The First Hole Transfer in Photocatalytic Water Oxidation by Transient Spectroscopy

David Herlihy^a, Xihan Chen^a, Matthias Waegele^a, Tanja Cuk^a

^aUniversity of California, Berkeley, US, Berkeley, California, EE. UU., Berkeley, United States

Proceedings of International Conference on New Advances in Materials Research for Solar Fuels Production (SolarFuel14)

Montréal, Canada, 2014 June 25th - 26th

Organizer: Thomas Hamann

Invited Speaker, Tanja Cuk, presentation 044
Publication date: 16th April 2014

Photo-electrochemical experiments have sought to map the water oxidation cycle by modeling the dependence of steady state O₂ evolution on free energy differences between reactants and products, tuned by the applied potential. In principle, the transient response of the photo-driven water oxidation reaction directly measures the disparate kinetic components of the catalytic cycle that go into those models. Here, we show how the transient optical spectroscopy of the photo-driven water oxidation reaction directly measures the time-dependent kinetics of interfacial hole transfer at the anode. At the n-SrTiO₃/water interface, the kinetics reveal rate constants spanning time scales of 10 ps to 1 ns that can be directly assigned to, and are exponential dependent on, the surface hole potential. At these fast time scales, and in basic solution, the reaction occurring at the anode is suggested to be h+ + OH- --> OH*, the initial hole transfer of the water oxidation reaction. In analogy to Tafel kinetics used by electrochemists, we obtain a coefficient, α=0.20±0.05, describing how the applied potential reduces the activation barrier for this reaction. A redox potential for OH-/OH* is also suggested. For other catalysts that cannot be excited directly by light, such as Co₃O₄, a unique photodiode configuration is used to inject holes at ultrafast time scales into the catalyst over-layer. Transient grating spectroscopy applied to this photodiode/catalyst configuration to investigate the interfacial kinetics are currently underway.

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