Electrostatic Potential-Driven Adsorption of Alkali Metal Cations on Graphene and Hexagonal Boron Nitride

Ion adsorption on the surfaces of 2D materials is crucial for applications in ion sieving, electrochemical sensing, and synthesis of abnormal 2D crystals. However, achieving tunable cation adsorption on 2D materials remains challenging. Using first principles calculations, we reveal that electrostatic potentials enable tunable adsorption of alkali metal cations on graphene and hexagonal boron nitride (hBN) surfaces. Negative potentials markedly strengthen the adsorption for all three cations on both substrates, whereas positive potentials weaken the adsorption. Remarkably, the adsorption enhancement under negative potentials is larger on hBN than on graphene, which is attributed to the much greater charge transfer from hBN to cations. Molecular orbital analysis indicates that in cation@hBN system the HOMO is mainly localized on the N atoms and LUMO centers on the adsorbed cation, making cations on hBN more prone to reduction, while the delocalized HOMO and LUMO in cation@graphene systems hinder the π electron departure from graphene. Moreover, electrostatic potentials can modulate interlayer spacing in cation-intercalated graphene and hBN bilayers: negative potentials contract the spacing and positive potentials expand it. These findings illuminate ion adsorption on 2D material surfaces and offer a feasible strategy to regulate ion adsorption and interlayer spacing in 2D layered membranes.