Graphene nucleation from amorphous nickel carbides: QM/MD studies on the role of subsurface carbon density

The mechanism and kinetics of graphene formation from amorphous nickel carbides have been investigated employing quantum chemical molecular dynamics (QM/MD) simulations. Amorphous Ni₃C, Ni₂C, and NiC were employed to elucidate the role of the subsurface carbon density (ρ_C) on graphene formation. In each case, the nickel carbide phase underwent rapid carbon precipitation, resulting in a segregated nickel-carbon structure. The kinetics of graphene formation was most favorable for high carbon densities. At low ρ_C, i.e., Ni₃C and Ni₂C, there was a tendency for the formation of a number of small carbon fragments that failed to coalesce due to their inability to diffuse over the nickel surface. Graphene formation was only observed in the presence of high carbon densities that were relatively localized. These simulations, therefore, suggest that graphene nucleation is not immediately related to the presence of catalyst carbide phases.