Location: Home >> News >> Research Progress  
  Research Progress  
    Revisiting the Coordination Chemistry for Preparing Manganese Oxide Nanocrystals in the Presence of Oleylamine and Oleic Acid
http://www.gaomingyuan.com  Thursday 27, 2014  10:00
    Manganese oxides have drawn considerable interest owing to their potential applications in high performance electrodes, catalysts, soft magnetic storage, and contrast agents for magnetic resonance imaging. Over the past years, great efforts have been paid for regulating the size and shape of manganese oxide especially Mn3O4 and MnO nanocrystals synthesized through different wet-chemical synthetic routes. Among them, pyrolyzing manganese-organic compounds at elevated temperature, known as thermal decomposition method, has been demonstrated as the most effective approach for achieving high quality manganese oxide nanocrystals. In this method, oleylamine and oleic acid are widely used ligands and it can be summarized that oleylamine was found to be in favor of the formation of Mn3O4 while oleic acid tended to give rise to MnO. When oleylamine and oleic acid were used simultaneously as surface capping agents for synthesizing manganese oxide nanocrystals, the form of resulting particles was quite dependent on synthesis parameters. In addition, there remains a lack of in-depth studies into the underlying molecular mechanism for such ligand-dependent composition of manganese oxide nanocrystals.
    In this research, we studied the synthesis of manganese oxide nanocrystals through thermal decomposition of Mn(Ac)2 in 1-octadecene (ODE) by using oleylamine and oleic acids as surface capping agents. To disclose the surface ligand-dependent formation of MnO and Mn3O4 nanocrystals, different heating procedures were purposely implemented. In addition, various reference experiments were designed to show the effects of oleylamine and oleic acid in achieving different manganese oxide nanocrystals to facilitate discussion of the underlying mechanisms. It was observed in the experiments 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. Upon investigating the coordination between oleic acid/oleylamine and Mn2+, we found 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 oleate ligand. Following these new insights, through properly balancing the coordination of oleic acid and oleylamine to Mn2+, the selective synthesis of MnO and Mn3O4 nanocrystals was successfully achieved. Moreover, the current investigations provide deeper understanding on the role of oleic acid and oleylamine for preparing transition metal oxide nanocrystals.
    The accepted article was benefited from the selfless help and inculcation of Professor Mingyuan Gao in the whole experiment process and the writing as well as revising of the article. I am sincerely grateful to his patient guidance and also appreciate the help from Dr. Lihong Jing and Mr. Jianfeng Zeng.
                                                                                                                                                                                                         Hongwei Zhang et al.