Revisiting the coordination chemistry for preparing manganese oxide nanocrystals in the presence of oleylamine and oleic acid

Abstract

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.