Internal electric fields enhance reductive reactivity at MoS2 boundaries revealed by ultrafast near-field nanospectroscopy.

Monolayer MoS2 is a promising low-cost and efficient catalyst for electrochemical or photochemical reduction reactions. Many previous works have demonstrated that 2D MoS2 edges or boundaries are active reaction sites; however, their structural and electron dynamic origins remain unclear. Here we report an aperture-type near-field ultrafast transient absorption spectroscopy to directly probe the electron dynamics at nanoscale MoS2 boundaries. Near-field transient absorption imaging reveals a Stark effect signature at boundaries, where a built-in electric field of 1.93 MV/cm is intrinsically formed due to the oxidation of crystal boundaries. This field drives ultrafast electron injection and accumulation at the boundaries, enhancing boundary reactivity over the interior. These findings uncover the nanoscale structure-activity correlation for 2D MoS2 and promote the rational design of catalysts based on transition metal dichalcogenides.