Aqueous Synthesis of Efficient Zinc Ferrite Thin Film Photoelectrodes for Water Oxidation

ABSTRACT Zinc ferrite (ZnFe ₂ O ₄ ) is a potential photoanode material for photoelectrochemical (PEC) water splitting due to its suitable properties, such as an ideal band gap, chemical robustness, low cost, and earth abundance. However, to date, ZnFe ₂ O ₄ struggles to achieve satisfactory performance. Here, we show the deposition of ZnFe ₂ O ₄ thin films using a simple aqueous sol‐gel method and unravel the role of processing conditions on their PEC performance. We obtained pure spinel zinc ferrite with a nanocrystalline porous morphology, as confirmed by X‐ray diffraction and electron microscopy, and a suitable optical band gap of 2.3 eV. We demonstrate how the PEC performance is greatly affected by the annealing temperature and environment. Annealing at high temperatures (700°C) under a nitrogen atmosphere provides the optimal conditions, achieving photoanodes exhibiting the highest water oxidation photocurrent density of 0.28 mA/cm ² at 1.23 V vs. RHE (reversible hydrogen electrode), ∼4 times higher than the best air‐annealed sample and also ∼4 times higher than the samples annealed in nitrogen below 600°C. The reasons behind this behavior are elucidated using spectroscopic methods, including optical, X‐ray photoelectron, and impedance, and attributed to enhanced electrical properties facilitating charge transport, providing guidelines for future improvement for these emerging ferrite photoanode materials.