Jahn-Teller Disproportionation Induced Exfoliation of Unit-Cell Scale ε-MnO2.

Exfoliation of nanosheet from non-layered bulk material is challenging because of the strong chemical bonds in lattice, rather than the weak van de Waals (vdW) interaction in the layered bulk counterparts. In this paper, we propose a top-down method to exfoliate e-MnO2 nanosheets from a family of charge-ordered La1-xAExMnO3 (AE=Ca, Sr, Ba) perovskites, taking advantage of Jahn-Teller disproportion effect of Mn3+ and bond strength difference. The e-MnO2 nanosheet (a thickness of 0.91 nm, two-fold of the unit cell along c-axis) crystalized into a nickel arsenide (NiAs) structure with ordered arrangement of 50% cation vacancy in reference to NiAs, which shows thermal metastability and the best water oxidation activity compared with other types of manganese oxide. The exfoliation mechanism has been reasoned as a synergistic proton-induced Mn3+ disproportionation and structural reconstruction. The stochastic surface walking (SSW) pathway calculation of energy and phase transformation suggested a proton-exchanged perovskite to birnessite route followed by the local reconstruction of MnO6 from perovskite skeleton. The synthetic method could also be potentially extended to the exfoliation of other two-dimensional nanosheet materials with non-layered crystal structures.