Unexpected Inner-Sphere Versus Outer-Sphere Redox in Bilayer Molybdenum Disulfide (MoS ₂ ) from Correlative Electrochemical Imaging

The rational design of electrode materials (e.g., electrocatalysts) requires nanoscale insights into surface structure–activity relationships, yet such understanding remains incomplete and is generally out-of-reach with conventional macroscopic electrochemical characterization. In this work, scanning electrochemical cell microscopy (SECCM) is employed to directly compare inner- and outer-sphere redox activity on bilayer molybdenum disulfide (MoS2) crystals. Using the hydrogen evolution reaction (HER) as an inner-sphere benchmark and [Ru(NH3)6]3+/2+ as a model outer-sphere probe, contrasting layer-dependent electrochemical behavior is observed on bilayer 3R MoS2. High-resolution activity maps reveal the expected attenuation of HER kinetics with increasing layer thickness, attributable to hindered through-plane conductivity and increased electron tunnelling barriers, alongside nanoscale “hotspots” consistent with defect-mediated activity. In stark contrast, the [Ru(NH3)6]3+/2+ couple is more facile and electrochemically reversible on the upper layer of bilayer 3R MoS2, despite increased electron tunnelling distances. This unexpected response highlights the influence of interfacial electronic structure (e.g., Fermi level pinning and screening effects) and demonstrates that outer-sphere redox mediators can yield misleading indications of “metal-like” behavior for 2D semiconductors. This correlative multimicroscopic approach (i.e., SECCM combined with colocated conductive atomic force microscopy and scanning electron microscopy) provides insight into layer-dependent electrochemistry in 2D semiconductors and underscores the need for caution when employing conventional redox probes as proxies for conductivity or redox activity.