Double Layer Simulation Studies in Ultrahigh Vacuum of Anion and Cation Hydration at Metal Surfaces

Abstract

Adsorption / Electrochemistry / Surfaces Using low energy electron diffraction (LEED), thermal desorption spectroscopy (TDS) and measurements of work function changes (A@), the coadsorption of water and ions on metal surfaces has been studied in ultrahigh vacuum. As an example of anion specific adsorption, the hydration of bromide on Cu(ll0) has been characterized with regards to inner layer composition, surface hydration number and the formation of long-range order. Simulation of non-specific adsorption of lithium ions has also been attempted. O n Ag(llO), a drastic reduction of the binding energy of lithium in the presence of water is attributed to this type of surface hydration. Such evidence is not observed on Cu(lll), where substantial fragmentation of the water seems to occur. The electrochemical relevance of these results is discussed.

Modeling Electrochemical Interfaces in Ultrahigh Vacuum: Influence of Progressive Cation and Surface Solvation upon Charge−Potential Double-Layer Behavior on Pt(111)

“…Distinctly non-monotonic ΔΦ−θ s * traces are uniformly obtained under these conditions, the work function initially increasing up to θ s * ∼ 1 to 2 EL before decreasing in a parallel fashion as for solvent dosing onto clean Pt(111) (Figure ). The initial Φ increases, which are largest for water and methanol, have been shown by vibrational spectroscopy to be associated with cation-induced solvent reorientation, featuring K + −oxygen coordination along with surface-hydrogen bonding interactions for these two solvents. 10a,11a, (Indeed, the degree of cation-induced water reorientation has been estimated from the initial Φ changes observed upon solvent dosing. 5a, More generally (or equivalently), these Φ increases can be considered to arise from solvent “dielectric screening” of the K + ···e - “surface dipole”, which is largely responsible for the very marked (up to 3 eV) K-induced Φ decreases seen on clean Pt(111). The notable attenuation of these Φ decreases brought about by water addition (Figure A) reflects the particularly efficient dielectric screening characteristic of this solvent.…”

“…We have recently been pursuing measurements involving ordered metal surfaces in ultrahigh vacuum (UHV), with one objective being the experimental elucidation of interfacial solvation effects of relevance to electrochemistry. − The general tactics, involving the sequential dosing onto the metal−vacuum interfaces of various double-layer components, including solvent, owe much to the pioneering efforts of Sass and co-workers in Berlin during the last decade; they are often referred to as “UHV electrochemical modeling” . In particular, we have employed infrared reflection−absorption spectroscopy (IRAS) along with work-function measurements.…”

“…While less frequently discussed in the surface-science literature, responses of the work function (Φ) to progressive alterations in the surface composition can provide unique insight into the interfacial potential profile and related electrostatic issues, of central importance in electrochemistry. In the context of “UHV electrochemical modeling”, examining Φ responses to controlled additions of solvent and ions (or ionizable species) can indeed constitute a powerful means of exploring double-layer interactions …”

Coverage-Dependent Infrared Spectroscopy of Carbon Monoxide on Iridium(111) in Aqueous Solution: A Benchmark Comparison between Chemisorption in Ordered Electrochemical and Ultrahigh-Vacuum Environments

“…Studies along these lines in our laboratory have included undertaking both in-situ electrochemical IRAS measurements for CO (and for the related chemisorbate NO) on various low-index Pt-group metals in nonaqueous as well as aqueous media ,− and UHV-based “electrochemical modeling” studies. ,, The latter entail sequential low-temperature dosing onto the clean ordered surface in UHV of the various components (chemisorbate, solvent, ionic/electronic charge) that together constitute the so-called electrochemical “double layer”. , Examining the response of the chemisorbate (and solvent) vibrational spectra to progressive alterations in the UHV-based interfacial composition can yield considerable insight into the nature of the inherently coupled solvation/electrostatic charge effects wrought by the double layer on the chemisorbate properties . These effects include solvent-induced alterations in the spatial chemisorbate structure, especially binding geometries, 11b-e and those attributable directly to the combined influence of solvated ionic charge and dipolar solvent on the electrostatic field experienced by the chemisorbate. , The latter influence, collectively constituting the so-called “Stark-tuning” effect, is commonly manifested for in-situ electrochemical systems as shifts in the chemisorbate vibrational frequency as a function of the electrode potential. , Indeed, near-identical ν CO frequency−surface potential behavior has been observed for saturated CO adlayers on Pt(111) in both in-situ electrochemical and UHV-based environments in the presence of various solvents once the potential values are converted to a common reference scale (vide infra) 11a.…”

Interpretation of IR Spectra of Ultrathin Films

“…The effect of solvent on IRRAS of adlayers has been studied using the so-called non situ strategy, or UHV double-layer modeling, developed by Sass and coworkers during the 1980s [223,224] and Weaver and co-workers (see review in Ref. [113]) and Ito and co-workers [225][226][227] during the 1990s.…”