Surface spectroscopic techniques

The course of a surface reaction can in principle be followed directly with the use of various surface spectroscopic techniques plus equipment allowing the rapid transfer of the surface from reaction to high-vacuum conditions see Campbell [232]. More often, however, the experimental observables are the changes with time of the concentrations of reactants and products in the gas phase. The rate law in terms of surface concentrations might be called the true rate law and the one analogous to that for a homogeneous system. What is observed, however, is an apparent rate law giving the dependence of the rate on the various gas pressures. The true and the apparent rate laws can be related if one assumes that adsorption equilibrium is rapid compared to the surface reaction. 

Comparison of SALI to other surface spectroscopic techniques. 

SALI compares fiivorably with other major surface analytical techniques in terms of sensitivity and spatial resolution. Its major advantj e is the combination of analytical versatility, ease of quantification, and sensitivity. Table 1 compares the analytical characteristics of SALI to four major surfiice spectroscopic techniques.These techniques can also be categorized by the chemical information they provide. Both SALI and SIMS (static mode only) can provide molecular fingerprint information via mass spectra that give mass peaks corresponding to structural units of the molecule, while XPS provides only short-range chemical information. XPS and static SIMS are often used to complement each other since XPS chemical speciation information is semiquantitative however, SALI molecular information can potentially be quantified direedy without correlation with another surface spectroscopic technique. AES and Rutherford Backscattering (RBS) provide primarily elemental information, and therefore yield litde structural informadon. The common detection limit refers to the sensitivity for nearly all elements that these techniques enjoy. 

The non situ experiment pioneered by Sass uses a preparation of an electrode in an ultrahigh vacuum through cryogenic coadsorption of known quantities of electrolyte species (i.e., solvent, ions, and neutral molecules) on a metal surface. " Such experiments serve as a simulation, or better, as a synthetic model of electrodes. The use of surface spectroscopic techniques makes it possible to determine the coverage and structure of a synthesized electrolyte. The interfacial potential (i.e., the electrode work function) is measured using the voltaic cell technique. Of course, there are reasonable objections to the UHV technique, such as too little water, too low a temperature, too small interfacial potentials, and lack of control of ionic activities. ... 

As mentioned previously, this can be attributed in part to the lack of structure-sensitive techniques that can operate in the presence of a condensed phase. Ultrahigh-vacuum (UHV) surface spectroscopic techniques such as low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and others have been applied to the study of electrochemical interfaces, and a wealth of information has emerged from these ex situ studies on well-defined electrode surfaces.15"17 However, the fact that these techniques require the use of UHV precludes their use for in situ studies of the electrode/solution interface. In addition, transfer of the electrode from the electrolytic medium into UHV introduces the very serious question of whether the nature of the surface examined ex situ has the same structure as the surface in contact with the electrolyte and under potential control. Furthermore, any information on the solution side of the interface is, of necessity, lost. 

Surface spectroscopic techniques must be separated carefully into those which require dehydration for sample presentation and those which do not. Among the former are electron microscopy and microprobe analysis, X-ray photoelectron spectroscopy, and infrared spectroscopy. These methods have been applied fruitfully to show the existence of either inner-sphere surface complexes or surface precipitates on minerals found in soils and sediments (13b,30,31-37), but the applicability of the results to natural systems is not without some ambiguity because of the dessication pretreatment involved. If independent experimental evidence for inner-sphere complexation or surface precipitation exists, these methods provide a powerful means of corroboration. 

X-Ray Photoelectron Spectroscopy (XPS). This technique is also known as electron spectroscopy for chemical analysis (ESCA), and as this name implies, it is a surface analytical technique. At present it is probably the most versatile and generally applicable surface spectroscopic technique. It is called XPS because of the type of beam used to study the interfacial region, that is, X-rays. These X-rays consist of monochromatic radiation—radiation of a given energy—emitted by a metal target bombarded by an electron beam of several kiloelectron volts of kinetic energy... 

This very short treatment of reversal techniques has the following basis. There are certainly treatments in the literature of chronopotentiometiy dealing with current reversal, or reversed-step voltammetry. However, their validity has to be diligently examined in each application. For example, is an assumption of a first-order reaction tacitly involved, when the actual solution may correspond to a fractional reaction order Another reason for the limited treatment has an eye on the future. There are those who see in the rapid development of in situ spectroscopic techniques (see, e.g., Section 6.3), together with advances in STM and AFM, the future of surface analysis in electrochemistry. If these surface spectroscopic techniques continue to grow in power, and give information on surface radicals in time ranges as short as milliseconds, transient techniques to catch intermediate radicals adsorbed on surfaces may become less needed. 

Most of the contemporary research areas that utilize surface chemistry techniques employ thin organic films that have been physically or chemically adsorbed onto a solid (usually metallic) substrate. The use of conducting metal surfaces is due not only to their relevance to many different fields, but also to the fact that many surface spectroscopic techniques (Table I) need such a surface in order to produce high-quality spectra. This criterion effectively eliminated many interesting non-metallic surfaces from study using modem surface-sensitive spectroscopic methods. 

Table 9.2. Common Surface Spectroscopic Techniques and Sample Preparation...

Bulk spectroscopic techniques such as x-ray fluorescence and optical and infrared spectroscopies involve minimal sample preparation beyond cutting and mounting the sample. These are discussed in Section 9.2.1. Spectroscopic techniques such as wavelength dispersive spectroscopy (WDS) and energy dispersive spectroscopy (EDS) are performed inside the SEM and TEM during microscopic analysis. Therefore, the sample preparation concerns there are identical to those for SEM and TEM sample preparation as covered in Section 9.3. Some special requirements are to be met for surface spectroscopic techniques because of the vulnerability of this region. These are outlined in Section 9.5. 

Table 9.3. Capability Comparison of Common Surface Spectroscopic Techniques That Involve Electron or Ion Detection...

Other surface spectroscopic techniques such as X-ray photoelectron spectroscopy (XPS) and sum-frequency generation (SFG) have been used in a very recent paper for the detection of self-assembled monolayers of a penicillanic acid featuring an anchoring group adapted for gold substrates <2007MI1071>. 

Electrochemical and surface spectroscopic techniques [iii, v] have shown that the NEMCA effect is due to electro chemically controlled (via the applied current or potential) migration of ionic species (e.g., Os, NalS+) from the support to the catalyst surface (catalyst-gas interface). These ionic species serve as promoters or poisons for the catalytic reaction by changing the catalyst work function O [ii, v] and directly or indirectly interacting with coadsorbed catalytic reactants and intermediates [iii—v]. 

Weighing the solid, inspecting it visually, and examining it by X-ray and/or surface spectroscopic techniques and/or by cyclic voltammetry both before and after the experiment. 

Physical Methods of Chemistry, Vol. IX. Investigations of Surfaces and Interfaces, B.W. Rosslter, Baetzold, Eds., Wiley-Intersclence, 2nd ed. (1993). (Authoritative reviews by various authors most of the chapters deal with surface spectroscopic techniques.)... 

A variety of methods have been used to characterize the solubility-limiting radionuclide solids and the nature of sorbed species at the solid/water interface in experimental studies. Electron microscopy and standard X-ray diffraction techniques can be used to identify some of the solids from precipitation experiments. X-ray absorption spectroscopy (XAS) can be used to obtain structural information on solids and is particularly useful for investigating noncrystalline and polymeric actinide compounds that cannot be characterized by X-ray diffraction analysis (Silva and Nitsche, 1995). X-ray absorption near edge spectroscopy (XANES) can provide information about the oxidation state and local structure of actinides in solution, solids, or at the solution/ solid interface. For example, Bertsch et al. (1994) used this technique to investigate uranium speciation in soils and sediments at uranium processing facilities. Many of the surface spectroscopic techniques have been reviewed recently by Bertsch and Hunter (2001) and Brown et al. (1999). Specihc recent applications of the spectroscopic techniques to radionuclides are described by Runde et al. (2002b). Rai and co-workers have carried out a number of experimental studies of the solubility and speciation of plutonium, neptunium, americium, and uranium that illustrate combinations of various solution and spectroscopic techniques (Rai et al, 1980, 1997, 1998 Felmy et al, 1989, 1990 Xia et al., 2001). 

Structural Reorganisation of the Hydrided Alloys.—The distortion of the lattice when H dissolves can allow structural changes to occur at low temperatures. Hemplemann and Wicke have observed that when TiFe is hydrided and dehydrided several times there is an irreversible change in the magnetic moment, which they attribute to the formation of iron clusters about 3.4 nm in diameter dispersed throughout the bulk. However, Schlapbach et using surface spectroscopic techniques on fresh and... 

The understanding of how chirality is introduced at surfaces via the adsorption of organic molecules has wimessed a real step-change in the past decade and a number of important parameters and phenomena have been revealed from the dramatic images obtained by STM and from the detailed insights attained by a range of surface spectroscopic techniques and theoretical calculations. A number of excellent... 

The exact value of <5 is not yet known, but useful information can be extracted from the surface spectroscopic techniques described in the continuation of this chapter. Both XPS [87] and dipole moment measurements [139] suggest so that O is O, at least for Pt [138]. Nevertheless it is still safer to maintain the symbolism 0 . The symbolism emphasizes that the backspillover oxygen... 

The electronic transitions from the valence or core states of the metal, caused by the excitation process, are depicted in Fig. 30. Table X gives the electronic transitions associated with some surface spectroscopic techniques and summarizes methods of detection. 

Use of pressure-jump relaxation and other relaxation techniques have been shown to offer much in the study of sorption measurements on soil components (Sparks and Zhang, 1991 Sparks, 1995). An especially attractive approach for ascertaining sorption mechanisms on soils would be to combine relaxation approaches with in situ surface spectroscopic techniques. However, there are a few examples in the literature of studies where sorption reactions on soil components have been hypothesized via kinetic experiments and verified in separate spectroscopic investigations (Fuller et al., 1993 Waychunas et al., 1993 Fendorf et al., 1997 Grossi et al., 1997 Scheidegger et al., 1997). 

X-Ray Photoelectron Spectroscopy (XPS). In order to get more information about the chemical changes of the polymer surface, high vacuum surface spectroscopic techniques were used. For XPS measurements the analytical depth is in the order of 5 nm and should therefore be more sensitive to surface modifications than FT-Raman spectroscopy. The XPS spectra before and after irradiation at 248 nm are shown in Fig. 16. 

Figure 17.3.1 General principle of ultrahigh vacuum surface spectroscopic techniques. [From A. J. Bard, Integrated Chemical Systems, Wiley, New York, 1994, p. 102, with permission.]...

Figure 17.3.2 Detection limits, sampling depth, and spot size for several surface spectroscopic techniques. XRP (x-ray fluorescence) EMP (electron microprobe) EEL (electron energy loss), SAM (scanning Auger microprobe) STEM (scanning transmission electron microscopy). Other abbreviations in Figure 17.3.1. This figure is meant to provide a graphic summary of the relative capabilities of different methods modem instmments have somewhat better quantitative performance characteristics than the 1986 values given here. [From A. J. Bard, Integrated Chemical Systems, Wiley, New York, 1994, pp. 103, with permission adapted from Texas Instmments Materials Characterizations Capabilities, Texas Instmments, Richardson, TX, 1986, with permission.]...

Results contrasting markedly with the foregoing and showing net radiation-induced desorption rather than adsorption, have been obtained not only in containment vessels subject to intense beams of ionising radiations [ 1 ], but also with well-characterised single-crystal surfaces exposed to the radiations employed in modern surface spectroscopic techniques (cf. Table 1). The phenomenon of radiation-induced desorption from the walls of containment vessels has acquired new technological interest from the probability that plasma-induced desorption from the... 

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