Nanotube nucleation versus carbon-catalyst adhesion—probed by molecular dynamics simulations

Abstract

Catalytic nucleation of carbon nanotubes (CNTs) remains a challenge for the theory: Which factors and forces decide if the gathering sp²-network of atoms will adhere to the catalyst particle and fully cover it or the graphitic cap will liberate itself to extend into a hollow filament? This intimate mechanism cannot be seen in experiment, yet it can be investigated through comprehensive molecular dynamics. We systematically vary the adhesion strength (W[sub ad]) of the graphitic cap to the catalyst and temperature T (and C diffusion rate). Observations allow us to build a statistically representative map of CNT nucleation and define the conditions for growth or metal encapsulation in a fullerene-shell (catalyst poisoning). It shows clearly that weak W[sub ad], sufficient thermal kinetic energy (high T) or fast C diffusion favor the CNT nucleation. In particular, below 600 K carbon-diffusion on the catalyst surface limits the growth, but at higher T it fully depends on cap lift-off. Informed choice of parameters allowed us to obtain the longest simulated nanotube structures. The study reveals a means of designing the catalyst for better CNT synthesis, potentially at desirably low temperatures.