EXAFS Techniques Research Articles

Structure of interfaces in GaN/AlN and Ge/Si multilayered heterosystems by XAFS spectroscopy

III-nitride heterostructures in the form of multilayered quantum wells (MQWs) or quantum dots (QDs) and interacting Ge QDs (“quantum molecules”) are promising candidates for high-speed intersubband (ISB) optical devices relying on the quantum confinement of electrons. Microstructural parameters (interatomic distances, coordination numbers, and Debye-Waller factors) were determined by means of EXAFS spectroscopy based on the Synchrotron Radiation, and the relationship between the variations in these parameters and the morphology of superlattices and symmetric assembles of QDs were established. The EXAFS technique has been used to study the local structure of thin hexagonal GaN/AlN MQWs grown by ammonia MBE at different temperatures. It is shown that the heterointerface intermixing leads to a decrease in the Ga-Al interatomic distance and the Ga-Ga coordination number in MQWs. The degree of intermixing in the boundary layers rises from 30% to 40% with increase of the growth temperature from 795 to 895 °C. It was found that in the first phase of quantum molecules growth Ge atoms concentration is 25%. With further growth (deposition of the base layers) Ge concentration increases up to 35-45%, depending on the temperature (from 610 to 550 °C) of deposition.

Crystal growth, structure, magnetic properties and theoretical exchange interaction calculations of Cu2MnBO5

Single crystals of ludwigite Cu2MnBO5 were synthesized by flux growth technique. The detailed structural and magnetic characterizations of the synthesized samples have been carried out. The cations composition of the studied crystal was determined using X-ray diffraction and EXAFS technique, the resulting composition differ from the content of the initial Mn2O3–CuO components of the flux. Magnetic susceptibility measurements and the calculations of the exchange integrals in frameworks of indirect coupling model revealed that monoclinic distortions strongly affect exchange interactions and appearance of magnetic ordering phase at the temperature T=93K. The hypothesis of the existence of several magnetic subsystems was supposed.

EXAFS investigations on amorphous GaSe9 thin films

The structural properties of amorphous GaSe9 thin films produced by vacuum evaporation were investigated using the EXAFS technique and the cumulant expansion method. Structural parameters such as average coordination numbers and interatomic distances, disorder and asymmetry of the partial pair distribution functions gij(r) were obtained and compared to those found in the amorphous GaSe9 alloy used as precursor in the evaporation technique. Results indicate that structural units present in GaSe9 evaporate without dissociation, showing the stability of this alloy.

Macroscopic and Microscopic Investigation of U(VI) and Eu(III) Adsorption on Carbonaceous Nanofibers.

The adsorption mechanism of U(VI) and Eu(III) on carbonaceous nanofibers (CNFs) was investigated using batch, IR, XPS, XANES, and EXAFS techniques. The pH-dependent adsorption indicated that the adsorption of U(VI) on the CNFs was significantly higher than the adsorption of Eu(III) at pH < 7.0. The maximum adsorption capacity of the CNFs calculated from the Langmuir model at pH 4.5 and 298 K for U(VI) and Eu(III) were 125 and 91 mg/g, respectively. The CNFs displayed good recyclability and recoverability by regeneration experiments. Based on XPS and XANES analyses, the enrichment of U(VI) and Eu(III) was attributed to the abundant adsorption sites (e.g., -OH and -COOH groups) of the CNFs. IR analysis further demonstrated that -COOH groups were more responsible for U(VI) adsorption. In addition, the remarkable reducing agents of the R-CH2OH groups were responsible for the highly efficient adsorption of U(VI) on the CNFs. The adsorption mechanism of U(VI) on the CNFs at pH 4.5 was shifted from inner- to outer-sphere surface complexation with increasing initial concentration, whereas the surface (co)precipitate (i.e., schoepite) was observed at pH 7.0 by EXAFS spectra. The findings presented herein play an important role in the removal of radionuclides on inexpensive and available carbon-based nanoparticles in environmental cleanup applications.

Magnetism and structure of Ni2MnBO5 ludwigite

Single crystals of ludwigite Ni2MnBO5 were synthesized by flux growth technique. The detailed structural and magnetic characterizations of the synthesized samples have been carried out. The cations composition of the studied crystal was determined using X-ray diffraction and EXAFS technique, the resulting composition is differ from the content of the initial Mn2O3–NiO components of flux. Magnetic susceptibility measurements and the calculations of the exchange integrals in frameworks of indirect coupling model revealed strong antiferromagnetic interactions and appearance of magnetic ordering phase at the temperature T=85K. The hypothesis of the existence of several magnetic subsystems was supposed.

Adsorption and desorption of U(VI) on functionalized graphene oxides: a combined experimental and theoretical study.

The adsorption and desorption of U(VI) on graphene oxides (GOs), carboxylated GOs (HOOC-GOs), and reduced GOs (rGOs) were investigated by batch experiments, EXAFS technique, and computational theoretical calculations. Isothermal adsorptions showed that the adsorption capacities of U(VI) were GOs > HOOC-GOs > rGOs, whereas the desorbed amounts of U(VI) were rGOs > GOs > HOOC-GOs by desorption kinetics. According to EXAFS analysis, inner-sphere surface complexation dominated the adsorption of U(VI) on GOs and HOOC-GOs at pH 4.0, whereas outer-sphere surface complexation of U(VI) on rGO was observed at pH 4.0, which was consistent with surface complexation modeling. Based on the theoretical calculations, the binding energy of [G(···)UO2](2+) (8.1 kcal/mol) was significantly lower than those of [HOOC-GOs(···)UO2](2+) (12.1 kcal/mol) and [GOs-O(···)UO2](2+) (10.2 kcal/mol), suggesting the physisorption of UO2(2+) on rGOs. Such high binding energy of [GOs-COO(···)UO2](+) (50.5 kcal/mol) revealed that the desorption of U(VI) from the -COOH groups was much more difficult. This paper highlights the effect of the hydroxyl, epoxy, and carboxyl groups on the adsorption and desorption of U(VI), which plays an important role in designing GOs for the preconcentration and removal of radionuclides in environmental pollution cleanup applications.

Exceptional adsorption-induced cluster and network deformation in the flexible metal-organic framework DUT-8(Ni) observed by in situ X-ray diffraction and EXAFS.

The "gate opening" mechanism in the highly flexible MOF Ni2(2,6-ndc)2dabco (DUT-8(Ni), DUT = Dresden University of Technology) with unprecedented unit cell volume change was elucidated in detail using combined single crystal X-ray diffraction, in situ XRD and EXAFS techniques. The analysis of the crystal structures of closed pore (cp) and large pore (lp) phases reveals a drastic and unique unit cell volume expansion of up to 254%, caused by adsorption of gases, surpassing other gas-pressure switchable MOFs significantly. To a certain extent, the structural deformation is specific for the guest molecule triggering the transformation due to subtle differences in adsorption enthalpy, shape, and kinetic diameter of the guest. Combined adsorption and powder diffraction experiments using nitrogen (77 K), carbon dioxide (195 K), and n-butane (272.5 K) as a probe molecules reveal a one-step structural transformation from cp to lp. In contrast, adsorption of ethane (185 K) or ethylene (169 K) results in a two-step transformation with the formation of intermediate phases. In situ EXAFS during nitrogen adsorption was used for the first time to monitor the local coordination geometry of the metal atoms during the structural transformation in flexible MOFs revealing a unique local deformation of the nickel-based paddle-wheel node.

Local structure around Ge in lithium germanate glasses analyzed by AXS and EXAFS techniques

Local structure around Ge in lithium germanate glasses analyzed by AXS and EXAFS techniques

Approaches to surface complexation modeling of Ni(II) on Callovo-Oxfordian clayrock

Callovo-Oxfordian formation (COX) is as potential host formation for emplacement of long-term nuclear waste repositories in France. The objective of this work is to assess whether a simplified “bottom-up” approach may explain the retention of Ni(II) by the COX considering two levels of ‘upscaling’: (i) from clay surfaces to rock clay fraction and (ii) from clay fraction to whole rock samples. To this end, Ni(II) adsorption was investigated by batch equilibrium, XPS, and EXAFS techniques on a representative sample extracted at the location where the storage is supposed to be built (clay content of about 50%) and on the corresponding carbonate-free <2μm fractions. The results showed that a simplified “bottom-up” approach based on published models available for illite and montmorillonite cannot explain Ni(II) adsorption on the <2μm fraction when the retention is controlled only by surface complexation on the reactive clay edge sites. An operational model based on the generalized composite modeling approach was used instead. The developed model considers an interaction between two Ni(II) species with one type of clay edge sites. The model developed for the clay fraction gives a satisfactory estimation of Ni(II) adsorption data for the representative clayrock sample. Complementary experiments were performed by X-ray photoelectron (XPS) and X-ray absorption (EXAFS) spectroscopies for both clay fraction and raw COX sample at high Ni(II) loadings. Spectroscopic data were characterized by similar fitting parameters considering formation of Ni phyllosilicate. This indicates that the clay fraction governs the retention of Ni(II), as it was concluded from the batch experiments. Complementary adsorption experiments preformed with COX samples having clay contents representative of the variability occurring at the formation scale (i.e. 1.5–47% in weight) show that one cannot neglect the retention properties of the non-clay phases, mainly dominated by calcite, when the clay content becomes the minority. Retention values in the range of 60–300L/kg can finally be given for describing adsorption properties of trace concentrations of Ni(II) for the clay contents representative of the majority of the Callovo-Oxfordian formation.

Probing the Giant Piezoelectric response of ferroelectric perovskites

By analogy with ferromagnetism and the hysteresis of the magnetic moment with a magnetic field, materials that exhibit a macroscopic spontaneous polarization Ps, which can be reversed under electric field E were defined as ferroelectrics. Ps, the directional order parameter can give rise to different polar structural phase transitions and finally disappear as a function of temperature T and/or hydrostatic pressure P in a transformation from a non-centrosymmetric to a centrosymmetric space group. The physical properties of ferroelectric materials are the basis of many technological applications based on their hysteretic properties (Ps / E in ferroelectric random access memories) or based on their coupled properties (η (mechanical strain)/ E in piezoelectric applications). In order to understand the origin and the mechanisms associated with the ferroelectric properties, "in-situ" structural studies as a function of E, T and P have to be performed. In addition ferroelectric materials exhibit based on their directional properties (Ps) a particular domain configuration which makes the structural understanding of these compounds much more complex. Different scales should be taken into account: from the atomic scale (individual polar displacements) to the macroscopic scale (macroscopic piezoelectric effect) and finally the mesoscopic scale in between, which is governed by the domain wall motion. High piezoelectric/ferroelectric properties in lead perovskite materials (PZT, PMN, PZN) are structurally linked to strong disorder which can be characterized by the presence of diffuse scattering in diffraction experiments and by nanosized domains. Here we will present "in-situ" characterization in lead perovskite materials as a function of the applied electric field based on X-ray and neutron diffraction and EXAFS techniques. A brief overview of the challenges to solve in future studies as a function of pressure and temperature will also be discussed.

A comprehensive facility for EXAFS measurements at the INDUS-2 synchrotron source at RRCAT, Indore, India

The EXAFS technique, which deals with fine structure oscillations observed in the X-ray absorption spectrum of an element from 50 eV to ~700 eV above its absorption edge, gives precise information regarding the short range order and local structure around the particular atomic species in the material. With the advent of modern bright synchrotron radiation sources, EXAFS has emerged to be the most powerful local structure determination technique, which can be applied to any type of material viz. amorphous, polycrystalline, polymers, surfaces and solutions etc. Over the last few years a comprehensive facility for carrying out EXAFS measurements with synchrotron radiation over variety of samples has been developed at the 2.5 GeV, Synchrotron Radiation Source (INDUS-2) at RRCAT, Indore, India. The facility consists of two operational beamlines viz., the energy dispersive EXAFS beamline (BL-08) and the Energy Scanning EXAFS beamline (BL-09).

Nature of WO4 tetrahedra in blue light emitting CaWO4 probed through the EXAFS technique

Blue emission due to self trapped exciton recombination in CaWO4 is believed to be very sensitive to the nature of the WO4 structural units. The present manuscript deals with the probing of structural differences existing in WO4 tetrahedra of CaWO4 particles having varying average crystallite sizes and different blue light emission characteristics. Based on XRD, W L1 edge XANES and W L3 edge EXAFS studies, it is inferred that factors like average W–O and Ca–O bond lengths, average number of oxygen atoms around W6+ ions and disorder in WO4 tetrahedra do not have any effect on the blue luminescence intensity from the samples. The lifetime value of excitons is lower for the nanocrystals/nanoparticles of CaWO4 compared to the bulk sample. The lower lifetime of self trapped excitons and the associated decrease in blue luminescence intensity for nanoparticles/nanocrystals (compared to bulk) has been explained based on competing non-radiative processes involving the interaction of holes with surface hydroxyl groups and an associated decrease in the extent of radiative exciton recombination.

XAFS, XPS characterization of cerium promoted Ti-TUD-1 catalyst and it's activity for styrene oxidation reaction

Titanosilicate and many others transition metals deposited mesoporous silica materials are well recognized oxidation catalysts in different types of oxidation reactions, such as propylene oxidation, styrene oxidation, CO oxidation, etc. To increase the electrophilic character of Ti-site in TUD-1 framework cerium was incorporated during the synthesis of Ti-TUD-1 material. The synthesized material was characterized by N2 adsorption–desorption isotherm, XPS, XANES and EXAFS techniques. From XPS result it is found that all Ti is present in framework position by the formation of TiOSi bond in Ti-TUD-1 matrix. From EXAFS experiment it is found that after cerium doping, the electrophilic character of the Ti site was increased resulting more styrene oxidation activity.

EXAFS and kinetic study of MnOx/γ-alumina in gas phase catalytic oxidation of toluene by ozone

This work addresses characterization of Mn2O3 materials by EXAFS technique and the mechanism of toluene oxidation by ozone over MnOx/γ-alumina (10%) catalyst. Bond Valence Model was used to model EXAFS signal of bulk and nano-particles of Mn2O3. The Bond Valence Model relates formal oxidation state of Mn atoms to the coordination number of oxygen atoms at the first Mn shell, adding restraints to the EXAFS theoretical models. Use of the Bond Valence Model resulted in distinguishing closely located oxygen atoms around each Mn site without any simplifying assumption on the structure of Mn2O3. Kinetic study of catalytic oxidation of toluene by ozone using MnOx/γ-alumina (10%) was performed at constant toluene or ozone partial pressures by varying the reactor space time in the range of 0–1212kgcat.moltol.−1s. Differential method of analysis was used to estimate the initial reaction rates. Apparent activation energy of the reaction was determined to be 31kJmol−1, obtained by a power law model. The power law model also determined the reaction orders with respect to toluene and ozone as −1 and 2, respectively. To explain the initial reaction rates, two Langmuir–Hinshelwood mechanisms based on different toluene activation steps were examined. The first mechanism was based on activation of adsorbed toluene molecule by atomic oxygen while the second model was based on activation of toluene molecule via abstraction of hydrogen from the methyl group. The second Langmuir–Hinshelwood mechanism led to the same rate equation as the power law model, explaining the effect of toluene and ozone partial pressures on toluene oxidation rate.

Nature and catalytic activity of bimetallic CuNi particles on CeO2 support

CuNi bimetallic alloy particles supported on CeO2 are prepared by coprecipitation of the corresponding salt precursors followed by calcination and hydrogen reduction. The supported alloy particles are characterized by PXRD, H2-TPR, XPS, XANES and EXAFS techniques. The PXRD study shows that copper rich oxide phases supported on CeO2 are reduced at 200°C. However, complete reduction of CuO–NiO phases occurs at 450°C forming CuNi alloy particles on CeO2. The H2-TPR results show that mixed oxides are reduced at higher temperatures than pure components indicating strong wetting between oxide components and CeO2. The Ni 2p XPS shows stronger interaction between Ni2+ and CeO2 support. XANES and EXAFS studies show the formation of CuNi alloy particles with small surface segregation of Cu. The catalytic activity of the supported CuNi alloy particles is studied by non-oxidative dehydrogenation of cyclohexanol to cyclohexanone reaction. The effect of temperature, contact time and time on stream are examined to gain insights into the catalytic activity of the CeO2 supported CuNi alloy catalysts. Among all the CuNi alloy compositions, CuNi(1:1)/CeO2 catalyst shows higher catalytic activity and selectivity to cyclohexanone. Formation of stable CuNi alloy particles on CeO2 by alloy–redox support interaction and segregation of Cu are crucial factors for catalytic activity.

Structural, vibrational and optical studies on an amorphous Se90P10 alloy produced by mechanical alloying

We investigated some physicochemical properties of an amorphous Se90P10 alloy produced by mechanical alloying through x-ray diffraction, Raman spectroscopy, optical absorption spectroscopy and EXAFS techniques. The total structure factor obtained from x-ray diffraction and the EXAFS χ(k) oscillations on the Se K edge were used in reverse Monte Carlo simulations to obtain structural information such as average coordination numbers and interatomic distances and the distribution of structural units present in the alloy. In addition, we also determined the vibrational modes and the optical band gap energy of the alloy.

Investigation of radionuclide 63Ni(II) sequestration mechanisms on mordenite by batch and EXAFS spectroscopy study

The sorption behavior and microscopic sequestration mechanisms of radionuclide 63Ni(II) on mordenite as a function of aging time, ionic strength, initial 63Ni(II) concentrations, solid content and coexistent electrolyte ions were investigated by the combination of batch and EXAFS techniques. Macroscopic experiment results show that the sorption of 63Ni(II) is dependent on ionic strength at pH 7. The sorption percentage of 63Ni(II) on mordenite increases with increasing solid content, while the sorption capacity decreases as solid content increases. The presence of different electrolyte ions can enhance or inhibit the sorption of Ni(II) on mordenite in various degrees. EXAFS analysis results of the samples under three different ionic strengths suggest that the retained 63Ni(II) in these samples exists in an octahedral environment with six water ligands. In the initial period of rapid uptake, the sorption of 63Ni(II) is dominated by the formation of inner-sphere surface complexes. As aging time increases, 63Ni(II) sequestration behavior tends to be mainly controlled by the formation of Ni phyllosilicate co-precipitates and/or Ni(OH)2(s) precipitates. Results for the second shell fit of the sample prepared at an initial 63Ni(II) concentration of 100 mg/L indicate the possible formation of Ni polynuclear surface complexes. Both the macroscopic sorption data and the molecular level evidence of 63Ni(II) surface speciation at the mordenite/water interfaces should be factored into better predictions of the mobility and bioavailability of 63Ni(II) in environment mediums.

Catalytic hydrotreatment using NiMo/MAS catalysts synthesized from ZSM-5 nano-clusters

Mesostructured alumino-silcate (MAS) materials were synthesized from ZSM-5 nano-clusters and used as catalysts supports for the hydrotreatment of a model compound and real feed stock. Supports and catalysts were thoroughly characterized by using XRD, N2 adsorption analysis, FT-IR, 27Al MAS NMR, Raman, EXAFS and HRTEM techniques. The alumino-silicate materials exhibited different acid strength and textural properties depending on the duration of hydrothermal treatment of the zeolite seeds. Materials synthesized with low seeding time of 4h showed properties similar to Al-SBA-15 prepared using the direct synthesis route, whereas material with high seeding time (tseed=24h) resembled ZSM-5. The EXAFS study of the sulfided catalysts revealed that sulfidation was highest in the NiMo catalyst supported on mesoporous alumino-silicate synthesized from ZSM-5 seeds with 16h of hydrothermal treatment (MAS-16). Hydrotreating experiments were conducted using model compound (DBT) at 325°C and 600psi as well as real feed stock (Coker light gas oil) at industrial conditions. Catalytic activity in the hydrodesulfurization (HDS) of dibenzothiophene (DBT) was found to be maximum for this particular catalyst (MAS-16). The same catalyst also gave higher hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) conversion than NiMo/γ Al2O3 in the hydrotreating of Light Gas Oil.

Macroscopic and Microscopic Investigation of Ni(II) Sequestration on Diatomite by Batch, XPS, and EXAFS Techniques

Sequestration of Ni(II) on diatomite as a function of time, pH, and temperature was investigated by batch, XPS, and EXAFS techniques. The ionic strength-dependent sorption at pH < 7.0 was consistent with outer-sphere surface complexation, while the ionic strength-independent sorption at pH = 7.0-8.6 was indicative of inner-sphere surface complexation. EXAFS results indicated that the adsorbed Ni(II) consisted of ∼6 O at R(Ni-O) ≈ 2.05 Å. EXAFS analysis from the second shell suggested that three phenomena occurred at the diatomite/water interface: (1) outer-sphere and/or inner-sphere complexation; (2) dissolution of Si which is the rate limiting step during Ni uptake; and (3) extensive growth of surface (co)precipitates. Under acidic conditions, outer-sphere complexation is the main mechanism controlling Ni uptake, which is in good agreement with the macroscopic results. At contact time of 1 h or 1 day or pH = 7.0-8.0, surface coprecipitates occur concurrently with inner-sphere complexes on diatomite surface, whereas at contact time of 1 month or pH = 10.0, surface (co)precipitates dominate Ni uptake. Furthermore, surface loading increases with temperature increasing, and surface coprecipitates become the dominant mechanism at elevated temperature. The results are important to understand Ni interaction with minerals at the solid-water interface, which is helpful to evaluate the mobility of Ni(II) in the natural environment.

An X-ray absorption spectroscopy study of FeCo alloy nanoparticles embedded in ordered cubic mesoporous silica (SBA-16)

Nanocomposites containing FeCo alloy nanoparticles dispersed in a highly ordered cubic mesoporous silica (SBA-16) matrix were prepared using two different synthetic methods, co-precipitation and impregnation. Extended X-ray Absorption Spectroscopy (EXAFS) technique at both Fe and Co K-edges was used to investigate the structure of FeCo nanoparticles and the presence of additional disordered oxide phases. EXAFS technique gives evidence of differences in the oxidation degree of the FeCo nanoparticles depending on the synthetic method used.