Observation of 4th-order water oxidation kinetics by time-resolved photovoltage spectroscopy.

Artificial photo-driven water oxidation has been proposed over half a century through a four-charge 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 third one under photoelectrochemical condition. In this work, we identified the fourth-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.