In-situ construction of photoanode with Fe2O3/Fe3O4 heterojunction nanotube array to facilitate charge separation for efficient water splitting

Hematite (α-Fe ₂O ₃ ), as a promising material in photoelectrochemical water splitting, has been given much attention to enhance its performance through a series of modifications. In this work, anodized Fe ₂O ₃ nanotubes were annealed in air and then in argon atmosphere. The samples annealed in argon underwent partial escape of oxygen atoms, thus generating Fe ₃O ₄ and eventually forming Fe ₂O ₃/Fe ₃O ₄ heterojunction in situ. The built-in electric field constructed by the heterojunction establishes a channel for effective charge transfer. In addition, the formation of heterojunction not only accelerates the charge transfer at the interface, but also shortens the band gap and promotes light absorption. As a result, the two-step annealed sample showed the best photoelectrochemical performance with a photocurrent of 2.5 mA cm ⁻² photocurrent at 1.6 V νs . RHE, threefold that of pure Fe ₂O ₃ nanotubes (0.75 mA cm ⁻² ). The highest hydrogen production rate and oxygen production rate reached 23.84 and 15.66 μmol cm ⁻² h ⁻¹, respectively. The facile two-step annealing method to generate Fe ₂O ₃/Fe ₃O ₄ heterojunction in-situ contributes to solving the problem of short cavity diffusion length and slow water oxidation kinetics.