Abstract
Covalently bonded in-plane two-dimensional (2D) transition metal dichalcogenide (TMD) heterojunctions with atomically sharp interfaces hold great promise for photocatalytic applications in solar energy conversion and environmental remediation; however, their spatially resolved charge distribution and transport, particularly under operando conditions, remain poorly understood. Here, we employ photoscanning electrochemical microscopy (photo-SECM) to directly visualize photoinduced charge separation in monolayer MoS(2)-WS(2) in-plane heterojunctions. Spatial separation of photogenerated carriers is observed, with electrons accumulating in MoS(2) and holes in WS(2), leading to strongly asymmetric interfacial kinetics: Fc(+) reduction proceeds rapidly on MoS(2) (0.6 cm s(-1)), whereas Fc oxidation on WS(2) is significantly slower (0.008 cm s(-1)). High-resolution surface photovoltage microscopy (SPVM) enables a quantitative comparison of charge-separation capacity across architectures. The in-plane MoS(2)-WS(2) heterojunction shows the largest photovoltage contrast (-35 mV in MoS(2), 20 mV in WS(2)), exceeding the vertical heterojunction (-18 mV in MoS(2), 11 mV in WS(2)) and the individual monolayers (-12 mV for MoS(2), - 1 mV for WS(2)), establishing the following trend: in-plane > vertical > monolayers. Ultraviolet photoelectron spectroscopy (UPS) indicates that this directional charge separation is driven by intrinsic type-II band alignment, while photoluminescence (PL) imaging shows that the interface acts as a recombination center that limits efficient carrier extraction. These results provide direct experimental evidence of type-II-driven charge separation in in-plane heterojunctions and offer critical insights for interface design in high-efficiency photocatalytic and optoelectronic systems.