Production of Large-Diameter Single-Wall Carbon Nanotubes by

SORST-JST, c/o NEC, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan, NEC, 34 Miyukigaoka,. Tsukuba, Ibaraki 305-8501, Japan, and Meijo UniVersity, ...
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J. Phys. Chem. B 2004, 108, 12757-12762

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Production of Large-Diameter Single-Wall Carbon Nanotubes by Adding Fe to a NiCo Catalyst in Laser Ablation Minfang Zhang,*,† Masako Yudasaka,†,‡ and Sumio Iijima†,‡,§ SORST-JST, c/o NEC, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan, NEC, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan, and Meijo UniVersity, Tempaku-ku, Nagoya 468-8502, Japan ReceiVed: March 5, 2004; In Final Form: June 7, 2004

We have found that the simple introduction of Fe into a conventional NiCo catalyst enables the production of single-wall carbon nanotubes (SWNTs) with controlled diameter distributions by laser ablation. The diameter of as-grown SWNTs can be made larger by increasing the atomic percentage of Fe in the target without changing any other synthesis parameters. We think that Fe helps increase the aggregation rate of metal in laser ablation and enlarges the crystal lattice constant of metal catalysts because of a change in the crystal structure from Ni to Ni3Fe, NiFe, and NiFe2, which then results in the enlargement of metal particles and the growth of larger-diameter SWNTs.

1. Introduction After more than 10 years of extensive research on carbon nanotubes, it is widely recognized that single-wall carbon nanotubes (SWNTs)1 have considerable potential as building blocks in future nanoscale electronics. Such applications require that SWNTs be produced with reasonably uniform properties and high quality. The physical and chemical properties of SWNTs depend strongly on their diameter and helicity.2-5 For example, SWNTs can be metallic or semiconducting, depending on their helicity, which is closely correlated with their diameters.2-5 Therefore, a primary goal in SWNT synthesis is to control the diameter distribution of as-grown SWNTs. So far, SWNTs have been produced by the methods of laser ablation,6,7 arc-discharge,8 and chemical vapor deposition (CVD).9,10 Several studies have shown that the diameters of SWNTs are proportional to the size of catalytic particles used in the CVD processes.11,12 However, the diameter distribution of SWNTs produced in this way is still broad. Although oxidation can narrow the diameter distribution of SWNTs,13,14 this treatment always causes defects in the SWNT structure and makes them chemically unstable.14 Compared to the CVD method, laser ablation produces SWNTs with a narrower diameter distribution that are also more chemically stable and freer of defects. The diameter distribution of SWNTs obtained by laser ablation is typically within 1.2-1.4 nm when the catalysts of Ni and Co are used. To produce SWNTs with different diameter distributions, especially larger diameters for transforming organic molecules, we need to improve the laser ablation synthesis method. It has been reported that the diameter distribution of SWNTs can be changed by adjusting parameters such as the process temperature, pressure, and laser power in laser ablation.15-19 However, such variations has always decreased the yield of nanotubes. Alternatively, the diameter distribution can be controlled by using different catalysts. It has been reported that Pd and Rh catalysts enabled the production of smaller-diameter * Corresponding author. E-mail: [email protected]. † SORST-JST. ‡ NEC. § Meijo University.

(about 0.8-1.2 nm) SWNTs, and Rh and Pt catalysts enabled diameters of 1.2-1.4 nm.20 The low SWNT yield, though, was still a problem. Another study showed that SWNTs with large diameters from 2 to 5.6 nm can be prepared by using a target of carbon rods doped with Co, Ni, and FeS in an atmosphere of Ar and H2.21 However, the mechanism leading to the synthesis of SWNTs with this diameter distribution is unclear. In our present work, we have tried to control the SWNT diameter distribution, without decreasing the yield, by altering the catalysts used in laser ablation. The idea is to add Fe to the conventional catalysts of Ni and Co for SWNT synthesis. We have found that different diameter distributions, for example, 1.2-1.45, 1.4-1.6, and 1.4-1.8 nm, can be obtained by simply adjusting the target composition. We also used bimetal catalysts of NiCo and NiFe with different ratios of Fe or Co to Ni to investigate the effect of Fe added to conventional catalysts of Ni and Co. Furthermore, we have studied the crystal structures of catalysts in the target and as-grown SWNTs to understand why SWNTs with relatively large diameters can be produced by adjusting the catalyst composition. 2. Experimental Section The targets for laser ablation were prepared by compressing mixtures of pure carbon (99.99% graphite) and nitrate metals of Ni(NO3)2‚6H2O, Co(NO3)2‚6H2O, and/or Fe(NO3)2‚9H2O. The atomic percentages of Ni and Co in each target were kept at 0.3%. The atomic percentage of Fe or Co was changed from 0 to 0.8% to study their effect on SWNT synthesis. The target was heat-treated by heating the mixture in argon from room temperature to 1200 °C at a rate of ∼10 °C/min and then keeping it at 1200 °C for 1 h before laser ablation. The laser power was about 25 W/cm2. The other processes and equipment used for the laser ablation were the same as previously reported.16,17,22,23 The abbreviations we used for the various targets and the as-grown SWNTs obtained from them are given in Table 1. For example, the as-grown SWNTs obtained from a target of C99.4Ni0.3Co0.3 without Fe are termed SWNTs (Fe 0%), and when the concentration of Fe in the target was 0.10.8 at. %, the as-grown SWNTs are termed SWNTs (Fe 0.1%), SWNTs (Fe 0.2%), ..., and SWNTs (Fe 0.8%), respectively.

10.1021/jp0490047 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/28/2004

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TABLE 1: Abbreviations of Various Targets and Their Obtained As-Grown SWNTs targets abbreviations CNiCoFe (0%) CNiCoFe (0.1%) CNiCoFe (0.2%) CNiCoFe (0.3%) CNiCoFe (0.4%) CNiCoFe (0.5%) CNiCoFe (0.6%) CNiCoFe (0.8%) CNi0.3 CNi0.3Fe0.3 CNi0.3Fe0.6 CNi0.3Fe0.8 CNi0.3Co0.6 CNi0.3Co0.8 CNi0.6Co0.3

as-grown SWNTs C, Ni, Co, and Fe (at. %)

C: C: C: C: C: C: C: C: C: C: C: C: C: C: C:

99.4, Ni: 99.3, Ni: 99.2, Ni: 99.1, Ni: 99.0, Ni: 98.9, Ni: 98.8, Ni: 98.6, Ni: 99.7, Ni: 99.4, Ni: 99.1, Ni: 98.9, Ni: 99.1, Ni: 98.9, Ni: 99.1, Ni:

0.3, Co: 0.3, Co: 0.3, Co: 0.3, Co: 0.3, Co: 0.3, Co: 0.3, Co: 0.3, Co: 0.3 0.3, Fe: 0.3, Fe: 0.3, Fe: 0.3, Co: 0.3, Co: 0.6, Co:

0.3 0.3, Fe: 0.3, Fe: 0.3, Fe: 0.3, Fe: 0.3, Fe: 0.3, Fe: 0.3, Fe: 0.3 0.6 0.8 0.6 0.8 0.3

We characterized the as-grown SWNTs by Raman spectroscopy, transmission electron microscopy (TEM), ultravioletvisible-near-infrared (UV-vis-NIR) absorption spectroscopy, and X-ray diffraction (XRD). Raman spectra were obtained with an NRS-2000 laser Raman spectrometer with a 514- and 488nm excitation of an Ar ion laser. The samples for TEM were prepared by directly dispersing as-grown SWNTs in hexane and dropping them onto perforated carbon grids. For the UV-visNIR observations, as-grown SWNTs were dispersed for 3 h in about 10 mL of D2O containing 0.5 wt % sodium dodecylbenzene sulfonate (NaDDBS) using a 400-W ultrasonic processor equipped with a titanium tip. The dispersions were then centrifuged at 200000g for 90 min, and the upper ∼2/3 volume of supernatant was collected and analyzed. This method is considered a good way to disperse SWNTs obtained by laser ablation.24 XRD was measured from as-grown SWNTs on a Si single-crystal substrate using an X-ray source of Cu KR with a conventional 2θ/θ scan. 3. Results 3.1. Quantities of SWNTs Obtained Using Various Targets. After laser ablation, as-grown SWNTs forming weblike deposits were produced near the outlet of the chamber. The quantities of as-grown SWNTs obtained when using various targets after laser ablation for 7 min are shown in Table 1. The quantities of as-grown SWNTs decreased only slightly when we increased the ratio of Fe to Ni and Co in the target. The quality and components of as-grown SWNTs were next investigated with respect to their Raman spectra and optical absorption spectra. 3.2. Raman Spectra of SWNTs Obtained with Various Targets. Raman spectra of as-grown SWNTs obtained using atomic percentages of Fe ranging from 0 to 0.8% with Ni (0.3%) and Co (0.3%) in the targets were measured with excitation wavelengths of 488 and 514.5 nm. Raman spectra (Figures 1 and 2) showed that all of the samples have characteristic peaks of SWNTs at about 1591 and 1567 cm-1 and radial breathing mode (RBM) peaks in the range from 100 to 200 cm-1.25 These Raman spectra also had extremely small peaks that are characteristic of C60 at 1470 cm-1 and of amorphous carbon at about 1350 cm-1.26 The Raman intensity ratio between the peaks at 1350 and 1592 cm-1 (D/G) is always used to gauge the amorphous carbon impurity in as-grown SWNTs. Figure 3 shows the values of D/G plotted as a function of the atomic percentage of Fe in the target. There was little change in D/G when the atomic percentage of Fe ranged from 0 to 0.5%, but it began to increase

0.1 0.2 0.3 0.4 0.5 0.6 0.8

abbreviations

weight (mg)

SWNTs (Fe 0%) SWNTs (Fe 0.1%) SWNTs (Fe 0.2%) SWNTs (Fe 0.3%) SWNTs (Fe 0.4%) SWNTs (Fe 0.5%) SWNTs (Fe 0.6%) SWNTs (Fe 0.8%) SWNTs (CNi0.3) NTs (CNi0.3Fe0.3) NTs (CNi0.3Fe0.6) NTs (CNi0.3Fe0.8) NTs (CNi0.3Co0.6) NTs (CNi0.3Co0.8) NTs (CNi0.6Co0.3)

5.28 6.28 4.86 5.53 4.52 5.10 4.78 2.57 2.11 5.76 4.22 4.31 5.18 4.87 4.27

Figure 1. Raman spectra (excitation wavelength of 488 nm) of asgrown SWNTs obtained using an atomic percentage of Fe ranging from 0 to 0.8% in the targets.

Figure 2. Raman spectra (excitation wavelength of 514 nm) of asgrown SWNTs obtained using an atomic percentage of Fe ranging from 0 to 0.8% in the targets.

when the atomic percentage of Fe was greater than 0.5%. This indicated that the SWNT yield was almost unchanged for an Fe concentration in the target of up to ∼0.5 at. % and decreased only slightly when the Fe concentration was greater than 0.5 at. %.

Production of Large-Diameter SWNTs

Figure 3. Raman intensity ratios between the peaks at 1350 and 1592 cm-1 (D/G), plotted as a function of the atomic percentage of Fe in targets of CNi0.3Co0.3.

The RBM frequency (ω) is related to the inverse diameter (Dt) of SWNTs, and SWNT diameters can be estimated by Dt ) 244/ω.25 Figure 2 shows the Raman spectra that were measured with a 488-nm excitation wavelength. When as-grown SWNTs were obtained from a target without Fe, the two peaks of the Raman spectra at 182 and 164 cm-1 corresponded to diameters of ∼1.34 and 1.49 nm, respectively. When 0.1 at. % Fe was added to the target, the Raman spectra of SWNTs (Fe 0.1%) showed that the RBM peaks appeared at the same position, but their intensities were different. The intensity of the peak at 182 cm-1 decreased, and the intensity of the peak at 164 cm-1 increased compared to those of SWNTs (Fe 0%). This indicated that the relative quantity of SWNTs with smaller diameters (about 1.34 nm) decreased, and those with larger diameters (about 1.49 nm) increased. When the concentration of Fe in the target increased to 0.2 and 0.3 at. %, the Raman spectra of SWNTs (Fe 0.2%) and SWNTs (Fe 0.3%), respectively, had only one peak at 164 cm-1, which corresponded to the 1.49-nm diameter SWNTs. When the concentration of Fe increased from 0.4 to 0.8 at. %, a new peak at 146 cm-1 appeared in the Raman spectra; this corresponded to SWNT diameters of ∼1.67 nm. These results indicate that the diameters of synthesized SWNTs increased as the atomic percentage of Fe in the target increased. Raman spectra obtained using a 514-nm excitation wavelength (Figure 2) also showed that the diameter distribution shifted toward larger diameters with an increase of Fe in the target. The diameter of SWNTs (Fe 0%) estimated from these Raman spectra was about 1.35 nm. When the atomic percentage of Fe in the targets was increased from 0.1 to 0.8%, the respective diameters of SWNTs (Fe 0.1%), SWNTs (Fe 0.3%), and SWNTs (Fe 0.8%) were 1.38, 1.51-1.65, and 1.51-1.81 nm, respectively. In addition, we kept the atomic percentage of Ni constant at 0.3% and varied the atomic percentage of Co or Fe from 0 to 0.8%. Raman spectra (Figure 4) indicated that increasing the atomic percentage of either Co or Fe can produce SWNTs with relatively large diameters, but the addition of Fe was more effective. When the total metal (Fe, Co, and Ni) percentage in the target was kept constant at 0.9 at. %, the Raman spectra showed that a larger atomic percentage of Fe produced largerdiameter SWNTs (Figure 5). All of the above results consistently demonstrated that the SWNT diameter increased when the atomic percentage of Fe in the target was increased relative to the Ni and Co content. 3.3. Optical Absorption Spectra. Figure 6 shows the UVvis-NIR absorption spectra of SWNTs obtained by using different concentrations of Fe with Ni (0.3%) and Co (0.3%) in

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Figure 4. Raman spectra (excitation wavelength of 514 nm) of asgrown SWNTs obtained when the atomic percentage of Fe or Co in the target was 0.8% and that of Ni was 0.3%.

Figure 5. Raman spectra (excitation wavelength of 514 nm) of asgrown SWNTs obtained when the total metal concentration in the target was 0.9%.

Figure 6. UV-vis-NIR absorption spectra of SWNTs (Fe 0%), SWNTs (Fe 0.2%), and SWNTs (Fe 0.3%).

the target. Three regions can be identified in Figure 5: the first interband transition for metals, M11 (600-800 nm), and two interband transitions for semiconductors, S11 (1200-1800 nm) and S22 (800-1200 nm). The spectra of SWNTs (Fe 0%), SWNTs (Fe 0.2%), and SWNTs (Fe 0.3%) show that the peak band in each interband transition is narrow, indicating that the SWNTs in these three samples have narrow diameter distribu-

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Figure 8. Diameter distribution of (a) as-grown SWNTs (Fe 0%), (b) as-grown SWNTs (Fe 0.3%), and (c) as-grown SWNTs (Fe 0.6%).

Figure 7. TEM images of (a) as-grown SWNTs (Fe 0%), (b) as-grown SWNTs (Fe 0.3%), and (c) as-grown SWNTs (Fe 0.6%).

tions. The spectrum of SWNTs (Fe 0%) showed that there was a peak at ∼1657 nm, a peak shoulder at 1556 nm in S11, and a peak at 940 nm in S22. Because the data obtained in our experiment did not completely match the calculation results, we estimated that the SWNTs have (12, 8) and (10, 9) structures based on the peaks in S11 and an (11, 9) structure based on the peak in S22.27 The diameters of the (12, 8), (10, 9), and (11, 9) tubes were 1.38, 1.30, and 1.37 nm, respectively, and their chiral angles were 23.4, 28.3, and 26.7°, respectively. The spectra of the as-grown SWNTs obtained with an Fe concentration of 0.20.3% showed that the absorption peaks had a red shift of ∼50 nm. One peak at 1702 nm and one peak shoulder at 1770 nm in S11 probably corresponded, respectively, to the (11, 10) and (12, 10) structures, and the peak at about 998 nm in S22 suggested that a structure of (13, 9) probably existed.27 The diameters of the (11, 10), (12, 10), and (13, 9) structures were 1.45, 1.51, and 1.52 nm, respectively, and the chiral angles of these structures were 28.4, 27.0, and 24.0°, respectively. The diameters of the metallic tubes were also estimated from the

peaks in the M11 region based on the study of Kataura et al.5 This showed that the diameters of the metallic tubes were 1.31 nm in a sample of as-deposited SWNTs (Fe 0%), whereas the diameters increased to 1.47 nm when the Fe content in the catalysts was increased to 0.2-0.3%. 3.4. TEM Observations. Panels a-c of Figure 7 show the typical TEM images of SWNTs (Fe 0%), SWNTs (Fe 0.3%), and SWNTs (Fe 0.6%), respectively. We measured the SWNT diameters directly from the TEM images. (This is a widely accepted method for estimating SWNT diameters.) Careful measurements of more than 50 sheet images for each sample (Figure 8) showed that the diameter distributions of SWNTs (Fe 0%), SWNTs (Fe 0.3%), and SWNTs (Fe 0.6%) were about 1.2-1.4, 1.4-1.6, and 1.45-1.8 nm, respectively. The average diameter clearly increased with an increase of the Fe concentration in the target. These results strongly supported the Raman spectra and UV-vis-NIR spectra results. 3.5. X-ray Diffraction. To investigate the effect of Fe on the synthesis of SWNTs, we measured the XRD of SWNTs to analyze the Fe, Ni, and Co structures. The 2θ range used to record the XRD profile was 35-58°, which is sensitive to the crystals of Ni, Co, and Fe. The XRD profiles of all of the samples (Figure 9) showed that the peak at about 54.51°, corresponding to the crystal faces of graphite (004) (graphite is the main impurity in as-grown SWNTs), did not change with the increased concentration of Fe. The XRD profile of SWNTs (Fe 0%) showed that there were two peaks at about 44.3 and 51.66 nm, corresponding to the Ni and Co crystal faces of (111) and (200), respectively. There were peaks at 44.12 and 51.40°, corresponding to the Ni3Fe crystal faces of (111) and (200), respectively, in the XRD profile of SWNTs (Fe 0.1%). The XRD profile of SWNTs (Fe 0.3%) showed peaks at 43.90 and 51.49° corresponding, respectively, to the diffraction of the NiFe crystal faces of (111) and (200). Peaks also appeared at 42.50 and 44.60°, which corresponded to NiCoO2 (200) and NiCo2O4 (400), respectively, when the atomic percentage of Fe increased to 0.1 and 0.3%. When the atomic percentage of Fe increased to 0.4-0.8%, peaks appeared in the XRD profile which were due to the diffraction of the NiFe2 crystal faces of (111) and (400). These results indicate that the respective crystal lattice

Production of Large-Diameter SWNTs

Figure 9. XRD results for SWNTs (Fe 0%), SWNTs (Fe 0.1%), SWNTs (Fe 0.3%), SWNTs (Fe 0.4%), SWNTs (Fe 0.5%), SWNTs (Fe 0.6%), and SWNTs (Fe 0.8%), labeling as Fe 0, Fe 0.1, ..., and Fe 0.8, respectively.

constants of the catalysts increased because of structural changes in the metals in as-grown SWNTs from Ni and/or Co to Ni3Fe, NiFe, and NiFe2 when the atomic percentage of Fe in the targets was increased from 0 to 0.4-0.8%. The XRD profiles of Fe, Ni, and Co in the targets (not shown) were very similar to those for the as-grown SWNTs (Figure 9). This indicated that the arrangement of the metals did not change during the process of the SWNT synthesis. 4. Discussion The Raman spectra, optical absorption spectra, and TEM measurements that we have obtained all indicate that the diameters of SWNTs can be increased by increasing the ratio of Fe to NiCo or Ni in the target. Most researchers who have produced SWNTs by laser ablation have always used the bimetallic catalysts of Ni and Co with the same atomic percentages of Ni and Co (e.g., Ni0.3Co0.3 or Ni0.6Co0.6). However, Kataura et al. found that they could obtain thinner SWNTs by changing the composition of the catalyst to Ni0.6Co0.3.20 Also, Lebedkin et al. used trimetallic catalysts of Ni, Co, and FeS to produce SWNTs by laser ablation in Ar and H2 and obtained SWNTs with diameters of up to 6 nm.21 (We have not observed such large diameters in our samples because we did not use S and H2 in our experiment.) These experimental results also suggest that changing the atomic percentage of Ni, Co, and Fe in the target is a simple way to control the diameters of SWNTs, and that large-diameter SWNTs can be obtained by increasing the ratio of Fe to Ni or NiCo. Many studies on the synthesis of SWNTs using CVD have demonstrated that the SWNT diameter depends on the size of the catalysts.11,12 However, as-grown SWNTs obtained by laser ablation are very long and always tangled with each other; therefore, no direct evidence has been found to clarify the relation between the SWNT diameter and the catalyst size. A series of studies on the laser ablation growth mechanism of SWNTs by our group16,17,22 have shown that the nanometerscale metal-cluster-catalyst model is highly consistent with the experimental results. In this model, graphite initially absorbs the laser beam, is heated to 3000-4000 K, and transforms into

J. Phys. Chem. B, Vol. 108, No. 34, 2004 12761 molten C. The metal particles in the target then gain heat and form a solution with the molten C. Droplets of the molten C containing metal catalysts (C-metal droplets) are expelled from the target due to the recoil pressure. As the expelled droplets cool from 3000-4000 K to the ambient temperature of 1470 K, metal clusters with diameters of 1-2 nm segregate and catalyze the formation of SWNTs with diameters of 1-2 nm.22 A reasonable conclusion is that the SWNT diameter is determined by the size of the segregated metal clusters. It has been reported that the dissolution rate of the metal in molten C follows the order Ni > NiCo > Co; therefore, the segregation rate of metals might follow the order Co > NiCo > Ni.28 Metals with a faster segregation rate should form larger metal particles for use in the SWNT growth. The phase diagram of FeNi shows that its melting point is close to that of NiCo.29 This suggests that the aggregation rate of NiCo and NiFe should also be similar. The aggregation rate with a metal and a bimetal catalyst (i.e., Fe and NiCo) will probably be faster than that for a NiFe or NiCo catalyst alone because of the increase of collision probability among metals. As a result, larger metal clusters will be formed. In addition, when the total metal percentage in the target was the same, increasing the Fe concentration was more effective than increasing the Co concentration with regard to producing larger-diameter SWNTs (Figures 4 and 5). Therefore, another possible explanation for the growth of larger-diameter SWNTs is related to the lattice constant of metals. On the basis of the XRD results, where the arrangement of metals in the target was the same as that in the as-grown SWNTs, we deduced that the arrangement of metals in the growth of SWNTs was also the same as that in as-grown SWNTs. XRD data indicated that the lattice constants of the metals increased as the atomic percentage of Fe increased. The increased lattice constant was due to the changes of the crystal structures from NiCo to Ni3Fe, NiFe, and NiFe2. We can also easily deduce that the lattice constant also determines the volume of metal clusters. Metal clusters with a larger lattice constant should be larger when the quantity of metal atoms in the cluster is the same, and this will cause the growth of larger-diameter SWNTs according to the nanometer-scale metal-cluster-catalyst model. Moreover, our optical absorption spectra (Figure 6) showed that the chiral angles of SWNTs obtained by laser ablation were typically close to 30°, indicating that armchair SWNTs might be more easily produced. XRD showed that only the crystallographic faces of (111) and (200) were detected in our experiment (Figure 9). A previous study28 pointed out that if a Ni, NiCo, or NiFe particle maintained a crystallographic face of (111) parallel to the SWNT wall, it would promote steady growth of the SWNT. Audier et al. reported that the preferred crystallographic orientation of metals to carbon affects the growth of multiwall carbon nanotubes,30 and we can reasonably infer that this might also be true for SWNT growth. Since most SWNTs obtained by laser ablation are the armchair type, we think that the crystallographic face of metals (111) parallel to the armchair graphite (200) might be a preferential case (Figure 10). Although further study is needed, we believe that the crystal structure of metals is related to the structure of SWNTs. 5. Conclusion We have found that SWNTs with relatively large diameters (for example, 1.4-1.6 and 1.4-1.8 nm) can be obtained by simply introducing Fe into the conventional CNiCo target used in laser ablation. Our experimental results have shown that the

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Figure 10. Scheme of the metal crystal face (111) parallel to the nanotube axis.

SWNT diameter could be increased by increasing the atomic percentage of Fe in the target, while keeping the other synthesis parameters unchanged. On the basis of previously reported research and X-ray diffraction profiles, we think that the enlargement of the crystal lattice constant and an increase in the aggregation rate of the metal cluster might be the two main factors in the production of large-diameter SWNTs. The relationship between the crystal structure of metal catalysts and the structure of SWNTs appears to be a very significant consideration in the structure-controlled synthesis of SWNTs. Acknowledgment. We thank Mr. Alan Maigne for his help in the TEM experiments. References and Notes (1) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (2) Mintmire, J. W.; Bunlap, B. L.; White, C. T. Phys. ReV. Lett. 1992, 68, 1579. (3) Saito, R.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Appl. Phys. Lett. 1992, 60, 2204. (4) Hamada, N.; Sawada, S.; Oshiyama, A. Phys. ReV. Lett. 1992, 68, 1579. (5) Kataura, H.; Kumazawa, Y.; Maniwa, Y.; Umezu, I.; Suzuki, S.; Ohtsuka, Y.; Achiba, Y. Synth. Met. 1999, 103, 2555. (6) Guo, T.; Nikolaev, P.; Thess, A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1995, 243, 49.

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