Monolayer formation underpotential deposition

The electrodeposition of alloys at potentials positive of the reversible potential of the less noble species has been observed in several binary alloy systems. This shift in the deposition potential of the less noble species has been attributed to the decrease in free energy accompanying the formation of solid solutions and/or intermetallic compounds [61, 62], Co-deposition of this type is often called underpotential alloy deposition to distinguish it from the classical phenomenon of underpotential deposition (UPD) of monolayers onto metal surfaces [63],... 

One way to view UPD is as formation of a surface compound. In other words, deposition of the first atomic layer of an element on a second element involves a larger deposition driving force than subsequent layers, as it benefits from the AG of compound formation. For deposits formed at underpotential, once the substrate is covered the deposition stops because the reaction is surface limited. No more of the substrate element is available to react, unless it can quickly diffuse to the surface through or around the initially deposited monolayer (an example would be amalgam formation at a mercury electrode surface). Subsequent deposition is then only observed when the bulk deposition potential has been exceeded. 

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. 

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]. 

Pitner and Hussey studied the electrochemistry of tin in acidic and basic AICI3/I-ethyl-3-methyl-imidazolium chloride-based ionic liquids by using voltammetry and chronoamperometry at 40 °C [15]. They reported that the Sn(II) reduction process is uncomplicated at a platinum substrate, where in the atidic ionic liquid the reduction wave was observed at +0.46 V on the Pt electrode and the oxidation at +0.56 V. When they used a gold electrode instead of platinum, they observed an underpotential deposition of a tin monolayer and an additional underpotential deposition process that was attributed to the formation of tin-gold alloy at the surface. The deposition of tin on glassy carbon was controlled by nudeation. 

Noble metal electrodes include metals whose redox couple M/Mz+ is not involved in direct electrochemical reactions in all nonaqueous systems of interest. Typical examples that are the most important practically are gold and platinum. It should be emphasized, however, that there are some electrochemical reactions which are specific to these metals, such as underpotential deposition of lithium (which depends on the host metal) [45], Metal oxide/hydroxide formation can occur, but, in any event, these are surface reactions on a small scale (submonolayer -> a few monolayers at the most [6]). 

Cathodic deposition of magnesium from various chloride melts on different substrates has been studied by several authors [288-290], In dilute solutions of Mg(II) species the cathode process has been found to be controlled by diffusion of the reactant. Alloy formation has been observed on platinum, as reported by Tunold [288] and Duan et al. [290], The rate constant of the charge transfer process on a Mg/Ni electrode in molten NaCl-CaCl2-MgCl2 was reported by Tunold to have a value of about 0.01 cm s 1. This author also reported underpotential deposition of a monolayer on iron electrodes, at potentials approximately 100 mV positive to the Mg deposition potential. 

The high mass sensitivity of ETSM sensors renders them particularly suited for the analysis of monolayer and submonolayer films. In fact, the earliest applications of the ETSM involved studying the electrochemical deposition of monolayers, including the formation of metal oxides [207], electrosorption of halides [208], and the underpotential deposition of metal atoms [209-213]. In some cases, the electrovalency (i.e., the ratio of moles of electrons transferred at the electrode to moles of adsorbate deposited) was found to vary with adsorbing species the adsorption of iodide onto gold, for example, occurs with complete charge transfer from the halide to the electrode, whereas the adsorption of bro-... 

Another important area in which X-ray surface scattering is applied is the underpotential deposition of metals. Underpotential deposition is the phenomenon by which one metal deposits on another at a potential positive of its normal reduction potential. For example, Pb deposits at underpotentials on Ag. This is due to the fact that the Gibbs energy for formation of a Pb-Ag bond is less than that for formation of a Pb-Pb bond. Other metals which undergo underpotential deposition on Ag, Au, and Pt are T1 and Bi. On the basis of the electrochemistry observed in formation of the metal monolayers, there is good reason to expect that they are well ordered. Tl, Pb, and Bi all form an incommensurate monolayers on Au(lll). On Au(lOO), Tl and Bi form an incommensurate monolayer with a c(2 X 2) surface structure [14]. On the other hand, underpotential deposition of Pb on Ag(lll) leads to an incommensurate monolayer [13]. These studies demonstrate clearly that the nature of the monolayer formed depends on both the nature and structure of the substrate metal. 

The electrochemical form of ALE makes use of underpotential deposition (UPD). the electrochemical phenomena where an atomic layer of one element frequently deposits on a second element at a potential prior to (under) that needed to deposit the element on itself fhe driving force for UPD can be thought of as resulting from the free energy of formation of a surface compound. These surface limited reactions are then used in a deposition cycle, where atomic layers of each element are deposited in turn, in order to form a monolayer of the deposit. The number of cycles performed detennines the number of compound monolayers and the thickness of the deposit. One of the main advantages of this methodology is that the electrochemical formation of a compound is broken down into a series of individually addressable steps. Each step in the cycle becomes a point of control over the deposition process. 

Summary. The potential of in-situ scanning probe techniques for the local investigation of surface properties and reactions at "nonideal" electrodes is presented in a typical example in the field of metal underpotential deposition, the essential role of the step dislocations for the local progress of adsorbate formation and also for the longterm adsorbate stability is shown and discussed for the adsorption of Pb and TI monolayers at stepped Ag(l 11) electrodes. 

In earlier voltammetric experiments [17] it has been found that Tl underpotential deposition occurs in two distinctly separated potential intervals that have been associated with the successive formation of two monolayers prior to Tl bulk deposition, whereby the voltammogram in the more anodic potential range (assigned to the formation of a first monolayer) exhibits a very similar splitting into three distinct peaks Al/Dl, A2/D2, A3/D3 as observed in the system Pb/Ag(l 11) (see Fig. 2). 

Recently, the structure of the solid/liquid interface has been studied with a wide range of in-situ structural techniques. In particular, scanned probe microscopes [1-5] and synchrotron-based methods [6-9] have yielded a wealth of structural information. The ultimate goal of this work is an understanding of the structure and reactivity of the electrode surface at the atomic level. One of the most extensively studied processes is metal underpotential deposition (UPD) [10], which involves the formation of one or more metal monolayers at a potential positive of the reversible Nemst potential for bulk deposition. 

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). 

The surface structure of support metal exerts a significant influence on the underpotential deposition. This phenomenon is similar to that observed for hydrogen adsorption on various noble metal surfaces. The similarity is evident as no difference can be expected between the reactions +e => and any Me" -t-e" <=> Megds- Striking differences were found, for instance, for the underpotential deposition of Cu on Pt(lll), Pt(llO), and Pt(100) crystal faces. In some cases, two (or more) distinct steps in the monolayer formation can be observed. The multistep adsorption is considered as evidence for ordered adsorption and a proof of this assumption can be obtained by ex situ electron diffraction experiments. 

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. 

A main field of activities is focused on structure and reactivity in two-dimensional adlayers at electrode surfaces. Significant new insights were obtained into the specific adsorption and phase formation of anions and organic monolayers as well as into the underpotential deposition of metal ions on foreign substrates. The in situ application of structure-sensitive methods with an atomic-scale spatial resolution, and a time resolution up to a few microseconds revealed rich, potential-dependent phase behavior. Randomly disordered phases, lattice gas adsorption, commensurate and incommensurate (compressible and/or rotated) stmctures were observed. Attempts have been developed, often on the basis of concepts of 2D surface physics, to rationalize the observed phase changes and transitions by competing lateral adsorbate-adsorbate and adsorbate-substrate interactions. 

This chapter concerns the state of development of electrochemical atomic layer epitaxy (EC-ALE), the electrochemical analog of atomic layer epitaxy (ALE). EC-ALE is being developed as a methodology for the electrodeposition of compound semiconductors with nanoscale control. ALE is based on the formation of compounds, one monolayer (ML) at a time, using surface-limited reactions. An atomic layer of one element can be electrodeposited at a potential under that needed to deposit the element on itself, and this process is referred to as underpotential deposition (UPD). EC-ALE is the use of UPD for the surface-limited reactions in an ALE cycle. 

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