π conjugation driven bulk and interfacial charge modulation in triazine based and heptazine based COF@ZnIn2S4 heterojunctions for efficient uranium removal.

The limited π-conjugation in covalent organic frameworks (COFs) often restricts exciton diffusion and active site exposure for photocatalysis. To address this, a heptazine-based donor-acceptor COF (TPc) was synthesized via Schiff base condensation and coupled with ZnIn2S4 (ZIS) to construct a TPc@ZIS heterojunction, employing a triazine based TPa@ZIS as a control material. The extended π-conjugation of the TPc core not only narrows the band gap and enhances visible-light absorption but also facilitates the formation of abundant interfacial Zn-N bonds with ZIS. This unique structure establishes a synergistic dual modulation, the intramolecular D-A framework creates a built-in electric field for bulk charge separation, while the interfacial Zn-N bonds reinforce an S-scheme heterojunction, collectively enabling highly efficient charge separation and transfer. As a direct result, TPc@ZIS achieves a uranium removal rate nearly double that of TPa@ZIS under challenging conditions (100 ppm, pH ∼5), and attains up to 98.2% removal at 50 ppm, outperforming most previously reported photocatalysts under similar conditions. This work demonstrates that extended π-conjugation simultaneously amplifies the intramolecular D-A polarization for bulk charge separation and the interfacial Zn-N coupling for heterojunction charge transfer, establishing a synergistic dual modulation strategy for high-performance photocatalytic uranium removal.