Decomposition of Methylthiolate Mono layers on Au(111) Prepared from Dimethyl Disulfide in Solution Phase

We investigated the formation and stability of layers of methylthiolate prepared on the Au(111) surface by the method of immersion in an ethanolic solution of dimethyl disulfide (DMDS). The surface species were characterized by electrochemical reductive desorption and high-resolution photoelectron spectroscopy. Both techniques confirmed the formation of a methylthiolate monolayer at short immersion times (around 1 min). As the immersion time increased, the electrochemical experiments showed the disappearance of the methylthiolate reductive desorption current peak and the appearance of a current peak at ca. -0.9 V which was attributed to sulfur species. At long immersion times, the XPS measurements showed two main components for the S 2p signal: a component at ca. 161 eV which corresponds to atomic sulfur and a component at ca. 162 eV which we attributed to polysulfide species. We propose that the breakage of the S-C bond of methylthiolate is responsible for the appearance of sulfur species on the surface. Density functional theory (DFT) calculations were performed to identify the elementary steps that may lead to the decomposition of methylthiolate. We found that the cleavage of the S-C bond is only activated by the oxidative dehydrogenation of the methyl group of methylthiolate. Thio-oxymethylene, SCH(2)O, is the key intermediate leading to the breakage of the S-C bond because it decomposes into atomic sulfur and formaldehyde with an activation energy barrier of only 1.1 kcal/mol.