Superior photocatalytic activity of Mn vanadate/reduced graphene oxide magnetic nanocomposite for the oxidation of methylene blue dye under sunlight irradiation†

Abstract

A magnetic photocatalyst based on reduced graphene oxide and semiconducting MnV₂O₆ (rGO/Fe₃O₄/MnV₂O₆) was synthesized by magnetizing the rGO/MnV₂O₆ composite for the efficient degradation of methylene blue (MB), a biodegradation-resistant dye. The prepared magnetic photocatalyst was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Raman spectroscopy, Brunauer–Emmett–Teller surface area analysis, Barrett–Joyner–Halenda (BET-BJH) pore analysis and vibrating sample magnetometry (VSM). Its photocatalytic properties and activity were investigated using UV–Vis diffuse reflectance spectroscopy (DRS), fluorescence spectroscopy and inductively-coupled plasma analysis. The synthesized rGO/Fe₃O₄/MnV₂O₆ nanocomposite exhibited a bandgap of 2.59 eV and a specific surface area of 201.5 m² g⁻¹. The optimum photocatalytic conditions for efficient MB photodegradation were determined, and the photocatalytic efficiency was investigated under different light irradiations, including blue and yellow light as well as sunlight. The rGO/Fe₃O₄/MnV₂O₆ photocatalyst demonstrated excellent MB degradation efficiency, achieving up to 94% under sunlight irradiation. Additionally, the catalyst exhibited remarkable reusability, maintaining 88% degradation efficiency after the 6th cycle, with negligible structural changes. The magnetic properties of the employed photocatalyst played a key role in facilitating their separation and recycling. The kinetics and mechanism of the photodegradation process were evaluated, revealing a pseudo-first-order rate constant of 0.0882 min⁻¹ for a hydroxyl radical-based mechanism. The MB degradation was driven by the generation of superoxide (O₂˙⁻) and hydroxyl (˙OH) free radicals during the reaction.