Short communicationMicrowave-modified sol-gel process of NaY(WO4)2: Ho3+/Yb3+ phosphors and the upconversion of their photoluminescence properties
Introduction
The photoluminescence particles have evolved in their applications, such as fluorescent lamps, cathode ray tubes, solid-state laser, amplifiers for fiber optics communication and new optoelectronic devices, which show high luminescence quantum yields, since usually more than one metastable excited state exists, multiple emissions are observed [1], [2], [3]. The double tungstates of MR(WO4)2 (M=Li+, Na+, K+; R=La3+, Gd3+, Y3+) possess the tetragonal scheelite structure with the space group I41/a, and belong to the family of double tungstates compounds. It is possible for the trivalent rare-earth ions in the disordered tetragonal-phase to be partially substituted by Ho3+ and Yb3+ ions, these ions are effectively doped into the crystal lattices of the tetragonal phase due to the similar radii of the trivalent rare-earth ions in R3+, this results in high red emitting efficiency, and superior thermal and chemical stability. In these compounds, W6+ is coordinated by four O2− at a tetrahedral site, which makes [WO4]2− relatively stable. R3+ and M+ are randomly distributed over the same cationic sublattice, and they are coordinated by eight O2− from near four [WO4]2− with a symmetry S4 without an inversion center [4], [5], [6]. The [WO4]2− group has strong absorption in the near ultraviolet region, so that energy transfers process from [WO4]2− group to rare-earth ions can easily occur, which can greatly enhance the external quantum efficiency of rare-earth ions doped materials. Among rare-earth ions, the Ho3+ ion is suitable for converting infrared to visible light through the UC process due to its appropriate electronic energy level configuration. The co-doped Yb3+ ion and Ho3+ ion can remarkably enhance the UC efficiency for the shift from infrared to visible light due to the efficiency of the energy transfer from Yb3+ to Ho3+. The Yb3+ ion, as a sensitizer, can be effectively excited by an incident light source energy. This energy is transferred to the activator from which radiation can be emitted. The Ho3+ ion activator is the luminescence center of the UC particles, while the sensitizer Yb3+ enhances the UC luminescence efficiency [7], [8], [9].
NaY(WO4)2:Ho3+/Yb3 phosphors have been developed to prepare including solid-state reactions [10], [11], the hydrothermal method [12], [13], and the Czochralski method [14], [15]. For practical application of UC photoluminescence in products morphology features need to be well defined. Usually, double tungstates are prepared by a solid-state method that requires high temperatures, lengthy heating process and subsequent grinding, which results in loss of the emission intensity and an increase in cost. Sol-gel process provides some advantages over the conventional solid-state method, including good homogeneity, low calcination temperature, small particle size and narrow particle size distribution optimal for good luminescent characteristics. However, the sol-gel process has a disadvantage in that it takes a long time for gelation. As compared with the usual methods, microwave-modified sol-gel synthesis has the advantages of very short reaction time, homogeneous morphology features and high purity of final polycrystalline samples [16], [17], [18]. Microwave heating is delivered to the material surface by radiant and/or convection heating, which is transferred to the bulk of the material via conduction [19]. Microwave-modified sol-gel process is a cost-effective method that provides high-quality luminescent materials with easy scale-up in short time periods. However, the synthesis of NaY(WO4)2:Ho3+/Yb3 phosphors by the microwave-modified sol-gel method has not been reported.
In this study, NaY1−x(WO4)2:Ho3+/Yb3+ phosphors with doping concentrations of Ho3+ and Yb3+ (x=Ho3++Yb3+, Ho3+=0.05, 0.1, 0.2 and Yb3+=0.2, 0.45) phosphors were prepared by the microwave-modified sol-gel route followed by heat treatment. The synthesized particles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). The optical properties were examined comparatively using photoluminescence (PL) emission and Raman spectroscopy.
Section snippets
Experimental
Appropriate stoichiometric amounts of Na2WO4∙2H2O (99%, Sigma-Aldrich, USA), Y(NO3)3∙6H2O (99%, Sigma-Aldrich, USA), (NH4)6W12O39∙xH2O (99%, Alfa Aesar, USA), Ho(NO3)3∙5H2O (99.9%, Sigma-Aldrich, USA), Yb(NO3)3∙5H2O (99.9%, Sigma-Aldrich, USA), citric acid (99.5%, Daejung Chemicals, Korea), NH4OH (A.R.), ethylene glycol (A.R.) and distilled water were used to prepare NaY(WO4)2, NaY0.8(WO4)2:Ho0.2, NaY0.7(WO4)2:Ho0.1Yb0.2 and NaY0.5(WO4)2:Ho0.05Yb0.45 compounds with doping concentrations of Ho3+
Results and discussion
Fig. 1 shows the X-ray diffraction patterns of the (a) JCPDS 48-0886 data of NaY(WO4)2, the synthesized (b) NaY(WO4)2, (c) NaY0.8(WO4)2:Ho0.2, (d) NaY0.7(WO4)2:Ho0.1Yb0.2, and (e) NaY0.5(WO4)2:Ho0.05Yb0.45 particles. All of the XRD peaks could be assigned to the tetragonal-phase NaY(WO4)2 with the space group of I41/a, which was in good agreement with the crystallographic data of NaY(WO4)2 (JCPDS 48-0886). No impurity phases were detected. This finding means that the tetragonal-phase NaY1−x(WO4)
Conclusions
NaY1−x(WO4)2:Ho3+/Yb3+ phosphors with doping concentrations of Ho3+ and Yb3+ (x=Ho3++Yb3+, Ho3+=0.05, 0.1, 0.2 and Yb3+=0.2, 0.45) were successfully synthesized by the microwave-modified sol-gel method. Well-crystallized particles formed after heat treatment at 900 °C for 16 h showed a fine and homogeneous morphology with particle sizes of 2–5 μm. Under excitation at 980 nm, the UC intensities of NaY0.7(WO4)2:Ho0.1Yb0.2 and NaY0.5(WO4)2Ho0.05Yb0.45 particles exhibited yellow emissions based on a
Acknowledgments
This study was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2014-046024).
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