Nanocrystalline oxalate/carbonate precursors of Ce and Zr and their decompositions to CeO2 and ZrO2 nanoparticles

Article abstract of DOI:10.1111/j.1551-2916.2007.01484.x

The oxalate and carbonate precursors of cerium and zirconium have been prepared using reverse micelles as nanoreactors. Cerium oxalate precursor on thermal decomposition leads to a mixture of nanorods and nanoparticles of cerium oxide (nanoparticles of 10 nm and nanorods with 7 nm diameter and 30 nm length). Cerium oxide with crystallite size of 10 nm was obtained from cerium carbonate precursor. Monodispersed nanoparticles of zirconia with an average size of 3-5 and 12 nm were obtained from the oxalate and carbonate precursor, respectively. Detailed dielectric properties of sintered discs of nanocrystalline ceria and zirconia have been studied with variation of frequency and temperature.

Full text of DOI:10.1111/j.1551-2916.2007.01484.x

J. Am. Ceram. Soc., 90 [3] 863–869 (2007)
DOI: 10.1111/j.1551-2916.2007.01484.x
r 2007 The American Ceramic Society
ournal
J
Nanocrystalline Oxalate/Carbonate Precursors of Ce and Zr and Their
Decompositions to CeO2 and ZrO2 Nanoparticles
Sonalika Vaidya, Tokeer Ahmad, Suman Agarwal, and Ashok K. Ganguliw
Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India
The oxalate and carbonate precursors of cerium and zirconium
have been prepared using reverse micelles as nanoreactors. Ce-
rium oxalate precursor on thermal decomposition leads to a
mixture of nanorods and nanoparticles of cerium oxide (nano-
particles of 10 nm and nanorods with 7 nm diameter and 30 nm
length). Cerium oxide with crystallite size of 10 nm was obtained
from cerium carbonate precursor. Monodispersed nanoparticles
of zirconia with an average size of 3–5 and 12 nm were obtained
from the oxalate and carbonate precursor, respectively. Detailed
dielectric properties of sintered discs of nanocrystalline ceria
and zirconia have been studied with variation of frequency and
temperature.
reverse micelles have been found to be a versatile route to syn-
thesize a variety of nanomaterials viz. nanorods,8,9 nanoparti-
cles,10 and core shell nanostructures.11 Reverse micelles act as a
nanoreactor and the reactants are contained in the aqueous
core. The precipitation of the product is confined to the small
core of the reverse micelle which gives rise to highly monodis-
persed and homogeneous particles. We have earlier obtained
several oxides of transition metals with variable oxidation states
using metal oxalate nanorods as precursors12 by the reverse mi-
cellar route. In this report we discuss our efforts to utilize this
method to synthesize nanocrystalline (oxalate and carbonate)
precursors, which on decomposition led to the formation of
ceria and zirconia nanoparticles. Dielectric properties of the
sintered discs of ceria and zirconia have been investigated.
I. Introduction
OTH CeO2 and ZrO2 are refractory oxides with several ap-
plications. CeO2 has a high refractive index, strong adhe-
II. Experimental Procedure
B
CeO2 nanoparticles were synthesized by the thermal decompos-
ition of cerium oxalate and cerium carbonate precursors. The
precursors were synthesized by the reverse micellar (microemul-
sion) route with CTAB (cetyl trimethyl ammoniumbromide;
Spectrochem, Mumbai, India) as the surfactant, 1-butanol
(Qualigen, Mumbai, India) as the co-surfactant and iso-octane
(Spectrochem) as the nonpolar solvent. The weight fraction of
various constituents in the microemulsion was 16.76% of
CTAB, 13.9% of n-butanol, 59.29% of isooctane, and 10.05%
of aqueous phase. For the synthesis of cerium oxalate precursor
two different microemulsions, one containing 0.1M aqueous so-
lution of cerium nitrate hexahydrate (CDH, New Delhi, India)
and the other containing the aqueous solution of ammonium
oxalate (Merck, Mumbai, India), were slowly mixed and stirred
for 15 h. The product was separated from microemulsions by
centrifugation and washed with 1:1 mixture of chloroform and
methanol and dried at room temperature. The product was de-
composed at 5001C for 6 h to obtain cerium oxide. The synthetic
procedure of obtaining the cerium carbonate precursor is similar
as discussed above for cerium oxalate except that the second
microemulsion contains 0.1M aqueous solution of ammonium
carbonate (Merck) instead of ammonium oxalate. Cerium car-
bonate precursor was heated at 3501C for 6 h and subsequently
at 5001C for 8 h to obtain pure CeO2 nanoparticles.
ZrO2 nanoparticles were also obtained from two different
precursors (a) zirconium oxalate and (b) zirconium carbonate.
The precursors were synthesized by the reverse micellar route
with CTAB as the surfactant. The compositions of the micro-
emulsions were same as mentioned above for the cerium pre-
cursors. Two different microemulsions, one containing zirconyl
oxychloride and the other containing ammonium oxalate, were
used to synthesize zirconium oxalate. For obtaining the carbon-
ate one of the microemulsion was prepared with ammonium
carbonate. The precursors were decomposed at 5001C for 6 h to
obtain ZrO2. Effect of temperature on the structural transfor-
mation was studied by further heating the oxides at higher tem-
peratures.
sion, and stability toward high temperature, chemical attack,
and mechanical abrasion. It has been used for a variety of ap-
plications such as fuel cells,1 gas sensors,2 NO removal,3 counter
electrodes in smart window devices, and humidity sensors.4 Na-
nocrystalline CeO2 particles have been synthesized earlier by
precipitation method.5 Morphology of the particles changed
from square-like faces to hexagonal faces with increase in tem-
perature (hexagonal-shaped particles were also observed in an
atmosphere with low oxygen content). The size of the particle
also varied with temperature and atmospheric condition. It was
found to increase from 7 to 18 nm with temperature, while the
size decreased with increase in the oxygen content (16 nm in 5%
O2 and 9 nm in pure O2).5
Zirconia is known to exist in three different structures viz.
monoclinic (thermodynamically most stable form), tetragonal,
and cubic. The monoclinic phase exists below 11701C, tetrago-
nal between 11701 and 23701C, while above 23701C the cubic
form becomes stable. Zirconia has been used to replace Si as the
dielectric material in metal-oxide–metal capacitors for dynamic
random access memory devices6 because of its high dielectric
constant, thermal stability, and large band gap. Tetragonal
ZrO2 shows good ionic conductivity and possesses high strength.
Tetragonal ZrO2 with minor impurity of the monoclinic phase
has been synthesized via the sol–gel process from zirconyl ox-
alate.7 Owing to interesting applications of both CeO2 and
ZrO2, we have investigated the structure and properties of na-
nocrystalline CeO2 and ZrO2. These nanoparticles were ob-
tained by the thermal decomposition of the precursors namely,
cerium oxalate, cerium carbonate, zirconium oxalate, and zir-
conium carbonate. The oxalate and carbonate precursors have
been synthesized using reverse micelles. Chemical reactions in
P. Paranthaman—contributing editor
Manuscript No. 22070. Received August 1, 2006; approved November 6, 2006.
A. K. G. thanks the Department of Science & Technology, India, for financial support.
S. V. and T. A. thank CSIR, Government of India, for a fellowship.
wAuthor to whom correspondence should be addressed. e-mail: ashok@chemistry.
iitd.ernet.in
Powder X-ray diffraction (PXRD) studies were carried out
on a Bruker D8 Advance diffractometer (Bruker AXS GmbH,
Karlsruhe, Germany) using Ni-filtered CuKa radiation. Normal
863

864
Journal of the American Ceramic Society—Vaidya et al.
Vol. 90, No. 3
scans were recorded with a step size of 0.021 and step time of 1 s.
Raw data was subjected to background corrections and Ka2
lines were stripped off. The grain size was calculated using the
Scherrer’s formula13 (t 5 0.9l/Bcosy) where t is the diameter of
the grain, l is the wavelength of the radiation (l for CuKa is
˚
1.5418 A) and B is the line broadening which is measured from
the full-width at half-maximum (FWHM) and calculated by the
Warren’s formula14,15; B2 5 (BM2 –BS2) where BM is the FWHM
of the sample and BS is the FWHM of the standard quartz with
a grain size of around 2 mm. The cell parameters were deter-
mined using a least square fitting procedure on all reflections
using quartz as the external standard. TGA/DTA experiments
were carried out on Perkin Elmer Pyris Diamond TGA/DTA
system (Wellesley, MA) on well-ground samples in flowing ni-
trogen atmosphere with a heating rate of 101C/min. TEM and
HRTEM studies were carried out on JEOL JEM 3010 electron
microscope (Tokyo, Japan) operated at 300 kV.
Scanning electron microscopy (SEM) studies were carried out
on Carl Zeiss EVO 50 WDS electron microscope (Oberkochen,
Germany). Dielectric measurements were carried on discs of (a)
CeO2 nanoparticles (synthesized from cerium oxalate) sintered
at 8001 and 10001C, (b) CeO2 nanoparticles (synthesized from
cerium carbonate) sintered at 8001C, (c) ZrO2 nanoparticles
(synthesized from zirconium oxalate) sintered at 10001C, and (d)
ZrO2 nanoparticles (synthesized from zirconium carbonate) sin-
tered at 8001 and 10001C. The measurements were carried out
using Hewlett Packard 4284L multifrequency LCR meter (Santa
Clara, CA) in the range of 50–500 kHz. Temperature variation
studies of the dielectric constant and dielectric loss were carried
out in the range of 351–3001C.
Fig. 2. TGA/DTA plot for the decomposition of cerium oxalate. The
bold line represents the TGA curve. The dotted line represents the DTA
curve.
dried at room temperature. PXRD studies show that all the re-
flections could be indexed on the monoclinic cell corresponding
to cerium oxalate decahydrate with refined lattice parameters of
˚
˚
˚
a 5 11.311(3) A, b 5 9.653(3) A, c 5 10.390(3) A, and b 5 114.51
(JCPDS No. 20-0268) (Fig. 1(a)). The first weight loss in the
TG/DTA plot corresponds to the loss of 10 water molecules
whereas the second weight loss corresponds to the loss of CO
and CO2 from the decomposition of cerium oxalate to form
cerium oxide (CeO2) (Fig. 2).
III. Result and Discussion
The product obtained after centrifugation of the mixture of
microemulsions containing Ce(III) ions and C2O24À ions was
a
80
70
60
50
40
30
20
10
0
SEM studies (Fig. 3) showed that the cerium oxalate particles
are spherical in shape and are agglomerated. HRTEM studies
confirm the nanocrystalline nature of nanoparticles (Fig. 4). The
particle size obtained for the cerium oxalate nanoparticles varied
from 4–6 nm as seen from the HRTEM image. Note that in our
earlier8,9,12 studies we have reported the formation of nanorods
(not nanoparticles) of transition metal oxalates. However, in the
current studies we have synthesized cerium oxalate in spherical
shape as observed from the SEM image (Fig. 3). The metals (Cu,
Ni, Mn, and Zn), used in the earlier reports pertaining to metal
oxalate nanorods, belong to the first row of the transition metal
series and were divalent, whereas in the current study trivalent
Ce has been used as the metal source. The availability of a di-
valent metal ion (in the earlier8,9,12 studies) allows a 1:1 stoichi-
5
10
20
30
2 Theta-Scale
190
b
180
170
160
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
10
20
30
40
50
60
70
80
90
2-Theta - Scale
Fig. 1. Powder X-ray diffraction pattern of (a) monoclinic cerium
oxalate decahydrate and (b) cubic cerium oxide.
Fig. 3. Scanning electron microscopy image for cerium oxalate synthe-
sized at room temperature.

March 2007
Nanocrystalline Oxalate/Carbonate Precursors of Ce and Zr
865
<111>
<111>
<110>
<111>
5 nm
Fig. 4. TEM images for cerium oxalate synthesized at room temperature.
Fig. 6. HRTEM images of cerium oxide (from the oxalate precursor)
showing the presence of nanorods and nanoparticles. Inset shows a
nanorod of cerium oxide obtained from cerium oxalate.
ometry with the oxalate ion and also is appropriate for the for-
mation of a linear geometry between oxalate ligands and the
M21 ion (Fig. 5). In the case of cerium this type of 1:1 coord-
ination is not possible (due to its trivalent oxidation state) and
this probably affects the formation of the linear geometry and
hence the rod-like feature as known for other metal oxalates. An
earlier study on the formation of cerium oxalate nanoparticles
has been reported with different microemulsion systems16 using
methyl oxalate as the oxalate source. The average size of cerium
oxalate nanoparticles at W 5 14 (W 5 [water]/[surfactant]) with
a reaction time of 30 min was reported as B25 nm. The authors
report that these values have been calculated by TEM (no TEM
images were given) and DLS. The authors have also not men-
tioned the shape of the particles obtained. The cerium oxalate
precursor, obtained in our study, was decomposed at 5001C for
6 h to obtain nanocrystalline CeO2. The PXRD pattern
lated using X-ray line broadening studies and was found to be
around 10 nm.
The PXRD pattern of zirconium oxalate precursor shows a
broad hump, indicating the amorphous nature of the precursor.
From the TGA plot (Fig. 8) for zirconium oxalate, two regions
of weight loss are observed. The first is due to the loss of one
water molecule at 1251C. The second weight loss is due to the
loss of oxides of carbon from the decomposition of the oxalate.
In the DTA curves we observe three endotherms. The first cor-
responds to the loss of water molecule while the second peak at
B3401C corresponds to the decomposition of the oxalate to
form oxide. However, the oxide formed at this temperature was
amorphous (from XRD studies). Note that XRD studies of the
oxide obtained at 500 C suggests its crystalline nature and was
indexed as mainly tetragonal ZrO2. This suggests that decom-
˚
(Fig. 1(b)) could be indexed on cubic cell with a 5 5.411(3) A
(JCPDS No. 81-0792). The crystallite size calculated from the
Scherrer’s formula was 15 nm. HRTEM studies show the for-
mation of both particles and nanorods for CeO2 (Fig. 6). The
nanoparticles of CeO2 are 10 nm in size; however the dimensions
of the nanorods are 7 nm in width and around 30 nm in length
as seen from the HRTEM image (Fig. 6). It seems that the par-
ticles are aggregating to form rods as observed in Fig. 7. Earlier
study17 report the synthesis of cerium oxide nanoparticles (3.7
nm, no sign of any rods). Note that this study employed differ-
ent microemulsion systems.17 PXRD pattern of CeO2, taken
after sintering at 8001 and 10001C, showed the presence of cubic
CeO2. The cerium carbonate precursor (which was amorphous
at room temperature) was decomposed at 3501C to obtain ce-
rium oxide nanoparticles. The oxide nanoparticles were again
heated at 5001C to improve the crystallinity of the nanoparticles.
The crystallite size of the cerium oxide nanoparticle (obtained
from cerium carbonate precursor) heated at 5001C was calcu-
<111>
O
O
O
O
O
O
C
C
M
C
M
C
C
C
5 nm
O
O
O
O
O
O
Fig. 5. Schematic diagram showing the linkages of a divalent, tetra-
coordinated metal ion with oxalate ligand.
Fig. 7. HRTEM images of cerium oxide (from cerium oxalate) showing
the growth of nanorods.

866
Journal of the American Ceramic Society—Vaidya et al.
Vol. 90, No. 3
a
Tetragonal
Monoclinic
100
80
60
40
20
0
%
500
800
1000
Temperature (°C)
Tetragonal
Monoclinic
b
Fig. 8. TGA/DTA plot for the decomposition of zirconium oxalate.
The bold line represents the TGA curve. The dotted line represents the
DTA curve.
100
75
50
25
0
position of the oxalate initially leads to an amorphous oxide,
which transforms to a crystalline form at higher temperature.
The exact temperature is evident from the third endotherm in
the DTA curve (at B4751C) and appears to be a structural
phase transition (amorphous to crystalline) as no weight loss is
seen in the TGA curve. Tetragonal ZrO2 with minor impurity of
monoclinic phase (10%) was obtained when the oxalate precur-
sor was heated at 5001C for 6 h (Fig. 9(a)). The refined unit cell
%
500
800
1000
Temperature
Fig. 10. Variation in the phases of ZrO2 with temperature for the oxide
obtained by the decomposition of (a) zirconium oxalate and (b) zirco-
nium carbonate.
a
parameters were calculated for the tetragonal cell as a 5 3.957(9)
#
˚
˚
A, c 5 5.16(3) A. The inset of Fig. 9(a) shows the tetragonal
splitting of ZrO2. It is to be noted that we have been successful in
stabilizing the higher temperature tetragonal phase (tetragonal
phase is stabilized between 11701 and 23701C) at low tempera-
ture. An earlier report on the synthesis of zirconia from zirco-
nium oxalate by sol–gel process also show that a minor amount
of monoclinic phase appears on the decomposition of oxalate7
which is similar to our study. Further heating at 8001C led to
increase in the monoclinic phase and at 10001C pure monoclinic
phase was obtained. Variation in the concentration (with tem-
perature) of the two modifications of zirconia formed from the
oxalate precursor is shown in Fig. 10(a).
200
* Monoclinic
# Tetragonal
#
100
#
#
#
#
#
#
#
*
0
Zirconium carbonate precursor was also amorphous as ob-
served from the PXRD pattern. A mixture of tetragonal and
monoclinic ZrO2 phases (1:1 ratio) was obtained when the car-
bonate precursor was heated at 5001C for 6 h (Fig. 9(b)). Figure
10(b) shows the variation in the amount of the two phases of
ZrO2 with temperature using carbonate precursor. Tetragonal
ZrO2 is reported to degrade to the monoclinic phase in presence
of water when annealed at low temperature.18 It is possible that
since the carbonate precursor has more water content as ob-
served from thermogravimetric studies, a mixture of monoclinic
and tetragonal phase was formed when the carbonate precursor
was heated at 5001C. The crystallite size of zirconia particles,
obtained by the thermal decomposition of zirconium oxalate at
5001C, was found to be 12 nm from the X-ray line broadening
studies, however TEM studies show the spherical grains of sizes
in the range of 3–5 nm (Fig. 11(a)). TEM studies, as shown in
Fig. 11(b), for the ZrO2 particles obtained from the carbonate
precursor shows the monodispersed nanoparticles of sizes 12
nm. HRTEM images of the ZrO2 nanoparticles synthesized
from the decomposition of zirconium oxalate and carbonate
precursor is shown in Fig. 12(a) and (b), respectively. Tetragonal
phase could be seen from the HRTEM image of zirconia ob-
tained by the thermal decomposition of the oxalate precursor.
10
20
30
40
50
60
70
80
90
2 Theta - Scale
600
400
200
0
b
*
Monoclinic
*
+ Tetragonal
*
+
*
+
+
*
+
*
*
*
*
*
+
*
*
*
*
*
*
*
20
30
40
50
60
70
2 Theta Scale
Fig. 9. PXRD pattern for ZrO2 obtained by the decomposition of (a)
zirconium oxalate and (b) zirconium carbonate at 5001C for 6 h. The
inset of Fig. 6(a) shows the tetragonal splitting confirming the presence
of tetragonal ZrO2.

March 2007
Nanocrystalline Oxalate/Carbonate Precursors of Ce and Zr
867
a
a
<101>
Tetragonal ZrO
2
50 nm
5 nm
b
b
<111>
monoclinic
<100>
Tetragonal
50 nm
Fig. 11. TEM images for ZrO2 synthesized by the decomposition of (a)
zirconium oxalate. The inset shows the area containing number of na-
noparticles. (b) ZrO2 nanoparticles obtained from zirconium carbonate.
2 nm
Fig. 12. HRTEM images for ZrO2 synthesized by the decomposition of
(a) zirconium oxalate and (b) zirconium carbonate.
Figure 12(b) shows the presence of both tetragonal and mono-
clinic grains for the zirconia obtained from the carbonate pre-
cursor. This also corroborates with the fact that 1:1 mixture of
tetragonal and monoclinic phase was obtained from the car-
bonate precursor. Microemulsion based synthesis of zirconium
oxalate and subsequently the formation of zirconia nanoparti-
cles have been reported earlier.19 The oxalate precursor in the
above report was synthesized from single microemulsion as well
as double microemulsion systems using a mixture of two non-
ionic surfactants (poly (oxyethylene)9 nonyl phenol ether
(NP9)1poly (oxyethylene)5 nonyl phenol ether (NP5)). The de-
composition of the oxalate precursors at 6001C obtained from
these two methods led to zirconia particles of larger size (559
and 295 nm, respectively). However, we were successful in ob-
taining 3–5 nm zirconia nanoparticles using the oxalate precur-
sor (synthesized using a cationic surfactant). Zirconia powder
(crystallite size 8.5 nm) has also been synthesized by the calci-
nation of zirconium hydroxide precursor20 where hydroxide
precursor (5–10 nm in size) was prepared by the reverse micel-
lar route. However, there are no TEM images to confirm the size
for the zirconia nanoparticles in the above study.
also studied on disc sintered at 10001C (Fig. 13(a)). The dielec-
tric constant and the loss were found to be stable with fre-
quency. The dielectric constant was nearly stable with
temperature, while the dielectric loss shows a constant value
till 2001C beyond which it rises sharply (Fig. 13(b)). The dielec-
tric constant for bulk CeO2 was earlier reported21 to be 26.
However, there was no mention of the grain size and the sin-
tering temperature. Dielectric properties of CeO2 obtained from
the carbonate precursor were studied on a disc sintered at 8001C
(Table I). Dielectric constant at room temperature was found to
be stable with frequency (B21 at 500 kHz). Temperature vari-
ation study shows that dielectric constant at 500 kHz attained
minima at 601C with a value of 24.5 and thereafter it increased
to a value of 27 at 3001C. Dielectric loss at room temperature
Table I. Dielectric Constant and Loss at 500 kHZ and Room
Temperature for CeO2 and ZrO2
Sintering
Dielectric properties have been measured on sintered disks of
nanocrystalline CeO2 and ZrO2. Sintered discs of CeO2 (ob-
tained from the oxalate precursor) at 8001C show a dielectric
constant of B19 at 500 kHz and dielectric loss of B0.11 at 500
kHz (Table I). Both the dielectric constant and dielectric loss
were stable with frequency at room temperature. Temperature
variation studies show that the dielectric constant was stable
with temperature at 500 kHz, however the dielectric loss shows a
minimum at 1501C with a value of 0.028. Dielectric properties of
CeO2 nanoparticles synthesized from the oxalate precursor were
Sl.
temperature
no. Compound
Precursor
(1C)
e
D
1.
2.
3.
4.
5.
6.
CeO2
CeO2
CeO2
ZrO2
ZrO2
ZrO2
Cerium oxalate
Cerium oxalate
Cerium carbonate
Zirconium oxalate
Zirconium carbonate
Zirconium carbonate
800
1000
800
1000
800
1000
19 0.11
15 0.08
21 0.59
12 0.31
10 0.012
11 0.099

868
Journal of the American Ceramic Society—Vaidya et al.
Vol. 90, No. 3
a
a
0.9
0.6
0.3
60
40
20
0
1
120
80
40
0
0.8
0.6
0.4
0.2
0
D
ε
ε
D
0
0
200000
400000
600000
0
200000
400000
600000
Frequency (Hz)
Frequency (Hz)
At 500 kHz
b
16
0.45
0.3
0.15
0
30
0.15
0.1
b
D
ꢀ
ε
D
500 kHz
12
8
20
10
0
0.05
4
0
0
0
100
200
300
400
0
100
200
300
400
Temperature
Temperature
Fig. 14. Variation of the dielectric constant and dielectric loss of ZrO2
obtained by the decomposition of zirconium oxalate with (a) frequency
and (b) temperature (at 500 kHz) for the pellets sintered at 10001C.
Fig. 13. Variation of the dielectric constant and dielectric loss of
CeO2 (obtained from the oxalate precursor) with (a) frequency and (b)
temperature (at 500 kHz) for the pellets sintered at 10001C.
increased to a maximum (B0.59) at 500 kHz. A minima at
1001C (B0.076 at 500 kHz) was obtained for the loss, in the
temperature variation studies.
ever, the doping of CaO to ZrO2 led to the formation of 95%
tetragonal phase22 and the dielectric constant increases to 34.
These studies indicate the increase of dielectric constant in Y and
Ca-stabilized ZrO2.
For ZrO2 nanoparticles synthesized from the oxalate precur-
sor, the dielectric properties on the discs sintered at 10001C are
shown in Figs. 14(a) and (b). The dielectric constant (B12 at 500
kHz) and the loss (B0.31 at 500 kHz) were highly stable with
frequency at room temperature. Temperature variation studies
show that the dielectric constant was stable after 1001C, how-
ever the dielectric loss falls to B0.03 at 1001C from 0.31 at room
temperature.
IV. Conclusions
Nanocrystalline ceria (crystallite size of 15 nm) and zirconia (3–5
nm) could be obtained at 5001C from oxalate and carbonate
precursors, which were synthesized using reverse micelles. Mix-
ture of nanoparticles and nanorods of ceria were obtained by the
thermal decomposition of cerium oxalate precursor. Zirconium
oxalate precursor was found to be more suitable for the synthe-
sis of nearly pure (90%) nanocrystalline ZrO2 (3–5 nm) in the
tetragonal modification, while the carbonate precursor gave an
equal amount of tetragonal and monoclinic zirconia at 5001C.
Both the precursors of Zr led to the formation of the pure mon-
oclinic phase of ZrO2 at 10001C.
The dielectric properties of nanocrystalline zirconia (synthe-
sized by the decomposition of the carbonate precursor) have
been carried out at sintering temperatures of 8001 and 10001C
(Table I). Dielectric constant and loss at room temperature was
found to be 10 and 0.012, respectively, at 500 kHz, for the pellets
sintered at 8001C. A minima in the dielectric loss was observed
at 1501C (B0.015 at 500 kHz). Note that the dielectric loss of
0.015 is very low for an oxide sintered at a low temperature of
8001C. The dielectric constant was found to be 11 at 500 kHz for
the ZrO2 discs sintered at 10001C and it was stable with fre-
quency and temperature. Temperature variation studies on
ZrO2 disc obtained from both carbonate and oxalate precursor
showed that the loss increased with temperature for the particles
obtained from the carbonate precursor. However, the loss stead-
ily decreases with temperature for the particle obtained by the
decomposition of oxalate precursor. The dielectric loss for the
ZrO2 particles obtained from the carbonate precursor (B0.099
at room temperature at 500 kHz) was also stable with frequency
but increased with temperature. It may be noted that the ZrO2
obtained from the carbonate precursor shows a much lower
value of dielectric loss (B0.015) at a low sintering temperature
of 8001C. The dielectric constant for pure monoclinic zirconia
(from the carbonate precursor), obtained when the sintering of
the discs was done at 10001C, was found to be B11 at 100 kHz.
(Pure tetragonal ZrO2 is not known at room temperature and so
its dielectric constant cannot be compared.) At sintering tem-
perature of 8001C, a mixture of monoclinic and tetragonal ZrO2
in the ratio of 80:20 was obtained and the dielectric constant was
found to be B11. It has been reported22 that the dielectric con-
stant for pure monoclinic zirconia is B22 at 10 kHz. Addition of
14 wt% CeO2 to zirconia results in a mixture of monoclinic
(44%) and tetragonal phase and the dielectric constant for this
Ce-stabilized zirconia was reported to be 27 at 10 kHz. How-
Acknowledgment
The authors also thank Prof. T. Pradeep, Department of Chemistry, IIT
Madras, for the HRTEM studies.
References
1B. C. H. Steele and A. Heinzel, ‘‘Materials for Fuel-Cell Technologies,’’ Nature,
414, 345–52 (2001).
2F. H. Garzon, R. Mukundan, and E. L. Brosha, ‘‘Solid-State Mixed Potential
Gas Sensors: Theory, Experiments and Challenges,’’ Solid State Ionics, 136/137,
633–8 (2000).
3R. Dimonte, P. Fornasiero, M. Graziani, and J. Kagpar, ‘‘Oxygen Storage and
Catalytic NO Removal Promoted by CeO2-Containing Mixed Oxides,’’ J. Alloys
Compd., 275–277, 877–85 (1998).
4B. Elidrissi, M. Addou, M. Regragui, C. Monty, A. Bougrine, and A. Kacho-
uane, ‘‘Structural and Optical Properties of CeO2 Thin Films Prepared by Spray
Pyrolysis,’’ Thin Solid Films, 379, 23–7 (2000).
5H. I. Chen and H. Y. Chang, ‘‘Synthesis of Nanocrystalline Cerium Oxide
Particles by the Precipitation Method,’’ Ceram. Int., 31, 795–802 (2005).
6J. P. Chang and Y. S. Lin, ‘‘Highly Conformal ZrO2 Deposition for Dynamic
Random Access Memory Application,’’ J. App. Phys., 90, 2964–9 (2001).
7J. Etienne, A. Larbot, A. Julbe, C. Guizard, and L. Cot, ‘‘A Microporous
Zirconia Membrane Prepared by the Sol–Gel Process from Zirconyl Oxalate,’’
J. Membr. Sci., 86, 95–102 (1994).
8T. Ahmad, R. Chopra, K. V. Ramanujachary, S. E. Lofland, and A. K.
Ganguli, ‘‘Nanorods of Copper and Nickel Oxalates Synthesized by the Reverse
Micellar Route,’’ J. Nanosci. Nanotech., 5, 1840–5 (2005).

March 2007
Nanocrystalline Oxalate/Carbonate Precursors of Ce and Zr
869
9T. Ahmad, S. Vaidya, N. Sarkar, S. Ghosh, and A. K. Ganguli, ‘‘Zinc Oxalate
Nanorods: A Convenient Precursor to Uniform Nanoparticles of ZnO,’’ Nano-
technology, 17, 1236–40 (2006).
16Y. He, ‘‘Study on the Formation Mechanism of Cerium Oxalate Nanoparti-
cles from the Coupling Route of Homogeneous Precipitation with Microemul-
sion,’’ Mater. Lett., 59, 3010–3 (2005).
10T. Ahmad and A. K. Ganguli, ‘‘Reverse Micellar Route to Nanocrystalline
Titanates (SrTiO3, Sr2TiO4 and PbTiO3): Structural Aspects and Dielectric Prop-
erties,’’ J. Am. Ceram. Soc., 89, 1326–32 (2006).
17S. Sathyamurthy, K. J. Leonard, R. T. Dabestani, and M. P. Paranthaman,
‘‘Reverse Micellar Synthesis of Cerium Oxide Nanoparticles,’’ Nanotechnology, 16,
1960–4 (2005).
11F. Teng, Z. Tian, G. Xiong, and Z. Xu, ‘‘Preparation of CdS–SiO2 Core Shell
Particles and Hollow SiO2 Sphere Ranging from Nanometers to Microns in the
Nonionic Reverse Microemulsion,’’ Catalysis Today, 93–95, 651–7 (2004).
12T. Ahmad, K. V. Ramanujachary, S. E. Lofland, and A. K. Ganguli,
‘‘Nanorods of Manganese Oxalate: A Single Source Precursor to Different
Manganese Oxide Nanoparicles (MnO, Mn2O3, Mn3O4),’’ J. Mater. Chem., 14,
3406–10 (2004).
18X. Guo, ‘‘Property Degradation of Tetragonal Zirconia Induced by Low-
Temperature Defect Reaction With Water Molecules,’’ Chem. Mater., 16, 3988–94
(2004).
19J. Fang, J. Wang, S. C. Ng, C. H. Chew, and L. M. Gan, ‘‘Ultrafine Zirconia
Powders Via Microemulsion Processing Route,’’ Nanostruct. Mater., 8, 499–504
(1997).
20J. Wang, L. S. Ee, S. C. Ng, C. H. Chew, and L. M. Gan, ‘‘Reduced
Crystallization Temperature in a Microemulsion-Derived Zirconia Precursor,’’
Mater. Lett., 30, 119–24 (1997).
13L. S. Birks and H. Friedman, ‘‘Particle Size Determination from X-Ray Line
Broadening,’’ J. Appl. Phys., 17, 687–92 (1946).
14B. E. Warren and B. L. Averbach, ‘‘The Effect of Cold-Work Distortion on
X-Ray Patterns,’’ J. Appl. Phys., 21, 595–9 (1950).
21S. Logothetidis, P. Patsalas, E. K. Evangelou, N. Konofaos, I. Tsiaoussis, and
N. Frangis, ‘‘Dielectric Properties and Electronic Transitions of Porous and
Nanostructured Cerium Oxide Films,’’ Mater. Sci. Eng. B, 109, 69–73 (2004).
22D. P. Thompson, A. M. Dickins, and J. S. Thorp, ‘‘The Dielectric Properties
15B. E. Warren and B. L. Averbach, ‘‘The Separation of Cold-Work Dis-
tortion and Particle Size Broadening in X-Ray Patterns,’’ J. Appl. Phys., 23, 497
(1952).
of Zirconia,’’ J. Mater. Sci., 27, 2267–71 (1992).
&