Progressive cation solvation at Pt(111) model electrochemical interfaces in ultrahigh vacuum as probed by infrared spectroscopy and work-function measurements

Abstract

The utility of infrared reflection-absorption spectroscopy (IRAS) for exploring ionic solvation at model electrochemical interfaces in ultrahigh vacuum (uhv) is briefly reviewed and illustrated for the specific cases of K + solvation by water, methanol, acetonitrile, acetone, and ammonia on Pt(111). The solvents were selected so to span a range of physicochemical properties, while K + is an alkali cation typical of conventional electrochemical double layers which can reliably be dosed (as K atoms) to yield known low coverages on Pt(111). Along with the infrared spectra, changes in the surface work function (ΔΦ) are evaluated for progressive solvent dosages onto both clean and K +-predosed Pt(111), both as a means of exploring the solvation-dependent interfacial potential profile and also to provide the required link between the uhv-based and electrochemical potential scales. In each case, low solvent dosages onto K +-predosed Pt(111) yielded infrared spectra for intramolecular solvent modes, especially for dipolar functional groups, which are markedly different from those obtained on the clean surface, and consistent with the occurrence of progressive cation solvation. Analyses of solvent dosage-band intensities as well as frequencies enable solvent structures within the primary, and in some cases secondary, solvation shell to be suggested. The important role of the metal surface in modifying the nature of such cation solvation is intriguingly evident upon comparing the present IRAS data with corresponding vibrational spectra for progressive solvation of alkali-metal cations by methanol and ammonia in the gas phase. The ΔΦ responses to solvent dosage in the presence of predosed K + are typically non-monotonic, and feature significant initial Φ increases which are consistent with cation-induced solvent reorientation. The marked Φ decreases uniformly observed for the solvents dosed onto unmodified Pt(111), however, do not correlate simply with the solvent orientation as deduced by IRAS. Some more general implications of such combined IRAS/work-function measurements for exploring double-layer structural issues are pointed out.