Preparation and characterization of a novel bulk ceo2quercetin fluorescent nanocomposites

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 CeO 2 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 - 57 http://www.ivypub.org/rms

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.).

OH OH

2' 1

8

HO 7

' 2 1

O

A

C

5

HO

4

5' 6'

3

6

B

4'

OH

O

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 - 58 http://www.ivypub.org/rms

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 66.77m2/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 140

120

100

Volume (cc/g)

90 80 70 60

0.30

Desorption Dv (log d) (cc/g)

110

Adsorption Desorption

0.35

130

0.25

0.20

0.15

0.10

0.05

50 0.00

40

0

10

20

30

40

50

Pore Diameter (nm)

30 20 10 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Relative Pressure（P / P0）

FIG. 4 N2 ADSORPTION-DESORPTION ISOTHEMS AND PORE DIAMETER DISTRIBUTION OF CeO2 POROUS SOLIDS, ACCORDING TO THE BARRETT-JOYNER-HALENDA (BJH) METHOD.

657

1048

2281

1566

a

4000

3500

619

538

1070

990 864

1653

3442 3382

2026

c

1386 1263

527

1081 1054 981

1386

1652 1560

2026

3415

Transmittance(%)

b

3000

2500

2000

1500

1000

500

-1

Wavenumbers(cm )

FIG. 5 FTIR SPECTRA OF CeO2 POROUS SOLIDS (a), CeO2/QUERCETIN POROUS NANOCOMPOSITES (b) AND QUERCETIN (c) - 59 http://www.ivypub.org/rms

Transmittance(%)

1566

a

1142

1386

1505

1560 1537

1652

1696

b

1700

1600

1500

1263

1386

1457

1521

1653

1559

c

1400

1300

1200

1100

-1

Wavenumbers(cm )

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 C of C C O 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 C between O in =C-O-C group and Ce atoms. The peak of skeleton vibration of C C O moves from 1070cm−1 to 1081 −1 cm due to this new bond [23]. b

Intensity(a.u.)

12

9 c

a

6

3

0 300

350

400

450

500

550

Wavelength(nm)

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 - 60 http://www.ivypub.org/rms

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 CeO 2 porous nanosolid and quercetin.

Intensituy(a.u.)

50

40

30

20 b a

10

0

250

c

300

350

400

450

Wavelength(nm)

FIG. 8 THE EXCITATION SPECTRA OF CeO2 POROUS SOLID (a), C-4(b) AND QUERCETIN (c)

d

12

c

Intensituy(a.u.)

Intensity （a.u.）

10

b

8 6 a

4

e f

2 0

350

400

450

500

550

600

Wavelength（nm）

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, - 61 http://www.ivypub.org/rms

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

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