The Bologna Stone: history's first persistent luminescent material

In 1603, the Italian shoemaker Vincenzo Cascariolo found that a stone (baryte) from the outskirts of Bologna emitted light in the dark without any external excitation source. However, the calcination of the baryte was needed prior to this observation. The stone later named as the Bologna Stone was among the first luminescent materials and the first documented material to show persistent luminescence. The mechanism behind the persistent emission in this material has remained a mystery ever since. In this work, the Bologna Stone (BaS) was prepared from the natural baryte (Bologna, Italy) used by Cascariolo. Its properties, e. g. impurities (dopants) and their valences, luminescence, persistent luminescence and trap structure, were compared to those of the pure BaS materials doped with different (transition) metals (Cu, Ag, Pb) known to yield strong luminescence. The work was carried out by using different methods (XANES, TL, VUV-UV-vis luminescence, TGA-DTA, XPD). A plausible mechanism for the persistent luminescence from the Bologna Stone with Cu+ as the emitting species was constructed based on the results obtained. The puzzle of the Bologna Stone can thus be considered as resolved after some 400 years of studies.

Quantum Mechanics Insight into the Microwave Nucleation of SrTiO3 Nanospheres

An extensive investigation of strontium titanate, SrTiO3 (STO), nanospheres synthesized via a microwave-assisted hydrothermal (MAH) method has been conducted to gain a better insight into thermodynamic, kinetic, and reaction phenomena involved in STO nucleation and crystal growth processes. To this end, quantum chemical modeling based on the density functional theory and periodic super cell models were done. Several experimental techniques were employed to get a deep characterization of structural and optical features of STO nanospheres. A possible formation mechanism was proposed, based on dehydration of titanium and strontium clusters followed by mesoscale transformation and a self-assembly process along an oriented attachment mechanism resulting in spherical like shape. Raman and XANES analysis renders a noncentrosymmetric environment for the octahedral titanium, while infrared and first order Raman modes reveal OH groups which are unsystematically incorporated into uncoordinated superficial sites. These results seem to indicate that the key component is the presence of distorted TiO6 clusters to engender a luminescence property. Analysis of band structure, density Of states, and charge map shows that there is a close relationship among local broken symmetry, polarization, and energy split of the 3d orbitals of titanium. The interplay among these electronic and structural features provides necessary conditions to evaluate its luminescent properties under two energy excitation.

Reforming of a model biogas on Ni and Rh-Ni catalysts: Effect of adding La

Ni catalysts supported on gamma-Al2O3 modified by Rh and La were prepared and evaluated on the reforming of a model biogas. The catalysts were characterized by EDS, XRD, TPR, XANES and surface area estimation (BET). The results showed that in the original Ni catalyst, the Ni interacted strongly with the alumina support, exhibiting high reduction temperatures in TPR tests. In the catalytic tests, the addition of Rh on Ni catalysts improved CH4 conversion but also increased carbon deposition, possible by causing the segregation of Ni species under the reaction conditions. The presence of La on Ni catalysts reduced the carbon deposition by favoring the gasification of carbon species. Addition of synthetic air to the process improved the CH4 conversion and also decreased the carbon formation. The catalysts Ni, Rh-NiLa, and Rh showed good results in the conversion of model sulfur-free biogas, which suggests that they are promising catalysts to be tested in conversion of real biogas. (C) 2012 Elsevier B.V. All rights reserved.

Ni catalyst on mixed support of CeO2-ZrO2 and Al2O3: Effect of composition of CeO2-ZrO2 solid solution on the methane steam reforming reaction

In this study, catalysts containing 5 wt.% Ni deposited on a support composed of a CeO2-ZrO2 solid solution deposited on alumina were tested in the steam reforming of methane. The supports, with various ratios of Ce to Zr, were prepared by co-precipitation of the oxide precursors, followed by calcination in synthetic air. The catalysts were then prepared by Ni impregnation of the supports. The prepared solids were characterized by temperature-programmed reduction with H-2 (TPR-H-2), in situ X-ray diffraction (XRD) and X-ray absorption near-edge structure (XANES) spectroscopy. The XRD analysis confirmed the formation of a solid solution between ZrO2 and CeO2. In the catalytic tests, it was found that catalysts with higher Ce content did not exhibit deactivation during 6 h of reaction. The catalyst with highest Ce content, Ni(0.8Ce0.2Zr)AI, provided the best result, with the highest rate of conversion of methane and the lowest carbon deposition, which may be partly due to the smaller Ni-0 crystallites in this sample and also the segregated CeO2 particles may have favored H2O adsorption which could lead to higher C gasification. (C) 2012 Elsevier B.V. All rights reserved.

Study of CuO/CeO2 catalyst with for preferential CO oxidation reaction in hydrogen-rich feed (PROX-CO)

The CuO/CeO2 system was investigated as a catalyst for preferential CO oxidation reaction in hydrogen-rich feed (PROX-CO). The catalysts were prepared by deposition-precipitation (DEP) and co-precipitation (COP) methods and the catalytic performance reveals that the preparation method influences the properties of solids prepared, where a direct consequence is the difference in behavior of the catalysts in the PROX-CO reaction. A high specific area and a better dispersion of the metallic phase were obtained in the catalyst prepared by co-precipitation. The redox properties during the reaction were reported by measures of temperature programmed reduction (TPR), OSC measurements and X-ray absorption near edge structure (XANES-TPR) in situ showed the relationship between the preparation method, the physicochemical characteristics and redox properties in the PROX-CO reaction. By this means, the good dispersion of CuO and the best oxygen capacity are the response of the high performance of CuO/CeO2-COP catalysts for the PROX-CO reaction. Crown Copyright (c) 2012 Published by Elsevier B.V. All rights reserved.

Cluster Coordination and Photoluminescence Properties of alpha-Ag2WO4 Microcrystals

In this paper, we report our initial research to obtain hexagonal rod-like elongated silver tungstate (alpha-Ag2WO4) microcrystals by different methods [sonochemistry (SC), coprecipitation (CP), and conventional hydrothermal (CH)] and to study their cluster coordination and optical properties. These microcrystals were structurally characterized by X-ray diffraction (XRD), Rietveld refinements, Fourier transform infrared (FT-IR), X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopies. The shape and average size of these alpha-Ag2WO4 microcrystals were observed by field-emission scanning electron microscopy (FE-SEM). The optical properties of these microcrystals were investigated by ultraviolet-visible (UV-vis) spectroscopy and photoluminescence (PL) measurements. XRD patterns and Rietveld refinement data confirmed that alpha-Ag2WO4 microcrystals have an orthorhombic structure. FT-IR spectra exhibited four IR-active modes in a range from 250 to 1000 cm(-1). XANES spectra at the W L-3-edge showed distorted octahedral [WO6] clusters in the lattice, while EXAFS analyses confirmed that W atoms are coordinated by six O atoms. FE-SEM images suggest that the alpha-Ag2WO4 microcrystals grow by aggregation and the Ostwald ripening process. PL properties of alpha-Ag2WO4 microcrystals decrease with an increase in the optical band-gap values (3.19-3.23 eV). Finally, we observed that large hexagonal rod-like alpha-Ag2WO4 microcrystals prepared by the SC method exhibited a major PL emission intensity relative to alpha-Ag2WO4 microcrystals prepared by the CP and CH methods.

Study of La2-xCaxCuO4 perovskites for the low temperature water gas shift reaction

The effects of small fractions of calcium (x = 0, 0.05, 0.1, 0.15, and 0.20) on the structure and the catalytic properties of La2-xCaxCuO4 peroviskites have been investigated. The samples have been synthesized using the co-precipitation method. Perovskite-type oxides were characterized by XRD, TPR, XPS, XANES, SEM, and TEM. Catalytic tests for the water gas shift reaction (WGSR) were carried out in a tubular reactor at 290 degrees C. All samples showed a well-defined perovskite structure with surface areas between 6 and 18 m(2) g(-1). The partial substitution of La by Ca enhanced the stability of the perovskites and increased their reduction temperature. All catalysts were actives for WGSR, and the best catalytic performance was obtained for the La1.85Ca0.15CuO4 catalyst, but the samples with 5 and 10% of Ca had the best TOF values for reaction. These results can be associated to promoter effect of calcium, the high surface area, and the reducible species Cu-0 and Cu1+. (C) 2011 Elsevier B.V. All rights reserved.

Local order and electronic structure of Pb1-xLaxZr0.40Ti0.60O3 materials and its relation with ferroelectric properties

The knowledge of electronic and local structures is a fundamental step towards understanding the properties of ferroelectric ceramics. X-ray absorption near-edge structure (XANES) of Pb1-xLaxZr0.40Ti0.60O3 ferroelectric samples was measured in order to know how the local order and electronic structure are related to their ferroelectric property, which was tailored by the substitution of lead by lanthanum atoms. The analysis of XANES spectra collected at Ti K- and L-edges XANES showed that the substitution of Pb by La leads to a decrement of local distortion around Ti atoms on the TiO6 octahedron. The analysis of O K-edge XANES spectra showed that the hybridization between O 2p and Pb 6sp states is related to the displacement of Ti atoms in the TiO6 octahedra. Based on these results, it is possible to determine that the degree of ferroelectricity in these samples and the manifestation of relaxor behavior are directly related to the weakening of O 2p and Pb 6sp hybridization. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4720472]

Discovery of the Persistent Luminescence Mechanism of CdSiO3:Tb3+

The persistent luminescence of CdSiO3:Tb3+ was investigated with photoluminescence, thermoluminescence (TL), synchrotron radiation X-ray absorption (XANES and EXAFS) and UV-VUV spectroscopies. Only the typical intraconfigurational 4f(8)-4f(8) transitions of the Tb3+ ion were observed with no traces of band emission in either the conventional UV excited or persistent luminescence spectra. The trap structure from TL with three traps from 0.65 to 0.85 eV is ideal for room-temperature persistent luminescence similar to, e.g., Sr2MgSi2O7:Eu2+,R3+. Despite the rather low band gap energy, 5.28 eV, the persistent luminescence from Tb3+ is produced only under UV irradiation due to the inauspicious position of the F-7(6) ground level deep in the band gap of CdSiO3. This confirms the role of electrons as the charge carriers in the mechanism of Tb3+ persistent luminescence. The XANES spectra indicated the presence of only the trivalent Tb3+ species, thus excluding the direct Tb3+ -> Tb-IV oxidation during the charging process of persistent luminescence. Eventually, a unique persistent luminescence mechanism for Tb3+ in CdSiO3 was constructed based on the comprehensive experimental data.

X-ray absorption spectroscopy study on La0.6Sr0.4CoO3 and La0.6Sr0.4Co1¡yFeyO3 nanotubes and nanorods for IT-SOFC cathodes.

In the last years, extensive research has been devoted to develop novel materials and structures with high electrochemical performance for intermediate-temperatures solid-oxide fuel cells (IT-SOFCs) electrodes. In recent works, we have investigated the structural and electrochemical properties of La0:6Sr0:4CoO3 (LSCO) and La0:6Sr0:4Co1¡yFeyO3 (LSCFO) nanostructured cathodes, finding that they exhibit excellent electrocatalytic properties for the oxygen reduction reaction [1,2]. These materials were prepared by a pore-wetting technique using polycarbonate porous membranes as templates. Two average pore sizes were used: 200 nm and 800 nm. Our scanning electronic microscopy (SEM) study showed that the lower pore size yielded nanorods, while nanotubes were obtained with the bigger pore size. All the samples were calcined at 1000oC in order to produce materials with the desired perovskite-type crystal structure. In this work, we analyze the oxidation states of Co and Fe and the local atomic order of LSCO and LSCFO nanotubes and nanowires for various compositions. For this pur- pose we performed XANES and EXAFS studies on both Co and Fe K edges. These measurements were carried out at the D08B-XAFS2 beamline of the Brazilian Synchrotron Light Laboratory (LNLS). XANES spectroscopy showed that Co and Fe only change slightly their oxidation state upon Fe addition. Surprisingly, XANES results indicated that the content of oxygen vacancies is low, even though it is well-known that these materials are mixed ionic-electronic conductors. EXAFS results were consistent with those expected according to the rhombohedral crystal structure determined in previous X-ray powder dffraction investigations. [1] M.G. Bellino et al, J. Am. Chem. Soc. 129 (2007) 3066 [2] J.G. Sacanell et al., J. Power Sources 195 (2010) 1786

Fe valence fluctuations and magnetoelastic coupling in Pb-based multiferroics perovskites

Lead-based multiferroics perovskites with nominal compositions Pb(Fe1/2Nb1/2)O3 and Pb(Fe2/3W1/3)O3 were synthesized following a two-stage method. Magnetic proprieties were investigated and correlated to anelastic proprieties, measured by the conventional pulse-echo method. The discussions are focused in the region around 250 K, where magnetoelectroelastic instabilities have been observed. X-ray absorption nearedge structure (XANES) study further indicates that the edge position varies with temperature revealing a fluctuation on the valence of iron ions with the temperature, which can be related to a variation in anelastic and magnetic proprieties.

Synthesis and properties of substituted Hg-based superconductors

This thesis is focused on studies of substituted Hg-based superconducting copper oxides ((Hg1-xMx)Ba2Can-1CunO2n+2+δ). These compounds are promising objects of investigation, not only from a fundamental point of view but also because of their high values of superconducting transition temperature (Tc) and irreversibility field (Hirr). The first part of the thesis is devoted to optimization of the synthesis procedure for Hg-based cuprates. The influence of different parameters (T, t, p(Hg), p(O2)) on the synthesis of these compounds in sealed silica tubes was studied. Optimal conditions yielded samples containing up to 95% of HgBa2Ca2Cu3O8+δ (Hg-1223). The formation of solid solutions with the formula (Hg1-xCux)Ba2Ca2Cu3O8+δ (where x <= 0.5) was also established. Another technique was developed, using LiF as a flux, for synthesis of samples containing up to 90% of the HgBa2CaCu2O6+δ (Hg-1212) phase. The second part concerns synthesis and studies of oxyfluorides using Hg-1212 and Hg-1223 as starting materials together with XeF2 as a fluorinating agent. It was found that oxyfluorides of both phases have a parabolic dependence of Tc vs. a parameter as well as enhanced Tc values (ΔT ≈ 3-4 K) in comparison with optimally doped non-fluorinated analogues. The crystal structure of Hg-1223 oxyfluoride was studied by X-ray powder and neutron diffraction methods. It is suggested that chemical modification of the crystal structure leads to a decrease in Cu-O distance without noticeable change in Cu-O-Cu angle (in the (CuO2) layers), which may be the significant factors influencing this Tc increase. Hg-1223 oxyfluoride was also studied under high pressure for first time. It was found that this compound has a record-high Tc value (≈ 166 K) at P ≈ 23 GPa. The last part describes the investigation of substituted Hg-based superconductors in the series (Hg0.9M0.1)Ba2CuO4+δ {(Hg,M)-1201}, where M = Tl, Pb, W, Mo, Nb and V. A comprehensive study of these compounds by various methods (X-ray powder diffraction, EDX, IR-, EXAFS- and XANES -spectroscopy) indicated that the change of charge carrier doping level is a crucial factor determining the irreversibility line. (Hg0.9Mo0.1)Ba2CuO4+δ showed the most improved irreversibility line position among the (Hg,M)-1201 compounds studied in this series.

Analytische und funktionale Charakterisierung von ultradünnen kohlenstoffbasierten Schichten in der Magnetspeichertechnologie

Ein Schwerpunkt der in dieser Arbeit durchgeführten Analysen konzentrierte sich auf den Einfluss der Kohlenstoffschichtdicke auf die resultierenden Schichteigenschaften. Dies erfolgte vor dem Hintergrund, dass ein Anstieg der Speicherdichte auf 100 Gbits/in2 mit einer Dickenreduzierung der magnetisch inaktiven Kohlenstoffschutzschichten auf 2 nm verbunden ist. Im Rahmen der Charakterisierung von wasserstoffhaltigen, mittels einem CVD-Netzhohlkathodenreaktor abgeschiedenen Schichten, wurde eine zuverlässige Anwen-dung der XRR zur Dickenbestimmung ultradünner Kohlenstoffschichten im Bereich von 2-20 nm demonstriert. Die konsistente Verwendung eines optischen Einschichtmodells deutet auf eine homogene Tiefenstruktur ohne deutliche Ausprägung von scharf defi-nierten Grenzflächenbereichen zum Substrat und zur Oberfläche hin. Die Analyse des Schichtdickeneinflusses ergab eine leichte Verminderung der Kratz-härte sowie einen Rückgang des Elastizitätsmoduls von 90 GPa auf etwa 60 GPa, wenn die Schichtdicken bei Raumtemperaturabscheidung von 17 nm auf 2 nm reduziert wer-den. Diese Ergebnisse stehen in guter Übereinstimmung mit gewonnen ramanspektro-skopischen Resultaten, die einen ansteigenden sp2-Bindungsanteil für abnehmende Schichtdicken belegen. Die Charakterisierung von wasserstofffreien, gepulsten Vakuumbogen-Schichten ergab ein heterogenes Tiefenprofil, bestehend aus Oberflächen-, Bulk- und Interfacebereich. Diese Struktur führt zu einer Schichtdickenabhängigkeit von mechanischen und struktu-rellen Parametern, die über die gesamte Schichtdicke gemittelt werden. XRR-Analysen ergaben Bulkdichten von 2.8-3.0 g/cm3 bei Raumtemperaturabscheidung und 2.2 g/cm3 für eine Beschichtung von Speicherplatten bei einer Temperatur von 200 °C. Durchgeführte AFM-Messungen an Schichten auf Silizumsubstraten zeigten nahezu kei-ne Abhängigkeit der RMS-Oberflächenrauhigkeit von der Schichtdicke bis zu 18 nm und lieferten konstante Werte im Bereich der Substratrauhigkeit von etwa 0.1 nm. Bei der Beschichtung von Speicherplatten bewirkt eine Dickenabnahme der aufgebrachten Filme sogar einen glättenden Einfluss auf die Oberflächentopographie. Der Elastizitätsmodul und die mittels Mehrwellenlängen-Ramanspektroskopie ermittelte Raman-G-Peak-Dispersion als Maße für Schichthärte und Schichtstruktur zeigen einen korrelierenden Abfall für verringerte Schichtdicken. Der Elastizitätsmodul fällt im Fall ei-ner Raumtemperaturabscheidung von etwa 450 GPa bei 20 nm Dicke auf ca. 50 GPa bei weniger als 2 nm Dicke ab. Diese Resultate werden durch röntgenabsorptionsspektro-skopische Untersuchungen der Kohlenstoff-K-Kante bestätigt. Kombinierte XPS- und XANES-Analysen der Bedeckungsqualität von Vakuumbogen-Schichten ergaben eine kritische Dicke von nur ca. 1 nm. Bei einer Unterschreitung die-ses Dickenwertes kann eine vollständige Bedeckung bzw. der Korrosionsschutz des un-terliegenden metallischen Mediums nicht mehr uneingeschränkt gewährleistet werden. XPS-Referenzmessungen an konvenzionellen CNx-Schichten weisen demgegenüber oxidische Cobaltsignale bereits im Bereich von 2-3 nm nach.

Speciation of Pu(III) in the environmental system humic substances-groundwater-kaolinite

For the safety assessments of nuclear waste repositories, the possible migration of the radiotoxic waste into environment must be considered. Since plutonium is the major contribution at the radiotoxicity of spent nuclear waste, it requires special care with respect to its mobilization into the groundwater. Plutonium has one of the most complicated chemistry of all elements. It can coexist in 4 oxidation states parallel in one solution. In this work is shown that in the presence of humic substances it is reduced to the Pu(III) and Pu(IV). This work has the focus on the interaction of Pu(III) with natural occurring compounds (humic substances and clay minerals bzw. Kaolinite), while Pu(IV) was studied in a parallel doctoral work by Banik (in preparation). As plutonium is expected under extreme low concentrations in the environment, very sensitive methods are needed to monitor its presence and for its speciation. Resonance ionization mass spectrometry (RIMS), was used for determining the concentration of Pu in environmental samples, with a detection limit of 106- 107 atoms. For the speciation of plutonium CE-ICP-MS was routinely used to monitor the behaviour of Pu in the presence of humic substances. In order to reduce the detection limits of the speciation methods, the coupling of CE to RIMS was proposed. The first steps have shown that this can be a powerful tool for studies of pu under environmental conditions. Further, the first steps in the coupling of two parallel working detectors (DAD and ICP_MS ) to CE was performed, for the enabling a precise study of the complexation constants of plutonium with humic substances. The redox stabilization of Pu(III) was studied and it was determined that NH2OHHCl can maintain Pu(III) in the reduced form up to pH 5.5 – 6. The complexation constants of Pu(III) with Aldrich humic acid (AHA) were determined at pH 3 and 4. the logß = 6.2 – 6.8 found for these experiments was comparable with the literature. The sorption of Pu(III) onto kaolinite was studied in batch experiments and it was determine dthat the pH edge was at pH ~ 5.5. The speciation of plutonium on the surface of kaolinite was studied by EXAFS/XANES. It was determined that the sorbed species was Pu(IV). The influence of AHA on the sorption of Pu(III) onto kaolinite was also investigated. It was determined that at pH < 5 the adsorption is enhanced by the presence of AHA (25 mg/L), while at pH > 6 the adsorption is strongly impaired (depending also on the adding sequence of the components), leading to a mobilization of plutonium in solution.

Speciation of tetravalent plutonium in contact with humic substances and kaolinite under environmental conditions

Plutonium represents the major contribution to the radiotoxicity of spent nuclear fuel over storage times of up to several hundred thousand years. The speciation of plutonium in aquifer systems is important in order to assess the risks of high-level nuclear waste disposal and to acquire a deep knowledge of the mobilization and immobilization behavior of plutonium. In aqueous solutions, plutonium can coexist in four oxidation states and each one of them has different chemical and physical behavior. Tetravalent plutonium is the most abundant under natural conditions. Therefore, detailed speciation studies of tetravalent plutonium in contact with humic substances (HS) and kaolinite as a model clay mineral have been performed in this work. Plutonium is present in the environment at an ultratrace level. Therefore, speciation of Pu at the ultratrace level is mandatory. Capillary electrophoresis (CE) coupled to resonance ionization mass spectrometry (RIMS) was used as a new speciation method. CE-RIMS enables to improve the detection limit for plutonium species by 2 to 3 orders of magnitude compared to the previously developed CE-ICP-MS. For understanding the behavior of Pu(IV) in aqueous systems, redox reactions, complexation, and sorption behavior of plutonium were studied. The redox behavior of plutonium in contact with humic acid (HA) and fulvic acid (FA) was investigated. A relatively fast reduction of Pu(VI) in contact with HS was observed. It was mainly reduced to Pu(IV) and Pu(III) within a couple of weeks. The time dependence of the Pu(IV) complexation with Aldrich HA was investigated and a complex constant (logßLC) between 6.4 - 8.4 of Pu(IV) was determined by means of ultrafiltration taking into account the loading capacity (LC). The sorption of tetravalent plutonium onto kaolinite was investigated as a function of pH in batch experiments under aerobic and anaerobic conditions. The sorption edge was found at about pH = 1 and a maximum sorption at around pH = 8.5. In the presence of CO2 at pH > 8.5, the sorption of plutonium was decreased probably due to the formation of soluble carbonate complexes. For comparison, the sorption of Th(IV) onto kaolinite was also investigated and consistent results were found. The Pu(IV) sorption onto kaolinite was studied by XANES and EXAFS at pH 1, 4, 9 and the sorbed species on kaolinite surface was Pu(IV). Depending on the pH, only 1 - 10 % of the sorbed plutonium is desorbed from kaolinite and released into a fresh solution at the same pH value. Furthermore, the sorption of HS onto kaolinite was studied as a function of pH at varying concentrations of HS, as a prerequisite to understand the more complex ternary system. The sorption of HA onto kaolinite was found to be higher than that of FA. The investigation of the ternary systems (plutonium-kaolinite-humic substances) is performed as a function of pH, concentration of HS, and the sequences of adding the reactants. The presence of HS strongly influences the sorption of Pu(IV) onto kaolinite over the entire pH range. For comparison, the influence of HS on the sorption of Th(IV) onto kaolinite was also investigated and a good agreement with the results of Pu(IV) was obtained.