Ultrafast optical spectroscopy of thermodynamics & kinetics of reaction steps at an electrode surface*

Computationally, often the energetics of intermediate reaction steps differentiate the efficiency of heterogeneous catalysts for product evolution.  Yet, when compared to experiment, kinetic models are applied.  For example, a material’s activity measured by one rate of product evolution is plotted as a function of the calculated formation energies of intermediate chemical forms.  A critically important reaction for which this dichotomy between experiment and theory exists is the oxygen evolution reaction (OER) from water.  In the laboratory group, we employ time-resolved electronic (visible) and vibrational (infrared) spectroscopy to deconstruct OER into its individual reaction steps on an electrode surface. In the presentation, I will describe the experimental methodology and the chosen model system, the n-doped SrTiO₃/aqueous interface.  I’ll show how these experiments identify both the rates and energetics of the first reaction step, the release of a proton and electron from an absorbed water species, denoted by OH* or O*. A recent success was to isolate a Langmuir isotherm of the intermediate population arising within < 2 ps on the SrTiO₃ surface.  This < 2 ps population also engenders interfacial strain that leads to coherences in the visible spectroscopy.  Current work connects the decay of these intermediates at microsecond timescales, presumably related to later reaction steps of OER, to their pH-dependent formation.  During the course of the presentation, connections between ultrafast optical and x-ray probes of this interface will be suggested. A growing area is to apply the particular experimental methodology to a surface of similar electronic structure but diverse crystal geometries, namely the photo-electrochemistry of rutile TiO₂.