Spontaneous water oxidation at hematite (α-Fe2O3) crystal faces.

Hematite (α-Fe2O3) persists as a promising candidate for photoelectrochemical water splitting, but a slow oxygen evolution reaction (OER) at its surfaces remains a limitation. Here we extend a series of studies that examine pH-dependent surface potentials and electron-transfer properties of effectively perfect low-index crystal faces of hematite in contact with simple electrolyte. Zero-resistance amperometry (ZRA) was performed in a two electrode configuration to quantify spontaneous dark current between hematite crystal face pairs (001)/(012), (001)/(113), and (012)/(113) at pH 3. Exponentially decaying currents initially of up to 200 nA were reported between faces over 4 min experiments. Fourth-order ZRA kinetics indicated rate limitation by the OER for current that flows between (001)/(012) and (001)/(113) face pairs, with the (012) and (113) faces serving as the anodes when paired with (001). The cathodic partner reaction is reductive dissolution of the (001) face, converting surface Fe(3+) to solubilized aqueous Fe(2+), at a rate maintained by the OER at the anode. In contrast, OER rate limitation does not manifest for the (012)/(113) pair. The uniqueness of the (001) face is established in terms of a faster intrinsic ability to accept the protons required for the reductive dissolution reaction. OER rate limitation inversely may thus arise from sluggish kinetics of hematite surfaces to dispense with the protons that accompany the four-electron OER. The results are explained in terms of semiquantitative energy band diagrams. The finding may be useful as a consideration for tailoring the design of polycrystalline hematite photoanodes that present multiple terminations to the interface with electrolyte.