Preparation and Photoluminescence of Yttrium Hydroxide and Yttriumoxide Doped with Europiumnanowires

ARTICLE IN PRESS

Journal of Crystal Growth 277 (2005) 643–649 www.elsevier.com/locate/jcrysgro

Preparation and photoluminescence of yttrium hydroxide and yttriumoxide doped with europiumnanowires

Xingcai Wua,b,Ã, Yourong Taoa, Fei Gaoa, Lin Donga, Zheng Hua aDepartment of Chemistry, Laboratory of Mesoscopic Materials Science, Nanjing University, Nanjing 210093, People’s Republic of China bState Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, People’s Republic of China

Received 17 July 2004; accepted 27 January 2005 Available online 10 March 2005 Communicated by J.M. Redwing

Abstract

Single-crystalline Y(OH)3:Eu nanowires have been prepared by polymer-assisted hydrothermal method. The yields of the nanowires were higher than 95%. XRD patterns showed that the products were the hexagonal Y(OH)3 structure. SEM images displayed that the nanowires consisted of nanobelts and nanorods. The nanorods have the diameter of about 85 nmand length up to several tens of microns,and the nanobelts have the width of 95 nm, and the thickness of 20 nm, and the length of several tens of microns. Y(OH)3:Eu nanowires were converted to Y2O3:Eu nanowires by thermal decomposition at 500 1C for 3 h. The morphologies still remained. The photoluminescence (PL) properties of Y(OH)3:Eu and Y2O3:Eu nanowires were studied. There are four strong peaks at 592, 616, 651 and 696 nmfor the Y(OH)3:Eu nanowires under the excitation of 396 nm. There is only strong peak at 611 nm for the Y2O3:Eu nanowires under the excitation of 465 nm. r 2005 Elsevier B.V. All rights reserved.

PACS: 61.46; 78.55; 81.10

Keywords: A1. Low dimensional structures; A1. X-ray diffraction; A2. Hydrothermal crystal growth; B1. Nanomaterials; B1. Yttrium compounds; B2. Phosphors

1. Introduction

Phosphors are essential materials in display applications. The high performance of display requires high-quality phosphors for sufﬁcient Ã Corresponding author. Department of Chemistry, Labora- brightness and long-termstability. To enhance tory of Mesoscopic Materials Science, Nanjing University, the luminescent characteristic of phosphors, Nanjing 210093, People’s Republic of China. Tel.: +86 25 83594945; fax: +86 25 83317761. extensive research has been carried out on E-mail address: [email protected] (X. Wu). rare-earth-activated oxide phosphors. Among these

644 X. Wu et al. / Journal of Crystal Growth 277 (2005) 643–649 oxide-based phosphors, Y2O3:Eu nanoparticles are clave with 40 ml. The autoclave was maintained at one of the most promising oxide-based red phos- 170 1C for 24 h, and cooled to roomtemperature. hors systems due to its excellent luminescence The products were collected and washed with the efﬁciency, color purity, and stability [1].Inthe distilled water and ethanol, and dried at 80 1C for preparation of Y2O3:Eu nanoparticles, many dif- 4 h. They were converted into Y2O3:Eu nanowires ferent techniques have been reported such as spray after the as-prepared Y(OH)3:Eu nanowires were pyrolysis [2], chemical vapor deposition [3], sol–gel calcined in alumina crucible at 500 1C at a rate of [4] and so on. One-dimensional (1D) quantum wires 5 1C/min and maintained for 3 h in air. The have attracted much attention for both their hydroxide samples of the different times such as importance in mesoscopic physics and potential 2 and 12 h were prepared for investigation into applications in fabricating novel nanoelectronic, growth process of the nanowires. Bulk Y(OH)3:Eu optoelectronic, electrochemical, and electromecha- was synthesized according to the above similar nical device [5–8]. Therefore, it is important to method, without PEG 20000, but reaction time fabricate Y(OH)3:Eu and Y2O3:Eu quantumwires was prolonged to 36 h. Bulk Y(OH)3:Eu was to meet demand in fabricating nanodevices. There converted to bulk Y2O3:Eu at 500 1C. have also been a few reports related to the synthesis The products were characterized on a Shimadzu of Y2O3:Eu 1D nanostructures such as Y2O3:Eu XD-3A X-ray diffractometer with graphite mono- nanowire array with anodic aluminum oxide chromatized Cu Ka-radiation ðl ¼ 0:15418 nmÞ template [9],Y2O3:Eu nanotubes [10],Y(OH)3 and nickel ﬁlter. TEM images and SAED (selected and Y2O3 nanotubes [11] and Y2O3:Eu nanobelts area electron diffraction) pictures were recorded [12].However,Y(OH)3:Eu nanowires and their on a JEOL-JEM 200CX transmission electron photoluminescence (PL) properties have not been microscope, using an accelerating voltage of reported until now. Here, we report the synthesis of Y(OH)3:Eu nanowires via (polyethylene glycol) PEG 20000-assisted hydrothermal reaction. Y2O3:Eu nanowires were prepared by thermal decomposition of Y(OH)3:Eu nanowires, and their morphologies were characterized in detail, and their PL properties were studied.

2. Experimental section

All the reagents used in our experiments were of analytical purity and were used without further puriﬁcation. Yttriumoxide, sodiumhydroxide and PEG 20000 were purchased fromShanghai (Chi- na) chemical reagent company. Nitric acid was purchased fromNanjing (China) chemicalre- agents factory. In a typical procedure, 0.6 mmol Y2O3 and 0.03 mmol Eu2O3 powders were dissolved in 2 ml concentrated nitric acid, and were adjusted to pHE13 with sodiumhydroxide. The solution was added with 0.5 g PEG 20000 and distilled water, and the total volume of the solution was 30 ml.

After the solution mixture was stirred for 20 min, it Fig. 1. XRD patterns of the samples. (a) Y(OH)3 nanowires, was transferred into a Teﬂon-lined stainless auto- (b) Y(OH)3:Eu nanowires, and (c) Y2O3:Eu nanowires. ARTICLE IN PRESS

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200 kV. SEM pictures were, respectively, taken by reported data in JCPDS cards (JCPDS 24-1422). LEO-1530VP scanning electron microscope. The Perhaps, because Eu content of Y(OH)3:Eu PL spectra were taken by time resolved ﬂuorescence nanorods is low or because radius of Y3+ spectrometer. (SLM 48000DSCF/AB2, SLM Inc.) approaches that of Eu3+, there is almost no change in the lattice parameters. Y(OH)3:Eu nanowires were calcined at 500 1C to be converted 3. Results and discussion to Y2O3:Eu nanowires. The powder XRD pattern of the samples is shown as Fig. 1c, which is a cubic 3.1. Structure characterization phase with the cell constants of a ¼ 1:0604 nm (JCPDS 25-1200).

The powder XRD pattern of Y(OH)3 and Y(OH)3:Eu nanowires have been shown respec- 3.2. Morphologies tively as Fig. 1a and b, which can be indexed to be a pure hexagonal phase with the cell constants of Fig. 2a is a low-magniﬁcation SEM micrograph a ¼ 0:6268 nmand c ¼ 0:3547 nm; identical to the of Y(OH)3:Eu nanowires, which shows that the

Fig. 2. SEM images of the samples. (a) Overviews of Y(OH)3:Eu nanowires, (b) morphologies of Y(OH)3:Eu nanorods, (c) micrograph of Y(OH)3:Eu nanobelts, (d) low-magniﬁcation images of Y2O3:Eu nanowires, (e) images of Y2O3:Eu nanobelts, and (f) morphologies of Y2O3:Eu nanorods. ARTICLE IN PRESS

646 X. Wu et al. / Journal of Crystal Growth 277 (2005) 643–649

Fig. 3. TEM images of the samples. (a) Y(OH)3:Eu nanowires, (b) single Y(OH)3:Eu nanowires, inset is its SAED pattern, (c) Y2O3:Eu nanowires, (d) SAED pattern of Y2O3:Eu nanowires, and (e) Y2O3:Eu nanobelts.

samples are composed of the nanowire bundles Y(OH)3:Eu nanowires, the morphologies of with the diameter of 1–3 mmand length up to Y2O3:Eu nanowires almost remain the same, but several tens of microns. Further observation shows the diameter (width) decreases slightly. the coexistence of nanorods and nanobelts. As Fig. Fig. 3a is TEM micrograph of a bundle of 2b shows, a single nanowire has the diameter of Y(OH)3:Eu nanowires. Fig. 3b shows TEM image about 85 nm. Fig. 2c reveals the Y(OH)3:Eu of a single nanowire, and inset is its SAED pattern nanobelts with the width of 95 nm, and the along ½110¯ direction, which indicates the nano- thickness of 20 nm. Fig. 2d is a low-magniﬁca- wire grows along [0 0 1] direction. Fig. 3c is TEM tion SEM image of Y2O3:Eu nanowires. Fig. 2e image of Y2O3:Eu nanowires. The SAED pattern reveals that the Y2O3:Eu nanobelts have a width of (Fig. 3d) of a single Y2O3:Eu nanowire supported about 53 nm, and thickness of about 20 nm. Fig. 2f the results indexed by XRD. Fig. 3e shows TEM indicates morphologies of Y2O3:Eu nanorods with images of a bundle of Y2O3:Eu nanobelts, and the diameters of about 80 nm. Compared with typical belt was indicated by the arrow in Fig. 3e. ARTICLE IN PRESS

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3.3. Photoluminescence properties transitions to even J-numbers have much higher intensity than those to corresponding neighboring Fig. 4a presents the room-temperature excita- odd J-numbers, however, the intensity of the 5 7 tion spectrumof Y(OH) 3:Eu nanowires, which the D0– F1 transition is higher than that of the 5 7 strongest peak is at 396 nm, so it is selected as the D0– F2 as for Y(OH)3:Eu nanowires, which is excited light. Fig. 4b is the emission spectrum of an abnormal phenomenon, and the same phenom- the sample under 396 nm excitation. The peak at enon also appears on bulk samples, therefore 592 nmderives fromthe allowed magnetic dipole investigation into themwill remainfurther. Fig. 5a 5 7 3+ transition ( D0– F1) (Eu site with inversion presents the room-temperature excitation spec- symmetry). Peak at 616 nm is due to the forced trumof Y 2O3:Eu nanowires, which shows the 5 7 electric dipole transition ( D0– F2), which is double excitation peaks at 394 and 465 nm, and allowed on condition that the europiumion the strongest peak is at 465 nm, so we select it as occupies a site without an inverse center. Its the excited light. Fig. 5b is the room-temperature intensity is hypersensitive to crystal environments. emission spectrum of the sample under 465 nm Peaks at 651 and 696 nmare, respectively, excitation. The strongest peak at 611 nmis 5 7 5 7 attributed to D0– F3 and D0– F4 transitions attributed to the forced electric dipole transition 5 7 [13]. In accordance with Judd–Ofelt theory, ( D0– F2) (the europiumion occupies a site without an inverse center). The PL spectrumof

Fig. 4. Photoluminescence spectrum of Y(OH)3:Eu nanowires. Fig. 5. Photoluminescence spectrum of Y2O3:Eu nanowires. (a) (a) Excitation spectrum, and (b) emission spectrum under Excitation spectrum, and (b) emission spectrum under 465 nm 396 nmexcitation. excitation. ARTICLE IN PRESS

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Fig. 6. Photoluminescence spectra of (a) Y(OH)3:Eu bulk at excitation of 396 nm, and (b) Y2O3:Eu bulk at excitation of 466 nm.

Y2O3:Eu nanowires is similar to that of Y2O3:Eu nanocrystallites [14], which shows their Eu coordination environments are similar. In order to compare PL properties of bulk Y(OH)3:Eu and Y2O3:Eu with corresponding nanowires, we synthesized bulk Y(OH)3:Eu and Y2O3:Eu, however, there are no differences. PL spectra of bulk Y(OH)3:Eu or Y2O3:Eu are shown in Fig. 6.

3.4. Growth process Fig. 7. Formation of Y(OH)3:Eu nanowires. (a) 2 h, (b) 12 h, and (c) 24 h without PEG 20000. Fig. 7a and b shows the TEM images of the samples taken at different stages of the hydro- were added, there were more the particles (Fig. 7c). thermal reaction: (a) 2 h and (b) 12 h. A large It shows that PEG plays a polymer-template role. number of nanoparticles appeared after hydro- The growth process could be included as follows: thermal reaction for 2 h. The particles and rods (1) PEG coordinated with Y3+ to forma long coexisted after hydrothermal reaction for 12 h. chain complex—PEG–Y(NO3)3 [11] (Fig. 8b); (2) After 24 h, the particles were almost dissolved, and in the alkaline solution, the unstable complex were the nanowires were formed completely. If no PEG converted into Y(OH)3–PEG gel and amorphous ARTICLE IN PRESS

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4. Conclusions

We have demonstrated an effective approach to prepare single-crystalline Y(OH)3:Eu and Y2O3:Eu nanowires. Growth process of Y(OH)3:Eu nanowires has been thought as SLS mechanism under the direction of polymer-templates. Photolumines- cence (PL) properties of Y(OH)3:Eu and Y2O3:Eu nanowires are obviously different. It can be attributed to change of the symmetry of Eu3+ coordination environments.

Acknowledgment

The work was supported by Nanjing University Fig. 8. Schematic growth mechanism of Y(OH)3 nanowires. Talent Development Foundation. Dark circles represent OH, white circles represent Y3+. particles, and under the present hydrothermal References condition, the gel could convert to Y(OH)3 molecule chains along PEG long chains (Fig. 8c); [1] G. Wakefied, E. Holland, P.J. Dobson, T.L. Hutchison, (3) as the reaction proceeded, some little amor- Adv. Mater. 13 (2001) 1557. phous particles dissolved, and Y3+ and OH ions [2] Y.C. Kang, H.S. Roh, S.B. Park, J. Electrochem. Soc. 147 (2000) 1601. transferred to the surface of Y(OH)3 molecule [3] L.D. Sun, J. Yao, C. Liu, C. Liao, C.H. Yan, J. Lumin. chains, condensed and formed ultrafine nanowires. 87–89 (2000) 447. If there were less polymer chains connected with [4] M.H. Lee, S.G. Oh, S.C. Yi, J. Colloid Interface Sci. 226 the side of the ultrafine nanowires, as Y3+ and (2000) 65. OH ions attacked around ultrafine nanowires, [5] M. Morales, C.M. Lieber, Science 279 (1998) 208. [6] B. Mayers, B. Gates, Y.D. Yin, Y.N. Xia, Adv. Mater. 13 the section of the nanowires approached a circle. If (2001) 1380. there were more polymer chains connected with [7] D.P. Yu, Z.G. Bai, J.J. Wang, Y.H. Zou, W. Qian, H.Z. the side of the ultrafine nanowires, as polymer Zhang, Y. Ding, G.C. Xiong, S.Q. Feng, Phys. Rev. B 59 chains resisted Y3+ and OH ions to attack (1999) R2498. around the ultrafine nanowires, the nanobelts [8] S.M. Prokes, K.L. Wang, MRS Bull. 24 (1999) 13. [9] J. Zhang, G. Hong, J. Solid State Chem. 177 (2004) 1292. were formed. Such a process is illustrated as Fig. [10] C. Wu, W. Qin, G. Qin, D. Zhao, J. Zhang, S. Huang, S. 8. In fact, it is a process of solid–liquid–solid (SLS) Lu¨, H. Liu, H. Lin, Appl. Phys. Lett. 82 (2003) 520. under direction with PEG 20000 long chain. If no [11] Q. Tang, Z. Liu, S. Li, S. Zhang, X. Liu, Y. Qian, J. polymers were added, a little of the nanowires Crystal Growth 259 (2003) 208. could be formed. Because of the anisotropy [12] Y. He, Y. Tian, Y. Zhu, Chem. Lett. 32 (2003) 862. [13] K. Riwotzki, M. Haase, J. Phys. Chem. B 102 (1998) hexagonal structure of Y(OH)3, there was an 10129. intrinsic tendency to grow into rod-like morphol- [14] M. Kwak, J. Park, S. Shon, Solid State Commun. 130 ogies. (2004) 199.