Metal monolayer

Peterson I R 1980 Defect density in a metal-monolayer-metal cell Aust. J. Chem. 33 1713-6... 

At potentials positive to the bulk metal deposition, a metal monolayer-or in some cases a bilayer-of one metal can be electrodeposited on another metal surface this phenomenon is referred to as underiDotential deposition (upd) in the literature. Many investigations of several different metal adsorbate/substrate systems have been published to date. In general, two different classes of surface stmetures can be classified (a) simple superstmetures with small packing densities and (b) close-packed (bulklike) or even compressed stmetures, which are observed for deposition of the heavy metal ions Tl, Hg and Pb on Ag, Au, Cu or Pt (see, e.g., [63, 64, 65, 66, 62, 68, 69 and 70]). In case (a), the metal adsorbate is very often stabilized by coadsorbed anions typical representatives of this type are Cu/Au (111) (e.g. [44, 45, 21, 22 and 25]) or Cu/Pt(l 11) (e.g. [46, 74, 75, and 26 ]) It has to be mentioned that the two dimensional ordering of the Cu adatoms is significantly affected by the presence of coadsorbed anions, for example, for the upd of Cu on Au(l 11), the onset of underiDotential deposition shifts to more positive potentials from 80"to Br and CE [72]. 

Figure 2 displays a qualitative correlation between the increase or decrease in CO desorption temperature and relative shifts in surface core-level binding energies (Pd(3d5/2), Ni(2p3/2), or Cu(2p3/2) all measured before adsorbing CO) [66]. In general, a reduction in BE of a core level is accompanied by an enhancement in the strength of the bond between CO and the supported metal monolayer. Likewise, an opposite relationship is observed for an increase in core-level BE. The correlation observed in Figure 2 can be explained in terms of a model based on initial-state effects . The chemisorption bond on metal is dominated by the electron density of the occupied metal orbital to the lowest unoccupied 27t -orbital of CO. A shift towards lower BE decreases the separation of E2 t-Evb thus the back donation increases and vice versa. 

Figure 9.16 ORR activity of two mixed-metal monolayer electrocatalysts supported on Pd(l 11), expressed as the kinetic current density at 0.85 V as a function of the M Pt ratio in the Pd-supported Pt-M monolayer. (Reproduced with permission from Zhang et al. [2005b].)...

Brankovic SR, Wang JX, Adzic RR. 2001a. Metal monolayer deposition by replacement of metal adlayers on electrode surfaces. Surf Sci 474 L173-L179. 

Kolb, D. M., Physical and Electrochemical Properties of Metal Monolayers on Metallic Substrates, in Advances in Electrochemistry and Electrochemical Engineering, H. Gerischer and C. W. Tobias, Editors. 1978, Wiley New York. p. 125. 

Similar increases of the surface second harmonic intensity have been observed during the formation of alkali metal monolayers on Ge and Rh surfaces in vacuo (10-11). ... 

Properties of 3d Transition Metal Monolayers on Metal Substrates. 

Work Function Changes Following Methanol Adsorption on Clean Metals Monolayer Coverages)... 

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]. 

The composition and the structure of electrodeposited metallic monolayers have always been a subject of intensive electrochemical investigations. However, the sources of information were limited by the number of available electrochemical methods. The UPD process on singlecrystal surfaces (including Ag) has been summarized by Jtlttner and Lorenz [299]... 

In Table 5.1 the surface structures of ordered metal monolayers adsorbed on metal surfaces are listed. For each substrate, the crystallographic structure, the distance between nearest neighbors, and the heat of sublimation (that is proportional to the surface free energy) are given. For each metal adsorbate the identical information is provided along with the technique of deposition and all the ordered surface structures that form with increasing coverage. 

The available data are inadequate to permit a detailed analysis of the various factors that control the ordering of metal monolayers on metal surfaces. It is likely that both the electronic interaction between the two metals and the relative atomic sizes should be important in determining the nature of ordering in the monolayer. 

Table 5.1. Adsorption properties of metal monolayers on metal substrates. The clean substrate properties are also given for comparison. Substrates are ordered by lattice type (fee, bcc, hep, cubic, diamond and rhombic). The structures, nearest neighbor distances and heats of vaporization refer to the bulk material of the substrate or the adsorbate. VD, ID and S stand for vapor deposition, ion beam deposition and surface segregation, respectively. TD, WF and TED stand for thermal desorption, work function measurements and transmission electron diffraction, respectively...

Wolstenholme, G. A., and Schulman, J. H. Metal-monolayer interactions in aqueous systems. Part I. The interaction of monolayers of longchain polar compounds with metal ions in the underlying solution. Trans. Faraday Soc. 46, 475—487 (1950). 

Despite these potential difficulties, Vuillaume and co-workers have reported electrical characteristics of octadecyl (Cig) monolayers on Si(lll) using evaporated A1 contacts [25,78]. These metal/monolayer/silicon junctions exhibit the expected capacitance-voltage characteristics for these types of junctions. Figure 13 shows the measured C-V curves for moderately... 

Types of electrode/solution interface studied include oxide films on metals, monolayer deposits obtained by underpotential deposition, adsorption, and spectroelectrochemistry in thin-layer cells. 

See also the main entry -+ underpotential deposition. Refs. [i] Haissinsky M (1933) J Chim Phys 30 27 [ii] Frumkin AN (1934) Zh Fiz Khimii 5 240 [iii] Kolb DM (1978) Physical and electrochemical properties of metal monolayers on metallic substrates. In Gerischer H, Tobias CW (eds) Advances in electrochemistry and electrochemical engineering, vol. 11. Wiley New York, p 125 [iv] Conway B (1984) Progr Surf Sci 16 1 [v] Ye S, Uosaki K (2003) Atomically controlled electrochemical deposition and dissolution of noble metals. In BardAJ, Stratmannn M, Gileadi E, Urbakh M (eds) Thermodynamics and electrified interfaces. Encyclopedia of electrochemistry, vol. 1. Wiley-VCH, p 471 [vi] Adzic R (2003) Electrocatalysis on surfaces modified by metal monolayers deposited at underpotentials. In Bard AJ, Stratmannn M, Gileadi E, Urbakh M (eds) Thermodynamics and electrified interfaces. Encyclopedia of electrochemistry, vol. 1. Wiley-VCH, p 561... 

Zhang, J. et al., Platinum and mixed platinum-metal monolayer fuel cell electrocatalysts design, activity and long-term performance stability, ECS Trans., 3, 31, 2006. 

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