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Charge reduction in ions in the ionic liquid 1-ethy-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide on the Au(111) surface

Abstract

Room-temperature ionic liquids (RTILs) are one type of electrolytes which have promising applications in batteries, catalysts, supercapacitors, etc. Computational simulations have played an essential role in elucidating many of the RTILs’ properties. Because Coulomb interactions dominate in RTILs, it is important to understand charge fluctuations, i.e., when ions’ charges are significantly less than unity. In this work, we perform calculations from density functional theory methods and the Bader charge analysis to examine ionic charges of 1-ethy-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ([EMMIM]+[TFSI]) thin film on the gold (111) surface. For cation and anion of similar sizes, we first identify the most stable ionic arrangement on Au(111) to be a checkerboard pattern. We build the thin film with up to four layers of ions in the most stable configuration and analyze the charge of each ion. In addition to cases of equal numbers of cations and anions, we also study systems where there is one more cation or anion but no net charge. We find that ions mostly maintain a near unity charge. However, when the ion numbers are not equal, and therefore the system is kept neutral by accruing counter charge on the gold, the counter charge is much less than unity, indicating charge reduction in ions. We further find that the charge deviation mostly occurs on the ion in the top layer and the reduction increases with the number of layers. We explain the results through a model of state mixing that includes charge transfer between the gold and ions.

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Acknowledgements

This research is supported by the Center for Functional Nanomaterials, which is a US DOE Office of Science Facility, and used resources at the Scientific Data and Computing Center, a component of the Computational Science Initiative, at Brookhaven National Laboratory under Contract No. DE-SC0012704.

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Correspondence to Qin Wu.

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Liu, M., Shao, Y. & Wu, Q. Charge reduction in ions in the ionic liquid 1-ethy-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide on the Au(111) surface. Theor Chem Acc 139, 24 (2020). https://doi.org/10.1007/s00214-019-2527-x

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Keywords

  • Ionic liquid
  • Charge analysis
  • Charge configuration model