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 of M–OH*.

Water oxidation is considered as one of the most important reactions in solar-to-fuel generation. The initial catalytic intermediates formed on an ultrafast timescale play a great role in controlling water oxidation reaction. Here, we use ultrafast in situ infrared attenuated total reflectance spectroscopy to study the initial water oxidation intermediates at a state-of-the-art TiO2 P25/aqueous interface.

The conversion of diffusive forms of energy (electrical and light) into short, compact chemical bonds by catalytic reactions regularly involves moving a carrier (electron or hole) from an environment that favors delocalization to one that favors localization. While delocalization lowers the energy of the carrier through its kinetic energy, localization creates a polarization around the carrier that traps it in a potential energy minimum. The trapped carrier and its local distortion—termed a polaron in solids—can play a role as a highly reactive intermediate within energy-storing catalytic reactions but is rarely discussed as such. Here, we present this perspective of the polaron as a catalytic intermediate through recent in-situ and time-resolved spectroscopic investigations of photo-triggered electrochemical reactions at material surfaces.

Theoretical descriptors differentiate the catalytic activity of materials for the oxygen evolution reaction (OER) by the strength of oxygen binding in the reactive intermediate created upon electron transfer. Recently time-resolved spectroscopy of (photo)-electrochemically driven OER followed the vibrational and optical spectra of this intermediate, denoted M-OH*. However, these inherently kinetic experiments have not been connected to the relevant thermodynamic quantities. Here, we discover that picosecond optical spectra of the Ti-OH* population on lightly doped SrTiO3 are ordered by the surface hydroxylation.

Understanding the equilibrium conditions at the metal oxide/aqueous interface is a key component toward visualizing the structure of water in confined environments and differentiating the catalytic activity of transition-metal oxides. While ambient pressure X-ray photoelectron spectroscopy (AP-XPS) has been the primary technique to investigate the formation of a hydration layer on many surfaces, results over the extended relative humidity (RH) range accessible experimentally have not been compared quantitatively to theoretical predictions. With the use of first-principles theoretical methods and accumulated knowledge of AP-XPS spectral analysis, we do so here for a model surface, TiO2-terminated undoped SrTiO3(100) (STO).