Residual‐density mapping and site‐selective determination of anomalous scattering factors to examine the origin of the Fe K pre‐edge peak of magnetite

The electron‐density distribution and the contribution to anomalous scattering factors for Fe ions in magnetite have been analyzed by X‐ray resonant scattering at the pre‐edge of Fe K absorption. Synchrotron X‐ray experiments were carried out using a conventional four‐circle diffractometer in the right‐handed circular polarization. Difference‐Fourier synthesis was applied with a difference in structure factors measured on and off the pre‐edge (E_on = 7.1082 keV, E_off = 7.1051 keV). Electron‐density peaks due to X‐ray resonant scattering were clearly observed for both A and B sites. The real part of the anomalous scattering factor f′ has been determined site‐independently, based on the crystal‐structure refinements, to minimize the squared residuals at the Fe K pre‐edge. The f′ values obtained at E_on and E_off are ?7.063 and ?6.682 for the A site and ?6.971 and ?6.709 for the B site, which are significantly smaller than the values of ?6.206 and ?5.844, respectively, estimated from the Kramers–Kronig transform. The f′ values at E_on are reasonably smaller than those at E_off. Our results using a symmetry‐based consideration suggest that the origin of the pre‐edge peak is Fe ions occupying both A and B sites, where p–d mixing is needed with hybridized electrons of Fe in both sites overlapping the neighbouring O atoms.     

Extended X‐ray absorption fine structure and multiple‐scattering simulation of nickel dithiolene complexes Ni[S2C2(CF3)2]2n (n = −2, −1, 0) and an olefin adduct Ni[S2C2(CF3)2]2(1‐hexene)

A series of Ni dithiolene complexes Ni[S₂C₂(CF₃)]₂ⁿ (n = ?2, ?1, 0) ( 1 , 2 , 3 ) and a 1‐hexene adduct Ni[S₂C₂(CF₃)₂]₂(C₆H₁₂) ( 4 ) have been examined by Ni K‐edge X‐ray absorption near‐edge structure (XANES) and extended X‐ray absorption fine‐structure (EXAFS) spectroscopies. Ni XANES for 1 – 3 reveals clear pre‐edge features and approximately +0.7 eV shift in the Ni K‐edge position for `one‐electron' oxidation. EXAFS simulation shows that the Ni—S bond distances for 1 , 2 and 3 (2.11–2.16 Å) are within the typical values for square planar complexes and decrease by ～0.022 Å for each `one‐electron' oxidation. The changes in Ni K‐edge energy positions and Ni—S distances are consistent with the `non‐innocent' character of the dithiolene ligand. The Ni—C interactions at ～3.0 Å are analyzed and the multiple‐scattering parameters are also determined, leading to a better simulation for the overall EXAFS spectra. The 1‐hexene adduct 4 presents no pre‐edge feature, and its Ni K‐edge position shifts by ?0.8 eV in comparison with its starting dithiolene complex 3 . Consistently, EXAFS also showed that the Ni—S distances in 4 elongate by ～0.046 Å in comparison with 3 . The evidence confirms that the neutral complex is `reduced' upon addition of olefin, presumably by olefin donating the π‐electron density to the LUMO of 3 as suggested by UV/visible spectroscopy in the literature.     

The competitive growth of cubic domains in Ti1–xAlxN films studied by diffraction anomalous near‐edge structure spectroscopy

Titanium and aluminium nitride films deposited by magnetron sputtering generally grow as columnar domains made of oriented nanocrystallites with cubic or hexagonal symmetry depending on Al content, which are embedded in more disordered grain boundaries. The substitution of Al atoms for Ti in the cubic lattice of the films improves their resistance to wear and oxidation, allowing their use as protective coatings. Ti K‐edge X‐ray absorption spectroscopy, which probes both crystallized and more disordered grain boundaries, and X‐ray diffraction anomalous fine structure, which is sensitive to short‐ and long‐range order within a given crystallized domain, are carried out on a set of Ti_1–xAl_xN films deposited by magnetron sputtering on Si substrates. Attention is paid to the shape of the pre‐edge region, which is sensitive to the symmetry of the site occupied by Ti atoms, either octahedral in face‐centred‐cubic Ti‐rich (TiN, Ti_0.54Al_0.46N) samples or tetrahedral in hexagonal‐close‐packed Al‐rich (Ti_0.32Al_0.68N) films. In order to obain information on the titanium environment in the well crystallized areas, subtraction of the smooth part of the energy‐dependent structure factor for the Bragg reflections is applied to the pre‐edge region of the diffraction anomalous data in order to restore their spectroscopic appearance. A flat pre‐edge is related to the typical octahedral environment of Ti atoms for cubic reflections. The difference observed between pre‐edge spectra associated with face‐centred‐cubic 200 and 111 Bragg reflections of Ti_0.54Al_0.46N is assigned to Ti enrichment of 111 large well ordered domains compared with the more disordered 200 ones. The sharp peak observed in the spectrum recorded from the hexagonal 002 peak of Ti_0.32Al_0.68N can be regarded as a standard for the pure tetrahedral Ti environment in hexagonal‐close‐packed nitride.     

Au/TiO2 nanorod‐based Schottky‐type UV photodetectors

TiO₂ nanorods (NRs) were synthesized on fluorine‐doped tin oxide (FTO) pre‐coated glass substrates using hydrothermal growth technique. Scanning electron microscopy studies have revealed the formation of vertically‐aligned TiO₂ NRs with length of ～2 µm and diameter of 110–128 nm, homogenously distributed over the substrate surface. 130 nm thick Au contacts using thermal evaporation were deposited on the n‐type TiO₂ NRs at room temperature for the fabrication of NR‐based Schottky‐type UV photodetectors. The fabricated Schottky devices functioned as highly sensitive UV photodetectors with a peak responsivity of 134.8 A/W (λ = 350 nm) measured under 3 V reverse bias. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Titanium local structure in tektite probed by X‐ray absorption fine structure spectroscopy

The local structure of titanium in tektites from six strewn fields was studied by Ti K‐edge X‐ray absorption near edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS) in order to provide quantitative data on Ti—O distance and Ti coordination number. The titanium in tektites possessed different coordination environment types. XANES spectra patterns revealed resemblance to high‐temperature TiO₂–SiO₂ glass and TiO₂ anatase. All samples showed that the valence of Ti is 4+. Based on the Ti—O distances, coordination numbers and radial distribution function determined by EXAFS analyses, the tektites were classified into three types: type I, Ti occupies a four‐coordinated tetrahedral site with Ti—O distances of 1.84–1.79 Å; type II, Ti occupies a five‐coordinated trigonal bipyramidal or tetragonal pyramidal site with Ti—O distances of 1.92–1.89 Å; type III, Ti occupies a six‐coordinated octahedral site with Ti—O distances of 2.00–1.96 Å. Although Ti occupies the TiO₆ octahedral site in most titanium minerals under ambient conditions, some tektites have four‐ and five‐coordinated Ti. This study indicated that the local structure of Ti might change in impact events and the following stages.     

Solid energy calibration standards for P K‐edge XANES: electronic structure analysis of PPh4Br

P K‐edge X‐ray absorption near‐edge structure (XANES) spectroscopy is a powerful method for analyzing the electronic structure of organic and inorganic phosphorus compounds. Like all XANES experiments, P K‐edge XANES requires well defined and readily accessible calibration standards for energy referencing so that spectra collected at different beamlines or under different conditions can be compared. This is especially true for ligand K‐edge X‐ray absorption spectroscopy, which has well established energy calibration standards for Cl (Cs₂CuCl₄) and S (Na₂S₂O₃·5H₂O), but not neighboring P. This paper presents a review of common P K‐edge XANES energy calibration standards and analysis of PPh₄Br as a potential alternative. The P K‐edge XANES region of commercially available PPh₄Br revealed a single, highly resolved pre‐edge feature with a maximum at 2146.96 eV. PPh₄Br also showed no evidence of photodecomposition when repeatedly scanned over the course of several days. In contrast, we found that PPh₃ rapidly decomposes under identical conditions. Density functional theory calculations performed on PPh₃ and PPh₄⁺ revealed large differences in the molecular orbital energies that were ascribed to differences in the phosphorus oxidation state (III versus V) and molecular charge (neutral versus +1). Time‐dependent density functional theory calculations corroborated the experimental data and allowed the spectral features to be assigned. The first pre‐edge feature in the P K‐edge XANES spectrum of PPh₄Br was assigned to P 1s → P‐C π* transitions, whereas those at higher energy were P 1s → P‐C σ*. Overall, the analysis suggests that PPh₄Br is an excellent alternative to other solid energy calibration standards commonly used in P K‐edge XANES experiments.     

Electronic structure effects on B K‐edge XANES of minerals

In order to assess the usability of X‐ray absorption near‐edge structure (XANES) for studying the structure of BO_n‐containing materials, the dependence of theoretical XANES at the B K‐edge on the way the scattering potential is constructed is investigated. Real‐space multiple‐scattering calculations are performed for self‐consistent and non‐self‐consistent potentials and for different ways of dealing with the core hole. It is found that in order to reproduce the principal XANES features it is sufficient to use a non‐self‐consistent potential with a relaxed and screened core hole. Employing theoretical modelling of XANES for studying the structure of boron‐containing glasses is thus possible. The core hole affects the spectrum significantly, especially in the pre‐edge region. In contrast to minerals, B K‐edge XANES of BPO₄ can be reproduced only if a self‐consistent potential is employed.     

Electronic structure and enhanced visible‐light absorption of N,B‐codoped TiO2

We present GGA+U calculations to investigate the electronic structure and visible‐light absorption of N,B‐codoped anatase TiO₂. The N_sB_i (substitutional N, interstitial B) codoped TiO₂ produces significant Ti 3d and N 2p mid‐gap states when the distance of N and B atoms is far, whereas the N_iB_i (interstitial N and B) and N_sB_s (substitutional N and B) codoped TiO₂ prefer to form localized p states at 0.3–1.2 eV above the valence band maximum. Further, the optical band edges of the three codoped systems shift slightly to the visible region, but only the far‐distance N_sB_i codoped TiO₂ clearly shows an optical transition. These results indicate that N_sB_i codoped TiO₂ has a dominant contribution to the optical absorption of N,B‐codoped TiO₂. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Zr and Ba edge phenomena in the scintillation intensity of fluorozirconate‐based glass‐ceramic X‐ray detectors

The energy‐dependent scintillation intensity of Eu‐doped fluorozirconate glass‐ceramic X‐ray detectors has been investigated in the energy range from 10 to 40 keV. The experiments were performed at the Advanced Photon Source, Argonne National Laboratory, USA. The glass ceramics are based on Eu‐doped fluorozirconate glasses, which were additionally doped with chlorine to initiate the nucleation of BaCl₂ nanocrystals therein. The X‐ray excited scintillation is mainly due to the 5d–4f transition of Eu²⁺ embedded in the BaCl₂ nanocrystals; Eu²⁺ in the glass does not luminesce. Upon appropriate annealing the nanocrystals grow and undergo a phase transition from a hexagonal to an orthorhombic phase of BaCl₂. The scintillation intensity is investigated as a function of the X‐ray energy, particle size and structure of the embedded nanocrystals. The scintillation intensity versus X‐ray energy dependence shows that the intensity is inversely proportional to the photoelectric absorption of the material, i.e. the more photoelectric absorption the less scintillation. At 18 and 37.4 keV a significant decrease in the scintillation intensity can be observed; this energy corresponds to the K‐edge of Zr and Ba, respectively. The glass matrix as well as the structure and size of the embedded nanocrystals have an influence on the scintillation properties of the glass ceramics.     

A Li K‐edge XANES study of salts and minerals

The first comprehensive Li K‐edge XANES study of a varied suite of Li‐bearing minerals is presented. Drastic changes in the bonding environment for lithium are demonstrated and this can be monitored using the position and intensity of the main Li K‐absorption edge. The complex silicates confirm the assignment of the absorption edge to be a convolution of triply degenerate p‐like states as previously proposed for simple lithium compounds. The Li K‐edge position depends on the electronegativity of the element to which it is bound. The intensity of the first peak varies depending on the existence of a 2p electron and can be used to evaluate the degree of ionicity of the bond. The presence of a 2p electron results in a weak first‐peak intensity. The maximum intensity of the absorption edge shifts to lower energy with increasing SiO₂ content for the lithium aluminosilicate minerals. The bond length distortion of the lithium aluminosilicates decreases with increasing SiO₂ content, thus increased distortion leads to an increase in edge energy which measures lithium's electron affinity.     

Spin‐polarized electron gas in Co2M Si/SrTiO3 (M = Ti,V, Cr,Mn, and Fe) heterostructures

Spin‐polarized density functional theory is used to study the TiO₂ terminated interfaces between the magnetic Heusler alloys Co₂Si (M = Ti, V, Cr, Mn, and Fe) and the non‐polar band insulator SrTiO₃. The structural relaxation at the interface turns out to depend systematically on the lattice mis‐ match. Charge transfer from the Heusler alloys (mainly the M 3d orbitals) to the Ti d_xy orbitals of the TiO₂ interface layer is found to gradually grow from M = Ti to Fe, resulting in an electron gas with increasing density of spin‐polarized charge carriers. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

An unconventional method for measuring the Tc L3‐edge of technetium compounds

Tc L₃‐edge XANES spectra have been collected on powder samples of SrTcO₃ (octahedral Tc⁴⁺) and NH₄TcO₄ (tetrahedral Tc⁷⁺) immobilized in an epoxy resin. Features in the Tc L₃‐edge XANES spectra are compared with the pre‐edge feature of the Tc K‐edge as well as other 4d transition metal L₃‐edges. Evidence of crystal field splitting is obvious in the Tc L₃‐edge, which is sensitive to the coordination number and oxidation state of the Tc cation. The Tc L₃ absorption edge energy difference between SrTcO₃ (Tc⁴⁺) and NH₄TcO₄ (Tc⁷⁺) shows that the energy shift at the Tc L₃‐edge is an effective tool for studying changes in the oxidation states of technetium compounds. The Tc L₃‐edge spectra are compared with those obtained from Mo and Ru oxide standards with various oxidation states and coordination environments. Most importantly, fitting the Tc L₃‐edge to component peaks can provide direct evidence of crystal field splitting that cannot be obtained from the Tc K‐edge.     

Temperature dependence of the Ho L2‐edge XMCD spectra of Ho6Fe23

An X‐ray magnetic circular dichroism (XMCD) study performed at the Ho L_2,3‐edges in Ho₆Fe₂₃ as a function of temperature is presented. It is demonstrated that the anomalous temperature dependence of the Ho L₂‐edge XMCD signal is due to the magnetic contribution of Fe atoms. By contrast, the Ho L₃‐edge XMCD directly reflects the temperature dependence of the Ho magnetic moment. By combining the XMCD at both Ho L₂‐ and L₃‐edges, the possibility of determining the temperature dependence of the Fe magnetic moment is demonstrated. Then, both μ_Ho(T) and μ_Fe(T) have been determined by tuning only the absorption L‐edges of Ho. This result opens new possibilities of applying XMCD at these absorption edges to obtain quantitative element‐specific magnetic information that is not directly obtained by other experimental tools.     

Phosphorus L2,3‐edge XANES: overview of reference compounds

Synchrotron‐based X‐ray absorption near‐edge structure (XANES) spectroscopy is becoming an increasingly used tool for the element speciation in complex samples. For phosphorus (P) almost all XANES measurements have been carried out at the K‐edge. The small number of distinctive features at the P K‐edge makes in some cases the identification of different P forms difficult or impossible. As indicated by a few previous studies, the P L_2,3‐edge spectra were richer in spectral features than those of the P K‐edge. However, experimentally consistent spectra of a wide range of reference compounds have not been published so far. In this study a library of spectral features is presented for a number of mineral P, organic P and P‐bearing minerals for fingerprinting identification. Furthermore, the effect of radiation damage is shown for three compounds and measures are proposed to reduce it. The spectra library provided lays a basis for the identification of individual P forms in samples of unknown composition for a variety of scientific areas.     

Ru‐doped TiO2 nanotubes: Improved performance in dye‐sensitized solar cells

In the present work we use a series of Ti–Ru alloys, with minor amounts of Ru (0.01, 0.02, 0.05 and 0.2 at%) to grow anodic self‐organized Ru‐doped TiO₂ nanotube layers. When used in dye‐sensitized solar cells (DSSCs), the nanotube layers with an optimum amount of Ru (0.02 at% Ru in the alloy) show a considerable increase in solar cell efficiency (η = 5.2%) under AM1.5 (100 mW/cm²) conditions compared with non‐doped TiO₂ nanotubes (η = 4.3%).

     

Room‐Temperature Synthesis of High Surface Area Anatase TiO2 Exhibiting a Complete Lithium Insertion Solid Solution

The synthesis of highly divided anatase TiO₂ nanoparticles displaying 300 m² g^?1 surface area is achieved by following a two‐step synthetic process at room temperature. The particles exhibit a needle‐like morphology composed of self‐assembled 4 nm nanoparticles. The crystallization process from amorphous TiO₂.1.6H₂O to oriented aggregation of anatase TiO₂ proceeds according to a slow solid dehydration process taking place in a large range of pH in deionized water (1 < pH < 12) or alternatively when including a low amount of NH₄F_(aq) in solution. Driven by their high surface area enhancing the chemical/electrochemical reactivity, it is reported in the case of the anatase TiO₂ that a modification in the lithium insertion mechanism is no longer attributable to a two‐phase reaction between the two‐end members Li_εTiO₂ and Li_0.5±αTiO₂ when downsizing the particle size, but instead through a complete solid solution all along the composition range.     

Effect of irradiation on ethyl 2‐amino‐4,5,6,7‐tetrahydrobenzo[b]thiophene‐3‐carboxylate: an ESR study

Ethyl 2‐amino‐4,5,6,7‐tetrahydrobenzo[b]thiophene‐3‐carboxylate [C₁₁H₁₅NO₂S] was synthesized by the Gewald method. Its single crystals were grown from an alcohol/ethyl acetate solution at 15 °C and characterized using IR and ¹H‐NMR. These single crystals were irradiated for 72 h at 298 K by a ⁶⁰Co gamma source with a dose speed of 0.864 kGy/h. After irradiation, electron spin resonance (ESR) measurements were carried out to study radiation‐induced radicals in the temperature range from 120 to 450 K. Additionally, for the single crystal, ESR angular dependencies were measured in the xy, xz and yz planes of the substance. This irradiated single crystal was analyzed based on the ESR spectra. Analysis of the spectra revealed that the radical was formed by a C–H bond fission at the carbon end of the substance. It was also observed that the color of the sample changed after irradiation. The hyperfine and g parameters were determined from the experimental spectra. It was inferred from these results that the hyperfine parameters and g value exhibited anisotropic behavior. The average values of these parameters were calculated as follows: g = 2.0088, A_H1=H2 = 20.70 G, A_H3=H4 = 10.80 G, A_Ha = 4.59 G, A_Hb = 3.24 G and, A_N = 6.10 G. Copyright © 2011 John Wiley & Sons, Ltd.     

Improved surface‐enhanced Raman scattering properties of TiO2 nanoparticles by Zn dopant

In this paper, pure and Zn‐doped TiO₂ nanoparticles (NPs) with various content of Zn were prepared by a sol–hydrothermal method and were employed as active substrates for surface‐enhanced Raman scattering (SERS). On the 3% Zn‐doped TiO₂ substrate, 4‐mercaptobenzoic acid(4‐MBA) molecules exhibit a higher SERS intensity by a factor of 6, as compared with the native enhancement of 4‐MBA adsorbed on undoped TiO₂ NPs. Moreover, the higher SERS activity was still observed on the 3% Zn‐doped TiO₂ NPs at temperature even up to 125 °C. These results indicate that an appropriate amount of Zn doping can improve the SERS performances of TiO₂ SERS‐active substrates. The introduction of Zn dopant can enrich the surface states (defects) of TiO₂ and improve the separation efficiency of photo‐generated charge carriers (electrons and holes) in TiO₂, according to measurements of X‐ray diffraction, UV‐visible diffuse reflectance spectroscopy, and photoluminescence, which are responsible for the influence of Zn dopant on the improved SERS performances of TiO₂ NPs. Copyright © 2009 John Wiley & Sons, Ltd.     

Thickness and angular dependence of the L‐edge X‐ray absorption of nickel thin films

We report on the near‐edge X‐ray absorption fine structure spectroscopy of the L₃ (2p_3/2) and L₂ (2p_1/2) edges for ferromagnetic pure nickel transition metal and show that the L_2,3 edge peak intensity and satellite feature at ~6 eV above the L₃ edge in nickel increase with increasing nickel film thickness both in the total electron yield and transmission modes. The absorption spectra of nickel metal, however, exhibit strong angular‐dependent effects when measured in total electron yield mode. In addition, we calculated the mean electron escape depth of the emitted electrons (λ_e), which was found for pure nickel metal to be λ_e=25 ± 2 Å. We point out the advantages of the total electron yield technique for the study of the L‐edge of 3d transition metals. Copyright © 2011 John Wiley & Sons, Ltd.     

K‐edge XANES analysis of sulfur compounds: an investigation of the relative intensities using internal calibration

Sulfur K‐edge XANES (X‐ray absorption near‐edge structure) spectroscopy is an excellent tool for determining the speciation of sulfur compounds in complex matrices. This paper presents a method to quantitatively determine the kinds of sulfur species in natural samples using internally calibrated reference spectra of model compounds. Owing to significant self‐absorption of formed fluorescence radiation in the sample itself the fluorescence signal displays a non‐linear correlation with the sulfur content over a wide concentration range. Self‐absorption is also a problem at low total absorption of the sample when the sulfur compounds are present as particles. The post‐edge intensity patterns of the sulfur K‐edge XANES spectra vary with the type of sulfur compound, with reducing sulfur compounds often having a higher post‐edge intensity than the oxidized forms. In dilute solutions (less than 0.3–0.5%) it is possible to use sulfur K‐edge XANES reference data for quantitative analysis of the contribution from different species. The results show that it is essential to use an internal calibration system when performing quantitative XANES analysis. Preparation of unknown samples must take both the total absorption and possible presence of self‐absorbing particles into consideration.