Atmosphere-Dependent Crystallization of Carbon Nitride: Balancing Activity and Stability for Efficient Photocatalysis

Abstract

Crystalline carbon nitride (CCN) represents a superior alternative to graphitic carbon nitride (g-C3N4) for photocatalysis, yet its structure and performance are highly dependent on synthesis conditions. This study elucidates the crystallization mechanism of carbon nitride and reveals how the synthesis atmosphere dictates the crucial trade-off between photocatalytic activity and stability. Through a systematic investigation under N2, air, and vacuum, we identify a universal three-stage transformation pathway: structural collapse of g-C3N4, K+-assisted reorganization into CCN, and eventual thermal decomposition. In situ mass spectrometry corroborates that KCl catalyzes the structural rearrangement at lower temperatures (450-550 oC), accompanied by H2 release. Although the N2 atmosphere suppresses decomposition to yield a highly crystalline, metastable phase (CCN-N550) with an exceptional initial hydrogen evolution rate of 2.00 mmol﹒g-1﹒h-1 (8-fold higher than g-C3N4), it suffers from rapid deactivation, with the rate dropping to 1.18 mmol﹒g-1﹒h-1 after 10 hours. In contrast, the material derived from an air atmosphere (CCN-A550) possesses moderate crystallinity but exhibits superior stability, maintaining its activity at 1.61 mmol﹒g-1﹒h-1 over 24 hours. This work demonstrates that high crystallinity alone does not guarantee a practical photocatalyst and establishes synthesis in air at 550 oC as the optimal strategy to fabricate CCN that harmonizes high activity and long-term operational stability, offering key guidance for the rational design of robust photocatalytic systems.