Direct and real-time observation of hole transport dynamics in anatase TiO₂ using X-ray free-electron laser

Carrier dynamics affects photocatalytic systems, but direct and real-time observations in an element-specific and energy-level-specific manner are challenging. In this study, we demonstrate that the dynamics of photo-generated holes in metal oxides can be directly probed by using femtosecond X-ray absorption spectroscopy at an X-ray free-electron laser. We identify the energy level and life time of holes with a long life time (230 pico-seconds) in nano-crystal materials. We also observe that trapped holes show an energy distribution in the bandgap region with a formation time of 0.3 pico-seconds and a decay time of 8.0 pico-seconds at room temperature. We corroborate the dynamics of the electrons by using X-ray absorption spectroscopy at the metal L-edges in a consistent explanation with that of the holes.