Salt-Mediated Structural Transformation in Carbon Nitride: From Regulated Atomic Configurations to Enhanced Photocatalysis

Molten salts-assisted synthesis is widely used in the construction of high efficiency graphitic carbon nitride (g-C3N4) photocatalysts, and two isotypes of g-C3N4 have been synthesized by such method, namely poly (heptazine imide) (PHI) and poly (triazine imide) (PTI). However, the understanding of the structural changes taking place during the molten salt process and the structure–activity relationship of g-C3N4 polymorphs remain blurred. Herein, by regulating the treatment duration of g-C3N4 nanosheets (melon) in molten salts, we successfully synthesized g-C3N4 with phases of PHI, PHI/PTI and PTI. A continuous structural transformation induced by ions, in which melon transforms to a stable phase PTI via PHI, an intermediate state, was unveiled for the first time. In addition, results reveal that atomic configurations play a vital role in photo absorption, and charge carrier transfer and surface reaction, leading to significant differences in photocatalytic degradation. Among them, PHI with K+ and cyan groups modification, as well as high crystallinity, exhibits remarkable degradation efficiency, with 90% removal of tetracycline in 10 min and 80% removal of phenol in 60 min. This study sheds light on a deeper understanding for the molten salt-assisted synthesis and provides new ideas for preparing efficient organic semiconductor photocatalysts.