Diffusion of the catalysis-relevant S-vacancies on defective 2H-phase MoS2

Background: S-vacancies play an essential role in various MoS2-catalyzed processes. Knowing that the size and shape of the S-vacancies affect the catalytic properties, this work investigates the diffusion of S-vacancies along different directions and whether these diffusion processes lead to the formation of specific surface patterns on a defective MoS2 monolayer. Methods: A first-principles-based kinetic Monte Carlo model is used to study how the S-vacancies of a vacancy cluster, i.e., a region where all surface S atoms are removed, migrate when the diffusion of the S-vacancies is limited to the [100], [120] and [120] directions, respectively. Significant findings: At up to 600 K, the diffusion processes transform a vacancy cluster into a pattern of single Satoms surrounded by six S-vacancy sites. Breaking this pattern requires high-barrier diffusion steps, only accessible at high temperatures where defective MoS2 sublimates. We have further found that the diffusion along the considered directions is very different, with the diffusion along the [120] direction being the slowest. In general, when diffusion is confined to a specific direction, the transformation of the vacancy cluster is slower than in the case with no constraints. This finding may help stabilize vacancy clusters on a catalyst.