Incorporation of Sb5+ into CeO2 : local structural distortion of the fluorite structure from a pentavalent substituent

Hydrothermal crystallisation of CeO2 from aqueous sodium hydroxide solution at 240oC using CeCl3·7H2O in the presence of hydrogen peroxide with addition of either SbCl3 or SbCl5 yields polycrystalline samples of antimony-containing ceria directly from solution. Powder X-ray diffraction shows a contraction of the cubic lattice parameter with increasing Sb content, and also a broadening of Bragg peaks, from which Scherrer analysis yields crystallite domain sizes of 5 - 20 nm. Scanning transmission electron microscopy provides consistent results with observation of highly crystalline particles of a few nm in diameter. X-ray absorption near edge structure spectroscopy at the Ce LIII and Sb K edges reveals the presence of Ce4+ and Sb5+ in the solids. To balance charge the presence of co-included Na is proposed, corroborated by elemental analysis. The general chemical formula of the materials can thus be written as (Ce1 xSbx)1 yNayO2-δ (where x < 0.4 and y ≥ x/3). Sb K-edge extended X-ray absorption fine structure spectroscopy of the substituted ceria samples shows that the local structure of Sb resembles that in NaSbO3, where six-coordinate metal sites are found, but with evidence of a longer interatomic correlation due to surrounding Ce/Sb atoms in the fluorite structure; this implies that the Sb is displaced from the ideal eight-coordinate site of the fluorite structure. This structural distortion gives materials that are unstable under reducing conditions, coupled by the ease of reduction to elemental antimony, which is extruded leading to phase separation

Incorporation of square-planar Pd2+ in fluorite CeO2 : hydrothermal preparation, local structure, redox properties and stability

The direct hydrothermal crystallisation at 240 °C of Pd2+-containing ceria is investigated to study the extent to which precious metal dopants may be introduced into the cubic fluorite lattice. Samples of composition Ce1−xPdxO2−δ, where 0 ≤ x ≤ 0.15 can be produced in which Pd is included within the CeO2 structure to give a linear lattice expansion. Attempts to produce higher Pd2+-substitution result in the formation of PdO as a secondary phase. Ce and Pd were determined to be in the +4 and +2 oxidation states, respectively, by X-ray absorption near edge structure, suggesting oxide deficiency as the mechanism of charge balance. Extended X-ray absorption fine structure (EXAFS) analysis at the Pd K-edge reveals that Pd2+ has local square-planar coordination, as expected, and that a structural model can fitted in which the average fluorite structure is maintained, but with Pd2+ sitting in the square faces of oxide ions present in the local cubic geometry of Ce. This model, consistent with previous modelling studies, gives an excellent fit to the EXAFS spectra, and explains the observed lattice expansion. Transmission electron microscopy analysis shows that Pd is well dispersed in the nanocrystalline ceria particles, and in situ powder XRD shows that upon heating in air the samples remain stable up to 800 °C. H2-TPR shows that Pd-substitution leads to low temperature (<200 °C) reduction of the oxide, which increases in magnitude with increasing Pd-substitution. On prolonged heating, however, the Pd is lost from the ceria lattice to give dispersed Pd metal, suggesting an inherent instability of Pd-doped CeO2

Zigzag HgTe nanowires modify the electron–phonon interaction in chirality-refined single-walled carbon nanotubes

Atomically thin nanowires (NWs) can be synthesized inside single-walled carbon nanotubes (SWCNTs) and feature unique crystal structures. Here we show that HgTe nanowires formed inside small-diameter (<1 nm) SWCNTs can advantageously alter the optical and electronic properties of the SWCNTs. Metallic purification of the filled SWCNTs was achieved by a gel column chromatography method, leading to an efficient extraction of the semiconducting and metallic portions with known chiralities. Electron microscopic imaging revealed that zigzag HgTe chains were the dominant NW geometry in both the semiconducting and metallic species. Equilibrium-state and ultrafast spectroscopy demonstrated that the coupled electron–phonon system was modified by the encapsulated HgTe NWs, in a way that varied with the chirality. For semiconducting SWCNTs with HgTe NWs, Auger relaxation processes were suppressed, leading to enhanced photoluminescence emission. In contrast, HgTe NWs enhanced the Auger relaxation rate of metallic SWCNTs and created faster phonon relaxation, providing experimental evidence that encapsulated atomic chains can suppress hot carrier effects and therefore boost electronic transport

In situ XAFS of acid-resilient iridate pyrochlore oxygen evolution electrocatalysts under operating conditions

Pyrochlore iridates (Na,Ca)2-xIr2O6?H2O are acid-stable electrocatalysts that are candidates for use in electrolysers and fuel cells. Ir LIII-edge X-ray absorption fine structure spectroscopy in 1 M H2SO4 at oxygen evolution conditions suggests the involvement of the electrons from the conduction band of the metallic particles, rather than just surface iridium reacting

Nanocrystalline transition-metal gallium oxide spinels from acetylacetonate precursors via solvothermal synthesis

The synthesis of mixed-metal spinels based on substituted γ-Ga2O3 is reported using metal acetylacetonate precursors in solvothermal reactions with alcohols as solvents at 240 °C. New oxides of Cr, Mn and Fe have been produced, all of which are formed as nanocrystalline powders, as seen by high-resolution transmission electron microscopy (HR-TEM). The first chromium-gallium mixed oxide is thus formed, with composition 0.33Ga1.87Cr0.8O4 ( = vacant site). X-ray absorption near-edge spectroscopy (XANES) at the chromium K-edge shows the presence of solely octahedral Cr3+, which in turn implies a mixture of tetrahedral and octahedral Ga3+, and the material is stable on annealing to at least 850 °C. An analogous manganese material with average chemical composition close to MnGa2O4 is shown to contain octahedral Mn2+, along with some Mn3+, but a different inversion factor to materials reported by conventional solid-state synthesis in the literature, which are known to have a significant proportion of tetrahedral Mn2+. In the case of iron, higher amounts of the transition metal can be included to give an Fe:Ga ratio of 1:1. Elemental mapping using energy dispersive X-ray spectroscopy on the TEM, however, reveals inhomogeneity in the distribution of the two metals. This is consistent with variable temperature 57Fe Mössbauer spectroscopy that shows the presence of Fe2+ and Fe3+ in more than one phase in the sample. Variable temperature magnetisation and electron paramagnetic resonance (EPR) indicate the presence of superparamagnetism at room temperature in the iron-gallium oxides. View Full-Tex

Resonance Raman Spectroscopy of extreme nanowires and other 1D systems

This paper briefly describes how nanowires with diameters corresponding to 1 to 5 atoms can be produced by melting a range of inorganic solids in the presence of carbon nanotubes. These nanowires are extreme in the sense that they are the limit of miniaturization of nanowires and their behavior is not always a simple extrapolation of the behavior of larger nanowires as their diameter decreases. The paper then describes the methods required to obtain Raman spectra from extreme nanowires and the fact that due to the van Hove singularities that 1D systems exhibit in their optical density of states, that determining the correct choice of photon excitation energy is critical. It describes the techniques required to determine the photon energy dependence of the resonances observed in Raman spectroscopy of 1D systems and in particular how to obtain measurements of Raman cross-sections with better than 8% noise and measure the variation in the resonance as a function of sample temperature. The paper describes the importance of ensuring that the Raman scattering is linearly proportional to the intensity of the laser excitation intensity. It also describes how to use the polarization dependence of the Raman scattering to separate Raman scattering of the encapsulated 1D systems from those of other extraneous components in any sample

Structures of mixed manganese ruthenium oxides (Mn1−xRux)O2(Mn_{1−x}Ru_x)O_2 crystallised under acidic hydrothermal conditions

A synthesis method for the preparation of mixed manganese–ruthenium oxides is presented along with a detailed characterisation of the solids produced. The use of 1 M aqueous sulfuric acid mediates the redox reaction between KRuO$_4$, KMnO$_4$ and Mn$^{2+}$ to form ternary oxides. At reaction temperature of 100°C the products are mixtures of α-MnO$_2$ (hollandite-type) and β-MnO$_2$ (rutile-type), with some evidence of Ru incorporation in each from their expanded unit cell volumes. At reaction temperature of 200°C solid-solutions β-Mn$_{1−x}$Ru$_x$O$_2$ are formed and materials with x ≤ 0.6 have been studied. The amount of Ru included in the oxide is greater than expected from the ratio of metals used in the synthesis, as determined by elemental analysis, implying that some Mn remains unreacted in solution. Powder X-ray diffraction (XRD) shows that while the unit cell volume expands in a linear manner, following Vegard's law, the tetragonal lattice parameters, and the a/c ratio, do not follow the extrapolated trends: this anisotropic behaviour is consistent with the different local coordination of the metals in the end members. Powder XRD patterns show increased peak broadening with increasing ruthenium content, which is corroborated by electron microscopy that shows nanocrystalline material. X-ray absorption near-edge spectra show that the average oxidation state of Mn in the solid solutions is reduced below +4 while that of Ru is increased above +4, suggesting some redistribution of charge. Analysis of the extended X-ray absorption fine structure provides complementary local structural information, confirming the formation of a solid solution, while X-ray photoelectron spectroscopy shows that the surface oxidation states of both Ru and Mn are on average lower than +4, suggesting a disordered surface layer may be present in the materials

Hydrothermal synthesis of iridium-substituted NaTaO3 perovskites

Iridium-containing NaTaO3 is produced using a one-step hydrothermal crystallisation from Ta2O5 and IrCl3 in an aqueous solution of 10 M NaOH in 40 vol H2O2 heated at 240 °C. Although a nominal replacement of 50 of Ta by Ir was attempted, the amount of Ir included in the perovskite oxide was only up to 15 mol. The materials are formed as crystalline powders comprising cube-shaped crystallites around 100 nm in edge length, as seen by scanning transmission electron microscopy. Energy dispersive X-ray mapping shows an even dispersion of Ir through the crystallites. Profile fitting of powder X-ray diffraction (XRD) shows expanded unit cell volumes (orthorhombic space group Pbnm) compared to the parent NaTaO3, while XANES spectroscopy at the Ir LIII-edge reveals that the highest Ir-content materials contain Ir4+. The inclusion of Ir4+ into the perovskite by replacement of Ta5+ implies the presence of charge-balancing defects and upon heat treatment the iridium is extruded from the perovskite at around 600 C in air, with the presence of metallic iridium seen by in situ powder XRD. The highest Ir-content material was loaded with Pt and examined for photocatalytic evolution of H2 from aqueous methanol. Compared to the parent NaTaO3, the Ir-substituted material shows a more than ten-fold enhancement of hydrogen yield with a significant proportion ascribed to visible light absorption

Atomically Resolved Imaging of Highly Ordered Alternating Fluorinated Graphene

One of the most desirable goals of graphene research is to produce ordered 2D chemical derivatives of suitable quality for monolayer device fabrication. Here we reveal, by focal series exit wave reconstruction, that C2F chair is a stable graphene derivative and demonstrates pristine long-range order limited only by the size of a functionalized domain. Focal series of images of graphene and C2F chair formed by reaction with XeF2 were obtained at 80 kV in an aberration-corrected transmission electron microscope. EWR images reveal that single carbon atoms and carbon-fluorine pairs in C2F chair alternate strictly over domain sizes of at least 150 nm^2 with electron diffraction indicating ordered domains >/= 0.16 square micrometer. Our results also indicate that, within an ordered domain, functionalization occurs on one side only as theory predicts. Additionally we show that electron diffraction provides a quick and easy method for distinguishing between graphene, C2F chair and fully fluorinated stoichiometric CF 2D phases