Coherent Acoustic Interferometry During the Photo-Driven Oxygen Evolution Reaction Associates Strain Fields with the Reactive Oxygen Intermediate

The oxygen evolution reaction (OER) from water requires the formation of meta-stable, reactive oxygen intermediates to enable oxygen-oxygen bond formation. On the other hand, such reactive intermediates could also structurally modify the catalyst. A descriptor for the overall catalytic activity, the first electron and proton transfer OER intermediate from water, (M-OH*), has been associated with significant distortions of the metal-oxygen bonds upon charge-trapping. Time-resolved spectroscopy of in-situ, photo-driven OER on transition metal oxide surfaces has characterized M-OH* for the charge trapped and the symmetry of the lattice distortions by optical and vibrational transitions, respectively, but had yet to detect an interfacial strain field arising from a surface coverage M-OH*. Here, we utilize picosecond, coherent acoustic interferometry to detect the uniaxial strain normal (100) to the SrTiO₃/aqueous interface directly caused by Ti-OH*. The spectral analysis applies a fairly general methodology for detecting a combination of the spatial extent, magnitude, and generation time of the interfacial strain through the coherent oscillations’
phase. For lightly n-doped SrTiO₃, we identify the strain generation time (1.31 ps), which occurs simultaneously with Ti-OH* formation, and a tensile strain of 0.06% (upper limit 0.6%). In addition to fully characterizing this intermediate across visible, mid-infrared, and now GHz-THz probes on SrTiO₃, that strain fields occur with the creation of some M-OH* modifies design strategies for tuning material properties for catalytic activity and provides insight into photo-induced degradation so prevalent for OER. To that end, the work put forth here provides a unique methodology to characterize intermediate-induced interfacial strain across OER catalysts.