High-Resolution Imaging of Photocatalytic Activity and Charge Separation by Multimodal Photo-SECM

Photocatalytic processes are central to many energy and environmental applications. Understanding the fundamental parameters that govern these reactions at the nanoscale is essential for the rational design of solar energy conversion systems and sustainable chemical synthesis. In nanostructured photocatalysts, the charge-transfer rate and direction and catalytic activity can vary within nanometer-scale distances across different sites─even within a single nanoparticle─due to structural, compositional, and electronic inhomogeneities. Traditional techniques provide bulk-averaged data, thereby obscuring such intrinsic spatial variability and limiting mechanistic insight. Photo-scanning electrochemical microscopy (photo-SECM) has proven useful for mapping local catalytic activity through current-based measurements, but it lacks the capacity to directly probe surface potentials and the driving force for light-induced charge-transfer and light-driven catalytic processes. The high complexity of underlying mechanisms, which include photogeneration, separation, and transport of charge carriers, as well as interfacial charge-transfer processes, necessitates multimodal correlative experimental characterizations. However, widely employed characterization techniques such as surface photovoltage spectroscopy (SPV) and photoemission electron microscopy (PEEM) operate under vacuum or in air, failing to capture the steady-state driving forces for light-induced redox reactions. A recently developed nanoscale photo-SECM methodology has enabled simultaneous interrogation of interfacial electron-transfer reactions and charge separation in particulate and two-dimensional photo(electro)catalysts. In this Account, we discuss the development of a multimodal SECM technique that combines surface potentiometry with amperometric reaction rates mapping to directly resolve the steady-state energetic landscape, along with the spatial activity distribution within individual catalytic domains. This correlative approach enables simultaneous access to both kinetics (reaction flux) and thermodynamics (local potential), providing a more complete description of photocatalytic function. The surveyed applications include probing charge separation in a monolayer in-plane heterojunction, high-resolution activity mapping of heterogeneous photocatalysts by the tunnelling mode of photo-SECM, and amperometric/potentiometric photo-SECM measurements of photoelectrochemical processes in nanostructured photocatalysts in operando. The ability of multimodal SECM to resolve steady-state interfacial energetics and kinetics under catalytic turnover was essential for probing a novel planar photocatalyst in which flux-responsive charging of cocatalyst surface dynamically sustains the high local driving force while maintaining efficient separation, accumulation, and utilization of photogenerated electrons and holes. Future developments may further extend this platform toward time-resolved and dynamic measurements of charge separation and interfacial processes, as well as perturbing the system with dynamically applied bias and illumination conditions, thus offering additional insight into transient photophysical and photochemical phenomena on the μs-s time scale and bridging the gap between the short time scale of ultrafast spectroscopies (fs-ps) and steady-state photo-SECM kinetic measurements. We envision this technique to enable nanoscale interrogation of surface potentials, reaction rates, band energetics, and driving forces, thereby guiding the design of photocatalysts and active sites with tailored geometries and compositions to access a wide range of redox chemistries. Along with these localized probing techniques, integrating machine-learning algorithms for autonomous probe control, approach-curve analysis, and real-time data-driven tip positioning may further transform potential-sensing photo-SECM into a self-driving platform for highly reproducible, autonomous interrogation of photocatalytic interfaces.