  The most popular way to produce graphene nowadays is chemical vapor
deposition, where, surprisingly, H$_2$ gas is routinely supplied even though it
is a byproduct itself. In this study, by identifying dominant growing pathways
via multiscale simulations, we unambiguously reveal the central role hydrogen
played in graphene growth. Hydrogen can saturate the edges of a growing
graphene island to some extent, depending on the H$_2$ pressure. Although
graphene etching by hydrogen has been observed in experiment, hydrogen
saturation actually stabilizes graphene edges by reducing the detachment rates
of carbon-contained species. Such a new picture well explains some puzzling
experimental observations and is also instrumental in growth protocol
optimization for two-dimensional atomic crystal van der Waals epitaxy.
