Carbon nanotube formation exemplifies atomically precise self-assembly, where atomic interactions dynamically engineer nanoscale architectures with emergent properties that transcend classical material boundaries. However, elucidating the transient molecular intermediates remains a critical mechanistic frontier. This study investigates the atomic-scale nucleation process of single-walled carbon nanotubes (SWCNTs) from acetylene on iron (Fe) clusters, utilizing GFN-xTB-based nanoreactor molecular dynamics simulations. The simulations reveal a consistent nucleation pathway, regardless of iron cluster size (Fe13, Fe38, Fe55), where the chemisorption and dissociation of acetylene molecules on the Fe clusters lead to the formation of C2H and C2 intermediates. These species then undergo oligomerization, initiating the growth of carbon chains. As the chains cross-link and cyclize, five-membered carbon rings are preferentially formed, which eventually evolve into six-membered rings and more complex sp2-hybridized carbon networks, resembling the cap structures of nascent SWCNTs. Although the nucleation mechanism remains similar across all cluster sizes, larger clusters show enhanced catalytic activity, leading to higher molecular weight hydrocarbons and more extensive carbocyclic networks due to their higher density of active sites per reacting molecule. Crucially, the study highlights the role of C2H as the key active species in the carbon network formation process. These findings offer critical insights into the initial stages of SWCNT nucleation, contributing to a deeper understanding of the mechanisms driving SWCNT growth and guiding the development of optimized synthetic strategies.
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