Green Synthesized α-MnO₂ As a Photocatalytic Reagent for Methylene Blue and Congo Red Degradation

Pure α-MnO₂ materials with interconnected nanowires in an urchin shape were synthesized by a green synthesis method. Purified extracts of cinnamon and moringa were used as benign and economical reducing agents to synthesize C-MnO₂ and M-MnO₂, respectively, from reduction of KMnO₄. X-ray diffraction (XRD) investigation confirmed the phase purity of the as-prepared α-MnO₂ without any additional impurities. Further characterization by thermogravimetric analysis (TGA) showed the thermal behavior of α-MnO₂ which decomposed to Mn₂O₃ at about 500°C as a result of thermal reduction and loss of oxygen. Transmission electron microscopy (TEM) images showed an urchin shape of as-prepared oxides. Based on a Kubelka–Munk plot, the band gaps of C-MnO₂ and M-MnO₂ nanowires were found to be 1.42 eV and 1.34 eV, respectively. The room temperature dielectric constant values were estimated to be 1980 and 1130 for C-MnO₂ and M-MnO₂ at applied frequency of 42 Hz, respectively. The dielectric constant values of 2.02 × 10⁶ at 140°C and 1.05 × 10⁶ at 220°C were noted for M-MnO₂ and C-MnO₂, respectively. The results of photocatalytic experiments showed that both M-MnO₂ and C-MnO₂ exhibited high visible light photocatalytic activities for methylene blue and Congo red degradation with efficiencies above 90% after irradiation for 140 min.