Preparation and characterization of monodispersed YSZ nanocrystals

Article abstract of DOI:10.1021/jp010334q

A novel method for large-scale preparation of yttria-stabilized zirconia (YSZ) nanocrystals is presented. The hydrous YSZ colloidal nanoparticles are self-assembled on the surface of SrCO₃ nanoparticles by a sonochemical method. After calcination, fully crystalline monodispersed YSZ nanoparticles are obtained, and the SrCO₃ is washed out by 10% HNO₃ solution. The agglomeration of YSZ particles is inhibited as the crystallization occurs on the surface and interface of SrCO₃ nanoparticles. The nanocrystals are monodispersed with an average particle size of 4.7 nm and a high surface area of 165 m²/g. The quantum confinement effect is observed: the band gap increases from 4.13 eV for the agglomerated sample to 5.44 eV for the monodispersed YSZ nanocrystals.

Full text of DOI:10.1021/jp010334q

J. Phys. Chem. B 2001, 105, 4647-4652
4647
Preparation and Characterization of Monodispersed YSZ Nanocrystals
†
Guangsheng Pang, Siguang Chen, Yingchun Zhu, Oleg Palchik, Yuri Koltypin,
Afie Zaban, and Aharon Gedanken*
Department of Chemistry, Bar-Ilan UniVersity, Ramat-Gan 52900, Israel
ReceiVed: January 25, 2001
A novel method for large-scale preparation of yttria-stabilized zirconia (YSZ) nanocrystals is presented. The
hydrous YSZ colloidal nanoparticles are self-assembled on the surface of SrCO nanoparticles by a
sonochemical method. After calcination, fully crystalline monodispersed YSZ nanoparticles are obtained,
and the SrCO is washed out by 10% HNO solution. The agglomeration of YSZ particles is inhibited as the
crystallization occurs on the surface and interface of SrCO nanoparticles. The nanocrystals are monodispersed
3
3
3
3
2
with an average particle size of 4.7 nm and a high surface area of 165 m /g. The quantum confinement effect
is observed: the band gap increases from 4.13 eV for the agglomerated sample to 5.44 eV for the monodispersed
YSZ nanocrystals.
Introduction
decomposes in air at 350 °C resulting in SiO2 particles which
serves as the “pinning” particles. Washing the precipitate with
The development of reliable and reproducible methods for
preparing large quantities of high crystallinity and monodis-
persivity of inorganic nanocrystals remains an area of extensive
interest. Monodispersed nanoscale zirconia powder is essential
for most of the applications in ceramics and catalysis. The
uniform distribution of the size of particles is very important
for optical spectroscopy. The optical absorption of yttria-
stabilized zirconia (YSZ) has been reported as having a band
gap in the range from 4 to 5.8 eV.1-3 This wide range may
result from the fact that it is difficult to obtain good optical
quality specimens with well-controlled microstructure. The
electronic properties of nanoparticles differ from those of bulk
ethanol affects the crystallization process of the hydrous zirconia
and the textural properties of the calcined zirconia.18 Lowering
the agglomeration is ascribed to the presence of an ethoxide
1
9
group on the surface, which is formed by replacing hydroxyl
group and the coordinated water on the surface by ethanol. It
has also been reported that weakly agglomerated nanoparticles
of YSZ can be prepared by the decomposition of metal nitrate
2
0
coated on carbon powder.
Recently, several publications reported on the coating of
nanoparticles on the surfaces of submicrospherical silica and
2
1-25
alumina particles using the sonication method.
We extend
4
this method to preparing YSZ nanocrystals by self-assembling
the hydrous YSZ colloidal particles on the surface of SrCO3
nanoparticles. SrCO3 particles are selected as the substrate for
two reasons: first, they are thermally stable and their decom-
position temperature is ca. 1100 °C, and second, they dissolve
in dilute acid and can easily be washed out from the products.
After drying and calcination at higher temperature, the hydrous
YSZ particles dehydrate and crystallize on the surface of SrCO3
nanoparticles. Crystallized YSZ particles are separated by SrCO3
particles and aggregation is inhibited. This novel method is
suitable for most cases in which inorganic nanocrystals are
prepared by the precipitation method. It will yield in all cases
a monodispersed calcined product.
materials. Quantum confinement is expected, and the absorption
edge is shifted to a higher energy when the grain size decreases.
For nanocrystalline YSZ thin films, only a small band gap
energy shift of 0.25 eV is observed when the particles size
3
changes from 100 to 1 nm.
A large variety of methods have been used to prepare
5,6
nanocrystalline zirconia or YSZ, such as sol-gel precipita-
7,8
9
10
11
tion, hydrothermal, spray pyrolysis, and plasma synthesis.
Precipitation of inorganic salt solutions provides an economical
route for large quantity production of nanoscale zirconia particles
since this approach uses inexpensive inorganic salts as a starting
7,12,13
material.
Formation of hydrous zirconia particles by a low
temperature aqueous chemical method has been studied
13-15
extensively.
The as-prepared samples are usually amorphous
Experimental Section
and contain water, and further drying and calcination at high
temperature are therefore indispensable for crystallization and
dehydration. The calcined product is made up of aggregated
crystalline particles. The particle size depends on the calcination
The reactants used for the preparation of YSZ are Y2O3, ZrO-
(NO3)2.xH2O, Sr(NO3)2, and NH4HCO3 (Aldrich). The experi-
mental procedure is schematically described in Figure 1. Y2O3
and ZrO(NO3)2‚xH2O are dissolved in nitric acid and deionized
water under heating and stirring, respectively. The solutions are
then mixed together. The mole composition of the starting
reaction mixture is 0.08Y2O3:0.92ZrO(NO3)2.xH2O, and the
concentration of zirconium in the mixture solution at this point
is 0.05 M. Ammonium hydroxide is dropped into the solution
under sonication using a direct immersion titanium horn
16
temperature and the concentration of the stock solution. Several
attempts have been made to prepare monodispersed zirconia
nanocrystals. Changing the surface properties by replacing the
hydroxyl group with methyl siloxyl surface group has been used
to avoid the crystal growth.17 Methyl siloxyl surface group
*
Corresponding author. Fax: 972-3-535-1250; E-mail: gedanken@
mail.biu.ac.il.
2
†
(Vibracell, 20 kHz, 100 W/cm ). The product is centrifuged and
Permanent address: Department of Chemistry, Jilin University,
Changchun, 130023, China.
washed with deionized water. At this stage, the product is
1
0.1021/jp010334q CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/02/2001

4648 J. Phys. Chem. B, Vol. 105, No. 20, 2001
Pang et al.
Figure 1. . Preparation procedure of YSZ nanocrystals.
referred to as as-prepared hydrous YSZ. The as-prepared
hydrous YSZ is washed with ethanol and sonicated in ethanol
for another 30 min. At this stage, the product is referred to as
an ethanol-treated sample. Nano SrCO3 particles are prepared
by dropping NH4HCO3 solution into 0.05 M Sr(NO3)2 solution
under sonication and washed with water and ethanol. The SrCO3
nanopartiles are dissolved in ethanol and formed a suspension
solution. The SrCO3 nanoparticles suspension solution is added
into the hydrous YSZ colloidal solution under stirring. The
proportion of hydrous YSZ and the SrCO3 is 1:2.5 in weight.
The mixture solution is further sonicated for 10 min. The
mixture solution is evaporated by heating and stirring, dried at
the freshly prepared sample is ca. 2 nm according to the
transmission electron micrographs (TEM) result, but it is not
stable. In aqueous solution, the particle size increases after
storage a few hours, which is due to coagulation and polym-
erization. When the as-prepared hydrous YSZ particles are
dissolved in ethanol under sonication, the large shear gradients,
which are generated by the liquid motion in the vicinity of the
cavitation bubbles, prevent the coagulation of the particles.
The YSZ particles are separated in ethanol and a very stable
colloidal solution is formed. When the hydrous YSZ colloidal
2
6
solution is mixed with the SrCO suspension, a homogeneous
3
mixture is formed. The hydrous YSZ particles are self-assembled
on the surface of SrCO3 nanoparticles.
1
20 °C, and calcined at 600 °C. SrCO3 is washed out by
dissolving it in 10% HNO3 solution under sonication. The solid
product at this stage is referred to as sample C. The calcined
sample of the as-prepared hydrous YSZ and the ethanol-treated
hydrous YSZ are referred to as sample A and sample B,
respectively.
Thermogravimetric analysis (TGA) results show that both the
as-prepared and ethanol-treated hydrous YSZ contain water. For
the as-prepared sample, there is a weight loss of 30.9 wt %
when the sample is heated from room temperature to 600 °C.
Weight loss of 21.2 wt % when the sample is heated from room
temperature to 230 °C is due to the loss of surface water and
hydroxide. Weight loss of 9.7 wt % when the sample is heated
from 230 °C to 600 °C is due to the loss of intraparticle water.
The total weight loss of the ethanol-treated hydrous YSZ sample
is 18.7 wt % from room temperature to 600 °C, which is
significantly lower than that of the as-prepared sample. For the
ethanol-treated sample, the weight losses from room temperature
to 300 °C are mainly due to the loss of water. The weight losses
from 300 °C to 400 °C are due to the oxidative decomposition
of chemisorbed ethoxy group, according to the TG and DTA
The transmission electron micrographs (TEM) are obtained
by using JEOL-JEM 100SX microscopes. Samples for TEM
are prepared by placing a drop of suspension solution on a
copper grid (400 mesh, electron microscopy sciences) coated
with carbon film and then being allowed to dry in air. Powder
X-ray diffraction (XRD) is performed on a Rigaku 2028
diffractometer, with nickel filtered CuKR radiation. The particle
size is calculated from the X-ray line broadening, using the
Debye-Scherrer equation. Thermogravimetric analysis (TGA)
is performed using a Mettler Toledo TGA/SDTA851 in the
temperature range of 30-1100 °C in nitrogen atmosphere at a
heating rate of 10 °C per minute. The particle size of sample C
is measured by dynamic light scattering (DLS) with ethanol as
solvent. The DLS measurement is performed with a Coulter
N4 plus instrument at a wavelength of 632.8 nm of He-Ne
at 23°. The particle size is estimated by SDP weight analysis.
The specific surface area of the powders is measured via
the Brunauer-Emmett-Teller (BET) method with nitrogen
absorption. UV-vis diffuse reflectance spectroscopy (DRS) is
measured by Cary 500 UV-visible spectophotometer at room
temperature in the wavelength region between 200 and 800 nm.
1
8
study of ethanol washing samples. The decrease of water
content of the ethanol-treated sample indicates that there is a
partial dehydrating process taking place during the sonication
in ethanol. The pure SrCO3 nanoparticles decompose at 1000
°
C. TGA curve of the mixture of ethanol-treated hydrous YSZ
and SrCO3 reveals both the dehydration of YSZ and the
decomposition of SrCO3. There is no weight loss between 400
°
C and 600 °C. This means that the hydrous YSZ can be
dehydrated and crystallized at this temperature range, whereas
the SrCO3 nanoparticles do not decompose.
Figure 2 shows the TEM micrographs of the YSZ samples.
Sample A is ground before the TEM measurement. Figure 2a
shows that sample A consists of aggregated particles with a
particle size from a few nanometers to tens of nanometers
(Figure 2a). On the contrary, sample B consists of weakly
aggregated particle with a narrow distribution of particle sizes
of ca. 6 nm (Figure 2b). The most important reasons for the
aggregation of the hydrous YSZ are the coagulation of the
Results and Discussion
The hydrous YSZ nanoparticles are precipitated by dropping
ammonium hydroxide into a mixture of yttrium and zirconium
solution under sonication. The as-prepared hydrous YSZ nano-
particles are amorphous and contain water. The particle size of

Preparation of Monodispersed YSZ Nanocrystals
J. Phys. Chem. B, Vol. 105, No. 20, 2001 4649
Figure 2. TEM micrographs of (a) sample A; (b) sample B; (c) SrCO
3
coated with ethanol-treated hydrous YSZ; and (d) sample C.
hydrous particles in aqueous solution and the condensation of
the surface hydroxyl groups. After sonication in ethanol, the
surface groups are replaced by ethoxide. Washing the precipitate
with ethanol affects the crystallization process of the hydrous
hydrous YSZ nanoparticles are coated on the surface of the
SrCO3 nanoparticles. After calcination at 600 °C and washing
with HNO3 solution, single-phase monodispersal YSZ nano-
particles with a particle size of ca. 4-5 nm are obtained (Figure
2d).
1
8
zirconia and the textural properties of the calcined zirconia.
The degree of aggregation is significantly lower than that of
sample A, which is prepared without washing and sonication
in ethanol, but there is still obvious agglomeration as confirmed
by the TEM micrograph. Figure 2c shows the TEM micrographs
of the mixture of ethanol-treated hydrous YSZ and SrCO3. The
The particle size of sample C was also checked by dynamic
light scattering (DLS). When sample C dissolves in ethanol, it
can be sustained for a few hours without precipitation. The DLS
result of sample C indicates that the average particle size is 4.9
nm with a very narrow distribution from 3 to 6 nm. The particle

4650 J. Phys. Chem. B, Vol. 105, No. 20, 2001
Pang et al.
TABLE 1: BET Surface Areas and the Average Particle
Sizes Estimated by BET, XRD, and DLS
BET surface
area (m /g)
2
a
b
c
d
BET (nm)
d
XRD (nm)
d
DLS (nm)
sample A
sample B
sample C
60
113
165
10
17.4
8.8
6.0
16.4
6.3
4.7
4.9
SrCO
3
a
Derived from the BET surface area assuming a spherical shape of
the particles. b Calculated from the X-ray line broadening. Estimated
by SDP weight analysis.
c
due to the agglomeration of the particles after calcination.
2
Sample C has a very high surface area of 165 m /g. The
equivalent dBET of 6.0 nm is also larger than dXRD of 4.7 nm.
The reason for relative larger dBET is due to a soft-agglomeration
existing in sample C. The soft-agglomeration of sample C can
be broken by sonication in ethanol according to the DLS results.
On the contrary, the hard-agglomeration in sample B cannot
be broken. The soft-agglomeration in sample C can be inter-
preted in terms of the van der Waals and electrostatic forces of
the crystalline YSZ nanoparticles. For normal size particles, the
van der Waals and electrostatic forces are negligibly small.
However, when the particle size is on the nanometer scale,
particles prefer to agglomerate since the van der Waals forces
27
and electrostatic forces far exceed the single particle weight.
2
The surface area of SrCO3 nanoparticles is 10 m /g. When
the hydrous YSZ nanoparticles and SrCO3 nanoparticles are
mixed in the proportion 1:2.5 by weight, the hydrous YSZ
nanoparticles are coated on the surface of SrCO3 nanoparticles
in a mutilayer structure. It is possible to decrease the particle
size of the dehydrated YSZ by increasing the proportion of
SrCO3 in the mixture. The calcination temperature and the
concentration of the stock solution have also been reported as
influencing the particle size of zirconia. In our novel method,
the particle sizes are insensitive to the preparation conditions.
The average particle size, estimated by XRD, is in the range
4.9 ( 0.2 nm even as changing the concentration of zirconium
from 0.01 to 0.1 M, the calcination temperature from 500 °C
to 600 °C, and the proportion of hydrous YSZ and the SrCO3
from 1:2.5 to 1:10.
Figure 3. XRD patterns of (a) the as-prepared hydrous YSZ; (b) the
ethanol-treated sample; (c) sample A; (d) sample B; (e) sample C; and
3
(f) the calcined mixture of hydrous YSZ and SrCO .
size of the DLS result is consistent with the Powder X-ray
diffraction (XRD) and TEM results. DLS result confirmed that
the YSZ particles are monodispersed.
The XRD patterns of the ethanol-treated sample (Figure 3b)
are the same as those of the as-prepared hydrous YSZ sample
1
6
(Figure 3a), which shows two broad humps at ca. 30 and 50
degree (2θ), respectively. The amorphous hydrous YSZ consists
of tetrameric species of zirconium, which have a structure
similar to that of cubic ZrO2. After partial dehydration of the
as-prepared hydrous YSZ by sonication in ethanol, the colloidal
particles retain their amorphous structure. This is in contrast to
boiling the zirconium solution, which results in a crystalline
The band gaps of the YSZ sample are studied by UV-vis
diffuse reflectance spectroscopy (DRS). The Kubelka-Munk
2
function, F(R) ) (1-R) /2R, is used to determine the band gap
1
4
phase. During the sonication process, partial dehydration of
the hydrous YSZ particles takes place at the interfacial region
around the cavitation bubbles. The high temperature in this
region causes the formation of amorphous hydrous YSZ particles
instead of the crystalline phases. After calcination at 600 °C,
the hydrous amorphous YSZ is fully dehydrated and crystallizes
in a cubic phase. Figure 3c-e shows the XRD patterns of sample
A, B, and C, respectively. The particle sizes estimated from
the broadening of the XRD are 17.4, 6.3, and 4.7 nm for the
samples A, B and C, respectively. Figure 3f shows the XRD
patterns of the calcined mixture of ethanol-treated hydrous YSZ
and SrCO3. After calcination at 600 °C, the crystalline YSZ
and SrCO3 retain as a simple mixture of the two solids.
Brunauer-Emmett-Teller (BET) specific surface area results
and the average particle sizes estimated by BET, XRD, and DLS
are listed in Table 1. Sample A has a moderate surface area of
by analyzing the DRS results. Figure 4 shows the plots of F(R)
vs wavelength of sample A, B, and C. The band gaps are defined
by extrapolation of the rising part of the plots to X axis (dotted
line in Figure 4). The band gaps of sample A, B, and C are
4.13 eV (300 nm), 5.10 eV (243 nm), and 5.44 eV (228 nm),
respectively. The band gap of the agglomerated YSZ (4.13 eV)
is similar to the literature value of the single-crystal YSZ.1
Although impurities, defects, and surface states also affect the
,2
2
9
change of band gap, these effects are neglected here because
all the samples are precipitated in the same composition and
calcined at the same temperature (600 °C). The inset of Figure
4 shows the DRS results of the YSZ samples. The absorption
edge is significantly shifted to higher energy when the particle
sizes decrease from 17.4 nm of sample A to 6.3 nm of B and
further to 4.7 nm of C. It is worth noting that there are two
obvious absorption bands for sample B. This phenomenon is
2
16
28
6
0 m /g, which is consistent with the reported result. After
also reported in the literature and is attributed to color centers
2
9
treated in ethanol by sonication, the surface area of sample B
of polaron-like defects. Our result, however, shows that this
phenomenon is more likely to be due to the quantum confine-
ment effect. Weakly agglomerated YSZ nanoparticles exist in
sample B according to the TEM results. The DRS curve of
2
significantly increases to 113 m /g. The equivalent BET
diameter (dBET) of 8.8 nm, which is derived by assuming a
spherical particle shape, is larger than dXRD of 6.3 nm. This is

Preparation of Monodispersed YSZ Nanocrystals
J. Phys. Chem. B, Vol. 105, No. 20, 2001 4651
for 10 min. Our results indicate that an additional absorption
band observed in the zirconia spectrum is due to the quantum
confinement effect. Under certain preparing conditions, the
zirconia product can be identified as consisting of two distinct
types of particles, nonagglomerated and agglomerated. As a
result, the observed DRS spectrum is composed of two different
absorption bands. The heavily agglomerated particles have a
band gap similar to that of the bulk material, whereas the
nonagglomerated particles exhibit a quantum confinement effect
and the increase in the band gap energy.
Conclusion
In summary, fully crystalline yttria-stabilized zirconia (YSZ)
nanocrystals are prepared by a novel method. The agglomera-
tions of YSZ particles are inhibited, while crystallization occurs
on the surface and interface of SrCO3 nanoparticles. This method
is simple, inexpensive, and easy to scale-up. The YSZ nano-
crystals are monodispersed with an average particle size of 4.7
Figure 4. Plots of F(R) vs wavelength of (a) sample A; (b) sample B;
and (c) sample C. The inset show the DRS result.
2
nm and a high surface area of 165 m /g. The quantum
confinement effect is observed, and the band gap increases from
4
.13 eV for the heavily agglomerated YSZ sample to 5.44 eV
of the monodispersed YSZ nanoparticles with a particle size
4
.7 nm.
Acknowledgment. Prof. A. Gedanken thanks the BMBF,
Germany, for financial support through the Energy Program.
Dr. Chen, Dr. Zhu, and Dr. Pang thank the Kort 100 Scholarship
Foundation for supporting their postdoctoral fellowship. The
authors thank Antje V o¨ lkel for the assistance in analytical
ultracentrifuge (AUC) measurements. The authors also thank
Prof. Arlene Wilson-Gordon for editorial assistance.
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