Decoupling the Effects of Ion Gating from Photoinduced Quasi-Fermi Level Splitting at Photoelectrochemical Interfaces

In this paper, we investigate the local electric field and surface potential at semiconductor-electrolyte interfaces using the surface reporter molecule 4-mercaptobenzonitrile (4-MBN) via surface enhanced Raman scattering (SERS) spectroscopy and the vibrational Stark effect. Using this approach, we evaluate the photovoltages produced at a semiconductor-oxide-metal-electrolyte junction. Two models are considered, including a semiconductor-oxide-electrolyte (SOE) system and a semiconductor-oxide-metal-electrolyte (SOME) junction, in which the metal layer forms a Schottky-like contact with the semiconductor. Using this SERS approach, both interfaces must be considered in parallel, which adds complexity. Essentially, these measurements allow us to map the surface potential (i.e., the photovoltage generated) as a function of both electrode potential and light intensity. This two-dimensional parameter space reveals, for the first time, a clear separation between ion-gating effects and photoinduced quasi-Fermi level splitting. In the SOME configuration, ion accumulation dynamically modulates the Schottky-like barrier height, effectively operating as an ion-gated metal-semiconductor junction under illumination. This interplay between ionic screening and photovoltage generation governs band bending and interfacial electric fields. Together, these results establish a unified framework for understanding coupled electrostatic and photophysical processes at semiconductor-electrolyte interfaces and provide a quantitative platform for engineering interfacial energetics in photoelectrocatalysis and optoelectronic systems.