Surface characterization techniques

Surface spectroscopy is the most common tool used to characterize and analyze surfaces. Assuming that a technique of sufficient sensitivity can be found, another major problem that needs to be addressed in surface spectroscopy is distinguishing between signals from the surface and the bulk of the sample. It is important that the spectroscopic technique chosen be surface specific. The technique must be able to distinguish between signals from the bulk and the surface phase. To illustrate this, we shall look at one way in which surface specificity can be achieved that makes use of the special properties of low energy electrons. It is an approach employed in common surface spectroscopic techniques such as Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS). 

Most analytical techniques used in chemistry measure all the atoms within a typical sample. By contrast, a surface-specific technique is more sensitive to those atoms which are located near the surface than it is to atoms in the bulk which are well away from the surface. Electron spectroscopic techniques are not completely surface specific. 

Most of the signal comes from within a few atomic layers of the surface but a small part comes from much deeper into the material. AES and XPS are really surface-sensitive techniques. 

To solve this problem, consider only the electrons emitted normal to the surface. The probability of escape from a given depth, P d), is determined by the likelihood of the electron not being inelastically scattered before being emitted from the surface. The probability will decrease the deeper into the sample from which the electron is emitted. In order to quantify this, the inelastic scattering process must be examined in detail. 

Let us now consider many sources of electrons uniformly distributed at all distances from the surface of the solid and detect those unscattered electrons which emerge normal to the surface. What sort of distribution of the depths of these electrons can be detected This new function, P d), will have the same exponential form as P d) since the detection of electrons from different depths in the solid is directly proportional to the probability of electron escape from each depth. What percentage of electrons will have come from within a distance of one IMFP from the surface Recall from our definition of probability that this is simply the integral between the limits of 0 and 1 in the exponential function divided by the integral over all space 

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It is now a practice to use a variety of surface characterization techniques in the study of chemisorption and catalysis. The examples given here are illustrative most references in this section as well as throughout the chapter will contain results from several techniques. 

Although the teclmiques described undoubtedly provide valuable results on various materials, the most useful infonuation almost always comes from a combination of several (chemical and physical) surface characterization techniques. Table B1.25.1 gives a short overview of the techniques described in this chapter. 

The development of modern surface characterization techniques has provided means to study the relationship between the chemical activity and the physical or structural properties of a catalyst surface. Experimental work to understand this reactivity/structure relationship has been of two types fundamental studies on model catalyst systems (1,2) and postmortem analyses of catalysts which have been removed from reactors (3,4). Experimental apparatus for these studies have Involved small volume reactors mounted within (1) or appended to (5) vacuum chambers containing analysis Instrumentation. Alternately, catalyst samples have been removed from remote reactors via transferable sample mounts (6) or an Inert gas glove box (3,4). 

SURFACE CHARACTERIZATION TECHNIQUES EX SITU VERSUS IN SITU... 

XPS has typically been regarded primarily as a surface characterization technique. Indeed, angle-resolved XPS studies can be very informative in revealing the surface structure of solids, as demonstrated for the oxidation of Hf(Sio.sAso.5)As. However, with proper sample preparation, the electronic structure of the bulk solid can be obtained. A useful adjunct to XPS is X-ray absorption spectroscopy, which probes the bulk of the solid. If trends in the XPS BEs parallel those in absorption energies, then we can be reasonably confident that they represent the intrinsic properties of the solid. Features in XANES spectra such as pre-edge and absorption edge intensities can also provide qualitative information about the occupation of electronic states. 

Colloidal nanoparticles can be employed as heterogeneous catalyst precursors in the same fashion as molecular clusters. In many respects, colloidal nanoparticles offer opportunities to combine the best features of the traditional and cluster catalyst preparation routes to prepare uniform bimetallic catalysts with controlled particle properties. In general, colloidal metal ratios are reasonably variable and controllable. Further, the application of solution and surface characterization techniques may ultimately help correlate solution synthetic schemes to catalytic activity. 

Current surface characterization techniques fall into two broad categories those that focus on the outermost few layers (to within the 10-20 layer boundary) and those whose focus includes components present to several thousand angstroms into the solid (hundred to several hundred layers). 

Sections 13.2.1 through 13.2.5 provide a brief description of some of these modern surface characterization techniques. 

Since the thickness and properties of the interphase strongly influence the characteristics of composites and the strength of the interaction determines the dominating micromechanical deformation process, many attempts have been made to characterize them quantitatively. Many various techniques are used for this purpose, and it is impossible to give a detailed account here. As a consequence a general overview of the most often used techniques is given with a more detailed account of some specific methods which have increased importance. A more detailed description of the surface characterization techniques can be found in a recent monograph by Rothon [15],... 

Inspired by these Surface Science studies at the gas-solid interface, the field of electrochemical Surface Science ( Surface Electrochemistry ) has developed similar conceptual and experimental approaches to characterize electrochemical surface processes on the molecular level. Single-crystal electrode surfaces inside liquid electrolytes provide electrochemical interfaces of well-controlled structure and composition [2-9]. In addition, novel in situ surface characterization techniques, such as optical spectroscopies, X-ray scattering, and local probe imaging techniques, have become available and helped to understand electrochemical interfaces at the atomic or molecular level [10-18]. Today, Surface electrochemistry represents an important field of research that has recognized the study of chemical bonding at electrochemical interfaces as the basis for an understanding of structure-reactivity relationships and mechanistic reaction pathways. 

Table 3. Comparison of primary elemental surface characterization techniques used to determine the locus of failure in adhesion systems 159). (Reprinted from Ref. 59, p. 136, by courtesy of Plenum Press)...

Surface characterization techniques, such as photoelectron spectroscopy, can be used to verify the quality of the surface of such layers. 

Surface and interfacial phenomena of importance in mineral processing are reviewed. Examples of a fundamental and an applied nature are taken from the recent literature to illustrate how the use of several different surface characterization techniques makes it possible to delineate a detailed molecular-scale picture of interfaces. Lack of... 

In mineral processing, surface characterization techniques are used primarily to study mechanisms of various subprocesses. These studies are carried out mostly in research laboratories using model systems so as to keep the system simple and amenable to interpretation by known laws of physics and chemistry. For these very reasons, some of the newer surface characterization techniques have been used to investigate pure solids, often single crystals. In mineral processing operations, one necessarily deals with particles of complex ores with an objective to recover the valuable minerals contained in the ore. Experience, both in industry and laboratory, shows that complex ore particles behave differently from simple solids in many ways. In process evaluation and in optimization of operating plants, it is necessary to characterize the ore particles as they undergo various treatments. In recent years ESCA has been found to be a useful technique for... 

The effects of conditioning layers of two important blood serum proteins, albumin and fibrinogen were investigated. Protein adsorption was studied using bovine serum albumin (BSA) and fibrinogen (F) from Sigma. The samples were incubated for 3 h at 37°C in solutions of albumin (1 mg/mL) and fibrinogen (0.2 mg/mL) prepared in phosphate buffered saline (PBS, 0.01 M phosphate buffer, 0.0027 M KC1, 0.137 MNaCl, pH 7.4). After the incubation period, the samples were rinsed 3 times with PBS and analyzed by the various surface characterization techniques. 

Boujday S, Methivier C, Beccard B, Pradier C-M (2009) Innovative surface characterization techniques applied to immunosensor elaboration and test comparing the efficiency of Fourier transform-surface plasmon resonance, quartz crystal microbalance with dissipation measurements, and polarization modulation-reflection absorption infrared spectroscopy. Anal Biochem 387 194-201... 

In the following sections an account of the origin and measurement of electroosmosis is elicited, Furthermore, it is shown how to employ its measurement as a characterization technique. The discussion will focus on the measurement of electro-osmosis in cylindrical chambers and in a novel rectangular chamber whereby electro-osmosis can be measured at small sample plates. Examples of using the measurement of electro-osmosis as a surface characterization technique are discussed in terms of interpretation of the source of electro-osmosis according to classical electrokinetic theory. 

Successfully developing a surface engineering strategy based on surfactant behavior at interfaces requires surface characterization techniques that can validate and quantify surface chemistry changes. This review describes the role of two surface chemistry analysis techniques that have proven highly successful in surfactant analysis x-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectrometry (SSIMS). In Section II, the methods by which these techniques analyze surface chemistry are described. In Section III, recent examples of their application in surfactant-based surface engineering are described. 

Many of the surface characterization techniques listed in Table 1 are based on the analysis of electrons emitted from the surface region. The emission of these electrons can come as a result of temperature, applied electrical field, or the interaction of the surface species with incident photons or electrons. 

The structure and reactivity of ethylene chemisorbed on transition-metal surfaces are of fimdamental importance in surface science and heterogeneous catalysis. HREELS has been foremost among the surface characterization techniques employed in fact, the first vibrational spectroscopic study of ethylene chemisorbed on Pt(lll) was carried out with electron energy-loss spectroscopy (EELS) almost a decade before IRAS was employed. ... 

A number of modern physical techniques are used to characterize heterogeneous catalysts. These methods range from techniques probing the interaction of catalysts with probe molecules, to in situ surface characterization techniques as well as structural elucidation under both in situ and ex situ conditions. In general, interaction of catalysts with probe molecules is followed using some spectroscopic property of the probe molecule itself and/or the changes induced by the heterogeneous catalyst. The spectroscopic techniques used include vibrational spectroscopies, NMR spectroscopy, UV-Vis spectroscopy and mass spectrometry to name a few examples. Similarly, in situ techniques tend to use properties of probe molecules but also combined with structural techniques such as X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). In recent years XAS has been widely used in the characterization of catalysts and catalyst surfaces. 

Figure 3. Integrated UHV-EC instrument, where UHV signifies AES, HREELS, LEED and TPD surface characterization techniques.

Consequently, STM quickly became a pillar among the many powerful techitiques employed in surface science. While such advances may tempt a few to regard EC-STM as the elixir of the myriad problems in interfacial electrochemical science, the enthusiasm has to be tempered by the realization that tmmeling microscopy is unable to probe other fundamental issues such as surface energetics, composition, and electronic structure EC-STM will always require additional surface characterization techniques if a more complete understanding of complex heterogeneous processes is desired. 

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