Phase transformation in the surface region of zirconia detected by UV Raman spectroscopy

Article abstract of DOI:10.1021/jp010526l

The phase transformation of zirconia from tetragonal to monoclinic is characterized by UV Raman spectroscopy, visible Raman spectroscopy, and XRD. Electronic absorption of ZrO₂ in the UV region makes UV Raman spectroscopy more sensitive at the

Full text of DOI:10.1021/jp010526l

J. Phys. Chem. B 2001, 105, 8107-8111
8107
Phase Transformation in the Surface Region of Zirconia Detected by UV Raman
Spectroscopy
Meijun Li, Zhaochi Feng, Guang Xiong, Pinliang Ying, Qin Xin, and Can Li*
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
P.O. Box 110, Dalian 116023, China
ReceiVed: February 9, 2001; In Final Form: June 18, 2001
The phase transformation of zirconia from tetragonal to monoclinic is characterized by UV Raman spectroscopy,
visible Raman spectroscopy, and XRD. Electronic absorption of ZrO2 in the UV region makes UV Raman
spectroscopy more sensitive at the surface region than XRD or visible Raman spectroscopy. Zirconia changes
from the tetragonal phase to the monoclinic phase with calcination temperatures elevated and monoclinic
phase is always detected first by UV Raman spectroscopy for the samples calcined at lower temperatures
than that by XRD and visible Raman spectroscopy. When the phase of zirconia changes from tetragonal to
monoclinic, the slight changes of the phase at very beginning can be detected by UV Raman spectroscopy.
UV Raman spectra clearly indicate that the phase transition takes place initially at the surface regions. It is
found that the phase change from tetragonal to monoclinic is significantly retarded when amorphous
Zr(OH)4 was agglomerated to bigger particles and the particle agglomeration of amorphous zirconium hydroxide
is beneficial to the stabilization of t-ZrO2 phase.
Introduction
Ultraviolet Raman spectroscopy (UVRS) is a powerful tool
for the study of catalysts and other solids.24-29 Raman scattering
Recent research progress in zirconia has greatly enhanced
the prospects for applying this material as catalyst, catalyst
support, ceramic, engineering gemstone, fuel cell, pigment,
automotive gas sensor, etc.1-6 Its varied chemical properties
including reducing, oxidizing, and acidic and basic properties
make zirconia an attractive catalyst or catalyst support for a
number of reactions.1,5,7-9 For example, zirconia impregnated
with sulfate ions has superacidic behavior and shows high
activity for hydrocarbon isomerization and methanol conversion
to hydrocarbons.10-13
Zirconia exhibits three different phases: monoclinic (m-
ZrO2), which is stable below 1170 °C; tetragonal (t-ZrO2), which
is stable between 1170 °C and 2370 °C; and cubic (c-ZrO2),
stable from 2370 °C to its melting temperature at 2680 °C.
Besides m-ZrO2, metastable t-ZrO2 can be stable at room
temperature.2,14,15 Metastable t-ZrO2 can be obtained by the
thermal treatment of suitable starting materials (zirconium salt,
zirconium alkoxides, or zirconium hydroxide).1,2,15,16 Stabilized
tetragonal zirconia is considered an important structure for
ceramics because of its excellent mechanical properties such
as fracture toughness, strength, and hardness. However, a phase
transformation from the metastable tetragonal (t) phase to the
monoclinic (m) phase of crystalline ZrO2 prevents its applica-
tions from a broader temperature ranges. Many studies have
addressed the factors that affect the phase transformation of
tetragonal zirconia, such as the amount of stabilizer,17,18 the pH
values of precipitation in the synthesis,2 the grain size of the
tetragonal powder,19,20 water vapor in its crystalline growth,21
and intrinsic defects such as vacancies in the anionic sublat-
tice.22,23 However, these investigations provided the information
mainly on the bulk of ZrO2.
comes usually from both the surface and the bulk of a solid
sample, but the signal from the bulk is attenuated when the
sample strongly absorbs the excitation laser light and scattering
light. Therefore, UV Raman spectroscopy is more sensitive to
the surface region of zirconia because zirconia shows a strong
absorbance in the UV region. In this work, we have studied the
phase transformation of zirconia using UVRS together with
visible Raman spectroscopy and XRD. It is found that the results
from UV Raman spectroscopy are different compared with those
from XRD and visible Raman spectroscopy, particularly when
the tetragonal phase begins to change into the monoclinic phase.
On the basis of the results of UV Raman spectroscopy, visible
Raman spectroscopy, and XRD, it is concluded that the phase
transformation of zirconia takes place initially at surface regions,
then develops into the bulk.
Experimental Section
Zr(OH)4 was obtained by adding aqueous ammonia slowly
to the solution of zirconium oxychloride at room temperature
under stirring until the pH value of the solution reached ∼10.
The precipitate thus obtained was washed thoroughly with
distilled water until chloride ions were not detected, and then
was dried at 110 °C for 12 h. The dried precipitate sample, in
the forms of powder pressed and unpressed, was calcined at
different temperatures for 2 h. The pressure for making the
pressed pellet was about 5 MPa. UV, VIS Raman spectra, and
XRD patterns were recorded at room temperature for the
calcined powder unpressed and pressed samples.
UV Raman spectra were recorded on a homemade UV Raman
spectrograph, which has four main parts: an UV cw laser, a
Spex 1877d triplemate spectrograph, a CCD detector, and an
optical collection system. The 244-nm line from Innova 300
FRED was used as the excitation source. The laser power of
the 244-nm line at the samples was below 2.0 mw. Visible
* Author to whom correspondence should be addressed. Tel: 8-411-
4671991, ext. 728, 726. Fax: +86-411-4694447. E-mail: canli@ms.dicp.ac.cn.
Homepage: http://www.canli.dicp.ac.cn.
10.1021/jp010526l CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/08/2001

8108 J. Phys. Chem. B, Vol. 105, No. 34, 2001
Li et al.
Figure 2. XRD patterns of unpressed powder Zr(OH)4 calcined at 400
°C, 500 °C, and 700 °C.
Figure 1. UV Raman spectra of unpressed powder Zr(OH)4 calcined
at 400 °C, 500 °C, and 700 °C.
Figure 2 shows the XRD patterns of unpressed samples
calcined at different temperatures. The “t” and “m” in the figure
denote the tetragonal and monoclinic phases, respectively. After
the sample was calcined at 400 °C, the diffraction peaks of the
tetragonal phase are predominant, and the peaks of the mono-
clinic phase are relatively weak. Besides, small portion of ZrO2
may be still remained as amorphous phase after the sample was
calcined at 400 °C because the XRD peaks are somewhat broad.
This means that the zirconia is mainly in the tetragonal phase.
However, from UV Raman spectra, it is seen that the zirconia
is a mixture of phases m-ZrO2 and t-ZrO2. The diffraction peaks
of t-ZrO2 decline and the peak intensities of m-ZrO2 increase
after the sample was calcined at 500 °C. But the XRD peaks of
t-ZrO2 are still very strong, indicating that the t-ZrO2 phase is
still a big fraction in the zirconia. On the contrary, the UV
Raman spectra (Figure 1) suggest that zirconia is nearly all in
the monoclinic phase. After the sample was calcined at 700
°C, it can be observed that the diffraction peaks of m-ZrO2 are
predominant, and the diffraction peaks of t-ZrO2 become weak.
The results of XRD suggest that some t-ZrO2 phase still exist
even after the sample was calcined at 700 °C.
From the comparison between Figure 1 and Figure 2, it can
be seen that there are distinct differences between the results
from UV Raman spectroscopy and XRD. According to UV
Raman spectra, the Raman bands due to the mixed phases (t-
ZrO2 and m-ZrO2) are observed for the zirconia calcined at 400
°C, and only the Raman bands of m-ZrO2 are detected after the
sample was calcined at 500 °C. But the results from XRD
indicate that t-ZrO2 is the dominant phase after the calcination
at 400 °C and it can be detected even after the calcination at
700 °C.
Raman spectra were recorded on a Jobin-Yvon U1000 scanning
double monochromator with the spectrum resolution of 4 cm-1
using a laser at 532 nm line as the excitation source. The Raman
signal was collected with a 90o-geometry.
UV-Vis diffuse reflectance spectra were recorded on a
JASCO V-550 UV-Vis Spectrophotometer. XRD patterns were
collected on a Rigaku Rotaflex (RU-200B) powder diffracto-
metry equipped with a Cu target and a Ni filter.
Results and Discussion
Figure 1 displays the UV Raman spectra of unpressed powder
samples calcined at different temperatures. It can be seen in
the Raman spectrum that the major bands are at 181, 269, 312,
472, and 634 cm-1 with weak bands at 149, 377, and 556 cm-1
for the zirconia sample calcined at 400 °C. The Raman bands
at 149, 269, and 312 cm-1 are assigned to the Raman-active
modes for the tetragonal phase of ZrO2, B1g at 149 cm-1, Eg at
269 cm-1, and B1g at 312 cm-1. The bands at 181, 337, 472,
556, and 634 cm-1 are assigned to the Raman-active modes
for the monoclinic phase of ZrO2, Ag at 181 cm-1, Bg at 377
cm-1, Ag at 472 cm-1, Ag at 558 cm-1, and Ag at 634 cm-1 30
.
The band at 269 cm-1 appears only for t-ZrO2 and the band at
181 cm-1 appears only for m-ZrO2. In addition, there are two
main differences in the Raman spectra between the monoclinic
phase and the tetragonal phase of zirconia, i.e., the band at 472
cm-1 is stronger than the band at 634 cm-1 for the monoclinic
phase and the reverse for the tetragonal phase, and there are
some small bands between 472 and 634 cm-1 for the monoclinic
phase, but these weak bands are absent for the tetragonal phase.
Obviously, the UV Raman spectrum of the sample calcined at
400 °C is due to mixed phases of t-ZrO2 and m-ZrO2.
Figure 3 shows the visible Raman spectra of unpressed
zirconia samples calcined at different temperatures. It is
observed that Raman bands of t-ZrO2 are dominant in the
spectrum after the sample was calcined at 400 °C, because the
characteristic bands of t-ZrO2 at 149, 269, and 313 cm-1 are
relatively strong, and the band at 640 cm-1 is stronger than the
band at 469 cm-1. After the sample was calcined at 500 °C,
the intensities of the bands at 178, 189, and 379 cm-1 attributed
to m-ZrO2 increase somewhat. The band at 469 cm-1 is slightly
stronger than the band at 640 cm-1. This suggests that the
portion of m-ZrO2 phase in zirconia is increased, but there is
After the sample was calcined at 500 °C, the Raman bands
due to t-ZrO2 (149, 269, and 312 cm-1) disappear as shown in
Figure 1, while the bands of m-ZrO2 (181, 337, 472, 560, and
634 cm-1) develop. In addition, some Raman bands appear at
219, 305, and 337 cm-1, which are assigned to the Raman-
active modes for m-ZrO2, Bg at 220 cm-1, Ag at 305 cm-1, and
Bg at 337.30 The Raman spectrum of the sample calcined at
500 °C shows the bands mainly attributed to m-ZrO2. The
Raman bands of the monoclinic phase increase in intensity
further after the sample was calcined at 700 °C (see Figure 1).

Phase Transformation in Surface Region of Zirconia
J. Phys. Chem. B, Vol. 105, No. 34, 2001 8109
Figure 3. Visible Raman spectra of unpressed powder Zr(OH)4
calcined at 400 °C, 500 °C, and 700 °C.
Figure 4. UV-Visible diffuse reflectance spectra of zirconia.
still very much t-ZrO2. For the sample calcined at 700 °C, the
Raman bands at 178, 189, 220, 307, 339, 379, and 635 cm-1
due to monoclinic phase become considerably stronger. Very
interestingly, the results from visible Raman spectroscopy are
in good agreement with the results from XRD (Figure 2).
Generally speaking, Raman scattering comes from both the
surface and the bulk of a solid sample. But the signal from the
bulk is attenuated when the sample strongly absorbs the
excitation laser light and the scattering light, and the Raman
spectra then contain more signal from the surface top region
than the bulk of a sample. In particular, most transition metal
oxides strongly absorb UV laser light, so UV Raman spectros-
copy is more sensitive to the surface region than to the bulk
for these samples. Visible Raman spectroscopy gives the
information from both the bulk and the surface of a sample
because there is usually no electronic absorption in the visible
region for most samples.
UV-Visible diffuse reflectance spectra of zirconia show two
broad bands centered at 210 and 230 nm (Figure 4). The laser
line at 244 nm is near the electronic absorption of ZrO2.
Accordingly, the phase transformation in the surface region
could be detected selectively by UV Raman spectroscopy. In
contrast, it is hard to separate the information of the surface
phase from the bulk phase of zirconia using visible Raman
spectroscopy, because there is no electronic absorption in the
visible region for zirconia.
The m-ZrO2 phase is detected by UV Raman spectroscopy
when the samples are calcined at relatively lower temperatures
compared with XRD and visible Raman spectroscopy. The fact
that the results from UV Raman spectra disagree with those
from XRD and visible Raman can be interpreted only as that
the phase transformation of zirconia starts from the surface
region and then progresses to the bulk. The initial change of
the phase at the surface region can be sensitively detected by
UV Raman spectroscopy but not by XRD and visible Raman
spectroscopy.
Figure 5. UV Raman spectra of pressed Zr(OH)4 calcined at 400 °C,
500 °C, 600 °C, and 700 °C.
UV Raman spectrum gives strong bands at 149, 269, 312, 462,
and 640 cm-1. The band at 640 cm-1 is stronger than the band
at 462 cm-1. These features are readily attributed to the t-ZrO2
phase, but a tiny band at 181 cm-1 due to m-ZrO2 phase can be
observed. As the calcination temperature was increased to 500
°C, the bands at 149, 269, and 312 cm-1 attributed to t-ZrO2
phase become slightly stronger, and the weak bands at 181, 270,
and 556 cm-1 attributed to the m-ZrO2 phase begin to appear.
After the sample was calcined at 600 °C, the intensities of the
bands at 149, 269, and 312 cm-1 decrease, while those of bands
at 181, 376, and 556 cm-1 increase. The typical Raman bands
at 220 and 337 cm-1, attributed to m-ZrO2, become evident.
The Raman spectrum of the pressed sample calcined at 700 °C
gives the strong bands of m-ZrO2 at 181, 220, 306, 337, 376,
472, and 634 cm-1, and the bands of t-ZrO2 disappear.
The influence of mechanical treatment on the phase trans-
formation from t-ZrO2 to m-ZrO2 was also investigated using
UV Raman spectroscopy and XRD. Figure 5 displays the UV
Raman spectra of pressed samples calcined at different tem-
peratures. After the pressed sample was calcined at 400 °C, the
Figure 6 shows the XRD patterns of the pressed samples
calcined at different temperatures, to compare with the UV
Raman spectra (Figure 5). It is found that only the tetragonal
phase is present after the pressed sample was calcined at 400

8110 J. Phys. Chem. B, Vol. 105, No. 34, 2001
Li et al.
however, the XRD peak of t-ZrO2 is still quite strong. On the
contrary, from UV Raman spectra (Figure 5), it can be observed
that the m-ZrO2 phase forms the major portion of the zirconia.
When the sample was calcined at 700 °C, the XRD peaks of
m-ZrO2 are predominant and the peaks of t-ZrO2 become weak,
while the UV Raman bands of t-ZrO2 disappear (Figure 5). From
UV Raman spectra (Figure 5) and XRD (Figure 6) of the pressed
samples, on one hand, it is found that the m-ZrO2 phase can be
first detected by UV Raman spectroscopy when the samples
were calcined at temperatures relatively lower than that by XRD.
On the other hand, the t-ZrO2 phase disappear at relatively lower
temperatures deduced from UV Raman spectra than from XRD.
This obvious discrepancy again suggests that the phase trans-
formation from t-ZrO2 to m-ZrO2 take place initially at surface
region.
Comparing Figure 1 with Figure 5, differences can be found
for the phase changes between the unpressed and pressed powder
samples. After the samples were calcined at 400 °C, the UV
Raman bands at 181, 269, 312, 377, 472, and 634 cm-1 due to
the mixture with nearly equally amount of t-ZrO2 and m-ZrO2
are detected for the unpressed powder sample (Figure 1), but,
for the pressed sample, the bands of t-ZrO2 at 149, 269, 312,
462, and 640 cm-1 are predominant (Figure 5). After the
samples were calcined at 500 °C, mainly the Raman bands of
m-ZrO2 are observed for the unpressed sample (Figure 1), while
the bands attributed to t-ZrO2 are still dominant for the pressed
sample (Figure 5). The UV Raman bands of t-ZrO2 can be
observed even after the pressed sample was calcined at 600 °C
(Figure 5).
Figure 6. XRD patterns of pressed Zr(OH)4 calcined at 400 °C, 500
°C, 600 °C, and 700 °C.
°C (Figure 6). But in the UV Raman spectra (Figure 5), a
relatively weak band of m-ZrO2 at 181 cm-1 is observed. As
the sample was calcined at 500 °C, some weak diffraction peaks
of m-ZrO2 appear, but zirconia is still mainly in the tetragonal
phase. From UV Raman spectra (Figure 5), the zirconia is a
mixture of phases of t-ZrO2 and m-ZrO2. As the calcination
temperature was increased to 600 °C, the diffraction peaks of
t-ZrO2 decline and the peak intensities of m-ZrO2 increase,
Figure 7. Schematic description of the phase evolution of zirconia calcined at different temperatures and the information obtained from UV
Raman spectroscopy, visible Raman spectroscopy, and XRD. The inset is the UV-visible absorbance of m-ZrO2 and t-ZrO2.

Phase Transformation in Surface Region of Zirconia
J. Phys. Chem. B, Vol. 105, No. 34, 2001 8111
From the comparison between Figure 1 and Figure 5, it is
clearly seen that there are distinct differences in UV Raman
spectra between the unpressed and pressed powder samples. In
the spectra of the unpressed powder sample, the Raman bands
due to both t-ZrO2 and m-ZrO2 are equally observed for the
sample calcined at 400 °C. However, for the pressed sample
calcined at 400 °C, Raman spectrum indicated that the zirconia
is nearly all in t-ZrO2 phase. As the calcination temperature
was increased to 500 °C, only the Raman bands of m-ZrO2 are
observed for the unpressssed powder sample (Figure 1), but the
Raman bands of t-ZrO2 are still strong for the pressed sample
(Figure 5). This suggests that the t-ZrO2 phase is more stable
for the pressed sample, and its phase change from tetragonal to
monoclinic is obviously retarded.
The difference between unpressed and pressed samples can
be seen in XRD patterns in Figure 2 and Figure 6. After the
unpressed sample was calcined at 400 °C, the diffraction peaks
of t-ZrO2 are dominant, but with relatively weak peaks of
m-ZrO2 (Figure 2), while only the diffraction peaks of t-ZrO2
are observed for pressed sample (Figure 6). When the unpressed
sample was calcined at 500 °C, the diffraction peaks of both of
t-ZrO2 and m-ZrO2 are detected with equal intensities (Figure
2); but the diffraction peaks are due to t-ZrO2 phase are mainly
observed for the pressed sample calcined at 500 °C (Figure 6).
On the basis of the results presented above, it can be
concluded that t-ZrO2 phase is more stable after the powder of
ZrO2 sample was pressed before its calcination. It turns out that
pressing the sample will retard the transformation from the
tetragonal phase to the monoclinic phase. This can be interpreted
as that the sample pressing may induce the particle agglomera-
tion of amorphous zirconium hydroxide. The agglomerated
particle may experience a slower phase transformation than
smaller particles. It was reported that a slow addition of base
during the precipitation process led to the polymerization of
zirconia, that makes the t-ZrO2 stable.31 These results indicated
that the agglomerated bigger particle of amorphous zirconium
hydroxide is beneficial to the formation and stabilization of
t-ZrO2 phase.
to 400 °C. It is found that UV Raman spectroscopy is more
surface-sensitive than XRD and visible Raman spectroscopy for
ZrO2 because zirconia absorbs UV light. The m-ZrO2 phase is
detected by UV Raman spectroscopy for the samples calcined
at lower temperatures than when it is detected by XRD or visible
Raman spectroscopy. The results of UV Raman spectroscopy,
visible Raman spectroscopy, and XRD suggest that the phase
transformation of ZrO2 from tetragonal to monoclinic takes place
initially at the surface regions of ZrO2 particle. Agglomeration
of amorphous zirconium hydroxide particle favors stabilization
of t-ZrO2, because the portion of surface region in the whole
particle is decreased when the particle size is increased.
Acknowledgment. This work was financially supported by
the State Key Project (Grant 19999022407) of the Ministry of
Science and Technology, People’s Republic of China, and the
National Science Foundation of China(NSFC) for Distinguished
Young Scholars (Grant 29625305).
References and Notes
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A schematic description of the phase transformation of
zirconia as deduced from UV Raman spectra, visible Raman
spectra and XRD is shown in Figure 7. When the samples are
excited by a UV laser, e.g., 244-nm laser light, the Raman
signals are mostly from the surface region of ZrO2 because ZrO2
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