Ionic Liquids Achieve the Exfoliation of Ultrathin Two-Dimensional VOPO₄·2H₂O Crystalline Nanosheets: Implications on Energy Storage and Catalysis

Two-dimensional (2D) nanosheets have been widely reported and applied. Among them, 2D VOPO4·2H2O (HVPO) is widely used in the chemical industry and energy storage field because of its polyanionic laminates. However, its layered structure is difficult to be damaged because of the strong hydrate hydrogen bonds and a small interlayer spacing. Herein, the ionic liquids (ILs) are utilized to achieve the physicochemical exfoliation of HVPO. Also, the ultrathin two-dimensional (2D) crystalline nanosheets are successfully obtained with fully exposed crystal planes and a monoatomic or few-atomic-layer structure. Further, the exfoliation mechanism of hydrogen bond destruction and recombination is proposed by combining the density functional theory (DFT) and characterization analysis. The strong hydration hydrogen bonding of an HVPO interlayer is destroyed and a system of hydrogen bonds between the cations of the ILs with a layered plate of VOPO4 and the anions of the ILs with crystal water is reformed. Besides, reassembly of the 2D nanosheets was prohibited due to the recombination of the hydrogen-bonding network. It is found that the 2D crystalline nanosheets have a quenching effect on the photoluminescence of ILs and have good electrochemical properties such as reducing the battery impedance (about 5 times less than that of the blank) and reducing the redox potential difference.