Decomposition of natural gas hydrocarbons using thermal plasma produced by arc discharge is a promising pathway for large-scale production of hydrogen in combination with high-value carbon nanotubes (CNTs). Metal(s) evaporating from an electrode and condensing to form seed nanoparticles that nucleate and grow CNTs. However, lack of understanding of the mechanisms underlying the CNT nucleation and growth on metal nanocatalysts in such a process makes control of the diameter, chirality, length and yield of CNTs difficult. We debundled and separated CNTs from soots produced using iron (Fe) nanocatalysts, and distributed the CNTs on a monolayer graphene support for high-resolution transmission electron microscopy (HRTEM) imaging to gain mechanistic insights into how Fe nanocatalysts nucleated and grew CNTs in a thermal plasma. Full graphene encapsulation was found for relatively small Fe nanoparticles that were molten at high temperatures. Zigzag single-wall CNTs (SWCNTs) or zigzag double-wall CNTs (DWCNTs) appeared to have grown out directly from the graphene covering on the conical or cylindrical bodies of small molten Fe nanodroplets with high curvature. Also, single-wall carbon nanocones (SWCNCs) were observed on conical Fe nanoparticles, but were likely etched by H or O atoms in the reactor causing growth of a more stable SWCNT. TEM images indicated that a SWCNT or DWCNT could also be generated from the cracked opening of the graphene covering on a face-centered cubic (FCC) Fe nanoparticle. A simple, plausible pathway is proposed for growth of an open, H-passivated, zigzag SWCNT involving reaction of CH2 and CH radicals at high temperatures.
Gu et al. (Tue,) studied this question.