron carbide, BC x .. From numbers of analysis on the similar tip structures we conclude that the composition of the BC x particle is not uniform and ranges widely from the pure boron Ž x s 0. to the B 4 C Žthe carbon-saturated boron carbide, x s 0.25.. On the other hand, the body of the nanotube consists of pure carbon as shown in the line-profiles ŽFig. 2c. recorded along the indicated line y 1 –y 2 in the Fig. 2a. After numerous investigations of the obtained nanotubes, it is concluded that the boron can be found particularly at the tip and never detected in the body of nanotubes. Since the detection limit for the boron concentration of the present technique can be at most 0.5 ; 1.0 at%, the result suggests that the incorporation of boron into the graphite layer hardly occurs at least under the present experimental condition and therefore indicates that the estimated doping rate of the boron into the nanotube is nearly zero Ždiscussed later.. Fig. 2d illustrates a scheme for the typical structure of the boron-catalyzed nanotube derived from the analysis described above. Fig. 3a shows a TEM image of a nanocapsule which is the dominant product of the experiment. This nano-capsule is composed of about 10 graphite layers and encapsulates a crystalline particle, whose diameter is about 20 nm. Chemical analysis by means of EELS ŽFig. 3b. also indicates that the layers are made of pure carbon and that the crystalline particle is the boron carbide ŽBC x .. The EELS near-edge structure ŽFig. 3c. confirms that the encapsulated
particle is the boron carbide while the outer layers of the particle are made of graphite. The boron and carbon K-edges obtained from the encapsulated particle resemble to those for the B 4 C reference spectrum and are not likely to that for the pure amorphous boron ŽFig. 3d., suggesting that the encapsulated BC x particle is almost saturated with carbon, namely, the x is close to 0.25.
Fig. 4. Ža. Distribution of the number of walls for the produced carbon nanotubes by the present experiment. It shows that multiwalled carbon nanotubes consisting of 2 or 3 layers are selectively synthesized. Žb. Distribution of the inner diameter Ž2.1 nm in average., which is relatively large compared with the typical one of nanotubes by laser ablation Ž0.9–1.5 nm.. Žc. The size distribution of the boron carbide particle encapsulated at the nanotube tip ŽI. or in the spherical nano-particle ŽII.. Smaller boron carbide particle is indispensable for the nanotube formation. This critical size of the boron carbide particle is around 5 nm.
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Fig. 4 presents results of numerical analyses made on the produced nanotubes and nanocapsules. Statistics for the number of layers and the inner diameter of the nanotubes are given in Fig. 4a,b, respectively. The former indicates that the nanotubes with the reduced number of the layers are exclusively produced by the present method and the double- and triple-walled nanotubes are dominant by more than 75% frequency. The inner diameter of the nanotubes is 2.1 nm in average and obviously larger than the previously reported ones Ž0.9–1.5 nm for the typical metal catalyzed single-walled nanotubes. w2–4x. More intriguing is a distinction in the size distribution of the boron carbide ŽBC x . particle at the tip of the nanotubes and in the nanocapsules shown in Fig. 4c. While the mean size of the BC x particle encapsulated in the nanocapsules is around 5–11 nm, the typical size of the BC x particle at the nanotube tip is 2–5 nm. This clear difference in the size distribution confirms that the particle size of catalyst is an important factor for the nanotube growth.
4. Discussion The optimal furnace temperature for the nanotube growth with the boron catalyst during the laser ablation is around 6008C. It is considerably lower than that for the other metal catalyst Ž800–15008C for the typical metal-catalyzed laser ablation experiments.. So far, in such a low temperature range, multi-walled carbon nanotubes have not been produced by laser ablation method. This result seems to be consistent with the previous work reporting the substantially lower graphitization temperature when the boroncatalyst was used for the carbon graphitization w7x. The BC x particle is frequently found at the nanotube tip and the composition of the BC x particle widely ranges from the pure boron to the B 4 C Ž0 - x - 0.25.. This suggests that boron acts as a catalyst through the formation of the boron carbide during the nanotube growth. Therefore we infer the catalytic behavior of the boron as follows. At first, the target surface is heated to the high temperature momentarily by the laser shot and the boron carbide droplet is formed. Then it dissociates upon cooling and the pure carbon graphite layers segregate at the surface of the particle. Since the amount of carbon initially
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involved in a B 4 C particle can afford a nanotube of only a few nanometer length, the carbon supply should be continuously fed to the catalytic particles during the nanotube growth. The similar situation has been reported for the BNrC nanotube growth in which the HfB x metallic particle acts as a catalyst and the chemical composition widely ranges from the pure hafnium to the HfB 2 w15x. The wide-range solubility of the constituent within the catalytic particle might be essential for this kind of nanotube growth. Judging from the Fig. 4c, smaller boron carbide particle in diameter seems to be indispensable for the nanotube formation. A critical size of the particle leading to the nanotube growth is estimated around 5 nm. The boron carbide particles with a larger diameter than 5 nm lead to the formation of the nanocapsule in round shape. The smaller particles have larger curvatures on their surfaces. Therefore the graphite layers covering them have often hexagon network discontinuity, involving the bent and distorted regions. Such unstable places could act as growth nuclei for the one-dimensional growth of the graphite sheets. This could be the reason why the initial nanotube is formed at the surface of the small particle. If the boron particle size in the vapor phase can be well controlled at a few nanometer scale, it might be possible to obtain multi-walled carbon nanotubes with a uniform diameter in higher yield. The statistic analysis of the number of layers and the inner diameter of produced nanotubes presented in the Fig. 4a,b clearly demonstrates the distinct features of the boron-catalyzed carbon nanotubes. The reduced number of layers Ždominantly 2–3 layers. and the larger inner diameter ; 2.1 nm are unique to the boron-catalyzed nanotubes and have not been reported so far for the other laser ablated nanotubes with the metal catalysts. The specific features found for the boron-catalyzed nanotubes may be commonly expected in the boron-catalyzed graphite. The fact might be a clue to explain the reason why the boron-catalyzed graphite shows the good performance for the lithium battery anode w8x, i.e., this crude quality of the graphitization, the reduced number of the stacking layers and the large pores, can be beneficial for this application use. In the present work, no boron concentration can be detected within the graphitic layers of the nan-
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otubes and the boron Žin the form of carbide. is found to be segregated at the nanotube tip. This result contradicts the previous works reporting the boron-doped carbon nanotubes within the layers w9– 13x. The authors of these papers have claimed that the maximum doping rate of boron into the nanotubes are found to be 5 at% w11x or 10 at% w12,13x, or even 25 at% w9x, despite the solubility of boron into the graphite is limited to 2.35 at% w16x. From our viewpoint, their chemical compositions derived from single EELS spectrum are not sufficient to conclude the entire doping rate because the boron can be segregated at nanometer scale and may not be homogeneously distributed in the graphite layers w17–19x, which is more likely in the view of the present results. We do not deny the possibility of synthesizing the boron-doped carbon nanotubes, however, more careful analysis is indeed required to confirm the high doping rate of boron into the graphite layer and should be demonstrated more clearly.
5. Conclusion Multi-walled carbon nanotubes with the reduced number of layers are produced by laser ablation at a relatively lower growth temperature Žaround 6008C.. The results of the chemical analysis suggest that the boron additives act as a catalyst through the formation of boron carbide at the tip during the nanotube growth and that the doping the graphite layers with boron can never be realized under the present condition.
Acknowledgements This work is supported by an international cooperative research project ŽICORP. established be-
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