Showing papers on "Chemical state published in 2006"

XRD and XPS Study of Cu−Ni Interactions on Reduced Copper−Nickel−Aluminum Oxide Solid Solution Catalysts

Abstract: Copper−nickel−aluminum oxide solid solutions were reduced in hydrogen to produce alumina-supported copper−nickel alloy catalysts. XRD patterns of reduced oxides showed that the type of active metals which emerged upon reduction were sensitive to the reduction temperature and the copper content. Variations from +0.8 to +1 eV were found in the experimental Ni 2p3/2 binding energy (BE) of nickel in the solid solutions compared to the experimental Ni 2p3/2 BE of bulk nickel, attributed to the Ni−Ni arrangements in the solid solution. Also, when the curve-fitted BE values of Ni 2p3/2 and Cu 2p3/2 of different reduced solid solutions were compared, it was found that copper and nickel were in different chemical states depending on reduction temperature and the amount of the copper. Changes in Ni 2p3/2 BE in the reduced solid solutions were also discussed in terms of the filling of nickel d-hole bands due to nickel−copper d−d band interactions. At both high copper content and reduction temperature, copper had a t...

Chemical Natures and Distributions of Metal Impurities in Multicrystalline Silicon Materials

Abstract: We present a comprehensive summary of our observations of metal-rich particles in multicrystalline silicon (mc-Si) solar cell materials from multiple vendors, including directionally-solidified ingot-grown, sheet, and ribbon, as well as multicrystalline float zone materials contaminated during growth. In each material, the elemental nature, chemical states, and distributions of metal-rich particles are assessed by synchrotron-based analytical x-ray microprobe techniques. Certain universal physical principles appear to govern the behavior of metals in nearly all materials: (a) Two types of metal-rich particles can be observed (metal silicide nanoprecipitates and metal-rich inclusions up to tens of microns in size, frequently oxidized), (b) spatial distributions of individual elements strongly depend on their solubility and diffusivity, and (c) strong interactions exist between metals and certain types of structural defects. Differences in the distribution and elemental nature of metal contamination between different mc-Si materials can largely be explained by variations in crystal growth parameters, structural defect types, and contamination sources. Copyright © 2006 John Wiley & Sons, Ltd.

Chemical state study of palladium powder and ceria-supported palladium during low-temperature CO oxidation.

Abstract: The chemical state of Pd at the surfaces of two sizes of Pd powders and ceria-supported Pd during low-temperature CO oxidation has been studied using X-ray photoelectron spectroscopy (XPS). During oxidation in O2, metallic Pd powder is converted into PdO, and the thickness of the PdO layer increases with increasing reaction temperature. A similar Pd oxidation process occurs during the catalytic CO oxidation reaction, but the extent of the Pd oxidation is less due to the presence of CO which is a reducing agent. The reaction rate data indicate that the larger size Pd powder is about three times more active than the smaller size Pd powder on a surface area basis at 160 °C. Catalytic CO oxidation data obtained from 10 wt % Pd supported on nanocrystalline ceria powder indicate that there is a strong chemical interaction between the ceria and the supported Pd. The Pd is present as PdO on the fresh catalyst. During reaction, small amounts of Pd metal and PdO2 are formed at 50 °C while less Pd metal and only a s...

XPS and XAES analysis of copper, arsenic and sulfur chemical state in enargites

Abstract: Enargite, a copper arsenic sulfide with the formula Cu3AsS4 is of environmental concern due to its potential to release toxic arsenic species. The oxidation and dissolution of enargite are governed by the composition and chemical state of the outermost surface layer. Qualitative and quantitative analysis of the enargite surface can be initially obtained on the basis of X-ray photoelectron spectroscopy (XPS) binding energy and intensity data. However, a more precise determination of the chemical state of the principal elements of enargite (copper, arsenic and sulfur) in the altered surface layer and in the bulk of the mineral requires a combined analysis based on XPS photoelectron lines and the corresponding X-ray excited Auger lines. On the basis of results obtained on natural and synthetic enargite samples and on standards of sulfides and oxides, the Auger parameter α′ of different compounds was calculated and the Wagner chemical state plots were drawn for arsenic, copper and sulfur. Arsenic in enargite is found to be in a chemical environment similar to that of arsenides or elemental arsenic, whereas copper in enargite is in a chemical state that corresponds to copper sulfide, Cu2S, for all samples irrespective of surface treatment (natural or freshly cleaved). Only sulfur changed from a chemical state similar to that of copper or iron sulfide in freshly cleaved samples to another state in natural enargite in the as-received state. Thus, it is the sulfur atom at the surface of enargite that is most susceptible to changes in the enargite surface state and composition. A more detailed interpretation of this behavior, based on differences in the initial and final state effects, is proposed here. The concept of Auger parameter and chemical state plot, used here for the first time for investigating enargite, has proved to be a method to unambiguously assign the chemical state of the principal elements copper, arsenic and sulfur in these minerals. Copyright © 2006 John Wiley & Sons, Ltd.

XPS and XAES analysis of copper, arsenic and sulfur chemical state in enargites; ; Surface and interface analysis;

Abstract: Enargite, a copper arsenic sulfide with the formula Cu3AsS4 is of environmental concern due to its potential to release toxic arsenic species. The oxidation and dissolution of enargite are governed by the composition and chemical state of the outermost surface layer. Qualitative and quantitative analysis of the enargite surface can be initially obtained on the basis of X-ray photoelectron spectroscopy (XPS) binding energy and intensity data. However, a more precise determination of the chemical state of the principal elements of enargite (copper, arsenic and sulfur) in the altered surface layer and in the bulk of the mineral requires a combined analysis based on XPS photoelectron lines and the corresponding X-ray excited Auger lines. On the basis of results obtained on natural and synthetic enargite samples and on standards of sulfides and oxides, the Auger parameter ′ of different compounds was calculated and the Wagner chemical state plots were drawn for arsenic, copper and sulfur. Arsenic in enargite is found to be in a chemical environment similar to that of arsenides or elemental arsenic, whereas copper in enargite is in a chemical state that corresponds to copper sulfide, Cu2S, for all samples irrespective of surface treatment (natural or freshly cleaved). Only sulfur changed from a chemical state similar to that of copper or iron sulfide in freshly cleaved samples to another state in natural enargite in the as-received state. Thus, it is the sulfur atom at the surface of enargite that is most susceptible to changes in the enargite surface state and composition. A more detailed interpretation of this behavior, based on differences in the initial and final state effects, is proposed here. The concept of Auger parameter and chemical state plot, used here for the first time for investigating enargite, has proved to be a method to unambiguously assign the chemical state of the principal elements copper, arsenic and sulfur in these minerals.

Characterization of As-doped, p-type ZnO by x-ray absorption near-edge structure spectroscopy

Abstract: The x-ray absorption near-edge structure (XANES) spectroscopy has been used as a “fingerprint” to address the unresolved issues related to the changes in the local structure around As and to identify its chemical state in the As-doped, p-type ZnO The spectral features of both AsK- and OK-edge XANES spectra strongly suggest that in the p-type state As substitutionally replaces O in the ZnO lattice, thereby forming AsO, which is the acceptor responsible for p-type conduction in the As-doped, p-type ZnO

Surface core-level shifts of strontium observed in photoemission of barium strontium titanate thin films

Abstract: Angle resolved x-ray photoelectron spectroscopy was used to investigate the surface electronic structure of barium strontium titanate (BST) films. In contrast to previous photoemission studies which identified two chemical states associated with only Ba in the near surface region, the authors have resolved core-level features from surface Sr atoms which provide new insight into the surface electronic structure of BST thin films. The surface Sr 3d features are found to lie approximately 1eV higher in binding energy than the bulk derived peaks. The effects of aqueous and annealing surface treatments and the origin of the Sr surface core-level shifts are discussed.

Chemical states and electrical properties of a high-k metal oxide/silicon interface with oxygen-gettering titanium-metal-overlayer

Abstract: The authors report on the chemical bonding structure of the HfO2∕Si (001) stack after the SiO2 interfacial layer (IL) is partially removed by a reactive titanium metal overlayer. Using synchrotron photoelectron spectroscopy, they found that ultrathin SiO2-like IL ∼6.5A thick, which is significantly less than the initial SiO2 IL thickness of ∼15A, exists at the HfO2∕Si interface with an overlying Ti electrode. The dissociated Si from SiO2 IL is believed to go onto Si substrate where it regrows epitaxially. The interfacial trap density of the Ti-electrode sample was extracted to be ∼1.6×1011eV−1cm−2 near the midgap of Si, which was comparable to that of the control sample with W electrode.

Chemical vapor infiltration method for deposition of gold nanoparticles on porous alumina supports

Abstract: Knudsen’s effusion method with mass spectral analysis of the composition of the gas phase was used to measure the temperature dependence of the saturated vapor of several (CH3)2AuL chelate complexes and to determine the thermodynamic parameters of their sublimation. Based on the results of this study, conditions for chemical vapor deposition of gold using dimethylgold(III) chelates were chosen. Gold nanoparticles were synthesized by chemical vapor deposition (infiltration) of (CH3)2Au(acac) on porous granules of γ-Al2O3 with subsequent calcination in air at 325°C. Particle size and the chemical state of gold in Au/γ-Al2O3 systems were evaluated by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). A vapor infiltration procedure is suggested to prepare metallic gold particles ≤5 nm in diameter from Au/γ-Al2O3. It is shown that Au/γ-Al2O3 systems obtained by chemical vapor infiltration and containing small gold crystallites possess high catalytic activity in CO oxidation reactions at 40°C.

Adsorption free energy of variable-charge nanoparticles to a charged surface in relation to the change of the average chemical state of the particles

Abstract: Variable-charge nanoparticles such as proteins and humics can adsorb strongly to charged macroscopic surfaces such as silica and iron oxide minerals. To model the adsorption of variable-charge particles to charged surfaces, one has to be able to calculate the adsorption free energy involved. It has been shown that the change in the free energy of variable-charge particles is related to the change in their average chemical state upon adsorption, which is commonly described using surface complexation models. In this work, expressions for the free-energy change in variable-charge particles due to changes in chemical binding are derived for three ion-binding models (i.e., the Langmuir, Langmuir-Freundlich, and NICA models) and for changes due to nonspecific binding for the Donnan model. The expressions for the adsorption free energy of the variable-charge particles to a charged surface are derived on the basis of the equality of the (electro)chemical potential of the particles in the bulk solution and adsorption phase. The expressions derived are general in the sense that they account for the competition between charge-determining ions that bind chemically to the particles, and they also apply in case of the formation of chemical bonds between particle ligands and surface sites. The derived expressions can be applied in the future to model the adsorption of variable-charge nanoparticles to charged surfaces. The results obtained for the NICA-Donnan model make it possible to apply this advanced surface complexation model to describe the adsorption of humics to minerals.

Oxidation Behavior of Tungsten in H2O2- and Fe ( NO3 ) 3-Base Aqueous Slurries

Abstract: By using potentiodynamic scanning and cyclic voltametry, the oxidation and dissolution kinetics of tungsten/tungsten-oxides were investigated in the chemical mechanical planarization (CMP) slurries containing either H 2 O 2 or Fe(NO 3 ) 3 as an oxidant. The microstructure of the tungsten/tungsten-oxide surface and its temporal and depth variation of the chemical state were also examined by scanning electron microscopy and X-ray photoelectron spectroscopy. Through the analysis, it is shown that the oxide layer formed in the slurry containing Fe(NO 3 ) 3 is relatively dense and therefore passive, whereas that formed in the H 2 O 2 slurry is a porous tungsten oxide film which would not protect the underlying tungsten from further chemical etching. The electrochemical responses of tungsten surfaces in the two different oxidant conditions were compared, and a significant difference in the CMP performance due to the microstructure and chemical state variation of the tungsten surface was observed.

Supported Binary Oxide Catalysts Containing CuO Coupled with Ga2O3 and SnO2

Abstract: Two series of binary oxide catalysts (CuGa/SA and CuSn/SA) containing CuO coupled with Ga2O3 or SnO2 were prepared by dispersing the metal phases onto a high surface area acidic silica−alumina (SA) support by an adsorption method. Similar total amounts of metals (about 1.6 atomMet·nm-2) were deposited onto the support, in different proportions, to obtain samples with weight percent of copper varying from 3.3 to 6.5% The SA support was first covered by the copper precursor, Cu(C2H3O2)2, and then gallium (Ga(NO3)3·H2O) or tin (SnCl4·5H2O) precursors were deposited on the dried Cu-containing sample. The calcined materials were characterized by surface techniques (N2 adsorption and XPS), to detect the surface morphology and chemical state of metal species, and by FT-IR and adsorption calorimetry after CO adsorption. Nanosized metal phases were observed in every case. Besides Cu(II), the surfaces contained Cu(I) as well as Cu(δ+) ions (with 1 < δ < 2) stabilized by the interaction with the acid centers of the ...

Dependence with the oxidation state of X-ray transition energies, intensities and natural line widths of CrKβ spectra

Abstract: The Kβ emission spectrum of chromium was experimentally analyzed in different compounds and compared with previous data. Measurements of whole Kβ spectra were performed with a wavelength dispersive commercial XRF equipment. To study possible effects of the chemical state in the width and position of the main Kβ1,3 line, high resolution measurements were also performed. In the latter measurements, a spectrometer based on a backdiffracting crystal analyzer with spherical focalization and synchrotron radiation monochromatic excitation was used. Kβ1,3 line shifts in relation to metallic Cr were observed, both to higher energies (≅+1 eV) for CrIII and to lower energies (≅−0.5 eV) for CrVI. It was also found that the natural width of CrKβ1,3 line, the ionization energy of the 3p orbital of Cr, and the relative intensities of Kβ″ and Kβ2,5 lines with respect to the main Kβ1,3 line increase as the oxidation state increases. The use of these features as an index for chemical state analysis is discussed.

Study on Iron Distribution and Electrical Activities at Grain Boundaries in Polycrystalline Silicon Substrate for Solar Cells

Abstract: To clarify the role of grain boundaries in iron sinks and carrier recombination centers, iron distributions and their chemical states were studied before and after gettering. They were measured by the X-ray microprobe fluorescence and the X-ray absorption in the near-edge structure using the beamline 37XU at the SPring-8 third-generation synchrotron facility. To determine the crystallographic orientation of the grain boundaries, electron backscatter diffraction measurements were performed. The distribution of electric active defects was characterized by electron-beam-induced current measurements. Before gettering, the iron was distributed in the small grain and its chemical state was similar to that of iron oxide. After gettering, the iron was redistributed along the small angle grain boundary, and its chemical state was similar to the iron silicide complexed with the iron oxide. Regarding the electrical activity, high carrier recombination was observed along the small-angle grain boundary. On the contrary, Σ3 grain boundaries were relatively weak impurity sinks and showed low recombination activity.

X-ray photoelectron spectroscopy studies of non-stoichiometric superconducting NbB2+x

Abstract: Polycrystalline samples of NbB2+x with nominal composition (B/Nb) = 2.0, 2.1, 2.2, 2.3, 2.4 and 2.5 were studied by x-ray photoelectron spectroscopy (XPS). The spectra revealed Nb and B oxides on the surface of the samples, mainly B2O3 and Nb2O5. After Ar ion etching the intensity of Nb and B oxides decreased. The Nb 3d5/2 and B 1s core levels associated with the chemical states (B/Nb) were identified and they do not change with etching time. The binding energies of the Nb 3d5/2 and B 1s core levels increase as boron content increases, suggesting a positive chemical shift in the core levels. On the other hand, analysis of valence band spectra showed that the contribution of the Nb 4d states slightly decreased while the contribution of the B 2pπ states increased as the boron content increased. As a consequence, the electronic and superconducting properties were substantially modified, in good agreement with band-structure calculations.

Bond nature of oxygen-deficient HfO2∕Si(100) film

Abstract: The authors investigate the bonding environment of an oxygen-deficient HfO2∕Si film grown by means of pulsed laser deposition, by analyzing the Hf L3-edge extended x-ray absorption fine structure. The local characteristics around the Hf atom, such as the bond length or the number of nearest neighbors, are found to depend on the oxygen supply during film growth. The chemical states of these samples are also probed in situ by x-ray/ultraviolet photoelectron spectroscopies. The core-level binding energy and the work function for each sample are found to be correlated with the mean Hf–O bond length, implying a close connection between the chemical environment and bond nature.

XPS study of pulsed Nd:YAG laser oxidized Si

Abstract: X-ray photoelectron spectra (XPS) of thin SiO2 layers grown by pulsed Nd:YAG laser at a substrate temperature of 748 K are presented The peak decomposition technique combined with depth profiling is employed to identify the composition and chemical states of the film structure It is established that the oxide is non-stoichiometric, and contains all oxidation states of Si in different amounts throughout the film The interface Si/laser-grown oxide is not abrupt, and the coexistence of Si2O3 and Si2O suboxides in a relatively wide interfacial region is found It is concluded that post-oxidation annealing is necessary in order to improve the microstructure of both oxide and near interface region

Nitridation of InP(1 0 0) substrates studied by XPS spectroscopy and electrical analysis

Abstract: The nitrides of group III metals as InN are very important materials in optoelectronic (light-emitting diodes and laser diodes) and microelectronic areas. It is essential for the realisation of such devices to grow high quality nitride single crystals. In this paper, the nitridation of InP(1 0 0) surfaces has been studied in situ using X-ray photoelectron spectroscopy (XPS). After ionic cleaning by Ar + ions, metallic indium crystallites are created and the nitridation of the substrates is performed using a plasma glow discharge cell reaction with these indium clusters. We used the In 4d and P 2p core levels to monitor the chemical state of the surface and the coverage of the present species. We observed the creation of InN and PN bonds while the In–In metallic bonds decrease. This confirms the reaction between indium clusters and nitrogen species. A theoretical model based on stacked layers allows us to assert that we have produced almost two monolayers of indium nitride. In order to determine the quality of the elaborated nitride films and the electrical properties of the InN/InP interface we have used the I ( V ) electrical method. Analysis of the measured characteristics at room temperature allows the determination of the electrical parameters. The saturation current I s , the ideality factor η , and the serial resistance R s are evaluated to 1.92 × 10 −6 A, 3.07 and 375 Ω, respectively. The barrier height is determined at room temperature and is equal to 0.547 eV.

Chemical speciation via X‐ray emission spectra

Abstract: Since the early days of X-ray spectrometry, X-ray emission and fluorescence spectra have been used to investigate chemical speciation, e.g. the dependence on the formal oxidation state. Laboratory wavelength-dispersive spectrometers have adequate resolution for these measurements. However, almost all studies have employed empirical methods to interpret the spectra. We aim to place such methods on a quantitative basis by means of efficient ab initio calculations of the X-ray emission line shapes based on a self-consistent, real-space Green's function approach, as implemented in the X-ray spectroscopy code FEFF8.2. Calculations are presented for the phosphorus K-M2, 3, and the chromium L-series emission lines for a selection of simple compounds. These lines exhibit changes depending on the oxidation state and on the neighboring atoms in the compounds that can be observed with instruments available in many XRF laboratories. The calculated spectra, as modified by convolution with a model monochromator response function, are compared with measured spectra. Simulated and measured spectra are found to be in reasonable agreement, and show that the approach has the potential to yield quantitative information about the chemical state. Copyright © 2006 John Wiley & Sons, Ltd.