Nanotechnology’s Wonder Material: Synthesis of Carbon Nanotubes - Hierarchical Nanostructures for Energy Devices - page 49

where K ¼ k 1 /k 2 and t r is the residence time of the ethylene gas in the GPR
reaction zone, determined mostly by the dynamics of gas flow through the
CVD reactor.

d n 3 r 4 n g | 6

3.2.4 Termination of CNT Growth
Experimental observations have consistently shown that CNT growth slows
down and eventually terminates. The termination mechanism is ambiguous,
and remains yet another active topic in the CNT growth field. Understanding
this phenomenon should be highly beneficial in improving growth yields
and controllability of the CVD process for CNT growth. We discuss several
potential growth termination mechanisms in the next section.

3.2.4.1 Termination Mechanisms
It is clear that the catalyst deactivation mechanism plays a critical role in
CNT growth termination. Particularly, this mechanism should be able to
explain the abrupt growth cessation that is a very intriguing feature of the
growth termination of VACNTs. 18 One of the biggest innovations was the
development of growth enhancers such as water, alcohol, and other oxygen-
containing molecules. Indeed, numerous experimental reports demon-
strated that these enhancers remarkably increase the CVD growth time and
thereby enable millimetre-scale (or even cm-scale) growth of carbon nano-
tubes. Futaba et al. 43 examined the effects of various enhancers on the
growth kinetics and concluded that their growth-promoting properties are
indeed related to the presence of the oxygen-containing molecules in the
gas mixture. Therefore, understanding the effect of enhancers will help to
elucidate growth termination mechanisms.
The prevalent opinion in the literature is that these mild oxidants remove
amorphous carbon from the catalyst to sustain the catalytic activity. Modern
catalysis studies reveal that hydrocarbon decomposition leaves carbonaceous
by-products that have longer residence time on the metal catalyst and easily
get stabilized at high temperatures. 59 Strongly adsorbed carbon residues
could poison catalysts by blocking their active sites. A similar approach was
accepted by Hata et al.; 73 they proposed that catalyst nanoparticles can be
deactivated by accumulation of amorphous carbon on the catalyst surface
during the CVD growth of CNTs. Therefore, they suggested that the role of
water vapor is mainly restricted to removal of such poisonous carbon.
Yet, a similar effect with water could result from a different mechanism.
Based on an iron-alumina catalyst system, Amama et al. 49 suggested that
Ostwald ripening of small iron catalyst particles could be the main cause of
termination and the role of water is related to prevention of ripening by
surface hydroxylation of the alumina support. This hypothesis can also ex-
plain different eciencies of various catalyst support layers that have different
catalyst-support interactions. 74-77 Other studies underscore the importance of
the support layer in preventing growth termination. Kim et al. 78 demonstrated

.