Combined STM, AFM, and DFT Study of the Highly Ordered Pyrolytic Graphite/1-Octyl-3-methyl-imidazolium Bis(trifluoromethylsulfonyl)imide Interface

Abstract

The highly ordered pyrolytic graphite ( H O P G ) / 1 -o c t y l -3 -m e t h y l i m i d a z o l i u m b i s -(trifluoromethylsulfonyl)imide ([OMIm]Tf 2 N) interface is examined by ultrahigh vacuum scanning tunneling microscopy (UHV-STM), atomic force microscopy (UHV-AFM) (and as a function of potential by in situ scanning tunneling microscopy (STM)), in situ atomic force microscopy (AFM), and density functional theory (DFT) calculations. In situ STM and AFM results reveal that multiple ionic liquid (IL) layers are present at the HOPG/electrode interface at all potentials. At open-circuit potential (OCP), attractions between the cation alkyl chain and the HOPG surface result in the ion layer bound to the surface being cation rich. As the potential is varied, the relative concentrations of cations and anions in the surface layer change: as the potential is made more positive, anions are preferentially adsorbed at the surface, while at negative potentials the surface layer is cation rich. At −2 V an unusual overstructure forms. STM images and AFM friction force microscopy measurements both confirm that the roughness of this overstructure increases with time. DFT calculations reveal that [OMIm] + is attracted to the graphite surface at OCP; however, adsorption is enhanced at negative potentials due to favorable electrostatic interactions, and at −2 V the surface layer is cation rich and strongly bound. The energetically most favorable orientation within this layer is with the [OMIm] + octyl chains aligned "epitaxially" along the graphitic lattice. This induces quasi-crystallization of cations on the graphite surface and formation of the overstructure. An alternative explanation may be that, because of the bulkiness of the cation sitting along the surface, a single layer of cations is unable to quench the surface potential, so a second layer forms. The most energetically favorable way to do this might be in a quasi-crystalline/multilayered fashion. It could also be a combination of strong surface binding/orientations and the need for multilayers to quench the charge.

The interface between HOPG and 1-butyl-3-methyl-imidazolium hexafluorophosphate

“…16). Ordering could be detected on HOPG in OMITf2N at very negative potentials in a combined STM, AFM and DFT study 50 . However, at these potentials, apparently, the HOPG and the BMI cations do not fit to each other and the double-row ordering of the cations is not possible for this electrode-cation combination.…”

In situ PM-IRRAS of a glassy carbon electrode/deep eutectic solvent interface

“…Layering of organic solvents 73,74 and ionic liquids [75][76][77][78] near solid surfaces at OCP have been detected by in situ AFM of metal/liquid interfaces. Molecular ordering at solid surfaces has been found a common phenomenon for organic solvents and ionic liquids.…”

Topological defects in electric double layers of ionic liquids at carbon interfaces

“…Therefore, it is important to have techniques capable of determining the interfacial structure of ionic liquids at the molecular level in a 3D nature. Scanning probe techniques are well suited for this type of study due to their high lateral resolution, and scanning tunneling microscopy (STM) and AFM have been used to image the 2d molecular level structure of the adsorbed layer of ionic liquids on various substrates [9,28,29] and has also been extended to image near surface layers providing information in 3 dimensions [30]. Similarly 3D mapping of liquid structure at the solid-liquid interface has been done for aqueous electrolytes on graphite [31], mica [32], and Al 2 O 3 [33] surfaces.…”