Synthesis of YVO4:Eu3+/YBO3Heteronanostructures with Enhanced Photoluminescence Properties
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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.
KeywordsCore/shell heteronanostructures Nanophosphors Photoluminescence Yttrium vanadate Yttrium borate
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 higher-resolution 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 . 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 . 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 (YVO4:Eu3+) 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 YVO4:Eu3+ nanophosphors [16, 17, 18], but few measures have been taken to improve their luminescence efficiency. In this paper, we propose novel YVO4:Eu3+/YBO3 core/shell heteronanostructures that exhibit enhanced photoluminescence efficiency. Compared to the reported heteronanostructures of YVO4:Eu3+ such as Y2O3:Eu3+@SiO2@YVO4:Eu3+, SiO2@YVO4:Eu3+ and YV0.7P0.3O4:Eu3+,Bi3+@SiO2 [19, 20, 21], yttrium borate (YBO3), is used as shell material in this new heteronanostructures. YBO3 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.
The YVO4:Eu3+/YBO3 core/shell heteronanostructures were prepared by a facile two-step method. The YVO4:Eu3+ nanocrystals (cores) doped with 5 mol% europium were prepared by hydrothermal method. The YBO3 nanocoatings (shells) were grown onto the cores by the sol–gel method reported in our previous literature . The shell ratio (SR) is molar percentage of the shell material (YBO3) in the core/shell heteronanostructures. In this study, different shell ratios such as 1/9, 1/8, 1/7, 1/5, 1/3, 1/2 and 2/3 were adopted, so a total of seven heteronanostructures were prepared.
Preparation of YVO4:Eu3+Nanocrystals
To 130 mL of deionized water, 30.4 mL of Y(NO3)3solution (0.15 mol/L), 1.6 mL of Eu(NO3)3solution (0.15 mol/L) and 0.758 g of NaVO3·2H2O 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 YVO4:Eu3+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(NO3)3·6H2O and 0.68 g of H3BO3(~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(NO3), 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 YVO4:Eu3+/YBO3Heteronanostructures
Herein, we take the heteronanostructures (SR = 1/7) as an example to present their detailed procedures. The YVO4:Eu3+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 YVO4:Eu3+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 YVO4:Eu3+/YBO3heteronanostructures (SR = 1/7) were obtained.
In this paper the YVO4:Eu3+(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 “YVO4:Eu3+/YBO3core/shell heteronanostructures (SR = 0)”. In addition, YBO3powder 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 Kα radiation (λ = 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 MgKα (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
YVO4:Eu3+/YBO3core/shell heteronanostructures with different shell ratios (SRs) were successfully prepared by sol–gel growth of YBO3nanocoating onto the YVO4:Eu3+nanocrystals. Characterizations by means of XRD, TEM and XPS confirmed the formation of the YVO4:Eu3+/YBO3core/shell heteronanostructures. The heteronanostructures exhibited much stronger photoluminescence (PL) than the YVO4:Eu3+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 (5D0–7F2emission) was 27% higher than that of the YVO4:Eu3+nanocrystals. YBO3is 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.
This work was supported by the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of Zhejiang Province. Authors also thank financial supports from the Doctoral Science Foundation of Zhejiang Sci-Tech University (no. 0803611-Y).