J. Phys. Chem. C 2007, 111, 3241-3245
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Synthesis and Photoluminescence Properties of Truncated Octahedral Eu-Doped YF3 Submicrocrystals or Nanocrystals Feng Tao, Zhijun Wang, Lianzeng Yao,* Weili Cai, and Xiaoguang Li Department of Materials Science and Engineering, UniVersity of Science and Technology of China, Hefei 230026, People’s Republic of China ReceiVed: September 11, 2006; In Final Form: December 5, 2006
Highly uniform and monodisperse Eu:YF3 nanosized and submicrosized truncated octahedra are successfully prepared in large quantities using a facile hydrothermal approach assisted by a capping reagent, ethylenediaminetetraacetic acid disodium salt (Na2H2EDTA). X-ray diffraction, field emission scanning microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, and selected area electron diffraction have been used to study the morphologies and crystal structure. The photoluminescence spectra at room temperature show that, in addition to the characteristic red emission peaks of Eu3+, an intense blue emission band centered at about 445 nm originating from the 4f65d to 4f7 configuration in Eu2+ is also observed when the crystals are excited at 393 nm, which is quite different from those reported previously. The results reveal that there should be a redox reaction during hydrothermal preparation of YF3:Eu at 180 °C, and some of the Eu3+ ions should have been reduced to the divalent state in the preparation process.
Introduction al.1
Since Bhargava et reported that doped nanocrystalline phosphors yielded high luminescence efficiencies in 1994, there has been an increasing interest in the doping of nanocrystal hosts with rare earth ions.2-7 With rapid shrinking in size, rare earth doped nanocrystalline materials may play an outstanding role as phosphors in lamps and display devices,8 as components in optical telecommunication,9 as active materials in solid-state lasers,10 in new optoelectronic devices,11 and as biological labels12 in the near future. Such applications rely on the luminescence of lanthanide ions (sharp lines and high efficiency). It is well-known that the shape and size of inorganic submicrocrystals and nanocrystals have great influence on their physical properties.13-15 Therefore, it is important to synthesize inorganic submicrocrystals and nanocrystals with a controlled shape and size. A number of synthesis techniques such as the sol-gel route,16-18 chemical vapor synthesis,19 combustion,20 coprecipitation,21,22 and hydrothermal techniques23 have been developed so far to synthesize submicrocrystals and nanocrystals with a controlled size and shape. Yttrium trifluoride (YF3) doped with rare earth ions is a potential candidate as an efficient phosphor owing to its capability of producing efficient visible emission under vacuum UV irradiation.24,25 So far, optical properties of rare earth ion doped YF3 nanocrystalline materials have been investigated extensively. Unique luminescence properties of rare earth ions incorporated in glass matrixes have long been recognized.26,27 Ln3+:YF3 nanocrystals with down/up conversion have also been synthesized using a sonochemistry-assisted hydrothermal route.28 Fan et al. studied the luminescence properties of Ln3+-doped YF3 nanoparticles synthesized using a hydrothermal technique.29 Furthermore, Pankratov et al. investigated the intrinsic luminescence for YF3:Eu at 10 K.24 However, the as-obtained YF3 crystals were almost irregular nanoparticles. Since the properties * Corresponding author. Telephone: +86-551-3600249. E-mail: yaolz@ ustc.edu.cn.
of inorganic nanomaterials can greatly be changed by tailoring their morphology and crystallinity, some recent efforts have been devoted to the synthesis of inorganic submicrocrystals and nanocrystals with well-defined nonspherical morphologies. Recently, YF3 nanoparticles with quadrilateral and hexagonal shapes have been synthesized by a reverse microemulsion technique.30 In this paper, we report the preparation of high-quality YF3: Eu truncated octahedral nanocrystals using a simple hydrothermal approach, and a study of their room-temperature photoluminescence behavior. Experimental Section The initial chemicals used in this work were yttrium oxide (Y2O3, 99.99%), europium oxide (Eu2O3, 99.99%), ethylenediaminetetraacetic acid disodium salt (Na2H2EDTA, AR), ammonium fluoride (NH4F, AR), and HNO3 (AR) without further purification. In a typical reaction, 0.429 g of Y2O3 and 0.0352 g of Eu2O3 (5% in molar ratio) were completely dissolved in dilute nitric acid to form a clear aqueous solution and the pH was adjusted to about 2-3. Then, 0.729 g of Na2H2EDTA was added to the solution to form a chelated yttrium complex. After magnetic stirring for 10 min, 0.51 g of NH4F was added to the solution. Subsequently, the solution was transferred into a 30 mL Teflonlined autoclave. The Teflon vessel was then filled with distilled water to about 80% of its volume. After the autoclave was tightly sealed, it was heated at 180 °C for 48 h and then cooled to room temperature naturally. The products were washed several times with distilled water and absolute ethanol in turn, and finally dried in air at about 70 °C for 12 h. X-ray diffraction (XRD) of the samples was performed using an X-ray diffractometer (D/MAX-γA) with Cu KR radiation (λ ) 0.154 18 nm) in the range 20° e 2θ e 80°. The morphology of the YF3:Eu submicrocrystals and nanocrystals was investigated on a transmission electron microscope (TEM; H800) with an accelerating voltage of 200 kV and a field
10.1021/jp065905z CCC: $37.00 © 2007 American Chemical Society Published on Web 02/06/2007
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Figure 1. X-ray diffraction pattern of the as-obtained product.
emission scanning electron microscope (FESEM; JEOL-JSM6700F). A high-resolution transmission electron microscope (HRTEM; JEOL-2010) and selected area electron diffraction (SAED) attached to it were employed to characterize the crystal structure. The photoluminescence spectra were measured at room temperature with a steady-state/lifetime fluorescence spectrometer (FLUOROLOG-3-TAU) (excitation (EX) slits ) 2 nm; emission (EM) slits ) 5 nm). Results and Discussion The X-ray diffraction pattern of the as-obtained product is shown in Figure 1. It can be seen from Figure 1 that all of the diffraction peaks can be indexed according to the orthorhombic YF3 [space group Pnma (62)] with lattice constants (a ) 0.6353, b ) 0.6853, and c ) 0.4393 nm) very close to the reported data in the literature (JCPDS 74-0911). No impurity can be identified from the XRD pattern, suggesting that our synthesis is a promising method to prepare pure and single-phase YF3. Typical FESEM images of the YF3:Eu crystals prepared at 180 °C for 48 h are presented in Figure 2. An abundance of almost uniform and regular truncated octahedra with an average length of about 700 nm and smooth surfaces can be seen in Figure 2a, and the inset is the magnification of some YF3:Eu truncated octahedra. Figure 2b is the magnified FESEM image of a single truncated octahedron with sharp edges. Well-defined truncated octahedron morphology is the characteristic of singlecrystalline orthorhombic-structured YF3:Eu crystals bound by eight {111} planes (see Figure 2b). The rhombus base of the truncated octahedron is indexed to the {010} lattice plane. It is also noticed in Figure 2b that some small particles are attached on the surfaces of the octahetra, which suggests that these small particles may serve as building blocks for growing the nanooctahedra. Figure 2c is a schematic crystallographic diagram of the truncated octahedron. Figure 3 displays a typical TEM image of YF3:Eu crystals and SAED pattern of a single particle. Well-defined nanoparticles with different shapes are observed in Figure 3a, most of which are rhombuses, and the others are hexagons. Comparing Figure 3a with the FESEM image in Figure 2, it is known that the hexagons and rhombuses represent, in fact, the same particle morphology observed from different facets. Figure 3b is the TEM image of a single truncated octahedron showing a rhombic projection, and the inset is the selected area electron diffraction (SAED) pattern corresponding to the single truncated octahedron. The diffraction spots are indexed to the (002), (101), and (200) planes, respectively, of the orthorhombic YF3:Eu, which indicates that the octahedron is a well-developed single crystal. Figure 4 shows a typical HRTEM image of a relatively small truncated octahedron with a rhombus base. The measured lattice spacing is about 0.32 and 0.44 nm corresponding to the distances of the {200} and {001} planes, respectively, of the orthorhombic
Figure 2. Typical FESEM images of Eu:YF3 crystals prepared at 180 °C for 48 h. (a) FESEM image of the Eu:YF3 crystals; the inset is the magnification of some Eu:YF3 truncated octahedra. (b) Magnified FESEM image of a single truncated octahedral. (c) Schematic diagram of the truncated octahedron.
Figure 3. Typical TEM image of Eu:YF3 crystals and SAED pattern of a single particle. (a) Typical TEM image of Eu:YF3 crystals with different shapes. (b) TEM image of a single truncated octahedron Eu: YF3 nanocrystal; the inset is the selected area electron diffraction (SAED) pattern corresponding to the single truncated octahedron Eu: YF3 nanocrystal.
YF3:Eu. This image also reveals the single-crystal nature of the product.
Truncated Octahedral Eu-Doped YF3 Nanocrystals
Figure 4. HRTEM image of the rhombus base of a relatively small truncated octahedron.
Figure 5. Excitation and emission spectra of YF3:Eu3+ (5%) (excitation (EX) slits ) 2 nm; emission (EM) slits ) 5 nm). (a) Room-temperature excitation spectrum monitored at 593 nm (550 filter). (b) Emission spectrum of YF3:Eu3+ nanocrystals excited at 393 nm.
Figure 5 shows the room-temperature excitation (monitored at 593 nm) and emission (excited at 393 nm) spectra of the YF3:Eu3+ (5%) submicrocrystals and nanocrystals suspended in absolute ethanol. Figure 5a is the excitation spectrum, which consists of the characteristic absorption peaks of Eu3+ corresponding to the direct excitation from the europium ground state into the higher excited states of the europium f-electrons. The most intense peak appears at 393 nm with a width of only 3 nm at half-maximum. The position of these sharp lines is practically identical to the characteristic absorption for the f-f transition in Eu3+,31 and the excitation spectra reported for other Eu3+-doped materials.32 Some small and sharp lines can also be observed in the range of 200-300 nm in the excitation
J. Phys. Chem. C, Vol. 111, No. 8, 2007 3243 spectrum. This is quite different from those excitation spectra reported previously.28,29 It is proposed that there should be a charge-transfer (CT) transition when λ < 200 nm. Krupa and Queffelec33 have reported the CT transition energy from F- to Eu3+ is about 8.15 eV (∼152 nm) in LiYF4:Eu3+ due to the high electronegativity for pure fluoride systems, and the threshold of the CT absorption is raised to a much higher energy into the vacuum ultraviolet (VUV) region. Unlike in the oxide systems, the CT transition energy in the Eu-doped systems strongly depends on the electronegativity of the ligand. Figure 5b is the emission spectrum of the YF3:Eu submicrocrystals and nanocrystals excited at 393 nm, which shows sharp lines mainly in the red spectrum range from 575 to 630 nm corresponding to the Eu3+ transition from the excited 5D0 and 5D levels to the 7F (j ) 1, 2, 3, 4) levels. The most intense 1 j peaks in Figure 5b are centered at 588 and 593 nm. There is no notable shift in positions of the emission peaks compared to other Eu3+-doped systems since the 4f energy levels of Eu3+ are hardly affected by the crystal field because of the shielding effect of the 5s25p6 electrons. It is well-known that the 5D0-7F2 transition is parity forbidden and can be observed only when the lattice environment is distorted and contains a noninversion symmetry. Being a forced electric dipole transition, this transition is hypersensitive to the environment. However, the 5D0-7F1 transition is insensitive to the environment due to allowed magnetic dipole consideration and the ratio of the two intensities is a good measure for the symmetry of the Eu3+ site. The 5D0-7F1 magnetic dipole transition in the inversion symmetry site is dominating, while the 5D0-7F2 electric dipole transition in the noninversion symmetry site is the strongest. Therefore, it is inferred by comparing the intensity at 593 nm (5D0-7F1) with that at 616 nm (5D0-7F2) in Figure 5b that the Eu3+ ions in YF3:Eu3+ occupy a site with a small deviation from the inversion symmetry. A very strong emission band centered at about 445 nm in Figure 5b can also be seen when the crystals are excited at 393 nm, which probably corresponds to the transition from the 4f65d configuration to the 4f7 configuration in Eu2+. A similar result was also observed by Hong in the CaF2:Eu and MgF2:Eu phosphors using a sol-gel synthesis.34 Efficient Eu2+ emission has been obtained in a number of compounds,35-37 and many of them have found applications. UV-emitting phosphors are useful in erythermal and photocopying lamps. Blue-emitting phosphors such as Sr4Al14O25:Eu38 and BaAl12O19:Eu39 have found applications as lamp phosphors and blue components of color television phosphors. Efficient Eu2+ emission has also been observed in europium doped fluoride materials. Dujardin et al. studied the fluorescence properties and the f-d and f-f transitions of Eu2+ in M1-xNxF2 mixed fluoride crystals (M, N ) Ca, Sr, and Ba).40,41 Moine et al. investigated luminescence characteristics of Eu2+ in the BaF2 crystal.42 Dorenbos summarized the energy of the 4f7 f 4f65d transition of Eu2+ in inorganic compounds.43 The Eu2+ emission in different hosts lies in characteristically different ranges due to different structures. The characteristic excitation and emission spectra of Eu2+ ions are illustrated in Figure 6. Figure 6a is the excitation spectrum of the as-prepared YF3:Eu nanocrystals when monitored at about 445 nm. The excitation band with a maximum at about 323 nm corresponds to the 4f-5d transition of Eu2+. It can be seen from Figure 6b that a very intense emission band at about 390 nm is observed when the nanocrystals are excited at 323 nm, which corresponds to the 5d-4f emission of Eu2+. Figure 7 shows the emission spectra of the
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Figure 8. Relationship between the luminescence peak and excitation wavelength.
Figure 6. Excitation and emission spectra of YF3:Eu2+ (5%) (EX slits ) 2 nm; EM slits ) 5 nm). (a) Excitation spectrum of YF3:Eu3+ nanocrystals with emission wavelength at about 445 nm. (b) Emission spectrum of YF3:Eu3+ nanocrystals excited at 323 nm.
Figure 7. Emission spectra of Eu2+ ions in as-prepared YF3:Eu nanocrystals under different excitation wavelengths at room temperature.
Eu2+ ions in the as-prepared YF3:Eu nanocrystals under different excitations at room temperature. Table 1 gives the peak positions of the emission band varying with the excitation wavelength from 300 to 393 nm. Figure 7 and Table 1 clearly show that the emission band of Eu2+ moves from 445 to 390 nm when the excitation wavelength changes from 393 to 300 nm. The relationship between the peak position of the luminescence band and the excitation wavelength is displayed in Figure 8. It is found in Figure 8 that the red shift of the luminescence band is mild when the excitation wavelength changes from 300 to 350 nm. This is due to the fact that the Eu2+ emission originates
from the formation of an impurity-trapped exciton state which gives rise to a broad red-shifted emission. A sharp increase in the red shift of the emission band is observed as the excitation wavelength is longer than 350 nm. When the excitation wavelength is longer than 380 nm, however, the red shift of the emission band turns mild again, and the luminescence of Eu3+ begins to appear. The red shift of the emission band with the increase of the excitation wavelength strongly suggests the energy transfer from Eu2+ to Eu3+.44 The energy transfer from the excited Eu2+ ions to the neighboring Eu3+ ions through a nonradiative relaxation seems to affect the fluorescence of Eu3+. It is observed from Figure 5b that some of the Eu3+ ions have been reduced to the divalent state during hydrothermal preparation of YF3:Eu at 180 °C. The europium anomalies in the hydrothermal synthesis may arise from the possibility of reduction of Eu3+ to Eu2+, which corresponds to the photoluminescence spectra of the as-prepared YF3:Eu. Sverjensky45 described the europium redox equilibria in an aqueous solution in 1984 and found that the trivalent state of europium should be dominating at low temperatures except possibly in the most reducing, alkaline pore waters of anoxic marine sediments. However, at temperatures higher than about 250 °C and elevated pressures, europium in aqueous solution should be divalent, such as under most hydrothermal and metamorphic conditions. At intermediate temperatures, around 100 °C, significant activities of both Eu2+ and Eu3+, and related complexes, can occur in aqueous solutions, depending on the oxidation state and the pH of the solutions, and the activities of potential ligands such as sulfate, carbonate, and chloride. The predicted stability of divalent europium in an aqueous solution at elevated temperatures is consistent with our result. It is also found that Eu3+ can easily change to Eu2+ in KCaF3 in the presence of minimal water.46 A similar result was also observed by Dhopte in the CaSO4:Eu system.47 The mechanism responsible for the reduction process should be as follows: -(n-1)H2O 1 2Eu3+‚nH2O 98 2Eu2+ + 2H+ + O2v 2
Conclusion In summary, high-quality Eu:YF3 truncated octahedral submicrocrystals and nanocrystals have been synthesized using a simple hydrothermal approach, and their photoluminescence properties at room temperature have been studied. The submi-
TABLE 1: Eu2+ Ion Luminescence Peaks Corresponding to Different Excitation Wavelengths excitation wavelength (nm) peak position of emission band (nm)
300 391
323 393
340 397
350 401
360 409
370 426
380 441
393 445
Truncated Octahedral Eu-Doped YF3 Nanocrystals crocrystals and nanocrystals can be produced in a sample procedure with high yields, and the dopant ions can easily be changed for different applications. In addition to the characteristic red emission peaks of Eu3+, a stronger wide blue emission band centered at about 445 nm is also observed when the crystals are excited at 393 nm, which is quite different from those reported previously. The blue emission band originates from the 4f65d-4f7 transition in Eu2+. The results reveal that there should be a redox reaction in the hydrothermal preparation of YF3:Eu3+ at 180 °C which should have caused some Eu3+ ions to reduce to the divalent state. The intense 390-nm emission for YF3:Eu may find applications in the near future. This preparation method provides a simple route to synthesize other fluoride nanophosphors. References and Notes (1) Bhargava, R. N.; Gallaghar, D.; Hong, X.; Nurmikko, A. Phys. ReV. Lett. 1994, 72, 416. (2) Tissue, B. M. Chem. Mater. 1998, 10, 2837. (3) Yin, M.; Duan, C.; Zhang, W.; Lous, L.; Xia, S.; Krupa, J. C. J. Appl. Phys. 1999, 86, 3751. (4) Soo, Y. L.; Huan, S. W.; Kao, Y. H.; Chabra, V.; Kulkarni, B.; Veliadis, J. V. D.; Bhargava, R. N. Appl. Phys. Lett. 1999, 75, 2464. (5) Hebbink, G. A.; Stouwdam, J. W.; Reinhoudt, D. N.; vanVeggel, F. C. J. M. AdV. Mater. 2002, 14, 1147. (6) Wang, X.; Li, Y. D. Angew. Chem. 2002, 114, 4984. (7) Huignard, A.; Buissette, V.; Franville, A. C.; Gacoin, T.; Boilot, J. P. J. Phys. Chem. B 2003, 107, 6754. (8) Justel, T.; Nikol, H.; Ronda, C. Angew. Chem., Int. Ed. 1998, 37, 3085. (9) Barber, D. B.; Pollock, C. R.; Beecroft, L. L.; Ober, C. K. Opt. Lett. 1997, 22, 1247. (10) Reisfeld, R.; Jorgensen, C. K. Lasers and Excited States of Rare Earths; Springer: Berlin, 1977. (11) Kawano, K.; Arai, K.; Yamada, H.; Hashimoto, N.; Nakata, R. Sol. Energy Mater. Sol. Cells 1997, 48, 35. (12) Dahan, M.; Laurence, T.; Pinaud, F.; Chemla, D. D.; Alivisators, A. P.; Sauer, M.; Weiss, S. Opt. Lett. 2001, 26, 825. (13) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (14) Alivisatos, A. P. Science 1996, 271, 933. (15) Williams, F.; Nozik, A. J. Nature 1984, 21, 312. (16) Soo, Y. L.; Huang, S. W.; Ming, Z. H.; Kao, Y. H.; Smith, G. C.; Goldburt, E.; Hodel, R.; Kulkarni, B.; Veliadis, J. V. D.; Bhargava, R. N. J. Appl. Phys. 1998, 83, 5404.
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