Synergistic adsorption-photocatalysis via rGO-mediated electron shuttling in ultrasonically synthesized NiO/g-C3N4-based ternary nanocomposites for Safranin O removal

Abstract

This study reports the facile ultrasound-assisted synthesis of a ternary rGO/NiO/g-C₃N₄ nanohybrid (rGO-GN10) for efficient safranin O (SAF) removal via synergistic adsorption and photocatalysis. The composite was strategically engineered to transcend the limitations of binary systems by establishing an rGO-bridged direct Z-scheme heterojunction, which not only achieves exceptional charge separation but also uniquely synergizes multi-mechanistic adsorption with photocatalytic mineralization at the shared interface. Advanced characterization confirmed successful integration: XRD identified crystalline NiO and g-C₃N₄ phases, while HRTEM revealed hierarchical heterostructures with intimate interfacial contact enabling efficient charge transfer. The nanocomposite exhibited a significantly narrowed bandgap (2.19 eV vs. g-C₃N₄'s 2.75 eV), extending the light absorption edge to 565 nm. Remarkable charge separation was evidenced by 92% PL quenching and 79% reduction in charge-transfer resistance, validating an rGO-bridged Z-scheme mechanism. XPS confirmed covalent bonding via pyridinic N–Ni bonds, Ni^δ+ states, and rGO-mediated electron delocalization. The nanohybrid demonstrated multi-mechanistic SAF adsorption (capacity: 23 mg g⁻¹) through electrostatic attraction, π–π stacking, hydrogen bonding, and Ni–O–SO₃⁻ coordination, with 4.2× selectivity over anionic IC dye. Adsorption was exothermic, entropy-driven, and followed PFO kinetics, with equilibrium adsorption data best fitted by the Sips isotherm model. Crucially, pre-adsorbed SAF underwent rapid photocatalytic mineralization (95% in 120 min; k = 0.0316 min⁻¹) under visible light via Z-scheme charge separation—rGO shuttled electrons from g-C₃N₄ to recombine with NiO holes, preserving high-potential holes (+1.4 eV) for direct oxidation while generating ˙O₂⁻/˙OH radicals. Adsorption preconcentration shortened radical diffusion pathways, accelerating degradation kinetics 2.5× versus non-adsorptive controls. The hybrid maintained 92% efficiency over 5 cycles and exhibited outstanding performance even in the presence of coexisting species (NaCl and SDS surfactant), highlighting its practical robustness.