Foreign substrate

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. 

Nucleation The formation of sohd crystalhne nuclei on a foreign substrate is basically subject to the same laws as the formation of noncrystalhne nuclei. Specific features found in the case of crystalline nuclei are (1) considerably higher ESE values (2) a faceted rather than spherical shape and (3) anisotropic properties (i.e., different ESE values o, for different crystal faces /) ... 

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. 

Most often, these disperse metal catalysts are supported by an electronically conducting substrate or carrier that should provide for uniform supply or withdrawal of electrons (current) to or from all catalyst crystallites. The substrate should also serve to stabilize the disperse state of the catalyst and retard any spontaneous coarsening of the catalyst crystallites. Two situations are to be distinguished (1) the disperse metal catalyst is applied to a substrate consisting of the same metal, and (2) it is applied to a chemically different substrate (a foreign substrate). Platinized platinum is a typical example of the former situation. 

Experimental observations exist according to which a foreign substrate may influence the catalytic properties of the microcrystalline metal supported by it, and the supported metal conversely may influence the catalytic properties of the substrate. 

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]. 

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]. 

Fig. 7.146. Formation of 2D and 1D Meads phases on stepped foreign substrates, (a) 2D nucleation on atomically flat terraces. (b) 2D nucleation at monatomic steps, (c) 1D Meads phase formation along monatomic steps at AE > AP for FMend- 3d. (Reprinted from E. Budevski, G. Staikov, and W. J. Lorenz, Electrochemical Phase Formation and Growth, p. 116, copyright 1996 John Wiley Sons. Reproduced by permission of John Wiley Sons, Ltd.)...

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 ... 

A microbial transformation is the conversion of one substance (substrate) to another (product) by a micro-organism. It is a chemical reaction, catalyzed by a particular cellular enzyme or by an enzyme originally produced within cells. Most such enzymes are necessary for the normal functioning of the biological processes of cellular metabolism and reproduction. In microbial transformation, however, these enzymes simply act as catalysts (biocatalysts) for chemical reactions. In addition to their natural substrates, many of these enzymes can utilize other structurally related compounds as substrates and therefore occasionally catalyze unnatural reactions when foreign substrates are added to the reaction medium. Thus, microbial transformation constitutes a specific category of chemical synthesis. 

Obtaining an image of the first layer deposited on a foreign substrate can be rightly considered to be not completely representative of bulk crystallization. However, this is only the first layer of a thin polymer film, the structure of which can be investigated by electron diffraction. The two techniques are indeed very complementary. AFM probes the first layer, whereas electron diffraction determines the structure of the thin film as a whole - the structure of the film interior. 

Electron diffraction therefore makes it possible to establish that the structural continuity of the film is ensured. For example, it can differentiate the chiral and the racemic crystal polymorph of a given polymer (it can tell if the selection of helical hands observed in the first layer is still operative in layers deposited subsequently, away from the foreign substrate). As such, electron diffraction probes growth processes taking place in the polymer itself, as opposed to growth on a foreign substrate. Recall that deposition of,... 

Liquid droplets — The condition for the - equilibrium form of a liquid droplet formed on a solid foreign substrate (the working electrode) is obtained from... 

Solid crystals — If a crystal contacts a foreign substrate (the working electrode) the total free surface energy O of the system crystal-solution-foreign substrate is given by [i]... 

Here y and Sj are the specific free surface energies (- surface energy) and the surface areas of the crystal faces, which contact only the solution y and are the specific free interfacial energies and the surface areas of the faces which contact the foreign substrate and ys is the substrate specific free surface energy (the quantities ys, y, and y2 correspond to yi2, yo, and y23 from -> Equilibrium form of crystals and droplets -> liquid droplets). Substituting the difference y - ys for y - according to the rule of Dupre (- Dupre equation) [ii] one obtains... 

Equilibrium form of crystals and droplets — Solid crystals — Fig-Equilibrium form of crystals and droplets — Liquid droplets — Fig- ure. Equilibrium form of a cubic crystal formed on a foreign substrate ure. Cross section of a liquid droplet formed on a flat foreign substrate (cross section)... 

Wetting angle — A liquid droplet formed on a flat solid foreign substrate has the cap-shaped form of a spherical segment (Figure) with a volume,... 

Microbial transformations of foreign substrates, often referred to as precursor fermentation. It may be preferred to perform such transformations with whole cells rather than isolated enzymes when the latter approach would involve the recycling of expensive cofactors. Hence, precursor fermentations are often preferred for conducting redox transformations (see Chapter 6). 

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