Page 174 - Carbon Nanotubes

Onion-like graphitic particles 165

is an example of the graphitization of a "hard carbon"
(when subjected to heat treatment, it yields irregularly
shaped pores or particles instead of extended flat gra-
phitic planes)[14,21].
In the case of carbon melting experiments[l6] and
electric arc[5,6], it has been suggested that the onion-
like particles are generated by the graphitization of a
liquid carbon drop. The growth of graphite layers is
supposed to begin at the surface and progress toward
the center (see Fig. 3a-d). Saito et d[7] has suggested
a similar mechanism but, instead of liquid carbon,
they considered a certain carbon volume on the elec-
trode surface, which possesses a high degree of struc-
tural fluidity due to the He ion bombardment.
The progressive ordering from the surface to the
center has been experimentally observed in the case of
the electron irradiation-induced formation of the
quasi-spherical onion-like particles[25]. In this case,
the large inner hollow space is unstable under electron
Fig. 2. HREM image of a quasi-spherical onion-like graphitic bombardment, and a compact particle (innermost shell
particles generated by electron irradiation (dark lines repre- 460) is the final result of the graphitization of the
sent graphitic shells, and distance between layers is 0.34 nm). carbon volume (see Fig. 3e-h).
The large inner hollow space observed in polyhe-
dral particles is supposed to be due to the fact that
able spheroidal shape[l9], and a compact structure the initial density of the carbon volume (drop) is lower
very similar to the bucky-onions generated by electron than graphite[7]. Then, in order to prepare more
irradiation[ 1 11. compact graphitic particles (smaller inner shell), the
starting carbon phase should have a density closer
to graphite (2.25 gr/cm2). This basic hypothesis has
3. FORMATION MECHANISM
been confirmed by subjecting nanodiamonds to a
In the preceding section, we have described differ- high-temperature treatment (diamond is much denser
ent experiments generating graphitic nanoparticles. As than graphite, 3.56 gr/cm2)[ 191. Experimentally, it
for the case of fullerene synthesis, the procedures are has been observed that the formation of graphitic
rather violent (electric arc, plasma torch, shock waves, layers begins at the (1 11) diamond facets, then gener-
high-temperature treatment, electron irradiation, etc.) ates closed-surface graphitic layers, and subsequently
and clearly display the present incapacity of generat- follows the formation of concentric shells epitaxially
ing nanometric curved and closed graphitic systems by towards the center. At an intermediate stage, the onion-
standard chemical techniques. For the synthesis of like graphitic particles contain a tiny diamond core
c60, it has been found that a temperature on the or- (see Fig. 2 in ref. [19]). This process yields carbon on-
der of or higher than 1200°C are necessary to anneal
the carbon clusters in the gas phase and efficiently
form Cso molecules[20]. In the case of the graphitiza-
tion process, the dewrinkling and elimination of de-
fects in graphitic layers begins at 2O0OoC[21]. The
extremely stable carbon-carbon bonds are responsible
for the high-energetic process necessary to anneal gra-
phitic structures. a b C d
The formation mechanism of fullerenes and related
structures is not well understood. The fascinating high
aspect ratio of nanotubes is associated to the electric
field of the arc[7,22], but this fact has not yet been
confirmed and/or been applied to control their growth. f h
Moreover, the arc process generates simultaneously a
large quantity of polyhedral graphic particles. The for- Fig. 3. Schematic illustration of the growth process of a
mation of multi-shell graphitic particles from the gas graphitic particle: (a)-(d) polyhedral particle formed on the
phase by a spiral growth mechanism has also been sug- electric arc; (d)-(h) transformation of a polyhedral particle
gested[23,24], but no convincing experimental data of into a quasi-spherical onion-like particle under the effect of
a spiral structure has been reported. high-energy electron irradiation; in (f) the particle collapses
and eliminates the inner empty space[25]. In both schemes,
The thermal treatment of fullerene black generates the formation of graphite layers begins at the surface and
nanometric polyhedral particles[ 131. This experiment progresses towards the center.