New complete assignment of X-ray powder diffraction patterns in graphitic carbon nitride using discrete Fourier transform and direct experimental evidence.

To date, there have been only a few studies focusing on the assignment of X-ray diffraction (XRD) patterns in graphitic carbon nitrides (g-C3N4) and contradictory determination for a broad peak around 12°-14° has been perplexing. In this study, assignments are carried out both theoretically and experimentally. The cell parameters for g-C3N4 are determined as a = b = 8.1 Å, c = 6.5 Å, α = β = 90°, and γ = 120°. Qualitative and semi-quantitative methods such as fast Fourier transform and residual after 1st and 2nd derivatives are used to confirm and search the hidden peaks. Discrete Fourier transform is applied for the extraction of peak profiles and separation of overlapping peaks. In the broad peak around 12°-14° (with Cu Kα as referring source), two peaks are selected and determined as (100) and (001), which is fairly consistent with the (200) diffraction peak and (002) diffraction peak obtained by 2nd derivative method, respectively. In addition, g-C3N4 nanorods, MOF-doped g-C3N4 nanorods, and oxidized bulk g-C3N4 are successively investigated to present the 7.0 Å d-spacing of (100), hexagonal system of bulk g-C3N4, defect (1/2 0 0) structure with 14.0 Å d-spacing, and ABA stacking sequence. The structural transition in the oxidation of bulk g-C3N4 is presented by XRD to show accordance with the interpretation. Specific phenomena reported in other studies are also reinterpreted successfully, such as the appearance of peak at ∼12.4°.