Novel two-dimensional crystalline carbon nitrides beyond g-C₃N₄: Structure and applications

Two-dimensional (2D) crystalline carbon nitrides (CxNy) with graphene-like atomic structures but semiconducting nature are new appealing materials, and increasing interest has been focused on their synthesis, properties, and applications. Apart from the well-known graphitic carbon nitride (g-C3N4), other types of 2D CxNy nanocrystals including C2N, C3N and C5N2 are also successfully fabricated in recent years and their properties and potential uses in various fields have been widely investigated. Meanwhile, g-CN, C2N2, C2N3 and C5N have been theoretically predicted, and their properties and potential applications have also been researched. Until now, 2D crystalline CxNy compounds have demonstrated promising applications in photocatalysis, electrocatalysis, gas adsorption/separation, and energy storage devices, from both experimental and theoretical aspects, stemming from their unique geometric structures, tunable electronic properties, and high specific surface area. Herein, recent advances in the synthesis of 2D CxNy nanocrystals and their applications in various fields are summarized. Moreover, future challenges and opportunities for developing 2D CxNy nanocrystals for more broad applications including environmental remediation are also discussed.