OVERVIEW ON THE MAJOR RESEARCH ACTIVITIES ON CARBON NANOTUBES BEING DONE IN AMERICA, EUROPE AND ASIA - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 270

In order to compare a range of catalyst compositions, CNTs have been
grown using Fe, Co, Ni and Co/Fe spin-on-catalyst (SOC) systems, involving
the metal salt dispersed with a spin-on-glass precursor. 112 During initial
growth runs (CH 4 /H 2 /900°C), the CNT yield followed the order Co-SOC >
Fe-SOC >> Ni-SOC. The Fe catalysts produced the longest nanotubes at the
expense of a larger average CNT diameter. A series of binary Co/Fe-SOCs was
also prepared. After initial CNT growth, the original samples were subjected
to additional growth runs. Four individual reactions were observed in the
Fe-SOC and binary Co/Fe-SOC (Fig. 9.17): regrowth (ampliication), double
growth (a second CNT growing from a previously active catalyst), CNT
etching, and nucleation from initially inactive catalysts (new growth). CNT
etching was observed for the mixed catalyst systems (Co/Fe-SOC) but not for
either Fe-SOC or Co-SOC. During the regrowth experiments, some CNTs were
observed in positions where they were not present after the initial growth
run, thus suggesting that the catalysts that were initially inactive towards
nucleation of CNTs in the original growth run became activated when placed
back into the furnace and submitted to regrowth under identical conditions.
Figure 9.17 Schematic representations of (a) initial growth, (b) regrowth
(ampliication), (c) double growth, (d) new growth (nucleation from initially inactive
catalysts) and (e) CNT etching . The red CNT represents material grown during the
irst growth run, whereas the green CNT represents new material formed during
subsequent growth runs. Reproduced from Crouse et al . 112 with permission. See also
Colour Insert.