Nanotubules

CATALYTIC PRODUCTION AND PURIFICATION OF NANOTUBULES HAVING FULLERENE-SCALE DIAMETERS... 

Abstract—Carbon nanotubules were produced in a large amount by catalytic decomposition of acetylene in the presence of various supported transition metal catalysts. The influence of different parameters such as the nature of the support, the size of active metal particles and the reaction conditions on the formation of nanotubules was studied. The process was optimized towards the production of nanotubules having the same diameters as the fullerene tubules obtained from the arc-discharge method. The separation of tubules from the substrate, their purification and opening were also investigated. 

Fig. 7. Graphite nanotubule on Co-SiOj with the fragments of amorphous carbon (arrowed) at the...

It is also important to point out that pure cobalt oxide, alone or finely dispersed in Si02 (i.e. Co-Si02, Co-Si02-l and Co-Si02-2 in Table 1), zeolite HY, fullerene (i.e. C q/C-,0 80/20) is at least as effective as the reduced oxides for the production of nanotubules in our experimental conditions. In fact, the catalysts studied in this work are also active if the hydrogenation step is not performed. This important point, is presently being investigated in our laboratory in order to elucidate the nature of the active catalyst (probably a metal carbide) for the production of nanotubules. 

Fig. 9. Carbon nanotubules on Co-Si02 (a) HREM image showing defects in tubules (b) helical tubules of various pitches between the straight tubules.

Fig. 10. H NMR spectra (a) coronene (b) Co-SiOj covered by carbon nanotubulcs (c) Co-SiOj covered by carbon nanotubules and evacuated to 10 torr for the NMR measurement (d) Co-SiO evacuated to 10 torr for the NMR measurement.

Fig. 11. The loss of carbon rapidly increases with the increase of temperature. Heating of the catalysts in open air for 30 minutes at 973 K leads to the total elimination of carbon from the surface. The gasification of amorphous carbon proceeds more rapidly than that of filaments. The tubules obtained after oxidation of carbon-deposited catalysts during 30 minutes at 873 K are almost free from amorphous carbon. The process of gasification of nanotubules on the surface of the catalyst is easier in comparison with the oxidation of nanotubes containing soot obtained by the arc-discharge method[28, 29]. This can be easily explained, in agreement with Ref [30], by the surface activation of oxygen of the gaseous phase on Co-Si02 catalyst.

For the physico-chemical measurements and practical utilisation in some cases the purification of nanotubules is necessary. In our particular case, purification means the separation of filaments from the substrate-silica support and Co particles. 

The carbon-containing catalyst was treated by ultra-sound (US) in acetone at different conditions. The power of US treatment, and the time and regime (constant or pulsed), were varied. Even the weakest treatments made it possible to extract the nanotubules from the catalyst. With the increase of the time and the power of treatment the amount of extracted carbon increased. However, we noticed limitations of this method of purification. The quantity of carbon species separated from the substrate was no more than 10% from all deposited carbon after the most powerful treatment. Moreover, the increase of power led to the partial destruction of silica grains, which were then extracted with the tubules. As a result, even in the optimal conditions the final product was never completely free of silica (Fig. 12). 

In this study we have shown that the catalytic method—carbon deposition during hydrocarbons conversion—can be widely used for nanotubule production methods. By variation of the catalysts and reaction conditions it is possible to optimize the process towards the preferred formation of hollow... 

Fig. 12. Carbon nanotubules after separation from the substrate by ultra-sound treatment. Note the Si02...

The characteristics of nanotubules obtained by catalytic reaction are better controlled than in the arc-discharge method. By varying the active particles on the surface of the catalyst the nanotubule diameters can be adjusted. The length of the tubules is... 

Carbon nanotube research was greatly stimulated by the initial report of observation of carbon tubules of nanometer dimensions[l] and the subsequent report on the observation of conditions for the synthesis of large quantities of nanotubes[2,3]. Since these early reports, much work has been done, and the results show basically that carbon nanotubes behave like rolled-up cylinders of graphene sheets of bonded carbon atoms, except that the tubule diameters in some cases are small enough to exhibit the effects of one-dimensional (ID) periodicity. In this article, we review simple aspects of the symmetry of carbon nanotubules (both monolayer and multilayer) and comment on the significance of symmetry for the unique properties predicted for carbon nanotubes because of their ID periodicity. 

As distinct from ihe ideal connection of Dunlap, we now describe the series of nanotubule knees (9 ,0)-(5m,5 ), with n an integer. We call this series the perfectly graphitizahle carbon nanotuhules because the difference of diameter between the two connected segments of each knee is constant for all knees of the series (Fig. 4). The two straight tubules connected to form the = 1 knee of that series are directly related to Cfio, the most perfect fullerene[15], as shown by the fact that the (9,0) tubule can be closed by 1/2 Qo cut at the equatorial plane perpendicular to its threefold rotation symmetry axis, while the (5,5) tubule can be closed by 1/2 Qo cut at the equatorial plane perpendicular to its fivefold rotation symmetry axis [Fig. 5(a)]. 

Fig. 5. Model structure of the (9,0)-(5,5) curved nanotubule ended by two half C, caps (a) and of the (12,0)-(7,7) curved nanotubule (b). A knee angle of 36" is observed in both models.

The inner (outer) diameter of the observed curved or coiled nanotubules produced by the catalytic method[8] varies from 20 to 100 A (150 to 200 A), which corresponds to the graphite layer order 3< <15 (Table 1). 

Model based on the variation of the active catalyst perimeter. To form the (5,5)-(9,0) knee represented in Fig. 13(c) on a single catalyst particle, the catalyst should start producing the (5,5) nanotubule of Fig. 13(a), form the knee, and afterwards the (9,0) nanotubule of Fig. 13(b), or vice versa. It is possible to establish relationships between... 

Fig. 13. Model of the growth of a nanotubule bonded to the catalyst surface, (a) Growth of a straight (5,5) nanotubule on a catalyst particle, with perimeter I5ak (b) growth of a straight (9,0) nanotubule on a catalyst particle whose perimeter is 18ak (k is a constant and the grey ellipsoids of (a) and (b) represent catalyst particles, the perimeters of which are equal to 5ak and 18a/t, respectively) (c) (5,5)-(9,0) knee, the two sides should grow optimally on catalyst particles having perimeters differing by ca. 20%.

When the active perimeter of the catalyst particle matches perfectly the values 5nak or 18nu/c (where n is the layer order, a is the side of the hexagon in graphite and k is a constant), the corresponding straight nanotubules (5n,5n) or (9n,0) will be produced, respectively [Fig. 13(a) and (b)]. 

Third, the units are inserted between the catalyst coordination sites and the growing nanotubule (Fig. 14). The last unit introduced will still be bonded to the catalyst coordination sites. From the catalyst surface, a new C2 unit will again displace the previous one, which becomes part of the growing tubule, and so on. 

The most exciting challenge is probably the preparation of BN nanostructures, including nanofibers and nanotubules, using the template-assisted PDCs route. Such an approach could allow us to control the morphology and size of the nanostructured BN materials to be incorporated into the BN matrix. This should significantly enhance the mechanical performance of the resulting composites compared to composites reinforced by BN microfibers. 

Halloysite Nanotubules, a Novel Substrate for the Controlled Delivery of Bioactive Molecules... 

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