Underpotential deposition process

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.. 

When the potential step method is employed to elucidate the mechanism of the underpotential deposition process, the current j against time t is readily found to be in the form of... 

Pitner and Hussey studied the electrochemistry of tin in acidic and basic AICI3/I-ethyl-3-methyl-imidazolium chloride-based ionic liquids by using voltammetry and chronoamperometry at 40 °C [15]. They reported that the Sn(II) reduction process is uncomplicated at a platinum substrate, where in the atidic ionic liquid the reduction wave was observed at +0.46 V on the Pt electrode and the oxidation at +0.56 V. When they used a gold electrode instead of platinum, they observed an underpotential deposition of a tin monolayer and an additional underpotential deposition process that was attributed to the formation of tin-gold alloy at the surface. The deposition of tin on glassy carbon was controlled by nudeation. 

Finally, it should be mentioned that by the presence of certain additives the underpotential deposition process can be inhibited. Upd of copper on Pt(lll), Pt(lOO), and Pt(llO) can be inhibited by thiourea and dithiadecyldisodium sulfonate. The results of a study on the effects of organic adsorbates on the underpotential deposition of silver on Pt(lll) electrode show that the presence of coadsorbates (2,2-bipiridyl, 4-mercapto-pyridine, etc.) can have a pronounced effect on the underpotential deposition. It has been found that adsorbates that bind primarily through a ring nitrogen atom inhibit the second, but not the first, silver monolayer. In contrast, the sulfur-containing adsorbates inhibit the formation of the first monolayer owing to the formation of the Pt-S bond. 

A third approach is the layer-by-layer deposition of monolayers. t xhis process is a combination of two sequential underpotential deposition processes. In the first process a metal is deposited as UPD film. Then after rinsing, the electrolyte is changed, and by an anodic oxidation (a type of anodic underpotential deposition) the anionic component is deposited as a second layer on the primary UPD film. In both deposition processes epitaxial film growth is expected and has been partially confirmed. 

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