Journal of Colloid and Interface Science 316 (2007) 85–91
www.elsevier.com/locate/jcis
Synthesis and photocatalytic activity of mesoporous TiO2 with
the surface area, crystallite size, and pore size
Dong Suk Kim, Shin Jung Han, Seung-Yeop Kwak ∗
Department of Materials Science and Engineering, Seoul National University, San 56-1, Sillim-dong, Gwanak-gu, Seoul 151-744, South Korea
Received 8 March 2007; accepted 20 July 2007
Available online 29 August 2007
Abstract
Mesoporous TiO2 materials with various pore-size distributions were synthesized by using diblock copolymers via a sol–gel process in aqueous
solution. The properties of these materials were characterized by FE-SEM, HR-TEM, XRD, DRS, BET, and BJH analysis. All particles have
spherical morphology with a diameter range of 1–3 µm. The mesoporous TiO2 materials calcined at 400 ◦ C were found to have different specific
surface areas—186, 210, and 192 m2 g−1 —and average pore sizes depending on the type of diblock copolymer—5.1, 6.1, and 6.4 nm—and
their crystallite sizes were found to be 8.1, 8.3, and 8.8 nm. The photocatalytic activity of each sample was investigated by measuring the
photodecomposition of methylene blue (MB), and the small crystallite size, large surface area, and small pore size were found to exhibit better
photocatalytic activities. In addition, the photocatalytic activities of all the mesoporous TiO2 materials were found to be better than that of
commercial TiO2 .
© 2007 Elsevier Inc. All rights reserved.
Keywords: Mesoporous TiO2 ; Photocatalytic activity; Calcination temperature; Pore size; Porous structure
1. Introduction
Titanium dioxide (TiO2 ) is a very useful semiconducting
transition metal oxide material and exhibits unique characteristics such as low cost, easy handling, nontoxicity, and resistance to photochemical and chemical erosion. These advantages make TiO2 a material in solar cell, chemical sensor, hydrogen gas evolution, and environmental purification applications [1–3]. The photocatalytic activity of TiO2 is one of its
most distinctive features, and is largely determined by properties such as the crystalline phase, crystallite size, specific surface area, and porous structure. Some studies of the effects of
these properties on the photocatalytic activity of TiO2 photocatalysts have been conducted [4]. High photocatalytic activity
is associated with crystallite sizes because small crystallite size
can lead to quantum size effects in semiconductors. In addition,
TiO2 with a large surface area and a porous structure is expected
to exhibit outstanding photodegradation properties [5–7], be* Corresponding author.
E-mail address: [email protected] (S.-Y. Kwak).
0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcis.2007.07.037
cause TiO2 with a large surface area has many active sites so
that substances can be adsorbed in large quantities onto the
TiO2 surface, and porosity facilitates pollutant access, adsorption, and decomposition [8–11].
Peng et al. recently prepared mesoporous TiO2 nanosized
powders with high specific surface areas and anatase walls by
using cetyltrimethylammonium bromide (CTAB) as a surfactant-directing and pore-forming agent. In their study, it was
found that the high photocatalytic activity of the resulting material was related to its large surface area and small anatase
crystallites [12]. Zhan et al. fabricated long TiO2 hollow fibers
with mesoporous walls and large surface areas by combining
a sol–gel process with a two-capillary spinneret electrospinning technique using a triblock copolymer (Pluronic, P123) as
a pore-directing agent, and compared the photocatalytic activities of the fibers with those of Degussa P25 and mesoporous
TiO2 powders [13]. Jimmy et al. synthesized three-dimensional
and thermally stable mesoporous TiO2 without the use of any
surfactants via treatment with high intensity ultrasound irradiation, and its high photocatalytic activity was attributed to its
high surface area and the three-dimensional connectivity of its
mesoporous wormhole frameworks [14]. However, few precise
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D.S. Kim et al. / Journal of Colloid and Interface Science 316 (2007) 85–91
investigations of the effect of pore-size distribution and porous
structure of mesoporous TiO2 on the photocatalytic activity
have previously been carried out.
In this paper, we report the preparation and characterization
of mesoporous TiO2 materials with various pore sizes synthesized by using surfactants with ethylene oxide groups of various
lengths, and focus on the effects of the calcination temperature,
porous structure, and pore size on their photocatalytic activity. The mesoporous TiO2 materials are synthesized in aqueous
solution with a sol–gel process from various diblock copolymers as templates and titanium tetraisopropoxide mixed with
2,4-pentanedione, which is a retarding agent that limits the rate
of hydrolysis and condensation. The resulting materials have a
spherical morphology with various pore sizes, and exhibit high
thermal stabilities up to 600 ◦ C maintaining mesoporous structures. They show better photocatalytic activities compared to
commercial TiO2 , Ishihara ST-01.
radiation) at room temperature. An accelerating voltage of
50 kV and an emission current of 100 mA were used. The
crystallite sizes were calculated with the Scherrer equation
(Φ = Kλ/β cos θ ), where Φ is the crystallite size, K is usually taken as 0.89, λ is 0.154 nm, which is the wavelength of
the X-ray radiation, β is the full width at half-maximum intensity (FWHM), and θ is the diffraction angle of the (101)
peak for anatase (2θ = 25.3◦ ). Nitrogen adsorption–desorption
isotherms were collected at 77 K on a Micrometrics ASAP
2000 apparatus after the samples were degassed at 100 ◦ C for
10 h. The specific surface areas were estimated by using the
Brunauer–Emmett–Teller (BET) method, and the pore-size distributions were determined with the Barrett–Joyner–Halenda
(BJH) method by using the nitrogen desorption branches of
the isotherms. Ultraviolet/visible diffuse reflectance spectra
(DRS) of the samples were obtained by using a UV–Vis spectrophotometer (Varian Cary5000) with BaSO4 as the reference.
2. Experimental
2.3. Photocatalytic activity
2.1. Preparation of mesoporous TiO2 samples
Titanium tetraisopropoxide (Ti(Oi Pr)4 , 97%, Aldrich) and
the diblock copolymer surfactants (Lutensol AT 18, Lutensol
AT 25, Lutensol AT 50, BASF) were used without further purification. The diblock copolymers (RO(CH2 CH2 O)x H) consist
of 16 or 18 carbon units in hydrophobic alkyl groups, where
x is the number of ethylene oxide units, for example, x = 18
for Lutensol AT 18. To optimize the synthetic conditions, many
parameters including the surfactant concentration, sulfuric acid
concentration, Ti precursor to surfactant ratio, and reaction temperature were controlled. First, 3 g of each surfactant (Lutensol
AT 18, 2.9 mmol; Lutensol AT 25, 2.2 mmol; Lutensol AT
50, 1.2 mmol) was dissolved in 100 ml of distilled water, and
then 1.5 g (15.3 mmol) of sulfuric acid was added. Next, titanium tetraisopropoxide (AT-18, 28.6 mmol; AT-25, 27.7 mmol;
AT-50, 27.5 mmol) mixed with 2,4-pentanedione (AT-18, 28.6
mmol, AT-25, 27.7 mmol, AT-50, 27.5 mmol) was dropped into
each surfactant solution under magnetic stirring. The resulting solutions were kept at 50 ◦ C. Light yellow powders formed
within several minutes, and then hydrothermal treatment was
carried out at 90 ◦ C for about 10 h. The solids in the solutions
were filtered out, dried in air, and ground to fine powders. The
resulting materials are denoted MT-18, MT-25, and MT-50, according to the number of ethylene oxide units in the surfactants.
Finally, the materials were calcined at 400, 500, and 600 ◦ C at
a rate of 1 ◦ C min−1 in air.
2.2. Characterization
The morphologies and pore structures of the mesoporous
TiO2 particles were determined from using field emission-gun
scanning electron microscope (FE-SEM, JEOL JSM-6330F)
and transmission electron microscope (TEM, JEOL JEM2000EXII). The crystal compositions and crystallite sizes of
each sample were determined by using wide-angle X-ray diffraction (XRD, MAC/Sci. MXP 18XHF-22SRA with CuKα
The photocatalytic activities of the mesoporous TiO2 samples were evaluated by measuring the decomposition rate of
methylene blue (MB) at room temperature, and compared with
the activity of commercial TiO2 , Ishihara ST-01. A quartz cell
containing both aqueous MB solution and TiO2 powder was
placed in the center of the reactor, with four UV lamps located at each corner inside the reactor. Each quartz cell contained 25 ml of the aqueous MB solution with 50 ppm dye and
25 mg of the catalyst. In order to provide sufficient UV radiation, four UV lamps providing UV-A (320 < λ < 400 nm)
were used as light sources. In each experiment, the reaction
solutions were magnetically stirred in the dark for 30 min until adsorption/desorption equilibrium was reached. The solution
was then irradiated under UV light with continuous magnetic
stirring. A fixed quantity of each MB solution was taken at regular intervals during the illumination period and filtered through
a syringe filter to analyze the amount of MB remaining in the
solution. The UV–Vis absorption spectra of the taken solutions
were obtained using a Shimadzu 2101PC UV–Vis scanning
spectrophotometer in the range 500–700 nm, because λmax of
methylene blue is approx 664 nm.
3. Results and discussion
3.1. FE-SEM and TEM analyses
FE-SEM images of the mesoporous TiO2 materials (MT-18,
MT-25, and MT-50) calcined at 400 ◦ C are shown in Figs. 1a,
1b, and 1c, respectively. The particles of these materials all have
spherical morphologies, and increase in size with increases in
the number of ethylene oxide units. The magnified FE-SEM
images of the materials are shown in Figs. 1d, 1e, and 1f. It
can be observed that discernible pores are present at the surface of mesoporous TiO2 material among the nanosized TiO2
particles. In order to investigate the properties of the pores, HRTEM images were obtained. The HR-TEM images of MT-18,
D.S. Kim et al. / Journal of Colloid and Interface Science 316 (2007) 85–91
87
Fig. 2. HR-TEM images of (a) MT-18, (b) MT-25, and (c) MT-50 calcined at
400 ◦ C and (d) magnified image of MT-18 (the inset is the electron diffraction
pattern of MT-18).
Fig. 1. FE-SEM images of as-synthesized (a) MT-18, (b) MT-25, and (c) MT-50,
and magnified images of mesoporous materials (d) MT-18, (e) MT-25, and
(f) MT-50 calcined at 400 ◦ C.
MT-25, and MT-50 calcined at 400 ◦ C are shown in Figs. 2a, 2b,
and 2c, respectively. The magnified HR-TEM image and electron diffraction pattern of the material are shown Fig. 2d. The
pores can be seen as black spots with nonordered wormholelike structures, and the nanosized TiO2 particles appear white.
This result is consistent with the FE-SEM images. From FESEM and TEM results, it is found that the pore walls consist of
aggregated TiO2 nanoparticles.
3.2. XRD analysis
Figs. 3a, 3b, and 3c show XRD patterns of as-synthesized
MT-18, MT-25, and MT-50, and of MT-18, MT-25, and MT-50
calcined at various temperatures in the range of 400–600 ◦ C.
Relatively broad diffraction peaks due to anatase structures are
present for all these materials, even for the as-synthesized materials. As the calcination temperature increases, the diffraction
peak (101) due to the anatase structure becomes sharper with
increases in all the samples in the crystallite size [15]. Table 1
represents crystallite sizes of as-synthesized mesoporous TiO2
and mesoporous TiO2 calcined at various temperatures. It was
estimated with the Scherrer equation, and the crystallite sizes
of the as-synthesized MT-18, MT-25, and MT-50 are about 6.9,
7.1, and 7.5 nm, respectively; i.e., the crystallite sizes gradually
increase with increases in the calcination temperature (Table 1).
Hydrothermal treatment at 90 ◦ C for 10 h is used to ensure the
formation of an anatase structure before calcination [16,17].
The phase transformation from anatase to rutile does not occur in TiO2 materials during calcination at high temperatures,
so only the anatase phase is present at 600 ◦ C, without any other
crystalline by-products. In addition, thermodynamic phase stability is generally dependent on particle size, and the anatase
structure is more stable thermodynamically than the rutile structure at particle sizes below approx 14 nm [18], which is similar
to the crystal sizes of MT-18, MT-25, and MT-50 calcined at
600 ◦ C, as shown in Table 1. Thus it is possible that the three
mesoporous TiO2 materials are thermally stable up to 600 ◦ C
because of these effects [19].
3.3. N2 adsorption–desorption analysis
N2 adsorption–desorption isotherms of all the samples calcined at various temperatures are shown in Fig. 4. All the
isotherms of the samples are of classical type IV with H2 hysteresis between the adsorption and the desorption curves, indicating the existence of ink-bottle-type pore structure with a narrow entrance and a large cavity [20,21]. For MT-18, MT-25, and
MT-50 calcined at 400 ◦ C, the surface areas determined with
the BET method are similar: 186, 210, and 192 m2 g−1 , respectively. When the calcination temperature is 600 ◦ C, the specific
surface areas of these materials decrease to approx 90 m2 g−1
due to the crystallization of TiO2 , subsequent crystal growth,
and the collapse of the mesoporous structure [12]. However, a
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D.S. Kim et al. / Journal of Colloid and Interface Science 316 (2007) 85–91
Fig. 4. N2 adsorption–desorption isotherms of (a) MT-18, (b) MT-25, and
(c) MT-50 calcined at various temperatures.
Fig. 3. XRD patterns of (a) MT-18, (b) MT-25, and (c) MT-50 as-synthesized
and calcined at various temperatures.
large proportion of the mesopores is maintained because all the
samples have isotherms with well-defined hysteresis loops up to
600 ◦ C, which indicates their thermal stability [6]. The thermal
stability of mesoporous TiO2 can be achieved with hydrothermal treatment [16,17,22], which results in a higher degree of
polymerization of the inorganic precursors [23] and the crys-
tallization of the mesoporous wall into anatase so that there is
no phase transformation during the calcination process. As the
calcination temperature is increased, the hysteresis loop of a
given sample shifts to the high pressure (P /P0 ) range and its
slope becomes steeper. It is well known that the average pore
diameter increases with increases in the TiO2 crystallite size, as
confirmed by the XRD analysis, and the pore-size distribution
becomes narrower with increases in the calcination temperature, indicating that the pores formed from the assembly of
TiO2 nanoparticles have similar sizes [24].
Fig. 5 shows the pore-size distributions of MT-18, MT-25,
and MT-50 calcined at 400 ◦ C, as estimated with the BJH
method from the desorption branches. As the number of ethylene oxide units in the surfactants is increased, the average
D.S. Kim et al. / Journal of Colloid and Interface Science 316 (2007) 85–91
89
Table 1
Crystallite sizes, surface areas, and pore sizes of the mesoporous TiO2 materials (MT-18, MT-25, and MT-50) calcined at various temperatures
Sample
MT-18
MT-25
MT-50
Surface area (m2 g−1 )
Crystallite size (nm)
Pore size (nm)
400 ◦ C
500 ◦ C
600 ◦ C
400 ◦ C
500 ◦ C
600 ◦ C
400 ◦ C
500 ◦ C
8.1
8.3
8.8
10.0
11.1
16.5
15.8
16.5
17.6
186
210
192
135
145
177
95
99
96
5.1
6.1
6.4
6.0
7.1
9.7
600 ◦ C
9.2
9.7
11.7
Fig. 5. Pore-size distributions of MT-18, MT-25, and MT-50 calcined at 400 ◦ C.
pore sizes of the mesoporous TiO2 materials increase: MT-18
(5.1 nm), MT-25 (6.1 nm), and MT-50 (6.4 nm). The change of
pore size with increase of the ethylene oxide units may be considered as the influence of the hydrophilic ethylene oxide units
of the copolymer which is dependent on the temperature [25].
At the reaction temperature of 50 ◦ C, the surfactant can form a
micelle in aqueous solution, and the ethylene oxide units are
expected to interact more strongly with the titanium species
and thus be more closely associated with the TiO2 nanoparticles than the hydrophobic alkyl chain. However, at the higher
temperature of 90 ◦ C, the ethylene oxide units become more
hydrophobic, resulting in increased hydrophobic domain volumes, and increased pore sizes. As increases in the lengths of
the PEO chains in the surfactants result in increases during the
hydrothermal treatment in the size of the hydrophobic portions,
the pore sizes of the mesoporous TiO2 vary. The results for the
surface areas and pore sizes of the samples calcined at various
temperatures are shown in Table 1.
Fig. 6. UV–Vis diffuse reflectance spectra of (a) MT-18 calcined at various temperatures, and (b) MT-18, MT-25, and MT-50 calcined at 400 ◦ C.
quantum size effects of semiconductors. In the quantum size
effect, the band-gap energy decreases as the crystallite size increases [28].
3.5. Photocatalytic activities of the mesoporous TiO2 samples
3.4. DRS analysis
To investigate the absorption band shifts of TiO2 , DRS was
analyzed. Fig. 6 shows DRS of MT-18 materials calcined at various temperatures and mesoporous TiO2 materials calcined at
400 ◦ C, respectively. The Kubelka-Munk formula was used for
the analysis of diffuse reflectance spectra obtained from absorbing samples [26]. The absorption band shifts toward the shorter
wavelength region with decreases in the crystallite sizes of the
samples, which indicates an increase in the band-gap energy [4,
27]. As found in the XRD results, the crystallite sizes increase
as the calcinations temperature increases. The absorption band
shifts toward the longer wavelength region with the increase in
the crystallite sizes of samples, which indicates a decrease in
the band-gap energy. This phenomenon could be caused by the
The photocatalytic activity of TiO2 is evaluated by measuring the amount of methylene blue remaining in solutions under
UV illumination at regular intervals. In each experiment, prior
to UV irradiation, the reaction solution was magnetically stirred
in the dark for 30 min until adsorption/desorption equilibrium
was reached. The MB solutions containing the mesoporous
TiO2 samples were almost fully degraded and became transparent within 45 or 60 min under UV light as shown in Fig. 7.
In plots of ln(C/C0 ) versus UV irradiation time, straight lines
were found for all materials, indicating that the degradation
of methylene blue is a first-order process. The photodecomposition rate constants are shown in Table 2. In comparison
with MT-18 calcined at different temperature, MT-18 calcined
at 400 ◦ C has the highest photodecomposition rate constant
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D.S. Kim et al. / Journal of Colloid and Interface Science 316 (2007) 85–91
Fig. 7. Kinetics of the photocatalytic degradation of MB solutions containing
(a) MT-18 calcined at various temperatures and (b) MT-18, MT-25, and MT-50
calcined at 400 ◦ C and Ishihara ST-01.
Table 2
Photodecomposition rate constants of mesoporous TiO2 and the commercial
TiO2 material, ST-01
Sample
MT-18
MT-25
MT-50
ST-01
Calcination
temperature
(◦ C)
400
500
600
400
400
–
Degradation
rate
(min−1 )
Degradation rate
per surface area
(g m−2 min−1 )a
R2
0.093
0.091
0.067
0.081
0.066
0.016
0.50 × 10−3
0.99
0.99
0.98
0.99
0.98
0.99
0.67 × 10−3
0.70 × 10−3
0.38 × 10−3
0.34 × 10−3
0.06 × 10−3
we performed the photocatalytic activity of MT-18, MT-25,
and MT-50, which were similar in surface area and crystallite size except for different pore-size distributions (Table 1).
The ordering of decreasing photodecomposition rate constants
of the mesoporous TiO2 materials is MT-18, MT-25, and MT50 (Fig. 7b and Tables 1 and 2), indicating that small pore
size has a much greater photocatalytic activity. This result indicates that the adsorption of MB onto the active sites of the
pore walls closer is enhanced, and so photocatalysts with small
pore sizes decompose adsorbed substances more readily and
effectively. In comparison with photocatalytic activity of commercial TiO2 , Ishihara ST-01, all samples calcined at 400 ◦ C
have better photocatalytic activity. Fig. 7b shows the kinetics
of the photocatalytic degradation of MB of these TiO2 materials. The photodecomposition rate constants were found to be
0.093 min−1 (MT-18), 0.081 min−1 (MT-25), and 0.066 min−1
(MT-50), which were higher than that of ST-01 (0.016 min−1 ).
It is considered that the differences in their rate constants are
attributed to the crystallite, pore size, and surface area of the
mesoporous TiO2 materials. If the surface area is only considered, ST-01 (ca. 290 m2 g−1 ) has a larger surface area than
MT-18 (ca. 186 m2 g−1 ), indicating that the ST-01 should have
better photocatalytic activity. However, the crystallite size of
ST-18 is 8.7 nm, which is larger than that of MT-18 (ca. 8.1 nm).
In addition, the pore size of ST-01 (ca. 7.6 nm) is larger than that
of MT-01 (5.1 nm), indicating that the photocatalytic activity of
MT-18 can be higher than that of ST-01. Therefore, we think
that the photocatalytic activity of mesoporous TiO2 is strongly
dependent on the crystallite size, pore size, and surface area,
but the effects of these factors on the photocatalytic activity are
interdependent. Further investigation of how the porous structure of ST-01 and MT-18 affects the photocatalytic activity is
necessary.
4. Conclusions
a Degradation rate is divided into surface area.
(Fig. 7a and Table 2). This result can be explained in terms of
surface area, crystallite size, and pore-size distribution. A large
surface area means that many substances are adsorbed onto the
active sites of the catalysts which can decompose them [27,29].
Regarding the effect of pore size, smaller pore size shows
higher photocatalytic activity (Tables 1 and 2). It has been commonly accepted that smaller crystallite size corresponds to more
powerful redox ability because smaller crystallite size induces
a larger band gap, which agrees with XRD results for crystallite
sizes (Table 1) and DRS results for the absorption band edges
(Fig. 6). Furthermore, in the effect of the pore size, mesoporous
TiO2 with smaller pore size is likely to exhibit better photocatalytic activity. To precisely investigate the effect of pore size,
This paper has shown the synthesis and photocatalytic activity of mesoporous TiO2 with various surface area, crystallite
size, and pore-size distributions. The sol–gel synthesis with diblock copolymer and titanium precursor in aqueous solution
gave a spherical morphology of mesoporous TiO2 with different
pore sizes, crystallite sizes, and surface areas according the calcination temperature. The photocatalytic activity was increased
with decrease of crystallite size, larger surface area, and smaller
pore-size distribution. In comparison with commercial TiO2 ,
Ishihara ST-01, the mesoporous TiO2 materials exhibit better
photocatalytic activities in the decomposition of MB.
Acknowledgment
This study was supported by the Ministry of Environment,
Republic of Korea, through the Eco-Technopia 21 project.
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