Elektrochemische Untersuchungen zur Struktur und Dynamik der Grenzfläche zwischen ionischen Flüssigkeiten und Au(111)

We have carried out fundamental investigations of the IL/Au(111) interface by combining EIS, in-situ STM and in-situ AFM. The aim of the study was to obtain deeper insights into the structur and dynamics of this interface, which is relevant for numerous electrochemical applications of ILs. For our studies, we have used the ultra pure, custom made neat ILs [EMIm]FAP and [Pyr1,4]FAP. The (111) metal plane of single crystalline Au was chosen as the electrode surface. For both systems and at almost every WE-potential, the recorded EIS data point to the existence of two capacitive processes taking place at the interface on different time scales. The fast process showing a time constant of milliseconds at RT was identified with the electrostatic charging of the IL side of the interface via the bulk resistance to screen the electrode’s charge. For the slow capacitive process showing a time constant of seconds we proposed the existence of an additional activation barrier. In the potential regime, where the Au(111) surface is negatively charged, we assumed that the process can be identified with the potential-induced spatial re-orientation of cations in the innermost interfacial layer. Since the Au(111) surface also reconstructs in this potential regime, this process might also be accompanied with a phase transition of the cationic layer. At potentials where the surface is positively charged, a site exchange between the cations in the innermost layer and anions from the outer layers has to be performed to screen the electrode’s charge. The site exchange might also afford the surmounting of an activation barrier, which would slow down the process and lead to an additional capacitive contribution observable on longer time scales. Besides the before-mentioned capacitive processes, a third process, which seems to be faradaic, was observable on time scales of several minutes. By performing EIS experiments, structural information can only be obtained indirectly. The potential dependence of the differential interfacial capacitance can be determined and compared with the predictions made by theories which are based on well-defined structural models. The obtained potential dependence of the differential capacitance of the fast capacitive process and of the overall differential interfacial capacitance did not match with the theoretically predicted curves. The experimental curves were neither bell nor camel shaped, and the values were much lower than the predicted ones. Temperature-dependent EIS measurements of the system [Pyr1,4]FAP/Au(111) have finally confirmed that (i) the fast capacitive process can be identified as electrostatic charging of the interface governed by bulk ion transport and that (ii) the slow capacitive process is decoupled from the ionic conductivity. The temperature dependence of the time constant of the fast process as well as the specific ionic conductivity both show VFT behaviour, while the time constant of the slow process can better be described by an Arrhenius equation. The force-distance curves obtained by in-situ AFM measurements pointed to the existence of a layered structure at any WE potential. The number and the force needed to disrupture the layers increases with increasing negative or positive WE potential. Furthermore, at the ocp and at negative WE potentials, an innermost layer exists whose thickness is comparable with the diameter of the IL cation. At positive WE potentials, the force-distance curves exhibit an innermost layer with a thickness of the IL anion. The in-situ STM investigations of the interface [Pyr1,4]FAP/Au(111) at negative WE potentials demonstrated that the Au(111) surface undergoes a reconstruction resulting in the famous herringbone motive best described by a Au(111)-(22 x sqr(3)) structure. Another observation deserves to be mentioned: Even at WE potentials, at which the CV does not show any hints for faradaic processes, the formation of holes across the Au surface on a time scale of minutes to hours can be depicted. Most likely, the hole formation is related to the ultra slow faradaic process observable in the EIS experiments. In-situ STM images of the Au surface at positive WE potentials show a rather structureless ad-layer.