Facile synthesis of ceria nanoparticles by precipitation route for UV blockers

Article abstract of DOI:10.1016/j.jallcom.2013.12.121

Homogeneous ceria (CeO₂) nano particles of approximately 4 nm have been successfully synthesized via a simple precipitation route by employing the mixed solvent method. X-ray diffraction analysis revealed the precipitate particles to be of highly crystalline nature with face centered cubic structure along (1 1 1) (2 0 0) (2 2 0) (3 1 1) (4 0 0) (3 3 1) (4 2 2) (5 1 1) planes. Cerium oxide nanoparticles exhibits enhanced specific surface area of about 139.116 m²/g. The mono-dispersed spherical shape morphology of approximately 4 nm particles was confirmed using TEM analysis and its chemical composition by SEM-EDS analysis. Surface morphology reveals the smooth surface with an average roughness of 14.9 nm with the help of AFM. Raman studies show a characteristic peak at 464 cm^-1. The UV absorption edge was found at 314 nm i.e. In the Ultra Violet region suggesting that the material has a good absorption of UV light. Also, it shows an excellent transparency in the visible region.

Full text of DOI:10.1016/j.jallcom.2013.12.121

Journal of Alloys and Compounds 590 (2014) 406–410
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Facile synthesis of ceria nanoparticles by precipitation route
for UV blockers
⇑
K. Anupriya, E. Vivek, B. Subramanian
ECMS Division, Central Electrochemical Research Institute, Karaikudi 630 006, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 October 2013
Received in revised form 13 December 2013
Accepted 16 December 2013
Available online 21 December 2013
Homogeneous ceria (CeO ) nano particles of approximately 4 nm have been successfully synthesized via
a simple precipitation route by employing the mixed solvent method. X-ray diffraction analysis revealed
the precipitate particles to be of highly crystalline nature with face centered cubic structure along (111)
2
(
200) (220) (311) (400) (331) (422) (511) planes. Cerium oxide nanoparticles exhibits enhanced spe-
2
cific surface area of about 139.116 m /g. The mono-dispersed spherical shape morphology of approxi-
mately 4 nm particles was confirmed using TEM analysis and its chemical composition by SEM–EDS
analysis. Surface morphology reveals the smooth surface with an average roughness of 14.9 nm with
the help of AFM. Raman studies show a characteristic peak at 464 cm . The UV absorption edge was
found at 314 nm i.e. In the Ultra Violet region suggesting that the material has a good absorption of
UV light. Also, it shows an excellent transparency in the visible region.
Keywords:
Cerium oxide nanoparticles
Mixed solvent
Precipitation route
UV absorption
ꢀ1
Ó 2013 Elsevier B.V. All rights reserved.
1
. Introduction
Due to the ozone layer depletion, there is presently a strong
Ceria nanoparticles can be synthesized by different methods
such as Homogenous precipitation [11–13], Sol–gel process
[14,15], Sonochemical synthesis [16,17], Thermal decomposition
[18], hydrothermal synthesis [19,20], Microwave synthesis [21]
and Reverse emulsion [22].
Hu et al. [23] engaged a new approach called alcohol/water
2
mixed solution as a base for preparing nanosized ZrO particles.
This is due to the dielectric property of the mixed solvent which
affects the nucleation and growth of Zirconia particles. Li and
Gao [24] synthesized ZrO (Y O ) metal oxide nanoparticles by
2 2 3
demand for UV shielding materials which protect against the
harmful effect of UV solar radiation which causes hazardous
diseases to human such as skin cancer, premature aging of skin
and cataract [1]. In the current era, shielding from UV irradiation
by using metal oxide nanoparticles have been attracted much for
its remarkable physical and chemical properties which are signifi-
cantly different from their bulk counterparts [2]. Metal oxide nano-
particles such as titanium dioxide (TiO
cerium oxide (CeO ), were often studied as inorganic UV-Shilders
3]. However, the UV-blocking property of ceria nanoparticles
show good shielding effect in comparison with ZnO and TiO [4].
Especially, cerium oxide (CeO ) is one of the most reactive rare
2
), zinc oxide (ZnO) and
using an ethanol/water mixed solvent. The reaction rate was
much higher and also the particle precipitation time was reduced
as the reaction temperature increased. According to Huey-Ing
Chen et.al. [25], that the particle size decreases in the following
order: pure water (6.5 ± 0.9)>t-BuOH/water (7.4 nm)>EG/water
(3.7 nm). Specifically, from the above reports it is observed that
the diameter of the particles can be controlled by varying the ratio
of alcohol in the reaction medium.
In this work, cerium oxide nanoparticles were synthesized by
homogeneous precipitation route which has the advantages of
low cost, mild synthesis conditions and easy scale-up process in
which the mixer of ethylene glycol/water serves as a reaction med-
ium. The structure, composition and particle size were character-
ized using XRD, Raman, SEM with EDS, TEM and AFM. Finally,
the UV shielding effect of ceria nanoparticles using UV–Visible
spectroscopy was examined.
2
[
2
2
earth metal oxides which possesses wide band gap energy and
high refractive index in the visible region (2.1 ± 2.2) [5] which
mainly made them as most prominent candidate for UV shielders.
Also, Ceria changes between Ce3 and Ce states to inactivate most
toxic super hydroxide radicals, hydrogen peroxide and nitroxyl
radicals [6–9], and the high concentration of oxygen deficiency
leads to the production of free charge carriers rapidly upon UV irra-
diation which decreases the photocatalytic effect [10].
+
4+
⇑
Corresponding author. Tel.: +91 4565 241538; fax: +91 4565 227713.
E-mail addresses: subramanianb3@gmail.com, bsmanian@cecri.res.in
(
B. Subramanian).
0
925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.12.121

K. Anupriya et al. / Journal of Alloys and Compounds 590 (2014) 406–410
407
2
. Experimental
2.1. Materials required
Chemicals used in this synthesis were Cerium (III) Nitrate hexa hydrate (purity
of 99.99%,Ce(NO ) as a precursor, CTAB (cetyltrimethylammonium bromide) as
3 2
)
surfactant and Monoethanolamine as a reducing agent, all chemicals being pur-
chased from CDH chemicals and Merck TM. Ethylene glycol (>99.99% pure, Merck
TM) was used as a solvent Conc. HCl was used to adjust the pH. All these chemicals
were used without any further purification.
2.2. Synthesis
In a typical synthesis, 0.2 M of Cerium Nitrate and 0.2 g of CTAB were mixed in
ethylene glycol (EG)/water (1:1) mixed solvent system. The reaction was performed
at room temperature at a constant stirring rate. The initial pH was noted to be 7. Few
drops of concentrated HCl were added to the mixture to reduce the pH to 1. Mono-
ethanolamine was then dripped to the above solution until the pH got increased to
1
3
2. Initially a yellow solution of cerium hydroxide Ce(OH) was obtained. Slowly, this
yellow color disappears and turns to purple. This is due to absorption of dissolved
oxygen from air which is an intermediate mixed hydrate state. Stirring was carried
out in an ambient environment for 12 h. Finally, yellow precipitate was obtained
showing the complete conversion of cerium oxide nanoparticles. Then, the solution
was centrifuged at 7500 rpm and washed with acetone and double distilled water
for three times. It was then dried at 100 °C for 2 h in hot air oven. Further the dried
powder was calcinated at 400 °C for 2 h to obtain a crystalline product.
2.3. Materials characterization
Powder XRD analysis was carried out using a Bruker AXS D8 Advance X-ray dif-
fractometer using Cu Ka radiation (k = 0.15406 nm) with a scintillation counter. The
ꢀ
1
scanning rate was 0.05 s in the 2h range from 20° to 100°. The specific surface
area was measured with Quantachrome Nova Win 3200e equipment by de-gassing
the sample at 200 °C for 2 h and specific area was calculated by nitrogen adsorption
keeping the relative pressure between 0.04 and 0.2. TEM studies were performed
with a JEOL JEM-200CX instrument operating at 200 kV. EDS analysis was per-
formed with a HITACHI S-3700N scanning electron microscope with an accelerating
voltage of 20 kV. Surface morphology was examined using a XE-100 Park systems
AFM, operated in a non-contact mode with a scan rate of 1 Hz. Scan size in X and
Fig. 1. XRD pattern of cerium oxide nanoparticles prepared by chemical precipi-
tation route. (a) Reference JCPDS card 43–1002, (b) before calcination of ceria NP’s,
Y axis was made as 1 ꢁ 1
lm. Raman spectra were collected using a Renishaw Invia
(
c) after calcinated at 400 °C.
Raman microscopy at room temperature. He–Ne laser beam with an excitation of
6
1
33 nm was focused under 50ꢁ objective lens and the laser spot size was about
ꢀ1
l
m. Raman spectra were recorded in the 200–600 cm range with the spectral
pattern as in Fig. 2(b). Like the XRD profile, the reflections were in-
dexed to 111, 200, 220, 311, 331, 422, 611 planes respectively with
ꢀ
1
resolution of 1 cm . The UV–Visible spectra were recorded using V-650 UV–Vis
instrument from JASCO operated at room temperature in the wavelength between
2
00–700 nm with a scan speed of 400 nm/min.
2
a cubic CeO crystal structure. In addition, all the diffraction rings
match with CeO
system.
The specific surface area is about 139 m /g exhibited for cerium
oxide nanoparticles measured by nitrogen adsorption with a rela-
tive pressure of 0.04–0.2. The corresponding particle size (dBET) can
be estimated as [11]
2
powder showing absence of impurity in the
3
. Results and discussion
2
3.1. Structural analysis
The X-ray diffraction (XRD) pattern obtained from as-prepared
and calcinated ceria is as shown in Fig. 1, the improvement in crys-
tallinity after calcination. These peaks are in good agreement with
the standard JCPDS card 43–1002 for face centered cubic structure.
Before calcination as in Fig. 1b shows poor crystallinity with weak
d
BET ¼ 6=
q
A
ð1Þ
3
where
q is the true density of CeO , i.e. 7.28 g/cm and A is the spe-
2
cific surface area calculated from BET. The particle size estimated
from BET indicates that the average crystalline size is about
5
peaks viewing the presence of Ce(OH)
was calcinated at 400 °C [26] where Ce(OH)
rectly to form CeO . Thus, the effect of calcination temperature
4
. So the resultant sample
2
.9 nm for CeO nanoparticles.
4
will be dehydrated di-
2
shows a good crystalline growth of cerium oxide NP’s with a crys-
talline sharp and strong diffraction peak. The diffraction peaks
were all relatively broader because of the smaller size of cerium
oxide nanoparticles. The crystallite size was about 9 nm calculated
by using the Debye–Schrrer’s formula.
From the Fig. 1c, the characteristic peaks of cerium oxide nano-
particles after calcination at 400 °C are in consistent with face cen-
tered cubic structure.
The average particle size of cerium oxide nanoparticles was
examined by using TEM. The results confirmed that the particles
were in spherical shape and also the size distribution was uniform
with an average crystallite size of 4 nm as in Fig. 2(a). The highly
3.2. Morphological studies
Structural morphological studies were done using SEM as
shown in Fig. 3(a). It shows that nanostructured CeO was nearly
2
spherical in shape with a variety of particle sizes due to the calci-
nation which makes the particles condense faster. Aggregates seen
above are due to the accumulation of nanoparticles. It can be seen
that the average diameter of the secondary particles were ꢂ36 nm
to ꢂ50 nm respectively, along with fine particles below ꢂ10 nm. As
crystallite size was not clearly identified by using SEM further
investigation was done by using TEM. The energy dispersive X-
ray (EDAX) spectrum gives information about the presence of Ce
and O. This confirms that the resultant sample was cerium oxide
and is shown in Fig. 3(b).
2
crystalline nature of CeO powder leads to the Debye–Scherrer
diffraction rings in the Selected Area Electron Diffraction (SAED)

4
08
K. Anupriya et al. / Journal of Alloys and Compounds 590 (2014) 406–410
Fig. 3. (a) SEM image of the cerium oxide nanoparticles, (b) EDSspectrum.
Fig. 2. (a) TEM image of cerium oxide nanoparticle calcinated at 400 °C, (b) SAED
pattern.
The Surface roughness study is obtained by AFM. The obtained
sample scanned in the non-contact mode of AFM across the scan
size of 1
l
m ꢁ 1
lm. Average Surface roughness were found to
be 14.9 nm as determined by AFM. From the Fig. 4, the 3D image
shows well defined particles with a granular morphology and also
indicates the presence of small crystalline grains of cerium oxide
nanoparticles.
3.3. Raman studies
For a comparison we did studies on resultant cerium oxide
nanoparticle and commercial cerium oxide powder (size: 14 mi-
cron) from 99.99% (REO), Alfa Aesar™ (named as COP-14 sample)
Fig. 4. AFM – three dimensional (3D) morphology of cerium oxide coated on glass
substrate.
both shows cubic structure of CeO
2
with a single Raman mode at
ꢀ1
4
64 cm , which is an F g symmetric band obtained from the space
2
described by the dependence of the full width at half maximum,
group Fm3m of a cubic fluorite structure. This is due to the sym-
metrical stretching mode of Ce–O8 vibrational unit [27]. From
the Fig. 5, it is inferred that the increase in the asymmetry and
broadening of the line features shows that the particles are of
nanocrystalline size but for COP sample only narrow peak was
C
, on the inverse of grain size, d
g
, which follows a linear behavior
[
28]. Thus the obtained peaks were similar for both the samples
which are in good agreement.
24:7
1
C
ðcmꢀ Þ ¼ 10 þ 1
ð2Þ
d
g
2
obtained. The Raman line broadening of CeO nanoparticle can be

K. Anupriya et al. / Journal of Alloys and Compounds 590 (2014) 406–410
409
¼ 2
: 303Abs
q
¼
ꢀ lnðT=100Þ
tðcmÞ
ð4Þ
a
Lc
where Abs is the absorbance of the sample, c is the sample loading
and the L is the path length (L = 1 cm). The density
ken as 7.28 g/cm , t = thickness.
2
q of CeO , is ta-
3
ꢀ
+
Fig. 6. Charge carrier separation (electron e and hole h ) and recombination in
CeO
2
by UV light irradiation (adapted from [29]).
Fig. 5. Raman spectrum of cerium oxide nanoparticles and cerium oxide bulk.
Using the above relation, the grain size was calculated to be 9.5 nm
which matches with the crystallite size calculated from Scherrer
formula.
3.4. UV-shielding mechanism
The UV absorption mechanism of cerium oxide is based on the
charge separation and recombination processes which is schemat-
ically illustrated as reported in [29] (Fig. 6). The process can be de-
scribed through the following equations,
CeO
2
þ h
m
! CeO
2
! eꢀ þ hþ
ðaÞ
ðbÞ
ꢀ
! O Å2 ꢀ ! HOÅ
e
þ O
2
2
þ
þ OHꢀ ! OH
Å
h
ðcÞ
where (a) Incident UV photon exits the electron from valence band
to conduction band. (b) Exited electron reacts with oxygen to
hydrogen ions to form superoxide. (c) Hydroxyl radicals formed
by reaction of positive holes with hydroxyl ions.
Fig. 7. UV absorption spectra of CeO2 nanoparticles and CeO2 bulk.
The resulting cerium oxide nanoparticles (size: 4 nm) were sub-
jected to UV–Visible measurements and for a comparison COP-14
samplewas also examined. The results show that the strong
absorption has taken place in the UV region with an absorption
edge located at 314 nm for cerium oxide nanoparticle and for
COP-14 there is no such absorption is observed between 200–
8
00 nm, shown in Figs. 7 and 8 compares UV transmittance spectra
which points out the UV-shielding effect occurs between 200–
3
3
80 nm with 0% transparency and it increases gradually from
80 nm reaching 85% up to the visible region besides no shielding
effect have been observed in the UV–Visible region for COP-14
sample.
The direct band gap energy was calculated by the Estimating
the value of absorption coefficient (a) to zero with
2
ð
aE
photonÞ ¼ Cdir
ð
aEphoton ꢀ Edir
Þ
ð3Þ
where Cdir is the absorption constant and Edir is the band gap energy
for direct transition. Conventionally, the absorption coefficient (
a)
is defined as [18,30]
2
Fig. 8. Optical transmittance spectra of cerium oxide nanoparticles and CeO bulk.

4
10
K. Anupriya et al. / Journal of Alloys and Compounds 590 (2014) 406–410
Acknowledgement
This work was supported by the Molecules to Materials to
Devices (M2D) program, CSC 0134, funded by Council of Scientific
and Industrial Research (CSIR) New Delhi.
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ꢀ1
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2
strong mode at 464 cm with F g symmetric band cubic structure.
[
cerium oxide nanoparticle shows an excellent UV absorption and
transparency properties in the visible region due to the increase
in surface-to-volume ratio.These facts suggest that ceria nano-
structures were good applicant for UV-shielders.
(