Topotactic Growth of Edge-Terminated MoS₂ from MoO₂ Nanocrystals

Layered transition metal dichalcogenides have distinct physicochemical properties at their edge-terminations. The production of an abundant density of edge structures is, however, impeded by the excess surface energy of edges compared to basal planes and would benefit from insight into the atomic growth mechanisms. Here, we show that edge-terminated MoS₂ nanostructures can form during sulfidation of MoO₂ nanocrystals by using in situ transmission electron microscopy (TEM). Time-resolved TEM image series reveal that the MoO₂ surface can sulfide by inward progression of MoO₂(202̅):MoS₂(002) interfaces, resulting in upright-oriented and edge-exposing MoS₂ sheets. This topotactic growth is rationalized in the interplay with density functional theory calculations by successive O-S exchange and Mo sublattice restructuring steps. The analysis shows that formation of edge-terminated MoS₂ is energetically favorable at MoO₂(110) surfaces and provides a necessary requirement for the propensity of a specific MoO₂ surface termination to form edge-terminated MoS₂. Thus, the present findings should benefit the rational development of transition metal dichalcogenide nanomaterials with abundant edge terminations.