Ultrathin Hematite on Mesoporous WO₃ from Atomic Layer Deposition for Minimal Charge Recombination

α-Fe₂O₃ is considered as one of the most suitable materials for photoelectrochemical (PEC) water oxidation due to its appropriate band gap energy, band positions, and abundance. However, the short hole diffusion length (2–4 nm) of α-Fe₂O₃ is a long-term problem to be solved. Here, a core–shell-structured ultrathin α-Fe₂O₃ on mesoporous WO₃ photoanode was fabricated by atomic layer deposition (ALD) to achieve minimal charge recombination, which is the main reason for the lower PEC performance of α-Fe₂O₃. A facile drop-casting method was adopted to produce mesoporous WO₃ scaffolds, and the subsequent ALD process formed uniform and ultrathin α-Fe₂O₃ layers that act as both light absorption and protective layers. The ultrathin α-Fe₂O₃ layer with a thickness of only ∼8 nm on mesoporous WO₃ scaffolds has been proven to achieve a reasonable photocurrent density, which originates mainly from enhanced light absorption and minimized charge recombination. The PEC performance of the as-fabricated m-WO₃/Fe₂O₃ photoanode was measured, and the effect of an oxygen evolution cocatalyst (OEC) and a TiO₂ overlayer on the performance was also studied. Consequently, the photoanode with ∼8 nm thick α-Fe₂O₃ showed an ∼4 mA/cm² photocurrent density at 1.6 V vs reversible hydrogen electrode (RHE) under 1 sun illumination.