Protonation of Ionic Squaraine Covalent Organic Frameworks With Double Reinforced Built‐In Electrical Field for Efficient Photocatalytic Hydrogen Evolution

ABSTRACT Low efficiency of charge separation and migration seriously restrain the practical application of 2D covalent organic frameworks (2D COFs) in the conversion of solar energy into various chemicals. To address this issue, an ionic building block, squaraine tetraaldehyde (SQ‐4CHO), has been designed and developed for fabricating 2D imine kgm COFs. The polarization effect from ionic squaraine units and the protonated imine bonds of COFs induces the generation of double‐reinforced built‐in electrical field and thus the efficient charge separation and migration. In addition to the modulation of light absorption capability, these two ionic sites make COFs with excellent hydrophilicity, reduced exciton binding energy, and the photo‐induced charge transfer from squaraine to protonated imine for the hydrogen evolution according to the in situ X‐ray Photoelectron Spectroscopy (XPS) and theoretical investigations. In particular, USTB‐63 exhibits an impressive hydrogen evolution reaction (HER) rate as high as 66.8 mmol g −1 h −1 with the help of a 3 wt.% Pt cocatalyst under visible light illumination. The present results open a new gate to establish the high‐performance HER photocatalysts by introducing the multiple ionic sites to enhance the charge separation efficiency.