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The oxygen evolution reaction is crucial to sustainable electro- and photo-electrochemical approaches to chemical energy production (for example, H2). Although mechanistic descriptions of the oxygen evolution reaction have been proposed, the frontier challenge is to extract the molecular details of its elementary steps. Here we discuss how advances in spectroscopy and theory are allowing for configurations of reaction intermediates to be elucidated, distinguishing between experimental approaches (static and dynamic) across a range of surface oxygen binding energies on catalysts (from ruthenium to titanium oxides).

Reaction intermediates buried within a solid-liquid interface are difficult targets for physiochemical measurements. They are inherently molecular and locally dynamic, while their surroundings are extended by a periodic lattice on one side and the solvent dielectric on the other. Challenges compound on a metal-oxide surface of varied sites and especially so at its aqueous interface of many prominent reactions. Recently, phenomenological theory coupled with optical spectroscopy has become a more prominent tool for isolating the intermediates and their molecular dynamics.

Oxygen evolution catalysis fuels the planet through photosynthesis and is a primary means for hydrogen storage in energy technologies. Yet the detection of intermediates of the oxygen evolution reaction (OER) central to the catalytic mechanism has been an ongoing challenge. This tutorial and minireview covers the relevance of ultrafast electronic and vibrational spectroscopy of the electrochemical transformations of a metal-oxide surface undergoing OER. Here, we highlight the ultrafast trigger and probes of the electron-doped SrTiO3/electrolyte as the primary example in which light probes across the electromagnetic spectrum have detected intermediate forms.

One of the most reactive intermediates for oxidative reactions is the oxyl radical, an electron-deficient oxygen atom. The discovery of a new vibration upon photoexcitation of the oxygen evolution catalysis detected the oxyl radical at the SrTiO3 surface. The vibration was assigned to a motion of the sub-surface oxygen underneath the titanium oxyl (Ti–O●−) created upon hole transfer to (or electron extraction from) a hydroxylated surface site. Evidence for such an interfacial mode is derived from its spectral shape, which exhibited a Fano resonance—a coupling of a sharp normal mode to continuum excitations. Here, this Fano resonance is utilized to derive precise formation kinetics of the oxyl radical and its associated potential energy surface (PES). 

The oxygen evolution reaction (OER) from water, while more stable on transition metal oxide surfaces than others, has nonetheless proved to be concomitant with charge-induced surface degradation. Since heterogeneous and nanostructured electrodes are often used and with a large excitation area, the degradation can be difficult to quantify. Here, we utilize single crystalline SrTiO3, highly efficient photoexcitation of the OER, and a focused laser to spatially define the degradation. A repetitive, ultrafast laser pulse above the band gap energy is employed, which allows for highly varied exposure of the surface using different scan methods. It also connects the work to the OER and its time-resolved mechanisms.

The oxygen evolution reaction (OER) from water is a critical component of a sustainable energy future; however, its mechanism has proved difficult to identify experimentally. This complexity is due to the elusive nature of electron and proton transfer intermediates that form within an interfacial water network and are buried at the solid–liquid interface. Here, we summarize recent measurements identifying the first two electron and proton transfer intermediates, e.g., OH∗ and O∗, on metallic oxide surfaces prior to the OER cycle, and the first electron and proton transfer intermediate, OH∗, as a metastable species during OER on a photo-excited, semiconducting oxide surface.

Water dissociation on transition metal oxide (TMO) surfaces regulates their catalytic activity in aqueous media. Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) has differentiated TMO surfaces by the population of their first hydration layer on a scale between water molecularly absorbed and water fully dissociated into hydroxyl groups. Here, we show that electron-doping a single TMO (SrTiO3: STO) can also span this range, with the data on lightly (0.1 wt % Nb) and moderately (0.7 wt % Nb) doped STO suggestive of partial and full water dissociation, respectively.

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.