[en] Dark brown ceria, CeO₂, with excellent photocatalytic properties, was synthesized by constructing a crystalline-core/disordered-shell heterostructure using NaBH₄ reduction. The structure, crystallinity and morphology of the as-prepared samples were characterized using x-ray diffraction, transmission electron microscopy, and high-resolution transmission electron microscopy. The x-ray photoelectron spectra confirm a high concentration of Ce³⁺ and oxygen vacancies in the CeO₂ sample. The dark brown compound shows excellent properties especially with respect to solar-energy absorption but also the transfer and separation of charge carriers—due to its heterostructure-related features. The photocatalytic activity of the treated samples is obviously higher than that of pure CeO₂ and reaches the maximum with Ce³⁺/Ce molar ratio of 51.7%. (paper)