A van der Waals–Covalent Bonding-Inspired Typical Coordination with Ultrahigh Lattice Mismatch as Active Sites for Hydrogen Electrosynthesis

We report a van der Waals–covalent bonding interface with boosted hydrogen evolution reaction (HER) catalytic activity compared to the well-known edge defects for two-dimensional catalysts. The central region of a chemical-vapor-deposition-grown multilayer MoS₂ is transformed to Mo₅N₆, thus forming a van der Waals–covalent (v–c) interface that has high structural strain due to the large lattice mismatch of 10.5% between the two different phases. The large structural distortion creates several kinds of stretched sites that show ideal HER catalytic activities theoretically, such as the S sites coordinated by 2 Mo atoms and the 3-coordinated N atoms. In experiments, the v–c interface bonding and coordination variations were observed, and a number-of-layers-dependent MoS₂-to-Mo₅N₆ transformation mechanism was found. Using the on-chip electrochemical micromeasurements, the catalytic activity of the atoms at the v–c interface was demonstrated to be higher than the edge atoms of either MoS₂ or Mo₅N₆ sheets. Further macrotests using the as-synthesized powder materials showed greatly enhanced HER performance of the v–c structure than MoS₂ or Mo₅N₆ sheets. This comprehensive study of the v–c structure shows the synthesis route and a clear bonding interface, establishes the number-of-layers-dependent transformation principles, exhibits high-priority HER activities, and explains the strain/coordination-variation-induced catalytic mechanism.