Substrate-stabilized FeS monolayer on Au(111): Identification by combined experimental and theoretical approaches

Two-dimensional materials have emerged as promising platforms for electrocatalysis since the discovery of graphene, particularly in the search for efficient and earth-abundant catalysts for the hydrogen evolution reaction. Here, we investigate a FeS monolayer supported on Au(111) as a model FeS x -based two-dimensional system for HER applications. The film was characterized by scanning tunneling microscopy and X-ray photoelectron spectroscopy, revealing the formation of a structurally distinct and previously unreported phase. The combined experimental and density functional theory analyses allow us to assign the system to a stoichiometric 1:1 Fe:S monolayer exhibiting a 3 × 3 moiré modulation with respect to the Au(111) substrate. The determination of this structure enables a detailed assessment of its electronic properties and provides a robust foundation for evaluating the potential of two-dimensional FeS as an electrocatalyst for the hydrogen evolution reaction.