Synthesis and characterization of zirconia nanorods

Article abstract of DOI:10.1111/j.1151-2916.2002.tb00596.x

ZrO₂ nanorods are prepared by annealing precursor powders produced in the novel inverse microemulsion system. The length and diameter of ZrO₂ nanorods are a few micrometers and 40-100 nm, respectively. The microstructure of the resultant nanorods are studied by XRD, TEM, selected area electron diffraction, and Raman spectroscopy. The ZrO₂ nanorods are single crystalline and have monoclinic structure. The formation of ZrO₂ nanorods is discussed.

Full text of DOI:10.1111/j.1151-2916.2002.tb00596.x

J. Am. Ceram. Soc., 85 [12] 3120–22 (2002)
journal
Synthesis and Characterization of Zirconia Nanorods
Yingkai Liu,†,‡ Chenglin Zheng,† Wengzhong Wang,† Yiongjie Zhan,† and Guang Hou Wang†,§
National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China
Department of Physics, Yunnan Normal University, Kunming 650092, People’s Republic of China
phase.18 Unlike conventional emulsions, however, I␮E domains
fluctuate in size and shape and undergo spontaneous coalescence
and breakup. They can exhibit water-continuous structure, with
typical equilibrium domain sizes ranging from 10 to 100 nm.
These domains are ideal for the preparation of extremely fine
particles.18 In the last decades, nanoparticles of metal oxides have
been prepared in I␮E.19,20 Most recently, I␮E systems have found
utility as templates for synthesis of hollow polymer shells21 and
nanocrystal superlattices.22 Here, we employ I␮E systems to
prepare the precursor powders of ZrO2 nanorods.
ZrO2 nanorods are prepared by annealing precursor powders
produced in the novel inverse microemulsion system. The
length and diameter of ZrO2 nanorods are a few micrometers
and 40–100 nm, respectively. The microstructure of the result-
ant nanorods are studied by XRD, TEM, selected area electron
diffraction, and Raman spectroscopy. The ZrO2 nanorods are
single crystalline and have monoclinic structure. The forma-
tion of ZrO2 nanorods is discussed.
I. Introduction
IRCONIA (ZrO2) has attracted much attention in the fields of
Z
electrochemical devices, structural ceramics, oxygen sensors,
II. Experimental Procedures
catalytic systems, and magnetic and thermal fields.1–3 For exam-
ple, ZrO2, with a melting point of 3100 K, is one of the most
important refractories, and fine-grained ZrO2 ceramics have im-
portant applications where superplasticity and transformation
toughening are highlighted.4–6 Today, ceramic fibers and whiskers
have been increasingly used as reinforcements for advanced
composite materials. The mechanical properties of these reinforce-
ments significantly affect the strength of composite materials.
Single-crystal fibers show very high mechanical strength because
of their crystalline perfection and small dimensions, which mini-
mize the occurrence of defects. It is well-known that the strength
of ceramic fibers and whiskers is size-dependent.7–10 As the
diameter decreases, the strength of ceramic fibers and whisker
increases. Meanwhile, the fracture of a single crystal of a brittle
material is initiated through the formation of high stress concen-
trations at a particular imperfection or combination of imperfec-
tions in the structure. However, the smaller whiskers approach a
condition of being almost free of imperfections.11–13 If ZrO2
nanorods and whiskers can be synthesized, their strength is bound
to be excellent. Although many methods have been developed to
synthesize ZrO2 powders, including precipitation,14 sol–gel,15
thermal decomposition,16 and hydrothermal treatment,17 there are
few methods for preparing ZrO2 ceramic nanofibers and
nanowhiskers. In the present work, the synthesis of ZrO2 nanorods
is accomplished by annealing precursor powders in which NaCl
and Zr(OH)4 particles are homogeneously mixed. Inverse micro-
emulsion techniques have been used to prepare the precursor
powders.
The starting materials used in the present work included a
high-purity zirconium oxychloride (ZrOCl2⅐8H2O), sodium chlo-
ride (NaCl), and sodium hydroxide (NaOH). A high-purity cyclo-
hexane, mixed polyoxyethylene (5) nonylphenol ether (NP5) with
volume ratio 1:1, polyoxyethylene (9) nonylphenol ether (NP9),
p-octyl polyethylene glycol phenyl ether (OP), and polyoxyethyl-
ene alkyl ether (AEO) were used in the volume ratio 4:2:1:1. All
chemical reagents were analytical grade. Three microemulsion
compositions were prepared: (a) 48 wt% cyclohexane ϩ 48 wt%
NP5/NP9/OP/AEO (hereafter referred to as NPPA) ϩ 4 wt% 1
mol/L ZrOCl2 aqueous solution (hereafter expressed as “A”); (b)
46 wt% cyclohexane ϩ 46 wt% NPPA ϩ 8 wt% 2 mol/L NaOH
aqueous solution (hereafter expressed as “B”); and (c) 41.7 wt%
cyclohexane ϩ 41.7 wt% NPPA ϩ 16.6 wt% 2 mol/L NaCl
aqueous solution (hereafter expressed as “C”); First, 26 mL of “A”
was added to 36 mL of “C” while being stirred magnetically to
produce “D.” Subsequently, 27 mL of “B” were added to “D”
while being vigorously magnetically stirred to produce mixture
“E.” Mixing was achieved by vigorously magnetically stirring “E”
Ͼ30 min to produce precipitates. Next, the precipitates were
repeatedly washed by using anhydrous acetone then centrifuged.
The precursor powders were then dried in an oven at 80°C before
they were calcined at 820°C for 3 h to form ZrO2 nanorods.
The samples prepared were examined by XRD using a diffrac-
tometer (Model D/MAX-RA, Rigaku, Japan) with CuK␣ radiation
and a graphite monochromatic crystal. TEM was performed using
a micrometer (Model JEM–200CX, JEOL, Tokyo, Japan) to
characterize the ZrO2 nanorods. The samples for TEM measure-
ments were prepared by dispersing some products in ethanol,
immersing them in an ultrasonic bath for 30 min, then putting a
few drops of the resulting suspensions onto a copper grid coated
with a layer of amorphous carbon. Crystal structures were deter-
mined by selected area electron diffraction (SAD) using an
accelerating voltage of 200 kV and a double-tilt sample holder.
Measurements of Raman spectra were performed on a spectrom-
eter (Model 1403, Spex Industries, Edison, NJ) under backscattering
geometry. The blue line (514.5 nm) of an Arϩ laser was taken as the
excitation source. ZrO2 nanorods were weighed and compressed into
disks 12 mm in diameter and 0.5 mm in thickness under a pressure of
3 ϫ 108 Pa.
An inverse microemulsion (I␮E), which consists of an oil
phase, a surfactant, and an aqueous phase, is a thermodynamically
stable isotropic dispersion of the aqueous phase in a continuous oil
T. W. Coyle—contributing editor
Manuscript No. 187778. Received April 6, 2001; approved January 2, 2002.
Supported by National Nature Science Foundation of China, under Grant Nos.
29890210, 10074024, and 10023001.
†Nanjing University.
‡Yunnan Normal University.
§Author to whom correspondence should be addressed.
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December 2002
Communications of the American Ceramic Society
3121
Fig. 2. XRD pattern of ZrO2 nanorods annealed at 820°C for 3 h.
given in figure). The diffraction data are consistent with Powder
Diffraction File 37-1484, National Centre for Diffraction Data,
Newtown Square, PA. No tetragonal ZrO2 is detected.
Figure 3 shows the Raman spectra of the as-prepared ZrO2
nanorods; they are similar to those for bulk monoclinic ZrO2,23,24
with the exception of a redshift in frequency and the Raman line at
142 cmϪ1 belonging to tetragonal ZrO2. The Raman lines at
186.65, 302.80, 343.95, 471.54, 554.32, and 634.21 cmϪ1 can be
assigned as the Ag modes of the monoclinic phase. The Raman
lines at 215.31 and 329.18, 379.30, 498.14, and 611.76 cmϪ1 can
be ascribed to the Bg modes, and the line at 173.72 cmϪ1 is derived
from the Ag ϩ Bg mode of monoclinic ZrO2. This further proves
that the as-prepared nanorods have a monoclinic phase and show
long-range order structure. XRD usually reveals the long-range
order of the materials and gives structural information within
several unit cells. Raman scattering, as a local probe, is very
sensitive to the crystallinity and microstructures of the materials.
The very weak Raman line at 142 cmϪ1 implies that ZrO2
nanorods contain a little tetragonal phase and that ZrO2 nanorods
are not phase-pure, although no tetragonal ZrO2 is detected by the
XRD measurement. However, from the XRD measurement and the
weak peak of the Raman spectrum for the monoclinic ZrO2, the
major phase of the ZrO2 nanorods is a monoclinic structure.
Fig. 1. TEM and SAD images of ZrO2 nanorods. (a) Pictures of ZrO2
nanorods annealed at 820°C for 3 h, (b) morphology of one single ZrO2
nanorod annealed at 820°C for 3 h, and (c) SAD of one such single-
crystalline ZrO2 nanorod.
III. Results and Discussion
Figure 1(a) shows a typical micrograph of ZrO2 nanorods, in
which ZrO2 nanorods are straight with diameters of 40–100 nm
and lengths up to a few micrometers. One of the nanorods with 40
nm diameter is shown in Fig. 1(b), and its SAD pattern is shown
in Fig. 1(c), indicating that it is a single crystal. The SAD patterns
can be indexed according to the monoclinic structure.
Figure 2 shows XRD patterns of ZrO2 nanorods. All peaks are
indexed as the monoclinic lattice of ZrO2 (their hkl indexes are
Fig. 3. Raman spectra of ZrO2 nanorods annealed at 820°C for 3 h.

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Communications of the American Ceramic Society
Vol. 85, No. 12
3C. K. Nantla, J. E. Allison, D. R. Baure, and H. S. Gandhi, “Materials Chemistry
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Temperature has a stronger effect on the formation of ZrO2
nanorods. After annealing at Ͻ780°C, no ZrO2 nanorods are
detected, whereas annealing at Ͼ820°C produces a few ZrO2
nanorods and bulk ZrO2 crystals.
In addition, the influence of surfactants on the formation of
ZrO2 nanorods is investigated. Under the same conditions, without
AEO or OP, only very short ZrO2 whiskers are observed. The
development of the morphology of the products is related to the
growth environments. There is a strong correlation between the
formation of ZrO2 nanorods and the existences of OP, NP, and
AEO. The effects of AEO or OP on the formation of ZrO2
nanorods are not clear. Further study is in progress.
Based on these experiments, it is suggested that the formation
mechanism of ZrO2 nanorods is similar to that occurring in molten
salt synthesis (MSS) approaches,25 which are similar to those in
VLS and SLS approaches.17 Nanorod growth requires a fluid
phase in which elements of the crystal phase can easily move for
a certain distance. Another advantage of the fluid phase is that the
materials for nanorod formation are homogeneously distributed.
Such homogeneity, which is ascribed to the microemulsion, leads
to uniform nanorod water/oil (W/O) microemulsions consisting of
nanosized water droplets dispersed in a continuous oil medium and
stabilized by surfactant molecules accumulated at the W/O inter-
face. The highly dispersed water pools are an ideal nanostructural
reactor18 for producing monodispersed nanoparticles, which make
the precursors decompose easily to form ZrO2 nanorods. The
formation process of the ZrO2 nanorod is as follows: the precur-
sors are fired at a temperature above the melting point of the salt
to form a flux of the salt composition. At this temperature, the
oxides rearrange and then diffuse rapidly in the liquid salt. In the
heating process, the ZrO2 nanorods are formed through nucleation
and growth. At higher synthesis temperatures, the ZrO2 nanorods
become larger in diameter and even become bulk ZrO2. The
formation reaction for the ZrO2 nanorod in MSS is very fast. The
action can be completed in a very short time because of the short
diffusion distance and the high mobility of species in the liquid
state. In this case, below 780°C, NaCl does not melt. Therefore, no
molten phase forms, and no nanorods are detected.
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