Theoretical Studies on the Adsorption of 1-Butyl-3-methyl-imidazolium-hexafluorophosphate (BMI/PF $$_6$$ 6 ) on Au(100) Surfaces

Abstract

In order to gain a systematic understanding of electrode/electrolyte interface between gold electrodes and ionic liquids, the adsorption of 1-butyl-3-methyl-imidazolium-hexafluorophosphate (BMI/PF $$_6$$ ) on Au(100) was determined for a wide range of surface coverages using density functional theory (DFT) calculations. At low coverages BMI is exceptionally mobile (translational and rotational barriers of $$<$$ 0.06 eV) and lies flat on the surface. For higher coverages the repulsive interactions between neighboring BMI adsorbates are minimized as much as possible, while attractive interactions between oppositely charged BMI and PF $$_6$$ are maximized by coordinating PF $$_6$$ with positively charged carbons in the imidazole rings. At coverages up to 1/20 monolayers (ML per Au surface atoms) this results in adsorbate islands with an internal coverage of 1/20 ML in a checkerboard pattern of BMI and PF $$_6$$ , all lying in a plane parallel to the surface. For coverages between 1/20 and 2/35 ML the BMI ions are twisted out of this plane. Beyond 2/35 ML a new strategy is needed to accommodate excess BMI/PF $$_6$$ on the surface and a second checkerboard plane of BMI/PF $$_6$$ forms above the first. This strategy is successful until the coverage reaches 1/10 ML, at which the BMI adsorbates stand end on in order to squeeze even more adsorbates onto the surface.