Hydrothermal synthesis, structure study and luminescent properties of YbPO4:Tb3+ nanoparticles

JOURNAL OF RARE EARTHS, Vol. 28, Spec. Issue, Dec. 2010, p. 299

Hydrothermal synthesis, structure study and luminescent properties of YbPO4:Tb3+ nanoparticles ZHANG Wenyan (ᓴ᭛ཡ), NI Yaru (‫׾‬Ѯ㤍), HUANG Wenjuan (咘᭛࿳), LU Chunhua (䰚᯹ढ), XU Zhongzi (䆌ӆṧ) (State Key Laboratory of Materials-Orient Chemical Engineering, College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, China) Received 31 July 2010; revised 15 October 2010

Abstract: YbPO4:Tb3+ were synthesized by mild hydrothermal method. The luminescent properties, morphologies and structure of the obtained powders were characterized by photoluminescence (PL) spectra, FESEM, X-ray diffractometer (XRD) and FTIR. The results showed that the prepared YbPO4:Tb3+ nanoparticles were pure tetragonal phase and the average grain size varied with increasing of Tb3+ concentration. Hydrothermal temperature was revealed to be the key factor to enhance the emission intensity of YbPO4:Tb3+ phosphors. The spherical nanoparticles could be effectively excited by near UV (369 nm) light and exhibited green performance at 543 nm (5D4ĺ7F5), 489 nm (5D4ĺ7F6) and 586 nm (5D4ĺ7F4). The CIE chromaticity was calculated to be x=0.298, y=0.560. The YbPO4:Tb3+ nanoparticles exhibited potential to act as UV absorber for solar cells to enhance the conversion efficiency. Keywords: luminescence; hydrothermal; YbPO4:Tb3+; nanoparticle; rare earths

Compounds with rare-earth (RE) metal atoms are widely known as potential laser host materials, oxygen ion conductors, and fluorescent lamp phosphors due to their unique physical and chemical properties, which are attributed to the electronic transitions of rare earth ions between the 4f energy level[1–4]. However, sometimes the applications of RE compounds are limited because of their poor thermal stability and mechanical properties[5]. To overcome these drawbacks, it is important to choose the appropriate host to introduce RE ions. Phosphate compound is an appropriate host matrix for rare earth ions to fabricate luminescent materials. Rare earth phosphate, which introduces rare-earth metal atoms into phosphate compound, has attracted much attention to be used as phosphors, catalyst, biological detection and so on for their excellent performance such as high quantum efficiency, expected low toxicity and stability at high temperatures[6,7]. For the purpose of improving the conversion efficiency of solar cells, it is feasible to modify the solar spectrum by transferring near-ultraviolet light (UV) in the sunlight to visible light (VIS) so that the spectrum could fit with the spectroscopic sensitivity curve of the solar cell[8]. The conversion from UV to VIS could be realized by using a wavelength shifting material doped with rare earth ions[9]. Rare earth phosphates were usually synthesized by solid-phase reaction at high temperature[10]. Hydrothermal synthesis offers some advantages over the conventional solid state reaction method, such as its simplicity, relatively low tempera-

ture for reaction and the capability to control crystal growth[11]. Therefore, in this work, YbPO4:Tb3+ nanoparticles were prepared through hydrothermal way, and the photoluminescent properties under the excitation of near UV light were investigated.

1 Experimental 1.1 Preparation of TbxYb1–xPO4 (x=0.05, 0.9, 0.85) The solution was prepared by dissolving Tb(NO3)3 (99.99%), Yb(NO3)3 (99.99%), and NH4H2PO4 (99.5%) into distilled water under magnetic stirring, and urea was dissolved into the solution as co-precipitated reagent. The mol ratios of Tb(NO3)3 to Yb(NO3)3 were 5:95, 10:90, and 15:85, respectively, and the mixture was stirred constantly for 40 min. Then the obtained precursor were transferred into Teflon-lined autoclave and heated at 140–180 ºC for 24–72 h. After the autoclave cooled down to room temperature naturally, the obtained suspension was centrifuged and washed with distilled water for six times. The prepared white powder was then dried at 60 °C for 24 h for further characterization. 1.2 Characterization The structure and phase purity were checked by powder X-ray diffraction (XRD) on a Thermo ARL XTRA diffractometer employing Cu KĮ radiation (Ȝ=0.15406 nm). The

Foundation item: Project supported by National Natural Science Foundation of China (20901040/B0111), the Natural Science Foundation of the Jiangsu Higher Education Institutions, China (08KJB480001), the Natural Science Foundation of Jiangsu Province (BK2004121), the major program for the Natural Science Fundamental Research of the Jiangsu Higher Education Institutions of China and the Innovation Foundation for Graduate Students of Jiangsu Province (CX10B_159Z) Corresponding author: NI Yaru (E-mail: [email protected]; Tel.: +86-25-83587220) DOI: 10.1016/S1002-0721(10)60337-7

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morphology and microstructure were analyzed by a field emission scanning electronic microscope (FESEM, LEO1550). Fourier transform infrared spectra (FTIR) of the samples which were suspended in KBr pallets were acquired from 400 to 4000 cm–1 using a Nicolet 360 spectrometer. The photoluminescence properties were characterized with a JOBINYVON FL3-221 fluorescence spectrophotometer.

2 Results and discussion XRD patterns of the samples are shown in Fig. 1. The reflections demonstrated that the samples could all be indexed to tetragonal phase (JCPDS 76-1643), and space groups are I41/amd. The result indicates that the incorporation of Tb3+ ions has little influence on the crystal structure of YbPO4, due to the similar ionic radius of Tb3+ (0.104 nm) and Yb3+ (0.098 nm). No peak of any other phases was detected, showing that the simple hydrothermal method is a feasible route to prepare pure phase TbxYb1–xPO4 phosphors. The broadened peaks of the XRD patterns indicate that the grain size is small. The grain size can be roughly estimated by Scherrer equation, D=0.89Ȝ/ȕcosș, where ș is the diffraction angle, ȕ is the full-width at half-maximum (FWHM, in radian) of an observed peak, Ȝ is the wavelength of X-ray (0.15406 nm), and D is the average grain size, respectively[12]. The calculated average grain size are 25, 30, and 34 nm for Tb0.05Yb0.95PO4, Tb0.1Yb0.9PO4 and Tb0.15Yb0.85PO4, respectively. Since XRD is not adequate to fully characterize amorphous or poorly crystalline materials, in order to complete the structural and micro-structural analysis of as-prepared

YbPO4: Tb3+ powders, FTIR spectroscopy was also used. Fig. 2 shows the FT-IR absorption spectra of Tb0.05Yb0.95PO4, Tb0.1Yb0.9PO4 and Tb0.15Yb0.85PO4, respectively. The bands at 3445 and 1637 cm–1 (H–O–H bond bending) showed the presence of surface water molecules. The bands near 1008 cm–1 were attributed to the Ȟ3 anti-symmetric stretching of the PO43– groups[13]. The bands at 645 and 521 cm–1 were assigned to Ȟ4 bending vibration of the PO43– groups. It can also be presented that with the addition of Tb ions, the bands near 1000 cm–1 shifted to higher wavenumber slightly because of strong interaction between Tb ions and PO43– groups. Fig. 3 depicts the FESEM images of Tb0.05Yb0.95PO4, Tb0.1Yb0.9PO4 and Tb0.15Yb0.85PO4 nanoparticles obtained at 140 ºC for 72 h. It could be observed that the nanoparticles were in the range of 50–100 nm and the prepared particles appeared to become larger with the incorporation of Tb3+ ions. It could also be found from the FESEM images that some smaller nanocrystals agglomerated. Comparing with the bulk material, the small size of YbPO4:Tb3+ nanoparticles may increase the absorption area for UV light and therefore enhance the absorption. Fig. 4 presents the emission and excitation spectra of the prepared Tb0.15Yb0.85PO4. Under the excitation of 369 nm, the emission spectrum exhibited three sharp emission bands at 489, 543 and 586 nm, which were the typical terbium emissions due to 5D4ĺ7F6, 5D4ĺ7F5 and 5D4ĺ7F4 transitions, respectively. The strongest one appeared at 543 nm (5D4ĺ7F5). Emissions from 5D3 to 7Fj (j=3, 4, 5 and 6) were

Fig. 2 Infrared spectra of TbxYb1–xPO4 (1) x=0.05; (2) x=0.10; (3) x=0.15 Fig. 1 XRD patterns of TbxYb1–xPO4 (1) x=0.05; (2) x=0.10; (3) x= 0.15

Fig. 3 FESEM images of TbxYb1–xPO4 (a) x=0.05; (b) x=0.10; (c) x=0.15

ZHANG Wenyan et al., Hydrothermal synthesis, structure study and luminescent properties of YbPO4: Tb3+ nanoparticles

 not detected due to the cross-relaxation effect of Tb3+ ions[14]. The excitation spectrum was obtained by monitoring the 5 D4ĺ7F5 transition of Tb3+ at 543 nm (Fig. 4 (a)). The sharp peaks from 351 to 377 nm, which matches the emission wavelengths of near-UV chips, are due to the f-f transitions within the 4f8 configuration of Tb3+ [15]. The strongest one at 369 nm is attributed to the 7F6ĺ5G6 transition of Tb3+ and implies that the particles can be excited by near UV light and convert the near-ultraviolet light (UV) to visible light (VIS). The conversion for UV light implies that the Tb0.15Yb0.85PO4 could be used as UV absorber for solar cells to enhance the conversion efficiency. The luminescent color calculated from the spectra in Fig. 4 is plotted in the Commission International de l’Eclairage (CIE) 1931 chromaticity diagram, as shown in Fig. 5. The CIE coordinate was (0.298, 0.560), which showed that Tb0.15Yb0.85PO4 emit yellow-green light under the excitation of 369 nm. The color temperature (Tc) was 6268 K. Therefore, the Tb0.15Yb0.85PO4 nanoparticles could also have potential to be used for LED. To examine the effect of hydrothermal temperature on the luminescent intensity, we measured the emission spectra of Tb0.15Yb0.85PO4, as shown in Fig. 6. They were synthesized at different hydrothermal temperatures. The positions of emission peaks did not change with different hydrothermal

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Fig. 6 Emission spectra of Tb0.15Yb0.85PO4 prepared at different hydrothermal temperatures (1) 140 °C; (2) 180 °C; (3) 220 °C

temperature whereas the intensity decreased with the rising of hydrothermal temperature. The emission intensity reached the maximum under the hydrothermal condition of 140 °C. It is desirable to prepare nanoparticles of phosphors at relatively low temperatures so that both the particle aggregation and segregation of lanthanide ions can be prevented[16]. Our results suggest that segregation of Tb3+ ions in the YbPO4 host may be inhibited with the decreasing of hydrothermal temperature and the emission intensity of YbPO4:Tb3+ phosphors could be enhanced by preparing them at low temperature.

3 Conclusions



Fig. 4 Emission (1) and excitation (2) spectra of Tb0.15Yb0.85PO4 (Ȝex=369 nm, Ȝem=543 nm)

Mild hydrothermal method was adopted to synthesize YbPO4:Tb3+ nanoparticles with tetragonal phase. The average grain size could be controllable by adjusting the doping concentrations of Tb3+ in the phosphate host. The nanoparticles emitted strong yellow-green light under near UV light excitation and indicated the efficient conversion of UV light. The optimum synthesis temperature was 140 °C, which was much lower than the traditional solid-state reaction method (over 1100 °C). The small size of YbPO4:Tb3+ nanoparticles may be helpful to enhance the absorption of UV light. The YbPO4:Tb3+ nanoparticles could have potential as UV absorber for solar cells to increase the conversion efficiency or as phosphors for LED. Further work is in progress to investigate the effect of calcination on the luminescent properties of YbPO4:Tb3+.

References:

Fig. 5 Coordinates diagram of Tb0.15Yb0.85PO4 calcined at 1000 ºC plotted in the CIE1931

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