J. Indian Chem. Soc.,
Vol. 91, January 2014, pp. 47-52
Synthesis and characterization of zirconium oxide nanoparticle prepared by
aqueous gelation method
Seena Kurakaran*, Abraham George and A. Sreekumaran Nair
Department of Chemistry, Mar Ivanios College, University of Kerala, Thiruvananthapuram-695 015,
Kerala, India
E-mail : seenakurakaran@gmail.com
Manuscript received online 18 February 2013, revised 22 March 2013, accepted 29 March 2013
Abstract : Tetragonal zirconium oxide (ZrO 2 ) nanostructure was synthesized by a novel aqueous gelation route. Surface
morphology analysis depicts the formation of cylindrical nanocrystals. The structural analysis confi rm that the assynthesized ZrO2 product is mainly of tetragonal phase with minor fraction of monoclinic phase having crystallite size
of less than 25 nm. The tetragonal phase of ZrO 2 is strongly evidenced by XRD and Raman analysis. Thermal analysis
shows that the gel is continuously dehydrated in the temperature range between room temperature and 500 ºC. The
DSC analysis coupled with TG and structural information, indicate that the exothermic processes between 349 ºC and
460 ºC can be attributed to the nucleation process of the formation of tetragonal zirconia, with phase transformation
at 460 ºC. The UV-Vis absorption spectrum showed a strong absorption peak at 226 nm and the estimated optical band
gap was found to be 5.6 eV.
Keywords : Aqueous gelation, ZrO 2 , SAED, HRTEM, tetragonal.
Introduction
Zirconia nanoparticles find applications in various
fields. It has been reported as a better catalyst1 and catalyst support compared to classical materials such as Al2O3,
SiO2 and TiO2. The modified versions of zirconium dioxide viz. the sulfated ZrO2, zirconia substituted mixed
oxides, various transition metal stabilized zirconia and
hydrous zirconium oxide have been reported to be effective for several organic reactions, and gas phase reactions2. The partial substitution of ZrO2 by CeO2 improves
the thermal stability3, oxygen storage capacity and redox
properties of ceria considerably. The catalytic performance
of ZrO2 depends also on the structural, textural properties, surface area, synthesis method adopted and calcination temperatures. ZrO2 nanostructures are of significant
current interest in preparing piezoelectric, electrooptic,
dielectric and nanocomposite materials4–6. It tends to become more conductive with increasing temperatures.
Pure ZrO2 exists in three polymorphic phases at different temperatures : monoclinic, tetragonal and cubic.
At very high temperatures (>2370 ºC) the material has a
cubic structure. At intermediate temperatures (1150–2370 ºC)
it has a tetragonal structure. At low temperatures (below
1150 ºC), the material transforms into the monoclinic
structure, which is a thermodynamically stable phase7.
The versatility of ZrO2 nanoparticles has prompted researchers to find convenient and cheap methods to prepare the nanoparticles. Different methods have been
adopted for the preparation of phase pure ZrO2. The sol
gel route using zirconium alkoxide8 has been proved to
be one of the best methods to obtain phase pure ZrO2
nanoparticles and composite nanostructures9–12.
Aqueous gelation method has turned out to be a simple
and excellent route for the preparation of zirconium
nanoparticles13. The aqueous gelation method has many
advantages viz. a highly homogenous crystalline product
can be obtained directly at a relatively lower reaction
temperature (< 150 ºC), favors a decrease in agglomeration between particles, narrow particle size distribution, phase homogeneity, uniform composition, high product purity and controlled particle morphology14. Moreover the reaction takes place at ordinary laboratory conditions with less expensive chemicals. There is a wide
scope of using this method to produce other oxide materials with required properties for catalysis.
47
J. Indian Chem. Soc., Vol. 91, January 2014
In the present work, we discuss the synthesis of ZrO2
nanoparticles by aqueous gelation method using zirconyl
oxychloride as the precursor. We present the synthesis of
t-ZrO2 nanoparticles with controlled morphology and high
crystallinity. Structural analysis of the nanoparticle was
performed by transmission electron microscopy, X-ray
diffractometry, Raman and IR and UV-Visible spectroscopies and thermal analysis.
Results and discussion
Preparation : The aqueous gelation method using sodium acetate can be considered as a highly versatile
method. When sodium acetate was added to the zirconyl
oxychloride solution, the pH increased from 2 to 4. This
pH is conducive for the formation of the gel. The acetate
ions function as a good ligand coordinating to Zr controlling its hydrolysis, leading to the formation of ZrO2-x(OH)2x.yH2O15.
Infrared spectrum : The IR spectrum of the gel was
recorded (Fig. 1a) and shows two prominent peaks. A
broad band around 3500 cm–1 corresponds to the stretching vibrations of the O-H bonds. The sharp bands around
1500 cm–1 are due to the water molecules and also the
vibrations of the Zr-O bonds. The IR spectra of the ZrO2
nanoparticles (Fig. 1b) essentially show the various stretching frequencies at 500, 572, 740, 1104 and 1187 cm–1
respectively as reported earlier16. The features particularly at 740 cm–1 and 500 cm–1 due to Zr-O-Zr asymmetric and Zr-O stretching modes respectively, confirm the
formation of phases.
X-Ray diffraction analysis :
XRD profile of ZrO2 nanoparticle synthesized at 600 ºC
is given in Fig. 2. The diffraction peaks in the spectrum
are composed of t-ZrO2 with minor quantities of m-ZrO2.
Almost all the diffraction peaks with similar relative intensity as suggested in the JCPDS data are observed in
the experimental XRD pattern and are significantly sharp,
hence confirming the purity and high crystallinity of the
synthesized ZrO2 sample. The XRD of the as-prepared
gel after drying was recorded and exhibits no characteristic peaks showing that the dried gel is amorphous.
The strongest diffraction peak at 30.1º corresponds to
the (111) plane of t-ZrO2. Small diffraction peaks, are
attributed to the monoclinic phase (reflections at 28.3º,
48
Fig. 1. (a) IR spectrum of ZrO2-x(OH)2x.yH2O gel and (b) IR spectrum of ZrO2 powder annealed at 600 ºC.
indexed as [111] and 55.2º, indexed as [130], PDF #
811314 and PDF # 652357). A particle with tetragonal
structure has lower surface energy than that of monoclinic structure when the size is very small. As a result,
the smaller particle is preferentially stabilized with the
tetragonal structure, and a larger one with the monoclinic
structure. Results of Garvie showed that below 30 nm the
metastable t-ZrO2 is stable in nanocrystal ZrO2 and above
this value m-ZrO2 will be stable17. The results of Shukla
reveals that metastable t-ZrO2 can exist in large size (500–
600 nm) at room temperature18 much larger than the critical size reported in literature. For most of the reported
Kurakaran et al. : Synthesis and characterization of zirconium oxide nanoparticle prepared etc.
Fig. 2. XRD profile of ZrO2 powder synthesized at 600 ºC.
ZrO2 samples synthesized by hydrothermal techniques,
the as synthesized products are of mainly tetragonal or
cubic or mixed phases19–21 and they attain monoclinic
phase only after calcinations or annealing at higher temperature (>1000 ºC). In this case, the soft conditions
prefer the formation of t-ZrO2.
Thermal analysis :
The gel prepared as such was subjected to thermal
analysis. The TG curve (Fig. 3a) shows, three distinct
inflections, corresponding to the loss of adsorbed and
embedded water molecules and also the dehydration of
the gel leading to the formation of zirconia nanoparticles.
The total weight loss is 27.5%. The DSC curve (Fig. 3b)
shows one endothermic peak at 110 ºC and an exothermic peak at 460 ºC. The DSC and TG analysis compared
with XRD results indicate that the exothermic process
between 349 and 460 ºC can be associated to the nucleation of tetragonal phase and the transformation into this
phase from amorphous state. The endothermic peak at
110 ºC, corresponds to the dehydration process.
Raman spectroscopy :
Amorphous, monoclinic, tetragonal and cubic phases
have been observed in ZrO2. These phases may be identified from one another by means of Raman scattering.
On the basis of experimental results22,23, tetragonal ZrO2
is expected to yield a spectrum consisting of six bands
Fig. 3. (a) TG curve during dehydration process of ZrO2-x(OH)2x.
yH2O between RT and 800 ºC at a heating rate of 10 ºC
min–1 and (b) DSC profile of ZrO2-x(OH)2x.yH2O between
RT and 700 ºC at a heating rate of 10 ºC min–1.
49
J. Indian Chem. Soc., Vol. 91, January 2014
with frequencies at about 146, 267, 315, 456, 607 and
645 cm–1.
In Fig. 4, the Raman spectrum of the ZrO2 nanoparticles, annealed at 600 ºC, in the 100–800 cm–1 region is presented. At 600 ºC, the tetragonal phase is
clearly evidenced with bands at 148, 268, 317, 462 and
647 cm–1. One small band at 187 cm–1 can be observed
due to the monoclinic phase.
Fig. 4. Raman spectrum of ZrO2 powder annealed at 600 ºC.
Crystal structure analysis by TEM :
Further, the structural information of the ZrO 2
nanostructure was studied by TEM, high resolution TEM
(HRTEM), and selected area electron diffraction (SAED).
TEM image of ZrO2 showing tiny cylindrical particles
with diameters below 25 nm and particles with regular
cylindrical shaped nanostructures. Average aspect ratio
of ZrO2 nanopowder is found to be 5 : 15.
The SAED pattern and HRTEM image of ZrO 2
nanostructures in the form of cylindrical shaped particles
are shown in Fig. 5(a) and (b) with TEM image in the
inset respectively. The HRTEM images show well resolved lattice fringes indicating the single crystalline nature and high crystallinity of the synthesized product.
The lattice fringes are of equidistance and clear all along
the length of the crystal, without any lattice mismatch.
These fringes are separated by 0.316 nm, which agrees
well with the interplanar spacing corresponding to the
(111) plane of t-ZrO2. The equally spaced reflections in
the SAED pattern and lattice fringes correspond to the
50
Fig. 5. (a) SAED pattern and (b) HRTEM image of ZrO2 annealed
at 600 ºC with TEM image in the inset.
lattice planes of t-ZrO2, hence suggesting the purity and
high crystallinity of the synthesized ZrO2 product.
UV-Visible absorption spectroscopy :
UV-Vis absorption spectrum recorded for the ZrO2
nanomaterial, in the wavelength range 200–400 nm is as
shown in Fig. 6. UV-Vis spectrum shows a sharp absorption peak centered at around 226 nm. This can arise due
to the transition between the valence band and the conduction band24. The optical band gap determined for the
ZrO2 nanoparticle from the absorption spectrum is 5.6
eV.
Experimental
Synthesis of ZrO2 nanoparticles :
ZrO2 nanoparticle was synthesized by a simple aqueous gelation technique. Zirconyl oxychloride hexahydrate
Kurakaran et al. : Synthesis and characterization of zirconium oxide nanoparticle prepared etc.
Fig. 6. UV-Vis spectrum of ZrO2 nanomaterial annealed at 600 ºC.
ZrOCl 2.8H2O (E. Merck, India) and sodium acetate
(CH3COONa, E. Merck, India) were used as the starting
materials for the synthesis. All the chemicals were of
analytical grade and used without further purification.
ZrOCl2.8H2O (0.01 M) was taken in an autoclavable bottle
and was stirred with a magnetic stirrer. An equal volume
of CH3COONa (0.01 M) solution was added dropwise to
the zirconium salt with constant stirring. The clear homogeneous mixture obtained was then maintained at 110 ºC
for 6 h, resulting in the formation of a gel. The gel was
centrifuged using a high speed centrifuge and washed
several times with distilled water and dried in air. The
white powder so obtained was calcined in an alumina
crucible in a muffle furnace at 600 ºC for 4 h. Fine white
powder of zirconia thus obtained was subjected to instrumental analysis.
Structural characterization :
The X-ray diffractogram (XRD) was recorded on a
Siemens D-5000 X-ray diffractometer with a Cu K radiation (wavelength 1.54 Å). The observed data was compared with JCPDS standards. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) were done on a JEOL 3010 machine operating at an accelerating voltage of 300 kV. For
TEM analysis, the ZrO2 was ultrasonically dispersed in
ethanol and then spread on carbon-coated copper grids
and dried under vacuum. UV-Visible spectrum was recorded on a Shimadzu UV-3600 instrument. For the spectroscopic analysis, the nanomaterial was dispersed in ethaJICS-7
nol by sonication. Thermal analysis was done on a
Shimadzu DPG-60 in the range of RT to 800 ºC at a
heating rate of 10 ºC/min in nitrogen atmosphere at a
flow rate of 100 cm3 min–1. The differential scanning
calorimetry (DSC) traces were obtained using a Dupont
DSC 2010 differential scanning calorimeter attached to
Thermal Analyst 2100 data solution. The samples were
heated from RT to 700 ºC in N2 atmosphere at a heating
rate of 10 ºC/min. The infrared (IR) spectrum of the
samples were recorded in the range 400–4000 cm–1 on a
Fourier transform infrared (FTIR) spectrometer (model
Thermo-Nicolet Avatar 370) using the KBr pellet method.
Raman spectroscopy measurements were performed at
room temperature on a computerized Spex 1403 double
monochromator in combination with the 514.5 nm line
on an Argon Laser at a power level of 50 mW at the
Laser head.
Conclusion
ZrO2 nanoparticles were synthesized by aqueous gelation using zirconium oxychloride as the starting material. Surface morphology analysis confirms the formation
of cylindrical crystalline nanostructure. XRD, HRTEM
and SAED studies show that the synthesized ZrO2 is tetragonal with minor monoclinic phase. Tetragonal phase
is clearly evidenced by Raman spectrum. The DSC and
TG analysis compared with XRD results indicate that the
exothermic process about 349 and 460 ºC can be associated to the nucleation process of tetragonal phase and the
transformation to this phase from amorphous state. The
transformation of ZrO2-x(OH)2x.yH2O into tetragonal zirconia by aqueous gelation process has been proved. The
present technique provides a simple and efficient route
for the synthesis of cylindrical nanoparticles with controlled morphology, purity, uniform composition and high
crystallinity.
Acknowledgement
The authors are grateful to the Chemistry Department
of Mar Ivanios College, Thiruvananthapuram.
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