Electrochemical Synthesis of Carbyne-Like Materials and Other Nanocarbons - Polyynes: Synthesis, Properties, and Applications - page 67

These precursors provided fullerene C 60 in yields of ca. 0.1% [4]. Sometimes,
small amounts of C 70 and graphite onions of diameters 20-100 nm were also
detected [4]. A variant of reaction 4.25 was reported by Lee et al. [93], who
produced carbon onions from hexachlorocyclopentadiene and sodium.
Interestingly, hexachlorobenzene gave only graphite in the same reaction
[93]. This confirms that C 60 can be constructed from sole pentagons C 5 as
building blocks. Still another variant of reaction 4.25 was reported by
Lu et al. [94]. They have produced C 60 from hexachlorocyclopentadiene
and potassium in the yield of ca. 3%. However, this reaction required the
presence of Ni catalyst and temperatures 550-600 C. (The amalgam-driven
dehalogenation of perfluorocyclopentene (reaction 4.25) amalgam pro-
ceeded at room temperature without a catalyst.)

4.4 ELECTROCHEMICAL SYNTHESIS OF CARBON NANOTUBES

Kroto et al. [95-98] have reported on the synthesis of multiwalled carbon
nanotubes and carbon onions during electrolysis on carbon electrodes in
molten LiCl (at 600 C). The nanotubes were 2-10 nm in diameter and
>500 nm long, and contained occasionally encapsulated lithium oxide,
lithium chloride or lithium metal. The yield of nanotubes was 20-30% and,
by tuning the electrolysis conditions, the morphology of nanotubes varied
between straight, bent and spring-like ones [96-98]. The process was further
optimized by Chen et al. [99], while nanotube yields up to 50% were
reported during electrolysis on graphite electrodes in molten LiCl, NaCl and
KCl. Further optimization by Bai et al. [100] even allowed demonstration
of single walled carbon nanotubes in NaCl melt at 810 C.
Nanotubes and onions were also prepared by electrolysis of acetylene at

40 C in liquid ammonia, without deliberately added supporting electrolyte
[101]. This is the lowest temperature record for the production of nanotubes.
In contrast to reaction 4.14, this carbonization was not aided by a catalyst.
The mechanism started from dissociation of NH 3 :

NH 2 þ NH 4 2NH 3 $ ð 4 : 26 Þ

followed by the formation of atomic hydrogen:

NH 4 þ e ! NH 3 þ H ð 4 : 26a Þ

which finally carbonized acetylene:

C 2 H 2 þ 2H ! 2H 2 þ 2C ð nanotubes Þ ð 4 : 26b Þ