▎ 摘　　要

Metal nanoclusters can be anchored at defective sites of graphene sheets to strengthen their thermal stability for potential device applications. A previous transmission electron microscopy (TEM) experimental study on the morphology change of an ultrafine iron cluster embedded in a graphene nanocavity suggests that the underlying reaction mechanism is likely due to solid-solid transformation [Sci. Rep. 2012, 2, 995]. The morphology change of the Fe cluster may also assist the enlargement of the graphene nanocavity. This TEM experiment reminds us that if the anchoring Fe nanocluster within the graphene nanocavity can efficiently catalyze graphene etching at a certain operation temperature, the device application of graphene-metal nanocluster composites would be largely limited. Herein, we have performed ab initio molecular dynamics (AIMD) simulations of a triangular hexagonal close-packed (HCP) Fe-53 cluster in contact with either the edge of the graphene nanocavity or graphene nanoribbon to investigate its structural evolution and catalytic behavior at an elevated temperature (1173 K). Contrary to the previous TEM experiment, we suggest an alternative reaction mechanism, namely, the melting recrystallization for the structural transformation of Fe cluster. Moreover, we find that the molten iron cluster can etch and enlarge the graphene nanocavity. At the high temperature of 1173 K without H and O atoms, the Fe 53 cluster undergoes a phase transition from the HCP structure to a liquid-like nanodroplet while in contact with the edge of either graphene nanocavity or graphene nanoribbon. Interestingly, the Fe 53 cluster tends to saturate the graphene edges via forming Fe-C bonds but without breaking any C-C bonds within the time scale of AIMD simulations. Our reactive MD simulations show that the HCP Fe 53 cluster can complete with the reaction of carbide formation within 10 ps. Independent climbing-image nudged elastic band calculations offer additional insight into the Fe-catalyzed reaction mechanism of C-C bond dissociation, C-C displacement, or C-C rotation during graphene etching. We find that the Fe cluster can only efficiently catalyze the C-C dissociation at the armchair edge, following the C-C displacement mechanism, because of the formation of strong bonds between Fe and dangling C atoms. We also find that the catalytic ability of Fe atoms seems less effective compared with that of Ni atoms, in part because Fe clusters tend to change their shapes during the reaction. Lastly, we perform AIMD simulations of the Fe-53 cluster in contact with smaller-sized sp(2)-C flakes. We observe that the cluster can soak the carbon flake on its surface, followed by breaking the C-C bonds through C-C displacement or C-C rotation. It appears that the catalytic ability of the Fe-53 cluster that is in contact with graphene C atoms depends on the size of the carbon species.