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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: lianxiaoxues@163.com

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: lianxiaoxues@163.com