Control of phase and pore structure of titania powders using HCl and NH4OH catalysts

Article abstract of DOI:10.1111/j.1151-2916.2001.tb00613.x

Porous titania powders were prepared by hydrolysis of titanium tetraisopropoxide (TTIP) and were characterized at various calcination temperatures by nitrogen adsorption, X-ray diffraction, and microscopy. The effect of HCl or NH₄OH catalysts a

Full text of DOI:10.1111/j.1151-2916.2001.tb00613.x

J. Am. Ceram. Soc., 84 [1] 92–98 (2001)
journal
Control of Phase and Pore Structure of Titania
Powders Using HCl and NH OH Catalysts
4
†
Ki Chang Song and Sotiris E. Pratsinis*
Institute of Process Engineering, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Zurich, Switzerland
1
0
Porous titania powders were prepared by hydrolysis of tita-
nium tetraisopropoxide (TTIP) and were characterized at
various calcination temperatures by nitrogen adsorption,
X-ray diffraction, and microscopy. The effect of HCl or
et al. also produced monodisperse titania particles of 0.7 ␮m
diameter after 10 min of reaction at neutral pH. They investigated
the effect of water concentration and pH on the particle size and
1
1
dispersion stability. Ding et al. synthesized titania powders from
titanium sec-butoxide (TSB) by controlling the amount of water
added during the hydrolysis reaction. They reported that the grain
size of the powder prepared at low water concentration (H O/
TSB ϭ 1) was smaller than that prepared at high water concen-
NH OH catalysts added during hydrolysis on the crystallinity
4
and porosity of the titania powders was investigated. The HCl
enhanced the phase transformations of the titania powders
from amorphous to anatase as well as anatase to rutile, while
2
NH OH retarded both phase transformations. Titania pow-
tration (H O/TSB ϭ 4). As a result, the phase transformation
4
2
ders calcined at 500°C showed bimodal pore size distributions:
one was intra-aggregated pores with average pore diameters of
temperature of anatase to rutile of the former powder was lower
1
2
than that of the latter powder. Terabe et al. prepared titania
powders by the hydrolysis of TTIP and investigated the effect of
water and hydrochloric acid addition on the molecular structure
and crystallization behavior of the powders. They revealed that the
molecular structure of prepared amorphous precursors resembled
3
–6 nm and the other was interaggregated pores with average
pore diameters of 35–50 nm. The average intra-aggregated
pore diameter was decreased with increasing HCl concentra-
tion, while it was increased with increasing NH OH concen-
4
tration.
that of the anatase phase with increased H O and HCl content.
2
1
3
Bacsa and Gratzel studied the effect of peptization on the phase
transformation of titania powders precipitated from titanium
alkoxide and found that the anatase–rutile phase transformation in
peptized titania was critically dependent on the water/alkoxide
I. Introduction
1
–4
ITANIA (TiO ) powders find diverse applications
and they
14
2
molar ratio and the size of the alkoxide. Vorkapic and Matsoukas
T
are typically made in the liquid or gas phase by the sulfate and
synthesized titania powders from titanium alkoxides and investi-
gated the effect of temperature and alcohols on the size of the
titania particles. They reported that the particle size was not
sensitive to the hydrolysis temperature, but the peptization tem-
perature and that the presence of alcohols resulted in larger
particles.
chloride processes, respectively. In the liquid phase, these powders
are usually prepared from TiO(SO ), Ti(SO ) , or TiCl . However,
4
4 2
4
it is known that the counteranions of the starting titanium salts
5
remain in the product and deteriorate the purity of the powders.
To avoid counteranion contamination, titanium alkoxides can be
1
used as the starting material for the titania powders.
Recently, preparation of porous powders with controlled pore
size and high surface area for uses such as filters, adsorbates,
Sol–gel processing can be used to make nanoparticles by a
chemical reaction in solution starting with metal alkoxides as a
15
catalysts, and catalyst supports has attracted much interest.
6
precursor. An advantage of the sol–gel processing is that it is
Sol–gel processing is a very effective method to control pore sizes
possible to obtain large amounts of powders with a high level of
16,17
18
of inorganic oxide powders.
Lu et al. have prepared silica
6
chemical purity. In sol–gel synthesis of titania, two simultaneous
particles with hexagonal, lamellar, or cubic mesostructured poros-
ity by aerosol processing. Although many research groups have
explored the properties of titania powders prepared by controlling
reaction variables during hydrolysis, to the best of our knowledge,
the influence of hydrolysis catalysts on the pore properties of
titania powders has received little attention. This paper describes
reactions—hydrolysis and polycondensation—take place when
7
titanium tetraisopropoxide (TTIP) reacts with water. The poly-
condensation reaction induces polymerization forming higher
8
molecular weight products (nuclei and subsequently particles).
These two reactions are sensitive to the reaction variables, namely,
water concentration, type and amount of catalysts, type and
amount of solvents, reaction temperature, and mixing conditions.
Thus, the properties of titania powders depend strongly on the
reaction variables.
the effect of hydrolysis catalysts (HCl and NH OH) on the
4
characteristics and especially the porosity of the titania powders
made by hydrolysis of TTIP.
9
Barringer and Bowen synthesized monodisperse titania parti-
cles by controlled hydrolysis of alcoholic solutions of titanium
ethoxide and showed that the average particle size of the titania
powders decreased for increased water concentration, while it
increased slightly with increasing alkoxide concentration. Ikemoto
II. Experimental Procedure
Titanium tetraisopropoxide (TTIP; Ti(OC H ) , 97%, Aldrich)
3
7 4
was used as a precursor. To moderate its high reactivity, the TTIP
was first diluted in anhydrous ethyl alcohol (molar ratio of ethyl
alcohol/TTIP of 5) before reaction with water. In this study, HCl
and NH OH were used as hydrolysis catalysts. The molar ratio of
4
catalyst/TTIP was varied in the range of 0 to 0.5. Different
amounts of the hydrolysis catalysts were mixed with a constant
amount of water (molar ratio of water/TTIP of 20). The hydrolysis
was conducted at room temperature by adding distilled water
containing various catalyst concentrations to the mixed solution
(ethyl alcohol and TTIP) with a vigorous stirring for 1 h. On
adding water to the mixed solution, a slurry containing white
titanium hydroxide precipitate was formed. Clean liquid was
R. H. French—contributing editor
Manuscript No. 188947. Received November 16, 1999; approved July 31, 2000.
This research was sponsored by the SNF (Swiss National Science Foundation) and
the KOSEF (Korea Science and Engineering Foundation).
*
Member, American Ceramic Society.
†
On leave from Konyang University, South Korea.
9
2

January 2001
Control of Phase and Pore Structure of Titania Powders
93
Table I. Effect of Catalyst Amounts on the Structure of Crystalline Phase and Pore of Titania Powders Dried at 150°C
Catalyst/TTIP
molar ratio
BET surface area
Pore volume
2
3
Sample
Type of catalyst
pH
Crystal structure
(m /g)
(cm /g)
HN0.0
H0.05
H0.10
H0.15
H0.20
H0.30
N0.05
N0.10
N0.30
N0.50
†
No catalyst
HCl
0.00
0.05
0.10
0.15
0.20
0.30
0.05
0.10
0.30
0.50
4.0
3.3
2.7
2.5
2.2
1.8
6.8
7.2
9.9
11.7
Amorphous
Anatase
491.8
384.5
296.2
221.8
174.8
143.2
560.1
551.0
457.6
344.9
0.40
0.35
0.27
0.16
0.11
0.08
0.41
0.40
0.36
0.31
HCl
Anatase
HCl
Anatase
HCl
Anatase
HCl
Anatase
NH OH
Amorphous
Amorphous
Amorphous
Amorphous
4
NH OH
4
NH OH
4
NH OH
4
The pH of the solution just after hydrolysis reaction has finished.
removed from the slurry by filtering through a filter paper (Grade
The specific surface area of the powders was performed on a
BET surface area analyzer (ASAP2010, Micromeritics) using
nitrogen as adsorbent at liquid-nitrogen temperature. All powders
were degassed at 150°C before measurement. The specific surface
area was determined by the multipoint BET method using the
5
␮
0 hardened, Whatman) with openings less than or equal to 2–3
m in diameter. The filtered powder was dried at 150°C for 24 h,
1
9
followed by calcination at different temperatures ranging from
00° to 800°C for 1 h at a heating rate of 10°C/min.
3
Fig. 1. XRD patterns of the (a) HN0.0, (b) H0.30, and (c) N0.50 powders (Table I) as a function of calcination temperature.

9
4
Journal of the American Ceramic Society—Song and Pratsinis
Vol. 84, No. 1
Fig. 2. Average grain size of anatase in the HN0.0, H0.30, and N0.50
powders (Table I) as a function of calcination temperature.
Fig. 4. Nitrogen adsorption isotherms of the powders dried at 150°C and
Fig. 3. Calculated rutile weight fractions of the titania powders prepared
prepared with (a) HCl and (b) NH OH catalysts.
4
with HCl or NH OH catalysts as a function of catalyst concentration at
4
various calcination temperatures.
where ␭ is the wavelength and Bhkl is the full width at half-
maximum. The value of K varies from 0.9 to 1.4 and the value used
in this study was 1.4. The powder morphology was obtained by
scanning electron microscopy (JSM-6400, Jeol).
adsorption data in the relative pressure (P/P ) range of 0.05–0.25.
0
The desorption isotherm was used to determine the pore size
distribution using the Barrett, Joyner, and Halenda (BJH) method
2
0
with cylindrical pore size. An X-ray diffractometer (D5000,
Siemens) was used for identification of crystalline species in the
calcined powders. Ni-filtered CuK␣ radiation was used as the
III. Results and Discussion
(1) Evolution of Phase Composition
o
X-ray source for diffraction angles 2␪ between 20° to 60 . The
Table I summarizes the XRD analysis of titania powders dried
weight fractions of rutile present in the tiatnia powders were
at 150°C and made with different catalyst concentrations. The type
of catalysts used during hydrolysis influences the crystal structure
of the dried powders: powders made without catalyst (HN0.0) and
with NH OH catalyst are amorphous Ti(OH) as confirmed by
2
1
calculated by
X ϭ 1/͑1 ϩ 0.8I /I ͒
A
R
4
4
2
3
FT-IR analysis. However, when HCl is used as the hydrolysis
catalyst, the dried powders are anatase. This result is in good
where X is the weight fraction of rutile in the powders, while IA
and IR are the X-ray integrated intensities of the strongest peaks
corresponding to anatase (2␪ ϭ 25.3° for the (101) reflection of
anatase) and rutile (2␪ ϭ 27.5° for the the (110) reflection of
rutile), respectively. The crystallite sizes (Dhkl) were calculated
1
2
agreement with Terabe et al., who also found anatase in the
powders prepared with HCl catalyst. From the measurement of
XRD and Raman spectroscopy, they concluded that with high HCl
concentration, the molecular structure of the titania precursor
resembled that of the crystalline anatase because the anatase phase
is more stable than the amorphous phase in low pH solutions. This
2
2
using Scherrer’s equation:
Dhkl ϭ K␭/͑Bhkl cos ␪͒

January 2001
Control of Phase and Pore Structure of Titania Powders
95
completed until 800°C. As a result, the transformation temperature
of anatase to rutile is 100°C higher for the NH OH-catalyzed than
4
the noncatalyzed powder. Clearly, the presence of HCl enhances
the amorphous-to-anatase and anatase-to-rutile phase transforma-
tions, while the NH OH catalyst inhibits them.
4
Figure 2 shows the evolution of the average grain size of
anatase in the HN0.0, H0.30, and N0.50 powders. It is evident that
the H0.30 powder shows the smallest average grain size, but N0.50
powder exhibits the largest average grain size at all calcination
2
5
temperatures except 600°C. According to Ding et al., with
decreasing grain size, the nucleation sites increased because of the
increase of the surface area, and consequently the transformation
rate was increased. Based on this result, it is possible to explain
that the H0.30 powder showed the lowest phase transformation
temperature of amorphous to anatase, while the N0.50 powder
showed the highest transformation temperature. It is interesting to
note that the average grain size in the HN0.0 powder is larger than
that in the N0.50 powder at 600°C. This was due to the rapid
growth of anatase crystallite in the HN0.0 powder during phase
2
2
transformation of anatase to rutile at 600°C. Kumar et al. found
that sol–gel-derived nanostructured titanias showed rapid growth
of anatase crystals during the anatase-to-rutile phase transforma-
tion.
Figure 3 shows the calculated rutile weight fraction of the
powders calcined at various temperatures as a function of the
reactant ratios of catalyst/TTIP. In this figure, it is seen that the
rutile weight fraction increases with increasing calcination tem-
perature. Also the rutile weight fraction increases with increasing
HCl concentration, but it decreases with increasing NH OH
4
concentration. This reconfirms that the HCl catalyst enhances the
transformation of anatase to rutile, while NH OH retards it. This is
4
2
6
in good agreement with Sasamoto et al., who found that the
anatase–rutile transformation was accelerated under acidic condi-
tions.
(2) Texture Property
Table I also summarizes the effect of catalyst concentration on
the specific surface area and pore volume of the powders dried at
50°C. Among all powders prepared under HCl and NH OH
1
4
catalysts, the N0.05 powder shows the largest surface area and
pore volume. The isoelectric point (IEP), the pH at which no net
charge exists in the particle/liquid interface region, is between 4.5
2
7
and 6.7 for titania powders. It is thought that because the pH of
the solution at the end of the hydrolysis reaction is 6.8 (close to the
IEP) for the N0.05 powder, the particles in the powder are highly
aggregated, causing nonuniform particle packing, and resulting in
the powder with the highest specific surface area and pore volume.
It is also shown that for all powders prepared under HCl and
Fig. 5. Pore size distributions of the powders dried at 150°C and prepared
NH OH catalysts, the specific surface area and pore volume
with (a) HCl and (b) NH OH catalysts.
4
4
decrease with increasing catalyst concentration.
To compare the pore structure of the powders dried at 150°C,
their nitrogen adsorption isotherms are presented in Figs. 4(a,b) for
2
4
is also in agreement with So et al., who investigated by XRD the
initial phase of titania powders peptized with HNO after being
powders hydrolyzed at various HCl and NH OH concentrations,
3
4
hydrolyzed at room temperature. They found that the powders
respectively. The isotherms corresponding to the H0.05, H0.10,
and H0.15 powders in Table I are of type IV (BDDT classifica-
were anatase or rutile at low pH as determined by the HNO /TTIP
3
2
8,29
molar ratio during peptization.
tion)
and have two hysteresis loops, indicating bimodal pore
The XRD patterns of the HN0.0, H0.30, and N0.50 powders in
Table I, respectively, are shown in Figs. 1(a–c), as a function of
calcination temperature. Figure 1(a) shows that the HN0.0 powder
is amorphous at 150°C, while anatase appears at 300°C. A phase
transformation from anatase to rutile begins at 600°C and is not
completed until 800°C. For the H0.30 powder, anatase peaks are
already present at 150°C in Fig. 1(b). The phase transformation
from anatase to rutile occurs at 400°C and is completed at 700°C.
It is apparent that the transformation temperature of anatase to
rutile is 200°C lower for the H0.30 than for the HN0.0 powder. On
the other hand, the N0.50 powder remains amorphous until 300°C
and is crystallized to anatase at 400°C in Fig. 1(c), which is about
size distributions. The shapes of the two hysteresis loops are
different from each other. At low relative pressures between 0.4 to
0.7, the hysteresis loops are of type H2 which can be observed in
2
8
the pores with narrow necks and wider bodies (ink-bottle pores).
However, at high relative pressures between 0.8 to 1.0, the shape
of the hysteresis loops is of type H3 associated with aggregates of
2
8
platelike particles giving rise to slitlike pores.
From the information on the shape of the isotherms, it can be
inferred that the powders prepared at low HCl concentrations
(HCl/TTIP ϭ 0.15) are mesoporous and the pores are of two types
(i.e., one is ink-bottle-shaped pores, and the other is a narrow
slitlike shape). However, the H0.20 and H0.30 powders prepared
2
8,29
1
00°C higher than the crystallization temperature of the HN0.0
with excess HCl exhibit a type I isotherm with no hysteresis.
This type of isotherm is associated with microporous powders with
powder. It is also seen in the N0.50 powder that the phase
transformation from anatase to rutile begins at 700°C and is not
2
8
uniform distribution of pores 2 nm or less in diameter. The

9
6
Journal of the American Ceramic Society—Song and Pratsinis
Vol. 84, No. 1
Fig. 6. Scanning electron micrographs of the (a) H0.05, (b) H0.30, (c) N0.05, and (d) N0.50 powders in Table I dried at 150°C.
absence of a hysteresis loop indicates the absence of pores larger
than 2 nm in diameter (mesopores). Figure 4(b) shows isotherms
made with excess HCl (HCl/TTIP Ն 0.20) can be explained by the
influence of the peptization step on the dispersion of primary
2
8
31
of the powders hydrolyzed at various NH OH/TTIP ratios. All
particles in the sol. For the powder prepared under excess HCl
concentration, protons (H ) from HCl are adsorbed on the surface
4
ϩ
powders show a type IV isotherm with a hysteresis loop, indicating
2
8,29
the presence of mesopores.
is of type H3, associated with narrow slitlike pores.
The shape of the hysteresis loops
of the primary particles. Assuming that all particles have the same
surface charge, the particles are repelled and a stable sol results in
a minimum degree of aggregation. On the other hand, the sol
consisting of powders prepared at low HCl concentrations contains
nonuniform, highly aggregated clusters of primary particles. From
this result, it can be inferred that the powders prepared at low
HCl/TTIP ratios contain nonuniform, highly aggregated clusters of
primary particles, while the powders prepared with excess HCl
concentration do not have aggregates, and the primary particles
within the powders are packed more uniformly.
2
8,29
Figures 5(a,b) show the pore size distributions of the titania
powders dried at 150°C made in the presence of HCl and NH OH,
4
respectively. The powders made at low HCl concentrations (HCl/
TTIP ϭ 0.15) show bimodal pore-size distributions consisting of
fine intra-aggregated pores with a constant maximum pore diam-
eter of 3.3 nm and larger interaggregated pores with pore diameters
3
0
of 20–150 nm (Fig. 5(a)). According to Kumar et al., a bimodal
pore size distribution is due to the hard aggregates in the powders.
They also stated that there are two types of pores present in the
bimodal pore-size distributions. One is fine intra-aggregated pores
The dried powders prepared using NH OH catalyst exhibit
4
bimodal pore size distributions, consisting of fine intra-aggregated
pores and larger interaggregated pores (Fig. 5(b)). All powders
except N0.50 show constant average diameters of 2.3 and 70 nm
for the intra-aggregated and interaggregated pores, respectively,
(
represented by the hysteresis loop in the lower P/P range) and
0
the other is larger interaggregated pores (hysteresis loop in the
higher P/P range). It is interesting to note that even though the
0
amount of HCl catalyst increases, the average pore size of the
intra-aggregated pores shows a constant diameter of 3.3 nm.
However, the H0.30 powder prepared with excess HCl concentra-
tion shows only intra-aggregated pores having an average diameter
in the microporous region.
regardless of the NH OH concentration. The pore volumes of the
average diameters of the intra-aggregated pores decrease with
4
increasing the NH OH concentration. It is also observed that the
4
N0.50 powder has shifted its average pore diameter of intra-
aggregated pores to the microporous region (pore diameter less
than 2 nm).
The difference in the pore size distributions between powders
prepared at low HCl/TTIP ratios (less than 0.15) and powders
Figures 6(a–d) show scanning electron micrographs of the

January 2001
Control of Phase and Pore Structure of Titania Powders
97
Table II. Effect of Catalyst Concentration on the Specific
Surface Area and XRD Anatase Crystallite Size of Titania
Powders Calcined at 500°C
BET surface area
Pore volume
Crystallite size
(nm)
2
3
Sample
(m /g)
(cm /g)
H0.05
H0.10
H0.15
H0.20
N0.05
N0.10
N0.30
N0.50
HN0.0
97.4
93.0
68.9
44.4
58.8
64.1
59.5
55.5
58.7
0.21
0.19
0.12
0.07
0.10
0.12
0.12
0.14
0.11
9.89
9.89
11.23
11.30
12.36
13.18
14.64
19.77
13.18
prepared with various catalyst/TTIP molar ratios. The size of
anatase crystals increases with increasing catalyst concentration.
On the other hand, the surface area and the pore volume decrease
with increasing HCl concentration. For the powders prepared with
NH OH catalyst, the pore volume increases with increasing
4
NH OH content, while the specific surface area is nearly constant
4
2
(
56–64 m /g). The pore structure of the titania powders prepared
by the sol–gel synthesis is determined by the size and shape of the
3
1
primary particles. In Fig. 7(b), the fact that the average pore
diameter of the intra-aggregated pores increases with increasing
NH OH concentration can be explained by the growth in crystal-
4
lite size with increasing NH OH concentration because the pore
4
size is proportional to the primary particle size. However, it is
interesting to note that the average pore size of the intra-
aggregated pores in Fig. 7(a) decreases with increasing HCl
concentration despite the growth of crystallite size. This may be
explained by the fact that the primary particles in the powders
prepared under HCl catalyst are more closely packed with increas-
ing HCl concentration; thus the average pore diameter becomes
smaller.
IV. Conclusions
The phase and texture of titania powders prepared by hydrolysis
of titanium tetraisopropoxide (TTIP) are strongly influenced by the
presence of catalysts. When HCl was used as a catalyst, both the
phase transformation of amorphous Ti(OH) to anatase TiO and
4
2
that of anatase to rutile were accelerated. In contrast, when
NH OH was added as a catalyst, both transformations were
retarded.
4
Fig. 7. Pore size distributions of the titania powders calcined at 500°C for
1
h prepared with (a) HCl and (b) NH OH catalysts.
4
The porosity of titania powders can be controlled through the
catalyst concentration. Titania powder prepared near the IEP and at
low NH OH concentration (NH OH/TTIP ϭ 0.05) showed the
4
4
H0.05, H0.30, N0.05, and N0.50 powders in Table I dried at
50°C, respectively. The H0.30 powder shows a dense structure
highest specific surface area, pore volume, and high degree of
aggregation. Titania powders calcined at 500°C showed bimodal
pore size distributions: one was intra-aggregated pores with an
average diameter of 3–6 nm and the other was interaggregated
pores with an average diameter of 35–50 nm. The average pore
diameter of the intra-aggregated pores decreased with increasing
1
without aggregates, resulting in the absence of the large interag-
gregated pores in Fig. 5(a). The N0.05 powder prepared at near
IEP shows highly aggregated, nonuniform structure, resulting in
the highest surface area and pore volume in Table I. On the other
hand, the N0.50 powder shows a microstructure of uniform,
compact secondary particles with smaller size.
HCl concentration and increased with increasing NH OH concen-
4
tration.
Figures 7(a,b) represent the pore size distributions of the titania
powders calcined at 500°C for 1 h after being prepared with HCl
and NH OH catalysts, respectively, and dried at 150°C for 24 h.
The average intra-aggregated pore diameter ranging from 3.5 to
4
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