Ethanol-promoted growth of dense vertically aligned small-diameter carbon nanotubes

We report the use of a small concentration of ethanol in addition to ethylene as the carbon source for growth of vertically aligned CNT ``forests.'' In our system, adding ethanol promotes the catalyst lifetime from approximately 20 minutes to nearly 60 minutes, and accordingly increases the forest height from 2.5 mm to over 5 mm, with CNT diameters of approximately 5 nm (2-3 walls). Spread composition films deposited by gradient sputtering, combined with non-destructive mapping of CNT diameter and alignment by small-angle X-ray scattering, enable high-throughput discovery of necessary and sufficient conditions for growth of small-diameter CNTs. Compared with the widely known water-assisted ``super growth'' process, we find that ethanol enables much finer control of the dewpoint and thus offers more consistent and tunable results. Also, ethanol is a weaker oxidant than water and therefore creates fewer structural defects due to unwanted etching of the CNT walls.