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

Pure α-MnO 2 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-MnO 2 and M-MnO 2 , respectively, from reduction of KMnO 4 . X-ray diffraction (XRD) investigation confirmed the phase purity of the as-prepared α-MnO 2 without any additional impurities. Further characterization by thermogravimetric analysis (TGA) showed the thermal behavior of α-MnO 2 which decomposed to Mn 2 O 3 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-MnO 2 and M-MnO 2 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-MnO 2 and M-MnO 2 at applied frequency of 42 Hz, respectively. The dielectric constant values of 2.02 × 10 6 at 140°C and 1.05 × 10 6 at 220°C were noted for M-MnO 2 and C-MnO 2 , respectively. The results of photocatalytic experiments showed that both M-MnO 2 and C-MnO 2 exhibited high visible light photocatalytic activities for methylene blue and Congo red degradation with efficiencies above 90% after irradiation for 140 min.