Adsorption of Halides on Au (111) Electrode in Aprotic Solvents: AC- Voltammetry, EIS, XPS and Surface-Enhanced Infrared Spectroscopy

Abstract

Specific adsorption of ions on the electrode surface has a significant impact on the electric double layer (EDL) structure and the kinetics of electrode processes[1]. The ion adsorption at aqueous solution interfaces[2] on various metals has been examined intensively, but much fewer studies from non-aqueous solvents have been conducted[3]. Such studies are important because the metal-solvent [4] and ion-solvent interaction energy [5] depend on the chemical nature of the solvent.In this study, three organic solvents (propylene carbonate (PC), diglyme (DG), and dimethyl sulfoxide (DMSO)) were investigated for the adsorption of iodide and bromide ions on Au(111) single crystal electrode. This analysis was conducted by using differential capacity measurements, cyclic voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and Surface-Enhanced Infrared Spectroscopy (SEIRAS).The magnitude of anion adsorption on Au(111) electrode increases in the series of halides I- ˃ Br- due to better solvation [6] and in the solvent order PC ˃ DG ˃ DMSO[7]. As well, the influence of water content on the adsorption rate of iodide on Au(111) electrode in PC and DMSO was studied. This effect, and the much higher adsorption rate in water is explained by the closer approach of the solvated halogen ion to the surface and the resulting stronger interaction of the transition state with the electrode. Quantification of the adsorbed iodide amount from XPS spectra confirms the coverage estimated from the adsorption charge in CVs. During SEIRAS measurements, a bipolar band is observed at around 1805 cm−1 which correlates to the C-O-vibration of PC as reported on silver electrodes from SERS measurements[8]. A stronger electrical field with decreasing potential is observed in absence of iodine leads to higher absorbance of PC molecules on the surface. Acknowledgments Financial support by the German Federal Ministry of Education and Research (BMBF) of the “LuCaMag” project (Fkz: 03EK3051A) under the framework of “Vom Material zur Innovation” program is acknowledged.