JOURNAL OF RARE EARTHS, Vol. 29, No. 12, Dec. 2011, P. 1170
Structural and luminescent properties of (Eu,Tb)PO4·H2O nanorods/ nanowires prepared by microwave technique Nguyen Thanh Huong1, Nguyen Duc Van1, Dinh Manh Tien1, Do Khanh Tung1, Nguyen Thanh Binh1, Tran Kim Anh1, Le Quoc Minh1, 2 (1. Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Road, Cau Giay Distr., Hanoi, Vietnam; 2. University of Engineering and Technology, National University Hanoi, Vietnam, 144 Xuan Thuy Road, Cau Giay Distr., Hanoi, Vietnam) Received 15 August 2011; revised 5 September 2011
Abstract: Nanowires/nanorods of europium/terbium orthophosphate monohydrate with Eu3+ concentration of 6, 11, and 20 at.% were prepared by microwave synthesis method. The effects of Eu3+ doping concentration on structure, morphology and optical properties of nanomaterials were also investigated. The results showed that, for all studied Eu3+ doping concentrations, a single-crystalline phase of rhabdophane-type (Eu,Tb)PO4·H2O nanowires/nanorods was obtained by using microwave heating of an aqueous solution of terbium(III) nitrate, europium(III) nitrate and NH4H2PO4 with pH=2. The length and width of these nanowires/nanorods ranged from 150 to 300 nm and from 10 to 50 nm, respectively. The evidence of energy transfer from Tb3+ to Eu3+ due to the energy overlap between the donor Tb3+ and the acceptor Eu3+ was observed obviously via a significant enhancement in the luminescent intensity of Eu3+. Keywords: microwave technique nanowires/nanorods energy transfer fluorescence spectroscopy; rare earths
Recently, nanosized inorganic luminescent materials have been studied intensively due to their high potential applications such as nanobiophotonics, biological fluorescence labeling[1– 4]. Among them, rare-earth orthophosphates (LnPO4 with Ln: Y, Sc, and La-Lu) nanomaterials exhibit a number of fascinating properties such as very high thermal stability, low water solubility[5] and, especially, their luminescent properties[6,7]. Numerous researches on preparation and luminescent property of these compounds with or without dopants have been carried out[8–17]. Many preparative methods have been used to synthesize rare-earth orthophosphates such as conventional solid-state reaction[18], sonochemical synthesis[11,19], or wet chemistry routes[20,21]. For the case of terbium orthophosphate, the doping Eu3+ of ions into the host lattice with the concentration ranging from 0.1 at.% up to 5 at. was reported to enhance the energy transfer efficiency of both anhydrous TbPO4 and TbPO4·H2O[11,12]. However, effects of higher concentrations of doped Eu3+ ions on structure, morphology and optical properties of TbPO4·H2O nanowires/nanorods have not been not studied to date. In our work, (Eu,Tb)PO4·H2O nanorods/nanowires were prepared by microwave heating and characterized by field-emission scanning electron microscopy (FESEM) and X-ray diffraction. The microwave-assisted synthesis technique was employed for the reasons of its high possibility of providing low dimensional nanomaterials in a simple, fast, clean, efficient, economical, non-toxic, and eco-friendly way.
PL spectra of (Eu,Tb)PO4·H2O nanorods/nanowires were measured in the UV region under 370 nm excitation. The effects of the Eu3+ doping concentration on structure and photoluminescent properties of prepared samples were also discussed.
1 Experimental 1.1 Synthesis Terbium(III) nitrate pentahydrate, europium(III) nitrate pentahydrate and NH4H2PO4 with purity of 99% were purchased from Aldrich Co. and used as received without further purification. (Eu,Tb)PO4·H2O nanomaterials were prepared by microwave heating of an aqueous solution of terbium(III) nitrate, europium(III) nitrate and NH4H2PO4 at ambient pressure in an open system. In a typical synthesis procedure, 20 ml of 0.25 mol/L NH4H2PO4 solution were added to a 50 ml round-bottomed flask containing 20 ml of a 0.25 mol/L aqueous solution of Tb(NO3)3 and Eu(NO3)3 during stirring. A colloidal precipitate was obtained upon the addition of NH4H2PO4 to Tb(NO3)3 and Eu(NO3)3 solution at the pH value of 2. The Eu3+ doping concentration was intentionally selected in the range of 6 at.%–20 at.%. The reacting solution was then microwave irradiated using a MAS-II microwave synthesis extraction workstation, Sino Co., China, for 30 min with an irradiated power of 500 W. The resulted products were collected, centrifugated, and washed several
Foundation item: Project supported by Vietnamese National Foundation for Science and Technology Development (NAFOSTED) (103.06-2010.16) Corresponding author: Nguyen Thanh Huong (E-mail:
[email protected]; Tel.: +84 4 66599000) DOI: 10.1016/S1002-0721(10)60619-9
Nguyen Thanh Huong et al., Structural and luminescent properties of (Eu,Tb)PO4·H2O nanorods/nanowires prepared by…
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times with ethanol and distilled water. The final products were dried at 60 ºC for 24 h in air. 1.2 Characterization The crystalline phase identification of the as-synthesized samples are carried out by X-ray diffraction (XRD) analysis with a Siemens D5000 diffractometer (using Cu KĮ radiation with Ȝ=0.15406 nm). The morphology of the products was characterized by using a field emission scanning electron microscope, Hitachi, S-4800. The excitation spectroscopy measurements were carried out with a Varian Carry eclipse fluorescence spectrometer. The emission spectra of studied samples were recorded on luminescence spectrophotometer system, Horiba Jobin Yvon IHR 550.
2 Results and discussion SEM images of (Eu,Tb)PO4·H2O samples synthesized by using microwave heating with different Eu3+ doping concentrations are shown in Fig. 1. For EuPO4·H2O sample, nanorods are found with lengths from 150–200 nm and widths of about 50 nm. The decrease in Eu3+ concentration led to the increase in length of nanorods/nanowires from 150 to 300 nm and, at the same time, the decrease in their width from 50 to 10 nm. XRD patterns of the as-synthesized (Eu,Tb)PO4·H2O nanorods/nanowires indicate that only single crystalline phase of (Eu,Tb)PO4·H2O existed in obtained samples (Fig. 2). All reflections can be distinctly indexed to a rhabdophane-type pure hexagonal phase. This implies that the crystal structures of all Eu3+-doped terbium orthophosphate monohydrates are isostructural to that of pure TbPO4·H2O (space group: P3121, PDF card No. 20–1244). These results are the same as those reported previously[3]. As shown in Fig. 2, no impurity phases were observed for all measured samples with different Eu3+ concentrations. Thus, by using microwave synthesis method and an aqueous solution containing nitrates of
Fig. 1 FESEM images of EuPO4·H2O (a) TbPO4·H2O:20 at.% Eu3+ (b), TbPO4·H2O:11 at.% Eu3+ (c) and TbPO4·H2O:6 at.% Eu3+ (d) nanorods/nanowires synthesized by microwave-assisted method
Fig. 2 XRD patterns of EuPO4·H2O (1) TbPO4·H2O:20 at.% Eu3+ (2), TbPO4·H2O:11 at.% Eu3+ (3) and TbPO4·H2O:6 at.% Eu3+ (4) nanorods/nanowires synthesized by microwaveassisted method
trivalent rare-earth ions and NH4H2PO4 at a suitable pH value of 2 the hexagonal phase of europium/terbium orthophosphate monohydrate, (Eu,Tb)PO4·H2O, was obtained as a unique product with Eu3+ concentration ranging from 6 up to 20 at.%. The photoluminescence excitation (PLE) spectrum of the as-synthesized TbPO4·H2O sample is shown as an example in Fig. 3. For this sample, excitation bands of 310, 350, 370 and 480 nm were observed in PLE spectra monitored at 542 nm. This result coincided to that of the previous work reported by Yang M. and co-workers[2]. In order to investigate the luminescent emission of prepared samples in the visible and infrared regions that were required for biomedical fluorescence labeling as well as to study the energy transfer from Tb3+ to Eu3+ ions, the excitation wavelength of 370 nm was selected for emission (PL) spectroscopy measurements. The PL spectra of (Eu,Tb)PO4·H2O nanorods/nanowires of all investigated Eu3+ doping concentrations were recorded under 370 nm excitation (Fig. 4). The intensity of four emission peaks found at 589, 615, 650, and 695 nm varied as a function of the Eu3+ doping concentration and reached a maximum value with the Eu3+ concentration of 11 at.%. This originated from the energy transfer from Tb3+ to Eu3+ ions
Fig. 3 Photoluminescence excitation spectrum monitored at 542 nm of TbPO4 ·H2O
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Fig. 4 Photoluminescence spectra of EuPO4·H2O (1) TbPO4·H2O: 20 at.% Eu3+ (2), TbPO4·H2O:11 at.% Eu3+ (3) and TbPO4·H2O: 6 at.% Eu3+ (4) nanorods/nanowires synthesized by microwave-assisted method
due to the energy overlap between the donor Tb3+ and the acceptor Eu3+. The emission spectra with characteristic red emission of Eu3+ ions corresponding to the transitions from 5D0 to the ground states 7Fj (j=1, 2, 3, 4) are observed, respectively. It is quite interesting that the strongest energy transfer from Tb3+ to Eu3+ ions was found with the Eu3+ concentration of 11 at.%. This value is significantly higher than that reported in previous works (about 5 at.%)[22,23]. The characteristic fluorescence emission peak of 543 nm (5D4ĺ7F5) for all samples containing Tb3+ ions was observed (Fig. 5) while it disappears for EuPO4·H2O sample. The intensity of this peak, however, was unchanged with the concentration of Eu3+ ranging from 6 to 11 at.% and was almost suppressed when the Eu3+ concentration reached to 20 at.% (Figs. 4 and 5) due to the inhibition of spontaneous emission[23]. Fig. 6 presents the emission spectra of EuPO4·H2O, TbPO4·H2O:11 at.% Eu3+, and TbPO4·H2O samples. The enhancement in intensity of four emission peaks of TbPO4·H2O:11 at.% Eu3+ sample at 589, 615, 650 and 695 nm with respect to that of the pure EuPO4·H2O sample confirms once again the energy transfer in (Eu,Tb)PO4·H2O nanorods/nanowires.
Fig. 5 Photoluminescence spectra of EuPO4·H2O (1) TbPO4·H2O:6 at.% Eu3+ (2) and TbPO4·H2O:11 at.% Eu3+ (3) samples
JOURNAL OF RARE EARTHS, Vol. 29, No. 12, Dec. 2011
Fig. 6 Photoluminescence spectra of EuPO4·H2O, TbPO4·H2O:11 at.% Eu3+ and TbPO4·H2O samples synthesized by microwave-assisted method
3 Conclusions Nanorods/nanowires of (Eu,Tb)PO4·H2O were successfully fabricated using microwave techniques. The length and width of these nanowires/nanorods were 150–300 nm and 10–50 nm, respectively. Structure of these (Eu,Tb)PO4·H2O materials was corresponding to rhabdophane-type hexagonal phase. (Eu,Tb)PO4·H2O nanowires/nanorods exhibited the characteristic narrow emission peaks of trivalent europium ions. The evidence of energy transfer from Tb3+ to Eu3+ due to the energy overlap between the donor Tb3+ and the acceptor Eu3+ was observed clearly via a significant enhancement in the luminescent intensity of Eu3+ together with the suppression in intensity of the characteristic fluorescence emission peak of Tb3+ ion at 543 nm. The emission intensity of Eu3+ ions reached a maximum value with the Eu3+ concentration of 11 at.%. Acknowledgements: The authors are also thankful to the Key Laboratory of Electronic Materials and Devices, Institute of Materials Science, Vietnam Academy of Science and Technology.
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