Phase control of hierarchically structured mesoporous anatase TiO
2
microspheres covered with {001} facets†
Zhao Zhao,
ab
Zaicheng Sun,
*
a
Haifeng Zhao,
a
Min Zheng,
a
Peng Du,
ab
Jialong Zhao
a
and Hongyou Fan
*
cd
Received 29th July 2012, Accepted 24th August 2012
DOI: 10.1039/c2jm35045a
The controlled synthesis of anatase titanium dioxide (TiO
2
) with both high surface area and high energy
facets is technologically important for its application in photocatalysis, photoelectrochemical cells, and
solar cells. Here we report a simple and fluorine free hydrothermal method to synthesize hierarchically
nanostructured mesoporous anatase TiO
2
spheres (MATS), which were covered with {001} facets.
Mild H
2
SO
4
was used as both a phase-inducer for the formation of the anatase phase and a capping
agent to promote oriented growth and formation of {001} facets. Detailed XRD and SEM studies
suggested that formation of MATS follows a typical nucleation and growth process. The refining or
reconstruction of TiO
2
crystal structure during growth resulted in a mesoporous crystalline framework
that exhibits enhanced adsorption and photocatalytic degradation of rhodamine B in comparison with
that of commercial Degussa P25 TiO
2
.
1. Introduction
Anatase titanium dioxide (TiO
2
) has attracted intensive research
interests during the past decades due to its wide applications in
photocatalysis, photoelectrochemical cells, solar cells, and smart
coatings.
1–10
The photocatalytic property of anatase TiO
2
crys-
tals has significantly relied on the access of high-energy facets
such as {001} and {100}. Thus, controlled synthesis of anatase
TiO
2
with both exposure of high-energy facets and high surface
area is technologically very important. The anatase TiO
2
crystal
has a tetragonal structure. The surface free energy of different
facets in anatase TiO
2
crystals has been calculated to be: 0.44 J
m
2
for {101} < 0.53 J m
2
for {100} < 0.90 J m
2
for {001}.
6,11
Both theoretical and experimental studies have demonstrated
that the high surface energy {001} facet exhibits the most active
photocatalytic properties. However, due to its high surface
energy, {001} facets often diminish rapidly during the growth
process of the anatase TiO
2
crystals. Consequently, anatase TiO
2
crystals are usually dominated by the low energy {101} facet.
Recently, it has been shown that by using a capping agent, or
with a co-capping agent containing HF, anatase TiO
2
micro-
crystals with a high percentage exposure of {001} facets were
synthesized.
6,12–18
Based on theoretical calculation and surface
analysis,
13,14
it has been understood that the fluorine atoms
preferentially bind or absorb on the {001} facet, which results in
exceptional stabilization of the {001} facet in fluorine-terminated
anatase TiO
2
crystals. Later, this idea was extended for utiliza-
tion of other fluorine or amine capping agents such as
NH
4
HF
2
,
19
NH
4
F,
16
BF
4
,
20
amine,
21,23
etc. Despite these
successes, highly toxic and corrosive fluorinated compounds
have been extensively employed in the synthesis of TiO
2
crystals
to achieve high-energy facets. In addition, removal of surface F
from exposed {001} facets was difficult due to strong interactions
between F
and TiO
2
facets. In order to avoid the use of fluo-
rinated compounds, development of a simple and fluorine free
synthesis route to achieve anatase TiO
2
crystals with exposed
high energy facets is highly desirable.
21–24
Although the micro-
crystal TiO
2
with {001} facets showed a higher photo-catalytic
activity, low surface area still is another barrier for practical
application.
14
Control of nanoporous crystal frameworks with
high surface area, such as hollow and/or mesoporous parti-
cles,
16,25–28
will be also technologically very important for
photocatalytic applications.
29–31
Herein, we report a simple and fluorine free hydrothermal
method to synthesize mesoporous anatase TiO
2
spheres
(MATS), with a surface covered solely with {001} facets. We
performed the hydrolysis and condensation of tetrabutyl titanate
(TBT) in 2 mol L
1
H
2
SO
4
solution during hydrothermal
processes. We found that H
2
SO
4
acted as both a phase inducer
for the anatase phase and a capping agent that effectively
stabilizes only the {001} facet, which leads to a high coverage of
a
State Key Laboratory of Luminescence and Applications, Changchun
Institute of Optics, Fine Mechanics and Physics, Chinese Academy of
Sciences, 3888 East Nanhu Road, Changchun 130033, People’s Republic
of China. E-mail: sunzc@ciomp.ac.cn
b
Graduate University of Chinese Academy of Sciences, Beijing, 100039,
People’s Republic of China
c
The University of New Mexico/NSF Center for Micro-Engineered
Materials, Department of Chemical and Nuclear Engineering,
Albuquerque, New Mexico 87131, United States
d
Sandia National Laboratories, Advanced Materials Laboratory, 1001
University Blvd. SE, Albuquerque, New Mexico 87106, United States.
E-mail: hfan@sandia.gov
† Electronic supplementary information (ESI) available: More XRD
patterns, SEM and TEM images. See DOI: 10.1039/c2jm35045a
This journal is ª The Royal Society of Chemistry 2012 J. Mater. Chem., 2012, 22, 21965–21971 | 21965
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