  In recent years, liquid metal catalysts have emerged as a compelling choice
for the controllable, large-scale, and high-quality synthesis of
two-dimensional materials. At present, there is little mechanistic
understanding of the intricate catalytic process, though, of its governing
factors or what renders it superior to growth at the corresponding solid
catalysts. Here, we report on a combined experimental and computational study
of the kinetics of graphene growth during chemical vapor deposition on a liquid
copper catalyst. By monitoring the growing graphene flakes in real time using
in situ radiation-mode optical microscopy, we explore the growth morphology and
kinetics over a wide range of CH4-to-H2 pressure ratios and deposition
temperatures. Constant growth rates of the flakes' radius indicate a growth
mode limited by precursor attachment, whereas methane-flux-dependent flake
shapes point to limited precursor availability. Large-scale free energy
simulations enabled by an efficient machine-learning moment tensor potential
trained to density-functional theory data provide quantitative barriers for key
atomic-scale growth processes. The wealth of experimental and theoretical data
can be consistently combined into a microkinetic model that reveals mixed
growth kinetics that, in contrast to the situation at solid Cu, is partly
controlled by precursor attachment alongside precursor availability. Key
mechanistic aspects that directly point toward the improved graphene quality
are a largely suppressed carbon dimer attachment due to the facile
incorporation of this precursor species into the liquid surface and a
low-barrier ring-opening process that self-heals 5-membered rings resulting
from remaining dimer attachments.
