Controlled synthesis and luminescence properties of doped NaLa(WO4)2 microstructures

Abstract

In this work, well-defined NaLa(WO4)2 microstructures were prepared by a facile hydrothermal process in the presence of oleylamine and oleic acid as a mixed surfactant. The phase, morphology, size and luminescence properties were well characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL) spectra, and decay time, respectively. The results reveal that the morphology and size of the as-synthesized samples can be tuned by altering the reaction time and the amounts of oleic acid. A possible formation mechanism of tetragonal NaLa(WO4)2 is proposed on the basis of oleic acid amount-dependent experiments. Upon ultraviolet (UV) excitation, NaLa(WO4)2:Eu3+ and NaLa(WO4)2:Tb3+ exhibit the characteristic down-conversion emissions of Eu3+ and Tb3+. Yb3+/Er3+ and Yb3+/Tm3+ co-doped NaLa(WO4)2 samples show strong green (Er3+, 4S3/2, 2H11/2→4I15/2) and blue (Tm3+, 1G4→3H6) up-conversion emission under 980nm laser diode excitation. Upconversion emission mechanisms of NaLa(WO4)2:Ln (Ln=Yb/Er, Yb/Tm) are also presented based on the emission spectra and the plot of luminescence intensity to pump power. It should be noted that the intensity of the upconversion emissions can be dramatically enhanced compared with sodium free tungstates, which can be attributed to the charge compensation and means the introduction of Na+ ions in WO42− causes less distortion in the crystal structure.