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

Highly monodisperse amorphous manganese oxide (MnOx) nanospheres with diameter of ca. 300nm have been obtained from ammonia aqueous solution of KMnO4 at room temperature. The amorphous MnOx 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 MnOx has been a challenge because fewer reports are available to examine amorphous structure. Thus, shape, crystallinity, and structure of the amorphous and crystalline MnOx 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 MnOx nanospheres based on the investigations. The obtained MnOx 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 MnOx nanospheres was about one hundred times faster than that of K-OMS-2 nanoraspberries.