Green and sol-gel synthesis of Cr₂O₃ nanoparticles: comparative photocatalytic degradation of methylene blue using a UV photoreactor

Abstract This study presents a comparative analysis of Cr₂O₃ nanoparticles synthesized by two different methods: a green route using Cordyline fruticosa extract (Cr₂O₃-G) and a chemical route via a solvent-controlled sol–gel process with ethanol (Cr₂O₃-C). Their photocatalytic performance in degrading methylene blue (MB) was evaluated under UV irradiation. Comprehensive characterization was performed using FTIR, XRD, UV–vis spectroscopy, particle size analysis, zeta potential measurements, SEM, and EDX to examine structural, optical, and morphological features. Crystallite sizes estimated by the Scherrer equation were 38 nm (Cr₂O₃-G) and 44 nm (Cr₂O₃-C), while Williamson–Hall analysis gave slightly larger values of 48 nm and 58 nm, respectively. Specific surface area (SSA) calculations from XRD indicated a higher SSA for Cr₂O₃-G (30 m²/g) compared to Cr₂O₃-C (26 m²/g). The optical band gap was slightly lower for Cr₂O₃-G (3.05 eV) than for Cr₂O₃-C (3.07 eV). Zeta potential values showed better stability for Cr₂O₃-G (–44.3 mV) than Cr₂O₃-C (–21.8 mV). SEM analysis revealed hexagonal structures in Cr₂O₃-G and cubic structures in Cr₂O₃-C. Both catalysts demonstrated excellent photocatalytic activity, achieving nearly complete MB degradation within 20 minutes (97% for Cr₂O₃-G and 96% for Cr₂O₃-C). Although their efficiencies were comparable, notable differences in stability, morphology, surface area, and band gap highlight the strong influence of synthesis methods on the physicochemical properties of Cr₂O₃ nanoparticles. Graphical Abstract