Metal deposition foreign substrate

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). 

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. 

Underpotential Deposition of Metal Atoms Because of the energy of interaction between a foreign substrate and the adsorbed metal atoms formed by discharge, cathodic discharge of a limited amount of metal ions producing adatoms is possible at potentials more positive than the equilibrium potential of the particular system, and also more positive than the potential of steady metal deposition. 

After formation of a primary deposit layer on foreign substrates, further layer growth will follow the laws of metal deposition on the metal itself. But when the current is interrapted even briefly, the surface of the metal already deposited will become passivated, and when the current is turned back on, difficulties will again arise in the formation of first nuclei, exactly as at the start of deposition on a foreign substrate (see Section 14.5.3). This passivation is caused by the adsorption of organic additives or contaminants from the solution. Careful prepurification of the solution can prolong the delay with which this passivation will develop. 

The initial stages, notably the formation of a monolayer on a foreign substrate at underpotentials, were mainly studied by classical electrochemical techniques, such as cyclic voltammetry [8, 9], potential-step experiments or impedance spectroscopy [10], and by optical spectroscopies, e.g., by differential reflectance [11-13] or electroreflectance [14] spectroscopy, in an attempt to evaluate the optical and electronic properties of thin metal overlayers as function of their thickness. Competently written reviews on the classic approach to metal deposition, which laid the basis of our present understanding and which still is indispensable for a thorough investigation of plating processes, are found in the literature [15-17]. 

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. 

Such behavior is similar in this respect to the electrochemical deposition of metal on a foreign substrate, in which an overpotential is required for nucleation, after which further growth of the metallic layer occurs at the characteristic redox potential of the metal, leading to a trace-crossing in the reverse sweep. However, recent voltammetric studies have shown that such trace-crossings still appear even if deposition processes or insoluble film formation cannot be detected... 

Beside O P D it is well known that metal deposition can also take place at potentials positive of 0. For this reason called underpotential deposition (UPD) it is characterized by formation of just one or two layer(s) of metal. This happens when the free enthalpy of adsorption of a metal on a foreign substrate is larger than on a surface of the same metal [ 186]. This effect has been observed for a number of metals including Cu and Ag deposited on gold ]187]. Maintaining the formalism of the Nernst equation, deposition in the UPD range means an activity of the deposited metal monolayer smaller than one ]183]. 

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. 

Unlike anions that specifically adsorb at electrodes, cations normally do not lose their solvation shell due to their smaller size and are electrostatically adsorbed at electrodes at potentials negative to the pzc. However, depending on the affinity with the foreign substrate, cations can be reduced to a lower oxidation state or even discharged completely to the corresponding metal atom at the sub-monolayer or monolayer level at potentials positive to the equilibrium Nernst potential for bulk deposition. This deposition of metal atoms on foreign metal electrodes at potential positive to that predicted by the Nernst equation for bulk deposition has been called underpotential deposition and has been extensively investigated in recent years. Detailed discussion of the... 

Underpotential deposition occurs because the upd metal has a stronger interaction with the foreign substrate than with the corresponding bulk metal and such potential difference is a measure of the binding energy of the upd layer on the foreign substrate. 

In Chapter 6 we have seen that metal M will be deposited on the cathode from the solution of M"+ ions if the electrode potential E is more negative than the Nemst potential of the electrode M/M"+. However, it is known that in many cases metal M can be deposited on a foreign substrate S from a solution of M"+ ions at potentials more positive than the Nemst potential of M/M"+. This electrodeposition of metals is termed underpotential deposition (UPD). Thus, in terms of the actual electrode potential E during deposition and the Nemst equilibrium potential (M/M"+) and their difference AE = E — (M/M"+), we distinguish two types of electrodeposition ... 

Figure 29. Different nanostructures of foreign metals deposited on different substrates.

Figure 1.1 Schematic representation of different growth modes in metal (Me) deposition on foreign substrate (S) depending on the binding energy of Mcgds on S, compared to that of Meads...

In contrast to the bulk metal stability range, which covers the supersaturation or OPD range, a metal deposit can exist on a foreign substrate even in the undersaturation or UPD range in systems with strong Me-S interaction. In this last case, however, the deposit consists of 2D Meads overlayers and/or 2D Me-S surface alloy phases (cf. Chapter 1 and Chapter 3). 

The formation of ultrathin Me films on foreign substrates S (metals, superconductors, and semiconductors), S/Me, plays an important role in modern fields of technology such as micro- and nano-electronics, sensorics, electrocatalysis, etc. The process is often carried out by physical or chemical vapor deposition (PVD or CVD) of metals [6.152]. However, the difficult adjustment and control of the supersaturation via the gas flux is a great disadvantage of vapor deposition techniques. The situation becomes even more complicated, if more than one metal is deposited to form metallic sandwich layers and/or surface alloys. Therefore, electrochemical processes for the formation of ultrathin metal films and heterostructures became of great interest in modern thin layer technology. 

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... 

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