Gadolinium luminescent materials obtained by spray pyrolysis, co-precipitation, and non-hydrolytic sol-gel route: Structure and optical properties

cited By 0International audienceImproving luminescent materials has been the focus of researchers working in the areas of solid-state lasers, scintillators for medical imaging, and phosphors, among others. Lanthanide materials have wide application in optical fibers, amplifiers, and display devices: the inner shell electronic transitions between the 4f-4f energy levels of the lanthanide ions (Ln3+) provide them with excellent luminescent properties. We prepared the inorganic matrixes GdAlO3, Gd2O3, GdCaAl3O7, GdVO4, GdNbO4, and Gd2O2S doped with europium ions (Eu3+) in three different ways: (1) non-hydrolytic sol-gel route, by reacting the precursor metal chlorides in ethanol; (2) spray pyrolysis, using aqueous solution; and (3) co-precipitation synthesis, by reacting the precursor metal nitrates with urea in water, followed by various heat treatments. We characterized the resulting powders by thermal analyses, X-ray diffraction (XRD) analysis, scanning electron microscopy, and Eu3+ photoluminescence. The thermogravimetric curves showed mass loss, attributed to water molecules weakly bound to the oxide surface, solvent molecules, pyrolysis of organic matter remaining from the synthesis, and structural arrangement. The differential thermogravimetric curves evidenced exothermic peaks at different temperatures, associated with the structural rearrangement of the matrixes. The samples were thermally treated; the XRD patterns presented peaks ascribed to the crystalline phases of GdAlO3, GdCaAl3O7, Gd2O3, Gd2O2S, GdVO4, and GdNbO4, depending on the matrix. These results confirmed that the methodologies used to synthesize the materials were efficient. The morphological study revealed particles with different sizes and shapes: spray pyrolysis, co-precipitation synthesis, and the non-hydrolytic sol-gel methodology furnished particles measuring around 500 nm, spherical particles of around 100 nm, and irregular particles with less than 100 nm, respectively. For all the matrixes, the excitation spectra of Eu3+ displayed a broad band at shorter wavelength, assigned to the charge transfer between the ligand and the metal (O2-→ Eu3-or S2-→ Eu3-) and to the f-f transitions of the Eu3-excited state. Some spectra presented a band relative to the excited state of the Gd3+ ion, indicating Gd3+ → Eu3-energy transfer. The emission spectra of Eu3+, excited at different wavelengths, exhibited narrow lines between 500 and 750 nm, relative to the typical transition from the excited level to manifold level (5D0 → 7F2, with J = 0, 1, 2, 3, and 4). The more intense band of the matrixes corresponded to the hypersensitive transition 5D0 → 7F2, with dipole-electric character, detected with maximum at approximately 615 nm. The synthetic processes used here furnished gadolinium matrixes with excellent luminescent red emission; in some matrixes, Gd3+ efficiently transferred energy to Eu3+. Therefore, these materials have promising technological applications in luminescent devices