Activating MoS₂ basal planes for hydrogen evolution through direct CVD morphology control

Monolayer MoS₂ has emerged as an active and non-precious electrocatalyst for electrochemical hydrogen production. The atomic thinness and ultrahigh surface-to-volume ratio of the chemical vapor deposition (CVD)-grown monolayers result in an ideal material to facilitate efficient electrochemical hydrogen evolution and explore the mechanism at the atomic level. However, the active sites of pristine monolayer MoS₂ are reported to locate at the edges, leaving the basal planes inert, which limits their hydrogen evolution reaction performance. Here, we synthesize monolayer MoS₂ hexagonal flakes with high surface coverage and abundant highly distributed S vacancies as active reaction sites directly by CVD. The catalytic performance of the hexagonal MoS₂ flakes presented here is superior to that of the existing as-grown MoS₂, exhibiting a current density of 100 mA cm^-2 at -353 mV versus the reversible hydrogen electrode (RHE) with an extraordinarily low onset potential of only 41 mV. And an outstanding exchange current density of 0.091 mA cm^-2 stands as the highest ever reported for all kinds of non-precious-metal doped MoS₂ catalysts. It is proved by a variety of tests that abundant S vacancies are distributed in the highly crystalline basal plane of hexagonal MoS₂, which leads to a remarkably improved catalytic efficiency compared with that of the triangular one. The monolayer MoS₂ is further evidenced to have excellent long-term stability and high durability, maintaining a stable performance for nine months in air and retaining the initial performance after 6000 cyclic voltammetry scans. On-site defect engineering of monolayer MoS₂ for excellent hydrogen evolution reaction (HER) performance enriches insights into the structure-performance relationship in 2D materials and provides new opportunities for the design of highly active catalysts.