Mechanical compaction of melamine controls carbon nitride structure and enhances AO7 photodegradation

Graphitic carbon nitride (g-C₃N₄) is a promising metal-free photocatalyst; however, its performance strongly depends on the synthesis pathway and resulting structure. In this study, a simple compaction-assisted method for the preparation of melamine-derived carbon nitride materials in air is proposed. Mechanical compaction of the melamine precursor before thermal treatment enables improved packing density and more homogeneous heat transfer, leading to controlled structural evolution during polycondensation. The transformation from melamine to melem, polymeric melon, and finally g-C₃N₄ was systematically investigated using XRD, FTIR, SEM, UV-VIS DRS, and photoluminescence spectroscopy. The results show that the intermediate melem-rich structure obtained at 450 °C exhibits the highest photocatalytic activity toward Acid Orange 7 degradation. Under UV irradiation, nearly complete degradation (99%) was achieved with the highest reaction rate (k = 74.2 × 10^-3 min^-1), while under VIS light the same sample reached 88% removal (k = 15.9 × 10^-3 min^-1). The enhanced performance is attributed to a favourable balance between structural organization, defect-related electronic states, and charge separation efficiency at an intermediate stage of condensation. In contrast, fully polymerized g-C₃N₄ shows reduced activity, particularly under visible light, due to increased charge recombination and limited surface accessibility. This work demonstrates that mechanical pre-treatment provides a simple, scalable, and cost-effective strategy for tuning the structure and photocatalytic properties of carbon nitride materials, offering potential for environmental remediation applications while highlighting the importance of controlled condensation pathways.