Abstract
Mesoporous spherical Yb–Er-doped \(\hbox {TiO}_{2}\) nanoparticles were prepared by sol–gel method. The structure and morphology of the nanoparticles were characterized using Raman, Fourier transform infrared spectroscopies, transmission electron microscopy and by low-temperature \(\hbox {N}_{2}\) adsorption. It is shown that both anatase (tetragonal) and brookite (orthorhombical) phases are present in the titania nanoparticles. Their diameter size is between 12 and 15 nm and an average surface area of 136 \(\hbox {m}^{2}\)\(\hbox {g}^{-1}\). Under infrared irradiation, the nanoparticles show luminescence by an upconversion process of the ytterbium and erbium ions, the green emission corresponds to \(^{2}\hbox {H}_{11/2}+{}^{4}\hbox {S}_{3/2} \rightarrow {}^{4}\hbox {I}_{15/2 }\) transition and for the red emission, the transition energy is: \(^{4}\hbox {F}_{7/2}\)–\({}^{4}\hbox {I}_{15/2}\). The green and red photoluminescence intensities are highly dependent on the OH amount, which is produced during the hydrolysis and condensation processes and depends on the reaction time, nanoparticles wash and annealing temperature. The influence of synthesis parameters on the properties of porosity and luminescence was studied by the Plackett–Burman experimental design.
