Electrochemical oxidative adsorption of ethanethiolate on Ag(111)

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 voltammetric methods. In aqueous 0.2 M NaOH solutions containing 5 mM ethanethiolate, the voltammetric response at Ag(111) displays two well-resolved 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 reaction steps, a conclusion supported by in-situ SERS and EQCM measurements of the potential-dependent adsorption isotherm. The voltammetric wave located at more negative potentials (E-1/2 = -1.17 V) has a nearly ideal Nernstian shape and is associated with the initial rapid oxidative adsorption of ethanethiolate at submonolayer coverage. The second wave at more positive potentials (E-1/2 = -0.95 V) corresponds to additional, but kinetically slower, oxidative adsorption of ethanethiolate resulting in an essentially complete monolayer (similar to 7.8 x 10(-10) moll cm(2)). The slower kinetics associated with this wave, as well as general features of the wave shape, suggest that structural ordering of the monolayer occurs during the second oxidation. A thermodynamic expression for the dependence of E-1/2 On the bulk solution concentration of ethanethiolate is derived and employed to determine the number of electrons transferred, n = 1.06 +/- 0.06, per molecule of ethanethiolate adsorbed on the Ag(111) surface. A value of n = 0.90 +/- 0.14 has also been independently determined by EQCM and coulometric measurements of the mass adsorbed and electrical charge consumed, respectively, during monolayer deposition.