Impact of Intrinsic Defects and Tungsten Doping on the Catalytic Properties of Two-Dimensional Cu₂S

Understanding how intrinsic defects govern dopant activity is central to the rational design of catalytic two-dimensional materials. Here, combining first-principles defect thermodynamics with hydrogen adsorption descriptors, we investigate the defect landscape and catalytic behavior of a Cu₂S monolayer. Our calculations show that sulfur-on-copper antisites (S_Cu) are the most thermodynamically favorable intrinsic defects, with defect formation energies ranging from approximately -0.12 to 0.35 eV depending on the chemical potential environment. In contrast, copper vacancies (V_Cu) exhibit higher defect formation energies in the range of 0.45 to 0.85 eV, but become increasingly stabilized under Cu-poor conditions. Importantly, we demonstrate that thermodynamic stability does not directly correlate with catalytic activity. Although S_Cu is energetically favored, it shows weak hydrogen binding with a hydrogen adsorption free energy of ΔG _H* = -0.66 eV, whereas V_Cu-based configurations significantly improve hydrogen adsorption toward more optimal values. Single tungsten substitution at Cu sites is found to preferentially stabilize a V_Cu+W defect complex with a formation energy of approximately 0.28 eV, which modifies the electronic structure by introducing acceptor-like states near the valence band edge while avoiding strongly localized defect levels that may hinder charge transport. This defect-mediated electronic restructuring leads to near-thermoneutral hydrogen adsorption, with ΔG _H* improving from -0.66 eV for pristine Cu₂S to -0.08 eV for the V_Cu+W complex. Overall, these results highlight how the interplay between intrinsic defects and extrinsic dopants can be exploited to tune the electronic structure and catalytic behavior of Cu₂S, providing general design principles for defect-engineered electrocatalysts for the hydrogen evolution reaction.