In Situ EC-STM Study and DFT Modeling of the Adsorption of Glycerol on Cu(111) in NaOH Solution

Organic coatings are often required for the protection of metals against environmental degradation. Despite their extensive use, only limited information is available on the initial stages of adsorption and formation of protective films, especially at the molecular and atomic levels. As a model system for coating formation, this study investigates the adsorption of glycerol molecules on the single-crystalline Cu(111) surface. A combined electrochemical and scanning tunneling microscopy study in NaOH aqueous solution enabled following the adsorption process in detail, providing molecular information on the glycerol film structure. A potential-driven adsorption of glycerol was observed, suppressing the adsorption of hydroxyl molecules and copper oxidation. The adsorbed species assembled in a nearest neighbor arrangement fitting a (√3 × √3) R30° hexagonal structure with respect to the Cu(111) lattice. This experimentally observed configuration was confirmed by density functional theory (DFT) calculations. DFT modeling indicates that a mixed adsorption mode involving the two primary alcohol groups adsorbed at different z-positions relative to the surface is the most favorable. This mixed configuration enabled the formation of an extended network of hydrogen bonds that aids in stabilization of the glycerol film. This implies that interactions between glycerol molecules have a non-negligible effect on the growth process of such an organic film, allowing the formation of organic layers in the absence of strong interfacial interactions.