Manipulation of band gap and hydrophilicity in vinylene-linked covalent organic frameworks for improved visible-light-driven hydrogen evolution by end-capping strategy

Since the first example was prepared via Knoevenagel condensation in 2016, vinylene-linked covalent organic frameworks (COFs) have attracted increasing interest owing to the high in-plane π-conjugation and robust stability. However, such COFs can only produce few amounts of hydrogen under visible-light irradiation due to the wide band gap and low hydrophilicity. Herein, we developed a facile end-capping strategy to manipulate the band gap, hydrophilicity and morphology of vinylene-linked COF (g-C18N3-COF). The COFs end-capped with variable positions of hydroxyl groups showed 4.5 times improvement in visible-light-driven hydrogen evolution activity compared to pristine COF. Both experimental and computational studies reveal that optimizing the contents and positions of hydroxyl groups narrowed the band gap and improved the hydrophilicity of pristine COF. The end-capping strategy efficiently reduced the charge recombination and energy barrier for the formation of H intermediate species (H*) explaining the improved hydrogen evolution properties.