ELECTROCHEMICALLY INDUCED TRANSFORMATIONS OF MONOLAYERS FORMED BY SELF-ASSEMBLY OF MERCAPTOETHANOL AT GOLD

The structural stability of monolayers formed by the self-assembly of mercaptoethanol (HOCH2CH2SH) at annealed, mica-supported gold (ME/Au) has been characterized using electrochemical, infrared spectroscopic, and X-ray photoelectron spectroscopic techniques. The study was motivated by the observation that the one-electron reductive desorption of long-chain n-alkanethiolates by linear sweep voltammetry generally exhibits a single well-defined wave,1 while that of ME/Au produces two waves. The relative magnitudes of the waves for ME/Au are dependent on the extent of exposure to the laboratory ambient. Electrochemical oxidation of ME/Au shows that three oxidative processes occur sequentially. In the first process, ME/Au is catalytically oxidized by a four-electron process to a sulfidoacetic acid monolayer (adsorbed mercaptoacetic acid, MAA/Au). In the second process, the electrogenerated MAA/Au undergoes C-S bond cleavage, which gives rise to the second reductive desorption wave. In this case, as the extent of oxidation increases, a third desorption wave is also produced as a result of gradual formation of adsorbed polysulfur species. In the third oxidative process, the remaining sulfur monolayer is oxidatively desorbed from the surface. Our results, coupled with earlier literature reports,1a,2 suggest that electrochemical oxidation of thiolates on gold is a multipath process with the favored path dependent on the reaction conditions and the chemical composition of the thiolate. The spontaneous oxidation of ME/Au may proceed via the same path as the electrochemical oxidation (ME/Au to MAA/Au to S/Au), or adsorption of thiolate may activate the C-S bond, leading to direct conversion from ME/Au to S/Au.