A green chemistry approach was successfully used in this work to synthesize manganese oxide nanoparticles with different shapes and crystalline phases. This approach is based on the visible light irradiation (445 nm) at room temperature of an aqueous solution of a manganese(II) salt in the presence of an alkaline hydroxide (NaOH, KOH or LiOH). Several experimental parameters, i.e., the nature of the precursor salt, hydroxide concentration, hydroxide type, irradiation time and the atmosphere, were tuned and their influence on the morphology and the structure of the nanoparticles was studied by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. A diversity of manganese oxide phases (λ-MnO2, γ-Mn3O4 and MnO(OH)) were obtained. It must be emphasized that these compounds were easily and fairly quickly synthesized at room temperature, without surfactants and, moreover, by using an environmentally friendly method. In addition, the successful control of the size and the shape of the nanoparticles allowed to obtain a variety of nanoparticles morphologies ranging from one-dimensional (1-D) to three-dimensional (3-D) nanostructures (i.e., spherical, nanorods, nanoflowers, nanocubes). This synthetic approach could be readily extended to other transition metal oxides.
