Electrochemical Synthesis of Carbyne-Like Materials and Other Nanocarbons - Polyynes: Synthesis, Properties, and Applications - page 68

Perfluorocyclopentene, perfluorodecalin and perfluoronaphthalene were
carbonized by Li-amalgam and gave, besides small amounts of fullerenes
and onions (Section 4.3), also carbon nanotubes in ca. 1-2% yield [4,5]. The
tubes were capped and multiwalled, typically 15 nm in diameter and about
50-200 nm long. Whereas the tubes from C 5 F 8 were straight, the tubes from
C 10 F 18 and C 10 F 8 were curly and substantially longer [4,5].
Wang et al. [102] prepared carbon nanotubes through a reaction of
tetrachloroethylene, C 2 Cl 4 with potassium at 200 C in the presence of
Au/Fe catalyst. Jiang et al. [103] have reported on the formation of multi-
walled carbon nanotubes from hexachlorobenzene and potassium at 350 C
in the presence of a Co/Ni catalyst. The authors claimed their reaction tem-
perature to be the lowest record for nanotube production [103]. However,
by cross-checking the older literature [4], this claim was, apparently, not
correct. A similar reaction of hexchalorobenzene with sodium did not pro-
duce nanotubes, but graphite only [93].
Carbon nanotubes and nested fullerenes were also prepared by reductive
carbonization of CO 2 with magnesium metal. Motiei et al. [104] reported
on the production of nanotubes in ca. 0% yield and nested fullerenes in
ca. 1-2% yield at 1000 C by the reaction of dry ice and magnesium. The
reductive carbonization of CO 2 is reminiscent of the HiPco process [105,
106], based on disproportionation of carbon monoxide (the Boudouard
reaction):

2CO ! CO 2 þ C ð nanotubes Þ ð 4 : 27 Þ

It occurs catalytically on the surface of Fe nanoparticles grown from
Fe(CO) 5 . Also, the conventional synthesis of nanotubes by catalytic CVD
from acetylene or methane can be formally considered as redox reaction.
Nevertheless, the electrochemical model of carbonization (Sections 4.1.1
and 4.1.2) is hardly applicable for CVD and HiPco, since the nanotubes
grow on the catalyst particle by apposition from the gas phase, and not from

the barrier film ( Figure 4.1 ) . The yield and quality of electrochemically

made nanotubes are usually not competitive to those of catalytic processes
in carbon arc, laser ablation, CVD and HiPco. However, this methodology
demonstrated that nanotubes (and also fullerenes and onions (Section 4.3))
can be prepared by ''soft chemistry'' at room or sub-room temperatures
[4,5,101]. Secondly, some electrochemical syntheses of nanotubes do not
require a catalyst [4,5,95-98,100,101]. This might be attractive if high-purity,
metal-free tubes are required.

4.4.1 I NDIRECT S YNTHESIS OF N ANOTUBES FROM P OLYYNE

Although the mechanism of gas phase growth of fullerenes and nanotubes
is not clearly understood, the short-chain sp-bonded carbon molecules,
formed for example during laser vaporization of graphite {A214}, surely