Salt-assisted activation of n → π* electronic transition in orange carbon nitride for enhanced visible-light-driven H2 generation†

Abstract

Metal salts demonstrate significant advantages in promoting the polymerization of highly crystalline carbon nitrides for meeting the demands of green photocatalytic hydrogen production. However, the enhanced light absorption arising from n → π* electronic transition is difficult to achieve within symmetric and orderly planar heptazine-based structures, which are typically induced by molten-salt treatments. Herein, a novel orange carbon nitride (OCN) with activated n → π* electronic transition is synthesized via a NaCl-assisted melamine assembly of melem and a secondary calcination approach. Na⁺ ions are crucial for the formation of hexagonal melem flakes, which are exfoliated from rod-like structures connected by interlayer van der Waals forces. The cyano groups within OCN can widen the visible light harvesting ability and adjust the band structure. The introduction of nitrogen defects suppresses the radiative recombination of photogenerated charge carriers by creating a midgap energy level, thereby facilitating efficient electron–hole separation and migration. Benefiting from the n → π* electronic transition and optimized carrier dynamics, the OCN presents an enhanced hydrogen evolution rate of 1043 μmol g⁻¹ h⁻¹ under visible light (λ > 420 nm). Our low-temperature NaCl-assisted polymerization process not only reduces crystallinity but also endows unique optical features to carbon nitrides, which broadens the function of metal salts in synthesizing catalysts for solar energy applications.