Unexpected Inner-Sphere Versus Outer-Sphere Redox in Bilayer Molybdenum Disulfide (MoS₂) from Correlative...

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.