Polyol-mediated syntheses and characterizations of NaYF4, NH4Y3F10 and YF3 nanocrystals/sub-microcrystals

Article abstract of DOI:10.1016/j.materresbull.2007.09.019

In this paper, NaYF₄ nanocrystals and NH₄Y₃F₁₀ sub-microcrystals were prepared using a polyol method. After being annealed, NH₄Y₃F₁₀ can convert into YF₃, which is a very efficient host matrix for upconversion. The structures of obtained NaYF₄, NH₄Y₃F₁₀ and YF₃ samples are pure cubic phase, a mixture of cubic and hexagonal phase and pure orthorhombic phase, respectively. The size of obtained NaYF₄ nanocrystals is about 20 nm, and those of NH₄Y₃F₁₀ and YF₃ sub-microcrystals are both about 200 nm. When co-doped with Er³⁺ and Yb³⁺, NaYF₄ and YF₃ samples can emit bright light under 978 nm excitation. Upconversion mechanisms in Yb³⁺/Er³⁺ co-doped NaYF₄ and YF₃ samples were discussed.

Full text of DOI:10.1016/j.materresbull.2007.09.019

Materials Research Bulletin 43 (2008) 2130–2136
www.elsevier.com/locate/matresbu
Polyol-mediated syntheses and characterizations of NaYF4,
NH Y F and YF nanocrystals/sub-microcrystals
4
3 10
a,b
3
a,
a,b
a,b
*
Ruifei Qin , Hongwei Song , Guohui Pan , Lanying Hu ,
Hongquan Yu , Suwen Li , Xue Bai , Libo Fan , Qilin Dai ,
a,b
a,b
a,b
a,b
a,b
a
a
a
Xinguang Ren , Haifeng Zhao , Tie Wang
a
Key Laboratory of Excited State Physics, Changchun Institute of Optics, Fine Mechanics and Physics,
Chinese Academy of Sciences, 16 Eastern Nan-Hu Road, Changchun 130033, PR China
b
Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China
Received 20 March 2007; received in revised form 18 July 2007; accepted 15 September 2007
Available online 29 September 2007
Abstract
In this paper, NaYF
4
nanocrystals and NH
4 3
Y F10 sub-microcrystals were prepared using a polyol method. After being annealed,
NH 10 can convert into YF
4
Y
3
F
3
, which is a very efficient host matrix for upconversion. The structures of obtained NaYF ,
4
NH Y F and YF samples are pure cubic phase, a mixture of cubic and hexagonal phase and pure orthorhombic phase,
4
3
10
3
respectively. The size of obtained NaYF
4
nanocrystals is about 20 nm, and those of NH
4
Y
3
F
10 and YF
samples can emit bright light under 978 nm excitation.
Upconversion mechanisms in Yb /Er co-doped NaYF and YF samples were discussed.
3
sub-microcrystals are both
3
+
3+
4 3
about 200 nm. When co-doped with Er and Yb , NaYF and YF
3
+
3+
4
3
#
2007 Elsevier Ltd. All rights reserved.
Keywords: A. Fluorides; B. Nanostructures; C. Luminescence
1
. Introduction
As a very efficient host matrix for green and blue upconversion, sodium yttrium tetrafluoride (NaYF ) in the
4
nanometer size range has attracted considerable interest recently due to its potential use as biolabels [1–11].
Upconversion is the generation of higher energy light from lower energy radiation. At ambient temperature and pressure,
the NaYF exists in two polymorphs: the hexagonal (Na Y F type) structure and the cubic (fluorite type) one [12]. In
4
1.5 1.5 6
the past few years, many papers reported thesynthesis ofnanoscale NaYF . NaYF :Yb, Er nanocrystals were synthesized
4
4
using a co-precipitation method and demonstrated to be promising upconversion biolabels [3]. Cubic NaYF4
nanoparticles were prepared hydrothermally in the presence of surfactants or chelators [12,13]. Li’s group synthesized
hexagonal NaYF :Yb, Er nanoparticles using a solvothermal route, and combining magnetic separation and
4
concentration technology and fluorescence resonant energy transfer technology they applied these nanoparticles to the
sensitive detection of trace amounts of targeted biomolecules [4–6]. Very recently, high-quality cubic and hexagonal
NaYF nanocrystals were synthesized via the co-thermolysis of Na (CF COO) and Y (CF COO) precursors [7–11].
4
3
3
3
*
Corresponding author. Fax: +86 431 86176320.
E-mail address: hwsong2005@yahoo.com.cn (H. Song).
0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2007.09.019

R. Qin et al. / Materials Research Bulletin 43 (2008) 2130–2136
2131
+
MLn3F10 (M = alkaline metal, NH4 and Ln = rare earth) is a big family of compounds including many useful
materials, such as KY F , which has great potential as a host of the solid-state laser. Regarding the ionic radii and
3
10
crystal structures of the MLn3F10 family, ammonium compounds are particularly interesting, because the ionic radius
+
+
(
(
Pauling) of ammonium (1.48 Å) is comparable with those of K (1.33 Å) and Rb (1.48 Å) [14]. NH Ln F
4 3 10
Ln = Dy, Ho, Y, Er, Tm) has been synthesized using a hydrothermal method [14]. Cubic NH Ho F , NH Er F and
4
3
10
4
3 10
NH Tm F could be obtained by precipitation from a solution containing ammonium ions [15].
4
3 10
The polyol method is well-suited for the preparation of nanoparticles, in which a high-boiling alcohol (e.g.
diethylene glycol, DEG, bp 246 8C) serves as both a solvent and a stabilizer that limits particle growth and prohibits
agglomeration [16,17]. Many kinds of materials, such as metal [18], oxides [19–21], phosphates [22,23], and binary
fluorides [24–26] in the nanometer size range have been synthesized through this method. In this paper, we synthesized
NaYF nanocrystals and NH Y F sub-microcrystals using this method. After being annealed, NH Y F can convert
4
4
3
10
4 3 10
into YF , which is a very efficient host matrix for upconversion.
3
2
. Experimental
2
.1. Syntheses
All chemicals were of analytical grade and used as received without further purification. Deionized water was
used throughout. In a typical synthesis of NaYF nanocrystals, 0.672 g (0.016 mol) NaF was added to the mixture
4
of 6 mL H O and 80 mL DEG. Afterwards a solution of 1.532 g (0.004 mol) Y(NO ) ꢀ6H O in 20 mL DEG
2
3 3
2
was added under vigorous stirring. After stirring for 12 h, it became transparent. The suspension was then
transferred to a 250 mL round-bottomed flask and heated at 180 8C in a silicon oil bath under vigorous stirring and
reflux for 2 h. After cooling to room temperature, the product was washed with absolute ethanol and water four
times to remove DEG and reactants possibly remaining and was dried at 80 8C for 12 h in vacuum. It was referred to
sample A.
The synthetic procedure of NH Y F sub-microcrystals was similar to that of NaYF nanocrystals, just using
4
3
10
4
NH F instead of NaF. The molar ratio of reactants was NH F:Y(NO ) ꢀ6H O = 10:3. The product was referred to
4
4
3 3
2
sample B. After being annealed at 500 8C for 2 h, the yielded product was referred to sample C.
2
.2. Methods
Crystal structure, morphology and size were obtained by powder X-ray diffraction (XRD, Rigaku D/max-rA
diffractometer with CuKa radiation resource, l = 1.54078 Å), field emission scanning electron microscopy (FE-SEM,
Hitachi S-4800). The FTIR spectra were recorded on a Bio-Rad FTS-3000 (EXCALIBUR SERIES) spectrometer.
Upconversion fluorescent spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer using a
continuous 978 nm diode laser as the excitation source.
3
. Results and discussion
3
.1. Crystal structures and morphologies
The XRD patterns of the as-obtained products are depicted in Fig. 1. The diffraction peaks in curve 1a for sample A
can be indexed to the pure cubic NaYF , which is in good agreement with that in literature (JCPDS card No. 77-2042).
4
Estimated according to the widths of the diffraction peaks by Scherrer equation, the average crystallite size of sample
A is 23 nm. We also synthesized NaGdF nanocrystals using the polyol method. Although the same synthetic
4
procedure is used, the as-obtained NaYF and NaGdF nanocrystals are cubic and hexagonal in structure, respectively.
4
4
3
+
This difference may be ascribed to kinetic factors resulting from the slightly larger size of Gd (1.107 Å) compared to
3
+
Y
(1.075 Å) [27]. The diffraction peaks in curve 1b for sample B indicate that the product is a mixture of the cubic
and the hexagonal (marked with asterisks) NH Y F , which is consistent with that in literature [14]. Generally,
4
3 10
NH Y F cannot be obtained by conventional solid-state reaction due to decomposition at high temperature. Our
4
3 10
above results show that NH Y F can be synthesized via the simple polyol method. After being annealed at 500 8C for
4
3 10
2
h, NH Y F can convert into pure orthorhombic YF (sample C), as is confirmed by the XRD pattern (JCPDS card
4 3 10 3

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R. Qin et al. / Materials Research Bulletin 43 (2008) 2130–2136
Fig. 1. XRD patterns of samples A (a), B (b) and C (c).
No. 74-0911). According to the widths of the diffraction peaks, samples B and C nearly have the same crystallite size
with sample A based on the estimation by Scherrer equation.
Fig. 2 shows typical FE-SEM images of all samples. NaYF nanocrystals (see Fig. 2a) aggregate to some degree and
4
is ꢁ20 nm in size, which is in good agreement with the result obtained from the XRD. As seen in Fig. 2b, NH Y F
4
3 10
sub-microcrystals are characterized by rough spheres and are about 200 nm in size. The size is much larger than that of
NaYF nanocrystals. According to the result obtained from XRD analysis, it is suggested that these NH Y F sub-
4
4 3 10
microcrystals should be composed of smaller crystallites. A higher magnification image of sample B (Fig. 2c) appears
that NH Y F sub-microcrystals are the assembly of nanocrystals. YF sub-microcrystals obtained by annealing
4
3
10
3
NH Y F sub-microcrystals have the similar shape and nearly same size with NH Y F sub-microcrystals (see
4
3
10
4 3 10
Fig. 2d).
Fig. 2. FE-SEM images of samples A (a), B (b), B (c) and C (d).

R. Qin et al. / Materials Research Bulletin 43 (2008) 2130–2136
2133
Fig. 3. FTIR spectra of samples A (a), B (b) and C (c).
3
.2. FTIR spectra
Fig. 3 shows the FTIR spectra of all samples. In Fig. 3a, the bands around 3400 and 1650 cm , respectively,
ꢂ1
ꢂ
ꢂ1
originate from the stretching and bending vibrations of the OH groups of adsorbed H O. The peak at 1050 cm is
2
attributed to the C-OH stretching vibration mode, indicating the adsorption of a small amount of DEG on the surface of
ꢂ
NaYF nanocrystals. In Fig. 3b, besides absorption bands arising from the OH and C-OH vibrations, there exist
4
ꢂ1
absorption bands corresponding to the vibrations of the ammonium group. The bands at 1430 and 3306 cm can be
assigned to the bending and stretching vibrations of the ammonium group, respectively. In Fig. 3c, the bands arising
ꢂ
from the ammonium group disappear due to the conversion of NH Y F to YF and those arising from the OH and
4
3
10
3
C-OH vibrations still exist. Therefore after these samples are annealed at 500 8C for 2 h, some H O and DEG are still
2
adsorbed on the surface of them.
3
.3. Upconversion luminescence properties
To examine the feasibility of the as-obtained nanocrystals/sub-microcrystals as efficient upconversion host
3
+
3+
materials, the most frequently used upconversion ion Er and the sensitizer ion Yb were co-doped into samples A–
C. Doped samples were prepared using the same synthetic procedure, except for adding 6% Yb(NO ) ꢀ6H O and 1%
3
3
2
Er(NO ) ꢀ6H O into Y(NO ) ꢀ6H O. The upconversion emission of doped NH Y F sample is invisible to the naked
3
3
2
3 3
2
4 3 10
eye due to the quenching effect of ammonium with high-energy vibrations. Doped NaYF and YF samples both emit
4
3
bright light under 978 nm excitation. Figs. 4 and 5 show upconversion emission spectra of doped NaYF and YF
4
3
samples in the range of 500–700 nm, respectively, under 978 nm excitation. Green luminescence corresponding to the
2
4
4
4
4
H11/2/ S ꢂ I
transitions locates in the wavelength region of 500–575 nm, and the F – I
transition-derived
15/2
3/2
15/2
9/2
3
+
red luminescence ranges from 625 to 700 nm. Figs. 4 and 5, respectively, agree with what have been reported for Er
upconversion in cubic NaYF nanocrystals and orthorhombic YF sub-microcrystals prepared using other methods
4
3
[
1,8,9,28].
In the upconversion processes, the visible output intensity is in proportion to the nth power of the NIR excitation
n
intensity, i.e., Iup / (I ) , where I and Iexc represent the upconversion luminescence intensity and the NIR
exc
up
excitation intensity, respectively, and n is the number of photons required to produce an upconversion photon [29]. To
2
4
4
understand how the H / S and F excited states are populated under 978 nm excitation, power dependence of
1
1/2
3/2
9/2
upconversion luminescence intensity is plotted on a double logarithmic scale, as shown in the insets of Figs. 4 and 5.
For doped NaYF sample, the slopes are 1.7 and 2.0 for the red and green emissions, respectively, and for doped YF
4
3
sample, the slopes are 2.0 and 1.9 for the red and green emissions, respectively. These results indicate that two IR
photons are required to produce a visible photon in doped NaYF and YF samples. Fig. 6 illustrates the energy level
4
3
2
4
4
diagram and the possible mechanisms by which the H / S and F excited states are populated. Following
1
1/2
3/2
2
9/2
3
+
2
3+
9
78 nm irradiation, the Yb ion is excited from the ground state F to the excited state F5/2. Then the excited Yb
7/2

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R. Qin et al. / Materials Research Bulletin 43 (2008) 2130–2136
3
+
3+
Fig. 4. Upconversion emission spectrum of sample A co-doped with 1% Er and 6% Yb under 978 nm excitation. The inset shows the
upconversion luminescence intensity as a function of the laser power.
3
+
3+
Fig. 5. Upconversion emission spectrum of sample C co-doped with 1% Er and 6% Yb under 978 nm excitation. The inset shows the
upconversion luminescence intensity as a function of the laser power.
3
+
4
4
returns to the ground state and transfers its energy to Er in the ground state I
and promotes it up to the I
1
5/2
11/2
3+
3
+
intermediate excited state. Following this process, immediate energy transfer from another excited Yb ion to Er
ion results in population of the F state. Subsequent nonradiative relaxations of F – S / H
as well as the F states. Er in the I state can also relax to the I
11/2
emission and can subsequently be excited to the F state through energy transfer, which is another possible path to
4
4
4
2
4
4
and S – F
3/2 9/2
7
/2
7/2
3/2
11/2
4
2
4
3+
4
4
populate the S / H
state by phonon
3
/2
11/2
9/2
13/2
4
9
/2
4
populate the F state and is more possible in nanocrystals [30–32].
9
/2
The GRR value is defined as the intensity ratio between the green to red emission. From Figs. 4 and 5, it is obvious that
the GRR value of NaYF nanocrystals is smaller than that of YF sub-microcrystals. It is known that the GRR value
4
3
depends on preparation method, particle size, host matrix, crystallographic phase, doping level, excitation density, and so
3
+
3+
on [33–35]. For example, following excitation with 978 nm, the GRR value of nanocrystalline Y O :Er , Yb is much
2
3
3
+
3+
smaller than that of bulk Y O :Er , Yb [31,32]. The smaller particles often involve more surface defects, such as
surface adsorption of H O and CO , which can effectively bridge the nonradiative transition channels of I – I as
13/2
well as S – F9/2, leading to the decrease of GRR value. Generally, particles with lower upconversion efficiency show a
2
3
4
4
2
2
11/2
4
4
3
/2
lower GRR value [33]. For bulk materials, upconversion emission of cubic NaYF is about half of that of YF [34,36].
4
3
Compared with YF sub-microcrystals, the smaller GRR value of NaYF nanocrystals can mainly be attributed to the
3
4
much smaller size and the lower upconversion efficiency of cubic NaYF host matrix.
4

R. Qin et al. / Materials Research Bulletin 43 (2008) 2130–2136
2135
3+
3+
Fig. 6. The energy level diagrams for Yb /Er systems and upconversion luminescence processes under 978 nm excitation. The full, dotted, and
curly arrows represent emission, energy transfer and multiphonon relaxation processes, respectively.
4
. Conclusions
We synthesized NaYF , NH Y F and YF3 nanocrystals/sub-microcrystals via the polyol method and
4
4 3 10
3
+
3+
characterized them by XRD patterns, FE-SEM images, FTIR spectra. Yb /Er co-doped NaYF nanocrystals and
4
YF sub-microcrystals emit bright red and green light under 978 nm excitation and have potential to be used in the field
3
of bioassays. This method should be desirable to other ternary lanthanide fluoride compounds in the nanometer/sub-
micrometer size range.
Acknowledgements
This work is supported by the National Nature Science Foundation of China (Grants 10374086 and 10504030) and
Talent Youth Foundation of JiLin Province (Grants 20040105).
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