Carbon-Sulfur Bond Cleavage During Adsorption of Octadecane Thiol to Copper in Ethanol

The effect of the solvent on the formation of thiol self-assembled monolayers (SAMs) on oxide-covered, reactive metals is more involved than in the well-studied gold-thiol system. In this work, copper covered with a native oxide was modified with 1-octadecanethiol (ODT) in either tetrahydrofuran or ethanol. Infrared spectroscopy indicated the formation of crystalline chain packing of alkyl chains from both solvents. Surface coverage was approximately equal in both systems, with differences in tilt angles of the chains. A detailed analysis by X-ray photoelectron spectroscopy showed the formation of Cu₂S and copper-bound carbon when the adsorption was carried out in ethanol. This observation can be explained by the cleavage of the C-S bond in ODT during adsorption. Based on the analogy of preparations, we reason that the solvation of ODT in ethanol must be such that it weakens the C-S bond in ODT, thus enabling the cleavage of this bond. Based on the evidence presented here, it is not possible to distinguish between surface solvation and bulk solvation. Electrochemical linear sweep voltammetry shows that SAMs from both solvents have an enhancing protective effect compared to the native oxide layer. The results from this work show interesting possibilities for the preparation of adsorbed monolayers with chemical interaction to reactive metals, with some similarities to carbene-based SAMs.