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Microwave-assisted synthesis of hydrophilic BaYF5:Tb/Ce,Tb greenfluorescent colloid nanocrystals†

Yongqian Lei,a,b Min Pang,a,b Weiqiang Fan,a,b Jing Feng,a,b Shuyan Song,a,b Song Danga,b andHongjie Zhang*a

Received 20th July 2010, Accepted 22nd September 2010DOI: 10.1039/c0dt00873g

Hydrophilic Ce, Tb doped BaYF5 nanocrystals with uniform size were synthesized by a microwave-assisted route. The synthesized nanocrystals can be well dispersed in hydrophilic solutions (DMSO,DMF, EG, H2O). This synthesis procedure represents a less time consuming method, with high productyield and without using any assistant or/and template reagents, which may be expected to be a generalmethod for rapid synthesis of other hydrophilic RE doped fluoride fluorescent nanocrystals. The Ce3+,Tb3+ codoped BaYF5 nanocrystals show bright green fluorescence emission. The Ce3+ acts as aneffective energy transfer medium and the emission at the high 5D3 energy level of Tb is enhanced in thishost material.

Introduction

Rare-earth (RE) fluorescent nanocrystals (NCs) with sharp emis-sion lines, long lifetimes, and superior photostability are nowreceiving wide attention for potential applications in biologicallabeling, medical imaging and multicolor displays.1 The fluoridesof rare earth ion doped materials are regarded as ideal hostfor the lower phonon energies.2 In the past decade, a varietyof RE3+ doped fluoride nanocrystals were synthesized throughdifferent routes. They were firstly obtained through decompositionof metal trifluoroacetate precursors in the presence of oleic acidand octadecene.3 The hydrophobic fluoride nanocrystals was alsoobtained by hydro(solvo)thermal route.4 Recently, RE3+ dopedupconversion NaYF4 multicolor nanocrystals were synthesizedvia a hydrophilic polyethylenimine (PEI) assisted solvothermalprocess.5 However, the thermal decomposition methods requirehigh temperatures (about 300 ◦C) and long reaction times, becauseof the high stability of the trifluoroacetate precursor. Additionally,the hydrophobic nanocrystals need further surface modification,which restricts their potential applications. Simultaneously, thereare few reports on the direct synthesis of hydrophilic RE nanocrys-tals. To avoid the above disadvantages, we introduced a microwavesynthesis method for the synthesis of fluoride nanocrystals, whichprovides a unique opportunity for the large-scale synthesis ofcolloid nanocrystals without suffering thermal gradient effectsand rapid heating in a short time interval.6

aState Key Laboratory of Rare Earth Resource Utilizations, ChangchunInstitute of Applied Chemistry, Chinese Academy of Sciences, Changchun,130022, Jilin, China. E-mail: hongjie@ciac.jl.cnbGraduate School of the Chinese Academy of Sciences, Beijing, 100039, P.R. China† Electronic Supplementary Information (ESI) available: EDX spectrumand decay curve of different nanocrystals. See DOI: 10.1039/c0dt00873g/

Barium yttrium fluoride (BaYF5) is expected to be a promisinghost material in the energy transfer of RE but gets less attention.It was reported that Tm and Yb codoped BaYF5 upconversionnanocrystals showed strong emission in the near-infrared to blueregion.7 However, there are no reports on Ce3+ and Tb3+ codopedBaYF5 colloidal nanocrystals. Here we report a microwave-assisted route for the synthesis of Ce3+, Tb3+ doped BaYF5

hydrophilic nanocrystals. The different doping concentrationsindicate that Ce3+ acts as an effective energy transfer medium inthis host material.

Experimetal

Chemical and synthesis

All chemicals were of analytical grade and were used with-out further purification. Ethylene glycol (EG), Ba(CH3COO)2,Y(NO3)3, NH4F, Ce(NO3)3 and Tb(NO3)3 were purchased fromSinopharm Chemical Reagent Co., Ltd (Shanghai, China) andused as starting materials. The synthesis of Ce3+/Tb3+ dopedBaYF5 NCs were present in the ethylene glycol (EG) solution.In a typical experiment, 0.2 mmol stoichiometry reagents ofBa(CH3COO)2, Y(NO3)3, Tb(NO3)3, Ce(NO3)3 were dissolved in20 mL EG solution and 1 mmol NH4F was dissolved in 1 mLwater to get a transparent solution. This aqueous solution wasadded to the above EG solution. The obtained mixture was stirredfor 30 min and transferred to microwave reaction equipment. Themixture was reacted for 10 min at a defined temperature and thencooled to room temperature naturally (for detailed information seeESI†). The product was collected by centrifugation, and washedwith ethanol several times and dispersed in ethanol for furthercharacterization.

142 | Dalton Trans., 2011, 40, 142–145 This journal is © The Royal Society of Chemistry 2011

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Characterization

TEM and HRTEM measurements were carried out on an FEItransmission electron microscope at an operating voltage of200 kV. The crystallinity and phase purity of the product wereexamined by XRD measurement, which was performed on aBruker X-ray diffractometer with CuKa radiation. The fluores-cence spectra were measured on a Horiba Jobin Yvon Fluorolog-3fluorescence spectrophotometer, which was equipped with a xenonlamp (450 W) as the excitation source. The fluorescence images andphotographs were obtained by a Panasonic FS25 camera.

Results and discussion

In our synthesis procedure, the high boiling point solvent of EGplayed an important role in the synthesis of the NCs. The highviscosity reduced the diffusion rate of ions to get a uniformnucleation, and it is also beneficial to prevent the aggregationof nanocrystals. Transmission electron microscopy (TEM) showstypical morphology of Tb (5%) doped BaYF5 NCs. Fig. 1a showsthat the obtained NCs are of rectangular morphology. The NCspresent a narrow size distribution of 12 ± 2 nm (Fig. 1b). Allthe peaks in the XRD pattern in Fig. 1c can be well indexedcorresponding to the tetragonal BaYF5 phase (JCPDS file number46-0039). The sharp peaks suggest the high crystallinity of thesample. High-resolution TEM (Fig. 1d) shows lattice fringes withd-spacing of 0.33 nm, which is in good agreement with the strong(131) planes in the XRD pattern. Different concentrations ofcodopant or dopant (5%) did not affect the host crystal structure(Fig. 2).

Fig. 1 (a) TEM image of BaYF5 : Tb NCs. (b) Size distribution patternof the product. (c) XRD pattern of BaYF5 : Tb NCs. (d) High-resolutionTEM image of an individual NC.

The BaYF5:Ce3+,Tb3+ NCs were obtained with the same crystalphase. The energy-dispersive X-ray spectroscopy (EDX) of anindividual particle of BaYF5:Ce3+,Tb3+ reveals the presence of

Fig. 2 The XRD patterns of the different sample accompanied by thestandard card, a) BaYF5 NCs, b) BaYF5:Tb NCs, c)BaYF5:Ce,Tb NCs.

the doped elemental Ce and Tb (Fig S1†), and the ratio is closeto 1 : 1. The synthesized doped BaYF5 NCs also show goodsolubility. The NCs can be dispersed in various solvents includingwater, methanol, dimethylsulfoxide (DMSO), dimethylformamide(DMF), and ethylene glycol (Fig. 3) to get nearly transparentsolution.

Fig. 3 The sample dispersed in water, methanol, DMSO, DMF and EG.

Fig. 4a and 4b show the room-temperature emission spectra ofa colloidal solution of the BaYF5:Tb3+ and BaYF5:Ce3+,Tb3+ inethanol. It can be seen that both of the spectra consist of fourmain sharp peaks between 450 nm and 650 nm, corresponding tothe typical 5D4-7FJ transitions of Tb3+ (5D4-7F6 at 488 nm; 5D4-7F5 at 544 nm; 5D4-7F4 at 584 nm;5D3-7F3 at 620 nm). However,the photograph (inset) shows that there is a great differencebetween the two kinds of colloidal nanocrystals. The Ce3+ andTb3+ codoped BaYF5 sample shows bright green emission, whilethe Tb3+ doped sample shows weaker emission. The excitationspectrum of BaYF5:Tb3+ shows a typical absorption of Tb3+. Themulti-peak indicates the complex energy level absorption of Tb3+

ion. While the excitation spectrum of BaYF5:Ce3+,Tb3+ shows atypical absorption of Ce3+, the broad band with a maximum at274 nm, which corresponds to the 4f-5d absorption of Ce3+.8 Itcan be seen that the Ce3+ has played an important role in the hostof BaYF5. It acts as an effective energy absorption and transfermedium in BaYF5 nanocrystals, which results in a high effectiveemission of Tb3+ ion. This difference is also reflected in their

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Fig. 4 The excitation (dashed line) and emission (green line) spectra of(a) BaYF5:Tb NCs, (b) BaYF5:Ce,Tb NCs. The inset graphs show thefluoresce samples under 254 nm UV light irradiation.

lifetime. Fig S2a and S2b† show the luminescence decay curveof the BaYF5:Tb3+ and BaYF5:Ce3+,Tb3+ at 544 nm emission. Thecurves can be well fitted into a single exponential function as I =I0exp(-t/t) (t corresponds to Tb3+ lifetime) (red line). The fittedlifetimes are 2.13 ms and 5 ms for BaYF5:Tb3+ and BaYF5:Ce3+,Tb3+ respectively.

The energy transfer is also changed with varied Tb3+ and Ce3+

concentrations. Fig. 5a shows the normalized emission spectra ofthe BaYF5 NCs with 5% Ce and Tb 5%, 2%, 1%, 0.5%, respectively.The broad emission at 360 nm can be attributed to the 5d-4ftransition of Ce3+ ion.9 It can be seen that the emission intensity isstrongly dependent on the concentration of Tb3+. The ratio of themaximum emission peak of Tb at 544 nm to the peak at 360 nmis decreased remarkably from 4.5 (5% Tb) to 0.32 (0.5% Tb). Fig.5b shows the normalized emission spectra with 5% Tb and Ce5%, 2%, 1%, 0.5% respectively. Unlike the case of BaYF5:Tb3+

(5%) with different concentrations of Ce, an emission intensitychange of Tb3+ is not obvious on the change of the concentrationof Ce3+. It can be seen that the ratio of the maximum emissionpeak of Tb at 544 nm to the peak at 360 nm decreased from 4.76(Ce 5%) to 3.61 (Ce 0.5%). The Ce, Tb codoped BaYF5 showsbright emissions even at concentrations of Ce3+ as low as 0.5%.The low concentration indicated that the Ce3+ acts as an effective

Fig. 5 The normalized emission spectra of (a) the BaYF5:Ce (5%) withdifferent concentration of Tb3+, (b) the BaYF5:Tb (5%) with differentconcentration of Ce3+.

energy transfer medium for Tb3+ in this host material. Interestingly,the emission spectra show that the doped NCs show high energylevel emission at 5D3 energy level. The absorption and emissionband of Ce3+ is usually overlapped in most matrixes due to theallowed d-f transition and forbidden f -f transition. The energylevels of Tb3+ are suitable to accept energy from the excited stateof Ce3+, which results in the efficient energy transfer from Ce3+

to Tb3+ ion.10 According to Dexter’s theory,11 an efficient energytransfer requires a partial overlap between the excitation spectrumof the activator and the emission spectrum of the sensitizer. Thewell matched emission peak of Ce3+ at 360 nm (Fig. 5) and theexcitation peak of Tb3+ between 300 nm and 400 nm indicates theefficient energy transfer from Ce3+ to Tb3+.

The energy-transfer process of Ce3+ and Tb3+ is schematicallydemonstrated in Scheme 1 as previous reported.12 Initially, theCe3+ ions are excited by UV irradiation, then, the energy transferhappens from the 5D3/2 state of Ce3+ to the acceptor energy states ofTb3+, which decays nonradiatively to 5D4 and 5D3 states followedby radiative decay to various lower levels of 7FJ (J = 0–6). Inprevious Ce, Tb codoped systems, like NaYF4,13 phosphate6,14 andfluoride,15 the weak emission of Tb3+ at the 5D3 energy level isusually self-quenched, for the so-called cross-relaxation betweeneach Tb3+ at 5D3-5D4 and 7F0-7F6.16 The host material of BaYF5

may restrict the cross-relaxation process between 5D3-5D4 and7F0-7F6, which result in high energy emission even at high Tb3+

concentration.

Conclusions

In conclusion, we reported a microwave-assisted route for thesynthesis of fluorescent nanocrystals with high hydrophilic and

144 | Dalton Trans., 2011, 40, 142–145 This journal is © The Royal Society of Chemistry 2011

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Scheme 1 The energy-level diagram of Ce3+ and Tb3+ in BaYF5 NCsand energy-transfer process. Dashed arrows: excitation, dash-line arrows:nonradiative decay, solid arrows: radiative decay.

narrow size distribution. This synthesis procedure presents a lesstime consuming method with high product yield and without usingany assistant or/and template reagents, which may be extendedto the synthesis of other fluoride nanocrystals. The Ce3+, Tb3+

codoped BaYF5 nanocrystals also show bright green fluorescenceemission. The Ce3+ acts as an effective energy transfer mediumand the emission at the high 5D3 energy level of Tb3+ is enhanced.

Acknowledgements

This work was supported by NSFC (Grant No.: 20631040) andthe MOST of China (Grant No.: 2006CB601103).

Notes and references

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2 H. Schafer, P. Ptacek, O. Zerzouf and M. Haase, Adv. Funct. Mater.,2008, 18, 2913.

3 J-C. Boyer, F. Vetrone, L. A. Cuccia and J. A. Capobianco, J. Am.Chem. Soc., 2006, 128, 7444.

4 L. Y. Wang, P. Li and Y. D. Li, Adv. Mater., 2007, 19, 3304.5 F. Wang and X. G. Liu, J. Am. Chem. Soc., 2008, 130, 5642.6 (a) G. Buhler and C. Feldmann, Angew. Chem., Int. Ed., 2006, 45, 4864;

(b) A. B. Panda, G. Glaspell and M. Samy El-Shall, J. Phys. Chem. C,2007, 111, 1861.

7 F. Vetrone, V. Mahalingam and J. A. Capobianco, Chem. Mater., 2009,21, 1847.

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11 (a) D. L. Dexter, J. Chem. Phys., 1962, 37, 511; (b) X. Liu, C. Lin andJ. Lin, Appl. Phys. Lett., 2007, 90, 081904.

12 Q. Li and V. W-W. Yam, Angew. Chem., Int. Ed., 2007, 46,3486.

13 X. Yu, Y. H. Wang and J. D. Liu, Electrochem. Solid-State Lett., 2010,13, J18.

14 (a) K. Kompe, H. Borchert, J. Storz, A. Lobo, S. Adam, T. Moller andM. Haase, Angew. Chem., Int. Ed., 2003, 42, 5513; (b) L. Ma, L. M.Xu, W. X. Chen and Z. D. Xu, Mater. Lett., 2009, 63, 1635.

15 (a) X. Zhu, Q. Zhang, Y. Li and H. Wang, J. Mater. Chem., 2008, 18,5060; (b) C. Li, X. Liu, P. Yang, C. Zhang, H. Lian and J. Lin, J. Phys.Chem. C, 2008, 112, 2904.

16 (a) L. G. Van Uitert and L. F. Johnson, J. Chem. Phys., 1966, 44, 3514;(b) F. Auzel, Chem. Rev., 2004, 104, 139.

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