Synthesis of ¹³C-,¹⁵N-Labeled Graphitic Carbon Nitrides and NMR-Based Evidence of Hydrogen-Bonding Assisted Two-Dimensional Assembly

Graphitic carbon nitride (g-C₃N₄) has gained great attention as a material of promise for artificial photosynthesis. In place of synthesis of traditional three-dimensional g-C₃N₄ via polymerization of melamine or melem, recent studies seek to establish an alternative synthetic approach for two-dimensional g-C₃N₄ using a smaller precursor such as urea. However, the effectiveness of such a synthetic approach and resultant polymeric forms of g-C₃N₄ in this approach are still largely unknown. In this study, we present that solid-state NMR (SSNMR) analysis for ¹³C- and ¹⁵N-labeled g-C₃N₄ prepared from urea offers an unparalleled structural view for the heterogeneous in-plane structure of g-C₃N₄ and most likely for its moieties. We revealed that urea was successfully assembled in melem oligomers, which include extended oligomers involving six or more melem subunits. SSNMR, transmission electron micrograph, and ab initio calculation data suggested that the melem oligomer units were further extended into graphene-like layered materials via widespread NH-N hydrogen bonds between oligomers.