Monodisperse manganese oxide nanoparticles: Synthesis, characterization, and chemical reactivity

Highly monodisperse amorphous manganese oxide (MnO_x) nanospheres with diameter of ca. 300nm have been obtained from ammonia aqueous solution of KMnO₄ at room temperature. The amorphous MnO_x nanospheres successfully converted to monodisperse K-OMS-2 (cryptomelane) and K-OMS-2/Mn₂O₃ nanoraspberries through calcination process at 600 and 800°C, respectively. Analyzing the structure of such amorphous MnO_x has been a challenge because fewer reports are available to examine amorphous structure. Thus, shape, crystallinity, and structure of the amorphous and crystalline MnO_x nanostructures were characterized in detail by X-ray diffraction (XRD), thermogravimetry/differential thermal analysis (TG/DTA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray Photoelectron Spectroscopy (XPS), and energy dispersive spectroscopy (EDS). We discussed a plausible formation mechanism of amorphous MnO_x nanospheres based on the investigations. The obtained MnO_x nanostructures have been demonstrated to possess oxidative degradation ability of Rhodamine B (RhB) under acidic aqueous condition without any additives such as chemical oxidizing agents and UV and/or visible light irradiation. RhB degradation rate of amorphous MnO_x nanospheres was about one hundred times faster than that of K-OMS-2 nanoraspberries.