Navegando por Autor "Almeida, Cláudio Romero Rodrigues de"
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Artigo 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 High energy mill processing of polymer based nanocomposites(SAGE Publications, 2008-08-07) Melo, José Daniel Diniz; Almeida, Cláudio Romero Rodrigues de; Paskocimas, Carlos Alberto; Mendonça, Rannier Marques; Medeiros, Antonio M.Polymer matrix nanocomposites have attracted growing attention due to the potential for significantly improving the properties of the polymer by adding a very small amount of nanoparticles. However, the improvement in properties has been related to the degree of dispersion of the nanoparticles in the polymer matrix and, due to their enormous specific surface area, nanoparticles tend to agglomerate. Thus, processing techniques able to produce complete particle dispersion in polymer matrix are of great interest. The purpose of this work is to present a new processing technique for polymer matrix nanocomposites using a high energy mill as an effective approach to disperse ceramic nanoparticles in a polymer matrix. SiO2/epoxy nanocomposites were processed with various SiO2 contents using the proposed approach. Transmission electron microscopy (TEM) micrographs of the nanocomposites processed indicated good particle dispersion. In addition, agglomerates were not observed on the scanning electron microscopy (SEM) fractographs of the nanocomposites, up to 3wt% SiO2. The processed nanocomposites were also characterized by dynamic mechanical analysis (DMA) to investigate the effect of nanoparticles content on the viscoelastic properties and on the glass transition temperature. In summary, the technique was found promising in achieving good levels of particle dispersion in a thermoset polymer matrixArtigo One-step synthesis of CaMoO4: Eu3+ nanospheres by ultrasonic spray pyrolysis(Springer, 2017-08-02) Paskocimas, Carlos Alberto; Almeida, Cláudio Romero Rodrigues de; Lovisa, Laura Ximena; Santiago, Anderson de Azevedo Gomes; Li, Maximo Siu; Silva, Elson Longo da; Motta, Fabiana Villela da; Delmonte, Maurício Roberto BomioIn this paper, CaMoO4 and of Eu3+-doped CaMoO4 nanospheres were prepared by a spray pyrolysis method and characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Raman spectra and photoluminescence spectroscopy. XRD patterns indicate that these crystals have a scheelite-type tetragonal structure. The morphology of the CaMoO4: Eu3+ nanospheres was investigated from the FESEM results and it was observed the formation of spherical particles peculiar to the synthesis method used. Sample of pure CaMoO4 thermally treated 1000 °C exhibit a photoluminescence behavior broadband type with emission in white light according to the calculated values of the CIE coordinates: x=0.37, y=0.37. The emission intensity in the red for samples doped with Eu3+ (5 D0 → 7 F2 in 615 nm) increases directly with the dopant concentrationArtigo Photoluminescent properties of the Ba1−xZnxMoO4 heterostructure obtained by ultrasonic spray pyrolysis(Elsevier, 2018-03) Paskocimas, Carlos Alberto; Santiago, Anderson de Azevedo Gomes; Almeida, Cláudio Romero Rodrigues de; Tranquilin, Ricardo Luis; Nascimento, Rubens Maribondo do; Silva, Elson Longo da; Motta, Fabiana Villela da; Bomio, Mauricio Roberto BomioIn this study, Ba1−xZnxMoO4 (x = 0, 0.25, 0.5, 0.75 and 1) microspheres were the first time described in the literature the by ultrasonic spray pyrolysis method. Powders were characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM-FEG) and UV–visible and Photoluminescence (PL) spectroscopies. The XRD patterns showed that the samples with x = 0 and x = 1 displayed pure tetragonal and triclinic structures, respectively. On the other hand, samples with x = 0.25, 0.5 and 0.75 revealed the formation of tetragonal/ triclinic heterojunction structure. The SEM-FEG images showed that the Ba1−xZnxMoO4 particles present the spherical morphology with the decrease of particle sizes when the value of “x” increase. The gap energies vary between 4.56 eV and 4.17 eV, being influenced by the degree of structural order-disorder. The PL emission spectra of the samples showed a broad band behavior with emission intensity predominant in the orange-red region and of a lower intensity in the blue-green region. The chromaticity coordinates x and y showed that the samples with x ≤ 0.75 present emission in orange, while the samples with x = 1 showed emission in whiteArtigo Presence of excited electronic states on terbium incorporation in CaMoO4: Insights from experimental synthesis and first-principles calculations(Elsevier, 2021-02) Tranquilin, Ricardo Luis; Oliveira, Marisa Carvalho de; Santiago, Anderson de Azevedo Gomes; Lovisa, Laura Ximena; Ribeiro, Renan Augusto Pontes; Silva, Elson Longo da; Lazaro, Sergio R. de; Almeida, Cláudio Romero Rodrigues de; Paskocimas, Carlos Alberto; Motta, Fabiana Villela da; Delmonte, Maurício Roberto BomioWe present a combined experimental and theoretical study to understand the structure and electronic and optical properties of CaMoO4:xTb3+ (x = 1 mol%, 2 mol%, and 4 mol%) microspheres. The microspheres were prepared by ultrasonic spray pyrolysis and characterized by X-ray diffraction (XRD), field-emission gun scanning electron microscopy (FEG-SEM), micro Raman spectroscopy, and photoluminescence (PL) spectroscopy. First-principles quantum mechanical calculations were performed at the density functional theory level to obtain the geometry and electronic properties of CaMoO4:xTb3+ microspheres in the ground electronic state and excited electronic states (singlet and triplet). These results, combined with XRD patterns, indicate that these crystals have a scheelite-type tetragonal structure. The morphology of the CaMoO4:xTb3+(x = 1 mol%, 2 mol%, and 4 mol%) samples was investigated by FEG-SEM, and a spherical shape was found. The optical properties were investigated by UV–vis spectroscopy and PL spectroscopy, and the chromaticity coordinates of these compounds were obtained. The relationships between the PL properties and the Raman spectra indicate that Tb3+-doped CaMoO4 microspheres constitute promising photoluminescent materials for use in new lighting devices. This also allowed us to understand the charge transfer process that happens in the singlet (s) ground state and the singlet (s*) and triplet (t*) excited states, which generates the photoluminescent emissions of the Tb3+-doped CaMoO4 microspheresTese Síntese e caracterização das propriedades fotoluminecentes do CaMoO4 com terras raras obtidos pela técnica de spray pirólise ultrassônico(2017-08-31) Almeida, Cláudio Romero Rodrigues de; Paskocimas, Carlos Alberto; http://lattes.cnpq.br/2365059843175411; http://lattes.cnpq.br/2030208218019912; Silva, Elson Longo da; http://lattes.cnpq.br/9848311210578810; Motta, Fabiana Villela da; http://lattes.cnpq.br/9918299069511517; Delmonte, Mauricio Roberto Bomio; http://lattes.cnpq.br/9558299312183852; Melo, Maxymme Mendes de; http://lattes.cnpq.br/3779445657082353Os materiais que pertencem a família dos molibdatos com estrutura cristalina tipo scheelita têm sido objeto de pesquisas ao longo do tempo. O amplo interesse nestes compostos reside em suas excelentes propriedades ópticas, que formam a base de sua utilização como substâncias fosforescentes, materiais para lasers, e detectores de cintilação entre outros. A técnica de spray pirólise ultrassônica, por sua vez, é uma técnica de produção contínua de pós, de apenas uma etapa, de fácil controle dos parâmetros e apesar de ser uma metodologia amplamente utilizada, não há referências de seu uso para a produção de molibdatos de cálcio puro e com dopagem de terras raras. Neste trabalho foi estudado a obtenção de partículas de molibdato de cálcio (CaMoO4) puro, dopado com diferentes porcentagens em mol de 1%, 2% e 4% das terras raras Eu+3, Tb+3, Tm+3, e a mistura das três terras raras (TR) a partir da técnica de spray pirólise ultrassônico (SPU). O tempo de permanência dentro do reator de fluxo laminar, foi de 27 segundos Os materiais obtidos foram caracterizados pelas técnicas de difração de raios-X, espectroscopia de micro Raman, espectroscopia ótica nas regiões do ultravioleta-visível, microscopia eletrônica de varredura com canhão de elétrons por emissão de campo e espectroscopia de fotoluminescência e coordenadas de cor do CaMoO4 com TR. Os resultados indicaram que a técnica de spray pirólise ultrassônico produziu material de alta pureza, nas dimensões nano e micro, com morfologia esférica. Com a adição das terras raras com diferentes porcentagens em mol de 1%, 2% e 4% e sua TR na estrutura do CaMoO4, foi possível observar as emissões de fotoluminescência características das terras raras e a obtenção de materiais com emissões no branco para as amostras de CaMoO4 com 2% e 4% da TR.Artigo Understanding the white-emitting CaMoO4 Co-Doped Eu3+, Tb3+, and Tm3+ phosphor through experiment and computation(American Chemical Society, 2019-07-09) Paskocimas, Carlos Alberto; Tranquilin, Ricardo Luis; Lovisa, Laura Ximena; Almeida, Cláudio Romero Rodrigues de; Li, Maximo Siu; Oliveira, Marisa Carvalho de; Gracia, Lourdes; Andres, Juan; Silva, Elson Longo da; Motta, Fabiana Villela da; Delmonte, Maurício Roberto BomioIn this article, the synthesis by means of the spray pyrolysis method, of the CaMoO4 and rare-earth cation (RE3+)-doped CaMoO4:xRE3+ (RE3+ = Eu3+, Tb3+, and Tm3+; and x = 1, 2, and 4% mol) compounds, is presented. The as-synthesized samples were characterized using X-ray diffraction, Rietveld refinement, field emission scanning electron microscopy (FE-SEM), Raman spectroscopy, and photoluminescence (PL) spectroscopy. To complement and rationalize the experimental results, first-principles calculations, at the density functional theory level, have been performed to analyze the band structure and density of states. In addition, a theoretical method based on the calculations of surface energies and Wulff construction was applied to obtain the morphology transformation of the CaMoO4 and CaMoO4:RE3+ microstructures. The experimental morphologies can be observed in the FE-SEM images. The PL behavior of the Co-doped samples exhibited well-defined bands in the visible region. The samples with 2 and 4% of RE3+ released white emission according to the chromaticity coordinates (0.34, 0.34) and (0.34, 0.33), respectively. The present results provide not only a deep understanding of the structure−property relationships of CaMoO4-based phosphor but also can be employed as a guideline for the design of the electronic structure of the materials and the fabrication of photofunctional materials with optimal properties, which allows for the modeling of new phosphors for applications in solid-state lighting