Electrodes metal deposition

FIGURE 2.41 Palladium electrode (metal deposited onto gold). Coulometric determinations in a solution 1 molL" Csl in DMF for a surface polarized at -2.1 V versus SCE electrode during 60 s (full line) and then discharge at -0.5 V. The discharge amount Q, is quasi-instantaneous. By comparison, the broken line shows the same chronocoulometry at the bare gold electrode (Cougnon, C., Ph.D. thesis, Universite de Rennes 1, 2002). 

In the many reports on photoelectron spectroscopy, studies on the interface formation between PPVs and metals, focus mainly on the two most commonly used top electrode metals in polymer light emitting device structures, namely aluminum [55-62] and calcium [62-67]. Other metals studied include chromium [55, 68], gold [69], nickel [69], sodium [70, 71], and rubidium [72], For the cases of nickel, gold, and chromium deposited on top of the polymer surfaces, interactions with the polymers are reported [55, 68]. In the case of the interface between PPV on top of metallic chromium, however, no interaction with the polymer was detected [55]. The results concerning the interaction between chromium and PPV indicates two different effects, namely the polymer-on-metal versus the metal-on-polymer interface formation. Next, the PPV interface formation with aluminum and calcium will be discussed in more detail. 

Reaction overpotential. Both overpotentials mentioned above are normally of higher importance than the reaction overpotential. It may happen sometimes, however, that other phenomena, which occur in the electrolyte or during electrode processes, such as adsorption and desorption, are the speed-limiting factors. Crystallization overpotential. This exists as a result of the inhibited intercalation of metal ions into their lattice. This process is of fundamental importance when secondary batteries are charged, especially during metal deposition on the negative side. 

The major types of interferences in ASV procedures are overlapping stripping peaks caused by a similarity in the oxidation potentials (e.g., of the Pb, Tl, Cd, Sn or Bi, Cu, Sb groups), the presence of surface-active organic compounds that adsorb on tlie mercury electrode and inhibit the metal deposition, and the formation of intermetallic compounds (e.g., Cu-Zn) which affects the peak size and position. Knowledge of these interferences can allow prevention through adequate attention to key operations. 

Figure 5.45 shows a Pt electrode (light) deposited on YSZ (dark). There are three circular areas of bare YSZ connected via very narrow bare YSZ channels. The rest of the surface is Pt. Note that, as will be discussed in Chapter 7, the Fermi levels of the Pt film and of the YSZ solid electrolyte in the vicinity of the Pt film are equal. The YSZ, however, appears in the PEEM images much darker than the Pt film since YSZ has a negligible density of states at its Fermi level in comparison to a metal like Pt. 

A monolith was made from YSZ and the surface of the monolith channels was covered with a Ru02 catalyst. Two terminal Au electrodes were deposited on the outside surface of the monolith. Potential application between the two terminal Au electrodes was found to induce NEMCA on the Ru02 catalyst which is not in electrical contact with any metal wire.9... 

Experience shows that in the deposition of a number of metals (mercury, silver, lead, cadmium, and others), the rate of the initial reaction is high, and the associated polarization is low (not over 20 mV). For other metals (particularly of the iron group), high values of polarization are found. The strong inhibition of cathodic metal deposition that is found in the presence of a number of organic substances (and which was described in Section 14.3) is also observed at mercury electrodes (i.e., it can be also associated with the initial step of the process). 

In metal deposition, the primary products form adsorbates on the electrode surface rather than a supersaturated solution. Their excess chemical potential is directly related to polarization and given by nFAE. The total excess surface energy = 2 S,o,. Otherwise, all the results described above remain valid. 

One must realize that once complete metal deposition has been attained, the emf across the electrodes cannot be switched off before the cathode has been taken out of the solution and rinsed with water, otherwise the metal deposit may start to redissolv e in the solution as a consequence of internal electrolysis by the counter emf. After disconnection the electrode is rinsed with acetone and dried at 100-110° C for 3-4 min. The analytical result is usually obtained from difference in weight of the dry cathode before and after electrolysis. In a few instances a copper- or silver-plated Pt cathode or even an Ag cathode is used, e.g., Zn and Bi are difficult to remove entirely from Pt, as they leave black stains and on heating form an alloy with the noble metal for this and other reasons (see below) the experimenter should consult the prescriptions in handbooks149. 

As a second experimental condition, the metal deposit should be pure and adhere well to the electrode, for which the following two factors are of most importance ... 

Sections 5.6.2 and 5.6.3 dealt with the deposition of metals from complexes these processes follow the simple laws dealt with in Sections 5.2 and 5.3, particularly if they take place at mercury electrodes. The deposition of metals at solid electrodes (electrocrystallization) and their oxidation is connected with the kinetics of transformation of the solid phase, which has a specific character. A total of five different cases can be distinguished in these processes ... 

It was mentioned on page 306 (see Fig. 5.24) that, even at room temperature, a crystal plane contains steps and kinks (half-crystal positions). Kinks occur quite often—about one in ten atoms on a step is in the half-crystal position. Ad-atoms are also present in a certain concentration on the surface of the crystal as they are uncharged species, their equilibrium concentration is independent of the electrode potential. The half-crystal position is of basic importance for the kinetics of metal deposition on an identical metal substrate. Two mechanisms can be present in the incorporation of atoms in steps, and thus for step propagation ... 

Electrolytic recovery systems work best on concentrated solutions. For optimal plating efficiency, recovery tanks should be agitated ensuring that good mass transfer occurs at the electrodes. Another important factor to consider is the anode/cathode ratio. The cathode area (plating surface area) and mass transfer rate to the cathode greatly influence the efficiency of metal deposition. 

Electrochemical reactions are driven by the potential difference at the solid liquid interface, which is established by the electrochemical double layer composed, in a simple case, of water and two types of counter ions. Thus, provided the electrochemical interface is preserved upon emersion and transfer, one always has to deal with a complex coadsorption experiment. In contrast to the solid/vacuum interface, where for instance metal adsorption can be studied by evaporating a metal onto the surface, electrochemical metal deposition is always a coadsorption of metal ions, counter ions, and probably water dipols, which together cause the potential difference at the surface. This complex situation has to be taken into account when interpreting XPS data of emersed electrode surfaces in terms of chemical shifts or binding energies. 

More than a decade ago, Hamond and Winograd used XPS for the study of UPD Ag and Cu on polycrystalline platinum electrodes [11,12]. This study revealed a clear correlation between the amount of UPD metal on the electrode surface after emersion and in the electrolyte under controlled potential before emersion. Thereby, it was demonstrated that ex situ measurements on electrode surfaces provide relevant information about the electrochemical interface, (see Section 2.7). In view of the importance of UPD for electrocatalysis and metal deposition [132,133], knowledge of the oxidation state of the adatom in terms of chemical shifts, of the influence of the adatom on local work functions and knowledge of the distribution of electronic states in the valence band is highly desirable. The results of XPS and UPS studies on UPD metal layers will be discussed in the following chapter. Finally the poisoning effect of UPD on the H2 evolution reaction will be briefly mentioned. 

The rotating hemispherical electrode (RHSE) was originally proposed by the author in 1971 as an analytical tool for studying high-rate corrosion and dissolution reactions [13]. Since then, much work has been published in the literature. The RHSE has a uniform primary current distribution, and its surface geometry is not easily deformed by metal deposition and dissolution reactions. These features have made the RHSE a complementary tool to the rotating disk electrode (RDE). 

The principal evidence in favor of the electrochemical mechanism is the observation that metal deposition can occur, albeit at a much reduced rate, in a two-compartment cell. When one of the compartments contains only the metal ions and the other only the sodium hypophosphite, metal deposition occurs on the electrode in the first compartment, being driven by hypophosphite oxidation at the electrode in the other half-cell [36, 37],... 

Limiting currents are usually associated with cathodic reactions (e.g., in metal deposition), although anodic reactions are by no means excluded. Whenever the supply of a dissolved species from the solution to the electrode surface becomes the rate-limiting factor, limiting-current phenomena may be observed. Anodic limiting currents can be obtained, for example, in the oxidation of ferrous to ferric ion, or ferro- to ferricyanide ion (El). Diffusion of H20 limits 02 evolution in fused NaOH (A2). In these examples the limiting current is caused by depletion of the reactant species at the anode. 

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