Changes on iron electrode surface during hydrogen permeation in borate buffer solution

Hydrogen interaction with oxide films grown on iron electrodes at open circuit potential (E-oc) and in the passive region (+0.30 V-ECS) was studied by chronopotentiometry, chronoamperometry and electrochemical impedance spectroscopy techniques. The results were obtained in deaerated 0.3 mol L-1 H3BO3 + 0.075 mol L-1 Na2B4O7 (BB, pH 8.4) solution before, during and after hydrogen permeation. The iron oxide film modification was also investigated by means of in situ X-ray absorption near-edge spectroscopy (XANES) and scanning electrochemical microscopy (SECM) before and during hydrogen permeation. The main conclusion was that the passive film is reduced during the hydrogen diffusion. The hydrogen permeation stabilizes the iron surface at a potential close to the thermodynamic water stability line where hydrogen evolution can occur. The stationary condition required for the determination of the permeation parameters cannot be easily attained on iron surface during hydrogen permeation. Moreover, additional attention must be paid when obtaining the transport parameters using the classical permeation cell. (c) 2007 Elsevier Ltd. All rights reserved.

Photoluminescence behavior in MgTiO3 powders with vacancy/distorted clusters and octahedral tilting

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Relationship between Crystal Shape, Photoluminescence, and Local Structure in SrTiO3 Synthesized by Microwave-Assisted Hydrothermal Method

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Structure study of donor doped barium titanate prepared from citrate solutions

Barium titanates doped with Nb5+, and Y3+, were prepared. The starting powders were synthesized from citrate solutions by the Pechini process and partial Pechini process in two steps. Sintering was performed in the range from 1310° up to 1380°C for 2 hours in air atmosphere. The structural study concerning the incorporation of Nb and Y ions in the barium titanate crystal lattice was performed by XRD, XANES and EXAFS techniques. The dielectric properties were analyzed and the relationship between properties and structure of doped barium titanate was established.

Correlation between local structure and ionic conductivity of potassium-doped siloxane poly(propyleneoxyde) ormolytes

The local order around K for K(CF3SO3) doped Siloxane-Poly(propyleneoxide) hybrids at different doping concentration was investigated by x-ray absorption spectroscopy (EXAFS and XANES) at the potassium K-edge. The results indicate that the use of HCl as hydrolytic catalyst for gelation induces the precipitation of KCl. The ionic conductivity is strongly related to the presence of oxygen atoms as first neighbours around potassium and to the amount of KCl precipitate. © Physica Scripta 2005.

Structural refinement and photoluminescence properties of cube-like (Ca 1-xCu x)TiO 3 crystals synthesized by the microwave-hydrothermal method

In this work, (Ca 1-xCu x)TiO 3 crystals with (x = 0, 0.01 and 0.02), labeled as CTO, CCTO1 and CCTO2, were synthesized by the microwave-hydrothermal method at 140°C for 32 min. XRD patterns (Fig. 1), Rietveld refinement and FT-Raman spectroscopy indicated that these crystals present orthorhombic structure Pbnm. Micro-Raman and XANES spectra suggested that the substitution of Ca by Cu in A-site promoted a displacement of the [TiO6]-[TiO6] clusters adjacent from its symmetric center, which leads distortions on the [CaO 12] clusters neighboring and consequently cause the strains into the CaTiO3 lattice. FE-SEM images showed that these crystals have an irregular shape as cube like probably indicating an Ostwald-ripening and self-assemble as dominant mechanisms to crystals growth. The powders presented an intense PL blue-emission.

Correlation between photoluminescence and structural defects in Ca 1+x Cu 3 - X Ti 4 O 12 systems

Ca1+xCu3-xTi4O12 powders were synthesized by a conventional solid-state reaction. X-ray diffraction (XRD) was performed to verify the formation of cubic CaCu3Ti4O 12 (CCTO) and orthorhombic CaTiO3 (CTO) phases at long range. Rietveld refinements indicate that excess Ca atoms added to the Ca 1-xCu3-xTi4O12 (x = 1.0) composition segregated in a CaTiO3 secondary phase suggesting that solubility limit of Ca atoms in the CaCu3Ti4O12 lattice was reached for this system. The FE-SEM images show that the Ca 1+xCu3-xTi4O12 (0 < x < 3) powders are composed of several agglomerated particles with irregular morphology. X-ray absorption near-edge structure spectroscopy (XANES) spectra indicated [TiO5Vo z]-[TiO6] complex clusters in the CaCu3Ti4O12 structure which can be associated with oxygen vacancies (Vo z = V o x, Vo •, and Vo ••) whereas in the CaTiO3 powder, this analysis indicated [TiO6]-[TiO6] complex clusters in the structure. Ultraviolet-visible (UV-vis) spectra and photoluminescence (PL) measurements for the analyzed systems revealed structural defects such as oxygen vacancies, distortions, and/or strains in CaCu3Ti4O12 and CaTiO3 lattices, respectively. © 2012 The American Ceramic Society.

Local electronic structure, optical bandgap and photoluminescence (PL) properties of Ba(Zr0.75Ti0.25)O3 powders

Ba(Zr0.75Ti0.25)O3 (BZT-75/25) powders were synthesized by the polymeric precursor method. Samples were structurally characterized by X-ray diffraction (XRD), Rietveld refinement, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques. Their electronic structures were evaluated by first-principle quantum mechanical calculations based on density functional theory at the B3LYP level. Their optical properties were investigated by ultraviolet-visible (UV-Vis) spectroscopy and photoluminescence (PL) measurements at room temperature. XRD patterns and Rietveld refinement data indicate that the samples have a cubic structure. XANES spectra confirm the presence of pyramidal [TiO5] clusters and octahedral [TiO6] clusters in the disordered BZT-75/25 powders. EXAFS spectra indicate distortion of Ti-O and Ti-O-Ti bonds the first and second coordination shells, respectively. UV-Vis absorption spectra confirm the presence of different optical bandgap values and the band structure indicates an indirect bandgap for this material. The density of states demonstrates that intermediate energy levels occur between the valence band (VB) and the conduction band (CB). These electronic levels are due to the predominance of 4d orbitals of Zr atoms in relation to 3d orbitals of Ti atoms in the CB, while the VB is dominated by 2p orbitals related to O atoms. There was good correlation between the experimental and theoretical optical bandgap values. When excited at 482 nm at room temperature, BZT-75/25 powder treated at 500 C for 2 h exhibited broad and intense PL emission with a maximum at 578 nm in the yellow region. © 2013 Elsevier Ltd. All rights reserved.

Dielectric and structural characterization of Pb1-xAxZr0.40Ti0.60O3 (A = Sr, Ca) ferroelectric ceramics

This paper reports the influence of Sr- and Ca-substitution on the structural and ferroelectric properties of Pb1-xSrxZr0.40Ti0.60O3 (PSZT) and Pb1-xCaxZr0.40Ti0.60O3 (PCZT) ceramic systems. The dielectric measurements show that these substitutions cause a diffuse behavior in the dielectric permittivity curves for all samples. According to the X-ray absorption near-edge structure (XANES) spectra collected at Ti K- and LIII-edge, when Pb was replaced by Sr or Ca, a decrease in the local distortion around Ti atoms in the TiO6 octahedron could be observed. The O K-edge XANES spectra also revealed that the hybridization between O 2p and Pb 6sp states decreased as the amount of Sr or Ca atoms increased. Based on these results, it was possible to ascertain that the ferroelectric behavior in PSZT and PCZT samples bears a close correlation to the hybridization weakening between O 2p and Pb 6 sp states. © 2013 by American Scientific Publishers.

Soft-chemical synthesis, characterization and humidity sensing behavior of WO3/TiO2 nanopowders

Tungsten oxide/titania (WO3/TiO2) nanopowders were synthesized by the polymeric precursor method which varied the WO3 content between 0 and 10 mol%. The powders were thermally treated in a conventional furnace and their structural, microstructural and electric properties were evaluated by X-ray diffraction (XRD), Raman spectrometry, N 2 physisorption, NH3 chemisorption, temperature-programmed reduction (TPR), X-ray absorption near-edge spectroscopy (XANES) in situ XANES and extended X-ray absorption fine structure spectroscopy (EXAFS) and transmission electron microscopy (TEM). XRD and Raman spectrometry confirmed the homogeneous distribution of an amorphous WO3 phase in the TiO 2 matrix which stabilized the anatase phase through the generation of [TiO5·V0] or [TiO5·V 0] complex sites. Conventional TPR-H2 (temperature programmed reduction) along with XANES TPR-H2 and XANES TPR-EtOH showed that WO3/TiO2 sample reduction occurs through the formation of these complex clusters. Moreover, the addition of WO3 promoted an increase in the surface acidity of doped samples as revealed by NH3 chemisorption. The WO3/TiO2 bulk-ceramic samples were further used to estimate their potential application in a humidity sensor in the range of 15-85% relative humidity. Probable reasons that lead to the different humidity sensor responses of samples were given based on the structural and surface characterizations. Correlation between the sensing performance of the sensor and its structural features are also discussed. Although all samples responded as a humidity sensor, the W2T sample (2 mol% added WO3) excelled for sensitivity due to the increase in acid sites, optimum mean pore size and pore size distribution. © 2013 Elsevier B.V.

Filmes finos preparados a partir da matriz vítrea a base de WO3. Propriedades e aplicações

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Morfologias de óxido de cobre (II) na mesoescala: síntese hidrotérmica assistida por micro-ondas, mecanismo de crescimento e atividade catalítica na reação de desidrogenação do etanol

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Caracterização estrutural de pós de 'SRTIO IND.3' puro e dopado com samário

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Propriedade fotoluminescente dos materiais 'CA''TI''O IND.3' e 'CA IND.1-x''EU IND.x''TI''O IND.3'

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Preparação pelo processo sol-gel de pós redispersáveis, filmes finos densos e materiais mesoporosos de óxido de titânio

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)