Abstract:

Covalent organic frameworks (COFs), recognized for their precisely tunable microstructures and high surface area, are promising photocatalysts for H₂O₂ production. However, the critical influence of pH on the stability of COF during the photocatalytic H₂O₂ production remains poorly understood. In this work, the photocatalytic H₂O₂ production performance of an imine-linked COF is significantly enhanced through a simple protonation strategy. Crucially, the protonated COF exhibits excellent stability under weakly acidic conditions (pH ≥ 3), but undergoes irreversible hydrolyzed under strongly acidic conditions (pH < 3). The protonation occurs specifically at the nitrogen atoms of imine units and serves a dual function: it suppresses ultrafast charge recombination (as revealed by femtosecond transient absorption spectroscopy) and directly provides a proton source for H₂O₂ generation. Moreover, fluoride ions (F⁻) are introduced into the photocatalytic system to further improve the photocatalytic H₂O₂ production rate. The strong electronegativity of F⁻ facilitates electron transfer from COF to F⁻, thus realizing the spatial separation of photogenerated carriers. Mechanistic studies confirm that H₂O₂ production follows a two-electron oxygen reduction reaction pathway. These findings elucidate the pH-dependent stability and activity of protonated COFs, provide fundamental insights into charge carrier dynamics, and establishe design principles to develop highly efficient and stable COF-based photocatalysts for solar-driven H₂O₂ generation.

Key words: Covalent organic frameworks, Structural stability, Protonation, Fluoride ion surface modification, Femtosecond transient absorption spectroscopy