Carbon coating precedes SWCNT nucleation on silicon nanoparticles: Insights from QM/MD simulations

Nucleation of single-walled carbon nanotubes (SWCNTs) from C₂ molecules adsorbed on silicon nanoparticles (SiNPs) has been investigated using quantum chemical molecular dynamics (QM/MD) simulations. The SWCNT nucleation mechanism was observed to be strikingly different from that of traditional, transition metal catalyzed SWCNT growth. Most notably, neither a bulk Si carbide phase nor the precipitation of carbon from the nanoparticle bulk was observed to precede SWCNT nucleation on SiNPs. Instead, the intermediate stage during SWCNT nucleation featured the formation of a "carbon-coated" Si surface, i.e., one covered with networks of sp²-hybridized carbon. In addition, the QM/MD simulations indicate that the growth of these sp ²-carbon networks was dependent exclusively on the dynamic motion of the polyyne chains formed on the catalyst surface. Analysis of the SiNP phase during SWCNT nucleation also indicated that nucleation proceeded while the Si catalyst remained in the solid phase. Thus, it is concluded that the SWCNT nucleation mechanism presented here was consistent with a vapor-solid-solid mechanism, rather than a vapor-liquid-solid mechanism. This conclusion correlates with recent findings concerning SWCNT nucleation on SiO₂ catalyst nanoparticles (Page et al. J. Am. Chem. Soc. 2011, 133, 621).