Electronically-Coupled Phase Boundaries in α-Fe₂O₃/Fe₃O₄ Nanocomposite Photoanodes for Enhanced Water Oxidation

Photoelectrochemical (PEC) water splitting reactions are promising for sustainable hydrogen production from renewable sources. We report here, the preparation of α-Fe₂O₃/Fe₃O₄ composite films via a single-step chemical vapor deposition of [Fe(O^tBu)₃]₂ and their use as efficient photoanode materials in PEC setups. Film thickness and phase segregation was controlled by varying the deposition time and corroborated through cross-section Raman spectroscopy and scanning electron microscopy. The highest water oxidation activity (0.48 mA/cm² at 1.23 V vs RHE) using intermittent AM 1.5 G (100 mW/cm²) standard illumination was found for hybrid films with a thickness of 11 μm. This phenomenon is attributed to an improved electron transport resulting from a higher magnetite content toward the substrate interface and an increased light absorption due to the hematite layer mainly located at the top surface of the film. The observed high efficiency of α-Fe₂O₃/Fe₃O₄ nanocomposite photoanodes is attributed to the close proximity and establishment of 3D interfaces between the weakly ferro- (Fe₂O₃) and ferrimagnetic (Fe₃O₄) oxides, which in view of their differential chemical constitution and valence states of Fe ions (Fe²⁺/Fe³⁺) can enhance the charge separation and thus the overall electrical conductivity of the layer.