Fe-doped Ni nanocrystals as a cocatalyst for efficient BiVO₄ photoanode: Mechanism insights into Fe-accelerated in-situ reconstruction

To address the severe carrier recombination in BiVO₄-based photoelectrochemical (PEC) water splitting, we developed Fe-doped Ni nanocrystals (Fe:Ni NCs) through an organic liquid phase synthesis as an efficient cocatalyst. Through a dip-coating and annealing modification process, the optimized Fe:Ni/BiVO₄ photoanode achieved a remarkable photocurrent density of 4.72 mA/cm² at 1.23 V vs. RHE, representing a 4.07-fold enhancement over pristine BiVO₄. In-situ Raman characterization revealed that Fe:Ni NCs on BiVO₄ underwent in-situ surface reconstruction to form a unique Fe:Ni@Fe:NiOOH core–shell structure during PEC reaction. The presence of Fe accelerated the conversion of Ni to active NiOOH at lower potential (0.25 V vs. RHE) compared to pure Ni NCs modified BiVO₄ photoanode (0.30 V vs. RHE). Mechanistic studies identified a dual activation pathway involving both applied potential and BiVO₄ valence band oxidation. Meanwhile, Fe:Ni NCs cores with high conductivity accelerated the migration and accumulation of holes into Fe:NiOOH shells, resulting in improved carrier injection efficiency and water oxidation kinetics. This work not only elucidates fundamental differences between PEC and electrocatalytic water splitting mechanisms but also provides a reference for designing high-performance PEC photoanodes.