Increasing oxidation time during fabrication of hematite (Fe 2O 3) films reduces the amount of grain boundaries, resulting in lower flat band potential and onset potential for water oxidation.
Hematite (α-Fe 2O 3) is a very promising material for solar water splitting that requires a high anodic potential to initiate the oxygen evolution reaction (OER). In this work, we explore the correlation between the downshift in flat band potential of hematite, V fb, and in onset potential of OER, V onset, caused by prolonged annealing. We observed a cathodic shift ( i.e., towards lower potentials) of 200 mV of V onset on model photoanodes consisting of ultra-thin hematite films, upon increasing the oxidation time during fabrication and without any further modifications. Detailed physical characterization, electrochemical impedance spectroscopy, and Mott–Schottky analysis revealed a quantitative correlation between the cathodic shift of V onset and a lowering of V fb. We identified a reduction in concentration of grain boundaries with increasing oxidation time, as the mechanism behind the observed shift of the V fb. The approach presented here can be seen as a complementary strategy to co-catalysts and other post-fabrication treatments to lower V onset. Moreover, it is generically applicable to photoelectrodes used to carry out oxidation and reduction half–cell reactions.