Dw. Hatchett et al., ELECTROCHEMICAL OXIDATIVE ADSORPTION OF ETHANETHIOLATE ON AG(111), Journal of the American Chemical Society, 119(28), 1997, pp. 6596-6606
The electrochemical deposition of ethanethiolate monolayers on Ag(111)
has been investigated by in-situ surface-enhanced Raman spectroscopy
(SERS), electrochemical quartz crystal microbalance (EQCM), and voltam
metric methods. In aqueous 0.2 M NaOH solutions containing 5 mM ethane
thiolate, the voltammetric response at Ag(111) displays two well-resol
ved surface waves with half-wave potentials (E-1/2) Of -1.17 and -0.95
V vs Ag/AgCl. The two-wave voltammetric response suggests a mechanism
of monolayer deposition that consists of two energetically-distinct r
eaction steps, a conclusion supported by in-situ SERS and EQCM measure
ments of the potential-dependent adsorption isotherm. The voltammetric
wave located at more negative potentials (E-1/2 = -1.17 V) has a near
ly ideal Nernstian shape and is associated with the initial rapid oxid
ative adsorption of ethanethiolate at submonolayer coverage. The secon
d wave at more positive potentials (E-1/2 = -0.95 V) corresponds to ad
ditional, but kinetically slower, oxidative adsorption of ethanethiola
te resulting in an essentially complete monolayer (similar to 7.8 x 10
(-10) moll cm(2)). The slower kinetics associated with this wave, as w
ell as general features of the wave shape, suggest that structural ord
ering of the monolayer occurs during the second oxidation. A thermodyn
amic expression for the dependence of E-1/2 On the bulk solution conce
ntration of ethanethiolate is derived and employed to determine the nu
mber of electrons transferred, n = 1.06 +/- 0.06, per molecule of etha
nethiolate adsorbed on the Ag(111) surface. A value of n = 0.90 +/- 0.
14 has also been independently determined by EQCM and coulometric meas
urements of the mass adsorbed and electrical charge consumed, respecti
vely, during monolayer deposition.