Ag underpotential deposition

Fig. 29. Ag3d core level spectra of Ag underpotential deposited onto polycrystalline Pt at various electrode potentials. 0.1 molLHC104 + 0.001 molL-1 AgN03.

Fig. 31. Work function change, referred to the clean Pt electrode surface, as a function of electrode potential for Ag underpotential deposition onto Pt. The work function of bulk Cu would correspond... 

Ag underpotential deposition inlM HCIO4 was observed by FEED,... 

STM images of Ag underpotential deposition on Pt(lll) were observed as (1 X 1) and (VJx V3) structures at the potentials of Ag mono-layer formation and between two underpotential deposition peaks, respectively." The STM image at Pt(lOO) was examined for Cu underpotential deposition in the presence of halide ions in sulfuric acid solution... 

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. ... 

For example, the voltammogram in Fig. 1 depicts Ag UPD on an I-coated Pt(lll) electrode [26], Three features can be attributed to the UPD of Ag, each of which results in the formation of a new structure on the surface, as indicated by the FEED patterns diagrammed in the circles. It was concluded in that work that UPD involved more than a single monolayer of Ag. Ag depositing at underpotential reacted with the Pt substrate as well as with the adsorbed I atom layer. It is also interesting to note that Ag underpotentially deposited in Fig. 1 reacted with the adsorbed atomic layer of I atoms to form a monolayer of the I-VII compound Agl on the Pt surface. 

Carnal D, Oden P I, Muller U, Schmidt E and Siegenthaler FI 1995 In situ STM investigation of T1 and Pb underpotential deposition on chemically polished Ag(111) electrodes E/ecfroc/r/m. Acta 40 1223-35... 

Alois GD, CavaUini M, Innocent M, Foresti ML, Pezzatini G, GuideUi R (1997) In situ STM and electrochemical investigation of sulfur oxidative underpotential deposition on Ag(lll). J Phys Chem B 101 4774 780... 

Herrero E, Buller LJ, Abruna HD (2001) Underpotential deposition at single crystal surfaces of Au, Pt, Ag and other materials. Chem Rev 101 1897-1830... 

Although not the subject of this article, double layer studies are briefly discussed in this paragraph in order to demonstrate that ex situ XPS studies indeed provide information about the state of the electrode exposed to an electrochemical environment at a defined potential. A crucial step in any ex situ experiment is the emersion of the electrode. Here the question arises whether the electrochemical double layer or part of it is preserved at the interface after emersion and transfer. Winograd et al. [10,11] first demonstrated that the electrode under UHV conditions still remembers the electrode potential applied at the time of emersion. These authors investigated oxide formation on Pt and the underpotential deposition of Cu and Ag on Pt by means of XPS and proved that the electrochemically formed oxide layer and... 

Melroy and co-workers88 recently reported on the EXAFS spectrum of Pb underpotentially deposited on silver (111). In this case, no Pb/Ag scattering was observed and this was ascribed to the large Debye-Waller factor for the lead as well as to the presence of an incommensurate layer. However, data analysis as well as comparison of the edge region of spectra for the underpotentially deposited lead, lead foil, lead acetate, and lead oxide indicated the presence of oxygen from either water or acetate (from electrolyte) as a backscatterer. 

A similar effect was observed in our work and in the work of others (5), where voltammetry curves changed after extended cycling, particularly if the cathodic sweep was reversed before the full Pb deposition coverage. The observed "cathodic memory effect" may be due to the proposed structural transformation phenomenon and subsequent step density growth, initially facilitated by a high step density on a UHV-prepared or chemically polished (6) Ag(lll) substrate. Post electrochemical LEED analysis on Ag(lll)-Pb(UPD) surfaces provided additional evidence of a step density increase during Pb underpotential deposition, which will be discussed later in this text. (See Figure 3.)... 

Underpotential deposition is described as less than monolayer metal deposition on a foreign metal substrate, which occurs at more positive potentials than the equilibrium potential of a metal ion deposed on its own metal, expressed by the Nemst equation. Kolb reviewed state-of-the-art Underpotential deposition up to 1978. As Underpotential deposition is a process indicative of less than a monolayer metal on a substrate, it is expected to be quite sensitive to the surface stmcture of the substrate crystal a well-defined single-crystal electrode preparation is a prerequisite to the study of Underpotential deposition. In the case of Au and Ag single-crystal electrodes, Hamelin and co-workers extensively studied the necessary crystal surface structure, as reviewed in Ref. 2. 

Figure 34. STM image of Pb underpotential deposition on (a) Ag(l II) in 0.0IAfHCIO4 + lOmAf at A = 0.24 V and (b) on Ag(JOO), (From Ref, 112 with permission of Electrvchimica Acta.)...

Figure 36. An STM image of underpotential deposition Tl on Ag(l 11) in O.OIA/ HCIO4 + 5mW Tl taken at A = 0.359 V. The image size is 6.33 X 6.33 nm and the details are in the text. (From Ref. 113 with permission of Electrochimica Acta.)...

Underpotential deposition of Zn ions on polycrystalline Ag was investigated in IM KOH to form two kinds of alloys."" ... 

Equations (27) and (28) give mountainlike curves of j against t, and s against t of underpotential depositions of Pb on Ag single crystals were... 

The characteristics of monatomic height step formation ofCu under-potential deposition on Ag(l 11) in sulfuric acid solution were observed to be dependent on electrode potentials/ At more positive underpotential deposition potential regions, STM revealed the frizzy edges of Cu underpotential deposition at the comers of Cu islands growing on the Ag( 111) terrace STM. The kink site mobility was roughly estimated as 3000 nm s . Monatomic height steps on Au(l 11) electrodes did not show any sign of frizziness under the experimental condition. ... 

The overpotential three-dimensional nucleation process is affected by the change of prepolarization potential in the underpotential deposition region of Pb on Ag. When A is low (potential is close to feq), the rate of the... 

The effect of acetate, citrate, and thiourea on Pb underpotential deposition on Ag(lll) has been studied. The effects of the anionic character of the additives were discussed in relation to changes in voltammograms. The two-dimensional phase transformation was also discussed. The influence of various additives such as dimethylfluoride (DMF) and pyridine on cyclic voltammograms of Cu underpotential deposition on Pt(lll) was observed. Cu underpotential deposition on Pt(lll) was studied in the presence of crystal violet, coumarin, and hydroquinone. ... 

In this chapter, hydrogen adsorption, particularly observed on Pt electrodes, was not treated as an underpotential deposition phenomenon. However, from a theoretical point of view, it may provide a breakthrough insight into underpotential deposition. Since the underpotential deposition ofM on M is quite characteristic among different combinations of M and M, together with a change in the kind of anions in the solution, a theoretical approach, which requires simplification, is still limited, and more experimental clarification is needed for theoretical work. However, the jellium model was successfully used to describe the lattice contraction of adsorbate T1 and Pb on Ag(lll) as underpotential deposition. ... 

Beside O P D it is well known that metal deposition can also take place at potentials positive of 0. For this reason called underpotential deposition (UPD) it is characterized by formation of just one or two layer(s) of metal. This happens when the free enthalpy of adsorption of a metal on a foreign substrate is larger than on a surface of the same metal [ 186]. This effect has been observed for a number of metals including Cu and Ag deposited on gold ]187]. Maintaining the formalism of the Nernst equation, deposition in the UPD range means an activity of the deposited metal monolayer smaller than one ]183]. 

Discharge of H+ ions reveals the role played by the structure of Ag surface in the underpotential deposition (UPD) of hydrogen. Figure 4 shows that in the acidic medium, the activity of the electrode surfaces toward H+ discharge decreases in the series Ag(lll) > Ag(poli) > Ag(lOO) > Ag(llO), following the decrease in the surface density of atoms [48,189]. 

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