Near-Thermoneutral Hydrogen Adsorption at Sulfur Dangling-Bond Sites of an Experimentally Observed MoS2 Grain Boundary

Grain boundaries (GBs) in monolayer 1H-MoS2 can provide favorable hydrogen-adsorption sites relevant to the hydrogen evolution reaction (HER), but the atomistic origin of this behavior remains unclear for experimentally realized boundary structures. Here, spin-polarized density functional theory calculations are performed to investigate an experimentally observed ∼18° MoS2 GB containing an apparently periodic sequence of five-, seven-, and four-membered rings, i.e., 5–7–4 GB motif. Four local configurations (α1, α2, β1, and β2) are examined as candidate structures of the 5–7–4 GB motif. The β-oriented supercell exhibits a lower average GB formation energy (11.02 eV nm–1) than the α-oriented supercell (12.86 eV nm–1), while β1 most closely reproduces the experimentally inferred bond connectivity. GB reconstruction generates boundary-localized Mo 4d and S 3p states near the Fermi level. Among six distinct sulfur environments, the undercoordinated h2 (α1) and h4 (β1) sites exhibit near-thermoneutral hydrogen adsorption free energies of −0.20 and −0.05 eV, respectively. Occupied nonbonding states localized on these sulfur sites provide a plausible electronic origin for their favorable hydrogen adsorption thermodynamics.