Transparent and Homogeneous Hybrids of Epoxy Resin/ZnO
Quantum Dots
Dinh Huong Nguyen, Gwang Seok Song and Dai Soo Lee*
Department of Semiconductor and Chemical Engineering,
Chonbuk National University, Jeonju 561-756, Korea
ZnO quantum dots (QDs) were synthesized by sol-gel method from zinc acetate using
monoethanol amine as base catalys and complexing agent. Photoluminescence of ZnO QDs
solution showed strong blue emission at 440 nm under the excitation of 360nm. Epoxy
resin/ZnO QDs hybrids obtained from as-prepared ZnO QDs and epoxy resin exhibited high
transmittance, strong blue emission, and UV absorption.
Keywords: ZnO, Quantom dots, epoxy, photoluminescence
1. INTRODUCTION
Zinc oxide, ZnO, with its wide band gap (3.4 eV)
and large exciton binding energy (60 meV) is an
environmentally friendly and inexpensive oxide
semiconductor
compared
with
other
semiconductors such as CdSe and CdTe. The
advantages made it more attractive for practical
applications.1,2 The luminescence characteristic
of ZnO quantum dots (QDs) depend on their size
as well as surface properties.3,4 In general, solgel process became one of the most successful
approaches to obtain ZnO QDs in dispersion.5
ZnO QDs typically synthesized from zinc actate
dihydrate (Zn(Ac)2.2H2O) in alcoholic solution
under alkaline base catalyst (KOH, NaOH),6 or
amines base catalysts such as monoethanol amine
(MEA) or diethanol amine (DEA),7,8 in which
amine compounds not only act as catalyst but also
as ligands to stable ZnO QDs.
Nanocomposite base on semiconductor quantum
dots (QDs) of metal oxide have been extensively
studied for use in lens, lasers, light emitting
diodes, and encapsulations of LED. 9,10 The
nanocomposites not only inherit optoelectric
properties of inorganic semiconductor QDs but
also possess advantages of polymers such as light
weight, flexibility, UV stability, and conformity.
Furthermore incorporation of inorganic QDs in
∗
Corresponding author. Tel:82.63.270.2310 fax:
E-mail address: daisoolee@chonbuk.ac.kr
nanoparticles (NPs) also improves mechanical and
thermal proprieties of the composites.11,12 Ability
to control the fine dispersion of NPs in polymers
is one of the big issues for archiving optimal
performance of these organic/inorganic composite.
Significant phase segregation occurs through
direct mixing because inorganic NPs often possess
hydrophilic surface characteristics.13 Generally,
the optical properties of nanocomposites depends
on the degree of nanoparticles dispersion in
medium. Optical properties of poor dispersion of
QDs in polymer exhibit low transmittance and
red-shift compared to fully dispersed QDs. 14
Epoxy resin/ZnO QDs hybrids were prepared
with ZnO QDs synthesized using the catalyst of
MEA.7 Optical properties of the nanocomposites,
especially UV resistance and photoluminescence,
were studied and highlights are reported in this
paper.
2. EXPERIMENTAL DETAILS
2.1 Materials
Zinc acetate dihydrate, monoethanol amine,
hexahydro-4-methyl phthalic anhydride (HMA),
and methanol were obtained from Aldrich. All
the chemicals were used without further
furification. A cycloaliphatic epoxy resin, Cell
2010 (epoxy equivalent weight: 126, Mn=252.3)
was purchased from Deicel Chemical.
Removal of MeOH
Epoxy resin
Curing agent (HMA)
Vacuum
0.5g-Zn(Ac)2.2H2O
100g - MeOH
1g - MEA
reflux
ZnO QDs +
Epoxy resin
in MeOH
ZnO QDs
in MeOH
ZnO QDs in
Epoxy resin
Epoxy/ZnO
QDs hybrids
Curing
Scheme 1. Flow chart to prepare ZnO QDs and nanocomposites of epoxy resin/ZnO QDs hybrids
O
O
O
H3C
Zn+ C
C
OH2O
Zn2+
OH
HO
CH3
O
ZnO
H
O
ZnO
O
H
Zn2+
NH2
N
H2
H2N
O
+
H
O
H2
N
Sol-gel
H 3C
O
NH2
O
N
H2
ZnO
Curing
OH
Scheme 2. Scheme of the chemical equilibria during sol-gel process and curing process of epoxy resin
with amine on the surface of ZnO QDs
3.0
500
400
2.0
300
1.5

200
1.0
100
0.5
2nm
0.0
250
PL intensity
UV absorption
2.5
300
350
400
450
500
550
0
600
Wave length (nm)
Figure 1. HR-TEM image of ZnO QDs
2.2 Samples preparation
ZnO QDs was prepared according to previous
report.7 Epoxy resin/ZnO QDs hybrids was
prepared following the Scheme 1. The transparent
solution of ZnO in methanol obtained by heating
with refluxing the mixture containing 0.5g
Zn(Ac)2.2H2O, 100g methanol, and 1g MEA for 3
days. Then the epoxy resin was directly added into
the ZnO QDs solution. Methanol was removed
at room temperature under reduced pressure
before adding HMA. Both HMA and MEA was
considered as curing agents and the stoichiometric
ratio of Cell 2010 and HMA was determined.
Different samples were obtained by varying the
concentration of ZnO QDs from 0 % ~ 0.5%.
Figure 2. UV/vis and photoluminescence spectra
excited at 360nm of ZnO QDs.
The samples were casted on Teflon molds and
cured at 120 oC for 2 hours and 150 oC for 1 hour.
2.3 Characterization
Optical absorption and photoluminescence spectra
were obtained with a UV-670 Jasco
spectrophotometer
and
FP-6500
Jasco
spectrofluorometer.
High
resolution
transmittance electron microscopy (HR-TEM)
observations of ZnO QDs were carried out
employing a JEOL JEM-2010 operated at 200 kV.
X-ray mapping of samples was done using
scanning electron microscopy (SEM, JSM-6400).
5000 nm
3. RESULTS AND DISCUSSION
Figure 1 shows HR-TEM image of ZnO QDs.
Highly crystalline and uniform particle of ZnO
QDs was clearly observed. Figure 2 shows UV
absorption and photoluminescence emission of asprepared ZnO QDs solution. The absorption at
near 330 nm shows a significant blue shift
compare with bulk ZnO (370nm), which is
attributed to the quantum confinement effect.13
Based on the experimental relationship between
average diameter (d) and wavelength at which the
absorption is equal to 50% of the excitonic peak
(or shoulder) (λ1/2), the following equation was
used to determine average diameter of QDs14 ;
1240/λ1/2 = 3.301 + 294.0/d2 + 1.09/d
Particle size calculated from this equation was 2.1
nm. The photoluminescence emission of ZnO
QDs solution showed blue emission centered at
440 nm under excitation of 360nm. This is due to
complexing effect of MEA on the surface of ZnO
QDs. It have been reported that ZnO QDs
synthesized from zinc acetate in alcoholic without
surface modification show green or yellow
emission because of surface defects related with
OH- on the surface.5 When OH- was displaced by
ligands such as diethanolamine and oleic acid,8 or
acetate17 the green emissions were quenched and
enhanced blue emission is observed.
The dispersion of ZnO QDs in epoxy resin was
confirmed by TEM and X-ray mapping images
showing in Figure 3. TEM image shows fine
dispersion of ZnO QDs in epoxy resin in small
area. X-ray mapping image shows uniform
dispersion of ZnO in epoxy rein in large area. The
fine dispersion was attributed to the presence of
MEA. .
0% ZnO
0.25% ZnO
0.5% ZnO
80
60
40
20
0
300
400
500
600
700
800
Wave length (nm)
Figure 4. UV/vis spectra of epoxy resin/ZnO QDs
hybrids with different concentration.
Figure 4 shows UV/vis spectra of epoxy
resin/ZnO QDs hybrids. High transmittances was
observed not only for pure epoxy resin but also for
the epoxy resin/ZnO hybrids. The transmittance of
epoxy resin/ZnO hybrids containing 0.5%ZnO
QDs maintains 99.2% at 680nm with only slightly
reduction (0.3%) compare with pure epoxy resin
matrix, because of homogeneous and fine
dispersion of ZnO QDs in epoxy resin. Moreover,
the hybrids of epoxy resin/ZnO QDs exhibit UV
blocking ability. This phenomenon can be
attributed by the characteristic absorption of ZnO
QDs dispersed in epoxy resin. 18
In Figure 5, photoluminescence (PL) of the
hybrids under excitation of 360 nm are given. An
emission band around 427 nm has been observed
for pure epoxy resin, which is similar to the
previous report.10 When ZnO was incorporated
into epoxy resin, the PL intensity increase with
increasing ZnO QDs content but no change in
peak position was observed.
0% ZnO
0.25% ZnO
0.5% ZnO
Intensity
0.5%
Figure 3. TEM (a) and X-ray mapping (b) images
of epoxy resin containing 0.5% ZnO QDs.
Transmittance (T, %)
100
350
400
450
500
550
600
Wave length (nm)
Figure 5. Photoluminescence of epoxy resin/ZnO
QDs hybrids excited at 360nm
2.
0%
025 %
0.5 %
0%
025 % 0.5 %
3.
4.
Figure 6. Optical images of epoxy resin/ZnO
QDs hybrids under 365nm UV radiation (a) and
visible light (b)
5.
It is postulated that ZnO QDs improve PL of
epoxy resin. Optical images of epoxy resin/ZnO
QDs hybrids under UV radiation and visible light
are shown in Figure 6. High transparency and
strong blue emission of epoxy resin/ZnO QDs
hybrids could be confirmed in Figure 6..
7.
4. CONCLUSION
ZnO QDs with 2.1 nm diameter were successfully
prepared in methanol in the present of MEA.
Epoxy resin/ZnO QDs hybrids were prepared by
direct mixing epoxy resin with as prepared ZnO
QDs TEM and X-ray mapping images of the
hybrids confirmed fine dispersion of ZnO in
epoxy resin. It was attributed to the presence of
MEA, playing the role of complexing agent and
coupling
agent.
The
hybrids
exhibited
characteristics of highly transparent and UV
resistant. Photoluminescence of epoxy resin/ZnO
QDs increase with increasing ZnO QDs content.
Acknowledgment:
This
research
was
financially supported by the Ministry of
Education, Science Technology (MEST) and
Korea Industrial Technology Foundation
(KOTEF) through the Human Resource
Training Project for Regional Innovation
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