Solution-mediated nanometric growth of α-Fe₂O₃ with electrocatalytic activity for water oxidation

This paper describes a simple, low-temperature, and environmentally friendly aqueous route for the layer-by-layer nanometric growth of crystalline α-Fe₂O₃. The formation mechanism involves alternative sequences of the electrostatic adsorption of Fe²⁺ ions on the surface and the subsequent onsite oxidation to Fe³⁺. A combination analysis of X-ray diffraction, scanning electron microscopy, UV-Vis spectroscopy, and X-ray photoelectron spectroscopy revealed that α-Fe₂O₃ is directly formed without post-growth annealing via designed chemical reactions with a growth rate of ca. 1.7 nm per deposition cycle. The obtained α-Fe₂O₃ layer exhibits electrocatalytic activity for water oxidation and, at the same time, insignificant photo-electrocatalytic response, indicating its defective nature. The electrocatalytic activity was tailored by annealing up to 500 °C in air, where thermal diffusion of Sn⁴⁺ into the α-Fe₂O₃ lattice from the substrate probably provides an increased electrical conductivity. The subsequent surface-modification with Ni(OH)₂ lowers the overpotential (250 mV at 0.5 mA cm^-2) in a 1 M KOH solution. These findings open direct growth pathways to functional metal oxide nanolayers via liquid phase atomic layer deposition.