Atomic Layer Deposition of Epitaxial Iron Oxides for Photoelectrochemical Water Oxidation

Abstract

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.