X-ray Absorption Studies Research Articles

Probing disorder in high-pressure cubic tin (IV) oxide: a combined X-ray diffraction and absorption study.

The transparent conducting oxide, SnO2, is a promising optoelectronic material with predicted tailorable properties via pressure-mediated band gap opening. While such electronic properties are typically modeled assuming perfect crystallinity, disordering of the O sublattice under pressure is qualitatively known. Here a quantitative approach is thus employed, combining extended X-ray absorption fine-structure (EXAFS) spectroscopy with X-ray diffraction, to probe the extent of Sn-O bond anharmonicities in the high-pressure cubic (Pa\bar{3}) SnO2 - formed as a single phase and annealed by CO2 laser heating to 2648 ± 41 K at 44.5 GPa. This combinational study reveals and quantifies a large degree of disordering in the O sublattice, while the Sn lattice remains ordered. Moreover, this study describes implementation of direct laser heating of non-metallic samples by CO2 laser alongside EXAFS, and the high quality of data which may be achieved at high pressures in a diamond anvil cell when appropriate thermal annealing is applied.

(Invited) Structural Design Via Group Energy Materials for High-Efficient Photoelectrochemical Hydrogen Evolution Reaction

Photoelectrochemical hydrogen evolution reaction (HER) and electrochemical reduction of carbon dioxide (CO2RR) to chemical fuels have the potential to relieve the energy problem. The performance of these devices depends intimately on the properties of their materials. Herein we modify the inorganic sulfide by organic polymer molecules and thus improve the electronic and band structure, optimization the overall hydrogen evolution performance. Meanwhile, earth-abundant first-row (3d) transition-metal-based catalysts have been developed for the oxygen-evolution reaction (OER); however, they operate at overpotentials significantly above thermodynamic requirements. Density functional theory suggested that non-3d high-valency metals such as tungsten can modulate 3d metal oxides, providing near-optimal adsorption energies for OER intermediates. We developed a room-temperature synthesis to produce gelled oxy-hydroxide materials with an atomically homogeneous metal distribution. These gelled FeCoW oxy-hydroxide exhibits the lowest overpotential (191 mV) reported at 10 milliamperes per square centimeter in alkaline electrolyte. The catalyst shows no evidence of degradation following more than 500 hours of operation. X-ray absorption and computational studies reveal a synergistic interplay between W, Fe and Co in producing a favorable local coordination environment and electronic structure that enhance the energetics for OER. We also used in operando soft X-ray absorption to quantify the ratio of the Ni4+ and Ni2+ active site, achieved an OER overpotential of only 330 mV under neutral condition, better than novel metal oxide catalyst, IrO2, with ideal stability within 100 h. With the modulating of 3d coordination environment and electronic structure, maximum optimization the energetics of oxygen evolution reaction, we finally achieved the lowest over potential for OER and hence improved the HER efficiency. Furthermore, we engineered the metal sites by using non-metal element and which significantly improved the efficiency of CO2RR to formate.

Tuning the Structure of Pt Nanoparticles through Support Interactions: An in Situ Polarized X-ray Absorption Study Coupled with Atomistic Simulations.

Interactions of nanoparticles (NPs) with their environment may have a pronounced effect on their structure and shape as well as on their functionality in applications such as catalysis. It is therefore crucial to disentangle the particle–adsorbate and particle–support interaction effects on the particle shape, its local structure, atomic dynamics, and its possible anisotropies. In order to gain insight into the support effect, we carried out an X-ray absorption fine-structure spectroscopy (XAFS) investigation of adsorbate- and ligand-free size-selected Pt NPs deposited on two different supports in ultrahigh vacuum. Polarization-dependent XAFS measurements, neural network-based analysis of X-ray absorption near-edge structure data, and reverse Monte Carlo (RMC) simulations of extended X-ray absorption fine structure (EXAFS) were used to resolve the 3D shape of the NPs and details of their local structure. A synergetic combination of advanced in situ XAFS analysis with atomic force microscopy and scanning tunneling microscopy (STM) imaging provides uniquely detailed information about the particle–support interactions and the NP/support buried interface, not accessible to any experimental technique, when considered alone. In particular, our combined approach reveals differences in the structure of Pt NPs deposited on TiO2(110) and SiO2/Si(111). Pt NPs on SiO2 assume a spherical-like 3D shape and weakly interact with the support. In contrast, the effective shape of analogously synthesized Pt NPs on TiO2(110) after annealing at 600 °C is found to be a truncated octahedron with (100) top and interfacial facets that are encapsulated by the TiO2 support. Modeling disorder effects in these NPs using an RMC approach reveals differences in bond-length distributions for NPs on different supports and allows us to analyze their anisotropy, which may be crucial for the interpretation of support-dependent atomic dynamics and can have an impact on the understanding of the catalytic properties of these NPs.

Novel catalytically active Pd/Ru bimetallic nanoparticles synthesized by Bacillus benzeovorans

Bacillus benzeovorans assisted and supported growth of ruthenium (bio-Ru) and palladium/ruthenium (bio-Pd@Ru) core@shell nanoparticles (NPs) as bio-derived catalysts. Characterization of the bio-NPs using various electron microscopy techniques and high-angle annular dark field (HAADF) analysis confirmed two NP populations (1–2 nm and 5–8 nm), with core@shells in the latter. The Pd/Ru NP lattice fringes, 0.231 nm, corresponded to the (110) plane of RuO2. While surface characterization using X-ray photoelectron spectroscopy (XPS) showed the presence of Pd(0), Pd(II), Ru(III) and Ru(VI), X-ray absorption (XAS) studies of the bulk material confirmed the Pd speciation (Pd(0) and Pd(II)- corresponding to PdO), and identified Ru as Ru(III) and Ru(IV). The absence of Ru–Ru or Ru–Pd peaks indicated Ru only exists in oxide forms (RuO2 and RuOH), which are surface-localized. X ray diffraction (XRD) patterns did not identify Pd-Ru alloying. Preliminary catalytic studies explored the conversion of 5-hydroxymethyl furfural (5-HMF) to the fuel precursor 2,5-dimethyl furan (2,5-DMF). Both high-loading (9.7 wt.% Pd, 6 wt.% Ru) and low-loading (2.4 wt.% Pd, 2 wt.% Ru) bio-derived catalysts demonstrated high conversion efficiencies (~95%) and selectivity of ~63% (~20% better than bio-Ru NPs) and 58%, respectively. These materials show promising future scope as efficient low-cost biofuel catalysts.

Metal phthalocyanines interaction with Co mediated by a moiré graphene superlattice.

The assembling of metal phthalocyanines on the rippled moiré superlattice of graphene/Ir(111) intercalated with one Co layer is driven by the site-dependent polarization field induced by the incommensurate graphene-Co interface. We have performed an X-ray absorption and photoemission study to unveil the role of the metallic centers and of the organic ligands in the molecule-Co interaction process mediated by graphene. Notably, we consider different electronic molecular orbitals, i.e. phthalocyanines with Cu and Mn metallic ions. The spectroscopic response suggests almost unaltered CuPc molecular states upon adsorption, and the rippled graphene carpet decouples completely the electronic interaction between the molecules and the Co layer, while a slight hybridization is present for MnPcs. MnPc molecules, trapped in the valleys of the moiré graphene superlattice, slightly intermix, through the orbitals protruding out of the molecular plane, with the underlying Co, while the organic ligands are almost unaltered. Graphene acts as an interlayer and mediates the interaction between metal phthalocyanines and the metallic substrate, preventing a strong chemical intermixing and enabling the assembly of almost unaltered molecules, preserving their electronic/magnetic state.

A laboratory-based double X-ray spectrometer for simultaneous X-ray emission and X-ray absorption studies

A newly developed laboratory-based double X-ray spectrometer enables systematic and simultaneous X-ray emission (XES) and X-ray absorption (XAS) measurements.

Differential uranyl(v) oxo-group bonding between the uranium and metal cations from groups 1, 2, 4, and 12; a high energy resolution X-ray absorption, computational, and synthetic study.

The uranyl(vi) 'Pacman' complex [(UO2)(py)(H2L)] A (L = polypyrrolic Schiff-base macrocycle) is reduced by Cp2Ti(η2-Me3SiC[triple bond, length as m-dash]CSiMe3) and [Cp2TiCl]2 to oxo-titanated uranyl(v) complexes [(py)(Cp2TiIIIOUO)(py)(H2L)] 1 and [(ClCp2TiIVOUO)(py)(H2L)] 2. Combination of ZrII and ZrIV synthons with A yields the first ZrIV-uranyl(v) complex, [(ClCp2ZrOUO)(py)(H2L)] 3. Similarly, combinations of Ae0 and AeII synthons (Ae = alkaline earth) afford the mono-oxo metalated uranyl(v) complexes [(py)2(ClMgOUO)(py)(H2L)] 4, [(py)2(thf)2(ICaOUO)(py) (H2L)] 5; the zinc complexes [(py)2(XZnOUO)(py)(H2L)] (X = Cl 6, I 7) are formed in a similar manner. In contrast, the direct reactions of Rb or Cs metal with A generate the first mono-rubidiated and mono-caesiated uranyl(v) complexes; monomeric [(py)3(RbOUO)(py)(H2L)] 8 and hexameric [(MOUO)(py)(H2L)]6 (M = Rb 8b or Cs 9). In these uranyl(v) complexes, the pyrrole N-H atoms show strengthened hydrogen-bonding interactions with the endo-oxos, classified computationally as moderate-strength hydrogen bonds. Computational DFT MO (density functional theory molecular orbital) and EDA (energy decomposition analysis), uranium M4 edge HR-XANES (High Energy Resolution X-ray Absorption Near Edge Structure) and 3d4f RIXS (Resonant Inelastic X-ray Scattering) have been used (the latter two for the first time for uranyl(v) in 7 (ZnI)) to compare the covalent character in the UV-O and O-M bonds and show the 5f orbitals in uranyl(vi) complex A are unexpectedly more delocalised than in the uranyl(v) 7 (ZnI) complex. The Oexo-Zn bonds have a larger covalent contribution compared to the Mg-Oexo/Ca-Oexo bonds, and more covalency is found in the U-Oexo bond in 7 (ZnI), in agreement with the calculations.

Expression of concern: Ligand discrimination of myoglobin in solution: an iron L-edge X-ray absorption study of the active centre.

Expression of concern for 'Ligand discrimination of myoglobin in solution: an iron L-edge X-ray absorption study of the active centre' by Kathrin M. Lange et al., Chem. Commun., 2013, 49, 4163-4165.

Structural studies of spray pyrolysis synthesized oxygen deficient anatase TiO2 thin films by using X-ray absorption spectroscopy.

The present work focuses on synthesis and X-ray absorption studies of single phase oxygen deficient anatase TiO2 thin films. These films are prepared in a two-step method viz. the synthesis of near stoichiometric anatase TiO2 films using an open atmospheric spray pyrolysis method followed by vacuum annealing at their corresponding synthesis temperatures (Ts = 450 °C, 500 °C) for different time durations (t = 2, 4, 6, 8 hours). XRD and Raman studies of these films ascertained the formation and retention of the anatase phase post annealing, indicating that there was no phase change due to prolonged annealing. Extended X-ray absorption spectra (EXAFS) and X-ray absorption near edge spectra (XANES) revealed the presence of an oxygen vacancy and its effect on the local co-ordination. The co-ordination number obtained from EXAFS analysis revealed that the number density of the oxygen vacancy is higher in the case of thin films synthesized at 450 °C than in the case of the films synthesized at 500 °C. As the oxygen vacancy leads to changes in local co-ordination, which in turn have a profound effect on the pre-edge features of the X-ray absorption spectra (XAS), theoretically simulated XAS spectra of pure anatase TiO2 and oxygen deficient anatase TiO2 were generated using FEFF and were found to match with the experimentally observed spectra. In addition, the ambiguities in whether a change in the metal-oxygen bond length has any effect on the pre-edge features or not were delineated in the present study by comparing the pre-edge peak positions of the oxygen deficient TiO2 films. Our results matched with those of some of the researchers who have studied the rutile phase TiO2, wherein it was concluded that in the case of the titanium-oxygen system, the mean Ti-O bond length does have an effect on the pre-edge peak position. It was observed that as the Ti-O mean bond length increases, the pre-edge peak positions shift towards lower energy, which is in concurrence with the literature available for other Ti-O systems with similar geometry. The second pre-edge peak intensity, which is a measure of disorder, is higher for TiO2450 °C-2 h and TiO2500 °C-2 h thin films. This is attributed to the annealing effect, which suggests that TiO6 octahedrons are the most disordered for thin films annealed for 2 h and become more ordered upon annealing for longer times.

Nanofocused synchrotron X-ray absorption studies of the intracellular redox state of an organometallic complex in cancer cells.

Synchrotron nanoprobe X-ray absorption (XAS) studies of a potent organo-osmium arene anticancer complex in ovarian cancer cells at subcellular resolution allow detection and quantification of both OsII and OsIII species, which are distributed heterogeneously in different areas of the cells.

High-Resolution Near-Edge X-ray Absorption Fine Structure Study of Condensed Polyacenes

We present a systematic study of high-resolution near-edge X-ray absorption fine structure (NEXAFS) spectra of well-ordered films of condensed benzene and polyacenes, namely naphthalene, anthracene, tetracene, and pentacene. Increased spectral complexity with increasing molecular size is observed: NEXAFS features decrease in intensity and move to lower photon energy (red shift) as the size of the aromatic system grows. Moreover, a second group of transitions arises. The dichroism in the C K-edge spectra increases with molecular size. While benzene molecules are randomly oriented, the polyacenes preferentially lie flat with increasing molecular size. Vibrational fine structures coupled to the C 1s π* transitions are apparent for all investigated molecules. The energy position of the onset of the first resonance decreases from 284.86 eV for benzene to 283.26 eV for pentacene. Calculations of absolute band envelopes with time-dependent density functional theory (TDDFT), followed by analysis of the transition...

Role of oxygen vacancies in Co/Ni Substituted CeO2: A comparative study

Single phase Co/Ni substituted CeO2 nanoparticles reveal the importance of oxygen vacancies on the electronic properties of the materials. The effect of Co/Ni substitution on the structural, optical, and photoluminescence properties of CeO2 have been studied systematically. Lattice shrinks and hence strain increase owing to incorporation of Co/Ni in CeO2. Optical absorption analysis shows a red shift in band-gap with Co/Ni substitution. Photoluminescence studies reveals increase in defect concentration which causes quenching of PL emission. Vibration modes at ~460 cm−1 in Raman spectra indicates incorporation of Co/Ni in CeO2 lattice whereas a broad peak appearing at ~ 540–640 cm−1 with substitution depicts the defects related to the increase in oxygen vacancy. X-ray absorption (XAS) studies show that Co/Ni substitution maintains local structure while Ce4+ → Ce3+ concentration increases to preserve the charge neutrality in lattice.

Structure and Transport Properties of Nickel-Implanted CoSb3 Skutterudite Thin Films Synthesized via Pulsed Laser Deposition

The present study reports the effect of Ni ion implantation on the structural, compositional, electrical, and thermoelectric properties and electronic structures of CoSb3 skutterudite thin films deposited on Si substrate by pulsed laser deposition. Ni ions were implanted at 200 keV in CoSb3 thin films at three different fluences: 3 × 1015, 6 × 1015, and 1.5 × 1016 ions/cm2. X-ray diffraction of Ni-implanted films shows an additional phase of Co0.75Ni0.25Sb3. The electrical measurement of pristine films exhibits typical semiconductor behavior, while the Ni-implanted films show an abrupt increase in resistivity, which may be attributed to the formation of Co0.75Ni0.25Sb3 and material modification by the energetic ion beam. The Seebeck coefficient measurements imply that all the films are n-type with maximum Seebeck coefficient of ∼75 μV/K at ∼410 K for 2% Ni-implanted film which is about 7 times higher than the pristine CoSb3 film. The X-ray absorption study of Ni, Co, and Sb spectrum confirms that Ni ions ...

XAS studies of brain-sponge CNClZnO nanostructures using polyaniline as dual source for solar light photocatalysis

Carbon, nitrogen and chlorine triple doped zinc oxide (CNClZnO) nanostructures have been synthesized from zinc acetate dihydrate and polyaniline by in-situ thermal oxidation method. PANI acts as both a soft template and a dopant source. In this study, PANI was taken in three different weight percentages 1%, 5% and 10% and the prepared samples are named as PZnO-1, PZnO-5 and PZnO-10 respectively. X-Ray Diffraction (XRD) analysis shows the formation of pure wurtzite structure in all the samples. Synchrotron based X-Ray Absorption (XAS) studies have been done to study the doping process in ZnO. These confirm the doping of ZnO lattice with carbon, nitrogen and chlorine. The Scanning Electron Microscopy (SEM) images show the formation of porous structures with the surface resembling that of a brain sponge. The band gap of the samples keeps decreasing with increase in the PANI weight %, with the band gap reducing to 2.51 eV for PZnO-10. Finally, these prepared samples are used as photocatalysts for the degradation of methylene blue dye under direct sunlight. Among all the three doped ZnO samples, PZnO-5 showed the highest photocatalytic efficiency with the kinetics rate constant k as 0.0153 min−1.

Carbon-supported Pt during aqueous phenol hydrogenation with and without applied electrical potential: X-ray absorption and theoretical studies of structure and adsorbates

Adsorbed hydrogen and phenol on Pt nanoparticles during (electro)catalytic hydrogenation are explored by combining X-ray absorption spectroscopy and ab initio simulations. Direct evidence for two types of Pt-C bonds at the surface of the metal particles detected by X-ray absorption spectroscopy suggest strong bonding between metal and the carbon support as well as adsorption of phenol nearly parallel to the surface. Hydrogen and phenol compete for accessible Pt sites. The surface concentrations are compatible with the proposal that atomic hydrogen and chemisorbed phenol are the species reacting in the rate-determining step of hydrogenation in the presence and absence of an external cathodic potential. During electrocatalytic hydrogenation the external electric potential controls the concentration of species on the surface, but does not impose structural or electronic property changes of the Pt compared to Pt particles in presence of hydrogen gas. Increasing reaction rates with increasing cathodic potential are attributed to the increase in chemical potential of adsorbed H.

X-ray absorption, photoemission and electron spin resonance studies of Ar+ ion implanted ZnO

Defect induced room temperature ferromagnetism (RT-FM) in Ar+ implanted ZnO single crystal (ZnO-SC) is observe from the electron Spin Resonance (ESR) spectra and is correlated with their electronic structure studies using synchrotron radiation based X-ray absorption near edge structure (XANES) spectroscopy and valence band photoemission spectroscopy (VB-PES). Enhancement of absorption intensity at the edge of XANES spectra of Ar+ implanted ZnO indicates the increase of local density of states (DOS); which arises from the surface defects and/or dangling bonds in ZnO. VB-PES spectra change upon Ar+ implantation, becoming broader, implying the induced surface defects in ZnO-SC. X-ray photoelectron spectroscopy studies confirm that the Zn 2p3/2 core level peak shifts in lower energy side in Ar+ implanted ZnO-SC that is associated with the formation of oxygen vacancies/defects in ZnO-SC which is responsible for the RT-FM.

X-ray absorption study of platinum nanoparticles on an ion-irradiated carbon support

The chemical state and local structure of 2.6-nm-sized platinum (Pt) nanoparticles on an ion-irradiated glassy carbon (GC) substrate were investigated by X-ray absorption fine structure measurements. The partial oxidation of the Pt nanoparticles was confirmed by the peak intensity in the near-edge region of the absorption spectrum. The analysis of the extended region revealed a higher coordination number and shorter bond length of Pt–Pt compared to those of the Pt nanoparticles on the non-ion-irradiated GC. Thus, Pt nanoparticles on the ion-irradiated GC substrate were found to hold a rigid metallic coordination during the oxidation.

Promising light converting BaMoO4:Er3+-Tm3+-Yb3+ phosphors for display and optical temperature sensing

Er3+-Tm3+-Yb3+ tri-doped BaMoO4 phosphors were synthesized by co-precipitation technique and characterized by X-ray diffraction analysis, absorption study and field emission scanning electron microscopy analysis. Upconversion as well as downconversion luminescence studies were performed by using near infrared (980 nm) and ultraviolet (380 nm) excitations. Energy level diagram, pump power dependence and colour coordinate study were utilized to describe the multicolor upconversion emission properties. Under single 980 nm diode laser excitation the dual mode sensing behaviour is realized via Stark sublevels and thermally coupled energy levels of the Tm3+ and Er3+ ions in the prepared tri-doped phosphors. A comparative fluorescence intensity ratio analysis for integrated emission intensities arising from the Stark sublevels {1G4(a) and 1G4(b)} and thermally coupled energy levels {2H11/2 and 4S3/2} of the Tm3+ and Er3+ ions, respectively was carried out in the prepared tri-doped BaMoO4 phosphors. The maximum sensitivity for thermally coupled energy levels of the Er3+ and Stark sublevels of the Tm3+ ion was reported. The developed phosphors could be useful in the display devices and optical thermometric applications.

A novel electrochemical cell for operando X-ray absorption measurements at low energies: Probing electrochemically induced electronic changes in palladium

Operando assessment of electronic properties near the Fermi level is essential to gain new insights into the mechanisms of electrochemical reactions as well as for the development of more efficient electrocatalysts. However, the high vacuum ambient needed for X-ray absorption measurements at low energies has made studies under electrochemical conditions quite challenging. Here, we describe an out-of-chamber setup with a new electrochemical cell that allowed us to performed operando X-ray absorption studies at the low energy of the Pd L3 edge. Using the new electrochemical cell, we were able to probe, for the first time, the changes in electronic properties of carbon-supported Pd nanoparticles induced by the electrochemical oxidation of the Pd surface. Our results demonstrate that the oxidation process produces an increase in the Pd 4d band vacancy, which indicates that charge is transferred from the metal to the adsorbed oxygenated species.

Kr environment in feldspathic glass and melt: A high pressure, high temperature X-ray absorption study

Noble gases are used to trace the evolution of the Earth's atmosphere and magmatic processes. However little is known about their solubility mechanisms in silicate melts or glasses, whereas it could be fundamental to interpret these data. We report here the first in situ X-ray absorption spectroscopy data at the Kr K-edge on a feldspathic Kr-bearing glass and melt (0.5 wt% Kr) up to 2.7 GPa and 1086 °C. Kr fitting of the EXAFS signal shows that Kr is surrounded by oxygen atoms. Two Kr-O distances are identified: 2.49 ± 0.1 Å and 3.32 ± 0.1 Å. Results can be interpreted as two Kr populations. Kr surrounded by O atoms at 3.32 ± 0.1 Å suggests a filling of the large cavities available in the silicate network, in agreement with previous studies. On the contrary, Kr surrounded by O atoms at 2.49 ± 0.1 Å, and the observed +0.6 eV shift of the edge position at high pressure, suggest Kr bonding to O atoms and Kr oxidation inside cages formed by the largest aluminosilicate rings (i.e. ~12-membered-rings). Present results show that heavy noble gases incorporation in silicate melts can no longer be considered as a passive filling of the voids.