1. Introduction

The optical materials with high purity, single phase, and uniform particle size are very important for the development of photonic devices due to their high resolution and high luminescence efficiency [1–3]. Moreover, the synthesis method of the optical materials has direct impact on their performance. The molten salt method is a low-temperature synthesis method, which uses a molten salt as solvent for the constituent oxides to enable molecular level mixing of different reactants, and lead to a homogeneous structure of the final product [4–6]. Because of the short diffusion distances among different reactants, and the relatively high mobility of reactant species (10⁻⁵ to 10⁻⁸ cm² s⁻¹) in the molten salt media (compared to as low as 10⁻¹⁸ cm² s⁻¹ in the solid state), a full reaction can be accomplished in a relatively short time [4–6]. Compared with the conventional methods, MSS is a simplest technique and a well-established process of forming a desirable compound in a flux of low melting point and characterized by low formation temperature and short synthesis time due to the accelerated diffusivities of reactants [7, 8]. For these reasons, MSS method can give better chemical homogeneity and finer particles together with better controlling of particle morphology, improving purity, sinterability and compositionally uniformity of optical materials than the solid-state reaction.

Due to the intra 4fN electronic transition of rare earth ion, rare earth-doped materials exhibit fascinating luminescence properties such as sharp emission bands, long-lived excited electronic states, higher photostability, long emission lifetimes, etc [9]. Particularly, rare earth-doped materials can create a variety of colors by proper selection of rare earth dopants, which cover almost the entire visible spectrum, and multicolor emission could be realized in a single-phase host matrix [10]. Yttrium orthovanadate (YVO₄) is one of the most promising inorganic luminescence materials which have a zirconia-type tetragonal structure and have wide practical applications in many devices involving the lighting display, polarizer and laser host material [11–13]. Compared with other oxide luminescence compounds such as Y₂O₃, YVO₄ has lower formation energy and higher crystallinity, which makes it easier to form regular morphology and shows higher luminous efficiency [14–24]. Meanwhile, rare earth ions having emission in the primary color (red, blue and green) range can be combined to get a white light emitter. The incorporation of the rare earth ions into YVO₄ matrix and using their energy transfer can effectively achieve the white emission, which is very meaningful to the practical applications especially to white LEDs. On the other hand, there are many ways to synthesize the YVO₄ matrix materials, such as solid-state reaction [25], hydrothermal process [26], sol–gel process [27], co-precipitation method [28] and so on. However, the current synthesis methods still cannot meet the application requirements. To improve particle uniformity and luminescence efficiency, it is still necessary to develop new synthesis methods and further regulate the luminescence properties of YVO₄ matrix materials.

In this study, we aimed to produce rare earth ions (Eu³⁺, Sm³⁺, Dy³⁺, Tm³⁺) doped YVO₄ by the MSS method and investigate into the crystal structure and luminescence properties in order to achieve better practical applications.

2. Experimental

2.1. Powder preparation

YVO₄:RE (RE = Eu³⁺, Sm³⁺, Dy³⁺, Tm³⁺) phosphors have been prepared by MSS method. The starting materials with the purity of 99.99% were supplied from Aldrich Chemical Company and used without further purification. At first, the stoichiometric amounts of Y₂O₃, Eu₂O₃, Sm₂O₃, Dy₂O₃, Tm₂O₃ and NH₄VO₃ (A.R.) were mixed together in an agate mortar. Secondly, NaNO₃ (A.R.) was used as molten salt, then added into the above mixture. After this, the mixture was lapped until they became homogenous system. Then, the mixture was moved into a crucible and heated in an electric muffle furnace for 5 hours at different temperatures (from 350 to 500 °C). After cooling to room temperature, the reacted mass was washed with diluted HNO₃ to get pure phase. Finally, the powder was washed in hot distilled water followed by filtration to remove the salts and this washing process will be repeated about five times. The resultant powder was oven-dried at 70 °C for about 6 h prior to characterization.

2.2. Physical measurements

Phase purity and crystal structures of products were performed with Japan D/Max-3B X-ray diffraction (XRD) with Cu Kα (λ = 1.5418 Å) radiation generated at 30 kV/30 mA. The morphology of powders was observed using scanning electron microscope (SEM, Japan JSM-35CF environmental scanning electron microscope). Both the excitation and emission spectra of these phosphors were analyzed with a Spex spectrofluorometer (Fluoromax-4P, JOBIN YVON) equipped with a 150 W xenon lamp as the excitation source. To evaluate the material performance for color luminescent emission, one computer program was used to calculate CIE chromaticity coordinates. All the measurements were carried out at room temperature.

3. Results and discussion

3.1. Structures

Figure 1 shows the XRD patterns of YVO₄ doped with different Eu³⁺ ions calcined at different temperatures for 5 h in air by MSS method. From the XRD patterns of the samples after reaction at 350 °C (Fig. 1(a)), it could be found that the main phase was YVO₄, but the impurity peak of Y₂O₃ has been detected, which was marked with ●, this result indicated that the solubility of the raw materials and the diffusion rate of the ions were very low in molten salt mixture due to the low calcined temperature, although most reactants have reacted and transformed to the tetragonal YVO₄ crystalline phase during the process, minor impurity phases (Y₂O₃, V₂O₅) still existed. In order to attain pure YVO₄ phase, the samples were annealed at 400, 450 and 500 °C for 5 h, respectively. As can be seen in Fig. 1, with the temperature increased, the peak position was not changed but the intensity obtained enhanced (Fig. 1 (b), (c) and (d)). Except for the peak of Y₂O₃, all the other diffraction peaks of the selected samples were in good agreement with the JCPDS 17-0341 standard card of YVO₄ and no characteristic peaks from any other impurities were detected. It indicated that the main phase of the obtained samples adopted the same structure as YVO₄, illuminating that the dopants were dissolved in the YVO₄ host and did not cause any detectable change in the host structure.

 

Fig. 1 XRD patterns of YVO₄ doped with Eu³⁺ ions calcined at different temperatures in air for 5 h. (a) 350 °C, (b) 400 °C, (c) 450 °C, (d) 500 °C, (e) 500 °C + diluted HNO₃ wash.

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When the samples were heat-treated at 500 °C, there still showed the coexistence of Y₂O₃ phases (Fig. 1(d)). This phenomenon indicated that one of the raw material (NH₄VO₃) has been pyrolysis during the reaction process, leading to another raw material (Y₂O₃) overweight which could not react completely. The scheme of the pyrolysis process can be shown as below:

To solve this problem, after the powders calcined at 500 °C for 5 h and cooled to room temperature, the diluted HNO₃ was used to wash the powders to get the pure products. As could be seen in Fig. 1(e), all the prominent peaks (101), (200), (112), (220), (202), (301), (103), (321), (312), (400) and (420) were observed at the corresponding angles in the peak positions, which was in excellent accordance with the powder data of index card JCPDS No. 17-0341, and it also implied that the pure YVO₄ crystallite (space group I41/amd) without any other crystalline phase could be obtained after treated with diluted HNO₃. The temperature (500 °C) used for MSS method was much lower than other methods [15–17], this could be attributed to the enhanced diffusion in the molten chlorides liquid phase compared to that in the solid state. Meanwhile, it should be noted that the YVO₄:RE (RE = Sm³⁺, Dy³⁺, Tm³⁺) samples have been synthesized by the same process and all these samples showed pure phase which was confirmed by XRD results (not shown in figure).

3.2. SEM study

Figure 2(a) and 2(b) display the representative SEM images of YVO₄:Eu³⁺ samples calcined at 500 °C and treated with diluted HNO₃. As shown in the figure, the YVO₄:Eu³⁺ sample presented a smooth surface and had good dispersion. Interestingly, most particles showed two different morphologies, block and rod shapes, but both showed uniform distribution. This indicated that the particle uniformity of YVO₄:Eu³⁺ could be efficiently enhanced by MSS method. In addition, the YVO₄:RE (RE = Sm³⁺, Dy³⁺, Tm³⁺) samples which synthesized by MSS method showed similar morphology with YVO₄:Eu³⁺ (not shown in figure). The average grain size of phosphors was obtained by averaging the grain diameter of at least 200 grains observed from the SEM images at low magnification to ensure the measurement accuracy. As shown in the Fig. 2(c), the mean size of YVO₄:Eu³⁺ particle was approximately 700 nm.

 

Fig. 2 SEM images of YVO₄:Eu³⁺ phosphors (a) and (b), and the measured size distribution of the particle (c).

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3.3. Photoluminescence properties

Figure 3 represents the excitation and emission spectra of the YVO₄:RE (RE = Eu³⁺, Sm³⁺, Dy³⁺, Tm³⁺) samples prepared by MSS method heating at 500 °C for 5 h. The spectra had a broad excitation band with maximum values at around 313 nm (Fig. 3(a)) for YVO₄:Eu³⁺, 256 nm (Fig. 3(b)) for YVO₄:Sm³⁺, 256 nm (Fig. 3(c)) for YVO₄:Dy³⁺ and 310 nm (Fig. 3(d)) for YVO₄:Tm³⁺, respectively. It could be seen clearly from Fig. 3(a) that the short-wavelength excitation at 260 nm was due to the charge-transfer (CT) process involving the Y-O components and the long-wavelength excitation at 318 nm was due to the V-O components of the matrix. Also, the ⁷F₀ → ⁵L₆ excitation line of Eu³⁺ at about 394 nm was observed with very weak intensity. On the other hand, the broad band around 313 nm was assigned to the CT transition between Eu³⁺ and O^2-, i.e., an electron transfer from O^2- (2p⁶) orbital to the empty orbital of ⁴f₆ for Eu³⁺ [29–31]. It could be concluded that the broad band around 313 nm was assigned to the overlap of VO₄^3- absorption and CT transition between Eu³⁺ and O^2-. Figure 3(b) shows the spectrum of YVO₄: Sm³⁺. It could be found that there were two peaks which located at 256 and 293 nm at the short wavelength region from 200 to 350 nm monitored at an emission wavelength of 602 nm. From previous data reports [32, 33], the former was due to the absorption of VO₄^3- for the existing of two excitation bands, the reason might be ascribed to the distortion of the YVO₄. At first the particle size could induce the distortion of YVO₄, second YVO₄ belongs to the tetragonal zircon structure and Y³⁺ is located at a site (D2d) deviated from an inverse center in the host of YVO₄. So, they could arouse different V-O bond of VO₄^3-. The excitation spectrum of YVO₄:Dy³⁺ had the similar feature to that of YVO₄:Sm³⁺ (Fig. 3(c)). But in short ultraviolet region (200-350 nm), the excitation bands should correspond to the f-d transitions of Dy³⁺. As for Tm³⁺ ions (Fig. 3(d)), there were also CT transitions, but the maximum peak moved from 256 nm to 310 nm, the intensity of f-f transition at around 256 nm was relatively weaker compared to the former transitions. Figure 3(a) also shows the emission spectrum of YVO₄:Eu³⁺ under the 313 nm excitation. The spectrum was dominated by the emission from the trivalent europium ions, and mainly the ⁵D₀ → ⁷F_{2, 4} forced electric dipole transitions for which high intensities were at 613 and 618 nm. Other contributions of weaker importance were the ⁵D₀ → ⁷F_{1, 3} magnetic dipole transitions at 592, 608, 696 and 703 nm [29–31]. The emission spectrum of YVO₄:Sm³⁺ was measured in the 350-750 nm range. Upon 256 nm excitation, we could see clearly from Fig. 3(b) that the emission spectrum of YVO₄:Sm³⁺ consisted of four main peaks at 568, 602, 647 and 700 nm, which corresponded to ⁶G_5/2 → ⁶H_5/2, ⁴G_5/2 → ⁶H_7/2, ⁴G_5/2 → ⁶H_9/2 and ⁴G_5/2 → ⁶H_11/2 transitions, respectively [32, 33]. Interestingly, there was a peak splitting at about 618 and 655 nm in YVO₄:Eu³⁺ and YVO₄:Sm³⁺ spectrum, respectively. It was the same phenomenon as the literature reported [34]. The local symmetry of the crystal-field (CF) around the Eu³⁺ and Sm³⁺ would cause peak splitting. Moreover, generally the YVO₄:Sm³⁺ phosphor could emit orange light prepared by traditional methods, but by MSS method, the magnetic dipole transition (⁴G_5/2 → ⁶H_7/2 and ⁴G_5/2 → ⁶H_9/2) was the strongest emission peak for YVO₄:Sm³⁺, led the phosphor to emit bright red light, which was different from other methods. This phenomenon provided a new technology for synthesis red phosphors. Upon UV excitation, we also obtained the emission spectra of YVO₄:Dy³⁺ and YVO₄:Tm³⁺, which contained exclusively the characteristic transition lines of Dy³⁺ and Tm³⁺. As for Dy³⁺ ion, the emission peaks were centered at 483 nm (⁴F_9/2 → ⁶H_15/2) and 574 nm (⁴F_9/2 → ⁶H_13/2), which made YVO₄:Dy³⁺ phosphor emit yellow light and the characteristic emission of Tm³⁺ was only at 477 nm (¹G₄ → ³H₆) that made YVO₄:Tm³⁺ phosphor emit blue light. This indicated that an efficient energy transfer from YVO₄ matrix to the rare earth ions of Dy³⁺ and Tm³⁺ also occurred [35, 36].

 

Fig. 3 Excitation and emission spectra of YVO₄:Eu³⁺ (a), YVO₄:Sm³⁺ (b), YVO₄:Dy³⁺ (c), YVO₄:Tm³⁺ (d) phosphors.

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In order to further investigate the luminescence behaviors of YVO₄:RE phosphors, a series of YVO₄:xRE (RE = Eu³⁺, Sm³⁺) and (RE = Dy³⁺, Tm³⁺) phosphors were prepared via MSS method. The emission spectra of YVO₄:xRE (RE = Eu³⁺, Sm³⁺) (x = 0.5%, 1.0%, 2.0%, 3.0%, and 5.0%) are shown in Fig. 4. It illustrated that all of the emission spectra exhibited the similar profile with different relative intensities. In YVO₄:Eu³⁺ host lattice, the ⁵D₀ → ⁷F₂ electric dipole transition became strongest among all these transitions due to the lack of inversion symmetry and the break of parity selection rules (Fig. 4(a)), which was benefit for obtaining pure red phosphors with good CIE chromaticity coordinates. The inset in Fig. 4(a) shows the dependence of the peak intensity of Eu³⁺ centered at 613 nm on concentration in YVO₄ host. As same as most results of the researches, it indicated that the emission intensity increased initially with the increase of Eu³⁺ doping concentration and reached to the maximum at x = 1.0%, then gradually decreased due to the internal concentration quenching [37]. As for Sm³⁺ ion (Fig. 4(b)), we used the same concentrations for study, the peak maximum was at x = 0.5% for the Sm³⁺-doping concentration between 0.5% ~5.0%, as the Sm³⁺ ions concentration increased, the emission intensity gradually decreased also due to the concentration quenching as same as the Eu³⁺ ions.

 

Fig. 4 Emission spectra of YVO₄:xRE (RE = Eu³⁺, Sm³⁺) (x = 0.5%, 1.0%, 2.0%, 3.0%, 5.0%), the inset shows the dependence of integrated emission intensity on the concentration of RE ions in YVO₄ with λ = 313 nm for Eu³⁺ (a) and λ = 256 nm for Sm³⁺ (b).

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Figure 5 presents the excitation and emission spectra of YVO₄:2%(Dy_xTm_1-x) (x = 0.1~0.6) as a function of Dy³⁺ contents (x). There were two dominant bands in the emission spectrum of YVO₄:Dy³⁺ (Fig. 2(c)). The yellow band (574 nm) corresponded to the hypersensitive transition ⁴F_9/2 → ⁶H_13/2 and the blue band (483 nm) originated from the ⁴F_9/2 → ⁶H_15/2 transition [38]. Because the Dy³⁺ was located at a site (D_2d) deviated from an inverse center in the host of YVO₄, the intensity of yellow emission (574 nm) was stronger than that of its blue emission (483 nm) (Fig. 3(c)). In this case, if co-doped Dy³⁺ and Tm³⁺ in YVO₄ host, as changing the ratio of Dy³⁺/Tm³⁺, the intensity of blue emission was increased with the enhanced concentration of Tm³⁺. When the value of x reached 0.3, the intensity of yellow emission was nearly equal to that of blue emission (Fig. 5).

 

Fig. 5 The excitation and emission spectra of YVO₄:Dy_xTm_1-x (x = 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) under λ_em of 475 nm and λ_ex of 320 nm at room temperature.

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The CIE 1931 chromaticity coordinates of YVO₄:RE (RE = Eu³⁺, Sm³⁺, Dy³⁺ and Tm³⁺) phosphors which were calculated based on the corresponding emission spectra are represented in Fig. 6 and Tab. 1. Compared with the Fig. 6 and Tab. 1, it was clearer to see that the coordinate of YVO₄:1% (Dy_0.3Tm_0.7) was (0.32, 0.31), very close to the equi-energy white point (x = 0.33, y = 0.33). These results indicated that the white emission could be gained by suitably adjusting the ratio of Dy³⁺/Tm³⁺ in the host of YVO₄.

 

Fig. 6 The CIE 1931 chromaticity coordinates of YVO₄:RE (RE = Eu³⁺, Sm³⁺, Dy³⁺, Tm³⁺) and YVO₄:Dy_xTm_1-x (x = 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) phosphors.

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Tab. 1. Characteristic and International commission on illumination (CIE) chromaticity coordinates of YVO₄:RE (RE = Eu³⁺, Sm³⁺, Dy³⁺, Tm³⁺) and YVO₄:Dy_xTm_1-x (x = 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) phosphors

View Table

4. Conclusions

In this paper, we have demonstrated MSS method for the synthesis of YVO₄:RE (RE = Eu³⁺, Sm³⁺, Dy³⁺, Tm³⁺) phosphors with a broad range of regular and uniform shapes. The resulting samples presents a smooth surface and had good dispersion. Optical studies provide hints that the f-f characteristic transitions could all confirmed by the luminescent spectra. By tuning the doping concentration of Eu³⁺ and Sm³⁺ ions, optimum concentrations were confirmed at 1.0% Eu³⁺ ion doping and 0.5% Sm³⁺ doping in YVO₄:xRE (RE = Eu³⁺, Sm³⁺) phosphors. Meanwhile, efficient energy transfer could be confirmed from Dy³⁺ to Tm³⁺ ions in YVO₄:Dy³⁺/Tm³⁺ sample. It could achieve good white emission which was demonstrated by luminescent spectra and CIE coordinates. These results indicated that YVO₄:Dy³⁺/Tm³⁺ phosphors would be the potential white-emitting phosphors for light emitting devices.