Y. Zhang, J. Gregoire, R.B. van Dover, A.J. Hart. Journal of Physical Chemistry C 114(14):6389-6395, 2010. [http://dx.doi.org/10.1021/jp100358j]
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We report the use of a small concentration of ethanol in addition to ethylene as the carbon source for growth of dense vertically aligned “forests” of few-walled carbon nanotubes (CNTs). Through a detailed comparison of CNTs grown with and without ethanol added to the C2H4/H2 feedstock, we quantify several important effects of the ethanol addition. We show that ethanol selectively reduces the number of CNT walls without changing the outer diameter, increases the catalyst lifetime more than 3-fold, and increases the rate of carbon conversion more than 5-fold. Online dewpoint and mass spectrometry measurements of the exhaust stream suggest that ethanol decomposes into active carbon species that enhance growth, and into H2O, which counteracts the accumulation of amorphous carbon and thus prolongs the catalyst lifetime. We performed a systematic study of the effect of the catalyst film thickness, and identify a set of conditions that provides growth of millimeter-tall double-walled CNT forests. Importantly, our study reveals that the chemistry of the CVD atmosphere alone plays a critical role in controlling the structure of CNTs, and that addition of ethanol results in few-walled CNTs over a broad range of growth conditions. These findings are an important step toward the ultimate goal of control of CNT chirality during synthesis as well as toward realization of important large-scale applications of aligned CNT films having high monodispersity and structural quality.
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