Synthesis of YVO4:Eu3+/YBO3Heteronanostructures with Enhanced Photoluminescence Properties

Novel YVO4:Eu3+/YBO3core/shell heteronanostructures with different shell ratios (SRs) were successfully prepared by a facile two-step method. X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy were used to characterize the heteronanostructures. Photoluminescence (PL) study reveals that PL efficiency of the YVO4:Eu3+nanocrystals (cores) can be improved by the growth of YBO3nanocoatings onto the cores to form the YVO4:Eu3+/YBO3core/shell heteronanostructures. Furthermore, shell ratio plays a critical role in their PL efficiency. The heteronanostructures (SR = 1/7) exhibit the highest PL efficiency; its PL intensity of the5D0–7F2emission at 620 nm is 27% higher than that of the YVO4:Eu3+nanocrystals under the same conditions.


Introduction
Rare-earth (RE)-doped phosphors have a broad range of applications in cathode ray tubes (CRTs), plasma display panels (PDPs), field emission displays (FEDs), X-ray detectors, fluorescent lamps and so on [1][2][3]. In recent years, RE-doped nanophosphors have received a great deal of research attention due to the unique applications in higherresolution displays, drug delivery system and biological fluorescence labeling [4][5][6][7][8]. Furthermore, fluorescent lamps made from small-sized phosphors always have high-packing density and low loading [9]. RE-doped nanophosphors are expected to have high brightness and luminescence quantum yield for practical applications. Unfortunately, high specific surface area and surface defects of the nanophosphors always result in serious surface recombination, which is a pathway for nonradiative relaxation [10]. Consequently, RE-doped nanophosphors have lower luminescence efficiency compared to their corresponding bulk powder phosphors [11,12]. More attention should be paid to improve the luminescence efficiency of RE-doped nanophosphors.
During the past decade, core/shell heteronanostructures have been widely investigated to obtain better properties [13,14]. Luminescence efficiency of RE-doped nanophosphors can be improved by forming core/shell heteronanostructures, because surface defects and surface recombination of the nanophosphors (cores) are greatly reduced by the nanocoatings (shells) [11,15]. Among RE-doped phosphors, europium ions-doped yttrium orthovanadate (YVO 4 :Eu 3? ) is an important red phosphor, which has been commercially used in CRTs, high-pressure mercury lamps and color television due to its excellent luminescence properties [2,3]. Many literatures have reported the preparation and luminescence properties of YVO 4 :Eu 3? nanophosphors [16][17][18], but few measures have been taken to improve their luminescence efficiency. In this paper, we propose novel YVO 4 [19][20][21], yttrium borate (YBO 3 ), is used as shell material in this new heteronanostructures. YBO 3 has excellent properties such as high VUV transparency, high stability, low synthesis temperature and exceptional optical damage threshold [22,23], so the new core/shell heteronanostructures proposed here may have promising applications in the fields of display, lighting and bio-nanotechnology.

Preparation of YVO 4 :Eu 3? Nanocrystals
To 130 mL of deionized water, 30.4 mL of Y(NO 3 ) 3 solution (0.15 mol/L), 1.6 mL of Eu(NO 3 ) 3 solution (0.15 mol/L) and 0.758 g of NaVO 3 Á2H 2 O were added under vigorous magnetic stirring for 30 min. The pH value of the solution was adjusted to 9.5 using ammonia under stirring. Then, the above solution was transferred into a Teflon-lined stainless steel autoclave (capacity 200 mL) and sealed. The autoclave was heated at 200°C for 16 h and cooled naturally to room temperature. Finally, the YVO 4 :Eu 3? nanocrystals were collected by centrifugation.

Preparation of Sol-Gel Solution
To 100 mL of water-ethanol solution (the volume ratio is 1:4) 3.83 g of Y(NO 3 ) 3 Á6H 2 O and 0.68 g of H 3 BO 3 (*10 mol% of excess) were added under stirring. To the above solution, 6.30 g of citric acid (CA) and 12.00 g of PEG 6000 (the molar ratio of Y(NO 3 ), CA, and PEG was 5:15:1) were added. Herein, CA and PEG were used as the chelating and cross-linking reagents respectively. The above solution was stirred for 5 h and subsequently aged for 24 h. Finally, highly transparent sol-gel solution with yttrium concentration of 0.1 mol/L was obtained.

Preparation of YVO 4 :Eu 3? /YBO 3 Heteronanostructures
Herein, we take the heteronanostructures (SR = 1/7) as an example to present their detailed procedures. The YVO 4 :Eu 3? nanocrystals (4.56 mmol) obtained in the first step were heated to 120°C in a petri dish. Then, 6.51 mL of the sol-gel solution was slowly dropped onto the heated YVO 4 :Eu 3? nanocrystals. The obtained sample was annealed at 700°C in air for 2 h with a heating rate of 1°C/min. The furnace was cooled to room temperature naturally and the white YVO 4 :Eu 3? /YBO 3 heteronanostructures (SR = 1/7) were obtained.
In this paper the YVO 4 :Eu 3? (5 mol% Eu) nanocrystals obtained in the first step are called ''the original sample''. To avoid the influence of annealing on the photoluminescence property, the original sample was also annealed at 700°C for 2 h under the same conditions. The annealed original sample is denoted as ''YVO 4 :Eu 3? /YBO 3 core/ shell heteronanostructures (SR = 0)''. In addition, YBO 3 powder was prepared by the above-mentioned sol-gel approach, for comparison.

Characterization and Photoluminescence Property
Phase identification of the products was carried out using a Thermo ARL X'TRA X-ray diffractometer (XRD) with Cu Ka radiation (k = 1.54178 Å ). Morphology observation of the original sample was observed using a JEOL JEM 200 CX transmission electron microscope (TEM). In addition, a Philips CM200 high-resolution transmission electron microscope (HRTEM) with an accelerating voltage of 200 kV was also employed to investigate the morphology and structure of the core/shell heteronanostructures (SR = 1/2). X-ray photoelectron spectroscopy (XPS) measurement was performed on a X-ray photoelectron spectrometer (Model Axis Ultra DLD, Kratos Corp., UK) with a standard MgKa (1,256.6 eV) X-ray source operating at 150 W. All binding energies were referenced to the C 1 s peak at 284.6 eV of the surface adventitious carbon. Photoluminescence (PL) excitation and emission spectra of all the powder products were obtained on a Hitachi fluorescence spectrophotometer (Model F-4600, Hitachi Corporation, Japan) under the same conditions.

Results and Discussion
All as-synthesized products were characterized by XRD, and their data were analyzed by a Thermo ARL WinXRD software package. Figure 1 shows XRD patterns of the original sample (the YVO 4 nanocrystals obtained in the first step), typical core/shell heteronanostructures and YBO 3 powder. As shown in Fig. 1a, all XRD peaks are in good agreement with the values of YVO 4 (JCPDS no. 72-0274) confirming that the core material was YVO 4 :Eu 3? . Likewise, the XRD pattern of the YBO 3 prepared by the sol-gel method is in good agreement with the standard card of YBO 3 (JCPDS no. 16-0277). Therefore, pure YBO 3 can be successfully obtained by the sol-gel approach. As shown in Fig. 1b-f, the YVO 4 :Eu 3? /YBO 3 core/shell heteronanostructures exhibit two series of XRD patterns, namely, those of YVO 4 and YBO 3 . In addition, the intensities of the peaks of YBO 3 increase with the shell ratio. Figure 2 shows enlarged XRD patterns of some typical products, which clearly demonstrate that the XRD peaks of both YVO 4 and YBO 3 could be found in the heteronanostructures. Therefore, the heteronanostructures are composed of the YVO 4 :Eu 3? and YBO 3 .
Transmission electron microscope images of the original sample (the YVO 4 :Eu 3? nanocrystals obtained in the first step) and YVO 4 :Eu 3? /YBO 3 core/shell heteronanostructures (SR = 1/2) are shown in Fig. 3. Figure 3a reveals that the original sample used as the core is nanocrystals. The inset of Fig. 3a clearly shows that the YVO 4 :Eu 3? nanocrystals are around 20 nm in diameter. The core/shell heteronanostructures were obtained by sol-gel growth of YBO 3 nanocoatings onto the YVO 4 :Eu 3? nanocrystals, so their particle sizes were larger than that of the YVO 4 :Eu 3? nanocrystals. Figure 3b shows TEM image of the core/ shell heteronanostructures (SR = 1/2). As shown in Fig. 3b, the heteronanostructures have a similar morphology to the original sample, while the average particle size of the heteronanostructures is approximately twice larger than the original YVO 4 :Eu 3? nanocrystals. This phenomenon indirectly verifies that the YBO 3 nanocoatings have been grown onto the YVO 4 :Eu 3? nanocrystals by the sol-gel process. Figure 3c is HRTEM image of a single particle of the heteronanostructures (SR = 1/2). Interestingly, two lattice fringes of different spacing appear in a single nanoparticle. The lattice fringes with a d-spacing of about 0.473 nm are found at the center of the particle, while the lattice fringe spacing is 0.308 nm in the peripheral zones of the particle. The two different types of the lattice fringes correspond well to the {101} planes of YVO 4 (JCPDS no. 72-0274) and the {101} planes of YBO 3 (JCPDS no. 16-0277), respectively. Therefore, YVO 4 :Eu 3? /YBO 3 core/shell heteronanostructures were formed by the two-step process.
X-ray photoelectron spectroscopy is the most commonly used technique for investigating the elemental composition of surface layers 1-5 nm in depth. Herein, XPS was used to further determine the formation of the YVO 4 :Eu 3? /YBO 3 core/shell heteronanostructures. If the YVO 4 :Eu 3? cores were effectively coated with the shell material (YBO 3 ), the XPS peak intensities of the core material (YVO 4 :Eu 3? ) would be very low. In other words, whether or not the product was the YVO 4 :Eu 3? /YBO 3 core/shell heteronanostructures could be determined by the XPS bands of vanadium. Figure 4 shows XPS spectra of the YVO 4 :Eu 3? / YBO 3 heteronanostructures (SR = 1/2), YVO 4 :Eu 3? nanocrystals and YBO 3 powder. XPS spectra in the range of 135-210 eV (Fig. 4a)    XPS band of vanadium is located at 515.6 eV, which is assigned to V 2p [25]. As shown in Fig. 4b, the V 2p band of the YVO 4 :Eu 3? nanocrystals is very strong, while that of the YVO 4 :Eu 3? /YBO 3 heteronanostructures (SR = 1/2) is much lower. This is because the YVO 4 :Eu 3? cores have been coated with YBO 3 nanocoatings and no enough V 2p XPS signal from the cores was generated by X-ray source.
High photoluminescence (PL) efficiency is important for practical applications of YVO 4 :Eu 3? nanophosphors. The YVO 4 :Eu 3? /YBO 3 core/shell heteronanostructures reported here are expected to exhibit enhanced PL efficiency under the same conditions. All PL excitation and emission spectra of the samples were measured in powder form using the same measurement parameters, so their respective PL emission intensity can relatively represent their PL efficiency. Figure 5a, b shows PL excitation and emission spectra of the YVO 4 :Eu 3? /YBO 3 core/shell heteronanostructures and original YVO 4 :Eu 3? nanocrystals and annealed YVO 4 :Eu 3? nanocrystals respectively. As shown in the excitation spectra (Fig. 5a), the heteronanostructures and YVO 4 :Eu 3? nanocrystals exhibit a similar broad excitation band in the range of 200-360 nm with a maximum value at 320 nm, which is ascribed to a charge transfer from the oxygen ligands to the central vanadium   4 3ion [5,26]. As shown in Fig. 5b, both the YVO 4 :Eu 3? nanocrystals and the heteronanostructures show two well-known PL emission bands in the range of 550-650 nm. The two emission bands at 596 nm and 620 nm are assigned to the magnetic-dipole transition 5 D 0 -7 F 1 of Eu 3? (596 nm) and the forced electric-dipole transition 5 D 0 -7 F 2 of Eu 3? (620 nm), respectively [27]. Herein, the 5 D 0 -7 F 2 emission at 620 nm (red emission) is selected as a criterion to determine their relative PL efficiency. The annealed YVO 4 :Eu 3? nanocrystals exhibit a little stronger PL emission than the original sample, because the crystallinity of the nanocrystals was improved by the annealing process. However, the influence of annealing on the photoluminescence properties can be avoided by comparison between the annealed sample and the heteronanostructures. Figure 5 reveals that all the heteronanostructures except for those with the shell ratios of 1/2 and 2/3 exhibit much stronger photoluminescence than the annealed YVO 4 :Eu 3? nanocrystals under the same conditions. Furthermore, the shell ratio plays a critical role in the PL efficiency of the heteronanostructures. When SR = 1/7, the heteronanostructures exhibit the highest PL efficiency, whose photoluminescence intensity of the 5 D 0 -7 F 2 emission is 27% higher than that of the annealed YVO 4 :Eu 3? nanocrystals. Therefore, PL efficiency of YVO 4 :Eu 3? nanophosphor can be improved by forming YVO 4 :Eu 3? /YBO 3 core/shell heteronanostructures.
Nanostructured materials have a high surface area-tovolume ratio, and this characteristic inevitably results in high surface defects density and serious surface recombination. Therefore, RE-doped nanophosphors suffer more serious nonradiative relaxation than corresponding bulk power phosphors. Consequently, RE-doped nanophosphors always have lower luminescence efficiency. In this paper, the nonradiative decay of the YVO 4 :Eu 3? nanocrystals was greatly reduced by the YBO 3 nanocoating on the YVO 4 :Eu 3? nanocrystals, so PL emission of the heteronanostructures was enhanced. YBO 3 has excellent properties such as high VUV transparency, high stability, low synthesis temperature and exceptional optical damage threshold [22,23]; so, it is an ideal shell material for composite phosphors with core/shell heterostructures. The YBO 3 shell ratio is a critical factor in photoluminescence enhancement of the heteronanostructures. Figure 6 shows the plot of change of PL intensity of the 5 D 0 -7 F 2 emission at 620 with the shell ratio. The change exhibits a parabola-like curve that reaches the peak at SR = 1/7. When SR \ 1/7, PL intensity increases with increasing SR. This is because the surface recombination, surface defects density and surface state density of YVO 4 :Eu 3? nanocrystals decrease with increasing the YBO 3 coating. When SR = 1/7, the surface recombination, surface defects density and surface state density have been decreased to the maximum level, so  Conclusions YVO 4 :Eu 3? /YBO 3 core/shell heteronanostructures with different shell ratios (SRs) were successfully prepared by sol-gel growth of YBO 3 nanocoating onto the YVO 4 :Eu 3? nanocrystals. Characterizations by means of XRD, TEM and XPS confirmed the formation of the YVO 4 :Eu 3? /YBO 3 core/shell heteronanostructures. The heteronanostructures exhibited much stronger photoluminescence (PL) than the YVO 4 :Eu 3? nanocrystals under the same conditions. The shell ratio is a critical factor in PL enhancement of the heteronanostructures. When SR = 1/7, the heteronanostructures exhibited the highest PL efficiency, whose PL intensity ( 5 D 0 -7 F 2 emission) was 27% higher than that of the YVO 4 :Eu 3? nanocrystals. YBO 3 is an ideal shell material for composite phosphors with core/shell heterostructures due to its high VUV transparency, high stability, low synthesis temperature and exceptional optical damage threshold.