Spatially resolved visualization of long-lived charge carriers in Al-doped SrTiO₃ by time-resolved microscopy

Al-doped SrTiO₃ (SrTiO₃:Al) exhibits exceptional performance for photocatalytic overall water splitting, yet the microscopic origins of its long-lived charge carriers remain insufficiently understood. Pattern-illumination time-resolved phase microscopy (PI-PM) was applied to directly visualize the spatiotemporal dynamics of electrons and holes in SrTiO₃, SrTiO₃:Al, and Rh-loaded SrTiO₃:Al thin films. PI-PM revealed that Al doping suppresses fast electron-hole recombination pathways associated with Ti³⁺ defect states and introduces a new hole-trapping state with a markedly delayed decay extending over two orders of magnitude compared with pristine SrTiO₃. Clustering analysis of all the local responses distinguished multiple kinetic categories and demonstrated that this Al-induced hole population is selectively quenched by hole scavengers, confirming its assignment as a long-lived, reactive hole species. Rh deposition introduced an additional slower electron response, attributed to electron trapping at Rh cocatalyst sites. Kinetic simulations reproduced these experimental features only when deep Al-induced hole traps and Rh-induced electron traps were incorporated. These results establish that Al-doping and Rh-cocatalyst loading generate spatially heterogeneous trap states that stabilize long-lived carriers at specific surface domains, providing the mechanistic basis for enhanced charge separation and reactivity in SrTiO₃:Al-based photocatalysts.