Revisiting the coordination chemistry for preparing manganese oxide nanocrystals in the presence of oleylamine and oleic acid†
By pyrolyzing manganese(II) acetate in 1-octadecene in the presence of oleylamine and oleic acid, manganese oxide nanocrystals were prepared. It was observed that both MnO and Mn3O4 nanocrystals were simultaneously formed by quickly heating the reaction mixture up to 250 °C, while a preheating procedure carried out at 100 °C led to uniform MnO nanocubes that developed into eight-arm MnO nanocrystals upon prolonged reaction. To understand the mechanisms for forming these two different kinds of manganese oxide nanocrystals, i.e., Mn3O4 and MnO, the coordination between oleic acid/oleylamine and Mn2+ was investigated. The detailed investigations suggest that Mn2+–oleylamine coordination is kinetically driven and favorable for the formation of Mn3O4 nanocrystals due to the relatively low electronegativity of N from oleylamine, while Mn2+–oleate coordination is thermodynamically driven and can prevent the central metal ion (Mn2+) from being oxidized owing to the relatively high electronegativity of O from the oleate ligand. Following these new insights, by properly balancing the coordination of oleic acid and oleylamine to Mn2+, the selective synthesis of MnO and Mn3O4 nanocrystals with uniform shapes was successfully achieved.