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19.2.5. Nanotube Synthesis

There are currently three main methods used to synthesize carbon nan-otubes: carbon arc synthesis, chemical vapor deposition, and ion bombardment, although some nanotube synthesis has been reported using flames [19.54,55].

Most of the work thus far reported has used the carbon arc method for the preparation of carbon nanotubes [19.2,3], which is similar to the carbon arc method for the synthesis of fullerenes (see §5.1). Typical synthesis conditions for nanotubes employ a dc current of 50-100 A and a voltage of 20-25 V operating in an inert (e.g., He) atmosphere (see Fig. 5.1). The magnitude of the current required should scale with the electrode diameter (i.e. larger currents are needed to vaporize larger electrodes); the same rule

Fig. 19.16. (a) The conelike tip of a carbon nanocone prepared by the vapor-growth method [19.17], (b) A structural model of a conical nanotube based on a rolled-up hexagonal network [19.53].

applies to the arc synthesis of fullerenes. Some workers report optimum results for an He pressure of ~500 torr while others use lower pressures [19.56]. Once the arc between the cathode and anode is struck, the rods (typically about 6-7 mm diameter for the translatable anode and 9-20 mm for the fixed cathode) are kept about 1 mm apart. The deposit forms at the rate of ~1 mm per minute on the larger negative electrode (cathode), while the smaller positive electrode (anode) is consumed. The nanotubes form only where the current flows. The inner region of the electrode, where the most copious tubule harvest is made, has an estimated temperature of 2500-3000°C. In this inner region of the electrode, columnar growth tex-

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Fig. 19.17. Schematic diagram of the fractal-like organization of the carbon nanotube bundles, from the largest bundles down to individual carbon nanotubes [19.3].

ture of nanotube bundles containing smaller bundles of nanotubes (see Fig. 19.17) has been reported by many workers [19.27,37,38]. The smallest bundle is the microbundle, which consists of 10 to 100 aligned nanotubes of nearly the same length [19.3,9]. Along with the tubule bundles is a growth of well-ordered carbon particles and other disordered carbonaceous material. The arc deposit typically consists of a hard gray outer shell (composed of fused nanotubes and nanoparticles) and a soft fibrous black core which contains about two thirds nanotubes and one third nanoparticles. Adequate cooling of the growth chamber is necessary to maximize the nanotube yield and their ordering. The growth of carbon tubules appears to be unfavorable under the conditions that are optimized to synthesize fullerene molecules.

The nanotubes can be separated from each other by sonication in solvents such as ethanol. A TEM picture of the core material of the carbon arc deposit is shown in Fig. 19.18(a), where both nanoparticles and nanotubes (20-200 A outer diameter, length ~1 /xm) can be seen. The separation of the nanoparticles from the nanotubes can be accomplished by burning

Fig. 19.18. TEM pictures of standard core material from the deposit (top) containing both nanotubes and nanoparticles and (bottom) purified nanotubes [19.9].

away the nanoparticles in oxygen while still leaving behind some of the nanotubes, which also vaporize by oxygen treatment but at a slower rate than the nanoparticles [19.9]. The oxygen-burning technique which is valuable for the elimination of nanoparticles also tends to eliminate most of the nanotubes, so that only about 1% of the initial deposit remains after the oxygen treatment.

The synthesis of single-shell nanotubes utilizes a variant of the carbon arc method. In this case, a hole is made in the carbon anode (6 mm diameter), which is filled with a composite mixture of transition metal material (Co, Fe, or Ni) [19.31,33,57-59] and graphite powder, while the 6-mm-diameter cathode rod is pure carbon. The transition metal serves as a catalyst and yields single shell tubules of small diameter with a narrow size distribution (see §19.2.2). Best results for the Co-catalyzed tubules [see Fig. 19.6(a)] were obtained when the Co-graphite mixture had a 4% Co stoichiometry. Typical operation conditions for the arc are a dc current of 95-115 A, a voltage of 20-25 V, with 300-500 torr He gas, and a flow rate of 5-15 ml/s [19.31], For the Fe-catalyzed single-wall nanotubes, two vertical electrodes were used, and Fe filings were inserted into a cup-shaped indentation in the lower-lying cathode electrode (20 mm diameter). The arc was operated between the cathode and a 10 mm diameter anode at a dc current of 200 A at 20 V, and a gas mixture of 10 torr methane and 40 torr argon was used [19.33]. In this system the Fe and carbon were vaporized simultaneously. The single-wall tubules are found in web-like rubbery sooty deposits on the walls of the evaporation chamber and away from the electrodes. In the arc process, the iron or cobalt catalyst also forms nanometer-size carbide particles, around which graphene layers form, as well as metal clusters encapsulated within graphene layers [19.31,33]. Single-wall carbon tubules are seen to grow out from the carbon nanoparticles.

The second synthesis method for carbon nanotubes is from the vapor phase and utilizes the same apparatus as is used for the preparation of vapor-grown carbon fibers (see §2.5), with the furnace temperature also held at 1100°C, but with a much lower benzene gas pressure [19.17,21,32,60]. Carbon nanotubes can grow at the same time as conventional vapor-grown carbon fibers, as is seen in Fig. 19.4. Vapor-grown carbon nanotubes also grow in bundles. These bundles have been studied by high-resolution TEM in both their as-grown form and after heat treatment in argon at 2500-3000° C. The as-grown nanotubes generally show poor crystallinity. The crystallinity, however, is much improved after heat treatment to 2500-3000° C in argon, as seen in high-resolution TEM studies [19.17]. On the basis of the very large difference in the diameter of the hollow core between typical vapor-grown carbon fibers and carbon nanotubes and the appearance of internal bamboo-shaped structures [see Fig. 19.12(a)], it is suggested that the growth mechanism for the nanotubes may be different from that of the vapor-grown carbon fibers. Referring to the bamboo structure of Fig. 19.19 for carbon nanotubes, Endo [19.17] argues that the capping off of an inner layer terminates its growth, so that the exposed cap layer provides growth along the length and the epitaxial layers follow this growth while at the same

Fig. 19.19. (a) Transmission electron micrograph of a cone containing only single conical shells. The nearly periodic structures of the conical shells appear inside the cone tips [see part (a)], which are attributed to overshooting growth on the basis of the open tube growth model [19.37]. (b) Commonly observed nanotube structure for the cap region of vapor-grown carbon nanotubes heal treated at 2800°C in Ar. Here a number of bamboo-like structures are observed in the core region near the cap [19.17],

Fig. 19.19. (a) Transmission electron micrograph of a cone containing only single conical shells. The nearly periodic structures of the conical shells appear inside the cone tips [see part (a)], which are attributed to overshooting growth on the basis of the open tube growth model [19.37]. (b) Commonly observed nanotube structure for the cap region of vapor-grown carbon nanotubes heal treated at 2800°C in Ar. Here a number of bamboo-like structures are observed in the core region near the cap [19.17], time adding to the tubule diameter. Bending of the growth axis of the nanotubes has been reported by a number of workers [19.17,37] and is related to the introduction of a heptagon-pentagon defect pair at the bend location.

A third method of nanotube synthesis relates to the use of carbon ion bombardment to make carbon whiskers [19.61,62], In this method, carbon is vaporized in vacuum using either an electron beam [19.24] or resistive heating, and the deposit is collected on a cold surface. The deposit contains carbon nanotubes, along with other structures. Of the three techniques, less is known about the optimization of the ion bombardment technique as well as the characteristics of the material that is prepared. With all three methods, sample characterization and separation of the nanotubes from other structures remain challenges for workers in the field.

A totally different approach to the physical realization of single-wall carbon nanotubes has been through oxidation of capped multiwall nanotubes in C02 at ~850°C [19.63] and in air in the 700-800°C range [19.64], much higher than the temperature range for oxidation of C50 (500-600° C). Carbon nanoparticles and graphite both oxidize at higher temperatures than nanotubes, presumably due to the curvature of the tubes and the consequent presence of lattice strain. In fact, amorphous carbon and poorly ordered carbons are removed at even lower temperatures than any of the ordered structures, because of their weaker carbon-carbon bonding. Thus oxidation at elevated temperatures can be used to clean up a carbon tubule sample, removing amorphous carbon and carbon onions (see §19.10) [19.9,65]. Both of these studies show that the cap is more reactive and is etched away first. Much slower layer-by-layer removal of the cylindrical layers follows, until eventually a region of single-wall tubules is seen under TEM imaging [19.63,64], An increase in the surface area by ~50% was observed in the oxidized tubules, which may be due to a reduction in average tubule diameter, and an increased accessibility of the inner tubule surface to probing gas molecules, after the cap has been removed and the tube is opened [19.63], The oxidation reaction is thermally activated with an energy barrier of 225 kJ/mol in air [19.64], When the carbon nanotubes open up, carbonaceous material may be sucked up the tube.

Tube opening can also occur at lower temperatures (~400°C), using, for example, a lead metal catalyst in air [19.66]. Lead metal in the absence of air is not sucked up the tubule, from which it is concluded that the material that is sucked up is likely an oxide of lead [19.9].

The stability of a carbon nanotube relative to a graphene sheet or a carbon onion (see §19.10) has been widely discussed [19.67-71]. For small clusters of carbon atoms, closed surfaces or closed rings are preferred to reduce dangling bonds, thus favoring fullerenes and onions. The growth geometry is also believed to be involved in the stabilization of carbon tubules, including both kinetic and space-filling considerations. In this context, the diameter of the smallest tubule has been calculated by balancing the energy gained by stitching together the dangling bonds along the generator element of the tubule cylinder with the energy lost by the strain in bending the graphene sheet to form the cylinder [19.68,69,72]. The diameter which results from these estimates is 6.78 A, which is very close to the diameter of C60.

19.2.6. Alignment of Nanotubes

For a variety of experiments and applications it is desirable to align the carbon nanotubes parallel to each other. Two approaches to nanotube alignment have been reported [19.73,74]. In the first method, the nanotubes are dispersed in a polymer resin matrix to form a composite, which is then sliced with a knife edge, causing the nanotubes to align preferentially along the direction of the cut [19.73]. In the second method nanotube films are prepared by dispersing the deposit on the cathode of the carbon arc in ethanol to prepare a suspension which is then passed through a 0.2 /¿m pore ceramic filter. The deposit left in the filter is transferred to a Del-rin or Teflon surface by pressing the tube-coated side of the filter on the surface. After lifting the filter, the tubules remain attached to the surface. The surface is then lightly rubbed with a thin Teflon sheet or aluminum foil to produce tubules on the surface aligned in the direction of the rubbing [19.74], Ellipsometry and resistivity measurements show large anisotropics, with maximum conduction in the tubule direction (aN), and lower but different values of conductivity normal to but within the plane (aL) and normal to the plane surface (/3).

High-resolution STM studies of tubule bundles show that all the outer shells of the tubes in nanotube bundles are broken, suggesting that the nanotubes are strongly coupled through the outer shell of the bundles [19.15]. The inner tubes of the bundles, however, were not disturbed, indicating stronger intratube interaction in comparison with the intertube interaction. After a tubule of a certain diameter is reached it may be energetically favorable to grow adjacent tubules, leading to the generation of tubule bundles [19.15],

19.3. Growth Mechanism

The growth mechanism for cylindrical fullerene tubules is especially interesting and has been hotly debated. One school of thought [19.35,75] assumes that the tubules are always capped (see §19.2.3) and that the growth mechanism involves a C2 absorption process that is assisted by the pentagonal defects on the caps. The second school [19.37,51,76] assumes that the tubules are open during the growth process and that carbon atoms are added at the open ends of the tubules. Since the experimental conditions for forming carbon nanotubes vary significantly according to growth method, more than one mechanism may be operative in producing carbon nanotubule growth.

The first school of thought focuses on tubule growth at relatively low temperatures (~1100°C) and assumes that growth is nucleated at active sites of a vapor-grown carbon fiber of about 1000 A diameter. Although the parent vapor-grown carbon fiber is itself nucleated by a catalytic transition metal particle [19.32], the growth of the carbon nanotube is thought to be associated with the absorption of a C2 dimer near a pentagon in the cap of the tubule. Referring to the basic model for C2 absorption in Fig. 6.3(b), we see that sequential addition of C2 dimers results in the addition of a row of hexagons to the carbon tubule. To apply the C2 absorption mechanism described in §6.1, it is usually necessary to use the Stone-Wales mechanism to bring the pentagons into their canonical positions, as necessary for the execution of each C2 absorption, in accordance with Figs. 6.2(b) and 19.20. For example, in Fig. 19.20(b) the pentagonal defect labeled 2 is the active pentagon for C2 absorption, but after one C2 dimer absorption has occurred, the active pentagon becomes site 3, as shown in Fig. 19.20(c). Thus, Fig. 19.20 shows a sequence of five C2 additions, which result in the addition of one row of hexagons to a carbon nanotube based on a C60 cap. [19.17].

For the growth of carbon nanotubes by the arc discharge method, it has been proposed that the tubules grow at their open ends [19.51,67], If the tubule has chirality [see Fig. 19.21(a)], it is easily seen that the absorption of a single C2 dimer at the active dangling bond edge site will add one hexagon to the open end. Thus the sequential addition of C2 dimers will result in continuous growth of the chiral tubule. If carbon atoms should be added out of sequence, then addition of a C2 dimer would result in the addition of a pentagon, which could lead to capping of the tubule, while the addition of a C3 trimer out of sequence as shown in Fig. 19.21(a) merely adds a hexagon. In the case of an armchair edge, here again a single C2 dimer will add a hexagon, as shown in Fig. 19.21(b). Multiple additions of C2 dimers lead to multiple additions of hexagons to the armchair edge as shown in Fig. 19.21(b). Finally, for the case of a zigzag edge, initiation of growth requires one trimer C3 [see Fig. 19.21(c)], which then provides the necessary edge site to complete one row of growth for the tubule through the addition of C2 dimers, except for the last hexagon in the row, which requires only a C, monomer. If, however, a C2 dimer is initially bonded at a zigzag edge, it will form a pentagon. Because of the curvature that is introduced by the pentagon, the open end of the tubule will likely form a cap, and growth of the tubule by the open end process will be terminated.

A schematic diagram for the open tube growth method is shown in Fig. 19.22 [19.37], While the tubes grow along the length, they also grow in diameter by an epitaxial growth process, as shown in Fig. 19.22. The large aspect ratio of the tubules implies that growth along the tube axis is more likely than growth along the tubule diameter. Referring to Fig. 19.19, Iijima argues that the inner tubes are capped first with the capping providing a

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Crystal Growth of A Chiral Fiber

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