ABSTRACT Nanosized TiO 2 with anatase structure has been prepared by a non-alkoxide sol-gel route from an aqueous solution of titanyl sulfate. This paper investigated the influence of stirrer speed on anatase precipitation carried out in batch mode. Stirrer speed was varied between 400 and 1000 rpm. The intensity of stirring determined the final particle size of anatase aggregates in suspension. Changes to primary agglomerate and crystal population due to varying stirrer speed were reported. In our study, the smallest aggregates (mean size of 6-8 nm) were obtained at a stirrer speed of 1000 rpm. It was also found that the anatase to rutile phase transformation began at 800°C of calcination. A calcination treatment, from 400 to 1000°C, was applied. The degree of crystallinity and purity of the synthesized materials were reported. The crystalline anatase particles had an average particle size of 10-20 nm between the calcination temperature 400-600°C. The powders were characterized by powder X-ray diffraction (XRD), the morphology and grain size of the titania particles were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). XRD patterns indicated that the samples were crystalline titania which had very broad and small peaks of anatase phase. The TEM observations showed the vast majority of particles were quasispherical; the average size distribution of particle was about 20 nm. The TEM results supported the conclusions derived from the XRD data. Finally, the amorphous hydrous and crystalline titania particles were characterized by thermogravimetric (TGA) and differential thermal analysis (DTA)

Chinda Ngernsuttichaiporn

Chiravadee Sritara

Kornvalai Panpae

Suppalak Angkaew

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Kasetsart J. (Nat. Sci.) 41 : 178 - 185 (2007)
An Alkoxide Free Sol-Gel Synthesis of Nanosized TiO2
Kornvalai Panpae*, Suppalak Angkaew, Chiravadee Sritara
and Chinda Ngernsuttichaiporn
ABSTRACT
Nanosized TiO2 with anatase structure has been prepared by a non-alkoxide sol-gel route from
an aqueous solution of titanyl sulfate.  This paper investigated the influence of stirrer speed on anatase
precipitation carried out in batch mode.  Stirrer speed was varied between 400 and 1000 rpm.  The
intensity of stirring determined the final particle size of anatase aggregates in suspension.  Changes to
primary agglomerate and crystal population due to varying stirrer speed were reported.  In our study, the
smallest aggregates (mean size of 6-8 nm) were obtained at a stirrer speed of 1000 rpm.  It was also
found that the anatase to rutile phase transformation began at 800°C of calcination. A calcination treatment,
from 400 to 1000°C, was applied.  The degree of crystallinity and purity of the synthesized materials
were reported.  The crystalline anatase particles had an average particle size of 10-20 nm between the
calcination temperature 400-600°C.  The powders were characterized by powder X-ray diffraction (XRD),
the morphology and grain size of the titania particles were examined by scanning electron microscopy
(SEM) and transmission electron microscopy (TEM).  XRD patterns indicated that the samples were
crystalline titania which had very broad and small peaks of anatase phase. The TEM observations showed
the vast majority of particles were quasispherical; the average size distribution of particle was about 20
nm. The TEM results supported the conclusions derived from the XRD data. Finally, the amorphous
hydrous and crystalline titania particles were characterized by thermogravimetric (TGA) and differential
thermal analysis (DTA).
Key words: sol-gel, nanosized, anatase, titania, titanyl sulfate, non-alkoxide
Department of Chemistry, Faculty of Science, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand.
* Corresponding author, e-mail: kornvalai.pan@kmutt.ac.th
INTRODUCTION
The production of particles with a
specific size and morphology is of primary
importance for the development of new materials.
Mesoscale spheres of ceramic nanomaterials are
of particular interest for fundamental research in
order to interpret physical properties or surface
interactions quantitatively as a function of the
morphology and size of the spheres. Recently, the
importance of tailored particles has been
recognized in a number of applications such as
ceramics (Wei et al., 1999; Assmann et al., 2004;
Sivakumar et al., 2004), solar cells, photonic
crystals, pigments (Lee and Chen, 2001; Allen et
al., 2002; Assmann et al., 2004) and catalysts
(Yamazaki et al., 2001; Yasir et al., 2001;
Sathyamoorthy et al., 2001; Sivakumar et al. 2002;
Ma et al., 2002; Kandavelu et al., 2004; Baiju et
al., 2005). Among the many metal oxides and their
inorganic-organic hybrid materials used for the
above applications, one of the most important and
complex in this regard is titanium dioxide (titania).
To date, research activities into the photochemical
properties of titania relate to a number of areas.
Titania exists in three crystal structures (anatase,
rutile and brookite) and each behaves differently.
Even the same form if prepared by different
methods can also show different properties due to
different surface area, porosity, and functional
groups at the surface. Two morphologies of titania,
anatase and rutile, exhibit 6 : 3 coordination
whereas the anatase forms a distorted octahedral
structure the rutile forms slender primatic crystal
that is often twinned. Differences in the
photoactivities of these two modifications of
titania depends markedly upon the history of the
synthetic methods. Thus, different approaches to
their synthesis have been developed (Wang and
Ying, 1999) : (i) inert gas condensation, (ii) flame
synthesis by TiCl4 oxidation, (iii) oxidation-
hydrothermal synthesis of metallic Ti, and (iv)
hydrolysis precipitation of titanium alkoxides of
chlorides. Of particular interest is synthetic method
(iv), the wet-chemical synthesis, or the sol-gel
approach. The hydrolytic sol-gel method allows
compositional and microstructural tailoring
through controlling the precursor chemistry and
processing conditions. Sol-gel processing provides
for excellent chemical homogeneity and the
possibility of deriving unique metastable structures
at low reaction temperatures. It involves the
formation of a metal-oxo-polymer network from
molecular precursors such as metal oxides of metal
salts. Sivakumar et al. (2002; 2004) have reported
that the sol-gel method becomes one of the
successful and simple techniques for preparing
nanosized titania. Most of earlier attempts used
titanium alkoxides as precursors in the sol-gel
route. As precursors of monodispersed oxide
powders, however, inorganic compounds may be
more economical than alkoxides and a few
attempts were reported in this direction
(Sivakumar et al., 2002). Recently, Wei et al.
(1999) prepared nanodispersed spherical TiO2
powder, of average particle size 80 nm, by forced
hydrolysis of Ti(SO4)2 aqueous solution containing
sulfuric acid under boiling reflux condition.
Sivakumar et al. (2002) synthesized
nanocrystalline anatase with high surface area by
homogeneous precipitation of aqueous titanyl
sulfate solution at room temperature. In this study,
the processing parameters in an alkoxide free sol-
gel synthesis of anatase titania nanoparticles were
investigated. Factors affecting the sol-gel process
include the reactivity of inorganic titanium salt,
pH of the reaction medium, calcination
temperature, the influence of varying stirrer speeds
on the formation of anatase particles, and the
nature of hydrolytic solvent and peptisation
reagent. By varying these processing parameters,
materials with different microstructure and surface
chemistry can be obtained.
MATERIALS AND METHODS
Synthesis
Titanyl sulfate (98% Ti(SO4)2), obtained
from Sigma-Aldrich Laborchemikalien Co., was
dissolved in 500 ml of deionized water (0.2 M)
and subjected to precipitation by the slow addition
of ammonium hydroxide solution (10% NH4OH)
under constant stirring (400 rpm) by a mechanical
stirrer at room temperature (30°C). The hydrolysis
was controlled with the addition of NH4OH, until
the reaction mixture attained a pH between 7 and
8. White precipitate of hydrous oxide was
produced instantly, and the mixture was stirred at
the constant rate for at least 1 hour. The precipitate
obtained was separated from the mother liquor by
vacuum filtration with 0.2 µm micropore filter
(diameter = 47 mm) and washed repeatedly with
deionized water until the precipitate became free
from sulfate ion (confirmed by BaCl2 test). The
precipitate was converted into a stable sol by
dispersing it into 1000 ml of boiling (100°C)
distilled water, followed by peptisation with
addition of 10% HNO3. The stable sol of pH 1.8 –
2.2 was aged 12 hours at room temperature. The
sol to gel conversion was achieved by raising the
Kasetsart J. (Nat. Sci.) 41(5) 179
180 Kasetsart J. (Nat. Sci.) 41(5)
pH to 6, by dropwise addition of 10% NH4OH
solution. The gel was again kept at 40°C for 24
hours and then dried at 90°C in an oven over a
period of 24 hours. The amorphous precipitate was
then subjected to either calcination or
hydrothermal treatment. The dried gel was
calcined at various temperatures i.e. 400, 600, 800,
and 1000°C in separate batches for 2 hours with a
heating rate of 10 – 15°C min-1. The influence of
varying stirrer speeds on the crystallinity of titania
particles was studied by precipitating 0.2 M
Ti(SO4)2 with 10 % NH 4OH under various stirrer
speeds from 400 to 1000 rpm for 1 hour at room
temperature.
Methods of characterization
Thermal analysis and the water content
of the gel was determined thermogravimetrically
using a Perkin Elmer – thermonalyzer TGA7 (O2
atmosphere, heating rate 10°C min-1) combining
thermogravimetry (TG) and differential scanning
calorimeter DSC7. The gel and the calcined
powders were characterized by powder X-ray
diffraction (XRD) using a diffractometer of Rigaku
(Model Miniflex) with graphite monochromated
Cu Kα radiation at a scanning rate of 4.0 deg.min-
1 in 2θ range 5.0 – 80.0°. To determine the average
crystallite size, peak broadening analysis was
applied to anatase (101) diffractions using
Scherrer’s equation (Bai et al., 1999; Wang and
Ying, 1999; Sivakumar et al., 2002). The
morphology and grain size of the titania particles
were examined by a Joel JSm-5800 scanning
electron microscope (SEM) and a Joel JEM-2010
transmission electron microscope (TEM)
operating at 200 kV. Drops of a powder suspension
were added to carbon-coated copper girds in TEM
sample preparation.
RESULTS AND DISCUSSION
The precipitates from sol-gel processing
of titania were amorphous in nature. A significant
weight loss was noted in the TGA curves (Figure
1a) for the dried titania gels at 90-95°C undergoing
a total weight loss of 7.36% with two
decomposition steps between the analysed
temperature of 30-800°C. The weight loss below
150° was due to the removal of loosely bound
water in the gel network. The decomposition step
between 230 and 300°C (55.67%wt. loss) was due
to the decomposition of nitrate ions as well as the
dehydroxylation of the gel. The DTA pattern
(Figure 1b) was complementary to the TGA
observations. The very small exothermic peak
centered around 390°C represented the
decomposition of nitrate species along with
dehydroxylation of titanium species, which was
supported by the corresponding weight loss in the
TGA curve. A calcination treatment (from 400 to
1000°C) was applied : these temperatures were
also necessary to eliminate water and other
inorganic species from the hydrous gel (Figure 2).
The change in titania gel particle size
with constant stirring time (1 hour) at different
stirrer speeds was investigated. SEM images of
gel particle sizes are illustrated in Figure 3 and
the TEM micrograph of the sample prepared under
the same experimental conditions except stirrer
speed at 1000 rpm is described in Figure 4. This
was clear evidence that stirrer speed had a
significant influence over the final aggregate size.
At 400 rpm, aggregates appeared to be larger, and
gradually reduced in size with higher stirrer speed.
If we compare SEM images for 400, 600 and 800
rpm, we notice that the reduction in size is followed
by an increase in the number of aggregates.
Sathyamoorthy et al. (2001) have reported that the
kinetics of precipitation remains unchanged for
varying stirrer speeds. Therefore, the difference
in size and number of aggregates at different stirrer
speeds must maintain equal mass in all cases.
Figure 4 illustrated the maximum reduction in
mean particle size of aggregates for stirrer speed
of 1000 rpm from TEM microphotograph. In the
case of anatase, similar observations were deduced
from experimental results of Sathyamoorthy et al.
(2001). They described the attractive force as
Kasetsart J. (Nat. Sci.) 41(5) 181
Figure 1 Thermogravimetric (a) and differential
thermal (b) analysis curves of
amorphous titania gel (0.2 M TiO2, with
stirrer speed 1000 rpm).
Figure 2 Thermogravimetric (a) and differential
thermal (b) analysis curves of 400oC
calcined  titania gel (0.2 M TiO2, with
stirrer speed 1000 rpm).
Figure 3 Scanning electron micrographs of amorphous titania gel (0.2 M TiO2) in the form of aggregates
recovered at the end of 1 hour; precipitation for different stirrer speeds : (a) 400 rpm  (b)  600
rpm and (c) 800 rpm.
182 Kasetsart J. (Nat. Sci.) 41(5)
mainly physical since the breakage of these
aggregates to primary agglomerates is possible via
changes to the surface chemistry alone. So, it is
possible that beyond a certain stirrer speed, shear
forces acting on the aggregates have the maximum
effect, and the aggregates simply break into their
minimum size.
Figure 5 showed the TEM micrographs
of the gel and the calcined powders at various
temperatures. It was evident that in the great
majority of particles, the largest size was not over
50 nm and the average size was about 20 nm.
Calcination at higher temperatures increased the
mineralization accompanied by an increase in the
crystallite size (Table 1). Further calcination of the
powder at 600°C for 2 hours showed the marginal
increase in the crystallite size to 20 nm with only
Figure 4 TEM microphotograph of amorphous
titania gel (0.2 M TiO2)  in the form of
aggregates recovered at the end of 1
hour precipitation  for a stirrer speed of
1000 rpm.
Figure 5   TEM microphotograph of titania particles (0. 2 M TiO2, stirrer speed 1000 rpm for 1 hour
precipitation) synthesized by (a) no calcination and calcination at (b) 400oC (c) 600oC and
(d) 800oC.
Table 1 Crystallite size from TEM micrograph
of titania from different calcination
temperatures.
Temperature (°C) Crystallite size (nm)
90 <9
400 10
600 20
800 40
Kasetsart J. (Nat. Sci.) 41(5) 183
anatase phase (XRD results).
X-ray diffraction patterns of the gel dried
at 90°C and calcined at 400-1000°C showed well-
defined diffraction peaks corresponding to the
anatase phase (Figure 6). The amorphous gel began
to crystallize upon heat treatment at 200°C, as
indicated by the emergence of a very broad, low-
intensity XRD peak associated with anatase (101)
diffraction (Wang and Ying, 1999). Calcination at
low temperature could not completely crystallize
the material, giving a mixture of amorphous and
nanocrystalline titania. To convert the sol-gel
derived sample fully to crystalline TiO2, a
calcination treatment of 400 and 600°C were
applied. Anatase was the metastable phase of
titania, typically associated with high surface
areas; it underwent a transformation to the rutile
phase upon thermal treatment typically at 800-
1200°C (Sivakumar et al., 2002).
The crystallite sized (D) were calculated
using Scherrer’s eqution as :
D = kλ
β θ π cos 
360



Where λ is the wavelength of the Cu Kα1 radiation,
θ is the diffraction angle of the (hkl) reflection, k
is a unit cell geometry dependent constant whose
value is typically between 0.85 and 0.99 (for this
study, k = 0.9) and β is the half-width at the half
maximum of a step-scan profile of the (hkl) peak
after correction for Kα1 - Kα2 separation and
instrumental broadening. The calculated sizes are
glown in Table 2.
Figure 6 (c) showed that the gel calcined
at 800°C was a mixture of anatase and rutile
phases, with a rutile fraction of 10.68%, having a
crystallite size larger than the anatase phase. The
rutile fraction in the sample was determined by
measuring the XRD intensities of anatase (101)
and rutile (110) from the equation :
WR = 
1
1 0 8+





.
I
I
A
R
Where WR is the weight fraction of the rutile
present, IA and IR are the X-ray integrated
intensities of anatase and rutile reflection,
respectively (Sivakumar et al., 2002). X-ray
intensities of anatase (101) and rutile (110) are
presented in Figure 7. Calcination at 1000°C
resulted in pure rutile phase with a crystallite
average size of 31 nm.
Figure 6 XRD patterns of nanocyrstalline anatase titania gel calcined at (a) 400oC (b) 600oC (c) 800oC
and (d) 1000oC, The samples (0.2 M TiO2 with stirrer speed 1000 rpm for 1 hour) were
characterized after calcination for 2 hours.
184 Kasetsart J. (Nat. Sci.) 41(5)
CONCLUSION
In this study, we have reported a method
for the synthesis of nanosized anatase by
homogeneous precipitation of aqueous titanyl
sulfate solution at room temperature. The observed
enhancement in the properties such as anatase-
rutile phase stability, crystallite size and
morphology fraction prepared by the present
method indicates the novelty of the synthesis
procedure over the alkoxide-free sol-gel method.
In particular, we have successfully synthesized the
monodisperse titania particles with diameters of
10-32 nm in a reproducible way by using the sol-
gel route. We have shown that both primary
agglomerates and crystals of anatase are
significantly affected by variation in stirrer speed.
The smallest aggregates (6-8 nm) are formed when
the highest stirrer speed is used (1000 rpm).
Nanocrystalline anatase of crystallite size of 6-8
nm formed as low as 90°C, has been prepared by
this method. The gel retains the pure anatase phase
with a 14.2 nm crystallite size even after
calcination at 600°C for 2 hours.
ACKNOWLEDGEMENT
The authors would like to thank the
Faculty of Science, King Mongkut’s University
of Technology Thonburi for financial support of
this research work.
Table 2 Crystalline phase and crystallite size of titania from different calcination temperatures.
Temperature (°C) Phase Crystallite size, D (nm)
D101 D200 D
90 Anatase 6.8 6.9 6.8 – 6.9
400 Anatase 10.7 11.2 10.7 – 11.2
600 Anatase 13.6 14.8 13.6 – 14.8
800 Anatase + Rutile 26.4 28.3 26.4 – 28.3
1000 Rutile 29.6 32.1 29.6 – 32.1
Figure 7 XRD intensities of nanocrystalline titania sample calcined at 800oC (0.2 M TiO2, stirrer speed
1000 rpm for 1 hour).
Kasetsart J. (Nat. Sci.) 41(5) 185
LITERATURE CITED
Allen, N.S., M. Edge, A. Ortega, C. M. Liauw, J.
Stratton and R. B. McIntyre. 2002. Behavior
of nanoparticle (ultrafine) titanium dioxide
pigments and stabilisers on the photooxidative
stability of water based acrylic and isocyanate
based acrylic coatings. Polym. Degrad. Stab.
78: 467-478.
Assmann, S.E., J. Widoniak and G. Maret. 2004.
Synthesis and characterization of porous and
nonporous monodisperse colloidal TiO2
particles. Chem. Mat. 16: 6-11.
Bai, W., K.L. Choy, N.H.J. Stelzer and J.
Schoonman. 1999. Thermophoresis-assisted
vapour phase synthesis of CeO2 and CexY1-
xO2-δ nanoparticles. Solid State Ionics 116:
225-228.
Baiju, K.V., C. P. Sibu, K. Rajesh, P. Krishan Pillai,
P. Mukundan, K.G.G. Warrier and W.
Wunderlich 2005. An aqueous sol-gel route
to synthesize nanosized lanthana-doped titania
having an increased anatase phase stability for
photocatalytic application. Mat. Chem. Phys.
90: 123-127.
Kandavelu, V., H. Kastien and K. R. Thampi. 2004.
Photocatalytic degradation of isothiazolin-3-
ones in water and emulsion paints containing
nanocrystalline TiO2 and ZnO catalysts. Appl.
Cat. B : Environmental. 48: 101-111.
Lee, L. H. and W. C. Chen. 2001. High-refractive-
index thin films prepared from
trialkoxysilane-capped poly (methyl
methacrylate)-titania materials. Chem. Mat.
13: 1137-1142.
Ma, Z., Y. Yue, X. Deng and Z. Gao. 2002.
Nanosized anatase TiO2 as precursor for
preparation of sulfated titania catalysts. J.
Mol. Cat. A : Chemical 178: 97-104.
Sathyamoorthy, S., M. J. Hounslow and G.D.
Moggridge. 2001. Influence of stirrer speed
on the precipitation of anatase particles from
titanyl sulphate solution. J. Cryst. Growth.
223: 225-234.
Sivakumar, S., P. Krishna Pillai, P. Mukundan and
K.G.K. Warrier. 2002. Sol-gel synthesis of
nanosized anatase from titanyl sulfate. Mat.
Lett. 51: 330-335.
Sivakumar, S., C. P. Sibu, P. Mukundan, P.
Krishnapillai and K.G.K. Warrier. 2004.
Nanoporous titania-aulmina mixed oxides –
an alkoxide free sol-gel synthesis. Mat. Lett.
58: 2664–2669.
Wang, C.-C. and J. Y. Ying. 1999. Sol-gel synthesis
and hydrothermal processing of anatase and
rutile titania nanocrystals. Chem. Mat. 11:
3113–3120.
Wei, Y., R. Wu and Y. Shang. 1999. Preparation
of monodispersed spherical TiO2 powder by
forced hydrolysis of Ti(SO4)2 solution. Mat.
Lett. 41: 101-103.
Yamazaki, S., N. Jujinaga and K. Araki. 2001.
Effect of sulfate ions for sol-gel synthesis of
titania photocatalyst. Appl. Catal. A :
General 210: 97-102.
Yasir, V.A., P.N. Mohandas and K.K.M. Yusuff.
2001. Preparation of high surface area TiO2
(anatase) by thermal hydrolysis of titanyl
sulfate solution. Inter. J. Inorg. Mat. 3: 593-
593.


An Alkoxide Free Sol-Gel Synthesis of Nanosized TiO 2

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An Alkoxide Free Sol-Gel Synthesis of Nanosized TiO 2

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