Co@Co₃O₄ Prepared in Situ from Metallic Co as an Efficient Semiconductor Catalyst for Photocatalytic Water Oxidation

This paper reported the first attempt of using Co@Co₃O₄ core–shell nanoparticles obtained in situ from a metallic Co precursor as a highly active and stable catalyst for the photocatalytic water oxidation. Co nanoparticle precursor was prepared through a hydrothermal process. The components of precursor and catalyst were confirmed by multiple measurements (X-ray diffraction, field emission scanning electron microscopy, scanning transmission electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, line scanning analysis, UV–vis diffuse reflectance spectroscopy, Mott–Schottky curve). The Co@Co₃O₄ semiconductor catalyst exhibited excellent activity for the photocatalytic water oxidation without any addition of photosensitizer or cocatalyst, with an average O₂ evolution rate of 2778 μmol h^–1 g^–1, and the Co@Co₃O₄ maintained 90% of the initial activity even after the sixth run; its oxygen evolution reaction performance under λ = 600 and 765 nm still remained 16% and 7.2% of λ ≥ 420 nm, respectively. The high activity of this photocatalyst was strongly dependent on the generation of Co₃O₄ nanoclusters on the surface of metallic Co. The synergistic effect between Co₃O₄ and metallic Co was helpful for electron transfer and separation and catalytic performance improvement, because metallic Co played a crucial role during the water oxidation process