JOURNAL OF RARE EARTHS, Vol. 28, Spec. Issue, Dec. 2010, p. 222

Foundation item: Project supported by the China Postdoctoral Science Foundation (20080430216) and the Science Technology Project of Zhejiang Province (2008C21162)Corresponding author: WANG Xiuli (E-mail: xlwang_hztc@hotmail.com; Tel.: +86-571-28865312) DOI: 10.1016/S1002-0721(10)60306-7

Controlled synthesis and tunable luminescence of NaYF4:Eu3+

WANG Xiuli ( )1, ZHAO Shilong ( )2, ZHANG Yijian ( )1, Sheng Guoding ( )1 (1. College of Materials and Chemistry, Chemical Engineering, Hangzhou Normal University, Hangzhou 310036, China; 2. College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China)

Received 28 July 2010; revised 21 October 2010

Abstract: High quality NaYF4:Eu3+ luminescent materials were successfully synthesized via a facile template technique by hydrothermal method. The samples were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and fluorescence spectroscopy (FS). The incorporating of Eu3+ ions into NaYF4 crystal lattice influenced the symmetry types of NaYF4 crystals, resulting in phase transformation of NaYF4 crystals between and phase. The pure hexagonal phase of branched NaYF4: Eu3+ was obtained as the Eu3+ concentration reached 15 mol.%. In addition, the luminescence color was tuned by changing the doping concentration of Eu3+ ions.

Keywords: NaYF4:Eu3+; controlled synthesis; luminescence; rare earths

Luminescent materials activated by lanthanide ions (Ln3+) have attracted extensive attention because of their wide range of emission colors, high photochemical stability, and sharp emission bandwidths, compared to organic phosphors and semiconducting fluorescent materials[1–5]. The trivalent europium ion (Eu3+) is well-known as a red-emitting activa-tor due to its 5D0–7FJ transitions (J=0, 1, 2, 3, 4). In addition to the above emission lines, those from higher 5D levels, such as 5D1 (green), 5D2 (green, blue), and 5D3 (blue), are of-ten observed depending upon the host lattice, e.g. the phonon frequency as well as the crystal structure, and the doping concentration of Eu3+ [6]. Since the emission properties of Eu3+ are very sensitive to the crystal lattice type, the selec-tion of a suitable host lattice is important to obtain excellent luminescent materials. NaYF4, an excellent luminescent host material, has many applications in lighting and display de-vices, optical telecommunications, and solid-state lasers be-cause of its high radiative emission rate, narrow emission bands, and stability and durability under high temperature and intense excitation energy[7–9]. NaYF4 usually adopts face-centered cubic ( phase) or hexagonal crystal structures ( phase). Previous investigations have shown that -NaYF4 is a much better host lattice for the luminescence of various optically active Ln3+ ions than -NaYF4

[10,11]. Consequently, how to obtain pure -NaYF4 via a facile and effective route is crucial in successfully achieving an excellent luminescent material. Conventional techniques for controlling crystal phase generally impose stringent control over a set of ex-perimental variables, such as nature of solvent, temperature, reaction time and concentration of metal precursors, etc.[12–19]. Lin group has tuned the emission colors by chang-ing the doping concentration of Eu3+ [20]. But little work has been explored about simultaneous control of crystal phase

and luminescent properties through lanthanide doping[21]. In this work, we introduce Eu3+ ions into the lattice of NaYF4 via a facile hydrothermal method, and control the crystal phase and luminescent properties simultaneously by chang-ing the doping concentration of Eu3+.

1 Experimental

NaYF4:Eu3+ luminescent materials were synthesized via a facile template technique by hydrothermal method. All the reagents were of analytical grade without further purification. Stoichiometric weights of Y2O3 dissolved in diluted hydro-chloric acid (HCl) was used as the Y3+ source and EuCl3 6H2O was used as the Eu3+ source. The typical synthetic steps were as follows: 7.2 g CTAB (cetyltriethylammnonium bromide) was dissolved in 40 ml of deionized water, fol-lowed by adding stoichiometric amount of Y3+ source and Eu3+ source, and then a stoichiometric amount of NaF was added. The resulting mixture with the molar ratios of 1YCl3·6H2O:4NaF:10CTAB was kept under vigorous stir-ring for 0.5 h. The final mixture was sealed in Teflon-lined stainless steel autoclaves at 180 ºC for 4 d under autogenous pressure. Thereafter, the suspension was cooled down to room temperature. The powders were obtained by centrifu-gation and washed with plenteous deionized water at room temperature, then dried at 60 ºC for 24 h in air and kept dry-ing at room temperature.

The crystal structure was analyzed with a Rigaku D/max2550V/PC X-ray powder diffractometer with Cu K radiation ( =0.154 nm). The size and morphology of the nanocrystals were determined by a transmission electron microscope (TEM, Philips-FEI-Tecnai G2 F30 S-Twin). The photoluminescence excitation and emission spectra were

WANG Xiuli et al., Controlled synthesis and tunable luminescence of NaYF4:Eu3+ 223

measured with a Jobin-Yvon Fluorolog-3 fluorescence spec-trophotometer with a Xe-lamp. All measurements were taken at room temperature.

2 Results and discussion

2.1 XRD analysis

Generally, material with certain crystal phase was synthe-sized by controlling experimental conditions, such as raw resource, type of solvent, reaction temperature and time, and so forth. In this article, we synthesized target product (pure NaYF4) successfully by changing the doping concentration of Eu3+. Fig. 1 shows the XRD patterns of NaYF4 with dif-ferent doping concentration of Eu3+. From the XRD results we can see that the samples with 1 mol.%, 3 mol.%, 5 mol.% and 10 mol.% Eu3+ concentration are all mixtures of phase and phase, and that the relative amounts of the two phase vary with the changing of doping concentration of Eu3+. The pure hexagonal phase of NaYF4 was obtained as the Eu3+ concentration reached 15 mol.%. The position and relative intensity of all diffraction peaks are in good agreement with the standard values for hexagonal NaYF4 (JCPDS 16-0334). No impurity is identified from the XRD pattern.

2.2 Morphology analysis

The morphology of the 15 mol.% Eu3+ sample was ob-served by TEM image, as shown in Fig. 2. It shows that the samples we synthesized are composed of crystals with branched morphology and with a diameter of about 200 nm. Considering that the morphology of NaYF4 usually adopted spherical particles, cubic shape, six prismatic microrods, etc. The branched morphology of the synthesized sample is unique, about which few work is reported[21]. Branched structures on the nanometer scale have received much con-sideration in recent years because of their intrinsic electronic, magnetic, photonic, and catalytic properties and their poten-tial to be crucial building blocks for future nanodevices by a “bottom-up” self-assembly process[22,23]. The appearance of

Fig. 1 XRD patterns of NaYF4:Eu3+ with different doping concen-

trations of Eu3+

branched structures greatly increases the diversity of build-ing blocks. Branching and further branching of nanocrystals can effectively increase the structural diversity of building blocks and create more complex structural architectures[24]. Therefore, the synthesis of branched NaYF4 nanocrystals should be of great significance for both research and applica-tions.

2.3 Photoluminescence properties

The photoluminescence spectra of NaYF4:xEu3+ (x=1, 3, 5, 10, and 15 mol.%) excited under 397 nm are shown in Fig. 3. It indicates that the spectra of all the above samples consist of all the emission lines associated with the Eu3+ transitions from the excited 5D0,1,2,3 levels to the 7FJ level, i. e., 5D3–7F3 (445 nm), 5D2–7F0 (464 nm), 5D2–7F2 (489 nm), 5D2–7F3 (511 nm), 5D1–7F0 (525 nm), 5D1–7F1 (536 nm), 5D1–7F2 (555 nm), 5D1–7F3 (583 nm), 5D0–7F1 (590 nm), 5D0–7F2

(614 nm), 5D0–7F3 (648 nm), and 5D0–7F4 (693 nm)[25]. When the doping concentration of Eu3+ is 1 mol.%, the emission lines of Eu3+ cover the whole visible spectral region with comparable intensity, resulting in a white light emission. The relative intensity of low wavelength becomes weaker and that of long wavelength becomes stronger with the dop-ing concentration of Eu3+ increasing. Finally, the emission color is almost only red as the Eu3+ concentration reached 15 mol.%. On the basis of the above analysis, it can be con-cluded that the luminescence color of NaYF4:Eu3+ phosphors

Fig. 2 TEM image of NaYF4:Eu3+ with doping concentration of

15 mol.% Eu3+

224 JOURNAL OF RARE EARTHS, Vol. 28, Spec. Issue, Dec. 2010

Fig. 3 Photoluminescence spectra of NaYF4:Eu3+ with different

doping concentration of Eu3+ can be finely tuned by changing the doping concentrations of Eu3+.

3 Conclusions

The pure hexagonal phase of branched NaYF4:Eu3+ was successfully synthesized via a facile template technique by hydrothermal method. The relative amounts of phase and

phase varied with the doping concentration of Eu3+ chang-ing. The pure phase of NaYF4: Eu3+ was obtained as the Eu3+ concentration reached 15 mol.%. Not only the phase was controlled but the photoluminescence properties was simultaneously tuned from white to red emission as the doping concentration of Eu3+ changed from 1 mol.% to 15 mol.%. In particular, the branched morphology of NaYF4:Eu3+ was unique since branched NaYF4 nanocrystals should be of great significance for both research and applications.

References:

[1] Wang Guofeng, Qin Weiping, Wang Lili, Wei Guodong, Zhu Peifen, Zhang Daisheng, Ding Fuheng. Synthesis and upcon-version luminescence properties of NaYF4:Yb3+/Er3+ micro-spheres. Journal of Rare Earths, 2009, 27(3): 394.

[2] Wang Zhenling, Li Min, Wang Chang, Chang Jiazhong, Shi Hengzhen, Lin Jun. Photoluminescence properties of LaF3: Eu3+ nanoparticles prepared by refluxing method. Journal of Rare Earths, 2009, 27(1): 33.

[3] Wang F, Liu X. Upconversion multicolor fine-tuning: visible to near-infrared emission from lanthanide-doped NaYF4 nanopar-ticles. J. Am. Chem. Soc., 2008, 130: 5642.

[4] Li C, Hou Z, Zhang C, Yang P, Li G, Xu Z, Fan Y, Lin J. Con-trolled synthesis of Ln3+ (Ln=Tb, Eu, Dy) and V5+ ion-doped YPO4 nano-/microstructures with tunable luminescent colors. Chem. Mater., 2009, 21: 4598.

[5] Qian H, Zhang Y. Synthesis of hexagonal-phase core-shell NaYF4 nanocrystals of tunable upconversion fluorescence. Langmuir, 2008, 24: 12123.

[6] Blasse G, Grabmaier B C. Luminescence Materials. Berlin: Springer-Verlag, 1994, Chapters 4-5.

[7] Yan Y C, Faber A J, Waal H, Kik P G, Polman A. Erbium-

doped phosphate glass waveguide on silicon with 4.1 dB/cm gain at 1.535 m. Appl. Phys. Lett., 1997, 71: 2922.

[8] Giesber H, Ballato J, Chumanov G, Kolis J, Dejneka M. Spec-troscopic properties of Er3+ and Eu3+ doped acentric LaBO3 and GdBO3. J. Appl. Phys., 2003, 93: 8987.

[9] Xia H R, Li L X, Zhang H J, Meng X L, Zhu L, Yang Z H, Liu X S, Wang J Y. Raman spectra and laser properties of Yb-doped yttrium orthovanadate crystals. J. Appl. Phys., 2000, 87: 269.

[10] Heer S, Kömpe K, Gödel H U, Haase M. Highly efficient mul-ticolour upconversion emission in transparent colloids of lan-thanide-doped NaYF4 nanocrystals. Adv. Mater., 2004, 16(23-24): 2102.

[11] Yi G S, Chow G M. Synthesis of Hexagonal-phase NaYF4:Yb, Er and NaYF4:Yb,Tm nanocrystals with efficient up-conver-sion fluorescence. Adv. Funct. Mater., 2006, 16(18): 2324.

[12] Liu C, Wang H, Zhang X, Chen D. Morphology- and phase- controlled synthesis of monodisperse lanthanide-doped NaGdF4 nanocrystals with multicolor photoluminescence. J. Mater. Chem., 2009, 19: 489.

[13] Lim Byungkwon, Jiang Majiong, Camargo Pedro H C, Cho Eun Chul, Tao Jing, Lu Xianmao, Zhu Yimei, Xia Younan. Pd-Pt bimetallic nanodendrites with high activity for oxygen reduction. Science, 2009, 324: 1302.

[14] Sun S, Murray C B, Weller D, Folks L, Moser A. Monodis-perse FePt nanoparticles and ferromagnetic FePt nanocrystal superlattices. Science, 2000, 287: 1989.

[15] Tang Z, Zhang Z, Wang Y, Glotzer S C, Kotov N A. Self-as-sembly of CdTe nanocrystals into free-floating sheets. Science, 2006, 314: 274.

[16] Wang X, Zhuang J, Peng Q, Li Y. A general strategy for nanocrystal synthesis. Nature, 2005, 437: 121.

[17] Wang L, Li Y. Controlled synthesis and luminescence of lan-thanide doped NaYF4 nanocrystals. Chem. Mater., 2007, 19: 727.

[18] Mai Haoxin, Zhang Yawen, Si Rui, Yan Zhengguang, Sun Lingdong, You Liping, Yan Chunhua. High-quality sodium rare-earth fluoride nanocrystals: controlled synthesis and opti-cal properties. J. Am. Chem. Soc., 2006, 128: 6426.

[19] Yi G S, Chow G M. Synthesis of hexagonal-phase NaYF4:Yb, Er and NaYF4:Yb,Tm nanocrystals with efficient up-conver-sion fluorescence. Adv. Funct. Mater., 2006, 16: 2324.

[20] Li Chunxia, Zhang Cuimiao, Hou Zhiyao, Wang Lili, Quan Zewei, Lian Hongzhou, Lin Jun. -NaYF4 and -NaYF4:Eu3+ Microstructures: Morphology Control and Tunable Lumines-cence Properties. J. Phys. Chem. C, 2009, 113: 2332.

[21] Wang Feng, Han Yu, Lim Chin Seong, Lu Yunhao, Wang Juan, Xu Jun, Chen Hongyu, Zhang Chun, Hong Minghui, Liu Xiaogang. Simultaneous phase and size control of upconver-sion nanocrystals through lanthanide doping. Nature, 2010, 463: 1061.

[22] Huynh W U, Dittmer J J, Alivisatos A P. Hybrid nanorod- polymer solar cells. Science, 2002, 295: 2425.

[23] Cozzoli P D, Manna L. Asymmetric nanoparticles: Tips on growing nanocrystals, Nature Mater., 2005, 4(11): 801.

[24] Wang D L, Lieber C M. Inorganic materials: Nanocrystals branch out. Nature Mater., 2003, 2(6): 355.

[25] Yu M, Lin J, Fang J. Silica spheres coated with YVO4:Eu3+ layers via sol-gel process: a simple method to obtain spherical core-shell phosphors. Chem. Mater., 2005, 17(7): 1783.