Synthesis and stabilization of nanocrystalline zirconia with MSU mesostructure

Article abstract of DOI:10.1021/jp048190d

In the presence of nonionic block-copolymer surfactant, nanocrystalline zirconia particles with MSU mesostructure were synthesized by a novel solid-state reaction route. The zirconia particles possess a nanocrystalline pore wall, which renders higher thermal stability compared to an amorphous framework. To further enhance its stability, laponite, a synthetic clay, was introduced. Laponite acts as an inhibitor to crystal growth and also as a hard template for the mesostructure. High surface area and ordered pore structure were observed in the stabilized zirconia. The results show that the formation of the MSU structure is attributed to reverse hexagonal micelles, which are the products of the cooperative self-assembly of organic and inorganic species in the solid-state synthesis system with crystalline water and hygroscopic water present.

Full text of DOI:10.1021/jp048190d

J. Phys. Chem. B 2004, 108, 15523-15528
15523
Synthesis and Stabilization of Nanocrystalline Zirconia with MSU Mesostructure
X. M. Liu,†,‡ G. Q. Lu,* and Z. F. Yan
,‡
†,‡
Key Laboratory for HeaVy Oil Processing, Key Laboratory of Catalysis, CNPC, UniVersity of Petroleum,
Dongying 257061, China, and ARC Centre for Functional Nanomaterials, UniVersity of Queensland,
QLD 4072, Australia
ReceiVed: April 26, 2004; In Final Form: July 25, 2004
In the presence of nonionic block-copolymer surfactant, nanocrystalline zirconia particles with MSU
mesostructure were synthesized by a novel solid-state reaction route. The zirconia particles possess a
nanocrystalline pore wall, which renders higher thermal stability compared to an amorphous framework. To
further enhance its stability, laponite, a synthetic clay, was introduced. Laponite acts as an inhibitor to crystal
growth and also as a hard template for the mesostructure. High surface area and ordered pore structure were
observed in the stabilized zirconia. The results show that the formation of the MSU structure is attributed to
reverse hexagonal micelles, which are the products of the cooperative self-assembly of organic and inorganic
species in the solid-state synthesis system with crystalline water and hygroscopic water present.
Introduction
Zirconia is a special transition-metal oxide that possesses
bifunctional characteristics of weak acid and base properties.
The P-type semiconductivity exhibits abundant oxygen vacan-
cies on its surface. The high ion-exchange capacity and redox
activities make it possible to be used in many catalytic processes
as the catalyst, the supporter, and the promoter. In addition,
the superior chemical stability, mechanical strength, and ion-
exchange capacity are favorable for applications in ceramic
als, owing to the extremely small crystallite dimension.18,19 They
exhibit increased strength or hardness, improved ductility or
1
,2
toughness, enhanced diffusivity, and increased edges and
corners, which all contribute to favorable properties for use in
catalysis, electronics, and ceramics. This is why nanocrystalline
zirconia has attracted much research attention in the last two
2
0-23
decades.
Though various nanosized or mesoporous metal
oxides have successfully been exploited, there are few reports
on mesostructured materials with nanocrystalline walls or
nanocrystallites. These materials should have more advantages
over those materials having only nanocrystalline or ordered
mesoporous structures.
toughening, thermal-barrier coating, electronics, and oxygen
sensors.3-7
In recent years, many synthesis routes have been proposed
for the preparation of mesoporous or nanosized zirconia because
of its promising applications. Using a cationic surfactant, Hudson
For mesostructured or nanocrystalline zirconia, thermal
stability is always a challenging area. Many researchers have
attempted to improve the thermal stability of these materials.
For example, sulfate or phosphate anions are incorporated into
the framework to enhance the stability of the mesostructure
first synthesized mesoporous zirconia with high surface area
via a scaffolding mechanism.8,9 Anionic surfactants have also
been used to synthesize mesostructured zirconia, but a disordered
1
2,24-25
3+
product was obtained.1
0,11
12
framework.
Aliovalent and tetravalent dopants, such as
Ying et al. obtained the lamellar
+
2+
4+
4
4+
Na , Ca , Y , Si , Ce, Th , and so forth, have been used
to reinforce the nanocrystalline structures.
and disordered hexagonal structure in the presence of an
amphiphilic surfactant and proposed a ligand-assisted templating
route. The lamellar and hexagonal mesostructure can also be
observed by interaction between long-chain-length primary
26-30
The stabilization
was believed to be achieved by strong surface interaction
between dopant ions and zirconia or by higher coordination
number via the addition of large-size dopants.
13
amine and inorganic species. Using block-copolymer tem-
plating, Zhao and co-workers1 demonstrated that very-ordered
mesoporous zirconia could be synthesized in nonaqueous
solution. They thought that the mesophase could be formed
through a mechanism that combines block-copolymer self-
assembly with alkylene oxide complexation of the inorganic
metal species, whereas Blin et al.1 proposed that the hydrolysis
and polymerization of precursors occurred around surfactant
micelles, which resulted in supermicropores formed first and
then the supermicropores transformed to mesopores after
hydrothermal treatment. Recently, Zhao et al.17 further devel-
oped an “acid-base pair” route to prepare cubic zirconia.
Many properties of nanocrystalline materials are fundamen-
tally different from those of conventional polycrystalline materi-
4,15
In this work, for the first time, nanocrystalline zirconia with
the MSU mesostructure was synthesized using a block-
copolymer surfactant as the structure-directing agent via a novel
solid-state reaction route. To improve the thermal stability of
the mesostructure, laponite was introduced into the inorganic
framework. The study demonstrates that this novel synthesis
methodology is also promising for synthesizing other meso-
porous metal oxides and that laponite is a good stabilizer for
the mesostructure.
6
Experimental Section
Chemicals. Block copolymer HO(CH CH O) (CH CH(CH )-
2
2
x
2
3
O)y(CH2CH2O)xH (fw ) 5800) and zirconyl chloride (ZrOCl2‚
H2O) were obtained from Aldrich and used as received.
Laponite (Na0.7 [(Si8Mg5.5Li0.3) O20 (OH) 4] ) was supplied
by Fernz Specialty Chemicals, Australia.
*
Corresponding author. E-mail: maxlu@cheque.uq.edu.au. Tel: +61-
8
7
-33653885. Fax: +61-7-33656074.
0
.7
†
‡
University of Petroleum.
University of Queensland.
1
0.1021/jp048190d CCC: $27.50 © 2004 American Chemical Society
Published on Web 09/01/2004

15524 J. Phys. Chem. B, Vol. 108, No. 40, 2004
Liu et al.
Figure 1. XRD patterns of zirconia samples. (Inset) High-angle peak.
Synthesis. In a typical synthesis, 12.8 g of zirconyl chloride
and a certain amount of block-copolymer surfactant were milled
together in a mortar and then reacted with 6.4 g of milled sodium
hydroxide under vigorous stirring. The resulting product was
transferred to an autoclave to crystallize. The crystallizing time
and temperature varies for the different inorganic system. After
crystallization, the product was washed with deionized water
Figure 2. Effect of crystallization temperature on the crystal phases
of zirconia.
demonstrates that the broadened diffraction here is attributed
to the particle-size effect because there are sharp lines for the
calcined sample whose size is increased to 6.0 nm. The
mesostructure tends to be less ordered after calcination at 873
K because the low-angle peak almost disappeared and the high-
angle lines become narrower and sharper. This shows that the
crystallization of the inorganic wall during the calcination
process resulted in a more disordered mesostructure. We note
that the particle was still nanocrystalline and the lattice structure
was more ordered upon further crystallizing at higher calcination
-
until it was free of Cl ions and then washed with ethanol twice
to remove water; the surfactant remained in the solid. Finally,
the samples were dried at 383 K overnight and calcined at 450
and 600 °C.
The method to prepare the stabilized samples is similar to
that above. The only difference is that a certain amount of
laponite was milled together with zirconyl chloride and the block
copolymer and then reacted with the alkali. Crystalline water
in the zirconyl chloride and hygroscopic water during the milling
of sodium hydroxide contribute to the micellization process of
the surfactant. This methodology can also be applicable to other
salt systems for nanocrystalline metal oxide synthesis.
Characterization. Nitrogen adsorption and desorption iso-
therms at 77.3 K were carried out with Autosorb-1C (Quan-
tachrome, USA) after degassing at 200 °C for 10 h. The
mesopore size distribution was calculated from the desorption
branch of the isotherms. Low-angle and wide-angle X-ray
powder diffraction (XRD) patterns were obtained with a Bruker
Axs (Germany) diffractometer using Co KR radiation with a
tube voltage of 40 kV and a tube current of 20 mA. Transmis-
sion electron microscopy (TEM) images were taken using a
Philips CM200 microscope in bright-field transmission mode.
The samples for TEM were prepared by directly dispersing the
zirconia particles treated by ultrasonic dispersion onto holey
carbon grids. Energy-disperse X-ray (EDX) spectra were taken
on a Gatan detector connected to the electron microscope.
Scanning electron microscopy (SEM) examinations were per-
formed on a JEOL 6400.
.32
temperatures, which is confirmed by TEM examination
The nanocrystallite size and mesopore structure can be
33
tailored by varying the crystallization temperature. It can be
seen from Figure 2 that the crystallite size increased with the
increase in crystallization temperature and that the phase is
transformed from tetragonal to monoclinic. The tetragonal phase
(t phase) is stable up to 200 °C with some monoclinic phase
(
m phase). Beyond this temperature, transformation of the t
phase into the m phase starts, and at 250 °C, complete
transformation occurs. The nanocrystalline sizes at these tem-
peratures are 5.0 and 54.2 nm, respectively. This confirms the
previous conclusion that the stability of tetragonal ZrO2 is due
34-36
to nanocrystallite size.
The crystal growth with increasing
crystallization temperature is faster and results in larger nano-
crystallites.
Nitrogen adsorption/desorption isotherms (Figure 3) indicate
that the isotherms of mesoporous zirconia synthesized at
different crystallization temperatures are typically IV type with
large hysteresis loops except for the sample prepared at 250
°
C, which means that the materials obtained with the solid-
state reaction using block copolymers as structure-directing
agents have mesoporous structure. The capillary condensation
partial pressure and the size of the hysteresis loop increases
with temperature. This indicates that the pore size is enlarged,
which is evidenced by the mesopore distribution shown in Figure
3b. From Figure 3b, we can also see that the mesopore of
zirconia prepared with this novel method possesses a mono-
modal mesopore distribution. The mesoporous diameter can be
tuned in the range of 3.8-12.0 nm, and the pore volume
increases with temperature before 150 °C. However, nonporous,
monoclinic-phase zirconia can be obtained at 250 °C, which
testifies to the fact that the mesostructure is destroyed by the
Results and Discussion
The powder XRD patterns of as-synthesized and calcined
zirconia are shown in Figure 1. The broader primary low-angle
peak is observed in the as-synthesized sample, which is
indicative of MSU mesostructure, as also confirmed by the TEM
images (Figure 6). The inorganic framework is periodic because
it consists of nanocrystallites in the tetragonal phase according
to the XRD peaks at high angles. The crystallite size calculated
by the Scherrer equation is around 1.5 nm. Upon calcination at
3
7
723 K, the intensity of the low-angle XRD peak is substantially
phase transformation. The increased pore size at higher
crystallization temperature is attributed to the increase in
aggregation of micelles. For the PEO-block-PPO surfactant, the
hydrophobic capacity is relatively enhanced because the affinity
between the hydrophilic groups and water is weakened at higher
greater than that of the as-synthesized sample, suggesting that
the mesostructure was retained. The broadening of diffraction
is thought to arise from a lack of long-range crystallographic
31
order or finite-size effects. The TEM image (Figure 6)

Synthesis and Stabilization of Nanocrystalline Zirconia
J. Phys. Chem. B, Vol. 108, No. 40, 2004 15525
Figure 3. Effect of crystallization temperature on the isotherms and mesopore distribution of nanocrystalline zirconia.
upon calcination. When incorporating laponite into the synthesis,
the mesopore distribution is stabilized by an 0.88% molar ratio
of laponite (Figure 4, inset). The results show that the presence
of laponite not only enhances the stability of mesostructure but
also increases the mesopore volume. The pore-size distribution
is almost the same after calcination at 450 °C, and the pore
diameter increases very little for the sample calcined at 600
°
C. It is worthy to note that the addition of laponite led to
narrower pore distribution compared to that of the pure zirconia.
The data in Table 1 indicate that the surface area and pore
volume can also be retained compared to the data for the as-
made sample after it was calcined at 450 °C. Overall, the N2
adsorption/desorption results showed that laponite is a better
stabilizer than other dopants previously reported in the literature.
The XRD patterns shown in Figure 5 demonstrate that
laponite can also resist the crystallization of the inorganic
framework and inhibits the growth of nanocrystallites. The
diffraction peaks of the calcined, stabilized sample are more
broadened than the as-made pure zirconia. This means that
laponite must have been incorporated into the zirconia frame-
work and formed a strong interaction with Zr . This interaction
inhibits crystal growth during crystallization in the calcination
process; consequently, it forms more fine nanocrystallites, which
leads to high surface area. This result is also confirmed by the
TEM images shown in Figure 6.
Figure 4. Mesopore-size distribution of zirconia calcined at different
temperatures. (Inset) Pore-size distribution of samples stabilized by
laponite.
4+
The evidence for disordered, hexagonal-like packing of
channels in MSU mesostructures is also evident in the TEM
images. The micrograph shows that the nanocrystallites of
zirconia formed around the wormholelike pores are regular in
diameter although lacking long-range packing order. These short
channels and nanocrystallite size are conducive to high activity
and conductivity in catalytic and electrode materials, respec-
tively, because of less resistance to mass transport and the
charge-transfer process. From Figure 6, it can be seen that the
nanocrystallite size of the stabilized samples is much smaller
than that of the pure zirconia, particularly for calcined samples.
The crystal EDX fringes are shown in Figure 6c. It is observed
that for stabilized samples calcined at 600 °C there are merely
nanocrystallites about 2.0 nm in size. These results further
confirm that the laponite in the inorganic framework played a
role in inhibiting crystal growth.
Figure 5. XRD pattern of calcined zirconia before and after stabiliza-
tion.
temperatures. To decrease the entropy and free energy of the
water-organic system, the aggregation number of micelles is
increased, which results in the zirconia sample with larger
nanocrystallites, thus leading to products with larger mesopores.
The above results show that nanocrystalline zirconia with
MSU mesostructure can be obtained using the solid-state
reaction route. However, the surface area (Table 1) is dramati-
cally reduced at higher calcination temperatures. In addition,
the spread of pore-size distribution (Figure 4) tends to be larger,
and the average pore size also shifts to larger values. It is
therefore important to improve the structural stability further
The TEM studies of the pure zirconia calcined at 450 °C
showed an apparently mesoporous structure with pores about
6.2 nm in diameter, which corresponds to the value of 6.1 nm
calculated by the BJH method. Interestingly, the mesostructure
of the laponite-stabilized samples looks more ordered than that

15526 J. Phys. Chem. B, Vol. 108, No. 40, 2004
Liu et al.
Figure 6. TEM images of pure and stabilized zirconia before and after calcination: (a) pure zirconia as-made, (b) zirconia stabilized by 0.88%
laponite, (c) pure zirconia calcined at 450 °C, and (d) zironia stabilized with 0.88% laponite calcined at 600 °C.
Figure 7. SEM images for as-made zirconia: (a) pure zirconia and (b) stabilized zirconia.
TABLE 1: Effect of Laponite Stabilization on the Surface Area and Pore Structure of Zirconia
not stabilized
stabilized
sample
as-made
450 °C
600 °C
as-made
450 °C
600 °C
2
-1
BET/m g
354.2
0.366
45.8
103.3
0.279
61.2
51.19
0.335
178.0
356.5
0.454
40.8
360.9
0.472
41.0
231.2
0.411
45.6
3
-1
pore volume/cm g
pore size/Å
in the pure zirconia. The local lamellar structure can be observed
in the mesostructure of the calcined sample. This could be
attributed to the interaction between the precursor of zirconia
and the inorganic species in laponite. This interaction is achieved
via hydrogen bonding between the PEO segments and various
inorganic species and may be extended along the direction of
the laponite-platelet layer. The Zr-O-Mg-O-Si network is
formed upon the condensation of the inorganic pore wall at high
temperature, which is the reason that the mesostructure of the
calcined laponite-stabilized sample is more ordered. The more
ordered mesostructure can be also observed in the SEM images
shown in Figure 7. The morphology of the stabilized sample
possesses a similar lamellar structure, whereas for pure zirconia,
only a disordered structure is observed.
with MSU mesostructure. It is necessary to investigate the
optimum amount of laponite that can be incorporated into the
mesostructure. The mesopore distributions of zirconia calicined
at 450 °C for various amounts of laponite are shown in Figure
8. It is seen that the samples all possess a monomodal pore-
size distribution centered around 3.6 nm when the laponite
addition is less than 0.88% (molar ratio). For higher amounts
(>0.88%) of laponite, a bimodal mesopore distribution is
observed. Table 2 shows that the pore volume is drastically
increased with the increase in the amount of laponite added,
whereas the crystallite size is decreased. The sample that is
stabilized by 0.88% laponite exhibits the highest surface area.
The optimal amount of laponite addition in nanocrystalline
zirconia can be said to be around 0.88%.
The above discussion demonstrates that the introduction of
laponite strongly affects the stability of nanocrystalline zirconia
The solid-state reaction method using a block copolymer as
the structure-directing agent to synthesize nanocrystalline zir-

Synthesis and Stabilization of Nanocrystalline Zirconia
J. Phys. Chem. B, Vol. 108, No. 40, 2004 15527
The zirconia precursors interact strongly with the platelets
of laponite via the bridge of PEO segments with hydrogen
bonding. Thus, Zr-O-Mg-O-Si networks can form upon the
condensation of the framework during the calcination process.
In addition, the interaction among various inorganic species can
be extended along the direction of the laponite platelet layer.
This is the reason that local lamellar structures can be observed
in the zirconia mesostructure. The Zr-O-Mg-O-Si networks
with large covalent bonds induce stronger interactions for the
framework. It is believed that the introduction of laponite not
only renders the lamellar structure but also forms the solid
mesoporous skeleton. It is the large covalent-bond network that
contributes higher stability to the mesostructure. With the
increase in laponite amount, the interaction among the inorganic
species results in the larger pillared particles, which induce the
larger mesopore size and broaden the pore-size distribution. We
also found that there is an optimum amount of laponite as a
hard template to stabilize the mesostructure and to achieve
maximum porosity.
Figure 8. Effect of the amount of laponite on the mesopore distribution
of zirconia calcined at 723 K.
The formation of the MSU mesostructure should be attributed
to the cooperative self-assembly of the block-copolymer sur-
factant with the inorganic species. For the nonionic surfactant,
the shape of the micelles changes in the sequence of spherical
to hexagonal to lamellar to reverse hexagonal as the concentra-
tion increases. In a solid-state reaction, the mass concentration
of the surfactant is higher than 70% under all synthesis
conditions used because the solvent is supplied only by the
crystalline water in zirconyl chloride and a small amount of
hydroscopic adsorbed water during milling. This means that the
aggregation of the structure-directing agent is similar to the high
surfactant concentration situation in aqueous synthesis systems.
Thus, the lamellar and reverse hexagonal structures in the
micelles are favored. In this unique synthesis system, according
to the studies in this work, the mesophase of inorganic and
organic templates is most likely to be of the reverse hexagonal
structure. The N2 adsorption/desorption data also showed that
the pore diameter of the ziconia samples thus formed is
independent of the chain length of the polymer surfactant but
depends on the size of the hydrophobic head. This is more
evidence confirming the existence of the reverse-hexagonal
mesophase.
Figure 9. Schematic of the solid-state reaction route involving laponite
as the stabilization agent for nanocrystalline zirconia formation.
TABLE 2: Effect of Laponite on the Mesostructure of
Zirconia Calained at 450 °C
nanocrystallite BET pore volume pore diameter
2
-1
3
-1
sample
size/nm
/m g
/cm g
/Å
Zr-lap-0.38%
Zr-lap-0.63%
Zr-lap-0.88%
Zr-lap-1.25%
Zr-lap-3.75%
2.7
2.1
2.0
1.8
1.5
287.9
282.2
360.9
309.7
276.2
0.383
0.334
0.472
0.582
0.914
40.8
41.0
41.0
41.2, 58.0
63.4, 98.7
Conclusions
Nanocrystalline zirconia with MSU mesostructure is synthe-
sized via a novel solid-state reaction route using a block
copolymer as the structure-directing agent. Laponite as a hard
template and stablizer for the mesostructure has proved to be
very effective in tailoring the nanocrystallite size and thermal
stability of the zirconia product. The addition of laponite inhibits
the crystallization of the zirconia precursor and induces the
formation of the more ordered mesostructure. In this novel solid-
state synthesis system, the reverse-hexagonal mesophase can
be formed, which leads to the MSU mesostructure. In addition,
using this novel route, the surface area and pore structure of
zirconia can be easily tuned by the synthesis conditions. This
methodology using a solid-state reaction could be applied to
the synthesis of other metal oxides with high crystallinity and
mesostructure stability. Because it is a simple process, the cost
for the final-product zirconia is expected to be lower than that
of the aqueous routes.
conia with MSU mesostructure is the first attempt in this area.
Pure zirconia without the addition of a stabilizer can be obtained
with a high surface area and a nanocrystallite pore wall. The
MSU mesostructure with a nanocrystalline pore wall is periodic
in lattice structure, and the pore diameter can be easily tailored
by synthesis conditions. This MSU mesostructure with nano-
crystalline walls prevents the mesostructure from collapsing.
However, the structure becomes more disordered and the surface
area decreased dramatically at high calcination temperature
14,15
(>600 °C). To apply this material to high-temperature applica-
tions such as solid-oxide fuel cells, we used laponite to improve
its stability. The main composition of laponite is SiO2 and MgO.
SiO2 and MgO can play a stabilization role for the zirconia
mesostructure according to their electronic properties and charge
sizes. The presence of laponite platelets induces the more
ordered and more stable mesostructure. The analogous lamellar
structure can also be observed in this mesostructure, particularly
in the calcined sample. Laponite can be considered to be a hard
template as well. As illustrated in Figure 9, the mechanism of
the stabilization of the mesostructure by laponite is proposed
as follows.
Acknowledgment. Financial support from the Australian
Research Council is gratefully acknowledged. We thank Dr. J.
Riches and Ms. A. Yago for their assistance on TEM and XRD
analysis.

15528 J. Phys. Chem. B, Vol. 108, No. 40, 2004
Liu et al.
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