Halide adlayer structures

Ref [360]). The halide adlayer structures at intermediate Cu coverages between the deposition peaks A and 61, and the Cu coverage within the respective bilayers are also indicated. 

Surface X-ray scattering (SXS) The first SXS study of an underpotentially deposited metal monolayer was reported more than ten years ago. In a recent review [132] it is demonstrated that this method is well suited for the study of the structure of metals, halides, and metal-halide adlayers on single-crystal electrodes. As another example, the study of the distribution of water at Ag(lll) surface can be mentioned [133]. 

Phase Transitions in Two-dimensional Adlayers at Electrode Surfaces 405 Tab. 2 Adlayer structures of halides on Au(hkl) and Ag(hkl), adapted from Ref [21]... 

The adsorption of anions such as halides, cyanide, and sulfate/bisulfate on electrode surfaces is currently one of the most important subjects in electrochemistry [1 - 3]. It is well known that various electrochemical surface processes such as underpotential deposition of hydrogen and metal ions are strongly affected by co-adsorbed anions. Particularly, structures of the iodine adlayers on Pt, Rh, Pd, Au, and Ag surfaces have... 

Considering the adsorption of anions induced by adatoms, it is quite evident that anions can have a profound effect on the metal monolayer formation. In some cases even anions can determine the structure of the metal adsorbate. For instance, the coadsorption structure of upd copper and halides on Pt(lll) and Au(lll) was studied extensively. These studies revealed the bilayer coadsorption structure of copper and halides. An ordered structure of halide ions is formed on an adlayer of copper deposited on the Pt(l 11) surface. 

The adsorption of anions on metal electrodes has been one of the major topics in surface electrochemistry. Specific adsorption of anions occurs when the anion loses aU or part of its solvation shell and forms a direct chemical bond with the substrate. In this situation the surface coverage by anions can be high and the adlayer tends to form a close-packed structure that depends critically on the surface atomic geometry of the underlying substrate and the balance between the anion-metal and anion-anion interaction energies. The structures of halide anions adsorbed onto Au(Jtkl), Ag(hkl), and Pt(hkl) low-index surfaces have been the most widely studied systems by SXS, and a comprehensive review of ordered anion adlayers on metal electrodes is given by Magnussen [57]. 

Introduction Anions have a strong tendency to adsorb specifically at metal surfaces, for example, to estabKsh a direct bond with the electrode by partial loss of their hydration shell. As a consequence of the contact with the electrode, the ionic character of the anions is markedly reduced, resulting in a higher surface concentration than in case of nonspecific adsorption. This effect was first observed in double-layer studies on mercury [229, 230] and later confirmed and studied in detail on single-crystal solid electrodes [231-234]. Specifically adsorbed anions can form various types of ordered structures, either more open (cf. sulfate on Au(hld) [235, 236]) or close-packed as reported for halides on different solid electrodes [21]. Cyclic current-potential curves often reveal sharp current peaks, indicative of phase transitions within the anionic adlayers and hence of the existence of ordered phases [21, 237]. Thermodynamic data of specific anion adsorption was obtained in surface tension studies (on mercury only [229,238-240]), capacitance measurements [231-233], cyclic voltammetry, and chronocoulometry [234]. As an... 

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