Deciphering the Limitations of γ-Fe₂O₃ as a Photoanode via Electrochemical and Near-Field Heterodyne Transient Grating Spectroscopic Analyses

We fabricated a maghemite (γ-Fe2O3) electrode to investigate the reasons why γ-Fe2O3 cannot be utilized as a photoanode. The γ-Fe2O3 electrode did not generate a photocurrent under illumination. We identified the reasons for the PEC inactivity of γ-Fe2O3 through various electrochemical analyses and the NF-HD-TG technique, one of the time-resolved spectroscopic methods. We confirmed that the recombination between free holes and trapped electrons in the γ-Fe2O3 electrode is faster than that in hematite (α-Fe2O3), which is considered to be accelerated by abundant oxygen vacancies, and we also observed the accelerated recombination between many detrapped electrons and photogenerated holes at the surface. Additionally, we demonstrated that the long-lived holes at the surface of the γ-Fe2O3 electrode do not contribute to charge transfer in water oxidation, which reveals a critical reason why γ-Fe2O3 cannot be used as a photoanode.