Reversible and Cyclical Transformations between Solid and Hollow Nanostructures in Confined Reactions of Manganese Oxide and Silica within Nanosized Spheres

Abstract

Annealing of MnO@SiO(2) 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 Mn(3)O(4) 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 Mn(3)O(4) was readily reduced to the chemically reactive MnO, even at low temperature, which enabled reconversion of the solid nanospheres with a Mn(3)O(4) core to hollow nanostructures during reductive annealing. Therefore, when MnO@SiO(2)/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.

Mechanistic Insight into the Yolk@Shell Transformation of MnO@Silica Nanospheres Incorporating Ni2+ Ions toward a Colloidal Hollow Nanoreactor

“…The MnO@SiO 2 /Ni 2 + nanospheres were made through modifying previously reported reverse microemulsion technique. [5a] Igepal CO‐520 (1.2 ml) was dispersed in a round bottom flask containing cyclohexane (22 ml) by sonication. Next, 10 mg of MnO nanoparticles dispersed in cyclohexane were added to the reaction solution.…”

“…This approach has been the most successfully employed in the recent production of hollow nanoparticles that have received considerable attention as attractive candidates for biomedical and catalytic applications . In this regard, our research was devoted to developing a simple and versatile solid‐state conversion strategy that can contribute to the development of nanoparticles of increasing complexity, by exploiting an understanding of the transformation of nanoparticles during their dimensionally confined reaction within a nanosized solid matrix . In a previous study on the conversion chemistry of manganese oxide nanocrystals inside a silica nanosphere, we observed the evolution of the hollow structure into a thermodynamically more stable silicate phase of the shell as a result of an outward diffusion of the MnO species.…”

“…In a previous study on the conversion chemistry of manganese oxide nanocrystals inside a silica nanosphere, we observed the evolution of the hollow structure into a thermodynamically more stable silicate phase of the shell as a result of an outward diffusion of the MnO species. [5a] An interesting phenomenon was that the incorporation of Ni 2+ ions left a single Ni nanocrystal inside the newly generated void space and therefore produced a yolk@shell‐type nanostructure, quite distinct from those including Pt 2+ and Pd 2+ complexes. This paper reports recent findings that can contribute toward a more complete understanding of the pathway for this transformation, revealing the existence of sequential steps, including the formation of a mixed‐metal oxide and segregation of the reduced metallic species, all within the silica nanosphere.…”

Supramolecular nano-assemblies with tailorable surfaces: recyclable hard templates for engineering hollow nanocatalysts

“…The most commonly used templates can broadly be divided into two categories: hard templates and soft templates. Monodisperse silica, polymers, carbon nanospheres, and nanoparticles of metals and metal oxides have been used as hard templates for preparation of hollow structures [17][18][19][20]. The synthesis of designed materials with hollow structures requires the templates to have additional surface functionalization.…”

Colloids of Holey Gd2O3 Nanosheets Converted from Exfoliated Gadolinium Hydroxide Layers

“…In order to develop a colloidal suspension of crystalline 2D Gd 2 O 3 that could avoid the need to anneal step the LGdH after deposition on the substrate, we employed a confined solid‐state reaction approach that involved the encapsulation of the exfoliated LGdH layers in thermally stable SiO 2 . The SiO 2 coating was expected to provide an antistacking and antisintering environment for the LGdH layer during the high temperature reaction, and to be facilely removed to liberate the transformed product with its 2D morphology and colloidal properties intact . The silica shell could also obstruct atomic diffusion between layers, which could allow for a spatially and dimensionally confined phase transition within an isolated 2D layer, which cannot be achieved in a bulk layered material.…”