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Redox Chemistry of Vanadium in Soils and Sediments
Published in Jörg Rinklebe, Vanadium in Soils and Plants, 2023
Jörg Rinklebe, Vasileios Antoniadis, Sabry M. Shaheen
Beyond such an approach, V speciation can be addressed with advanced methods, that is, XAS (synchrotron-powered X-ray absorption spectroscopy), a technique that has the major advantage of using whole soil (Kelly et al., 2009). This is achieved at core 1s electron (K-edge), with excitation energy of 5,465 eV. Such a low energy, however, may exhibit problems if soils are of low V content (Shahen et al., 2019). X-ray absorption near edge structure (XANES) is also a powerful advanced technique often employed for V analysis (Wisawapipat and Kretzschmar, 2017). This analysis requires the fitting of the produced spectra of the tested sample to known reference spectra of given “clean” constituents. An example of XANES is presented in Figure 5.2. One can observe (a) a pre-edge feature at ca. 5,470 eV being increased and (b) a 1–2 eV increase in the absorption edge position (E1/2) for the step-wise V oxidation state from V(III) to V(V) (Chaurand et al., 2007).
Detector Characterization
Published in Alan Owens, Semiconductor Radiation Detectors, 2019
X-ray Absorption Fine Structure measurements (XAFS) are routinely carried out at synchrotron facilities to probe both short- and long-range order in materials. XAFS is a generic term and can be broken down into structure originating far from an absorption edge and structure originating close to the edge, which arise from different processes. Extended X-ray Absorption Fine Structure (EXAFS) is a diagnostic of short-range order by means of which details in the local geometry (atom types, bond lengths and bond angles) around the photo-absorbing atom can be extracted from far-edge spectra. X-ray Absorption Near Edge Structure (XANES), on the other hand, is a diagnostic of long-range order through which details of atom types and how they are structured collectively (the coordination environment) can be extracted from near-edge spectra. XAFS measurements, both EXAFS and XANES, can also be applied to detector metrology. For example, several authors have noted the wide spread in the radiation detection properties of Cd1-xZnxTe crystals and have attempted to use the structural information embedded in XAFS to find a link between performance and structural perfection.
Chemical mapping with x-ray absorption spectroscopy
Published in Elaine DiMasi, Laurie B. Gower, Biomineralization Sourcebook, 2014
Yannicke Dauphin, Murielle Salomé
Absorption edge spectroscopy techniques are based on the measurement of the absorption coe cient variation by tuning the energy of the probing photons through an absorption edge of an element. This variation is physically related to the excitation cross section of the core electrons into unoccupied electronic states or into vacuum continuum, following dipole selection rules (l = ±1). The spectral features observed close to the absorption edge—known as XANES or near-edge x-ray absorption fine structure (NEXAFS)—re ect the molecular environment of a given absorbing atom and provide the basic mechanism for imaging with chemical sensitivity (Bianconi 1988, Stöhr 1992). Information on different chemical states within systems having the same elemental composition is therefore possible. Figure 6.3 shows a typical x-ray absorption spectrum acquired in transmission through a thin Fe foil. As the energy of the incident x-ray beam is scanned near and above the binding energy of the core level electrons of an element, a sharp jump in absorption, known as the absorption edge, is observed and accompanied with complex structures that provide information about the molecular environment of the atom. XANES allows probing transitions to unoccupied or partially lled bound electronics states or to the continuum and is sensitive to oxidation state and coordination environment. The x-ray absorption spectrum can be divided into three different regions. (1) e so-called pre-edge region, preceding the absorption edge, corresponds to incident beam energies
Science and technology of a transformational multifunctional ultrananocrystalline diamond (UNCDTM) coating
Published in Functional Diamond, 2022
Confirmation that the dominant sp2-C bonds, observed by Raman in UNCD films, are not from an impurity graphite phase (i.e. parallel planes formed by hexagonal unit lattice with C atoms on the corner), but from dangling sp2-C bonds in grain boundaries, was provided by near edge X-ray absorption fine structure spectroscopy (NEXAFS) done on UNCD, MCD, and a crystal diamond gem. This technique is also defined as X-ray absorption near edge spectroscopy (XANES). Its sensitivity to the local atomic bond order in a material arises from the dipole-like electronic transitions from core atoms electronic states, which have well-defined orbital angular momenta, into empty electronic (e.g. antibonding) states. The symmetry of the final state can be determined, and thus the difference between sp2 and sp3 bonding can be readily observed in covalent, low-z materials like diamond. The NEXAFS spectra show the characteristic spectrum of crystalline diamond for UNCD, MCD and a diamond gem, which is completely different from the NEXAFS spectrum from a pure graphite film (see Figure 5 (a)) [19]. In addition, NEXAFS analysis showed that UNCD films can be grown at substrate temperatures from 800 °C to 250 °C (see Figure 5 (b)).
Carbon nanotubes: a review on green synthesis, growth mechanism and application as a membrane filter for fluoride remediation
Published in Green Chemistry Letters and Reviews, 2021
Bayisa Meka Chufa, H. C. Ananda Murthy, Bedasa Abdisa Gonfa, Teketel Yohannes Anshebo
A SAXS characterization technique is the most important technique when the use of imaging techniques becomes difficult and impossible. It gives structural and morphological information and correlations on CNTs and their mutual orientation. The result obtained from this technique is in agreement with the qualitative ones from SEM and TEM. X-ray absorption near edge structure (XANES) spectroscopy has been a powerful tool (39) which provides information on the local environment around carbon but also investigates the absorption and adsorption of hydrocarbon molecules (41) radicals and atoms with a specific selectivity for the orientation of these compounds. XANES is a local probe, sensitive to chemical impurities, defects, chemical adsorption and curvature induced orbital re-hybridization.
Evolution of intermetallic GaPd2/SiO2 catalyst and optimization for methanol synthesis at ambient pressure
Published in Science and Technology of Advanced Materials, 2019
Elisabetta M. Fiordaliso, Irek Sharafutdinov, Hudson W. P. Carvalho, Jan Kehres, Jan-D. Grunwaldt, Ib Chorkendorff, Christian D. Damsgaard
While XANES describes the local geometry, type of atoms and oxidation state of the intermetallic catalysts, EXAFS captures the local structure and complements the knowledge gained from the XRD results. Figure 5 shows the experimental and adjusted Fourier transform of the k2-weighted EXAFS spectra collected in a 25% H2/He mixture at the Pd-K edge and Ga-K edge from the GaPd2/SiO2 catalyst with 23 wt.% metal content at three temperatures: 25°C, 320°C and 550°C. The structural parameters obtained from the fitting procedures are presented in Table 1. The experimental and fitted data for GaPd2 crystal structure are in good agreement, as the ρ factor is within the acceptable range [26]. Figure 5(a,b) show EXAFS spectra acquired at 25°C in a 25% H2/He mixture for the Ga-edge and Pd-edge, respectively. The Ga and the Pd atoms are situated in two different sites: the Ga atoms are surrounded by O atoms, indicating that the Ga is in an oxidic state. The fitting required the inclusion of two O distances, which suggests a distorted oxide structure. The Pd atoms are surrounded by O atoms, but a fraction of metallic Pd was observed both at the first coordination shell. Thus, PdO was partially reduced at room temperature. The intensity ratio between the Pd-Pd peak and the Pd-O peak and particularly the high Pd-Pd coordination number with typical values for bulk Pd of 2.76 Å, indicates a dominating metallic Pd phase over a PdO phase at room temperature in the presence of H2. This correlates with results from XRD shown in Figure 3(a). Figure 5(c) and (d) show EXAFS spectra acquired at 320°C in a 25% H2/He mixture for the Ga-edge and Pd-edge, respectively. We find that the Ga atoms have three shells of neighbors, namely O, Pd and Ga atoms, while the Pd atoms are surrounded by Ga and Pd atoms. The results indicate that at this intermediate temperature a mixture of metallic Pd and Ga oxide coexists with a Pd-Ga compound. This observation correlates with the XANES. In contrast, XRD analysis shows no crystalline GaPd structure below 400°C, i.e. it remains rather X-ray amorphous. This be explained by either very small crystallites or by an amorphous nature of the GaPd at this intermediate state. Figure 5(e,f) show EXAFS spectra acquired at 550°C in a 25% H2/He mixture for the Ga-edge and Pd-edge, respectively. At this temperature, the Ga atoms are only surrounded by Pd atoms and the Pd atoms are surrounded by Pd and Ga atoms, supporting the formation of the GaPd2 phase.