A simple route to synthesize hierarchical porous ZrO2

Article abstract of DOI:10.1039/c3ra47967f

A simple method for preparing porous ZrO₂ has been proposed by directly decomposing a Zr(NO₃)₄·5H₂O ethanol sol-gel solution. Contrary to other traditional sol-gel methods, porous ZrO₂ with multi-sized macro- and meso-pores are synthesized via only one simple step. The size of the macropores ranges from 100 nm to 500 nm. Remarkably, we also isolated some highly dispersed ZrO₂ nanoparticles (～60 nm), which on their surface or even in the whole body are fully filled with well-sized ever-smaller spherical mesopores with an average diameter around ～5 nm. A further investigation of low-temperature nitrogen absorption has shown a high surface area over 162 m² g^-1 for the fully crystallized ZrO₂. No more additional modifiers or complicated preparations are needed in the present method, which promises a new potential option in fabricating high-porosity materials.

Full text of DOI:10.1039/c3ra47967f

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A simple route to synthesize hierarchical porous
ZrO2†
Cite this: RSC Adv., 2014, 4, 8039
a
a
a
b
b
b
Yu Chen,* Hui Xia, Daiguo Zhang, Zilai Yan, Fangping Ouyang, Xiang Xiong
and Xiaoyi Huang*c
Received 25th December 2013
Accepted 13th January 2014
DOI: 10.1039/c3ra47967f
www.rsc.org/advances
A simple method for preparing porous ZrO
directly decomposing a Zr(NO $5H
Contrary to other traditional sol–gel methods, porous ZrO
2
has been proposed by template-based methods need to build a porous framework
)
3 4
2
O ethanol sol–gel solution. rst, then insert cared materials into the template, and nally
with remove the pre-created template away if necessary. This is a
2
multi-sized macro- and meso-pores are synthesized via only one complicated project and even in its rst step, to a large extent, is
simple step. The size of the macropores ranges from 100 nm to sometimes a difficult and costly task. In addition, removing the
5
00 nm. Remarkably, we also isolated some highly dispersed ZrO
2
pore-forming frameworks is another big challenge. In contrast,
nanoparticles (ꢀ60 nm), which on their surface or even in the whole self-assembly processes are comparatively simple. But the
body are fully filled with well-sized ever-smaller spherical mesopores method usually requires specic surfactants or pore-forming
with an average diameter around ꢀ5 nm. A further investigation of agents. These surfactants and agents are expensive and how to
low-temperature nitrogen absorption has shown a high surface area remove them at the end of the fabrication is also a big problem.
2
À1
over 162 m
g
2
for the fully crystallized ZrO . No more additional
2
ZrO is one of the most important ceramic materials, which
modifiers or complicated preparations are needed in the present is widely used in oxygen sensors, fuel cells, catalysts and catalyst
11–13
method, which promises a new potential option in fabricating high- supports.
These applications can be attributed to its unique
porosity materials.
physical properties. It is the only traditional metal oxide having
both acid and base active centers on its surface, which shows
excellent catalytic activity on some processes of the alkylation
Hierarchical porous metal oxides on multiple scales have
attracted considerable attention due to their interesting phys-
4,14,15
and oxidation.
2
ZrO , as a widely used catalyst or air sensor,
requires high specic surface area and strong adsorptive
capacity. Increasing the porosity of ZrO or decreasing its
1,2
ical and chemical properties. These types of materials oen
exhibit high specic surface area, good permeability and low
relative density, promising potential application in sensors,
2
particle size is a candidate option. However different from SiO2,
TiO , reports on the preparing and manufacturing of porous
2
3–6
catalyzers as well as gas purication. Thus, any approaches
that could achieve these materials efficiently and inexpensively
are of great interests. Up to now, many typical technologies –
ZrO are very limited. Fujita et al. successfully prepared a porous
2
crystalline zirconia by combining a sol–gel and solvothermal
3
process. They achieved a high surface area precursor of ZrO
2
7,8
such as those based on hard or so templates, self-assembly
gels. However, as mentioned by Fujita et al. themselves, the
3,9,10
processes with a help of specic surfactants,
etc. – have been
collapse of microstructures in the high temperature of sintering
developed to meet this requirement. However, traditional
approaches above need either additional modiers or compli-
cated pre-preparation and post-treating processes. For example,
ꢁ
(
>400 C) results in a following great loss of surface area. Hong
et al. fabricated a ZrO porous ceramic using a camphene-based
2
6
freeze-casting method. But the method requires some
comparatively special preparation conditions and procedures.
Here, we proposed a new and simple method with which
a
Hunan Key Laboratory for Super-microstructure and Ultrafast Process, Central South
hierarchical porous ZrO
posing step of Zr(NO $5H
decomposition products, we found some micron-sized ZrO
blocks that full lled with spherical air pores. Aer a further
2
was fabricated via only one decom-
University, 932 Lushan Nan Rd, Changsha, Hunan 410083, P.R. China. E-mail:
3
)
4
2
O ethanol sol–gel solution. In the
chenyu_8323@csu.edu.cn
b
Powder Metallurgy Research Institute, State Key Laboratory of Powder Metallurgy,
2
9
c
32 Lushan Nan Rd, Changsha, Hunan 410083, P.R. China
Physics Department, Xiangnan University, Chenzhou, Hunan 423000, P.R. China. separation process, we also isolated some of highly dispersed
E-mail: jghxy@126.com
ZrO nanoparticles (ꢀ60 nm), which on their surface or even in
2
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c3ra47967f
the whole body are full of well-sized ever-smaller spherical
1
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mesopores with the average diameter around ꢀ5 nm. The
The achieved ZrO
2
gels were dried in the air for another 2
successful fabrication of these porous ZrO suggests a new hours and then transferred to muffle furnace annealing at
2
optional approach for preparing high-porosity metal oxides.
different temperature. Fig. 2 is the XRD results and shows a
The entire fabrication process is described as illustrated in tetragonal crystalline ZrO if the sintering temperature is higher
2
ꢁ
Fig. 1(a). The concentration of the sol–gel precursor, chemical than 400 C.
reactions to generate porous ZrO
2
gels and the nal annealing
The morphologies and microstructures of the achieved ZrO
2
process were all nished in the same alumina crucible. The new aer crystallization were carefully investigated by FESEM and
method is, to a large extent, able to avoid the introduction of the results are shown in Fig. 3. A large area of macro-pores on a
unwanted impurities. Furthermore, in order to fabricate a micron-sized ZrO block can be clearly observed. It is not a
2
porous ZrO , air bubbles emitted in the thermal decomposition cross-linked air channel as reported in the ref. 3, but regular
2
process (NO2 bubbles) were utilized as a so template. This spherical air bubbles on the surface or embedded inside of the
kind of so template also has been successfully applied in the ZrO . Generally, polymerization-induced bicontinuous pores
2
16
fabricating of porous hierarchical In
2
O
3
.
The biggest advan- (in ref. 3) are easily to be collapsed sintering in high-tempera-
tage of using air bubbles as a template is that it requires no ture. However, air bubbles utilized as pore-forming template
further treatments at the end of the synthesis. The precursor, can be well preserved in present new method (see in Fig. 3).
however, is another important factor which provides a growth- From FESEM images of Fig. 3(c), we can infer that these
2
environment for both air bubbles and ZrO . This requires a
liquid of high concentration, low owability and a fast gelation
process accompany with abundant air emission. But the solu-
bility of Zr(NO ) $5H O powder in ethanol at room temperature
3
4
2
is much lower than this requirement, and there is no signicant
changes even at the boiling point of this solution. But we found
if we rstly prepared a diluent solution, and then let it being
ꢁ
volatilized slowly at 60 C, it will be able to form a transparent
gel–sol solution with a very high concentration. Thus in this
article, 10 g Zr(NO ) $5H O powder was rstly mixed with 30 mL
3
4
2
of anhydrous ethanol to form a relatively diluent solution. Then
ꢁ
the solution was heated up to 60 C just a little lower than its
boiling point and volatilizing at this temperature slowly to
achieve a high concentration. As we know, the volatilizing
ꢁ
process is usually very slow. Thus we volatilized at 60 C is for
the purpose to accelerate this step. The heating rate is well
controlled at a very slow speed because present Zr(NO ) sol–gel
3 4
solution is easily to be hydrolyzed directly at a high-speed of
ꢁ
heating or if the temperature is higher than 80 C. The reaction
process is depicted in Fig. 1(a). As the precursor has been Fig. 2 XRD results of the as-prepared ZrO2 annealed at different
temperature.
concentrated to a very high level, creating a liquid of poor

owability, air bubbles (with a small size) in this circumstance
will be jammed and aggregated where it was created. These air
bubbles are emitted simultaneously and utilized as a so
ꢁ
template in the gelation when the temperature reached 90 C.
Fig. 1(b) shows the synthesized amorphous ZrO gels.
2
Fig. 3 Morphologies and microstructures of the porous ZrO
2
block.
2
block
(a) SEM images at a considerable small magnification rate. ZrO
Fig. 1 Schematic representation of the reaction process and the in micron-size was full filled with well-sized spherical pores. The inset
photo of synthesized porous ZrO sample. (a) A illustration of the pore- is a close-up view of the region selected by the red rounded rectangle.
2
forming mechanism. The precursor has been concentrated to a very (b) A further magnification of the selected area in (a), from which the
high level, creating a liquid of poor flowability. Air bubbles (with a small pore size distribution can be estimated ranging from 100 nm to
size) in this circumstance will be frozen and aggregated where they 500 nm. (c) A SEM investigation on a crack where bubbles inside of this
ꢁ
were created. (b) A photo of amorphous ZrO
2
gels formed at 90 C.
material can be identified.
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spherical air pores are not only existed on the materials' surface,
but also inside of the ZrO . The size distribution estimated from
2
Fig. 3(b) is ranged from 100 nm to 500 nm. It should be pointed
out that this bubble-embedded ZrO is not the only product
2
obtained in the reaction. There are other reactants that have
distinct micro-structures (please see the details in the ESI†). In
other works, we will focus on the factors, such as pressure,
solution concentration, etc., that could determine the formation
of pores.
The achieved ZrO in the last step was further pulverized into
2
powder by an agate mortar. Remarkably, we isolated some of
mesoporous ZrO particles this time. The powder was ultrasonic
2
oscillated in anhydrous ethanol for another 1 hour and then le
untouched until a stable and homogeneous suspension was
obtained. As we know that pure ethanol has a very low surface
tension. Only those materials with high surface area can be
suspended stably in this solution. Thus we can infer that the
isolated ZrO particles possess a considerable high surface area.
2
A
further investigation of the low-temperature nitrogen
absorption has conrmed these conclusions which shows a
2
À1
high BET surface area of over 162 m g . Fig. 5(a) shows the
nitrogen adsorption–desorption isotherm curve. This kind of
isotherm indicates the existence of closed pores. A copper grid
2
is then put into the ZrO suspension and li up for the next SEM
and TEM investigations to further examine this assumption.
The detailed SEM and FETEM analysis results are shown in
Fig. 4. Fig. 4(a) shows SEM images of the highly dispersed ZrO2
nanoparticles. Red circle selected area was a possible region
that will be further characterized using FETEM. The FETEM
results are presented in Fig. 4(b)–(f). Nanopores which are
spherical and with average diameter around ꢀ5 nm were Fig. 4 SEM and TEM analysis of highly dispersed ZrO particles which
2
were isolated from the products in present fabrication. (a) SEM images
of the highly dispersed ZrO nanoparticles. Red circle select area is the
2
possible region for the next investigation of TEM. (b) TEM test results in
a comparative low magnification. Inset of (b) shows diffraction rings
founded spreading on the surface of a comparative bigger ZrO
2
seeds (ꢀ60 nm). Pore size distribution obtained from nitrogen
˚
absorption data shows that it is centered at 50 A. In the present
experiment, the decomposing process is simultaneously along
2
and EDX results of tetragonal ZrO . Copper peaks originate from the
with the gelation process which emits abundant red-brown NO2 TEM sample grid. (c and d) HRTEM images of the morphology of one
isolated ZrO particle, which sub-nanostructures of spherical nanopits
can be clearly observed. Red arrows labeled the irregular edge surface.
e and f) Another two distinct ZrO particles which also shown a
2
gases. The emitted NO will form a mass of bubbles in the liquid
2
by the action of surface tension. However, because the liquid at
this moment is concentrated to a very high level and the phase
transformation is nished so fast that NO bubbles, which have
2
(
2
irregular or a wave shape edge. This phenomena indicates that the
surface of these particles are embedded with hemispheric nanopits.
not enough time to be grown up and moved out of the liquid,
are frozen and aggregated where they were created and nally to
be utilized as a so pore-forming template. The color of ach-
distinct ZrO
of edge surface irregularity is not a single case, but widely
existed in these isolated ZrO nanoparticles.
2
nanoparticles. It is indicated that the phenomena
ieved ZrO gels (as show in Fig. 1(b)) are of red-brown at the
2
2
beginning and beige some hours latter. This phenomenon
indicates that ZrO
of NO gases in the gelation. However, aer dried in air for some
hours, the absorbed NO have nally been released.
2
gels have absorbed a considerable amount
2
2
From the results in Fig. 4 and 5, we were still unable to
conrm whether there are air bubbles or pores embedded
inside of these isolated ZrO nanoparticles. But what we can
2
make sure is that hemispheric nanopits are full lled on their
surface. Fig. 4(d) is a lower magnication FETEM image for the
same particle investigated in Fig. 4(c) which can gives a full view
of the particle edge. Red arrows in these two gures marked
clearly the signs of hemispheric nanopits because of its irreg-
Fig. 5 Nitrogen adsorption characterizations of the isolated porous
ZrO powder. (a) Nitrogen adsorption–desorption isotherms. (b) The
2
ular edge surface. Fig. 4(e) and (f) is the image of another two pore size distribution data.
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Only Zr, O and Cu (from Cu grid) were detected in the energy (high-resolved transmission electron microscopy) measure-
dispersive spectrum (EDS) tests (see Fig. 4(a)), which conrmed ments were taken on a JEM-2100f instrument under an accel-
the chemical elements. The electron diffraction pattern (see eration voltage of 200 kV. Energy dispersive X-ray spectra (EDS)
Fig. 4(a)) matches well with the tetragonal ZrO phase, which were also utilized to conrm the chemical components of the
2
the diffraction rings {011}, {002}, {112}, {013} and {121} has samples. The size distribution of the pore size was calculated
been indexed.
In summary, a novel and simple approach for preparing Halenda (BJH) method, and the surface are was obtained by the
hierarchical porous ZrO has been proposed using air bubbles Brunauer–Emmett–Teller (BET) method.
as a template. ZrO with multi-sized macro- and meso-pores can
from the nitrogen adsorption data using the Barrett–Joyner–
2
2
be easily synthesized via directly decomposing highly concen-
ꢁ
Acknowledgements
trated Zr(NO ) $5H O ethanol sol–gel solution at 90 C. Air
3
4
2
bubbles originated from this fast decomposing process were
simultaneously utilized as the so template. The bulk ZrO
This work is nancially supported by the National Science
Foundation of China the (no. 51002009), the Fundamental
Research Funds for the Central Universities (no. 2012QNZT055)
and Hunan provincial Natural Science Foundation of China
2
material fabricated using this template is full lled with
spherical air bubbles and, this bubble framework can still be
well preserved even aer a high temperature of annealing.
Macro-pores analyzed by SEM are ranged from 100 nm to
(no. 13jj3005). We greatly appreciate the support from Instru-
mental Analysis Center of Shanghai Jiao Tong University,
Shanghai, P. R. China.
500 nm. Remarkably, we also isolated some of highly dispersed
ZrO nanoparticles (ꢀ60 nm) which on their surface or even in
2
the whole body are distributed meso-pores with average diam-
eter around ꢀ5 nm. No more additional modiers or compli-
cated preparations are needed in the present method, which
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