Novel fabrication of high-quality ZrO2 ceramic thin films from aqueous solution

Article abstract of DOI:10.1111/j.1551-2916.2005.00522.x

We report on the direct deposition of high-quality ZrO₂ thin films on various kinds of substrates by the liquid phase deposition method. After reaction for 24 h, thin films formed on various kinds of substrates, and the obtained thin film was comprised of densely packed nano-sized particles. The film annealed at 500°C showed a tetragonal phase at room temperature and this phenomenon has been discussed from the viewpoint of crystallite size effect. The result of optical transmittance measurement revealed that high transparency, more than 70% transmittance, has been achieved for the film after annealing at 900°C.

Full text of DOI:10.1111/j.1551-2916.2005.00522.x

J. Am. Ceram. Soc., 88 [10] 2923–2927 (2005)
DOI: 10.1111/j.1551-2916.2005.00522.x
r 2005 The American Ceramic Society
ournal
J
Novel Fabrication of High-Quality ZrO2 Ceramic Thin Films from
Aqueous Solution
Kentaro Kuratani
Department of Molecular Science, Graduate School of Science and Technology, Kobe University, Nada, Kobe
657-8501, Japan
Masayasu Uemura, Minoru Mizuhata, Akihiko Kajinami, and Shigehito Dekiw
Department of Chemical Science and Engineering, Faculty of Engineering, Kobe University, Nada, Kobe 657-8501,
Japan
We report on the direct deposition of high-quality ZrO2 thin
films on various kinds of substrates by the liquid phase deposi-
tion method. After reaction for 24 h, thin films formed on vari-
ous kinds of substrates, and the obtained thin film was comprised
of densely packed nano-sized particles. The film annealed at
5001C showed a tetragonal phase at room temperature and this
phenomenon has been discussed from the viewpoint of crystallite
size effect. The result of optical transmittance measurement re-
vealed that high transparency, more than 70% transmittance,
has been achieved for the film after annealing at 9001C.
In our previous studies, the following equilibrium reaction
was presumed:
MFxðxꢀ2nÞꢀ þ nH2O , MOx þ xFꢀ þ 2nHþ
H3BO3 þ 4HF , BFꢀ4 þ H3Oþ þ 2H2O
Al þ 6HF , H3AlF6 þ 3=2H2
(1)
(2a)
(2b)
The equilibrium reaction of (1) shifts to the right-hand side by
adding boric acid or aluminum metal, which readily react with
Fꢀ ions to form more stable complex ions (Eqs. (2a) and (2b)).
The addition of boric acid or aluminum metal accelerates the
hydrolysis reaction. Then, metal oxide directly deposits at the
solid/liquid interface on substrate, followed by continuous self-
growth of the film. As a result, we can obtain thin films directly
from aqueous solution close to room temperature. A large
amount of energy is not required for the LPD method because
this process relies on self-growth processes such as bio-mineral-
ization or electroplating reaction. Moreover, this process uses an
aqueous solution as a reaction field, which is a typical homoge-
neous mixing system, whereby uniform thin films can readily be
fabricated on the various kinds of substrates with large surface
areas and complex morphologies.
I. Introduction
ERAMICS are useful materials in engineering as advanced
C
materials such as catalysts,1 transformation-toughened ma-
terials,2 solid oxide fuel cells,3,4 oxygen sensors,5 and optics.6
For example, stabilized-ZrO2 ceramics are used as a thermal
barrier coating to protect the shrouds, even under harsh envi-
ronments such as in gas turbines and aero engines, because of
their low thermal diffusivity and excellent mechanical proper-
ties. For many of these applications, high-quality ceramic thin
films are required to obtain the desired properties.
Ceramic thin films have been prepared by sputtering,7,8 chem-
ical vapor deposition,9 atomic layer deposition,10–13 and other
methods.14–18 Among these methods of synthesizing ceramic
thin films, soft solution chemistry for synthesizing ceramic thin
films has received increasing interest because of its simplicity,
low energy consumption, and low cost.19–22
In the present contribution, we report on the extension of this
process for fabrication of ZrO2 thin films. We also show the
optical transmittance of the film before and after annealing.
The sol–gel method has been widely used to fabricate ceramic
thin films in solution. This process, however, includes hydro-
lysis and condensation reactions of molecular precursors such
as metal alkoxides. Moreover, this method needs a spin- or dip-
coating process and heat treatment to fabricate thin film on
substrate.
Recently, we have developed a novel aqueous solution proc-
ess to fabricate ceramic thin films by the liquid phase deposition
(LPD) process using the chemical equilibrium reaction between
metal-fluoro complex and metal oxide in the solution.23–25 This
method is based on direct deposition of metal oxide at the solid
(substrate)/liquid (reaction solution) interface, which is quite
different from the sol–gel process.
II. Experimental Procedure
(1) Preparation of ZrO2 Thin Films
For preparation of ZrO2 thin films, 3.0 mol/dm3 fluorozirconic
acid (H2ZrF6; Morita Chemical Co. Ltd., Osaka, Japan) was
used as a starting material. In the LPD solution, H2ZrF6 solu-
tion was diluted at a concentration of 0.06 mol/dm3, and an
aluminum metal plate (purity: 99.9%) with an area from 4 to
48 cm2 was dipped into the solution as an Fꢀ scavenger. The
total volume of the reaction solution was 50 m/dm3. Mainly an
Si (100) wafer (Shin-Etsu Chemical Co. Ltd., Tokyo, Japan) was
used for depositing, and soda-lime glass (Matsunami Glass Co.
Ltd., Osaka, Japan), fused silica glass (Fujiwara Scientific Co.
Ltd., Tokyo, Japan), and Au wire (Nilaco Corp. Tokyo, Japan)
were also used as substrates for the quantitative analysis, optical
transmission measurement, and cross-sectional transmission
electron microscope (TEM) observation, respectively. These
substrates were cleaned ultrasonically and then immersed into
LPD solution for 24 h at 301C. The films deposited onto these
substrates were washed with distilled water, and then dried
overnight at room temperature. Subsequently, the films were
annealed for 1 h at various temperatures (from 5001 to 9001C).
K. J. Bowman—contributing editor
Manuscript No. 11115. Received June 17, 2004; approved April 28, 2005.
This study was supported by Grant-in-Aid for Scientific Research (A) (No.15205026)
from the Japan Society for the Promotion of Science and Grant-in-Aid for Scientific Re-
search of Priority Areas (No. 16080211) for ‘‘Panoscopic Assembling and High Ordered
Functions for Rare Earth Materials (440)’’ from the MEXT in Japan.
wAuthor to whom correspondence should be addressed. e-mail: deki@kobe-u.ac.jp
2923

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Journal of the American Ceramic Society—Kuratani et al.
Vol. 88, No. 10
(2) Instruments
X-ray diffraction (XRD) analysis for the samples was carried
out on a Rigaku RINT-2100 diffractometer (Rigaku, Tokyo,
Japan) equipped with thin film attachment. CuKa radiation was
used as the X-ray source. The Zr content in the deposited films
was determined by inductively coupled plasma-atomic emission
microscopy (ICP-AES; 1500VF, Seiko Instruments Int., Chiba,
Japan). For the ICP-AES analysis, the samples were dissolved in
a dilute hydrochloric acid solution. Raman spectra were meas-
ured with a Jobin-Yvon U-1000KX double monochromator
(Jobin-Yvon, Longjumeau, France), with a spectrum resolution
of 1 cmꢀ1. The emission line at 488.0 nm from an Ar1 ion laser
was used as the excitation source. The Raman signal was col-
lected with a backscattering geometry. Atomic force microscopy
(AFM: SPI-300, Seiko Instruments Int.) in the tapping mode
was used to investigate the surface morphology of the films. A
cantilever with a spring constant of 2 N/m was used for scan-
ning. Microstructure of the deposited films was investigated us-
ing a cross-sectional TEM (JEM-2010, JEOL, Tokyo, Japan)
operated at 200 kV. Thin cross-sections of the sample for TEM
observations were obtained by the following procedure. The
samples deposited on the Au wire were immersed in an epoxy
matrix, curried at 301C for 24 h, and sliced with a Leica Ultracut
UCT microtome (Wein, Austria) using a diamond knife. The
thin sections were then mounted on a conventional carbon-coat-
ed copper grid. X-ray photoelectron spectra (XPS) were also
measured with an ESCA 3400 (Shimadzu Corp., Kyoto, Japan)
spectrometer by using MgKa (1253.6 eV) radiation as the exci-
tation source. Charging of samples was corrected by setting the
binding energy of adventitious carbon (C 1s) at 284.6 eV.
The XPS analysis was carried out at ambient temperature and
pressures typically below 5 ꢁ 10ꢀ6 Pa. Optical transmissions
for the ZrO2 films were recorded by a UV-visible spectrometer
(U-3000, Hitachi Ltd., Tokyo, Japan) in the wavelength range of
400–800 nm.
Fig. 1. Plot of Zr content in the deposited films as a function of the area
of aluminum metal added into the reaction solution.
samples have a dense and homogeneous microstructure. As
shown in Fig. 2(a), an as-deposited ZrO2 film consists of dense-
ly packed small spherical particles approximately 50 nm in di-
ameter. This dense structure is one of the features of the LPD
process, because saturated nano-sized metal oxides directly de-
posit at the solid/liquid interface in the reaction solution. Such a
uniform distribution is very important for the optical property.
At high temperature, the aggregation of these particles occurs.
The particle sizes of the films annealed at 5001, 7001, and
9001C were approximately 90, 120, and 250 nm (Fig. 2(b)–(d)),
respectively.
Figure 3 shows visible transmission spectra of the films de-
posited on fused silica glass before and after annealing at 9001C.
Before annealing, the film appeared optically clear and demon-
strated more than 80% of transmittance in the spectral range of
400–800 nm. After annealing at 9001C, the film also shows high
III. Results and Discussion
In the LPD method, the equilibrium reaction (Eq. (1)) shifts to
the right-hand side on addition of boric acid or aluminum metal,
which readily consumes Fꢀ ions in the reaction solution. Thus,
investigation of the relationship between the concentration of
the metal-fluoro complex and the amount of Fꢀ scavenger is one
of the important tasks carried out in the LPD process. In this
system, the Al31 ion content in the reaction solution is an im-
portant factor for film deposition. The actual amount of Al31
ions in the reaction solution is, however, unknown because there
Al31 ions are generated by the dissolution of the aluminum
metal plate. Thus, we represent the variation of Al31 ion in
the reaction solution as the area of the aluminum metal plate.
Figure 1 shows a plot of the Zr content in the films deposited on
soda-lime glass as a function of the aluminum metal area. No
deposition of Zr on the substrate is observed at less than 20 cm2.
However, an abrupt increase of the Zr content in the deposited
film on the substrate can be seen from 20 to 40 cm2, indicating
that addition of the aluminum metal, whose area is larger than
20 cm2, into the reaction solution leads to the supersaturation of
ZrO2 in the reaction solution, and the resulting saturated ZrO2
deposits on the substrate. The amount of Zr deposited on the
substrate finally reaches a constant value (1.5 ꢁ 10ꢀ7 mol/cm2).
From this result, we concluded that the optimum condition of
the area of aluminum metal for ZrO2 film deposition was more
than 40 cm2. Thus, aluminum metal plates that have 48 cm2 of
area are adequate for the film deposition in all the experiments
described in this paper.
Surface morphology of the deposited ZrO2 films was ob-
served by using AFM. Figure 2 shows AFM images of the ZrO2
thin films annealed at various temperatures. The films were de-
posited on a Si wafer. AFM images reveal that the substrate is
entirely covered with small particles, indicating the formation of
a thin film on the substrate. AFM images also reveal that all the
Fig. 2. Atomic force microscopy images of the deposited films annealed
at various temperatures: (a) as-deposited film, (b) 5001C, (c) 7001C, and
(d) 9001C.

October 2005
Novel Fabrication of High-Quality ZrO2 Ceramic Thin Films
2925
Fig. 3. Visible transmittance spectra of the deposited film before and
after annealing at 9001C.
transparency (more than 70%), although a decrease of trans-
mittance could be observed. The decrease of transmittance ob-
served in the shorter wavelength region is owing to scattering of
light, which is caused by aggregated particles, confirmed by
AFM. From these results, it is concluded that ZrO2 thin films
fabricated by the LPD process have high transparency because
the films consist of uniformly dispersed nano-sized particles.
Raman spectroscopy is one of the useful techniques generally
used to investigate the structure of ZrO2 because most XRD
reflection peaks of cubic and tetragonal ZrO2 overlap. Figure 4
shows Raman spectra of the deposited film annealed at (a)
6001C and (b) 9001C, respectively. The Raman spectrum of the
film annealed at 5001C could not be obtained because of fluo-
rescence from the sample. The results reveal that the character-
istic peaks of the cubic phase of ZrO2 did not appear. It can be
seen in Fig. 4(a) that the main bands are at 259, 329, 339, 466,
607, and 639 cmꢀ1. The strong peak at 520 cmꢀ1 was because of
Si of the substrate. The bands at 259, 329, 466, 607, and 639
cmꢀ1 are assigned to the Raman-active vibration modes for the
tetragonal phase of ZrO2 (indicated by solid arrows),26–28 while
the band at 339 cmꢀ1 is assigned to the Raman-active vibration
modes for the monoclinic phase of ZrO2 (indicated by dashed
arrows),29–31 which indicates that the film annealed at 6001C is
formed mainly by the tetragonal phase and a small amount of
the monoclinic phase. On the contrary, the characteristic peaks
corresponding to the monoclinic phase of ZrO2 (179, 191, 305,
Fig. 4. Raman spectra of the deposited films annealed at (a) 6001C and
(b) 9001C, respectively. (solid arrows, tetragonal; dashed arrows, mono-
clinic).
become strong, which indicates that the phase transformation
from tetragonal to monoclinic occurs with increasing annealing
temperature. After the sample was annealed at 9001C, the film
was formed by a large amount of the monoclinic phase and a
small amount of the tetragonal phase, corresponding to the re-
sult obtained from Raman measurement (Fig. 4). From these
results, it is concluded that ZrO2 thin films prepared by the
LPD method mainly show a tetragonal phase after annealing
at 5001C, and a phase transformation from tetragonal to mono-
clinic occurs during annealing.
335, 344, 379, 475, 503, 535, 560, 616, and 634 cmꢀ1 29–31
(in-
)
dicated by dashed arrows) are clearly observed from the film
annealed at 9001C (Fig. 4(b)). A very weak peak at 263 cmꢀ1 is
assigned to the vibration of the tetragonal ZrO2 structure (indi-
cated by solid arrows). This result indicates that a phase trans-
formation from tetragonal to monoclinic occurs during
annealing from 6001 to 9001C.
The continuous phase transformation of the tetragonal to
monoclinic phase was observed by XRD measurement. Figure 5
shows XRD patterns of the deposited thin films annealed at
different temperatures. The ‘‘t’’ and ‘‘m’’ in the figure denote the
tetragonal and monoclinic phases, respectively. No peak shift
was observed between the film annealed at 5001C and the film
annealed at 6001C. Thus, we conclude that the films annealed at
5001 and 6001C have the same crystal phase (tetragonal). As
shown in Fig. 5, an as-deposited film has an amorphous struc-
ture. After the sample was annealed at 6001C, the diffraction
peaks of the tetragonal phase were predominant and the peaks
of monoclinic phase were relatively weak. This means that the
tetragonal phase occurs in a large fraction in the film after an-
nealing at 6001C. With increasing annealing temperature, the
peak intensities assigned to the tetragonal phase become weak.
In contrast, the peak intensities assigned to the monoclinic phase
Fig. 5. X-ray diffraction patterns of the deposited films annealed at
various temperatures: (a) as-deposited film, (b) 5001C, (c) 6001C, (d)
7001C, (e) 8001C, and (f) 9001C.

2926
Journal of the American Ceramic Society—Kuratani et al.
Vol. 88, No. 10
The crystallite size of the film annealed at various temperatures
was determined by using XRD data and the Scherrer equation
kl
b cos y
D ¼
(3)
where k is a constant value, y and b are the diffraction angle and
full-width at half-maximum (fwhm) of the observed peak, re-
spectively, and D is the average crystallite size. For calculation,
k 5 0.90 and b of the peak at 2y 5 30.31 (tetragonal (111) plane)
ꢀ
and 2y 5 28.21 (monoclinic (111) plane) were taken. The calcu-
lated results show that nanocrystals gradually grow from 18 to
22 nm as the annealing temperature is elevated from 5001 to
9001C in the tetragonal phase. Nanocrystals that formed a
monoclinic phase also grew from 28 to 32 nm with increasing
annealing temperature. Such a crystallite growth is a common
phenomenon in the annealing behavior of the film or powder.
Figure 6 shows high-resolution TEM images of the deposited
film annealed at (a) 5001C and (b) 9001C, respectively. TEM
observation reveals that the thickness of the film deposited on
the Au wire was ca. 100 nm. As shown in Fig. 6, both samples
consist of many nano-sized crystallites.
In Fig. 6(a), the average crystallite size of the tetragonal phase
was determined to be 16–20 nm, in good accordance with that
calculated by the Scherrer equation. Uniform lattice fringes were
observed all over the film. The crystallite in the figure shows
˚
well-defined (111) lattice fringes with 2.96 A spacing, indicating
that the film consists of very small tetragonal crystallites. In
˚
Fig. 6(b), on the other hand, the lattice fringes with 3.16 A
ꢀ
spacing assigned to (111) of monoclinic phase can be observed.
This provides proof that both samples after annealing at 5001
and 9001C have high crystallinity and consist of dense, uniform,
randomly oriented nanocrystals (B40 nm).
In order to understand the chemical state of Zr and O in the
deposited film, the sample annealed at various temperatures has
been investigated using the XPS technique. The photoelectron
peaks of Zr 3d and O 1s are depicted in Figs. 7(a) and (b), re-
spectively. The binding energies for Zr 3d5/2 and Zr 3d3/2 are
182.1 and 184.4 eV, respectively. These values are identified as
Zr(IV) in the oxide.32 The shape of Zr 3d peaks, being inde-
pendent of the annealing temperature, indicates that Zr is in a
single oxidation state in the film. The O 1s spectra consist of two
branches: the stronger peaks at 530.2 eV correspond to ZrO2,
and the smaller shoulder at 532.25 eV is attributed to SiOx.15
The binding energy difference between O 1s and Zr 3d5/2 is 348.1
eV, which is similar to the value found from the oxidation of
bulk zirconium.33 Thus, the obtained zirconium oxide thin film
mainly consists of ZrO2.
Fig. 7. X-ray photoelectron spectra of the (a) Zr 3d line and (b) O 1s
line in the deposited films annealed at various temperatures.
about the stabilization of this metastable phase of ZrO2 at low
temperature have been carried out.34–36 Among these discus-
sions, the main reason for this stabilization was the crystallite
size effect, as proposed by Garvie et al.
They have proposed that the stabilization of small particles of
t-ZrO2 at temperatures below those required for the tetragonal-
to-monoclinic transformation of bulk ZrO2 (11701C) reflects the
higher surface-free energy of m-ZrO2.37,38 In Garvie,38 the fol-
lowing equation of radius of the critical crystallite size, rc, for the
tetragonal to monoclinic transformation in thin film has been
proposed:
Pure ZrO2 has three polymorphs under ambient pressure, and
the monoclinic phase is the most stable phase from room tem-
perature to 11701C. In our case, however, the crystallization of
the amorphous phase into t-ZrO2 up to 5001C and the subse-
quent transformation of this phase to m-ZrO2 at 9001C were
confirmed by Raman and XRD measurement.
rc ¼ ꢀ3Ds=½qð1 ꢀ T=TbÞ þ Deꢂ
(4)
The existence of this high-temperature phase at low temper-
ature is confirmed by various conditions, and many discussions
where De is the change in strain energy/unit volume for a par-
ticle, q is the heat of the tetragonal to monoclinic transforma-
tion, Ds is the difference in surface-free energies between the
monoclinic and tetragonal phases of ZrO2, Tb is the temperature
at which the tetragonal to monoclinic transformation occurs for
an infinite crystal of ZrO2 (11701C), and T is the temperature at
which crystallites of a given radius rc undergo the tetragonal to
monoclinic transformation. To calculate the value of rc at room
temperature, the following parameter values were used for cal-
culation: Ds5 0.91 J/m2, q 5 ꢀ2.82ꢁ 108 J/m3, Tb 5 1443 K,
T 5 298 K, and De 5 0.46 ꢁ 108 J/m3.38 The value of rc obtained
from Eq. (4) is ca. 15 nm.
As summarized in Table I, for our samples, the radius of
crystallite (r) of the tetragonal phase from Scherrer and TEM
analysis is below 15 nm, in good agreement with the rc calcu-
lated from Eq. (4), while other factors of the stabilization of the
metastable phase at room temperature such as retention of wa-
Fig. 6. High-resolution transmission electron microscope images of the
deposited films annealed at (a) 5001C and (b) 9001C, respectively.

October 2005
Novel Fabrication of High-Quality ZrO2 Ceramic Thin Films
2927
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for the Deposited Films Annealed at Various Temperatures
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¨
Crystallite size (nm)
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Sample (1C)
Scherrer (r)
TEM (r)
500
700
t: 18.1 (9.05)
t: 20.4 (10.2)
m: 28.6 (14.3)
t: 22.4 (11.2)
m: 32.2 (16.2)
t:16 (8)
t: 18 (9)
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m: 27 (13.5)
t: 20 (10)
m: 30 (15)
900
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A
ter,39,40 or the presence of reticular defects,39,40 have been at-
tributed by several authors. However, as shown in Fig. 5(b), no
component corresponding to H2O has been confirmed from
XPS analysis. Moreover, the existence of reticular defects in the
film could not be confirmed from high-resolution TEM obser-
vation. From these results, it can be concluded that LPD-ZrO2
thin films show a metastable tetragonal phase after annealing
below 9001C because of their small crystallite size.
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IV. Conclusions
In this report, we showed the first experimental evidence for the
preparation of high-quality ZrO2 thin films on various kinds of
substrates by the LPD method. Tetragonal ZrO2 thin films that
had high crystallinity could be obtained after annealing at
5001C. This film is stable up to a calcination temperature of
6001C; beyond this, it starts to transform into monoclinic ZrO2.
The tetragonal to monoclinic transformation occurred when the
size of the tetragonal ZrO2 crystallites reached close to a critical
value, which is determined by thermodynamics. The obtained
films showed high transparency in the visible region before and
after annealing at 9001C because the films were composed of
densely packed small particles. This technique, which can be
described as an aqueous solution-based direct deposition proc-
ess, is relatively simple and allows preparation of homogeneous,
high-quality ZrO2 thin films on various kinds of substrates (e.g.,
substrates with complex shapes and powders), without expen-
sive equipment or high energy consumption.
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