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

Solving
the structure of carbon nitrides has been a long-standing
challenge due to the low crystallinity and complex structures observed
within this class of earth-abundant photocatalysts. Herein, we report
on two-dimensional layered potassium poly­(heptazine imide) (K-PHI)
and its proton-exchanged counterpart (H-PHI), obtained by ionothermal
synthesis using a molecular precursor route. We present a comprehensive
analysis of the in-plane and three-dimensional structure of PHI. Transmission
electron microscopy and solid-state NMR spectroscopy, supported by
quantum-chemical calculations, suggest a planar, imide-bridged heptazine
backbone with trigonal symmetry in both K-PHI and H-PHI, whereas pair
distribution function analyses and X-ray powder diffraction using
recursive-like simulations of planar defects point to a structure-directing
function of the pore content. While the out-of-plane structure of
K-PHI exhibits a unidirectional layer offset, mediated by hydrated
potassium ions, H-PHI is characterized by a high degree of stacking
faults due to the weaker structure directing influence of pore water.
Structure–property relationships in PHI reveal that a loss
of in-plane coherence, materializing in smaller lateral platelet dimensions
and increased terminal cyanamide groups, correlates with improved
photocatalytic performance. Size-optimized H-PHI is highly active
toward photocatalytic hydrogen evolution, with a rate of 3363 μmol/gh
H2 placing it on par with the most active carbon nitrides.
K- and H-PHI adopt a uniquely long-lived photoreduced polaronic state
in which light-induced electrons are stored for more than 6 h in the
dark and released upon addition of a Pt cocatalyst. This work highlights
the importance of structure–property relationships in carbon
nitrides for the rational design of highly active hydrogen evolution
photocatalysts.