Ionic liquid/metal salt mixtures at the graphene interface: A density functional theory approach

Abstract

The interaction of graphene sheets with metal cations is very relevant because of the modifications that are induced in the electrochemical properties of the 2D material. Ionic liquids are a promising kind of materials with several applications in electrochemical devices, so understanding how they affect the graphene-metal interaction is key for their practical implementation. Therefore, we have studied the adsorption of a mixture of an ionic liquid and a metal salt on a graphene surface by means of first-principles calculations. Several metals were chosen to analyze the effect they have on the optoelectronic properties of the graphene monolayer and to understand the trends in their adsorption behavior. We have characterized the ground state configurations in terms of their binding energies and the distance between the metal atom and the graphene layer. From the analysis of the charge transfer behavior, calcium and magnesium have been identified as the species that transfer the highest and the lowest amount of charge, respectively, which is related to their ionization energies. Band structure diagrams and projected density of states calculations also show that the energy shift of the Dirac cone increases with the degree of charge transfer. In addition, a stronger interaction of magnesium with the ionic liquid compared to that of other metal atoms was observed. The analysis of several electromagnetic parameters indicated an anisotropic behavior for electric fields polarized both perpendicular and parallel to the graphene layer. Our density functional theory study offers fundamental insights into the adsorption behavior of ionic liquids mixed with metal ions on monolayer graphene.