Interplay of structural and magnetic properties in hematite/magnetite anodic nanotubes for photoelectrochemical water splitting

Hematite (α-Fe₂O₃) is a promising photoanode material for solar water splitting due to its abundance, stability, suitable band gap and low cost. However, its photoelectrochemical (PEC) performance is often limited by poor electrical conductivity and high charge carrier recombination. In this work, self-organized anodic iron oxide nanotubes were synthesized by electrochemical anodization and systematically studied as a function of annealing temperature (400–700 °C) and nanotube length with different anodization times (leading up to 4 μm) to establish a clear structure–property–performance relationship. Structural and magnetic characterization revealed the coexistence of hematite (α-Fe₂O₃) and magnetite (Fe₃O₄) phases, confirmed by Morin and Verwey magnetic transitions. Annealing at 600 °C promotes predominantly hematite (∼80%) with controlled magnetite contribution and improved crystallinity. Electrochemical impedance spectroscopy and Mott–Schottky analysis demonstrate reduced semiconductor resistance, lower interfacial charge transfer resistance, faster charge transfer kinetics and suppressed recombination for shorter nanotubes (2 min anodization). The optimized photoelectrodes exhibit the highest photocurrent density, lowest onset potential and highest intrinsic solar-to-chemical (ISTC) efficiency. These results demonstrate that controlling phase balance of, defect density and nanotube length is more critical than maximizing optical absorption for improving hematite-based photoelectrodes for solar fuel production.