Amorphous Exsolution of Fe₃O₄ Nanoparticles in SrTiO₃: A Path to High Activity and Stability in Photoelectrochemical Water‐Splitting

Exsolution creates metal nanoparticles embedded within perovskite oxide matrices, promoting optimal exposure, even distribution, and robust interactions with the perovskite structure. Fe 3 O 4 , an oxidized form of Fe, is an attractive catalyst for photoelectrochemical (PEC) water‐splitting due to its strong light absorption, excellent electrical conductivity, and chemical stability. However, exsolving Fe is challenging, often requiring harsh reduction conditions that can decompose the perovskite. Herein, hybrid composites are fabricated for PEC water‐splitting by reductively annealing a solution of SrTiO 3 photoanode and Fe cocatalyst precursors. In situ transmission electron microscopy reveals uniform, high‐density Fe particles exsolving from amorphous SrTiO 3 films, followed by film‐crystallization at elevated temperatures. This innovative process extracts entire Fe dopants while maintaining structural stability, even at doping levels exceeding 50%. Upon air exposure, the embedded Fe particles oxidize to Fe 3 O 4 , forming a Schottky junction and enhancing light absorption. These conditions yield a high activity of 5.10 mA cm −2 at 1.23 V versus reversible hydrogen electrode (an 11.86‐fold improvement over SrTiO 3 ) from the 30% Fe‐doped SrTiO 3 , with excellent stability (97% retention) over 24 h. Theoretical calculations indicate that in the amorphous state, FeO bonds weaken while TiO bonds remain strong, promoting selective exsolution. The mechanisms driving amorphous exsolution versus crystal exsolution are elucidated.