Green-mediated synthesis of bixbyite α-Mn2O3 nanoparticles via Moringa oleifera: impact of calcination on properties and visible-light photocatalytic degradation of Eosin yellow and eriochrome black T dyes

The leaf extraction of Moringa oleifera was used to synthesize α-Mn₂O₃ nanomaterials. The synthesized manganese oxide nanostructures were subjected to various annealing temperatures (500 °C, 600 °C, and 700 °C). Powder X-ray diffraction (PXRD) was employed to verify the phase purity and crystallinity characteristics. The PXRD analysis revealed the pristine phase of a cubic bixbyite structure. Diffuse reflectance spectroscopy (DRS) showed that the optical band gap of α-Mn₂O₃ NPs were 1.91 eV, 1.75 eV, and 1.65 eV, respectively for 500 °C, 600 °C, and 700 °C. Photoluminescence spectroscopy showed multicolour emissions in the UV and visible regions because of Mn₂O₃ defect centers. Fourier Transformed Infrared Spectroscopy (FTIR) was used to identify the functional groups that are liable for the nanoparticle formation. The SEM and TEM analyses revealed that the α-Mn₂O₃ nanoparticles exhibited a spherical morphology with particle sizes ranging approximately from 50 to 100 nm. The characteristic hysteresis graph observed in VSM measurements illustrated the paramagnetic behaviour at room temperature (RT). Impedance analysis, AC conductivity, and frequency-dependent electrical characteristics were investigated. The photocatalytic performance of α-Mn₂O₃ was assessed for the degradation of Eosin Y (EY) and Eriochrome Black-T (EB-T) dyes under visible light irradiation. At a calcination temperature of 600 °C, the degradation efficiencies achieved within 90 min were 98% for Eosin Y (EY) and 88% for Eriochrome Black-T (EB-T) dyes. The photodegradation mechanism of EY and EBT dyes was investigated using radical scavenger agents. Furthermore, the synthesized nanoparticles demonstrated excellent reusability, retaining their photocatalytic activity over five consecutive cycles under the same optimal conditions.