High photoactivity in ultrathin as-grown hematite films prepared by atomic layer deposition*

Nanostructured hematite ($\alpha$-Fe$_2$O$_3$) has been widely studied for use in a variety of thin film applications including solar energy conversion, water oxidation, catalysis, and gas sensing. Among established deposition methods, atomic layer deposition (ALD) is a leading technique for large-scale, controlled synthesis of a wide range of nanostructured materials. In this work, ALD of Fe$_2$O$_3$ is demonstrated using FeCl$_3$ and H$_2$O precursors at growth temperatures between $200-350^{\circ}$C. Self-limiting growth of Fe$_2$O$_3$ is observed with a growth rate of $\sim0.06$ nm/cycle. As-deposited, films are nanocrystalline with low Cl impurities and a mixture of $\alpha$- and $\gamma$-Fe$_2$O$_3$. Post-deposition annealing in O$_2$ leads to phase-pure hematite 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$^{\circ}$C) exhibit remarkably high photocurrent densities $\sim0.71$ mA/cm$^{2}$ at 1.53 V vs. the reversible hydrogen electrode (RHE) without further processing. Films annealed in air at 500$^{\circ}$C show current densities of up to 0.84 mA/cm$^{2}$ (1.53V vs. RHE).