Effect of nanocrystallite structure on the lower activation energy for Sm2O3-doped ZrO2

Article abstract of DOI:10.1016/j.molstruc.2004.05.010

Pure and 2mol% Sm₂O₃-doped ZrO₂ nanopowders were synthesized via the precipitation method. The very lower activation energy (～25.979 kJ/mol) related to the bulk counterpart is observed for the grain growth within the calcination temperature range of 600-1000°C. Experimental results show that two factors were contributed to the lower activation energy of grain growth. Firstly, the introduction of oxygen vacancies in nano-2SmSZ grain surface reduces the activation energy of the formation of necks between grains. Secondly, the rotation process between coherent grains has very lower activation energy or even a zero-kinetic barrier.

Full text of DOI:10.1016/j.molstruc.2004.05.010

Journal of Molecular Structure 703 (2004) 19–23
www.elsevier.com/locate/molstruc
Effect of nanocrystallite structure on the lower activation energy
for Sm2O3-doped ZrO2
a,b
c
a,b
Shou-Gang Chena,b, , Yan-Sheng Yin , Dao-Ping Wang , Xin Wang
*
aInstitute of Materials Science and Engineering, Ocean University of China, Qingdao, Shandong Province 266003, China
bKey Lab of Liquid Structure and Heredity of Materials, Ministry of Education, School of Materials, Shandong University, Jinan 250061, China
cCollege of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, China
Received 2 March 2004; revised 7 May 2004; accepted 17 May 2004
Available online 2 July 2004
Abstract
Pure and 2 mol% Sm2O3-doped ZrO2 nanopowders were synthesized via the precipitation method. The very lower activation energy
(,25.979 kJ/mol) related to the bulk counterpart is observed for the grain growth within the calcination temperature range of 600–1000 8C.
Experimental results show that two factors were contributed to the lower activation energy of grain growth. Firstly, the introduction of
oxygen vacancies in nano-2SmSZ grain surface reduces the activation energy of the formation of necks between grains. Secondly, the
rotation process between coherent grains has very lower activation energy or even a zero-kinetic barrier.
q 2004 Elsevier B.V. All rights reserved.
Keywords: Crystal structure; Defect; X-ray diffraction; Zirconia
1. Introduction
several factors such as chemical effects (the presence
of anionic impurities) [10], structural similarities
between the tetragonal phase and the precursor amorphous
phase [11,12], as well as particle size effects [13], which is
based on the lower surface energy in the tetragonal phase
compared to the monoclinic phase. Studies on new dopants
capable of producing powders with improved physical and
chemical properties are still of great interest. Doping of
aliovalent oxides leads to both an increase in the oxygen
vacancy concentration and an enhancement of the oxygen-
ion conductivity, which enables the usage of this stabilized
zirconia as an electrolyte in the fuel cells. Sm2O3 has been
chosen being one of the materials under investigation in the
field of oxygen-storage materials, the electrical conductivity
materials and luminescent materials among the doped-
zirconia materials [14], since Sm3þ-doping induces the least
distortion of the parent lattice.
Zirconia is an extremely important material with wide-
ranging applications including as a high-performance
transformation-toughened structural engineering ceramic
[1] and as a solid electrolyte [2] (leading to applications as
an oxygen sensor and as a fuel cell electrolyte). Zirconia has
also been found application as a catalyst and catalyst
support [3–5], especially when durability to chemical attack
is required. Pure zirconia (ZrO2) is monoclinic ðmÞ at zero
temperature (space group P21/c [6]); upon increase of the
temperature (at zero pressure) the material transforms to
tetragonal ðtÞ and then to a cubic ðcÞ fluorite structure (space
groups P42/nmc and Fm3m, respectively) [7,8]. These
phase transitions induce large volume changes and make the
pure material unsuitable for applications. The addition of
lower-valence oxides like CaO, MgO, or Y2O3 disfavor the
m phase, stabilizing more symmetric structures with cubic
and tetragonal symmetry [9]. Generally, the presence of the
tetragonal phase at low temperature can be attributed to
Recently, Leite et al. [15] and Oliveira et al. [16] studied
the grain growth process of nanocrystalline materials and
proposed that the lower crystallite size of doped particles is
due to the surface effect, with dopants preventing the
formation of necks and the process of coalescence between
particles. The similar phenomenon of reduced particle size
for mixed ZrO2–Sm2O3 oxide is also observed in our
present results related to pure ZrO2 particles obtained in
*
Corresponding author. Address: Key Lab of Liquid Structure and
Heredity of Materials, Ministry of Education, School of Materials,
Shandong University, Jinan 250061, China. Tel./fax: þ86-531-8392439.
E-mail address: sgchen2000@yahoo.com.cn (S.-G. Chen).
0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2004.05.010

20
S.-G. Chen et al. / Journal of Molecular Structure 703 (2004) 19–23
the same way, and the very lower activation energy for
2SmSZ powders is also observed in this paper. Thus, the
dope-induced small size effect must relate to the process of
grain growth.
0SmSZ and 2SmSZ samples at 600 8C calcination tempera-
ture were further confirmed by the Hitachi H-800 trans-
mission electron microscope (TEM).
Based on the above reasons, not only synthesizing
nanocrystalline oxide is important but also understanding its
grain growth characteristics at the nanolevel is essential.
3. Results and discussion
An XRD of each sample is measured, and relative
tetragonal volume fraction and crystalline size of each
sample are shown in Table 1. Fig. 1 shows the XRD results
of 0SmSZ samples under different calcination temperatures
(600 and 800 8C). Fig. 2 shows the XRD results of 2SmSZ
samples at different calcination temperatures (600, 700, 800,
and 1000 8C). Fig. 3 shows the TEM result of 0SmSZ
powders dried at 100 8C. Fig. 4 shows the TEM results of
0SmSZ(a) and 2SmSZ(b) powders at 600 8C calcination
temperature, which agrees well with the crystalline sizes
calculated by Scherrer formula.
2. Experimental procedures
Mixed stock solution of Zr4þ and Sm3þ of 0.1 M
concentrations with 2 mass% polyglycol (PEG,
MWpeg ¼ 20,000 g/mol) were prepared in the required
proportions to attain pure and 2.0 mol% Sm2O3-doped
precursor solutions. These samples (denoted 0SmSZ and
2SmSZ) were prepared using the precipitation method by
the slower addition of the NH4OH (25%) solution until
produce a desired pH value of 9.5 ^ 0.2. The resulting
precipitate was repetitious centrifugal washing with deio-
nized water at 8000 n/min until a negative test for Cl2, and
then continues centrifugal washing two times with anhy-
drous ethanol at 8000 n/min, and eventually dried at 100 8C
for 24 h. Subsequently, the hydrous oxide was calcined for
2 h at different temperatures, always heating with a rate of
20 K/min to the desired temperatures.
The precipitation-synthesized nanosized 0SmSZ and
2SmSZ powders are mainly tetragonal zirconia at 600 8C
calcination temperature. At this temperature, the tetragonal
volume fraction and crystalline size of 0SmSZ sample are
64.52 vol.% and 20.14 nm, and the tetragonal volume
fraction and crystalline size of 2SmSZ sample are
100 vol.% and 16.48 nm, respectively. However, the
tetragonal volume fraction decreased to 23.85 vol.% and
crystalline size increased to 48.13 nm for 0SmSZ sample,
and the tetragonal volume fraction reduced to 73.53 vol.%
and crystalline size increased to 32.80 nm for 2SmSZ
sample, at 800 8C calcination temperature. Calcination at
higher temperatures (1000 8C) for 2SmSZ sample, the
tetragonal volume fraction still contains 72.80 vol.% and
crystalline size further increased to 50.77 nm. These results
suggest that the growth in the average crystalline size is due
to the aggregation of freestanding powder at higher
calcination temperature, which also agrees well with the
TEM observation of Figs. 3 and 4(a). As it can be observed
from the change of crystalline size, the stabilization of
tetragonal phase in the nanocrystalline 0SmSZ powders
appears to be due to the critical size effect proposed by
Garvie [13]. In Fig. 4(a), the distribution of crystalline size
is not uniform, and the bigger particles (.30 nm) denote
monoclinic zirconia while relatively smaller particles
(,30 nm) denote tetragonal zirconia, which has occurred
in previous study [19]. In Fig. 4(b), the distribution of
Crystalline species in zirconia powders were identified
using an X-ray diffraction (XRD: Model D/MAX-RB12X,
Rigaku Co., Tokyo, Japan). The volume fraction of the
monoclinic phase ðVmÞ was determined by the empirical
formula [17]
Vm ¼ ½Imð111Þ þ Imð111Þꢀ=½Imð111Þ þ Imð111Þ þ Itð111Þꢀ
ð1Þ
where Im denotes the intensities of the monoclinic peaks,
and It denotes the intensities of the tetragonal peaks. The
mean crystallite sizes were calculated using the Scherrer
and Warren equation [18]
Dt ¼ 0:9l=ðB2 2 b2Þ1=2cos uB
ð2Þ
where Dt is the mean crystallite size, l(0.154 nm) and uB
denote the wavelength of X-rays and the Bragg diffraction
angle, respectively, and B and b are the full width at half-
maximum observed for the sample and the standard,
respectively. The crystallite sizes and morphology of
Table 1
Crystallite size and phase volume fraction at different calcination temperature (pH ¼ 9.5 ^ 0.2)
600 8C
700 8C
800 8C
1000 8C
PC (vol.%)
CS (nm)
PC (vol.%)
CS (nm)
PC (vol.%)
CS (nm)
PC (vol.%)
CS (nm)
ZrO2
tð64:52Þ
tð100Þ
20.14
16.48
–
–
tð23:85Þ
tð73:53Þ
48.13
32.80
–
–
2Sm–ZrO2
tð80:44Þ
24.85
tð72:80Þ
50.77
PC, denotes phase volume fraction; CS, denotes crystalline size.

S.-G. Chen et al. / Journal of Molecular Structure 703 (2004) 19–23
21
Fig. 3. TEM image of nanosized 0SmSZ powder at 100 8C temperature for
24 h via the precipitation method.
Fig. 1. Typical broad scan XRD patterns obtained reference-ZrO2 powder
calcination at different temperatures.
process, is dependent on the calcination temperature
according to Eq. (3)
crystalline size is almost uniform, and all the crystalline
sizes smaller than 30 nm. So those particles should be
assigned to tetragonal zirconia according to the critical size
effect [13]. At the same time, this fact has also been
confirmed by the phase analysis of XRD results in Figs. 1
and 2. Thus, the stabilization of tetragonal phase in the
nanocrystalline 2SmSZ powders, within the calcination
temperature range of 600–1000 8C, is a result of doping the
ZrO2 lattice with Sm3þ cations.
Dt ¼ D0 eð2Q=RTÞ
ð3Þ
where Do and Dt denote the initial and final nanocrystallite
sizes (nm), R is the gas constant (kJ/degree mol), T is the
calcination temperature (K). Hence, the activation energy of
grain growth can be obtained from the graph of Fig. 5(b)
and is calculated to be ,25.979 kJ/mol, which is much
lower than that observed for bulk zirconia (,580 kJ/mol)
[20]. Similar observation has also been reported for other
ceramics, namely ZnO [21], where the growth activation
energy in the nanocrystalline grains is observed to be
,20 kJ/mol, while that for microcrystalline grains is
observed to be ,275 kJ/mol. Moreover, this result accords
well with the activation energy (,32–44 kJ/mol) of nano-
SnO2 growth [22], and this fact is further confirmed by
The effect of calcination temperature on the average
2SmSZ nanocrystallite size is presented in Fig. 5(a). The
average 2SmSZ nanocrystallite size increases from ,16 to
50 nm as the calcination temperature increases from 600 to
1000 8C. The limited growth in the average 2SmSZ
nanocrystallite size with increasing calcination temperature
indicates that the synthesized nanosized 2SmSZ powder
contains hard aggregates of size ,50 nm under the given
processing conditions.
To determine the activation energy (Q; kJ/mol) of grain
growth within an agglomerated nanosized 2SmSZ powders,
we replot Fig. 5(a) into Fig. 5(b). It assumes that the grain
growth in the nanosized 2SmSZ, being a thermally activated
Fig. 2. Typical broad scan XRD patterns obtained 2SmSZ powder
calcination at different temperatures.
Fig. 4. TEM images of nanosized 0SmSZ (a) and 2SmSZ (b) powders after
calculation at 600 8C for 2 h via the precipitation method.

22
S.-G. Chen et al. / Journal of Molecular Structure 703 (2004) 19–23
Fig. 5. (a) Effect of calcination temperature on the average nanocrystallite size for nanocrystalline 2SmSZ powder; (b) activation energy plot for
nanocrystalline 2SmSZ.
the experimental result of ,40 kJ/mol activation energy for
pure nano-ZrO2 growth [23]. It is obviously that the very
lower activation energy of grain growth appears to be a
characteristic feature of nanocrystalline ceramic oxides, and
the introduction of dopant further reduces the activation
energy of grain growth.
forming a single larger grain. During the calcination
treatment of such a free-standing powder at low tempera-
ture, the increase in the particle size is mainly due to the
elimination of the boundaries between the nanocrystallites
within the hard aggregates [31]. At the same time, the bulk
vacancies move toward grain surface [32] during the
increase of calcining temperature. As a result, the very
low activation energy value of ,25.979 kJ/mol observed in
this study mainly comes from two aspects contribution for
the grain growth behavior. First, the introduction of oxygen
vacancies for nanograin reduces the activation energy of the
formation of necks between grains. After this step, a rotation
process, which presents very low activation energy or even a
zero-kinetic barrier according to the GRIGC mechanism
[29,30], may occur to decrease the angle of misorientation
and change the grains’ orientation. When the grains assume
the same orientation, the grain boundary must migrate
toward the smaller particle, resulting in a single larger
crystalline nanocrystal.
How to elucidate the above phenomenon on the reduced
activation energy? Based on the assumption of a hard-
sphere approximation, the instability of c-ZrO2 originates
from the close-packed oxygen ions in the fluoride structure,
because of the ionic radii ratio, RZr4þ=RO22 < 0:564; is too
small for eightfold coordination [24]. So the strong covalent
nature of the Zr–O bond within the ZrO2 lattice favors a
seven-fold coordination number, which is offered by the
monoclinic crystal structure. As a result, monoclinic phase
is the most stable crystal structure at room temperature for
undoped ZrO2 (assuming no particle size effect). When a
low-valence cation dopant, such as Sm3þ, is introduced into
the ZrO2 lattice, oxygen vacancies are created to maintain
local electrical neutrality [9,25]. Due to the large size of
Sm3þ cations relative to Zr4þ cations, the produced oxygen
vacancies tend to be associated with Zr4þ cations [26],
which reduce the effective coordination number of Zr4þ
cations below seven. Furthermore, the crystal chemistry
model also postulates that the dopant Sm3þ cations would
also favor this eight-fold coordination with oxygen [27].
Hence, these effects are reflected in the stabilization of the
tetragonal phase in 2SmSZ. The presence of large numbers
of oxygen vacancies on grain surface, which mainly comes
from the doping of Sm3þ and nanometer small size effect
[28], makes the surface energy increase drastically and
decrease the growth activation energy of nanoparticles.
The presence of vacancy defects must have an influence
on the growth mechanism of grains, but the problem is more
complicated. Leite et al. [29] thought that the grain-rotation-
induced grain coalescence (GRIGC) mechanism [30] must
be considered during the growth of nanocrystalline colloidal
oxides. According to this model, the rotation of grains
among neighboring grains results in a coherent grain–grain
interface (the grains assume the same crystallographic
orientation), which leads to the coalescence of neighboring
grains via the elimination of common grain boundaries, thus
4. Conclusions
Nanocrystalline pure and 2 mol% Sm2O3-doped ZrO2
nanopowders were synthesized via the precipitation method
using mixed 0.1 M ZrOCl2·8H2O and SmCl3 precursor
solution under the processing conditions of pH 9.5 ^ 0.2
and 2 wt% polyglycol (PEG, MWpeg ¼ 20,000 g/mol). The
very lower activation energy (,25.979 kJ/mol) is observed
for the grain growth in the nanocrystalline 2SmSZ powder
within the calcination temperature range of 600–1000 8C,
and this fact was attributed to two factors. Firstly, the
introduction of oxygen vacancies in nano-2SmSZ grain
surface reduces the activation energy of the formation of
necks between grains. After this step, a rotation process with
very lower activation energy or even a zero-kinetic barrier
may occur to decrease the angle of misorientation and
change the grains’ orientation. When the grains assume the
same orientation, the grain boundary must migrate toward
the smaller particle, resulting in a single larger crystalline
nanocrystal.

S.-G. Chen et al. / Journal of Molecular Structure 703 (2004) 19–23
23
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This work was sponsored by the National Science
Foundation (No. 50242008) and the Education Ministry
Doctorate Foundation (No. 20020422001) of China.
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