The initial step in the photo-catalytic water oxidation reaction at the metal oxide/aqueous interface involves radicals formed by trapping photo-generated charges into surface-bound radicals. Subsequently, these one-electron intermediates of the catalytic cycle will react with water molecules to produce the first O-O bond of O2 evolution. In SrTiO3, these radicals are expected to be located in Ti-O● (oxyl) and Ti-O●-Ti (bridge) groups arranged perpendicular and parallel to the surface respectively, and form mid-gap states in the band gap of SrTiO3. Using an ultrafast sub band-gap visible light probe of 400 nm wavelength and white light, we excite transitions specifically from the radical states to the conduction band. By measuring the time evolution of surface reflectivity following photo-excitation, we determined a characteristic radical formation time of 1.3 ± 0.2 ps. Further, this formation time coincides with the population growth of the Ti-O● (oxyl) radical observed by its vibrational signature at 800 cm-1. A polarization dependence of the transition optical dipole, together with the mapping o the oxyl’s vibrational signature to its mid-gap state in time, allows us assign the 1.3 ps time constant to the production of both O-site radicals. Our results, together with the fact that a 1.3 ps time constant also describes hydroxyl stretch vibrational energy transfer to neighboring molecules in water, led us to propose a new kinetic mechanism for generating O-site radicals based on their solvation in a water environment. Recently, by following the dynamics of radicals to microsecond time scales, we found two new characteristic time constants. Unlike the formation time, these time constants depend sensitively on the electrolytes’ ionic strength and pH and exhibit a kinetic H/D isotope effect. These dependencies suggest that we can probe the later steps of the catalytic cycle that involve O-O bond formation.

Dynamics of photoelectrochemical water splitting on SrTiO3

253rd National Meeting (2017)

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