Direct and real-time observation of hole transport dynamics in anatase TiO2 using X-ray free-electron laser
Sang Han Park, Abhishek Katoch, Keun Hwa Chae, Sanjeev Gautam, Piter S. Miedema, Sang Wan Cho, Minseok Kim, Ru‐Pan Wang, Masoud Lazemi, Frank M. F. de Groot, Soonnam Kwon
Abstract
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.