High-Yield Synthesis of Sodium Chlorides of Unconventional Stoichiometries

Abnormal salt crystals with unconventional stoichiometries, such as Na₂Cl, Na₃Cl, K₂Cl, and CaCl crystals that have been explored in reduced graphene oxide membranes (rGOMs) or diamond anvil cells, hold great promise in applications due to their unique electronic, magnetic, and optical properties predicted in theory. However, the low content of these crystals, only <1% in rGOM, limits their research interest and utility in applications. Here, a high-yield synthesis of 2D abnormal crystals with unconventional stoichiometries is reported, which is achieved by applying negative potential on rGOM. A more than tenfold increase in the abnormal Na₂Cl crystals is obtained using a potential of −0.6 V, resulting in an atomic content of 13.4 ± 4.7% for Na on rGOM. Direct observations by transmission electron microscopy and piezoresponse force microscopy demonstrates a unique piezoelectric behavior arising from 2D Na₂Cl crystals with square structure. The output voltage increases from 0 to ≈180 mV in the broad 0–150° bending angle regime, which meets the voltage requirement of most nanodevices in realistic applications. Density functional theory calculations reveal that the applied negative potential of the graphene surface can strengthen the effect of the Na⁺–π interaction and reduce the electrostatic repulsion between cations, making more Na₂Cl crystals formed.