Underpotential deposition adsorption

When underpotential deposition adsorption/desorption takes place randomly at any substrate site M, the following random adsorptioncontrolling treatment is to be employed, and when the process is controlled by a two-dimensional nucleation-growth mechanism, the process analysis should be carried out according to Section ni.l.(b). 

Types of electrode/solution interface studied include oxide films on metals, monolayer deposits obtained by underpotential deposition, adsorption, and spectroelectrochemistry in thin-layer cells. 

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]. 

Hydrogen adsorption from solution Oxygen adsorption from solution Underpotential deposition of metals Adsorption of probe molecules from solution ... 

Clavilier J, Feliu JM, Aldaz A. 1988. An irreversible structure sensitive adsorption step in bismuth underpotential deposition at platinum electrodes. J Electroanal Chem 243 419-433. 

The energetic aspects of underpotential deposition can be investigated by a slow (i.e., a few millivolts per second) potential scan starting at a potential so high that no adsorption takes place. As the potential is lowered, one or more current peaks axe observed, which are caused by the adsorption of the metal ions (see Fig. 4.9). According to the usual convention, the adsorption current is negative (i.e., cathodic). Different peaks may correspond to different adsorption sites, or to different structures of the adsorbate layer. If the potential is scanned further past the equilibrium potential bulk deposition is observed. 

As an example [13] we consider the underpotential deposition of thallium on silver (Fig. 15.13). At potentials above the onset of the upd of thallium the SHG signal decreases, at first slowly, then more rapidly. The adsorption of thallium causes a strong rise in a(o ), because the region in which the electronic density decays to zero becomes more extended with an angle of incidence of 45° this shows up as a drastic increase in the signal. A similar behavior is seen in other systems, and often even fractions of a monolayer can be detected. 

Figure 15.13 Cyclic voltammogram (top) and SHG signal for the underpotential deposition of thallium on silver The letters in the voltammogram denote various adsorption (A) and desorption (D) peaks. Reprinted with permission from Ref. 13.

We have previously employed such techniques in the study of iodide adsorption onto Pt(lll) electrodes (2 5 > as well as in the in-situ structural characterization of underpotentially deposited copper and silver on Au(lll) electrodes. (23.)... 

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. 

An in situ EUR study of bisulfate and sulfate adsorption indicated that the coverage of adsorbed sulfate increased in the presence of Cu underpotential deposition on polycrystalline Au. ... 

EQCM frequency of 20 Hz, which corresponds to a one-third monolayer of sulfate species adsorption/desorption. However, the electricity from the above cyclic voltammogram current is calculated to be about 1 x 10 C ctn i.e., 6 x 10 molecules cin" which is about one-tenth of a monolayer. This may indicate that sulfate adsorption on Au(lll) is associated with a partial charge transfer process. In Fig. 25b, an increase in EQCM frequency was observed as for (a), and a decrease in the frequency was observed at the Cu underpotential deposition region. The frequency change due to Cu underpotential deposition is determined to be 35 Hz,... 

Underpotential deposition of Zn on R was seen as adsorption of Zvi ions in 1970, since is as high as 1V, a value that well exceeds the one expected from previous results on various metal combinations of underpotential deposition systems. Recently, Zn underpotential deposition on R was reported in acidic solutions, and then in alkaline solutions. Zn underpotential deposition on Pd was also found in acidic and alkaline solutions. Then cyclic voltammograms of underpotential deposition on three low-index, single-crystal Rs were observed in sulfuric and perchloric acid solutions, as shown in Figs. 27 and 28. " In comparing Figs. 27 and 28, Zn underpotential deposition on R seans to take place only on Pt(llO), but not on Pt(lll) and Pt(lOO). Therefore it can be said that a cyclic voltammogram on polycrystalline R was observed on a R(110)-like surface. 

A cyclic voltammogram ofZn underpotential deposition on Pt(llO) in sulfuric acid solution is more reversible than that in perchlorie acid solution with respect to the change of potentials, as shown in Fig. 27. The was evaluated as about 1 V. Since Zn underpotential deposition in acidic solutions was observed in the adsorbed hydrogen region, the pH was increased to separate Zn underpotential deposition from the hydrogen adsorption region. At pH 4.6, Zn underpotential deposition was observed, as shown in Fig. 29. The different cyclic voltammogram appearances of Zn underpotential deposition on Pt(lll) at different pHs is discussed in Section n.4. 

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.. 

A kinetic study of Cu underpotential deposition was carried out to determine if it is best described by adsorption processes or by nucleation processes. The nucleation growth process is classified into two categories instantaneous nucleation growth and progressive nucleation growth. In the case of instantaneous nucleation growth, where nucleation site formation is so fast that no other following nucleation sites are created, the number of nucleation sites N(t) is expressed as... 

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