Detection of nanocrystallinity by X-ray absorption spectroscopy in thin film transition metal/rare-earth atom,elemental and complex oxides

Nanocrystallinity has been detected in the X-ray absorption spectra of transition metal and rare-earth oxides by (i) removal of d-state degeneracies in the (a) Ti and Sc L₃ spectra of TiO₂ and LaScO₃, respectively, and (b) O K₁ spectra of Zr(Hf)O₂, Y₂O₃, LaScO₃ and LaAlO₃, and by the (ii) detection of the O-atom vacancy in the O K₁ edge ZrO₂–Y₂O₃ alloys. Spectroscopic detection is more sensitive than X-ray diffraction with a limit of ∼2 nm as compared to >5 mm. Other example includes detection of ZrO₂ nanocrystallinity in phase-separated Zr(Hf) silicate alloys.     

Suppression of Jahn–Teller term-split band edge states in the x-ray absorption spectra of non-crystalline Zr silicates and Si oxynitride alloys,and alloys of ZrO2 with Y2O3

Jahn–Teller (J–T) term-split states in nanocrystalline transition metal and trivalent rare earth elemental and complex oxides reduce the band gap, and tunnelling barrier height at interfaces with crystalline Si substrates. These states are identified by x-ray absorption spectroscopy and spectroscopic ellipsometry. Alloys for suppression of J–T d-state degeneracy removal are identified as: (i) non-crystalline Zr/Hf silicates and Si oxynitrides and (ii) ZrO₂–Y₂O₃ alloys with high concentrations of randomly distributed O-vacancies that promote cubic crystalline symmetry.     

Stability,Infrared Spectra and Electronic Structures of (ZrO2)n(n=3-6)Clusters:DFT Study

The stability, infrared spectra and electronic structures of (ZrO₂)_n (n=3–6) clusters have been investigated by using density‐functional theory (DFT) at B3LYP/6‐31G* level. The lowest‐energy structures have been recognized by considering a number of structural isomers for each cluster size. It is found that the lowest‐energy (ZrO₂)₅ cluster is the most stable among the (ZrO₂)_n (n=3–6) clusters. The vibration spectra of Zr? O stretching motion from terminal oxygen atom locate between 900 and 1000 cm^?1, and the vibrational band of Zr? O? Zr? O four member ring is obtained at 600–700 cm^?1, which are in good agreement with the experimental results. Mulliken populations and NBO charges of (ZrO₂)_n clusters indicate that the charge transfers occur between 4d orbital of Zr atoms and 2p orbital of O atoms. HOMO‐LUMO gaps illustrate that chemical stabilities of the lowest‐energy (ZrO₂)_n (n=3–6) clusters display an even‐odd alternating pattern with increasing cluster size.     

X-ray absorption spectroscopy of rare earth intermediate oxides

The M_IV–V and L_III absorption spectra (between 850 and 7500 eV) of intermediate rare earth oxides (Pr₇O₁₂ and Pr₉O₁₆) were studied. These oxides required careful preparation and handling in order to assure their composition.The spectra are characterized by multiplet features that are interpreted as having contributions from both trivalent and tetravalent sites. In the M_IV–V spectra the appearance of distinct multiplet lines and additional weaker features demonstrate clearly the increasing ratio of the tetravalent sites as the oxygen content increases from Pr₇O₁₂ to PrO₂. Similar behavior has been observed for the CeO_x and TbO_x systems. These observations show that in these oxides, the trivalent and tetravalent sites are inequivalent and that the evidence of valence transition is seen in the appearance of the complex spectral features originating from the tetravalent sites.     

Nano-crystalline zirconia: a viable adsorbent for the column chromatographic separation of 58Co from neutron irradiated nickel targets

Owing to the high chemical reactivity of molten uranium alloys, the use of traditional graphite crucibles for casting fuel slugs for a sodium-cooled fast reactor (SFR) is problematic. Moreover, rare earth (RE) elements retained in the fuel slugs for an SFR, which are extracted from the spent fuel by pyro-processing, are more reactive than uranium melt. Therefore, in this study, Y₂O₃ single-layer coatings with thicknesses of approximately 50, 70, and 120 μm and double-layer coatings of TaC/Y₂O₃ and Y₂O₃/TaC were plasma-sprayed onto niobium substrates and tested for thermal shock resistance and compatibility against U–10 wt% Zr and U–10 wt% Zr–5 wt% RE melt. The Y₂O₃ single-layer coating, regardless of coating thickness, and the TaC/Y₂O₃ double-layer coating showed good contact at the interface between the coating and the niobium substrate, with no deterioration after the thermal cycling test. In the interaction studies, the single- and double-layer coatings showed good compatibility with the U–Zr melt. However, the Y₂O₃ coatings with thicknesses of approximately 50 and 70 μm showed severe penetration of the U–Zr–RE melt and reacted with the niobium substrate. The single-layer Y₂O₃ coating with a thickness of 120 μm and the double-layer TaC/Y₂O₃ coating exhibited the most promising performance among the candidate coatings.     

Hydrogen Production by Steam Reforming of Ethanol Over Mesoporous Ni–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–ZrO<Subscript>2</Subscript> Catalysts

This review paper reports the recent progress concerning the application of nickel–alumina–zirconia based catalysts to the ethanol steam reforming for hydrogen production. Several series of mesoporous nickel–alumina–zirconia based catalysts were prepared by an epoxide-initiated sol–gel method. The first series comprised Ni–Al₂O₃–ZrO₂ xerogel catalysts with diverse Zr/Al molar ratios. Chemical species maintained a well-dispersed state, while catalyst acidity decreased with increasing Zr/Al molar ratio. An optimal amount of Zr (Zr/Al molar ratio of 0.2) was required to achieve the highest hydrogen yield. In the second series, Ni–Al₂O₃–ZrO₂ xerogel catalysts with different Ni content were examined. Reducibility and nickel surface area of the catalysts could be modulated by changing nickel content. Ni–Al₂O₃–ZrO₂ catalyst with 15 wt% of nickel content showed the highest nickel surface area and the best catalytic performance. In the catalysts where copper was introduced as an additive (Cu–Ni–Al₂O₃–ZrO₂), it was found that nickel dispersion, nickel surface area, and ethanol adsorption capacity were enhanced at an appropriate amount of copper introduction, leading to a promising catalytic activity. Ni–Sr–Al₂O₃–ZrO₂ catalysts prepared by changing drying method were tested as well. Textural properties of Ni–Sr–Al₂O₃–ZrO₂ aerogel catalyst produced from supercritical drying were enhanced when compared to those of xerogel catalyst produced from conventional drying. Nickel dispersion and nickel surface area were higher on Ni–Sr–Al₂O₃–ZrO₂ aerogel catalyst, which led to higher hydrogen yield and catalyst stability over Ni–Sr–Al₂O₃–ZrO₂ aerogel catalyst.     

EPMA of interfaces applied to the solid oxide fuel cell

Chemical interactions at the phase boundaries of materials applied for the solid oxide fuel cell (SOFC) have been studied by EPMA. The chemical reactivity at the interface of La_y-xSr_xMnO₃/ZrO₂-Y₂O₃ is dependent on the stoichiometry (y) and the Sr content (x) of the perovskite. Typical reaction products (zirconates) and a diffusion zone in the ZrO₂–Y₂O₃ have been observed. The extension of cation release (Mn) is related to the increasing chemical activity of Mn oxide in the perovskite by the Sr substitution for La. The wettability of the metal/oxide interface in the anode cermet (Ni/ZrO₂–Y₂O₃) has been found to be influenced by chemical reactions resulting from the applied reducing atmosphere with high carbon activity. The disintegration of ZrO₂–Y₂O₃ in contact with molten Ni or Ni-Ti and Ni-Cr alloys leads to the redeposition of Y₂O₃-enriched oxides and also to Zr-rich intermetallic compounds and eutectics.     

Structure and optical properties of [Ba1–xY2x/3](Zr0.25Ti0.75)O3 powders

[Ba_1–xY_2x/3](Zr_0.25Ti_0.75)O₃ powders with different yttrium concentrations (x = 0, 0.025 and 0.05) were prepared by solid state reaction. These powders were analyzed by X-ray diffraction (XRD), Fourier transform Raman scattering (FT-RS), Fourier transform infrared (FT-IR) and X-ray absorption near-edge (XANES) spectroscopies. The optical properties were investigated by means of ultraviolet–visible (UV–vis) absorption spectroscopy and photoluminescence (PL) measurements. Even with the addition of yttrium, the XRD patterns revealed that all powders crystallize in a perovskite-type cubic structure. FT-RS and FT-IR spectra indicated that the presence of [YO₆] clusters is able to change the interaction forces between the O–Ti–O and O–Zr–O bonds. XANES spectra were used to obtain information on the off-center Ti displacements or distortion effects on the [TiO₆] clusters. The different optical band gap values estimated from UV–vis spectra suggested the existence of intermediary energy levels (shallow or deep holes) within the band gap. The PL measurements carried out with a 350 nm wavelength at room temperature showed that all powders present typical broad band emissions in the blue region.     

Synthesis of Er3+:Y3Al5O12 and its effects on the solar light photocatalytic activity of TiO2–ZrO2 composite

The Er³⁺:Y₃Al₅O₁₂ as an upconversion luminescence agent, which can transform visible light into ultraviolet light, was synthesized by nitrate?Ccitrate acid and calcined method. Then, a novel photocatalyst, Er³⁺:Y₃Al₅O₁₂/TiO₂?CZrO₂, was prepared using ultrasonic dispersion and liquid boiling method. The samples were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). In succession, the degradation process of organic dye was monitored by UV?CVis spectrum and ion chromatography for verifying the photocatalytic activity of Er³⁺:Y₃Al₅O₁₂/TiO₂?CZrO₂. The influences on its photocatalytic activity such as Ti/Zr molar ratio, heat-treated temperature, and time were studied. In addition, the influences of initial concentration, Er³⁺:Y₃Al₅O₁₂/TiO₂?CZrO₂ amount, solar light irradiation time, and organic dye category on the photocatalytic degradation efficiency were also investigated. It was found the photocatalytic activity of Er³⁺:Y₃Al₅O₁₂/TiO₂?CZrO₂ was superior to Er³⁺:Y₃Al₅O₁₂/TiO₂ and Er³⁺:Y₃Al₅O₁₂/ZrO₂. Therefore, the Er³⁺:Y₃Al₅O₁₂/TiO₂?CZrO₂ is a useful photocatalytic material for the wastewater treatment duo to efficient utilization of solar light.     

Crystal growth of SnO2 and other MO2 (M = Ti,Zr, Hf) oxides by flux of B2O3?V2O5 system

Single crystals of SnO₂ and MO₂ (M = Ti, Zr, Hf) oxide were grown from flux of B₂O₃? V₂O₅ system. Mixtures of the flux and the starting powder of Zn₂SnO₄, TiO₂, ZrO₂, or HfO₂ were soaked at a temperature of 1030–1340°C for 10–72 hr and then were cooled down to 900°C at a rate of 5°C/hr. Grown crystals of SnO₂ were pale brown needles. An increase in V₂O₅ content of the flux (up to V₂O₅/B₂O₃ ratio equal to 2) or in the soaking temperature increases the crystal size. A largest crystal with the size of 15.0 × 0.4 × 0.4 mm was obtained in the case of V₂O₅/B₂O₃ = 2. Crystals of TiO₂ were black needles or platelets, and those of ZrO₂ and HfO₂ were yellowish, transparent needles or blocks. The maximum size of TiO₂, ZrO₂ or HfO₂ crystal was 12.0 × 0.1 × 0.1 mm, 4.0 × 0.3 × 0.3 mm or 11.0 × 0.6 × 0.6 mm, respectively. The long axis of the crystals was all C-axis and main faces on the crystals were of {100} and/or {110} families. All these crystals were found to include the impurities of boron and vanadium. The electrical resistivities of SnO₂ and TiO₂ crystals were measured to be 1.4 × 10⁶ and 5.6 × 10⁴ Ω · cm at 25°C, respectively.     

Nanostructuring phenomena in oxygen-conducting complex oxides of heavy REE

In complex oxides of REE (Ln₄M₃O₁₂ (Ln = Tm, Lu; M = Zr, Hf), Ln₂TiO₅ (Ln = Er-Yb)) and Ho₂TiO₅, the following phase transitions of the order-disorder type are studied for different cooling rates: rhombohedral δ-phase-defective fluorite in Ln₄M₃O₁₂ (Ln = Tm, Lu; M = Zr, Hf), pyrochlor-like phasedefective fluoride in Ln₂TiO₅ (Ln = Er-Yb), and hexagonal β-phase-pyrochlor in Ho₂TiO₅. The presence of nanostructuring phenomena typical of fluorite-like polymorphous modifications of complex oxides in the Ln₂O₃-MO₂ (Ln = Ho-Lu; M = Ti, Zr, Hf) systems is confirmed. The conductivity of polymorphous modifications of Ln₄Zr₃O₁₂ (Ln = Tm, Lu;) and Ln₂TiO₅ (Ln = Ho-Yb) with different thermal prehistory is studied. The comparative studies of the oxygen-ionic conductivity of fluorite- and pyrochlor-like Ln₂TiO₅ (Ln = Ho-Yb), pyrochlor Ho₂TiO₅, and β-Ho₂TiO₅ and also of the conductivity of fluorite-like compounds and δ-Ln₄Zr₃O₁₂ (Ln = Tm, Lu) are carried out. The oxygen-ionic conductivity of complex oxides in the Ln₂O₃-MO₂ (Ln = Er-Lu; M = Ti, Zr, Hf) system is shown to decrease in the following series: defective pyrochlor-defective fluorite-rhombohedral δ-phase ∼ hexagonal β-phase.     

Activation of Sintering of Zirconia Ceramics by Gamma Irradiation

It was studied how gamma irradiation affects the formation of the phase composition and the properties of ZrO₂-based ceramics containing various contents of Y₂O₃, which stabilizes the tetragonal ZrO₂ phase. Gamma irradiation favored sintering and a decrease in the content of the monoclinic ZrO₂ modification. At a sintering temperature of 1250°C, ZrO₂–3 mol % Y₂O₃ ceramic with a strength of 225 MPa was obtained; this strength is half as high again as that of ceramic synthesized without irradiation.     

Highly efficient UV–visible absorption of TiO2/Y2O3 nanocomposite prepared by nanosecond pulsed laser ablation technique

Nanostructured TiO₂-based composites are promising materials because of their superior optical, structural, and electronic properties relative to pristine nanostructured TiO₂. The enhanced properties of TiO₂-based composites have been used in several important applications such as gas sensors, solar cells, and photocatalytic applications. In the past, numerous materials have been coupled with TiO₂ to enhance their optical properties. In this work, full-spectrum (UV and Visible) responsive TiO₂ /Y₂O₃ nanocomposite has been synthesized via pulsed laser ablation in liquid (PLA) to study the impact of Y₂O₃ on the structural, morphology, and optical property of the TiO₂. The nanostructured composites prepared were characterized by XRD, Raman spectroscopy, Field-Emission Scanning Electron Microscope (FESEM) attached with Energy-Dispersive X-ray spectroscopy (EDX), Photoluminescence, XPS, and UV–Vis absorbance spectra. The result demonstrates that the coupling Y₂O₃ with TiO₂ not only changes the structural, optical, and morphology of the TiO₂ but also significantly amplified the light absorption characteristics of the TiO₂ within the UV and visible region. The synthesized TiO₂ /Y₂O₃ nanocomposite could potentially be useful for visible-light responsive applications.     

The structure and ordering of zirconium and hafnium containing garnets studied by electron channelling, neutron diffraction and Mössbauer spectroscopy

Garnets, A₃B₂C₃O₁₂, are an important group of minerals and have potential uses in the safe immobilisation of high-level nuclear waste. They have been found naturally to incorporate Zr, Ti and Fe, three elements of interest in the safe storage of nuclear waste. Kimzeyite, Ca₃(Zr,Ti)₂(Si,Al,Fe)₃O₁₂, is a naturally occurring garnet that contains Zr in a high percentage∼30 wt%. For such a material to be of potential immobilisation for nuclear waste the structure needs to be completely understood. Electron channelling studies have shown that the Zr/Ti cations are located on the Y-site, with the Al/Fe cations located on the Z-site. This work has investigated synthetic analogues of kimzeyite, Ca₃(Zr,Hf)₂(Al,Fe,Si)₃O₁₂, by neutron powder diffraction, using the C2 spectrometer at the Chalk River nuclear facility, coupled with ⁵⁷Fe Mössbauer spectroscopy. Such work has allowed the structure of the synthetic material to be determined along with the distribution of cations across the X (CN=8), Y (CN=6), and Z (CN=4) sites. Results have shown that it is possible to synthesise Ca₃(Zr,Hf)₂(Al,Fe,Si)₃O₁₂ with a range of Al/Fe ratios containing Zr and Hf. The Mössbauer data has indicated the Fe is located on the Z site. The structural analyses show that the unit cell changes linearly as a function of composition, and analysis of the disorder indicates that the Zr, Hf reside on the Y site and the Al, Fe, and Si reside on the Z site.     

XANES Spectroscopic Studies of the Phase Transition in Gd2Zr2O7

The structural phase transition from fluorite to pyrochlore and the strength of the coordination bond of Zr–O in Gd₂Zr₂O₇ were investigated by XANES spectra of both O and Zr K‐edge. The energy difference of the O K‐edge absorption spectra at 532 and 536 eV was assigned to the crystal field splitting energy of the 4d orbital (ΔE_4d, t_2g and e_g) of the Zr ion. Also, in the samples prepared at higher temperatures, the 536 eV peak moves slightly to higher energy, whereas the absorption energy of 532 eV peak does not shift. A correlation between ΔE_4d and the strength of interaction between Zr (4d) and O (2p) orbitals has been found. Furthermore, two Zr K‐edge absorptions at 18020 and 18030 eV of Gd₂Zr₂O₇ have been observed; the splitting energy (ΔE), peak intensity ratio (I₁₈₀₃₀/I₁₈₀₂₀), and FWHM of the first derivative of the absorption curve depend on the preparation temperatures. The effect of crystal symmetry and Zr‐O bonding character on the XANES spectral profile was discussed.     

Efficiency of various semiconductor catalysts for photodegradation of Safranin-T

Semiconductor photocatalysis often leads to partial or complete mineralization of organic pollutants. In this study, photocatalytic degradation of Safranin-T, a hazardous textile dye, has been investigated using various semiconductors such as titanium dioxide (TiO₂), zinc oxide (ZnO), bismuth oxide (Bi₂O₃), cerium oxide (CeO₂), yttrium oxide (Y₂O₃), and zirconium oxide (ZrO₂). The experiments were carried out by irradiating the aqueous solution of Safranin-T containing photocatalysts with UV and air. Maximum decolorization of Safranin-T occurred with TiO₂ (99.8%), followed by ZnO (80.3%), Bi₂O₃ (57.1%), CeO₂ (13.1%), Y₂O₃ (12.2%), and ZrO₂ (10.2%). The rate of photocatalytic degradation varied with increasing concentration of Safranin-T. The equilibrium degradation data of Safranin-T by TiO₂ and ZnO were fitted to the Langmuir and Freundlich isotherm models. The Freundlich and Langmuir model showed satisfactory fit to the equilibrium degradation data for TiO₂ and ZnO, respectively. Photocatalytic degradation of Safranin-T followed pseudo second-order kinetics.     

Structural features of ZrO2-Y2O3 and ZrO2-Gd2O3 nanoparticles formed under hydrothermal conditions

We synthesized nanoparticles of variable composition based on zirconium dioxide in the ZrO₂-Y₂O₃ (or Gd₂O₃)-H₂O systems under hydrothermal conditions. By X-ray diffraction and small-angle X-ray scattering studies revealed that the nanoparticles consist of crystalline core and amorphous shell. Increase of Y₂O₃ (or Gd₂O₃) content yields increases of shell size and decreases of core size. The effect is due to suppressed ZrO₂ crystallites growth caused by development of the shell preventing zirconium ions transport.     

Straightforward synthesis of Ti-doped YSZ gels by chemical modification of the precursors alkoxides

Translucent, homogeneous, and monolithic gels of the [(ZrO₂)_0.92(Y₂O₃)_0.08]_1?x(TiO₂)_x system, where x = 0, 0.05, 0.08, and x = 0.10 (mol), have been reliably obtained, for the first time, by a sol–gel route from zirconium (IV) n-propoxide (Zr(OPrⁿ)₄), yttrium acetate hydrate and titanium (IV) isopropoxide (Ti(OPrⁱ)₄). Chemical modification of both alkoxides, zirconium (IV) n-propoxide and titanium (IV) isopropoxide, by acetic acid allows us to change the hydrolysis and condensation behavior of them. Their modification implies the formation of chelating and bridging acetates avoiding the formation of precipitates. The line width and some shoulders in the FT-IR spectra of the solution, resulting of the mixture of the precursors and the catalysts, during the hydrolysis reaction suggest that both coordinations, chelating and bridging, should occur. Furthermore, the separation of the steps of hydrolysis and condensation allows to achieve conditions under which hydrolysis of the molecular precursor is slowed, whereas condensation is promoted under chemical reversibility to ensure a crystalline product at low calcination temperatures. In addition, the formation of metalloxane bondings (M–O–M’, M and M’ = Zr, Y, and Ti) has been confirmed by FT-IR throughout the sol–gel process. At about 630 °C, the crystallization of yttria stabilized zirconia (YSZ) for x = 0 or a titania-doped yttria stabilized zirconia solid solution (Ti-doped YSZ) for x = 0.05, 0.08, and 0.10 is detected by DTA-TG. By SEM-EDX and TEM-EDX the presence of Zr, Y, and Ti elements, in the adequate proportions according to the nominal compositions, has been proven in both dried and calcined gels.     

Synthesis of camphorquinoxaline and quinoxaline derivatives over metal oxides as catalyst

Under solvent-free conditions, the synthesis of camphorquinoxaline and quinoxaline derivatives catalyzed by various solid metal oxides (ZnO, TiO₂, ZrO₂, MgO, acidic and basic Al₂O₃, and CaO) and salts (K₂CO₃, CaCO₃) is described. In the cases of ZnO, TiO₂, and ZrO₂, the catalysts can be recovered and reused several times without losing activity.     

A density‐functional theory approach to the separation of K2ZrF6 and K2HfF6 via fractional crystallization

Because of the low absorption cross‐section for thermal neutrons of zirconium (Zr) as opposed to hafnium (Hf), Zr‐metal must essentially be Hf‐free (<100 ppm Hf) to be suitable for use in nuclear reactors. However, Zr and Hf always occur together in nature, and due to very similar chemical and physical properties, their separation is particularly difficult. Separation can be achieved by traditional liquid–liquid extraction or extractive distillation processes, using Zr(Hf)Cl₄ as feedstock. However, the production of K₂Zr(Hf)F₆ via the plasma dissociation route, developed by the South African Nuclear Energy Corporation Limited (Necsa), could facilitate the development of an alternative separation process. In this theoretical study, the results of density‐functional theory (DFT) simulations of K₂Zr_(1‐z)Hf_zF₆ solid solutions [using Cambridge Serial Total Energy Package (CASTEP)] are presented, for which the supercell approach was applied in an attempt to determine whether solid solution formation during crystallization from aqueous solutions (fractional crystallization) is thermodynamically possible, which would hinder the separation efficiency of this method. Consequently, the calculated thermodynamic properties of mixing were used to evaluate the separation efficiency of Zr and Hf by fractional crystallization using a thermodynamic model to calculate the relative distribution coefficients. The small mixing enthalpies that were calculated from the DFT results, indicates that lattice substitution of Zr(IV) by Hf(IV) in K₂ZrF₆ could occur with relative ease. This is not surprising, considering the close similarities between Zr and Hf, and it was therefore concluded that K₂Zr_(1‐z)Hf_zF₆ solid solution formation might well restrict the separation efficiency of Zr and Hf by fractional crystallization of K₂Zr(Hf)F₆. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2011