Doped Magnesium Ferrite Thin Films as Photoanodes for Water Oxidation

Magnesium ferrite (MgFe₂O₄) is a promising photoanode material for photoelectrochemical (PEC) water oxidation, combining a suitable bandgap, chemical stability, low cost, and earth abundance. However, to date, MgFe₂O₄ has remained largely underexplored. Here, we report the deposition of MgFe₂O₄ thin films via a simple aqueous route and systematically examine the effects of processing conditions and aliovalent doping on PEC performance. The fabricated films exhibit a nanocrystalline spinel structure with an optical bandgap of ~2.2 eV. We demonstrate that the PEC activity is strongly governed by annealing conditions, with high-temperature treatment (650 °C) under nitrogen rather than air yielding optimal performance. Under these conditions, the photoanodes deliver photocurrent densities that already surpass previously reported values for MgFe₂O₄. Titanium doping further enhances performance, with a photocurrent of 0.14 mA/cm² at 1.23 V versus reversible hydrogen electrode (RHE). Spectroscopic analyses (optical, X-ray photoelectron spectroscopy (XPS), and impedance) link these improvements to changes in electronic structure and charge transport. Additional gains are achieved through the incorporation of a surface cocatalyst, achieving a photocurrent of 0.18 mA/cm². Our results not only set the current performance record for magnesium ferrite photoanodes, establishing it as a viable PEC material, but also provide general practical guidelines for advancing ferrite-based photoelectrodes.