Silver underpotential deposition

Our work was generously supported by the Materials Science Center at Cornell University, the National Science Foundation, the Office of Naval Research, the Army Research Office, and the Dow Chemical Company. Special thanks to Dr. Michael J. Bedzyk (CHESS) and to Dr. James H. White as well as to Michael Albarelli, Mark Bommarito, Dr. Martin McMillan, and David Acevedo. The work on the copper and silver underpotentially deposited on gold... 

Based on the experimental conditions the gold electrode is most likely covered with underpotential deposited (upd) silver. Consequently the value of iip c should be compared with the corresponding value for a silver electrode. 

Fedchenfeld, H. and Weaver, M.J. (1989) Binding of alkynes to silver, gold, and underpotential deposited silver electrodes as deduced by surface-enhanced Raman spectroscopy. The Journal of Physical Chemistry, 93, 4276—4282. 

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. 

For the case of surface truncation rods, the technique is based on the detection of diffraction peaks between Bragg peaks. Although this requires careful alignment and some a priori knowledge of the structure, monolayer sensitivity can be achieved. In fact, Samant et a/.138 have recently performed an in situ surface diffraction study of lead underpotentially deposited on silver employing this technique along with grazing incidence diffraction. It is clear that this technique will also find widespread use in the near future. 

As an example [13] we consider the underpotential deposition of thallium on silver (Fig. 15.13). At potentials above the onset of the upd of thallium the SHG signal decreases, at first slowly, then more rapidly. The adsorption of thallium causes a strong rise in a(o ), because the region in which the electronic density decays to zero becomes more extended with an angle of incidence of 45° this shows up as a drastic increase in the signal. A similar behavior is seen in other systems, and often even fractions of a monolayer can be detected. 

Figure 15.13 Cyclic voltammogram (top) and SHG signal for the underpotential deposition of thallium on silver The letters in the voltammogram denote various adsorption (A) and desorption (D) peaks. Reprinted with permission from Ref. 13.

Lead Underpotential Deposition on Silver Single Crystals... 

Figure 2. Voltammograms for the underpotential deposition of lead on silver single crystal surfaces.

The authors acknowledge very helpful discussions with Dr. R. Adzic of the Institute of Electrochemistry, Belgrade, concerning the underpotential deposition of lead on single crystal silver substrates after chemical polishing. The authors also acknowledge support of the research by the U.S. Office of Naval Research. 

We have previously employed such techniques in the study of iodide adsorption onto Pt(lll) electrodes (2 5 > as well as in the in-situ structural characterization of underpotentially deposited copper and silver on Au(lll) electrodes. (23.)... 

Effect of Underpotentially Deposited Lead on the Surface-Enhanced Raman Scattering of Interfacial Water at Silver Electrode Surfaces... 

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. ... 

The underpotential deposition of T1 on Cu(l 11) has been examined by Shannon et al. [133] under static conditions, and Richmond and Robinson in the time domain [117]. A large mismatch between the copper and the thallium lattice constants makes it an interesting case for comparison with the Tl/Ag(lll) SH study described above [122] where the lattice constants are more closely matched and the overlayer has been shown to form in an ordered manner. From strictly geometrical considerations, the silver lattice is able to accommodate a thallium overlayer without buckling. On the copper substrate, a close-packed thallium overlayer must be either buckled or rotated with respect to the substrate lattice. 

N. Kimizuka, K. Itaya, In situ scanning tunnelling microscopy of underpotential deposition—silver adlayers on Pt(lll) in sulfuric acid solutions. Faraday Disc. 1992 (94), 117-126. 

A unique feature observed in metal deposition on a foreign substrate is underpotential deposition. It is found that a metal can be deposited on a foreign substrate at potentials positive with respect to the reversible potential for deposition of the metal in the same solution. Considering, for example, the deposition of silver on platinum, which can be written as... 

Underpotential deposition, as defined by Eq. (6), should be inherently terminated when a full monolayer has been formed, since the second layer is no longer deposited on the substrate. This is usually the case, but in certain instances (notably, for UPD of Ag on Pt) as much as two atomic layers can be deposited before the reversible potential is reached and bulk deposition takes over. This observation can be rationalized, considering that the properties of Ag atoms in the first layer on top of a Pt surface could be quite different from those in the bulk of silver. In other words, the surface of Pt below a single atomic layer of Ag could influence the chemical properties of this layer and its energy of bonding to the second layer. The effect is, however, short ranged, and it is not expected to extend further. 

Melroy and co-workers have obtained the EXAFS spectrum of Pb underpotentially deposited on a silver (111) electrode. In this case, no Pb/Ag... 

The most thoroughly characterized surface diffraction study at a single crystal surface is that of underpotentially deposited lead on a silverfl 11) single crystal electrode (epitaxial film on mica). " These investigators began by measuring the CTR of the silver substrate since, unless these were clearly observed, it would be unlikely that diffraction from an electrodeposited monolayer would be detected. 

Having characterized the substrate, they then investigated the structure of a monolayer of lead underpotentially deposited on the silver(l 11) electrode surface. Figures 42a and b, respectively, show radial and azimuthal scans from a lead monolayer underpotentially deposited on a Ag(lll) electrode surface at a potential of —0.40 V vs. Ag/AgCl. That the diffraction arises from the lead monolayer was conclusively demonstrated by the fact that at an applied potential of 0.0 V vs. Ag/AgCl, where the monolayer is stripped, no diffraction peaks were observed (Fig. 42c). For the deposited monolayer, diffraction peaks spaced at 60° intervals were observed, consistent with the anticipated hexagonal lead overlayer. From these measurements, it was concluded that the electrodeposited lead forms a hexagonal monolayer that is incommensurate with the silver substrate and that at this potential, the Pb-Pb distance is 3.45 A. 

Figure 47. Difference diffractograms from roughened silver electrodes. (A) Potential modulation between — 0.35 and — 1.05 V (bulk Tl deposition). (B) Potential modulation between — 0.25 and — 0.70 V (underpotential deposition of two Tl monolayers). (C) Potential modulation between — 0.25 V and 0.40 V (double-layer region). (From Fleischmann, M., and Mao, B. W., J. Electroanal. Chem. 247, 297 (1988), with permission.)...

Underpotentially deposited ultra-thin layers of silver or copper on gold substrates are also suitable for monolayer assembly. Monolayers prepared on such surfaces are different from those that form on the parent bulk metal surfaces and show enhanced stability against thermal desorption or exchange with thiol-containing solutions91 92. 

Summary. Scanned probe methods for imaging electrochemical deposition on surfaces are now well established. For such methods the smface structure at the atomic scale can be measured so that surface strains may be inferred. Here we demonstrate how extremely sensitive and fast stress sensors can be constructed from atomic force microscope (AFM) cantilevers for studies of interfacial processes such as adsorption and reconstruction. The surface stress sensor has submonolayer sensitivity for use in electrochemistry, whereby simultaneous cyclic voltammograms and stress changes can be recorded. This is demonstrated with measurements of the electrocapillary curve of gold, and stress changes associated with the underpotential deposition of silver on gold (111). 

The underpotential deposition of sulfur on Ag(l 11) is just the first step towards the realization of mono- and multilayers of semiconducting sulfides, such as CdS and PbS, by the alternated underpotential deposition of S and Cd (or Pb) atomic monolayers on the low-index faces of silver. This procedure, cdled electrochemical atomic layer... 

The deposition begins at potentials more positive than values where deposition of R occurs on bulk R. Consider, for example, the deposition of Ag on a 1-cm Pt electrode from a 0.01-L solution containing 10 M Ag". Let A = 1.6 X 10 cm and yo = yR. The potential for deposition of one-half of the silver (which forms about 0.05 monolayer) is = 0.35 V, compared to E = 0.43 V required for the same amount of deposition on a silver electrode. Deposition at potentials before that predicted by the Nernst equation with R = 1 is called underpotential deposition. The situation is much more complicated than the above treatment suggests, since the deposition potential depends on the nature of the substrate (material and pretreatment) and on adsorption of O. Also, the treatment assumes that formation of a second layer does not start until the first is complete. However, this is frequently not the case atoms of metal will often aggregate, rather than deposit on a foreign surface, and dendrites will form. Reviews on the nature of underpotential deposition and the deposition of solids in general are available (6-10). 

Finally, it should be mentioned that by the presence of certain additives the underpotential deposition process can be inhibited. Upd of copper on Pt(lll), Pt(lOO), and Pt(llO) can be inhibited by thiourea and dithiadecyldisodium sulfonate. The results of a study on the effects of organic adsorbates on the underpotential deposition of silver on Pt(lll) electrode show that the presence of coadsorbates (2,2-bipiridyl, 4-mercapto-pyridine, etc.) can have a pronounced effect on the underpotential deposition. It has been found that adsorbates that bind primarily through a ring nitrogen atom inhibit the second, but not the first, silver monolayer. In contrast, the sulfur-containing adsorbates inhibit the formation of the first monolayer owing to the formation of the Pt-S bond. 

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