Characterizing two surface states and their role in the photoinduced oxygen evolution reaction on hematite via photocurrent kinetics

Hematite (α−Fe2O3) is a promising photoanode for solar water splitting whose efficiency is limited by rapid charge recombination, sluggish hole transport and slow oxygen evolution reaction kinetics. Understanding which of these factors actually leads to inefficiency, i.e. nonunitary photon conversion, is challenging. Here we show, for a model hematite photoanode, that analysis of wavelength dependent (405–645 nm) photocurrent kinetics as a function of bias (0.9–1.65 V vs. RHE) reveals two surface states. The observed bias dependence and relative size of the charge transfer resistances associated with each state are most easily explained by the following scenario. Our α-Fe2O3(0001) anode is characterized by a mixed Fe/O termination that results in populations of monodentate and bidentate coordinated surface oxygens. Bidentate coordinated surface O(H) are the active site for the photo-induced OER but populations of monodentate surface OH change in response to applied bias/illumination in a manner that controls surface charge. At potentials where OER occurs in the dark both sites are catalytically active.