Hydrophilic/hydrophobic heterojunctions for enhanced photocatalytic hydrogen evolution via gas release dynamics

Covalent-organic frameworks (COFs), characterized by their exceptional light absorption and ordered architecture, have emerged as potential candidates for photocatalytic hydrogen production. In this work, we discovered that the incorporation of fluorine into the sub-nanocavity of azine-linked COF (TF-COF) not only augments its hydrophobicity but also strengthens the interaction between Pt cocatalysts and COFs. In an effort to enhance photocatalytic water splitting efficiency, we integrated the hydrophobic TF-COF with the hydrophilic carbon nitride (CN) to construct a hydrophilic/hydrophobic heterojunction (CTF-x heterojunction). Both experimental results and density functional theory (DFT) calculations reveal that the hydrophilic side, CN, aids in the adsorption and transfer of water molecules, whereas the hydrophobic side, TF-COF, generates hydrogen and promotes its overflow, thereby achieving space charge separation. The hydrogen evolution activity of CTF-50 % (with a CN content of 50 %) reached an optimal value of 2428 μmol g^-1h^-1_, with an apparent quantum yield (AQY) of 2.6 % at 400 nm. This is approximately four times higher than that of pure CN and ten times greater than that of TF-COF. We believe this work will provide valuable insights for developing efficient heterojunction photocatalysts.