Surface enhanced Raman spectroscopy of the ionic liquid - metal interface

Abstract

When a charge is applied to an electrode in the metal – Ionic Liquid (IL) interface, an electrochemical double layer is expected to form due to the arrangement of ions to counter the charge on the electrode surface. However, this arrangement of ions in ILs can be complicated by effects such as specific adsorption, ion re-orientation and superoxide ion and Au oxide formation. Traditional techniques used in the study of metal-IL interfaces, have provided a good indication of underlying processes. However, additional proof from new methods is required, as interpretations of the results sometimes vary.

In this work, surface enhanced Raman (SERS) spectra have been acquired from the electrochemically controlled interface between Au and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP TFSI). By analysing the intensity and positions of peaks in the spectra corresponding to specific vibrations in the different ions of the IL, useful information has been obtained about the processes occurring at the interface at a molecular level.

By using impedance spectroscopy, the potential of zero charge (PZC) was tentatively assigned to -0.85 V vs. PQRE. Cathodic features in cyclic voltammograms and in current-potential data of the SERS experiment, at potentials negative to the PZC, have been assigned to the re-orienting of the BMP cations, increase in axial BMP conformers and superoxide ion formation, tying together varying interpretations from the literature. Au oxide formation from trace water was detected in the SERS spectra and corresponded to a small increase in current at positive potentials.

Due to the high concentration of ions in ILs, the effect of the bulk signal on Raman and SERS spectra of ILs has also been examined. The depth resolution of the spectrometer, the SERS signal decay with distance from the substrate and the concentration of molecules in the analyte, have all been taken into account.

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Identifiers

ORCID for Andrea E. Russell: orcid.org/0000-0002-8382-6443

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