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Controlled production of aligned-nanotube bundles

Abstract

Carbon nanotubes1,2 might be usefully employed in nanometre-scale engineering and electronics. Electrical conductivity measurements on the bulk material3,4, on individual multi-walled5,6 and single-walled7 nanotubes and on bundles of single-walled nanotubes8,9 have revealed that they may behave as metallic, insulating or semiconducting nanowires, depending on the method of production—which controls the degree of graphitization, the helicity and the diameter. Measurements of Young's modulus show10 that single nanotubes are stiffer than commercial carbon fibres. Methods commonly used to generate nanotubes—carbon-arc discharge techniques1,2,4, catalytic pyrolysis of hydrocarbons11,12 and condensed-phase electrolysis13,14—generally suffer from the drawbacks that polyhedral particles are also formed and that the dimensions of the nanotubes are highly variable. Here we describe a method for generating aligned carbon nanotubes by pyrolysis of 2-amino-4,6-dichloro-s-triazine over thin films of a cobalt catalyst patterned on a silica substrate by laser etching. The use of a patterned catalyst apparently encourages the formation of aligned nanotubes. The method offers control over length (up to about 50 μm) and fairly uniform diameters (30–50 nm), as well as producing nanotubes in high yield, uncontaminated by polyhedral particles.

Main

In a typical experiment, a thin film of cobalt (10–100 nm) was deposited on a silica plate (1 mm thick, 5 mm wide and 20 mm long) using the following technique. A laser beam (Nd : YAG, wavelength 266 nm, 40 mJ per pulse, 10 Hz) was focused (spot size 2 mm outer diameter) on a rotating cobalt target (1.5 cm2, 0.25 mm thick) for 10–30 min under vacuum (1 × 10−6 torr). The silica plate, which was heated to 350 °C during the laser ablation process, was aligned 40 mm from and parallel to the target. Following ablation, the plate was exposed to air and etched with a single laser pulse (5 mJ) using cylindrical lenses (65 mm focal length) to create linear tracks (widths 1–20 μm, lengths 5 mm; Fig. 1)16.

Figure 1: SEM image showing uniform tracks etched by the laser beam.
figure 1

It is believed that along these channels/tracks uniform cobalt nanoparticles are deposited evenly by the laser striking over the thin film. Scale bar, 100 μm.

A sample of vacuum-dried 2-amino-4,6-dichloro-s-triazine (0.02–0.15 g), prepared from cyanuric chloride and ammonia15, was introduced into one end of a silica tube (6 mm outside diameter, length 60 cm), and the silica plate coated with etched cobalt was placed face downwards at the other end. The tube was inserted into a two-stage furnace17 fitted with independent temperature controllers (Fig. 2). Argon gas (20 cm3 min−1) was passed through the system and the temperature of the second furnace was set at 950 °C. The temperature of the first furnace was then raised at 15 °C min−1to 200 °C and then, by 100 °C increments (5 min per increment) to 1,000 °C. After 5 min at this temperature, the first furnace was allowed to cool to room temperature, but the second furnace was maintained at 950 °C for an additional 15 min in order to complete the annealing process. At this stage, the silica plate was covered with dark tracks visible to the naked eye; those areas where the cobalt film had not been laser-etched were transparent (see below).

Figure 2
figure 2

Pyrolysis device, illustrating the two-stage furnace system.

A series of silica plates, treated in this way, were coated with gold and examined by scanning electron microscopy, SEM (Leo 5420 operated at 20 keV). The black deposit on a second series of plates was removed by scraping, dispersed in acetone, and analysed by the following techniques: transmission electron microscopy (TEM, using a JEOL JEM 100CX at 100 keV), high resolution TEM (HRTEM, using a JEOL JEM 2010 at 200 keV and a JEM 4000 at 400 keV, and a Gatan GIF system operated at 200 kV with 0.3-eV dispersion for fine structure study) and energy dispersive X-ray analysis, EDX (using a NORAN Instruments detector attached to the latter microscope). Residues from the acetone exit bubblers (Fig. 2) were analysed by mass spectrometry, MS (VG autospec, electron impact, 70 eV).

SEM studies of the gold-coated plates revealed the presence of nanotube bundles closely aligned with the nanotracks (length 1–5 mm) created by laser etching. The tubes within these bundles were of uniform length (50 μm) and external diameter (30–50 nm; Fig. 3). The residual cobalt on the plate was removed, probably by the action of HCl, and was collected in the exit bubblers as cobalt chloride. We note that no traces of encapsulated or polyhedral particles were detected. Aligned-nanotube films also were observed in other etched regions.

Figure 3: SEM images of aligned nanotube bundles.
figure 3

a, Low-magnification of adjacent bundles in which nanotubes appear aligned. Scale bar, 100 μm. b, c, Higher magnification of one bundle, showing aligned nanotubes of uniform length (40 μm) and diameter (30–50 nm). Scale bars: in b, 10 μm; in c, 1 μm.

TEM and HRTEM observations (Fig. 4a, b) confirm the presence of multi-layered graphitic tubules (30–50 nm outer diameter, 60–80 layers). In most cases, cobalt (particles 50 nm diameter) was detected by EDX analysis within the nanotube tips. These particles appear to be responsible for the nanotube growth, but they were absent from a significant number (5%) of the closed end-caps. Occasionally, substantial sections of tubes were filled with cobalt. Analyses (using electron energy-loss spectroscopy, EELS) show that the nanotubes consist of pure carbon accompanied by traces of nitrogen (sp2hybridized carbon. A very broad weak feature at 400 eV may be due to nitrogen, generated during triazine decomposition and trapped inside the tubules. Both EELS and EDX measurements indicate that chlorine is entirely absent.

Figure 4: TEM image of a typical region filled with pure nanotubes dispersed by sonication.
figure 4

Encapsulated and polyhedral particles or other graphitic nanostructures are absent. Scale bar, 500 nm. b, HRTEM image showing good graphitization within the nanotubes (interlayer spacing 3.4 Å). c, Electron diffraction pattern from the group of nanotubes, exhibiting a collective feature. The tubes are grown in the direction indicated by the headed arrow. The centre of the first ring corresponds to 3.4 Å in real space, the rings revealing the presence of hexagonal graphite (armchair arrangement) which has an orientational relationship with the nanotubes. The walls of the tubes are parallel to (0001) reflections.

The deposition of a cobalt thin film on silica by laser ablation seems to be an efficient method for producing a uniform distribution of the metal catalyst. Subsequent laser-etching generates tracks or areas free of cobalt (Fig. 1), leaving cobalt particles evenly positioned along the edges of the eroded tracks or stripes16. These particles (50 nm), which exhibit a large surface/volume ratio, appear to be responsible for the carbon agglomeration and the unique nanotube growth behaviour reported here. An important factor in our experiments is the ratio of organic precursor (2-amino-4,6-dichloro-s-triazine) to cobalt. If this exceeds 2,700 : 1 (by weight) the cobalt is completely removed as cobalt chloride and the catalyst is destroyed.

It is not clear at this stage why the particle alignment occurs. The nanotubes appear to grow preferentially through the aligned cobalt crystals. Overcrowding may be responsible for simultaneous tube growth from the surface. The experiment was conducted with the cobalt catalyst on the lower (inverted) silica surface. No aligned tube growth occurred when the cobalt-coated/etched surface was on the upper face, so that gravitational effects may well be significant.

An electron diffraction pattern (Fig. 4c) recorded for a group of nanotubes reveals a highly ordered graphitic arrangement within the bundles, especially with respect to the (001) plane. In addition, the outer rings indicate the presence of hexagonal graphite, commensurate with a preferential direction for nanotube growth (related to a non-helical arrangement corresponding to an armchair configuration). These patterns are usually observed (20–30%) within analysed samples.

While this work was in progress a report appeared describing the large-scale synthesis of aligned carbon nanotubes by passage of acetylene over iron nanoparticles embedded in mesoporous silica18. Pyrolytic formation of nanotubes in high yield that are substantially free from pyrolytic carbon overcoatings have been reported by Hyperion Inc.19,20, but full details of the methods and product characterization have not been published, and the nanotubes do not appear to be aligned21.

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Acknowledgements

We thank J. Thorpe, D. P. Randall, S. Tehuacanero, R. Hernández, P. Mexía, R. Guardián and L. Rendón for providing electron microscope facilities, and D. Bernaerts for discussions. We thank CONACYT-México (M.T. and H.T.), the ORS scheme for scholarships (M.T. and W.K.H.), DGAPA-UNAM IN 107-296 (H.T.), EU-TMR grant (J.O.), the Royal Society (London) and the EPSRC for financial support.

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Correspondence to D. R. M. Walton.

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Terrones, M., Grobert, N., Olivares, J. et al. Controlled production of aligned-nanotube bundles. Nature 388, 52–55 (1997). https://doi.org/10.1038/40369

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