Page 48 - Carbon Nanotube Fibres and Yarns

Carbon nanotube fibers spun directly from furnace 41

Fig. 3.2 Poisoning-reactivation model explaining the growth of MWNT. (Reproduced
with permission from Gspann T, Smail F, Windle A. Spinning of carbon nanotube fibres using
the floating catalyst high temperature route: purity issues and the critical role of sulphur.
Faraday Discuss. 173 (2014) 2–7.)

identified by Raman spectra of split G peak, RBM signal, two- dimensional
(2D) peak downshift and higher I G /I D ratio (from 5 to 200). Gspann et al.
[11] suggested that sulfur could make the catalyst particle less “sticky,” which
limits the catalyst size by lowering the particle collision rate and also pre-
vents the iron deposited on the reactor wall in the high-temperature reac-
tion zone. Another analysis [21, 33] suggests a poisoning-reactivation model
which explains the formation of MWNT with higher sulfur concentration
(Fig. 3.2). At lower sulfur concentration, the partial sulfur coating enhances
the catalyst activity and prevents the particle coarsening. However, an ex-
cess amount of sulfur will initially make the iron particle inactive for CNT
growth, which grows larger due to collision. When these larger catalyst
particles enter the high-temperature growth zone, the sulfur gets partially
removed possibly by forming H 2 S with hydrogen. This reactivates the larger
catalyst particles and promotes growth of MWNTs. Based on this analysis,
two possible routes can be used to promote SWNT synthesis. The first is to
lower the amount of sulfur, which produces small active particles nucleating
SWNTs. The other method is to introduce carbon atoms in the earlier stage
where catalyst particles are still small; this could promote CNT nucleation
which prevents further growth of the iron particles.
The timing of sulfur availability is also essential. Earlier arrival of sulfur
could produce more active catalysts by limiting the carbon solubility in
the catalyst, which prevents the encapsulating graphene layer and pro-
motes the carbon reconstruction and edge growth [8]. This was used to
explain the low percentage of active iron particles due to the lower chance