The Formation Time of Ti-O^• and Ti-O^•-Ti Radicals at the n-SrTiO₃/Aqueous Interface during Photocatalytic Water...

The initial step of photocatalytic water oxidation reaction at the metal oxide/aqueous interface involves intermediates formed by trapping photogenerated, valence band holes on different reactive sites of the oxide surface. In SrTiO_3, these one-electron intermediates are radicals located in Ti-O^• (oxyl) and Ti-O^•-Ti (bridge) groups arranged perpendicular and parallel to the surface respectively, and form electronic states in the band gap of SrTiO₃. Using an ultrafast sub band gap probe of 400 nm and white light, we excited transitions between these radical states and the conduction band. By measuring the time evolution of surface reflectivity following the pump pulse of 266 nm light, we determined an initial radical formation time of 1.3 ± 0.2 ps, which is identical to the time to populate the surface with titanium oxyl (Ti-O^•) radicals. The oxyl was separately observed by a subsurface vibration near 800 cm^-1 from Ti-O located in the plane right below Ti-O^•. Second, a polarized transition optical dipole allows us to assign the 1.3 ps time constant to the production of both O-site radicals. After a 4.5 ps delay, another distinct surface species forms with a time constant of 36 ± 10 ps with a yet undetermined structure. As would be expected, the radicals' decay, specifically probed by the oxyl's subsurface vibration, parallels that of the photocurrent. Our results led us to propose a nonadiabatic kinetic mechanism for generating radicals of the type Ti-O^• and Ti-O^•-Ti from valence band holes based on their solvation at aqueous interfaces.