Various-Shaped Uniform Mn3O4 Nanocrystals Synthesized at Low Temperature in Air Atmosphere

We report a novel and facile method for the synthesis of manganese oxide (Mn3O4) nanocrystals with various sizes and shapes. Mn3O4 nanocrystals were synthesized via a reaction of manganese(II) acetate with water in xylene in the presence of surfactants at the temperature of as low as 90 degrees C in air atmosphere. Structural characterizations revealed that the synthesized nanocrystals were tetragonal Mn3O4 structure and that they were highly crystalline in spite of the low reaction temperature. The size and shape of the nanocrystals were readily controlled by varying the experimental conditions such as precursors, surfactants, and injection temperature of water. Nanoplates with a thickness of 5 nm and side dimensions of 9, 15, and 22 nm were synthesized using oleylamine as the surfactant. When carboxylic acid was used as the cosurfactant along with oleylamine, spherical nanocrystals were obtained with sizes of 5.5, 6.2, 7.2, 8.5, and 15 nm. Interesting anisotropic nanostructures including nanowires and nanokites were also prepared by changing the injection temperature of water. Mechanistic studies revealed that in situ generated manganese hydroxide (Mn(OH)(2)) mainly contributes to the nucleation, whereas the manganese-oleylamine complex contributes to the shape-controlled growth process. The current procedure can be readily applicable to large-scale synthesis because of their facile and mild reaction conditions including low reaction temperature and air environment and the use of nontoxic and inexpensive reagents. For example, under optimized reaction conditions, we were able to synthesize as much as 4.5 g of 15 nm sized Mn3O4 nanoplates using a 1 L reactor. Water-dispersible 9 nm sized Mn3O4 nanoplates exhibited specific relaxivity (r(1)) value of 0.13 mM(-1) s(-1), demonstrating the potential application of the nanocrystals to T-1 contrast agent for magnetic resonance imaging (MRI).