Hydrated surface structure and its impacts on the stabilization of t-ZrO2

Article abstract of DOI:10.1016/j.jssc.2007.08.015

We first optimized the preparation conditions to 3.6-6.0 nm ZrO₂ in a pure tetragonal structure (t-phase). All samples were characterized by X-ray diffraction, high-resolution transmission electron microscope, thermal analysis, Raman spectra, and infrared spectra. It is found that the surfaces of t-ZrO₂ nanostructures were terminated by an amorphous hydration layer co-existing with small amounts of carbonate molecules. With the removal of hydrated surface layers under hydrothermal conditions at T>150 °C, t-ZrO₂ nanostructures became thermodynamically unstable, which partially transformed into monoclinic ZrO₂ (m-phase). Such a transformation occurs initially at surface regions and then develops into the bulk. High-temperature annealing in air could also remove the hydrated surface layers, which is however followed by a gradual transformation of t-ZrO₂ into m-ZrO₂ in both bulk and surface regions. These observations are explained in terms of the difference in surface free energies of m-ZrO₂ and t-ZrO₂ upon H₂O adsorption.

Full text of DOI:10.1016/j.jssc.2007.08.015

ARTICLE IN PRESS
Journal of Solid State Chemistry 180 (2007) 2790–2797
www.elsevier.com/locate/jssc
Hydrated surface structure and its impacts on the stabilization of t-ZrO2
Ã
Hui Wanga, Guangshe Lia, Yanfeng Xueb, Liping Lia,
aState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter,
Graduate School of Chinese Academy of Sciences, Fuzhou 350002, PR China
bDepartment of Physics, Jilin University, Changchun 130023, PR China
Received 22 May 2007; received in revised form 26 July 2007; accepted 10 August 2007
Available online 29 August 2007
Abstract
We first optimized the preparation conditions to 3.6–6.0 nm ZrO2 in a pure tetragonal structure (t-phase). All samples were
characterized by X-ray diffraction, high-resolution transmission electron microscope, thermal analysis, Raman spectra, and infrared
spectra. It is found that the surfaces of t-ZrO2 nanostructures were terminated by an amorphous hydration layer co-existing with small
amounts of carbonate molecules. With the removal of hydrated surface layers under hydrothermal conditions at T4150 1C, t-ZrO2
nanostructures became thermodynamically unstable, which partially transformed into monoclinic ZrO2 (m-phase). Such a transformation
occurs initially at surface regions and then develops into the bulk. High-temperature annealing in air could also remove the hydrated
surface layers, which is however followed by a gradual transformation of t-ZrO2 into m-ZrO2 in both bulk and surface regions. These
observations are explained in terms of the difference in surface free energies of m-ZrO2 and t-ZrO2 upon H2O adsorption.
r 2007 Elsevier Inc. All rights reserved.
Keywords: Tetragonal ZrO2; Hydrothermal process; Hydrated surface structure
1. Introduction
chemical uniformity of pure t-phase of ZrO2 are still very
difficult to achieve.
ZrO2 exhibits three primary polymorphs that are
stabilized at different temperature ranges: monoclinic
(room temperature—1175 1C), tetragonal (1175–2370 1C),
and cubic (2370–2680 1C) [1]. Among these polymorphs,
tetragonal phase has received considerable attention due to
its wide applications in a number of technologies, including
fuel cell electrolytes, catalysts, oxygen sensor, damage-
resistant optical coatings, and gate dielectric [2–4]. Tetra-
gonal ZrO2 is thermodynamically metastable at ambient
conditions and is thus unfavorable for applications.
Scientific activities have been devoted to stabilize tetra-
gonal ZrO2 [5]. It is reported that high-temperature
tetragonal phase can be stabilized to room temperature
provided ZrO2 particles show a dimension less than a
critical size [6]. Various chemical methods [7–10] including
sol–gel and precipitation, have been extensively studied to
prepare small sizes of ZrO2. A precise phase control and
Hydrothermal synthesis has been proved to be advanta-
geous in homogenous nucleation for size processing and
property tailoring. For example, high-temperature (above
180 1C) hydrolysis of aqueous solution of zirconium salts
including zirconyl nitrates and zirconium oxyacetate leads
to the formation of almost pure monoclinic structure under
hydrothermal conditions [11,12]. While when zirconium
hydroxide was used as precursor and treated by a similar
hydrothermal condition, the product composed of pre-
dominately tetragonal phase and minor monoclinic phase
[12]. There are two strategies that are frequently used to
obtain phase-pure t-ZrO2: one is adding some organic
additives like polyhydric alcohols and tri-ethanolamine
[13,14], another one is adopting appropriate mineralizers
[15–17]. However, using these strategies, certain ionic
species could be strongly chemically bonded to the sample
surfaces, as a result, the roles that these additives have
played in the crystallization of zirconia are poorly under-
stood, which limits the systematic study of stabilization
and further applications of pure t-ZrO2.
Ã
Corresponding author. Fax: +86 591 83714946.
E-mail address: lipingli@fjirsm.ac.cn (L. Li).
0022-4596/$ - see front matter r 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2007.08.015

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In this work, we optimized the preparation conditions to
pure phase t-ZrO2. All particle sizes were controlled in the
range of 3.6–6.0 nm. We found that the surfaces of all
t-ZrO2 nanoparticles were terminated with hydration
layers. With removal of the surface hydration layers, all
t-ZrO2 nanoparticles became thermodynamically unstable
and partially transformed into a monoclinic phase.
spectrometer with a He–Ne laser. The excitation wave-
length is 632.8 nm and output powder is 20 mW.
3. Results and discussion
3.1. Optimized preparation conditions to t-ZrO2
NH4OH, KOH, LiOH, and NaOH could act as the
mineralizers for stabilization of tetragonal ZrO2. When
using NH4OH as the mineralizer, the product was
amorphous after hydrothermal reactions at 130 1C for
2 h. When using KOH, similar reaction conditions gave rise
to a mixture of tetragonal and monoclinic phases.
Comparatively, LiOH has already been reported not
suitable for stabilization of tetragonal ZrO2 [17]. In this
work, we explored the preparation of tetragonal ZrO2
using NaOH as the mineralizer.
Firstly, we studied the phase evolution of t-ZrO2 as a
function of NaOH concentrations at a fixed reaction
temperature of 110 1C. As indicated in Fig. 1, when NaOH
concentration is 0.5 M, the product was a mixture of
tetragonal and monoclinic phases. When the NaOH
concentration is higher than 1 M, the product is tetragonal
ZrO2, showing XRD profiles matching well the standard
diffraction data of tetragonal ZrO2 (JCPDS No. 42-1164).
Further increasing NaOH concentration did not give any
extra diffraction peaks but the broadened diffraction
peaks, which indicated the formation of small particle
sizes of tetragonal ZrO2.
Secondly, we studied the phase evolution of ZrO2 as a
function of reaction temperature at a fixed NaOH
concentration of 4 M. As indicated in Fig. 2, when
the reaction temperature is 90 1C, the products are
amorphous as characterized by several dispersed peaks.
When the reaction temperature was increased beyond
100 1C, tetragonal ZrO2 was formed with its diffraction
intensity being increased with the reaction temperature.
The stabilization of tetragonal ZrO2 was also confirmed by
2. Experimental section
Pure tetragonal ZrO2 nanocrystals were prepared by
hydrothermal conditions. Firstly, zirconium hydroxide
precursor was prepared by adding 50 mL of 0.5 mol/L
ZrOCl2 ꢀ 8H2O solution to ammonia solution (5 wt%) to
reach pH ¼ 10.5. The obtained white precipitate was
filtered and sufficiently washed with deionized water till
no chlorine could be detected. This precipitate along with
25 mL of 4 M NaOH solution (for certain parallel runs,
KOH and ammonia solutions with given concentrations
were also used) was transferred to Teflon-lined autoclaves
which were allowed to react at varied temperatures from 90
to 220 1C in an oven for 3 h. The products were collected
after washing with distilled water until pH ¼ 7, and dried
at 60 1C for 3 h. The final samples thus obtained were
named as Z90, Z100, Z110, and so on, in which the
numbers denote the reaction temperatures. The NaOH
concentration was also varied from 0.5 to 16 M to study the
influence of mineralizer concentration on the formation of
tetragonal ZrO2. To investigate the structural stabilities of
pure tetragonal ZrO2 nanocrystals, sample Z100 was
divided into several parts, which were annealed at 400,
500, 600, and 700 1C in air for 2 h, respectively.
The chlorine contents of our samples were examined
using precipitation method, in which parts of the as-
prepared samples were dissolved in the concentrated HNO3
solution under ultrasonication and then reacted with 3 M
AgNO3 solution. No turbid precipitates were observed,
indicating the absence of chlorine in our samples. X-ray
diffraction (XRD) patterns of the samples were recorded
on Rigaku D/MAX 2500 diffractometer with CuKa
radiation. Nickel powders with a purity of 99.9% were
used as the internal standard for peak position determina-
tion. The crystalline sizes were calculated from the peak
broadening of the most intense XRD peak (101) for
¯
tetragonal phase and ð1 1 1Þ for monoclinic phase according
to Scherrer formula, D ¼ 0:9l=b cos y, where l is the
X-ray wavelength used and b is the half-width. The particle
shape and size were examined by high-resolution transmis-
sion electron microscope (HRTEM) image, and the
chemical compositions were determined by energy-disper-
sive X-ray analysis (EDXA). Thermal behaviors of the
samples were investigated using STA449C Jupiter-QMS
403C Aeolos at a heating rate of 15 1C/min. Fourier
¨
transform infrared spectra of the samples were measured
on Perkin-Elmer Spectrum One FTIR Spectometer at a
resolution of 4 cmꢁ1 using KBr pellet technique. Raman
spectra of the samples were recorded using a JY-HR800
Fig. 1. XRD patterns of the samples obtained at 110 1C using different
NaOH concentrations.

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TEM observations (Fig. 3), in which highly crystalline
ZrO2 was seen in spite of an obvious agglomeration.
Average particle size of the samples obtained at 110 1C was
4 nm, which is compatible with that of 4.8 nm calculated by
XRD peak broadening. The interplanar spacing for (101)
plane measured by HRTEM (Fig. 3b) is almost the same as
that obtained for tetragonal ZrO2 by XRD structural
refinements. Compositional analysis for all single-phase
samples by EDXA indicated the presence of Zr only. These
results showed that our tetragonal ZrO2 nanocrystals are
chemically pure at ambient conditions. Tetragonal phase
was stable below 130 1C, which transformed into mono-
clinic phase with further increasing the reaction tempera-
ture higher than 150 1C. This phase transformation is also
accompanied by changes in particle sizes. The qualitative
phase analysis and particle sizes are presented in Table 1. It
is seen that the grain size of t-ZrO2 increased from 3.6 to
7.2 nm when temperature varied from 100 to 150 1C and
then decreased with increasing temperature. Correspond-
ingly, the grain size of m-ZrO2 increased from 7.2 to
20.6 nm when temperature was elevated from 150 to 200 1C
and then varied little with further increasing temperature.
These observations demonstrated that the critical size for
tetragonal-monoclinic transition is about 7 nm under the
present hydrothermal conditions.
Z90
Z100
Therefore, mineralizers and reaction temperature play
the critical role in the phase composition and particle sizes
of ZrO2. This could be understood by taking into account
the crystallization processes of zirconia. The crystallization
of ZrO2 nanocrystals mainly consists of nucleation and
grain growth processes. When ZrOCl2 ꢀ 8H2O was dis-
solved into an aqueous solution, tetramer complexes,
[Zr4(OH)8(H2O)16]8+, would form as a major species.
The tetramer has 8 hydroxo bridges and 16 coordinate
Z150
Z170
Table 1
Phase compositions and particle sizes of the samples synthesized at
different reaction temperatures at the presence of 4 M NaOH
Z200
Z220
Samples
Tetragonal ZrO2
Content (%)
Monoclinic ZrO2
Content (%)
Size (nm)
Size (nm)
Z100
Z110
Z130
Z150
Z170
Z200
Z220
100
100
100
91
3.6
4.9
6.0
7.2
5.9
5.7
5.2
0
0
–
20
30
40
50
60
70
–
2θ (˚)
0
–
9
7.2
12.2
20.6
19.3
Fig. 2. XRD patterns of the samples obtained at given temperatures using
a fixed NaOH concentration of 4 M. Vertical bars below the patterns
represent the standard diffraction data from JCPDS file for monoclinic
ZrO2 (No. 37-1484).
85
69
15
31
35
65
Fig. 3. TEM photo (a) and HRTEM images (b) of Z110.

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water molecules [18]. When this solution was heated or
dropped into strong basic solution, tetramer would release
certain amounts of H+ ions from the coordinated water in
terms of the following equation:
Z90
8þ
½Zr4ðOHÞ8ðH2OÞ16ꢂ ! ½Zr4ðOHÞ8þxðH2OÞ16ꢁxꢂð8ꢁxÞþ
Z130
Z160
þ xHþ:
Polymerization might proceed by olation among
[Zr4(OH)8+x(H2O)16ꢁx]
(8ꢁx)+. When the concentration of
Z220
polymeric species reaches a critical level, ZrO2 nuclei would
occur [19]. This procedure depends on the mineralizers
used. For the solution containing neutral mineralizers,
such as NaCl and KBr, high temperature was usually
necessary to promote the polymerization [15]. However,
high temperature is not beneficial for the stabilization of
t-ZrO2 since tetragonal phase is thermodynamically me-
tastable. As stated above, NaOH when involving in the
hydrothermal processes can lead to the formation of
crystalline ZrO2 at 100 1C, therefore it is probable that
hydrolysis and condensation of tetramers were promoted
by high concentration of OHꢁ in the solution.
Apart from anions (OHꢁ), alkali ions from the
mineralizers MOH (M ¼ Na+, K+, and NH4+) also affect
the crystallization process of ZrO2. A previous investiga-
tion [17] has shown that increasing radius size of cationic
would cause the decrease of crystalline rate of ZrO2. In our
experiments, we found that a mixture of m- and t-ZrO2 was
formed when 4 M KOH was employed as the mineralizer.
Compared with K+, Na+ favors the formation of
tetragonal structure. Chen et al. [20] studied the effect of
Na+ on thermal behavior of hydrous ZrO2 and also found
that Na+ can promote the formation of t-ZrO2 while
preventing the transformation from tetragonal to mono-
clinic phase. They concluded that Na+ species might
incorporate into the surface vacant sites and thus stabilize
t-ZrO2. From the chemical compositions of our samples
experimentally measured by EDX analysis and precipita-
tion method, we may conclude that all these adsorbed
Na+ species are loosely bonded to the surfaces of as-
prepared ZrO2 nanoparticles and therefore can be
removed after sufficient washings, although these Na+
species have already played significant impacts on the
crystallization of t-ZrO2. Furthermore, reaction tempera-
ture seems to have certain impacts on the tetragonal
stabilization since when reaction temperature is increased
up to 160 1C or higher, phase transformation from
tetragonal to monoclinic occurred, regardless of the Na+
concentration.
200
400
600
800
Wavenumber (cm-1)
Fig. 4. Raman spectra for typical samples obtained at different
temperatures.
and 130 1C, three broad peaks were observed in the Raman
shift region from 100 to 900 cmꢁ1, which are associated
with long-range disordering arrangement in amorphous
state. These observations seemed not consistent with our
XRD data and TEM images since for the latter cases, ZrO2
nanoparticles were seen to show a rather good crystallinity
at the absence of amorphous phases (Figs. 2 and 3).
Therefore, it is most likely that there exists a hydrated
amorphous layer at the surface of tetragonal ZrO2.
This hydrated amorphous layer was also indicated by the
broad Raman peaks when the reaction temperature was
increased up to 160 1C. When the reaction temperature is
beyond 160 1C, several weak Raman peaks appeared at
about 185, 345, and 383 cmꢁ1, which are assigned to the
intrinsic vibration modes of monoclinic ZrO2 [21,22].
When the reaction temperature was further increased up
to 220 1C, broad Raman peaks associated with the
hydrated amorphous layers were completely gone except
for the strong Raman peaks for monoclinic ZrO2.
It is noted that the Raman spectra of these samples
did not indicate the presence of any traces of tetragonal
ZrO2, which seems not consistent with our XRD analysis
(Table 1), since for the latter case, the composition of
tetragonal component is calculated to be at least 65% when
the reaction temperature is increased up to 220 1C. There-
fore, it is likely that the samples prepared in the
temperature range from 160 to 220 1C may have a
sandwich structure, in which the monoclinic phase existed
in the intermediate layers between the inner tetragonal
ZrO2 core and the top amorphous hydrated layers. The
transformation of ZrO2 from tetragonal to monoclinic
structure might take place initially at surface regions and
then develop into the bulk with increasing the reaction
temperatures, which is consistent with the results reported
by Li et al. [23].
3.2. Surface hydration structure of t-ZrO2 nanocrystals
Raman spectroscopy is a surface-sensitive technique.
Here, we used Raman spectra to monitor the surface
structure of ZrO2 nanoparticles. Fig. 4 shows Raman
spectra of the samples obtained at different temperatures.
When the reaction temperature is intermediate between 90
Surface hydration structure of t-ZrO2 nanocrystals was
also investigated by IR spectrum. Fig. 5 shows the IR
spectrum of a typical sample Z100. There are a strong
broad absorption centered around 3413 cmꢁ1, three sharp
absorption bands at about 1630, 1352, and 960 cmꢁ1, and

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100
95
Z150
Z130
Z110
Z100
90
85
4000
3000
2000
1000
200
400
600
800
Wavenumber (cm-1)
Temperature (˚C)
Fig. 5. IR spectrum for Z100.
0.6
0.5
0.4
0.3
0.2
16
12
8
two weak absorption bands at 580 and 454 cmꢁ1. The
bands at about 580 and 455 cmꢁ1 correspond to Eu mode
of Zr–O vibration of tetragonal structure [24]. The
absorption band located around 3413 cmꢁ1 is associated
with the O–H stretching vibration of adsorbed water and
hydroxyl group, while the absorption band at 1630 cmꢁ1 is
due to the bending mode of associated water [25]. The
observation of a strong broad absorption at 3400 and
sharp absorption band at 960 cmꢁ1 implied that the
hydration molecules could be in several different energe-
tically bonding states. The hydrated amorphous termina-
tions are highly reactive in forming carbonate species on
the surfaces by adsorbing atmospheric CO2 [26], as
indicated by the presence of absorption band at
1352 cmꢁ1. Mass spectra of the exhaust gases of the as-
prepared samples during heating processes confirmed the
presence of CO23ꢁ species absorbed on surfaces of t-ZrO2
nanocrystals, which will be described latter.
Thermal behaviors associated with the surface hydration
of t-ZrO2 nanocrystals were examined by thermogravimetric
analysis (TGA) technique. All t-ZrO2 nanocrystals showed a
similar mass loss in a wide temperature range from room
temperature to 900 1C (Fig. 6a). These surface hydration
terminations were completely removed when the tempera-
tures is higher than 900 1C, as is indicated by the almost
constant mass. The total amount of mass loss for hydration
removal was determined to be 13.5 wt% for sample Z100,
while the surface terminations for Z220 became less, as
indicated by a significant decrease of mass loss at 6.7 wt%.
The exhaust gases released during the heating process
were examined by mass spectroscopy (MS). Fig. 6b shows
the MS signals for sample Z100. In the temperature range
from room temperature to 1000 1C, only water and CO2,
corresponding to the m/Z ¼ 18 and 44, respectively, were
detected. It should be emphasized that the ionic current for
CO2 signal is only about 25th of that for H2O signal, which
implied that the amount of CO2 is qualitatively very low in
the exhaust gas in comparison with water. Therefore, the
total mass loss observed at temperatures up 600 1C should
CO2 (m/Z = 44)
H2O (m/Z = 18)
4
200
400
600
800
1000
Temperature (˚C)
Fig. 6. TGA curves (a) of t-ZrO2 and MS curves (b) of Z100.
be contributed mainly from surface hydration termination.
Careful analysis of MS signals can help us to determine the
bonding states of H2O and CO2 with the surfaces of t-ZrO2
nanocrystals. As shown in Fig. 6b, a broad and relatively
strong peak between 30 and 650 1C as well as a weak peak
above 650 1C were observed for CO2 signal. The MS signal
of CO2 could be evidently deconvoluted into four Gaussian
curves centered at 150, 310, 560, and 760 1C, which
indicated at least four kinds of energetically nonequivalent
group that surface carbonate molecules are coordinated
with nanocrystals. Similarly, H2O signal can also be
deconvoluted into four components with peak positions
at 140, 200, 330, and 530 1C, respectively. The first one with
low-temperature peak at 140 1C is attributed to physi-
sorbed water, while the components with temperature peak
at 200 and 330 1C are from chemisorbed water [27]. The
last one centered at 530 1C is very weak, which is associated
with the low concentration of residual surface hydroxyls at
high temperatures. This reasoning is consistent with the
general observation that most of the surface hydration
layers of almost the other oxide nanoparticles, just like
TiO2, Fe2O3, and MgO, have already been removed at
temperatures below 530 1C [28–30]. Basing on the desorbed

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Z100
400˚C
600˚C
700˚C
20
40
60
80
2θ (˚)
Fig. 8. XRD patterns of the ZrO2 samples obtained after sintering at
different temperatures.
Fig. 7. Structure model proposed for the surface hydration and
carbonates of t-ZrO2.
temperatures of H2O and CO2, we proposed a coordinated
model to describe bonding states of surface hydration and
carbonate molecules. In Fig. 7a, configuration I denotes
the physisorbed water molecules that would be dehydrated
at 140 1C, while both configurations II and III represent the
chemisorbed water molecules that would be dehydrated at
200 and 310 1C, respectively. Among these three kinds of
surface hydration molecules, configurations I and III are
highly reactive to CO2 to give two kinds of surface
carbonate molecules (Figs. 7a and b). The carbonate
molecules bounded with physisorbed water (configuration
I) would first decompose at 150 1C (Fig. 7a), while those
associated to chemisorbed water (Fig. 7b) would be
desorbed at 310 1C. The other two kinds of carbonate
molecules interacted with the surface lattice oxygen and Zr
ions through Figs. 7c and d to form comparatively stable
absorptions, which would decompose at as high as 560 and
760 1C, respectively. From Fig. 6b, it is seen that, for
sample Z100, the carbonate molecules bonding to the
chemisorbed water are the main component (taking about
79.8% of total surface carbonate molecules). On the
contrary, the carbonate molecules bonded to physisorbed
water is estimated to be only about 3.8%, indicating a very
weak bonding state that could be easily desorbed. It is also
noted that the proportions of carbonate molecules directly
bonded with Zr ions and lattice oxygen anions were almost
the same (i.e., 8.3% and 8%, respectively), which should be
related to the similar amounts of the Zr and oxygen ions
exposed on surfaces of t-ZrO2 nanocrystals.
700˚C
600˚C
400˚C
Z100
200
400
600
800
Wavenumber (cm-1)
Fig. 9. Raman spectra of the ZrO2 samples after sintering at different
temperatures.
Raman spectrum. Fig. 8 shows the XRD patterns of the
samples obtained after heating Z100 at 400, 600, and
700 1C in air for 2 h, respectively. It is seen that tetragonal
structure could be stabilized up to 700 1C, which is
followed by a grain growth such as from 6.6 to 19.6 nm.
While when the treatment temperature is above 700 1C,
t-ZrO2 nanocrystals were partially transformed into
monoclinic phase as indicated by the weak diffraction
peaks at 281.
Fig. 9 shows the Raman spectra of the samples after heat
treatments. Besides the two broad peaks at 160 and
547 cmꢁ1 for Z100, the sample obtained after heating at
400 1C exhibited several weak peaks at 278, 325, 474, and
638 cmꢁ1 that are responsible for t-ZrO2 [31]. The existence
of Raman peaks at 160 and 547 cmꢁ1 indicates the presence
of hydrated amorphous zirconia on surfaces. Further
increasing the temperature to 600 1C, the Raman peaks
for amorphous structure disappeared and all Raman peaks
3.3. Surface hydration structure evolution and stabilization
of t-ZrO2
Surface structure evolution of t-ZrO2 during heating
processes was investigated by XRD in combination with

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are assigned to the t-ZrO2. The variation of Raman spectra
during the heating process is consistent with the dehydra-
tion process as revealed by TGA and MS measurement, in
which the dehydration almost finished at 600 1C. However,
when the temperature was increased to 700 1C, three weak
Raman peaks at 180, 190, and 387 cmꢁ1 characteristic of
m-ZrO2 are observed in addition to characteristic Raman
peaks for t-ZrO2, which indicated that a small part of
t-ZrO2 has transformed into m-ZrO2 [32], consistent with
our XRD analysis stated above.
standing of the stabilization nature of tetragonal ZrO2
nanostructures for technological applications.
Acknowledgments
This work was financially supported by NSFC under the
contract (No. 20671092), a grant from Hundreds Youth
Talents Program of CAS (Li GS), National Basic Research
Program of China (2007CB613306) and in part by a
Science and Technology Program from Fujian Province
(No. 2005HZ01-1, Z0513026).
Based on these observations, we conclude that surface
hydration structure is a key factor that is responsible for
the stabilization of t-ZrO2 nanocrystals. There are two
primary ways to alter the surface hydration structure: one
is via a direct hydrothermal reaction and another is via a
high-temperature treatment, though their impacts on the
phase evolution could be significantly different. For
example, with increasing hydrothermal reaction tempera-
ture, the transformation of ZrO2 from tetragonal to
monoclinic structure could prefer to occur at surface
regions, while high-temperature annealing might lead
to the homogeneous formation of monoclinic structure
in bulk and surface of t-ZrO2. These distinct phase
evolutions are attributed to a decrease in the difference
between the surface free energy of m-ZrO2 and t-ZrO2
upon surface adsorption of H2O, since the stabilization
temperature range and critical size of t-ZrO2 in hydro-
thermal condition were both lower than those of the
samples after high-temperature annealing. Therefore, the
tetragonal-to-monoclinic phase transformation occurs
more easily in solutions. Similarly, Bell et al. [33] also
found that t-ZrO2 transforms into m-ZrO2 when immersed
in water.
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