COADSORPTION OF CU2+ AND SO4(2-) AT THE AU(111) ELECTRODE

We employed thermodynamics of the perfectly polarized electrode to measure the Gibbs excess of copper coadsorbed with SO42- at the Au(111) electrode interface. We combined the Gibbs excess data for adsorbed copper with the earlier determined Gibbs excesses for coadsorbed sulphate to give a complete description of the coadsorption of the two ions. Our results suggest that copper and sulphate form an ordered overlayer structure for the copper coverages 0.25 < ΘCu(a) < 0.7 in which the ratio of the adsorbed copper to the adsorbed sulphate is equal to two. Our data fit well into a statistical model developed by Blum and Huckaby [18] which assumes that copper atoms are packed to form a honeycomb √3 × √3 structure with the centers of the honeycomb occupied by sulphate ions. This model brings into harmony the thermodynamic data given in this work with the earlier structural studies of this system by LEED and RHEED [5] STM [7,8] and AFM [9]. The model suggests that the ordered corrugation seen in the STM [7,8] and AFM [9] experiments corresponds to the images of coadsorbed sulphate ions. We have compared copper coverages given by the Gibbs excesses with the direct estimates based on the coulometric measurements of the charge flowing to the interface which ignores the contribution from the coadsorbed sulphate ions. We have shown that the direct coulometric estimate gives too low values for the copper coverage. Finally, we have estimated the number of electrons flowing to the interface per one adsorbed copper ion. The number is close to two and is weakly dependent on potential. This result shows that the interpretation of the two state character of Cu adsorption at the Au(111) surface given in terms of the two stage charge transfer reaction: Cusol2+ + e- ⇌ Cua+ (first step) and Cua+ + e- ⇌ Cua 0 (second step) may not be correct. © 1994.