Reversible and Cyclical Transformations between Solid and Hollow Nanostructures in Confined Reactions of Manganese Oxide and Silica within Nanosized Spheres
-Tae-Lin Ha †, Jin Goo Kim ‡, Soo Min Kim ‡, and In Su Lee *‡

Annealing of MnO@SiO2 nanospheres in a reducing gas environment resulted in the transformation of the core–shell structure into a hollow structure as a result of outward diffusion of MnO species into the thermodynamically more stable silicate phase. When the hollow silicate nanospheres were oxidized, the interior cavities were refilled with a Mn3O4 phase segregated from the silicate phase, and the hollow structure reverted to the initial core–shell structure. More interestingly, when catalytically active Pt nanocrystals were introduced into the manganese oxide/silica system, the Mn3O4 was readily reduced to the chemically reactive MnO, even at low temperature, which enabled reconversion of the solid nanospheres with a Mn3O4 core to hollow nanostructures during reductive annealing. Therefore, when MnO@SiO2/Pt(II) nanospheres were subjected to an oxidation/reduction cycle by repeatedly switching the flowing gas between air and hydrogen, the nanospheres underwent a reversible change between solid and hollow structures, depending on the gas environment. The solid-to-hollow-to-solid transformation was successfully cycled many times simply by repeatedly switching the flowing gas during annealing.