Synthesis and characterization of mesoporous c-ZrO2 microspheres consisting of peanut-like nano-grains

Article abstract of DOI:10.1016/j.jallcom.2007.10.039

Smooth and spherical ZrO₂ particles were successfully synthesized by a novel spray technique and calcination at 600-800 °C. The synthesized ZrO₂ microspheres were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD). It was found that the synthesized ZrO₂ microspheres, which were mainly existed as c-phase, possessed a homogeneously mesoporous microstructure consisting of peanut-like nano-grains around 8-13 nm in width diameter.

Full text of DOI:10.1016/j.jallcom.2007.10.039

Journal of Alloys and Compounds 464 (2008) 569–574
Synthesis and characterization of mesoporous c-ZrO microspheres
2
consisting of peanut-like nano-grains
a,∗
b
c
b
d
c
Hui Zhang , Zhentao An , Fan Li , Qing Tang , Kathy Lu , Wenchao Li
a
School of Science, Beijing Jiaotong University, Beijing 100044, PR China
b
Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, PR China
c
Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, PR China
d
Materials Science and Engineering Department, Virginia Tech, Blacksburg, VA 24061, USA
Received 10 September 2007; received in revised form 8 October 2007; accepted 11 October 2007
Available online 22 October 2007
Abstract
Smooth and spherical ZrO
◦
particles were successfully synthesized by a novel spray technique and calcination at 600–800 C. The synthe-
2
sized ZrO microspheres were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution
transmission electron microscopy (HRTEM), and X-ray diffraction (XRD). It was found that the synthesized ZrO microspheres, which were
2
2
mainly existed as c-phase, possessed a homogeneously mesoporous microstructure consisting of peanut-like nano-grains around 8–13 nm in width
diameter.
©
2007 Elsevier B.V. All rights reserved.
Keywords: Spray reaction synthesis; Mesoporous c-ZrO2 microspheres; Peanut-like nano-grains; TEM/HRTEM characterizations
1
. Introduction
tals with a novel photonic band structure. Lerot [6], Cantfort [7],
and Yan [8] have synthesized the monodispersed ZrO2 spheres
by the hydrolysis of zirconium alkoxides, which is the only
available technique to obtain ZrO2 spheres with a narrow size
distribution today, and assembled them into ordered structures
by natural sedimentation. However, the production cost is high
because of the expensive zirconium alkoxide starting materials.
In this work, we present a new spray approach for synthesiz-
ing ZrO2 microspheres with the aim of exploring possible route
to produce monodisperse ZrO2 spheres, which will be used for
the construction of high quality ZrO2 opal photonic crystals via
self-assembly. Besides this application, high pure, mesoporous,
andsphericalZrO2 particlesarealsousefulinadvancedceramics
production, catalyst support, controlled drug release, etc.[9–12].
Self-assembly of monodisperse spheres is an extremely
promising method to fabricate photonic crystals in the visible
and near-infrared wavelength ranges [1–4]. By this method,
SiO2 and polystyrene spheres are widely used to assemble opal-
like photonic crystals because these two monodisperse spheres
with a size distribution of less than 5% are easy to attain. How-
ever, the properties of the prepared photonic crystals from either
SiO2 or polystyrene microspheres are not impressive due to rel-
atively low refractive indices of SiO2 (∼1.45) and polystyrene
(
∼1.59). Up to date, only a few groups have achieved the synthe-
sis and crystallization of monodisperse colloidal spheres made
from materials with a higher refractive index, like TiO2 [5].
These new opals exhibit photonic band structures different from
those of conventional opals owing to their assembly materials
higher refractive indices relative to that of SiO2 and polystyrene.
ZrO2, with a lower absorption in the visible and near-infrared
regions and a relatively high refractive index (2.13–2.20), is
another excellent candidate for use to create opal photonic crys-
2
. Experimental
An aqueous solution of ZrOCl2·8H2O (purity grade of 91.5%) was prepared
by mixing the metal salt with distilled water and filtered with a filter paper to
remove the solid impurity. NH3 gas (purity grade of 99.9%) from ammonia
cylinder was used as received.
A novel reaction method was used to produce ZrO2 spheres. Fig. 1 shows
−
1
the schematic reaction diagram. An aqueous solution (c = 0.49–1.34 mol L
)
∗
Corresponding author. Tel.: +86 010 51683627; fax: +86 010 51688433.
E-mail address: zhanghui14305@sohu.com (H. Zhang).
of ZrOCl2·8H2O was first atomized in the aerosol generator. After turning on
the vacuum device, the aerosols of zirconium salt solution were carried into
0
925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.10.039

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H. Zhang et al. / Journal of Alloys and Compounds 464 (2008) 569–574
Fig. 1. A schematic diagram of the production system for ZrO2 precursor
spheres.
the reactor, into which NH3 gas (qv = 8.33 × 10− –31.94 × 10 m3 s ) from
the cylinder with a pressure regulator was supplied. Thus, in the reactor quasi-
gaseous phase reaction took place at room temperature between droplets of
zirconium salt solution and NH3 gas, which was also used as carrier gas in this
method, forming the spherical ZrO2 precursor (hydrous ZrO2) particles. The
produced ZrO2 precursor spheres were carried into particle collector under a
reduced pressure (P = 92.50–100.37 kPa). The particle collector was filled with
distilled water and used simultaneously as the buffering bottle. The collected
ZrO2 precursor spheres were washed several times with distilled water until
no chloride ions could be detected in the supernatant solution. Afterwards, the
cleaned ZrO2 precursor spheres were washed twice with anhydrous ethanol to
remove absorbed water molecules. Finally, the precursor spheres were dried in
6
−6
−1
◦
Fig. 3. SEM image of ZrO2 precursor spheres after dried at 80 C for 2 h.
particles with a smooth surface. And SEM (Fig. 3) image of pre-
cursor spheres after drying illustrates an average size of 2.37 ␮m
(relative standard deviation 0.22). These images demonstrated
that the new technique could be successfully used to synthesize
the ZrO2 precursor spheres with a smooth surface, which was
important for the construction of opal photonic crystal via self-
assembly. However, some ZrO2 precursor spheres were broken
after drying. It is possible that these broken spheres were mostly
formed from the larger aerosols that were mainly aggregates
of smaller aerosols, and from those that were suspended in the
upper section of the reactor. It can be reasonably proposed that
duringtheprecursorsphereformation, twostepscouldtakeplace
after the aerosols flew into the reactor. The first step was the dis-
solution of ammonia molecules in the zirconium salt solution
droplets that acted as the microreactor. The second step was the
precipitation of zirconium salt that occurred first at the surface
of droplets. The speed of particle formation chiefly depended
on the diffusion rates of the aqueous ammonia molecules and
subsequently dissociated hydroxide ions at the droplet surface
and the rate of precipitation. The reaction rate of precipitation
should be much greater than that of diffusion because ZrO2 pre-
◦
◦
air at 80 C for 2 h and calcined in air at 350 C for 15 min, and at 600, 700, and
◦
8
00 C for 2.5 h, respectively.
Sphere size and morphology were determined using HITACHI H-8100 TEM
and Cambridge Stereoscan 250 MK 2 SEM. Average sphere sizes were estimated
by counting 100 particles from representative TEM or SEM images of samples.
The sphere microstructures were observed on a JEOL JEM2010 TEM. Spec-
imens for TEM observation were prepared by embedding ZrO2 precursor or
ZrO2 spheres in copper film by electrical chemical reaction, fixing the copper
film wrapping spheres on the grid, and ion milling. Thermal analysis of ZrO2 pre-
cursor powder was conducted on the thermal analyzer NETZSCH STA 449C to
simultaneously obtain thermogravimetry and differential scanning calorimetry
(TG/DSC). The phases in the powders were identified by Rigaku D/Max-2400
XRD using CuK␣1 radiation.
3
. Results and discussion
TEM (Fig. 2) and SEM (Fig. 3) images of ZrO2 precursor
spheres before and after drying clearly show fairly spherical
Fig. 2. TEM image of colloidal ZrO2 precursor spheres.
Fig. 4. TG/DSC curves of ZrO2 precursor powder.

H. Zhang et al. / Journal of Alloys and Compounds 464 (2008) 569–574
571
◦
◦
◦
Fig. 5. SEM images of ZrO2 microspheres after calcined at (a) 600 C, (b) 700 C, (c) 800 C.
cursor (Zr(OH)4) produced at the surface of droplets prevented
the diffusions of the aqueous ammonia molecules and hydrox-
ide ions into the inside of droplets. Therefore, some of the larger
aerosols and those floating in the upper section of the reactor
were pumped into particle collector before they were reacted
completely, resulting in the formation of spheres with a loose
core and dense shell (hollow spheres). When these imperfect
spheres were dried, the gas produced from the vaporization of
a large amount of ethanol and residual water could not escape
from the dense shells, resulting in sharp increase of the pres-
sure inside the spheres and subsequent breaking of the spheres.
On the contrary, for those completely precipitated spheres with
a homogeneous structure, the small amount of gas inside the
spheres could be successfully released without damaging the
spheres after drying. Moreover, Fig. 3 demonstrated that aggre-
gations happened among spheres after drying. Aggregates were
formed possibly due to hydrogen bonds between the hydroxyl
groups of adjacent hydrous ZrO2 spheres. After dehydration dur-
ing drying, chemical bonds (Zr-O-Zr) between particles could
be formed, resulting in hard aggregates [13]. Additionally, the
residual chloride ions could also cause the formation of hard
aggregates.
Fig. 6. Structures of the prepared ZrO2 precursor (a) and ZrO2 (b)–(d) microspheres. (a)–(b) the homogenous structure, (c) the structure with a loose core and dense
shell, (d) the hollow structure.

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H. Zhang et al. / Journal of Alloys and Compounds 464 (2008) 569–574
Thermal analysis (Fig. 4) of ZrO2 precursor particles
illustrated their decomposition characteristics and phase trans-
formationpropertiesduringheating(heatingrateof10.0 K/min).
SEM micrographs (Fig. 5) of ZrO2 microspheres calcined at
600, 700, and 800 C showed that the ZrO2 particles after cal-
◦
cinations still kept the spherical shape and the sphere surface
was smooth after crystallization. Average diameters of ZrO2
microspheres calcined at different temperatures were 1.71 ␮m
The DSC curve revealed an endothermic peak at around
◦
1
03.6 C, which corresponded to the volatilization and vapor-
◦
◦
ization of the absorbed ethanol and water in the powder. The TG
curveshowedasignificantweightlossofabout19.71%ataround
(600 C, relative standard deviation 0.29), 1.63 ␮m (700 C,
◦
relative standard deviation 0.34), and 1.66 ␮m (800 C, rela-
◦
the corresponding peak temperature of 103.6 C. Additionally,
tive standard deviation 0.30), respectively. In comparison with
precursor spheres, ZrO2 spheres calcined in the temperature
◦
the DSC curve exhibited an exothermic peak at about 533.8 C,
◦
which corresponded to the crystallization of ZrO2. It should be
noted that the TG curve did not display a rapid weight loss at
range of 600–800 C shrunk by 28–31% in size, stemming from
the dehydration decomposition and the structural densification.
Densification happened to those spheres with a porous structure.
A detailed discussion about the shrinkage will be given later in
this report.
◦
temperatures higher than 103.6 C, nor the DSC curve presented
any endothermic peak corresponding to structural dehydration.
Thesedatasuggestedthatstructuraldehydrationoccurredalmost
through the whole heating process, even probably at around
As discussed, three kinds of structures of the precursor
spheres were likely present, the homogenous, the soft core, and
◦
1
03.6 C.
◦
◦
◦
Fig. 7. Microstructures of the ZrO2 spheres before (a) and after calcination at (b) 600 C, (c) 700 C, (d) 800 C.

H. Zhang et al. / Journal of Alloys and Compounds 464 (2008) 569–574
573
◦
the hollow structures. Similar structures of ZrO2 microspheres
after calcinations were also found. Typical TEM micrographs
stable phase for pure ZrO2 is m-ZrO2 below 1170 C and subse-
◦
quently t-ZrO2 and c-ZrO2 above 1170 C. It is understandable
(
6
Fig. 6b–d) of cross sections of ZrO2 microspheres obtained at
00–800 C clearly demonstrated those three kinds of interior
that the precursor spheres were not crystalline phase before cal-
cined, since not enough energy was provided for the precursor to
dehydrate structurally and crystallize during synthesis at room
◦
structures that ZrO2 spheres possess, the homogeneous struc-
ture (Fig. 6b), the structure with a loose core and dense shell
◦
temperature and drying at 80 C for 2 h. An abnormal obser-
(
Fig. 6c), and the hollow structure (Fig. 6d). The loose core and
vation was that without doping with any bivalent or trivalent
impurities, all the ZrO2 microspheres calcined at 600–800 C
◦
hollow structures were produced from those precursor spheres
incompletely precipitated. As a comparison with Fig. 6b, we also
presented a representative TEM image (Fig. 6a) of the cross sec-
tion of a ZrO2 precursor sphere with a homogeneous structure,
where the central dark area was due to thick specimen. Fig. 6a,b
revealed that those well-developed precursor and ZrO2 spheres,
formedfromthecompletelyprecipitatedprecursorspheres, were
a homogeneously porous structure. In order to further under-
stand the interior microstructure and their formation of the
were not the stable monoclinic form, but mostly the phase at
high temperature, c-ZrO2. This nonequilibrium behavior was
also found in the published report [14], where pure ZrO2 fine
powders were produced by spray pyrolysis. It was most likely
caused by the kinetics of the phase transformation process. It was
shown from DSC curve that crystallization of ZrO2 mainly took
◦
place at around 533.8 C in this investigation. When the precur-
◦
sor was calcined at 600 C, ZrO2 generated by dehydration of
well-developed ZrO2 spheres before and after calcination at
the precursor gained kinetic energy high enough to crystallize.
The transition from the highest-energy state to an intermediate
energy state occurred with a much lower activation energy than
the transition to the most stable state [15]. Another reason for the
formation of c-ZrO2 can be found in the structural relationships
between the starting precursor and the final product. In general,
high-temperature phases have a more open structure than low-
temperature crystalline phases and consequently are more like
the structure of a glassy precursor [15]. Additionally, the size
of grains of nanometer scale in ZrO2 microspheres is proba-
bly another reason for the metastable phases existing in lower
temperature range. These factors tend to favor crystallization
of c-ZrO2 from amorphous precursor, even in the temperature
range of stability of a lower temperature form. Therefore, a pro-
cess of transformation from c-ZrO2 to t-ZrO2 to m-ZrO2 can be
expected with rising calcination temperature.
◦
6
00–800 C, we conducted TEM/HRTEM examination of the
cross sections of microspheres at high magnifications. The typi-
cal TEM micrographs (Fig. 7) of cross sections of spheres before
◦
and after calcinations at 600–800 C evidently showed that both
ZrO2 precursor and heat-treated ZrO2 microspheres were com-
posed of peanut-like nano-particles/grains. The average width
diameter (∼16 nm) of nano-particles in precursor microspheres
was larger than those (∼8–13 nm) of nano-grains after heat
◦
treated at 600–800 C. With the increase of calcination temper-
ature, grain size grew from around 8 to 13 nm in width-diameter
direction. Obviously, the pore size in precursor was smaller than
those in heat-treated microspheres. This kind of mesoporous
ZrO2 microspheres is likely useful in advanced ceramics prepa-
ration, catalyst support, and controlled drug release.
XRD patterns (Fig. 8) of the spherical ZrO2 precursor and
ZrO2 powders revealed the phase compositions in powders
Figs. 9 and 10 are representative HRTEM images of cross
sections of the precursor and heat-treated microspheres, respec-
tively. It again demonstrated that the precursor was amorphous
though some tiny crystalline particles were formed in the
amorphous matrix, as indicated by arrowheads in Fig. 9. This
◦
before and after calcination at 600–800 C. It showed that ZrO2
precursor spheres before calcination were amorphous, while all
◦
the ZrO2 spheres calcined at 600–800 C consisted of major
c-ZrO2 and minor t-ZrO2, and t-ZrO2 content gradually rose
with the increase of calcination temperature. It is known that the
Fig. 8. XRD patterns of the spherical powders of ZrO2 precursor (a) and ZrO2
prepared at (b) 600 C, (c) 700 C, (d) 800 C.
Fig. 9. A typical HRTEM picture of the cross section of the synthesized ZrO2
precursor spheres.
◦
◦
◦

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H. Zhang et al. / Journal of Alloys and Compounds 464 (2008) 569–574
enlargement in sizes, resulting from the increase of t-ZrO2
content in microspheres (crystalline phase densities:c-ZrO2
(
6.27) > t-ZrO2 (6.10) > m-ZrO2 (5.68)). The size of micro-
◦
spheresproducedat800 Cwasalittlebiggerthanthatofspheres
calcined at 700 C. This was because the size enlargement of
microspheres produced at 800 C, caused by further increase
◦
◦
of the tetragonal phase content, was larger than the shrinkage
caused by structural densification during calcination.
It should be pointed out that the colloidal ZrO2 precursor
spheres synthesized here were not monodisperse yet. There-
fore, they cannot be used as building blocks to construct ordered
photonic crystal structure. These problems will be further inves-
tigated and the synthetic process will be improved. However,
mesoporous ZrO2 spheres obtained here could still find other
high-potential applications in advanced ceramics production,
catalyst support, controlled drug release, chromatography sepa-
ration, etc.
4. Conclusions
In summary, a novel spray technique for synthesizing ZrO2
microspheres was developed. The prepared ZrO2 spheres were
characterized by SEM, TEM/HRTEM, and XRD. It was shown
that fairly spherical ZrO2 particles, with a smooth surface and
an average diameter from 1.63 to 1.71 ␮m, were obtained by
◦
using the new process and calcination at 600–800 C. The
well-developed ZrO2 microspheres had a homogeneously meso-
porous microstructure with peanut-like nano-grains around
8
–13 nm in width diameter. The phase compositions after calci-
◦
nation at 600–800 C were mainly c-ZrO2 with minor t-ZrO2.
Acknowledgments
This work has been financed by both National Science Foun-
dation of China (Grant No. 59974026) and Paper Foundation of
Beijing Jiaotong University (Grant No. PD 282).
Fig. 10. Two typical HRTEM pictures of cross sections of the ZrO2 micro-
spheres obtained at 600–800 C.
◦
observation suggested that some crystals were formed during
the preparation of precursor even though they cannot be detected
by X-ray. Fig. 10 clearly showed the formation of c-ZrO2 with
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◦
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◦
comparing with that calcined at 600 C. This size decrease
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◦
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