Synchrotron-based characterization technique

Advanced synchrotron based characterization techniques of solid state applied to macromolecules are reported in Chapter 4. After an introduction to the physics and principles of NEXAFS and XPS spectroscopy, the main features of these techniques that allow a non conventional assessment of the electronic and chemical structure are depicted. The study of macromolecular organization and self-assembly can be nicely obtained by these spectroscopic tools. For example the formation of SAMs (Self Assembled Monolayers) of a variety of molecules arranged in supramolecular assemblies can be detected as well as the behaviour of biomolecules bound to surfaces mimicking biological substrates. Many examples of macromolecules studied with NEXAFS and XPS highlight the potential of these spectroscopic methods to give insight into the molecular and supramolecular structure which in turn determine the most desired properties. 

Techniques of microscopic XRF ( j,-XRF) developed in the last 20 years provide 2D images and elemental maps of each element present in the target material. Portable/in situ p-XRF, j,-XRF spectrometers synchrotron-based ( -SRXRF) and micro-x-ray absorption spectroscopy/micro-x-ray absorption near-edge structure spectroscopy (XAS/ J,-XANES) have improved the mineralogical characterization, as well as the elemental and chemical imaging of samples at the submicrometer scale [61]. 

The table is organized as follows Column 2 lists methods used to complement the XRD experiments. It is obvious from the analysis of the literature that most investigations have required (within the same report) the availability of complementary data for the interpretation of the XRD results. By far the most common complementary techniques are EXAFS and XAS to characterize the evolution of short- and long-range ordering simultaneously. The pairing of this very important basic information about the constitution of a catalyst has even led to the construction of a combined synchrotron-based experiment whereby both techniques were used with catalysts in complex reaction atmospheres (Clausen et al., 1993 Dent et al., 1995 Grunwaldt and Clausen, 2002 Sankar et al., 2000). 

To date, standard bulk XAFS has been the most widely used synchrotron-based technique used to characterize heavy metals in environmental samples. However, in soils and sediments, microenvironments exist that have isolated phases in higher concentrations relative to the average of the total matrix.53 For example, the microenvironment of oxides, minerals, and microorganisms in the rhizosphere has been shown to have a quite different chemical environment compared to the bulk soil.60 Often these phases may be very reactive and of significance in the partitioning of... 

In addition to using X-rays to irradiate a surface, ultraviolet light may be used as the source for photoelectron spectroscopy (PES). This technique, known as ultraviolet photoelectron spectroscopy (UPS, Figure 7.38), is usually carried out using two He lines (Hel at 21.2 eV and Hell at 40.8 eV), or a synchrotron source. This technique is often referred to as soft PES, since the low photon energy is not sufficient to excite the inner-shell electrons, but rather results in photoelectron emission from valence band electrons - useful to characterize surface species based on their bonding motifs. It should be noted that both UPS and XPS are often performed in tandem with an Ar" " source, allowing for chemical analysis of the sample at depths of < 1 J,m below the surface. 

NEXAFS is a synchrotron-based spectroscopic tool routinely used as a complementary technique with XPS for surface characterizations. This method probes the adsorption of X-rays by the excitation of core (K-shell) electrons into unoccupied electronic states near the ionization limit. Subsequent emission of Auger electrons results in the formation of an NEXAFS electron yield the observed spectmm. Because the source of Auger electrons can extend only up to 10 nm and the spectral peak positions and intensities are directly related to the nature of unoccupied electronic states, NEXAFS spectroscopy provides an important tool for studying stmctural and chemical features of various surface thin films and coatings (Hemraj-Benny et al., 2006 Hahner, 2006). 

In the past years advanced characterization techniques such as synchrotron-based X-ray tomography have allowed to accurately determine the structure of the GDL materials including the effects of compression in the cell [4]. In combination with electrochemical methods [5, 6], this has lead to accurate determination of the effective diffusion coefficients as defined in Eq. 2. in dry GDL, depending on the compression and the direction (in- or through-plane), is in the range between 0.6 and 0.2 times the diffusion coefficient in the free volume (see Fig. 3). 

This chapter will highlight Li-ion battery research using 2D and 3D synchrotron based X-ray imaging techniques and show examples from characterizations of full electrodes as well as single particles. We will give a brief introduction into the available techniques and designated set-ups to make room for the results that... 

The availability of synchrotron radiation sources has stimulated the development of interfacial characterization techniques based on scattmng of x-rays and photoelectrons. For example, near-edge x-ray absorption fine structure (NEXAFS) analysis of surface monolayer films gives information on molecular orientation that is complementary to that usually obtained by vibrational spectroscopy. X-ray diffraction from interfaces, in particular x-ray standing wave techniques, now appear to be very promising as tools for providing vertical profiles of well-ordered surface structures at angstrom resolutions. ... 

In this chapter we first provide an overview of eommonly applied synchrotron radiation-based X-ray techniques for determining metal speciation in powdered samples, including X-ray absorption spectroscopy, micro X-ray fluorescence, and micro X-ray diffraction (XRD). The seeond part of this ehapter will provide an example of the application of these teehniques to an investigation of lead (Pb) speciation in a house dust sample, characterized by elevated total and bioaccessible Pb concentrations. 

The synchrotron radiation-based X-ray techniques presented here are powerful tools for characterizing the speciation of metals in house dust samples because of the metal specificity, high spatial resolution and ability to examine elements of interest with Uttle to no sample preparation or pre-treatment. Bulk XAFS analysis can be used to infer the chemical speciation of an element of interest in both moist and dry samples while p-XEF and p-XRD can determine the spatial relationship between trace metals and associated mineral particles. 

In fact, X-ray Diffraction (XRD) is a powerful techniques used to uniquely identify the crystalline phases present in materials and to measure the structural properties of these phases. XRD offers unparalleled accuracy in the measurement of atomic spacing and is the technique of choice for determining strain states in thin films. XRD is non-contact and non-destructive, which makes it ideal for in situ studies. The intensities measured with XRD can provide quantitative and accurate information on the atomic arrangements at interfaces. Indeed, with lab-based equipment, surface sensitivities down to a thickness of 50 A are achievable, but synchrotron radiation allows the characterization of much thinner films and for many materials, monoatomic layers can be analyzed. 

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