Angew. Chem. Int. Ed. 56, 13445–13449 (2017)

2D polymers represent a new frontier for functional materials. Such materials offer robust mechanical properties yet high reactivities for important chemical transformations. Carbon nitride, a nitrogen-containing cousin to graphene, is a promising metal-free catalyst that harvests sunlight and produces hydrogen from water. However, carbon nitride exhibits a poor exciton-separation ability and therefore underperforms in comparison to inorganic solar-to-fuel materials.

Now, Wang and Antonietti report a synthetic means to improve the solar-to-hydrogen conversion abilities of carbon nitride. The researchers employed molten salt mixtures to condense urea, with or without oxamide, and produced crystalline carbon nitride. The molten salt mixture forced the carbon nitride sheets to crystallize more closely, reducing the typical 0.326 nm intersheet spacing to 0.292 nm.

The closely packed carbon nitrides absorbed UV–visible light more strongly and displayed enhanced exciton-splitting behaviours. These advanced carbon nitrides displayed a 30-fold hydrogen production enhancement under white light illumination and nearly 40-fold with green light. The carbon nitrides presented ultrahigh activities in 3% NaCl without any activity losses across four removal–reuse cycles.