Large-Diameter Single-Wall Carbon Nanotubes Formed Alongside Small-Diameter Double-Walled Carbon Nanotubes

Samples containing a majority of either single-wall carbon\udnanotubes (SWCNTs) or double-walled carbon nanotubes (DWCNTs) are\udprepared in the same catalytic chemical vapor deposition conditions but using\udslightly different catalytic materials, based on alumina impregnated with iron and\udmolybdenum salts. There is a sharp SWCNTs-to-DWCNTs transition. By\udcontrast to the usual findings, the selectivity is not correlated to the size of the\udiron-based catalyst nanoparticles, nor does the transition occur upon a decreasing\udcarbon/catalyst ratio. The result is attributed to the increasing MoO3\udconcentration inducing modifications of the gas atmosphere, such as the\udformation of more reactive C2 species through C2H4 dissociation, which thus\udfavors the nucleation and growth of a DWCNT. In the DWCNT sample, the\udaverage diameter of the SWCNTs is higher than the average outer diameter of\udthe DWCNTs, which is uncommon, as many authors stress that SWCNTs show\uda lower diameter than DWCNTs. The study could provide guidelines for the synthesis of very small diameter DWCNTs