Page 33 - Carbon Nanotubes

et
24 V. IVANOV al.

Fig. 11. Tips of carbon nanotubules grown on Co- SiO, (acetylene reaction at 973 K, 30 minutes after
oxidation in air for 30 minutes at 873 , K: (a) low magnification; (b) HREM.

Fig. 11. The loss of carbon rapidly increases with the bules could be preferable in comparison with oxida-
increase of temperature. Heating of the catalysts in tion, because of the easier control in the former case.
open air for 30 minutes at 973 K leads to the total
elimination of carbon from the surface. The gasifica- 3.5 Product purijication
tion of amorphous carbon proceeds more rapidly For the physico-chemical measurements and prac-
than that of filaments. The tubules obtained after tical utilisation in some cases the purification of
oxidation of carbon-deposited catalysts during 30 nanotubules is necessary. In our particular case,
minutes at 873 K are almost free from amorphous purification means the separation of filaments from
carbon. The process of gasification of nanotubules the substrate-silica support and Co particles.
on the surface of the catalyst is easier in comparison The carbon-containing catalyst was treated by
with the oxidation of nanotubes containing soot ultra-sound (US) in acetone at different conditions.
obtained by the arc-discharge method C28, 291. This The power of US treatment, and the time and regime
can be easily explained, in agreement with Ref. [30], (constant or pulsed), were varied. Even the weakest
by the surface activation of oxygen of the gaseous treatments made it possible to extract the nanotubules
phase on Co-SiO, catalyst. from the catalyst. With the increase of the time and
The gasification of graphite layers proceeds more the power of treatment the amount of extracted
easily at the tips of the tubules and at structural carbon increased. However, we noticed limitations of
defects. Typical images of the tips of catalytically this method of purification. The quantity of carbon
produced tubules after treatment in air are presented species separated from the substrate was no more
in Fig. 11. On graphite tubules grown from Co-SiO, than 10% from all deposited carbon after the most
catalyst, two types of tip were usually observed. In powerful treatment. Moreover, the increase of power
the first, the tubules are closed by graphite layers led to the partial destruction of silica grains, which
with the metal particle inside the tubules (Fig. 4(a)). were then extracted with the tubules. As a result,
In the second type, more generally observed, the even in the optimal conditions the final product was
tubules are closed with amorphous carbon. The open- never completely free of silica (Fig. 12).
ing of tubules during oxidation could proceed on For better purification, the tubule-containing cata-
both types of tip. lyst was treated by HF (40%) over 72 hours. The
The gasification of carbon filaments by high- resulting extract was purer than that obtained after
temperature hydrogen treatment was postulated as US treatment. The addition of nitric acid also makes
involving the activation of hydrogen on the metal it possible to free the tubules of metal particles on
surfaceC31-331. We observed a very slight effect of the external surface. The conditions of the acid treat-
catalyst hydrogenation, which was visible only after ment and tubule extraction have yet to be optimized.
the treatment of carbon-deposited catalyst for 5 hours
at 1173 K. We suppose that the activation of
hydrogen in our case could proceed on the non- 4. CONCLUSIONS
covered centers of Co or, at very high temperatures, In this study we have shown that the catalytic
it could be thermal dissociation on the graphite method-carbon deposition during hydrocarbons
surface layers of tubules. The result was similar to conversion-can be widely used for nanotubule pro-
that of oxidation but the process proceeded much duction methods. By variation of the catalysts and
slower. We called it “gentle” gasification and we reaction conditions it is possible to optimize the
believe that this method of thinning of the nanotu- process towards the preferred formation of hollow