Spectroscopic surface methods

Generally, the chemical composition of the surface of a solid differs, often significantly, from the interior or bulk of the solid. I hus far in this lext. we have focused on analytical methods that provide information about bulk composition of solids only. In certain areas of science and engineering, however, the chemical composition of a surface layer of a solid is much more important than is the bulk composition of the material. 

Spectroscopic surface methods provide both qualitative and quantitative chemical information about the composition of a surface layer of a solid that is a few tenths of nanometers (a few angstroms) to a few nanometers (lens of angstroms) thick. In this section we describe some of the most widely used of these spectroscopic techniques.  

The most effective surface methods are those in which the primary beam, the secondary beam, or both is made up of cither electrons, ions, or molecules and not photons because this limitation assures that the measurements are restricted to the surface of a sample and not to its bulk. For example, the maximum penetration depth of a beam of l-keV electrons or ions is 

FIGURE 21-1 General scheme for surface spectroscopy. Beams may be photons, electrons, ions, or neutral molecules. 

There are several ways to classify surface techniques. Many of these arc based on the nature of the primary and detected beams. Table 2M lists the niosi widely used spectroscopic techniques. These will be discussed further in this section. 

---

As described in Chapter 2, a number of spectroscopic surface methods give information relating to d band shifts [45]. Ross, Markovic and coworkers have developed synchrotron-based high resolution photoemission spectroscopy to directly measure d band centers giving results in good agreement with the DFT calculations [46]. Another possibility is to exploit the fact that in some cases a shift in the d states can be measured as a core-level shift, as the d states and the core levels shift... 

Surface Charaeferization by Spectroscopy and Microscopy 589 21A Introduction to the Study of Surfaces 589 21B Spectroscopic Surface Methods 590 21C Electron Spectroscopy 591 2 ID Ion Spectroscopic Techniques 602 21E Surface Photon Spectroscopic Methods 604 21F Electron-Stimulated Microanalysis Methods 607... 

Methods based on the study of the scattering of an ion beam also belong among techniques for analysis of surfaces that have been used only occasionally in electrochemistry. The ion-scattering spectroscopic (ISS) method studies the scattering of slow ions (with energy up to 1 keV). The... 

The mechanism of the cocatalytic effect is still a matter of investigation. For most of the systems of interest in electrocatalysis, data for characterization of the surface by means of spectroscopic UHV methods are still missing. Also measurements of changes in the electronic properties of the metal in the presence of adatoms in addition to more intensive application of in situ and on-line methods are desirable for a systematic search of new catalytic materials. 

Electrochemical (EC) detectors have been used for detection in CE. EC methods offer an advantage over the spectroscopic detection methods because electrochemistry that occurs directly at an electrode surface is not limited by the small dimensions inherent in The... 

Metal labels have been proposed to resolve problems connected with enzymes. Metal ions [13-16], metal-containing organic compounds [17,18], metal complexes [19-21], metalloproteins or colloidal metal particles [22-28] have served as labels. Spectrophotometric [22,25], acoustic [25], surface plasmon resonance, infrared [24] and Raman spectroscopic [28] methods, etc. were used. A few papers have been dealing with electrochemical detection. However, electrochemical methods of metal label detection may be viewed as very promising taking into account their high sensitivity, low detection limit, selectivity, simplicity, low cost and the availability of portable instruments. 

The whole field received a new impetus after the first oil crisis, when Fujishima and Honda reported on the photoelectrolysis of water at Ti02-electrodes [13], Whereas, before the oil crisis, most basic models and results had been published only by 3-4 research groups in the world, many other scientists entered the field after this crisis and studied solar applications, and hundreds of papers were published. Since then, many processes at semiconductor electrodes have been studied more quantitatively by using not only standard electrochemical methods, but also new techniques, such as spectroscopic surface analysis (see e.g. [12]). Naturally, photoeffects played a dominant role in these investigations. These were not only restricted to reactions induced by light excitation within the semiconductor electrode [11], but were also extended to the excitation of adsorbed dye molecules [14,15]. 

One important aspect of this collection of different researchers in the field of nanotechnology is the question for the future developments. In this context one author writes "the technology has concentrated so far on the long lasting questions of electrochemistry". This can be emphasized with the statement that many of the results were already assumed on the basis of classical integral measurements. However, many STM or AFM results are completely unexpected and surprizing. Discrepancies between classical integral and local information have to be cleared up by independent measurements. In this context many authors mention that the new technique must be considered as only one method of the entire ensemble of in-situ and ex-situ surface methods. This is an important statement, since different surface spectroscopic methods such as in-situ X-ray, Raman, NMR, etc. may act as such independent methods. 

In this section we discuss methods in which (Ihoions provide both the primary beam and the detected beam. The techniques discussed arc listed in lable 21-1 namely, surface plasmon resonance,sum frequency generation, and e.lUpsometry. The electron and ion spectroscopic surface techniques described previously all suffer from one disadvantage they require an ultra-high vacuum environment and provide no access to buried interfaces. The photon spectroscopic melhods described here can all deal with surfaces in contact with liquids and, in some cases, surfaces that are buried under transparent layers. 

Many spectroscopic techniques are nowadays used for surface investigations. Some of them (i.r., Raman, u.v.-VIS-NIR), do not need particular modifications of commercially available instruments and so their use is common. As a consequence, the number of papers that have appeared in the recent literature (1975-1981) describing i.r., Raman and u.v.-VIS-NIR spectra of adsorbed species is so large that an exhaustive review is practically impossible. In this review we shall not attempt to give a complete examination of all contributions and we focus our attention on those where the structure of both the surface sites and surface species is taken into consideration. The reasons for this can be summarized as follows (/) many papers only report the spectra of surface species for analytical purposes and so no detailed discussion is given about the assignment (ii) many papers report the spectra only as ancillary data, able (in principle) to support hypotheses obtained through other surface methods (Hi) in many papers the examined solids are so complex (because they are or are similar to industrial catalysts) that a detailed discussion of the surface structures is nearly impossible. The boundary between the two types of contributions is labile and as a consequence the choice quite subjective. 

The fate of organic contaminants in soils and sediments is of primary concern in environmental science. The capacity to which soil constituents can potentially react with organic contaminants may profoundly impact assessments of risks associated with specific contaminants and their degradation products. In particular, clay mineral surfaces are known to facilitate oxidation/reduction, acid/base, polymerization, and hydrolysis reactions at the mineral-aqueous interface (1, 2). Since these reactions are occurring on or at a hydrated mineral surface, non-invasive spectroscopic analytical methods are the preferred choice to accurately ascertain the reactant products and to monitor reactions in real time, in order to determine the role of the mineral surface in the reaction. Additionally, the in situ methods employed allow us to monitor the ultimate changes in the physico-chemical properties of the minerals. 

Spectroscopic reflectance methods are UV/vis reflectance spectroscopy and infrared reflection absorption spectroscopy (IRRAS) with several variations. For the application of these methods a mirror-Uke electrode surface is needed. This can be avoided if the scattered... 

This chapter is divided into several major parts. After an introduction to surface methods in Section 21B. we then discuss electron spectroscopic techniques, ion spectroscopic techniques, and photon spectroscopic techniques to identify the chemical species making up... 

The molecular structures of the hydrated surface metal oxides on oxide supports have been determined in recent years with various spectroscopic characterization methods (Raman [34,37,40 3], IR [43], UV-Vis [44,45], solid stateNMR [32,33], and EXAFS/XANES [46-51]). These studies found that the surface metal oxide species possess the same molecular strucmres that are present in aqueous solution at the same net pH values. The effects of vanadia surface coverage and the different oxide supports on the hydrated surface vanadia molecular structures are shown in Table 1.2. As the value of the pH at F ZC of the oxide support decreases, the hydrated surface vanadia species become more polymerized and clustered. Similarly, as the surface vanadia coverage increases, which decreases the net pH at PZC, the hydrated surface vanadia species also become more polymerized and clustered. Consequently, only the value of the net pH at PZC of a given hydrated supported metal oxide system is needed to predict the hydrated molecular structure(s) of the surface metal oxide species. 

A number of methods that provide information about the structure of a solid surface, its composition, and the oxidation states present have come into use. The recent explosion of activity in scanning probe microscopy has resulted in investigation of a wide variety of surface structures under a range of conditions. In addition, spectroscopic interrogation of the solid-high-vacuum interface elucidates structure and other atomic processes. 

If a surface, typically a metal surface, is irradiated with a probe beam of photons, electrons, or ions (usually positive ions), one generally finds that photons, electrons, and ions are produced in various combinations. A particular method consists of using a particular type of probe beam and detecting a particular type of produced species. The method becomes a spectroscopic one if the intensity or efficiency of the phenomenon is studied as a function of the energy of the produced species at constant probe beam energy, or vice versa. Quite a few combinations are possible, as is evident from the listing in Table VIII-1, and only a few are considered here. 

The various spectroscopic methods do have in common that they typically allow analysis of the surface composition. Some also allow an estimation of the chemical state of the system and even of the location of nearest neighbors. 

---