Underpotential Deposition and Anion Adsorption

As found in preceding sections, the underpotential deposition process is strongly influenced by anion adsorption, or controlled sometimes by the presence of the adsorbed anions. Anion-specific adsorption/desorption seems to take place as an electron transfer process and may be accompanied by the underpotential deposition formation/removal process.  

Let us consider the case that underpotential deposition takes place at potentials where specifically adsorbed anions depart from the surface, or the removal of underpotential deposition species induces specific adsorption of anions. Then the underpotential deposition process is taken to consist of three consecutive steps (1) desorption of specifically adsorbed anions from substrate M, (2) adsorption and electron transfer of metal ions M to form an underpotential deposition metal layer on the substrate metal M, and (3) readsorption of the anions on the underpotential deposition metal M on M, i.e.. 

For the Cu underpotential deposition peak at the positive potential-side on Au(l 11) in sulfuric acid solution, the slope of peak potentials with 

was given as 1 for the case of sulfate adsorption on Pt(lll). By applying such an argument to the case ofZn underpotential deposition on Pt(lll), with the assumption of = C (that the number of electron transfers of anion adsorption on M and on M/M is identical), y = about 2.5 from the slope of 25 mV decade . As = 2 for Zn, y is found to be nearly identical with n. Since the condition of y 2 is not plausible, the number of electron transfers will be identical to n y = n  

As shown above, the effect of anions on a cyclic voltammogram was 

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Copper electrodeposition on Au(111) Copper is an interesting metal and has been widely investigated in electrodeposition studies from aqueous solutions. There are numerous publications in the literature on this topic. Furthermore, technical processes to produce Cu interconnects on microchips have been established in aqueous solutions. In general, the quality of the deposits is strongly influenced by the bath composition. On the nanometer scale, one finds different superstmctures in the underpotential deposition regime if different counter-ions are used in the solutions. A co-adsorption between the metal atoms and the anions has been reported. In the underpotential regime, before the bulk deposition begins, one Cu mono-layer forms on Au(lll) [66]. 

The process of monolayer deposition of metal ions in underpotential deposition is strongly affected by anion-specific adsorption, and the two processes at the electrode interface must be elucidated if one is to understand underpotential deposition phenomena in a unified way. 

In this chapter, hydrogen adsorption, particularly observed on Pt electrodes, was not treated as an underpotential deposition phenomenon. However, from a theoretical point of view, it may provide a breakthrough insight into underpotential deposition. Since the underpotential deposition ofM on M is quite characteristic among different combinations of M and M, together with a change in the kind of anions in the solution, a theoretical approach, which requires simplification, is still limited, and more experimental clarification is needed for theoretical work. However, the jellium model was successfully used to describe the lattice contraction of adsorbate T1 and Pb on Ag(lll) as underpotential deposition. ... 

Underpotential deposition produces a change in the potential distribution at the interface, affects the organization of the solvent molecules at the interface, and shifts the potential of zero charge in the opposite direction to the effect observed with specific anion adsorption. 

Induced adsorption — Enhancement of the - adsorption of a component induced by another adsorbed species. These phenomena are treated in terms of induced anion and cation adsorption. Interrelation of anionic -> specific adsorption and -> underpotential deposition of metal ions is a typical example for the induced adsorption of anions [i]. 

Chapter 3, by Rolando Guidelli, deals with another aspect of major fundamental interest, the process of electrosorption at electrodes, a topic central to electrochemical surface science Electrosorption Valency and Partial Charge Transfer. Thermodynamic examination of electrochemical adsorption of anions and atomic species, e.g. as in underpotential deposition of H and metal adatoms at noble metals, enables details of the state of polarity of electrosorbed species at metal interfaces to be deduced. The bases and results of studies in this field are treated in depth in this chapter and important relations to surface -potential changes at metals, studied in the gas-phase under high-vacuum conditions, will be recognized. Results obtained in this field of research have significant relevance to behavior of species involved in electrocatalysis, e.g. in fuel-cells, as treated in chapter 4, and in electrodeposition of metals. 

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... 

Typical examples include studies of the underpotential deposition of various metals on metallic substrates. The structure of the upd-layer [33, 34], the position of adsorbed anions and water molecules on top of the upd-layer and the respective bond angles and lengths could be elucidated [35, 36]. Surface reconstruction caused by weakly adsorbed hydrogen [37], surface expansion effects of low-index platinum and gold surfaces correlated with adsorption/desorption of solution species [38] and... 

A main field of activities is focused on structure and reactivity in two-dimensional adlayers at electrode surfaces. Significant new insights were obtained into the specific adsorption and phase formation of anions and organic monolayers as well as into the underpotential deposition of metal ions on foreign substrates. The in situ application of structure-sensitive methods with an atomic-scale spatial resolution, and a time resolution up to a few microseconds revealed rich, potential-dependent phase behavior. Randomly disordered phases, lattice gas adsorption, commensurate and incommensurate (compressible and/or rotated) stmctures were observed. Attempts have been developed, often on the basis of concepts of 2D surface physics, to rationalize the observed phase changes and transitions by competing lateral adsorbate-adsorbate and adsorbate-substrate interactions. 

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