In Situ Imaging Reveals Efficient Charge Separation in Monolayer MoS₂-WS₂ Type-II Heterojunctions

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 MoS₂-WS₂ in-plane heterojunctions. Spatial separation of photogenerated carriers is observed, with electrons accumulating in MoS₂ and holes in WS₂, leading to strongly asymmetric interfacial kinetics: Fc⁺ reduction proceeds rapidly on MoS₂ (0.6 cm s^-1), whereas Fc oxidation on WS₂ 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 MoS₂-WS₂ heterojunction shows the largest photovoltage contrast (-35 mV in MoS₂, 20 mV in WS₂), exceeding the vertical heterojunction (-18 mV in MoS₂, 11 mV in WS₂) and the individual monolayers (-12 mV for MoS₂, - 1 mV for WS₂), 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.