Microwave-modified sol-gel process of NaY(WO₄)₂: Ho³⁺/Yb³⁺ phosphors and the upconversion of their photoluminescence properties

Abstract

NaY_1−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, and the upconversion of their photoluminescence properties was investigated. Well-crystallized particles showed a fine and homogeneous morphology with particle sizes of 2–5 μm. Under excitation at 980 nm, the UC intensities of NaY_0.7(WO₄)₂:Ho_0.1Yb_0.2 and NaY_0.5(WO₄)₂ Ho_0.05Yb_0.45 particles exhibited yellow emissions based on a strong 550-nm emission band in the green region and a strong 655-nm emission band in the red region, which were assigned to the ⁴S₂/⁵F⁴→⁵I₈ and ⁵F⁵→⁵I₈ transitions, respectively. The Raman spectra of the doped particles indicated the presence of strong disordered peaks at higher frequencies and weak peaks at lower frequencies induced by the incorporation of the Ho³⁺ and Yb³⁺ elements into the Y³⁺ site in the crystal lattice, which resulted in the unit cell shrinkage accompanying the new phase formation of the [WO₄]²⁻ groups.

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 I4_1/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, NaY_1−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.