Influence of ZnO Magnetron Sputtering on Controlled Buildout of Zirconium-Doped ZnFe₂O₄/Fe₂O₃ Heterojunction Photoanodes for Photoelectrochemical Water Splitting

The development of efficient photoanodes for solar fuel generation via photoelectrochemical (PEC) water splitting is becoming a bottleneck. These limitations necessitate the design of iron-containing metal oxides, like “ferrites-based electrode materials” with improved oxygen evolution kinetics, light absorptivity, and intrinsic stability, yet at a low cost. Herein, we report the in-situ formation of Zr–ZnFe2O4/Fe2O3 heterojunction (ZZFO/HT) photoelectrodes using a facile magnetron sputtering and hydrothermal processes. First, the ZnO is systematically sputtered on in-situ Zr-doped FeOOH electrodes and then the ZnO-sputtered electrodes are quenched at 800 °C, 13 min to form ZZFO/HT. Furthermore, the effect of ZnO sputtering and roles of Zr–ZnFe2O4 and Fe2O3 in the ZZFO/HT heterojunction as well as their structural and photoelectrochemical properties were studied in detail. The optimum biphasic 25.6 nm ZnO-sputtered ZZFO/HT (ZZFO/HT-2) photoelectrode exhibited a photocurrent density of 0.430 mA/cm2 at 1.23 V vs RHE with an appropriate fraction of Zr–ZnFe2O4 and Fe2O3. The enhanced PEC performance is attributed to the optimum fraction of Zr–ZnFe2O4 and Fe2O3 in ZZFO/HT-2 heterojunction, which provides efficient charge transport across the bulk and at heterojunction interface. Lastly, the integration of Al2O3 passivation layer and Co–Pi cocatalyst on the optimized ZZFO/HT-2 photoelectrode exhibited a high photocurrent density of 0.780 mA/cm2 at 1.23 V vs RHE and generated 11.8 and 6.2 μmol/cm2 of hydrogen and oxygen, respectively during PEC water splitting. Further, it is expected that by fine-tuning of Zr–ZnFe2O4 and Fe2O3 NC ratio, the photocurrent density can be improved for establishing a benchmark for ZnFe2O4-based photoelectrodes.