Femtosecond X-ray Tracking of Oxygen Vacancy-Driven Shallow-to-Deep Trap Transition and Its Impact on Charge Carrier Dynamics in Blue TiO2

Abstract Oxygen vacancies introduced by self-reduction are central to the enhanced light absorption and photocatalytic activity of blue TiO2, yet how they reshape the ultrafast dynamics of photogenerated charge carriers has remained unresolved. Here, we directly track these dynamics across femtosecond-to-microsecond time scales by combining X-ray free electron laser (XFEL)-based femtosecond X-ray transient absorption (fs-XTA) spectroscopy at the Ti K-edge with optical transient absorption (OTA) measurements. The fs-XTA kinetics reveal a sequential charge localization process: hot electrons cool into shallow trap states within ∼110 fs and ∼1.3 ps, and subsequently undergo a deeper trapping transition on the nanosecond time scale that is entirely absent in pristine TiO2. Density functional theory (DFT) calculations identify these deep traps as inter-band-gap states arising from energetically overlapping Ti 3d and O 2p orbitals distributed heterogeneously throughout the bulk lattice. Crucially, charge carriers localized in these deep traps undergo charge recombination approximately 2.5 times slower than those in pristine TiO2, as quantified by nanosecond OTA. This element-specific, time-resolved picture establishes a mechanistic link between oxygen vacancy density, deep trap formation, and the extended carrier lifetimes that govern macroscopic photocatalytic performance in reduced TiO2.