X-ray absorption near edge structure and extended X-ray absorption fine structure analysis of Fe(II) aqueous and acetone solutions

X-ray absorption spectroscopy measurements were used to determine the structure of the first coordination shell of Fe(II) ions in aqueous and acetone based solutions. Extended X-ray absorption fine structure analysis coupled with ab initio X-ray absorption near edge structure calculations confirms the octahedral coordination of the iron ion in water based solution. Data collected for acetone rich solutions can be reproduced assuming coexistence of the octahedral Fe(H(2)O)(6)(2+) and tetrahedral [FeCl(4)](2-) complexes. Distortion of the tetrahedral coordination of ion was detected in some of the acetone based solutions.     

Reduction of clay surface-sorbed organometallics during measurement of X-ray photoelectron spectra (XPS)

Some organometallic compounds, e.g. Ph₃SnCl, react on the surface of the smectite clay, laponite. Other compounds, e.g. Br₃TeC₆H₄CH=NCH₂CH₂N=CHC₆H₄TeBr₃, are sorbed onto the organophilic surface of cetylpyridinium-ion-exchanged Wyoming bentonite. X-ray photoelectron spectroscopy (XPS) is an appropriate technique with which to examine the nature of the surface-sorbed species; however, it is demonstrated that decomposition of the organometallic can occur when the clay surface is exposed over a period of time to energetic X-rays. Thus, care must be taken with the interpretation of data of which some features may be the result of the XPS experiment.     

Ab initio DFT studies of oxygen K edge NEXAFS spectra for the V2O3(0001) surface

Theoretical near edge X-ray absorption fine structure (NEXAFS) spectra describing oxygen 1s core excitation have been evaluated for the differently coordinated oxygen species appearing near the V₂O₃(0001) surface with half metal layer V^′OV termination. Adsorption of oxygen above vanadium centers of the V^′OV terminated surface (O_tV^′O termination) results in very strongly bound vanadyl oxygen, which has also been considered for core excitation in this study. The angle-resolved spectra are based on electronic structure calculations using ab initio density functional theory (DFT) together with model clusters. Experimental NEXAFS spectra for V₂O₃(0001) show a rather strong dependence of peak positions and relative intensities on the photon polarization direction. This dependence is well described by the present theoretical spectra and allows us to assign spectral details in the experiment to specific O 1s core excitations where final state orbitals are determined by the local binding environments of the differently coordinated oxygen centers. As a result, a combination of the present theoretical spectra with experimental NEXAFS data enables an identification of differently coordinated surface oxygen species at the V₂O₃(0001) surface.     

X-ray absorption near edge structure study of BN nanotubes and nanothorns

Two boron nitride (BN) nanostructures, the bamboo-like nanotubes and nanothorns where the nanosize h-BN layers are randomly stacked looking like thorns, were synthesized selectively via thermal chemical vapor deposition of B/B(2)O(3) under the NH(3) flow at 1200 degrees C. Electron energy-loss spectroscopy reveals the N-rich h-BN layers with a ratio of B/N = 0.75-0.85. Angle-resolved X-ray absorption near edge structure of these two N-rich nanostructures has been compared with that of h-BN microcrystals. The pi transition in the N K-edge shifts to the lower energy by 0.8-1.0 eV from that of h-BN microcrystals, and the second-order signals of N 1s electrons become significant. We suggest that the N enrichment would decrease the band gap of nanostructures from that of h-BN microcrystals. The Raman spectrum shows the peak broadening due to the defects of N-rich h-BN layers.     

Electronic structure of ZrCuSiAs and ZrCuSiP by X-ray photoelectron and absorption spectroscopy

The electronic structures of quaternary pnictides ZrCuSiPn (Pn=P, As) were analyzed by X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES). Shifts in the core-line XPS and the XANES spectra indicate that the Zr and Cu atoms are cationic, whereas the Si and Pn atoms are anionic, consistent with expectations from simple bonding models. The Cu 2p XPS and Cu L-edge XANES spectra support the presence of Cu¹⁺. The small magnitudes of the energy shifts in the XPS spectra suggest significant covalent character in the Zr-Si, Zr-Pn, and Cu-Pn bonds. On progressing from ZrCuSiP to ZrCuSiAs, the Si atoms remain largely unaffected, as indicated by the absence of shifts in the Si 2p_3/2 binding energy and the Si L-edge absorption energy, while the charge transfer from metal to Pn atoms becomes less pronounced, as indicated by shifts in the Cu K-edge and Zr K, L-edge absorption energies. The transition from two-dimensional character in LaNiAsO to three-dimensional character in ZrCuSiAs proceeds through the development of Si-Si bonds within the [ZrSi] layer and Zr-As bonds between the [ZrSi] and [CuAs] layers.     

Substituent effects in the iron 2p and carbon 1s edge near-edge X-ray absorption fine structure (NEXAFS) spectroscopy of ferrocene compounds

The iron 2p and carbon 1s near-edge X-ray absorption fine structure (NEXAFS) spectra of substituted ferrocene compounds (Fe(Cp-(CH3)5)2, Fe(Cp)(Cp-COOH), Fe(Cp-COOH)2, and Fe(Cp-COCH3)2) are reported and are interpreted with the aid of extended Hückel molecular orbital (EHMO) theory and density functional theory (DFT). Significant substituent effects are observed in both the Fe 2p and C 1s NEXAFS spectra. These effects can be related to the electron donating/withdrawing properties of the cyclopentadienyl ligands and their substituents as well as the presence of pi* conjugation between the cyclopentadienyl ligand and unsaturated substituents.     

Additional evidence for the migration of clay upon heating of clay-polypropylene nanocomposites from X-ray photoelectron spectroscopy (XPS)

The explanation for reduction in the peak heat release rate of polymer-clay nanocomposites which is normally accepted is that clay accumulates at the surface, forming a thermal shield which is also a barrier to mass transport. The process by which this clay arrives at the surface has never been described in print but the common assumption is that pyrolysis is required for clay accumulation to occur. In this work, X-ray photoelectron spectroscopy, a tool much more sensitive in surface analysis than conventional techniques, is used to probe the surface of polypropylene-clay nanocomposites that have been annealed at relatively low temperatures, well below that required for pyrolysis. The composition of the surface changes with time and temperature of annealing, which provide a strong indication that the clay at the surface undergoes chemical change at fairly low temperatures.     

Thermal and mechanical aging of self-assembled monolayers as studied by near edge X-ray absorption fine structure

Self-assembled monolayers (SAMs) enable significant changes in the surface energy and/or specific interactions of surfaces, which are desirable for microelectromechanical systems (MEMS), superhydrophobic coatings, sensors, and other applications. However, SAMs often exhibit poor durability and rapid degradation upon mechanical, thermal, or moisture exposure. The chemical and orientational changes in SAMs due to mechanical and thermal degradation were investigated using near-edge X-ray absorption fine structure (NEXAFS) and the water contact angle. SAMs were based on unfluorinated or fluorinated linear hydrocarbons that form highly oriented and densely packed structures on silicon substrates. Complex chemical and orientational changes were observed via NEXAFS following degradation. Under heating in a dry, oxygen-rich environment, unfluorinated SAMs tended to cleave at C-C bonds on the main chain; below 250 °C, CH(3) groups were sequentially cleaved toward the surface, whereas above 250 °C, remaining hydrocarbon groups were converted to a graphitic coating dominated by C═C bonds. Under similar conditions, fluorinated SAMs began their chemical degradation at 350 °C and above, although the orientation decreased steadily from 150 to 300 °C; at and above 350 °C, the preferential removal of F occurred and the SAM was slowly converted to a graphitic layer. By contrast, under vacuum the fluorinated molecules were very thermally stable, showing good stability up to 550 °C; when degradation occurred, entire molecules were removed. Mechanical degradation followed two routes; both unfluorinated and fluorinated SAMs that were mechanically rubbed with smooth surfaces exhibited severe chemical degradation of the molecules, leading to an amorphous and poorly defined layer with C═C, C-C, C-H, and C-F bonds. Unfluorinated and fluorinated surfaces that were mechanically rubbed in the presence of free silicon particulates showed the rapid and complete destruction of both the molecular orientation and the protective SAM layer, even for short exposure periods. The resulting NEXAFS spectra were very similar to those produced by heating to 550 °C, suggesting that the friction created by granular particles may lead to extreme local heating.     

Matrix effects in the carbon 1s near edge x-ray absorption fine structure spectra of condensed alkanes

The carbon 1s near edge x-ray absorption fine structure (NEXAFS) spectra of simple gaseous alkane molecules differ from the spectra of the same alkane molecules in the condensed phase. The origin of these large, systematic differences is poorly understood. The NEXAFS spectra of gaseous alkanes are interpreted as a progression of core-->Rydberg transitions with distinctive vibronic structure. The interpretation of the NEXAFS spectra of condensed phase alkanes is varied. Specifically, the degree of Rydberg character in the pre-edge core excited states of condensed alkanes is controversial. We determined the character of core excited states in condensed alkanes with a combination of experiment and computational study. From this, we have determined the nature of matrix effects for these species. The high-resolution carbon 1s NEXAFS spectrum of gaseous neopentane is dramatically different from its condensed phase spectrum, a striking illustration of the dramatic spectroscopic changes that occur upon condensation. High quality ab initio calculations of a cluster designed to model the solid phase environment provide definitive evidence for the reduction of Rydberg character and support the assignment of sigma*C-H) valence character in the pre-edge features in the NEXAFS spectra of condensed alkanes.     

Changes in the near-edge X-ray absorption fine structure spectra of long-chain diacetylene derivatives through photopolymerization in Langmuir–Blodgett films

Photopolymerization of cadmium 10,12-pentacosadiynoate (CdDA) in Langmuir–Blodgett (LB) films, with the molecular packing well arranged by moderate preannealing, was investigated with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Greenish films of polydiacetylene with an absorption wavelength of 705 nm were obtained through the photopolymerization of preannealed monomer LB films, and this resulted in an extended π-conjugate system based on the well-ordered monomer in a two-dimensional arrangement. The electronic structures of the polydiacetylenes were found to be correlated to the variation of the molecular arrangements in the films from the changes in the NEXAFS spectra through photopolymerization in the LB films. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2329–2336, 2004     

X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure study of water adsorption on pyridine-terminated thiolate self-assembled monolayers

Adsorption of water on self-assembled monolayers (SAMs) of 4-(4-mercaptophenyl)pyridine on gold at low temperatures under ultrahigh vacuum conditions is studied by synchrotron radiation X-ray photoelectron and absorption spectroscopy. Water adsorption induces a strong modification of the chemical state of the pyridine N atoms at the SAM/ice interface, indicative for strong H bonding and partial proton transfer between water molecules and pyridine moieties. Additionally, the initial molecular orientation within the SAM is changed upon formation of an adsorbed water multilayer.     

An integrated experimental and analytical approach to the chemical state imaging of iron in brain gliomas using X-ray absorption near edge structure spectroscopy

X-ray absorption near-edge structure spectroscopy is used for human neoplastic tissues in order to investigate distributions and chemical states of iron. The specimens used in this study were obtained intraoperatively from brain gliomas of different types and various grades of malignancy and from a control subject. An integrated experimental and analytical approach toward topographic and quantitative analysis in thin freeze-dried cryo-sections is presented. The full XANES spectra at the Fe absorption K edge show the presence of both chemical forms of Fe in the analyzed points of the tissues. The main goal of the work is the chemical state imaging of Fe in tissue areas. Topographic analysis of Fe speciation in the tissues investigated with the use of the XANES technique indicates the presence of microstructures where Fe²⁺ is dominant as well as those with a high abundance of the oxidized form of Fe. The quantitative analysis shows that for all cases the content of the oxidized form of Fe is significantly higher in comparison with Fe²⁺. The highest level of Fe³⁺ is found in the control sample, and the lowest one for the glioma of the highest grade of malignancy. The content of either Fe²⁺ or Fe³⁺ is increased in low grade gliomas in comparison to high-grade malignant tumors.     

Novel visualization studies of lignocellulosic oxidation chemistry by application of C-near edge X-ray absorption fine structure spectroscopy

The research presented herein is the first attempt to probe the chemical nature of lignocellulosic samples by the application of carbon near edge X-ray absorption fine structure spectroscopy (C-NEXAFS). C-NEXAFS is a soft X-ray technique that principally provides selective interrogation of discrete atomic moieties using photoelectrons of variable energies. The X1A beam line of the National Synchrotron Light Source was employed for the specific purpose of observing carboxylic acid moieties that display a signature absorption band centered at 289 eV. This study caps a larger effort to support the mechanistic basis for lignocellulosic fiber chemical degradation induced by the disproportionation of hydrogen peroxiduring fiber bleaching trials. It is shown that fibers that have been bleached with a hydrogen peroxide phase without removal of resident pendant metals (Mn, Cu, Fe) sustain significant macroscopic damage likely via classical Fenton-type radical reactions, as evidenced by a tensile reduction by over 30%. We present X-ray absorption spectra obtained using a scanning transmission X-ray microscope (STXM) at the end of a 2.5 GeV electron synchrotron that provided 1s ^* contrast-enhanced micrographs illustrating a random distribution of acid functionalities that were principally located on fiber surfaces. Control studies using non-bleached fibers demonstrated that very little signature carboxylic acid absorption patterns were present in the fibers, suggesting that these groups are an incriminating fingerprint for macroscopic fiber strength damage during non-radical suppressed bleaching trials.     

Electronic structure of lanthanum copper oxychalcogenides LaCuOCh (Ch=S, Se, Te) by X-ray photoelectron and absorption spectroscopy

X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES) have been applied to examine the electronic structures of lanthanum copper oxychalcogenides LaCuOCh (Ch=S, Se, Te), whose structure has been conventionally viewed as consisting of nominally isolated [LaO] and [CuCh] layers. However, there is evidence for weak La-Ch interactions between these layers, as seen in small changes in the satellite intensity of the La 3d XPS spectra as the chalcogen is changed and as supported by band structure calculations. The O 1s and Cu 2p XPS spectra are insensitive to chalcogen substitution. Lineshapes in the Cu 2p XPS spectra and fine-structure in the Cu L- and M-edge XANES spectra support the presence of Cu⁺ species. The Ch XPS spectra show negative BE shifts relative to the elemental chalcogen, indicative of anionic species; these shifts correlate well with greater difference in electronegativity between the Cu and Ch atoms, provided that an intermediate electronegativity is chosen for Se.     

Sulfur 1s near-edge x-ray absorption fine structure (NEXAFS) of thiol and thioether compounds

The speciation and quantification of sulfur species based on sulfur K-edge x-ray absorption spectroscopy is of wide interest, particularly for biological and petroleum science. These tasks require a firm understanding of the sulfur 1s near-edge x-ray absorption fine structure (NEXAFS) spectra of relevant species. To this end, we have examined the gas phase sulfur 1s NEXAFS spectra of a group of simple thiol and thioether compounds. These high-resolution gas phase spectra are free of solid-state broadening, charging, and saturation effects common in the NEXAFS spectra of solids. These experimental data have been further analyzed with the aid of improved virtual orbital Hartree-Fock ab initio calculations. The experimental sulfur 1s NEXAFS spectra show fine features predicted by calculation, and the combination of experiment and calculation has been used to improve assignment of spectroscopic features relevant for the speciation and quantification of the sulfur compounds.     

X-ray photoelectron spectra and structure of 2-(2-phenylhydrazono) acetoacetanilide

2-(2-Phenylhydrazono)acetoacetanilide, itsN-methyl derivatives, and model compounds were studied by X-ray photoelectron spectroscopy. The chemical shifts were obtained from the¹³C NMR spectra. A correlation between the calculated charges, the binding energies on N atoms, and the¹³C NMR chemical shifts was found. The analysis of the XPS data and the¹³C NMR chemical shifts led to the conclusion that crystalline 2-(2-phenylhydrazono)acetoacetanilide exists mainly in the oxo hydrazone form. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 488–491, March, 1999.     

Structural study of NO adsorbed on the reconstructed Pt(110)-(1 x 2) surface with X-ray photoelectron diffraction and near-edge X-ray absorption fine structure spectroscopy

The adsorption structure of NO on the reconstructed Pt(110)-(1 x 2) surface was studied with X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD), low-energy scanned-angle photoelectron diffraction (LESA-PD), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The experiments were performed at 180 K, where no surface lifting from (1 x 2) to (1 x 1) takes place after NO adsorption. XPS indicates that the (1 x 2) unit cell of the Pt(110) surface contains 1.5 NO molecules at the saturated coverage. XPD and LESA-PD analyses allow us to propose a structural model for the NO adlayer, where two-thirds of the NO molecules in the (1 x 2) unit cell are adsorbed on the atop site of the close-packed Pt rows (ridges) along the [10] direction with an inclined geometry and one-third of the NO molecules adsorb on the bridge site between the Pt ridges with an upright configuration. This model is supported by the N K-edge NEXAFS experiments and is consistent with the recently reported model based on the density functional theory (Orita, H.; Nakamura, I.; Fujitani, T. J. Phys. Chem. B 2005, 109, 10312).     

Characterization of lithium alkyl carbonates by X-ray photoelectron spectroscopy: experimental and theoretical study

Lithium alkyl carbonates ROCO2Li result from the reductive decomposition of dialkyl carbonates, which are the organic solvents used in the electrolytes of common lithium-ion batteries. They play a crucial role in the formation of surface layers at the electrode/electrolyte interfaces. In this work, we report on the X-ray photoelectron spectroscopy (XPS) characterization of synthesized lithium methyl and ethyl carbonates. Using Hartree-Fock ab initio calculations, we interpret and simulate the valence spectra of both samples, as well as several other Li alkyl carbonates involved in Li-ion batteries. We show that Li alkyl carbonates can be identified at the electrode's surface by a combined analysis of XPS core peaks and valence spectra.