Adsorption of bromide ions on single crystals of gold: Determination of thermodynamic parameters and electroreflectance investigations

Abstract

In this paper the adsorption of Br − ions from bromide solutions at constant ionic strength at (111), (100), (110) and (311) single crystals of gold was investigated quantitatively by combined chronocoulometric and admittance measurements. Furthermore, some structural information concerning the adsorbed layer was obtained from modulated electroreflectance measurements. It was shown that the isotherm is satisfactorily congruent with charge density σ. The results were fitted with a one-constant virial isotherm. The charge due to adsorbed Br −, σ i, the free energy of adsorption and the coefficient B characterizing the lateral interactions between adsorbed ions, were evaluated for each face and compared with data reported in the literature on other metals. The adsorption parameters do not differ as a function of the atomic structure of the gold surface in constrast with the behaviour of a neutral substance. The electrosorption valency γ was estimated from the electrode charge σ and from the adsorbed ions charge σ i at constant inner-layer potential drop and found to depend on this potential (i.e. on Br − coverage) A particular behaviour of the electrosorption valency was observed on the (110) face. The modifications of the electroreflection spectra and of the azimuthal anisotropy of (110) and (311) planes by adsorbed Br − provide evidence for influence of the grooved structure of the Au substrate on the reconstruction mechanism of the superficial atomic structure induced by the adsorption process. At saturation coverage, the anisotropy of the spectrum disappears, whereas it is maintained on the (110) plane. This different behaviour has been correlated to the atomic roughness of the bare substrates.