Growth of Nearly Monodisperse MnO Nanocrystals in a Two-Size Distribution System

Nearly monodisperse MnO nanocrystals were synthesized by the thermal decomposition of manganese oleate using the hot-injection method. The growth dynamics of the nanocrystals was studied by the time-sampling method, which revealed an interesting growing-to-shrinking behavior accompanied by a broadening-narrowing-broadening size distribution. The size evolution of nanocrystals is attributed to the kinetics of the monomer concentration in the solution as a result of the competition between crystal growth and Ostwald ripening, which favors the growth of another ensemble of MnO nanocrystals that coexisted ill the products with much larger sizes. The two ensembles of MnO nanocrystals with rather different characteristic sizes could be easily separated by their different dispersibilities in hexane, allowing an independent determination of their growth modes in the reaction process. The correlation between the growth of small MnO nanoparticles and that of the large ones indicates that the existence of large MnO nanocrystals help improve the monodispersity of the small ones by reducing the nucleation time and suppressing size coarsening in the Ostwald ripening process. The characteristic size evolution of the nanocrystals in a two-size distribution system suggests an interesting method to control the growth (or dissolution) of nanocrystals with high monodispersity.