Ultrasonically activated hydrothermal synthesis of fine TiO2 and ZrO2 powders

Article abstract of DOI:10.1023/B:INMA.0000046468.73127.f5

Fine TiO₂ and ZrO₂ powders were prepared by hydrothermal synthesis at 150 and 250°C (10 min, 1 h, 3 h) from aqueous titanyl and zirconyl salts and amorphous titanyl and zirconyl hydroxide gels with and without ultrasonic activation. The synthesized materials were characterized by x-ray diffraction, scanning and transmission electron-microscopic techniques, and nitrogen capillary adsorption measurements. The results demonstrate that ultrasonic activation of the hydrothermal process markedly accelerates the crystallization rates of the amorphous titanyl and zirconyl hydroxides and raises the content of thermodynamically stable phases in the reaction products. It is also shown that hydrothermal treatment of amorphous cobalt(II) hydroxide leads to its crystallization, whereas ultrasonic-hydrothermal treatment results in partial Co(II)-to-Co(III) oxidation, yielding a mixture of Co(OH)₂ and Co₃O ₄.

Full text of DOI:10.1023/B:INMA.0000046468.73127.f5

Inorganic Materials, Vol. 40, No. 10, 2004, pp. 1058–1065. Translated from Neorganicheskie Materialy, Vol. 40, No. 10, 2004, pp. 1208–1215.
Original Russian Text Copyright © 2004 by Meskin, Baranchikov, Ivanov, Afanas’ev, Gavrilov, Churagulov, Oleinikov.
Ultrasonically Activated Hydrothermal Synthesis
of Fine TiO and ZrO Powders
2
2
P. E. Meskin*, A. E. Baranchikov**, V. K. Ivanov**, D. R. Afanas’ev*,
A. I. Gavrilov*, B. R. Churagulov*, and N. N. Oleinikov*
*
Moscow State University, Moscow, 119899 Russia
*
* Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia
e-mail: churagulov@inorg.chem.msu.ru
Received March 24, 2004; in final form, April 12, 2004
Abstract—Fine TiO and ZrO powders were prepared by hydrothermal synthesis at 150 and 250°C (10 min,
2
2
1
h, 3 h) from aqueous titanyl and zirconyl salts and amorphous titanyl and zirconyl hydroxide gels with and
without ultrasonic activation. The synthesized materials were characterized by x-ray diffraction, scanning and
transmission electron-microscopic techniques, and nitrogen capillary adsorption measurements. The results
demonstrate that ultrasonic activation of the hydrothermal process markedly accelerates the crystallization rates
of the amorphous titanyl and zirconyl hydroxides and raises the content of thermodynamically stable phases in
the reaction products. It is also shown that hydrothermal treatment of amorphous cobalt(II) hydroxide leads to
its crystallization, whereas ultrasonic–hydrothermal treatment results in partial Co(II)-to-Co(III) oxidation,
yielding a mixture of Co(OH) and Co O .
2
3
4
INTRODUCTION
(2) heating;
One of the most attractive processes for the prepara-
(3) cavitation, i.e., generation of bubbles, which
tion of oxide powders is hydrothermal synthesis— then collapse, giving rise to high local temperatures and
chemical reactions in aqueous media at temperatures pressures.
above 100°C and pressures above 0.1 MPa. An impor-
It is reasonable to expect that the synthesis of oxide
tant point is that this method is environmentally
phases under hydrothermal conditions will also involve
friendly since the process is run in closed systems. It
effects peculiar to sonochemical processes:
should be emphasized that, in most instances, hydro-
(
1) formation of additional nucleation centers in the
thermal processes offer the possibility of controlling
the physicochemical properties of resulting oxide pow-
ders by adjusting the process parameters—synthesis
vicinity of bubbles;
(2) increased growth rate of the particles of the new
temperature and duration, solution concentration and phase owing to accelerated mass transport;
acidity (pH), and pressure in the system [1–4].
(3) disintegration of aggregates and agglomerates of
The use of additional external influences makes it
possible to extend the capabilities of hydrothermal syn-
thesis. For example, microwave–hydrothermal synthe-
sis was used successfully to produce fine powders of
various oxides, in particular Sr, Ba, and Pb titanates,
zirconates, and niobates [5–7]; Zn, Ni, Mn, and Bi fer-
rites [8, 9]; a number of bismuthates [10]; and other
primary crystallites by the shock waves resulting from
bubble collapse;
(4) increased rate of redox reactions owing to water
sonolysis, leading to the formation of hydrogen per-
oxide.
In this paper, we report a detailed study of hydro-
oxide phases. It was shown that microwave activation thermal processes under sonication, exemplified by the
of the hydrothermal process offers the possibility of synthesis of TiO , ZrO , and Co O from aqueous solu-
2
2
3
4
drastically (by several orders of magnitude) reducing tions of appropriate salts and amorphous hydroxides. In
the synthesis duration and that the resulting fine-parti- particular, we focus on the influence of high-power
cle oxide materials are extremely uniform in particle ultrasonic activation on the phase composition and
size [8].
In this work, we examine the influence of high-
microstructure of the resulting oxide powders.
power ultrasonic activation on the hydrothermal pro-
cess. Acoustic waves are known to cause the following
effects in liquids [11]:
EXPERIMENTAL
As starting reagents for the synthesis of TiO and
2
(
1) activation of mass transport;
ZrO , we used 0.28 M aqueous H TiO(C O ) , 0.25 M
2
2
2
4 2
0
020-1685/04/4010-1058 © 2004 MAIK “Nauka/Interperiodica”

ULTRASONICALLY ACTIVATED HYDROTHERMAL SYNTHESIS
1059
aqueous ZrO(NO ) , and amorphous TiO · nH O and
3
2
2
2
7
ZrO · nH O gels.
2
2
6
5
4
The amorphous zirconyl hydroxide gel was pre-
pared by adding 0.25 M aqueous ZrO(NO ) to an
3
2
8
excess of 5 M aqueous ammonia. The amorphous tita-
nyl hydroxide gel was prepared by adding a small
amount of hexachlorotitanic acid, H TiCl , to an excess
2
6
of 5 M aqueous ammonia with vigorous stirring. The
precipitated hydroxides were collected on a filter and
then washed with water.
1
As the Co precursor, we used Co(OH) gel, which
3
2
was prepared by adding 2.6 M aqueous ammonia to
0
.3 M aqueous Co(NO ) to give pH 8.0 ± 0.2 (the solu-
3
2
tion pH was determined by a Checker-1 pH meter with
an HI 1270 electrode).
The ultrasonic–hydrothermal treatment of appropri-
ate aqueous hydroxide solutions and suspensions was
carried out using the apparatus shown schematically in
Fig. 1. As an autoclave for the hydrothermal treatment
of the starting materials, we used a purpose-designed
titanium waveguide (1). The waveguide was coupled to
a PMS-1-1 magnetostrictive transducer (2), connected
to a UZG-3-04 400-W ultrasonic generator (3). In all
experiments, the ultrasound frequency was maintained
at 21.45 ± 0.03 kHz. The oscillation amplitude at the
front surface of the magnetostrictive transducer was
2
Fig. 1. Schematic of the apparatus for ultrasonic–hydrother-
mal synthesis: (1) titanium waveguide/autoclave, (2) PMS-
1
0.0 ± 0.7 µm. The reaction cell was heated by a tubular
1
-1 magnetostrictive transducer, (3) UZG-3-04 ultrasonic
resistance furnace (4).
generator, (4) tubular resistance furnace, (5) reaction cell,
(
6) lid, (7) Chromel–Alumel thermocouple, (8) titanium
The cavitation intensity under hydrothermal condi-
tions at 250°C was 70–80% of that at 25°C, as deter-
mined by acoustic noise measurements [12].
plug.
The size of coherently scattering domains (CSDs) in
The ultrasonic–hydrothermal treatment of aqueous
titanyl and zirconyl salts and amorphous titanyl, zirco-
nyl, and cobalt(II) hydroxides was carried out at 150
and 250°C for 10 min, 1h, and 3 h. The fill factor of the
reaction cell (5) was ꢀ0.70–0.75. Heating the autoclave
to the set temperature took 50–60 min. The reaction
system was ultrasonically activated during both the iso-
thermal and heating steps. Control experiments were
performed under identical hydrothermal conditions but
without sonication.
the synthesized oxide powders was evaluated from
XRD patterns using the Scherrer formula
0
.9λ
DCSD = ------------- -- ,
βcosθ
(1)
where β is the integral width of the intrinsic diffraction
line profile (λ = 1.54178 Å).
Cu
The instrumental broadening was assessed using
appropriate well-crystallized oxide samples. The rela-
tive uncertainty in the CSD size thus determined was
estimated at 10%.
After the hydrothermal run, the autoclave was with-
drawn from the furnace and cooled to room tempera-
ture in air (3–4 h). The precipitate was separated from
the mother liquor by centrifugation, washed with dis-
tilled water, and then dried in air at about 77°C.
The mole fractions of monoclinic (x ) and tetrago-
M
nal (x ) zirconia in the samples were evaluated from the
T
The phases present in the synthesized materials
were identified by x-ray diffraction (XRD) on a DRON-
integrated XRD intensities I (111) , I (111), and
M
å
3
M powder diffractometer (CuK radiation). Micro- IT(111), using the relations
α
structures were examined by scanning (SEM) and
transmission (TEM) electron microscopy on a JEOL
JEM-2000FX-II at an accelerating voltage of 200 kV.
The specific surface of the samples was determined by
nitrogen capillary adsorption measurements with a
Coulter SA 3100 instrument.
I (111) + I (111)
M
M
xM = -----------------------------------------------------------------------,
(2)
(3)
I (111) + I (111) + I (111)
M
M
T
x = 1 – x .
T
M
INORGANIC MATERIALS Vol. 40 No. 10 2004

1
060
MESKIN et al.
2
0 nm
20 nm
(
a)
(b)
Fig. 2. TEM micrographs of ZrO samples prepared by (a) hydrothermal and (b) ultrasonic–hydrothermal treatment of 0.25 M aque-
2
ous ZrO(NO ) at 250°C for 3 h.
3
2
According to Toraya et al. [13], Eq. (2) ensures Co O and Co(OH) , respectively, normalized by that
3
4
2
more accurate results at low contents of monoclinic zir- for corundum [16, 17].
conia in M-ZrO + T-ZrO mixtures.
2
2
The relative uncertainty in the phase compositions
The mole fractions of rutile and anatase in reaction thus determined was 5%.
products were evaluated by the procedure described by
Spurr and Myers [14], using the relation
RESULTS AND DISCUSSION
1
The main physicochemical characteristics of the zir-
conia and titania samples prepared by hydrothermal
synthesis with and without ultrasonic activation are
summarized in Table 1.
x = ------------------------------------,
(4)
where x is the mole fraction of anatase, and I and I
R
A
1
+ 1.26(I /I )
R
A
Ä
Ä
are the integrated intensities of the 101 reflection from
The XRD and TEM results for TiO and ZrO pow-
2
2
anatase and the 110 reflection from rutile, respectively.
ders indicate that sonication has an insignificant effect
on the particle size and microstructure of the samples
prepared under hydrothermal conditions (Fig. 2). At the
same time, ultrasonic activation may influence the crys-
tallinity and phase composition of the TiO and ZrO
The Co O and Co(OH) contents of the samples
3
4
2
were determined using an α-Al O internal standard
2
3
and the relations [15]
2
2
powders.
w
I
Co O
Co O
3
4
3
4
-
---------------- -- = k--------------- -- ,
(5)
Conventional hydrothermal treatment of amorphous
w
I
Co(OH)
Co(OH)
zirconyl hydroxide gel at 250°C for 3 h yields a mixture
2
2
of two zirconia polymorphs: ꢀ12% M-ZrO (thermody-
2
namically stable phase) and ꢀ88% T-ZrO (metastable
(
I/I
)
2
cor Co(OH)
2
k = --------------------------------,
I/I
(6) phase). The treatment of amorphous ZrO · nH O gel
2 2
(
)
cor Co O
3
4
under identical hydrothermal conditions but with ultra-
sonic activation ensures a higher crystallization rate,
and the M-ZrO content of the reaction products
where w
and wCo(OH)2 are the weight fractions of
4
2
Co O
3
increases to about 30%. Note that increasing the ultra-
sonic–hydrothermal treatment time from 1 to 3 h
Co O and Co(OH) , and (I/I )
and (I/Icor)
Co(OH)2
3
4
2
cor Co O
3
4
are the peak intensities of the strongest lines from increases the M-ZrO content from ꢀ8 to ꢀ30%.
2
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ULTRASONICALLY ACTIVATED HYDROTHERMAL SYNTHESIS
1061
Table 1. Synthesis conditions and the main physicochemical characteristics of ZrO and TiO powders
2
2
2
Starting reagents
.25 M aqueous
Synthesis conditions
Phase composition
DCSD, nm (±10%) d , nm (TEM) S, m /g (±5%)
av
0
150°C, 3 h
UH
H
100% M-ZrO2
100% M-ZrO2
100% M-ZrO2
100% M-ZrO2
AP
5.5
4.5
6.5*
7.5*
–
9.0
6.0
110
130
135
110
305
290
ZrO(NO3)2
2
50°C, 3 h
UH
H
15 × 5
15 × 4
–
Amorphous
150°C, 3 h
UH
H
ZrO · nH O gel
AP
–
–
2
2
2
2
50°C, 1 h
50°C, 3 h
UH
92% T-ZrO2
8
% M-ZrO + AP
6.0
–
–
–
–
–
2
H
AP
UH
70% T-ZrO2
7.5
12
200
3
0% M-ZrO2
88% T-ZrO2
2% M-ZrO + AP
H
6.5
11
13
300
85
1
2
0
.28 M aqueous
250°C, 10 min UH
95% A-TiO2
5% R-TiO2
14
9.0
11
H TiO(C O )
2
2
4 2
H
100% A-TiO2
90% A-TiO2
13
13
145
60
2
2
50°C, 1 h
50°C, 3 h
UH
14.5
14
1
0% R-TiO2
95% A-TiO2
% R-TiO2
80% A-TiO2
0% R-TiO2
90% A-TiO2
0% R-TiO2
H
15.5
16
12
13
11
110
100
140
5
UH
H
14
2
14
13
1
15
Amorphous
TiO · nH O gel
150°C, 3 h
UH
H
100% A-TiO + AP
12.5
–
–
320
390
70
2
AP
–
2
2
2
50°C, 3 h
UH
H
100% A-TiO2
100% A-TiO2
16
28
25
17
95
Note: UH = ultrasonic–hydrothermal treatment, H = hydrothermal treatment, AP = amorphous phase.
Inferred from the 111 and 111 reflections from M-ZrO2.
*
Ultrasonic activation has little effect on the phase
Examination of TiO samples prepared via high-
2
composition of the materials prepared from aqueous temperature hydrolysis of aqueous H TiO(C O ) at
2
2
4 2
zirconyl nitrate: after high-temperature hydrolysis of 250°C for 10 min shows that the only phase forming
aqueous ZrO(NO ) with and without ultrasonic activa- under these conditions is anatase (metastable phase). At
3
2
tion, the only phase present is M-ZrO2.
the same time, ultrasonic activation of this reaction
INORGANIC MATERIALS Vol. 40 No. 10 2004

1
062
MESKIN et al.
Ä
Ä
R
1
2
R
2
2
23
24
25
26
27
28
29
30
θ, deg
2
Fig. 3. XRD patterns of TiO samples prepared by (1) hydrothermal and (2) ultrasonic–hydrothermal treatment of 0.28 M aqueous
2
H TiO(C O ) at 250°C for 3 h; A = anatase, R = rutile.
2
2 4 2
Co O
4
*
3
Co(OH)2
1
*
*
*
*
2
2
0
30
40
50
60
70
2
θ, deg
Fig. 4. XRD patterns of samples prepared by (1) hydrothermal and (2) ultrasonic–hydrothermal treatment of amorphous Co(OH)2
gel at 250°C for 3 h.
INORGANIC MATERIALS Vol. 40 No. 10 2004

ULTRASONICALLY ACTIVATED HYDROTHERMAL SYNTHESIS
1063
1
µm
1 µm
(
a)
(b)
1
00 nm
(
c)
Fig. 5. SEM and TEM micrographs of samples prepared by (a) hydrothermal and (b, c) ultrasonic–hydrothermal treatment of amor-
phous Co(OH) gel at 250°C for 3 h.
2
leads to the formation of a small amount (5%) of rutile
Note that, when TiO was prepared from amorphous
2
(
thermodynamically stable phase). Increasing the ultra- TiO · nH O gel, ultrasonic activation markedly increased
2
2
sonic–hydrothermal synthesis time to 3 h increases the the crystallization rate but had little effect on the phase
rutile content to 20%, whereas the rutile content after composition of the reaction products: both hydrothermal
conventional hydrothermal treatment for 3 h is no and ultrasonic–hydrothermal treatments led to the for-
higher than 10% (Fig. 3).
mation of phase-pure nanocrystalline anatase.
Table 2. Synthesis conditions and the main physicochemical characteristics of samples obtained by hydrothermal treatment
of an amorphous Co(OH) precipitate (250°C, 3 h) with and without ultrasonic activation
2
2
Treatment
UH
Phase composition
80% Co(OH)2
dav, µm (SEM, TEM)
S, m /g (±5%)
1.7 (Co(OH)2)
0.2 (Co O )
8.7
2
0% Co O
3 4
3
4
H
100% Co(OH)2
13
2.2
INORGANIC MATERIALS Vol. 40 No. 10 2004

1
064
MESKIN et al.
To assess the effect of ultrasonic activation on redox vation accelerates the Co(II)
Co(III) oxidation rate
reactions and crystallization processes under hydro- and reduces the particle size of the reaction products.
thermal conditions, we carried out ultrasonic–hydro-
thermal treatment of amorphous cobalt(II) hydroxide.
ACKNOWLEDGMENTS
The main physicochemical characteristics of the syn-
thesized samples are listed in Table 2,
We are grateful to Yu.V. Kolen’ko for determining
the specific surface of the samples by nitrogen adsorp-
tion measurements. We are also indebted to A.V. Gar-
shev for performing the electron-microscopic work.
This work was supported by the Russian Foundation
for Basic Research (project nos. 04-03-32295 and
Our results demonstrate that, during hydrothermal
treatment under high-power sonication, amorphous
Co(OH) partially oxidizes to form Co O nanopowder.
2
3
4
The Co(OH) -to-Co O conversion under such condi-
2
3
4
tions is about 20%. In control experiments, with no
ultrasonic activation, amorphous cobalt(II) hydroxide
fully crystallized, but no oxide phases were detected
0
3-03-32813), the Leading Scientific Schools Program
project no. NSh-20332003.03), the Universities of
(
Russia Program (project no. UR.06.02.033), and the
Chemical Sciences and Materials Research Division of
the Russian Academy of Sciences through the program
Development of the Scientific Principles of Advanced
Chemical Technologies and Pilot-Scale Production of
Substances and Materials.
(Fig. 4).
The formation of nanocrystalline Co O powder upon
3
4
the ultrasonic–hydrothermal treatment of amorphous
Co(OH) can be accounted for by the fact that, under
2
such conditions, crystallization is accompanied by par-
tial Co(II) oxidation with the hydrogen peroxide forming
in solution as a result of ultrasonic activation [18]:
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2
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2
6
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6
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INORGANIC MATERIALS Vol. 40 No. 10 2004