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Thin Solid Films 510 (
Synthesis of nanocrystalline photocatalytic TiO2 thin films and particles
using sol–gel method modified with nonionic surfactants
Hyeok Choi a, Elias Stathatos b, Dionysios D. Dionysiou a,*
a Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, OH 45221-0071, USAb Engineering Science Department, University of Patras, GR-26500 Patras, Greece
Received 7 June 2005; received in revised form 6 December 2005; accepted 14 December 2005
Available online 7 February 2006
Abstract
A simple sol–gel route has been developed for the preparation of nanocrystalline photocatalytic TiO2 thin films and particles at 500 -C.The synthesis involved a novel chemistry method employing nonionic surfactant molecules as a pore directing agent along with acetic acid-based
sol–gel route without direct addition of water molecules. This study investigated the effect of surfactant type and concentration on the
homogeneity, morphology, light absorption, dye adsorption and degradation, and hydrophilicity of TiO2 films as well as on the structural
properties of the corresponding TiO2 particles. The method resulted in the synthesis of mesoporous TiO2 material with enhanced structural and
catalytic properties including high surface area, large pore volume, pore size controllability, small crystallite size, enhanced crystallinity, and active
anatase crystal phase. The prepared TiO2 thin films were super-hydrophilic and possessed thermally stable spherical bicontinuous mesopore
structure with highly interconnected network. Highly porous TiO2 films prepared with polyethylene glycol sorbitan monooleate surfactant
exhibited four times higher photocatalytic activity for the decoloration of methylene blue dye than the nonporous control TiO2 films prepared
without the surfactant. This sol–gel method modified with surfactant templates is useful in the preparation of nanostructured anatase TiO2 thin
films with high photocatalytic activity and desired pore structure.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Titanium oxide; Photocatalysis; Nanostructures; Nanomaterials; Sol–gel, Thin films; Particles; Surfactant; Dip-coating
1. Introduction
The synthesis of nanocrystalline anatase TiO2 material with
high surface area has accelerated its widespread use in
environmental remediation such as photocatalytic destruction
of toxic organic compounds and inactivation of microorgan-
isms in water and air [1–7]. In addition to the intrinsic
photocatalytic activity of TiO2, which is directly related with its
crystal properties, the structural properties of porous TiO2
catalyst such as its surface area, porosity, and pore size and
distribution are also of importance because of their potential
role in enhancing the light absorbance of TiO2 catalyst and the
accessibility of reactants to the active catalytic sites. An
interesting method to fabricate highly porous materials with
desired pore structure and size for target-specific applications is
the use of amphiphilic organic molecules such as surfactants
0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.tsf.2005.12.217
* Corresponding author. Tel.: +1 513 556 0724; fax: +1 513 556 2599.
E-mail address: [email protected] (D.D. Dionysiou).
and block copolymers as pore directing agents in sol–gel
methods [8–12]. Ionic surfactants such as alkyl phosphate,
dodecylamine, and cetyltrimethylammonium chloride were
initially used in the synthesis due to the strong and well-
organized incorporation of titania inorganic framework onto
surfactant micelles by electrostatic interactions [9,11,12].
However, the use of ionic surfactants showed limited potential
for such applications since the strong electrostatic binding
force makes it difficult to remove the templates completely
through extraction methods and even heat treatment at high
temperature.
Common sol–gel methods employing direct addition of
water molecules in a sol can lead to the immediate precipitation
of amorphous particles with uncontrolled structure due to the
rapid hydrolysis and condensation reactions between the highly
reactive alkoxide titanium precursors and water. During the
past decade, an interesting synthetic route of the Ti–O–Ti
network in the absence of externally added water molecules has
been studied using acetic acid as a titania sol modifier in
2006) 107 – 114
ww
H. Choi et al. / Thin Solid Films 510 (2006) 107–114108
alcohol solvent [13–17]. In such a synthesis method, the
following basic steps are involved: (1) replacement of alkoxy
groups in the titania organic precursor with acetate groups,
resulting in the formation of alcohol, (2) esterification reaction
between alcohol and acetic acid to form water molecules, and
(3) slow hydrolysis reaction caused by water released from the
esterification reaction or direct condensation reaction of
acetate-bonded titanium [13,14].
Thus, the presence of surfactant molecules in the sol is
expected to play a crucial role in synthesizing tailor-designed
TiO2 catalytic materials and further reducing the hydrolysis
and condensation reaction rates due to the capping effect of
surfactants around the titania precursor [17,18]. This might
induce a highly porous titania inorganic network. However,
many research studies have focused on either the surfactant
self-assembling techniques or the acetic acid assisted sol–gel
strategies. Little work has been devoted to understand the role
of surfactants in the acetic acid assisted sol–gel method on the
formation of nanocrystalline TiO2 material and investigate its
structural and photocatalytic properties. In addition to the
porous structure and catalytic activity of TiO2 films, sol
stability and film homogeneity are also important factors in
such fabrication procedures.
In order to overcome these concerns and challenges, this
research deals with the preparation of highly efficient
nanostructured photocatalytic TiO2 thin films and particles
via an acetic acid-assisted sol–gel method employing nonionic
surfactants. Systematic study is conducted to elucidate the
effect of surfactant type and concentration on the structural and
catalytic properties of TiO2 material as well as on the physical
properties of TiO2 thin films including homogeneity, repro-
ducibility, thickness, and hydrophilicity.
2. Experimental details
2.1. Sol synthesis
The organic molecules used as pore directing agents were
representative nonionic long chain surfactants including Tween
20 (T20, polyethylene glycol sorbitan monolaurate), Tween 80
(T80, polyethylene glycol sorbitan monooleate), and Triton X-
100 (X100, polyethylene glycol tert-octylphenyl ether) pur-
chased from Aldrich. Compared to other commonly used toxic
and ionic templating agents, these organics are relatively
inexpensive, biodegradable, non-toxic, and easily removable.
Such large amphiphilic molecules exhibit the existence of
ordered mesophase and the ability to synthesize tailor-designed
porous TiO2 catalytic materials [19,20]. Each surfactant was
dissolved in isopropanol (i-PrOH, Fisher). Before adding
alkoxide precursor, acetic acid (Fisher) was added into the
solution for the esterification reaction with alcohol. Then,
titanium tetraisopropoxide (TTIP, Aldrich) was added under
vigorous stirring. The molar ratio of surfactant/i-PrOH/acetic
acid/TTIP was R:45:6:1, where the surfactant concentration R
was varied in the range from 0.0 to 3.0. Regardless of
surfactant addition, the sol was transparent, homogeneous,
and stable.
2.2. Formation of TiO2 thin films and particles
For the synthesis of immobilized TiO2 thin films,
borosilicate glass (Micro slide, Gold Seal) substrate with an
effective surface area of 10 cm2 was cleaned with water
followed by acetone. The substrate was dip-coated with the
sol using a homemade dip-coating apparatus at a withdrawal
rate of 12.8 cm/min. After coating, the films were dried at
room temperature for 1 h, calcined at a ramp rate of 3 -C/min
in a programmable furnace (Paragon HT-22-D, Thermcraft),
stayed at 500 -C for 15 min, and cooled down naturally.
This procedure was repeated three times. For convenience, the
abbreviations, ‘‘filmsurfactant’’ and ‘‘filmcontrol’’ denote TiO2
film prepared with surfactant and control TiO2 film prepared
without surfactant, respectively. Because of the difficulty in
direct characterization of the porosity and crystal structure of
small quantity of TiO2 films immobilized on glass support,
material characterization was carried out on the corresponding
TiO2 particles obtained from thick films. Even though the
properties of TiO2 particles are not exactly the same as those
of TiO2 films, this technique is useful for quickly examining
and comparing the effect of sol conditions on the structural
properties of the final material [21,22]. For TiO2 powder
characterization, the sol was spread on the glass substrate,
dried, and heat-treated at 500 -C for 1 h (instead of 15 min as
in making thin films) to thoroughly remove all the organics,
resulting in the formation of a thick film. The TiO2 particles
were collected by scraping the thick films and ground for
further characterization.
2.3. Materials characterization
A Kristalloflex D500 diffractometer (Siemens) with Cu Ka
(k =1.5406 A) radiation was employed for X-ray diffraction
(XRD) study of the TiO2 catalyst. A porosimetry analyzer
(Tristar 3000, Micromeritics) was used to investigate structural
characteristics of TiO2 material including Brunauer, Emmett,
and Teller (BET) surface area, porosity, and pore size and
distribution after purging samples with nitrogen gas for 2 h at
150 -C using Flow prep 060 (Micromeritics). A UV–Vis
spectrophotometer (Hewlett Packard 8452A) was used to
measure the UV–Visible light absorption of TiO2 films for
determining their band gap energy and evaluating their light
utilization. For examining the morphology of TiO2 nanostruc-
ture, a JEM-2010F (JEOL) high-resolution transmission
electron microscope (HR-TEM) with field emission gun at
200 kV was used. The samples were dispersed in methanol
(HPLC grade, Pharmco) using an ultrasonicator (2510R-DH,
Bransonic) for 5 min and fixed on a carbon-coated copper grid
(LC200-Cu, EMS). An environmental scanning electron
microscope (ESEM, Philips XL 30 ESEM-FEG) at accelerat-
ing voltage of 10 K was used to measure the thickness of films
and examine film homogeneity. Elemental composition
analysis of TiO2 materials was performed using energy
dispersive X-ray spectroscope (EDX, Oxford Isis) connected
to the HR-TEM and ESEM. For measuring the contact angle
of the film surface, the sessile drop method was employed
H. Choi et al. / Thin Solid Films 510 (2006) 107–114 109
using a static angle goniometer (Rame-Hart). In order to
measure weight change of the material due to desorption of
water and solvent and decomposition of organics in TiO2
films, thermogravimetric analysis (TGA, TA instruments
2050) was performed in the presence of air at a ramp rate
of 3 -C/min.
2.4. Dye adsorption and photocatalytic activity
In order to measure dye adsorption of TiO2 films, the
prepared films were soaked into 3.0 mM methylene blue (MB,
Riedel-deHaen) solution, dried thoroughly, and analyzed using a
UV–Vis spectrophotometer (Hewlett Packard 8452A). During
the procedure, the initial blue color of MB-absorbed films was
changed to purple and the corresponding absorbance peak of
MB at 664 nm was shifted to around 564 nm. For photocatalytic
activity evaluation, the film with surface area of 10 cm2 was
placed into a borosilicate glass dish containing 8 ml of 30 AMMB solution at pH 3.0. Two 15 W low-pressure mercury UV
tubes (Spectronics) emitting near UV radiation with a peak at
365 nm were used at a light intensity of 3.48 mW/cm2 at the film
surface. The absorbance of MB solution at 664 nm was
monitored over time.
3. Results and discussion
3.1. Structural characteristics of TiO2 particles
In order to investigate the effect of surfactant type and
concentration on the structural properties of TiO2, porosimetry
and crystallographic analyses of TiO2 particles were employed,
and the results are summarized in Table 1. The structural
properties were significantly improved with the addition of
surfactant. In the absence of surfactant, the materials were
Table 1
Structural characteristics of TiO2 particles
Surfactant R SBET (m2/g) Vpore (cm3/g) Porosity (%) DBJH
Ads.
None 0.0 18.5 0.034 11.6 5.85
T20 0.5 37.0 0.054 17.4 4.82
1.0 50.7 0.071 21.6 4.50
2.0 98.0 0.257 50.0 8.52
2.5 104 0.309 54.6 9.68
3.0 109 0.372 59.1 11.5
T80 0.5 65.6 0.135 34.4 5.66
1.0 70.9 0.227 46.9 9.89
2.0 74.0 0.346 57.4 15.8
2.5 84.6 0.548 68.1 22.7
3.0 90.1 0.658 71.9 25.1
X100 0.5 60.5 0.098 27.6 5.16
1.0 85.6 0.144 35.9 5.35
2.0 85.8 0.148 36.5 6.08
2.5 87.5 0.240 48.3 9.16
3.0 87.3 0.238 48.1 11.3
a Calculated from BET surface area.b Estimated from XRD peak.c Measured from TEM image.
almost nonporous with surface area of 18.45 m2/g, pore
volume of 0.034 cm2/g, and porosity of 11.6%. Increasing R
from 0.0 to 1.0 resulted in 2.8–4.7 times increase in surface
area and 2.1–6.7 times increase in pore volume without
significant change in pore size. The structural characteristics
were consistent with the properties of the surfactants. It is well
known that increasing the surfactant chain length has a similar
effect as increasing surfactant concentration [23]. The higher
porosity and larger pore size of TiO2 materials prepared with
Tween surfactants compared to those prepared with X100 can
be ascribed to the fact that Tween surfactants have a chain of 20
ether groups while X100 bears a chain of only 10 ether groups.
Moreover, TiO2 prepared with T80 had the highest porosity
and largest pore size due to longer hydrophobic tail length of
T80 (C17) than T20 (C11). In addition, T80 had better
controllability of pore size in the mesoporous range.
The crystallite size (CSBET) was calculated from the BET
surface area, assuming all crystallites are spherical and separate
without aggregation [24]. The result was compared with the
crystallite size (CSXRD) estimated using Scherrer’s equation
from the XRD peak broadening analysis [25]. The CSBET was
always larger than the CSXRD, implying that most primary
particles were more or less aggregated. The actual crystallite
size (CSTEM) measured from TEM image slightly decreased
with increasing R, but it was within a similar range from 7 to
12 nm. In the case of R =0.0, the large discrepancy between
CSTEM and CSBET suggests the prepared particles were highly
aggregated. As a result, the addition of surfactants as pore
directing agents also inhibited the growth of crystallites as well
as the aggregation of adjacent primary particles, and thus made
the materials highly porous. The representative XRD patterns
of the TiO2 materials prepared with T80 are shown in Fig. 1.
All the peaks are designated to anatase crystal phase with size
less than 16 nm. Interestingly, more distinct peaks with higher
(nm) DBJH range (nm) CS, crystallite size (nm)
Des. Ads. Des. CSBETa CSXRD
b CSTEMc
5.24 3–11 2–9.1 83.4 15.6 11.9
4.36 2–11 2–8.5 41.5 10.8 9.87
4.28 2–10 2–8.1 30.3 10.0 8.85
7.35 4–23 4–18 15.7 9.45 7.65
8.12 6–29 6–22 14.7 8.25 7.42
9.45 8–30 8–23 14.2 8.25 7.45
5.23 3–12 3–8.9 20.6 12.4 9.65
9.24 4–18 4–15 20.8 12.0 9.23
13.2 8–36 8–22 20.0 10.7 8.89
18.6 14–40 14–24 18.2 10.6 9.34
20.1 17–48 17–28 17.1 11.2 9.54
4.26 3–11 3–8.0 25.4 12.0 10.7
4.65 3–12 3–8.4 18.0 10.5 9.05
5.04 3–14 3–8.9 16.6 12.0 8.53
7.62 5–32 5–15 17.6 12.0 8.89
8.64 7–33 7–20 17.5 12.3 9.32
Molar ratio of surfactant to TTIP, R0.0 0.5 1.0 1.5 2.0 2.5 3.0
Abs
orba
nce
at 3
65 n
m (
arb.
uni
ts)
0.18
0.21
0.24
0.27
0.30
0.33
0.36
T80
T20X100
Fig. 3. UV light absorption of TiO2 films at 365 nm.20 30 40 50 60
Inte
nsity
(ar
b. u
nits
)
R=0.0
0.5
1.0
2.0
2.5
3.0
(101)(004) (200)
(105) (211)
2 (degrees)θ
Fig. 1. XRD patterns of TiO2 particles prepared with T80. Inserted numbers are
Miller indices.
H. Choi et al. / Thin Solid Films 510 (2006) 107–114110
intensities were observed with increasing R, suggesting the
material crystallinity was enhanced by the addition of
surfactants [26]. The same trend was observed in the case of
the other two surfactants.
3.2. Thickness, mass, UV light absorption, and dye adsorption
of TiO2 films
According to ESEM observation, the prepared transparent
TiO2 thin films were very reproducible and homogeneous
without cracks and pin-holes. As shown in Fig. 2, the thickness
of films increased as surfactant concentration R increased due
to an increase in the viscosity of the sol by the surfactants. The
mass of TiO2 immobilized on the glass substrates was
calculated from the thickness and porosity of films. In spite
of the increased film thickness, the mass of TiO2 decreased
over R because of the high porosity of the materials, which
implies that the TiO2 inorganic structure in the films became
less dense upon addition of surfactants.
Regardless of surfactant type and concentration, the UV
absorption edges of the TiO2 films were within the range of
372–374 nm, corresponding to band gap energy of around
3.34–3.32 eV. This is slightly larger than the commonly
reported band gap energy (Eg=3.23 eV, k =385 nm) for TiO2
Molar ratio of surfactant to TTIP, R0.0 0.5 1.0 1.5 2.0 2.5 3.0
Thi
ckne
ss (
μm)
0.24
0.26
0.28
0.30
0.32
0.34
0.36
Mas
s of
TiO
2 in
film
(μg
/cm
2 )
30
45
60
75
90
105
T80T20X100 T20
X100
T80
Fig. 2. Thickness and mass of TiO2 films.
Molar ratio of surfactant to TTIP, R0.0 1.0 1.5 2.0 2.5 3.0
Abs
orba
nce
at 5
64 n
m (
arb.
uni
ts)
0.0
0.2
0.4
0.6
0.8
1.0T80
T20
X100
0.5
Fig. 4. Visible light absorption of MB-adsorbed TiO2 films at 564 nm.
anatase phase because of the nanostructured properties of the
material [27,28]. As shown in Fig. 3, the UV light absorption
spectra of TiO2 films prepared under different conditions were
studied at 365 nm as an indirect evaluation of the photo-
catalytic activity under near UV radiation (300–400 nm).
Increasing surfactant concentration up to R =1.0 resulted in a
significant increase in the UV light absorption of TiO2 films,
and then increase in the UV light absorption became less
significant over R. This might be because the specific surface
area of the materials increased while the mass of TiO2
immobilized on the substrate decreased upon the addition of
surfactants. Moreover, the UV light absorption properties of
TiO2 film is a complex function of physicochemical properties
of the TiO2 material itself such as crystallinity, crystal phase,
crystal size, and purity as well as the structural properties of the
film such as thickness, mass, and surface area. In spite of their
smallest mass as shown in Fig. 2, filmsT80 had the highest UV
light absorption properties among films prepared with other
surfactants.
The visible light absorption of TiO2 films with adsorbed MB
at 564 nm as an indication of the porosity and homogeneity of
TiO2 films is shown in Fig. 4. MB adsorption of the films
increased with increasing surfactant concentration up to
R =1.0–2.0. In spite of the increased surface area of TiO2
material and thickness of the film with increasing surfactant
concentration, further addition of surfactant slightly decreased
(a)
Relative Pressure (Ps/Po)
0.0 0.2 0.4 0.6 0.8 1.0
Vol
ume
Ads
orbe
d(c
m3 /g
ST
P)
0
20
40
60
80
100
120
140
160
R=0.0
R=1.0
Adsorption
Desorption
Por
evo
lum
e(c
m3 /
g)
R=1.0
R=0.0
(b)
Por
evo
lum
e(c
m3 /
g)
0.0
0.1
0.2
0.3
0.4
0.5
R=1.0
R=0.0
H. Choi et al. / Thin Solid Films 510 (2006) 107–114 111
the adsorption capacity due to a lower TiO2 mass immobilized
on the substrates. Another reason is perhaps associated with the
fact that the homogeneity of films prepared at too high
concentration of surfactants decreased significantly, most
probably, due to the high viscosity of the sol, pore coalescence,
and multi-micellar interactions.
3.3. Hydrophilicity of TiO2 films
Hydrophilicity (or wettability) of TiO2 films was investi-
gated by measuring the water contact angle. Surface hydro-
philicity is important for the accessibility of organic
compounds to catalytic sites [29]. It has been well observed
that the surface of TiO2 exhibits super-hydrophilicity under UV
irradiation due to hydrophilic groups introduced [30,31]. Prior
to coating with TiO2 films, the pre-cleaned glass substrate
showed water contact angle of around 16.1-, corresponding to
relatively hydrophilic surface. As shown in Fig. 5(a), the
contact angles after TiO2 coating significantly decreased to the
reliable detection limit of 4- (actually the contact angle reached
0-) without any UV radiation, indicative of super-hydrophilic
surface. The decreased contact angle may be associated with
the porous structure of TiO2 films and the increase in surface
hydrophilic groups, which was supported by TGA analysis of
TiO2 films. For the TGA analysis, approximately 10 mg of
particles were collected from the filmsT80 and the weight
change was monitored during thermal treatment at a ramp rate
(a)
Molar ratio of surfactant to TTIP, R
Molar ratio of surfactant to TTIP, R
0.0 0.5 1.0 1.5 2.0 2.5 3.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Con
tact
ang
le (
degr
ees)
4
6
8
10
12
14
16
T80X100T20
(b)
Wet
tabi
lity,
Cos
θ /
r
0.00
0.05
0.10
0.15
0.20
0.25
0.30
T80
X100
T20
Fig. 5. (a) Water contact angle and (b) wettability of TiO2 films. The contact
angle and pore size of TiO2 material are denoted as h and r, respectively.
Pore diameter (nm)
5 10 15 20 25 30
Pore diameter (nm)
5 10 15 20 25 30
Fig. 6. (a) Nitrogen adsorption–desorption isotherms and (b) pore size
distribution of TiO2 filmcontrol at R =0.0 and filmT80 at R =1.0. Inserted image
is side view of the filmT80 at R =1.0.
of 3 -C/min. The weight loss of the samples within the
temperature range 25–100 -C increased significantly from
1.98% for film prepared at R =0.0 to 2.31% for that at R =0.5.
Further increase in surfactant concentration up to R =3.0
caused slight increase in weight loss of 2.64%. This result
indicated that the TiO2 films prepared at high surfactant
concentration entrapped a large amount of water in the porous
structure.
In addition, according to the Laplace equation, the wet-
tability of porous materials is known to be proportional to the
surface tension of liquid and inversely proportional to the pore
size and surface energy (contact angle) of the material [32].
The increase in surfactant concentration resulted in decreased
contact angles as observed in Fig. 5(a) but increased pore size
as summarized in Table 1. Considering the same surface
tension of water used, Fig. 5(b) suggests that the optimum
surfactant concentration for preparing TiO2 films with better
hydrophilicity was at around R =0.5–1.0. FilmsT20 and
filmsX100 had a higher wettability than filmsT80 because more
organics were entrapped in films prepared with T80, which has
longer hydrocarbon chain length. According to TGA analysis
of films prepared with surfactants at R =1.0, negligible weigh
loss was observed after 100 -C in the case of filmT20 and
filmX100. On the other hand, filmT80 still had a considerable
Table 2
Structural characteristics of TiO2 filmcontrol at R =0.0 and filmT80 at R =1.0
Parameter R =0.0 R =1.0
SBET (m2/g) 22.7 147
Vpore (cm3/g) 0.037 0.221
Porosity (%) 12.6 46.2
DBJH from adsorption branch (nm) 5.65 4.04
DBJH from desorption branch (nm) 5.38 3.72
CSTEM (nm) 12.4 9.20
Film thickness (Am) 0.26 0.31
TiO2 mass (Ag/cm2) 88.4 62.2
H. Choi et al. / Thin Solid Films 510 (2006) 107–114112
weight loss of 1.7% between 100 -C and 500 -C due to the
decomposition of organics remaining in the materials. More-
over, EDX results showed that the carbon content of filmT80
was around 1.2%, significantly higher than in other films,
which was in the range 0.4–0.6%.
3.4. Porosity and morphology of TiO2 films
Based on the obtained results so far, filmT80 at R =1.0
among all conditions investigated was considered the most
effective. For determining the structural properties of filmT80,
the TiO2 material was collected by scrapping the thin film
surface cautiously and analyzed using a porosimetry analyzer
and TEM. Fig. 6(a) shows the nitrogen adsorption–desorption
isotherms of filmT80. Compared to N2 isotherms of the
filmcontrol, which represents a nonporous material, these type
IV isotherms of filmT80 were typical of those of a well-
Fig. 7. Morphology and pore structure of (a–b) filmcontrol at R =
developed mesoporous material. A hysteresis loop in the
isotherms was observed with dissimilar shapes for the
adsorption and desorption branches, implying a different size
of pore throat diameter. The sharp drop on the desorption
branch can be assigned to the presence of mesopore constric-
tions at the boundaries between the ordered domains and of
smaller pores in the titania walls [33]. The pore size
distribution shown in Fig. 6(b) was relatively narrow ranging
from 2 to 8 nm. The Barrett, Joyner and Halenda (BJH) pore
diameters measured from the adsorption and desorption
branches were 4.04 nm and 3.72 nm, respectively. These
results imply good homogeneity of the pores. The main
structural characteristics deduced from the isotherms are
reported in Table 2. In spite of the high heat treatment
temperature of 500 -C, the BET surface of 147 m2/g and
porosity of 46.2% were significantly high, compared to other
research results reported [5,17]. The film thickness of 0.31 Amwas measured using ESEM, and its mass of 62.2 Ag/cm2 was
calculated from the pore volume and density of the anatase
crystal phase. Even though the thickness of filmT80 was much
larger than that of filmcontrol, the amount of TiO2 catalyst in
filmT80 was smaller due to its high porosity.
As shown in Fig. 6(b), the cross-section of filmT80 on glass
substrate revealed that filmT80 was homogeneous and well-
incorporated to the glass substrates without cracks and pin-
holes. EDX elemental analysis of collected thin films showed
that the films were composed of mainly Ti and O elements
without any significant impurities. Fig. 7 shows the morphol-
0.0 and (c–d) filmT80 at R =1.0 at different magnifications.
Reaction time (h)
Nor
mal
ized
MB
abs
orba
nce,
I/I o
0.0
0.2
0.4
0.6
0.8
1.0Without TiO2
R=0.0
R=1.0R=2.0
R=3.0
R=0.5
1 2 3 4 5 6 70
Fig. 8. Photocatalytic decoloration of MB by TiO2 filmsT80.
H. Choi et al. / Thin Solid Films 510 (2006) 107–114 113
ogy of the nanostructured anatase TiO2 thin films. For
filmcontrol, no distinct mesopore structure was observed and
even the lattice fringes were not clear. On the other hand,
filmsurfactant were highly porous and exhibited distinct pore
structure. The films had slightly collapsed spherical bicontin-
uous structure with highly interconnected network [34]. The
image at high magnification showed many randomly oriented
nanocrystallites with size of 8.3–10.6 nm and sets of clearly
resolved lattice fringes giving evidence that the TiO2 material
was highly crystalline, which was in good agreement with the
XRD results. The porous structure was very strong and stable
since the structure still remained at large extent until heat
treatment at 700 -C.
3.5. Photocatalytic activity of TiO2 films
Photocatalytic activity of the TiO2 films was measured in
terms of MB decoloration. As shown in Fig. 8, there was no
direct photolysis of MB in the absence of TiO2 photocata-
lysts. In spite of its relatively high catalyst mass of 88.4 Ag/cm2, the filmcontrol at R =0.0 did not show significant
decoloration of MB due to the almost nonporous properties
of the film, suggesting the photocatalytic reaction occurred
only at the very surface of TiO2 film. Increasing the amount
of surfactant template up to R =1.0–2.0 resulted in a
significant improvement of the photocatalytic activity of the
films mainly due to the porous structure of the film with high
surface area of 147 m2/g and porosity of 46%, and partially
the enhanced material crystallinity as observed in the XRD
and TEM analyses. However, further addition of surfactant
beyond R =2.0 caused adverse effect on the photocatalytic
activity of films, which was consistent with the results on MB
adsorption. It was because the homogeneity of the films
prepared under too high concentration of surfactants (R >2.0)
decreased and the amount of TiO2 photocatalyst immobilized
TiO2 films also decreased. These results show the importance
of preparing highly porous and homogeneous TiO2 thin films,
which might facilitate MB adsorption and UV light utiliza-
tion. Moreover, considering the small amount of TiO2 catalyst
immobilized on the glass substrate, the TiO2 films were
highly efficient to decolorize the dye.
4. Conclusions
Highly porous and hydrophilic nanostructured TiO2 thin
films and particles were synthesized from nanocomposite
organic/inorganic sol–gel, composed of isopropanol, acetic
acid, titanium tetraisopropoxide, and nonionic surfactant
molecules as templates. Slow hydrolysis reaction and stable
incorporation of the inorganic network onto surfactant
molecules made it possible to control the subsequent porous
TiO2 nanostructure. The TiO2 particles and films had
enhanced structural and catalytic properties including high
surface area, large pore volume, pore size controllability, small
crystallite size, enhanced crystallinity, and active anatase
phase. Among the surfactants investigated, Tween 80 was
the most promising in terms of film homogeneity, UV
absorbance, methylene blue adsorption, and especially pore
volume and pore size controllability. The porous TiO2 thin
films were highly efficient to decolorize MB dyes due to their
high surface area and photocatalytic activity. This acetic acid-
based sol–gel method modified by varying the type and the
concentration of the surfactant template is useful in the
preparation of nanostructured anatase TiO2 thin films with
high photocatalytic activity and desired pore structure for
environmental applications.
Acknowledgement
This research was funded by a grant from the Office of
Biological and Physical Research of the National Aeronautics
and Space Administration (NRA Grant No. NAG 9-01475).
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