UPD phase

Finally, one should mention that Hg UPD on Au(lll) proceeds differently on the neighboring metals in the periodic table, such as Tl, Pb, and Bi. UPD layers of T1 and Pb, just prior to the bulk deposition on Au(lll), were found to be compressed by only about 3% as compared to the bulk values, and decreased with the decreasing electrode potential. At the same time, two ordered Hg UPD phases had expanded structures compared to the frozen bulk Hg [23]. 

The formation and dissolution of 2D Me UPD phases can involve positive and negative 2D nucleation and growth steps, respectively. 2D nucleation and growth represent a first order phase transition where an expanded overlayer is transformed into a condensed one (or vice versa) by a discontinuous change of r. Additionally, higher order (order-disorder) phase transitions, characterized by 7" = constant, but with a discontinuity in its partial derivative (dr / dE), may also take place within 2D Meads overlayers in the UPD range. However, clear experimental evidence for higher order phase transitions in Me UPD overlayers does not yet exist. 

Pangarov [3.219] theoretically treated UPD and OPD of Me the same way, which is not possible (cf. Chapter 1 and Section 3.3). Furthermore, the fundamental assumptions made are contradictionary. As a result, the formation of 2D Meads UPD phases was considered as a first order phase transition process. 

The formation of 3D Me phase takes place under these conditions on a modified substrate surface. In this section, the influences of structure and properties of 2D Me UPD phases on the subsequent nucleation and growth processes of a 3D Me bulk phase are treated. 

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. 

In systems with significant Me-S lattice misfit, the 2D Meads overlayers and/or 2D Me-S surface alloys formed in the UPD range have a different structure in comparison with the 3D Me bulk phase, and contain considerable internal strain (cf. Section 3.4). Thus, the nucleation and growth kinetics in the OPD range will be strongly influenced by the internal strain energy of 2D Me UPD phases. 

Also the reverse transition, the formation of a (1 x 1) structure on the domain network of the (v x y )R30° phase can be followed in-situ by STM. Figure 3 shows the evolution of the surface structure upon a potential step from the (v X v )R30° into the (1x1) Cu UPD phase. Starting from the domain... 

During the second UPD peak, B2, the sulfate ions are displaced from the honeycomb center, and a (1 x 1) pseu-domorphic ML is formed with the Cu residing in the threefold hollow sites of the Pt surface, on top of which the sulfate ions arrange in a ( 3 x /7) pattern, 0 = 0.20 [389, 396, 397.] The reverse processes take place during the dissolution of the UPD phase upon positive potential excursion. 

The completed Pb UPD is metallic, and represents an incommensurate, hexagonal ML that is compressed compared with the bulk metal by 0.1-3.2%, and rotated from the substrate (Oil[-directionby 4.5° [426, 427, 429-431]. The rotation of the adlayer with respect to the substrate lattice gives rise to a characteristic Moire pattern as observed in several in situ STM studies [360, 426, 427] (Fig. 28). The interaction between solvent molecules and the Pb adatoms does not influence the structure of the complete ML deposited in C104 or acetate-containing electrolyte, since the UPD phase is essentially identical to that of vapor-deposited Pb on Ag(lll) at full coverage [420, 435]. The monolayer compression in the vacuum experiment (1-2%) is slightly less than for... 

Based on this concept, thermodynamic functions of the UPD phase can be determined. 

The formation reaction (Eq. (4.62)) can be used to calculate thermodynamic functions of the UPD phase. From the cell voltage AB = E — Eq one can calculate the partial molar Gibbs energy AG pp of the UPD modification of the metal B... 

From the peak potential the Gibbs energy of formation Aj Gypp, of the UPD phase is obtained. 

Figure 4.34 Born-Haber cycle for calculation of sublimation enthalpy of UPD phases.

The Gibbs energy of formation is calculated for one mole Cu transferred in the neutral cell reaction from the bulk into the UPD phase (Eq. (4.66)). 

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