Sulfur-Induced Shape Compliance Modulates Fe Catalyst Agglomeration in Carbon Nanotube Growth

Large-scale carbon nanotube (CNT) synthesis based on floating catalyst chemical vapor deposition (FC-CVD), unlike conventional CVD, utilizes a growth promoter, commonly a sulfur-containing species, whose role in the overall growth process is still poorly understood, hindering more efficient reactor design and a better quality CNT product. By developing a machine-learning interatomic potential here, we conduct atomistic molecular dynamics collision simulations for pure Fe and Fe-S clusters that allow us to directly quantify their sticking probability. Sulfur is found to reduce the intrinsic sticking for small clusters, instead strongly enhancing it for larger sizes. We demonstrate that this crossover is driven by S-induced shape compliance, a mechanism where surface passivation leads to large shape fluctuations that efficiently absorb collision energy. These insights may help rationalize the diverse and sometimes conflicting experimental outcomes reported for S-assisted FC-CVD synthesis of CNTs.