Edge X-ray Absorption Near-edge Spectroscopy Research Articles

Electronic structure of FeO, γ−Fe2O3 , and Fe3O4 epitaxial films using high-energy spectroscopies

We study the electronic structure of well-characterized epitaxial films of FeO (wustite), $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ (maghemite), and ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ (magnetite) using hard x-ray photoelectron spectroscopy (HAXPES), x-ray absorption near-edge spectroscopy (XANES), and electron energy loss spectroscopy (EELS). We carry out HAXPES with incident photon energies of 12 and 15 keV in order to probe the bulk-sensitive Fe $1s$ and Fe $2p$ core level spectra. Fe $K$-edge XANES is used to characterize and confirm the Fe valence states of FeO, $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$, and ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ films. EELS is used to identify the bulk plasmon loss features. A comparison of HAXPES results with model calculations for an $M{\mathrm{O}}_{6}$ cluster provides us with microscopic electronic structure parameters such as the onsite Coulomb energy ${U}_{dd}$, the charge-transfer energy $\mathrm{\ensuremath{\Delta}}$, and the metal-ligand hybridization strength $V$. The results also provide estimates for the ground-state and final-state contributions in terms of the ${d}^{n}, {d}^{n+1}{\underline{L}}^{1}$, and ${d}^{n+2}{\underline{L}}^{2}$ configurations. Both FeO and $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ can be described as charge-transfer insulators in the Zaanen-Sawatzky-Allen picture with ${U}_{dd}\phantom{\rule{4pt}{0ex}}>\phantom{\rule{4pt}{0ex}}\mathrm{\ensuremath{\Delta}}$, consistent with earlier work. However, the $M{\mathrm{O}}_{6}$ cluster calculations do not reproduce an extra satellite observed in Fe $1s$ spectra of $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ and ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$. Based on simplified calculations using an ${M}_{2}{\mathrm{O}}_{7}$ cluster with renormalized parameters, it is suggested that nonlocal screening plays an important role in explaining the two satellites observed in the Fe $1s$ core level HAXPES spectra of $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ and ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$.

Identification of Mn site in Mn-doped SrTiO 3 : First principles study

Abstract Manganese can incorporate into perovskite SrTiO 3 at either on the Sr-site ( A -site) or Ti-site ( B -site) depending on the growth conditions. However, there is no clear explanation on what conditions lead to A -site or B -site occupations and what are the local structures around Mn. In this work, first-principles calculations were carried out to study Mn in SrTiO 3 under different growth conditions. The calculations showed that Mn prefers to substitute for Ti site under a wide range of growth conditions; indicating that the substitution it is difficult to avoid even when one intentionally tries to dope Mn into the A -site. Due to a large size difference, Mn cannot fit in the Sr site. Instead, a charged interstitial Mn has rather low formation energy, especially under O-poor conditions. In addition, Mn i can bind with Sr vacancy which has the compensating charge with a very high binding energy to form Mn i - V Sr complex. Mn i - V Sr has an equivalent chemical composition to Mn Sr . Therefore, one might view Mn i - V Sr as an off-center Mn Sr . In this view point, Mn A -site has a large cation off-centering consistent with experimental observations. We further relate our local structure models with the experiments by calculating the Mn K -edge x-ray absorption near-edge spectroscopy (XANES) spectra associated with Mn Ti , Mn Sr (on regular Sr lattice), and Mn i - V Sr or off-center Mn Sr defects to compare with the measured Mn K -edge XANES spectra. Our results show good agreement with the measured spectra.

Defect compensation by Cr vacancies and oxygen interstitials inTi4+-dopedCr2O3epitaxial thin films

Epitaxial thin films of $\mathrm{C}{\mathrm{r}}_{2\ensuremath{-}x}\mathrm{T}{\mathrm{i}}_{x}{\mathrm{O}}_{3}$ were deposited by oxygen-plasma-assisted molecular beam epitaxy for $0.04\ensuremath{\le}x\ensuremath{\le}0.26$. Ti valence is verified by both x-ray photoelectron spectroscopy (XPS) and Ti $K$-edge x-ray absorption near-edge spectroscopy (XANES) to be $\mathrm{T}{\mathrm{i}}^{4+}$. Substitution of Ti for Cr in the corundum lattice is verified by fitting the Ti $K$-edge extended x-ray absorption fine structure (EXAFS) data. Room temperature electrical transport measurements confirm the highly resistive nature of Ti-doped $\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{3}$, despite the presence of aliovalent $\mathrm{T}{\mathrm{i}}^{4+}$. For comparison, the resistivity of very pure, undoped $\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{3}$ was measured to be four orders of magnitude higher than for Ti-doped $\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{3}$. Analysis of the XPS and EXAFS data reveal the presence of Cr vacancies at intermediate and high Ti concentrations. This conclusion is corroborated by the results of density functional modeling. At low Ti concentrations, a strong increase of the XPS $\mathrm{Ti}\phantom{\rule{0.16em}{0ex}}2p$ core level peak width is observed as the Ti concentration decreases. In this limit, the formation of Cr vacancies becomes less favorable due to the increased distance between Ti dopants, and compensation by O interstitials contributes to broadening of the $\mathrm{Ti}\phantom{\rule{0.16em}{0ex}}2p$ XPS peak. The differences in electronic structure which render $\mathrm{T}{\mathrm{i}}^{4+}$-doped $\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{3}$ resistive due to the formation of compensating defects, but $\mathrm{T}{\mathrm{i}}^{4+}$-doped $\mathrm{F}{\mathrm{e}}_{2}{\mathrm{O}}_{3}$ conductive, are discussed. The defect compensation model developed here provides insight into previous, conflicting reports of $n$-type versus $p$-type conductivity in Ti-doped $\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{3}$ at high temperature, and will inform future studies to exploit the wide variety of electronic and magnetic properties of corundum structure oxides.

Electronic Structure Assessment: Combined Density Functional Theory Calculations and Ru L2,3-Edge X-ray Absorption Near-Edge Spectroscopy of Water Oxidation Catalyst

Density functional theory (DFT) is now widely used for analysis of the electronic structure and reactivities of transition metal complexes. However, large variability in how well different combinations of exchange-correlation potentials/basis sets reproduce real molecular geometries and electronic configurations remains a problem. Experimental X-ray absorption near-edge structure (XANES) spectra directly reflect the electronic structure of transition metal complexes. Combined analysis of theoretical calculations and experimental data is highly beneficial for DFT validation as well as for understanding limitations of the DFT. Ruthenium-based molecular water oxidation catalyst cis,cis-[(bpy)2(H2O)RuIIIORuIVO(OH)(bpy)2]4+ is a complex coordination compound with two Ru centers in different oxidation states bound by μ-oxo bridge. Multiple DFT calculations of this catalyst in different oxidation states have been reported previously but it was never clear whether DFT is truly capable of describing its geometry and electronic structure. We tested a variety of DFT potentials with two relativistic models for electronic structure calculations and simulated experimental Ru L2,3-edges XANES spectra. We found that the local density approximation (LDA) exchange-correlation potential reproduces the experimental geometry and XANES spectra, while more advanced potentials fail. We also report combined DFT calculations/Ru L2,3-edges XANES spectra for three protonation forms: cis,cis-[(bpy)2(H2O)RuIIIORuIVO(H2O)(bpy)2]5+, cis,cis-[(bpy)2(H2O)RuIIIORuIVO(OH)(bpy)2]4+, and cis,cis-[(bpy)2(OH)RuIIIORuIVO(OH)(bpy)2]3+. We found that selected DFT technique captures well small modifications of the electronic structure caused by changes in the protonation state of the ligand. Small differences in the Ru L-edges XANES for various protonation states are reflected in the corresponding theoretical spectra. We also observed that overall DFT-based XANES modeling is sensitive to various subtle changes in molecular geometries of Ru complex.

L2,3-edge x-ray absorption near-edge spectroscopy analysis of photoisomerism in solid ruthenium–sulfur dioxide complexes

Ru L2,3 edge x-ray absorption near-edge spectroscopy spectra of four compounds [Ru(SO2)(NH3)4X]Y (X = water, Y = (±)-camphorsulfonate2; X =water, Y = tosylate2; X =isonicotinamide, Y = tosylate2; X =pyridine, Y = triflate2) are reported before and after optical excitation, which causes photoinduced linkage isomerism of the SO2 ligand. Principal component analysis reveals consistent changes in the spectra upon photoisomerization, suggesting a partial oxidation of the Ru ion. These results demonstrate that x-ray absorption near-edge spectroscopy is an effective means of probing photoinduced linkage isomerism.

X-Ray Diagnosis of the Pressure Induced Mott Nonmetal-Metal Transition

The evolution of the K-edge x-ray absorption near-edge spectroscopy (XANES) spectrum is investigated for an aluminum plasma expanding from the solid density down to 0.5 g/cm{3}, with temperatures lying from 5 down to 2 eV. The dense plasma is generated by nanosecond laser-induced shock compression. These conditions correspond to the density-temperature region where a metal-nonmetal transition occurs as the density decreases. This transition is directly observed in XANES spectra measurements through the progressive formation of a preedge structure for densities around 1.6 g/cm{3}. Ab initio calculations based on density functional theory and a jellium model have been efficiently tested through direct comparison with the experimental measurements and show that this preedge corresponds to the relocalization of the 3p atomic orbital as the system evolves from a dense plasma toward a partially ionized atomic fluid.

Complementarity between high-energy photoelectron and L-edge spectroscopy for probing the electronic structure of 5d transition metal catalysts

We demonstrate the successful use of hard X-ray photoelectron spectroscopy (HAXPES) for selectively probing the platinum partial d-density of states (DOS) in a Pt-Cu nanoparticle catalyst which shows activity superior to pure Pt towards the oxygen-reduction reaction (ORR). The information about occupied Pt d-band states was complemented by Pt L(2)-edge X-ray absorption near-edge spectroscopy (XANES), which probes unoccupied valence states. We found a significant electronic perturbation of the Pt projected d-DOS which was narrowed and shifted to higher binding energy compared to pure platinum. The effect of this electronic structure perturbation on the chemical properties of the nanoparticle surface is discussed in terms of the d-band model. We have thereby demonstrated that the combination of L-edge spectroscopy and HAXPES allows for an experimental derivation of the valence electronic structure in an element-specific way for 5d metal catalysts.

Raman and x-ray absorption spectroscopy characterization of Zr-doped MOCVDYBa2Cu3O6+δ

Metal–organic chemical vapor depositedYBa2Cu3O6+δ (YBCO) films(about 0.9 µm thick) containing varying amounts of added zirconium were examined by Ramanmicroscopy and synchrotron x-ray absorption spectroscopy. The self-field and in-field (1 T,) Jc performance of the YBCO films at 77 K (reported by the group at Oak Ridge NationalLaboratory that fabricated the samples) exhibited an increase on going from 0 mol%(m/o) Zr-added to 2.5 m/o Zr-added but then decreased sharply with increasing Zrcontent. Raman measurements on these films showed that the added Zr had littleeffect on YBCO cation disorder up to about 7.5 m/o Zr-added. Cation disorderincreased while Ba–Cu–O content remained relatively constant for Zr additions≥7.5 m/o. In the region ofsharpest descent of Jc with increasing Zr content (2.5–7.5 m/o Zr-added) neither the cationdisorder nor the Ba–Cu–O content showed a systematic variation withJc. Zirconium K edge x-ray absorption near-edge spectroscopy revealedthat virtually all of the added Zr in each sample was present as aBaZrO3-like phase(BZO). The Jc performance of the Zr-added films showed a high correlation with the variations in the next-nearest-neighborZr–M (M = Zr, Y) scattering path amplitude from the extended x-ray absorption finestructure (EXAFS) and the critical temperature over the full range ofZr additions and with cation disorder at the higher Zr-added levels (>7.5 m/o). There was no obvious correlation with the amount of residual barium cuprate orCuO. Approximate ranges for the BZO particle dimensions estimated from the EXAFSdata indicated that the mean particle size gets larger with increasing Zr addition.

Uranium Transformations in Static Microcosms

Elucidation of complex biogeochemical processes and their effects on speciation of U in the subsurface is critical for developing remediation strategies with an understanding of stability. We have developed static microcosms that are similar to bioreduction process studies in situ under laminar flow conditions or in sediment pores. Uranium L(3)-edge X-ray absorption near-edge spectroscopy analysis with depth in the microcosms indicated that transformation of U(VI) to U(IV) occurred by at least two distinct processes. Extended X-ray absorption fine structure (EXAFS) analysis indicated that initial U(VI) species associated with C- and P-containing ligands were transformed to U(IV) in the form of uraninite and U associated with Fe-bound ligands. Microbial community analysis identified putative Fe(III) and sulfate reducers at two different depths in the microcosms. The slow reduction of U(VI) to U(IV) may contribute the stability of U(IV) within microcosms at 11 months after a decrease in bioreducing conditions due to limited electron donors.

Optimizing experimental design, overcoming challenges, and gaining valuable information from the Sb K-edge XANES region

There are many challenges associated with collecting, processing, and interpreting high-energy XAS data. The most significant of these are broad spectra, minimal separation of edge positions, and high background owing to the Compton tail. Studies of the Sb system are a particular challenge owing to its complex bonding character and formation of mixed oxidation-state minerals. Furthermore, in environmental samples such as stream sediment containing mine waste, different Sb phases may coexist. Ways to overcome these challenges and achieve accurate and useful information are presented. Our investigations used Sb K -edge X-ray absorption near-edge spectroscopy (XANES) to elucidate Sb geochemical behavior. Several Sb mineral spectra are presented, including Sb sulfosalts, and contrasted based on the different hosting and coordination environments around the Sb atom in the crystal structure. These comparisons lead to the recognition of how the different hosting and coordination environments are manifested in the shape of the Sb mineral spectra. In fact from the shape of the spectra, the occupation of the Sb atom in a single or in multiple crystallographic sites, regardless of whether multiple phases are present in the sample, is discernible. Furthermore, we demonstrate that quantitative information can be derived from the XANES region using linear combination fitting of the derivative spectra, rather than the energy spectra. Particularly useful to the advancement of Sb research is the demonstration that a significant amount of information can be gained from the Sb K -edge XANES region.

Roles of Pt and BaO in the Sulfation of Pt/BaO/Al2O3 Lean NOx Trap Materials: Sulfur K-edge XANES and Pt LIII XAFS Studies

The roles of barium oxide and platinum during the sulfation of Pt−BaO/Al2O3 lean NOx trap catalysts were investigated by S K edge XANES (X-ray absorption near-edge spectroscopy) and Pt LIII XAFS (X-ray absorption fine structure). All of the samples studied [Al2O3, BaO(x; x = 8 or 20 wt %)/Al2O3, Pt(2.5 wt %)/Al2O3, and Pt(2 wt %)-BaO(x; x = 8 or 20 wt %)/Al2O3] were pre-sulfated prior to the X-ray absorption measurements. It was found that barium oxide itself has the ability to directly form barium sulfate even in the absence of Pt and gas-phase oxygen. In the platinum-containing samples, the presence of Pt−O species plays an important role in the formation of sulfate species. For the case of the BaO(8)/Al2O3 sample, where the barium coverage is about 0.26 ML, both baria and alumina phases are available for sulfation. S XANES results show that barium sulfates are formed preferentially over aluminum sulfates. When oxygen is absent from the gas phase, the sulfation route that involves Pt−O is eliminated after the initially present Pt−O species are completely consumed. In this case, formation of sulfates is suppressed unless barium oxide is also present. Pt LIII XAFS results show that the first coordination sphere around the Pt atoms in the Pt particles is dependent upon the gas mixture used during the sulfation process. Sulfation under reducing environments (e.g., SO2/H2) leads to formation of Pt−S bonds, while oxidizing conditions (e.g., SO2/O2) continue to show the presence of Pt−O bonds. In addition, a reducing environment was found to cause Pt sintering in greater extent than an oxidizing one. This result explains why samples sulfated under reducing conditions had lower NOx uptakes than those sulfated under oxidizing conditions. Therefore, our results provide needed information for the development of optimum practical operation conditions (e.g., sulfation or desulfation) for lean NOx trap catalysts that minimize deactivation by sulfur.

Biotic and abiotic products of Mn(II) oxidation by spores of the marineBacillus sp.strain SG-1

Bacterial Mn(II) oxidization by spores of Bacillus, sp. strain SG-1 has been systematically probed over the time scale 0.22 to 77 days under in-situ conditions and at differing Mn(II) concentrations. Three complementary techniques, K -edge X-ray absorption near-edge spectroscopy (XANES), X-ray emission spectroscopy (XES), and in-situ synchrotron radiation-based X-ray diffraction (SR-XRD), have been utilized to examine time-dependent changes in Mn oxidation state, local-, and long-range structure in amorphous, crystalline, cell-bound, and solute Mn species. The primary solid biogenic product of Mn(II) oxidation is an X-ray amorphous oxide similar to δ-MnO2, which has a Mn oxidation state between 3.7 and 4.0. Reaction of Mn(II) with the primary biogenic oxide results in the production of abiotic secondary products, feitknechtite or a 10 A Na phyllomanganate. The identity of the secondary product depends upon the Mn(II) concentration as described by thermodynamic relations. A decrease in the dissolved Mn(II) concentration is followed by mineralogic transformation of the secondary products. Thus, Mn(II) appears to act as a reductant toward the biogenic oxide and to control the stability of secondary reaction products. Mineralogic changes similar to these are likely to be commonplace in natural settings where bacterial Mn(II) oxidation is occurring and may liberate sorbed metal ions or alter the rates of important Mn oxide surface-mediated processes such as the degradation of organic molecules. It is plausible that microbes may exploit such mineral transformation reactions to indirectly control specific chemical conditions in the vicinity of the cell.

X-ray absorption near-edge structure at the Mg and FeKedges in olivine minerals

Mg and Fe $K$-edge x-ray absorption near-edge spectroscopy spectra of the $\mathrm{Pbnm}$ olivine series end members forsterite, fayalite, monticellite, and kirschsteinite, as well as of a synthetic Fe-Mg olivine of intermediate composition (Fo50), have been recorded and compared with multiple-scattering simulations. A comparison of experimental and theoretical spectra shows that all transition features, energy positions, and relative intensities can be reproduced in the energy range up to 60 eV above threshold when using clusters containing at least 80 atoms and extending over a sphere at least 0.6 nm in radius away from the absorber. Mg and Fe are entirely in the expected structural sites in the four investigated end members. None of the models of Mg and Fe ordering in the octahedral site for the Fe50 olivine compound proposed so far is verified; thus Fe-Mg ordering still remains an open problem for olivines of intermediate compositions.

X‐ray absorption spectroscopy in the analysis of GaN thin films

AbstractStoichiometric amorphous GaN thin films have been grown by an ion‐assisted deposition method and examined by x‐ray photoelectron spectroscopy and x‐ray absorption near‐edge spectroscopy (XANES). The crucial question is the nature of the local structure around the N and Ga in the x‐ray amorphous films. The N K edge XANES has been used to determine coordination around the N centre and reveals substantial differences to crystalline GaN. Although the transitions observed mirror those of the crystalline material and are consistent with density of states calculations, the low‐energy peak at ∼402 eV is dominant in all films less than ∼150 nm in thickness. This peak, initially attributed to an sp2 environment, is associated with the presence of molecular nitrogen. For thicker films, a duplex‐type structure is observed with a surface layer much closer to the structure of the crystalline material. Copyright © 2003 John Wiley & Sons, Ltd.

Study of the activation process of Mg-based hydrogen storage materials modified by graphite and other carbonaceous compounds

A nanocomposite (Mg–V)nano made of 90 wt% Mg and 10 wt% V was prepared by high-energy ball-milling during 40 h. The activation characteristics of (Mg–V)nano are rather poor, the hydrogen content [H] reaching 4 wt% after more than 100 h (t4wt%) following the initial exposure of the material to H2. Adding 9 wt% graphite to (Mg–V)nano and resuming the milling operation for 30 min leads to the formation of (Mg–V)nano /G, which exhibits a t4wt% value of only 10 min. The addition of more than 9 wt% graphite to (Mg–V)nano does not lead to any significant reduction of the t4wt% value. However, extending the milling period with graphite over 30 min leads to a steady increase in t4wt% and, thus, to a deterioration of the activation characteristics. Comparison of the behavior of graphite with other C-based compounds revealed that perylene (C20H12) and pentacene (C22H14), which are made of linked benzene rings, and thus have a 2D structure similar to that of the graphene sheet, are as effective as graphite in improving the activation characteristics of (Mg–V)nano. A structural investigation of (Mg–V)nano /G as a function of the milling time through both C 1s core-level x-ray photoelectron spectroscopy and C K edge x-ray absorption near-edge spectroscopy has shown that the integrity of graphite is progressively lost as the milling period is extended over 30 min. On the basis of these results, it is hypothesized that the adsorption of graphene layer on freshly created Mg surfaces and the formation of highly reactive C species during milling prevents the re-formation of the surface oxide layer responsible for the poor activation characteristics of untreated (Mg–V)nano