Underpotential oxidation

In a previous paper we described the experimental conditions needed to obtain up to 5 sulfur layers and 4 cadmium layers of CdS. Sulfur layers were obtained by oxidative underpotential deposition from sulfide ion solutions [4-6], whereas cadmium layers were obtained by reductive underpotential deposition from cadmium ion solutions [7], Both precursors were dissolved in pyrophosphate plus sodium hydroxide of pH 12. The high pH was used to shift the hydrogen evolution towards very negative potentials, in order to ev idence the w hole underpotential oxidation process of sulfide ions which takes place between -1.35 and -0.8 V/SCE. A strong complexing agent such as phyrophosphate was used to keep cadmium ions in solution at this high pH. 

Another variant, the atomic layer deposition (ALD), formerly called atomic layer epitaxy, is a process similar to CVD, but the deposition is split into two half-reactions by sequential use of usually two precursors. The first one is typically an organometaUic compound after its application and subsequent purging the second precursor, e.g., a plasma, is applied, and any by-products are purged off again. ALD is applied to deposit oxide or nitride films, occasionally sulfides. The layers may be also electrochemically deposited (E-ALD), usually with underpotential deposition to restrict reactions to a single layer at the surface and alternating reduction-oxidation cycles, fri case of cadmium sulfide a monoatomic layer of cadmium is deposited from Cd after solution exchange sulfide is deposited as CdS by underpotential oxidation of the reduced Cd°. The cycles can be repeated. 

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. 

Colletti LP, Teklay D, Stickney JL (1994) Thin-layer electrochemical studies of the oxidative underpotential deposition of sulfur and its application to the electrochemical atomic layer epitaxy deposition of CdS. J Electroanal Chem 369 145-152... 

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... 

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. 

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... 

In the following chapter examples of XPS investigations of practical electrode materials will be presented. Most of these examples originate from research on advanced solid polymer electrolyte cells performed in the author s laboratory concerning the performance of Ru/Ir mixed oxide anode and cathode catalysts for 02 and H2 evolution. In addition the application of XPS investigations in other important fields of electrochemistry like metal underpotential deposition on Pt and oxide formation on noble metals will be discussed. 

The catalytic properties of a Pt/Sn combination were observed on different kinds of electrode materials alloys [90], electro co-deposits of Pt and Sn [89, 90], underpotential deposited tin [42] or a mixture of tin oxide and platinum deposited on glass [95], All different materials present a marked influence on methanol electrooxidation. 

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. 

Reductive UPD is the major atomic layer deposition processes used in EC-ALE, Equation 1. Many metals can be obtained in a soluble oxidized form, from which atomic layers can be deposited at underpotentials. Control points are the reactant... 

Electrodes modified by underpotential deposition of metal were subjected as electrocatalysts to reduction of oxygen,oxidation of formic acid, and other processes in which polycrystalline metal substrates were used (see review in Ref. 151). Electrocatalysis of single-crystal electrodes modified by underpotential deposition was also investigated, as reviewed by Ad2iC. ... 

Since a well-defined condition of the underpotential deposition metal at a definite coverage is better for studying its modified surface at a definite coverage, an irreversibly adsorbed underpotential deposition metal is desirable Sb on Pt(lll) for CO oxidation,Bi and Te on Pt(lOO) for formic acid oxidation, " Sb + Bi on Pt(lOO) for formic acid oxidation, and others found in these references. 

In addition, the adsorption of halides from solution on metal surfaces can be thought of as UPD Cl [297], Br [298-300], and I [299,301-304], Normally, UPD is considered a precursor to the formation of a bulk deposit of an element. Bulk deposits of the halides are generally soluble, but the first atomic layer is formed at an underpotential. Recent studies have indicated that oxidative UPD of As can be performed as well [252,253]. 

A special case is when the electrochem-ically active components are attached to the metal or carbon (electrode) surface in the form of mono- or multilayers, for example, oxides, hydroxides, insoluble salts, metalloorganic compounds, transition-metal hexacyanides, clays, zeolites containing polyoxianions or cations, intercalative systems. The submonolayers of adatoms formed by underpotential deposition are neglected, since in this case, the peak potentials are determined by the substrate-adatom interactions (compound formation). From the ideal surface cyclic voltammetric responses, E° can also be calculated as... 

Unlike anions that specifically adsorb at electrodes, cations normally do not lose their solvation shell due to their smaller size and are electrostatically adsorbed at electrodes at potentials negative to the pzc. However, depending on the affinity with the foreign substrate, cations can be reduced to a lower oxidation state or even discharged completely to the corresponding metal atom at the sub-monolayer or monolayer level at potentials positive to the equilibrium Nernst potential for bulk deposition. This deposition of metal atoms on foreign metal electrodes at potential positive to that predicted by the Nernst equation for bulk deposition has been called underpotential deposition and has been extensively investigated in recent years. Detailed discussion of the... 

Underpotential deposited layers have a strong effect on the electro-catalytic properties of electrodes for surface-sensitive reactions such as organic oxidations, hydrogen evolution, oxygen reduction, etc. A review on this subject has recently been published by Adzic [131a, b]. 

In the case of oxides or sulphides, poisoning has been observed to be very weak [99,168]. One reason is that these materials, especially oxides, are usually obtained as porous layers [169, 170]. But another, more exciting explanation is that metal deposition on non-metallic surfaces is more difficult than on metals, since the bond formed between the surface and deposited metal atoms is weaker [168], Therefore, no underpotential deposition takes place, which pushes the potential range of impurity deposition to more cathodic potentials. If deposition takes place, it occurs at a high overpotential, and the resulting discharged particles form clusters rather than monolayers, thus leaving most of the active surface uncovered. 

Oxide electrodes have been observed to be almost immune from poisoning effects due to traces of metallic impurities in solution [99]. This is undoubtedly due primarily to the extended surface area. It can be anticipated that the calcination temperature must have a sizable effect. But in addition, a different mechanism of electrodeposition must be operative. Chemisorption on wet oxides is usually weak because metal cations are covered by OH groups [479]. As a consequence, underpotential deposition of metals is not observed on Ru02, although metal electrodeposition does takes place. However, electrodeposited metals give rise to clusters or islands and not to a monomolecular layer like on Pt. Therefore, the oxide active surface remains largely uncovered even if metallic impurities are deposited [168]. Thus, the weak tendency of oxides to adsorb ions, and its dependence on the pH of the solution is linked to their favorable behavior observed as cathodes in the presence of metallic impurities. 

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