Structural dependence of oxidative properties of Mn-oxide nanoparticles prepared by a facile gel formation method

Abstract

The noble synthesis of Mn-oxides nanoparticles and their structural dependence of oxidative properties is a subject of intensive research for their imminent applications in different fields of science and engineering. In the present study, Mn-oxides nanoparticles with different crystal geometry have been synthesized by a facile gel formation route and their structural and oxidative characterizations have been demonstrated. During the synthesis process, a gel precursor is formed through the reduction of KMnO4 with glycerol. After the heat treatment of the gel precursor at different temperatures, Mn-oxides nanoparticles having different oxidation states, viz., Mn3O4, Mn5O8 and Mn2O3 were formed. The phase identification, chemical composition, crystal structure, crystallinity and surface morphology were analyzed using powder X-ray Diffraction (XRD), Energy Dispersive X-ray (EDX), Fourier Transform Infra Red (FT-IR) and Field Emission Scanning Electron Microscopy (FESEM) techniques. Mn3O4 nanoparticles with tetragonal crystal structure and average crystallite size of 42 nm were formed at 80 °C. Then the two other forms, Mn5O8 nanorods with monoclinic crystal structure and Mn2O3 nanoparticles with cubic crystal structure were formed after heat treatment the gel precursor at 350 °C and 700 °C respectively. The average crystallite size of Mn5O8 and Mn2O3 nanoparticles calculated from the XRD data was found to be 35 nm and 28 nm respectively. The structural dependence of oxidative property of the synthesized Mn-oxides nanoparticles was demonstrated from the decolorization of Methylene Blue (MB) using UV-visible spectroscopic technique. Effect of MB concentration, amount of nanoparticles required, influence of pH of the medium and function of time on MB decolorization were also investigated. The dye degradation experiments show that Mn3O4 nanoparticles decolorizes the dye through oxidative and adsorptive activity whereas Mn5O8 and Mn2O3 nanoparticles decolorize the dye through oxidative activity. Among the three oxides of Mn, Mn5O8 nanorods show superior dye decolorizing ability.