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Research of Materials Science September 2014, Volume 3, Issue 3, PP.57-63
Preparation and Characterization of a Novel
Bulk CeO2/Quercetin Fluorescent
Nanocomposites Xiaoxue Lian, Xiulin Liu, Yan Li
#, Dongmin An, Yunling Zou, Nan Zhang
College of Science, Civil Aviation University of China, Tianjin 300300, PR China
#Email: [email protected]
Abstract
Using CeO2 nanoparticles as the starting material, porous nano CeO2 solid has been successfully prepared with a hydrothermal
hot-pressing method. Furthermore, quercetin was assembled into the pores of the CeO2 solids. The results showed that the PL
intensity of the bulk CeO2/quercetin porous nanocomposites was higher than that of both porous nano CeO2 solids and quercetin
powders, and the peak wavelength was also different from that of each of the raw materials, which represented that the chemical
reaction occurred between CeO2 solids and quercetin molecules. In addition, the effect of the quercetin concentrations on the
photoluminescence of the nanocomposites was also analyzed.
Keywords: Bulk Porous CeO2; Solid; Nanocomposite; Photoluminescent Enhancement; Blue-shift.
1 INTRODUCTION
CeO2 has received extensive attention in recent years because of its excellent performances, especially in the
application of luminescent materials [1-4], catalyst [5], polishing agent [6], gas sensor [7], solid oxide fuel cells [8],
etc. It is an important prerequisite for the preparation of function ceramic [9] for its excellent properties, such as high
strength, high hardness, corrosion resistance and wear. At the same time, porous CeO2 has been intensively
investigated [10-13]. Among of these studies, fabrication of porous CeO2 has excited much interest since the
products exhibiting some superior performances, such as remarkable adsorption and large specific surface area.
Quercetin is a natural flavonoid compounds which is found in plants, flowers, leaves and fruit with the form of
glycosides [14]. The three ring structures of quercetin may occur complex reaction with metal ions which are due to
its higher of superdelocalizability, complete conjugation π electrons system, strong ligand oxygen atoms and suitable
of space configuration (as FIG. 2). Acid-base adducts of chelating reaction is usually easier, in which, metal ion as
Lewis acid and quercetin for Lewis alkali. Quercetin with chromophores >C=C< and >C=O which are containing
π-π* electronic transition of unsaturated bond, and auxochrome -OH, are the structural basis of fluorescence [15].
Quercetin and metal ions, such as Al (III), Fe (III), Cu (II) and Tin (II) [16-19], formed complexes whose
fluorescence and antioxidant activity have been reported.
To our knowledge, there have been no reports on the synthesis of functional materials by assembling quercetin into
the pores of porous CeO2. In this paper, we report the results of the preparation and characterization of the bulk CeO2
porous solids and CeO2/quercetin fluorescent nanocomposites.
2 EXPERIMENTAL
2.1 Preparation of the BulkCeO2 Porous Solids.
The bulk CeO2 porous solids was prepared with a hydrothermal hot-pressing method: 3 g CeO2 nanoparticles (with
an average particle size of 20 nm, purchased from Aladdin, shown in FIG.1) were added into a mortar at first, then
0.8 ml distilled water was added to the mortar. After being ground uniformly, the samples were quickly encased into
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a hydrothermal hot-pressing autoclave. Finally, the temperature of the autoclave was increased to 175 ℃ and kept
constant for 3 hours. At the same time, the autoclave was applied with a constant pressure of 90 MPa. After the
autoclave was cooled to the room temperature, the bulk CeO2 porous solids was formed.
FIG. 1 SEM PHOTOGRAPH OF CeO2 NANOPARTICLES
2.2 Preparation of CeO2/Quercetin Nanocomposites.
The starting materials in our experiments are CeO2 porous solids, quercetin (C15H10O7, its molecule structure is
shown in FIG.2) and anhydrous alcohol (A.R.).
HO
HO
O
O
OH
8
7
6
5
1
2 1'
2'
3
4
6'5'
4'
A C
B
OH
OH
FIG. 2 SCHEMATIC ATRUCTURE OF QUERCETIN MOLECULE
Six aliquots of CeO2 porous solids were firstly added into six conical flasks, and then mingled with 50 ml quercetin
alcohol solutions of 10−8
mol/L, 10−7
mol/L, 10−6
mol/L, 10−5
mol/L, 10−4
mol/L and 10−3
mol/L, respectively. Then
all of them were treated ultrasonically at 60 °C, 70 Hz for 6 h. The obtained samples were rinsed for several times
with alcohol to remove the excessive quercetin molecules. Eventually, they were dried in an oven at 60 °C for 12 h.
The obtained samples were marked as C-1, C-2, C-3, C -4, C-5 and C-6, respectively.
The morphology of the samples was examined with an S-4800 scanning electron microscope with the accelerating
voltage of 150 kV. The pore size distribution of the samples was measured on NOVA 200 (Quantachrome). The
photoluminescence spectra were obtained with a WGY-10 fluorescent spectrometer. The FTIR spectra were collected
on an Avatar 330 spectrometer (Nicolet). The samples were mixed with KBr in accordance with the weight ratio of
the sample to KBr of 1:100 and pressed into slices for characterization. All measurements were carried out at room
temperature.
3 RESULTS and DISCUSSION
It can be seen from FIG.3 that, CeO2 porous solids were composed of CeO2 nanoparticles and large numbers of small
pores and channels. Obviously, the CeO2 nanoparticles joined together when water escaped from the bulk during the
hydrothermal hot-pressing process and thus bulk CeO2 porous solids were formed.
Obviously, these isothermal adsorption and desorption curves (shown in FIG.4) are inconsistent because of the
different relative pressures during the process of agglomeration and depolymerization of the adsorbtion in the same
hole [20, 21]. According to International Union of Pure and Applied Chemistry (IUPAC), such isotherms are type-Ⅳ
mode. As a key parameter of bulk CeO2 porous solids, the pore size distribution is shown in FIG.4. It can be seen
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from the figure that, the pore size of the porous solids is mainly distributed in the range of 5-20 nm. The average
pore size, pore volume and specific surface area of the solids are 7.746 nm, 0.1293cm3/g and 66.77m
2/g, respectively.
Clearly, the diameters of the pores are larger than the size of quercetin molecules, so it can be easily assembled into
the pores of the solids.
FIG. 3 SEM PHOTOGRAPH OF CeO2 POROUS SOLIDS
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.010
20
30
40
50
60
70
80
90
100
110
120
130
140
0 10 20 30 40 500.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
Desorption Dv (log d) (cc/g)
Pore Diameter (nm)
Relative Pressure(P / P0)
Adsorption
Desorption
Vo
lum
e (
cc
/g)
FIG. 4 N2 ADSORPTION-DESORPTION ISOTHEMS AND PORE DIAMETER DISTRIBUTION OF CeO2 POROUS
SOLIDS, ACCORDING TO THE BARRETT-JOYNER-HALENDA (BJH) METHOD.
4000 3500 3000 2500 2000 1500 1000 500
Tra
ns
mit
tan
ce
(%)
Wavenumbers(cm-1)
2281
15
66
10
48
65
7
34
42
33
82
20
26
1653 1
38
61
26
3
10
70
99
0 86
4 61
95
38
34
15
20
26
16
52
15
60
13
86
10
81
10
54
98
1
527
a
b
c
FIG. 5 FTIR SPECTRA OF CeO2 POROUS SOLIDS (a), CeO2/QUERCETIN POROUS NANOCOMPOSITES (b) AND
QUERCETIN (c)
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1700 1600 1500 1400 1300 1200 1100
Tra
nsm
itta
nce(%
)
Wavenumbers(cm-1)
1566
1653
1386
1263
1652
1560
1386
a
b
c
1696
1537
1505
1142
1559
1521
1457
FIG. 6 PART OF THE FTIR SPECTRA OF CeO2 POROUS SOLIDS (a), CeO2/QUERCETIN POROUS
NANOCOMPOSITES (b) AND QUERCETIN (c)
The characteristic absorption peaks of CeO2 porous solids locate at 2281, 1566, 1048 and 657 cm−1
(attributed to
Ce-O) (as shown in FIG.5a) [22], whereas, the absorption peaks of quercetin molecules at 3442, 3382 cm−1
in FIG.5c
can be attributed to the vibration of O-H group, and the peak at 1653 cm−1
is attributed to the vibration of 4 C=O.
The other peaks at 1599, 1521, 1507 and 1457 cm−1
in Fig.6c can be attributed to the vibration mode of benzene
rings, and the peak at 1386 cm−1
in Fig.5c is the stretch vibration of hydroxyl [22]. The peak at 1263 cm−1
represents
the asymmetry stretch vibration of =C-O-C, and 1070 cm−1
comes from the absorption peak of the skeleton vibration
of C OC
C group.
Comparing curves b and c, it can be seen that the absorption peaks of C=O (1653 and 1070 cm−1
) and O-H
(1386cm-1
) are almost no changed. The phenomenon reveals that these groups have not formed new bonds. Whereas,
the peak at 1263 cm−1
(attributed to =C-O-C) disappears, which indicates that chemical bonds have been formed
between O in =C-O-C group and Ce atoms. The peak of skeleton vibration of C OC
C moves from 1070cm
−1 to 1081
cm−1
due to this new bond [23].
300 350 400 450 500 5500
3
6
9
12
Inte
nsity(a
.u.)
Wavelength(nm)
a
b
c
FIG. 7 THE PL SPECTRA OF CeO2 POROUS SOLIDS (a), C-4 (b) AND QUERCETIN (c)
In FIG.7, we present the PL spectra of CeO2 porous solids, C-4 and quercetin, which provides the evidence that
chemical bonds have been formed between the surface atoms of CeO2 porous solids and quercetin molecules. The
formation of the bonds improves the fluorescence efficiency of the porous nanocomposites. Obviously, the PL
intensity of the nanocomposites is higher than that of CeO2 porous solids and quercetin powders. The peak
wavelength (491 nm) of the nanocomposites shows a blue shift of 24 nm compared with that (515 nm) of quercetin
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powders. The reason of the above phenomena is that quercetin molecules are adsorbed on the wall of porous solid
tunnel. Due to the limitation of the tunnel, the molecular motion is restricted, which weakens the interaction between
the fluorescent molecules, reduces the formation of dimmers and polymers, changes the molecular excited state and
makes it close to the single-molecule light emitting. Thus the fluorescence intensity is enhanced and the peak
wavelength is blue shifted. In addition, it can be seen from the excitation spectra of FIG.8, composites excitation
spectra are different from which of quercetin and CeO2, and is not only the simple superposition of both, but appears
new absorption wavelength. This is owing to a new composite material formed between the CeO2 porous nanosolid
and quercetin.
250 300 350 400 4500
10
20
30
40
50
In
ten
situ
y(a
.u.)
Wavelength(nm)
a
b
c
FIG. 8 THE EXCITATION SPECTRA OF CeO2 POROUS SOLID (a), C-4(b) AND QUERCETIN (c)
FIG. 9 THE PL SPECTRA OF NANOCOMPOSITES C-1 (a), C-2 (b), C-3 (c), C-4 (d), C-5 (e) AND C-6(f)
On the other hand, it is known that the photoluminescence of an organic molecule strongly depends on its
concentration in solution, and the PL intensity will die down when the molecules become too concentrated. In order
to study the effect of the quercetin concentrations on the PL of the nanocomposites, the PL spectra of
nanocomposites with different quercetin concentrations was studied and the curves were shown in FIG. 9. With the
increase of quercetin concentrations from 10-8
mol/L to 10-5
mol/L, the PL intensities of the porous nanocomposites
become higher. By further increasing the concentrations to 10-4
mol/l, the PL intensities of the porous
nanocomposites decrease significantly. However, the peak positions remain unchanged. These results reveal that the
PL intensity of the nanocomposite has reached its optimum value in C-4. At low concentrations, quercetin molecules
are dispersed on the surface of the channels of CeO2 porous solids, which can prevent the interaction of the organic
molecules. So with the increase of quercetin concentrations, more and more quercetin molecules contribute to the PL
intensity. However, if a growing number of quercetin molecules are bonded on the surface of CeO2 porous solids,
350 400 450 500 550 6000
2
4
6
8
10
12
Inte
nsi
ty(
a.u
.)
Wavelength(nm)
a
b
c
d
ef
Inte
nsitu
y(a
.u.)
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and they interact with each other, so dimmers or polymers are formed. This would result in a decrease of the energy
band gap of the molecules and an increase in the interactions between electrons and phonons [24]. This explains why
the decrease in the PL efficiency is observed.
4 CONCLUSIONS
By using the hydrothermal hot-pressing method, bulk nano CeO2 porous solids can be successfully prepared using
the CeO2 nanoparticle as the starting material. Furthermore, this porous solid has been used as a precursor for
synthesizing CeO2/ quercetin porous nanocomposites by assembling quercetin into its pores by the ultrasonic method.
Because of the interaction between quercetin molecules and surface atoms of CeO2 porous solids, the PL properties
of the nanocomposites were significantly different from that of the starting materials. Additionally, the PL intensity
of the porous nanocomposites was also affected by quercetin concentrations.
ACKNOWLEDGMENT
The project was supported by the Fundamental Research Funds for the Central Universities (3122013p001) and the
Science and Technology Innovation Guide Funds of Civil Aviation Administration of China (2014).
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Authors
Xiaoxue Lian, female, was born in 1985. She obtained the Master degree from Civil Aviation University of China in field of materials
chemistry. Currently her research interests include function materials. Email: [email protected]