PROGRESSIVE CATION SOLVATION AT PT(111) MODEL ELECTROCHEMICAL INTERFACES IN ULTRAHIGH-VACUUM AS PROBED BY INFRARED-SPECTROSCOPY AND WORK-FUNCTION MEASUREMENTS
I. Villegas et Mj. Weaver, PROGRESSIVE CATION SOLVATION AT PT(111) MODEL ELECTROCHEMICAL INTERFACES IN ULTRAHIGH-VACUUM AS PROBED BY INFRARED-SPECTROSCOPY AND WORK-FUNCTION MEASUREMENTS, Electrochimica acta, 41(5), 1996, pp. 661-673
The utility of infrared reflection-absorption spectroscopy (IRAS) for
exploring ionic solvation at model electrochemical interfaces in ultra
high vacuum (uhv) is briefly reviewed and illustrated for the specific
cases of K+ solvation by water, methanol, acetonitrile, acetone, and
ammonia on Pt(lll). The solvents were selected so to span a range of p
hysicochemical properties, while K+ is an alkali cation typical of con
ventional electrochemical double layers which can reliably be dosed (a
s K atoms) to yield known low coverages on Pt(lll). Along with the inf
rared spectra, changes in the surface work function (Delta Phi) are ev
aluated for progressive solvent dosages onto both clean and K+-predose
d Pt(lll), both as a means of exploring the solvation-dependent interf
acial 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(lll) yielded infrared spectra for
intramolecular solvent modes, especially for dipolar functional groups
, which are markedly different from those obtained on the clean surfac
e, and consistent with the occurrence of progressive cation solvation.
Analyses of solvent dosage-band intensities as well as frequencies en
able solvent structures within the primary, and in some cases secondar
y, solvation shell to be suggested. The important role of the metal su
rface in modifying the nature of such cation solvation is intriguingly
evident upon comparing the present IRAS data with corresponding vibra
tional spectra for progressive solvation of alkali-metal cations by me
thanol and ammonia in the gas phase. The Delta Phi responses to solven
t dosage in the presence of predosed K+ are typically non-monotonic, a
nd feature significant initial Phi increases which are consistent with
cation-induced solvent reorientation. The marked Phi decreases unifor
mly observed for the solvents dosed onto unmodified Pt(lll), however,
do not correlate simply with the solvent orientation as deduced by IRA
S. Some more general implications of such combined IRAS/work-function
measurements for exploring double-layer structural issues are pointed
out.