Intentional Extrinsic Doping into ZnFe₂O₄ Nanorod Photoanode for Enhanced Photoelectrochemical Water Splitting

Abstract

Zinc ferrite (ZnFe 2 O 4 ) is a promising candidate photoanode material for photoelectrochemical water splitting, but its poor electronic properties deter it from achieving high performance. To improve the electronic properties, Ti 4+ and Sn 4+ are incorporated into Fe 3+ sites of ZnFe 2 O 4 as electron donors under annealing conditions of 550, 800 °C, or hybrid microwave annealing (HMA). The intentional external doping mainly promotes charge transport with increased charge carrier density and thereby enhances the bulk charge separation efficiency ( η bulk ). Effectiveness of the doping varies with annealing conditions. Both Ti‐doped and Sn‐doped ZnFe 2 O 4 annealed at 550 °C show the most significant improvement in η bulk with little effect on the surface charge separation efficiency ( η surface ). Doping under high‐temperature annealing (800 °C, HMA) improves η bulk to a lesser degree but creates a passivation layer of TiO x or SnO x , resulting in a higher η surface as well. Finally, loading the NiFeO x oxygen evolution catalyst (OEC) further improves η surface values, and especially 2% Ti‐doped ZnFe 2 O 4 annealed at 800 °C shows near 100% η surface at 1.23 V RHE , and its photocurrent generation under simulated 1 sun (0.312 mA cm −2 at 1.23 V RHE ) represents a ≈18 times increment from that of unmodified ZnFe 2 O 4 annealed at 550 °C.