Geometrical and electronic structure of small copper nanoclusters: XANES and DFT analysis

The geometrical and electronic structure of small copper nanoclusters was studied by density functional theory (DFT) and analysis of X-ray absorption spectra. It was shown that the icosahedral geometry of small copper nanoclusters of 13 atoms was energetically more stable than cuboctahedral geometry. The binding energies in these structures were compared; the theoretical XANES spectra were compared with experiment. The paper gives the results of ab initio calculations of the electronic structure of copper clusters differing in size.     

X-ray absorption near-edge spectroscopic study of nickel catalysts

Ni-isocyanide and Ni-acac complexes have been studied by X-ray absorption spectroscopy. Theoretical analysis has been done using self-consistent full multiple scattering (MS) approach within both muffin-tin (MT) model of the potential and non-MT finite deference method. For the isocyanide complex, it was shown that MS theoretical spectra reproduce all structural details of the X-ray absorption near-edge structure (XANES), but also that it is important to consider the non-MT effects in the potential for a correct simulation of the shape of the pre-edge structures. The contribution of a non-constant potential in the interstitial regions is extremely important for the interpretation of the XANES of Ni(acac)₂.     

Atomic structure of the copper bromide complex based on benzoin 1′-phthalazinylhydrazone: DFT and X-ray absorption spectroscopy analysis

A copper bromide complex based on benzoin 1′-phthalazinylhydrazone is synthesized. Within density functional theory (OPBE/TZP) the optimal structural parameters of the complex are determined. X-ray Absorption Near-Edge Structure (XANES) spectra above the K absorption edge of copper are measured in the copper bromide complex. Theoretical XANES spectra of the K absorption edge of copper are calculated based on the full multiple scattering method and in the full potential of the finite difference method. Good agreement between the theoretical XANES spectrum and the experimental data is obtained.     

Determination of CoSi2 self-aligned nanostructures with grazing incidence X-ray absorption spectroscopy

Self-aligned nanostructures (SAN) made by reacting Co nanoparticles with crystalline Si substrates at high temperatures were studied with grazing incidence X-ray absorption spectroscopy (GI-XAS). The results from extended X-ray absorption fine structure (EXAFS) analysis and X-ray absorption near-edge spectroscopy (XANES) were used to identify SAN as crystalline CoSi2. Theoretical calculations of EXAFS and XANES spectra of several crystalline cobalt silicides were performed with the FEFF8 package. On the basis of these studies, the SAN samples were determined to contain nearly pure CoSi2.     

Nitrogen defect levels in InN: XANES study

The local and electronic structure of nitrogen-related defects in thin film of InN (0 0 0 1) has been studied using synchrotron-based X-ray absorption near edge structure (XANES) spectroscopy. Several defect levels within the band gap and the conduction band of InN were clearly resolved in XANES spectra around the nitrogen K-edge. Theoretical analysis of XANES data includes advanced “ab initio” simulations: self-consistent full multiple scattering calculations using muffin-tin approximation, non-muffin-tin finite difference approach to study the influence of non-muffin-tin effects on XANES shape as well as advanced local density approximation scheme for optimization of initial geometry around nitrogen defects. Theoretical analysis of XANES data allows to attribute the level observed at 1.7 eV above the conduction band mimimum to antisite nitrogen and a sharp resonance at 3.2 eV above the conduction band minimum to molecular nitrogen.     

Si X-ray absorption near edge structure (XANES) of Si,SiC, SiO2, and Si3N4 measured by an electron probe X-ray microanalyzer (EPMA)

Silicon K X-ray emission spectra of Si, SiC, Si₃N₄, and SiO₂ are measured using a wavelength dispersive electron probe X-ray microanalyzer. It is shown that the fine structures in the line shape of the low energy tail of the Kα characteristic X-ray emission spectra resemble those of the K X-ray absorption near edge structure (XANES). XANES spectra of 1 μm² area can be obtained by this method.     

Electronic structure and conductivity of nanocomposite metal (Au,Ag, Cu,Mo)-containing amorphous carbon films

In this work, we study the influence of the incorporation of different metals (Me = Au, Ag, Cu, Mo) on the electronic structure of amorphous carbon (a-C:Me) films. The films were produced at room temperature using a species selective bias pulsed dual-cathode arc deposition technique. Compositional analysis was performed with secondary neutral mass spectroscopy whereas X-ray diffraction was used to identify the formation of metal nanoclusters in the carbon matrix. The metal content incorporated in the nanocomposite films induces a drastic increase in the conductivity, in parallel with a decrease in the band-gap corrected from Urbach energy. The electronic structure as a function of the Me content has been monitored by X-ray absorption near edge structure (XANES) at the C K-edge. XANES showed that the C host matrix has a dominant graphitic character and that it is not affected significantly by the incorporation of metal impurities, except for the case of Mo, where the modifications in the line shape spectra indicated the formation of a carbide phase. Subtle modifications of the spectral line shape are discussed in terms of nanocomposite formation.     

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.     

Modelling of substitutional defects in the structure of Ti-bearing hibonite

A local atomic structure around titanium positions in Ti-bearing hibonite (CaAl₁₂O₁₉) has been studied. The structural models of substitution of different substitution defects Ti–Al in hibonite by titanium atoms have been considered. Optimization of structural models of hibonite has been done by means of density functional theory calculations using pseudopotential approximation as implemented in VASP 5.3 code. Gibbs free energies analysis has shown that models of substitution of M2 and M4 aluminum positions by titanium atoms are the most probable. For the most probable structural models of Ti-bearing hibonite theoretical X-ray absorption near-edge structure (XANES) spectra near the titanium K edge have been calculated. Significant differences in theoretical XANES spectra calculated for different structural models with non-optimized and optimized atomic structure have been demonstrated. Changes in the intensity of pre-edge structure of TiK XANES spectra for different substitution models of aluminum by titanium have been observed which relate to different titanium coordination in structural models. Energy shift of spectral features towards lower energy for optimized models implies increase of interatomic distances in local surroundings of Ti absorbing atoms.     

An analysis of X-ray absorption spectra in the XANES region of platinum-based electrocatalysts for low-temperature fuel cells

The knowledge of the electronic state of Pt-based electrocatalysts used in low temperature fuel cells is very important for the understanding of the activity of the material. Near-edge X-ray absorption spectroscopy (XANES), a particular region of the X-ray absorption spectra (XAS), can provide the desired information, but the analysis is not straightforward, particularly for the non-specialist. Mansour et al. (J. Phys. Chem., 88:2330, 1984) and Shukla et al. (J. Electroanal. Chem., 563:181, 2004) presented methods to obtain information on electronic states from XANES spectra. In this work, procedures to implement the two methods are presented in detail and applied to XANES results for Pt₂Sn₁/C and Pt/C electrocatalysts prepared in this laboratory. The scope and limitations of the two methods are also discussed. The results show that the presence of Sn in the Pt₂Sn₁/C material promotes a partial filling of the Pt 5d band, in comparison to Pt/C, which contributes to explain the better activity of the bimetallic material. This paper is dedicated to Prof. Francisco Nart, in memoriam.     

Estimation of phase ratio for nano-sized TiO2 powders by Ti K-edge XANES

We estimated the crystallinity and phase ratio of anatase to rutile in TiO₂ nano-particles by Ti K-edge X-ray absorption near-edge spectroscopy (XANES) spectra. The nano-sized TiO₂ powders prepared by the homogeneous precipitation process at low temperature (HPPLT) were compared with the commercial TiO₂ powders with good crystallinity. From the comparison with the experimental and calculated Ti K-edge XANES spectra, we found the possibility of efficient estimation in the crystallinites and the phase ratio of anatase to rutile for nano-sized TiO₂ mixtures.     

Structure,optical and magnetic properties of LaFeO3 nanoparticles prepared by polymerized complex method

This work reports the study the structure, optical and magnetic properties of LaFeO₃ nanoparticles synthesized by the polymerized complex method. The LaFeO₃ nanoparticles were successfully obtained from calcination of the precursor at different temperatures from 750 to 1,050 °C in air for 2 h. The calcined LaFeO₃ nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV–Visible spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge spectroscopy (XANES) and vibrating sample magnetometry. The XRD and TEM results showed that all LaFeO₃ samples had a single phase nature with the orthorhombic structure. The estimated crystallite sizes were in the range of 44.5 ± 2.4–74.1 ± 4.9 nm. UV–Vis spectra showed strong UV and Vis absorption with small band gap energy. The valence states of Fe ions were in the Fe³⁺ and Fe⁴⁺ state, as confirmed by XPS and XANES results. The weak ferromagnetic behavior with specific saturation magnetization of 0.1 emu/g at 10 kOe was obtained for the small particle of 44.5 ± 2.4 nm. The uncompensated spins at the surface was proposed as playing a part in the magnetic properties of small sized LaFeO₃.     

Analysis of the atomic structure of colloidal quantum dots of the CdSe family: X-ray spectral diagnostics and computer modelling

Colloidal quantum dots of the CdSe family have been studied by X-ray absorption near edge structure (XANES) spectroscopy and computer modelling. CdK edge XANES spectra in colloidal quantum dots based on varisized CdSe nanoparticles have been recorded. Atomic structure of CdSe particles and also CdSe particles doped by transition metal atoms Mn and Co has been modelled based on the density functional theory. The embedding of the doping atoms is shown to result in considerable changes in the local atomic structure of CdSe particles. XANES spectra have been calculated above the CdK edge in CdSe particles, above the MnK edge in CdSe:Mn particles, above the CoK edge in CdSe:Co particles. The sensitivity of XANES spectroscopy to small changes in structural parameters of the nanoparticles of CdSe family has been demonstrated that furnishes an opportunity to apply it for the verification of atomic structure parameters around positions of cadmium and doping atoms of transition metals in quantum dots based on CdSe.     

X-ray absorption near edge structures (xanes) of Cu/ZnO/Al2O3 Co shift catalysts

X-ray absorption near edge structures (XANES) of some Cu/ZnO/Al₂O₃CO shift catalysts have been measured at high resolution using synchrotron radiation, and compared to reference compounds Prior to reduction. CuO and ZnO are present, after reduction ZnO and highly dispersed Cu, no indication was found for Cu₂O or the presence of spinels. The results are qualitatively confirmed by preliminary analysis of EXAFS spectra.     

First-principles calculations of K-edge X-ray absorption near edge spectra of some transition metal dimers

Theoretical X-ray absorption near-edge spectra have been evaluated for different bond lengths of Mn₂, Co₂ and Ni₂ using a modified Xα scattered-wave procedure.     

X-ray absorption study of the solvation structure of Cu2+ in methanol and dimethyl sulfoxide

The solvation structure of Cu(2+) in methanol (MeOH) and dimethyl sulfoxide (DMSO) has been determined by studying both the extended X-ray absorption fine structure (EXAFS) and the X-ray absorption near-edge structure (XANES) regions of the K-edge absorption spectra. The EXAFS technique has been found to provide a very accurate determination of the next-neighbor coordination distances, but it is inconclusive in the determination of the coordination numbers and polyhedral environment. Conversely, quantitative analysis of the XANES spectra unambiguously shows the presence of an average 5-fold coordination in both the MeOH and DMSO solution, ruling out the usually proposed octahedral Jahn-Teller distorted geometry. The EXAFS and XANES techniques provide coherent values of the Cu-O first-shell distances that are coincident in the two solvents. This investigation shows that the combined analysis of the EXAFS and XANES data allows a reliable determination of the structural properties of electrolyte solutions, which is very difficult to achieve with other experimental techniques.     

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.     

Np(IV)/Np(V) valence determinations from Np L3 edge XANES/EXAFS

X-ray absorption near edge structure (XANES) spectroscopy for in situ metal valence determination has become a powerful analytical tool in heterogeneous systems. This is in part because it is applicable without prior separation procedures. For some systems, however, determining the oxidation state from XANES spectra is not straightforward and caution must be used. We show that the analysis of L3,2 edge EXAFS (extended X-ray absorption fine structure) spectra is better suited to distinguish between Np(IV) and Np(V) than from their XANES spectra. Whereas evidence for the oxidation of Np(IV) in solution samples from their Np L3 XANES is unclear, their EXAFS data unequivocally reveals Np(V) formation in the solutions.     

Next-nearest neighbour contributions to the XPS binding energies and XANES absorption energies of P and As in transition-metal arsenide phosphides MAs1−yPy having the MnP-type structure

X-ray photoelectron spectroscopic (XPS) and X-ray absorption near-edge spectroscopic (XANES) measurements have been made for several series of metal arsenide phosphides MAs_1−yP_y (M=Co, Fe, Cr) that adopt the MnP-type structure. The P and As XPS binding energies (BEs) and XANES absorption energies of the metal arsenide phosphides do not follow the trend observed for the simple binary phosphides or arsenides, a deviation that arises from the combination of nearest and next-nearest neighbour contributions acting on the P or As photoemission or absorption site. The P 2p_3/2 BEs and K-edge absorption energies are lower in MAs_1−yP_y than in MP because the P atoms are more negatively charged and because the P photoemission or absorption site is screened to a greater extent by less positively charged nearest-neighbour M atoms and more negatively charged next-nearest neighbour P atoms. The As L₃- and K-edge absorption energies are higher in MAs_1−yP_y than in MAs primarily because the As atoms are less negatively charged. The M charge has been evaluated from analysis of the M 2p XPS spectra and the M L- and K-edge XANES spectra.     

离子液体[BMIM]Br与[BMIM][BF4]配比浓度对Br-离子电荷转移的调节

利用X射线吸收精细结构光谱(XAFS)及紫外吸收光谱两种方法, 分析了离子液体1-丁基-3-甲基咪唑溴盐([BMIM]Br)中逐渐掺入1-丁基-3-甲基咪唑四氟硼酸盐([BMIM][BF₄])时, Br^-阴离子与咪唑阳离子之间氢键作用及电荷偏移量的改变. 随着[BMIM][BF₄]加入量增多, Br 元素XAFS近边(XANES)显示吸收峰降低, 吸收边位置向低能端位移0.9 eV; 扩展边(EXAFS)算出径向结构显示Br 与近邻原子间平均配位数降低、平均键长增长; 紫外光谱也有明显蓝移减色效应. 这些结果都表明Br₄^-的掺入改变了Br^-与阳离子间的电荷偏移量, 负电荷更多地转移到Br^-上, 量化计算的数据同样支持该结论.