Papers by Keyword: EXAFS

Abstract: A new method for studying multilayer structure using angle resolved XAFS measurements is proposed. The integral equation describing the relation between the fluorescence intensity for selective spectrum, the incident beam energy, the incident angle and a selective concentration profile, depth-dependent EXAFS has been derived. This integral Fredholm equation of the first kind belongs to the class of ill-posed problems and for the solution it needs special methods. We use the regularization Tikhonov method. The effectiveness of the proposed method was tested using the numerical simulations for trilayer system Cr-Fe-Cr. Some first experimental results for the thin films of pure Cr are also presented.

Abstract: The effect of chemical composition on the bonding environment of Cu, in a series of Ti1-x Cux and TiN/Cu films, is studied using X-Ray Absorption Spectroscopies (XAFS) at the Cu-K-edge. The EXAFS analysis reveals that in all studied samples Cu is amorphous. However, its bonding environment depends on the chemical composition. More specifically, in the Ti1-xCux films, Cu is coordinated with Ti and Cu and belongs both to intermetallic TiCu and to an amorphous Cu matrix. The coordination number of Cu, i.e., the sum of Ti and Cu first neighbours, increases systematically from 6.3 ± 0.7 to 10.6 ± 0.9 when the Cu content increases from 24.1 to 52.7 at%. On the contrary, in the TiN/Cu films, the type of atoms that consists the 1st nearest neighbour shell of Cu varies as a function of the Cu concentration. More specifically, in the TiN/Cu film with the lowest Cu content (27.3 at%), intermetallic TiCu is detected. At intermediate Cu concentration (37.8 at%), Cu is bonded to both Ti and Cu atoms. Finally, in the TiN/Cu film with the highest Cu content (67.7 at%), Cu is metallic.

Abstract: X-Rays Absorption Fine Structure Spectroscopy (XAFS) is an analytical technique that can be used as a probe to characterize almost all elements, even if they appear in diluted or non-crystalline systems. This is due to the fact that the absorption probability of X-rays has a unique feature for each element, and is modulated by the chemical and physical state of that element, as well as by its neighborhood. This paper presents a brief description of the X-rays absorption phenomenon and the analytical technique involving this phenomenon, as well as the application of XAFS in biosorption studies. For more details on XAFS theory, refer to [1].

Abstract: X-ray standing wave technique has been used to measure the kinetics of CoSi intermetallic phase growth in a-Si/Co/a-Si sandwich structure. The a-Si/Co/a-Si arrangement were placed into a waveguide structure formed by two Ta films. X-ray fluorescence and extended X-ray absorption fine structure analysis has been used in a combination with X-ray standing wave technique for depth profiling with sub-nanometer resolution of specimens annealed at 493K for different annealing time. The position and the thickness of the growing CoSi intermetallic phase have been monitored.

Abstract: Effects of lattice defects on cathode properties of LiMn2O4 synthesized at low temperatures were investigated. LiMn2O4 powders were synthesized by a sol-gel method. The specific capacities of LiMn2O4 decreased from 134 to 81 mAh g-1 with decreasing heating temperature from 750 to 200°C. X-ray absorption spectroscopy showed that a large amount of lattice defects such as cation vacancies existed and cation mixing occurred in LiMn2O4 calcined at low temperatures. It was found that the low specific capacities of LiMn2O4 calcined at low temperatures were attributed to these lattice defects.

Abstract: The XANES studies at Mn, Ni and Ga K-edge of Ni2MnGa compound have been carried out at room and low temperatures. The Mn K-edge and Ni K-edge spectra shows modulation in the post edge features when the sample is cooled below martensitic transition temperature. It is strongly reflected in the XANES of Mn K-edge where the peak after the edge gets totally suppressed when the sample is in martensitic phase. This peak shows a hysteretic behaviour when thermal cycling was done across the martensitic transition temperature. This clearly shows that the peak height is a measure of austenitic phase present at a particular temperature. This demonstrates the strong correlations of electronic states and crystal structures in these compounds.

Abstract: X-ray absorption near edge structural (XANES) and X-ray absorption fine structural (EXAFS) spectroscopies have been used to study chemical structure of nanosize copper extracted in the channels of MCM-41. The component-fitted XANES spectra show that about 95% of copper are adsorbed on the surfaces of MCM-41 at 298 K. A small amount of nanosize CuO in MCM-41 is also found. The adsorbed copper (Cu(II)(ads)) in the channels of MCM-41 may be oxidized and form nanosize CuO as calcined at 573 and 923 K. By EXAFS, it is observed that the nanosize CuO in MCM-41 possesses a Cu-O bond distance of 1.97 Å with a coordination number (CN) of 2.6, indicating the existence of linear type Cu3O4 cluster. Interestingly, calcinations of the nanosize CuO in MCM-41 at 923 K has led to a formation nanowire-type CuO (in the channels of MCM-41) with a length of 50-100 nm and width of about 5-9 nm.

Abstract: In the present work, sensing of ethanol on the ZnO thin films doped with Fe (5-50%) have been investigated. Nature of the sensing species in the nanosize Fe-doped ZnO (FeZnO) thin films has also been studied by in situ extended X-ray absorption fine structure (EXAFS) spectroscopy. By XRD, it is found that ZnO and ZnFe2O4 are the main compounds in the ZnO-Fe thin films. The thin film containing 5% of Fe has a high sensitivity (Rair/Rethanol>80) when sensing of ethanol at 300 K. On the contrary, the thin films with Fe fractions of 20-50% have a very low sensitivity to ethanol (Rair/Rethanol<15). In the presence of ethanol, the EXAFS spectra show that the bond distances of Zn-O and Fe-O in the thin films are 1.90 and 1.98 Å, respectively and restored to 1.91 and 1.97 Å in the absence of ethanol.

Abstract: Carbon-supported palladium catalysts were prepared by an impregnation method using palladium chloride and different carbon supports such as activated carbons with different surface oxygen concentrations and a mesoporous carbon, CMK-5.. The different degree of surface oxidation was achieved by the nitric acid and high-temperature heat treatment. The molecular PdCl2 species was stabilized by adding an HCl in an impregnation step.. As the reduction temperature increased, the Pd dispersion decreased for all Pd catalysts. There was no noticeable difference in Pd dispersion among Pd catalysts supported on carbon supports with different physicochemical properties when the reduction temperature was 423 K. Pd catalysts supported on the carbon support with a high concentration of surface oxygen groups showed a better dispersion than did other Pd catalysts when they were reduced at 573 K. The maximum Pd dispersion was observed over Pd catalyst supported on carbon supports with the highest surface area when the reduction temperature was higher than 573 K.

Abstract: Studies on lattice change of a nickel-phosphorus amorphous alloy were carried out using not only high temperature X-ray diffraction but also extended X-ray absorption fine structure (EXAFS) analysis. Their thermal properties were characterized by differential scanning calorimetry (DSC). Since the results suggested that lattice relaxation occurred in the amorphous state by annealing, EXAFS measurements were carefully performed for characterizing the local structure of the amorphous alloys. The EXAFS analysis showed that the local structures around Ni atoms, which may be the Ni-Ni and Ni-P correlation, were changed by annealing. High temperature X-ray diffraction showed that small amount of crystal phase appeared by annealing up to 820 K, while clear diffraction peaks of Ni3P were observed above 1100 K. These micro/nanoscale structural changes did not correspond to that obtained by DSC which showed an exothermic reaction of atomistic reordering at 630 K. We need analysis of the nanostructures by small angle X-ray scattering or a high resolution transmission electron microscope.