Underpotential Deposition UPD

Metal UPD corresponds to the electrochemical adsorption, often of one mono-layer, that occurs at electrode potentials positive with respect to the Nernst potential below which bulk metal adsorption occurs [80]. Numerous experiments have shown that the UPD layer can dramatically alter the chemical and elec- 

To illustrate the complexity of structural behavior that can be observed in such systems. Fig. 1.17 summarizes X-ray diffraction results obtained during the UPD of H onto Au(lll) in the presence of bromide anions [81, 82). The top panel shows the cycHc voltammetry for Au(lll) in 0.1 M HCIO4 containing 1 mM HBr along with schematic models of the structures that were observed in 

5 mVs , except for the c(2.14x3) phase at (0.468, 0.468), where it is 0.2 mV s , started after holding the potential around -0.1 V for several hours. Coverages, in units of monolayers of the Au substrate, shown by the open and filled circles for Br and Tl, respectively, are calculated from the adlayer lattice constants (taken from Refs. [81, 82]). 

For the Pb/Pt(lll) system, some insight into the displacement mechanism was obtained by studying the temporal evolution of the Pb-(3xv/3) structure as CO was introduced to the solution. The inset to Fig. 1.18 a shows a rocking scan through the (4/6, 1/6, 0.2) position, where scattering from the (3xv/3) structure occurs. The solid line is a Lorentzian fit to the line shape which enables a coherent domain size of ca. 160 A to be calculated for this structure. The main part of Fig. 1.18 a shows the time dependence of the peak intensity after CO was introduced to the solution at r 200 s. The presence of CO in solution initially caused a large increase in the intensity due to the (3xi/3) phase. This could be due to displacement of Pb that is adsorbed on the Pt surface in defect 

2) reflection indicated an increase in integrated intensity by a factor of three and an increase in domain size to ca. 200 A. Secondly, a rocking curve at (0, 1, 

Underpotential deposition (UPD) is a process in which a monolayer of a metal is deposited on a different metal substrate, in the range of potential that is positive with respect to the reversible deposition of a metal in the same solution. Metal deposition is a reduction reaction that occurs at potentials negative with respect to the reversible potential. Thus it might appear that underpotential deposition defies the laws of thermodynamics, but careful analysis of the process shows that it does not. 

Hence the X-axis showing the potential could be replaced by one showing time, and the current under the anodic peak could be integrated to yield the corresponding 

Physical Eleclrochemistry fundamentals, Techniques and Applications. Eliezer Gileadi Copyright 2011 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31970-1 

How can a metal be deposited at a potential positive with respect to its reversible potential. Deposition of Pb on an electrode made of the same metal can be represented by the equation 

The two reactions are similar, but not identical. Equation (12.4) represents the formation of a bond between a Pb atom deposited and another Pb atom on the surface, while Eq. (12.5) represents the formation of the bond between a Pb atom deposited and an Ag atom on the surface. Hence, there is no reason a priori to assume that they should have the same reversible potential. Apparently the Pb-Ag bond is stronger than the Pb-Pb bond, and, hence, it can be formed at a less negative potential the laws of thermodynamics have not been violated  

The characteristic voltammetry features associated with UPD process are demonstrated by existence of one or more deposition (stripping) peaks in the underpotential region observed during the potential sweep in cathodic (anodic) direction (Fig. 3) The complexity of the UPD peaks is often dependent on the perfection and preparation of the substrate surface, existence of one or more UPD adlayer superstructures,  

The isotherm has a Langmuir basis and implies additivity of the various UPD adatom-substrate and adatom-adatom interactions. The AE q term represents the underpotential of the most positive stripping peak of the UPD adlayer where the UPD ML coverage approaches zero. The term is the Temkin parameter describing the UPD layer-substrate interactions such as the electrode work function change with the UPD adlayer coverage. The term is the Frumkin parameter representing the lateral adatom interactions within the UPD adlayer. 

Different computational approaches were also considered in order to describe UPD as the potential dependent adsorption. For example, the Monte Carlo (MC) simulations of Pb UPD on Ag(lll), preformed by Obretenov et al., showed that the experimental isotherms can be completely recovered from the MC simulations if the proper distribution of the adsorption energy states on substrate surface and the interaction energies among the adatoms in the UPD ML are taken into account. This work has also highlighted the importance of surface defects and strain in the UPD ML on specific voltammetry features observed experimentally. 

In some UPD systems like Pb and Cd on Ag(l 1Pb on Cu(lOO) and, T1 and Cd on Au(l 1the significant surface alloying and structural transformation 

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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. 

In a similar way, electrochemistry may provide an atomic level control over the deposit, using electric potential (rather than temperature) to restrict deposition of elements. A surface electrochemical reaction limited in this manner is merely underpotential deposition (UPD see Sect. 4.3 for a detailed discussion). In ECALE, thin films of chemical compounds are formed, an atomic layer at a time, by using UPD, in a cycle thus, the formation of a binary compound involves the oxidative UPD of one element and the reductive UPD of another. The potential for the former should be negative of that used for the latter in order for the deposit to remain stable while the other component elements are being deposited. Practically, this sequential deposition is implemented by using a dual bath system or a flow cell, so as to alternately expose an electrode surface to different electrolytes. When conditions are well defined, the electrolytic layers are prone to grow two dimensionally rather than three dimensionally. ECALE requires the definition of precise experimental conditions, such as potentials, reactants, concentration, pH, charge-time, which are strictly dependent on the particular compound one wants to form, and the substrate as well. The problems with this technique are that the electrode is required to be rinsed after each UPD deposition, which may result in loss of potential control, deposit reproducibility problems, and waste of time and solution. Automated deposition systems have been developed as an attempt to overcome these problems. 

In view of the overwhelming success of PS in surface science, it is not surprising that XPS has been used rather early for the study of electrochemically modified electrode surfaces. Winograd et al. [10-12] were the first to use this spectroscopy for the study of oxide formation on Pt electrodes and also for the investigation of metal underpotential deposition (UPD) on Pt. Although a standard surface analytical tool, XPS has not found a corresponding consideration in electrochemistry. 

Surface limited reactions are well known in electrochemistry, and are generally referred to as underpotential deposits (UPD) [83-88], That is, in the deposition of one element on a second, frequently the first element will form an atomic layer at a potential under, or prior to, that needed to deposit the element on itself. One way of looking at UPD is that a surface compound, or alloy, is formed, and the shift in potential results from the free energy of formation of the surface compound. 

When a metal is in contact with its metal ion in solution, an equilibrium potential is established commonly referred to as Nernst potential (Er). Metal deposition occurs at potentials negative of Er, and dissolution for E > Er. However, when a metal is deposited onto a foreign metal substrate, which will always be the case for the initial stages of deposition, it is frequently observed that the first monolayer on the metal is deposited at potentials which are positive of the respective Nernst potential [37, 38]. This apparent violation for Nernst s law simply arises from the fact that the interaction between deposit metal and substrate is stronger than that between the atoms of the deposit. This effect has been termed underpotential deposition (upd), to contrast deposition processes at overpotentials. (One should keep in mind, however, that despite the symmetrical technical terms the physical origins of both effects are quite different. While the reason for an overpotential is solely due to kinetic hindrance of the deposition process, is that for underpotential deposition found in the energetics of the adatom-substrate interaction.)... 

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],... 

The electrochemical atomic layer epitaxy (ECALE) technique, also known as electrochemical atomic layer deposition (EC-ALD), is based on layer-by-layer electrodeposition. Each constituent of the thin him are deposited separately using underpotential deposition (UPD) of that element. UPD is a process wherein an atomic layer of one element is deposited on the surface of a different element at a potential under that needed to deposit the element on itself. ECALE has been used to grow mainly II-VI and III-V compounds. A thorough review of ECALE research has been published by Stickney.144 A summary of the materials deposited using ECALE are given in Table 8.4, with a more detailed discussion for a few select examples given below. 

The underpotential deposition (UPD) of metals on foreign metal substrates is of importance in understanding the first phase of metal electrodeposition and also as a means for preparing electrode surfaces with interesting electronic and morphological properties for electrocatalytic studies. The UPD of metals on polycrystalline substrates exhibit quite complex behavior with multiple peaks in the linear sweep voltammetry curves. This behavior is at least partially due to the presence of various low and high index planes on the polycrystalline surface. The formation of various ordered overlayers on particular single crystal surface planes may also contribute to the complex peak structure in the voltammetry curves. 

Underpotential deposition (upd) originates from an adatom-substrate bond being formed using less energy than that required to form subsequent ada-... 

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 electrodeposition of zinc on polycrystalline Au, Pt, and tungsten electrodes was preceded by underpotential deposition (UPD) [172]. The formal potential of Zn(II)/Zn(0) couple and diffusion coefficient of Zn(II) were also determined [172]. 

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]. 

One of the subjects that is still quite intensively developed (using electrochemical methods frequently combined with nonelectrochemical techniques) concerns reduction of Hg compounds at various surfaces (e.g. Pt or Au), with the emphasis laid on underpotential deposition (UPD) of mercury. Deposition of mercury on other metals is generally important for better understanding of the mechanism of the formation of amalgams. Moreover, underpotential Hg deposition characteristics constitute a significant source of information on Hg-metal interactions. In turn, mercury film electrodes obtained by such deposition have a significant appKcation in electrochemical analysis ofvarious species. 

Bulk Pt alloys for the electrooxidation of formic acid have been less frequently studied compared to underpotential deposition (upd) modified Pt surfaces. The Pt50Ru5o surface was again found to be one of the most active Pt-Ru surfaces. Underpotentially deposited metals, such as Bi, Se, Sb, were studied as reaction modifiers for Pt surfaces and provided significant electrocatalytic activity increases. Electronic factors (ligand effects) rather than bifunctional effects were held responsible for these activity modifications, because the metal coverages that caused the activity gains were extremely small. 

The existence of a difference in electrode potential for the underpotential deposition (upd) at a given coverage, 0upd, equilibrium conditions with respect to the bulk deposition potential implies that there is a difference in the chemical potential between the upd layer and the corresponding bulk metal [14]. 

In Chapter 6 we have seen that metal M will be deposited on the cathode from the solution of M"+ ions if the electrode potential E is more negative than the Nemst potential of the electrode M/M"+. However, it is known that in many cases metal M can be deposited on a foreign substrate S from a solution of M"+ ions at potentials more positive than the Nemst potential of M/M"+. This electrodeposition of metals is termed underpotential deposition (UPD). Thus, in terms of the actual electrode potential E during deposition and the Nemst equilibrium potential (M/M"+) and their difference AE = E — (M/M"+), we distinguish two types of electrodeposition ... 

Underpotential deposition (UPD) is the electrochemical adsorption and (partial) reduction of a submonolayer or monolayer of cations on a foreign metal substrate at potentials more positive than the reversible potential of the deposited metal [141]. The UPD phenomenon is used in many fundamental and applied studies because it offers a means of controlling coverages during electrodeposition in a very concise manner. Until recently, most of the information obtained about the structure of the overlayers deposited on single crystal surfaces has come from indirect means such as current-voltage analysis or by analysis of the deposited films after transfer to a UHV chamber [141]. 

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