First Evidence and Role of γ-Fe2O3 Seeds in Low-Temperature-Processed Sn-Doped Hematite Photoanodes

Sn doping is an effective strategy to improve the photoelectrochemical response of hematite photoanodes, but its activation typically requires high-temperature treatments that can degrade conductive substrates and limit device integration. Considering photoelectrochemical water splitting as a case study, this work investigates the mechanism of photoelectrochemical activation of Sn-doped hematite-based photoanodes by oxidative annealing of thin metallic Fe-Sn alloy films at a low temperature. X-ray absorption spectroscopy, Raman spectroscopy, grazing-incidence X-ray diffraction, and electron paramagnetic resonance reveal that while hematite is the main crystalline phase, poorly ordered γ-Fe2O3 seeds selectively form in Sn-containing films annealed at 450 °C and progressively transform into Sn-doped α-Fe2O3 at higher annealing temperatures. Under solar illumination in 1 M KOH, photoanodes annealed at 450 °C exhibit photocurrent density (≈480 μA/cm2) and onset potential (≈0.9 V vs RHE) comparable to those obtained after annealing at a higher temperature (750 °C). Sn K-edge X-ray absorption spectroscopy identifies Sn4+ in local Sn-Fe coordination environments consistent with incorporation into the hematite lattice. The γ-Fe2O3 domains may act as transient Sn reservoirs, accommodating Sn beyond the hematite solubility limit and promoting the formation of well-ordered, photoactive Sn-doped α-Fe2O3 nanocrystals, while limiting SnO2 segregation at an Sn/(Fe + Sn) atomic ratio of approximately 3 at %. Control films deposited under an oxygen atmosphere contain a lower γ-Fe2O3 fraction and show poor activity, indicating that low-temperature activation arises not from Sn addition alone but requires oxidation-driven restructuring of the metallic Fe-Sn precursor. The γ-Fe2O3 seeds represent structural fingerprints of a synthesis pathway that enables efficient Sn incorporation and low-thermal-budget fabrication of hematite photoanodes for solar water splitting.