Control of Size and Composition of Colloidal Nanocrystals of Manganese Oxide

A comprehensive study on the effects of experimental parameters on the composition and size of manganese oxide nanocrystals was completed using colloidal chemistry. The reactions studied involved the thermolysis of Mn²⁺ acetate and Mn³⁺ acetylacetonate in oleylamine. Temperature was found to be the dominant factor affecting the composition and size of the products. Reactions completed below 200 °C favored the formation of nanocrystals smaller than 20 nm, with the presence of even impurity amounts of oxidizing agents leading to the formation of Mn₃O₄. Nanocrystals of MnO could only be synthesized below 200 °C if Mn²⁺ acetate was used, and the reaction was carefully controlled to have no O₂ and H₂O contamination. In turn, particle growth was rapid above this temperature. In this case, regardless of the oxidizing agents used or oxidation state of the Mn precursor, nanocrystals of MnO formed after annealing for at least 1 h at temperatures higher than 200 °C. This finding suggests the role of oleylamine as solvent, surfactant, and reducing agent at sufficiently high annealing temperatures. These results increase the understanding of redox stability of manganese during the colloidal synthesis of semiconductor metal oxide nanocrystals.