| dc.description.abstract | In situ scanning tunneling microscope (EC-STM) and cyclic voltammetry (CV) were used to examine the adsorption behavior and structure of carboxyl thiol molecules on Cu(111). In particular, three molecules with different chain length, mercaptoacetic acid (MAA), 3-Mercaptopropionic acid (MPA), 4-mercaptobutyric acid (MBA), were deposited onto Cu(111) electrode from aqueous solutions containing 100 and 1 μM of these three molecules. In situ STM provided molecular – resolution views of the spatial structures of these molecular adlayers as a function of potential.
The structures of carboxyl thiol molecules adsorbed on Cu(111) electrode were examined in pH1 sulfuric acid containing 100 μM MAA, MPA and MBA, respectively. These admolecules were adsorbed in compact, long - range ordered moiré structures at coverage () of 0.41 (number of admolecules/number of Cu atoms). MAA molecules were presumed to adsorb via their SH end, forming ionic bond with the Cu substrate. By contrast, MPA and MBA molecules tended to be adsorbed in multilayer in the presence of 100 μM molecules. High resolution STM imaging revealed largely disordered structures in these multilayer thin films. In electrolyte containing 1 μM of these molecules, results were nearly the same. They all formed ordered (√3 × √3)R30° ( = 0.33) between -0.25 ~ -0.35 V (vs. Ag/AgCl). Shifting potential negatively resulted in restructuring into a stripe phase before admolecules continued to desorb at E < -0.35 V. These phase transition events were reversible. Given the nearly same results observed with these molecules, chain length of thiol molecules had an insignificant effect on molecular structures.
The presence of chloride in the electrolyte appeared to complicate the adsorption of these molecules. Aided by molecular – resolution STM imaging, it is thought that MAA arranging in ordered moiré structure was adsorbed more strongly than chloride at positive potentials. However, shifting potential negatively resulted in displacement of the moire pattern with a striped phase attributable to adsorbed chloride.
The adsorption of MAA and MPA in pH 4 sulfate media was examined with in situ STM. MAA molecules were adsorbed in the compact moiré structure between 0 to -0.3 V, which was converted the (√ 7 × √ 7) R19.1 °( = 0.29) structure when the potential was adjusted to -0.3 V, followed by another phase transition to (2√7 × 2√7) R19.1°( = 0.11) at -0.5 V. These results are essentially the same as those observed with sulfur adatoms on Cu(111) electrode, which suggests that S-Cu interaction was of prime importance in guiding the adsorption of thiol molecules on Cu(111). Chain length seemed to be secondary under the present conditions.
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