Electrochemical techniques metal-oxide interface

WILLIAM H. SMYRL is Professor of Chemical Engineering and Materials Sciences and Associate Director of the Center for Corrosion Research at the University of Minnesota. He received his Ph.D. (chemistry) at the University of California, Berkeley, and spent 3 years at the Boeing Scientific Research Laboratories and 11 years at Sandia National Laboratories. He joined the faculty of the University of Minnesota in 1984. His research interests are modeling of corrosion processes, in situ techniques for metal-metal oxide interface studies, digital impedance for faradaic analysis, stress corrosion cracking, polymer-metal interfaces, and electrochemical processes. 

The scanning Kelvin probe, which measures the Volta potential difference between a specimen and the calibrated sensing probe, is introduced as the only electrochemical technique which allows nondestructive, real-time measurements of electrode potentials at adhesive/metal oxide interfaces in situ, even if they are covered with an adhesive layer. 

Principles and Characteristics Voltammetric methods are electrochemical methods which comprise several current-measuring techniques involving reduction or oxidation at a metal-solution interface. Voltammetry consists of applying a variable potential difference between a reference electrode (e.g. Ag/AgCl) and a working electrode at which an electrochemical reaction is induced (Ox + ne ----> Red). Actually, the exper-... 

On the other hand, the progress of wet-processes as preparative techniques of metal oxide films has been remarkable. The so-called soft solution process that provides oxide layers by means of electrochemical oxidation of a metal surface is expanding as a synthetic method of various mixed metal oxides with controlled thickness [2], The two-dimensional (2D) sol-gel process based on the hydrolysis of metal alkoxides at the air/water interface has been reported as a preparative technique of ultrathin oxide films (Fig. 6.1a) [3]. It is also known that LB films of metal complexes of long-chain alkyl carboxylic acid can be converted to metal oxide films after removal of organic component by oxygen plasma [4] and UV-ozone treatments (Fig. 6.1b) [5]. Preparation of metal oxide... 

Minerals, electrochemistry of — Many minerals, esp. the ore minerals (e.g., metal sulfides, oxides, selenides, arsenides) are either metallic conductors or semiconductors. Because of this they are prone to undergo electrochemical reactions at solid solution interfaces, and many industrially important processes, e.g., mineral leaching and flotation involve electrochemical steps [i-ii]. Electrochemical techniques can be also used in quantitative mineral analysis and phase identification [iii]. Generally, the surface of minerals (and also of glasses) when in contact with solutions can be charged due to ion-transfer processes. Thus mineral surfaces also have a specific point of zero charge depending on their sur-... 

Preparing a clean surface is often a prerequisite for surface-science studies. UHV-based methods of sample preparation and characterization are established, and these may be exploited for studies of surfaces immersed in solution by interfacing an electrochemical cell with an UHV chamber. Samples can then be transferred from UHV and immersed into electrolyte solution under a purified-Ar atmosphere. However, even under these clean conditions, some metals oxidize or get contaminated prior to immersion. Other techniques for the preparation of clean surfaces that do not require UHV techniques are available for some metals. For example, flame annealing and quenching have been successfully used, but this procedure is probably limited to Au, Pt, Rh, Pd, Ir, and Ag substrates. In this technique, substrates are annealed in an oxygen flame and quenched in pure water. 

For the investigation of adsorption/desorption kinetics and surface diffusion rates, SECM is employed to locally perturb adsorption/desorption equilibria and measure the resulting flux of adsorbate from a surface. In this application, the technique is termed scanning electrochemical induced desorption (SECMID) (1), but historically this represents the first use of SECM in an equilibrium perturbation mode of operation. Later developments of this mode are highlighted towards the end of Sec. II.C. The principles of SECMID are illustrated schematically in Figure 2, with specific reference to proton adsorption/desorption at a metal oxide/aqueous interface, although the technique should be applicable to any solid/liquid interface, provided that the adsorbate of interest can be detected amperometrically. 

The origin of NEMCA effects is also studied, and it is reported that the surface modification of reactant by ion back spillover of an effective double layer at the metal-gas interface is strongly related with unique improvement in reaction rate and selectivity [5, 7], In situ work function measurements are performed by the Kelvin probe technique [7] or UPS [8] for explanation of NEMCA effects. These measurements show that over a wide range of temperatures, work function of metal catalyst linearly changes with increasing potential and the supplied ion species like oxide ion, proton, or Na" spiU over [9] on the metal catalyst to form electric double layer. Schematic image of electrochemical modification is shown in Fig. 3 for the case of oxide ion conductor, in which 8 value is not still determined yet [5]. 

At noble metals, the growth of submonolayer and monolayer oxides can be studied in detail by application of electrochemical techniques such as cyclic-voltammetry, CV 11-20) and such measurements allow precise determination of the oxide reduction charge densities. Complementary X-Ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), infra-red (IR) or elUpsommetry experiments lead to elucidation of the oxidation state of the metal cation within the oxide and estimation of the thickness of one oxide monolayer 12,21-23), Coupling of electrochemical and surface-science techniques results in meaningful characterization of the electrified solid/liquid interface and in assessment of the relation between the mechanism and kinetics of the anodic process under scrutiny and the chemical and electronic structure of the electrode s surface 21-23). 

Ehiring corrosion (oxidation) process, both anodic and cathodic reaction rates are coupled together on the electrode surface at a specific current density known ds icorv This is an electrochemical phenomenon which dictates that both reactions must occur on different sites on the metal/electrolyte interface. For a uniform process under steady state conditions, the current densities at equilibrium are related as o = — c = ieorr Ecorr- Assume that corrosion is uniform and there is no oxide film deposited on the metal electrode surface otherwise, complications would arise making matters very complex. The objective at this point is to determine both Ecorr and icorr either using the Tafel Extrapolation or Linear Polarization techniques. It is important to point out that icorr cannot be measured at Ecorr since ia = —ic and current wfll not flow through an external current-measuring device [3]. 

The development of infrared reflection-absorption spectroscopy to study gas-phase/solid interface started as a necessary step to avoid the practical limitations imposed by the use of oxide-supported metals [20]. This improvement opened the possibility of studying adsorbed species on well-defined metal surfaces, from which a considerable knowledge of the vibrational properties at the gas-phase/metal interface has been gained [21]. This information from ultrahigh vacuum (UHV) systems provides the basis for the application of the infrared technique to studying the (more complex) electrochemical interface. 

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