Navegando por Autor "Araújo, Vinícius Dantas"
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Artigo Effect of calcium on the structural properties of Ba(1−x)Ca x TiO3 particles synthesized by complex polymerization method(Springer, 2014-01-14) Araújo, Vinícius Dantas; Motta, Fabiana Villela da; Marques, Ana Paula de Azevedo; Paskocimas, Carlos Alberto; Delmonte, Maurício Roberto Bomio; Silva, Elson Longo da; Varela, José A.Ferroelectric materials, such as barium titanate (BaTiO3), have been extensively studied for application in electronic and optical devices. The substitution of Ba by Ca is an effective method to improve the piezoelectricity temperature stability, as it can greatly lower the tetragonal–orthorhombic phase transition temperature, whereas the change of the Curie point is negligible. Ba(1−x)Ca x TiO3 (x = 0, 0.05, 0.10, 0.15, and 0.20) powders were prepared by complex polymerization method. The effect of calcium on the tetragonality of the BaTiO3 system was monitored using basic characterization techniques: X-ray diffraction, differential scanning calorimetry, and Raman spectroscopy. The results indicate that increased calcium contents raise the Curie temperature (T c) and that the addition of calcium in the BT matrix reduces tetragonalityArtigo Effect of polyvinyl alcohol on the shape, photoluminescence and photocatalytic properties of PbMoO4 microcrystals(Elsevier, 2014-10) Araújo, Vinícius Dantas; Tranquilin, Ricardo Luis; Motta, Fabiana Villela da; Paskocimas, Carlos Alberto; Bernardi, Maria Inês Basso; Cavalcante, L.S.; Andres, Juan; Silva, Elson Longo da; Delmonte, Maurício Roberto BomioFor this study, lead molybdate (PbMoO4) microcrystals were prepared by the co-precipitation method and processed using a conventional hydrothermal method at 100 °C for 10 min with polyvinyl alcohol (PVA) as the capping agent. These microcrystals were structurally characterized by X-ray diffraction (XRD) and micro-Raman spectroscopy, and their morphology was investigated by field-emission gun scanning electron microscopy (FEG-SEM). The optical properties were analyzed by ultraviolet–visible (UV–vis) absorption spectroscopy and photoluminescence (PL) measurements. XRD patterns and MR spectrum indicate that the PbMoO4 microcrystals have a scheelite-type tetragonal structure. FE-SEM images reveal that the PVA promotes the aggregation of several octahedrons and the formation of large porous stake-like PbMoO4 microcrystals which are related to the oriented attachment growth process. Moreover, the effect of the capping agent hinders the growth of a large amount of micro-octahedrons which can be verified with by several nanocrystals on large crystals. Intense green PL emission was observed at room temperature for PbMoO4 microcrystals which are related to structural defects at medium range and intermediary energy levels between the valence band (VB) and the conduction band (CB). Photocatalytic activity was observed for PbMoO4 as a catalyst in the degradation of the rhodamine B (RhB) dye, achieving total degradation after 90 min under UV-lightArtigo Enhancement of the photocatalytic activity and white emission of CaIn2O4 nanocrystals(Elsevier, 2016-02-15) Paskocimas, Carlos Alberto; Melo, M. M.; Araújo, Vinícius Dantas; Tranquilin, Ricardo Luis; Almeida, Cláudio Romero Rodrigues de; Delmonte, Maurício Roberto Bomio; Tavares, Mara T. S.; Silva, Elson Longo da; Motta, Fabiana Villela daCaIn2O4 (CIO) nanocrystals (NCs) were prepared by ultrasonic spray pyrolysis at 1223 K without the use of surfactants to investigate the influences of the nanostructure on the photoluminescence and photo-catalytic properties of this white-light emitter. The optical properties were analyzed by ultraviolet evisible (UVevis) absorption spectroscopy, which indicated a band gap energy of 3.83 eV, and photo-luminescence (PL) measurements at room temperature that showed a broad and intense emission band. X-ray diffraction (XRD) analysis confirmed that the CIO NCs adopted the orthorhombic crystalline phase. Field-emission gun scanning electron microscopy (FEG-SEM) micrographs demonstrated the spherical morphology of the CIO NCs, comprising aggregates of several CIO NCs. Furthermore, the as-synthesized CIO NCs exhibited enhanced activity for the photodegradation of methylene blue (MB) under UVevis irradiation. The chromaticity coordinates were calculated for the sample based on the PL spectrum; the CIO NCs had values of x 1⁄4 0.31 and y 1⁄4 0.38, and this point is located on the white region of the CIE diagram. Ultrasonic spray pyrolysis provides a feasible approach for preparing shape- and size-controlled CIO nanocrystals that hold great potential for photocatalytic applications and as photoluminescent white emittersArtigo Fast photocatalytic degradation of an organic dye and photoluminescent properties of Zn doped In(OH)3 obtained by the microwave-assisted hydrothermal method(Elsevier, 2014-11) Tavares, Marileide L.A.; Lovisa, Laura Ximena; Araújo, Vinícius Dantas; Silva, Elson Longo da; Li, Maximo Siu; Nascimento, Rubens Maribondo do; Paskocimas, Carlos Alberto; Delmonte, Maurício Roberto Bomio; Motta, Fabiana Villela daCrystalline zinc-doped indium hydroxide structures were prepared by the rapid and efficient microwave-assisted hydrothermal (MAH) method. X-ray diffraction (XRD), field emission gun-scanning electron microscopy (FEG-SEM), photoluminescence (PL) and UV–visible (UV–vis) spectroscopy were used to characterization the materials. FEG-SEM images revealed that pure In(OH)3 samples and Zn-doped samples exhibit nanocubic morphologies with a wide range of particle sizes. Relative intensities of PL emissions decreased as the Zn ion concentration increased from 0 to 4 mol%. UV–vis spectra indicate that Zn ion doping caused a band gap decrease with increased doping of 0% to 4% Zn, respectively, from 5.15 eV for 4.96 eV. Samples with Zn in the crystal lattice showed better photocatalytic activity and degradation of the RhB dye after 16 min of UV exposureArtigo Microwave-assisted hydrothermal synthesis of magnetite nanoparticles with potential use as anode in lithium ion batteries(UFSCar, 2014-09-09) Xavier, Camila Soares; Paskocimas, Carlos Alberto; Motta, Fabiana Villela da; Araújo, Vinícius Dantas; Aragón, Maria José; Tirado, José Luís; Lavela, Pedro; Silva, Elson Longo da; Delmonte, Maurício Roberto BomioRechargeable solid-state batteries have long been considered an attractive power source for a wide variety of applications, and in particular, lithium-ion batteries are emerging as the technology of choice for portable electronics. One of the main challenges in the design of these batteries is to ensure that the electrodes maintain their integrity over many discharge-recharge cycles. Fe3O4 deserves great attention as one of the most important electrode active materials due to its high theoretical capacity (926 mAhg - 1), low cost, being environmental-friendly and naturally abundance in worldwide. A simple strategy to synthesize magnetite nanoparticles (Fe3O4) by microwave-assisted hydrothermal method in a short processing time without further treatment is reported. The material obtained was tested as anode active material for lithium ions batteries. Impedance spectroscopy revealed that small differences in cell performance on cycling observed between samples cannot be strictly correlated to cell resistance. A high reversible capacity of 768.5 mAhg - 1 at 1C over 50 cycles was demonstrated, suggesting its prospective use as anode material for high power lithium ion batteriesArtigo Photoluminescent properties of ZrO2: Tm3+, Tb3+, Eu3+ powders — A combined experimental and theoretical study(Elsevier, 2017-02-25) Paskocimas, Carlos Alberto; Lovisa, Laura Ximena; Andrés, Juan; Gracia, Lourdes; Li, Maximo Siu; Delmonte, Maurício Roberto Bomio; Araújo, Vinícius Dantas; Silva, Elson Longo da; Motta, Fabiana Villela daRare-earth (RE) element-based materials for optical applications have received increasing attention owing to the emission properties of RE ions, which render these materials suitable for use in color displays, lasers, and solid-state lighting. In the present work, ZrO2:RE (RE = Tm3+, Tb3+, and Eu3+) powders were obtained via complex polymerization, and characterized by means of X-ray diffraction (XRD), Raman spectroscopy, UV–visible absorption spectroscopy, and photoluminescence measurements. The XRD patterns and Raman spectra revealed the tetragonal phase of ZrO2 co-doped with up to 4 mol.% RE3+ and stabilization of the cubic phase, for up to 8 mol.% RE3+. In addition, the photoluminescence measurements revealed simultaneous emissions in the blue (477 nm), green (496.02 nm and 548.32 nm), and red-orange (597.16 nm and 617.54 nm) regions. These emissions result from the Tm3+, Tb 3+, and Eu3+ ions, respectively. Energy transfers, such as 1G4 levels (Tm3+) → 5D4 (Tb3+) and 5D4 levels (Tb3+) → 5D0 (Eu3+), occurred during the emission process. Calculations based on density functional theory (DFT) were performed, to complement the experimental data. The results revealed that structural order/disorder effects were generated in the cubic and tetragonal ZrO2 phases in the ZrO2:Eu3+ powders, and changes in the electronic structure were manifested as a decrease in the band gap values. The chromaticity coordinates of all the samples were determined from the PL spectrum. The coordinates, x = 0.34 and y = 0.34, of the ZrO2:8%RE sample corresponded to a point located in the white region of the CIE diagram and color correlated temperature (CCT) was found to be 5181 K. More importantly, the present results indicate that ZrO2:RE powders constitute promising photoluminescent materials for use in new lighting devices.