Papers by Keyword: XAS

Abstract: VOXES is a Von Hamos X-ray spectrometer developed at the INFN National Laboratories of Frascati for high-resolution laboratory X-ray spectroscopy in the 5-20 keV range. It uses curved mosaic crystals and motorized positioning stages to perform wavelength-dispersive X-ray fluorescence (WD-XRF) with sub-10 eV tunable resolution for extended and dilute samples. Recent developments include the integration of an energy-dispersive X-ray fluorescence (ED-XRF) line based on a silicon pin-diode detector, which enables flux monitoring and simultaneous ED and WD measurements. In addition, a dedicated liquid-sample holder has been introduced, and a Y-shaped support geometry, crucial for switching to a transmission layout, provides mechanical compatibility with laboratory XAS, now under implementation. These upgrades expand the versatility and automation of VOXES, strengthening its role as a table-top platform for laboratory X-ray spectroscopy.

Abstract: An experimental study to determine the effect of Cu doping on structural properties of ZnO nanoparticle has been conducted. Zn_1-xCu_xO (CZO) were prepared by co-precipitation method by varying Cu concentration with x = 0.00, 0.10, 0.11 and 0.12. The structural properties of the CZO sample were characterized by X-Ray diffraction (XRD) and X-ray Absorption Spectroscopy (XAS), which consist of XANES (X-ray Absorption Near-Edge Spectroscopy) and EXAFS (Extended X-ray Absorption Fine Structure). XAS measurements were performed in the energy of Cu K-edge and Zn K-edge using synchrotron radiation at SLRI (Synchrotron Light Research Institute) in Thailand. Both Cu K-edge and Zn K-edge XANES spectra indicate that the oxidation states of Cu and Zn ions are 2+. Analysis of XRD data with refinement method using Rietica software confirms the presence of Cu as doping in ZnO host lattice with the crystallite size ranging between 26 and 44 nm. Moreover, it shows that the obtained CZO samples are polycrystalline with (002) predominant reflection and exhibit a hexagonal wurtzite structure.

Abstract: Cu-doped ZnO (CZO) with Cu %mole of 0, 13, 14, and 15% have been synthesized by the co-precipitation method with controlled pH at 9. The diffraction pattern of CZO shows that a single phase of ZnO with wurtzite structure (w-ZnO) was achieved in the parent compound. In contrast, small fractions of the secondary phase of CuO monoclinic (m-CuO) were identified in the doped compounds. Rietveld-refinement on the X-Ray Diffraction (XRD) patterns reveals that the crystallite size of CZO is estimated in the range of 84 - 148 nm with instrumental correction factor and that CZO14 exhibits the most considerable fraction of m-CuO among the samples. Interestingly, lattice constants decrease by Cu doping. The effects of Cu doping on the valence state and the local structure were investigated by X-Ray Absorption Spectroscopy (XAS). Based on our analysis on both Cu K-edge and Zn K-edge XANES spectra, oxidation states of Cu and Zn ions are about +2 with no evidence of other valence states. Cu atom is likely to incorporate into wurtzite crystal lattice by occupying Zn sites in a small portion, and the doped compounds create a few oxygen vacancies.

Abstract: Effects of Mn-substitution on magnetic properties of Zn_1-xMn_xO (MZO) nanoparticles with x= 0.00, 0.03, 0.05 and 0.07 have been investigated along with their local structure. Study on Mn K-edge XANES spectra of MZO reveals that the oxidation state increases by Mn-substitution, which further implies that MZO exhibits a mixed valence state of Mn³⁺/Mn⁴⁺. The local structure analysis on Mn K-edge EXAFS spectra shows that the coordination number (CN) of Mn reduces by increasing Mn concentration, thus the amount of oxygen vacancy (V_O) increases by Mn-substitution. Interestingly, the magnetization of MZO also tend to increase as the Mn concentration increases. The M(H) curves exhibit a linear (paramagnetic) behavior, showing no evidence of room-temperature ferromagnetism. Our results show that magnetism of MZO is related to the correlation between Mn magnetic moment and V_O.

Abstract: In order to study the evolution of local crystal and electronic structure of Sr₂CuO₂F_2+x, the synchrotron radiation x-ray absorption (XAS) and x-ray absorption fine structure (XAFS) techniques were both performed on different amount of Sr₂CuO₂F_2+x materials. The relationship between high T_c superconductivity and valence state of cooper was mainly discussed. And the effect of by-reproduce SrF₂ in non-superconductive Sr₂CuO₂F_2+x on physical property of this material was considered. The experimental results showed that the valence state of Cu and the amount of SrF₂ was mainly related with high T_c superconductivity of Sr₂CuO₂F_2+x material, indicating a good method for improving T_c.

Abstract: We examined structural properties of nitrogen doped (ZnO:N) thin films prepared by reactive RF magnetron sputtering technique in conjunction with gas timing method. The deposited films were polycrystalline ZnO in wurtzite structure. Morphology of the ZnO:N films could be modified by adjusting gas timing conditions. The x-ray photoelectron spectroscopy (XPS) and extended x-ray absorption fine structure (EXAFS) analysis showed that incorporation of nitrogen may cause structural distortion in the ZnO:N crystal.

Abstract: Rare-earth (RE) ions-doped oxyfluoride transparent glass-ceramics containing nano-crystalline phases with very low phonon energies like LaF₃, NaLaF₄ and KLaF₄ combine the superior optical performance of low phonon energy phases with the high mechanical, chemical and thermal stability of oxide silicates. The insertion of the doping RE ions in the fluoride nanocrystals enhances the radiative optical emission processes currently focused on up- and down-conversion emissions. Thus, a key point is to identify and quantify the RE ions in the fluoride nano-crystals. In this contribution an oxyfluoride glass and the corresponding glass-ceramics of composition 55 SiO₂. 20 Al₂O₃.15 Na₂O.10 LaF₃ mol % doped with 1 mol % Tm₂O₃ is presented. The Tm³⁺ incorporation in the precipitated LaF₃ nano-crystals has been analysed by two different techniques. Firstly, TEM and Energy Dispersive X-Ray Spectroscopy (EDXS) were used to perform elemental distribution mappings of RE elements that allow to directly localising RE ions in the glass and glass-ceramics. Additionally, X-ray absorption spectroscopy (XAS) measurements were performed to analyse the distribution and first coordination sphere of Tm³⁺ ions in the glass and glass–ceramics by using X-ray absorption near edge structure spectroscopy (XANES) and extended absorption X-ray fine structure spectroscopy (EXAFS), which have allow to quantify the Tm³⁺ incorporation in the LaF₃ nano-crystals.

Abstract: The composites of polycrystalline (La_1.85Sr_0.15CuO₄)_1-x(La_0.5Sr_0.5MnO₃)_x were prepared by solid state reaction method. By means of X-ray absorption spectroscopy, it is found that electrons can transit at grain boundaries from La_1.85Sr_0.15CuO₄ to La_0.5Sr_0.5MnO₃. Additional holes are formed in La_1.85Sr_0.15CuO₄ when the electron transitions happen. The experimental results of both X-ray absorption near edge structure and extended X-ray absorption fine structure excluded the possibility of that the electron transitions are induced by local lattice structure. According to the X-ray absorption experimental results of O K-edge of the La_1.85Sr_0.15CuO₄ and La_0.5Sr_0.5MnO₃ materials, a possible mechanism of the electron transitions and a phenomenological model of the band structure around Fermi level are proposed to explain the transition behavior.

Abstract: The La_1-xSr_xCoO₃ (x =0.0, 0.1, 0.2 and 0.3) samples were prepared by solid-state reaction method. The X-ray diffraction patterns show that the samples are of R-3c spacegroup and the relative intensity of (024) plane diffraction peak increases with the increase of Sr concentration. The x-ray absorption fine structure data of Co ions can be fitted with 2 and 3 different scattering paths for the samples with x=0, 0.1 and for x=0.2 and 0.3, respectively. A new peak arises in x-ray absorption spectra of O ions. These results suggest a relationship between the variations of electron densities and local structure distortion. Both the valence band photoemission and x-ray absorption spectra indicate the existence of possible phase separation scenario that comes from the different types of Co-O bonds which coexist in one sample.

Abstract: The precalcined and calcined silica supported cobalt catalysts at 15, 20, and 25%Co were investigated by X-ray Absorption Spectroscopy (XAS) including the X-ray absorption near edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS). The results showed the phase of Co(NO₃)₂.6H₂O in all precalcined catalysts, which corresponded to the XRD measurement. When increasing the amount of cobalt in the precalcined catalysts, there was the presence of ordered Co(NO₃)₂.6H₂O phase. After calcination in Ar at 600°C for 6 h, the Co₃O₄ phase was presented in all calcined catalysts. For the catalytic performance testing, the selected 20%Co/Aerosil_wi_calcined catalyst was reduced at 450°C in H₂ and operated at 190°C with a total pressure of 10 bar and H₂/CO flow rate of 20:10 ml/min for Fischer-Tropsch synthesis. After reaction testing, the used 20%Co/Aerosil_wi catalyst showed the main phase of Co₃O₄. The result showed high methane selectivity at the beginning of reaction. By increasing of reaction time, the methane selectivity tended to decrease, whereas the C₂-C₄ and C₅₊ selectivity was increased.