Probing Semiconductor/Electrolyte Interfaces with Surface Reporter Molecules: Evidence for Zero-Field Regions and Nonequilibrium Electrochemical Behavior

We investigate the local electric field at semiconductor–electrolyte interfaces using the surface reporter molecule 4-mercaptobenzonitrile (4-MBN), which enables monitoring of interfacial electric fields under varying electrochemical potentials via the vibrational Stark effect. In some cases, the semiconductor electrodes exhibit a persistent zero-field region over a wide range of applied potentials, particularly when the semiconductor is in the accumulation regime. This behavior arises from the shift in equilibration between majority and minority carriers with the electrolyte. It contrasts with the behavior of metal electrodes like fluorine-doped tin oxide (FTO) and indium tin oxide (ITO), which exhibit a linearly increasing electric field as the electrochemical potential rises─reflecting a transition from anion-dominated conditions at low (reducing) potentials to cation-dominated conditions at high (oxidizing) potentials. For semiconductor electrodes, two distinct behaviors are observed: (1) monotonic Stark shifts with electrochemical potential in leaky junctions, and (2) vibrational frequencies that saturate near ∼2225 cm–1─indicative of a zero electric field─in more diode-like junctions. These findings suggest a complex equilibrium (or nonequilibrium) balance between the semiconductor’s band structure and the ionic composition of the surrounding solution, with important implications for charge transfer and field-driven processes in electrochemical and photoelectrochemical systems.