Improved Photoelectrochemical Performance of Chemically Grown Pristine Hematite Thin Films

Abstract

The alpha phase of hematite (α-Fe2O3) is one of the most promising catalysts for photoelectrochemical (PEC) water splitting among several photoanode materials due to its suitable band gap and stability in aqueous solutions. The surface structure and morphology of films play pivotal roles in the enhancement of water oxidation reaction kinetics. In this work, α-Fe2O3 films were produced via either spray pyrolysis (SP), chemical vapor deposition (CVD), or aerosol-assisted chemical vapor deposition (AACVD). Their structural and morphological properties were subsequently characterized by powder x-ray diffraction (PXD), scanning electron microscopy (SEM), and Raman spectroscopy. High-quality thin films were best achieved by AACVD annealed at 525 °C, possessing an average thickness of 0.75 µm with 85% transmittance and an optical absorption onset at 650 nm. The results showed that the thermal oxidation process achieved at 525 °C eliminated undesired impurity phases, such as FeO and Fe3O4 , and enabled the microstructure to be optimized to facilitate the generation and transport of photogenerated charge carriers. The optimized α-Fe2O3 film showed a stable PEC water oxidation current density of ~1.23 mA cm-2 at 1.23 V (vs. RHE), with an onset potential of 0.76 V, under AM 1.5 irradiation. The obtained higher current density of pristine α-Fe2O3 thin films obtained by the AACVD method is unique, and the films presented good photocurrent stability with 92% retention after 6 h. Data from electrochemical impedance spectroscopy (EIS) corroborated these results, identifying fast charge transfer kinetics with decreased resistance and an electron lifetime of 175 µs. Quantitative measurements showed that 1.2 μmol cm-2 of oxygen could be produced at the photoanode in 6 h.Graphical Abstract