A van der Waals–Covalent\nBonding-Inspired Typical\nCoordination with Ultrahigh Lattice Mismatch as Active Sites for Hydrogen\nElectrosynthesis

We report a van der Waals–covalent bonding interface\nwith\nboosted hydrogen evolution reaction (HER) catalytic activity compared\nto the well-known edge defects for two-dimensional catalysts. The\ncentral region of a chemical-vapor-deposition-grown multilayer MoS<sub>2</sub> is transformed to Mo<sub>5</sub>N<sub>6</sub>, thus forming\na van der Waals–covalent (v–c) interface that has high\nstructural strain due to the large lattice mismatch of 10.5% between\nthe two different phases. The large structural distortion creates\nseveral kinds of stretched sites that show ideal HER catalytic activities\ntheoretically, such as the S sites coordinated by 2 Mo atoms and the\n3-coordinated N atoms. In experiments, the v–c interface bonding\nand coordination variations were observed, and a number-of-layers-dependent\nMoS<sub>2</sub>-to-Mo<sub>5</sub>N<sub>6</sub> transformation mechanism\nwas found. Using the on-chip electrochemical micromeasurements, the\ncatalytic activity of the atoms at the v–c interface was demonstrated\nto be higher than the edge atoms of either MoS<sub>2</sub> or Mo<sub>5</sub>N<sub>6</sub> sheets. Further macrotests using the as-synthesized\npowder materials showed greatly enhanced HER performance of the v–c\nstructure than MoS<sub>2</sub> or Mo<sub>5</sub>N<sub>6</sub> sheets.\nThis comprehensive study of the v–c structure shows the synthesis\nroute and a clear bonding interface, establishes the number-of-layers-dependent\ntransformation principles, exhibits high-priority HER activities,\nand explains the strain/coordination-variation-induced catalytic mechanism.