Formation mechanism of Mn Co O yolk-shell structures.

Formation of Mn Co O yolk-shell microspheres a solvothermal reaction of hydrated cobalt and manganese nitrates in ethanol is investigated. Spinel nanocrystals of cobalt oxide or cobalt-rich ternary oxide preferentially develop in the system, while manganese-rich hydroxide form Mn(OH)-type nanosheets. Instead of continuing to grow individually, the nanocrystallites and nanosheets aggregate into large microspheres due to their strong inter-particle interaction. When the proportion of Mn-rich nanosheets is high, therefore the overall density is low, dehydration of hydroxide nanosheets and a surface re-crystallisation lead to formation of a dense and rigid shell, which is separated from a solid or hollow core a further Ostwald ripening process. The proposed formation mechanism of the yolk-shell structures based on electron microscopic studies would help us to develop yolk-shell structure based multifunctional materials.