Long-lived photoexcitation probed by photo-induced enhanced Raman spectroscopy: unveiling charge dynamics in Ag–TiO2 nano-heterojunctions

This work explores Photo-Induced Enhanced Raman Spectroscopy (PIERS) as a tool to investigate charge carrier dynamics in nanometer-thick Ag-TiO₂ heterojunctions with a Schottky barrier. Due to the light-induced charge transfer process at the semiconductor-metal interface, PIERS provides a significant signal enhancement over traditional Surface-Enhanced Raman Spectroscopy (SERS). In turn, a remarkably stable PIERS signal lasting over 10 days after UVC light illumination cannot be explained exclusively by the presence and the lifetime of the induced oxygen vacancies, so other features of the Ag-TiO₂ heterojunction must be responsible for this effect. Time-resolved Raman spectroscopy, photoluminescence (PL), UV-Vis, XPS, and I-V characterization were used to explore charge migration mechanisms further to prove PIERS applicability. While PL showed rapid healing of oxygen vacancies, the correlation of the PIERS signal with changes in the Schottky barrier height and relative changes in the electron density under various lighting conditions indicates that both Hot Electron Injection (HEI) and Plasmon-Induced Resonance Energy Transfer (PIRET) are responsible for the Raman signal changes over time. We showed that both phenomena can be probed by in situ PIERS spectroscopy. This proof-of-principle paves the way for developing more advanced photoactive semiconductor-metal composites suitable for energy conversion or storage, as well as SERS and PIERS analytics.