Harnessing S-scheme junctions for enhanced CO2 photoreduction: molecular bonding of copper(II) complexes onto K-doped polymeric carbon nitride via microwave heating

Photocatalytic conversion of CO₂ is pivotal for mitigating the global greenhouse effect and fostering sustainable energy development. Nowadays, polymeric carbon nitride (PCN) has gained widespread application in CO₂ solar reduction due to its excellent visible light response, suitable conduction band position, and good cost-effectiveness. However, the amorphous nature and low conductivity of PCN limit its photocatalytic efficiency by leading to low carrier concentrations and facile electron–hole recombination during photocatalysis. Addressing this bottleneck, in this study, potassium-doped PCN (KPCN)/copper(II)-complexed bipyridine hydroxyquinoline carboxylic acid (Cu(II)(bpy)(H₂hqc)) composite catalysts were synthesized through a multistep microwave heating process. In the composite, the formation of an S-scheme junction facilitates the enrichment of more negative electrons on the conduction band of KPCN via intermolecular electron–hole recombination between Cu(II)(bpy)(H₂hqc) (CuPyQc) and KPCN, thereby promoting efficient photoreduction of CO₂ to CO. Microwave heating enhances the amidation reaction between these two components, achieving the immobilization of homogeneous molecular catalysts and forming amidation chemical bonds that serve as key channels for the S-scheme charge transfer. This work not only presents a new PCN-based catalytic system for CO₂ reduction applications, but also offers a novel microwave-practical approach for immobilizing homogeneous catalysts.