Quantitative Determination the Role of the Intra-bandgap States in the Kinetics of Water Photooxidation over Hematite Electrodes

Abstract

Abstract The intra-bandgap states on the hematite (α-Fe2O3) electrodes are believed to play an important role in water photooxidation. Yet, it is not fully understood how the intra-bandgap states are involved in the reaction. In this work, the role of the intra-bandgap states in water photooxidation on the α-Fe2O3 electrodes is investigated by a combination of multiple (photo-) electrochemical techniques and operando spectroscopic methods. Two kinds of surface states are observed on the electrodes during water photooxidation, and their role are quantitatively determined by the correlation with the steady state photocurrent. It is demonstrated that the intrinsic electronic surface states close to the conduction band can act only as the recombination center for the photocarriers while water photooxidation on the electrodes is dictated by the photogenerated surface states at higher energy level with a first reaction order for the surface holes. More importantly, the photogenerated surface states are revealed to be the reactant in the rate limiting step of water photooxidation. These findings in this study may be beneficial to elucidate the actual function of the surface states, and provide mechanistic insights into the oxygen evolution on the α-Fe2O3 photoanodes.