Structural, optical and EPR studies on ZnO:Cu nanopowders prepared via low temperature solution combustion synthesis

Cu (0.1 mol%) doped ZnO nanopowders have been successfully synthesized by a wet chemical method at a relatively low temperature (300 degrees C). Powder X-ray diffraction (PXRD) analysis, scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Fourier transformed infrared (FTIR) spectroscopy, UV-Visible spectroscopy, Photoluminescence (PL) and Electron Paramagnetic Resonance (EPR) measurements were used for characterization. PXRD results confirm that the nanopowders exhibit hexagonal wurtzite structure of ZnO without any secondary phase. The particle size of as-formed product has been calculated by Williamson-Hall (W-H) plots and Scherrer's formula is found to be in the range of similar to 40 nm. TEM image confirms the nano size crystalline nature of Cu doped ZnO. SEM micrographs of undoped and Cu doped ZnO show highly porous with large voids. UV-Vis spectrum showed a red shift in the absorption edge in Cu doped ZnO. PL spectra show prominent peaks corresponding to near band edge UV emission and defect related green emission in the visible region at room temperature and their possible mechanisms have been discussed. The EPR spectrum exhibits a broad resonance signal at g similar to 2.049, and two narrow resonances one at g similar to 1.990 and other at g similar to 1.950. The broad resonance signal at g similar to 2.049 is a characteristic of Cu2+ ion whereas the signal at g similar to 1.990 and g similar to 1.950 can be attributed to ionized oxygen vacancies and shallow donors respectively. The spin concentration (N) and paramagnetic susceptibility (X) have been evaluated and discussed. (C) 2011 Elsevier B. V. All rights reserved.

New uniform solid catalyst for the low-temperature oxidation of carbon monoxide: A triple-layered rare earth perovskite containing Co and Cu ions

Oxygen reactivity and catalytic activity of the cobalt-containing layered defect perovskites, YBa2Cu2CoO7+delta and LaBa2Cu2CoO7+delta, in comparison with LaBa2Cu3O7-delta have been investigated employing temperature-programmed desorption (TPD) and temperature-programmed surface reactions (TPSR) in the stoichiometric and catalytic mode using carbon monoxide as a probe molecule. TPD studies showed evidence for the presence of two distinct labile oxygen species, one at (0 0 1/2) sites and the other at (0 1/2 0) sites in LaBa2Cu2CoO7+delta against a single labile species at (0 1/2 0) in the case of two other oxides. The activation energies for the catalytic oxidation of carbon monoxide by oxygen over LaBa2Cu3O7-delta, YBa2Cu2CoO7+delta, and LaBa2Cu2CoO7+delta have been estimated to be 24.2, 15.9, and 13.6 kcal/mol, respectively. The reactivity and catalytic activity of the oxide systems have been interpreted in terms of the structural changes brought about by substituents, guided by a directing effect of the larger rare earth cation. TPSR profiles, structural analysis, and infrared spectroscopic investigations suggest that the oxygen present at (0 0 1/2) sites in the case of LaBa2Cu2CoO7+delta is accessible to catalytic oxidation of CO through a Mars-Van Krevelen pathway. Catalytic conversion of CO to CO2 over LaBa2Cu2CoO7+delta occurs at 200 degrees C. The enhanced reactivity is explained in terms of changes brought about in the coordination polyhedra around transition metals, enhanced basal plane oxygen diffusivity, and redox potentials of the different transition metal cations.

Strontium and calcium zirconyl citrates as precursors for the low-temperature synthesis of SrZrO3 and CaZrO3 fine powders

Synthesis and the thermal decomposition behavior of new molecular precursors, strontium, and calcium zirconyl citrates are presented. The pathway to the metazirconate formation has been found to proceed through a multistep process. The precursors yield SrZrO3 and CaZrO3 fine powders at temperatures as low as 650 degrees C. Physico-chemical, spectroscopic, thermoanalytical, and microscopic techniques have enabled the identification of the sequence of events leading to the perovskite formation and proposition of a thermolysis scheme. Retention of the molecular level mixing of the metal ions during the course of the precursor decomposition is supported by these techniques. Prior to the formation of MZrO3 (M = Sr and Ca) an ionic oxycarbonate, M2Zr2O5CO3 (M = SI. and Ca), intermediate is produced by the thermal decomposition of the citrate precursors.

Low temperature fabrication and impedance spectroscopy of PMN-PT ceramics

Single phase perovskite 0.9Pb(Mg1/3Nb2/3)O-3-0.1(PbTiO3) ceramics were prepared using the columbite precursor method after optimizing the synthesis conditions. X-ray diffraction (XRD) studies were carried out to verify the phase formation at each processing step. Scanning electron microscopy (SEM) was employed to observe the microstructure of the sintered ceramics. Impedance and modulus spectroscopic data were used to gain an insight into the electrical properties of the samples and with a view to observing the relaxations in them. (C) 1999 Elsevier Science Ltd.

Embrittlement of a bulk metallic glass due to low-temperature annealing

Embrittlement of a bulk La-based metallic glass due to isothermal and isochronal annealing below the T-g was investigated. Results show that the impact toughness decreases with increasing annealing time or temperature, accompanied by a change in fracture morphology. Reasons for this are discussed in terms of structural relaxation. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Synthesis of LiCo1-xNixO2 from a low temperature solution combustion route and characterization

Nickel substituted lithium-cobalt oxides, LiCo1-xNixO2 (0 < x < 0.4), have been synthesized in a very short time by a solution combustion method at 350 degreesC using diformyl hydrazine as a fuel. Pure phases with hexagonal lattice structure have been obtained. These compounds facilitate reversible insertion/extraction of Li+ ions with good discharge capacity between 3.0 and 4.4 V versus Li/Li+. Results of the studies by powder X-ray diffraction, scanning electron microscopy, cyclic voltammetry, galvanostatic charge-discharge cycling and ac impedance measurements are presented. (C) 2002 Elsevier Science B.V. All rights reserved.

Low temperature synthesis of ferromagnetic (LaK)MnO3 from KCl, KBr and KI fluxes

Potassium doped lanthanum manganites have been synthesized from KCl, KBr and KI fluxes at 900, 850 and 750 °C respectively. While all the flux-grown oxides are ferromagnetic metals (Tc=290–330 K), the oxides grown from KCl and KBr fluxes crystallize in the rhombohedral structure and that synthesized from KI flux adopts the cubic structure. The enhancement in Tc correlates with the increasing tendency of the flux to get oxidized and decreasing melting points of the flux. Nearly stoichiometric (LaK)MnO3 with 33 % of Mn4+ concentration could be prepared at temperature as low as 750 °C. Composition of all the phases have been obtained from the chemical analysis of the elements present.

Epitaxial Growth of Co3O4 Films by Low Temperature, Low Pressure Chemical Vapor Deposition

The growth of strongly oriented or epitaxial thin films of metal oxides generally requires relatively high growth temperatures or infusion of energy to the growth surface through means such as ion bombardment. We have grown high quality epitaxial thin films of Co3O4 on different substrates at a temperature as low as 450°C by low-pressure metal-organic chemical vapor deposition (MOCVD) using cobalt(II) acetylacetonate as the precursor. With oxygen as the reactant gas, polycrystalline Co3O4 films are formed on glass and Si(100) in the temperature range 350-550°C. Under similar conditions of growth, highly oriented films of Co3O4 are formed on SrTiO3(100) and LaAlO3(100). The film on LaAlO3(100) grown at 450°C show a rocking curve FWHM of 1.61°, which reduces to 1.32° when it is annealed in oxygen at 725°C. The film on SrTiO3(100) has a FWHM of 0.330 (as deposited) and 0.29° (after annealing at 725°C). The ø-scan analysis shows cube-on-cube epitaxy on both these substrates. The quality of epitaxy on SrTiO3(100) is comparable to the best of the pervoskite-based oxide thin films grown at significantly higher temperatures.

Low temperature crystallographic data on Kevlar 49 fibres

Using X-ray diffraction data, the behaviour of Kevlar 49 fibres at low temperatures, up to -100degreesC, has been analysed. During cooling, the basal plane of the monoclinic unit cell shrinks whereas the c- (unique, chain axis) length is not significantly affected. In contrast, in the return heating cycle to ambient temperature, the basal plane expands and contraction occurs along the chain direction. The unit cell registers a reduction in volume in both the cooling and heating cycles. Conspicuously, after a cycle of cooling and heating, the unit cell does not return to its initial volume. (C) 2003 Kluwer Academic Publishers.

Low temperature synthesis, structure and properties of alkali-doped La2NiO4, LaNiO3 and LaNi0.85Cu0.15O3 from alkali hydroxide fluxes

We report a low-temperature synthesis of La1.95Na0.05NiO4 from NaOH flux, La0.97K0.03NiO3 and La0.95K0.05Ni0.85Cu0.15O3 phases from KOH flux at 400 degreesC. Alkali-doped LaNiO3 can be prepared in KOH, but not in NaOH flux and La2NiO4 can be prepared in NaOH, but not in KOH flux. The flux-grown oxides were characterized by powder X-ray Rietveld profile analysis and electron microscopy. Sodium doped La2NiO4 crystallizes in orthorhombic structure and potassium doped LaNiO3-phases crystallizes in rhombohedral structure. La1.95Na0.05NiO4 is weakly paramagnetic and semiconducting while La0.97K0.03NiO3 and La0.95K0.05Ni0.85Cu0.15O3 show Pauli paramagnetic and metallic behavior. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

Conformation-Changing Aggregation in Hydroxyacetone: A Combined Low-Temperature FTIR, Jet, and Crystallographic Study

Aggregation in hydroxyacetone (HA) is studied using low-temperature FTIR, supersonic jet expansion, and X-ray crystallographic (in situ cryocrystallization) techniques. Along with quantum chemical methods (MP2 and DFT), the experiments unravel the conformational preferences of HA upon aggregation to dinners and oligomers. The O-H center dot center dot center dot O=C intramolecular hydrogen bond present in the gas-phase monomer partially opens upon aggregation in supersonic expansions, giving rise to intermolecular cooperatively enhanced O-H center dot center dot center dot O-H hydrogen bonds in competition with isolated O-H center dot center dot center dot O=C hydrogen bonds. On the other hand, low-temperature IR studies on the neat solid and X-ray crystallographic data reveal that HA undergoes profound conformational changes upon crystallization, with the HOCC dihedral angle changing from similar to 0 degrees in the gas phase to similar to 180 degrees in the crystalline phase, hence giving rise to a completely new conformation. These conclusions are supported by theoretical calculations performed on the geometry derived from the crystalline phase.

Low temperature synthesis of layered NaxCoO2 and KxCoO2 from NaOH/KOH fluxes and their ion exchange properties

We report a low temperature synthesis of layered Na0×20CoO2 and K0×44CoO2 phases from NaOH and KOH fluxes at 400°C. These layered oxides are employed to prepare hexagonal HCoO2, LixCoO2 and Delafossite AgCoO2 phases by ion exchange method. The resulting oxides were characterised by powder X-ray diffraction, X-ray photoelectron spectroscopy, SEM and EDX analysis. Final compositions of all these oxides are obtained from chemical analysis of elements present. Na0×20CoO2 oxide exhibits insulating to metal like behaviour, whereas AgCoO2 is semiconducting.

Shape Transformation of ZnO Nanorods/Nanotubes at Low Temperature

We report the shape transformation of ZnO nanorods/nanotubes at temperatures (similar to 700 degrees C) much lower than the bulk melting temperature (1975 degrees C). With increasing annealing temperature, not only does shape transformation take place but the luminescence characteristics of ZnO are also modified. It is proposed that the observed shape transformation is due to surface diffusion, contradicting the previously reported notion of melting and its link to luminescence. Luminescence in the green-to-red region is observed when excited with a blue laser, indicating the conversion of blue to white light.