Abstract

Predicting the photocatalytic hydrogen-evolution ranking of ferrite/g-C3N4 composites from routine X-ray diffraction (XRD) and scanning electron microscopy (SEM) descriptors is attractive but rarely scrutinised. Here, XRD and FE-SEM data for pure g-C3N4, pure NiFe2O4 and NiFe2O4:g-C3N4 composites (25:75, 50:50 and 75:25 by mass, prepared by sonication-assisted physical mixing) were re-analysed at the level of individual reflections. The composition trends published for the mean spinel lattice constant (8.2534, 8.2769 and 8.3484 Å) and mean Scherrer size (10.99, 10.25 and 13.01 nm) could not be reproduced as stated. The averages combine sharp Bragg reflections (FWHM 0.12–1°) with features 3–9° wide; two averages treat the g-C3N4 (002) reflection inconsistently; and β cosθ varies 23- to 39-fold within a single pattern, which is incompatible with size-only broadening (Williamson–Hall R2 ≤ 0.10). Per-peak lattice constants of two samples drift systematically with diffraction angle, a signature of specimen displacement. Nelson–Riley extrapolation gives a0 = 8.371 ± 0.013, 8.344 ± 0.021 and 8.325 ± 0.013 Å for the pure ferrite and the 25:75 and 50:50 composites, values that bracket the bulk value of 8.341 Å and show no resolvable composition trend. The g-C3N4 (002) spacing recomputed for the 25:75 composite is 3.175 Å, a contraction rather than the reported expansion. A benchmark of four verified studies places the optimum NiFe2O4 fraction between 12 and 49 wt% and shows four non-comparable hydrogen metrics. Averaged XRD descriptors from these data therefore cannot rank hydrogen-evolution activity. A minimal characterisation and reporting protocol is proposed. No hydrogen-evolution measurements were made in this work.

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