Multiscale engineering of COFs toward practical photocatalysis: From heterojunction design to membrane fabrication

Transitioning covalent organic frameworks (COFs) from laboratory-scale photocatalysts to practical technologies requires overcoming two fundamental bottlenecks: rapid charge recombination at the microscopic level and mass transfer limitations coupled with the engineering challenges of particulate suspensions at the macroscopic level. This review presents a multiscale engineering strategy to bridge this gap, progressing from heterojunction design to membrane fabrication. We first provide a systematic assessment of COF-based heterostructures, incorporating metals, metal oxides, and inorganic semiconductors, to elucidate the structure-activity relationships governing excitonic dynamics. Special emphasis is placed on S-scheme COF-based heterojunctions, highlighting their superior capability in facilitating interfacial charge transfer while preserving high redox potentials. Subsequently, the focus shifts to the engineering of COF membranes via techniques such as interfacial polymerization and in-situ growth, emphasizing their structural merits in macroscopic processability. Critically, we propose the integration of heterojunction concepts into membrane architectures as a pivotal strategy to resolve the inherent trade-offs between the aggregation of powder catalysts and the limited semiconductor properties of traditional polymer membranes. By analyzing recent advances in heterostructured COF membranes, we highlight how this synergistic approach simultaneously enhances charge separation efficiency, substrate flux, and operational stability. We further discuss the application of these heterojunction and membrane systems in key photocatalytic processes, including H 2 production, CO 2 reduction, H 2 O 2 production, and pollutant degradation. The review concludes by outlining a roadmap for overcoming scalability hurdles, offering a blueprint for the development of efficient, robust, and industrially relevant COF photocatalytic systems.