Ordered Binary Ionic Adlayers on a Au(110) Electrode:  Coadsorption of Alkali Metal Cations with Iodide Anions

Alkali metal cations are found, using X-ray diffraction, to coadsorb with iodide ions on a Au(110) electrode surface at potentials positive of the onset of the Au(110) reconstruction and negative of the formation of iodine monolayers. The adlayer exhibits a c(2 × p) in-plane diffraction pattern with p depending on the cation species and concentration. For the CsI adlayer, p = 4/3 independent of potential and the Cs+−I- spacing is close to that in bulk CsI. In KI, NaI, and LiI solutions, p increases with increasing potential. As a result, the coverages of both the cation and anion species decrease with increasing potential. For the anion, the slope of the potential-dependent coverage is opposite of its behavior in the absence of cation coadsorption. This unusual behavior is attributed to the net positive charge within the adlayer due to the partial discharge of chemisorbed iodides. Details of the diffraction features suggest the existence of antiphase domains and relaxation at domain walls.