First-principles modeling of the highly dynamical surface structure of a MoS 2 catalyst with S-vacancies.

Vacancy sites, e.g. , S-vacancies, are essential for the performance of MoS 2 catalysts. As earlier studies have revealed that the size and shape of the S-vacancies may affect the catalytic activity, we have studied the behavior and mobility of such vacancies on MoS 2 using DFT calculations and kinetic Monte-Carlo (kMC) simulations. The diffusion barriers for the S-vacancies are highly dependent on the immediate environment: isolated single S-vacancies are found to be immobile. In contrast, small n S-vacancies formed from n = 2 to 5 neighboring S-vacancies are often highly dynamic systems that can move within a confined area. Large extended n S-vacancies are generally unstable and transform quickly into alternating patterns of S-atoms and vacancy sites. These results illustrate the importance of recognizing MoS 2 (but also other catalysts) as dynamic structures when trying to tune their catalytic performances by introducing specific defect structures.