Tuning heterojunction pathways via transition metal oxides on fibrous silica iron for enhanced photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting offers a promising route to sustainable green hydrogen production by directly converting solar energy into hydrogen without consuming fossil fuels. However, inefficient charge separation and sluggish interfacial charge transfer remain major limitations in photoanode development. Herein, fibrous silica iron oxide (FSFe) was synthesized as a photoanode and modified with CuO, ZnO, and NiO to regulate heterojunction pathways for enhanced PEC water splitting. Characterization results confirmed the formation of high-surface-area spherical fibrous structures with well-dispersed metal oxide species, while the primary FSFe framework was preserved after metal loading. The electrochemical characterization revealed that transition-metal oxide modification increased the charge carrier density and reduced charge transfer resistance, indicating efficient charge separation and interfacial charge transport. Among the prepared photoanodes, Cu/FSFe exhibited the highest photocurrent density of 4.14 mA cm-2 at 1.23 VRHE and an applied bias photon-to-current efficiency of 0.78%, compared with 2.70 mA cm-2 and 0.54% for pristine FSFe. The superior performance of Cu/FSFe was attributed to the combined effects of Z-scheme heterojunction formation and the strong visible-light response of CuO. Compared with the Type-II pathways proposed for Ni/FSFe and Zn/FSFe, the Z-scheme pathway in Cu/FSFe promoted efficient charge separation while preserving strong redox potentials, leading to a higher charge carrier density of 1.92 × 1021 cm-3, improved interfacial charge transfer, and enhanced PEC water splitting performance. This work provides insight into how the selection of transition-metal oxide modifiers governs heterojunction formation and charge transfer pathways in FSFe-based photoanodes for PEC water splitting.