Microwave assisted sol-gel synthesis of tetragonal zirconia nanoparticles

Article abstract of DOI:10.1016/j.jallcom.2011.03.138

A simple and fast synthesis of zirconia nanoparticles by microware assisted citrate sol-gel method is reported. The formation of nanoparticles was characterized by performing powder X-ray diffraction (XRD) of the samples calcined at higher temperatures. The physico-chemical characterizations were done employing techniques of Raman spectroscopy and BET surface area measurements. Fourier transform infrared spectroscopy (FTIR) was performed to analyze the band structure and functional groups. High-resolution transmission electron microscopy (HRTEM) was done for analyzing the morphology of nanoparticles and estimating the crystallite sizes. The prepared material was found to have tetragonal symmetry. The average particle sizes were found ranging ～5-10 nm. Gelation and fast combustion seems to be the reason for smaller particle sizes.

Full text of DOI:10.1016/j.jallcom.2011.03.138

Journal of Alloys and Compounds 509 (2011) 6848–6851
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Microwave assisted sol–gel synthesis of tetragonal zirconia nanoparticles
Reena Dwivedia, Anjali Mauryab, Akrati Vermaa, R. Prasada, K.S. Bartwalc,∗
a School of Chemical Sciences, Devi Ahilya University, Indore, India
b Department of Chemistry, Sagar University, Sagar, India
c Laser Materials Development and Devices Division, RRCAT, Indore 452013, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple and fast synthesis of zirconia nanoparticles by microware assisted citrate sol–gel method is
reported. The formation of nanoparticles was characterized by performing powder X-ray diffraction
(XRD) of the samples calcined at higher temperatures. The physico-chemical characterizations were
done employing techniques of Raman spectroscopy and BET surface area measurements. Fourier trans-
form infrared spectroscopy (FTIR) was performed to analyze the band structure and functional groups.
High-resolution transmission electron microscopy (HRTEM) was done for analyzing the morphology of
nanoparticles and estimating the crystallite sizes. The prepared material was found to have tetragonal
symmetry. The average particle sizes were found ranging ∼5–10 nm. Gelation and fast combustion seems
to be the reason for smaller particle sizes.
Received 22 February 2011
Received in revised form 21 March 2011
Accepted 23 March 2011
Available online 31 March 2011
PACS:
75.50
81.07.Wx
81.16.Dn
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Nanocrystals
Zirconia
Sol–gel technique
XRD
Raman spectroscopy
TEM
1. Introduction
oral planting [10]. Recently it has been reported that the oxide
nanoparticles are promising for photonics, however principal role
The development of zirconia (ZrO2) nanoparticles has attracted
much attention due to their multifunctional characteristics.
Nanoparticles have been recognized to have potential in the area
of photonic applications. It is known that the main crystal phases
of ZrO2 are cubic (c), tetragonal (t) and monoclinic (m) and their
main IR frequencies are 480, 435 and 270 cm−1 for cubic, tetrago-
nal and monoclinic phases, respectively. This indicates that phonon
energy of the ZrO2 host varies on the crystal phases. The monoclinic
phase is thermodynamically stable up to 1100 ◦C, the tetragonal
phase exists in the temperature range 1100–2370 ◦C and the cubic
phase is found above 2370 ◦C. It has several applications such as
solid oxide fuel cell, bio-sensors, H2 gas storage material, oxygen
sensor, catalyst and catalyst support [1–5]. In addition, zirconia is
used as piezoelectric material, electro-optic material and dielectric
material [6–8]. It is also used as support to disperse various noble
and transition metals for distinct catalytic applications. Zirconia
is a well-known solid acid catalyst and an n-type semiconductor
material [9]. ZrO2 is used as toughening ceramics in thighbone and
from technological point of view play their mono-dispersion and
appropriate contact with the surrounding medium [11].
The existence of metastable tetragonal (t-ZrO2) at low tem-
perature has been reported and synthesized by several methods.
Some of the methodologies such as oxidation of ZrCl2 by molec-
ular oxygen [12], molten hydroxides method [13] non-hydrolytic
sol–gel reaction between isopropoxide and ZrCl4 [14], sol–gel tem-
plate technique [15,16] are developed to prepare nanocrystalline
ZrO2. Microwave assisted synthesis of nanocrystalline materials
has attracted much attention in recent years because of its several
advantages such as unique synthetic pathways, rapid heating rates,
short processing durations, uniformity and low power require-
ments [16–24]. Microwave assisted citrate gel combustion method
is similar to solution combustion process (SCP) reported earlier
[25].
In the present work combustion synthesis has been performed
under microwave using citric acid as fuel and zirconium oxychlo-
ride as oxidizer. To the best of our knowledge, there is no report
on the synthesis of tetragonal zirconium oxide (ZrO2) employing
microwave assisted technique. The present problem was therefore
undertaken with a view to develop a cheap and fast microwave
assisted synthesis of t-ZrO2 and to characterize the samples for
∗
Corresponding author. Tel.: +91 731 248 8656; fax: +91 731 248 8650.
E-mail address: bartwalks@yahoo.co.in (K.S. Bartwal).
0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.03.138

R. Dwivedi et al. / Journal of Alloys and Compounds 509 (2011) 6848–6851
6849
1500
1200
900
600
300
0
ZrO2
20
40
60
80
2θ
Fig. 1. Representative powder XRD pattern for ZrO2 nanoparticles calcined at 450 ◦C.
their structural and optical properties employing various physico-
chemical methods.
2. Experimental details
High purity chemicals, zironium oxychloride (S.D. Fine Chemicals) and anhy-
drous citric acid (LOBA Chemie Pvt. Ltd.) were used for the preparation of the zirconia
(ZrO2) nanoparticles. In the present set of experiment, 9.78 g of zirconium oxychlo-
ride (ZrOCl2·8H2O) was mixed with 7.68 g of citric acid (C2O4H2) in a 250 ml corning
glass beaker. De-mineralized water was added to have homogenous slurry of pH 2.
The solution was evaporated to dryness by exposing it to microwave for 2 min. The
material swells into a white colored gel. The product obtained was grinded and kept
for calcination in a tubular furnace at a temperature of 450 ◦C for 4 h. On calcination
a black colored residue was obtained which was grinded in a motor-pastel to make
a fine powder.
Fig. 2. (a) Representative TEM micrograph for ZrO2 sample annealed at 450 ◦C. (b)
High-resolution TEM micrograph for samples annealed at 450 ◦C. The corresponding
SAD patterns are inserted into micrographs.
The prepared zirconia nanoparticles were examined by powder X-ray diffrac-
˚
tion using Cu K␣ radiation (ꢀ = 1.5406 A, Rigaku Geiger Flex X-ray diffractometer)
to confirm the phase and structure. The diffuse reflectance Fourier transform
infrared spectroscopy, FTIR (Jasco-660 plus Fourier transform spectrophotometer)
was used to determine the functional groups present in the material. The morphol-
ogy and sizes of the zirconia nanoparticles were analyzed by transmission electron
microscopy in high-resolution mode. The sample for TEM observation was prepared
by suspending the particles in ethanol by ultrasonification and drying a drop of the
suspension on a carbon coated copper grid. Philips Technai G2-20 (FEI) electron
microscope operating at 200 kV was used for TEM experiments. BET surface area
was measured using ASAP 2020 V3.04 H, BET analyzer in a N2 absorption appa-
ratus, using the single point BET method. Raman records were taken on a Labram
HR800 micro Raman spectrometer using Labspec software. Raman spectrum in the
range 50–4000 cm−1 were recorded using 488 nm wavelength Ar+ laser source at
the energy of 2.53 eV with recording time of 10 s.
peaks have been indexed and match with the t-ZrO2 (JCPDF card
file, no. 79-1771). The lattice parameters were calculated for t-ZrO2
˚
˚
from the XRD data. The parameters were; a = 5.083 A, c = 5.185 A and
the tetragonality, c/a = 1.0201. The diffraction characteristic peaks
were obtained with the (h k l) values of (1 0 1), (1 1 0), (1 1 2), (2 1 1),
and (2 2 0). The particle sizes were calculated from FWHM (Full
Width Half Maximum) of reflections of t-ZrO2 structured zirconia
nanoparticles using Debye–Scherer formula [26]. The particle sizes
were found varying ∼5–10 nm range.
Transmission electronic microscopy (TEM) in high-resolution
mode is best tool to analyze the morphology and the sizes of the
prepared nanoparticles [27–29]. Fig. 2(a and b) shows the TEM
micrographs taken for the samples calcined at 450 ◦C. The corre-
sponding selected area diffraction patterns are inserted into the
micrographs. Fig. 2(a) shows a typical TEM image for the dried
powders. The powders are very fine and agglomerated. Electron
diffraction analysis reveals that they have amorphous characteris-
tics due to small particle sizes. The micrograph shown in Fig. 2(a)
indicates the formation of nanoparticles with sizes ranging from
few nanometers to few tens of nanometers. The corresponding
diffraction pattern shows the presence of few clear spots along with
connecting diffraction rings. The presence of spots along with the
streaks shows the presence of crystallite of reasonably sufficient
sizes to diffract. The connecting streaks indicating the short-range
order due to presence of some smaller size particles as well. The
high-resolution electron micrograph for the samples annealed at
450 ◦C is shown in Fig. 2(b). The clarity in the fringe patterns inside
the crystallite indicates the formation of single phase ZrO2 with the
long-range order in the structure. The distance between two adja-
cent lattice planes is about 0.28 nm (see Fig. 2(b)), which is close to
3. Results and discussion
Tetragonal zirconium oxide (t-ZrO2) nanoparticles were pre-
pared by microwave assisted citrate sol–gel technique using
zirconium oxychloride as oxidizer. Powder XRD patterns of the
prepared nanoparticles calcined at different temperatures were
recorded. The representative powder XRD for the sample calcined
at 450 ◦C is shown in Fig. 1. The XRD pattern reveals the fact that
the single tetragonal phase of ZrO2 is crystallized. The calcination
temperature has important role to play in formation of crystalline
phase and the particle size. The calcination temperature was opti-
mized, and 450 ◦C was found effective to crystallize the desired
tetragonal phase. It was observed that the full width at half maxi-
mum of the reflection peaks decreases and also becomes sharp as
the calcining temperature increases. This suggests that the crys-
tallinity of prepared zirconia nanoparticles is increasing at higher
temperatures. The representative XRD pattern obtained for the
samples calcined at 450 ◦C was taken into consideration and all

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R. Dwivedi et al. / Journal of Alloys and Compounds 509 (2011) 6848–6851
0.8
0.7
0.6
0.5
0.4
50000
2355
ZrO2
(A) ZrO2 , 600°C
(B) ZrO2 , 800°C
2323
40000
1554
30000
1523
123
146
20000
259
470
382
10000
0
643
(A)
(B)
0
1000
2000
Cm-1
3000
2000
Cm-1
1000
Fig. 4. Raman spectra of ZrO2 precipitated at pH 2 and calcined at different temper-
atures.
Fig. 3. FTIR spectrum of ZrO2 in the range of 3500–500 cm−1
.
modes on the simple reasoning that these two modes also does not
involve movement of Zr atoms. The remaining three modes namely
two B1g modes and one Eg mode are assigned to the remaining three
band appearing at 259, 146 and 123 cm−1, respectively. The spec-
trum of sample calcined at 600 ◦C is shown as curve (B) in Fig. 4. The
above bands appeared in the Raman spectra for both the samples
are assigned to t-ZrO2. In addition, few faint bands at 563, 536, 381,
293, 176 cm−1 have appeared which are due to the co-existence of
small amount of monoclinic phase.
the spacing of (1 0 1) planes of the t-ZrO2. The corresponding SAD
pattern is inserted into the micrograph. The clear spots in SAD pat-
tern suggest that the crystallites are of sufficiently large size. The
absence of rings in the SAD pattern is indicative of the crystalline
order, larger particle size and long-range order in the crystallites.
The TEM results also suggest the successful preparation of tetrag-
onal phase of ZrO2 nanocrystals with the crystallite sizes ranging
∼5–10 nm.
BET surface area was measured for the prepared zirconia sam-
ples calcined at 450 ◦C for 4 h. The average values calculated for
surface area was 38.7767 m2/g and pore size was 3.85247 nm.
FTIR spectra of the samples in the range 400–4000 cm−1 were
recorded. Fig. 3 shows the FTIR spectrum for the zirconia nanopar-
ticles in absorbance mode. The prominent band at 2355 cm−1 and
2323 cm−1 corresponds the structural O–H stretching of the nano-
materials. In the bending mode region two bands are observed at
1554 cm−1 and 1523 cm−1, which are due to the O–H bending. In
addition the broad band at 900–1000 cm−1 can be assigned to the
Zr–O bond for t-ZrO2.
Raman spectroscopy is sensitive to the polarizability of the oxy-
gen ions and therefore it can be used to determine the symmetry
of a crystal system. In fact Raman spectroscopy is a technique more
sensitive to short-range order than X-ray diffraction, and it can
show the peaks for anatase or rutile as well as monoclinic zirco-
nia along with that of tetragonal zirconia. The Raman spectra of
ZrO2 calcined at the temperatures of 600 ◦C and 800 ◦C are plot-
ted in Fig. 4. The assignment of the observed bands was made on
the basis of the comparison of the observed spectra with that of
reported in the literature [24,25,30]. The vibrational Raman active
modes are classified as:
4. Conclusions
Zirconia nanoparticles were synthesized by the simple and most
convenient microware assisted citrate sol–gel method. The for-
mation of tetragonal crystalline phase t-ZrO2 was confirmed by
powder XRD analysis on calcined samples. The morphology, par-
ticle size and microstructure were analyzed using high-resolution
transmission electron microscopy. The HRTEM data also confirms
the formation of single phase t-ZrO2. The high-resolution TEM
micrograph and SAD pattern show the crystalline perfection and
long-range order in the prepared zirconia nanoparticles. The crys-
tallite sizes were found in the range of ∼5–10 nm. Raman spectra
further supports and confirms the crystalline phase as well as the
specific bands to show the modes of vibration in Zr–O system.
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