The key to highly efficient growth of carbon nanotubes includes two essential ingredients in the growth ambient: a carbon source that does not contain oxygen and a minute quantity of a secondary gas, which does contain oxygen, that acts as a growth enhancer. These and other general rules governing the growth of carbon nanotubes and the fundamental reasons from which they arise are presented.

We report a new direction for highly efficient carbon nanotube (CNT) synthesis where, in place of conventional highly reactive carbon feedstocks at low concentrations, highly stable carbon feedstocks at high concentrations were shown to produce superior yields. We found that a saturated hydrocarbon that is considered to possess a low reactivity, delivered at high concentrations, could achieve an extremely high growth yield (2.5 times that when using ethylene). This result stems from the unique behavior where the CNT yield linearly increased with carbon concentration, in contrast to more reactive carbon feedstocks, where the yield peaks. We propose that the mechanisms for the growth kinetics for high- and low-reactivity carbon feedstocks are fundamentally different, where the latter benefits from a longer catalyst lifetime because of a relatively low production rate of carbon impurities.

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Growth enhancers, most notably water, have been reported to increase catalyst activity, despite constituting 0.02% of the growth ambient, resulting in dramatic increases in vertically aligned carbon nanotube (CNT) forests. We demonstrate a surprising second function of the growth enhancer, i.e., its ability to control the structure of the CNTs exemplified by highly selective double-walled carbon nanotube synthesis using oxygen-containing aromatics. This approach provides an easy and convenient method to grow various kinds of nanotubes from the same catalyst and same growth conditions.