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

The synergistic effects of Zr doping and surface modification achieved the three-fold improvement in photoelectrochemical performance of zinc ferrite photoanode for solar water splitting. [Display omitted] •Zr-doped ZnFe2O4 photoanodes were prepared by facile in situ doping and dropping method.•Controlling Zr doping and Sn diffusion improves the charge transfer in Zr-ZFO-15.•Al2O3 layer can act as trap site for holes and promotes charge separation in Zr-ZFO.•Co–Pi further enhances the hole transfer kinetics at the photoanode‖electrolyte interface.•Synergistic effects of Zr doping and surface modification in ZFO led to photocurrent density of 646 µA/cm2. 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.