Heterojunction and crystallinity-mediated charge separation in β-ketoenamine covalent organic frameworks for synergistically improved photocatalytic H2 evolution

Abstract

Covalent organic frameworks (COFs), as an emerging class of metal-free organic semiconductor photocatalysts, have garnered extensive attention for their potential in photocatalytic H₂ evolution. However, the weaknesses associated with low crystallinity and rapid charge recombination in single-component COFs have resulted in their low photocatalytic efficiency. To address these issues, we have developed a strategy for preparing a highly crystalline β-ketoamine-linked TpBD-COF (denoted as TpBD-COF(h)) by adjusting the reaction solvent and incorporating MoS₂ nanosheets to improve surface- and bulk-carrier separation. As a result, as-prepared x-MoS₂/TpBD-COF(h) composites exhibited superior visible-light-driven H₂ evolution performance. In particular, the optimized 8-MoS₂/TpBD-COF(h) photocatalyst achieves a maximum H₂ evolution rate of 2970.0 μmol g⁻¹ h⁻¹, representing an enhancement of 18.3-fold, 1.5-fold, and 4.8-fold compared to TpBD-COF(h), 8-MoS₂/TpBD-COF(l) (where TpBD-COF(l) denotes TpBD-COF with lower crystallinity), and Pt/TpBD-COF(h), respectively. Based on in-depth experimental characterization and theoretical studies, the significant enhancement in H₂ production activity can be attributed to the improved crystallinity of the TpBD-COF, which boosts the kinetic energy of photoelectrons and increases the carrier mobility within the TpBD-COF. Moreover, the integration of MoS₂ not only improves the light-harvesting ability but also dramatically stimulates the charge separation and migration efficiency in the TpBD-COF(h). This work paves the way for the further design and synthesis of efficient noble-metal-free COF-based catalysts for photocatalytic energy conversion.