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.