Structural and optical characterizations of Ca2Al2SiO7:Ce3+, Mn2+ nanoparticles produced via a hybrid route

Abstract

Pure, Ce3+ doped and Ce3+ and Mn2+ co-doped Ca2Al2SiO7 ceramic powders were prepared by two different methodologies which are the proteic sol–gel process and a new hybrid route combining the proteic sol–gel with solid state reaction processes. The second one is an eco-friendly method because it uses natural raw materials in replacement of the metal alkoxides used in the traditional sol–gel routes. X-ray diffraction showed that Ca2Al2SiO7 crystalline phase was obtained for both preparations. Differential thermal analysis indicated that the exothermic event around 850°C, for sample produced by proteic sol–gel method, and around 927°C, for ceramics prepared by hybrid synthesis, can be associated to crystallization of Ca2Al2SiO7. Transmission electron microscope indicates that regular and spherical nanoparticles were obtained with average sizes of about 12nm. The Scherrer’s method was used to determine the average crystallite sizes and it was shown that nanometric crystallites were obtained of about 74nm for samples produced via hybrid route. For all the single phase samples, the crystallite sizes are about the same and that agrees with TEM results. Diffuse optical reflectance measurements were used to estimate the Ca2Al2SiO7 optical band gap and the obtained value is about 6eV, photoluminescence (PL) spectra presented typical emissions of Ce3+ and Mn2+ ions. Upon excitation at 352nm the emission spectra showed a broad band centered at 415nm due to the Ce3+ 4f1→5d1 typical transition. This emission is resonant with Mn2+ excitation and it transfers energy to Mn ions generating a second broad emission band centered at 620nm due to the Mn2+. The PL results were used to obtain, as a fist approach, the Ce3+ energy levels diagram and, using the Tanabe–Sugano diagrams, the transitions due to the Mn2+ were calculated. X-ray excited optical luminescence measurements showed the same emission spectra as the PL emission spectra. Luminescence lifetime decay constants were measured for Ce and Mn co-doped and for Ce doped samples and the results indicate Ca2Al2SiO7:Ce3+, Mn2+ showed quite fast responses with main time constants below 30ns.