Structural Insights into Poly(Heptazine Imides): A\nLight-Storing Carbon Nitride Material for Dark Photocatalysis

Solving\nthe structure of carbon nitrides has been a long-standing\nchallenge due to the low crystallinity and complex structures observed\nwithin this class of earth-abundant photocatalysts. Herein, we report\non two-dimensional layered potassium poly­(heptazine imide) (K-PHI)\nand its proton-exchanged counterpart (H-PHI), obtained by ionothermal\nsynthesis using a molecular precursor route. We present a comprehensive\nanalysis of the in-plane and three-dimensional structure of PHI. Transmission\nelectron microscopy and solid-state NMR spectroscopy, supported by\nquantum-chemical calculations, suggest a planar, imide-bridged heptazine\nbackbone with trigonal symmetry in both K-PHI and H-PHI, whereas pair\ndistribution function analyses and X-ray powder diffraction using\nrecursive-like simulations of planar defects point to a structure-directing\nfunction of the pore content. While the out-of-plane structure of\nK-PHI exhibits a unidirectional layer offset, mediated by hydrated\npotassium ions, H-PHI is characterized by a high degree of stacking\nfaults due to the weaker structure directing influence of pore water.\nStructure–property relationships in PHI reveal that a loss\nof in-plane coherence, materializing in smaller lateral platelet dimensions\nand increased terminal cyanamide groups, correlates with improved\nphotocatalytic performance. Size-optimized H-PHI is highly active\ntoward photocatalytic hydrogen evolution, with a rate of 3363 μmol/gh\nH2 placing it on par with the most active carbon nitrides.\nK- and H-PHI adopt a uniquely long-lived photoreduced polaronic state\nin which light-induced electrons are stored for more than 6 h in the\ndark and released upon addition of a Pt cocatalyst. This work highlights\nthe importance of structure–property relationships in carbon\nnitrides for the rational design of highly active hydrogen evolution\nphotocatalysts.