Monodisperse Octahedral α-MnS and MnO Nanoparticles by the Decomposition of Manganese Oleate in the Presence of Sulfur

Octahedral monodisperse alpha-MnS and MnO nanoparticles have been synthesized by decomposing manganese oleate and elemental sulfur in octadecene at high (250-320 degrees C) temperature. The chemical composition of the obtained NPs depends on the Mn:S ratio in an unexpected way. Pure alpha-MnS NP samples are obtained when S:Mn >= 2:1, whereas pure MnO NPs require S:Mn <= 0.6. Variation of several parameters (concentration of sulfur, heating rate and aging temperature and time) resulted in a alpha-MnS NP size interval of 11-14 (from Mn monooleate) and 18-30 nm (from Mn dioleate). For MnO NPs only, size control is also possible by addition of free oleic acid (14-24 nm). Analysis of TEM tilting experiments and electron diffraction shows that both alpha-MnS and MnO nanoparticles have octahedral shape and spontaneously form ordered arrays with strong texture in the {111} direction. Measurement of the magnetic properties showed that alpha-MnS nanoparticles consist of an antiferromagnetic core and a ferromagnetic-like shell that are exchange coupled below the blocking temperature of the shell (23 K for 29 nm alpha-MnS NP).