Shape-selective synthesis, characterization and upconversion improvement of Yb3+/Er3+ doped LiYF4 microphosphors through pH tuning
Yttrium fluoride compounds with different crystal phases and morphologies, including rice-like orthorhombic-phase YF3:Yb3+/Er3+, and octahedral and quasi-spherical tetragonal-phase LiYF4:Yb3+/Er3+ microcrystals have been synthesized under hydrothermal conditions by tuning the pH value of the mother solution. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, and photoluminescence spectra were used to characterize the samples. It is found that the pH value in the initial reaction solution has significant effects on selective adsorption of the organic additive EDTA on different crystal facets controlling the final sizes and geometries of the particles. Meanwhile the pH value in conjunction with hydrothermal reaction time affects the solubility constants and thermodynamic stability of reactants and products, further triggering the phase transformation from orthorhombic-phase YF3 to tetragonal-phase LiYF4 and controlling the nucleation and anisotropic growth of fluoride microcrystals. The possible formation mechanisms for products with various architectures have been presented. The multicolor upconversion (UC) luminescence was successfully realized in a series of Yb3+/Er3+ codoped fluoride microcrystals by excitation in the NIR region, and the UC emissions were investigated as a function of the pH value. Results show that the UC luminescence of microparticles sharply increases with pH elevation. The enhancement was attributed to Li+ interstitials in the channels of LiYF4, which is consistent with the growth mechanism of LiYF4 microcrystals, while the possibility of doping Li+ in the cation vacancies of LiYF4 cell cannot be fully excluded because of the presence of the cation defects. The study further demonstrates that the pH tuning approach can be used as an efficient tool to control the crystal phase, size and the improvement of luminescence emission for fluoride microcrystals.