the effects of oxidizers on the diameters of the carbon nanotubes ...

different views, Futaba et al. put forward that highly efficient growth can be achieved
not only using water but also o<strong>the</strong>r growth enhancers (alcohols, e<strong>the</strong>rs, esters, ketones,
aldehydes, and even CO2) (Futaba, et al. 2009). Their extensive investigations showed
two essential ingredients in <strong>the</strong> growth ambient: i) a growth enhancer containing oxygen
and ii) a carbon source that does not contain oxygen (Futaba, et al. 2009).
Different levels <strong>of</strong> water had a significant effect on <strong>the</strong> catalyst lifetime: <strong>the</strong>
reason <strong>of</strong> a drastic decrease in <strong>the</strong> catalyst lifetime is that even a small amount <strong>of</strong> water
vapor in <strong>the</strong> reaction mixture can change <strong>the</strong> CNT growth kinetics (In, et al. 2011).
When water is flown with <strong>the</strong> carbon source, during CVD growth process, <strong>the</strong>
efficiency <strong>of</strong> <strong>the</strong> catalyst has been grown in a few minutes (Smajda, et al. 2009).
However, <strong>the</strong> experiments <strong>of</strong> Hasegawa et al. showed that water addition stabilizes <strong>the</strong>
growth <strong>of</strong> millimeter-tall SWNTs for various conditions. On <strong>the</strong> o<strong>the</strong>r hand, water
addition disturbed SWNT growth using a thinner catalyst layer (Hasegawa and Noda
2011a) .
When compared to <strong>the</strong> SWNTs grown without water vapor, <strong>the</strong> intensities <strong>of</strong>
large-diameter CNTs relative to small-diameter CNTs decrased by adding water from
<strong>the</strong> beginning <strong>of</strong> CVD growth process in a few minutes (Hu, et al. 2010).
Although <strong>the</strong> growth mechanism and <strong>the</strong> role <strong>of</strong> water in CVD growth are still
under debate, it has been accepted that a small amount <strong>of</strong> water leads to an enhancement
in both growth height and nucleation density (Pint, et al. 2009b). None<strong>the</strong>less, sudden
decreases may occur in <strong>the</strong> growth characteristics as <strong>the</strong> optimum water level is
exceeded resulting in <strong>the</strong> fact that both catalysts and CNTs are oxidized
(Futaba, et al. 2005), <strong>the</strong> defect <strong>of</strong> CNTs increases (Liu, et al. 2010), <strong>the</strong> walls <strong>of</strong>
SWNTs are damaged (Hasegawa and Noda 2011a).
No enhancing <strong>effects</strong> were observed when ammonia (NH3) was selected as a
growth enhancer. This result demonstrated that <strong>the</strong> fundamental requirement <strong>of</strong> <strong>the</strong>
growth enhancer is to contain oxygen (Futaba, et al. 2009).
When compared with water; CO2, a good weak oxidant, can be more easily
controlled, and also promotes <strong>the</strong> growth <strong>of</strong> CNTs (Wen, et al. 2007). By introducing
large amounts <strong>of</strong> CO2 (to 30.4 mol %), clean and bare CNTs on <strong>the</strong> top surface <strong>of</strong> CNT
forests can be formed, <strong>the</strong> wall numbers <strong>of</strong> CNTs can be decreased gradually, <strong>the</strong>
diameter <strong>of</strong> CNTs can also be smaller, and <strong>the</strong> carbonaceous impurities can be
eliminated (Huang, et al. 2009). In spite <strong>of</strong> large amounts <strong>of</strong> CO2, Pint et al. pointed out
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