Mechanism of Temperature-Programmed Photoelectron Emis-sion (TPPE) from Cu₂O/Cu Surfaces: The Role of Oxygen Va-cancies in Photoredox Activation

The performance of coatings, corrosion barriers, photocatalysts, and tribological materials is greatly influenced by in situ surface properties, requiring highly sensitive and reproducible operando surface characterization methods. We previously developed a temperature-programmed photoelectron emission (TPPE) method to clarify electron transfer behavior on light-irradiated metal surfaces. TPPE is sensitive to surface temperature and prior chemical exposure, which affect the total photoemitted electron count (N_T), the photothreshold, and the activation energy derived from Arrhenius plots of N_T obtained during heating–cooling cycles. This study examines the reproducibility of TPPE data and the TPPE mechanisms for Cu₂O/Cu surfaces subjected to mechanical abrasion, cleaning, plasma treatment, and subsequent immersion in organic liquids. The resulting Arrhenius plots reveal both positive and negative activation energies, distinctly depending on the treatment conditions. Negative activation energies during cooling are associated with photoredox-mediated emission. TPPE is attributed to oxygen vacancies within the Cu₂O surface layer interfaced with metallic Cu, serving as a direct probe of these vacancy-related states. The TPPE characteristics (N_T intensity and activation energy) following exposure to various polar and nonpolar organic molecules (e.g., acetone, toluene, hexane, ethanol) correlate with the electronic properties of these vacancies, consistent with previous observations for ambient air, alcohol, and water vapor exposure. Under illumination, Cu₂O vacancy states enhance photocarrier extraction (electrons and holes) and accelerate surface redox reactions within adsorbed thin films, thereby improving photocatalytic performance. Notably, the reciprocal dielectric constant of the solvents significantly influences TPPE, indicative of electrostatic surface–solvent interactions. Finally, the TPPE mechanism is discussed in the context of antiviral inactivation at the metallic copper–environment interface.