Thermal modulation of Melem/g-C3N4 composites: Structural evolution, optical tuning, and enhanced photocatalytic degradation of organic dyes

Synthetic dyes pose significant threats to aquatic ecosystems and human health, underscoring the need for effective, metal-free photocatalysts that harness solar energy. This study explores melem/g-C 3 N 4 composites synthesized through thermal condensation of melamine at 550°C, followed by mild post-synthetic heating at 70 and 150°C to assess the impact of low-temperature treatment on structural and photocatalytic properties. Analytical techniques such as XRD, FTIR/Raman, PL, UV–Vis DRS, and N 2 physisorption (BET) analysis confirmed that thermal treatment preserves the g-C 3 N 4 structure while enhancing ordering, mesoporosity, and accessible surface area. SEM revealed a transition from compact agglomerates to granular nanoscale domains with improved pore connectivity in g-CN-MM-150, enhancing adsorption and mass transport. Optical characterization indicated a stable band gap of 2.75–2.79 eV, with Mott–Schottky measurements showing a conduction-band shift that enhances O 2 activation. Among all samples, g-CN-MM-150 demonstrated the highest photocatalytic activity, with 95% degradation of Rhodamine B under UV light and significant removal of AO7 under visible light (∼80%) with 83% conversion under sequential VIS+UV excitation. Performance improvements resulted from thermally induced defect redistribution, enhanced charge-carrier separation, and increased reactive oxygen species formation. This study highlights low-temperature thermal modulation as an effective strategy for optimizing metal-free g-C 3 N 4 -based photocatalysts for solar-driven environmental remediation. • Mild thermal treatment enhances melem/g-C 3 N 4 crystallinity and porosity • Stable band gap energy with defect tuning enables visible-light photocatalysis • g-CN-MM-150 shows superior AO7 and RhB degradation efficiencies • Sequential VIS+UV irradiation yields highest AO7 removal rates • Thermal modulation optimizes charge separation and ROS generation