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.