ABSTRACT Manganese oxides were synthesized during 40 min at 140 ºC via Microwave-Assisted Hydrothermal (MAH) method and treated at different temperatures in order to evaluate the phase evolution using structure refinement (Rietveld method). The samples obtained were heat treated at temperatures defined by means of thermal analysis (160 ºC, 480 ºC, 715 ºC, 870 ºC, 920 ºC and 1150 ºC) and analyzed by X-Ray Diffractometry (XRD), X-Ray Fluorescence (XRF), Fourier Transform Infrared (FTIR) spectroscopy, Raman scattering, UV-Vis absorption and Scanning Electron Microscopy (SEM). Structural characterizations allowed to identify five distinct phases: α-MnO2, Mn3O4, Mn5O8, Na2Mn5O10 and Na4Mn9O18 with weight percentages dependent on the heat treatment. The hausmannite structure (average crystallite size ranging from 28.9 nm to 99.1 nm) is present in all samples and go through various oxidation and reduction processes from 160 ºC to 1150 ºC without any major variation in the lattice parameters. Chemical characterizations identifies the presence of Na+ ions in all samples, either as substitution defects or as components of specific crystalline structures (Na2Mn5O10 and Na4Mn9O18), showing that the synthesized manganese oxides works as Na+ intercalation compounds, important materials for energy storage devices optimization. The results presented enables a better interpretation of the thermal and structural characteristics of manganese oxides synthesized via MAH.

In this study, quantum-mechanical calculations in the framework of the Density Functional Theory (DFT) were performed to investigate the role of exchange-correlation functional in describing structural, electronic, and magnetic properties of ZnFe2O4. Herein B3LYP, PBE0, B1WC, and WC1LYP functionals implemented in the CRYSTAL17 code were considered due to the different amounts of the exact Hartree-Fock exchange fraction. In particular, the role of HF fraction on ZnFe2O4 properties was addressed for the first time. Indeed, structural, electronic, and magnetic properties indicate the dependence upon the exchange fraction, where WC1LYP with a 16% exact HF exchange exhibits the best performance compared to the other hybrid functionals. The obtained results reveal an excellent agreement for bandgap, local magnetic moment, long-range magnetic ordering, and unit-cell lattice parameters, overcoming previous theoretical studies based on local/semilocal exchange-correlation treatments. These results confirm the importance of hybrid HF/DFT with controlled HF term contribution to describe the essential features of strongly correlated materials.

RESUMO O material CuCr2O4 foi sintetizado pela reação de combustão em solução e tratado termicamente na faixa de 600 à 1100oC, afim de verificar a evolução das fases. O composto foi carcterizado estrutural e morfologicamente por meio das técnicas de Difração de Raios X (DRX), refinamentos pelo método de Rietveld, Microscopia Eletrônica de Varredura com canhão de elétrons por emissão de campo (MEV-FEG) e espectroscopia na região do Infravermelho com transformada de Fourrier. O comportamento da sinterização do material foi analisado por meio da dilatometria. Por meio dos padrões da difração de raios X (DRX) observou-se a formação da fase pura da Cromita de Cobre na faixa de 700-1000°C, a qual é corroborada pelo Refinamento Rietveld. Quanto a morfologia, as fases puras da cromita de cobre apresentaram formas de poliedros regulares. Por meio da dilatometria observou-se que o material apresentou uma expansão seguida de retração associada a mudança estrutural do material.

ABSTRACT CuCr2O4 material was subjected to solution combustion synthesis and heat-treated in the range of 600 to 1100oC in order to analyze its phase evolution. The structure and morphology of the composite were characterized by X-ray diffraction (XRD), Rietveld refinement, field emission gun scanning electron microscopy (FEG-SEM) and Fourier transform infrared spectroscopy (FTIR), while its sintering behavior was analyzed by dilatometry. The XRD patterns revealed the formation of pure phase copper chromite in the range of 700-1000°C, which was confirmed by Rietveld refinement. This pure phase copper chromite exhibited a regular polyhedral shaped morphology. The dilatometric analysis indicated that the composite underwent thermal expansion followed by shrinkage, which were attributed to structural changes in the material.

The microwave-assisted hydrothermal method was used to obtain α-Ag2WO4. Rietveld refinement confirmed that α-Ag2WO4 is stable in the orthorhombic phase, without secondary phase. However, field-effect scanning electron microscope analysis showed that α-Ag2WO4 nanorods surfaces contain silver nanoparticles, confirmed by the X-ray photoelectron spectroscopy by the peak observed at 374.39 eV. In addition to metallic Ag, other Ag oxidation states were also observed on the surface. Hence, Ag (I) as Ag2O and Ag (I) as Ag2WO4 also were identified. DC measurements exhibited a high capacity of charge storage, nevertheless, with a large loss tangent (0.12 µC.cm-2.V-1) and no residual polarization for the voltage range between -100 V and +100 V. AC measurements at frequencies less than 275 Hz, revealed that ionic polarization is dominant, whereas at frequencies higher than 275 Hz, the electronic behavior predominates. The potential of electromagnetic energy conversion in thermal was observed from loss tangent analysis.

ABSTRACT The difficulty in the powder reuse favors the study of materials in the form of thin films. ZnO based films have high photocatalytic potential. In this work, ZnO:xSm3+ (x = 0, 1, 2 and 4 %mol) thin films were prepared by spin coating method. The resins obtained to manufacture the thin films were prepared by complex polymerization method. The samples were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM) and spectroscopy UV-Vis. XRD patterns indicate that doping with 4%Sm forms Sm2O3 as the secondary phase. FE-SEM images of the cross section of thin films indicated a uniform thickness ranging from 354 to 367 nm between samples. The incorporation of Sm3+ ions in the ZnO lattice increases the Egap of the films. The photocatalytic performance of the films was tested with the photodegradation of methylene blue dye. Pure ZnO and ZnO:1%Sm3+ exhibited the best activity in the photodegradation. Thin films of pure ZnO exhibit the best photocatalytic results for the first cycle, but considerably reduce their efficiency with reuse. Sm3+ incorporation, without secondary phase formation, was efficient for the maintenance of the photocatalytic property after 3 cycles.

RESUMO O desenvolvimento de catalisadores eficientes e de baixo custo tem sido o foco de diferentes pesquisas e, nesse contexto, as argilas se destacam como matéria prima natural para serem empregadas como suportes catalíticos. Neste sentido, este trabalho teve como objetivo sintetizar e caracterizar materiais a base de estanho e nióbio suportados em argila maranhense. Os óxidos de estanho e nióbio foram sintetizados sobre a superfície da argila pelo método dos precursores poliméricos. Os catalisadores obtidos foram caracterizados por meio de difração de raios X (DRX), espectroscopia na região do Infravermelho (IV), microscopia eletrônica de varredura (MEV), fisissorção de nitrogênio (BET) e espectroscopia dispersiva de raios X (EDS). Os resultados mostraram que o ataque ácido contribuiu para o aumento da área superficial específica do argilomineral natural, passando de 83,53 m2 g−1 para 155,2 m2 g−1. No entanto, a deposição do óxido de nióbio sobre o argilomineral acarretou uma diminuição significativa em sua área superficial específica, passado a 14,68 m2 g−1. A ativação com ácido nítrico promoveu a lixiviação de alguns metais, aumentando a quantidade relativa de sílica presente na amostra. Nas análises por DRX e infravermelho pode-se identificar a presença do nióbio e estanho na matriz de argila. Por fim, verificou-se que processo de síntese adotado foi eficiente para preparação do catalisador suportado (SnO2.Nb/argila).

ABSTRACT The development and optimization of heterogeneous catalysts is a fundamental step to ensure the technical and economic feasibility of new materials created for a wide range of applications, including catalytic applications. The purpose of this study was therefore to synthesize and characterize clay materials from the state of Maranhão, Brazil, doped with tin and niobium oxides synthesized by the polymeric precursor method. The catalysts were characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM), nitrogen physisorption (BET), and energy dispersive X-ray spectroscopy (EDS). The results indicated that activating the clay mineral increased the specific surface area from 83.53 m2 g−1 to 155.2 m2 g−1, while doping with niobium reduced the specific surface area to 14.68 m2 g−1. The chemical composition of the material treated with nitric acid was found to limit the concentrations of some metals and increase the silica content. The XRD and IR analyses revealed the presence of niobium and tin in the clay matrix. Lastly, the method of synthesis adopted here proved to be efficient for the preparation of the supported catalyst (SnO2.Nb/clay).

The aim of this study was to develop a novel, simple and fast route to prepare ZnO nanorods (NRs) -Au nanoparticles (NPs) hybrids directly supported on a substrate to be used in gas sensor devices. The ZnO NRs were promptly grown on interdigitated Au electrodes Al2O3 substrates by chemical bath deposition at a low temperature. After that, Au NPs were deposited by sputtering. Results obtained by XRD, SEM, EDX and TEM showed the perpendicularly aligned growth of the ZnO NRs with a hexagonal base on the substrate and the Au NPs homogeneously covered the ZnO NRs surface. The ZnO NRs-Au NPs hybrids-based sensor exhibited an improved sensor response for H2 and O2 gases compared to the ZnO NRs at 300 ºC. Due to the ability to prepare homogeneous hybrids with high surface directly supported on the substrate; the developed route might provide a convenient approach to preparing gas sensor devices.

In this work barium titanate (BT) and strontium titanate (ST) were prepared by Microwave-Assisted Hydrothermal (MAH) method, using a Ti precursor different from those shown in the literature and various concentrations (1, 3 e 6 mol L-1) of KOH mineralizer. As a titanium precursor, an anatase complex with hydrogen peroxide were used. For the precursors of barium and strontium, chlorides were used. The materials resulting from the synthesis processes were characterized in room temperature by different techniques as like: X-Ray Diffraction (XRD), micro-Raman Spectroscopy, UV-Vis, Scanning Electron Microscopy (SEM) and Photoluminescence (PL). The results show that there were differences in crystallinity, morphology, number of phases and photoluminescence depending on the concentration of the mineralizer.

In the last ten years, stannates with perovskite structure have been tested as photocatalysts. In spite of the ability of perovskite materials to accommodate different cations in its structure, evaluation of doped stannates is not a common task in the photocatalysis area. In this work, Fe3+ doped BaSnO3 was synthesized by the modified Pechini method, with calcination between 300 and 800ºC/4 h. The powder precursor was characterized by thermogravimetry after partial elimination of carbon. Characterization after the second calcination step was done by X-ray diffraction, Raman spectroscopy and UV-visible spectroscopy. Materials were tested in the photocatalytic discoloration of the Remazol Golden Yellow azo dye under UVC irradiation. Higher photocatalytic efficiency was observed under acid media. As no meaningful adsorption was observed at this condition we believe that an indirect mechanism prevails. Fe3+ doping decreased the band gap and favored the photocatalytic reaction, which may be assigned to the formation of intermediate levels inside the band gap.

We present a study of the controlled synthesis and optical properties of single-crystals Gd(OH)3, GdOOH and Gd2O3 nanorods. In this work, Gd(OH)3 nanorods were synthesized by a simple and fast microwave-assisted hydrothermal method. This process combined with the thermal decomposition oxidation of Gd(OH)3 nanorods as precursors enabled the preparation of single-crystalline GdOOH and Gd2O3 structures with well-defined morphology at low temperatures. The crystal structure dependence on the optical properties was investigated. We observed a green shift effect on the photoluminescence (PL) emission spectra from Gd(OH)3 to Gd2O3 nanorods, which can be attributed to different types of surface defects, as well as intrinsic properties that contribute significantly to the modified PL behavior.

Inorganic compounds doped with rare earths (Ce3+, Pr3+, Nd3+, Sm3+, Eu 3+, Gd 3+, Tb 3+, Tm3+) have been used in various applications including light-emitting devices such as fluorescent lamps, cathode ray tubes, lasers and inorganic pigments. In this study, Pr3+ is doped in spinel Li2ZnTi3O8 system and synthesized by the polymeric precursor method, which is based on the process developed by the Pechini, and characterized by X-ray diffraction, UV-visible and CIE-L colorimetric measures * a * b *., in order to study the effect of doping and thermal treatment on its colorimetric properties. With three different samples of Pr3+ doping (0.01; 0.05 and 0.1 mol%) were prepared and calcined at 500°C, 600°C, 700°C, 800°C and 900°C for 4 h. The analysis of X-ray diffraction confirmed the formation of pure phases with spinel structure and average crystallite size of less than 46 nm. It was found that the colorimetric properties ranging from green to red, in accordance with the increase in the concentration of Pr3+ and thermal processing temperature.

Varistores são elementos que fazem parte do sistema de transmissão e distribuição de energia elétrica ou de instalações elétricas especiais. Estes dispositivos são amplamente produzidos variando a escala, dependendo de sua aplicação, como dispositivos de baixa voltagem, com poucos grãos, exibindo ruptura de alguns volts, a vários kilovolts como usado em para-raios em rede de distribuição de energia. A física de operação dos varistores tem sido amplamente estudada e tem elementos comuns como barreiras Schottky formado na junção metal/semicondutor. Vários estudos têm sido desenvolvidos a fim de obter cerâmicas varistoras para aplicação em alta e baixa tensão. Dentre esses estudos encontram-se os varistores a base de ZnO, SnO2, TiO2 e mais recentemente os de WO3. Diferente das três composições citadas, os a base de WO3 apresentam comportamento varistor intrínseco devido a presença das fases monoclínica e triclínica. A adição de dopantes doadores e aceitadores de elétrons e o tratamento térmico em diferentes atmosferas também alteram as propiedades não lineares desses sistemas, uma vez que influencia na formação da barreira Schottky. Neste estudo foi realizada uma revisão sobre trabalhos relacionados a nova composição de cerâmica varistora a base de óxido de tungstênio (WO3).

Varistors are elements that are part of electric power transmission and distribution systems or of special electrical installations. Varistors are manufactured in a wide variety of types, depending on their application, such as low-voltage devices with a layer of a few grains of thickness and low breakdown voltage, to varistors with a breakdown voltage of several kilovolts, such as those used in the lightning arresters of electric power distribution networks. Varistors, whose physical operation has been extensively studied, share common elements such as metal-semiconductor Schottky barrier junctions. Several studies have focused on the development of varistor ceramics for high and low voltage applications, including ZnO, SnO2, TiO2, and more recently, WO3-based ceramics. Unlike the first three compositions, WO3-based ceramics present an intrinsic varistor behavior because they contain monoclinic and triclinic phases. The addition of electron donor and acceptor dopants and heat treatments in different atmospheres also alter the non-linear properties of these systems, since they affect the formation of the Schottky barrier. This paper offers a review of the literature on the new varistor ceramic composition based on tungsten oxide (WO3).

This article discusses a simple approach for the microwave-assisted synthesis of good quality nanosized calcium titanate crystals doped with trivalent Tm and Yb lanthanide ions. The prepared nanoparticles had a microcube-like structure and a crystallite size of ca. 47 nm. The structure of the nanocrystals was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), and their spectroscopic behavior was examined at a laser excitation wavelength of 350.7 nm. It was found that doping CaTiO3 nanocrystals with Yb and Tm leads to about 250-fold brighter photoluminescence (PL) emissions in the blue and near-infrared regions than that emitted by pure CaTiO3 nanocrystals. The PL emission was correlated with the structural disorder.

Crystalline (BiO)2CO3 nanosheets were synthesized by a rapid one-step reaction via microwave-assisted hydrothermal method using urea as a morphology mediator and carbon source. The hydrothermal method combined with microwave heating allowed to obtain sheet-like (BiO)2CO3 particles at shorter reaction times when compared to the conventional heating hydrothermal method. The photocatalytic activity of the as prepared samples was evaluated towards degradation of Ponceau 4R (C.I. 16255) under artificial UV-Vis light irradiation. The results show that good photocatalytic efficiency can be obtained for powders prepared with reaction times as low as 2 minutes.

Crystalline europium-doped indium hydroxide (In(OH)3:Eu) nanostructures were prepared by rapid and efficient Microwave-Assisted Hydrothermal (MAH) method. Nanostructures were obtained at low temperature. FE-SEM images confirm that these samples are composed of 3D nanostructures. XRD, optical diffuse reflectance and photoluminescence (PL) measurements were used to characterize the products. Emission spectra of europium-doped indium hydroxide (IH:xEu) samples under excitation (350.7 nm) presented broad band emission regarding the indium hydroxide (IH) matrix and 5D0 → 7F0, 5D0 → 7F1, 5D0 → 7F2, 5D0 → 7F3 and 5D0 → 7F4 europium transitions at 582, 596, 618, 653 and 701 nm, respectively. Relative intensities of Eu3+ emissions increased as the concentration of this ion increased from 0, 1, 2, 4 and 8 mol %, of Eu3+, but the luminescence is drastically quenched for the IH matrix.