Formation of C H Ablation of Graphite, Coal or C Particles Suspended in Selected Solvents - Polyynes: Synthesis, Properties, and Applications - page 146

When the relative abundance of major C 8 H 2 polyyne in hexane at
355 nm laser irradiation is compared among the three carbon sources, that
from C 60 is largest. It is larger than that from graphite and coals by factors
of about 2 and 4-9, respectively. In coals, a large amount of O-containing
substituents such as OH, CO, and COOH are involved. One reason for the
low formation rates of polyynes from coals will be suppression of formation
reactions of polyynes due to various side reactions of precursor carbon
clusters with O-containing substituents.
Heath et al. [28] have studied the clustering reactions of carbons ablated
from a graphite disk by a focused Nd:YAG 532 nm (30-40mJ/pulse) laser.
Graphite was laser ablated into a He carrier gas containing simple molecules
such as H 2 ,H 2 O, NH 3 ,andCH 3 CN, supersonically expanded, and skimmed
into a molecular beam, and the beam was interrogated by photoionization
TOF spectrometry. Without added reactants in the He carrier gas, C n clus-
ters up to n 130 were readily observed. In the presence of added gases

(H 2 and H 2 O), C 2n H 2 polyynes were formed. They explained the production
of these carbon-chain molecules on the basis that a significant proportion of
the C 2n species initially formed is reactive radicals with linear carbon-chain
structures, which can readily add H at the ends to form relatively stable
polyynes. On the other hand, Kato et al. proposed that C 2n D 2 (n

¼

2-5)
polyynes from graphite in H 2 or D 2 gas are dominantly produced by
fragmentation reactions of large clusters of C m (m>30) by the thermally
decomposed hydrogen atoms produced under intense vaporization field. We
found that C 2n H 2 (n

¼

4-8) polyynes were produced by laser ablation of
graphite, coals, and C 60 particles suspended in solutions. Although reaction
conditions in this study (liquid phase) are different from those employed by
Heath et al. [28] and Kato et al. [24] (gas phase), the present observation for
three carbon materials is consistent with previous findings for graphite.
Thus, solvents play a similar role to added gases (H 2 ,H 2 O) in the gas phase.
Heath et al. [28] measured the dependence of n-distribution of polyynes
on the pressure of added gases. They found that the C 8 H 2 peak was
dominant at high added gas pressures of H 2 as this species did not undergo
any further reaction. A similar inertness of C 8 H 2 was found for H 2 O,
CH 3 CN, and NH 3 , though its tendency was not so severe as that for H 2 .
In all our experiments using graphite, coals, and C 60 in solutions, C 8 H 2 was
a dominant polyyne and small amounts of longer polyynes C 2n H 2 (n

¼

5-8)
were observed. These findings led us to conclude that the inertness of C 8 H 2
is also kept for clustering reactions in solutions and the reactivity of ablated
species in solutions is similar to that in the gas phase under the presence of
added gases (H 2 ,H 2 O, CH 3 CN, NH 3 ).
We found that major products of C 60 suspended in hexane and methanol
under laser ablation are graphite-like carbon. It probably arises from photo-
decomposition of fullerene network and photochemical and/or thermal
isomerization from fullerene structure to graphite-like one. Another forma-
tion route of graphite-like carbon is clustering of small carbon clusters.

¼