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J. Phys. Chem. B 1999, 103, 6224-6229
Mechanism of the Effect of NiCo, Ni and Co Catalysts on the Yield of Single-Wall Carbon Nanotubes Formed by Pulsed Nd:YAG Laser Ablation M. Yudasaka,*,† R. Yamada,† N. Sensui,† T. Wilkins,‡ T. Ichihashi,‡ and S. Iijima†,‡ Nanotubulites Project, ICORP-JST, c/o NEC Corporation, 34 Miyukigaoka, Tsukuba 305-8501, Ibaraki, Japan, and NEC Corporation, 34 Miyukigaoka, Tsukuba 305-8501, Ibaraki, Japan ReceiVed: March 10, 1999; In Final Form: May 13, 1999
We revealed that the yield of SWNTs formed by Nd:YAG laser ablation depends on the target composition with yields following the order CxNiyCoy > CxNiy . CxCoz. The SWNT bundles in the web formed when using the CxNiyCoy target (web-CxNiyCoy) is thicker and longer than those in the web-CxNiy. The diameters of the SWNTs in the web-CxNiyCoy were larger and more uniform than those of the SWNTs in the web-CxNiy. The NiCo particles in the web-CxNiyCoy and the Ni particles in the web-CxNiy were nanometer sized and were embedded in the amorphous carbon flakes that were dispersed throughout the weblike deposits. Filmlike deposits were formed when using the CxCoz targets, and nanometer-sized Co particles in these deposits were localized within sub-millimeter-sized areas. Examination of the target surfaces revealed that Ni emits from the CxNiy target more efficiently than NiCo from the CxNiyCoy target or Co from the CxCoz target during the laser ablation. On the basis of these results, we provide an explanation of how the yield and structure of SWNTs formed by laser ablation depend on the species of the metal catalysts.
Introduction The single-wall carbon nanotubes (SWNTs) found by Iijima1 are usually formed by either arc discharge2 or laser ablation.3 The physics4 and chemistry5 of SWNTs indicate the potential of SWNT materials in nanometer-scale-material science and applications. For this potential to be realized, though, the production of large amounts of SWNTs with high purity must be made possible and this has not yet been done. For largescale production to be realized, we believe the formation mechanism of SWNTs has to be made clear and have been conducting research with this goal.6-8 Whatever formation method is used, metal catalysts such as Ni are needed to form SWNTs. In SWNT formation by arc discharge or laser ablation, bimetal catalysts are used2,3 because such catalysts are believed to produce SWNTs more efficiently than monometal catalysts. However, the reason why this is so is not yet known. In this study, we have changed the composition of target composed of C, Ni, and Co, and we observed the influence of the metal catalysts on the SWNT formation to clarify why the NiCo bimetal catalyst increases the yield of SWNTs. Experimental The SWNTs were formed in a cylindrical chamber with an inside diameter of 3.6 cm and a length of 60 cm. The window for the Nd:YAG-laser beam inlet was a quartz-glass plate. A quartz-glass tube with an inside diameter of about 2.7 cm and a length of 50 cm was located at the center of the chamber, and a target was placed at the center of the tube. The target was a compressed powder that was a mixture of graphite, Ni, and Co particles. The target composition is described, for example, as CxNiyCoy indicating that the atomic percentages of C, Ni and Co were x, y, and y, respectively. The atomic percentages of C, Ni, and Co in the targets used are listed in Table 1. Before the * Corresponding author: E-mail:
[email protected]. † Nanotubulites Project. ‡ NEC Corporation.
laser ablation, the targets were heat-treated at 1470 K for 30 min. Sizes of the metal-particle aggregates in the heat-treated targets were 2-10 µm. A metal-particle aggregate in the CxNiyCoy target was a mixture of Ni and Co that had a Ni:Co atomic ratio of 0.4 to 1.3, as was determined by energy dispersion X-ray analyses (EDX). An electric furnace that surrounded the chamber raised the temperature at and around the target. Argon gas flowed through the chamber at 0.5 L/min, and the pressure inside the chamber was kept at about 600 Torr. The Nd:YAG-laser beam (wavelength, 532 nm; pulse width, 6-7 ns; frequency, 10 Hz; beam diameter, about 3.5 mm) irradiated the target surface perpendicularly usually for 60 s. The direction of the laser-beam irradiation was the same as that of the Ar gas flow. Experimental Results The laser ablation on the CxNiyCoy and CxNiy targets produced carbonaceous weblike deposits at the outlet of the inner quartzglass tube and/or carbonaceous films on the inner wall of the quartz-glass tube in front of and at the rear of the targets. The laser ablation of the CxCoz target also produced the carbonaceous films on the inner wall of the quartz-glass tube in front of and at the rear of the targets but produced almost no weblike deposits. The structures of these products are described below. Weblike Deposits. The quantities of web-like deposits obtained at various laser-beam intensities are shown in Figure 1 where most of the curves reach a maximum. The quantity of weblike deposits formed using the CxNiyCoy target (abbreviated as web-CxNiyCoy) exceeded that of the web-CxNiy. The webCxCoz quantity was zero except when the C91Co9 was laser ablated at 1 W. Raman spectra (excitation: argon ion laser, 488nm) of the web-C99.4Ni0.3Co0.3 deposits are shown in Figure 2. The peaks at 1592, 1570, and about 165 cm-1 are characteristic of SWNTs.9 A sharp peak ascribed to C60 is seen at 1468 cm-1, and a broad peak characteristic of amorphous carbon (a-C) appears at about 1350 cm-1.10 (Usually a-C shows two broad
10.1021/jp9908451 CCC: $18.00 © 1999 American Chemical Society Published on Web 07/13/1999
Single-Wall Carbon Nanotubes
J. Phys. Chem. B, Vol. 103, No. 30, 1999 6225
Figure 1. Quantities of obtained web-like deposits depending on the laser intensity and target compositions, (-2-) C99.7Ni0.15Co0.15, (-9-) C99.4Ni0.3Co0.3, (-b-) C98.8Ni0.6Co0.6, (-[-) C91Ni4.5Co4.5, (‚‚‚3‚‚‚) C99.85Ni0.15, (‚‚‚4‚‚‚) C99.7Ni0.3, and (‚‚‚0‚‚‚) C99.4Ni0.6.
Figure 2. Raman spectra (excitation: argon ion laser, 488nm) of weblike deposits formed by laser ablation using a C99.4Ni0.3Co0.3 target at laser-beam intensities of (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, and (f) 6 W.
peaks near 1350 and 1600 cm-1. In the Raman spectra shown in Figure 2, the 1600-cm-1 peak could not be seen clearly even though magnified because this peak overlaps with the foot of the 1592-cm-1 peak of SWNTs.) We estimated the height of the 1468-cm-1 peak due to C60 and the 1350-cm-1 peak due to a-C relative to that of the 1592-cm-1 peak due to SWNTs (If/ ISWNT, Ia-C/ISWNT) from the Raman spectra, for example, the peaks in Figure 2. These relative heights are shown in Figure 3. The values of If/ISWNT, and Ia-C/ISWNT for the web-C99.4Ni0.3Co0.3 (Figure 3a) decreased slightly when the laser-beam intensity increased from 0.5 to 3 or 4 W, then increased when the laser-beam intensity rose further. The other web-CxNiyCoy showed a similar tendency. The values of If/ISWNT, and Ia-C/ ISWNT for the web-CxNiy increased a little as the laser-beam intensity rose, as seen, for example, in Figure 3b. For each target, the optimum laser-beam intensity to obtain a large quantity of SWNTs with small quantity of a-C and C60 was determined by referring to the quantities of the weblike deposits (Figure 1) and the If/ISWNT and the Ia-C/ISWNT values (for example, Figure 3), which are listed in Table 1. We estimated the diameters of SWNTs from the positions of the peaks appearing near 160 cm-1.9 The diameters of SWNTs in the web-CxNiyCoy were little influenced by the laser-beam intensities as seen, for example, in Figure 2, and the estimated diameters were distributed over a narrow range that was similar for all the web-CxNiyCoz deposits (Table 1). The diameters of SWNTs in the web-CxNiy tended to be smaller than the diameters of SWNTs in the web-CxNiyCoy (Table 1). The diameters of SWNTs in the web-CxNiy depended on the target composition, the laser beam intensity, and the measurement location in the web-like deposit, and these diameters were dispersed over a wide range (Table 1). The SWNT diameters in the web-C91Co9 obtained at the laser-beam intensity of 1 W were close to those in the web-CxNiyCoy (Table 1). These trends would have to be confirmed by Raman spectra with several excitation wavelengths. We compared transmission electron microscope (TEM) images of the web-C98.8Ni0.6Co0.6 (Figure 4) and the web-C98.8-
Figure 3. Relative peak height of (-9-) the 1468-cm-1 peak of C60 and 1592-cm-1 peak of SWNTs (If/ISWNT) and that of (-O-) the 1350cm-1 peak of a-C and 1592-cm-1 peak of SWNTs (Ia-C/ISWNT) estimated from Raman spectra (excitation: argon ion laser, 488 nm) of weblike deposits formed by laser ablation at various intensities using (a) a C99.4Ni0.3Co0.3 and (b) a C99.4Ni0.6 target.
TABLE 1: Maximum Quantity (Mmax) of Web-Like Deposits and Laser-Beam Intensity (Imax) at Which the Maximum Was Attaineda x/y/z CxNiyCoz
Mmax (mg)
100/0/0 99.7/0.15/0.15 0.17 99.4/0.3/0.3 0.31 98.8/0.6/0.6 0.21 91/4.5/4.5 0.02 99.85/0.15/0 99.7/0.3/0 0.16 99.4/0.6/0 0.08 98.8/1.2/0 CxNiy . CxCoz. The SWNT bundles in the web-CxNiyCoy were thicker and longer than those in the web-CxNiy. The diameters of the SWNTs in the web-CxNiyCoy were larger and
more uniform than those of SWNTs in the web-CxNiy. The NiCo particles in the web-CxNiyCoy and the Ni particles in the webCxNiy were nanometer sized and embedded in the a-C flakes that were dispersed throughout the weblike deposits. The Co particles in the film-CxCoz deposited in front of and at the rear of the target were localized. During the laser ablation, the emission rate of Ni from the CxNiy target was higher than that of NiCo from the CxNiyCoy target or that of Co from the CxCoz target. Discussion and Conclusion Cluster-Catalyst Model for the SWNT Formation Mechanism.6,7 The SWNT growth models proposed so far are seaurchin (micrometer-scale metal-particle catalysts) growth model,11,12 one-metal atom catalyst growth model,13 and nanometer-scale metal-cluster catalyst growth model.3,6,7,14 Our series of studies on the formation mechanism of SWNTs by laser ablation indicate that the nanometer-scale metal-cluster catalyst growth is the most likely.6,7 The structures of the SWNTs, their bundles, the non-SWNT carbonaceous materials formed by laser ablation, and the target surface after the laser ablation depend on the ambient pressure, temperature, and laserbeam intensity.6,7 In our model of the SWNT formation by laser ablation using a CxNiyCoy target,6,7 graphite absorbs the laserbeam, heats up to 3000-4000 K,15,16 and transforms into molten C. Then the NiCo particles in the target gain heat from and form a solution with the molten C when the ambient pressure is high, for example, 600 Torr. Droplets of the molten C containing Ni and Co (C-NiCo droplets) are expelled from the target due to recoil pressure. (The target materials suddenly evaporate from the target surface due the laser-beam irradiation. Transfer of the evaporated materials is hindered by the ambient gas at atmospheric pressure, which causes the recoil pressure.17-19)
Single-Wall Carbon Nanotubes As the expelled droplets cool from 3000-4000 K to the ambient temperature of 1470 K, NiCo metal clusters with diameters of 1-2 nm segregate and catalyze the formation of SWNTs with diameters of 1-2 nm.6,7,14 This SWNT formation model also explains how the SWNT growth is influenced by the ambient temperature.7 When the ambient temperature falls from 1470 to 1170 K, the SWNT yield decreases, the SWNTs become shorter, and the SWNT diameters decrease and are distributed over a wider range. The low yield and the short length of the SWNTs are due to the low reactivity of C at temperatures below 1470 K, and the smaller and more widely distributed SWNT diameters would be caused by the rapid segregation of NiCo clusters in the molten droplets at the lower temperatures. Influence of Metal Species on SWNT Yields in Terms of the Nanometer-Scale Metal Cluster Model. The inhomogeneous distribution of Co in the film-CxCoz indicated that Co emits from the target surface without dissolving well in the molten C during the laser-beam irradiation (Figure 9a). On the other hand, the Ni or NiCo distribution in the web was more homogeneous indicating that the dissolution rate of Ni or NiCo in the molten C is higher than that of Co. Our examination of the target surface indicated the dissolution rate of Ni in the molten C is higher than that of NiCo or Co. From these, it is deduced that the dissolution rate of the metal in the molten C follows the order Ni > NiCo > Co. Since carbonaceous materials that had graphitic structure such as SWNTs or graphite shells appeared little in the films-CxCoz, we think that Co was not the active graphitization catalyst on C in our laser ablation experiments. The low dissolution rate of Co in C and/or the poor activity of Co as a graphitization catalyst compared to that of Ni, account for the extremely small yield of SWNTs by laser ablation. (The poor activity of Co is consistent with an earlier report that Co is a less active graphitization catalyst than Ni, and its use necessitates higher temperatures than with Ni to form graphite or multiwall carbon nanotubes (MWNTs).20) Since the dissolution rate of the metal in the molten C follows the order Ni > NiCo > Co, the segregation rate of metals would follow the order Co > NiCo > Ni. This means that the Ni cluster in the C-Ni droplets expelled from the target would segregate more slowly than the NiCo cluster in the C-NiCo droplets. The NiCo clusters with diameters of 1-2 nm segregate in the C-NiCo droplets at a fairy specific temperature, possibly about 1470 K, and SWNTs with a uniform diameter of about 1.31.4 nm grow efficiently (Figure 9b). At a lower temperature, the Ni clusters with diameters of 1-2 nm would segregate in the C-Ni droplet and the chemical reactivity of C and Ni would be reduced, leading to the reduced yield of SWNTs and the reduced length of the SWNTs (Figure 9c). We also think that the small but widely distributed diameters of the SWNTs formed using the CxNiy target are due to the segregation of the Ni clusters that occurrs outside of the 1470-K zone where there was a steep temperature decrease in our equipment. Due to the rapid drop in the temperature of the C-Ni droplets, the Ni cluster sizes would be small and not uniform (Figure 9c). This would lead to the formation of SWNTs with diameters that were small and not uniform. (These tendencies are similar to the influence of temperature on the diameter of SWNTs formed using CxNiyCoy targets at temperatures below 1470 K.8) When SWNTs grow from the C-Ni droplets emitting from the CxNiy target, temperatures would have to be low, so the a-C would not be fluid and would remain in the form of flakes.
J. Phys. Chem. B, Vol. 103, No. 30, 1999 6229 The morphologies of a-C in the weblike deposits support this explanation. When SWNTs grow from the C-NiCo droplets emitting from a CxNiyCoy target, the temperature would have to be high, therefore deformation of the a-C would be easier and could form the sheaths around the SWNTs in this case. To test the above explanation, we repeated our experiments using C98.8Fe1.2 targets. Fe dissolves in C and they remain a solid solution or carbide at room temperature.21 Therefore the segregation of the Fe clusters with diameters s of 1-2 nm in the C-Fe droplets expelled from the target during its cooling is unlikely, and SWNT formation cannot be expected (Figure 9d). As a matter of fact, no SWNTs were formed in our laser ablation experiments using the C98.8Fe1.2 targets. The C-Co and C-Ni phase diagrams are quite similar,21 but Ni and Co behaved quite differently in the laser ablation experiments as described above. The phase diagrams are applicable when the system is in an equilibrium or a quasiequilibrium state. The laser ablation process for SWNT formation, however, begins with the irradiation of a high-intensity laser beam with a pulse width of 6-7 ns and ends well before the next pulse in 0.1 s.8 Therefore, the metal behavior during the laser ablation process does not necessarily follow the thermodynamic phase diagram, and we think the dissolution rate of Ni and Co in C and/or the reaction rate of Ni and Co with C explicitly influence the SWNT production by laser ablation. These rates have a similar influence in chemical vapor deposition; that is, Ni enables the formation of graphite and MWNTs at a lower temperature than does Co.20 Acknowledgment. We thank Ms. Yohko Kasuya and Ms. Minfang Zhang for their help in the experiments. References and Notes (1) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (2) Journet, C.; Maser, W. K.; Bernier, P.; Loiseau, A.; Chapelle, M. L.; Lefrant, S.; Deniard, P.; Lee, R.; Fisher, J. E. Nature 1997, 388, 756. (3) Guo, T.; Nikolaev, P.; Thess, A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1995, 243, 49. (4) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical Properties of Carbon Nanotubes; Imperial College Press: London, 1998. (5) Wong, S. S.; Joselevich, E.; Woolley, A. T.; Cheung, C. L.; Lieber, C. M. Nature 1999, 394, 52. (6) Yudasaka, M.; Komatsu, T.; Ichihashi, T.; Achiba, Y.; Iijima, S. J. Phys. Chem. B 1998, 102, 4892. (7) Yudasaka, M.; Ichihashi, T.; Iijima, S. J. Phys. Chem. B 1998, 102, 10201. (8) Yudasaka, M.; Ichihashi, T.; Komatsu, T.; Iijima, S. Chem. Phys. Lett. 1999, 299, 91. (9) Rao, A.M.; Richter, E.; Bandoh, S.; Chase, B.; Eklund, P. C.; Williams, K. A.; Fang, S.; Subbaswamy, K. R.; Menon, M.; Thess, A.; Smalley, R. E.; Dresselhaus, G.; Dresselhaus, M. S. Science 1997, 275, 187. (10) Kinoshita, K. Carbon, Electrochemical and Physicochemical Properties, 3rd ed.; John Wiley Sons: New York, 1988; Chapter 2.2. (11) Ruoff, R. S.; Lorents, D. C.; Malhotra, R. Nature 1993, 366, 637. (12) Maiti, A.; Barbec, C. J.; Bernholc, J. Phys. ReV. B: Condens. Matter. 1997, 55, R6097. (13) Kiang, C. H.; Goddard, W. A.; Beyers, R.; Salem, J. R.; Bethune, D. S. Mater. Res. Soc. Symp. Proc. 1995, 359, 69. (14) Maiti, A.; Barbec, C. J.; Bernholc, J. Phys. ReV. B: Condens. Matter. 1995, 52, 14850. (15) Steinback, J.; Braunstein, G.; Dresselhaus, M. S.; Venkatesan, T.; Jacobson, D. C. J. Appl. Phys. 1988, 58, 4374. (16) Achiba, Y. Private communication. (17) Baldwin, J. M. J. Appl. Phys. 1973, 44, 3362. (18) Olstad, R. A.; Olander, D. R. J. Appl.Phys. 1975, 46, 1499. (19) Cheng, J.; Horwitz, J. MRS Bull. 1992, 18, 30. (20) Yudasaka, M.; Kikuchi, R. Supercarbon; Yoshimura, S., Chang, R. P. H., Ed.; Springer-Verlag: Heidelberg, 1998; p 99. (21) Massalski, T. B. Binary Alloy Phase Diagrams; American Society for Metals: Metal Park, OH 44073, 1986.
