Investigations of underpotential deposition (UPD) phenomena are of particular interest and importance from both fundamental and practical viewpoints. Recently, attention has been focused on the influence of anions in the electrolyte on the UPD process, especially in the case of sulphate and halide ions on Cu UPD at the Au(111) electrode. Numerous ex situ and in situ surface techniques such as LEED, RHEED and AES, STM, AFM, EXAFS and XANES have been brought to bear on the study of this system. These studies have revealed that anions coadsorb with the deposited Cu atoms (adatoms), affecting the adlayer structure, the coverage and the oxidation state of Cu atoms. Although such advanced measurements have provided an abundance of structural information, its interpretation was puzzling and ambiguous since information regarding the composition of the coadsorbed overlayer is still lacking.
This motivated us to develop a methodology to determine the surface concentrations of both deposited metal adatoms and coadsorbed anions. We have demonstrated that this goal can be achieved by the total charge density measurements with the chronocoulometric technique, employing the thermodynamics of the so-called perfectly polarized electrode. Using this method, quantitative data such as charge density, surface tension, Gibbs excess (or coverage), Gibbs free energy and the amount of charge flowing to the interface per one adsorbed ion (or atom) can be determined. This macroscopic information will be used to compliment the ongoing structural studies to gain a thorough understanding of the composition and structure of the interface at an atomic scale.
Coadsorption of copper with SO42-, Cl- and Br- anions onto Au(111) surface has been studied. This research consists of three milestones:
(i) Investigations of the specific adsorption of SO42-, Cl- and Br- anions at Au(111) in the absence of Cu UPD. The adsorption behaviour of the three anions was determined in the absence of Cu2+ in the electrolyte. It is concluded that the strength of the specific adsorption follows the sequence SO42- < Cl- < Br-.
(ii) Investigations of the adsorption of SO42-, Cl- and Br- anions at Au(111) in the presence of Cu UPD. Surface concentration of the anion was determined in the presence of underpotential deposition of copper. Our results indicate that the underpotential deposition of copper strongly promotes the adsorption of SO42-, Cl- and Br- anions.
(iii) Coadsorption of copper atoms with SO42-, Cl- or Br- anions at the Au(111) electrode. The surface concentration of deposited copper atom was determined in the presence of adsorbed anion. This information was then combined with the information obtained in milestone (i) and (ii) to give a complete description of the composition of the Au(111)/solution interface in the presence of the two coadsorbed species.
Our results reveal that coadsorbed copper adatoms and sulphate ions form a well-ordered monolayer. In contrast, coadsorbed copper and chloride or bromide ions form a bilayer in which copper adatoms are adsorbed on the gold surface and the halide ions form the top layer. The STM or AFM images represent the ordered structure of coadsorbed anions rather than copper atoms. The adsorbed sulphate ions preserve the ionic character while the adsorbed chloride or bromide ions are discharged as neutral atoms. These results are essential to the elaboration of accurate models for the copper-anion overlayer structure.