Spray Pyrolysis Deposition of ZnFe₂O₄/Fe₂O₃ Composite Thin Films on Hierarchical 3‑D Nanospikes for Efficient Photoelectrochemical Oxidation of Water

In this work, we study the role of nanotextured ZnFe₂O₄/Fe₂O₃composite thin films fabricated by ultrasonic spray pyrolysis (USP) on the photoelectrochemical water oxidation reactions. The ZnFe₂O₄/Fe₂O₃ composites with different molar ratios are deposited on three-dimensional nanospikes (NSP) substrate, and the results are compared with those for planar devices. It is observed that optical absorption and charge separation due to larger surface area is significantly enhanced in nanotextured photoactive ZnFe₂O₄/Fe₂O₃ films. After characterization of ZnFe₂O₄/Fe₂O₃ composite films with different molar ratios (ZF1, ZF2, and ZF3), we find that the nanotextured ZF1 composite with a molar ratio of 1:1 has the highest activity with photocurrent density of 2.19 mA/cm² in photoelectrochemical oxidation of water. This photocurrent density is 3.4 and 2.73 times higher than the photocurrent density values of pure hematite on planar fluorine-doped tin oxide (FTO) coated glass and the highest reported value of ZnFe₂O₄/Fe₂O₃ composite, respectively. In addition, the results of electrochemical impedance spectroscopy (EIS) and photoluminescence (PL) tests indicate lower charge transfer resistance and faster charge extraction for the nanotextured ZnFe₂O₄/Fe₂O₃ composite (ZF1). Overall, our new fabrication process for the ZnFe₂O₄/Fe₂O₃ composite together with the effect of nanostructured substrate shows a better charge separation and enhanced optical absorption, resulting in a highly efficient photoelectrochemical water-splitting device.