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.