Cobalt-based cubane molecular co-catalysts for photocatalytic water oxidation by polymeric carbon nitrides

[Display omitted] •A stable water-soluble cobalt-based cubane molecular co-catalyst has been synthesized and studied.•The molecular co-catalysts injected into carbon nitride polymers exhibit a superb activity for photocatalytic water oxidation.•The modification promotes interface contact, reduces over-potential, accelerates charge separation, and enhances catalysis. In artificial photocatalysis, sluggish kinetics of hole transfer and the high recombination rate of charge have been the Achilles’ heel of photocatalytic conversion efficiency. The development of co-catalysts for promoting exciton splitting and charge separation has therefore gained continuous attention. Herein, “Co4O4” cubane complex Co4O4(O2CMe)4(py)4 (1a, py = pyridine derivatives), serves as a homogeneous molecular co-catalyst which is engineered into the framework of pristine polymeric carbon nitride (PCN) for water oxidation with light illumination. This modification strategy maximizes the contact areas between co-catalysts and reactants, reduces the over-potential of oxygen producing and accelerates the interface transfer rate of electron-hole pairs, thus leading to enhanced reaction kinetics for photocatalytic water oxidation. Compared with pristine PCN, the photocatalytic O2 production rate up to 19-fold-enhanced in the presence of molecular co-catalyst. This work emphasizes the importance of developing effective, stable and earth-abundant molecular co-catalysts for the promotion of water-splitting photocatalysis.