Electrochemically grown Fe2O3/Fe3O4 heterostructure nanotubes with In2O3 induced tandem internal electric fields for enhanced photoelectrochemical water oxidation

Abstract

Nanostructured hematite (α-Fe₂O₃) shows promise as a semiconductor for photoelectrochemical (PEC) water oxidation. However, it suffers from inadequate charge separation, limited hole-collection efficiency and sluggish kinetics. Herein, a nanotubular Fe₂O₃/Fe₃O₄ p–n heterojunction is prepared via electrochemical anodization to in situ construct an internal electric field (IEF) that facilitates charge separation from photoactive hematite. Additionally, In₂O₃ clusters are introduced to form a second IEF with dual-phase iron oxides, exploiting their Fermi level difference. The unique configuration of the dual IEFs in a novel tandem way synergistically promotes charge carrier separation/migration, enhancing PEC performance. Specifically, the 1^st IEF between Fe₂O₃ and Fe₃O₄ accelerates electron migration from Fe₃O₄ to Fe₂O₃ (with holes transporting in the opposite direction), while the 2^nd IEF at the In₂O₃ and Fe₂O₃/Fe₃O₄ interface drives holes towards the In₂O₃ surface, enhancing the hole-collection efficiency. The composite photoanode achieves a state-of-the-art current density of 11.5 mA cm⁻² at 1.55 V_RHE and a superior applied bias photon-to-current efficiency of 0.44% at 0.95 V. DFT calculations reveal that In₂O₃ induces an electron-deficient surface, creating favorable adsorption sites for oppositely charged key intermediates (*OOH). This work presents a novel approach for modulating reaction kinetics via the construction of tandem IEFs and holds great significance for the rational design of efficient PEC catalysts.