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

Abstract

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 K_α 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°.