Study of interfacial structure

Richmond G L, Robinson J M and Shannon V L 1988 Second harmonic generation studies of interfacial structure and dynamics Prog. Surf. Sc/. 28 1-70... 

G.L. Richmond, J.M, Robinson and V.L. Shannon, Second Harmonic Generation Studies of Interfacial Structure and Dynamics, Prog. Surf Sci. 28 (1988) 1. (Review, theory and experiment investigations of a variety of solid materials in vacuo, air, solutions and in contact with other solids are considered, as well as studies at the liquid/liquid interface.)... 

G.L. Richmond, J.M. Robinson, and V.L. Shannon. Second Harmonic Generation Studies of Interfacial Structure and Dynamics. Prog. Surf. Sci. 28 1 (1988). 

The interface between aqueous solutions and metal surfaces plays an important role in many fields of science and technology including, most notably, the disciplines focused on corrosion, electrochemistry, and catalysis. Although much has been learned from many experimental and theoretical studies of these systems, a more detailed and deeper understanding of both the microscopic structure and dynamics of the constituents at the interface is still emerging. Studies of interfacial structure require the extraction of surface data. This is difficult because most systems are composed largely of bulk and bulklike particles, and most researchers use experimental methods designed to probe the microscopic structure of bulk solids and liquids. This limitation has been over-... 

Alloy surfaces and epitaxial growth. This topic covers a range of subtopics the formation and structure of surface alloy phases, even in immiscible systems the depth variation of alloy components in the near-surface region (especially oscillatory compositional variations in surface segregation) the study of interfacial structure and film perfection in metal heteroepitaxy the growth modes - i.e., island, layer-by-layer, etc. In several of these problem areas systems of potential interest for their magnetic properties are emphasized. 

In contrast to the case of lithium, where extensive efforts have been devoted to the study of interfacial phenomena, surface film composition, etc., very little is yet known about the composition and structure of the surface films covering magnesium electrodes in nonaqueous media. In addition, the mechanisms of Mg dissolution and plating in the Grignard solutions are unclear. 

Studies of interfacial reactions include analysis of both the bulk phases and their interface. Analysis means using not only qualitative and quantitative analytical methods, but also structural studies of the phases and an investigation of the chemical species present by means of thermodynamic calculations and/or experimental techniques. For the interfacial studies on rocks and soils, many different classical and novel methods can be used. In this chapter, the most important methods used for the analysis of solid, liquid, and interface will be presented. 

Monolayer and multilayer thin films are technologically important materials that potentially provide well-defined molecular architectures for the detailed study of interfacial electron transfer. Perhaps the most important attribute of these heterogeneous systems is the ease with which their molecular architecture can be synthetically varied to tailor the properties of the ensemble. Assemblies incorporating specifically designed structures can, in principle, meet the needs of a variety of technological applications and be used as models for understanding fundamental interfacial reaction mechanisms. In fact, molecular assemblies are nearly ideal laboratories for the fundamental study of electron-transfer reactions at interfaces. In this chapter, the use of monolayer and multilayer assemblies to probe fundamental questions regarding electron transfer in surface-confined molecular assemblies will be addressed. 

Summary. Scanned probe methods for imaging electrochemical deposition on surfaces are now well established. For such methods the smface structure at the atomic scale can be measured so that surface strains may be inferred. Here we demonstrate how extremely sensitive and fast stress sensors can be constructed from atomic force microscope (AFM) cantilevers for studies of interfacial processes such as adsorption and reconstruction. The surface stress sensor has submonolayer sensitivity for use in electrochemistry, whereby simultaneous cyclic voltammograms and stress changes can be recorded. This is demonstrated with measurements of the electrocapillary curve of gold, and stress changes associated with the underpotential deposition of silver on gold (111). 

Density profiles are the central quantity of interest in computer simulation studies of interfacial systems. They describe the correlation between atom positions in the liquid and the interface or surface . Density profiles play a similarly important role in the characterization of interfaces as the radial distribution functions do in bulk liquids. In integral equation theories this analogy becomes apparent when formalisms that have been established for liquid mixtures are employed. Results for interfacial properties are obtained in the simultaneous limit of infinitesimally small particle concentration and infinite radius for one species, the wall particle (e.g., Ref. 125-129). Of course, this limit can only be taken for a smooth surface that does not contain any lateral structure. Among others, this is one reason why, up to now, integral equation theories have not been able to move successfully towards realistic models of the double layer. 

It is obvious that specific adsorption will also introduce a capacitive component and should also be detectable by the study of Cd. In fact, we could anticipate that changes in the degree of specific adsorption with changes in potential would be highlighted by examining the derivative of Cd that is, dCJdE. Some of the most general approaches to the analysis of interfacial structure are based on these ideas. Since their details are beyond our scope, the interested reader is urged to pursue them in the literature. 

Most of the work in the previous sections of this chapter has dealt with mercury electrodes for the reasons discussed in Section 13.2.1. However, electrochemists are also interested in studying the interfacial structure of solids, because most electrochemical studies are carried out with solid electrodes (e.g., platinum or carbon). Such studies are difficult, because there are problems in reproducing a surface and in keeping it clean. Impurities in solution can diffuse to the electrode surface and adsorb, thereby significantly changing the interfacial properties. Moreover, the surfaces of solids, unlike those of mercury, are not atomically smooth, but have defects, such as dislocation lines, with a density of at least 10 to 10 cm. In comparison, a typical metal surface density has about 10 atoms cm. Especially important to the understanding of solid electrodes has been the use of so-called well-defined metal electrodes, that is, single crystal metals with very carefully prepared surfaces of known orientation (35). 

Of the many X-ray based techniques available, a very powerful approach for probing interfacial structures is based on the measurement of X-ray reflectivity. The X-ray reflectivity is simply defined as the ratio of the reflected and incident X-ray fluxes. In the simple case of the mirror-like reflection of X-rays from a surface or interface, i.e., specular reflectivity, the structure is measured along the surface normal direction. Lateral structures are probed by non-specular reflectivity. The measurement and interpretation of X-ray reflectivity data (i.e., the angular distribution of X-rays scattered elastically from a surface or interface) (Als-Nielsen 1987 Feidenhans l 1989 Robinson 1991 Robinson and Tweet 1992) are derived from the same theoretical foundation as X-ray crystallography, a technique used widely to study the structure of bulk (three-dimensional or 3D) materials (Warren 1990 Als-Nielsen and McMorrow 2001). The immense power of the crystallographic techniques developed over the past century can therefore be applied to determine nearly all aspects of interfacial structure. An important characteristic of X-ray reflectivity data is that they are not only sensitive to, but also specifically derived from interfacial structures. 

Heald SM, Chen H, Tranquada (1988) Glancing-angle extended X-ray-absorption fine structure and reflectivity studies of interfacial regions. Phys Rev 38 1016-1026... 

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