Infrared Spectroscopy at Surfaces and Interfaces

The frequencies of vibrational motion in crystals and molecules fall into the infrared (IR) spectral range. IR radiation illuminating the sample surface can therefore excite either phonons in crystals or vibrations of adsorbed molecules. Their excitation is most efficient when the frequency of the IR radiation is close to the internal vibrational frequencies of the sample. As a result, the optical response is a maximum at resonance and decreases when the detuning from resonance increases. This feature allows one to determine the vibrational frequencies or to identify molecules present at the surface if their frequencies are known. 

In this chapter we shall consider various experimental techniques based on application of IR radiation which are commonly used for surface and interface analysis. 

1) Such a homogeneous hlackbody radiator at constant temperature over the whole emitting area is called a Globar. 

Optics and Spectroscopy at Surfaces and Interfaces. Vladimir G. Bordo and Horst-Giinter Rubahn Copyright 2005 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-40560-7 

As we have seen in Section 3.1.1, the ratio of the reflection coefficients in p-and s-polarizations, p, can be represented in terms of the ellipsometric angles Y and A (Eq. (3.33)). These quantities are determined by the sample properties and being measured as a function of the incident radiation frequency provide spectroscopic information about the sample. This is the basic idea of spectroscopic ellipsometry or, in the IR region, infrared spectroscopic ellipsometry (IRSE). 

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Hirschmugl CJ (2002) Frontiers in infrared spectroscopy at surfaces and interfaces. Surf Sci 500 577-604 Hochella MF Jr (1988) Auger electron and photoelectron spectroscopies. Rev Mineral 18 573-637 Hochella MF Jr (1995) Mineral surfaces Their characterization and their chemical, physical, and reactive properties. In Vaughan DJ, Pattrick RAD (eds) Mineral Surfaces, Mineral Soc Ser 5, Chapman Hall, London, p 17-60... 

Hirschmugl CJ (2002) Frontiers in infrared spectroscopy at surfaces and interfaces. Surface Science 500 577-604. 

In oriented polymer samples, polarized Infrared and Raman spectroscopy are also very useful in determining bond orientations in the crystals and oriented amorphous regions (28-34). Furthermore, surface enhanced Raman scattering techniques can also be used to obtain information regarding the molecular orientation at surfaces and interfaces. 

Interfacial water molecules play important roles in many physical, chemical and biological processes. A molecular-level understanding of the structural arrangement of water molecules at electrode/electrolyte solution interfaces is one of the most important issues in electrochemistry. The presence of oriented water molecules, induced by interactions between water dipoles and electrode and by the strong electric field within the double layer has been proposed [39-41]. It has also been proposed that water molecules are present at electrode surfaces in the form of clusters [42, 43]. Despite the numerous studies on the structure of water at metal electrode surfaces using various techniques such as surface enhanced Raman spectroscopy [44, 45], surface infrared spectroscopy [46, 47[, surface enhanced infrared spectroscopy [7, 8] and X-ray diffraction [48, 49[, the exact nature of the structure of water at an electrode/solution interface is still not fully understood. 

Any species showing infrared active vibrational modes adsorbed on a reflecting surface can be studied with infrared spectroscopy. The beam of light will interact absorptively with the species when passing through the adsorbate layer before and after the point of reflection. This enables studies of all kinds of adsorbates on many surfaces. Of particular interest in electrochemistry are surfaces of metals and semiconductors employed as electrodes. Thus the following text deals only with reflection at these surfaces other surface and interfaces are not treated. Attempts to record infrared spectra of emersed electrodes (i.e. ex situ measurements) have been reported infrequently in studies of adsorption of hydroquinone and benzoquinone on a polycrystalline platinum electrode [174-177]. Further development of this approach has... 

Several recent overviews of principles and applications of Raman, FTIR, and HREELS spectroscopies are available in the literature [35-37, 124]. The use of all major surface and interface vibrational spectroscopies in adhesion studies has recently been reviewed [38]. Infrared spectroscopy is undoubtedly the most widely applied spectroscopic technique of all methods described in this chapter because so many different forms of the technique have been developed, each with its own specific applicability. Common to all vibrational techniques is the capability to detect functional groups, in contrast to the techniques discussed in Sec. III.A, which detect primarily elements. The techniques discussed here all are based in principle on the same mechanism, namely, when infrared radiation (or low-energy electrons as in HREELS) interacts with a sample, groups of atoms, not single elements, absorb energy at characteristic vibrations (frequencies). These absorptions are mainly used for qualitative identification of functional groups in the sample, but quantitative determinations are possible in many cases. 

M. Osawa, In-situ surface-enhanced infrared spectroscopy at the electrode/ solution interface , in Advances in Electrochemical Science and Engineering,... 

Iwasita, T. and Nart, F.C. (1997) In situ infrared spectroscopy at electrochemical interfaces. Progress in Surface Science, 55, 271-340. 

High quahty SAMs of alkyltrichlorosilane derivatives are not simple to produce, mainly because of the need to carefully control the amount of water in solution (126,143,144). Whereas incomplete monolayers are formed in the absence of water (127,128), excess water results in facile polymerization in solution and polysiloxane deposition of the surface (133). Extraction of surface moisture, followed by OTS hydrolysis and subsequent surface adsorption, may be the mechanism of SAM formation (145). A moisture quantity of 0.15 mg/100 mL solvent has been suggested as the optimum condition for the formation of closely packed monolayers. X-ray photoelectron spectroscopy (xps) studies confirm the complete surface reaction of the —SiCl groups, upon the formation of a complete SAM (146). Infrared spectroscopy has been used to provide direct evidence for the hiU hydrolysis of methylchlorosilanes to methylsdanoles at the soHd/gas interface, by surface water on a hydrated siUca (147). 

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

Surface forces measurement is a unique tool for surface characterization. It can directly monitor the distance (D) dependence of surface properties, which is difficult to obtain by other techniques. One of the simplest examples is the case of the electric double-layer force. The repulsion observed between charged surfaces describes the counterion distribution in the vicinity of surfaces and is known as the electric double-layer force (repulsion). In a similar manner, we should be able to study various, more complex surface phenomena and obtain new insight into them. Indeed, based on observation by surface forces measurement and Fourier transform infrared (FTIR) spectroscopy, we have found the formation of a novel molecular architecture, an alcohol macrocluster, at the solid-liquid interface. 

In situ infrared spectroscopy allows one to obtain stracture-specific information at the electrode-solution interface. It is particularly useful in the study of electrocat-alytic reactions, molecular adsorption, and the adsorption of ions at metal surfaces. 

Yamakata, A., Uchida, T., Kubota, J. and Osawa, M. (2006) Laser-induced potential jump at the electrochemical interface probed by picosecond time-resolved surface-enhanced infrared absorption spectroscopy./. Phys. Chem. B, 110, 6423-6427. 

Vibrational spectroscopies such as Raman and infrared are useful methods for the identification of chemical species. Raman scattering [4] is a second-order process, and the intensities are comparatively low. A quick estimate shows that normal Raman signals generated by species at a surface or an interface are too low to be observable. Furthermore, in the electrochemical situation Raman signals from the interface may be obscured by signals from the bulk of the electrolyte, a problem that also occurs in electrochemical infrared spectroscopy (see Section 15.3)... 

Spectroscopic techniques may provide the least ambiguous methods for verification of actual sorption mechanisms. Zeltner et al. (Chapter 8) have applied FTIR (Fourier Transform Infrared) spectroscopy and microcalorimetric titrations in a study of the adsorption of salicylic acid by goethite these techniques provide new information on the structure of organic acid complexes formed at the goethite-water interface. Ambe et al. (Chapter 19) present the results of an emission Mossbauer spectroscopic study of sorbed Co(II) and Sb(V). Although Mossbauer spectroscopy can only be used for a few chemical elements, the technique provides detailed information about the molecular bonding of sorbed species and may be used to differentiate between adsorption and surface precipitation. 

To put things into perspective, we. can broadly classify these analytical methods into bulk, dry surface, and in situ interfacial techniques. This chapter focuses on the last category, illustrating two in situ techniques used to study anion binding at the goethite (a-FeOOH)-water interface titration calorimetry and cylindrical internal reflection-Fourier transform infrared (CIR-FTIR) spectroscopy. In fact, CIR-FTIR could prove to be extremely powerful, since it allows direct spectroscopic observation of ions adsorbed at the mineral-water interface. 

We then designed model studies by adsorbing cinchonidine from CCU solution onto a polycrystalline platinum disk, and then rinsing the platinum surface with a solvent. The fate of the adsorbed cinchonidine was monitored by reflection-absorption infrared spectroscopy (RAIRS) that probes the adsorbed cinchonidine on the surface. By trying 54 different solvents, we are able to identify two broad trends (Figure 17) [66]. For the first trend, the cinchonidine initially adsorbed at the CCR-Pt interface is not easily removed by the second solvent such as cyclohexane, n-pentane, n-hexane, carbon tetrachloride, carbon disulfide, toluene, benzene, ethyl ether, chlorobenzene, and formamide. For the second trend, the initially established adsorption-desorption equilibrium at the CCR-Pt interface is obviously perturbed by flushing the system with another solvent such as dichloromethane, ethyl acetate, methanol, ethanol, and acetic acid. These trends can already explain the above-mentioned observations made by catalysis researchers, in the sense that the perturbation of initially established adsorption-desorption equilibrium is related to the nature of the solvent. 

Most of the above membrane-oriented studies were carried out for peptides in multilayer systems that were collapsed or transferred onto a sample cell surface. An alternative and very interesting way to study membrane systems is by IRRAS (infrared reflection absorption spectroscopy) at the air-water interface. In this way, unilamellar systems can be studied as a function of surface pressure and under the influence of various membrane proteins and peptides added. Mendelsohn et al.[136] have studied a model series of peptides, [K2(LA) ] (n = 6, 8, 10, 12), in nonaqueous (solution), multilamellar (lipid), and unilamellar (peptide-IRRAS) conditions. In the multilamellar vesicles these peptides are predominantly helical in conformation, but as peptide only monolayers on a D20 subphase the conformation is (1-sheet like, at least initially. For different lengths, the peptides show variable surface pressure sensitivity to development of some helical component. These authors further use their IR data to hypothesize the existence of the less-usual parallel (i-sheet conformation in these peptides. A critical comparison is available for different secondary structures as detected using the IRRAS data for peptides on H20 and D20 subphasesJ137 ... 

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