Overpotential deposition

In the overpotential deposition regime we observed that nanosized Al was deposited in the initial stages. Eurthermore, a transfer of Al from the scanning tip to the Al covered substrate was observed. We accidentally succeeded in an indirect tip-induced nanostructuring of Al on growing Al (Eigure 6.2-6). 

Figure 6.2-6 The initial stages of Al overpotential deposition result in nanosized deposits.

Figure 23. Al images of Au( 111) in 1 mAf + 0.1 A/ perchloric acid or sulfuric acid solutions. (A) At 0.7 V(SCE), (B) overpotential deposition at -0.1 V, (C) Close-packed overlayer of Cu at 0.114 V in O.IA/ perchloric acid solution with an atomic distance of 0.29 nm. (D) Scheme of close-packed overlayer of Cu on Au. (E) (3 /2 X 3 /2)R30° overlayer of Cu on Au(l 11) at 0.144 V(SCE). (F) Scheme of (3 /2 X 3 /2)R30° overlayer of Cu (striped circles) on Au (open circles). (From Ref. 68 with permission from the American Association for the Advancement of Science. Copyright 1991, American Association for the Advancement of Science.)...

Silver deposition on polycrystalline Pt electrodes at potentials positive to the equilibrium potential gave 2.5 atomic layers. Two binding types of Ag layers were found by anodic stripping the first Ag layer deposited on Pt, which seems to form an alloy of Ag-Pt, on which the second Ag deposition takes place in the Ag underpotential deposition region. STM images from the underpotential to the overpotential deposition region were observed for Cu underpotential deposition on Au(l 11) in sulfuric acid solution, where Cu underpotential deposition does not affect overpotential deposition, although the latter always takes place on the surface with Cu underpotential deposition and a metal. ... 

Implementation of upd in material synthesis has also been explored. A particularly interesting effort has focused on the production of E-VI compounds by successive upd reactions performed in two different electrolytes. Importantly, process development has been tightly coupled with STM studies of both upd and overpotential deposition (opd) of the constituents [299,304,365-369]. Similarly, the influence of upd on catalytic activity towards certain reactions is well known [370]. An STM study of the inhibition of four-electron oxygen reduction on Pt(l 11) by upd Cu clearly demonstrates the importance of upd structure on reactivity [371]. 

Underpotential and overpotential deposition of Bi on Au(lll) have been studied applying in situ STM [453]. It has been found that the adsorbed bismuth lifts the reconstruction of the Au(lll) surface, leading to the formation of Au islands at potentials more cathodic than 0.170 V versus SCE. Atomic images of UPD Bi layer have shown the formation of a nearly rectangular unit cell of dimension 0.39 0.02 X 0.43 0.02 nm. 

Wang et al. ]509] have reported overpotential deposition of Ag monolayer and bilayer on Au(lll) mediated by the Pb adlayer UPD/stripping cycles. 

Figure 6.2-6 The initial stages ofAl overpotential deposition result in nanosized deposits. A jump to contact transfer of Al from the scanning tip to the growing Al was observed (picture from [65] - with permission of the Peep owner societes).

In other words, it now includes the term t] = Ed — E which represents the difference between the two expressions, Eqs. (11.1) and (11.2), above. The factor av+ represents, again, the activity value of the cation being deposited (i.e., cation in the film or layer of the bath at the cathode face). Thus tj is the overpotential (deposition factor). It is the extra potential needed to maintain the deposition going at a given desired rate suitable to the nature and properties of the cathode film. In practice, then, calculating the metal deposition potential by the above means that the practitioner must know the values of av+ and 17 for a fixed plating condition, including bath parameters, such as current density and temperature, as well as ionic parameters, such as concentration, valence, and mobility. 

The relation between UPD currents, as observed, for example, in cyclic-voltammetry experiments (cf. Refs. 100,107) on H deposition and desorption, and the continuous currents that result in cathodic evolution when the reversible potential is exceeded in the negative direction is illustrated in Fig. 6. It is seen that the overpotential deposition (OPD) process, resulting in evolution, can pass very much larger currents than the UPD process since the rates of the Faradaic reactions involved are not limited by approach to full coverage by the adsorbed intermediate, here H. Thus, changes of coverage by that OPD H are not at all easily detectable under conditions of passage of... 

In the fourth part of the book the problem of three-dimensional phase formation and growth by overpotential deposition (OPD) is presented. Thermodynamic and kinetic aspects are considered. The atomistic approach is discussed and illustrated on bare and UPD modified substrates. 

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