An in situ STM/AFM and impedance spectroscopy study of the extremely pure 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate/Au(111) interface: potential dependent solvation layers and the herringbone reconstruction

Abstract

The structure and dynamics of the interfacial layers between the extremely pure air- and water-stable ionic liquid 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate and Au(111) has been investigated using in situ scanning tunneling microscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and atomic force microscopy measurements. The in situ scanning tunnelling microscopy measurements reveal that the Au(111) surface undergoes a reconstruction, and at -1.2 V versus Pt quasi-reference the famous (22 × √3) herringbone superstructure is probed. Atomic force microscopy measurements show that multiple ion pair layers are present at the ionic liquid/Au interface which are dependent on the electrode potential. Upon applying cathodic electrode potentials, stronger ionic liquid near surface structure is detected: both the number of near surface layers and the force required to rupture these layers increases. The electrochemical impedance spectroscopy results reveal that three distinct processes take place at the interface. The fastest process is capacitive in its low-frequency limit and is identified with electrochemical double layer formation. The differential electrochemical double layer capacitance exhibits a local maximum at -0.2 V versus Pt quasi-reference, which is most likely caused by changes in the orientation of cations in the innermost layer. In the potential range between -0.84 V and -1.04 V, a second capacitive process is observed which is slower than electrochemical double layer formation. This process seems to be related to the herringbone reconstruction. In the frequency range below 1 Hz, the onset of an ultraslow faradaic process is found. This process becomes faster when the electrode potential is shifted to more negative potentials.

Long-range electrostatic screening in ionic liquids

“…Our results suggest that a long-range, monotonic force may be a general feature of the electric double layers formed by RTILs in contact with macroscopic charged surfaces, whenever surface-bound ion layers are unable to fully compensate surface charges. For mica surfaces, we find this longrange force to be in series with a short-range, nonmonotonic "oscillatory" structural force that is in agreement with the nanoscale surface-induced ion ordering identified in previous work on RTILs (17,(20)(21)(22)(23)(24)(25). Further, we show that the long-range screening behavior of RTILs can be rationalized via a thermally activated population of either free ions or correlated charge domains, reminiscent of the free electron/hole approximation used in semiconductor physics.…”

“…These studies predict that the electric double layers formed by RTILs should be short-range and exhibit pronounced charge density oscillations, similar to the screening behavior identified in pioneering work on molten KCl (15). Recent theoretical studies emphasize that complex ion ordering in RTILs can intimately depend on the electrochemical potentials of the charged surfaces (5,(17)(18)(19), but these studies have left intact the picture that the electric double layers formed by RTILs should be very short-ranged.Experimentally, the short-range, surface-induced ordering of ions in RTILs has been confirmed with X-ray scattering experiments, atomic force microscopy (AFM) force measurements, and surface forces apparatus (SFA) measurements (17,(20)(21)(22)(23), with these studies highlighting that the ordering of RTIL ions at surfaces exhibits many subtleties. For example, Atkin et al (22) used AFM to demonstrate that gold electrode surfaces immersed Significance Liquid solutions with high concentrations of electrically charged ions are key elements of many energy storage technologies and are prevalent in biology.…”

“…Experimentally, the short-range, surface-induced ordering of ions in RTILs has been confirmed with X-ray scattering experiments, atomic force microscopy (AFM) force measurements, and surface forces apparatus (SFA) measurements (17,(20)(21)(22)(23), with these studies highlighting that the ordering of RTIL ions at surfaces exhibits many subtleties. For example, Atkin et al (22) used AFM to demonstrate that gold electrode surfaces immersed Significance Liquid solutions with high concentrations of electrically charged ions are key elements of many energy storage technologies and are prevalent in biology.…”

Quantum design of ionic liquids for extreme chemical inertness and a new theory of the glass transition

“…A dense layer of ions on Au(111) has also been successfully imaged by STM, as reported by Waldmann et al [15]. In a similar vein, Atkin et al, have performed STM /AFM and impedance studies of the interface between 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate and Au(111), and found potential-dependent adsorbed multilayers of ions (as well as surface reconstruction of the substrate) [16]. Lastly, Roling et al have carried out impedance studies on the same system, and found evidence of both fast and slow capacitive charging processes [17].…”

Perspective: Chemical reactions in ionic liquids monitored through the gas (vacuum)/liquid interface

“…We explicitly do address only studies where the targeted process is a chemical reaction between different species but do not address physical processes such as a phase transition, 59-61 adsorption or desorption, [62][63][64] and diffusion 65,66 or a reorganization. [67][68][69][70][71][72] We also exclude studies where the reaction was not carried out in situ, that is, spectra were not recorded while the reaction proceeded, or the sample was moved to a different site in the XPS setup or was even removed from the XPS setup in order to carry out the reaction. [73][74][75][76][77] We also do not address studies that incorporate electrochemical sample manipulation, 66,67,[69][70][71][78][79][80][81][82][83][84][85][86] electrodeposition of metal or organic layers from ILs, [87][88][89][90][91] and IL decomposition 84,[92][93][94][95][96][97][98][99]...…”