Low temperature atomic layer deposition of highly photoactive hematite using iron(iii) chloride and water

Abstract

Nanostructured hematite (α-Fe₂O₃) has been widely studied for use in a variety of thin film applications including solar energy conversion, water oxidation, catalysis, lithium-ion batteries, and gas sensing. Among established deposition methods, atomic layer deposition (ALD) is a leading technique for controlled synthesis of a wide range of nanostructured materials. In this work, ALD of Fe₂O₃ is demonstrated using FeCl₃ and H₂O precursors at growth temperatures between 200 and 350 °C. Self-limiting growth of Fe₂O₃ is demonstrated with a growth rate of ∼0.6 Å per cycle. As-deposited, films are nanocrystalline with low chlorine impurities and a mixture of α- and γ-Fe₂O₃. Post-deposition annealing in O₂ leads to phase-pure α-Fe₂O₃ with increased out-of-plane grain size. Photoelectrochemical measurements under simulated solar illumination reveal high photoactivity toward water oxidation in both as-deposited and post-annealed films. Planar films deposited at low temperature (235 °C) exhibit remarkably high photocurrent densities ∼0.71 mA cm⁻² at 1.53 V vs. the reversible hydrogen electrode (RHE) without further processing. Films annealed in air at 500 °C show current densities of up to 0.84 mA cm⁻² (1.53 V vs. RHE).