Sub-nanometer Co₃O₄ clusters anchored on TiO₂(B) nano-sheets: Pt replaceable Co-catalysts for H₂ evolution

Cobalt-based heterogeneous cocatalysts are important substitutions of noble metal cocatalysts in many important commercial chemical processes, but their efficiency is extremely low on a per metal atom basis, because only the atoms located at surface active-sites participate in the chemical reaction. Thus, cocatalysts with small cluster dispersions are highly desirable to maximize the amount of active-sites and enhance the per atom efficiency. Here, we report the synthesis of sub-nanometer Co₃O₄ clusters which are anchored to 2D ultrathin TiO₂(B) nanosheets, as a cocatalyst for H₂ evolution reaction (HER). It was found that the conduction type of Co₃O₄ clusters turns from P-type to N-type, and the heterojunction band structure between TiO₂(B) and Co₃O₄ clusters changes from type II to type I, when the cluster size is reduced from nanometer scale to the sub-nanometer scale. With a suitable energy band matching between TiO₂(B) and sub-nanometer Co₃O₄ clusters, the electrons generated in TiO₂(B) during the photocatalytic process reduce the Co ions into metallic Co atoms, which produce excellent photocatalytic stability and extremely high HER efficiency comparable to that of the noble Pt cocatalyst.