Core Level Data of Ionic Liquids: Monitoring Charging by In Situ Electrochemical X-ray Photoelectron Spectroscopy

Abstract

Ionic liquids (ILs) are reported to provide large electrochemical stabilitywindows(4‐6V)makingthemattractiveelectrolytesforelectrochemical processes and applications. 1 Most ILs are also ultra high vacuum (UHV) compatible. Therefore, these compounds allow for application of analytical tools to, for instance, study electrochemical processes in situ in the UHV. 2 From fundamental X-ray photoelectron spectroscopy (XPS) studies of ILs, sample charging is known to be a major challenge. 3,4 Recently, we showed that ILs prepared on an activated carbon (AC) support show significantly improved XPS data with respect to charging. This result suggested that such a preparation method may provide a straightforward opportunity for the determination of reliable binding energies (BE) of ILs in this research field. 5 The lack of charging observed on the AC support was referred to the high surface area and, therefore, high double layer capacitance of AC when compared to common supports such as metal foils. In the present letter we further investigate charging using our recently designed in situ electrochemical XPS cell. 6 This setup allows recording shifts of core level spectra of the IL and measuring the potential of the WE simultaneously. In particular, the OCP can be measured as a function of the irradiation time. In agreement with the fundamental concept of electrochemically shifted binding energies, 6 the XPS lines shift by −1.0 eV/V with the X-ray induced OCP shift. Furthermore, the in situ investigations confirm that the lack of charging previously observed on AC support can indeed be explained by the high double layer capacitance of the support compensating the X-ray induced OCP and BE shift observed on smooth substrates.