Atomic Layer Deposition of Epitaxial Iron Oxides for Photoelectrochemical Water Oxidation

Solar-to-hydrogen energy conversion is regarded as an integral component in the future renewable energy infrastructure. Photo-assisted electrochemical (PEC) water splitting remains one possible route towards a viable solar-to-hydrogen conversion technology. Of the candidate materials for PEC water splitting photoanodes, hematite (α-Fe₂O₃) remains one of the most promising semiconductors for the water oxidation half-reaction because of its suitable bandgap for solar absorption, its terrestrial abundance, and its stability in aqueous. However, α-Fe₂O₃ photoanodes still demonstrate relatively poor PEC behavior due to slow water-splitting kinetics, poor charge transport, and challenges in the fabrication of high-quality Fe₂O₃ nanostructures necessary for efficient PEC devices.

Here, we show how epitaxial atomic layer deposition (ALD) can allow for the control of Fe₂O₃ phase, enables directed crystal growth, and permits nanostructured epitaxy. First, we utilize a transparent conducting substrate, (tin-doped indium oxide, ITO) as an isomorphic template for the epitaxial stabilization of the rare bixbyite phase of iron oxide (β-Fe₂O₃) and explore the PEC water oxidation properties of this material. Then, we utilize the metastable nature of β-Fe₂O₃ to produce various epitaxial α-Fe₂O₃/ITO films via a topotactic phase transition. These epitaxial α-Fe₂O₃ films provide benefits over their polycrystalline counterparts, possibly due to their well-defined crystallite orientations and reduction in density of high-angle grain boundaries. Finally, we show that ALD enables epitaxy of α-Fe₂O₃ on the walls of ITO nanowires, thereby providing a route to high-quality core-shell nanostructures which orthogonalize light absorption and charge transport to interfaces.