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Query: UMLS:C0276640 (TEM)
 20,729 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

In order to observe anisotropically grown crystalline materials perpendicular to a certain preferred orientation, a standard cross-sectional TEM preparation method has been modified. The material is embedded in an organic epoxy resin between two Si-wafers. Plates, needles and tubes lay flat inside the resulting sandwich, which is then cut into slices perpendicular to the wafers. The slices are thinned by mechanical abrading and, finally, by ion milling. Crystals located near the central hole are electron-transparent, and their orientation often allows for an observation along the desired direction. The usefulness of this procedure is demonstrated on the examples of high-Tc superconductors and vanadium oxide nanotubes.
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PMID:A simple and fast TEM preparation method utilizing the pre-orientation in plate-like, needle-shaped and tubular materials 1094 25

Near-edge fine structures of the metal L(2,3) and O K-edges in transition metal-oxides have been studied with a transmission electron microscope equipped with a monochromator and a high-resolution imaging filter. This system enables the recording of EELS spectra with an energy resolution of 0.1eV thus providing new near-edge fine structure details which could not be observed previously by EELS in conventional TEM instruments. EELS-spectra from well-defined oxides like titanium oxide (TiO(2)), vanadium oxide (V(2)O(5)), chromium oxide (Cr(2)O(3)), iron oxide (Fe(2)O(3)), cobalt oxide (CoO) and nickel oxide (NiO) have been measured with the new system. These spectra are compared with EELS data obtained from a conventional microscope and the main spectral features are interpreted. Additionally, the use of monochromised TEMs is discussed in view of the natural line widths of K and L(2,3) edges.
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PMID:Electron energy-loss near-edge structures of 3d transition metal oxides recorded at high-energy resolution. 1287 9

A sol-gel chemistry approach was used to fabricate nanoparticles of TiO(2) in its anatase form. The particle size is shown to be sensitive to the use of HClO(4) or HNO(3) as acid catalyst. The gold-capped TiO(2) nanocomposites were processed by the reduction of gold on the surface of the TiO(2) nanoparticles via a chemical reduction or a photoreduction method. Different percentages of vanadium-doped TiO(2) nanoparticles, which extended the TiO(2) absorption wavelength from the ultraviolet to the visible region, were successfully prepared. The synthesized nanocomposites have a size of about 12-18 nm and an anatase phase as characterized by XRD, TEM, AFM, and UV-vis spectroscopy. The TiO(2) nanocomposite coatings have been applied on glass slide substrates. The antibacterial activity of TiO(2) nanocomposites was investigated qualitatively and quantitatively. Two types of bacteria, Escherichia coli (DH 5alpha) and Bacillus megaterium (QM B1551), were used during the experiments. Good inhibition results were observed and demonstrated visually. The quantitative examination of bacterial activity for E. coli was estimated by the survival ratio as calculated from the number of viable cells, which form colonies on the nutrient agar plates. The antimicrobial efficiency and inhibition mechanisms are illustrated and discussed.
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PMID:Anatase TiO2 nanocomposites for antimicrobial coatings. 1685 57

Crystalline vanadium pentoxide with hierarchical mesopores was synthesized by using a CTAB/BMIC cotemplate (CTAB = cetyltrimethylammonium bromide, BMIC = 1-butyl-3-methylimidazolium chloride). The material was fully characterized by SEM, TEM, N2 adsorption-desorption, XRD, XPS, and CV methods. By elaborate adjustment of the template proportions, the distribution and size of the hierarchical pores were tuned successfully. CTAB cationic surfactant contributed more to the larger mesopores, whereas BMIC ionic liquid was beneficial in forming the smaller nanopores. The vanadium-containing anions combined with CTA+ micelles and BMI+ rings through electrostatic interactions. The CTA(+)-O(VO)O(-)-BMI(+) entities built up an orderly array, which finally formed the hierarchical mesoporous framework during thermal treatment. The mesoporous vanadium pentoxide directed by the cotemplate of CTAB/BMIC = 1:1 showed many orderly crystalline structures and demonstrated a large capacitance (225 F g(-1)); it is thus a promising material for electrochemical capacitors. Two alternative solutions to the disappearance of capacitance due to insertion of K+ are proposed in view of possible future applications.
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PMID:Crystalline vanadium pentoxide with hierarchical mesopores and its capacitive behavior. 1744 Nov 11

The major objective of this research project was to reach a microscopic understanding of the structure, function and dynamics of V-Mo-(W) mixed oxides for the partial oxidation of acrolein to acrylic acid. Different model catalysts (from binary and ternary vanadium molybdenum oxides up to quaternary oxides with additional tungsten) were prepared via a solid state preparation route and hydrochemical preparation of precursors by spray-drying or crystallisation with subsequent calcination. The phase composition was investigated ex situ by XRD and HR-TEM. Solid state prepared samples are characterised by crystalline phases associated to suitable phase diagrams. Samples prepared from crystallised and spray-dried precursors show crystalline phases which are not part of the phase diagram. Amorphous or nanocrystalline structures are only found in tungsten doped samples. The kinetics of the partial oxidation as well as the catalysts' structure have been studied in situ by XAS, XRD, temperature programmed reaction and reduction as well as by a transient isotopic tracing technique (SSITKA). The reduction and re-oxidation kinetics of the bulk phase have been evaluated by XAS. A direct influence not only of the catalysts' composition but also of the preparation route is shown. Altogether correlations are drawn between structure, oxygen dynamics and the catalytic performance in terms of activity, selectivity and long-term stability. A model for the solid state behaviour under reaction conditions has been developed. Furthermore, isotope exchange experiments provided a closer image of the mechanism of the selective acrolein oxidation. Based on the in situ characterisation in combination with micro kinetic modelling a detailed reaction model which describes the oxygen exchange and the processes at the catalyst more precisely is discussed.
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PMID:Heterogeneously catalysed partial oxidation of acrolein to acrylic acid--structure, function and dynamics of the V-Mo-W mixed oxides. 1761 23

Ni3V2O8 catalyst was prepared by oxalate co-precipitation method with microwave heating in this paper. In order to study the relationship between the catalytic performance and the surface species, the catalyst was characterized by XRD, BET, H2-TPR, XPS, TEM and conductivity measurement. The surface property of Ni3V2O8 was studied by XPS and the catalytic performance of the oxidative dehydrogenation of propane to propylene was also investigated. The results of XRD showedthat pure Ni3V2O8 with nice structure was obtained. TEM experiments results demonstrated that the prepared Ni3V2O8 catalyst at 700 degrees C calcination showed uniform particle with the mean particle size of 30 nm. The surface area of the catalyst was 8.623 m2 x g(-1). The diagram of the relationship between electrical conductivity and oxygen partial pressure of Ni3V2O8 showed dsigma/dPO2, >0, implying that Ni3V2O8 catalyst was a p-type semiconductor. H2-TPR results showed that only one unsymmetrical reduction peak appeared at 663.5 degreesC within 300-900 degrees C region over Ni3V2O8 catalyst and no obvious shoulder peak was observed. It could also be found that the ratio of non complete reduction oxygen species was about 33.59% (O(-) 27.55%, O2(2-) 6.04%) from the O(1s) XPS result and more V4+ species existed on the Ni3V2O8 catalyst surface. The TPR and XPS results illustrated that the transformation of the lattice oxygen to non-complete reduction oxygen in NiV2O8 catalyst might promote the oxidation-reduction reaction between different valence vanadium and promoted the oxygen vacancy formation. This then led to abundant non-complete reduction oxygen O(-) and V4+ species formation on the surface of Ni3V2O8 catalyst. The active result of oxidative dehydrogenation of propane to propylene showed that the 60.02% propylene selectivity could be reached at 18.60% propane conversion. Compared with the reported results over the coexistent NiO and Ni3V2O8 system from the literature, pure Ni3V2O8 catalyst system in this present paper showed higher propylene selectivity than the coexistent NiO and Ni3V2O8 system under the same propane conversion condition, suggesting that the performance of propane to propene is correlated to the oxidation-reduction of V4+ / V5+ couple and non complete reduction oxygen species (O(-) or O2(2-)). This result further illustrated that NiV2O8 was active phase for oxidative dehydrogenation of propane to propylene. Combining the active and characterization results, it was found that catalytic activity was correlated to the surface non-complete reduction O(-) and V4+ species, which was beneficial to improving the propylene selectivity.
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PMID:[Study on performance of Ni3 V2O8 catalyst and analysis of X-ray photoelectron spectroscopy]. 1830 14

A nonaqueous liquid-phase route involving the reaction of vanadium oxychloride with benzyl alcohol leads to the formation of single-crystalline and semiconducting VO 1.52(OH) 0.77 nanorods with an ellipsoidal morphology, up to 500 nm in length and typically about 100 nm in diameter. Composition, structure, and morphology were thoroughly analyzed by neutron and synchrotron powder X-ray diffraction as well as by different electron microscopy techniques (SEM, (HR)TEM, EDX, and SAED). The data obtained point to a hollandite-type structure which, unlike other vanadates, contains oxide ions in the channels along the c-axis, with hydrogen atoms attached to the edge-sharing oxygen atoms, forming OH groups. According to structural probes and magnetic measurements (1.94 mu B/V), the formal valence of vanadium is +3.81 (V (4+)/V (3+) atomic ratio approximately 4). The experimentally determined density of 3.53(5) g/cm (3) is in good agreement with the proposed structure and nonstoichiometry. The temperature-dependent DC electrical conductivity exhibits Arrhenius-type behavior with a band gap of 0.64 eV. The semiconducting behavior is interpreted in terms of electron hopping between vanadium cations of different valence states (small polaron model). Ab initio density-functional calculations with a local spin density approximation including orbital potential (LSDA + U with an effective U value of 4 eV) have been employed to extract the electronic structure. These calculations propose, on the one hand, that the electronic conductivity is based on electron hopping between neighboring V (3+) and V (4+) sites, and, on the other hand, that the oxide ions in the channels act as electron donors, increasing the fraction of V (3+) cations, and thus leading to self-doping. Experimental and simulated electron energy-loss spectroscopy data confirm both the presence of V (4+) and the validity of the density-of-states calculation. Temperature-dependent magnetic susceptibility measurements indicate strongly frustrated antiferromagnetic interactions between the vanadium ions. A model involving the charge order of the V (3+) sites is proposed to account for the observed formation of the magnetic moment below 25 K.
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PMID:Oxygen self-doping in hollandite-type vanadium oxyhydroxide nanorods. 1867 92

All three classes of serine beta-lactamases are inhibited at micromolar levels by 1:1 complexes of catechols with vanadate. Vanadate reacts with catechols at submillimolar concentrations in aqueous buffer at neutral pH in several steps, initially forming 1:1, 1:2, and, possibly, 1:3 complexes. Formation of these complexes is followed by the slower reduction of vanadate (V (V)) to vanadyl (V (IV)) and oxidation of the catechol. Vanadyl-catechol complexes, however, do not inhibit the beta-lactamases. Rate and equilibrium constants of formation of the 1:1 and 1:2 complexes of vanadate with catechol itself and with 2,3-dihydroxynaphthalene were measured by stopped-flow spectrophotometry. Typical examples of all three classes of serine beta-lactamases (the class A TEM-2, class C P99, and class D OXA-1 enzymes) were competitively inhibited by the 1:1 vanadate-catechol complexes. The inhibition was modestly enhanced by hydrophobic substituents on the catechol. The 1:1 vanadate complexes are considerably better inhibitors of the P99 beta-lactamase than 1:1 complexes of catechol with boric acid and are likely to contain penta- or hexacoordinated vanadium rather than tetracooordinated. Molecular modeling showed that a pentacoordinated 1:1 vanadate-catechol complex readily fits into the class C beta-lactamase active site with coordination to the nucleophilic serine hydroxyl oxygen. Such complexes may resemble the pentacoordinated transition states of phosphyl transfer, a reaction also catalyzed by beta-lactamases.
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PMID:Inhibition of serine beta-lactamases by vanadate-catechol complexes. 1870 3

Controlled synthesis based on spectroscopic characterization, structure, and catalytic performance of mesoporous silica SBA-15-supported vanadium oxide model catalysts (see TEM image) are reviewed. The effect of water on the structure and dispersion of highly dispersed vanadium oxide is discussed in the light of recent results in multiple in situ spectroscopy.Characterization of the synthesis, structure, and catalytic performance of vanadium oxide catalysts supported on mesoporous silica SBA-15 for selective oxidation reactions are reviewed. Controlled synthesis on the basis of surface functionalization and ion exchange allows homogeneous deposition of vanadium oxide within the pores of SBA-15. Catalytic tests on the selective oxidation of methanol and propane demonstrate the full catalytic function of these model powder catalysts. Characterization of the dehydrated state by vibrational spectroscopy provides insight into the structure of SBA-15-supported vanadium oxide at low loadings. The effect of water on the structure and dispersion of highly dispersed vanadium oxide is discussed in light of recent results from multiple in situ spectroscopy.
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PMID:Nanostructured vanadium oxide model catalysts for selective oxidation reactions. 1913 65

The vanadium (V)-doped mesoporous titanium dioxide (TiO(2)) nanoparticles with V/Ti ratios from 0-2 wt% were prepared using sol-gel method in the presence of triblock polymers, Pluronic F127. SEM images showed that the V-doped TiO(2) nanoparticles were porous structures. The surface areas and pore sizes were in the range 85-107 m(2)/g and 12-14 nm, respectively. From XRPD, the V-doped mesoporous TiO(2) after calcination at 500 degrees C was mainly anatase phase, and the crystallite sizes were in the range 14-16 nm. TEM images showed that vanadia was doped both on the surface and in the lattice of anatase TiO(2). A slight red-shift in wavelength absorption was observed when V/Ti ratio increased from 0 to 2 wt%. Addition of vanadium ion slightly decreased the photocatalytic activity of TiO(2) toward the decolorization of MB under the illumination of UV light at 305 nm. However, a 1.6-1.8 times increase in rate constants for MB photodegradation was observed when 0.5-1.0 wt% V-doped TiO(2) was illumined by solar simulator at AM 1.5. These results demonstrated that the doping of low concentrations of V ion into mesoporous TiO(2) enhance the photocatalytic activity of mesoporous TiO(2) towards photodecomposition of azo dye in the visible range.
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PMID:Characterization and photocatalytic activity of vanadium-doped titanium dioxide nanocatalysts. 1921 7


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