The crucial role of electron transfer from interfacial molecules in the negative potential shift of Au electrode immersed in ionic liquids

Abstract

Potential of zero charge (PZC) is essential in electrochemistry to understand physical and chemical phenomena at the interface between an electrode and a solution. A negative potential shift from the work function to the PZC has been experimentally observed in a metal/ionic liquid (IL) system, but the mechanism remains unclear and controversial. In this paper we provide valuable insight into the mechanism on the potential shift in the Au/IL (1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide: [BMIM][TFSA]) system using a computational approach combining classical molecular dynamics simulations and first-principles calculations. By separately estimating some contributions to the potential shift, the shift is calculated in an easy-to-understand manner. The resultant PZC is shown to be in good agreement with the experimental one. Among the contributions, the electron redistribution at the Au/IL interface is found to provide the largest negative potential change. This indicates that the redistribution plays a crucial role in determining the potential shift of the Au electrode immersed in the IL. Detailed analyses suggest that the redistribution corresponds to the electron transfer not only from the anionic TFSA but also from the cationic BMIM molecules to the Au electrode surface. This unique observation is understood to originate from the interfacial structure where the IL molecules are in very close proximity to the electrode surface via dispersion interaction.