Surface cyano groups optimize the charge transfer of poly heptazine imide for enhanced photocatalytic H2 evolution.

Surface functionalization has been considered as an effective strategy to manipulate charge separation of carbon nitride and therefore to largely improve the photocatalytic H 2 evolution efficiency. Poly heptazine imide (PHI) is a new class of crystalline carbon nitride frameworks that exhibits remarkable photocatalytic performance for hydrogen evolution. To further improve the H 2 evolution performance of PHI and explore the reaction mechanism, ammonium thiocyanate was used as a precursor for the synthesis of poly heptazine imide at elevated temperatures under molten salt conditions. The optimized PHI with an abundance of surface cyano groups shows a significantly enhanced photocatalytic performance for H 2 evolution, which is 4.3 times that on pristine PCN. Most importantly, the surface cyano group adjusts the electron intensity of the polymeric framework, enhances the light absorbance, reduces the bandgap, and improves the charge separation efficiency. The synthetic technique also could be applied to other sulfur-containing precursors for the synthesis of PHI frameworks with excellent hydrogen evolution production performance. • Polymerization of sulfur-containing based precursors in the presence of NaCl/KCl generates poly heptazine imide. • The newly generated PHI(AT) bears an abundance of terminal cyano groups. • The terminal cyano groups promote the charge separation efficiency and enhance photocatalytic activities. • PHI(AT) showed 4.3 times higher HER than pristine PCN. [ABSTRACT FROM AUTHOR]