Two-phase synthesis of shape-controlled colloidal zirconia nanocrystals and their characterization

Article abstract of DOI:10.1021/ja0612145

We have developed a two-phase approach for the synthesis of shape-controlled colloidal zirconia nanocrystals, including spherical-, teardrop-, rod-, and rice grain-shaped particles. We found that the key factors for controlling the shape were the reaction time, the nature of the capping agent, and the monomer concentration. We have analyzed the morphologies, crystallinity, optical properties, and structural features of the as-prepared ZrO₂ nanoparticles by using transmission electron microscopy (TEM), high-resolution TEM, X-ray powder diffraction, and UV-vis absorption and fluorescence spectroscopy. The possible nucleation and growth process is also discussed.

Full text of DOI:10.1021/ja0612145

Published on Web 07/19/2006
Two-Phase Synthesis of Shape-Controlled Colloidal Zirconia
Nanocrystals and Their Characterization
Nana Zhao, Daocheng Pan, Wei Nie, and Xiangling Ji*
Contribution from the State Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of the Chinese
Academy of Sciences, 5625 Renmin Street, Changchun 130022, People’s Republic of China
Received February 20, 2006; E-mail: xlji@ciac.jl.cn
Abstract: We have developed a two-phase approach for the synthesis of shape-controlled colloidal zirconia
nanocrystals, including spherical-, teardrop-, rod-, and rice grain-shaped particles. We found that the key
factors for controlling the shape were the reaction time, the nature of the capping agent, and the monomer
concentration. We have analyzed the morphologies, crystallinity, optical properties, and structural features
of the as-prepared ZrO2 nanoparticles by using transmission electron microscopy (TEM), high-resolution
TEM, X-ray powder diffraction, and UV-vis absorption and fluorescence spectroscopy. The possible
nucleation and growth process is also discussed.
Introduction
or that can be dispersed completely in certain solvents. The
shapes of nanocrystals influence their potential applications. For
ZrO2 is an important ceramic material that has widespread
potential applicability in the fields of structural materials, solid-
state electrolytes, thermal barrier coatings,1 electro-optical
materials,2 gas sensing, corrosion resistance, and catalysis.3 Over
the past two decades, several methods have been developed for
the preparation of zirconia nanocrystals, including sol-gel,4-6
hydrothermal/solvothermal,7,8 thermal decomposition,9-11 emul-
sion precipitation,12 and microwave/sonication-assisted copre-
cipitation13 approaches. Another possible general method for
preparing zirconia nanocrystals was described recently; it is
based on the phase transfer and separation mechanism.14 Jin et
al.4 synthesized colloidal monodisperse zirconia nanocrystals
of 2.9 nm diameter through a nonhydrolytic sol-gel reaction
between zirconium(IV) isopropoxide and zirconium(IV) chloride
at 340 °C. Noh and co-workers demonstrated the formation of
anisotropic ZrO2 nanocrystalline powders when using a hydro-
thermal process to perform reactions at temperatures in the range
150-250 °C. Unfortunately, this method is not suitable for
producing nanocrystals that possess a narrow size distribution
example, spherical ZrO2 nanocrystals have been applied as fuel
cell electrolytes, oxygen sensors, and gate dielectrics.7 In
contrast, ZrO2 nanocrystals having anisotropic shapes, such as
rod, rice, and leaf morphologies, are expected to be of use in
the fabrication of fibers, films, ceramic coatings, and grain-
oriented ceramics.4 Thus, it is desirable to develop a single
synthetic approach to provide soluble, shape-controlled ZrO2
nanocrystals under mild conditions for practical large-scale
production. Recently, Pan and Wang developed a two-phase
and a two-phase thermal approach for the production of highly
luminescent CdS, extremely small CdSe, and CdSe/CdS core/
shell quantum dots under relatively mild conditions.15-17 In
addition to these II-VI semiconductors, Pan and co-workers
also synthesized luminescent TiO2 nanocrystals having control-
lable sizes.18 In this paper, as a part of our extended work, we
report a two-phase thermal route, that is, a two-phase approach
combined with an autoclave, for the synthesis of crystalline ZrO2
nanocrystals having controllable shapes within a narrow size
distribution. In this approach, we mixed solutions of zirconium-
(IV) n-propoxide and oleic acid (OA) (or other fatty acid) in
toluene and tert-butylamine in water and then heated the mixture
without stirring. This method exhibits a number of interesting
features: (i) The reaction temperature is less than 180 °C; that
is, it is much lower than the temperature required for the thermal
decomposition process. Indeed, we synthesized nanocrystals
even at temperatures below 120 °C. (ii) Nucleation and growth
appear to take place continuously without a clear boundary. (iii)
(1) Birkby, I.; Stevens, R. Key Eng. Mater. 1996, 122, 527-552.
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(3) Murase, Y.; Kato, E. J. Am. Ceram. Soc. 1982, 66, 196-200.
(4) Joo, J.; Yu, T.; Kim, Y. W.; Park, H. M.; Wu, F.; Zhang, J. Z.; Hyeon, T.
J. Am. Chem. Soc. 2003, 125, 6553-6557.
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A. J. Less-Common Met. 1986, 116, 301-306.
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982-985.
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(12) Woudenberg, F. C. M.; Sager, W. F. C.; Sibelt, N. G. M.; Verweij, H.
AdV. Mater. 2001, 13, 514-516.
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Eur. J. 2005, 11, 3843-3848.
(17) Pan, D. C.; Wang, Q.; Jiang, S. C.; Ji, X. L.; An, L. J. AdV. Mater. 2005,
17, 176-179.
(13) Liang, J.; Deng, Z.; Jiang, X.; Li, F.; Li, Y. Inorg. Chem. 2002, 41, 3602-
3604.
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17, 1991-1995.
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10118
J. AM. CHEM. SOC. 2006, 128, 10118-10124
10.1021/ja0612145 CCC: $33.50 © 2006 American Chemical Society

Shape-Controlled ZrO2 Nanocrystals via a Two-Phase Route
A R T I C L E S
The reaction occurred under a certain pressure at the interface
between the two phases; this process probably facilitates the
preparation of shape-controlled nanocrystals. (iv) The resulting
nanocrystals were capped with alkyl chains, and thus they were
soluble in nonpolar solvents.
Experimental Section
Materials. Oleic acid (OA, tech. grade, 90%) and myristic acid (MA,
99.5%) were purchased from Aldrich. Zirconium(IV) n-propoxide (25%
in alcohol) was purchased from Strem Chemical Co. Decanoic acid
(DA, 98%), dodecanoic acid (DDA, 98%), docosanoic acid (DCA),
stearic acid (SA, 98%), and tert-butylamine (98.5%) were purchased
from China Medicine Shanghai Chemical Reagent Corp.
Synthesis of ZrO2 Nanocrystals. An aqueous solution (15 mL) of
tert-butylamine (200-1000 µL) was loaded into a 30-mL Teflon-lined
stainless steel autoclave. A toluene solution (5 mL) containing
zirconium(IV) n-propoxide (150-900 µL), OA (1.0 mL), and SA (or
MA, DA, DDA, or DCA; 0.5 g) was then transferred to the autoclave
to provide a two-phase reaction system. Without stirring the mixture,
the autoclave was sealed and maintained at 120-180 °C for a fixed
period of time and cooled to ambient temperature using tap water. The
crude solution of ZrO2 nanocrystals was precipitated with methanol
and further isolated by centrifugation and decantation. The purified
nanocrystals were redispersed in organic solvents for optical and
structural characterization without any size sorting.
Characterization of Nanocrystals. UV-vis absorption spectra were
recorded on a Shimadzu UV-2450 double-beam recording spectropho-
tometer using 1-cm quartz cells. Fluorescence spectra were obtained
on a Shimadzu RF-5301 instrument operated at a resolution of 1.0 nm.
Transmission electron microscopy (TEM) images were obtained using
a JEOL-1011 electron microscope operated at an accelerating voltage
of 100 kV. Samples for TEM observation were prepared by dropping
10 µL of diluted toluene solution onto 400-mesh carbon-coated copper
grids. The particle size and size distribution diagrams were obtained
after careful measurement of more than 200 individual ZrO2 nanopar-
ticles. HRTEM measurements were performed using a JEOL-2010F
transmission electron microscope operated at an acceleration voltage
of 200 kV. X-ray powder diffraction (XRD) investigations were
performed using a Rigaku D/MAX-2500 instrument (Cu KR1 radiation)
operated at 50 kV and 250 mA over the range 20-80° by step scanning
with a step size of 0.02°. The crystallite sizes were estimated using
Scherrer’s method. Raman spectra were recorded at room temperature
using a FT-Raman 960 instrument equipped with a 1064-nm laser beam.
XRD and Raman samples were prepared through the evaporation of
one drop of concentrated nanocrystal solution that had been placed on
a glass plate.
Figure 1. (a-d) TEM images of ZrO2 nanocrystals prepared at 180 °C
and a monomer concentration [Zr] of 0.2 M for different reaction times:
(a) 12, (b) 24, (c) 72, and (d) 120 h. (e-h) HRTEM images obtained after
reaction times of (e) 12, (f) 24, (g) 72, and (h) 120 h.
Results and Discussion
(1) Effect of Reaction Time. The TEM images in Figure 1
indicate that the shapes of the nanocrystals could be tuned
through control of the reaction time. At the beginning of the
reaction, the formation of nuclei was predominant. If the
chemically stable precursors produced a small number of nuclei,
we would expect the nanocrystals to grow into an elongated
shape if the monomer concentration remained relatively high
after nucleation.19 After a reaction time of 12 h, we obtained
roughly spherical particles having an average diameter of 2.9
nm (Figure 1a); the corresponding HRTEM image of an isolated
3.7 nm ZrO2 nanoparticle is presented in Figure 1e. The clear
lattice image indicates the high crystallinity of the as-synthesized
nanoparticles. We clearly observe a lattice spacing of 0.318 nm
for the (1h11) plane of the monoclinic ZrO2 structure. Zirconium-
(IV) n-propoxide is a relatively inactive precursor because of
the presence of its inert n-propoxide groups; thus, the system
produces a relatively small number of nuclei. When the
monomer concentration decreases to a critical value, these nuclei
will consume the remaining monomers and the nanocrystals tend
to form elongated shapes.19-21 It appears that the particle growth
occurred selectively on one face of the rods such that almost
teardrop-shaped particles formed with minimal surface energy
(Figure 1b). The teardrop-shaped particles, which had an average
length of ca. 8.6 nm, amounted to no less than 36% in the
sample. Figure 1f presents a HRTEM image of an isolated
(19) Peng, X. G.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120,
5343-5344.
(20) Peng, Z. A.; Peng, X. G. J. Am. Chem. Chem. 2001, 123, 183-184.
(21) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2002, 124, 3343-3353.
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A R T I C L E S
Zhao et al.
Table 1. The Volume Fraction of Monoclinic Phase at Different
Reaction Time
reaction time
Xm
Vm
12 h
24 h
48 h
72 h
5 days
7 days
0.103
0.454
0.476
0.572
0.555
0.580
0.131
0.522
0.544
0.637
0.621
0.644
teardrop-shaped ZrO2 nanoparticle. Manna et al.22 were the first
to observe this shape during the synthesis of CdSe nanocrystals;
they considered that it resulted from an Ostwald ripening process
occurring at very low monomer concentrations, followed by a
rapid increase in the monomer concentration upon further
injection. Peng21,23 also observed tadpole-like CdSe nanorods
whose morphologies were similar to the teardrop-like nano-
crystals mentioned above. They suggested that rods of a certain
length grew into tadpole-shapes because of asymmetric growth
along the c-axis. Obviously, this explanation is not valid for
our samples because of the different reaction conditions that
we employed. It may be possible that, in our system, a dot
formed the body of a tear and then the droplet elongated along
only one facet during the growth process. A further increased
reaction time resulted in an increase in the lengths of the clusters
as a result of growth along the thin end. It was also possible
for two rods to become linked in a head-to-head manner to form
a new “hump-back” rod rather than a straight rod (Figure 1c).
The corresponding HRTEM image in Figure 1g is that of a
nanoparticle having a length of 16.0 nm and an aspect ratio of
6. The distances between the two adjacent planes are 0.294 nm
(top) and 0.316 nm (bottom), corresponding to the (111) and
(1h11) planes in tetragonal and monoclinic ZrO2 phases. Up to
a certain time, the rods gradually became relatively smoother
and their sizes decreased slightly. For example, the average
particle length in Figure 1d is 12.8 nm. The representative
HRTEM image in Figure 1h is that of an isolated ZrO2
nanoparticle having a length of 12.6 nm and an aspect ratio of
3. The distance between the two adjacent planes is 0.298 nm,
which corresponds to the (111) planes of the tetragonal phase
of ZrO2. At this stage, the remaining monomer concentration
in the system was lower than the solubility of the as-prepared
clusters, and thus the clusters dissolved. To minimize the surface
energy of a given nanocrystal, atoms from relatively higher
surface energy positions relocated to the lower-energy sites; this
process led to the formation of nanocrystals having relatively
smooth surfaces.21
Figure 2. Temporal evolution of XRD patterns of OA-capped ZrO2
nanocrystals: (a) 12, (b) 24, (c) 48, (d) 72, and (e) 120 h. The bulk
crystalline ZrO2 diffraction lines are indicated as vertical bars; the thinner
and thicker lines represent monoclinic and tetragonal structures, re-
spectively.
and cubic phases exist only at high temperature above 1170
and 2380 °C, respectively.11 These two high-temperature phases
are unstable in bulk forms at ambient temperature, but Garvie24
predicted that pure ZrO2 is stabilized in the tetragonal form at
room temperature when the (spherical) particle size is less than
30 nm. As far as our system is concerned, the size of the
obtained particle is in the range of critical size predicted in the
literature. If it obeys the surface-energy theory, the tetragonal
polymorph should be spontaneous stabilization in pure zirconia.
However, upon increasing the reaction time, the fraction of
monoclinic phase increased from 0.137 to 0.644 (Figure 2, Table
1). The occurrence of mixed phase indicates that the stability
of metastable phase cannot be explained adequately by the
surface-energy theory.
The correlated changes in shape and phase of ZrO2 particles
have been mentioned in the literature. Nishizawa et al.25 reported
that amorphous ZrO2 transforms into the crystalline form at 130
°C under hydrothermal treatment; however, at a reaction
temperature of 320 °C, the cubic crystallites abruptly transform
to needlelike monoclinic ZrO2. They suspected that the smaller
particles migrate and align themselves to form the larger
monoclinic rods. Noh et al.7 reported that ZrO2 nanocrystals
with anisotropic shapes and various crystal structures were
synthesized by a hydrothermal process. The spherical particles
with a tetragonal phase were changed into the long rodlike
particles with a monoclinic phase with increasing reaction
temperature. They speculate that the monoclinic rods form
through the dissolution and reprecipitation, or by coagulation
of very fine particles in a highly oriented fashion. Recently,
Brus et al.26,27 reported that roughly spherical particles with
tetragonal phase form, whereas at higher temperatures (400 °C),
the particles form as nanorods; meanwhile, the tetragonal phase
changes to the monoclinic phase. The authors confirm that this
concomitant phase and shape change is a result of martensitic
transformation of isolated nanocrystals in a hot liquid.
The volume fraction of monoclinic phase was summarized
in Table 1. It was calculated by the following equations.7
Vm ) 1.311Xm/(1 + 0.311Xm)
(1)
Xm ) [Im(111) + Im(1h11)]/[It(111) + Im(1h11) + Im(111)]
At present, we believed that the nuclei (the coexistence of
monoclinic and tetragonal) could form as a result of the
(2)
where Vm is the volume fraction of monoclinic phase, Xm is the
integrated intensity ratio, Im(111) is the (111) intensity of
monoclinic ZrO2, Im(1h11) is the (1h11) intensity of monoclinic
ZrO2, and It(111) is the (111) intensity of tetragonal ZrO2.
ZrO2 exists in three polymorphs: the monoclinic, tetragonal,
and cubic phases. Normally, the monoclinic phase is thermo-
dynamically stable at room temperature, while the tetragonal
(22) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122,
12700-12706.
(23) Peng, X. AdV. Mater. 2003, 15, 459-463.
(24) Garvie, R. C. J. Phys. Chem. 1978, 82, 218-224.
(25) Nishizawa, H.; Yamasaki, N.; Matsuoka, K.; Mitsushio, H. J. Am. Ceram.
Soc. 1982, 65, 343-346.
(26) Tang, J.; Fabbri, J.; Robinson, R. D.; Zhu, Y.; Herman, I. P.; Steigerwald,
M. L.; Brus, L. E. Chem. Mater. 2004, 16, 1336-1342.
(27) Tang, J.; Zhang, F.; Zoogman, P.; Fabbri, J.; Chan, S. W.; Zhu, Y.; Brus,
L. E. AdV. Funct. Mater. 2005, 15, 1595-1602.
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Shape-Controlled ZrO2 Nanocrystals via a Two-Phase Route
A R T I C L E S
(iv) The percentage of the monoclinic phase increases with
increased reaction time. (cf., Figure 2b-e). However, in the
terminal stage, we cannot obtain the pure monoclinic ZrO2; the
fraction of the monoclinic phase did not show the significant
increment. In the martensitic transformation, a larger driving
force may be needed for ZrO2 than for HfO2, etc.27 Brus et al.
reported they only observed the tfm transformation for HfO2
and Hf-rich HfxZr1-xO2 nanoparticle. Hence, the incomplete
tfm transformation resulting in the tetragonal phase remained.
In addition, the martensitic transformation is independent of time
at a fixed temperature and only changes by varying the
temperature.
Figure 3. (a) High-resolution TEM (HRTEM) image of one OA-capped
ZrO2 nanocrystal for 4 h. (b) Selected area electron diffraction pattern of
sample in (a).
In summary, an explanation based on the concept of a
martensitic transformation in the initial stages may be more
reasonable. It is clear that the shapes of the nanocrystals could
be controlled very well merely by adjusting the reaction time
for the subsequent growth. The shape change is the result of
both the martensitic transformation and the diffusion mechanism,
which more effect the nucleation and growth, respectively.
(2) Effect of Capping Agent. The nature of the capping agent
also played an important role in controlling the shapes of the
nanocrystals. Figures 4a-d and 1c indicate that we could obtain
branched structures, that is, leaf-, arrow-, rice-, rod-, and dot-
shaped nanocrystals, when using the same or different capping
agents. The HRTEM images in Figures 4f-i and 1g reveal
structure information about these nanocrystals and also indicate
the respective distances between their two adjacent planes.
Figure 4j displays the selected area electron diffraction (SAED)
pattern acquired from the corresponding DCA-capped ZrO2
nanoparticles. All of the detectable rings are indexed perfectly
to the same positions as those of standard tetragonal-phase ZrO2.
This result implies that the fraction of monoclinic phase was
very low. It is known that the presence of differently shaped
II-VI semiconductor nanocrystals can be attributed to the fact
that they are formed through a diffusion-controlled growth
process. During diffusion-controlled crystal growth, a diffusion
sphere surrounds each crystal. The monomer concentration
gradient between the bulk solution and the stagnant solution,
as well as the diffusion coefficient of the monomers, determines
the direction (out or into the diffusion sphere) of growth and
the diffusion flux. The monomer concentration in the stagnant
solution maintains the solubility of a given facet through rapid
growth onto or dissolution away from the facet.29 A longer
hydrocarbon chain results in a slower reaction rate because of
the smaller diffusion coefficient. When we increased the length
of the hydrocarbon chain of the capping agents from C10 to C22
(DA < DDA < MA < SA < DCA), we found that the shapes
of the nanocrystals tended to vary from branched to spherical
structures; this result is in good agreement with the diffusion
mechanism described above. The XRD patterns in Figure 5
imply that the fraction of the monoclinic phase decreased upon
increasing the length of the hydrocarbon chain (see Table 2);
this finding may be consistent with martensitic transformation.
Brus et al. considered the martensitic transformation kinetics
are controlled by nucleation and growth; in nature, these
transformation are nucleation-controlled. They believed that the
tfm transformation of nanoparticle is facilitated in the surfactant
(trioctylphosphine oxide, TOPO), and this is probably due to
the enhancement of the nucleation in the presence of surfac-
martensitic transformation and then subsequently come into the
growth stage, which is controlled by the diffusion mechanism,
based on the following evidence.
(i) We have obtained considerable structure information from
high-resolution TEM (HRTEM) for the relatively smaller nano-
particle. Figure S1 shows the high-resolution TEM (HRTEM)
image of OA-capped zirconia nanocrystal at the earliest stage
(reaction time is 2 h in the Supporting Information). The typical
HRTEM image in the inset is that of an isolated ZrO2
nanoparticle having a diameter of 2.8 nm. We clearly observe
a lattice spacing of 0.294 nm for the (111) plane of the tetragonal
ZrO2 structure. Figure 3a shows the HRTEM image of an
isolated 3.6 nm ZrO2 nanoparticle for 4 h. The clear lattice image
indicates the high crystallinity and single crystalline nature of
ZrO2. The lattice spacing of 0.296 nm for the (111) planes of
the tetragonal structure can be readily resolved. Figure 3b is
the selected area electron diffraction (SAED) pattern acquired
from the ZrO2 sample for 4 h. The detectable rings are indexed
to the same position as those from standard tetragonal ZrO2
(JCPDS File No. 14-0534). At the earliest stage, the ZrO2
nanocrystals are the pure or predominantly tetragonal phase
form. After a reaction time of 12 h, the size of as-prepared
nanocrystals slightly increases as compared to the nuclei, but
the structure changed a lot. Further structure information can
be obtained using the XRD patterns (Figure 2a). It indicates
that the obtained nanocrystals exist with a mixture of monoclinic
and tetragonal phases. Confessedly, martensitic transformation
is diffusionless and accompanied by a little change in volume.
Thereby, we supposed the martensitic transformation occurred
in the initial stages.
(ii) Figure 2 indicates that the as-prepared samples are
polymorphs. In addition, the monoclinic and tetragonal coexisted
in one particle (such as Figure 1g) and did not exist beneath
each other in different particles. As far as we know, the
coexistence of both polymorphs in one particle is an indication
for the martensitic transition character.28
(iii) Both high-resolution TEM images and X-ray diffraction
investigations show that twinning structures occur in the
monoclinic nanocrystals. The details will be discussed in the
section of the effect of capping agent. Generally, twinning is a
typical result of martensitic phase transformation. Therefore,
the twinning structures observed provide very important evi-
dence for the phase changes of the nuclei resulting from the
martensitic phase transformation.
(28) Nitsche, R.; Rodewald, M.; Skandan, G.; Fuess, H.; Hahn, H. Nanostruct.
Mater. 1996, 7, 535-546.
(29) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123, 1389-1395.
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Zhao et al.
Figure 5. XRD patterns of ZrO2 nanocrystals prepared at monomer
concentration [Zr] ) 0.2 M for 72 h in the presence of different capping
agents: (a) DA, (b) DDA, (c) MA, (d) SA, (e) OA, and (f) DCA.
Table 2. The Volume Fraction of Monoclinic Phase with Different
Capping Agent
capping agent
Xm
Vm
DDA
MA
SA
1.000
0.585
0.333
0.083
1.000
0.649
0.392
0.106
DCA
Figure 6. (a) HRTEM image of DDA-capped ZrO2 nanocrystals. (b) The
corresponding diffraction pattern obtained from Fourier transforming the
image in (a).
monoclinic nanorods. Similarly, we observed the analogous
structure in the sample of DDA-capped ZrO2 nanocrystals. The
representative HRTEM image in Figure 6a is that of monoclinic
nanocrystals, in which the twinning element is clearly identified.
Figure 6b is a Fourier transformation of the corresponding
HRTEM image, and the twinning is further confirmed by spot
splitting in the diffraction pattern. Definitely, the martensitic
phase transformation exists in our system.
We also obtained considerable structure information from the
Raman spectra. Figure 7 presents the reduced Raman spectra
of the OA- and DDA-capped ZrO2 samples after their annealing
at 400 °C for 1 h to remove the organic ligands. As compared
to the spectrum of bulk monoclinic ZrO2,30 we attribute the two
strong peaks at 148 and 270 cm-1 to the tetragonal phase.
Measured with one of the modes, the peak at 478 cm-1 is present
in both monoclinic and tetragonal phases. These results suggest
that a mixture of monoclinic and tetragonal phases coexists,
which is in good accordance with our XRD data.
Figure 4. (a-e) TEM images of ZrO2 nanocrystals prepared at a monomer
concentration [Zr] of 0.2 M for 72 h in the presence of different capping
agents: (a) DA, (b) DDA, (c) MA, (d) SA, and (e) DCA. (f-i) Cor-
responding HRTEM images: (f) DA, (g) DDA, (h) MA, and (i) SA. (j)
Selected area electron diffraction pattern of the DCA-capped ZrO2 nano-
crystal.
tants.27 For instance, the DDA-capped ZrO2 nanocrystals are
the pure monoclinic phase form and the DCA-capped ZrO2
nanocrystals are predominantly the tetragonal phase form. A
longer hydrocarbon chain results in a slower nucleation and
confines the martensitic phase transformation to some extent.
Brus and co-workers found the twinning structures in the
Figure 8 displays the UV-vis absorption and photolumines-
cence (PL) spectra of the DA-capped ZrO2 nanocrystals in
toluene. Figure 8a indicates that a clear absorption onset occurs
at ca. 3.2 eV. Figure 8b displays the PL spectrum obtained using
(30) Keramidas, V. G.; White, W. B. J. Am. Ceram. Soc. 1974, 57, 22-24.
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Shape-Controlled ZrO2 Nanocrystals via a Two-Phase Route
A R T I C L E S
Figure 9. (a,b) TEM images of ZrO2 nanocrystals prepared over 72 h at
[Zr] monomer concentrations of (a) 0.07 M and (b) 0.4 M. (c,d)
Corresponding HRTEM images: (c) [Zr] ) 0.07 M, (d) [Zr] ) 0.4 M.
(3) Effect of Monomer Concentration. The monomer
concentration is also a key factor in controlling the shape. At a
high monomer concentration, we observed rice-like nanocrystals
having an average length of 5.1 nm (Figure 9b). The typical
HRTEM image in Figure 9d is that of an isolated ZrO2
nanoparticle having a length of 7.2 nm. We readily resolve a
lattice spacing of 0.317 nm for the (1h11) planes of the
monoclinic ZrO2 structure. During the initial stages of the
synthetic procedure, the system produced a relatively large
number of nuclei because of the high monomer concentration.
As a result, the amount of monomer remaining for growth was
relatively low. When the monomers entering the diffusion sphere
are shared for growth in all three dimensions,21 the particles
gradually form into shapes resembling rice grains. When the
monomer concentration was relatively low, these nuclei con-
sumed the remaining monomers and the nanocrystals tended to
form with elongated shapes. The roughly rod-shaped nano-
crystals exhibited in Figures 9a and 1d possessed average lengths
of 15.5 and 14.5 nm, respectively.
Figure 7. Raman scattering spectra of ZrO2 nanocrystals prepared using
(a) OA and (b) DDA as capping agents.
In general, the nucleation process is a crucial step for the
formation of the rod-shaped and other elongated structures. A
relatively high monomer concentration is required during the
formation of the elongated shapes. An excessively high mono-
mer concentration, however, did not lead to the appearance of
elongated shapes because of the rapid nucleation process.
(4) Effect of Reaction Temperature. The reaction temper-
ature also influences the shapes of nanocrystals. Figure 10
displays TEM images of the nanocrystals produced after typical
syntheses at different temperatures, but with a fixed reaction
time (72 h). The nanocrystals prepared at 120 and 150 °C exhibit
roughly spherical shapes, similar to those in Figure 1a and b,
whereas the samples obtained at 180 °C possessed branched
structures. In addition, the percentage of the monoclinic phase
is decreased at lower temperature (the volume fraction of
monoclinic phase was 0.327). In fact, the interplay of chemical
and mechanical (stain) forces plays an important role on the
Figure 8. (Top) UV-vis absorption and (bottom) PL spectra of DA-capped
ZrO2 nanocrystals.
an excitation wavelength of 250 nm; the emission was centered
at ca. 365 nm. These features are probably related to the
involvement of a mid-gap trap state that results from incomplete
surface passivation.
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A R T I C L E S
Zhao et al.
Figure 10. TEM images of ZrO2 nanocrystals prepared for 72 h at [Zr] ) 0.2 M at reaction temperatures of (a) 120, (b) 150, and (c) 180 °C.
reaction process stepwise over a range of temperatures.31 The
martensitic transformation is conspicuous by their predominantly
athermal character, meaning that the phase transformation keeps
pace with the temperature and independent of the time at a fixed
temperature.27,31 In the first instance, it is much more difficult
to transform with the lack of driving force and the significantly
decreased transformation rate at low temperature. At the growth
stage, some small particles are unable to grow into large ones
due to a lower probability of their arrival at the interface from
the oil phase. It seems that a lower temperature can lead to
incomplete mixing between the aqueous phase and organic phase
and a lower degree of monomer diffusion when compared to
those reacted at higher temperatures. Apparently, it is unfavor-
able for nucleation, phase transformation, and growth at low
temperature. However, a high reaction temperature readily
facilitates control over the shape of nanocrystals only through
varying the reaction time.
shapes of these nanocrystals varied from branched structures
to spheres upon increasing the length of the hydrocarbon chain
of the capping agent due to a slower nucleation and confining
the martensitic phase transformation to some extent at the initial
stage. The XRD patterns and Raman spectra indicate that the
as-prepared particles existed as mixtures of monoclinic and
tetragonal phases. A fluorescence spectrum recorded after
excitation at 250 nm displayed a broad band at ca. 365 nm. We
believe that the nanocrystals possessing anisotropic shapes, for
example, the rod- and rice-shaped particles, might find applica-
tions in films, ceramic coatings, and related devices. Indeed,
we hope that this simple, reproducible, and low-cost method
will be exploited in the future for large-scale syntheses of
zirconia and other metal oxides nanocrystals.
Acknowledgment. We are grateful for the support for this
study provided by the National Natural Science Foundation of
China (90101001, 50073024), the Distinguished Young Fund
of Jilin Province (20050104), the Chinese Academy of Sciences
(project KJCX2-SW-H07), and the International Collaboration
Project (04-03GH268, 20050702-2) from Changchun City and
Jilin Province, China. X.J. thanks Dr. L. An’s group for
assistance in obtaining the UV and PL spectra. N.Z. thanks Dr.
L. Sun and Dr. R. Xie for their help in analysis of structure
information.
Conclusions
We have applied a two-phase approach to the successful
synthesis of colloidal zirconia nanocrystals having different
morphologies, including spherical-, teardrop-, rod-, and rice-
shaped particles. The shape change is the result of both the
martensitic transformation and the diffusion mechanism, which
more effect the nucleation and growth, respectively. It is clear
that the shapes of the nanocrystals could be controlled very well
merely by adjusting the reaction time, the nature of the capping
agent, the monomer concentration, and the reaction temperature
for the subsequent growth. TEM images suggested that the
Supporting Information Available: High-resolution TEM
(HRTEM) image of OA-capped zirconia nanocrystal (Figure
S1). This material is available free of charge via the Internet at
http://pubs.acs.org.
(31) Wolten, G. M. J. Am. Ceram. Soc. 1963, 46, 418-422.
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