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

Pure alpha-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 alpha-MnO2 without any additional impurities. Further characterization by thermogravimetric analysis (TGA) showed the thermal behavior of alpha-MnO2 which decomposed to Mn2O3 at about 500 degrees 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 x 10(6) at 140 degrees C and 1.05 x 10(6) at 220 degrees 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.