Nanoparticle shape conservation in the conversion of MnO nanocrosses into Mn₃O₄

The conversion of hexagonal-, square-, and cross-shaped MnO nanoparticles into mixed MnO - Mn₃O₄ nanoparticles occurs with retention of the nanoparticle shape. Upon aging, extra diffraction spots appear in the TEM analyses of both hexagonal- and cross-shaped nanoparticles (NPs). These extra diffraction spots can be assigned to the spinel form of Mn ₃O₄ (s-Mn₃O₄) and exhibit moiré interference patterns arising from the presence of two closely aligned, crystallographically similar phases. Examination of a variety of reaction conditions showed that the transformation of MnO into MnO/Mn ₃O₄ occurred while the particles are suspended in hexane at ambient temperature, by refluxing in hexadecane for 36 h, by heating to 200 °C in air, and by irradiating the NPs with a Raman laser beam. The crystal phase development and shape retention can be observed by using transmission electron microscopy (TEM). Single-crystal and polycrystalline selected area electron diffraction (SAED) patterns and dark-field TEM images confirm the coexistence of both MnO and s-Mn₃O₄ phases. Evaluation of the polycrystalline SAED patterns after irradiation in the Raman spectrometer indicated the presence of rings assignable to the tetragonal phase of Mn ₃O₄ (t-Mn₃O₄) as well as MnO and s-Mn₃O₄. The growth of the tetragonal phase by laser heating in the Raman experiment was confirmed by powder X-ray diffraction.