Electrochemical Synthesis of Carbyne-Like Materials and Other Nanocarbons - Polyynes: Synthesis, Properties, and Applications - page 69

play a role. Lagow et al. [21] have even suggested that fullerenes grow from
long chains in helical conformation via the spiral zipper mechanism. The
gas-phase assembling of polyynes into nanotubes is also possible, and it may
even lead to encapsulation of polyyne inside the nanotube [29,30]. Polyynes
may trigger the formation of graphene-based sp 2 structures also in conden-
sed phase at lower temperatures via cross-linking (Eq. 4.13). This process is
undesired for the synthesis of carbyne, but the spontaneous conversion of
sp-chains into graphene may lead to nanotubes and other interesting nano-
carbons in the solid phase. This subject was pioneered in 1998 by Yasuda
et al. [65-71]. They have generated polyyne from PTFE by electrochemical
dehalogenation with Mg anode (reaction 4.21). The nanotube growth was
promoted by irradiation with a 100 keV electron beam at 600-800 C. The
nanotubes had diameters about 10-50 nm and lengths ca. 1 m m [65,69].
Besides nanotubes, carbon nanocapsules and nanoparticles [70] were also
detected by TEM.
A similar strategy was adopted by Hlavaty et al. [108-110], but the
poorly defined ex-PTFE polyyne was replaced by pure low-molecular
weight oligoynes [108,110]. Nanotubes were obtained by carbonization of
1,3,5-hexatriyne, 1-iodo-1,3,5-hexatriyne [108] and dialkali hexatriynides
[110]. They were straight, multi-walled, with diameter of 10-20 nm, length of
100-200 nm, and end-capped [108]. Interestingly, the nanotubes grew exclu-
sively from six-membered oligoynes. Higher or lower homologs of hexa-
triyne do not provide nanotubes [109], although they also polymerize
spontaneously towards carbons [111,112].

ACKNOWLEDGMENT

This work was supported by the Academy of Sciences of the Czech Republic
(contract No. A4040306) and by the Czech Ministry of Education (contract
No. ME487).

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