Local Structure of Sulfur Vacancies on the Basal Plane of Monolayer MoS₂

The nature of the S-vacancy is central to controlling the electronic properties of monolayer MoS₂. Understanding the geometric and electronic structures of the S-vacancy on the basal plane of monolayer MoS₂ remains elusive. Here, operando S K-edge X-ray absorption spectroscopy shows the formation of clustered S-vacancies on the basal plane of monolayer MoS₂ under reaction conditions (H₂ atmosphere, 100-600 °C). First-principles calculations predict spectral fingerprints consistent with the experimental results. The Mo K-edge extended X-ray absorption fine structure shows the local structure as coordinatively unsaturated Mo with 4.1 ± 0.4 S atoms as nearest neighbors (above 400 °C in an H₂ atmosphere). Conversely, the 6-fold Mo-Mo coordination in the crystal remains unchanged. Electrochemistry confirms similar active sites for hydrogen evolution. The identity of the S-vacancy defect on the basal plane of monolayer MoS₂ is herein elucidated for applications in optoelectronics and catalysis.