Novel approaches of nanoceria with magnetic, photoluminescent, and gas-sensing properties

The modification of CeO2 with rare-earth elements opens up a wide range of applications as biomedical devices using infrared emission as well as magnetic and gas-sensing devices, once the structural, morphological, photoluminescent, magnetic, electric, and gas-sensing properties of these systems are strongly correlated to quantum electronic transitions between rare-earth f-states among defective species. Quantitative phase analysis revealed that the nanopowders are free from secondary phases and crystallize in the fluorite-type cubic structure. Magnetic coercive field measurements on the powders indicate that the substitution of cerium with lanthanum (8 wt %), in a fluorite-type cubic structure, created oxygen vacancies and led to a decrease in the fraction of Ce species in the 3+ state, resulting in a stronger room-temperature ferromagnetic response along with high coercivity (160 Oe). In addition to the magnetic and photoluminescent behavior, a fast response time (5.5 s) was observed after CO exposure, indicating that the defective structure of ceria-based materials corresponds to the key of success in terms of applications using photoluminescent, magnetic, or electrical behaviors.Fil: Rocha, Leandro S.R.. Universidade Federal do São Carlos; BrasilFil: Amoresi, Rafael A.C.. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Moreno, Henrique. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Ramirez, Miguel A.. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Ponce, Miguel Adolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Foschini, Cesar R.. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Longo, Elson. Universidade Federal do São Carlos; BrasilFil: Simões, Alexandre Z.. Universidade Estadual Paulista Julio de Mesquita Filho; Brasi

Direct preparation of standard functional interfaces in oxide heterostructures for 2DEG analysis through beam-induced platinum contacts

Two-dimensional electron gas (2DEG) in SrTiO3/LaAlO3 heterostructures has been extensively studied in the last few years; however, little attention has been given to a practical way to contact electrically the low dimensional gas at the interface. This work demonstrates a method to contact the 2DEG formed at the oxide interfaces connected by platinum electrodes which were made by the decomposition of organometallic gas using focused ion beams. On the surface, the electrodes were defined through photolithography, and at the interface, the electrodes were deposited through the focused ion beams and electrons, which were then evaluated. The quality of the interface electrodes was evaluated at two different partial oxygen pressures (pO2) used for the film deposition: low (10−4 mbar) and high (10−1 mbar). The electrode deposition conditions using electrons or ions have resulted in different rates of metal deposition and interaction with the interface leading to either metallic (2DEG) or insulating behavior

Structure, electronic properties, morphology evolution, and photocatalytic activity in PbMoO4 and Pb12xCaxSrxMoO4 (x = 0.1, 0.2, 0.3, 0.4 and 0.5) solid solutions

In this work PbMoO4 and Pb12xCaxSrxMoO4 (x = 0.1, 0.2, 0.3, 0.4 and 0.5) solid solutions have been successfully prepared, for the first time, by a simple co-precipitation method and the as-synthesized samples were subjected to a water-based reflux treatment. Structural characterization of these samples was performed using X-ray diffraction with Rietveld refinement analysis and Raman spectroscopy. Their optical properties were investigated by UV-Vis absorption spectroscopy and PL emissions, and the photocatalytic activity of the as-synthesized samples for the degradation process of Rhodamine B has been demonstrated. The surface structure and morphologies were characterized by field emission scanning electron microscopy. To complement and rationalize the experimental results, the geometry, electronic structures, and morphologies of as-synthesized samples were characterized by first-principles quantum-mechanical calculations at the density functional theory level. By using Wulff construction, based on the values of the surface energies for the (001), (100), (110), (111), (011) and (112) surfaces, a complete map of the available morphologies for PbMoO4 was obtained and a good agreement between the experimental and theoretical predicted morphologies was found. The structural and electronic changes induced by the substitution of Pb by Ca and Sr allow us to find a relationship among morphology, the electron-transfer process at the exposed surfaces, optical properties, and photocatalytic activity. We believe that our results offer new insights regarding the local coordination of superficial Pb/Ca/Sr and Mo cations (i.e., clusters) on each exposed surface of the corresponding morphology, which dictate the photocatalytic activities of the as-synthesized samples, a field that has so far remained unexplored. The present study, which combines multiple experimental methods and first-principles calculations, provides a deep understanding of the local structures, bonding, morphologies, band gaps, and electronic and optical properties, and opens the door to exploit the electrical, optical and photocatalytic activity of this very promising family of materials

Synthesis of Lamellar Structures of Magnesium (II), Aluminum (III) and Iron (III) Hydroxides Interchanged with Carbonate Ion through Precipitation in pH 11

The synthesis of layered double hydroxides has been investigated aiming innumerous applications, mainly as adsorbent, catalyst and catalyst support materials, due their ability to adsorb anionic species and several aqueous soluble compounds. The carbonated magnesium-aluminium hydrotalcites are known as the main class of the layered double hydroxides and new composition are often characterized under the view point of thermal stability, crystallinity and catalytic performance for many reactions. Few trivalent cations are able to replace the aluminum one due the severe restriction for oxidation state and ionic radii, but the iron (III) one seems to be high potential to improve some of the characteristics required for that materials, such as specificity for built-rebuilt bonds in organic molecules. In this work, we have synthesized carbonated magnesium-aluminum hydrotalcite samples through the coprecipitation at pH 11 and investigate the Fe(III) insertion at 10 and 20 mol%. Thermal analysis, FTIR spectrometry and X-ray diffractometry techniques were used to understand the influence of the Fe(III) co-substitution, keeping the Mg(II) molar fraction invariable among the samples. We show the iron (III) insertion affects the dehydration and dehydroxylation processes due the changes in M-OH bond energies Very homogeneous structures were obtained for all of the samples dried at 100 ºC and a consistent lattice volume expansion was observed as a function of iron (III) content, which can be required for catalyst or catalyst matrix applications. DOI: http://dx.doi.org/10.17807/orbital.v10i1.103

Metallic behavior in STO/LAO heterostructures with non-uniformly atomic interfaces

The search for new and low-power switching devices involving the integration of semiconductor thin films is of interest, and has led to renewed research because such devices may exhibit innovative properties. Here, we investigate the two-dimensional electron gas (2DEG) at the LaAlO3/SrTiO3 interface with metallic and insulator behavior. Insight is offered by quantifying the interface charge distribution associated with structural and electronic order-disorder effects. Variations in the electron conductivity were observed to be associated with different specific clustering arrangements of both Ti and Al cations of the co-exposed surfaces at the interface, i.e., structural and electronic connectivity among the undercoordinated [TiO ]5 and [AlO ]5 clusters. These results indicate facet control as a strategy for enhancing the electric and magnetic properties of a device via the quantum confinement of electrons

Pure and Ni2O3-decorated CeO2 nanoparticles applied as CO gas sensor: Experimental and theoretical insights

In the present work, the structural, morphological and electrical properties of CeO2 nanoparticles with spherical and rod-like morphologies and rods decorated with Ni2O3 were investigated. Morphological, structural and electronic analyses showed a relationship between the shape/size of particles and the respective surface types, number of carriers, density of vacancies and local structural atomic order, in addition to an influence on the electrical properties of the materials. Electrical analyses revealed that the rod-like morphology has a better CO sensor response than the spherical one. In order to complement and rationalize the experimental findings, simulations at the density functional theory (DFT) were carried out to investigate the two possible mechanisms (Langmuir˗Hinshelwood and Mars-Van Krevelen) acting on the CO adsorption process and elucidate the behavior of the sensor in heterojunction-type samples

Experimental and theoretical interpretation of the order/disorder clusters in CeO2:La

A spectroscopic study of nanostructured lanthanum-doped cerium oxide samples showed that a mild decrease by5.3% in the surface concentration of Ce(III) species and, a reduction by 0.2 eV in the so-called energy-gap (EF ?Ev) after lanthanum addition was sufficient to create the previously reported significant dual behavior whenexposed to carbon monoxide at 653 K. The observed X-ray diffraction patterns indicated the presence of a purefluorite-type crystalline structure. A higher presence of paramagnetic defects clusters, indicated by electronparamagnetic resonance spectroscopy measurements and confirmed by the X-ray photoelectron spectroscopy,was attributed as being responsible for its dual behavior in a CO(g) atmosphere. Theoretical studies have shownthe band structure and density of state of the pure and doped material, illustrating the orbitals that participate inthe valence band and conduction band. The addition of a small quantity of La3+ converted some Ce3+ into Ce4+,narrowed the ?effective band-gap? by 0.2 eV, and created a singly ionized oxygen vacancy species, thus changingthe total electrical resistance and creating its dual behavior. The exposure to a reducing atmosphere such ascarbon monoxide resulted in a significant increase in defects and converted some Ce4+ into Ce3+ and vice-versawith oxygen atmosphere exposure.Fil: Silva Rosa Rocha, Leandro. Universidade Federal do São Carlos; BrasilFil: Aparecido Ciola Amoresi, Rafael. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Marinho Duarte, Thiago. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Marana, Naiara Letícia. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Sambrano, Julio Ricardo. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Aldao, Celso Manuel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Zirpoli Simões, Alexandre. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Ponce, Miguel Adolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Longo, Elson. Universidade Federal do São Carlos; Brasi

Influence of Chromium (III) Insertion on the Thermal Stability of Carbonated Magnesium and Aluminum Hydrotalcites Synthesized by the Hydroxide Coprecipitation Method

Anionic clays are structures capable of adsorbing large quantities of water and other polar molecules in aqueous medium due to an ordered lamellar arrangement. Generally, the most stable composition contains magnesium and aluminum hydroxides, but other trivalent metal cations with similar ionic radii may also be inserted in co-substitution to the aluminum cation, targeting changes in the specificities of the adsorbed molecules, such as the chromium (III) one. The main objective of this work was to investigate the thermal stability of the carbonated magnesium-aluminum hydrotalcite when different amount of aluminium (III) cation is replaced by chromium (III) one. For that purposes, the hydrotalcite samples were obtained through the coprecipitation method at pH 11, followed by ageing stage at 80 °C, washing and drying at 100 °C for 24 h. The results obtained by thermal analysis, FTIR spectroscopy, and X-ray diffractometry techniques have permitted to conclude that chromium insertion leads to turbostratic disorder along the c-axis, which affects also the thermal stability and crystallinity of the precipitated materials. The chromium-based hydrotalcites showed to be more easily dehydrated at lower temperatures, with dependence predominantly kinetic, which leads to significant volume contraction and the hindering of the subsequent dehydroxylation stage at higher temperatures. DOI: http://dx.doi.org/10.17807/orbital.v10i1.1039<br /

Influence of Synthesis Time on the Morphology and Properties of CeO2Nanoparticles: An Experimental-Theoretical Study

Here we combined experimental and theoretical results to correlate the morphological, optical, and electronic properties of cerium oxide (CeO2) prepared by a microwave-assisted hydrothermal method with varying synthesis times. X-ray diffraction confirmed a cubic structure without deleterious phases. Density functional theory simulations confirmed an indirect (K-L) bandgap energy of 2.80 eV, with an electron transition between O-2p and Ce-4f orbitals, which agrees with the value obtained using diffuse reflectance. Raman spectroscopy shows that changing the synthesis times results in samples with different defect densities at a short range. Theoretical calculations confirmed that the deformations and changes in the experimental Raman spectra area result in oxygen displacement; as the displacement decreases, the crystallinity increases, and only one peak was observed. Scanning electron microscopy and high-resolution transmission electron microscopy show changes in the morphologies as the synthesis time varies. For shorter times, sheet and polyhedral morphologies were noted. With time increases, the sheets turn into nanorods and nanowires until the nanowires decrease and cubes are observed. In addition, an initial study regarding the influence of the surface on the electric response of CeO2 was completed. It was observed that the presence of different surface defects ([CeO62Vox] or [CeO7Vox]) can alter the material resistance.Fil: De Oliveira, Regiane Cristina. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Amoresi, Rafael Aparecido Ciola. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Marana, Naiara Letícia. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Zaghete, Maria Aparecida. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Ponce, Miguel Adolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Chiquito, Adenilson José. Universidade Federal do São Carlos; BrasilFil: Sambrano, Julio Ricardo. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Longo, Elson. Universidade Federal do São Carlos; BrasilFil: Simões, Alexandre Zirpoli. Universidade Estadual Paulista Julio de Mesquita Filho; Brasi