This talk will highlight recent developments in the application of ultrafast optical spectroscopy to charge transfer across and charge transport along electrode/electrolyte interfaces. The experimental configurations are chosen such that they highlight one-electron intermediates of the water oxidation reaction; they demonstrate the potential for direct spectroscopic techniques to reveal the dynamics behind heterogeneous catalysis. By time-resolved reflectance changes at the n-SrTiO3/aqueous interface during the photo-catalytic water oxidation reaction (initiated by a UV pulse), the full surface-limited charge transfer reaction to create intermediates is uncovered: the formation of the Oxyl (Ti-O*) and the Bridge (Ti-O*-Ti) radicals, with an identical time constant of 1.3 ps, out of a decaying VB hole population. The O-site radicals create mid-gap states, separating their optical signature from the VB holes; each have unique, polarized optical dipole transitions to the CB edge. Further, the population growth of the Oxyl, measured separately by a sub-surface vibration in the plane right below it, exhibits the 1.3 ps time constant. The results lead to a kinetic model where solvation rate-limits the formation of O-site radicals. Other experiments, which utilize ultrafast optical diffraction to measure hole diffusion at the n-GaN/aqueous interface, show that mid-gap states of trapped one-electron intermediates of water oxidation increase the interfacial hole mobility. This result for the first time demonstrates that, at surfaces, charge transport depends on the presence of catalytically active sites.

Ultrafast optical spectroscopy of charge transfer kinetics and charge transport at electrode/electrolyte interfaces

253rd National Meeting (2017)

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