Synthesis of transparent Zr-doped ZnFe₂O₄ nanocorals photoanode and its surface modification via Al₂O₃/Co–Pi for efficient solar water splitting

The Zr-doped ZnFe₂O₄ (Zr-ZFO) nanocorals photoanode have been synthesized from Zr-doped FeOOH (Zr–FeOOH) nanocorals on fluorine-doped tin oxide (FTO) substrate followed by Al₂O₃/Co–Pi surface modification for effective PEC water splitting. The hydrothermal method was adopted for the synthesis of in situ Zr doping in FeOOH, while Zr-ZFO nanocorals was fashioned by subsequent additional procedures, such as Zn(NO₃)₂ precursor dropping, first quenching, etching and second quenching. The Al₂O₃ layer decreases the over-potential of the Zr-ZFO photoanode‖electrolyte interface as well as lowers the charge transfer resistance which aids to increase in the photocurrent density from 0.24 mA/cm² to 0.36 mA/cm² at 1.23 V vs. RHE under 1 sun illumination condition. The subsequent loading of cobalt phosphate (Co–Pi) as a co-catalyst leads to an improved photocurrent density of 0.49 mA/cm² at 1.23 V vs. RHE. These synergistic effects of Zr doping and surface modification led to remarkable improvement in photocurrent density from 0.19 mA/cm² (at 1.4 V vs. RHE) for conventional ZFO nanorods to 0.65 mA/cm² for the Zr-ZFO/Al₂O₃/CoPi photoanode. A systematic investigation into the effect of Zr doping and Al₂O₃/Co–Pi overlayer reveals the reduction of two distinct recombination processes (bulk and surface) in ZFO photoanode.