Paula F. S. Pereira

Rietveld refinement, morphology and optical properties of (Ba1 xSrx)MoO4 crystals

In this article, the structural refinement, morphology and optical properties of barium strontium molybdate [(Ba1−xSrx)MoO4 with x = 0, 0.25, 0.50, 0.75 and 1] crystals, synthesized by the co-precipitation (drop-by-drop) method, are reported. The crystals obtained were structurally characterized by X-ray diffraction (XRD), Rietveld refinement, and Fourier transform–Raman (FT–Raman) and Fourier transform–infrared (FT–IR) spectroscopies. The shapes of the crystals were observed by means of field-emission scanning electron microscopy (FE-SEM). The optical properties were investigated using ultraviolet–visible (UV–Vis) absorption spectroscopy and photoluminescence (PL) measurements. XRD patterns, Rietveld refinement, and FT–Raman and FT–IR spectra showed that all of the crystals are monophasic with a scheelite-type tetragonal structure. The refined lattice parameters and atomic positions were employed to model the [BaO8], [SrO8] and [MoO4] clusters in the tetragonal lattices. The FE-SEM images indicate that increased x content produces a decrease in the crystal size and modifications in the crystal shape. UV–Vis spectra indicated a decrease in the optical band gap with an increase in x in the (Ba1−xSrx)MoO4 crystals. Finally, a decrease in the intensity of PL emission is apparent with an increase in x up to 0.75 in the (Ba1−xSrx)MoO4 crystal lattice when excited by a wavelength of 350 nm, probably associated with the degree of structural order–disorder.

Fenilsilicato dopado com Eu III obtido pelo método sol-gel

In this work, we report the synthesis and the photoluminescence features of a Eu(III)-doped modified silica matrix obtained by the sol-gel method. The matrix was prepared by reaction between tetraethylorthosilicate and phenyltriethoxysilane alkoxide. The hydrolysis occurred using basic catalysis. The solids were treated at 100, 200 and 300 oC during 4 h and the structure was determined by thermogravimetric analysis (TG/DTG), nuclear magnetic resonance (NMR 29Si and 13C), infrared spectroscopy (IR) and photoluminescence (PL). The PL spectra display the Eu(III) lines characteristic of the ion, 5D0 ® 7FJ (J=0, 1, 2, 3, 4), the blue emission as ascribed in the silica matrix. The NMR and TG showed the stability of hybrid silica.

Óxido misto de ítrio-alumínio dopado com Eu(III)

In this work, we report the synthesis and the photoluminescence features of Eu(III)-doped yttrium-aluminium oxide obtained by non-hydrolytic sol-gel routes. After heating the powders above 600 oC the XRD patterns show the presence of the Y4Al2O9 (YAM) and Y3Al5O12 (YAG) phases. At 800 and at 1500 oC the PL spectra display the Eu(III) lines characteristic of the YAM monoclinic phase. The 5D0®7F2 transition is favored relatively to the 5D0®7F1 lines. However, at 1100 oC the cubic YAG is the preferential phase and the 5D0®7F1 transition dominates the spectrum. The Eu(III) ions lie in a centrosymmetrical site. The different solvents used in the sol-gel synthesis also change the relative proportion between these two phases. This is monitored analyzing the modifications in the relative intensity between the 5D0®7F2 and the 5D0®7F1 transitions.

Mechanism of Antibacterial Activity via Morphology Change of α-AgVO3: Theoretical and Experimental Insights

The electronic configuration, morphology, optical features, and antibacterial activity of metastable α-AgVO3 crystals have been discussed by a conciliation and association of the results acquired by experimental procedures and first-principles calculations. The α-AgVO3 powders were synthesized using a coprecipitation method at 10, 20, and 30 °C. By using a Wulff construction for all relevant low-index surfaces [(100), (010), (001), (110), (011), (101), and (111)], the fine-tuning of the desired morphologies can be achieved by controlling the values of the surface energies, thereby lending a microscopic understanding to the experimental results. The as-synthesized α-AgVO3 crystals display a high antibacterial activity against methicillin-resistant Staphylococcus aureus. The results obtained from the experimental and theoretical techniques allow us to propose a mechanism for understanding the relationship between the morphological changes and antimicrobial performance of α-AgVO3.

Influence of the hydrolysis and condensation time on the preparation of hybrid materials

Received: January 9, 2010; Revised: January 12, 2011This work describes the hydrolytic sol-gel synthesis of silica particles prepared by a modified Stober route, using the organofunctionalized alkoxide 3-methacryloxypropyl trimethoxysilane and tetraethylorthosilicate. The silica particles were obtained through basic catalysis from a mixture of precursor alkoxides, water, and isopropyl alcohol. Samples were prepared with change to the pre-hydrolysis and condensation time of tetraethylortosilicate, followed by addition of the organosilane 3-methacryloxypropyl trimethoxysilane. Eu

α-Ag2–2xZnxWO4 (0 ≤ x ≤ 0.25) Solid Solutions: Structure, Morphology, and Optical Properties

A theoretical study was elaborated to support the experimental results of the Zn-doped α-Ag2WO4. Theses α-Ag2-2xZnxWO4 (0 ≤ x ≤ 0.25) solid solutions were obtained by coprecipitation method. X-ray diffraction data indicated that all α-Ag2-2xZnxWO4 (0 ≤ x ≤ 0.25) microcrystals presented an orthorhombic structure. The experimental values of the micro-Raman frequencies were in reasonable agreement with both previously reported and calculated results. Microscopy images showed that the replacement of Ag+ by Zn2+ promoted a reduction in the average crystal size and modifications in the morphology, from rod-like with hexagonal shape to roll-like with a curved surface. A theoretical methodology based on the surfaces calculations and Wulff constructions was applied to study the particle shapes transformations and the surface energy variations in α-Ag2-2xZnxWO4 (0 ≤ x ≤ 0.25) system. The decrease in the band gap value (from 3.18 to 3.08 eV) and the red shift in photoluminescence with the Zn2+ addition were associated with intermediary energy levels between the valence and conduction bands. First-principles calculations with density functional theory associated with B3LYP hybrid functional were conducted. The calculated band structures revealed an indirect band gap for the α-Ag2-2xZnxWO4 models. The electronic properties of α-Ag2WO4 and α-Ag2-2xZnxWO4 microcrystals were linked to distortion effects and oxygen vacancies (VOx) present in the clusters, respectively. Finally, photoluminescence properties of α-Ag2WO4 and α-Ag2-2xZnxWO4 microcrystals were explained by means of distortional effects and oxygen vacancies (VOx) in [AgOy] (y = 2, 4, 6, and 7) and [WO6] clusters, respectively, causing a red shift. Calculations revealed that the substitution for Ag+ with Zn2+ occurred randomly in the α-Ag2WO4 lattice, and it was more favorable on the Ag4 site, where the local coordination of Ag+ cations was four.

Characterization of the calcium-fluoroaluminosilicate glass prepared by a non-hydrolytic sol-gel route for future dental application as glass ionomer cement

Glass ionomer cements are widely employed in dentistry due to their physical, biological and mainly anti-caries properties. Glass ionomers consist of an aluminosilicate glass matrix modified with other elements, and they contain large quantities of fluorine. In this study, we report on the preparation of calcium-fluoroaluminosilicate glasses by a nonhydrolytic sol-gel route as an alternative approach to obtaining alumina-silica matrices. The glass powders were prepared via the non-hydrolytic sol-gel method, by mixing AlCl3, SiCl4, CaF2, AlF3, NaF, and AlPO4. The powders were studied by thermal analysis (TG/DTA/DSC), photoluminescence (PL), nuclear magnetic resonance (NMR27Al-29Si), and X ray diffraction (XRD). TG/DTA/DSC analyses revealed a constant mass loss due to structural changes during the heating process, which was confirmed by NMR and PL. A stable aluminosilicate matrix with potential future application as a glass ionomer base was obtained.