Defect- and Geometry-Controlled Fermi-Level Dynamics in TiO₂

The Fermi level dictates charge distribution and redox activity, yet its dynamic evolution under illumination remains poorly resolved. Using Kelvin probe force microscopy, we map time-dependent Fermi-level shifts in a model sample system, a single-crystal TiO₂, under front- and back-side ultraviolet irradiation at two-photon energies. High-energy ultraviolet irradiation lowers the surface Fermi level through the formation of persistent photoinduced surface oxygen vacancies, while low-energy ultraviolet irradiation induces only short-lived electron–hole generation with rapid surface recovery. Moreover, front-side irradiation yields a transient response, followed by a uniform surface modification, whereas back-side excitation produces slower, bulk-mediated shifts. In addition, gold nanoparticle decoration of the surface further modifies the Fermi-level kinetics through the Schottky barrier at the interface. By decoupling photon energy from illumination geometry, this work reveals the paired dynamics of carrier transport and defect-mediated formation that shape oxide energetics. These findings establish a foundation for Fermi-level engineering in photocatalysis, sensing, and optoelectronic applications.