Abstract
A series of red phosphors Y2O3:Eu3+ were prepared by the molten salt method with different surfactants. Their structures, morphologies, and the photoluminescent properties were investigated at room temperature. The particles size of Y2O3:Eu3+ can be controlled by adjusting the kinds of surfactants. The phosphor Y2O3:Eu3+ prepared with NP-10 [polyoxyethylene (10) nonyl phenyl ether] shows regular morphology and higher crystallinity, and its average particle size is about 200 nm. Bright red light can be observed by naked eyes from the red phosphor under 254 nm excitation.
1. Introduction
In recent years, luminescent nanocrystals (NCs) doped with rare earth ions were paid more attentions because of their interesting luminescent properties. They can be as components in displays [1], light emitting diodes [2], biological assays [3], and optoelectronic devices [4]. Cubic Y2O3:Eu3+ is one of the most important commercial red phosphors, which can be used in fluorescent lights, cathode ray tubes (CRTS), plasma display panel (PDP), and field emission display (FED) [5–9]. Many methods for synthesis of Y2O3:Eu3+ phosphor have been reported, including solution combustion method [10], sol-gel method [11], spray pyrolysis method [12], hydrothermalmethod [13], and precipitation method [14, 15].
Wet chemical methods have been widely developed to prepare the luminescent materials [16–18], since these processes have advantages of good homogeneity through mixing the starting materials at the molecular level in solution, a lower calcination temperature and a shorter heating time. However, sol-gel process often requires expensive (and environmentally unfriendly) organic precursors and solvents. Then a low-temperature technique, molten salt synthesis (MSS), is beginning to attract a great deal of interest. MMS is one of the simplest and most versatile approaches available to obtain single-phase powders at lower temperatures with shorter reaction times and little residual impurities as compared with conventional solid-state reactions [19]. The ionic fluxes molten salts possess high reactivity toward different inorganic species and relatively low melting points, which is convenient for preparation of inorganic materials. An the reaction medium, the inorganic molten salts exhibit favorable physicochemical properties such as a greater oxidizing potential, and high mass transfer, high thermal conductivity, as well as relatively lower viscosities and densities, as compared to conventional solvents [20]. Furthermore, MMS is one of the most effective ways to prepare nanoscale shape-controlled materials [21–23].
In the present study, a series of red emission phosphors Y2O3:Eu3+ were prepared by MSS with NaCl as the molten salt. Different surfactants such as polyoxyethylene (10) nonyl phenyl ether (NP-10), polyoxyethylene (5) nonyl phenyl ether (NP-5), octyl phenol together ethylene (10) ether oxygen (OP-10), and sodium dodecyl benzene sulfonate (LAS) were used to control the particles size of Y2O3:Eu3+.
2. Experimental
2.1. Preparation of Y2O3:Eu3+
Y(NO3)·6H2O and Eu(NO3)·6H2O were obtained from Aladdin Reagent limited company, China; NaOH, NaCl, and C2H5OH were obtained from Guangzhou Chemical Reagent Factory; polyoxyethylene (10) nonyl phenyl ether (NP-10), polyoxyethylene (5) nonyl phenyl ether (NP-5), octyl phenol together ethylene (10) ether oxygen (OP-10), sodium dodecyl benzene sulfonate (LAS) were purchased from Guangzhou Chi Hung Trade LTD. Co. All chemical reagents are of analytical reagent grade. Distilled water was used throughout.
The process to prepare Y2O3:Eu3+ nanocrystals with NP-10 (Y-NP10) is as follows: the raw materials Y(NO3)·6H2O (4.75 mmol), Eu(NO3)·6H2O (0.25 mmol), and NaOH (15 mmol) were mixed for 30 min in agate mortar, and then NaCl (10 g) and NP-10 (1 g) were put in the mixture and were ground for 10 min. At last, the mixture was heated at 850°C for 4 h. The final product was the solidified melt, which was washed with distilled water and ethanol at room temperature, and then the product was dried overnight in air at 110°C. The process for Y2O3:Eu3+ prepared by MMS is shown in Scheme 1.
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The methods for synthesis Y2O3:Eu3+ with NP-5 (Y-NP5) and Y2O3:Eu3+ with OP10 (Y-OP10) and Y2O3:Eu3+ with LAS (Y-LAS) are similar with that of Y-NP10 mentioned above.
2.2. Measurements
The structure of the final product was examined by X-ray powder diffraction (XRD) using Cu Kα radiation on Rigaku D/Max 2200 vpc X-ray diffractometer. The size and morphology of Y2O3:Eu3+ were observed by a scaning electron microscopy (SEM) on LEO-1530 electron microscope. The luminescent spectra of the sample were recorded on an FLS920 fluorescence spectrophotometer.
3. Results and Discussion
The XRD patterns of Y2O3:Eu3+ prepared by MSS with different surfactants were shown in Figure 1. Curve (a) is very consistent with JCPDS43-1036 [Y2O3]. This reveals that the phosphor Y-NP5 shares the cubic structure with the single phase, and Eu3+ ions have been effectively built into the Y2O3 host lattices and occupied the Y3+ sites. Other three curves are similar with curve (a), the result shows that the single phase Y2O3:Eu3+ can be prepared by MSS with different surfactants.
The SEM images of as-prepared samples Y-NP5, Y-NP10, Y-OP10, and Y-LAS are shown in Figure 2. Y2O3:Eu3+ NCs have been successfully prepared by MMS method with NP-5. The particles are about 100 nm (Figure 2(a)). The result shows that the introduction of the surfactant NP-5 can effectively prevent Y2O3 NCs growth. However their crystallinity is lower. Y-NP10 shows good crystallinity, and its size is about 200 nm. We also prepared Y2O3:Eu3+ with other surfactants. Y-OP10 and Y-LAS particles sizes are about 500 nm and 1 μm, respectively.
(a)
(b)
(c)
(d)
The excitation spectra of Y-NP5, Y-NP10, Y-OP10, and Y-LAS are shown in Figure 3. The excitation spectra of these samples exhibit similar features; the broad excitation band from 230 nm to 290 nm is attributable to the charge transfer (CT) band from the 2p orbital of O2− to the 4f orbital of Eu3+. The sharp lines in 350–550 nm range are intraconfigurational 4f-4f transitions of Eu3+ ions in the host lattices.
Figure 4 is the emission spectra of Y2O3:Eu3+ prepared by MSS with different surfactants under 254 nm excitation. The strongest peak is at 611 nm, which is due to 5D0 → 7F2 transition in C2 symmetry for Eu3+. The other f-f transitions of Eu3+, such as 5D0 → 7FJ in 570–720 nm and 5D1 → 7FJ transitions in 520–570 nm are very weak. Y-NP10 shows intense red emission. Bright red light can be observed from this phosphor under 254 nm excitation (seeing Scheme 1). Its CIE (Commission Internationale de l’Eclairage, International Commission on Illumination) chromaticity coordinates are calculated to be , .
4. Conclusion
In summary, Y2O3:Eu3+ NCs were prepared by MSS with different surfactants. The structures, morphologies, and the photoluminescent properties of the phosphors were studied. The particles size of Y2O3:Eu3+ can be controlled by adjusting the kinds of surfactants. The sample Y-NP10 shows regular morphology and intense red emission under UV excitation. Bright red light can be observed by naked eyes from this phosphor under 254 nm excitation.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (21001092), the program of Innovative Research Team (in Science and Technology) at the University of Yunnan Province (2011UY09), and Yunnan Provincial Innovation Team (2011HC008).