Navegando por Autor "Andres, Juan"
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Artigo Computational procedure to an accurate DFT simulation to solid state systems(Elsevier, 2019-12) Gomes, Eduardo O.; Fabris, Guilherme da Silva Lopes; Ferrer, Mateus M.; Motta, Fabiana Villela da; Delmonte, Maurício Roberto Bomio; Andres, Juan; Silva, Elson Longo da; Sambrano, Júlio RicardoThe density functional theory has become increasingly common as a methodology to explain the properties of crystalline materials because of the improvement in computational infrastructure and software development to perform such computational simulations. Although several studies have shown that the characteristics of certain classes of materials can be represented with great precision, it is still necessary to improve the methods and strategies in order to achieve more realistic computational modeling. In the present work, strategies are reported in a systematic way for the accurate representation of crystalline systems. The crystalline compound chosen for the study as a case test was BaMoO4, both because of its potential technological application and because of the low accuracy of the simulations previously reported in the literature. The computational models were carried out with the B3LYP and WC1LYP functionals selected from an initial set containing eight hybrid functionals in conjunction with an all-electron basis set. Two different strategies were applied for improving the description of the initial models, both involving atomic basis set optimization and Hartree-Fock exchange percentage adjustment. The results obtained with the two strategies show a precision of structural parameters, band gap energy, and vibrational properties never before presented in theoretical studies of BaMoO4. Finally, a flowchart of good calculation practices is elaborated. This can be of great value for the organization and conduction of calculations in new researchArtigo 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 Structural refinement, growth mechanism, infrared/Raman spectroscopies and photoluminescence properties of PbMoO4 crystals(Elsevier, 2013-02-13) Delmonte, Maurício Roberto Bomio; Cavalcante, L. S.; Almeida, Márcio Aurélio Pinheiro; Tranquilin, Ricardo Luis; Batista, Nouga Cardoso; Pizani, Paulo Sérgio; Li, Maximo Siu; Andres, Juan; Silva, Elson Longo daLead molybdate (PbMoO4) crystals were synthesized by the co-precipitation method at room temperature and then processed in a conventional hydrothermal (CH) system at low temperature (70 C for different times). These crystals were structurally characterized by X-ray diffraction (XRD), Rietveld refinement, micro-Raman (MR) and Fourier transformed infrared (FT-IR) spectroscopies. Field emission scanning electron microscopy images were employed to observe the shape and monitor the crystal growth process. The optical properties were investigated by ultraviolet–visible (UV–Vis) absorption and photoluminescence (PL) measurements. XRD patterns and MR spectra indicate that these crystals have a scheelite-type tetragonal structure. Rietveld refinement data possibilities the evaluation of distortions in the tetrahedral 1⁄2MoO4 clusters. MR and FT-IR spectra exhibited a high mode m1(Ag) ascribed to symmetric stretching vibrations as well as a large absorption band with two modes m3(Eu and Au) related to anti-symmetric stretching vibrations in 1⁄2MoO4 clusters. Growth mechanisms were proposed to explain the stages involved for the formation of octahedron-like PbMoO4 crystals. UV–Vis absorption spectra indicate a reduction in optical band gap with an increase in the CH processing time. PL properties of PbMoO4 crystals have been elucidated using a model based on distortions of tetrahedral 1⁄2MoO4 clusters due to medium-range intrinsic defects and intermediary energy levels (deep and shallow holes) within the band gapArtigo 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(Royal Society of Chemistry, 2020) Gomes, E. O.; Gracia, Lourdes; Santiago, Anderson de Azevedo Gomes; Tranquilin, Ricardo Luis; Motta, Fabiana Villela da; Amoresi, Rafael Aparecido Ciola; Silva, Elson Longo da; Delmonte, Maurício Roberto Bomio; Andres, JuanIn 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 materialsArtigo Structure, morphology and photoluminescence emissions of ZnMoO4: RE 3+=Tb3+ - Tm3+ - X Eu3+ (x = 1, 1.5, 2, 2.5 and 3 mol%) particles obtained by the sonochemical method(Elsevier, 2018-06-25) Paskocimas, Carlos Alberto; Lovisa, Laura Ximena; Oliveira, Marisa Carvalho de; Andres, Juan; Gracia, Lourdes; Li, Maximo Siu; Silva, Elson Longo da; Tranquilin, Ricardo Luis; Delmonte, Maurício Roberto Bomio; Motta, Fabiana Villela daZnMoO4 and ZnMoO4: RE3+ = 1% Tb3+, 1% Tm3+, x Eu3+ (x = 1, 1.5, 2, 2.5 and 3 mol%) particles were prepared by a sonochemical method. The influence of the dopant content on photoluminescent behavior was investigated. The X-ray diffraction results confirmed the formation of the α-ZnMoO4 phase with a triclinic crystalline structure. The influence of the chemical compositions on photoluminescence emissions has been studied and the results clearly show the specific emissions of Tb3+ and Eu3+, simultaneously, with a strong contribution of the matrix. Band gap values are in the range of 3.55–4.25 eV. From the values calculated for the CIE coordinates, it was observed that this material develops an emission tendency in the orange-red region. It has been demonstrated for the first time that the sample ZnMoO4: 1% Tb3+, 1% Tm3+, 2% molEu3+, presented higher photoluminescence intensity. At higher concentrations of RE3+, the quenching effect was observed. The morphology of samples are interpreted based on a comparative analysis of the calculated and experimental field emission scanning electron microscopy (FE-SEM) images. First-principle calculations at a density functional theory level were performed to obtain the values of surface energies and relative stability of the (120), (001), (011), (201), and (100) surfaces by employing the Wulff construction. A complete map of the available morphologies of ZnMoO4 and ZnMoO4:12.5%molEu3+ is obtained and a possible explanation for the transformation processes is provided in which the experimental and theoretical morphologies can match. The present study offers a fundamental knowledge that is expected to enable the fabrication of ZnMoO4-based phosphor materials with a controllable emission peak shift and intensityArtigo Toward understanding the photocatalytic activity of PbMoO4 powders with predominant (111), (100), (011), and (110) facets. A combined experimental and theoretical study(American Chemical Society, 2013) Delmonte, Maurício Roberto Bomio; Tranquilin, Ricardo Luis; Motta, Fabiana Villela da; Paskocimas, Carlos Alberto; Nascimento, Rubens Maribondo do; Gracia, Lourdes; Andres, Juan; Silva, Elson Longo daA complementary combination of experimental work and first-principle calculations, based on the density functional theory (DFT) method, has been used to increase our limited understanding of the enhanced photocatalytic activity of PbMoO4 powders with predominant (111), (100), (011), and (110) facets. In this work, PbMoO4 powders were prepared by the coprecipitation method and processed on a hydrothermal reactor at 100 °C/10 min. The variation of different types of modifiers such as acetylacetone (acac) or polyvinylpyrrolidone (PVP) is found to play a crucial role in controlling the particle size and morphology of products and their photocatalytic properties. The structure and morphology of these crystals were characterized by X-ray diffraction (XRD), micro-Raman (MR) spectroscopy, field-emission gun scanning electron microscopy (FEG-SEM), and ultraviolet visible (UV–vis) absorption spectroscopy. Furthermore, the as-synthesized PbMoO4 micro-octahedrons without the presence of the (001) surface exhibit enhanced activity for the photodegradation of rhodamine B (RhB) under ultraviolet–visible light irradiation. On the basis of the theoretical and experimental results, we provide a complete assignment of the micro-Raman spectra of PbMoO4, while a growth mechanism for the formation of PbMoO4 micro-octahedrons was systematically discussed. A schematic illustration of the probable formation of morphologies in the whole of the synthetic process was also proposed, which reveals that the high photocatalytic activity is attributed to the absence of the (001) facetArtigo 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