First-Principles Study of Defect—Interface Synergy in Two-Dimensional Transition Metal Dichalcogenide Heterostructures for the Hydrogen Evolution Reaction

Two-dimensional transition metal dichalcogenides (TMDs) are promising electrocatalysts for the hydrogen evolution reaction (HER), but their coordinatively saturated basal planes are intrinsically inert. Using density functional theory with the PBEsol functional and DFT-D3 dispersion corrections, this study examines monolayers, homobilayers, and six vertical van der Waals heterostructures assembled from MoS2, MoSe2, WS2, and WSe2, together with chalcogen and transition metal vacancies introduced at the reaction surface. Pristine surfaces bind hydrogen weakly and require more than 2.2 eV to dissociate water; neither an increase in layer number nor heterojunction formation relieves this limitation. Phonon dispersions and quasi-harmonic free energies confirm that all six heterostructures are dynamically stable. Surface chalcogen vacancies expose undercoordinated metal centres and reduce the water dissociation free energy by up to approximately 2 eV, while the composition of the adjacent layer—in particular its Se content—further modulates adsorption at the exposed site. Introducing an additional metal vacancy reveals a marked spatial dependence: intralayer vacancies overbind hydrogen (ΔGH* from −0.40 to −1.37 eV), whereas vacancies in the adjacent layer shift ΔGH* into a near-thermoneutral window of −0.33 to 0.25 eV, despite their higher formation energies (≈6.45 eV versus ≈4.2 eV). Interlayer defect–interface synergy, therefore, constitutes a transferable design principle for activating TMD basal planes.