Pb adlayer

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

Consequently, the formation of the Pb adlayer in this underpotential range can be considered as an 1/2 localized adsorption on a square lattice. In this case each adatom in the compact monolayer covers effectively two adsorption sites. Thus, domains with an Ag(100)-c(2 x 2) Pb structure located on different substrate sublattices Oike white and black fields of a chessboard) separated by mismatch boundaries are obtained as shown, for example, by Monte Carlo simulation (cf. Section 8.4) of 1/2 adsorption on a square lattice [3.214], The fit of experimental coverage data of the first Pb adsorption step on Ag(lOO) (cf. Fig. 3.9) by Monte Carlo simulation is illustrated in Fig. 3.30. From this fit, a lateral attraction energy between the Pb adatoms of V Pbads-Pbads 2.5 X 10 J (corresponding to 1.5 x 10 J mole ) can be estimated [3.184, 3.190, 3.191, 3.214]. Preferential Me adsorption on surface heterogeneities like monatomic steps was disregarded in the fit procedure. 

Desorption of the complete Pb adlayer within the three distinct desorption peaks D3, D2 and D1 (see Fig. 2) by step polarization proceeds in an analogous way to the adsorption sequence, except on the monoatomic islands in contrast to the complete adsorbate formation at the islands in peak A3, desorption in peak D3 only involves the outermost part of the monolayer at the island periphery, whereas the remaining adsorbate coverage is completely desorbed in peak D2. Desorption on the monoatomic islands occurs thus in the same way as at the stepped terrace domains, except for the missing step decoration coverage desorbed in D1. 

The work function of the metal surface is considerably altered by the adsorption of foreign atoms. This is well established at the metal-vacuum interface. Pb adlayers on Ag cause a pronounced decrease in the work function with increasing Pb coverage [5]. A similar change can be expected at the electrode surface. The potential of zero charge (PZC) of the electrode surface, which is directly related to the work function, has been determined for the Pb adlayer on polycrystalline Ag. A value close to that was found for bulk Pb [6]. 

Fig. 4 Quasi-stationary current-potential plots for HCOOH (0.5 M) oxidation on sputtered Pt modified by Pb adlayer and HCOOH adsorbates in 0.1 M HCIO4. Current taken 2 min after stepping the potential. The current density is given with respect to the free-Pt surface (16% of the whole surface).

Fig. 21 Proposed mechanism of the UPD of Pb-mediated deposition of Ag. The Ag and Pb atoms are light and dark shaded spheres, respectively. Arrows indicate depositing metal cations, (a) Pb mediation at potentials close to reversible Pb deposition, a Pb monolayer covers the surface. The deposited Ag adatoms undergo interlayer place-exchange with Pb adatoms (light green sphere) forming 2D islands below the Pb adlayer. (b) On the reverse cycle, the Pb adlayer is stripped from the surface as Ag continues to deposit resulting in island growth. (Reproduced with permission from Ref [198].)...

FIG. 27. A comparison between the in-plane dimensions, determined by SXS and STM, of the electrocompressive Pb upd adlayer formed on Ag(lll). The open circles are the STM data and the solid line is a least-mean-square fit to the data, while the dotted line is derived from SXS measurements (k = A/a when A is the period of the Moire pattern while a is the lattice spacing of the overlayer). (From Ref. 343.)... 

TTie interatomic distance of nearest Pb neighbors in this STM image is d = 0.35 + 0.01 nm. It can be assumed that in this UPD range the overlayer most probably consists of coexisting domains of Ag(l(K))-hcp Pb and Ag(l(X))-c(2 x 2)Pb overlayer structures. The structural transformation from a com-mensurate quadratic Ag(lOO)-c(2 X 2) Pb overlayer to an Ag(100)-hcp Pb monolayer, which most probably starts at mismatch boundaries between adlayer domains and/or monatomic steps of the substrate, is completed at relatively low AE. In the UPD range 0 mV < A < 30 mV, only the hep Pb overlayer structure could be observed by in situ STM. 

This indicates that the UPD-OPD transition obviously proceeds via the Stranski-Krastanov mechanism (cf. Fig. Ic) involving the formation and growth of 3D Pb crystallites on top of the 2D internally strained Pb UPD adlayers which act as precursors for the nucleation and growth process in the OPD range. The unstrained 2D hep surface structure of a 3D Pb(lll) crystal face is reached after deposition of about 10 Pb monolayers as shown in Fig. 4.18. The interatomic distance corresponds to [Pg.194]

The early UPD lead deposition on Au(lll) was reported by Green et lei.iez yjjggg studies indicated that a deposit developed a monoatomic high adlayer associated probably with Pb atoms in a (1x1) structure, and that during removal of this layer a series of surface pits were formed. Later on, Tao et al. showed that an (1x1) layer is fonned, indeed, in the UPD process showing images of atomic resolution. ... 

In a recent STM study by Carnal et al. [3], these assumptions have been confirmed by the observation that a hexagoiuilly close-packed adlayer with slightly compressed Tl-Tl interatomic distances is formed at more anodic potentials, followed by the formation of a second hep adlayer with slightly disordered domains at small undervoltages. The progress of the formation of the first adsorbate layer was studied in that work by more conventional STM imaging techniques and was restricted to investigations at file stepped terrace domains. As shown in detail in [3], the formation of the first adsorbate layer at the stepped terrace domains follows the same scheme as shown in Fig. 2 for the system Pb/Ag(l 11), i.e. 

A very similar transformation of the original hep adlayer to a surface alloy coverage with the same Tl-Tl interatomic distances and [V3 x V3]R30° symmetry has been observed in the system Tl/Ag(lll) during extended polarization of the incompletely formed first T1 adsorbate layer. As in the system Pb/Ag(lll), there is strong evidence that the transformations proceed from the boundaries of the peripheral adsorbate-free domains inwards on the terraces. However, in contrast to the system Pb/Ag(lll), the transformed coverages include both ordered and disordered domains, and then-desorption results in the formation of monoatomic pits in the substrate with widths of ca. 3 to 10 nm [3]. These pits diminish and finally vanish within a few minutes by coalescence and lateral displacement, at a rate that can be increased markedly by positive shift of the substrate potential. 

The nature of the Pb-CO interaction is, however, repulsive. Given that there is also a repulsive Pb-Pb interaction in the (3x /3) adlayer due to the uniaxial compression which decreases the Pb-Pb interatomic spacing [89], it is apparent that the repulsive forces weaken the Pt-Pb bond, which creates a kinetic pathway for the displacement [90]. Pb in the (3x /3) phase is then displaced from the surface by CO, and the intensity of the (3x /3) peak decreases until, at T X 3000 s, it has almost disappeared. 

The following sections are not meant to review the entire field of UPD. Emphasis will be rather given to three model systems, Cu UPD on Au(hkl), Pt(hkl) and Pb UPD on Ag(hH) to describe, based on these examples, typical structural aspects and kinetic processes of 2D phase formation with low-dimensional metallic adlayers on foreign substrates. 

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... 

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