Urea-Derived Carbon Nitride Fragmentation: A Route to Melem-Based Superstructures

Urea-derived graphitic carbon nitride (g-C3N4) underwent chemical treatment in a strongly oxidizing H2SO4/HNO3/H2O2 environment, followed by dialysis against water. XRD and SEM measurements revealed significant disruption of the layered polymeric framework, including loss of hydrogen-bond-mediated order, partial depolymerization into melem-based units, and the formation of rod-like melem hydrate superstructures. FTIR spectra indicated that chemical treatment resulted in the loss of the heptazine structure and the emergence of melem/triazine vibrations. Consistent disruption of the C–N framework was also observed in XPS spectra, which showed a reduced contribution of nitrogen species associated with inter-triazine bridging. Mott–Schottky analysis yielded potential values suggesting the presence of additional electronic states in the chemically treated sample. This aligns with EPR spin-trapping results, which showed approximately a 2.5-fold decrease in superoxide radical anion generation upon visible light irradiation compared with the parent compound, attributed to both reduced visible-light absorption and trapping of photogenerated electrons in defect states.