Formation of Gold Clusters on TiO2 from Adsorbed Au(CH3)2(C5H7O2): Characterization by X-ray Absorption Spectroscopy

Gold nanoclusters on TiO₂ powder were prepared from adsorbed Au^III(CH₃)₂(C₅H₇O₂) (dimethyl acetylacetonate gold(III)) and characterized by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies. The samples were tested as catalysts for CO oxidation at 298 K and atmospheric pressure and characterized by EXAFS and XANES with the catalysts in the working state. The XANES results identify Au(III) in the initially prepared sample, and the EXAFS data indicate mononuclear gold complexes as the predominant surface gold species in this sample, consistent with the lack of Au–Au contributions in the EXAFS spectrum. The mononuclear gold complex is bonded to two oxygen atoms of the TiO₂ surface at an Au–O distance of 2.16 Å. Treatment of this complex in He or in H₂ at increasing temperatures led to formation of metallic gold clusters of increasing size, ultimately those with an average diameter of about 15 Å. The data demonstrate the presence of metallic gold clusters in the working catalysts and also show these clusters alone are not responsible for the catalytic activity.     

Developing the properties of new blue phosphors: TiO2-doped Zn2SiO4

2ZnO + SiO₂ + X mol% TiO₂ (Zn₂SiO₄-X-TiO₂, 1 ≤ X ≤ 3) and 2ZnO + SiO₂ + 3 mol% MnO₂ (Zn₂SiO₄-3-TiO₂) compositions were prepared using nanoscale ZnO, SiO₂, TiO₂, and MnO₂ particles. The mixing powders were calcined between 1000 °C and 1300 °C in a N₂ atmosphere. Zn₂SiO₄ was the only phase in the calcined Zn₂SiO₄-X-TiO₂ phosphors. We found that the photoluminescence (PL) properties of synthesized Zn₂SiO₄-X-TiO₂ phosphors revealed these to be blue rather than green. The effects of TiO₂ content and calcining temperature on the PL properties of Zn₂SiO₄-X-TiO₂ phosphors were rigorously investigated.     

Synthesis of high efficiency Zn2SiO4:Mn green phosphors using nano-particles

In this study, Zn₂SiO₄:Mn²⁺ luminescent phosphors were prepared by mixing nano-scale ZnO, SiO₂, and MnO₂ particles at the compositions corresponding to 2ZnO + SiO₂ + X mol% MnO₂ (Zn₂SiO₄–X-MnO₂, 0.02 ≤ X ≤ 0.05). The mixing powders were calcined from 900 °C to 1300 °C in air and in N₂ atmosphere. No matter calcined in air or in N₂ atmosphere, Zn₂SiO₄ was the mainly crystalline phase in particles calcined at 900 °C and was the only phase in particles calcined at 1000 °C and higher. The influences of MnO₂ concentration and calcining atmosphere and temperature on wavelength of luminescence peak and the emission intensity were further intensively investigated. We would show that the calcining atmosphere had no apparent influences on the physical and photoluminescence (PL) characteristics of Zn₂SiO₄:Mn²⁺ phosphors. The MnO₂ content and the calcining temperature were the main reasons to influence the physical and PL characteristics of Zn₂SiO₄:Mn²⁺ phosphors.     

Structure and electrical behaviour in air of TiO2-doped stabilized tetragonal zirconia ceramics

The data obtained from X-ray absorption (XANES and EXAFS) were used to provide information on the environment of Ti atoms in the TiO₂-doped stabilized tetragonal zirconia solid solutions. The electrical conductivity of the Ti–YTZP ceramics decreases with increasing TiO₂ content. From the EXAFS results, i.e. Ti–O and Ti–Ti distances, the decrease in conductivity was attributed to the formation of two kinds of cation–oxygen vacancy associations with different diffusion dynamics, leading to a decrease in the global concentration of moving oxygen vacancies.     

In situ X-ray absorption spectroscopic study for the electrochemical delithiation of a cathode LiFe0.4Mn0.6PO4 material

The electronic and local atomic structural characterization of a promising cathode material, LiFe_0.4Mn_0.6PO₄, for a lithium rechargeable battery was performed by in situ X-ray absorption fine structure (XAFS) on both Mn and Fe K-edges. Upon delithiation, the X-ray absorption near edge structure (XANES) spectra analysis showed that the Fe²⁺/Fe³⁺ electrochemical reaction was two times faster than that of Mn²⁺/Mn³⁺. The Fe and Mn K-edge extended X-ray absorption fine structure (EXAFS) spectra were effectively altered with different spectral behaviors for the local atomic structure near Fe and Mn during delithiation. Alternatively, the EXAFS spectra of LiFePO₄ changed significantly and those of LiMnPO₄ were constant through all delithiations for the corresponding reference materials of LiFePO₄ and LiMnPO₄. The present study with XAFS characterization demonstrates that initially delithiated Fe-rich domains at 3.5 V can promote more effective local structural change of the neighboring Mn-rich domains during the next second plateau at 4.1 V, which can ease delithiation in the Mn-rich domains through more flexible reaction of the local structure in the Mn octahedra.     

In situ d electron density of Pt particles on supports by XANES

The dependency of d electron density of Pt in Pt/SiO₂ catalysts on the particle size was investigated by means of in situ X-ray absorption near-edge structure (in situ XANES) spectroscopy. The d electron density of Pt particles was measured under vacuum, H₂ and ethene, to gain information about ethene hydrogenation on Pt/SiO₂. The intensities of the white lines at L_III and L_II edges in XANES spectra, which are regarded to reflect the unoccupied density of state, varied with the change of particle size under both vacuum and reaction gas atmospheres. The interaction between Pt particle and adsorbates was weak with small particles below 1.5 nm. A new peak induced by Pt-H bonding in the XANES spectra under H₂ was observed for the samples with Pt particle size 1.5 nm. This is related to the change of the turnover frequency and activation energy for ethene hydrogenation by Pt particle size.     

Preparation and characteristics of sol–gel derived Zn2SiO4 doped with Ni2+

The changing presented during the heating of sol–gel derived Zn₂SiO₄ doped with Ni²⁺ have been investigated by X-ray diffraction (XRD) and differential thermal analysis (DTA). When calcining temperature <700 °C, the XRD patterns of the sample show the characteristic peaks of ZnO crystal and non-crystalline SiO₂. When calcining temperature >900 °C, XRD pattern of the sample shows the characteristic peaks of α-Zn₂SiO₄ crystal phase. Also, the excitation and emission spectra of the undoped and Ni²⁺-doped samples have been investigated. Stable green–yellow–red emission has been observed from Zn₂SiO₄ crystalline phase. A novel photoluminescence (PL) phenomenon has been observed from Ni²⁺-doped Zn₂SiO₄.     

Photoluminescence of (ZnO)X-Z(SiO2)Y:(MnO)Z green phosphors prepared by direct thermal synthesis: The effect of ZnO/SiO2 ratio and Mn2+ concentration on luminescence

Green light emitting Zn₂SiO₄:Mn²⁺ phosphors have been synthetised by the solid-state reaction in ambient atmosphere at 1300 °C for 2 h, with ZnO, SiO₂ and MnO₂ as the reagents. The ZnO/SiO₂ molar ratio varied from 2 to 0.5. The doping level was in a lower concentration range (0.01≤x≤0.05). The effect of both the Mn²⁺ concentration and ZnO/SiO₂ molar ratio on luminescence intensity and decay was investigated in detail. The microstructure and phase composition of prepared phosphors were characterised by scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). XRD results indicate that the pure α-Zn₂SiO₄ phase with rhombohedral structure was obtained after heat treatment. The prepared phosphors exhibit a strong green emission centred at 525 nm from the ⁴T₁→⁶A₁ forbidden transition. The highest emission intensity was observed for phosphors with ZnO/SiO₂ molar ratio equal to 1.0, and the Mn²⁺ concentration x=0.03 (ZSMn3). The emission intensity of the ZSMn3 phosphor is comparable with the commercial Zn₂SiO₄:Mn²⁺ phosphor. The decay curves can be characterised by double exponential function. After fitting a fast component τ₁∼2 ms and a slow component τ₂∼10 ms were obtained. The decay times decrease significantly with increasing Mn²⁺ concentration. The decay time and luminescence mechanism depend on the excitation light wavelength. Temperature dependent luminescence of the ZSMn3 phosphor in the temperature range of 25–200 °C was studied.     

Structural studies of a ZrO2–CeO2 doped system

The local structure around Zr, Ce and dopant atoms (Fe and Ni) in the ZrO₂–CeO₂ system investigated by X-ray absorption spectroscopy (XAS) is reported to better understand the tetragonal phase stabilization process of zirconia. The first coordination shell around Zr atoms is not sensitive to the introduction of dopants or to an increase in the ceria content (from 12 to 20 mol%). Ce ions maintain the eight-fold coordination as in CeO₂, but with an altered bond distance. The formation of vacancies resulting from reduction of Ce atoms can be discarded, because XANES spectra clearly show that Ce ions are preferentially in a tetravalent state. XANES and EXAFS experiments at the Fe K-edge evidence that the local order around Fe is quite different from that of the Fe₂O₃ oxide. On the one hand, ab initio EXAFS calculations show that iron atoms form a solid solution with tetragonal ZrO₂. The EXAFS simulation of the first coordination shell around iron evidences that the substitution of zirconium atoms by iron ones generates oxygen vacancies into the tetragonal network. This is a driven force for the tetragonal phase stabilization process. For Ni doped samples, EXAFS results show that Ni–O mean bond length is similar to the distance found in the oxide material, i.e., NiO compound. Besides this result, no evidence of similar solid solution formation for Ni-doped systems has emerged from the EXAFS analysis.     

In situ X-ray absorption spectroscopy study of lithium insertion in a new disordered manganese oxi-iodide

Mn K-edge X-ray absorption measurements were carried out on an amorphous manganese oxi-iodide (Li_0.60Na_0.16MnO_2.33I_≈0.05) as cathode material in plastic lithium batteries. X-ray absorption spectroscopy experiments were performed in situ for every lithium intercalation increment Δx≈0.10 per formula unit along a discharge with 0≤x≤0.67. The X-ray absorption near edge structure (XANES) spectra demonstrate a smooth decrease of Mn valence with in-situ reduction, confirmed by a monotonous increase in Mn-O distance from extended X-ray absorption fine structure (EXAFS). The first-shell octahedral coordination of manganese is hardly affected by lithium insertion, while the evolution of the Mn-Mn coordinence shows a trend towards the formation of MnO_n polyhedra chains on deep discharge. The material discharged to an average manganese valence 3.25+ showed negligible static Jahn-Teller effect. Together with the inherent flexibility of disordered structures towards intercalation and bond length changes, this is probably a major cause of the stability of this oxi-iodide on cycling in lithium batteries.     

Sol-gel synthesis and luminescence properties of Ba2SiO4:Sm3+ nanostructured phosphors

Ba₂SiO₄:Sm³⁺ nanostructure phosphors have been synthesized by a simple sol-gel method. Phase evaluation, structural characteristics and photoluminescence properties of the synthesized Ba₂SiO₄:Sm³⁺ powders were studied using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric and differential thermal analysis (TG-DTA), Fourier transform infrared spectroscopy (FTIR), and photoluminescence spectroscopy (PL). X-ray diffraction results showed that all synthesized samples were single-phase barium silicate (Ba₂SiO₄) and samarium (Sm) ions were incorporated into the lattice of Ba₂SiO₄. Adding samarium to barium silicate changed the microstructure from vermicular to spherical structures. The Photoluminescence spectrum of Ba₂SiO₄:Sm³⁺ phosphors exhibited characteristic emission peaks at 562?nm which is due to the ⁴G_{5/2 →}⁶H_7/2 transition of samarium ions and corresponds to the orange region. The results showed that the barium silicate activated with 0.08?mol samarium exhibited the highest PL intensity.     

X-ray absorption fine structure study on residue bromine in carbons with different degrees of graphitization

The structure of bromine residue compounds was investigated by X-ray absorption fine structure (XAFS) in order to interpret where and how bromine is present in carbons with different degrees of graphitization. The residue compounds can be classified into three groups, as obtained from X-ray absorption near edge structure (XANES) spectra and the values of the intramolecular distance r_Br–Br determined by extended X-ray absorption fine structure (EXAFS). In Group I, prepared from the host carbons heat treated at temperatures higher than 1900 °C, bromine exists in the interlayer space of graphite in the form of Br₂ molecules with interaction of the π electrons of graphite. In Group III, from carbon heat treated at 1000 °C, most of the bromine probably reacts with carbon atoms having a dangling bond or functional groups. For Group II, where the host carbons are heat treated at intermediate temperatures, it is likely that bromine exists in undeveloped defects with a unique electronic state.     

Thermal,Structural, and Enhanced Photoluminescence Properties of Eu3+‐doped Transparent Willemite Glass–Ceramic Nanocomposites

The precursor glass in the ZnO–Al₂O₃–B₂O₃–SiO₂ (ZABS) system doped with Eu₂O₃ was prepared by the melt‐quench technique. The transparent willemite, Zn₂SiO₄ (ZS) glass–ceramic nanocomposites were derived from this precursor glass by a controlled crystallization process. The formation of willemite crystal phase, size, and morphology with increase in heat‐treatment time was examined by X‐ray diffraction (XRD) and field‐emission scanning electron microscopy (FESEM) techniques. The average calculated crystallite size obtained from XRD is found to be in the range 18–70 nm whereas the grain size observed in FESEM is 50–250 nm. The refractive index value is decreased with increase in heat‐treatment time which is caused by the partial replacement of ZnO₄ units of ZS nanocrystals by AlO₄ units due to generation of vacancies. Fourier transform infrared (FTIR) reflection spectroscopy was used to evaluate its structural evolution. Vickers hardness study indicates marked improvement of hardness in the resultant glass‐ceramics compared with its precursor glass. The photoluminescence spectra of Eu³⁺ ions exhibit emission transitions of ⁵D₀→⁷F_j (j = 0, 1, 2, 3, and 4) and its excitation spectra show an intense absorption band at 395 nm. These spectra reveal that the luminescence performance of the glass–ceramic nanocomposites is enhanced up to 17‐fold with the process of heat treatment. This enhancement is caused by partitioning of Eu³⁺ ions into glassy phase instead of into the willemite crystals with progress of heat treatment. Such luminescent glass–ceramic nanocomposites are expected to find potential applications in solid‐state red lasers, phosphors, and optical display systems.     

XANES investigation of the enhanced chemical stability of Pd in supported Pd–Mo catalysts

The XANES (X-ray absorption near edge structure) technique was used to study Pd and Mo catalysts deposited on supported Al₂O₃/SiO₂ and Al₂O₃/Si-MCM-41 in the form of monometallic and bimetallic systems. The results indicate that Pd has stronger chemical stability when in the presence of Mo and is always in the metallic form, which is surprising, because the samples were not subjected to reducing conditions prior to the measurements. The increased stability was attributed to the formation of a core-shell structure with a Pd rich core and a Mo rich surface.     

Structural properties of the red-color overglaze for the HIZEN porcelains produced in the early Edo period of Japan

HIZEN porcelains made in 1650s to 1750s (early Edo period) in Arita areas sited in south Japan (SAGA) are famous Japanese porcelains, which are characterized by elegant and bright colors in the overglaze and the underglaze. Red-overglazes and transparent glazes of the HIZEN porcelains have been investigated by means of X-ray diffraction (XRD) and X-ray absorption spectra (XAS) using synchrotron radiation. The results suggest that the red-color brightness of the Hizen porcelains is mainly induced by micro-structural correlation between α-Fe₂O₃ fine particles of red-color emission element and the oxide complexes of SiO₂–Al₂O₃–CaO–KNaO or SiO₂ in the fritted overglazes. The stability of the red-color overglaze on the porcelain body of white-color results from interfacial fusion between both glass-states in the fritted overglaze and the transparent glaze coating the porcelain body. The refined local structures around Fe ions of the α-Fe₂O₃ structure taken EXAFS spectra give the technical and historical relation among four kinds of the HIZEN porcelains for fritted materials of the overglazes and thermal treatment at high-temperature in the porcelain kilns.     

An X-ray absorption study on the local structures of highly dispersed supported titanium oxides prepared by a CVD method

Titanium oxides supported on SiO₂ and Al₂O₃ prepared by a CVD method have been characterized by XANES/EXAFS technique. Titanium species on SiO₂ are highly dispersed in a tetrahedral coordination. Titanium species on Al₂O₃ are also highly dispersed in fivefold coordination. High dispersion was substantiated by their photoluminescence emitted by excitation at 300–350 nm.     

X-ray absorption fine structure spectroscopy and X-ray diffraction study of cementitious materials derived from coal combustion by-products

Cementitious materials derived from coal combustion by-products have been investigated by means of X-ray diffraction (XRD) and S and Ca K-edge X-ray absorption fine structure (XAFS) spectroscopy. The XRD analysis revealed that these materials are a complex mixture of a small amount of quartz [SiO₂] and three calcium-bearing compounds: hannebachite [CaSO₃·1/2H₂O], gypsum [CaSO₄·2H₂O] and ettringite [(Ca₆(Al(OH)₆)₂(SO₄)₃·26H₂O)]. Analysis of the S XAFS data focused on deconvolution of the X-ray absorption near-edge structure (XANES) regions of the spectra. This analysis established that sulfate and sulfite are the two major sulfur forms, with a minor thiophenic component contained in unburned carbon in the fly ash. Increasing sulfate and decreasing sulfite correlated well with increasing gypsum and ettringite and decreasing hannebachite content in the samples. Different calcium compounds were identified primarily through simple comparison of the Ca K-edge XANES and radial structure functions (RSFs) of the cementitious samples with those of reference compounds. Because of the complex coordination chemistry of calcium in these materials, it was difficult to obtain detailed local atomic environment information around calcium beyond the first CaO peak. Analysis of the extended X-ray absorption fine structure (EXAFS) and the RSF gave average CaO distances in the range 2.44-2.5 Å, with each calcium atom surrounded roughly by eight oxygen atoms. In certain samples, the average CaO distances were close to that in ettringite (2.51 Å), suggesting that these samples have higher ettringite content. The results of S and Ca K-edges XAFS and the XRD data were in reasonable agreement.     

Formation and Local Structure Analysis of High‐Valence Chromium Ion in Dicalcium Silicate

Dicalcium silicate (2CaO·SiO₂) is a typical silicate compound that exists even at elevated temperatures. Different types of elements can be dispersed in dicalcium silicate to form a solid solution phase. However, the dispersion behavior of transition elements that can display different ionic valences is not well understood. Herein, we investigate the local structure of chromium ions dispersed in dicalcium silicate of different phase states under inert and air atmospheres using Cr K‐edge X‐ray absorption near‐edge structure (XANES) spectroscopy and first‐principle calculations. The measured XANES spectra indicated that Cr ions exist as high‐valence states when dispersed in dicalcium silicate in air atmosphere. Specifically, Cr⁶⁺ ions were detected in γ‐dicalcium silicate, which was prepared by annealing a mixture of 2CaO·SiO₂ and Cr₂O₃ at 973 K in air atmosphere. In addition, the XANES spectra obtained via first‐principle calculations revealed that Cr ions, with high‐valence states between Cr⁴⁺ and Cr⁶⁺, in dicalcium silicate, occupy the tetrahedral Si⁴⁺ sites.     

Luminescent and structural properties of manganese-doped zinc aluminate spinel nanocrystals

Manganese-doped zinc aluminate spinel (ZnAl₂O₄:Mn; Mn=0–6.0 mol%) phosphor nanoparticles were prepared by the sol–gel process. The effects of thermal annealing and dopant concentration on the structure, microstructure and luminescence of the powder phosphors were investigated. The X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) results confirmed that a single-phase spinel started to crystallize at around 600 °C for the investigated powders. On heating at 600–1200 °C, the powders had the average crystallite sizes of around 12–33 nm. The crystallite size and lattice constant increased as the doping level of Mn increased. FT-IR spectra exhibited only absorption bands of the AlO₆ octahedral groups, which suggested that the powder phosphors mainly crystallized in a normal spinel structure. Scanning electron microscopy (SEM) investigations showed the primary particle sizes were around 20–25 nm for the powders annealed at 1000 °C, and less than ca. 50 nm for those annealed at 1200 °C. Photoluminescence (PL) spectra under UV or visible light excitation exhibited a strong green emission band centered at 510 nm, corresponding to the typical ⁴T₁(⁴G)—⁶A₁(⁶S) transition of tetrahedral Mn²⁺ ions. The most intense PL emission was obtained by exciting at 458 nm. The PL intensity was significantly enhanced by the improved crystallinity and diminished OH^? groups. Optimum brightness occurred at a doping of 3.0 mol% Mn.     

Preparation of Sr2SiO4:Eu phosphors by microwave-assisted sintering and their luminescent properties

An Eu³⁺ activated strontium silicate phosphor was synthesized using a microwave-assisted sintering with a flux NH₄Cl. X-ray powder diffraction analysis confirmed the formation of pure Sr₂SiO₄ phase without second phase or phases of starting materials as Sr_1.9SiO₄:Eu³⁺_0.1 powders sintered at various temperatures in microwave furnace for 1 h. Scanning electron microscopy showed smaller particle size and more uniform grain size distributions are obtained by microwave-assisted sintering. In the PL studies, the excitation spectrum of Sr_1.9SiO₄:Eu³⁺_0.1 phosphors exhibited a broad band in the UV region centered at about 270 nm which was consistent with the absorption spectra. Both microwave sintered and conventionally sintered powders emitted a maximum luminescence centered at 617 nm under excitation of 395 nm, with similar luminescent intensity. The results showed that microwave processing has the potential to decrease the sintering time and required energy input for the production of Sr_1.9SiO₄:Eu³⁺_0.1 phosphors without degrading photoluminescence.