Surface alloy formation

UPD process and surface Cd-Ag alloying were also studied on polycrystalline Ag electrode [285, 286]. The surface alloy formation rate was similar to that for Ag(lOO) [286]. The dynamics of surface alloying promoted by Cd UPD was studied on irregular Ag substrates, dendritic, and columnar Ag deposits [291]. 

In another work from this series [433], deposition of Pd on the unreconstructed Au(llO) has been studied. An ordered adlayer of PdCfi " was imaged with atomic resolution. Pd deposition started at monoatomic high steps. From the coulo-metric data, it follows that approximately three monolayer equivalents are deposited in the UPD range, what may, in turn, be a result of the surface alloy formation. 

A substitutional Au impurity also reduces the CO adsorption energy on Au/ Ni(lll) surfaces on the sites immediately surrounding the impurity. In this case, the incorporation of Au into the Ni(l 11) surfaces by exchanging with a Ni atom was found to be endothermic, and consequently surface alloy formation must be entropically driven. The Au atom center of mass sits about 0.5 A above the average location of the Ni atoms due to its larger size. The adsorption energy in the threefold sites that include the Au atom are only —1.18 eV, compared to —2.16 eV in neighboring three fold sites that only include Ni. On pure Ni(lll), the CO... 

Figure 3.57 Schematic representation of 2D Meads overlayer formation and two different mechanisms (a) and (b) of 2D Me-S surface alloy formation in the system Ag(lll)/5 x 10 M TICIO4 + 10 M HCIO4 corresponding to the adsorption peaks A with n = 1,2,3 at T = 298 K (cf. Figs. 3.3 and 3.53) [3.175, 3.177, 3.178]. (a) lateral site exchange (b) vertical site exchange.

Coexisting domains of expanded and condensed surface structures are observed as shown in Fig. 3.61. The expanded surface structure should be formally described by an Ag(lll)-(-s X VS) R 30° structure, but can be attributed to 2D Me-S surface alloy formation starting at monatomic steps. The condensed surface structure corresponds to the compressed and rotated hep structure of the 2D Meads overlayer. A similar model for 2D Me-S surface alloy formation in the system Au(lll)/Pb, H, CIO4 was suggested by Green [3.195]. 

In both studies [3.107, 3.109, 3.175, 3.177, 3.178, 3.330-3.336], the decrease of 9A2(0 of fho adsorption peak A2 with increasing polarization time was used as a measure for the rate of 2D Me-S surface alloy formation. Anodic stripping after extended polarization at A gives additional and direct information on the kinetics of this process. However, such stripping experiments were not systematically carried out. 

Figure 3.63 Influence of monatomic step density, L, on the kinetics of 2D Me-S surface alloy formation in the system Ag(lll)/5 x 10" M Pb(C104)2 + 10 M NaC104 + 5 x 10 M HCIO4 at T = 298 K [3.109]. Pbads coverage in peak A2 decays in desorption experiments after extended polarization at A = 120 mV. Experimental q AE,t) data were interpreted assuming quasi-Nemst behavior (eq.(3.18)) of the system studied. L/cm" 0 (1) < 2 x 10 (2) 1.7 x 10 (3) according to [3.107] (4).

Formation of 2D Me surface alloy and 3D Me bulk alloy have to be taken into account besides 2D Meads overlayer formation for Me UPD systems with nonvanishing Me solubility in S. Influences of crystallographic orientation and crystal imperfection density of S on the rate of 2D and 3D Me-S alloy formation are observed. 2D Me-S surface alloy formation processes are pronounced on S(lll). The mechanisms of 2D Me surface alloy and 3D Me bulk alloy formation processes are still not well understood. More realistic models are necessary to describe these processes... 

UPD-OPD transition experiments applying long-time polarization in the UPD range would reflect the influence of the slow 2D Me-S surface alloy formation in this system on the mechanism of 3D Ag bulk deposition in the OPD range. However, such measurements have not yet been published. 

Results on the influence of 2D Me-S surface alloy formation on the UPD-OPD transient behavior are not reported. 

The third part treats surface modification by underpotential deposition (UPD) of metals. Physical nature, thermodynamics, structural aspects, kinetics, as well as surface alloy formation are discussed. Experimental support is given based on classical electrochemical investigations as well as on some recent results from modern in situ surface analytical studies including atomic imaging by in situ STM and AFM. 

The sign of the curvature of the surface energy curve "+" corresponds to surface alloy formation, to island formation, and "=" to zero curvature. Columns are labelled by the deposited element and rows by the substrate. 

The surface orientation also plays a very important role in the surface alloy formation, since alloying is determined by effective interactions and the corresponding coordination numbers, Zi, which are surface specific. In some cases, especially when the substrate belongs to the IVb-VIIlb group in the periodic table, the multisite interactions may play an important role. In those cases, the surface energy curve will not have the simple parabolic shape presented in Fig. 4 and the alloying behavior may be more complex than described above. To have even a qualitative understanding one may therefore need further input. 

The objective of this section is to introduce the BFS-based methodology for a detailed study of the most important features of surface alloy formation. The methodology assumes no a priori information on the system at hand. The only input necessary consists of the basic parameterization of the participating elements and lattice structures needed, as described in Sec. 2, and a catalogue of atomic distributions, where each configuration represents a state accessible by the system under study. Each entry in the catalogue is a computational cell popu-... 

The abundant experimental results for Pd/Cu(100) make this system an ideal framework for modeling the many recognized features during the early stages of surface alloy formation [44-59]. The first question is whether Pd atoms deposited on Cu(lOO) do or do not penetrate in the surface layer, and if they do, what hap-... 

Fig. 36 Dominance of Au/Ni(110) and Cu/Ni(110) features in the early stages of (Cu+Au)/ Ni(l 10) surface alloy formation. The Au/Ni trend to form Au dimers in the surface dominates. The mixed states most likely to appear in the ternary system include the formation of bcb chains due to the favorable Cu-Au bonds thus established. The tendency of Cu to remain in the overlayer is a secondary effect, leading to Cu-anchored cp chains or a Au-Cu-Au (BCB) chain in the overlayer with higher energy than its equivalent in the surface.

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