Self-assembly of 2D MnO₂ nanosheets into high-purity aerogels with ultralow density

Self-assembling inorganic nanoparticles (NPs) into macroscopic three dimensional (3D) architectures often requires the assistance of organic components, leaving residual organics in the resultant. In this work, organic-free MnO₂ aerogels with ultralow density have been achieved by the self-assembly of two dimensional (2D) MnO₂ nanosheets via an ice-templating approach. To the authors' best knowledge, it is the first reported case of constructing a high-purity inorganic aerogel from preformed NPs without using any functionalization or stabilization agents. Moreover, it has been demonstrated that an ultralight MnO₂ aerogel with a density as low as ∼0.53 mg cm^-3, which is the lightest metal oxide aerogel to date, can be well obtained by such an approach. The successful formation of the aerogel can be attributed to the enhanced van der Waals force between the 2D building blocks that have been more orderly arranged by the squeezing of ice crystals during the freezing process. Hence, this work shows a pioneering example of assembling inorganic NPs into aerogels relying only on the weak interactions between NPs (e.g. van der Waals forces). It has also been demonstrated that the obtained MnO₂ aerogel can function as an effective absorbent for toxic reducing gas, owing to its strong oxidation ability and high porosity. The strategy presented herein holds good potential to be applied to the fabrication of other high-purity inorganic aerogels, especially those with 2D building blocks readily available.