Detecting the oxyl radical of photocatalytic water oxidation at an n-SrTiO3/aqueous interface through its subsurface vibration.

Although the water oxidation cycle involves the critical step of O-O bond formation, the transition metal oxide radical thought to be the catalytic intermediate for this step has eluded direct observation. The radical represents the transformation of charge into a nascent catalytic intermediate, which lacks a newly formed bond and is therefore inherently difficult to detect. Here, using theoretical calculations and ultrafast in situ infrared spectroscopy of photocatalysis at an n-SrTiO3/aqueous interface, we reveal a subsurface vibration of the oxygen directly below, and uniquely generated by, the oxyl radical (Ti-O(•)). Intriguingly, this interfacial Ti-O stretch vibration, once decoupled from the lattice, couples to reactant dynamics (water librations). These experiments demonstrate subsurface vibrations and their coupling to solvent and electron dynamics to detect nascent catalytic intermediates at the solid-liquid interface at the molecular level. One can envision using the subsurface vibrations and their coupling across the interface to track and control catalysis dynamically.