No abstract

The conversion of diffusive forms of energy (electrical and light) into short, compact chemical bonds by catalytic reactions regularly involves moving a carrier from an environment that favors delocalization to one that favors localization.

<p>Theoretical descriptions differentiate catalytic activity of material surfaces for the water oxidation reaction by the stability of the reactive oxygen (O*) intermediate. The underlying conjecture is that there are several meta-stable steps of the reaction, each connected by free energy differences critically dependent on O*. Recently <i>in-situ, </i>time-resolved spectroscopy of the (<i>photo<br> </i>)-electrochemical water oxidation reaction identified the vibrational and optical signatures of O* time-evolution. However, there has been little connection between these inherently kinetic experiments and the underlying thermodynamic parameters of the theory. Here, we discover that picosecond optical spectra of the O* population modulated by a shift in reaction equilibria defines an effective equilibrium constant (K_eff) containing the relevant free-energy differences. A Langmuir isotherm as a function of electrolyte pH extracts K_eff using a model titania system (SrTiO₃). The results show how to obtain equilibrium constants of individual reaction steps on material surfaces, which had not been experimentally accessible previously. Further, we find that for a photo-excited reaction on a semiconductor surface tuning past a pH defined by K_eff doubles the initial O* population. That the free energies of the catalytic surface are definable through a time-resolved spectroscopy, alongside the finding that the surface recollects its explicit equilibrium with the electrolyte, provides a new and critical connection between theory and experiment by which to tailor the pathway of water oxidation and other surface reactions.</p>

The oxygen evolution reaction (OER) from water requires the formation of metastable, reactive oxygen intermediates to enable oxygen–oxygen bond formation. Conversely, 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, photodriven OER on transition metal oxide surfaces has characterized M–OH* for the charge trapping 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 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, we show that strain fields occur with the creation of some M–OH*, which modifies design strategies for tuning 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.

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’<br>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.