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 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.