Formation of Monodisperse and Shape-Controlled MnO Nanocrystals in Non-Injection Synthesis: Self-Focusing via Ripening

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Abstract
Formation of nearly monodiperse MnO nanocrystals by simple heating of Mn stearate in octadecene was studied systematically and quantitatively as a model for non-injection synthesis of nanocrystals. For controlling the shape of the nanocrystals, that is, rice, rods, peanuts, needles, and dots, either an activation reagent (ocadecanol) or an inhibitor (stearic acid) might be added prior to heating. The quantitative results of this typical non-injection system reveal that the formation of nearly monodisperse nanocrystals did not follow the well-known “focusing of size distribution” mechanism. A new growth mechanism, self-focusing enabled by inter-particle diffusion, is proposed. Different from the traditional “focusing of size distribution”, self-focusing not only affects the growth process of the nanocrystals, but may also play a role in controlling nucleation. Because of the simplicity of the reaction system, it was possible to also identify the chemical reactions associated with the growth and ripening of MnO nanocrystals with a variety of shapes. Through a recycling reaction path, water was identified as a decisive component in determining the kinetics for both growth and ripening in this system, although the reaction occurred at around 300 °C.