Defect-Driven Catalysis and Phase Relaxation in MoS2 Nanoflower-Derived Nanosheets during Hydrogen Evolution

Engineering the electronic structure of molybdenum disulfide (MoS2) through phase composition and intrinsic defect chemistry offers a promising strategy to overcome its limited hydrogen evolution reaction (HER) activity in acidic media. Herein, we report a phase-relaxation-assisted mechanism in delaminated MoS2 nanoflower-derived nanosheets, where controlled cathodic biasing induces a partial transformation from the metallic 1T phase to the semiconducting 2H phase while the intrinsic structural defects present in the as-synthesized material are preserved. Under HER conditions, surface oxidation (formation of S–Ox species) increases, yet the overall S/Mo stoichiometry remains unchanged. The synergistic features enable efficient charge transfer and deliver an overpotential of 198 mV at 10 mA cm–2, a Tafel slope of 87 mV dec–1, and a hydrogen production rate of ∼11.2 mol g–1 at 50 mA cm–2. In situ electron paramagnetic resonance, combined with ex situ Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy, confirm the initial 1T-rich character, the progressive 1T → 2H relaxation under HER conditions, and the persistence of defect-enriched regions alongside surface oxidation. These findings highlight the subtle activation–deactivation interplay in MoS2 catalysts and underscore the importance of phase stability and intrinsic defect control for durable, high-performance HER MoS2-based electrocatalysis.