Page 109 - Carbon Nanotubes

Graphitizable coiled carbon nanotubes 99
have more than the 20 coordination sites used, at As can be seen from Fig. 16, 24 coordination sites
least for the decomposition of acetylene. of the catalyst are involved in the growth of the (9,O)
3.2.2.2 Growth mechanism of a (9n,0) tubule, tubule and, in general, 24n coordination sites will be
over 24n coordination sites of the catalyst. The involved in the growth of the (9n,0) tubule.
growth of a general (9n,O) tubule on the catalyst 3.2.2.3 Growth mechanism of a (9n,0)-
surface is illustrated by that of the (9,O) tubule in (5n,5n) knee, involtiing from ,7411 to 20n coordi-
Fig. 16 which shows the "unsaturated" end of a (9,O) nation sites of the catalyst. Across the knee, the
tubule in a planar representation. At that end, the growth of the tubule starts as explained for the (9n,0)
carbons bearing a vacant bond are coordinatively tubule (Fig. 16) and must end as explained for the
bonded to the catalyst (grey circles) or to a growing (5n,5n) tubule (Fig. 15) or eice versa.
cis-polyacetylene chain (oblique bold lines in Fig. 16). As a starting point, a growing (9,O) tubule on the
The vacant bonds of the six cis-polyacetylene chains catalyst surface is illustrated in Fig. 17(a). In that
involved are taken to be coordinatively bonded to growing structure, the C, units arrive and are inserted
the catalyst [Fig. 16( b)]. These polyacetylene chains parallel to the tubule axis. If there is enough space ~
are continuously extruded from the catalyst particle probably thanks to the distortion caused by the
where they are formed by polymerization of C2 units elastic folding of the growing tubule [Fig. 12(b)] -
assisted by the catalyst coordination sites. Note that two carbons (1") can be inserted instead of one
in order to reduce the number of representations of between a pair of coordination sites [Fig. 17(b)]. The
important steps, Fig. 16(b) includes nine new C2 units knee is started by the arrival of a C, unit perpendicu-
with respect to Fig. 16(a). lar to the tubule axis, each carbon of which has a
The 12 catalyst coordination sites - drawn further vacant bond for coordination. These two carbons
away from the surface of the particle (closer to the (instead of the carbon with two vacant bonds usually
tubule) - are acting in pairs, each pair being always involved) displace one carbon from a pair of coordi-
coordinatively bonded to one carbon of an inserted nation sites. This perpendicular C, unit constitutes
(1") or of a to-be-inserted (2") C, unit and to two the basis of the future seven-membered ring. Note
other carbons which are members of two neighbour- again that nine new C, units are shown in Fig. 17(b)
ing cis-polyacetylene chains (3"). It should be empha- as compared with Fig. 17(a).
sized that, as against the (5n,5n) tubule growth, the Among the three C, units arriving together, two
C, units extruded from the catalyst particle are are parallel to the tubule axis (2') and one is ortho-
positioned in this case parallel to the tubule axis gonal to the tubule axis [I" in Fig. 17(b)]. First, only
before their jnsertion. the parallel units are inserted into the tubule structure
Two coordinative bonds of two neighbouring cis- [Fig. 17(c)]. These two inserted C, units (2') were
polyacetylene chains of the growing tubule are speci- displaced by the arrival of two orthogonal C, units
fically displaced from a pair of catalyst coordination (3") and all further arriving C, units will be ortho-
sites by the insertion of one carbon of a C2 unit [2" gonal to the tubule axis.
in Fig. 16(b)]. The second carbon of that C, unit is Meanwhile, the coordinative bonding of the two
still1 bonded to the pair of coordination sites. It will C2 units within the pairs of coordinative sites forces
later be displaced from that pair of coordination sites the occupied sites to spread. As a result, in the
by the srrival of the next two C, segments of the two direction opposite to the place where the first ortho-
growing cis-polyacetylene chains considered [ 3" in gonal C, unit arrived, there is only enough room for
Fig. 16(c)]. The C, unit and cis-polyacetylene C2 a single site (") instead of for a pair of sites
segments are hence inserted into the growing tubule. [Fig. 17(c)]. That site now coordinates the two cis-
The further arriving C, units or cis-polyacetylene C2 polyacetylene C, segments, but without the possibility
segments will act as the latter one did, and so on of inserting a new C, unit. The displacement of that
[Fig. 16(c)]. site from the growing nanotubule leads to the closure

I I

I 10
I
I
I
I
t
I
1 I
I I
I I
Fig. 16. Growth of a (9n,O) tubule on the catalyst surface illustrated by that of the (9,O) tubule. The
normal and bold lines represent single and double bonds, respectively.