Evidence of oxygen evolution over sputtered zinc ferrite (ZnFe2O4) thin film by enhanced lattice oxygen participation

We report evidence of oxygen evolution over zinc ferrite (ZnFe2O4) thin films grown on indium tin oxide and quartz substrates using the RF sputtering. The thin films are deposited at ambient temperature with different argon/oxygen gas ratios, specifically 1:0 (Z–Ar), 1:1 (Z–Ar:O), and 0:1 (Z–O). Structural characterization using grazing incidence x-ray diffraction at a 0.400° grazing angle confirmed the polycrystalline nature of the films. X-ray photoelectron spectroscopy scans of Zn 2p, Fe 2p, and O1s were conducted to investigate the lattice oxygen vacancies. The lattice oxygen vacancies in the Z–Ar film resulted in a lower bandgap of 2.05 eV than the Z–O film of 2.36 eV. The oxygen evolution reaction (OER) performances of the thin films are investigated to understand the effect of oxygen vacancies on electrochemical activity and observed that the Z–Ar film, with oxygen vacancies, exhibits a decrease in overpotential by ∼12.5% at 10 mA cm−2, eightfold increase in current density at 520 mV overpotential deduced from linear sweep voltammetry, and a 71.9% increase in donor density inferred from the Mott–Schottky plot, as compared to the Z–O film. The findings suggest that the Z–Ar film follows a “lattice oxygen participation mechanism” for the OER, instead of an “adsorbate evolution mechanism” observed in the Z–O film. The results highlight the significant impact of argon/oxygen gas ratios on the structural, optical, and electrochemical properties of zinc ferrite thin films and provide insight into the role of oxygen vacancies in modulating the OER performance for potential applications.