Activation of a highly oriented columnar structure of ZnFe₂O₄ for photoelectrochemical water splitting: Orchestrated effects of two-step quenching and Sn⁴⁺ diffusion

In present work, we synthesized ZnFe₂O₄ nanorods on a fluorine-doped tin oxide substrate using spray coating method followed by two-step high-temperature quenching (HTQ). X-ray photoelectron spectroscopy (XPS) results indicate that Sn⁴⁺ is diffused from the FTO substrate after the second quenching, which could help in minimizing the recombination of photogenerated carriers. Photoelectrochemical measurements of the ZnFe₂O₄ nanorod photoelectrodes quenched at 780 °C, 800 °C, and 820 °C indicate that among the studied samples (ZFO1, ZFO2 and ZFO3), the highest photocurrent density was observed for nanotextured ZFO3 photoelectrodes (130 µA cm⁻² at 1.23 V vs RHE). The photoelectrochemical performances of the ZnFe₂O₄ nanorods after the second quenching were compared with those of the firstly quenched ZnFe₂O₄ nanorod samples; water-oxidation photocurrent density of the former (ZFO3) was increased by 6.9 times compared with that of the first quenching (PZFO). Intensity modulated photocurrent spectroscopy (IMPS) and photoluminescence (PL) results confirm the faster charge extraction was achieved for the ZFO3 photoelectrode. Thus, the overall photocurrent density during the second quenching process results from the effectively improved crystallinity, the reduced strain and suppressed charge–carrier recombination's on both the surface as well as in the bulk of the ZnFe₂O₄ nanorods. In terms of solar water splitting, these research findings provide an effective route for the synthesis of other nanostructures.