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2008, 112, 17042–17045 Published on Web 10/10/2008
Enhanced Photoluminescence of Water Soluble YVO4:Ln3+ (Ln ) Eu, Dy, Sm, and Ce) Nanocrystals by Ba2+ Doping Guofeng Wang,† Weiping Qin,*,† Daisheng Zhang,†,‡ Lili Wang,† Guodong Wei,† Peifen Zhu,† and Ryongjin Kim† State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin UniVersity, Changchun 130012, P. R. China, and College of Physics, Beihua UniVersity, Jilin 132011, P. R. China ReceiVed: August 25, 2008; ReVised Manuscript ReceiVed: September 27, 2008
Water soluble YVO4:Ln3+ and YVO4:Ln3+/Ba2+ (Ln ) Ce, Dy, Eu, and Sm) nanocrystals were synthesized by a polyvinylpyrrolidone-assisted hydrothermal method. Under the excitation of the host absorption, phosphors can emit blue light for YVO4:Ce3+/Ba2+, yellow light for YVO4:Dy3+/Ba2+, red light for YVO4:Eu3+/Ba2+, and reddish orange light for YVO4:Sm3+/Ba2+. In comparison with those of YVO4:Ln3+ nanocrystals, the emissions of YVO4:Ln3+/Ba2+ are greatly enhanced. Furthermore, the excitation spectra of YVO4:Eu3+/Ba2+ and YVO4:Dy3+/Ba2+ show the similar features, which are different from those of YVO4:Sm3+/Ba2+ and YVO4:Ce3+/Ba2+. 1. Introduction In recent years, rare earth (RE) ions doped yttrium orthovanadate (YVO4) phosphors have attracted intensive attention due to their wide applications in lamps and displays.1-5 Moreover, investigations show that nanosized YVO4:Ln3+ phosphors have a significant promise in high definition flat display panel and water soluble nanocrystals have potential applications in biology.6-8 Consequently, a great deal of research work has been devoted to the investigation of the YVO4:Ln3+ phosphors with high efficiency.2,9 For example, Tian’s group have reported the enhanced photoluminescence of YVO4:Eu3+ by codoping large-sized divalent ions, such as Sr2+, Ba2+, and Pb2+ ion.10 However, the YVO4:Ln3+ codoped with large divalent ions are usually synthesized by solid state reaction, which can result in the increase of particle sizes as well as the impure phases in samples. Therefore, how to obtain the pure phase of YVO4:Ln3+ nanocrystals codoped with large divalent ions is the key in successfully achieving a brighter phosphor. In this study, water soluble YVO4:Ln3+ and YVO4:Ln3+/Ba2+ nanocrystals were synthesized by a facile hydrothermal method. The luminescence properties were studied in detail. In comparison with those of the YVO4:Ln3+ nanocrystals, the emissions of the YVO4:Ln3+/Ba2+ are greatly enhanced. 2. Experimental Details Polyvinylpyrrolidone (PVP, 55 kDa), Na3VO4, BaCl2, and HCl were supplied by Beijing Chemical Reagent Company, and were of analytical grade. Y2O3 (99.99%), CeO2 (99.99%), Dy2O3 (99.99%), Sm2O3 (99.99%), and Eu2O3 (99.99%) were supplied by Shanghai Chemical Reagent Co. All of the reagents and * Corresponding author. Tel/fax: +86 431 85168240 8325. E-mail address:
[email protected]. † Jilin University. ‡ Beihua University.
10.1021/jp807577b CCC: $40.75
solvents were used as received without further purification. PVP stock solution (5%) and Na3VO4 stock solution (0.095 M) were prepared by dissolving PVP and Na3VO4 in deionized water, respectively. LnCl3 (Ln ) Y, Ce, Dy, Sm, and Eu) stock solutions (0.1 M) were prepared by dissolving the corresponding lanthanide oxides in dilute HCl. In a typical procedure for the preparation of YVO4:Ln3+ nanocrystals, 6 mL of Na3VO4 solution were added dropwise to a well-stirred solution of 6 mL of LnCl3 and 3 mL of PVP at 60 °C. The resulting mixture was stirred for another 10 min, then transferred to a 20-mL Teflon-lined autoclave and subsequently heated at 170 °C for 2 h (the pressure in the reaction vessel was ∼2 MPaG). The obtained nanoparticles were collected by centrifugation, washed with ethanol and deionized water several times, and dried in an oven at 50 °C for 24 h. YVO4:Ln3+/Ba2+ nanocrystals were prepared by the same procedure, except for adding additional 5% BaCl2 into the solution of LnCl3 at the initial stage. The crystal structure was analyzed by a Rigaku RU-200b X-ray powder diffractometer (XRD) using a nickel-filtered Cu KR radiation (λ ) 1.4518 Å). The size and morphology were investigated by transmission electron microscope (TEM, JEM 2010 with operating voltage of 200 kV). Excitation and emission spectra were recorded with a Hitachi F-4500 fluorescence spectrophotometer (2.5 nm for spectral resolution and 400 V for PMT voltage) at room temperature. Luminescence decay curves were recorded by a Spex 1403 spectrometer. 3. Results and Discussion 3.1. Crystal Structure and Morphology. Water soluble YVO4:Ln3+ and YVO4:Ln3+/Ba2+ nanocrystals were synthesized in aqueous media in the presence of PVP. Figure 1a shows the XRD patterns of YVO4:Eu3+(5%) and YVO4:Eu3+(5%)/ Ba2+(5%) nanocrystals. All the diffraction peaks can be easily indexed to a pure tetragonal YVO4 (JCPDS 17-0341). No other 2008 American Chemical Society
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J. Phys. Chem. C, Vol. 112, No. 44, 2008 17043
Figure 1. XRD patterns and TEM images of YVO4:Eu3+(5%) and YVO4:Eu3+(5%)/Ba2+(5%) nanoparticles. Figure 3. Excitation spectra of YVO4:Eu3+(5%) and YVO4:Eu3+(5%)/ Ba2+(5%) nanocrystals in aqueous solutions monitored at 616 nm.
Figure 2. Emission spectra of YVO4:Eu3+ and YVO4:Eu3+/Ba2+ nanocrystals in aqueous solutions under 310-nm excitation. The inset shows the photographs of the nanocrystal solutions under 254-nm excitation: (a) YVO4:Eu3+(1%), (b) YVO4:Eu3+(5%), and (c) YVO4: Eu3+(5%)/Ba2+(5%).
impurity peaks were detected. Figure 1b shows the TEM images of the as-synthesized YVO4:Eu3+(5%) and YVO4:Eu3+(5%)/ Ba2+(5%) nanocrystals with an average size of ∼20 nm. It is worth noting that the morphology of YVO4:Eu3+(5%) nanocrystals has not obvious change after doping with Ba2+ ions. 3.2. Enhanced Luminescence from YVO4:Ln3+/Ba2+ Nanocrystal Solutions. 3.2.1. YVO4:Eu3+/Ba2+. Figure 2 shows the room-temperature emission spectra of YVO4:Eu3+ and YVO4: Eu3+/Ba2+ nanocrystals in aqueous solutions (1 mM) under 310nm excitation. Corresponding visible emissions of the particle solutions (1 mM) upon excitation at 254 nm are shown in the inset of Figure 2. Obviously, the luminescence intensity increased with increasing Eu3+ concentration, up to about 5 mol %, and then decreased abruptly. The peak positions and the shape of emissions were independent of Eu3+ concentration. In comparison with those of the YVO4:Eu3+(5%) nanocrystal solution, the emissions from the YVO4:Eu3+(5%)/Ba2+(5%) nanocrystal solution are greatly enhanced. The enhanced emissions of YVO4:Eu3+/Ba2+ were attributed to the increased absorption coefficient of the UV pump light or the enhanced luminescence efficiency by changing the composition and lattice parameters.10 Similar enhancement phenomenon has also been observed in the YVO4:Dy3+/Ba2+, YVO4:Sm3+/Ba2+, and YVO4:Ce3+/Ba2+ nanocrystal solutions, which will be described later. The emission spectra of YVO4:Eu3+ and YVO4:Eu3+/Ba2+ nanocrystals show similar features. The 5D0 f 7FJ (J ) 1, 2, 3, 4) and 5D1 f 7F1 transitions were observed. Because the 4f energy levels of Eu3+ are hardly affected by the crystal field,11,12 there is no notable shift in the positions of the emission peaks compared to other Eu3+-doped systems.13 Some of these transitions satisfy magnetic dipole selection rules (∆J ) 0, (1
except 0 T 0). The 5D0 f 7F1 and 5D1 f 7F1 transitions are magnetic-dipole-allowed and their intensities are almost independent of the local environment around Eu3+ ions.14 The 5D0 f 7F2 transition is electric-dipole-allowed due to an admixture of opposite parity 4fn-15d states by an odd parity crystal-field component.15,16 Therefore, its intensity is sensitive to the local structure around Eu3+ ions. The 5D0 f 7F3 transition exhibits a mixed magnetic dipole and electric dipole character.14 The 5D0 f 7F4 is electric dipole transition. Figure 3 shows the room-temperature excitation spectra (monitored at 616 nm) of YVO4:Eu3+ and YVO4:Eu3+/Ba2+ nanocrystals in aqueous solutions. For the YVO4:Eu3+ nanocrystals, there is only one excitation band centered at 310 nm. However, for the YVO4:Eu3+/Ba2+ nanocrystals, two excitation bands centered at 250 and 310 nm were observed. It is wellknown that the 310-nm excitation band is attributed to the charge transfer from the oxygen ligands to the central vanadium atom inside the VO43- absorption.17,18 The 250-nm excitation band is assigned to the overlap of VO43- absorption and charge transfer transition between Eu3+ and O2-.18,19 Zhou et al. have reported that the VO43- absorption in YVO4:Eu3+ occurred much more easily than charge transfer of Eu3+ f O2- because of the large differences of charges and ionic radii between V5+ (+5, r ) 0.0355 nm) and Eu3+ (+3, r ) 0.107 nm).17 For the YVO4:Eu3+/Ba2+ nanocrystals, the substitution of the Y3+ site with the divalent ions Ba2+ results in a negative charge SrY′ in the lattice.10 When Ba2+ ions were incorporated into the YVO4: Eu3+ lattice, the energy transfers not only from VO43- to Eu3+ but also from excited states of O2- (the charge transfer states) to Eu3+ obviously enhanced. Therefore, the excitation spectra of YVO4:Eu3+ and YVO4:Eu3+/Ba2+ nanocrystal solutions are different. The strong luminescence of Eu3+ results from an efficient energy transfer from the VO43- group to Eu3+ in YVO4:Eu3+ as reported previously.19,20 Figure S1 in Supporting Information shows the schematic energy level diagram, energy transfer, and luminescence processes. To further prove the energy transfer process from VO43- to Eu3+, decay curve of Eu3+ (5D0 f 7F2) in YVO4:Eu3+(5%)/Ba2+(5%) nanocrystal solution was recorded (Figure S2 in Supporting Information). Obviously, the emission of Eu3+ increases from t ) 1 to 23 µs and then begins to decay. The initial increase for the luminescence of Eu3+ within 23 µs can be attributed to the finite transfer time involved in the VO43-/O2- f Eu3+ energy transfer processes and the population of the 5D0 level via the higher level (5D1) after energy transfer.1,20 The subsequent decay cannot be fitted by single- or secondexponential function, which is related to the complicated VO43-/ O2- f Eu3+ energy transfers in the YVO4:Eu3+/Ba2+ nanoc-
17044 J. Phys. Chem. C, Vol. 112, No. 44, 2008 rystals.1 We can see that the VO43- f Eu3+ energy transfer rate is much faster than the radiative rate of Eu3+, which is in good agreement with the result reported by Yu et al.20 3.2.2. YVO4:Dy3+/Ba2+. Besides the europium ion, Dy3+ ion is another good activator and Dy3+-doped YVO4 is a potential white phosphor because of the yellow (4F9/2 f 6H13/2) and blue (4F9/2 f 6H15/2) emissions of Dy3+. Figure 4a shows the excitation and emission spectra of YVO4:Sm3+(5%) and YVO4: Sm3+(5%)/Ba2+(5%) nanocrystals in aqueous solutions. Corresponding visible emissions of the particle solutions upon excitation at 254 nm are shown in the inset of Figure 4a. Under 310-nm excitation, the 4F9/2 f 6H13/2 (∼572 nm), 4F9/2 f 6H15/2 (∼482 nm), and 4F9/2 f 6H11/2 (∼660 nm) transitions were observed. 3.2.3. YVO4:Sm3+/Ba2+. Figure 4b (left) shows the excitation spectra (monitored at 600 nm) of YVO4:Sm3+(5%) and YVO4: Sm3+(5%)/Ba2+(5%) nanocrystals in aqueous solution, which consist of an intense band with two peaks at 260 and 290 nm. Figure 4b (right) shows the emission spectra of YVO4: Sm3+(5%)/Ba2+(5%) nanocrystal solution under 290-nm excitation. It can been seen that each emission spectrum consists of four peaks at 564, 600, 644, and 701 nm, which correspond to 4G 6 4 6 4 6 4 6 5/2 f H5/2, G5/2 f H7/2, G5/2 f H9/2, and G5/2 f H11/2 transitions, respectively. Corresponding visible emissions of the particle solutions upon excitation at 254 nm are shown in the inset of Figure 4b. 3.2.4. YVO4:Ce3+/Ba2+. Ce3+-doped ionic crystals have attracted increasing attention for their applications in scintillators and tunable lasers in the ultraviolet and visible ranges. The luminescence of Ce3+ ion is induced by the transitions from 5d to 4f states. The 5d states strongly couple to the host material because it is out of 5s and 5p shells, resulting in 5d state being broadened.21,22 Figure 4c (right) shows the typical doublet 5d-4f emission of Ce3+ in YVO4:Ce3+(5%) and YVO4:Ce3+(5%)/ Ba2+(5%) nanocrystals under 270-nm excitation. The corresponding excitation spectra are shown in Figure 4c (left). The YVO4:Eu3+/Ba2+ and YVO4:Dy3+/Ba2+ nanocrystals have similar excitation spectra, which are different from YVO4: Sm3+/Ba2+ and YVO4:Ce3+/Ba2+. It is well-known that Eu3+, Dy3+, and Sm3+ ions have abundant energy levels around 310 nm (VO43- absorption), which are beneficial for the energy transfers from VO43- to Eu3+, Dy3+, and Sm3+. For YVO4: Eu3+/Ba2+ and YVO4:Dy3+/Ba2+, the excitation intensity of the VO43- band was much stronger than that of the charge transfer band (O2--Eu3+/Dy3+), indicating that the energy transfer from VO43- to Eu3+/Dy3+ was more effective than that from excited states of O2- (the charge transfer states) to Eu3+/Dy3+. For YVO4:Sm3+/Ba2+, the excitation intensity of VO43- band was a little stronger than that of the charge transfer band (O2--Sm3+), indicating that there was no significant difference in energy transfer efficiencies from VO43- to Sm3+ and from O2- to Sm3+. For YVO4:Ce3+/Ba2+, Ce3+ ions have not abundant energy levels around 310 nm (VO43- absorption), resulting in inefficient energy transfer from VO43- to Ce3+. Corresponding visible emissions of the particle solutions upon excitation at 254 nm are shown in the inset of Figure 4c. As mentioned above, the emissions from YVO4:Ln3+/Ba2+ are stronger than those from YVO4:Ln3+. 4. Conclusion In summary, water soluble YVO4:Ln3+ and YVO4:Ln3+/Ba2+ nanocrystals were synthesized in aqueous media in the presence of PVP. Under the excitation of the host absorption, phosphors can emit blue, yellow, red, and reddish orange light for YVO4:
Letters
Figure 4. Excitation (left) and emission (right) spectra of nanocrystals in aqueous solutions. (a) YVO4:Dy3+(5%) and YVO4:Dy3+(5%)/ Ba2+(5%), (b) YVO4:Sm3+(5%) and YVO4:Sm3+(5%)/Ba2+(5%), and (c) YVO4:Ce3+(5%) and YVO4:Ce3+(5%)/Ba2+(5%). Inset shows the photographs of (a) YVO4:Dy3+, (b) YVO4:Dy3+/Ba2+, (c) YVO4:Sm3+, (d) YVO4:Sm3+/Ba2+, (e) YVO4:Ce3+ and (f) YVO4:Ce3+/Ba2+ nanocrystal solutions under 254-nm excitation.
Ce3+/Ba2+, YVO4:Dy3+/Ba2+, YVO4:Eu3+/Ba2+, and YVO4: Sm3+/Ba2+, respectively. In comparison with those of the YVO4: Ln3+ nanocrystals, the emissions of the YVO4:Ln3+/Ba2+ are greatly enhanced. Acknowledgment. This research was supported by Natural Science Foundation of China (Grant Nos. 50672030 and 10874058).
Letters Supporting Information Available: Figure S1. Decay curve of Eu3+ (5D0 f 7F2) in YVO4:Eu3+(5%)/Ba2+(5%) nanocrystal solution. Figure S2. Schematic energy level diagram, energy transfer, and luminescence processes. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Zhou, Y.; Lin, J. J. Alloy. Compd. 2006, 408, 856. (2) Pan, G.; Song, H.; Bai, X.; Fan, L.; Yu, H.; Dai, Q.; Dong, B.; Qin, R.; Li, S.; Lu, S.; Ren, X.; Zhao, H. J. Phys. Chem. C 2007, 111, 12472. (3) Wang, H.; Yu, M.; Lin, C.; Lin, J. J. Colloid Interface Sci. 2006, 300, 176. (4) Zhang, H.; Fu, X.; Niu, S.; Xin, Q. J. Alloy. Compd. 2008, 457, 61. (5) Li, G.; Chao, K.; Peng, H.; Chen, K. J. Phys. Chem. C 2008, 112, 6228. (6) Riwotzki, K.; Haase, M. J. Phys. Chem. B 1998, 102, 10129. (7) Zhang, H.; Fu, X.; Niu, S.; Sun, G.; Xin, Q. J. Solid State Chem. 2004, 177, 2649.
J. Phys. Chem. C, Vol. 112, No. 44, 2008 17045 (8) Wang, F.; Xue, X.; Liu, X. Angew. Chem., Int. Ed. 2008, 47, 906. (9) Park, W.; Jung, M.; Yoon, D. Sens. Actuators B 2007, 126, 324. (10) Tian, L.; Mho, S. J. Lumin. 2007, 122, 99. (11) Zhang, M.; Fan, H.; Xi, B.; Wang, X.; Dong, C.; Qian, Y. J. Phys. Chem. C 2007, 111, 6652. (12) Tao, F.; Wang, Z.; Yao, L.; Cai, W.; Li, X. J. Phys. Chem. C 2007, 111, 3241. (13) Li, C.; Quan, Z.; Yang, J.; Yang, P.; Lin, J. Inorg. Chem. 2007, 46, 6329. (14) Ray, S.; Pramanik, P. J. Appl. Phys. 2005, 97, 094312. (15) Judd, B. Phys. ReV. 1962, 127, 750. (16) Ofelt, G. J. Chem. Phys. 1962, 37, 511. (17) Zhou, Y.; Lin, J. Opt. Mater. 2005, 27, 1426. (18) Wang, Y.; Zuo, Y.; Gao, H. Mater. Res. Bull. 2006, 41, 2147. (19) Pan, G.; Song, H.; Bai, X.; Liu, Z.; Yu, H.; Di, W.; Li, S.; Fan, L.; Ren, X.; Lu, S. Chem. Mater. 2006, 18, 4526. (20) Yu, M.; Lin, J.; Fang, J. Chem. Mater. 2005, 17, 1783. (21) Ruan, Y.; Zhang, S.; Lu, S.; Li, G.; Li, W.; Liu, J. J. Rare Earths 2007, 25, 122. (22) Elias, L.; Heaps, W.; Yen, W. Phys. ReV. B 1973, 8, 4989.
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