- Email: [email protected]
Low-temperature wet chemical synthesis and photoluminescence properties of YVO4: Bi3+, Eu3+ nanophosphors
ARTICLE IN PRESS
Journal of Luminescence 128 (2008) 1515–1522 www.elsevier.com/locate/jlumin
Low-temperature wet chemical synthesis and photoluminescence properties of YVO4: Bi3+, Eu3+ nanophosphors Satoru Takeshitaa, Tetsuhiko Isobea,, Seiji Niikurab a
Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan b SINLOIHI Co. Ltd., 2-19-12 Dai, Kamakura 247-8550, Japan Received 30 September 2007; received in revised form 10 February 2008; accepted 15 February 2008 Available online 4 March 2008
Abstract YVO4: Bi3+, Eu3+nanophosphors are prepared by the citrate-assisted low-temperature wet chemical synthesis. When the colloidal solution is aged at 60 1C, the crystalline YVO4: Bi3+, Eu3+ nanorods are formed from the amorphous gel precursors, as confirmed by transmission electron microscopy and X-ray diffractometry (XRD). YVO4: Bi3+, Eu3+ nanophosphors emit red through energy transfer from Bi3+ to Eu3+ under near-UV-light excitation. The emission intensity increases with increasing the fraction of the crystalline phase during aging. The excitation peak corresponding to Bi3+–V5+ charge transfer relative to those of O2–V5+ and O2–Eu3+ charge transfers gradually becomes strong until the completion of the crystallization, although the contents of individual Bi3+ and Eu3+ ions incorporated into YVO4 keep constant. When the aging is continued after the completion of the crystallization, the content of incorporated Bi3+ gradually increases, and hence the emission intensity decreases as a result of the energy migration among Bi3+ ions. These results suggest that in addition to the fraction of the crystalline phase and the contents of incorporated Bi3+ and Eu3+ ions, the local chemical states around Bi3+ play significant roles in photoluminescence properties. r 2008 Elsevier B.V. All rights reserved. PACS: 78.55.Hx; 78.67.Bf Keywords: Nanophosphor; Photoluminescence; YVO4: Bi3+, Eu3+; Wet chemical synthesis
1. Introduction Luminescent materials emitting visible colors by the excitation of near-UV-light around 365 nm (so-called ‘‘black light’’) are extensively used in the fields of security and art, because it is possible to print them invisibly and black light is more harmless to the human bodies in comparison with UV-light [1–4]. Invisibly printed phosphors are applied to passports, many kinds of banknotes and tickets for simple inspection and prevention of forgeries [1]. Furthermore, the invisible optical bar codes are printed on most of postal matters in automatic delivery system in Japan [2]. These invisible fluorescent paints often need high transparency. In fact, solutions containing luminescent metal complexes such as Eu3+ complexes are Corresponding author. Tel.: +81 45 566 1554; fax: +81 45 566 1551.
E-mail address: [email protected] (T. Isobe). 0022-2313/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.02.012
used for this purpose. On the contrary, commercial inorganic phosphors of micrometers in size cannot be used because of opacity. In this work, we consider inorganic nanophosphors emitting red under the excitation of black light. Such nanophosphors combined with near-UV LED might be used for solid-state lighting devices [5]. Bi3+ acts as a sensitizer of Eu3+ through energy transfer from Bi3+ to Eu3+ in the systems such as YVO4: Bi3+, Eu3+ [6–9], Y2O3: Bi3+, Eu3+ [10–13], CaWO4: Bi3+, Eu3+ [14], Y3Al5O12: Bi3+, Eu3+ [15], (Y,Gd)BO3: Bi3+, Eu3+ [16], CaO Gd2O3 SiO2: Bi3+, Eu3+ [17] and silica glass [18]. YVO4: Bi3+, Eu3+ under the excitation of black light emits red through the allowed 6s26s6p transition of Bi3+ or the charge transfer from Bi3+ 6s to V5+ 3d, followed by energy transfer to Eu3+ [9,19,20]. Cheetham’s group [6] reported that the edge of this excitation peak shifts toward the longer wavelength from 350 to 400 nm with increasing the Bi3+ concentration. Based on this
ARTICLE IN PRESS 1516
S. Takeshita et al. / Journal of Luminescence 128 (2008) 1515–1522
result, it should be needed to incorporate at least more than 10 at% Bi3+ ions into YVO4 in order to excite them under black light. This concentration is beyond a general doping concentration of a sensitizer, i.e., 0.1several at%. Micrometer-sized YVO4: Bi3+, Eu3+ bulk phosphor is prepared by the conventional solid-state reaction [7,8]. Yan et al. [9] reported the in situ co-precipitation of YVO4: Bi3+, Eu3+, but the average particle size was 1–3 mm and the Bi3+ concentration was limited to several at%. Thus, no research on the preparation of YVO4: Bi3+, Eu3+ nanophosphors has been reported to our knowledge. Similar rare-earth compound GdVO4: Bi3+, Eu3+ was prepared by the combustion method [21] and the hydrolyzed colloid reaction [22], but the particles were micrometer in size and the Bi3+ concentrations were limited to several at%. On the other hand, wet chemical syntheses of YVO4: Eu3+ nanoparticles have been reported by several workers. Haase’s group [23] reported the hydrothermal synthesis of YVO4: Eu3+ nanoparticles. Boilot’s group [24] synthesized a colloidal solution of YVO4: Eu3+ nanoparticles with the mean particle size of about 10 nm by a citrate-assisted low-temperature precipitation. Here we report the preparation of YVO4: Bi3+, Eu3+ nanophosphors by the citrate-assisted low-temperature wet chemical synthesis after Boilot’s method. We also discuss the influence of the doping process during aging on photoluminescent (PL) properties. Especially, we carefully take the following points into consideration on doping a high concentration of Bi3+ ions into YVO4 nanoparticles without postheat treatment: (i) it is difficult to form a continuous solid solution between BiVO4 and YVO4, because their stable crystal structures are different [6]. (ii) The dissolution property is different between both salts of bismuth and yttrium. 2. Experimental 2.1. Preparation of YVO4: Bi3+, Eu3+ nanophosphors Yttrium(III) nitrate hexahydrate (2.60 mmol, Kanto, 99.99%) and europium(III) nitrate hexahydrate (0.20 mmol, Kanto, 99.95%) were dissolved in 40.0 mL of ultrapure water. After adding 21.0 mL of 0.100 mol L1 solution of sodium citrate dihydrate (Wako, 99.0%) to this solution, a white suspension of Y–Eu citrate was formed. Bismuth(III) citrate (1.20 mmol, Soekawa, 98%), which was partially soluble in water, was added to this suspension and dispersed by a treatment in a ultrasonic water bath for 1 min. We did not use bismuth nitrate because it was hardly soluble in water. In another vessel, sodium orthovanadate(V) (3.00 mmol, Alexis) was dissolved in 40.0 mL solution of sodium hydroxide at pH 12.5. Then this solution was added to the above-mentioned suspension at 60 1C with stirring. Resulting yellowish suspension was aged at 60 1C for the aging time (tag, 0ptagp420 min). After cooling to room
Fig. 1. Photographs of the colloidal solution of YVO4: Bi3+, Eu3+ nanophosphors: (a) under white light and (b) under irradiation by black light.
temperature, the yellow-tinged Bi3+-rich precipitate of by-products was removed from the suspension by a centrifugation at 10,000 rpm for 10 min to obtain the transparent colloidal solution of nanophosphors. Excess ions were also removed by dialysis for 48 h using a dialysis membrane tubing with a pore size of 2.4 nm. Fig. 1(a) and (b) shows the photographs of dialyzed colloidal solution under white light and under irradiation by black light, respectively. Its ionic conductivity was about 4 mS m1 and the concentration of nanophosphors was 2–3 g L1. The slightly yellow-tinged powder samples of nanophosphors were obtained by a rotary evaporator at 40 1C under reduced pressure. Y1xyBixEuyVO4 bulk samples at various doping concentrations were also prepared from yttrium(III) oxide (Kanto, 99.99%), europium(III) oxide (Kanto, 99.95%), bismuth(III) oxide (Kanto, 98.0%) and ammonium metavanadate(V) (Kanto, 99.0%) by conventional solid-state reaction at 800 1C for 11 h. 2.2. Characterization The particle morphology and the microstructure were observed by field emission transmission electron microscopy (TEM, FEI, Technai 12), where the colloidal solution diluted in ethanol was dropped on a copper microgrid and dried at 25 1C. The particle size distribution was measured by dynamic light scattering (DLS, Malvern, HPPS), where the colloidal solution was diluted in ultrapure water, and the refractive index of bulk YVO4, 1.993, was used.
ARTICLE IN PRESS S. Takeshita et al. / Journal of Luminescence 128 (2008) 1515–1522
1517
The crystalline phase was identified by X-ray diffractometry (XRD, Rigaku, Rint-2200), using Cu Ka radiation. The XRD patterns were decomposed by peak fitting using pseudo-Voigt function for the peaks corresponding to the crystalline phase and Gaussian function for the halos corresponding to the amorphous phase. The fraction of the crystalline phase was estimated by the following equation: F cryst ¼ I cryst =ðI cryst þ I amorph Þ 100ð%Þ
(1)
where Fcryst is the fraction of the crystalline phase (%); Icryst, the integral intensity of all the crystalline peaks; Iamorph, the integral intensity of the amorphous halos. Interplanar spacing of (2 0 0), d2 0 0, was estimated from the centroid of its XRD peak which was measured by slow scan in the region of 22.51p2yp27.51. Fig. 2 shows the (2 0 0) peaks of the bulk samples at various Bi3+ concentrations. The crystal radii at eight-fold coordination are 1.159 A˚ for Y3+, 1.206 A˚ for Eu3+ and 1.31 A˚ for Bi3+ [25]. When Y3+ ions are substituted by larger Bi3+ and Eu3+ ions, the XRD peaks shift toward the lower-angle side because of the increase in the interplanar spacings. The d2 0 0 values of the bulk samples at various concentrations of Bi3+ and Eu3+ ions were shown in Fig. 3. Relationship between their concentrations and d2 0 0 were found to be linear on the basis of Vegard’s law. As a result, an empirical formula was obtained by a multiple linear regression analysis as follows: ˚ ¼ ð9:102 0:759Þ 104 x d 2 0 0 ðAÞ þ ð7:953 1:885Þ 104 y þ 3:560 0:002
Fig. 3. d2 0 0 values of bulk samples with different Eu and Bi concentrations.
II) by means of the fundamental parameter (FP) method. PL and its excitation (PLE) spectra were measured by a fluorescence spectrometer (JASCO, FP-6500). The wavelength dependence of the detector response for each spectrum was corrected using an ethylene glycol solution of Rhodamine B (5.5 g L1) and a standard light source (JASCO, ESC-333). All the measurements were carried out at room temperature.
(2) 3+
where x and y were the concentrations (at%) of Bi and Eu3+, respectively. Hence, the contents of Bi3+ and Eu3+ ions incorporated into YVO4 were evaluated from the shift of the XRD (2 0 0) peak, i.e., the difference in the d2 0 0 value between pure and doped YVO4 using Eq. (2). The concentrations of Y, Eu, Bi and V were determined by an X-ray fluorescent analyzer (XRF, Rigaku, ZSXmini
3. Results and discussion 3.1. Formation of nanorods TEM images of samples with different tag are shown in Fig. 4. Indefinite-shaped precursor gels of 100–200 nm in size are observed at tag ¼ 0 min. Nanorods nucleate inside the gels and gradually grow up to 40 nm in length with increasing the aging time. Aggregation of the oriented nanorods is also observed at tagX180 min. The typical hydrodynamic size distributions of the colloidal solutions are shown in Fig. 5. The following dual modal distribution is observed: One size distribution around 100–300 nm at 0ptagp30 min, and another distribution around 10–40 nm at tagX15 min. The former possibly corresponds to the precursor gels, and the latter to dispersed nanorods. The mean size of each mode is plotted as a function of tag and shown in Fig. 6. 3.2. Structural and compositional changes of crystalline phase during aging
Fig. 2. (2 0 0) peaks of Y1xBixVO4 bulk samples. x: (a) 0, (b) 0.10, (c) 0.20, (d) 0.32 and (e) 0.40.
The XRD patterns of the samples with different tag are shown in Fig. 7A. The tetragonal zircon type gradually crystallizes from the amorphous phase with increasing the aging time. In comparison with TEM images, the
ARTICLE IN PRESS 1518
S. Takeshita et al. / Journal of Luminescence 128 (2008) 1515–1522
Fig. 4. TEM images of nanophosphors aged at 60 1C for different time. tag (min): (a) 0, (b) 30, (c) 60, (d) 180, (e) 300 and (f) 420. Scale bar: 50 nm.
Fig. 6. Changes in the hydrodynamic size for colloidal solutions with aging time. Fig. 5. Hydrodynamic size distributions of colloidal solutions aged at 60 1C for different time. tag (min): (a) 0, (b) 15 and (c) 420.
amorphous phase corresponds to the precursor gels and the crystalline phase to the nanorods. The nanorods possibly grow anisotropically along the a-axis, because the (2 0 0) peak is narrower than any other peaks of the crystalline
phase. The crystallite size along the a-axis estimated by Scherrer’s equation is 9 nm for the sample aged for tag ¼ 240 min, indicating that a nanorod of 40 nm in length consists of several crystallites. The XRD patterns are decomposed by peak fitting to estimate Fcryst. An example of the decomposed pattern for the sample aged for tag ¼ 45 min is shown in Fig. 8.
ARTICLE IN PRESS S. Takeshita et al. / Journal of Luminescence 128 (2008) 1515–1522
1519
Fig. 7. XRD patterns of powder samples aged at 60 1C for different time. tag (min): (a) 0, (b) 45, (c) 240 and (d) 420. A: 2y ¼ 10–601. B: expanded patterns around (2 0 0).
Fig. 9. Changes in Fcryst and d2 0 0 with aging time.
Fig. 8. XRD patterns decomposed by curve fitting for the powder sample (tag ¼ 45 min).
The change in Fcryst with the aging time is shown by circles in Fig. 9. The value of Fcryst reaches unity at tag200 min, showing the achievement of the perfect crystallization. Expanded XRD patterns around (2 0 0) of samples for different tag are shown in Fig. 7B. The change in d2 0 0 with the aging time is shown by triangles in Fig. 9 together with
the change in Fcryst. Aging does not affect d2 0 03.58 A˚ before the completion of the crystallization, i.e., at tagp200 min. This means that the composition of the crystalline phase does not change during aging at tagp200 min. On the other hand, d2 0 0 slightly increases at tagX200 min, as will be discussed below. 3.3. Compositional change during aging The changes in the concentrations measured by XRF analysis with aging time are shown in Fig. 10. The molar ratio of V/(Y+Bi+Eu) increases from 0.36 to 0.62 during aging at tagp200 min, and then levels off (see circles in
ARTICLE IN PRESS 1520
S. Takeshita et al. / Journal of Luminescence 128 (2008) 1515–1522
derived Bi3+-rich precursor precipitate, which are removed by centrifugation after aging. In addition, this ionic exchange is possibly promoted in the absence of gel precursors around crystalline nanorods. 3.4. Change in photoluminescence properties during aging
Fig. 10. Changes in the concentrations measured by XRF analysis with aging time. Molar ratio of V/(Y+Bi+Eu) (K), atomic percentage of Bi/(Y+Bi+Eu) (.), Eu/(Y+Bi+Eu) (m).
Fig. 10). The concentration of Bi measured by XRF decreases from 23 to 12 at% during aging at tagp100 min, and then increases up to 18 at% at tag ¼ 420 min (see inverted triangles in Fig. 10). In contrast, the concentration of Eu10 at% does not change during the whole aging (see triangles in Fig. 10). Before the completion of the crystallization, i.e., at tagp200 min, the concentrations measured by XRF are the summation of those of crystalline and amorphous phases. There is no change in the composition of the crystalline phase, judging from the constant d2 0 0 value before the completion of the crystallization. Therefore, the decrease in Bi/(Y+Bi+Eu) and the increase in V/(Y+Bi+Eu) at tagp200 min are attributed to the changes in the fraction and/or composition of amorphous gel derived from the mixture of bismuth citrate, Y–Eu citrate and orthovanadate. There is no amorphous phase at tagX200 min. Therefore, the composition of the crystalline phase at tagX200 min can be estimated from the concentrations measured by XRF. Using Eq. (2) and the concentrations (Bi: 14.28 at%; Eu: 11.00 at%) measured by XRF for the sample aged for tag ¼ 240 min, the d2 0 0 value is calculated to 3.58270.005 A˚. This value is in agreement with the d2 0 0 value, 3.58 A˚, determined from the XRD peak position within experimental errors. This verifies that after the completion of crystallization, i.e., at tagX200 min, the composition estimated by XRF is equal to that of the YVO4: Bi3+, Eu3+ crystalline phase, estimated by XRD. When tag increases from 240 to 420 min, the Bi3+ concentration measured by XRF increases from 14 to 18 at%, the Y3+ concentration decreases from 75 to 72 at%, and the Eu3+ concentration 10 at% does not change (see Fig. 10). At the same time, d2 0 0 increases from 3.58 to 3.59 A˚ (see Fig. 9). We attribute this increase in d2 0 0 to the replacement of Y3+ by Bi3+ in YVO4. This replacement could occur through ionic exchange by repeating dissolution and precipitation processes during aging in the presence of added bismuth citrate and its
Fig. 11A and B shows PL and PLE spectra of the samples prepared for 0ptagp240 min and for 240ptagp 420 min, respectively. The sharp peaks corresponding to the 4f–4f transitions of Eu3+ are observed in the PL and PLE spectra of all the samples. In addition, two broad excitation peaks in the regions between 250 and 300 nm and between 300 and 400 nm are also observed in the PLE spectra. As shown in Fig. 12(a), the valence band is composed of the 6s orbital of Bi3+ and the 2p orbitals of O2, whereas the conduction band is composed of the 3d orbitals of V5+ and the 6p orbitals of Bi3+ [19,20,27]. The excitation peak between 250 and 300 nm is assigned to the excitation of the YVO4 host crystal due to O2–V5+ charge transfer, followed by energy transfer to Eu3+. This peak is also overlapped by the excitation peak due to O2–Eu3+ charge transfer around 260 nm [26]. The excitation peak between 300 and 400 nm is likely to be assigned to the charge transfer transition from Bi3+ to V5+ rather than the 6s26s6p transition of Bi3+, followed by the energy transfer to Eu3+, based on the discussion in Ref. [19]. Before the completion of the crystallization, i.e., at 0 ptagp240 min, the intensity of each excitation peak gradually becomes strong with increasing the aging time (see Fig. 11A). The normalized PL intensities at 260 and 344 nm in the excitation spectra of the samples aged for 0ptagp240 min are plotted as a function of Fcryst, as shown in Fig. 13. The intensity at 260 nm corresponding to O2–V5+ charge transfer is proportional to Fcryst. This means that the crystalline phase mainly contributes to the emission. On the contrary, the intensity at 344 nm corresponding to the excitation corresponding to Bi3+–V5+ charge transfer gradually increases with increasing Fcryst. As already mentioned, the contents of incorporated Bi3+ and Eu3+ in the crystalline phase keeps constant at 0ptagp240 min, judging from the change in d2 0 0. We, therefore, conclude that in addition to Fcryst the change in coordination states around Bi3+ ions possibly determines the relative intensity of the excitation corresponding to Bi3+–V5+ charge transfer. Blasse’s group [28] and Silver’s group [29] reported that the energy level of ns2 ions is lowered by ns–np mixing in the coordination environment with lower symmetry. So if ns2 ions such as Bi3+are in more symmetrical coordination, ns electrons are more delocalized, and then Bi3+ ions are subject to larger influence of coordination environment on luminescence properties. We, therefore, attribute the increase in the relative PL intensity during aging to the higher symmetrical coordination. After the completion of the crystallization, i.e., at 240ptagp420 min, the PL intensity decreases with
ARTICLE IN PRESS S. Takeshita et al. / Journal of Luminescence 128 (2008) 1515–1522
1521
Fig. 11. PL and PLE spectra of powder samples aged at 60 1C for different time. tag (min): (a) 0, (b) 1, (c) 15, (d) 30, (e) 45, (f) 60, (g) 120, (h) 180, (i) 240, (j) 300 and (k) 420. A: (a)–(i), B: (i)–(k).
Fig. 12. Assignments of excitation peaks by a simple energy diagram. (a) low Bi3+ concentration and (b) high Bi3+ concentration.
increasing the aging time (see Fig. 11B). At the same time, the increase in the incorporated Bi3+ content is confirmed by XRD (see Fig. 9) and XRF (see Fig. 10), as already
mentioned. We, therefore, attribute the decrease in the PL intensity at tagX240 min to concentration quenching by energy migration among Bi3+ ions. The replacement of Y3+ by Bi3+ in YVO4 occurs through ionic exchange by repeating dissolution and precipitation processes during aging in the presence of the Bi3+-rich precipitate. However, Bi3+ ions cannot diffuse homogeneously inside particles at the temperature as low as 60 1C. Therefore, Bi3+ ions localized near surface causes concentration quenching after prolonged aging. On the other hand, the position of the excitation peak corresponding to Bi3+–V5+ charge transfer shifts from 344 nm (tag ¼ 240 min) to 351 nm (tag ¼ 420 min). As shown in Fig. 12, this red shift is possibly explained by
ARTICLE IN PRESS 1522
S. Takeshita et al. / Journal of Luminescence 128 (2008) 1515–1522
corresponding to Bi3+–V5+ charge transfer becomes strong more slowly than that of O2–V5+ charge transfer. (ii) After the completion of the crystallization, the content of Bi3+ incorporated into YVO4 gradually increases through ionic exchange between the nanorods and the Bi3+-rich precipitates by repeating dissolution and precipitation processes. The PL intensity for the excitation corresponding to Bi3+–V5+ charge transfer decreases as a result of energy migration among Bi3+ ions. References [1] [2] [3] [4] [5]
[6] Fig. 13. Change in normalized PL intensities at 260 nm (K) and 344 nm (.) with Fcryst.
the following two items: (i) the number of energy levels of Bi3+ ions in valence and conduction bands, and (ii) the number of Bi3+–V5+ pairing ions with interaction increases with increasing the incorporated Bi3+ concentration. Both items (i) and (ii) lead to the observed shift of the excitation energy to lower energy. 4. Conclusion YVO4: Bi3+, Eu3+ nanphosphors are prepared by the citrate-assisted wet chemical synthesis at 60 1C. The crystallization completes at tag200 min. The characteristics of the doping process can be summarized as follows: (i) Before the completion of the crystallization, amorphous gel precursors formed contain Y3+, Bi3+, Eu3+, VO3 and citrate ligands by ionic exchange. With 4 increasing the aging time, the crystalline YVO4: Bi3+, Eu3+ nanorods with 14 at% of Bi3+ and 11 at% of Eu3+ are formed inside the gel precursors. The PL intensity increases with increasing the fraction of the crystalline phase. At the early stage of aging, however, Bi3+ 6 s electrons are localized due to the distorted Bi3+ environments, which do not contribute to the excitation process. Therefore, the excitation peak
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
A.N. Becidyan, Color Res. Appl. 20 (1995) 124. K. Egami, Opt. Alliance 8 (1997) 6 (Japanese). J.T. Schmidt, Mater. Eval. 33 (1975) 21A. S. Tanaka, S. Miyahara, S. Niikura, Jpn. Kokai Tokkyo Koho 2002–121203 (2002). D.A. Steigerwald, J.C. Bhat, D. Collins, R.M. Fletcher, M.O. Holcomb, M.J. Ludowise, P.S. Martin, S.L. Rudaz, IEEE. J. Sel. Top. Quantum Electron. 8 (2002) 310. S. Neeraj, N. Kijima, A.K. Cheetham, Solid State Commun. 131 (2004) 65. S.Z. Toma, F.F. Mikus, J.E. Mathers, J. Electrochem. Soc. 114 (1967) 953. R.K. Datta, J. Electrochem. Soc. 114 (1967) 1057. B. Yan, X.-Q. Su, J. Non-Cryst. Solids 352 (2006) 3275. S. Qiang, C. Barthou, J.P. Denis, F. Pelle, B. Blanzat, J. Lumin. 28 (1983) 1. L.S. Chi, R.S. Liu, B.J. Lee, J. Electrochem. Soc. 152 (2005) J93. W.J. Park, S.G. Yoon, D.H. Yoon, J. Electroceram. 17 (2006) 41. T.-S. Chan, C.-C. Kang, R.-S. Liu, L. Chen, X.-N. Liu, J.-J. Ding, J. Bao, C. Gao, J. Comb. Chem. 9 (2007) 343. R.B. Pode, S.J. Dhoble, Phys. Stat. Sol. (b) 203 (1997) 571. J.-G. Zhou, F.-Y. Zhao, Z.-Q. Li, S.-P. Xia, L. Yang, S.-Y. Gao, J. Mater. Sci. 39 (2004) 4711. X. Zeng, S.-J. Im, S.-H. Jang, Y.-M. Kim, H.-B. Park, S.-H. Son, H. Hatanaka, G.-Y. Kim, S.-G. Kim, J. Lumin. 121 (2006) 1. B. Li, Y. Tian, Y. Bai, Mater. Res. Bull. 24 (1989) 961. M. Wang, X. Fan, G. Xiong, J. Phys. Chem. Solids 56 (1995) 859. G. Blasse, A. Bril, J. Chem. Phys. 48 (1968) 217. A. Newport, J. Silver, A. Vecht, J. Electrochem. Soc. 147 (2000) 3944. B.N. Mahalley, R.B. Pode, P.K. Gupta, Phys. Stat. Sol. (a) 177 (1999) 293. B.N. Mahalley, S.J. Dhoble, R.B. Pode, G. Alexander, Appl. Phys. A 70 (2000) 39. K. Riwotzki, M. Haase, J. Phys. Chem. B 102 (1998) 10129. A. Huignard, V. Buissette, G. Laurent, T. Gacoin, J.-P. Boilot, Chem. Mater. 14 (2002) 2264. R.D. Shannon, Acta Crystallogr. A 32 (1976) 751. Y. Wang, Y. Zuo, H. Gao, Mater. Res. Bull. 41 (2006) 2147. M.W. Stoltzfus, P.M. Woodward, R. Seshadri, J.H. Klepeis, B. Bursten, Inorg. Chem. 46 (2007) 3839. M. Wiegel, W. Middel, G. Blasse, J. Mater. Chem. 5 (1995) 981. X. Jing, C. Gibbons, D. Nicholas, J. Silver, A. Vecht, C.S. Frampton, J. Mater. Chem. 9 (1999) 2913.
Journal of Luminescence 128 (2008) 1515–1522 www.elsevier.com/locate/jlumin
Low-temperature wet chemical synthesis and photoluminescence properties of YVO4: Bi3+, Eu3+ nanophosphors Satoru Takeshitaa, Tetsuhiko Isobea,, Seiji Niikurab a
Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan b SINLOIHI Co. Ltd., 2-19-12 Dai, Kamakura 247-8550, Japan Received 30 September 2007; received in revised form 10 February 2008; accepted 15 February 2008 Available online 4 March 2008
Abstract YVO4: Bi3+, Eu3+nanophosphors are prepared by the citrate-assisted low-temperature wet chemical synthesis. When the colloidal solution is aged at 60 1C, the crystalline YVO4: Bi3+, Eu3+ nanorods are formed from the amorphous gel precursors, as confirmed by transmission electron microscopy and X-ray diffractometry (XRD). YVO4: Bi3+, Eu3+ nanophosphors emit red through energy transfer from Bi3+ to Eu3+ under near-UV-light excitation. The emission intensity increases with increasing the fraction of the crystalline phase during aging. The excitation peak corresponding to Bi3+–V5+ charge transfer relative to those of O2–V5+ and O2–Eu3+ charge transfers gradually becomes strong until the completion of the crystallization, although the contents of individual Bi3+ and Eu3+ ions incorporated into YVO4 keep constant. When the aging is continued after the completion of the crystallization, the content of incorporated Bi3+ gradually increases, and hence the emission intensity decreases as a result of the energy migration among Bi3+ ions. These results suggest that in addition to the fraction of the crystalline phase and the contents of incorporated Bi3+ and Eu3+ ions, the local chemical states around Bi3+ play significant roles in photoluminescence properties. r 2008 Elsevier B.V. All rights reserved. PACS: 78.55.Hx; 78.67.Bf Keywords: Nanophosphor; Photoluminescence; YVO4: Bi3+, Eu3+; Wet chemical synthesis
1. Introduction Luminescent materials emitting visible colors by the excitation of near-UV-light around 365 nm (so-called ‘‘black light’’) are extensively used in the fields of security and art, because it is possible to print them invisibly and black light is more harmless to the human bodies in comparison with UV-light [1–4]. Invisibly printed phosphors are applied to passports, many kinds of banknotes and tickets for simple inspection and prevention of forgeries [1]. Furthermore, the invisible optical bar codes are printed on most of postal matters in automatic delivery system in Japan [2]. These invisible fluorescent paints often need high transparency. In fact, solutions containing luminescent metal complexes such as Eu3+ complexes are Corresponding author. Tel.: +81 45 566 1554; fax: +81 45 566 1551.
E-mail address: [email protected] (T. Isobe). 0022-2313/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.02.012
used for this purpose. On the contrary, commercial inorganic phosphors of micrometers in size cannot be used because of opacity. In this work, we consider inorganic nanophosphors emitting red under the excitation of black light. Such nanophosphors combined with near-UV LED might be used for solid-state lighting devices [5]. Bi3+ acts as a sensitizer of Eu3+ through energy transfer from Bi3+ to Eu3+ in the systems such as YVO4: Bi3+, Eu3+ [6–9], Y2O3: Bi3+, Eu3+ [10–13], CaWO4: Bi3+, Eu3+ [14], Y3Al5O12: Bi3+, Eu3+ [15], (Y,Gd)BO3: Bi3+, Eu3+ [16], CaO Gd2O3 SiO2: Bi3+, Eu3+ [17] and silica glass [18]. YVO4: Bi3+, Eu3+ under the excitation of black light emits red through the allowed 6s26s6p transition of Bi3+ or the charge transfer from Bi3+ 6s to V5+ 3d, followed by energy transfer to Eu3+ [9,19,20]. Cheetham’s group [6] reported that the edge of this excitation peak shifts toward the longer wavelength from 350 to 400 nm with increasing the Bi3+ concentration. Based on this
ARTICLE IN PRESS 1516
S. Takeshita et al. / Journal of Luminescence 128 (2008) 1515–1522
result, it should be needed to incorporate at least more than 10 at% Bi3+ ions into YVO4 in order to excite them under black light. This concentration is beyond a general doping concentration of a sensitizer, i.e., 0.1several at%. Micrometer-sized YVO4: Bi3+, Eu3+ bulk phosphor is prepared by the conventional solid-state reaction [7,8]. Yan et al. [9] reported the in situ co-precipitation of YVO4: Bi3+, Eu3+, but the average particle size was 1–3 mm and the Bi3+ concentration was limited to several at%. Thus, no research on the preparation of YVO4: Bi3+, Eu3+ nanophosphors has been reported to our knowledge. Similar rare-earth compound GdVO4: Bi3+, Eu3+ was prepared by the combustion method [21] and the hydrolyzed colloid reaction [22], but the particles were micrometer in size and the Bi3+ concentrations were limited to several at%. On the other hand, wet chemical syntheses of YVO4: Eu3+ nanoparticles have been reported by several workers. Haase’s group [23] reported the hydrothermal synthesis of YVO4: Eu3+ nanoparticles. Boilot’s group [24] synthesized a colloidal solution of YVO4: Eu3+ nanoparticles with the mean particle size of about 10 nm by a citrate-assisted low-temperature precipitation. Here we report the preparation of YVO4: Bi3+, Eu3+ nanophosphors by the citrate-assisted low-temperature wet chemical synthesis after Boilot’s method. We also discuss the influence of the doping process during aging on photoluminescent (PL) properties. Especially, we carefully take the following points into consideration on doping a high concentration of Bi3+ ions into YVO4 nanoparticles without postheat treatment: (i) it is difficult to form a continuous solid solution between BiVO4 and YVO4, because their stable crystal structures are different [6]. (ii) The dissolution property is different between both salts of bismuth and yttrium. 2. Experimental 2.1. Preparation of YVO4: Bi3+, Eu3+ nanophosphors Yttrium(III) nitrate hexahydrate (2.60 mmol, Kanto, 99.99%) and europium(III) nitrate hexahydrate (0.20 mmol, Kanto, 99.95%) were dissolved in 40.0 mL of ultrapure water. After adding 21.0 mL of 0.100 mol L1 solution of sodium citrate dihydrate (Wako, 99.0%) to this solution, a white suspension of Y–Eu citrate was formed. Bismuth(III) citrate (1.20 mmol, Soekawa, 98%), which was partially soluble in water, was added to this suspension and dispersed by a treatment in a ultrasonic water bath for 1 min. We did not use bismuth nitrate because it was hardly soluble in water. In another vessel, sodium orthovanadate(V) (3.00 mmol, Alexis) was dissolved in 40.0 mL solution of sodium hydroxide at pH 12.5. Then this solution was added to the above-mentioned suspension at 60 1C with stirring. Resulting yellowish suspension was aged at 60 1C for the aging time (tag, 0ptagp420 min). After cooling to room
Fig. 1. Photographs of the colloidal solution of YVO4: Bi3+, Eu3+ nanophosphors: (a) under white light and (b) under irradiation by black light.
temperature, the yellow-tinged Bi3+-rich precipitate of by-products was removed from the suspension by a centrifugation at 10,000 rpm for 10 min to obtain the transparent colloidal solution of nanophosphors. Excess ions were also removed by dialysis for 48 h using a dialysis membrane tubing with a pore size of 2.4 nm. Fig. 1(a) and (b) shows the photographs of dialyzed colloidal solution under white light and under irradiation by black light, respectively. Its ionic conductivity was about 4 mS m1 and the concentration of nanophosphors was 2–3 g L1. The slightly yellow-tinged powder samples of nanophosphors were obtained by a rotary evaporator at 40 1C under reduced pressure. Y1xyBixEuyVO4 bulk samples at various doping concentrations were also prepared from yttrium(III) oxide (Kanto, 99.99%), europium(III) oxide (Kanto, 99.95%), bismuth(III) oxide (Kanto, 98.0%) and ammonium metavanadate(V) (Kanto, 99.0%) by conventional solid-state reaction at 800 1C for 11 h. 2.2. Characterization The particle morphology and the microstructure were observed by field emission transmission electron microscopy (TEM, FEI, Technai 12), where the colloidal solution diluted in ethanol was dropped on a copper microgrid and dried at 25 1C. The particle size distribution was measured by dynamic light scattering (DLS, Malvern, HPPS), where the colloidal solution was diluted in ultrapure water, and the refractive index of bulk YVO4, 1.993, was used.
ARTICLE IN PRESS S. Takeshita et al. / Journal of Luminescence 128 (2008) 1515–1522
1517
The crystalline phase was identified by X-ray diffractometry (XRD, Rigaku, Rint-2200), using Cu Ka radiation. The XRD patterns were decomposed by peak fitting using pseudo-Voigt function for the peaks corresponding to the crystalline phase and Gaussian function for the halos corresponding to the amorphous phase. The fraction of the crystalline phase was estimated by the following equation: F cryst ¼ I cryst =ðI cryst þ I amorph Þ 100ð%Þ
(1)
where Fcryst is the fraction of the crystalline phase (%); Icryst, the integral intensity of all the crystalline peaks; Iamorph, the integral intensity of the amorphous halos. Interplanar spacing of (2 0 0), d2 0 0, was estimated from the centroid of its XRD peak which was measured by slow scan in the region of 22.51p2yp27.51. Fig. 2 shows the (2 0 0) peaks of the bulk samples at various Bi3+ concentrations. The crystal radii at eight-fold coordination are 1.159 A˚ for Y3+, 1.206 A˚ for Eu3+ and 1.31 A˚ for Bi3+ [25]. When Y3+ ions are substituted by larger Bi3+ and Eu3+ ions, the XRD peaks shift toward the lower-angle side because of the increase in the interplanar spacings. The d2 0 0 values of the bulk samples at various concentrations of Bi3+ and Eu3+ ions were shown in Fig. 3. Relationship between their concentrations and d2 0 0 were found to be linear on the basis of Vegard’s law. As a result, an empirical formula was obtained by a multiple linear regression analysis as follows: ˚ ¼ ð9:102 0:759Þ 104 x d 2 0 0 ðAÞ þ ð7:953 1:885Þ 104 y þ 3:560 0:002
Fig. 3. d2 0 0 values of bulk samples with different Eu and Bi concentrations.
II) by means of the fundamental parameter (FP) method. PL and its excitation (PLE) spectra were measured by a fluorescence spectrometer (JASCO, FP-6500). The wavelength dependence of the detector response for each spectrum was corrected using an ethylene glycol solution of Rhodamine B (5.5 g L1) and a standard light source (JASCO, ESC-333). All the measurements were carried out at room temperature.
(2) 3+
where x and y were the concentrations (at%) of Bi and Eu3+, respectively. Hence, the contents of Bi3+ and Eu3+ ions incorporated into YVO4 were evaluated from the shift of the XRD (2 0 0) peak, i.e., the difference in the d2 0 0 value between pure and doped YVO4 using Eq. (2). The concentrations of Y, Eu, Bi and V were determined by an X-ray fluorescent analyzer (XRF, Rigaku, ZSXmini
3. Results and discussion 3.1. Formation of nanorods TEM images of samples with different tag are shown in Fig. 4. Indefinite-shaped precursor gels of 100–200 nm in size are observed at tag ¼ 0 min. Nanorods nucleate inside the gels and gradually grow up to 40 nm in length with increasing the aging time. Aggregation of the oriented nanorods is also observed at tagX180 min. The typical hydrodynamic size distributions of the colloidal solutions are shown in Fig. 5. The following dual modal distribution is observed: One size distribution around 100–300 nm at 0ptagp30 min, and another distribution around 10–40 nm at tagX15 min. The former possibly corresponds to the precursor gels, and the latter to dispersed nanorods. The mean size of each mode is plotted as a function of tag and shown in Fig. 6. 3.2. Structural and compositional changes of crystalline phase during aging
Fig. 2. (2 0 0) peaks of Y1xBixVO4 bulk samples. x: (a) 0, (b) 0.10, (c) 0.20, (d) 0.32 and (e) 0.40.
The XRD patterns of the samples with different tag are shown in Fig. 7A. The tetragonal zircon type gradually crystallizes from the amorphous phase with increasing the aging time. In comparison with TEM images, the
ARTICLE IN PRESS 1518
S. Takeshita et al. / Journal of Luminescence 128 (2008) 1515–1522
Fig. 4. TEM images of nanophosphors aged at 60 1C for different time. tag (min): (a) 0, (b) 30, (c) 60, (d) 180, (e) 300 and (f) 420. Scale bar: 50 nm.
Fig. 6. Changes in the hydrodynamic size for colloidal solutions with aging time. Fig. 5. Hydrodynamic size distributions of colloidal solutions aged at 60 1C for different time. tag (min): (a) 0, (b) 15 and (c) 420.
amorphous phase corresponds to the precursor gels and the crystalline phase to the nanorods. The nanorods possibly grow anisotropically along the a-axis, because the (2 0 0) peak is narrower than any other peaks of the crystalline
phase. The crystallite size along the a-axis estimated by Scherrer’s equation is 9 nm for the sample aged for tag ¼ 240 min, indicating that a nanorod of 40 nm in length consists of several crystallites. The XRD patterns are decomposed by peak fitting to estimate Fcryst. An example of the decomposed pattern for the sample aged for tag ¼ 45 min is shown in Fig. 8.
ARTICLE IN PRESS S. Takeshita et al. / Journal of Luminescence 128 (2008) 1515–1522
1519
Fig. 7. XRD patterns of powder samples aged at 60 1C for different time. tag (min): (a) 0, (b) 45, (c) 240 and (d) 420. A: 2y ¼ 10–601. B: expanded patterns around (2 0 0).
Fig. 9. Changes in Fcryst and d2 0 0 with aging time.
Fig. 8. XRD patterns decomposed by curve fitting for the powder sample (tag ¼ 45 min).
The change in Fcryst with the aging time is shown by circles in Fig. 9. The value of Fcryst reaches unity at tag200 min, showing the achievement of the perfect crystallization. Expanded XRD patterns around (2 0 0) of samples for different tag are shown in Fig. 7B. The change in d2 0 0 with the aging time is shown by triangles in Fig. 9 together with
the change in Fcryst. Aging does not affect d2 0 03.58 A˚ before the completion of the crystallization, i.e., at tagp200 min. This means that the composition of the crystalline phase does not change during aging at tagp200 min. On the other hand, d2 0 0 slightly increases at tagX200 min, as will be discussed below. 3.3. Compositional change during aging The changes in the concentrations measured by XRF analysis with aging time are shown in Fig. 10. The molar ratio of V/(Y+Bi+Eu) increases from 0.36 to 0.62 during aging at tagp200 min, and then levels off (see circles in
ARTICLE IN PRESS 1520
S. Takeshita et al. / Journal of Luminescence 128 (2008) 1515–1522
derived Bi3+-rich precursor precipitate, which are removed by centrifugation after aging. In addition, this ionic exchange is possibly promoted in the absence of gel precursors around crystalline nanorods. 3.4. Change in photoluminescence properties during aging
Fig. 10. Changes in the concentrations measured by XRF analysis with aging time. Molar ratio of V/(Y+Bi+Eu) (K), atomic percentage of Bi/(Y+Bi+Eu) (.), Eu/(Y+Bi+Eu) (m).
Fig. 10). The concentration of Bi measured by XRF decreases from 23 to 12 at% during aging at tagp100 min, and then increases up to 18 at% at tag ¼ 420 min (see inverted triangles in Fig. 10). In contrast, the concentration of Eu10 at% does not change during the whole aging (see triangles in Fig. 10). Before the completion of the crystallization, i.e., at tagp200 min, the concentrations measured by XRF are the summation of those of crystalline and amorphous phases. There is no change in the composition of the crystalline phase, judging from the constant d2 0 0 value before the completion of the crystallization. Therefore, the decrease in Bi/(Y+Bi+Eu) and the increase in V/(Y+Bi+Eu) at tagp200 min are attributed to the changes in the fraction and/or composition of amorphous gel derived from the mixture of bismuth citrate, Y–Eu citrate and orthovanadate. There is no amorphous phase at tagX200 min. Therefore, the composition of the crystalline phase at tagX200 min can be estimated from the concentrations measured by XRF. Using Eq. (2) and the concentrations (Bi: 14.28 at%; Eu: 11.00 at%) measured by XRF for the sample aged for tag ¼ 240 min, the d2 0 0 value is calculated to 3.58270.005 A˚. This value is in agreement with the d2 0 0 value, 3.58 A˚, determined from the XRD peak position within experimental errors. This verifies that after the completion of crystallization, i.e., at tagX200 min, the composition estimated by XRF is equal to that of the YVO4: Bi3+, Eu3+ crystalline phase, estimated by XRD. When tag increases from 240 to 420 min, the Bi3+ concentration measured by XRF increases from 14 to 18 at%, the Y3+ concentration decreases from 75 to 72 at%, and the Eu3+ concentration 10 at% does not change (see Fig. 10). At the same time, d2 0 0 increases from 3.58 to 3.59 A˚ (see Fig. 9). We attribute this increase in d2 0 0 to the replacement of Y3+ by Bi3+ in YVO4. This replacement could occur through ionic exchange by repeating dissolution and precipitation processes during aging in the presence of added bismuth citrate and its
Fig. 11A and B shows PL and PLE spectra of the samples prepared for 0ptagp240 min and for 240ptagp 420 min, respectively. The sharp peaks corresponding to the 4f–4f transitions of Eu3+ are observed in the PL and PLE spectra of all the samples. In addition, two broad excitation peaks in the regions between 250 and 300 nm and between 300 and 400 nm are also observed in the PLE spectra. As shown in Fig. 12(a), the valence band is composed of the 6s orbital of Bi3+ and the 2p orbitals of O2, whereas the conduction band is composed of the 3d orbitals of V5+ and the 6p orbitals of Bi3+ [19,20,27]. The excitation peak between 250 and 300 nm is assigned to the excitation of the YVO4 host crystal due to O2–V5+ charge transfer, followed by energy transfer to Eu3+. This peak is also overlapped by the excitation peak due to O2–Eu3+ charge transfer around 260 nm [26]. The excitation peak between 300 and 400 nm is likely to be assigned to the charge transfer transition from Bi3+ to V5+ rather than the 6s26s6p transition of Bi3+, followed by the energy transfer to Eu3+, based on the discussion in Ref. [19]. Before the completion of the crystallization, i.e., at 0 ptagp240 min, the intensity of each excitation peak gradually becomes strong with increasing the aging time (see Fig. 11A). The normalized PL intensities at 260 and 344 nm in the excitation spectra of the samples aged for 0ptagp240 min are plotted as a function of Fcryst, as shown in Fig. 13. The intensity at 260 nm corresponding to O2–V5+ charge transfer is proportional to Fcryst. This means that the crystalline phase mainly contributes to the emission. On the contrary, the intensity at 344 nm corresponding to the excitation corresponding to Bi3+–V5+ charge transfer gradually increases with increasing Fcryst. As already mentioned, the contents of incorporated Bi3+ and Eu3+ in the crystalline phase keeps constant at 0ptagp240 min, judging from the change in d2 0 0. We, therefore, conclude that in addition to Fcryst the change in coordination states around Bi3+ ions possibly determines the relative intensity of the excitation corresponding to Bi3+–V5+ charge transfer. Blasse’s group [28] and Silver’s group [29] reported that the energy level of ns2 ions is lowered by ns–np mixing in the coordination environment with lower symmetry. So if ns2 ions such as Bi3+are in more symmetrical coordination, ns electrons are more delocalized, and then Bi3+ ions are subject to larger influence of coordination environment on luminescence properties. We, therefore, attribute the increase in the relative PL intensity during aging to the higher symmetrical coordination. After the completion of the crystallization, i.e., at 240ptagp420 min, the PL intensity decreases with
ARTICLE IN PRESS S. Takeshita et al. / Journal of Luminescence 128 (2008) 1515–1522
1521
Fig. 11. PL and PLE spectra of powder samples aged at 60 1C for different time. tag (min): (a) 0, (b) 1, (c) 15, (d) 30, (e) 45, (f) 60, (g) 120, (h) 180, (i) 240, (j) 300 and (k) 420. A: (a)–(i), B: (i)–(k).
Fig. 12. Assignments of excitation peaks by a simple energy diagram. (a) low Bi3+ concentration and (b) high Bi3+ concentration.
increasing the aging time (see Fig. 11B). At the same time, the increase in the incorporated Bi3+ content is confirmed by XRD (see Fig. 9) and XRF (see Fig. 10), as already
mentioned. We, therefore, attribute the decrease in the PL intensity at tagX240 min to concentration quenching by energy migration among Bi3+ ions. The replacement of Y3+ by Bi3+ in YVO4 occurs through ionic exchange by repeating dissolution and precipitation processes during aging in the presence of the Bi3+-rich precipitate. However, Bi3+ ions cannot diffuse homogeneously inside particles at the temperature as low as 60 1C. Therefore, Bi3+ ions localized near surface causes concentration quenching after prolonged aging. On the other hand, the position of the excitation peak corresponding to Bi3+–V5+ charge transfer shifts from 344 nm (tag ¼ 240 min) to 351 nm (tag ¼ 420 min). As shown in Fig. 12, this red shift is possibly explained by
ARTICLE IN PRESS 1522
S. Takeshita et al. / Journal of Luminescence 128 (2008) 1515–1522
corresponding to Bi3+–V5+ charge transfer becomes strong more slowly than that of O2–V5+ charge transfer. (ii) After the completion of the crystallization, the content of Bi3+ incorporated into YVO4 gradually increases through ionic exchange between the nanorods and the Bi3+-rich precipitates by repeating dissolution and precipitation processes. The PL intensity for the excitation corresponding to Bi3+–V5+ charge transfer decreases as a result of energy migration among Bi3+ ions. References [1] [2] [3] [4] [5]
[6] Fig. 13. Change in normalized PL intensities at 260 nm (K) and 344 nm (.) with Fcryst.
the following two items: (i) the number of energy levels of Bi3+ ions in valence and conduction bands, and (ii) the number of Bi3+–V5+ pairing ions with interaction increases with increasing the incorporated Bi3+ concentration. Both items (i) and (ii) lead to the observed shift of the excitation energy to lower energy. 4. Conclusion YVO4: Bi3+, Eu3+ nanphosphors are prepared by the citrate-assisted wet chemical synthesis at 60 1C. The crystallization completes at tag200 min. The characteristics of the doping process can be summarized as follows: (i) Before the completion of the crystallization, amorphous gel precursors formed contain Y3+, Bi3+, Eu3+, VO3 and citrate ligands by ionic exchange. With 4 increasing the aging time, the crystalline YVO4: Bi3+, Eu3+ nanorods with 14 at% of Bi3+ and 11 at% of Eu3+ are formed inside the gel precursors. The PL intensity increases with increasing the fraction of the crystalline phase. At the early stage of aging, however, Bi3+ 6 s electrons are localized due to the distorted Bi3+ environments, which do not contribute to the excitation process. Therefore, the excitation peak
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
A.N. Becidyan, Color Res. Appl. 20 (1995) 124. K. Egami, Opt. Alliance 8 (1997) 6 (Japanese). J.T. Schmidt, Mater. Eval. 33 (1975) 21A. S. Tanaka, S. Miyahara, S. Niikura, Jpn. Kokai Tokkyo Koho 2002–121203 (2002). D.A. Steigerwald, J.C. Bhat, D. Collins, R.M. Fletcher, M.O. Holcomb, M.J. Ludowise, P.S. Martin, S.L. Rudaz, IEEE. J. Sel. Top. Quantum Electron. 8 (2002) 310. S. Neeraj, N. Kijima, A.K. Cheetham, Solid State Commun. 131 (2004) 65. S.Z. Toma, F.F. Mikus, J.E. Mathers, J. Electrochem. Soc. 114 (1967) 953. R.K. Datta, J. Electrochem. Soc. 114 (1967) 1057. B. Yan, X.-Q. Su, J. Non-Cryst. Solids 352 (2006) 3275. S. Qiang, C. Barthou, J.P. Denis, F. Pelle, B. Blanzat, J. Lumin. 28 (1983) 1. L.S. Chi, R.S. Liu, B.J. Lee, J. Electrochem. Soc. 152 (2005) J93. W.J. Park, S.G. Yoon, D.H. Yoon, J. Electroceram. 17 (2006) 41. T.-S. Chan, C.-C. Kang, R.-S. Liu, L. Chen, X.-N. Liu, J.-J. Ding, J. Bao, C. Gao, J. Comb. Chem. 9 (2007) 343. R.B. Pode, S.J. Dhoble, Phys. Stat. Sol. (b) 203 (1997) 571. J.-G. Zhou, F.-Y. Zhao, Z.-Q. Li, S.-P. Xia, L. Yang, S.-Y. Gao, J. Mater. Sci. 39 (2004) 4711. X. Zeng, S.-J. Im, S.-H. Jang, Y.-M. Kim, H.-B. Park, S.-H. Son, H. Hatanaka, G.-Y. Kim, S.-G. Kim, J. Lumin. 121 (2006) 1. B. Li, Y. Tian, Y. Bai, Mater. Res. Bull. 24 (1989) 961. M. Wang, X. Fan, G. Xiong, J. Phys. Chem. Solids 56 (1995) 859. G. Blasse, A. Bril, J. Chem. Phys. 48 (1968) 217. A. Newport, J. Silver, A. Vecht, J. Electrochem. Soc. 147 (2000) 3944. B.N. Mahalley, R.B. Pode, P.K. Gupta, Phys. Stat. Sol. (a) 177 (1999) 293. B.N. Mahalley, S.J. Dhoble, R.B. Pode, G. Alexander, Appl. Phys. A 70 (2000) 39. K. Riwotzki, M. Haase, J. Phys. Chem. B 102 (1998) 10129. A. Huignard, V. Buissette, G. Laurent, T. Gacoin, J.-P. Boilot, Chem. Mater. 14 (2002) 2264. R.D. Shannon, Acta Crystallogr. A 32 (1976) 751. Y. Wang, Y. Zuo, H. Gao, Mater. Res. Bull. 41 (2006) 2147. M.W. Stoltzfus, P.M. Woodward, R. Seshadri, J.H. Klepeis, B. Bursten, Inorg. Chem. 46 (2007) 3839. M. Wiegel, W. Middel, G. Blasse, J. Mater. Chem. 5 (1995) 981. X. Jing, C. Gibbons, D. Nicholas, J. Silver, A. Vecht, C.S. Frampton, J. Mater. Chem. 9 (1999) 2913.