Metallic substrate, foreign, deposition

Such effects are observed inter alia when a metal is electrochemically deposited on a foreign substrate (e.g. Pb on graphite), a process which requires an additional nucleation overpotential. Thus, in cyclic voltammetry metal is deposited during the reverse scan on an identical metallic surface at thermodynamically favourable potentials, i.e. at positive values relative to the nucleation overpotential. This generates the typical trace-crossing in the current-voltage curve. Hence, Pletcher et al. also view the trace-crossing as proof of the start of the nucleation process of the polymer film, especially as it appears only in experiments with freshly polished electrodes. But this is about as far as we can go with cyclic voltammetry alone. It must be complemented by other techniques the potential step methods and optical spectroscopy have proved suitable. 

When a metal is in contact with its metal ion in solution, an equilibrium potential is established commonly referred to as Nernst potential (Er). Metal deposition occurs at potentials negative of Er, and dissolution for E > Er. However, when a metal is deposited onto a foreign metal substrate, which will always be the case for the initial stages of deposition, it is frequently observed that the first monolayer on the metal is deposited at potentials which are positive of the respective Nernst potential [37, 38]. This apparent violation for Nernst s law simply arises from the fact that the interaction between deposit metal and substrate is stronger than that between the atoms of the deposit. This effect has been termed underpotential deposition (upd), to contrast deposition processes at overpotentials. (One should keep in mind, however, that despite the symmetrical technical terms the physical origins of both effects are quite different. While the reason for an overpotential is solely due to kinetic hindrance of the deposition process, is that for underpotential deposition found in the energetics of the adatom-substrate interaction.)... 

The underpotential deposition (UPD) of metals on foreign metal substrates is of importance in understanding the first phase of metal electrodeposition and also as a means for preparing electrode surfaces with interesting electronic and morphological properties for electrocatalytic studies. The UPD of metals on polycrystalline substrates exhibit quite complex behavior with multiple peaks in the linear sweep voltammetry curves. This behavior is at least partially due to the presence of various low and high index planes on the polycrystalline surface. The formation of various ordered overlayers on particular single crystal surface planes may also contribute to the complex peak structure in the voltammetry curves. 

Underpotential deposition is described as less than monolayer metal deposition on a foreign metal substrate, which occurs at more positive potentials than the equilibrium potential of a metal ion deposed on its own metal, expressed by the Nemst equation. Kolb reviewed state-of-the-art Underpotential deposition up to 1978. As Underpotential deposition is a process indicative of less than a monolayer metal on a substrate, it is expected to be quite sensitive to the surface stmcture of the substrate crystal a well-defined single-crystal electrode preparation is a prerequisite to the study of Underpotential deposition. In the case of Au and Ag single-crystal electrodes, Hamelin and co-workers extensively studied the necessary crystal surface structure, as reviewed in Ref. 2. 

In industrial applications of metal deposition a metal M is deposited either on the native metal substrate M or on a foreign metal substrate S. As an example of the former, Cu is electrodeposited on a Cu substrate formed by electroless Cu deposition on an activated nonconductor in the fabrication of printed circuit boards. As an example of the latter, Ni is electrodeposited on Cu in the fabrication of contact pads in the electronics industry. 

The procedure is depicted schematically in Figure 7.22. For example, uniform-size Cu clusters were produced and distributed evenly over an atomically flat Au(l 11) electrode surface (76). This example is of the type of deposition of a metal on a foreign metallic substrate and involves UPD (defined in Section 7.15) (80). The procedure was made fully automated through the use of microprocessors in controlling the x-, y-, and z-movements of the tip. Arrays of up to 400 individual clusters were made at a rate of 50 clusters per second (76). The average height of the Cu clusters was 0.8 nm (75,80). 

Underpotential deposition (UPD) is the electrochemical adsorption and (partial) reduction of a submonolayer or monolayer of cations on a foreign metal substrate at potentials more positive than the reversible potential of the deposited metal [141]. The UPD phenomenon is used in many fundamental and applied studies because it offers a means of controlling coverages during electrodeposition in a very concise manner. Until recently, most of the information obtained about the structure of the overlayers deposited on single crystal surfaces has come from indirect means such as current-voltage analysis or by analysis of the deposited films after transfer to a UHV chamber [141]. 

A unique feature observed in metal deposition on a foreign substrate is underpotential deposition. It is found that a metal can be deposited on a foreign substrate at potentials positive with respect to the reversible potential for deposition of the metal in the same solution. Considering, for example, the deposition of silver on platinum, which can be written as... 

Another intriguing approach to electrocatalysis involves the use of underpotential-deposited monolayers and submonolayers of foreign metal adatoms on metal substrates. Such layers afford unique electronic and morphological surface properties, not usually achievable with pure metal or alloys. Underpotential-deposited layers have been found to have high catalytic activity for such reactions as H2 generation, 02 reduction, and certain electro-organic reactions. 

Formation of Adatoms (upd) and the Adsorption of Anions on the Surfaces Modified by Adatoms. - Upd of metals has been described extensively in several reviews. ° When a metal is deposited onto a foreign metal substrate, the very first monolayer is often deposited at potentials that are positive of the respective potential at which bulk deposition takes place. Such a behavior has been termed upd. 

The effect of upd is most conveniently demonstrated in cyclic voltammetry. The very fact that the first monolayer is formed at potentials much more positive than that for bulk deposition simply means that the metal adatom is bound more strongly to the foreign metal substrate than to a substrate of its own kind. 

S. Szab6, Underpotential Deposition of Metals on Foreign Metal Substrates, Int. Rev. in Phys. Chem., Taylor and Francis Ltd., 1991, Vol. 10, p. 207. 

Underpotential deposition Underpotential deposition (upd) occurs when monolayers (or submonolayers) of a metal ad-atom are deposited on a foreign metal substrate at potentials positive of the reversible Nernst potential for bulk deposition [16]. Monolayers will only form when a low work function metal is deposited onto the surface of a higher work function substrate. In this case, the metal ad-atom-substrate bond is greater than the ad-atom-ad-atom bond formed in bulk metal deposition. Upd phenomena have been the subject of extensive work using SPMs and of particular interest is the role of coadsorbed anions on this process, as a function of electrode potential. 

The underpotential deposition (abbreviated to UPD) of a metal M on a foreign metal substrate M is a process that takes place according to the reaction... 

On a foreign metal substrate and in most of the cases on a substrate of the like metal, the first step of metal deposition is the formation of nuclei of the depositing metal. The kinetics of nucleation of the new metallic phase and its forms and rate of growth, in many cases play a dominant role in determining the overall deposition kinetics as well as the properties of the metal deposit. 

The development of the ultrasensitive potential sweep technique, capable of detecting submonolayer amounts of substance on electrode surfaces and its application to metal deposition studies, resulted in detailed investigations of the phenomenon of deposition of metals on foreign substrates at potentials more positive than the thermodynamic reversible potential for the given conditions.This phenomenon has been termed underpotential deposition (UPD). 

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