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

Nanostructured hematite ( alpha -Fe sub(2)O sub(3)) 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 Fe sub(2)O sub(3) is demonstrated using FeCl sub(3) and H sub(2)O precursors at growth temperatures between 200 and 350 degree C. Self-limiting growth of Fe sub(2)O sub(3) is demonstrated with a growth rate of similar to 0.6 Aa per cycle. As-deposited, films are nanocrystalline with low chlorine impurities and a mixture of alpha - and gamma -Fe sub(2)O sub(3). Post-deposition annealing in O sub(2) leads to phase-pure alpha -Fe sub(2)O sub(3) 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 degree C) exhibit remarkably high photocurrent densities similar to 0.71 mA cm super(-2) at 1.53 V vs.the reversible hydrogen electrode (RHE) without further processing. Films annealed in air at 500 degree C show current densities of up to 0.84 mA cm super(-2) (1.53 V vs.RHE).