Showing papers on "Chemical state published in 2019"

Nanostructured spinel cobalt ferrites: Fe and Co chemical state, cation distribution and size effects by X-ray photoelectron spectroscopy

Abstract: Nanostructured spinel cobalt ferrite samples having crystallite size ranging between 5.6 and 14.1 nm were characterized by X-ray photoelectron spectroscopy and X-ray induced Auger electron spectroscopy in order to determine the chemical state of the elements, the iron/cobalt ratio and the cation distribution within tetrahedral and octahedral sites. The presence of size-dependent trends in the binding energy of the main photoelectron peaks and in the kinetic energy of the X-ray induced O KLL signal was also investigated. The results showed that iron is present as FeIII and cobalt is present as CoII. The iron/cobalt ratio determined by XPS ranges between 1.8 and 1.9 and it is in very good agreement, within experimental uncertainty, with the expected 2 : 1 ratio. The percentage of Fe in octahedral sites ranges between 62% and 64% for all samples. The kinetic energy of the O KLL signals increases with crystallite size. These results are explained in terms of changes in the ionicity of the metal–oxygen bonds. The results of this investigation highlight how the XPS technique represents a powerful tool to investigate the composition, the chemical state and inversion degree of cobalt spinel ferrites, contributing to the comprehension of their properties.

Quantitatively Unraveling the Redox Shuttle of Spontaneous Oxidation/Electroreduction of CuOx on Silver Nanowires Using in Situ X-ray Absorption Spectroscopy

Abstract: Oxide-derived copper catalysts have been shown to enhance CO2 reduction reaction (CO2RR) activity with high selectivity toward hydrocarbon products. However, the chemical state of oxide-derived copper during the CO2RR has remained elusive and is lacking in situ observations. Herein, a two-step process was developed to synthesize Ag nanowires coated with various thicknesses of a CuO x layer for the CO2RR. By employing in situ X-ray absorption spectroscopy, a strong correlation between the chemical state under reaction conditions and the CO2RR product profile can be revealed to validate another competing reaction (i.e., the spontaneous oxidation of Cu(0) in aqueous electrolyte) that significantly governs the chemical state of active centers of Cu. In situ Raman spectroscopy reveals the existence of reoxidation behavior under cathodic potential, and the quantification analysis of reoxidized behavior is revealed to indicate that the reoxidation rate is independent of surface morphology and strongly proportional to the electrochemically surface area. The steady oxidation state of Cu in an in situ condition is the paramount key and dominates the products' profile of the CO2RR rather than other factors (e.g., crystal facets, atomic arrangements, morphology, elements) that have been investigated in numerous reports.

Low-temperature CO oxidation at persistent low-valent Cu nanoparticles on TiO2 aerogels

Abstract: We exploit interfacial charge transfer from titania (TiO2) to copper (Cu) to design catalytic Cu/TiO2 composite aerogels that shift the chemical state of Cu nanoparticles away from Cu2+, making them highly active for low-temperature CO oxidation. The high degree of interfacial contact between ∼2–3 nm–diameter Cu particles and the networked ∼10 nm–diameter TiO2 particles in ultraporous aerogel stabilizes a high ratio of Cu0/1+:Cu2+. The reduced nature of Cu in Cu/TiO2 aerogels is evidenced by a strong surface plasmon resonance in its diffuse reflectance UV–vis spectrum, by its X-ray photoelectron spectral features, and by infrared spectroscopic evidence of CO binding at the catalyst surface. In contrast, when larger diameter (∼50–60 nm), non–networked TiO2 particles are used to support Cu nanoparticles, the single planar nanoscale interface between Cu and the support particle stabilizes a much lower fraction of low-valent Cu. The Cu0/1+ speciation stabilized within the aerogel catalyzes low-temperature CO oxidation (

CO2 Activation on Ni(111) and Ni(100) Surfaces in the Presence of H2O: An Ambient-Pressure X-ray Photoelectron Spectroscopy Study

Abstract: Nickel-based catalysts play an important role in the chemical transformation of CO2. A fundamental understanding of the interaction between CO2 and Ni surfaces at atomic level is necessary. In this...

Large Scale Solid-state Synthesis of Catalytically Active Fe 3 O 4 @M (M = Au, Ag and Au-Ag alloy) Core-shell Nanostructures

Abstract: Solvent-less synthesis of nanostructures is highly significant due to its economical, eco-friendly and industrially viable nature. Here we report a solid state synthetic approach for the fabrication of Fe3O4@M (where M = Au, Ag and Au-Ag alloy) core-shell nanostructures in nearly quantitative yields that involves a simple physical grinding of a metal precursor over Fe3O4 core, followed by calcination. The process involves smooth coating of low melting hybrid organic-inorganic precursor over the Fe3O4 core, which in turn facilitates a continuous shell layer post thermolysis. The obtained core-shell nanostructures are characterized using, XRD, XPS, ED-XRF, FE-SEM and HR-TEM for their phase, chemical state, elemental composition, surface morphology, and shell thickness, respectively. Homogeneous and continuous coating of the metal shell layer over a large area of the sample is ascertained by SAXS and STEM analyses. The synthesized catalysts have been studied for their applicability towards a model catalytic hydrogen generation from NH3BH3 and NaBH4 as hydrogen sources. The catalytic efficacy of the Fe3O4@Ag and Ag rich alloy shell materials are found to be superior to the corresponding Au counterparts. The saturation magnetization studies reveal the potential of the core-shell nanostructured catalysts to be magnetically recoverable and recyclable.

Doping strategy to boost electromagnetic property and gigahertz tunable electromagnetic attenuation of hetero-structured manganese dioxide.

Abstract: A facile and simple chemical route has been used to synthesize novel three-dimensional (3D) architectures of nickel-doped e-MnO2 without the addition of any surfactant or organic template. Nickel salt is used directly as the reagent rather than as an additive to produce doped manganese dioxide, which is different from the overwhelming majority of previous synthetic methods for doped MnO2 for use as an electromagnetic wave absorption material. This method overcomes the shortcomings of the previously reported approaches of doping with a slight amount of metallic ion, which is sometimes hard to detect. The chemical composition of the samples is analyzed by electron-probe micro-analysis (EPMA) and energy dispersive spectroscopy (EDS). The chemical state of the elements in the composites is demonstrated with X-ray photoelectron spectroscopy (XPS). The structures of the micro-spheres are detected by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that the self-organized crystals are made up of walnut-like spheres and are arrays of polycrystals. The nickel ion is certified to have been successfully doped into the crystal based on the results of EPMA, EDS, and XPS as well as dark field scanning TEM. Thus, a multiple heterojunction structure is constructed. After nickel doping, the crystalline phase remains e type and the morphology turns into a walnut-like structure. Electromagnetic performances also exhibit significant variation with the introduction of nickel ions. Nickel-doped MnO2 has a decreased dielectric constant compared with that of commercial MnO2, while the nickel-doped MnO2 appears to have fascinating magnetic properties with a maximum magnetic loss tangent value of 0.37, which is 7 times greater than that of the dielectric loss tangent. Likewise, it is further presented that the optimized electromagnetic capacities are related to the mass fraction of the walnut-like MnO2 spheres in the composite. When the mass fraction is as high as 50%, the magnetic loss tangent goes up with a distinct increase in mutations as well as in relaxation times and in the real part and the imaginary part of the relative complex permeability. Furthermore, the mechanisms of the highlighted electromagnetic attenuation are explored in detail.

Study on distribution, chemical states and binding energy shifts of elements on the surface of gasification fine ash

Abstract: Using feed coal as a reference, a series of modern advanced analysis and measurement technologies such as X-ray photoelectron spectroscopy, scanning electron microscopy–energy dispersive X-ray spectroscopy and X-ray diffraction have been used to analyze the gasification fine ash obtained from a pulverized coal gasification unit. The results show that the fine ash mainly consists of elemental carbon, oxygen, silicon, and aluminum. The elemental carbon distributes primarily on the foams structure, while the spherical particles mainly consisted of silicon and aluminum are embedded in the foams structure. The C1s spectrum is composed of five components in which the content of graphitized carbon is up to 38.36%, and the content of aromatic C–C or C–H, the main existing form on coal surface, is only 25.75%. The 67.31% of elemental silicon is combined with bridging oxygen (Si–O) and 32.69% of that connected to non-bridging oxygen (Si–O2). The existence of aluminum is in the form of aluminum oxides with two coordinated modes ([AlO6] and [AlO4]), and the content of [AlO6] group is nearly double that of [AlO4]. Simultaneously, the binding energies of silicon and aluminum increase by approximately 3 eV, while that of carbon almost no change because of the number of carbon atoms is significantly higher than that of other elements. The silicon atoms and aluminum atoms are surrounded by masses of carbon atoms for the special microstructure of FA. The different chemical states of carbon with higher electronegativity along with the role of high temperature and pressure make the binding energies of silicon and aluminum changed dramatically.

NEXAFS spectroscopy study of lithium interaction with nitrogen incorporated in porous graphitic material

Abstract: Nitrogen-doped carbon nanomaterials have greater capacity and better cycling stability for Li-ion batteries as compared to undoped carbon materials. In situ near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in combination with quantum chemical modeling has been applied to determine the chemical states of the incorporated nitrogen after interaction with lithium. NEXAFS N K-edge spectra of nitrogen-doped porous carbon were measured before and after thermal deposition of Li vapors. The simulation and interpretation of NEXAFS data were carried out based on density functional theory calculations of initial and lithiated graphene fragments that contained different nitrogen species. The preferable interactions of Li with pyridinic and hydrogenated pyridinic nitrogen which are located at edges of atomic vacancies and graphene planes were revealed.

Experimental methods in chemical engineering: X‐ray photoelectron spectroscopy‐XPS

Abstract: X‐ray photoelectron spectroscopy (XPS) is a quantitative surface analysis technique used to identify the elemental composition, empirical formula, chemical state, and electronic state of an element. The kinetic energy of the electrons escaping from the material surface irradiated by an x‐ray beam produces a spectrum. XPS identifies chemical species and quantifies their content and the interactions between surface species. It is minimally destructive and is sensitive to a depth between 1–10 nm. The elemental sensitivity is in the order of 0.1 atomic %. It requires ultra high vacuum (1 × 10 7 − Pa) in the analysis chamber and measurement time varies from minutes to hours per sample depending on the analyte. XPS dates back 50 years ago. New spectrometers, detectors, and variable size photon beams, reduce analysis time and increase spatial resolution. An XPS bibliometric map of the 10 000 articles indexed by Web of Science identifies five research clusters: (i) nanoparticles, thin films, and surfaces; (ii) catalysis, oxidation, reduction, stability, and oxides; (iii) nanocomposites, graphene, graphite, and electro‐chemistry; (iv) photocatalysis, water, visible light, and TiO2; and (v) adsorption, aqueous solutions, and waste water.

Surface reactivity and cation non-stoichiometry in BaZr1-xYxO3-δ (x=0-0.2) exposed to CO2 at elevated temperature

Abstract: The reactivity of BaZr1−xYxO3−δ (x = 0–0.2) ceramics under 1 atm CO2 at 650 °C for up to 1000 h was investigated in order to elucidate possible degradation processes occurring when the material is applied as a proton-conducting electrolyte in electrochemical devices. The annealed ceramics were characterized by a range of techniques (SEM, TEM, GIXRD, XPS and SIMS) with respect to changes in the phase composition and microstructure. Formation of BaCO3 was observed on the surfaces of the annealed samples and the amount increased with time and was higher for the Y-doped compositions. The subsurface regions were found to be deficient in Ba and, in the case of the Y-doped compositions, enriched in Y in two distinct chemical states as identified by XPS. First-principles calculations showed that they were Y residing on the Zr and Ba-sites, respectively, and that local enrichment of Y both in bulk and on the surface attained a structure similar to Y2O3. Overall, it was substantiated that the reaction with CO2 mainly proceeded according to a defect chemical reaction involving transfer of Y to the Ba-site and consumption of BaZrO3 formula units. It was suggested that a similar degradation mechanism may occur in the case of Ba(OH)2 formation under high steam pressure conditions.

Enhanced Visible Light Sensitization of N-Doped TiO2 Nanotubes Containing Ti-Oxynitride Species Fabricated via Electrochemical Anodization of Titanium Nitride

Abstract: The concentration and chemical state of nitrogen represent critical factors to control the band-gap narrowing and the enhancement of visible light harvesting in nitrogen-doped titanium dioxide. In this study, photocatalytic TiO2–N nanoporous structures were fabricated by the electrochemical anodization of titanium nitride sputtered films. Doping was straightforwardly obtained by oxidizing as-sputtered titanium nitride films containing N-metal bonds varying from 7.3 to 18.5% in the Ti matrix. Severe morphological variations into the as-anodized substrates were registered at different nitrogen concentrations and studied by small-angle X-ray scattering. Titanium nitride films with minimum N content of 6.2 atom % N led to a quasi-nanotubular geometry, whereas an increase in N concentration up to 23.8 atom % determined an inhomogeneous, polydispersed distribution of nanotube apertures. The chemical state of nitrogen in the TiO2 matrix was investigated by X-ray photoelectron spectroscopy depth profile analysis ...

In Situ Observation of Metal to Metal Oxide Progression: A Study of Charge Transfer Phenomenon at Ru–CuO Interfaces

Abstract: Surface charge and charge transfer between nanoclusters and oxide supports are of paramount importance to catalysis, surface plasmonics, and optical energy harvesting areas. At present, high-energy X-rays and theoretical investigation are always required to determine the chemical state changes in the nanoclusters and the oxide supports, as well as the underlying transfer charge between them. This work presents the idea of using chrono-conductometric measurements to determine the chemical states of the Ru nanoclusters on CuO supports. Both icosahedral and single-crystal hexagonal close-packed Ru nanoclusters were deposited through gas-phase synthesis. To study the charge transfer phenomenon at the interface, a bias was applied to cupric oxide nanowires with metallic nanocluster decoration. In situ conductometric measurements were performed to observe the evolution of Ru into RuOx under heating conditions. Structural elucidation techniques such as transmission electron microscopy, X-ray photoelectron spectroscopy, and Kelvin probe force microscopy were employed to study the corresponding progression of structure, chemical ordering, and surface potential, respectively, as Ru(0) was oxidized to RuOx on the supporting oxide surface. Experimental and theoretical investigation of charge transfer between the nanocluster and oxide support highlighted the importance of metallic character and structure of the nanoclusters on the interfacial charge transfer, thus allowing the investigation of surface charge behavior on oxide-supported catalysts, in situ, during catalytic operation via conductometric measurements.

Synthesis of M-doped (M = Ag, Cu, In) Bi2Te3 nanoplates via a solvothermal method and cation exchange reaction

Abstract: Highly quality 2D Bi2Te3 nanoplates are obtained via solvothermal synthesis and cations with different chemical valences (Ag+/Cu2+/In3+) are doped into Bi2Te3 by a cation exchange reaction in ethanol solution at room temperature. By combining precise XRD, SEM, HRTEM and XPS characterization, the cations are shown to be doped into the lattice of Bi2Te3 bringing lattice defects and distortion, and cation-doping caused a change in chemical state but without obvious changes in the hexagonal morphology. This research provides a possible general strategy for obtaining cation-doped bismuth telluride.

Substitution mechanisms in In-, Au-, and Cu-bearing sphalerites studied by X-ray absorption spectroscopy of synthetic compounds and natural minerals

Abstract: Sphalerite is the main source of In – a ‘critical’ metal widely used in high-tech electronics. In this mineral the concentration of In is commonly correlated directly with Cu content. Here we use X-ray absorption spectroscopy of synthetic compounds and natural crystals in order to investigate the substitution mechanisms in sphalerites where In is present, together with the group 11 metals. All the admixtures (Au, Cu, In) are distributed homogeneously within the sphalerite matrix, but their structural and chemical states are different. In all the samples investigated In3+ replaces Zn in the structure of sphalerite. The In ligand distance increases by 0.12 A and 0.09–0.10 A for the 1st and 2nd coordination shells, respectively, in comparison with pure sphalerite. The In–S distance in the 3rd coordination shell is close to the one of pure sphalerite. Gold in synthetic sphalerites is coordinated with sulfur (NS = 2.4–2.5, RAu–S = 2.35 ± 0.01 A). Our data suggest that at high Au concentrations (0.03–0.5 wt.%) the Au2S clusters predominate, with a small admixture of the Au+ solid solution with an Au–S distance of 2.5 A. Therefore, the homogeneous character of a trace-element distribution, which is commonly observed in natural sulfides, does not confirm formation of a solid solution. In contrast to Au, the presence of Cu+ with In exists only in the solid-solution state, where it is tetrahedrally coordinated with S atoms at a distance of 2.30 ± 0.03 A. The distant coordination shells of Cu are disordered. These results demonstrate that the group 11 metals (Cu, Ag and Au) can exist in sphalerite in the metastable solid-solution state. The solid solution forms at high temperature via the charge compensation scheme 2Zn2+↔Me++Me3+. The final state of the trace elements at ambient temperature is governed by the difference in ionic radii with the main component (Zn), and concentration of admixtures.

Interface Solid-State Reactions in La0.8Sr0.2MnO3/Ce0.8Sm0.2O2 and La0.8Sr0.2MnO3/BaCe0.9Y0.1O3 Disclosed by X-ray Microspectroscopy

Abstract: The stability of the electrode/electrolyte interface is a critical issue in solid-oxide cells working at high temperatures, affecting their durability. In this paper, we investigate the solid-state chemical mechanisms that occur at the interface between two electrolytes (Ce0.8Sm0.2O2, SDC, and BaCe0.9Y0.1O3, BCY) and a cathode material (La0.8Sr0.2MnO3, LSM) after prolonged thermal treatments. Following our previous work on the subject, we used X-ray microspectroscopy, a technique that probes the interface with submicrometric resolution combining microanalytical information with the chemical and structural information coming from space-resolved X-ray absorption spectroscopy. In LSM/BCY, the concentration profiles show striking reactive phenomena at the interface with a variety of micrometer-sized secondary phases: in particular, X-ray absorption spectra reveal at least three different chemical states for manganese (from +3 to +6). Also in LSM/SDC, a couple previously reported as chemically stable, we found...

Crystallographic and electronic evolution of lanthanum strontium ferrite (La0.6Sr0.4FeO3-δ) thin film and bulk model systems during iron exsolution

Abstract: We study the changes in the crystallographic phases and in the chemical states during the iron exsolution process of lanthanum strontium ferrite (LSF, La0.6Sr0.4FeO3-δ). By using thin films of orthorhombic LSF, grown epitaxially on NaCl(001) and rhombohedral LSF powder, the materials gap is bridged. The orthorhombic material transforms into a fluorite structure after the exsolution has begun, which further hinders this process. For the powder material, by a combination of in situ core level spectroscopy and ex situ neutron diffraction, we could directly highlight differences in the Fe chemical nature between surface and bulk: whereas the bulk contains Fe(iv) in the fully oxidized state, the surface spectra can be described perfectly by the sole presence of Fe(iii). We also present corresponding magnetic and oxygen vacancy concentration data of reduced rhombohedral LSF that did not undergo a phase transformation to the cubic perovskite system based on neutron diffraction data.

Impact of the morphological and chemical properties of copper-zirconium-SBA-15 catalysts on the conversion and selectivity in carbon dioxide hydrogenation

Abstract: A hybrid catalyst consisting of Zr-doped mesoporous silica (Zr-SBA-15) supports with intergrown Cu nanoparticles was used to study the effects of a catalysts chemical states on CO2 hydrogenation. T ...

Investigation of surface reactions in metal oxide on Si for efficient heterojunction Si solar cells

Abstract: Recently, the transition metal oxide thin film has been actively investigated for doping-free heterojunction Si solar cells. However, most of the research on characterizing the chemical state and work function of the metal oxide thin film has been conducted on its surface, while there has been little work on the characterization on the subsurface of the metal oxide thin film. Here, we systematically investigate the chemical state and work function of the evaporated nickel oxide (NiOx) thin film on a Si substrate as a function of the depth position. We found that the chemical state of the NiOx thin film is highly affected by the surface chemical reaction. For instance, an air-exposed NiOx surface exists more in nickel hydroxide [Ni(OH)2] than in nickel monoxide (NiO). In addition, we discern that NiOx near the Si substrate exists in nickel silicide (NiSix). The changed chemical state of the NiOx thin film creates a high variation in the work function as a function of the depth position in the range of 4.4–5.4 eV. We also investigate the heterojunction Si solar cell with the NiOx thin film. We found that the performance of the heterojunction Si solar cell was determined according to the air exposure on the NiOx thin film inducing an undesirable chemical reaction. The heterojunction Si solar cell with the air-exposed NiOx thin film shows a relatively low efficiency of 11.84% by the reduced work function of the NiOx thin film, while one with the controlled NiOx thin film exhibits an enhanced efficiency of 14.23%.