Sunlight responsive photo-oxidation of methylene blue dye using MgO/MnO2 nanoparticles

Abstract

Despite extensive research on metal oxide-based photocatalysts, challenges remain in optimizing their structural and defect properties to enhance photocatalytic efficiency for wastewater treatment, while maintaining stability and scalability. Herein, the application of MgO/MnO₂ for the photo-oxidation of methylene blue dye (MB), has been investigated under sunlight irradiations. The MgO/MnO₂ was prepared by co-precipitation in a one pot synthetic route. The resulted samples were characterized by FTIR, XRD, diffuse reflectance and electrical conductivity. The XRD showed that α-MnO₂ crystallizes in tetragonal symmetry with a medium broadening, characteristic of Nano-crystallites. A direct optical transition at 1.85 eV was determined from the diffuse reflectance. The capacitance-potential graph (C⁻² - E) exhibits a positive slope, characteristic of n-type behavior with a flat band potential of $-0.027$ V_SCE. Hence, the photoholes in the valence band (1.7 V_SCE) oxidize water into reactive radicals ^•OH, involved in the MB mineralization. The photocatalytic capability of Nano-materials was assessed through photodegradation of MB by solar light. Results showed that the MB elimination rate rises with increasing in catalyst load and the declines in the initial MB concentration. The catalytic behavior of MgO/MnO₂ synthesized by this method exhibits excellent efficiency, achieving 87 % degradation of MB under optimal conditions: 10 mg/L dye concentration and 75 mg/L catalyst dosage using the MgMn-2 catalyst. This research proposes a promising strategy to enhance the photocatalytic activity of MnO₂ by doping it with MgO, thereby improving its performance and contributing to a deeper understanding of the underlying photocatalytic mechanisms.