Sm 3+ Doping‐Induced Suppression of Ce 3+ 4f 1 Trap States in CeO 2 for Efficient Photocatalytic Overall Water Splitting

ABSTRACT Defect engineering and electronic structure modification are central to the development of efficient metal oxide photocatalysts for overall water splitting. However, the role of specific electronic trap states that govern charge carrier dynamics remains incompletely understood. Herein, we report a lanthanide‐assisted solid‐state strategy to synthesize Sm 3+ ‐doped CeO 2 that selectively suppresses detrimental Ce 3+ 4f 1 trap states while preserving surface active sites. Structural and spectroscopic characterizations indicate that Sm 3+ preferentially segregates at the CeO 2 surface, where its incorporation suppresses Ce 3+ 4f 1 trap states that act as dominant non‐radiative recombination centers, thereby prolonging charge carrier lifetimes and enabling efficient migration to the catalytic surface. When integrated with dual cocatalysts (CoO x –Rh/Cr 2 O 3 ), the optimized Sm 3+ ‐doped CeO 2 achieves an apparent quantum yield of 12.6% ± 0.4% at 365 nm, representing the highest reported value for CeO 2 ‐based photocatalysts for overall water splitting. These findings establish surface lanthanide segregation as a versatile approach for regulating defect states and charge carrier dynamics, unlocking the intrinsic activity of CeO 2 for solar‐fuel production.