Nanostructured AlFeO₃ thin films as a novel photoanode for photoelectrochemical water splitting

The earth-abundant and robust aluminum ferrite, AlFeO₃ (AFO), is mainly studied in the context of ferroelectrics. Herein, we demonstrate that AFO can be used as a stable solar absorber in photoelectrochemical cells for solar water splitting, exhibiting attractive performance. This is the first report on the photoelectrochemical activity of AFO. AFO thin-film photoelectrodes prepared by solution-processing methods are composed of vertically oriented thin nanosheets, featuring the rhombohedral symmetry (R3c) and n-type conductivity. The as-prepared AFO photoanodes generate a photocurrent density of +0.78 mA·cm⁻² at 1.23 V vs. reversible hydrogen electrode (RHE) with the photocurrent onset potential (U_onset) close to the flat band potential of 0.5 V vs. RHE in the presence of hole scavengers. Remarkably, the U_onset of AFO for solar water splitting coincides with the flat band potential as well, which is rare in n-type inorganic absorbers. We also report other properties of AFO associated with photoelectrochemical performance. AFO films exhibit a band gap energy of 2.31 eV and positive band edges with low dispersion. Moreover, the carrier lifetimes in AFO films are up to millisecond timescales under the mediation of defect traps. Based on the photoelectrochemical behavior and optoelectronic properties, we believe that AFO has great potential for application in photoelectrochemical cells.