Underpotential initial

The additivity principle was well obeyed on adding the voltammograms of the two redox couples involved even though the initially reduced platinum surface had become covered by a small number of underpotential-deposited mercury monolayers. With an initially anodized platinum disk the catalytic rates were much smaller, although the decrease was less if the Hg(I) solution had been added to the reaction vessel before the Ce(lV) solution. The reason was partial reduction by Hg(l) of the ox-ide/hydroxide layer, so partly converting the surface to the reduced state on which catalysis was greater. 

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.)... 

A similar effect was observed in our work and in the work of others (5), where voltammetry curves changed after extended cycling, particularly if the cathodic sweep was reversed before the full Pb deposition coverage. The observed "cathodic memory effect" may be due to the proposed structural transformation phenomenon and subsequent step density growth, initially facilitated by a high step density on a UHV-prepared or chemically polished (6) Ag(lll) substrate. Post electrochemical LEED analysis on Ag(lll)-Pb(UPD) surfaces provided additional evidence of a step density increase during Pb underpotential deposition, which will be discussed later in this text. (See Figure 3.)... 

One way to view UPD is as formation of a surface compound. In other words, deposition of the first atomic layer of an element on a second element involves a larger deposition driving force than subsequent layers, as it benefits from the AG of compound formation. For deposits formed at underpotential, once the substrate is covered the deposition stops because the reaction is surface limited. No more of the substrate element is available to react, unless it can quickly diffuse to the surface through or around the initially deposited monolayer (an example would be amalgam formation at a mercury electrode surface). Subsequent deposition is then only observed when the bulk deposition potential has been exceeded. 

The next step was to alternate the deposition of Cd and Te. As Te is more noble than Cd, various amounts of Te were first deposited and then exposed to a Cd ion solution at underpotential. Figure 8 is a graph of the Cd coverages observed to form on a Cu electrode initially coated with various amounts of Te. The slope of the graph is 0.95 (the Cd/Te ratio), which is consistent with the Cd reacting 1 1 with the Te. Similar results were observed for deposits formed on Pt and Au electrodes. The graph indicates that the Cd reacted at underpotential quantitatively with the Te, even when multiple atomic layers of Te were present. 

Gichuhi etal. [439] have used Au(lll) electrode covered with the initial under-potentially deposited Cd layer. When H2S was electrolyzed at this surface, applying underpotential, an adlattice of the S—S interatomic spacing equal to 0.34 nm was obtained. The second monolayer of Cd and S had the same structure as the first CdS monolayer, showing that these two CdS monolayers were epitaxial. However, the third deposited monolayer of CdS exhibited interatomic spacing as observed for the bulk CdS. A direct fabrication of monodispersed, ultrasmall nanocrystals from the SAMs at Au(lll) substrate has also been described [440]. Reconstruction of CdS monolayers has been studied by Demir and Shannon [441]. 

So-called underpotential deposited species arise when an electrochemical reaction produces first, on a suitable substrate adsorbent metal, a two-dimensional array or in some cases two-dimensional domain structures (cf. Ref 100) at potentials lower than that for the thermodynamically reversible process of bulk crystal or gas formation of the same element. The latter often requires an overpotential for initial nucleation of the bulk phase. The thermodynamic condition for underpotential deposition is that the Gibbs energy for two-dimensional adatom chemisorption on an appropriate substrate must be more negative than that for the corresponding three-dimensional bulk-phase formation. Underpotential electrochemisorption processes commonly involve deposition of adatoms of metals, adatoms of H, and adspecies of OH and O. 

Me UPD processes involving formation of 2D Meads phases, 2D Me-S surface alloy phases and 3D Me-S bulk alloy phases in the underpotential range (cf. eq. (1.7)) are due to a strong Me-S interaction and represent the initial step of metal electrocrystallization. 

Only a numerical solution of this diffusion problem is possible [3.73, 3.201]. For of a potential step polarization from an initial, AEi to a final underpotential, AEf, the well-known Cottrell equation can be derived assuming f(Ti) f(/f) [3.310] ... 

Figure 3.46 Current density transient from a potentiostatic pulse experiment in the system Ag(lll)/ 5 X 10-3 M TI2SO4 + 5 X lO l M Na2S04 + lO M HCIO4 at T = 298 K [3.110], Initial and final underpotentials AJEi = 550 mV, A f = 10 mV.

Figure 3.47 Potential-charge density transients from galvanostatic pulse experiments in the system Au(111)/10-3 M Bi(C104)3 + 1 M HCIO4 at T= 298 K [3.117], Initial underpotentials A i/mV= 110 (1) 190 (2) 220 (3) 223 (4) 225 (5), anodic pulse current densitiy i = 20 mA cm. ...

Figure 3.49 Current density transients from potentiostatic pulse experiments in the system Ag(lll)/3 X 10 3 M Pb(CH3COO)2 + 5 x Ifrl M NaC104 + lO M Na2H-citrate at 7= 298 K [3.94]. Initial and final underpotentials AEj/mV = 180, AEf/mV = 124 (1) 122 (2) 120 (3).

Figure 3.66 Kinetics of 3D Me-S bulk alloy formation in the system A1 (poly)/molten LiCl-KCl at T = 698 K [3.345]. Current density transients for the formation of p phase of Li-Al alloy from potentiostatlc pulse experiments. Initial and final underpotentials hEJrsN = 305, A f/mV= 296 (1) 292 (2) 288 (3).

Me UPD on foreign substrates S involving the formation of well-ordered 2D Meads overlayers, 2D Me-S surface alloys, and/or 3D Me-S bulk alloy phases in the underpotential (undersaturation) range represent the initial stage of Me deposition in systems with high Me-S interaction energy. 

Information about the influence of 2D UPD phases on thermodynamics and kinetics of subsequent 3D Me nucleation and growth can be obtained by UPD-OPD transition experiments. In general, the experiment has two stages. In the initial stage i, a 2D Me UPD phase is formed and eventually equilibrated at a selected underpotential AE. The final stage f of the system is characterized by an external potentiostatic pulse to t]f into the OPD range. There are two possibilities for pulse excitation techniques potentiostatic or galvanostatic conditions. 

The number of nucleation sites, Zo, in eq. (4.56) can be influenced by the initial underpotential A i. Therefore, UPD-OPD transition experiments can give information on the influence of the bare substrate structure on the 3D Me nucleation and growth kinetics in the OPD range. 

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