Fe(III)‐Mediated Formation of Cu Nanoinclusions and Local Heterojunctions in CuWO₄ Photoanodes

Abstract Enhancing the photoelectrochemical (PEC) performance of CuWO₄ photoanodes has typically relied on doping or co‐catalyst strategies to improve charge carrier dynamics. In this work, an alternative approach is presented in which Fe(III) acts as a self‐assembly mediator during hydrothermal synthesis, enabling the formation of a core–shell heterostructure composed of a crystalline CuWO₄ core, a partially amorphous CuO/WO₃ shell, and embedded metallic Cu nanoinclusions. Rather than functioning as a dopant or co‐catalyst, Fe(III) is completely removed during post‐synthetic treatment, mediating a redox‐guided phase reorganization without being incorporated into the final material. This architecture establishes local heterojunctions that facilitate charge separation, suppress recombination, and enhance oxygen evolution reaction (OER) activity. A relative increase of ≈30‐fold in photocurrent is observed compared to pristine CuWO₄, as confirmed by structural, spectroscopic, and electrochemical analyses. While absolute photocurrents remain modest, this enhancement reflects intrinsic modifications in charge transport and recombination behavior driven by Fe(III)‐mediated structural reorganization. Complementary photocatalytic dye degradation experiments reveal that Fe‐activated particles act as highly efficient ROS‐generating catalysts in suspension, demonstrating functionality beyond thin‐film devices. These findings offer a new paradigm for oxide photoanode design, leveraging Fe(III)‐induced self‐assembly to engineer multifunctional heterostructures without relying on conventional doping.