The effects of Eu3+ doping concentration on structure, morphology and optical properties of nanoma-terials were also investigated.. The results showed that, for all studied Eu3+ doping c
Trang 1JOURNAL OF RARE EARTHS, Vol 29, No 12, Dec 2011, P 1170
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: nthuong@ims.vast.ac.vn; Tel.: +84 4 66599000)
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 pre-pared by microwave synthesis method The effects of Eu3+ doping concentration on structure, morphology and optical properties of nanoma-terials were also investigated The results showed that, for all studied Eu3+ doping concentrations, a single-crystalline phase of rhabdo-phane-type (Eu,Tb)PO 4 ·H 2 O nanowires/nanorods was obtained by using microwave heating of an aqueous solution of terbium(III) nitrate, europium(III) nitrate and NH 4 H 2 PO 4 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
applica-tions such as nanobiophotonics, biological fluorescence
la-beling[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
lu-minescent property of these compounds with or without
dopants have been carried out[8–17] Many preparative
meth-ods 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
effi-ciency 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
tech-nique 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 pur-chased from Aldrich Co and used as received without fur-ther purification (Eu,Tb)PO4·H2O nanomaterials were pre-pared by microwave heating of an aqueous solution of ter-bium(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 dur-ing stirrdur-ing 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 inten-tionally selected in the range of 6 at.%–20 at.% The reacting solution was then microwave irradiated using a MAS-II mi-crowave 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
Trang 2Nguyen Thanh Huong et al., Structural and luminescent properties of (Eu,Tb)PO 4 ·H 2 O nanorods/nanowires prepared by… 1171
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
radia-tion with Ȝ=0.15406 nm) The morphology of the products
was characterized by using a field emission scanning
elec-tron microscope, Hitachi, S-4800 The excitation
spectros-copy measurements were carried out with a Varian Carry
eclipse fluorescence spectrometer The emission spectra of
studied samples were recorded on luminescence
spectro-photometer 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
concen-trations are shown in Fig 1 For EuPO4·H2O sample,
nano-rods 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
differ-ent Eu3+ concentrations Thus, by using microwave synthesis
method and an aqueous solution containing nitrates of
Fig 1 FESEM images of EuPO4·H2O (a) TbPO 4 ·H2O:20 at.% Eu3+
(b), TbPO 4 ·H 2 O:11 at.% Eu3+ (c) and TbPO 4 ·H 2 O:6 at.%
Eu3+ (d) nanorods/nanowires synthesized by microwave-assisted
method
Fig 2 XRD patterns of EuPO 4 ·H 2 O (1) TbPO 4 ·H 2 O:20 at.% Eu3+ (2), TbPO4·H2O:11 at.% Eu3+ (3) and TbPO4·H2O:6 at.%
Eu3+ (4) nanorods/nanowires synthesized by microwave- assisted method
trivalent rare-earth ions and NH4H2PO4 at a suitable pH value of 2 the hexagonal phase of europium/terbium ortho-phosphate 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 lumi-nescent emission of prepared samples in the visible and in-frared 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 emis-sion peaks found at 589, 615, 650, and 695 nm varied as a function of the Eu3+ doping concentration andreached 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 TbPO 4 ·H 2 O
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Fig 4 Photoluminescence spectra of EuPO4·H2O (1) TbPO 4 ·H2O: 20
at.% Eu3+ (2), TbPO 4 ·H 2 O:11 at.% Eu3+ (3) and TbPO 4 ·H 2 O:
6 at.% Eu 3+ (4) nanorods/nanowires synthesized by
micro-wave-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
fluo-rescence emission peak of 543 nm (5D4ĺ7
F5) for all samples containing Tb3+ ions was observed (Fig 5) while it
disap-pears 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
nano-rods/nanowires
Fig 5 Photoluminescence spectra of EuPO 4 ·H 2 O (1) TbPO 4 ·H 2 O:6
at.% Eu3+ (2) and TbPO4·H2O:11 at.% Eu3+ (3) samples
Fig 6 Photoluminescence spectra of EuPO 4 ·H 2 O, TbPO 4 ·H 2 O:11 at.% Eu3+ and TbPO 4 ·H 2 O samples synthesized by micro-wave-assisted method
3 Conclusions
Nanorods/nanowires of (Eu,Tb)PO4·H2O were success-fully 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 accep-tor Eu3+ was observed clearly via a significant enhancement
in the luminescent intensity of Eu3+ together withthe sup-pression in intensity of the characteristic fluorescence emis-sion peak of Tb3+ ion at 543 nm The emission intensity of
Eu3+ ions reached a maximum value with the Eu3+ concen-tration of 11 at.%
Acknowledgements: The authors are also thankful to the Key
Laboratory of Electronic Materials and Devices, Institute of Materi-als Science, Vietnam Academy of Science and Technology
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