Underpotential final

Finally, one has to distinguish between underpotential deposition of M on S and alloy formation. Alloys can be formed electrochemically (Brenner, 1942). Underpotential deposition is usually a monatomic step affair. With the alloy, the foreign atoms go on building up until they form part of the new substrate, the alloy. 

Finally, the chemisorption of hydrogen and oxygen atoms on platinum, considered in Sect. 6.1, occurs in the underpotential region. 

Robinson and Richmond [117] also found that the time constant for adsorption of the first thallium overlayer depends on the final potential, E, of the potential step. They investigated the effect of underpotential and overpotential steps on the best fit values of z for exponential fits to /p p((). The under- and overpotentials, AE, are defined with respect to the maximum of the current peak, Ep, in the CVs for the first monolayer deposition. Fig. 5.22 displays the results for seven different potential steps. The time required to form the deposit increases at anodic potentials (A <0) closest to Ep, indicating that more time is required to deposit successively larger coverages. The form of the data suggests that z would reach a maximum near... 

Atomic layer epitaxy (electrochemical) — Electrochemical atomic layer epitaxy (ECALE) is a self-limiting process for the formation of structurally well-ordered thin film materials. It was introduced by Stickney and coworkers [i] for the layer by layer growth of compound semiconductors (CdTe, etc.). Thin layers of compound semiconductors can be formed by alternating - underpotential deposition steps of the individual elements. The total number of steps determines the final thickness of the layer. Compared to flux-limited techniques... 

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 3.49 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).

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. 

Finally, it should be mentioned that by the presence of certain additives the underpotential deposition process can be inhibited. Upd of copper on Pt(lll), Pt(lOO), and Pt(llO) can be inhibited by thiourea and dithiadecyldisodium sulfonate. The results of a study on the effects of organic adsorbates on the underpotential deposition of silver on Pt(lll) electrode show that the presence of coadsorbates (2,2-bipiridyl, 4-mercapto-pyridine, etc.) can have a pronounced effect on the underpotential deposition. It has been found that adsorbates that bind primarily through a ring nitrogen atom inhibit the second, but not the first, silver monolayer. In contrast, the sulfur-containing adsorbates inhibit the formation of the first monolayer owing to the formation of the Pt-S bond. 

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