Spatially Resolved Hydrogen Evolution on Two-Dimensional MoS ₂ with Varied Phases

Phase-engineered two-dimensional (2D) MoS2 has shown great potential for hydrogen evolution reactions (HER), yet the phase-dependent catalytic activity variations and active site distributions remain unclear. Using phase-pure 2D 2H-, 1T′-, and 1T‴-MoS2 single crystals, we implemented a multiscale characterization approach combining micrometer-scale electrochemical testing, conductivity measurements, and in situ imaging of hydrogen nanobubbles to investigate the HER performance across different phases. We revealed a clear activity trend: 1T′ edge > 1T′ basal plane ≈ 1T‴ edge > 1T‴ basal plane > 2H edge > 2H basal plane, correlating with phase-specific conductivity. Real-time fluorescence imaging of labeled H2 nanobubbles during HER uncovered contrasting active site distributions: 2H-MoS2 demonstrates edge-preferential activity, while metastable 1T′ and 1T‴ phases display more uniformly distributed reactivity. Density functional theory (DFT) calculations trace this basal-plane activation to sulfur coordination geometry alterations in metastable phases. Our findings elucidated the structure–activity relationships governing phase-dependent HER performance while demonstrating phase engineering as an important optimization strategy for 2D electrocatalysts.