Organosulfur monolayers at gold surfaces: Reexamination of the case for sulfide adsorption and implications to the formation of monolayers from thiols and disulfides

This paper presents the results from a reexamination of the adsorption of organosulfides on gold films. Our original goal was to explore the adsorption of different organosulfur precursors (e.g., sulfides) as a basis for addressing details related to gold-sulfur interactions in thiol-derived monolayers. The results of our previous work (J. Am. Chem. Sec. 1994, 116, 11616) were, however, complicated by the purity of most organosulfides as well as by the competitive adsorption of thiol and disulfide impurities. By using preparative gas chromatographic purification, we were able to reduce significantly the levels of both types of impurities from the organosulfide precursors, as confirmed by mass spectrometric analyses. Spectroscopic (infrared and X-ray photoelectron) and electrochemical characterizations of adlayers formed on gold from these highly purified organosulfides have led us to reach new conclusions regarding our earlier work. First, the amount of adsorbed sulfide corresponds to only a small Fraction of a monolayer. Second, the chemical identity of sulfur for the adsorbed sulfide remains unchanged. These results not only correct our earlier assessment but also illustrate the importance of purifying these precursors to unexpectedly high levels when probing issues related to the formation and structure of this adsorbate-substrate combination. The implications of these results, particularly those developed from the XPS data, to the nature of the interactions of the sulfur from thiols and disulfides with gold are discussed.