Crystalline Poly(Heptazine Imide) Photocatalysts for Overall Water Splitting with Visible Light

Poly(heptazine imide) (PHI), a semicrystalline form of polymeric carbon nitride, is a promising photocatalyst for photocatalytic overall water splitting (OWS). However, pristine PHI typically exhibits only marginal activity due to insufficient charge separation and limited charge transport. Herein, we demonstrate that annealing of low-condensed PHI in the presence of NaCl markedly increases its crystallinity by simultaneously improving in-plane packing and interlayer stacking order. This structural ordering enables efficient charge separation, leading to substantially accelerated charge migration. Consequently, the crystalline PHI exhibits outstanding visible-light-driven OWS performance, with H₂ and O₂ evolution rates of 1100 and 559 μmol h^-1, respectively, and an apparent quantum yields of 10.3% at 405 nm and 4.5% at 420 nm. This performance surpasses that of melon by orders of magnitude and represents a record for PHI-based photocatalysts. The results establish crystallinity engineering as an effective strategy for modulating charge-transfer dynamics in polymeric photocatalysts.