Scalable synthesis of water-dispersible 2D manganese dioxide monosheets

2D nanomaterials with atomic thickness usually exhibit high specific surface areas and atom exposure rates, which are suitable for surface reaction related applications. In this study, we selected the oxalate ions as the structure-inducing agent to synthesize δ-MnO₂ ultrathin nanosheets (~4.5 nm) via a facile hydrothermal method. Subsequently, an efficient exfoliation method to prepare single-layer MnO₂ nanosheets (~0.9 nm) with the major exposed {0 0 1} facets was successfully developed. We found that the oxalate ions play a major role in the growth and formation of δ-MnO₂ ultrathin nanosheets, and the formation process of the ultrathin structure was also investigated. The resulting single-layer MnO₂ nanosheets (monosheets) with exposed {0 0 1} facets showed much higher catalytic performance for carcinogenic airborne formaldehyde, better than few-layer ultrathin nanosheets and nanoflowers with exposed {1 0 0} facets. The reasons for the high catalytic activity of MnO₂ monosheets can be attributed to its higher surface areas and oxygen vacancy concentration. Moreover, the density-functional-theory (DFT) theoretical calculations showed that the oxygen vacancy in single-layer {0 0 1} facets exhibited the strongest adsorption/activation ability to O₂ and H₂O, which was very favorable for catalytic oxidation of formaldehyde. The synthesis strategy of ultrathin nanosheets described in this article may serve as reference and guidance for the preparation of other 2D ultrathin nanomaterials.