Phase composition controllable preparation of zirconia nanocrystals via solvothermal method

Article abstract of DOI:10.1016/j.jallcom.2012.03.050

Zirconia nanopowders are synthesized via solvothermal method at a temperature as low as 180 °C without any additives. The pure tetragonal zirconia transforms into nearly pure monoclinic zirconia gradually with the progressing of solvothermal reaction, and the morphology of as-prepared nanoparticles grows up from spherical to short rodlike. The results of crystallite size estimated from XRD patterns using Scherrer equation show that the size of monoclinic zirconia ranges from 10 nm to 50 nm, while the size of tetragonal zirconia is below 5 nm.

Full text of DOI:10.1016/j.jallcom.2012.03.050

Journal of Alloys and Compounds 532 (2012) 98–101
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Phase composition controllable preparation of zirconia nanocrystals via
solvothermal method
Guanghui Lia, Zhanglian Hongb, Hui Yangb,∗, Dongnan Lia
a Department of Materials Science and Engineering, Fujian University of Technology, Fuzhou 350108, China
b Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
a r t i c l e i n f o
a b s t r a c t
Zirconia nanopowders are synthesized via solvothermal method at a temperature as low as 180 ◦C without
any additives. The pure tetragonal zirconia transforms into nearly pure monoclinic zirconia gradually
with the progressing of solvothermal reaction, and the morphology of as-prepared nanoparticles grows
up from spherical to short rodlike. The results of crystallite size estimated from XRD patterns using
Scherrer equation show that the size of monoclinic zirconia ranges from 10 nm to 50 nm, while the size
of tetragonal zirconia is below 5 nm.
Article history:
Received 14 June 2007
Received in revised form 5 March 2012
Accepted 11 March 2012
Available online 18 March 2012
Keywords:
Nanostructured materials
Nanofabrications
© 2012 Elsevier B.V. All rights reserved.
Phase transitions
X-ray diffraction
Scanning and transmission electron
microscopy
1. Introduction
Hence, it is important to synthesis ZrO2 particles with ideal
phase composition, morphology and well dispersibility. Till now,
ZrO2 has three crystal forms as follows: monolithic zirconia (m-
ZrO2), tetragonal zirconia (t-ZrO2) and cubic zirconia (c-ZrO2). The
m-ZrO2 is stable below 1170 ◦C and the c-ZrO2 exists stable above
2370 ◦C. The pure t-ZrO2 is considered to be stable in the range
of 1170–2370 ◦C. Due to the martensitic phase transformation
originated from volumetric change effect when tetragonal-to-
monoclinic phase takes place [1,2], the partial stabilized t-ZrO2
has been used extensively in ceramic because of its transforma-
tion toughening performance. In general, pure t-ZrO2 is well known
as a metastable phase at room temperature, and some oxides like
Y2O3, Sc2O3 and MgO are usually used as additives to prepare
t-ZrO2 [3,4]. Nevertheless, previous study has revealed that the
pure t-ZrO2 can keep its stability at room temperature when pre-
pared through sol–gel method without additives [5]. Furthermore,
recent investigations show that the nanocrystalline m-ZrO2 exhib-
ited much more excellent behaviors than t-ZrO2 in some aspects
such as mechanic and catalytic properties due to its special crystal
structure and surface features [6–8]. However, m-ZrO2 is consid-
ered to be even more difficult to synthesis at low temperature
especially in a pure state [9,10].
there are many different physical, mechanical, chemical and other
methods to obtain ZrO2 nanoparticles [11–13]. Among the wet
chemical synthesis routes, the hydrothermal methods have great
potential to fabricate nanocrystals materials at a relatively low tem-
perature. The main advantages of this method is related to the
homogeneous nucleation and grow processes and very low grain
size due to the elimination of the calcinations step.
In present study, pure t-ZrO2 and m-ZrO2 nanopowders were
manufactured via solvothermal process of the gel precursors which
were prepared by adding KOH ethanol solution dropwise into
ZrOCl2·8H2O ethanol solution. Furthermore, the phase composition
and morphology of the nanoparticles obtained via solvothermal
method were determined, and phase transformation of t-ZrO2 to
m-ZrO2 was observed with increasing solvothermal reaction time.
2. Experiment procedures
A feedstock Zr4+ solution with fixed concentration of 0.2 M was prepared by dis-
solving ZrOCl2·8H2O (analytical reagent) into ethyl alcohol. The gel precursor was
then prepared by adding 1.25 M KOH ethanol solution dropwise into this solution.
Zirconia nonapowders were then prepared in Teflon autoclave by the choice of the
reaction time range from 0 to 600 min at 180 ◦C. The precipitates, named as Z1, Z2,
Z3, Z4, Z5, Z6 and Z7 respectively, were washed with deionized water to remove the
soluble chlorides and ethanol to depress agglomeration and dried in vacuum dry-
ing chamber at 85 ◦C for 5 h. Phase composition of particles was determined by XRD
measurement using Cu K␣ as radiation source. The amount of tetragonal zirconia has
been calculated from the ratios of the diffraction intensity of characteristic (0 1 1)t,
∗
Corresponding author. Tel.: +86 571 87951408; fax: +86 571 87951408.
E-mail address: yang.hui.168@hotmail.com (H. Yang).
¯
(1 1 1)m and (1 1 1)m peaks. The crystallite size were calculated according to the
0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2012.03.050

G. Li et al. / Journal of Alloys and Compounds 532 (2012) 98–101
99
Table 1
( −11)
1
Phase composition and crystallite size of as prepared nanoparticles.
m
(111)
m
Time of
solvothermal/min
Tetragonal phase
Monoclinic phase
Z7
Z6
Xt%
D0 1 1 (nm)
Xm
%
D1 1 1 (nm)
Z1
Z2
Z3
Z4
Z5
Z6
Z7
0
30
60
180
300
420
600
100
58.49
48.26
28.43
11.12
4.43
3.82
4.46
3.97
4.36
4.96
4.86
4.98
0
–
41.51
51.74
71.57
88.88
95.57
98. 25
13.73
22.32
32.13
34.13
34.41
44.64
Z5
Z4
1.75
The phase composition and the crystallite size of as-prepared
nanopowders as a function of solvothermal time are presented in
Table 1. As is shown, there are no marked differences on crystallite
size of t-ZrO2 among five samples, while the content and crystal-
lite size of m-ZrO2 increase with the progressing of solvothermal
reaction.
Z3
Z2
The formation mechanism of m-ZrO2 was investigated with
Avrami equation [Eq. (3)]:
(011)
t
ln(−ln(1 − x)) = n lnt + lnm
(3)
Z1
where x is the fraction crystallized at time t, m a rate constant
dependent on the frequency of the nucleation and growth rate, and
n a constant dependent on the geometry of the system.
Plots of the left side of Eq. (3) versus ln t yield roughly linear
fits with slopes n, these slopes may fall within any of three distinct
ranges that are characteristic of specific reaction mechanisms [16].
For n = 0.54–0.62, a diffusion-controlled dependence is implied. A
zero-order, first-order, or phase-boundary-controlled mechanism
is implied for n = 1.0–1.24. Control by nucleation and growth is
connoted for n = 2.0–3.0.
10
20
30
40
50
60
70
80
2-Theta
Fig. 1. XRD patterns of ZrO2 nanoparticles obtained from solvothermal methods
with different heat treating time at 180 ◦C.
Sherrer formula from the (0 1 1)t and (1 1 1)m characteristic peaks of the nanopow-
ders. The morphology of the particles was analyzed by TEM (JEM-1230, JEOL, Japan).
Fig. 2 shows the relationship between ln(−ln(1 − x)) and ln t.
As is shown, the growth process of zirconia nanopowders is com-
posed of two stages. Many authors limited their data to 0.15 < x < 0.5
primarily to ensure linear behavior when Avrami equation was
applied to investigate the crystal growth. However, linear regres-
sion of our complete range of data (Table 1) shows good linear
relationship over both of the resultant segments (Fig. 2), and regres-
sion yields n values of 0.48 and 0.98 for the early and late regimes,
respectively. Also, a similar phenomenon can be seen in hydrother-
mal synthesising of barium titanate [17]. As shown in Table 2, the n
value for the first regime does not fall into any of the three ranges
listed above, which implies a mechanism other than those 3 reac-
tion mechanisms discussed above. On the contrary, the value of n
3. Results and discussion
Fig. 1 shows the phase composition of nanopowders prepared
via solvothermal method. As is shown, the t-ZrO2 formed at the
early stage of solvothermal reaction, and along with the progressing
of solvothermal procedure, t-ZrO2 transformed into a nearly pure
phase m-ZrO2 gradually. Mun˜oz-Tabares et al. [14] applied several
techniques such as TEM, XRD, Raman spectroscopy, nanoinden-
tation and atomic force microscopy to investigate the evaluation
of hydrothermal degradation of tetragonal zirconia. Their results
indicated that monoclinic phase propagation during degrada-
tion was assisted by stress-induced martensitic transformation
mechanisms (autocatalytic propagation and self-accommodation
variants), where destabilization of tetragonal phase by water attack
was necessary to activate these mechanisms. Similarly, as a result
of lack of water, the products in our samples are mainly tetragonal
zirconia crystals in early stage of the solvothermal reaction. And
then, owing to the increasing of the water (as a product of the reac-
tion) in the reaction system, the monoclinic phase was increased
with extending reaction time.
1.5
1.0
The crystallite size was calculated according to the Sherrer for-
mula from the (0 1 1)t and (1 1 1)m characteristic peaks, and the
volume fraction of each phase of the as-prepared ZrO2 nanoparti-
cles was calculated with the empirical Eqs. (1) and (2) respectively
[15] as follow:
0.5
0.0
¯
-0.5
-1.0
Im(1 1 1) + Im(1 1 1)
xm
=
(1)
(2)
¯
Im(1 1 1) + Im(1 1 1) + It(0 1 1)
xt = 1 − xm
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
where xm and xt represent the volume fraction of monoclinic and
tetragonal ZrO2 nanopowders, respectively, and the intensity of
characteristic peaks are expressed as Im and It.
Int
Fig. 2. Relationship between ln(−ln(1 − x)) and ln t.

100
G. Li et al. / Journal of Alloys and Compounds 532 (2012) 98–101
Fig. 3. TEM images of as-prepared zirconia nanoparticles. a, b, c, d and e are corresponding to Z1, Z2, Z3, Z4 and Z7 sample, respectively.
Table 2
The morphology of as-prepared samples is shown in Fig. 3. It can
The values of n calculated from Eq. (3).
be seen the spherical particles with several nanometers grew up to
short rodlike ones with around 50 nm in diameter and 200 nm in
length, along with the progressing of solvothermal reaction. At the
same time, Fig. 3 shows lots of small spherical particles attached on
the surface of rodlike particles, which indicates that the growth of
larger rodlike particles are based on the dissolving of tiny particles
in this reaction system. In addition, both pure t-ZrO2 and pure m-
ZrO2 show perfect dispersibility.
Solvothermal reaction time/min
0–180
n
Stage 1
Stage 2
0.48
0.98
180–600
for the second stage is 0.98, very close to 1, which suggests rate con-
trol of the late regime of the reaction by a zero-order, first-order,
or phase-boundary-controlled mechanism. Additionally, Eq. (3) has
been derived for spheres, discs, and rods, representing three-, two-
, and one-dimensional forms of growth, and the constants were
summarized according to different crystallization mechanism and
different mode of crystal growth. So it can be deduced that the m-
ZrO2 grows in one dimension in this period, and it is in accordance
with the result of TEM analysis in Fig. 3.
4. Conclusion
In this study, tetragonal ZrO2 nanopowders transform progres-
sively into monoclinic phase during the solvothermal procedure
at 180 ◦C, and the transformation process can be divided into two

G. Li et al. / Journal of Alloys and Compounds 532 (2012) 98–101
101
regimes based on reaction progress. Kinetic analysis, according to
the Avrami equation, was inconclusive in the analysis of the early
reaction-rate regime. While a zero-order, first-order, or phase-
boundary-controlled mechanism implied in the late reaction-rate
regime. The TEM images show the particles grow up from spher-
ical to short rodlike, which indicates the particles grow along
one-dimension during the late regime of the reaction. It agrees
well with the mode of crystal growth which is derived from
the Avrami exponent. Crystallite size estimated from XRD pat-
terns using Scherrer equation shows that the t-ZrO2 is below
5 nm, and has no marked changes during the whole solvother-
mal procedure, while the size of the m-ZrO2 varies from 10 nm to
50 nm.
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