  In the synthesis of highly crystalline single-walled carbon nanotubes
(SWCNTs), high growth temperatures are preferred, while the formation of
impurity amorphous carbon (a-C) causes the termination of SWCNT growth and
degrades its properties. To remove this by-product, H2O and CO2 have been
employed in metal-catalyzed SWCNT growth systems because of their oxidizing
ability. Recently, nonmetallic nanoparticles have become one of the growth seed
candidates because of their high melting points and fewer metal impurities. In
this study, by using nanodiamond-derived carbon nanoparticles as the solid
growth seeds, we investigated the effect of CO2 and H2O on high-temperature
SWCNT growth with two types of carbon-source supplies: C2H2 and C2H4. In this
growth system, H2O showed oxidizing ability to etch a-C with either carbon
sources. CO2 exhibited a similar a-C formation-preventing role in C2H4-supplied
growth and achieved higher-purity SWCNTs with higher concentration of C2H4 than
the case of H2O. However, in contrast to the other combinations, CO2 injection
in the C2H2-supplied growth significantly enhanced the formation of a-C rather
than the removal of it while the yield of SWCNTs was also increased, indicating
the occurrence of the dehydration reaction between CO2 and C2H2. The present
findings will lead to efficient growth of high-quality SWCNTs from nonmetallic
growth seeds with the use of growth enhancers.
