2D Highly Crystalline and Porous Covalent Heptazine Frameworks for Efficient Hydrogen Evolution

In recent decades, polymeric graphitic carbon nitride (g-C₃N₄) has garnered significant attention as a class of metal-free semiconductor photocatalysts. However, inherent limitations such as inadequate visible light absorption, low specific surface area, moderate charge transfer efficiency, and poor crystallinity restrict its application. To address the constraints, three novel donor-acceptor type covalent heptazine frameworks (CHFs) are constructed through a bottom-up approach by intergrating heptazine and triazine, which are the fundamental active moieties of g-C₃N₄, with diverse donor spacers. Compared to g-C₃N₄, noteworthy enhancements in photocatalytic activity and hydrogen evolution efficiency are attributed to the increased specific surface areas, broadened visible-light absorption, and accelerated photogenerated charge transfer within the CHFs. Notably, high crystallinity shows a profound influence on the photocatalytic efficiency of the synthesized CHFs. Among the CHFs, highly crystalline CHF-3 stands out to present the highest hydrogen evolution rate of 15284 µmol g^-1 h^-1 under visible-light irradiation (420-780 nm) with ascorbic acid as the hole sacrificial agent. This remarkable achievement represents a 144-fold improvement over g-C₃N₄ and a noteworthy sevenfold enhancement compared to the low-crystalline CHF-3. These results not only offer valuable insights for the design of efficient heptazine-based CHF photocatalysts but also contribute toward the advancement of heptazine-based functional materials.