Effects of urea on the microstructure and photocatalytic activity of bimodal mesoporous titania microspheres

Article abstract of DOI:10.1016/j.molcata.2009.08.009

Bimodal mesoporous TiO₂ microspheres with high photocatalytic activity were prepared by a hydrothermal method using titanium sulfate as precursor in the presence of urea. The results indicate that all prepared samples show bimodal pore-size dis

Full text of DOI:10.1016/j.molcata.2009.08.009

Journal of Molecular Catalysis A: Chemical 313 (2009) 107–113
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Effects of urea on the microstructure and photocatalytic activity of bimodal
mesoporous titania microspheres
Minghua Zhoua,b,1, Jiaguo Yua,∗, Huogen Yua,1
a State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, PR China
b Staff Room of Chemistry, Yunyang Medical College, Shiyan 442000, Hubei, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 April 2009
Received in revised form 9 June 2009
Accepted 13 August 2009
Available online 21 August 2009
Bimodal mesoporous TiO2 microspheres with high photocatalytic activity were prepared by a hydrother-
mal method using titanium sulfate as precursor in the presence of urea. The results indicate that
all prepared samples show bimodal pore-size distributions in the mesoporous region: smaller intra-
aggregated pores with peak pore diameter of ca. 2 nm and larger inter-aggregated pores with peak pore
diameter of ca. 12.5 nm. The molar ratio of urea to Ti(SO4)2 (Ru) has an obvious influence on the mor-
phology, microstructure and photocatalytic activity of TiO2. With increasing Ru, specific surface areas
and porosity increase, contrarily, the crystallite size and relative anatase crystallinity decrease. The pho-
tocatalytic activity first increases with Ru. At Ru = 2.0, the photocatalytic activity reaches the highest
and is obviously higher than that of Degussa P25. With further increasing Ru, the photocatalytic activity
decreases. The formation rate of hydroxyl radicals during photocatalysis has a positive correlation with
the photocatalytic activity.
Keywords:
Mesoporous
TiO2 microspheres
Bimodal
Photocatalytic activity
Hydrothermal
Hydroxyl radicals
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
tion of amorphous to anatase at low temperatures in the absence
of templates or surfactants.
Titania is a very important multifunctional environment and
energy material because of its peculiar and fascinating physico-
chemical properties and a wide variety of potential uses in diverse
fields including solar energy conversion, environmental purifica-
tion, and waste water treatment [1–5]. Although titania powders
and sols can be readily prepared by conventional precipitation or
sol–gel methods, synthetic routes to mesoporous TiO2 spheres with
good crystallization, small crystallite size and high specific sur-
face area and pore volume are less common. Also, the mesoporous
TiO2 microspheres obtained by conventional template methods
are amorphous in nature and high-temperature calcination (higher
than 400 ◦C) is required to realize the phase transformation from
amorphous to anatase and to remove the organic templates. Thus,
such a high-temperature will cause the obvious growth of crys-
tallites, resulting in the decrease of specific surface areas and the
destruction of pore structure [4–7]. Therefore, in order to obtain
highly photoactive mesoporous TiO2 microspheres with good crys-
tallization, small crystallite size and high specific surface area and
pore volume, an optimal method is to realize the phase transforma-
Soler-Illia et al. [8,9] have in detail reported the urea method
as an efficient route to synthesize both amorphous and crystalline
metal (hydrous) oxide with uniform particle shape from an aqueous
media. The decomposition of urea in aqueous solution is accompa-
nied by slow and controlled supply of ammonia and carbon dioxide
into solution. The smooth pH increase obtained by the decompo-
sition of urea in synchrony with the active release of OH− and
2−
CO3
ions, usually leads to the precipitation of metal hydrous
oxide particles of controlled particle morphology. However, to the
best of our knowledge, there are few papers on the preparation
and photocatalytic activity of bimodal mesoporous TiO2 micro-
spheres for the photocatalytic decomposition of acetone in air.
Herein, we adopt a simple one-step template-free hydrothermal
method to synthesize bimodal mesoporous TiO2 microspheres by
using urea and Ti(SO4)2 as precursors at 160 ◦C. Urea is used as a
dual-function agent, as a pore-produced agent and pH adjusting
agent. The preparation conditions are much milder and simpler
than those of the conventional template methods, which require
high-temperature calcination procedures. Another advantage is
that the as-prepared samples show bimodal mesoporous structure,
which will endow them with better mass transport for reactant
molecules and larger light-harvesting ability due to hierarchi-
cally porous structures, resulting in an enhanced photocatalytic
activity.
∗
Corresponding author. Tel.: +86 27 87871029; fax: +86 27 87879468.
E-mail address: jiaguoyu@yahoo.com (J. Yu).
1
Tel.: +86 27 87871029; fax: +86 27 87879468.
1381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2009.08.009

108
M. Zhou et al. / Journal of Molecular Catalysis A: Chemical 313 (2009) 107–113
2. Experimental
2.1. Synthesis
All the chemicals used in this study were reagent-grade and
deionized water was used in the whole experiment. The bimodal
mesoporous TiO2 microspheres were synthesized by hydrother-
mal treatment of Ti(SO4)2 in a CO(NH2)2 aqueous solution. The
molar ratio of CO(NH2)2 to Ti(SO4)2 (Ru) was varied from 0, 1, 2,
4 to 8. After continuous stirring for 30 min, the mixed solution
was transferred into a Teflon lined stainless steel autoclave. Before
hydrothermal reactions, the pH values of all the mixed solutions
were about 1.0. The autoclave was incubated at 160 ◦C for 5 h and
then cooled to room temperature. After hydrothermal reaction, the
pH values of the corresponding mixed solution changed greatly and
were about 0.5, 2.0, 4.5 6.5 and 8.8, respectively. The white precipi-
tates were centrifuged, and washed with distilled water and alcohol
for five times. The washed precipitates were dried in a vacuum oven
at 80 ◦C for 12 h.
Fig. 1. XRD patterns of the TiO2 samples prepared at different Ru.
light irradiation. The initial concentration of acetone after adsorp-
tion equilibrium was controlled at 300 20 ppm, which remained
constant for about 5 min until a 15 W, 365 nm, UV-lamp in the reac-
tor was turned on. The initial temperature was 25 1 ◦C and each
set of experiment under UV irradiation was performed for 60 min.
The photocatalytic activity of the samples can be quantitatively
evaluated by comparing the removal efficiency of acetone (R(%)).
R(%) was calculated according to the following equation [10]:
2.2. Characterization
X-ray diffraction (XRD) patterns obtained on a D/Max-RB X-ray
diffractometer (Rigaku, Japan) using Cu K␣ irradiation at a scan
rate (2ꢀ) of 0.05◦ s−1 were used to determine the phase structure
of the obtained samples [10,11]. Morphology observation was per-
formed on an S-4800 field emission scanning electron microscope
(SEM, Hitachi, Japan). Transmission electron microscopy (TEM)
and high-resolution transmission electron microscopy (HRTEM)
observation were conducted using a JEM 2100F microscope. The
Brunauer–Emmett–Teller surface areas (SBET) of the samples were
analyzed by nitrogen adsorption with a Micromeritics ASAP 2020
nitrogen adsorption apparatus (USA).
c0 − ct
R(%) =
× 100%
(2)
c0
where c0 and ct represent the initial equilibrium and reaction con-
centration of acetone, respectively.
3. Results and discussion
2.3. Analysis of hydroxyl radical (•OH)
3.1. Crystal structure
The formation of hydroxyl radical (•OH) on the surface of
photo-illuminated TiO2 was detected by photoluminescence (PL)
technique using terephthalic acid as a probe molecule [12,13]. This
method relies on the PL signal at 425 nm of the hydroxylation of
terephthalic acid with •OH generated at the TiO2/water interface.
Furthermore, this method is rapid, sensitive and specific, and needs
only a simple standard PL instrumentation. Experimental proce-
dures are as follows: 0.10 g of the TiO2 sample was dispersed in
a 25 mL of the 5 × 10−4 M terephthalic acid aqueous solution with
a concentration of 2 × 10−3 M NaOH in a dish with a diameter of
about 9.0 cm. A 15 W, 365 nm UV-lamp was used as a light source.
PL spectra of generated 2-hydroxyterephthalic acid were measured
on a Fluorescence Spectrophotometer (F-7000, Hitachi, Japan).
Fig. 1 shows XRD patterns of the TiO2 samples prepared at dif-
ferent Ru. The diffraction peaks of all samples are easily indexed to
the anatase phase of TiO2, which is consistent with the pure anatase
phase of TiO2 (space group: I41/amd (1 4 1); JPCDS No. 21-1272)
[17,18]. Usually, the phase transformation temperature from amor-
phous to anatase is higher than 400 ◦C in air. It can be concluded that
hydrothermal treatment can promote the phase transformation
from amorphous to anatase at a low temperature (160 ◦C). Further
observations indicate that, with increasing Ru, the peak intensi-
ties of anatase become weaker and the width of the diffraction
peaks of anatase show slightly wider, indicating the formation of
smaller TiO2 crystallites. The relative anatase crystallinity is quanti-
tatively evaluated via the relative intensity of the (1 0 1) diffraction
peak [19]. Table 1 lists the average crystalline sizes and relative
anatase crystallinity of TiO2 samples prepared at different Ru. It
can be seen that the average crystalline sizes and relative anatase
crystallinity decrease with increasing Ru. This is attributed to the
fact that, in pure water, the preparation of anatase TiO2 powders
from the inorganic salt precursor Ti(SO4)2 may undergo the follow-
ing three consecutive reaction processes via hydrothermal method
[20,21]:
2.4. Measurement of photocatalytic activity
In this work, we chose acetone as a model compound to evaluate
the photocatalytic activity of the as-prepared samples. Photocat-
alytic oxidation of acetone is based on the following reaction
[14–16]:
CH3COCH3 + 4O2 → 3CO2 + 3H2O
(1)
The weight of catalysts used for each experiment was kept at
0.20 g and coated onto three dishes. After TiO2-coated dishes were
placed in the reactor, a 15 1 ␮L acetone was injected into the reac-
tor with a microsyringe. The analysis of acetone, carbon dioxide,
and water vapor concentration in the reactor was on-line con-
ducted with a Photoacoustic Field Gas-Monitor (INNOVA Air Tech
Instruments Model 1412). The acetone vapor was allowed to reach
adsorption equilibrium with catalysts in the reactor prior to UV
Ti(SO4)2 + 4H2O → Ti(OH)4 + 2H2SO4
Ti(OH)4 → TiO2(amorphous) + 2H2O
TiO2(amorphous) → TiO2(anatase)
Clearly, the pH value greatly affects above hydrolysis and con-
densation reactions. In general, Eq. (3) is a rate-determining step of
above three reactions and its rate is much slower at acid conditions,
Hydrolysis
Condensation
Crystallization
(3)
(4)
(5)

M. Zhou et al. / Journal of Molecular Catalysis A: Chemical 313 (2009) 107–113
109
Table 1
Effects of Ru on physical properties of bimodal mesoporous TiO2 microspheres.
Ru
aPhase
SBET (m2/g)
Pore volume (cm3/g)
Average pore size (nm)
Porosity (%)
bCrystallite size (nm)
0
A
A
A
A
A
A + R
152.8
166.6
179.0
195.9
215.9
63.0
0.36
0.39
0.43
0.49
0.55
0.06
13.2
12.4
11.9
11.5
11.2
3.8
58.1
60.0
62.3
65.3
67.9
18.6
13.2 (1.0)
11.3 (0.97)
10.6 (0.78)
8.6 (0.72)
7.6 (0.54)
30.0 (A)
1.0
2.0
4.0
8.0
P25
a
A and R denoted anatase and rutile, respectively.
Relative anatase crystallinity: the relative intensity of the diffraction peak from the anatase (1 0 1) plane (indicated in parentheses, reference: Ru = 0).
b
and can be rapidly enhanced at a basic environment. Therefore,
in the absence of urea, the hydrolysis of Ti(SO4)2 in pure water
can cause the production of H2SO4. Under such strong acid condi-
tion (pH = 0.5), the rate of hydrolysis and condensation reactions is
rather slow and the nucleation is suppressed. So, fewer metastable
tiny TiO2 nanoclusters are formed. This is beneficial to crystal
growth, rather than aggregation. The added urea has important
influence on the rate of reaction (3) due to the following reactions
[8,22–24]:
The urea is decomposed into ammonia and carbon dioxide (Eq.
(6)) at 160 ◦C and then the produced ammonia is neutralized by
H2SO4 produced in Eq. (3) (Eq. (7)). According to the Le Chatelier’s
principle, reaction (3) is promoted due to the decrease of concentra-
tion of produced H2SO4 or the increase of pH value of the solution.
Consequently, the rate of Eq. (4) is enhanced with increasing Ru.
Thus, in hydrothermal conditions, the faster condensation rate will
result in smaller crystalline sizes and weaker anatase crystallinity
[25].
3.2. SEM and TEM studies
(NH2)2CO + 3H2O → 2NH3·H2O + CO2
2NH3·H2O + H2SO4 → (NH4)2SO4 + 2H2O
Decomposition
(6)
Fig. 2 shows the typical SEM images of the TiO2 samples obtained
at different Ru. As shown in Fig. 2a, the irregular aggregated
Neutralization (7)
Fig. 2. SEM images of the samples prepared at different Ru. (a) Ru = 0, (b) Ru = 1.0, (c) Ru = 2.0, (d) Ru = 4.0 and (e) Ru = 8.0. Inset in (e) showing SEM images of corresponding
single microsphere.

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M. Zhou et al. / Journal of Molecular Catalysis A: Chemical 313 (2009) 107–113
Fig. 3. TEM (a and b) and HRTEM (c) images of the bimodal mesoporous TiO2 microspheres prepared at Ru = 2.0.
particles with rough surface are obtained in the absence of urea (in
pure water, Ru = 0). Furthermore, the pH value of the mixture solu-
tion is about 0.5, which is much lower than that of the isoelectric
points of TiO2 (pH = 5.5–6) and thus each primary TiO2 crystallite is
positively charged at such a strong acid solution [26,27]. So, these
small primary TiO2 crystallites cannot aggregate spontaneously to
large spherical aggregates because of their electrostatic repulsion.
Taking the above XRD results into account, the amount of urea
also plays an important role in controlling the morphology of the
final products. Urea can influence the microstructures of the TiO2
via altering the pH values of the reaction system. When urea is
added into the reaction solution, the microstructures of the TiO2
samples have an obvious change. With increasing urea concentra-
tion (Ru = 1.0), the shape of the particles approximately transforms
into spherical aggregates. This is probably due to the fact that
when the pH of the mixed solution is adjacent to the isoelectric
point of TiO2, the electrostatic repulsion from primary crystal-
lite decreases. So, the primary crystallites spontaneously aggregate
into large microspheres to minimize their surface Gibbs energy.
The average diameters of the microspheres slowly increase, while
the roughness of the surface of the samples gradually decreases
(Fig. 2b). Interestingly, with further increasing urea concentration
to some extent (Ru = 2.0), TiO2 spherical particles are obtained in
addition to a small fraction of ill-defined small particles (Fig. 2c).
At Ru = 8, the particles show the round external configuration with
average diameter of ca. 3 ␮m. The enlarged SEM images (Fig. 2e) of
the microspheres surface clearly reveal that these spherical archi-
tectures are composed of numerous secondary particles with many
nanoscale channels, and the average diameter of the secondary par-
ticles is ca. 30 nm. The configuration of spherical microstructures is
further investigated by the corresponding TEM images (Fig. 3a and
b).
The corresponding TEM and HRTEM images of the sample pre-
pared at Ru = 2.0 are presented in Fig. 3, which further confirm that
the microspheres ca. 1 ␮m in size (see Fig. 3a and b) consist of pri-
mary crystallites. It can be seen that the primary crystallite size
is about 11 1 nm (Fig. 3b), which is in agreement with the value
of the crystallite size determined by XRD (10.6 nm) (as shown in
Table 1). Further observation indicates that a large number of small
mesopores (intra-aggregated pores) come from the aggregation of
primary crystallites and larger inter-aggregated pores are from the
aggregation of secondary particles (marked by arrow in Fig. 3b).
Fig. 3(c) shows clear lattice fringes of primary crystallite (0.35 nm
corresponding to the (1 0 1) crystallographic plane of anatase) of
the same sample, which allows for the identification of crystallo-
graphic spacing and indicates good crystallization of the prepared
anatase TiO2 microspheres.
3.3. BET surface areas and pore distributions
Fig. 4(a) shows the nitrogen adsorption–desorption isotherms of
the bimodal mesoporous TiO2 samples prepared at different Ru. All
the samples show the isotherms of type IV [25,28]. With increasing
Ru, the hysteresis loops shift to a lower relative pressure region and
the adsorbed volume increases, which indicates that the average
pore size decreases and specific surface area increases. The cor-
responding pore-size distribution of the prepared TiO2 samples is
shown in Fig. 4(b). It is interesting to find that all the samples show
bimodal pore-size distributions. In pure water (Ru = 0), the powders
contain fine intra-aggregated pores with peak pore diameters of

M. Zhou et al. / Journal of Molecular Catalysis A: Chemical 313 (2009) 107–113
111
Fig. 5. PL spectral changes with irradiation time on the sample prepared at Ru = 2.0
in a 5 × 10−4 M basic solution of terephthalic acid.
interface [12,13]. Fig. 5 shows PL spectral changes with irradiation
time. A gradual increase in PL intensity at wavelength range of
360–540 nm is observed with increasing irradiation time. More-
over, the generated spectra have the identical shape and peak
position (at 425 nm).
Fig. 6 shows the plots of PL intensity at 425 nm against irra-
diation time. It can be seen that the PL intensity of terephthalic
acid solutions under UV light irradiation increase linearly with
time. Consequently, it can be reasonable to infer that the amount
of •OH radicals produced at the TiO2/water interface is propor-
tional to the light irradiation time [12,13]. The formation rate of
the •OH radicals can be expressed by the slop of these lines shown
in Fig. 6. The order of the formation rate of •OH radicals formed on
the surface of bimodal mesoporous TiO2 microshpheres is as fol-
lows: Ru = 2.0 > Ru = 4.0 > Ru = 1.0 > Ru = 0 > Ru = 8.0, which suggests
that the Ru influences the formation rate of •OH radicals and there
is an optimal Ru.
Fig. 4. Nitrogen adsorption–desorption isotherms (a) and corresponding pore-size
distribution curves (b) of the bimodal mesoporous TiO2 microshpheres prepared at
different Ru.
ca. 2.1 nm and larger inter-aggregated pores with peak pore diam-
eters of about 12.5 nm. According to our previous works [19], a
bimodal pore-size distribution is due to two different aggregates
in the powders. The smaller micro/meso-pores are usually related
to primary intra-agglomeration (the hysteresis loop in the lower
P/P0 range), while the larger ones are associated with secondary
inter-aggregation (the hysteresis loop in the higher P/P0 range).
It is these bimodal pores that can promote the rapid diffusion of
various reactants and products during the photocatalytic reaction
and enhance the rate of the photocatalytic reaction. With increas-
ing urea concentration (Ru from 1.0 to 8.0), the peak pore sizes
of the intra-aggregated pores shift into smaller meso/micropores
regions (from 2.1 to 1.9 nm) and the inter-aggregated peak pore
almost keeps un-change (12.5 nm), indicating the decrease of pri-
mary crystallite size. The quantitative details about the BET surface
areas, pore volume, average pore size and porosity of the samples
are presented in Table 1. It can be seen that with increasing Ru,
the specific surface areas of the samples increase from 152.8 to
215.9 m2/g and pore volume of the samples increase from 0.36 to
0.55 cm3/g. While the average pore sizes decrease slightly from 13.2
to 11.2 nm.
3.5. Photocatalytic activity
Fig. 7 shows the concentration changes of acetone and carbon
dioxide with time during photocatalytic reaction. It can be seen that
the concentration of the produced carbon dioxide is about three
3.4. Hydroxyl radical analysis
Terephthalic acid readily reacts with •OH to produce highly
active fluorescent product, 2-hydroxyterephthalic acid [12,13].
This technique has been used in radiation chemistry, sonochem-
istry and biochemistry [29–36] for the detection of •OH generated
in water. The intensity of the PL peak of 2-hydroxyterephtalic acid is
proportional to the amount of •OH radicals produced at TiO2/water
Fig. 6. Plots of the induced fluorescence intensity at 425 nm against irradiation time
for terephthalic acid on the bimodal mesoporous TiO2 microshpheres prepared at
different Ru.

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M. Zhou et al. / Journal of Molecular Catalysis A: Chemical 313 (2009) 107–113
the samples. At Ru = 0, the removal efficiency is 26.4%. With increas-
ing Ru, the photocatalytic activity of the prepared samples increases
slightly. At Ru = 2.0, the photocatalytic activity of the prepared sam-
ple reaches a maximum value, and its activity exceeds that of P25
by a factor of more than two times. With further increasing Ru, the
photocatalytic activity of the prepared sample obviously decreases
due to drastic decrease of relative anatase crystallinity.
4. Conclusions
Bimodal mesoporous spherical TiO2 photocatalysts with high
photocatalytic activity can be prepared by a one-pot hydrothermal
method using titanium sulfate as a precursor in the presence of
urea. All prepared TiO2 powders exhibit bimodal pore-size distri-
butions in the mesoporous region: smaller intra-aggregated pores
(peak pore diameter at ca. 2 nm) and larger inter-aggregated pores
(peak pore diameters at ca. 12.5 nm). Ru has a significant influ-
ence on the morphology, microstructureandphotocatalyticactivity
of TiO2. With increasing Ru, specific surface areas and porosity
increase, on the contrary, the crystallite size and relative anatase
crystallinity decrease. The photocatalytic activity of the samples
first increases with Ru. At Ru = 2.0, the photocatalytic activity of
the sample reaches the highest value and is obviously higher
than that of P25 due to relative large specific surface areas and
good crystallization. With further increasing Ru, the photocatalytic
activity decreases. The formation rate of hydroxyl radicals during
photocatalysis has a positive correlation with the photocatalytic
activity. At Ru = 2.0, the greatest formation rate of hydroxyl radicals
is achieved.
Fig. 7. Concentration–time plots of acetone and carbon dioxide (oxidation of ace-
tone on the illuminated the samples prepared at Ru = 0 and 2.0 and P25).
times greater than the amount of the acetone destroyed. It is also
observed that the concentration of the acetone and carbon diox-
ide linearly change with increasing irradiation time. Therefore, it
can be concluded that the prepared photocatalysts and P25 can
completely decompose acetone. Furthermore, the photocatalytic
oxidation of acetone on the surface of TiO2 powders in the initial
time is also a pseudo-zero-order reaction, which is in agreement
with the order of the formation rate of •OH radicals. Finally, the
ascending sequence of photocatalytic activity of the samples is:
P25 < Ru = 0 < Ru = 2.0 and the values of reaction kinetic constant are
1.07, 1.41 and 2.20 ppm/min for photocatalytic oxidation of ace-
tone, respectively. In order to further characterize the performance
of all photocatalysts, we also use R(%) to compare their photocat-
alytic activities.
Fig. 8 shows R(%) of acetone of all the samples prepared at
different Ru. It can be seen that all the samples prepared by the
hydrothermal method show better activities than P25. The higher
photocatalytic activity of as-prepared microspheres may be first
directly related to their bimodal mesoporous structures, which
allows more efficient transport for the reactant molecules to get to
the active sites on the framework walls, meanwhile, enhances the
adsorption of light and reduce reflection of light, hence enhancing
the efficiency of photocatalysis [37–44]. Of course, their large spe-
cific surface areas, pore volume and porosity are also beneficial to
enhance their photocatalytic activity [45–47]. Further observation
shows that Ru has an obvious influence on photocatalytic activity of
Acknowledgements
This work was partially supported by the National Natural
Science Foundation of China (50625208, 20773097, 20877061
and 20803055). This work was also financially supported by the
National Basic Research Program of China (2007CB613302 and
2009CB939704).
References
[1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995)
69.
[2] J.H. Park, S. Kim, A.J. Bard, Nano Lett. 6 (2006) 24.
[3] X.Z. Li, F.B. Li, Environ. Sci. Technol. 35 (2001) 2381.
[4] J.G. Yu, W. Liu, H.G. Yu, Cryst. Growth Des. 8 (2008) 930.
[5] J.G. Yu, S.W. Liu, M.H. Zhou, J. Phys. Chem. C 112 (2008) 2050.
[6] J. Lin, P. Liu, M.J. Meziani, L.F. Allard, Y.P. Sun, J. Am. Chem. Soc. 124 (2002)
11514.
[7] V. Chhabra, V. Pillai, B.K. Mishra, A. Moron, D.O. Shah, Langmuir 11 (1995) 3307.
[8] G.J. Soler-Illia, M. Jobbagy, R.J. Candal, A.E. Regazzoni, M.A. Blesa, J. Dispersion
Sci. Technol. 19 (1998) 207.
[9] G.J. Soler-Illia, R.J. Candal, A.E. Regazzoni, M.A. Blesa, Chem. Mater. 9 (1997)
184.
[10] J.G. Yu, M.H. Zhou, B. Cheng, H.G. Yu, X.J. Zhao, J. Mol. Catal. A 227 (2005) 75.
[11] J.G. Yu, M.H. Zhou, B. Cheng, X.J. Zhao, J. Mol. Catal. A 246 (2006) 176.
[12] K. Ishibashi, A. Fujishima, T. Watanabe, K. Hashimoto, Electrochem. Commun.
2 (2000) 207.
[13] J.G. Yu, W.G. Wang, B. Cheng, B.L. Su, J. Phys. Chem. C 113 (2009) 6743.
[14] J.G. Yu, J.C. Yu, M.K.P. Lenug, W. Ho, B. Cheng, X.J. Zhao, J.C. Zhao, J. Catal. 217
(2003) 69.
[15] M.H. Zhou, J.G. Yu, B. Cheng, H.G. Yu, Mater. Chem. Phys. 93 (2005) 159.
[16] A. Fernandez, G. Lassaletta, V.M. Jimenez, A. Justo, A.R. Gonzalez-Elipe, J.M.
Herrmann, H. Tahiri, Y. Ait-Ichou, Appl. Catal. B 7 (1995) 49.
[17] M.H. Zhou, J.G. Yu, S.W. Liu, P.C. Zhai, B.B. Huang, Appl. Catal. B 89 (2009) 160.
[18] J.G. Yu, J.F. Xiong, B. Cheng, S.W. Liu, Appl. Catal. B 60 (2005) 211.
[19] J.G. Yu, G.H. Wang, B. Cheng, M.H. Zhou, Appl. Catal. B 69 (2007) 171.
[20] R. Wu, Y. Wei, Y. Zhang, Mater. Res. Bull. 34 (1999) 2131.
[21] H. Wang, J. Miao, J. Zhu, H. Ma, J. Zhu, H. Chen, Langmuir 20 (2004) 11738.
[22] S. Banerjee, A. Santhanam, A. Dhathathreyan, P.M. Rao, Langmuir 19 (2003)
5522.
[23] R. Zanella, L. Delannoy, C. Louis, Appl. Catal. A 291 (2005) 62.
[24] J. Subrt, V. Stengl, S. Bakardjieva, L. Szatmary, Powder Technol. 169 (2006) 33.
[25] E.P. Barrett, L.G. Joyner, P.H. Halenda, J. Am. Chem. Soc. 73 (1951) 373.
[26] N. Kallay, S. Zalac, G. Stefanic, Langmuir 9 (1993) 3457.
Fig. 8. The removal efficiency R(%) of acetone on the irradiated bimodal mesoporous
TiO2 microshpheres prepared at different Ru.

M. Zhou et al. / Journal of Molecular Catalysis A: Chemical 313 (2009) 107–113
113
[27] M. Ksibi, S. Rossignol, J.M. Tatibouët, C. Trapalis, Mater. Lett. 62 (2008) 4204.
[28] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T.
Siemieniewska, Pure Appl. Chem. 57 (1985) 603.
[29] A.K. Collins, G.M. Makrigiorgos, G.K. Svensson, Med. Phys. 21 (1994) 1741.
[30] H.M. Khan, M. Anwar, G. Ahmad, J. Radioanal. Nucl. Chem. Lett. 200 (1995) 521.
[31] R.W. Matthews, Radiat. Res. 83 (1980) 27.
[32] S.C. Ashawa, U.R. Kini, U. Madhvanath, Int. J. Appl. Radiat. Isot. 30 (1979) 7.
[33] W.A. Armstrong, R.A. Facey, D.W. Grant, W.G. Humphreys, Can. J. Chem. 41
(1963) 1575.
[37] J.G. Yu, H.G. Yu, H.T. Guo, M. Li, S. Mann, Small 4 (2008) 87.
[38] J.G. Yu, S.W. Liu, H.G. Yu, J. Catal. 249 (2007) 59.
[39] K.N.P. Kumar, J. Kumar, K. Keizer, J. Am. Ceram. Soc. 77 (1994) 1396.
[40] O. Carp, C.L. Huisman, A. Reller, Prog. Solid State Chem. 32 (2004) 33.
[41] K. Tanaka, M.F.V. Capule, T. Hisanaga, Chem. Phys. Lett. 187 (1991) 73.
[42] M. Anpo, T. Shima, S. Kodama, Y. Kubokawa, J. Phys. Chem. 91 (1987) 4305.
[43] C. Kormann, D.W. Bahnemann, M.R. Hoffmann, J. Phys. Chem. 92 (1988)
5196.
[44] G. Rothenberger, J. Moser, M. Graetzel, N. Serpone, D.K. Sharma, J. Am. Chem.
Soc. 107 (1985) 8054.
[34] G.J. Price, E.J. Lenz, Ultrasonics 31 (1993) 451.
[35] G.M. Makrigiorgos, J. Baranowska-Kortylewicz, E. Bump, S.K. Sahu, R.M.
Berman, A.I. Kassis, Int. J. Radiat. Biol. 63 (1993) 445.
[36] X. Fang, G. Mark, C. Sonntag, Ultrason. Sonochem. 3 (1996) 57.
[45] Q.Y. Li, L. Chen, G.X. Lu, J. Phys. Chem. C 111 (2007) 11494.
[46] J.G. Yu, Y.R. Su, B. Cheng, Adv. Funct. Mater. 17 (2007) 1984.
[47] M.H. Zhou, J.G. Yu, B. Cheng, J. Hazard. Mater. 137 (2006) 1838.