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.