Silica spheres coated with YVO4:Eu3+ layers via sol-gel process:: A simple method to obtain spherical core-shell phosphors

Spherical SiO2 particles have been successfully coated with YVO4:Eu3+ phosphor layers through a Pechini sol-gel process. The resulted YVO4:Eu3+ @ SiO2 core-shell phosphors were characterized by X-ray diffraction (XRD), Fourier-transform IR spectroscopy, scanning electron microscopy, X-ray photoelectron spectra, transmission electron microscopy, UV/vis absorption spectra, general and time-resolved photoluminescence spectra, as well as kinetic decays. The XRD results demonstrate that the YVO4:Eu3+ layers begin to crystallize on the SiO2 particles after annealing at 400 degrees C, and the crystallinity increases with raising the annealing temperature. The obtained core-shell phosphors have perfect spherical shape with narrow size distribution (average size ca. 500 nm), nonagglomeration, and smooth surface. The thickness of the YVO4:Eu3+ shells on SiO2 cores could be easily tailored by varying the number of deposition cycles (60 nm for two deposition cycles). The Eu3+ shows a strong photoluminescence (PL) (dominated by D-5(0)-F-7(2) red emission at 617 nm) due to an efficient energy transfer from vanadate groups to Eu3+. The energy transfer process was further studied by the time-resolved emission spectra as well as kinetic decay curves of Eu3+ upon excitation into the VO43- ion. The PL intensity of Eu3+ increases with raising the annealing temperature and the number of coating cycles, and optimum polyethylene glycol concentration in the precursor solution was determined to be 0.08 g/mL for obtaining the strongest emission of Eu3+.