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.