Metal clusters catalyst embedded in semiconductors is the key factor for an efficient photocatalyst toward photocatalytic CO2 reduction. However, developing a simple method to synthesize such photocatalyst with these features has remained challenging. Herein, a rational one-step synthesis of cobalt clusters embedded in g-C3N4 layers is developed for an enhanced photocatalytic CO2 reduction. The tiny cobalt clusters are embedded into the g-C3N4 structure and stabilized through the interaction with NH2-bpy moieties. Besides, the ligands of cobalt complex can copolymerize with urea during the synthesis owing to the presence of amino groups, thus providing a direct electron transfer between carbon nitride and cobalt clusters, which act as the light harvester and active reduction site, respectively. Consequently, a volcano relationship between the CO generation rate and the cobalt clusters incorporated into the carbon nitride matrix was found. Under optimized conditions, representative C3N4-0.5CoO(x) exhibits the very high CO generation rate of 2.6 lmol.h(-1). Its apparent quantum yield of 3.2% at 420 nm and 1.0% at 460 nm, which is one of the highest values reported so far. The finding offers new opportunities for designing metal clusters catalysts toward photocatalytic CO2 reduction based on the judicious use of the metal complex. (C) 2020 Elsevier Inc. All rights reserved.