Observation of 4th-order Water Oxidation Kinetics by Time-resolved Photovoltage Spectroscopy

SUMMARY Artificial photo-driven water oxidation has been proposed over half a century through a four-charges involved multiple-step oxygen evolution process. However, the knowledge of the intrinsic activity, such as the rate-law of the water oxidation reactions, has been inadequately studied. Up to date, the highest order reported is the 3rd-one under photoelectrochemical condition. In this work, we identified the 4th-order charge decay reactions on hematite by using a time-resolved surface photovoltage probe technique. A theoretical turnover frequency (TOF) >100 nm-2∙s-1 can be expected for O2 molecules when the hole density > 0.1 nm-2. This work demonstrates a facile and robust method to investigate the high-order reaction kinetics. More excitingly, this research built the bridge between the rate-law, rate-determining step and energy barrier of intermediates.