Molten Salt Synthesis

A series of red phosphors Y 2 O 3 :Eu 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 Y 2 O 3 :Eu can be controlled by adjusting the kinds of surfactants. The phosphor Y 2 O 3 :Eu 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.

Wet chemical methods have been widely developed to prepare the luminescent materials [16][17][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][22][23].
In the present study, a series of red emission phosphors Y 2 O 3 :Eu 3+ 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 Y 2 O 3 :Eu 3+ .  (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 Y 2 O 3 :Eu 3+ prepared by MMS is shown in Scheme 1.

Measurements.
The structure of the final product was examined by X-ray powder diffraction (XRD) using Cu K radiation on Rigaku D/Max 2200 vpc X-ray diffractometer. The size and morphology of Y 2 O 3 :Eu 3+ 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.

Results and Discussion
The XRD patterns of Y 2 O 3 :Eu 3+ prepared by MSS with different surfactants were shown in The particles are about 100 nm (Figure 2(a)). The result shows that the introduction of the surfactant NP-5 can effectively prevent Y 2 O 3 NCs growth. However their crystallinity is lower. Y-NP10 shows good crystallinity, and its size is about  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 O 2− to the 4f orbital of Eu 3+ . The sharp lines in 350-550 nm range are intraconfigurational 4f -4f transitions of Eu 3+ ions in the host lattices. Figure 4 is the emission spectra of Y 2 O 3 :Eu 3+ prepared by MSS with different surfactants under 254 nm excitation. The strongest peak is at 611 nm, which is due to 5 D 0 → 7 F 2 transition in 2 symmetry for Eu 3+ . The other f -f transitions of Eu 3+ , such as 5 D 0 → 7 F in 570-720 nm and 5 D 1 → 7 F 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 = 0.592, = 0.359.

Conclusion
In summary, Y 2 O 3 :Eu 3+ NCs were prepared by MSS with different surfactants. The structures, morphologies, and Journal of Nanomaterials