A facile hydrothermal method for the controllable synthesis of TiO2 nanocrystals with tunable shapes

Article abstract of DOI:10.1039/c5ra13006a

The controllable preparation of anatase TiO₂ nanocrystals with tunable shapes has drawn much attention recently. The general method used to prepare such nano-TiO₂ involves the use of HF as a surfactant. However, the use of HF should be avoided due to its high risk. Therefore, the development of facile and friendly methods to controllably synthesize anatase TiO₂ nanocrystals is important. In this research, instead of general titanium precursors, titanium peroxide sol was used as the raw material to synthesize anatase TiO₂ nanocrystals due to its high stability in water. Oleic acid and oleylamine were used as capping surfactants. A hydrothermal method, without the use of high risk HF, was developed to prepare uniform anatase TiO₂ nanocrystals with various shapes, which can be tuned from octahedral bipyramidal, to truncated octahedral bipyramidal, and then to nano-sheets. The ratio between {001} and {101} surfaces can be easily tuned by changing the amounts of oleic acid and oleylamine used. The photocatalytic activity of these anatase TiO₂ nanocrystals was also evaluated through crystal violet photo-degradation under UV light illumination. It was shown that the anatase TiO₂ nanocrystals can present a photocatalytic activity comparable to that of commercial P25.

Full text of DOI:10.1039/c5ra13006a

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A facile hydrothermal method for the controllable
synthesis of TiO2 nanocrystals with tunable shapes
Cite this: RSC Adv., 2015, 5, 103386
*
Shengchao Nie, Xiujian Zhao and Baoshun Liu
The controllable preparation of anatase TiO2 nanocrystals with tunable shapes has drawn much attention
recently. The general method used to prepare such nano-TiO2 involves the use of HF as a surfactant.
However, the use of HF should be avoided due to its high risk. Therefore, the development of facile and
friendly methods to controllably synthesize anatase TiO2 nanocrystals is important. In this research,
instead of general titanium precursors, titanium peroxide sol was used as the raw material to synthesize
anatase TiO2 nanocrystals due to its high stability in water. Oleic acid and oleylamine were used as
capping surfactants. A hydrothermal method, without the use of high risk HF, was developed to prepare
uniform anatase TiO2 nanocrystals with various shapes, which can be tuned from octahedral bipyramidal,
to truncated octahedral bipyramidal, and then to nano-sheets. The ratio between {001} and {101}
surfaces can be easily tuned by changing the amounts of oleic acid and oleylamine used. The
photocatalytic activity of these anatase TiO2 nanocrystals was also evaluated through crystal violet
photo-degradation under UV light illumination. It was shown that the anatase TiO2 nanocrystals can
present a photocatalytic activity comparable to that of commercial P25.
Received 4th July 2015
Accepted 11th November 2015
DOI: 10.1039/c5ra13006a
www.rsc.org/advances
preferable growth along the [101] direction, in order to get
nano-TiO2 with highly exposed {001} facets. Aer the rst
Introduction
synthesis of TiO2 nanocrystals with highly exposed {001} facets
from TiF4 solution via a hydrothermal method,33 much effort
has been made in the area due to great interest. In addition to
pure materials, nitrogen doped anatase TiO2 nanocrystals with
highly exposed {001} facets were also prepared via the hydro-
thermal treatment of TiN powder in HF solution.31
Nowadays, nano-TiO2 has become a key functional material in
the elds of photocatalysis, dye-sensitized solar cells (DSSCs),
lithium batteries, and others.1–10 It was shown that its properties
are closely dependent on the morphology and strcuture.11–19 For
example, it was reported that the controllable synthesis of
anatase TiO2 nanocrystals with various exposed facets is
important for improving the photocatalytic activity.20–26 From
the viewpoint of thermodynamics, anatase TiO2 has a truncated
octahedral bipyramidal shape that is surrounded by eight {101}
facets and two {001} facets. This is a very prevalent crystal shape
in nature. Moreover, theoretical studies have shown that the
order of average surface energies is g{110} (1.09 J mÀ2) > g{001}
(0.90 J mÀ2) > g{100} (0.53 J mÀ2) > g{101} (0.44 J mÀ2).27 The
high energy {001} facets may be more reactive in the dissociative
adsorption of reactant molecules than the {101} facets, there-
fore it was shown that TiO2 with highly exposed {001} facets
should present better performances in both photocatalysis and
DSSCs.28–31 From this viewpoint, the synthesis of such anatase
TiO2 nanocrystals is still interesting for us.
Generally, highly concentrated HF solution is needed to grow
anatase TiO2 nanocrystals with highly exposed {101} facets. Lu
et al. obtained anatase TiO2 microcrystals with 64% {001} facets
exposed by using HF as a shape controlling agent;29 this method
can be used to further increase the percentage of exposed {001}
facets to 89%, resulting in the formation of nano-sheets.34,35 The
big problem is that HF is undesirable due to its high toxicity.
Also, the HF-induced TiO2 nanocrystals always have a large size,
which inevitably reduces the specic surface area and limits
their applications. In addition to HF, the use of other surfac-
tants has also been attempted. For example, Zhao et al. obtained
mesoporous TiO2 microspheres covered predominantly with
{001} surfaces using H2SO4 as the surfactant via a hydrothermal
method.36 Xia et al. obtained uniform tiny TiO2 nanocrystals
with exposed {101} facets by treating an electrospun TiO2–pol-
yvinylphenol (PVP) complexes in a hydrothermal reaction.37 It
was shown that the shape of the TiO2 nanocrystals depends on
the pH. In comparison with the HF-induced method, the TiO2
nanocrystals prepared by Xia et al. had a much smaller size.
With diethylenediamine and isopropyl alcohol being used as
surfactants, Chen et al. produced hierarchical spheres
The preparation of anatase TiO2 nanocrystals with highly
exposed {001} facets is difficult as high-energy facets generally
diminish during TiO2 nanocrystal growth.32 Some special
surfactants or raw materials are needed to prohibit the
State Key Laboratory of Silicate Materials for Architectures Wuhan University of
Technology, Luoshi Road 122, Wuhan City, Hubei Province, 430070, P. R. China.
E-mail: liubaoshun@126.com; Fax: +86-027-87669729
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composed of ultrathin anatase TiO2 nanosheets with nearly produce the nal product. The amounts of titanium peroxide
100% exposed {001} facets. This material had a specic surface sol and ethanol were kept unchanged, and the amounts of OA
area of 170 m2 gÀ1
.
Do et al. reported a non-aqueous sol- and OM were adjusted for various experiments, which were
38
vothermal method to prepare anatase TiO2 nanocrystals in classied into three groups, as shown in Table 1. Lastly, the
rhombic, truncated rhombic, spherical, and dog-bone shapes.39 TiO2 products were washed and cleaned using ethanol several
For this method, ethanol, titanium butoxide (TBT), oleic acid times to remove the physically adsorbed residual organic
(OA), and oleylamine (OM) were used as the solvent, the tita- surfactant, and then were dispersed in toluene for further
nium precursor, and the two capping surfactants, respectively. characterization.
Water vapour was used as the hydrolysis agent for TBT to
control the reaction rate.
Characterization
The controllable growth of TiO2 nanocrystals with perfect
shapes needs a low reaction speed, so non-aqueous conditions
were favorable. However, non-aqueous conditions affect the
product yield of TiO2 nanocrystals due to the low amount of
water and low reaction speed. From this viewpoint, the devel-
opment of a green and risk-less hydrothermal method to
prepare such TiO2 materials is necessary and attractive. This
means that the titanium precursor should have a low reaction
speed. Our previous research has shown that titanium peroxide
sol is very stable in air, and can be kept for several months.39–41 It
is a good titanium precursor for growing anatase TiO2 nano-
crystals without using any reaction inhibitors via a hydro-
thermal route. In the present research, using OA and OM as the
capping surfactants, a facile and harmless hydrothermal
method was invented to prepare anatase TiO2 nanocrystals with
exposed {101} facets. The {001}/{101} ratio can be well
controlled, and the shapes of the anatase TiO2 nanocrystals can
be tuned from octahedral bipyramidal, to truncated octahedral
bipyramidal, and then to nano-sheets. Their application in
photocatalysis was also studied through the photodegradation
of crystal violet (CV) under UV light illumination. It was shown
that the anatase TiO2 nanocrystals present a photocatalytic
activity comparable to that of commercial P25.
X-ray diffraction (XRD) patterns of the anatase TiO2 nano-
crystals were measured using a D/Max-IIIA X-ray diffraction
meter in a 2q mode from 10ꢀ to 80ꢀ. High-resolution trans-
mission electron microscope (HR-TEM) images were obtained
using transmittance electron microscopy (HITACHI, H-600
STEM/EDX PV910). The thermo-gravimetric analysis (TG-DTA)
was carried Ào1ut under a ow of air with a temperature ramp
ꢀ
of 5 C min using a thermal analyser (Germany, NETZSCH,
STA449c/3/G). Infrared absorption spectra were recorded using
Nexus Fourier transform infrared spectroscopy (FT-IR) in the
range of 400–2000 cmÀ1 (America, Thermo-Nicolet company).
Photocatalytic measurement
Firstly, 0.04 g of TiO2 nanocrystal powder was mixed with
ethanol in a f 100 mm glass container, and was then ultra-
sonically dispersed for ca. 30 min. The ethanol was removedÀb1y
heating at 70 ꢀC, and then 50 mL of crystal violet (CV, 6 mg L
)
was slowly added into the glass container. The adsorption and
photocatalytic abilities of the TiO2 samples were evaluated by
measuring the UV-Vis absorption spectra of the CV aqueous
solution at different reaction intervals. A 15 W (TOSHIBA, 365
nm) UV uorescent lamp was used as the light source. The UV
light intensity was measured to be ca. 1.0 mW cmÀ2
.
Experimental
Titanium peroxide sol preparation
Results and discussion
Fig. 1 shows pictures of the titanium peroxide sol (A), the mixed
solution aer the hydrothermal reaction (B), and the toluene
8 mL of TiCl4 was added to 500 mL of ice-cold distilled water (ca.
0–5 ꢀC) under continuous magnetron stirring. Several hours
later, the TiCl4–water solution became transparent. Then, 1 : 5
ammonia was slowly added to the TiCl4–water solution, to
produce a Ti(OH)4 precipitate, until a pH of ca. 7 was reached.
The Ti(OH)4 sedimentation was kept for ca. 12 h, and then it was
ltered and cleaned for several times using ionized water. The
cleaned Ti(OH)4 sedimentation was dispersed in cold distilled
water (ꢁ4 ꢀC) again, and then 80 mL of H2O2 was slowly added.
The mixed water solution was kept stirring until it became
a yellow transparent titanium peroxide sol.
Table 1 Experimental parameters for the preparation of TiO2 nano-
crystals using a hydrothermal method
Titanium peroxide
OA (mL)
OM (mL)
sol (mL)
Ethanol (mL)
Group-1
2
5
8.5
10
12
0
3
6
9
7
12
10
8.5
5
2
7
7
7
7
0
20
20
20
20
20
20
20
20
20
20
20
20
20
33
33
33
33
33
33
33
33
33
33
33
33
33
Synthesis of the TiO2 nanocrystals
Group-2
Group-3
20 mL of the yellow titanium peroxide sol was placed in
a Teon-closed steel autoclave. The OA and OM were dissolved
in 33 mL of ethanol. Aerwards, the OA and OM ethanol solu-
tions were mixed with the titanium peroxide sol in the auto-
clave, which was then kept stirring for some minutes. The
mixed solution was hydrothermally treated at 150 ꢀC for 18 h to
7
7
7
3
6
9
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Fig. 1 Digital pictures of the titanium peroxide sol (A), the mixed
solution after hydrothermal treatment (B), and the transparent toluene
solution containing as-prepared anatase TiO2 nanocrystals.
solution that contains the TiO2 nanocrystals (C). The titanium
peroxide sol is a yellow transparent water-based sol, which is
very stable and can be kept for several months at room
temperature and under an air atmosphere. Aer hydrothermal
treatment, it could be seen that the yellow transparent solution
changed to a mixture solution with three layers, as shown in
Fig. 1(B). The top, middle, and bottom layers are the OA and OM
ethanol solution, the white TiO2 nanocrystal solution, and the
water–ethanol solution, respectively. The TiO2 nanocrystals
should be capped with OA and OM molecules, so they are both
hydrophobic and oleophilic, which makes the TiO2 sample
locate between the oil and water layers. Aer the solutions of the
top and bottom layers were carefully removed, the middle layer
that contains the TiO2 sample was le, they were washed with
ethanol several times to remove the physically absorbed OA and
OM molecules. The white TiO2 sample was lastly dispersed in
toluene for further TEM checking; the dispersion can form an
optically transparent solution, as shown in Fig. 1(C).
Fig. 2 TEM images of TiO2 nanocrystals prepared according to group-
1: (A) OA: 2 mL, OM: 12 mL; (B) OA: 5 mL, OM: 10 mL; (C) OA: 8.5 mL,
OM: 8.5 mL; (D) OA: 10 mL, OM: 5 mL; (E) OA: 12 mL, oleylamine 2 mL;
(F) TEM image of TiO2 sample prepared by hydrothermal treatment
without using OA and OM.
Fig. 2 shows TEM images of the as-prepared TiO2 samples
that were prepared according to the parameters of group-1
(Table 1). In these experiments, the OA amount was increased
from 2 mL to 12 mL, and the OM amount was decreased from 12
mL to 2 mL. When 2 mL of OA and 12 mL of OM were used, it
can be seen from Fig. 2(A) that the hydrothermal product is
composed of very small uniform TiO2 nanocrystals, which have
an octahedral bipyramidal shape. Obviously, they belong to
anatase TiO2 single nanocrystals. The nanocrystals are sur-
rounded by eight {101} surfaces, and almost no {001} exposed
facets can be seen. The ratio of exposed {001} and exposed {101}
is ca. 0. When the OA amount was increased to 5 mL, and the
OM amount was decreased to 10 mL, the hydrothermal product
changed to uniform nanocrystals with a truncated octahedral
bipyramidal shape. These TiO2 nanocrystals contain both
exposed {101} and {001} facets (Fig. 2(B)). By selecting some of
the nanocrystals, the average ratio between the exposed {001}
and {101} facets was calculated to be ꢁ7%. The further increase
of the OA amount to 8.5 mL and the further decrease of the OM
amount to 8.5 mL changed the shape of the hydrothermal
sample to nanosheets with a thickness of ca. 6 nm (Fig. 2(C)).
The high-resolution TEM image of one stood nanosheet is
shown in the top-right corner of the gure. It can be seen that
the TiO2 nanosheets are in the shape of truncated octahedrons.
The {001}/{101} ratio for this sample is ꢁ60%. Aer this, the
continual increase of the OA amount and the decrease of the
OM amount can no longer change the shape of the TiO2
nanocrystals, and they are still in the form of nano-sheets,
although their sizes tend to become smaller. As no stood TiO2
nano-sheets can be seen, although the thicknesses of these
samples cannot be decided, they should be thinner than the
sample shown in Fig. 2(C). For comparison, the sample
prepared without using OA and OM is shown Fig. 2(F), and it
can be seen that the TiO2 nanoparticles have an irregular shape.
Therefore, the above results show that the OA and the OM are
important to control the shape evolution of the TiO2 nano-
crystals even in hydrothermal environments, in addition to the
non-aqueous methods. It is known that the OA tends to control
the formation of the {101} surface, and OM tends to limit the
growth of the TiO2 nano-crystals along the [001] direction. As
a result, by using the titanium peroxide sol as the titanium
precursor, the anatase TiO2 nanocrystals with the tunable
shapes and the controllable {001}/{101} ratio can be prepared by
just using a facile hydrothermal method.
Fig. 3 shows the XRD patterns of the TiO2 samples prepared
according to the experimental parameters of group 1. All of the
samples can be indexed as anatase TiO2. With the increase of
the OA amount and the decrease of OM amount, the X-ray dif-
fractive intensity of the (004) lattice becomes weaker and
weaker, showing that the thermodynamically preferable growth
of TiO2 nanocrystals along the [001] direction is limited. At the
same time, it can be seen that the intensity ratio of the (101)
peak and the (004) peak increases, suggesting that the shape of
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Fig. 3 XRD patterns of TiO2 nanocrystals prepared according to
group-1: (A) OA: 2 mL, OM: 12 mL; (B) OA: 5 mL, OM: 10 mL; (C) OA:
8.5 mL, OM: 8.5 mL; (D) OA: 10 mL, OM: 5 mL; (E) OA; 12 mL, OM: 2
mL.
the anatase TiO2 nanocrystal has altered from octahedral
bipyramidal, to truncated octahedral bipyramidal, and then to
nano-sheets, which accords with the TEM analysis.
To clearly show the role of the OM in the controllable growth
of the anatase TiO2 nanocrystals, the OM amount was kept
unchanged at 7 mL during the hydrothermal reaction, while the
OA amount was changed from 0 mL to 9 mL, as shown in Table 1
(group 2). Fig. 4 shows the TEM images of the TiO2 samples for
group 2. When the OA was not used, the as-prepared anatase
TiO2 nanocrystals were in the shape of an octahedral bipyramid
(Fig. 4(A)), with less {001} surfaces being exposed. As the OA
amount was increased to 3 mL, the shape of the anatase TiO2
nanocrystals transformed to a truncated octahedral bipyramid
(Fig. 4(B)). One nanocrystal is shown in Fig. 4(C), and the angle
labelled is 68.3ꢀ, which indicates a good anatase crystalline
phase. The magnication of Fig. 4(C) is shown in Fig. 4(D), three
sets of crystalline facets for anatase are labelled, which corre-
spond to the (À101), (002), and (101) crystalline lattices of
anatase according to XRD analysis. We roughly estimated the
percentage of exposed {001} facets for the TiO2 nanocrystal in
Fig. 4(C), which is ca. 8%. As the OA amount was increased to 6
mL, most of the TiO2 nanocrystals were nanosheets (Fig. 4(E)),
although there are also some in a truncated octahedral bipyra-
midal shape. When the OA amount was increased to 9 mL, it can
be seen from Fig. 4(F) that all of the TiO2 nanocrystals are in the
shape of nano-sheets. The above results gave us a clear conclu-
sion that OA plays an important role in the formation of exposed
{101} facets under the conditions of hydrothermal reactions.
Similarly, in order to conrm the function of the OA for
preparing the shape-controllable anatase TiO2 nanocrystals, the
amount of OA was kept unchanged at 7 mL, while the OM
amount was changed from 0 mL to 9 mL, as shown in Table 1
(group 3). Fig. 5 shows the TEM images of these TiO2 samples.
Conversely, it can be seen that all of the anatase TiO2
Fig. 4 TEM images of TiO2 nanocrystals prepared according to group-
2: (A) oleic acid 7 mL, oleylamine 0 mL; (B) oleic acid 7 mL, oleylamine 3
mL; (C) one crystal in (B); (B) magnification of (C); (E) oleic acid 7 mL,
oleylamine 6 mL; (F) oleic acid 7 mL, oleylamine 9 mL.
nanocrystals are in the shape of nano-sheets, almost indepen-
dent on the OM amount. Therefore, it seems that OA has
a stronger effect on the control of TiO2 nanocrystal growth,
compared to OM, because the use of 9 mL of OM cannot induce
an obvious change of the TiO2 nanocrystal shape, which is still
in the form of nano-sheets.
The TiO2 sample (Fig. 2(B)) was rstly dried at 30 ꢀC, and was
then balled as TiO2 powder for TG-DTA analysis, the results of
which are shown in Fig. 6. The rst ꢁ1.5% weight-loss
ꢀ
happening at 69.1 C corresponds to an endothermic process,
which is the evaporation of the organic toluene. The second
ꢁ16.18% weight-loss happening at 340.9 ꢀC is an exothermic
process, corresponding to the decomposition and oxidation of
the OA and the OM adsorbed on the TiO2 surface. The
ꢀ
exothermic peak at ꢁ652.5 C on the DTA curve is due to the
anatase-to-rutile transition as there is no weight-loss at this
temperature. In summary, the TG-DTA analysis showed that the
TiO2 sample contains ꢁ16.2% organic species. The FT-IR
spectra of the TiO2 samples prepared with and without using
the OA and the OM are shown in Fig. 7. If the OM and the OA
were used during the hydrothermal treatment, four additional
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Fig. 7 FT-IR spectra of the TiO2 samples prepared with and without
using OA and OM in the hydrothermal reactions.
Fig. 5 TEM images of TiO2 nanocrystals prepared according to group-
3: (A) oleic acid 0 mL, oleylamine 7 mL; (B) oleic acid 3 mL, oleylamine 7
mL; (C) oleic acid 6 mL, oleylamine 7 mL; (D) oleic acid 9 mL, oleyl-
amine 7 mL.
The theory of Wulff construction tells us that the shapes of
anatase TiO2 nanocrystals are mainly controlled by the growth
speed along the [001] and [101] directions. Due to the high
surface energy of the {001} surface (0.9 J mÀ2), anatase TiO2
nanocrystals generally prefer to grow along the [001] direction,
which leads to the disappearance of the {001} surfaces. As
indicated by our results and another study,38 the OA molecules
would prefer to adsorb onto the {001} surfaces, which prevents
the TiO2 nanocrystal growth along the [001] directions, resulting
in the formation of octahedral bipyramidal structures. If the
OM was also used during the hydrothermal reactions, our above
observations indicate that the OM molecules can be preferably
and strongly adsorbed onto the {101} surface,38 which limits the
anatase TiO2 nanocrystal growth along the [101] direction. In
this case, the anatase TiO2 nano-crystals will grow along both
Fig. 6 TG-DTA results of the TiO2 sample prepared using 5 mL of OA
and 10 mL of OM.
strong adsorption peaks at 1462, 1426, 2852, and 2922 cmÀ1
appear in the spectrum (red line). The FT-IR absorption peak at
1462 cmÀ1 corresponds to the bending vibration of a –CH bond,
and the peak at 1526 cmÀ1 is due to the vibration of a N–H
bond, which indicate the existence of the OM molecules on the
TiO2 nanocrystal surface. In addition, the peaks at 2852 and
1922 cmÀ1 are from the stretching vibrations of –CH3, CH2, or
CH groups, also indicating the presence of OA on the TiO2
surface. In order to remove these surface-capped organics, the
TiO2 sample (Fig. 2(B)) was annealed at 450 ꢀC for an hour; the
TEM image of the annealed TiO2 sample is shown in Fig. 8.
Although the annealing process can remove the adsorbed
organics, it can be seen that the shape of the truncated octa-
hedrons were destroyed.
Fig. 8 TEM image of the annealed TiO2 sample prepared by the
hydrothermal treatment using 5 mL of OA and 10 mL of OM.
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the [001] and [101] directions, which nally leads to the and can be kept for several months without any obvious
formation of the truncated octahedral bipyramidal shapes and changes. Under heating, titanium peroxide sol slowly decom-
the nano-sheets, as shown in Fig. 2. By altering the ratio poses and transforms into titanium oxide clusters, which can be
between the OA amount and the OM amount, the {001}/{101} uniformly crystallized during hydrothermal treatment under
ratio of anatase TiO2 nanocrystals can be altered. Further- neutral conditions without using an inhibitor to suppress the
more, our results show that the OA seems to be more highly reaction speed. The slow reaction speed allows the anchoring of
functional at controlling the anatase TiO2 nanocrystal shape OA and OM molecules on the TiO2 cluster surface, which nally
compared to the OM.
controls the shape of the anatase TiO2 nanocrystals. As a large
The other titanium precursors, such as tetrabutyl titanate amount of water exists during the hydrothermal reaction, the
and titanium isopropoxide, will suffer from fast hydrolysis reaction speed should be faster than that for the non-aqueous
during the hydrothermal reaction in the presence of a large solvothermal method and therefore a high production yield
amount of water if no acid is used. The hydrolysed product is can be realized. From this viewpoint, the facile hydrothermal
located at the bottom of the autoclave, which prevents good method without the use of highly toxic HF to control the anatase
contact between the titanium, OA, and OM. This disadvantage TiO2 nanocrystal shape is feasible if titanium peroxide sol is
limits the accurate controlling of the anatase TiO2 nanocrystal used as the titanium precursor. The formation mechanism for
shape. As we have discussed above, non-aqueous methods were the anatase TiO2 nanocrystals is shown in Fig. 9.
generally used to prepare these TiO2 samples. For example, Wu
The application of such TiO2 nano-crystals in photocatalysis
et al. used OA and titanium isopropoxide as capping surfactant was evaluated. Three anatase TiO2 nanocrystal samples with
and titanium precursor to prepare TiO2 nanosheets and TiO2 different exposed {001}/{101} ratios were chosen to study the
nanocrystals with highly exposed {001} and {001} facets via
a non-aqueous solvothermal method, with a small amount of
water being used as the oxygen source;34 Dinh et al. used tet-
rabutyl titanate as a titanium source to prepare TiO2 nano-
crystals with various shapes through
a
non-aqueous
solvothermal method, with the OA and the OM being also
used as the capping surfactants.38 As only a tiny amount of water
is allowed during the non-aqueous solvothermal reaction, the
controllable growth of anatase TiO2 nanocrystals can be real-
ized. However, it is difficult to achieve the high production yield
of anatase TiO2 nanocrystals due to the presence of a small
amount of water. Compared with tetrabutyl titanate and tita-
nium isopropoxide, titanium peroxide sol is much more stable,
Fig. 10 (A) CV dye adsorption for TiO2 sample A (OA: 2 mL, OM: 12
mL), B (OA: 5 mL, OM: 10 mL), C (OA: 8.5 mL, OM: 8.5 mL), and the
Fig. 9 Diagram of the anatase TiO2 growth along the [001] and [101] commercial P25; (B) their photocatalytic performance in the degra-
directions.
dation of CV in comparison with the commercial P25.
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photocatalytic activity, with commercial P25 being used as
2 X. Li, P. Liu, Y. Mao, M. Xing and J. Zhang, Appl. Catal., B,
2015, 164, 352–359.
a
reference. Considering that CV photodegradation is
a complicated process, in which adsorption onto the TiO2
surface is the rst step, an adsorption experiment was carried
out to compare the adsorption ability of the different samples.
Fig. 10(A) shows the change in the CV concentration aer
different periods of dark adsorption. The adsorption of CV onto
the TiO2 surface almost reaches equilibrium aer ꢁ2 h. The P25
TiO2 has the lowest adsorption capacity, while our samples
show obvious adsorptions. The adsorption abilities decrease as
the {001}/{101} ratio increases. It was considered that the cap-
ped surfactants on the anatase TiO2 nanocrystals should play an
important role in CV adsorption.
3 S. Banerjee, D. D. Dionysiou and S. C. Pillaic, Appl. Catal., B,
2015, 176–177, 396–428.
4 S. Horikoshi, H. Tsutsumi, H. Matsuzaki, A. Furube,
A. V. Emeline and N. Serpone, J. Mater. Chem. C, 2015, 3,
5958–5969.
5 J. Schneider, M. Matsuoka, M. Takeuchi, J. Zhang,
Y. Horiuchi, M. Anpo and D. W. Bahnemann, Chem. Rev.,
2014, 114, 9919–9986.
6 X. Lang, X. Chen and J. Zhao, Chem. Soc. Rev., 2014, 43, 473–
486.
7 D. M. Schultz and T. P. Yoon, Science, 2014, 343, 6174.
8 M. Wang, J. Ioccozia, L. Sun, C. Lin and Z. Lin, Energy
Environ. Sci., 2014, 7, 2182–2202.
9 S. Banerjee, S. C. Pillai, P. Falaras, K. E. O’Shea, J. A. Byrne
and D. D. Dionysiou, J. Phys. Chem. Lett., 2014, 5, 2543–2554.
Fig. 10(B) shows the dependencies of the C/C0 values of the
CV solutions on the time of UV light irradiation. All of the
samples show photocatalytic activity. It was observed that the
sample with the highest {001}/{101} ratio has the lowest pho-
tocatalytic activity, i.e., photocatalytic activity decreases as the 10 J. S. DuChene, B. C. Sweeny, A. C. Johnston-Peck, D. Su,
{001}/{101} ratio increases. This result is different from other
research, which reported that highly exposed {001} facets are
E. A. Stach and W. D. Wei, Angew. Chem., Int. Ed., 2014, 53,
7887–7891.
favourable for photocatalysis. In the present research, the OA 11 H. Xu, G. Li, G. Zhu, K. Zhu and S. Jin, Catal. Commun., 2015,
adsorbing onto the {001} surface of the anatase TiO2 nano- 62, 52–56.
crystals may change the surface energy and structures, which 12 X. Chen, L. Liu, P. Y. Yu and S. S. Mao, Science, 2011, 331,
limits the positive function of the {001} surfaces. Combined 746–750.
with the results in Fig. 10(A), it was also considered that the 13 S. Liu, J. Yu and M. Jaroniec, J. Am. Chem. Soc., 2010, 132,
adsorption of CV onto the TiO2 nanocrystals may also have an 11914–11916.
effect on the photocatalytic activity, as the high adsorption 14 Q. Xiang, J. Yu and M. Jaronie, Chem. Commun., 2011, 47,
ability can capture more CV molecules on the TiO2 surface. 4532–4534.
Fig. 10(B) also shows that the photocatalytic activity of our 15 J. Ye, W. Liu, J. Cai, S. Chen, X. Zhao, H. Zhou and L. Qi, J.
sample is comparable to that of commercial P25 TiO2.
Am. Chem. Soc., 2011, 133, 933–940.
16 X. Yu, B. Kim and Y. K. Kim, ACS Catal., 2013, 3, 2479–2486.
17 F. Zuo, L. Wang, T. Wu, Z. Zhang, D. Borchardt and P. Feng,
J. Am. Chem. Soc., 2010, 132, 11856–11857.
Conclusions
18 C. Hu, X. Zhang, W. Li, Y. Yan, G. Xi, H. Yang, J. Li and
H. Bai, J. Mater. Chem. A, 2014, 2, 2040–2043.
19 J. Zhang, L. Qian, W. Fu, J. Xi and Z. Ji, J. Am. Ceram. Soc.,
2014, 97, 2615–2622.
20 K. Chen, Z. Jiang, J. Qin, Y. Jiang, R. Li, H. Tang and X. Yang,
Ceram. Int., 2014, 40, 16817–16823.
21 M.-V. Soanou, M. Tassi, N. Boukos, S. Thanos, T. Vaimakis,
J. Yu and C. Trapalis, Catal. Today, 2014, 230, 125–130.
22 B. Li, Z. Zhao, F. Gao, X. Wang and J. Qiu, Appl. Catal., 2014,
147, 958–964.
23 N. Sutradhar, A. K. Biswas, S. K. Pahari, B. Ganguly and
A. B. Panda, Chem. Commun., 2014, 50, 11529–11532.
24 B. Li, Z. Zhao, Q. Zhou, B. Meng, X. Meng and J. Qiu, Chem.–
Eur. J., 2014, 20, 14763–14770.
In the present research, using titanium peroxide sol as the
titanium precursor, a facile and mild hydrothermal method was
invented to prepare anatase TiO2 nanocrystals with tunable
shapes and exposed {001}/{101} ratios under neutral conditions.
In comparison with other titanium precursors, the titanium
peroxide sol is stable under an air atmosphere, and it can slowly
decompose and transform into TiO2 under hydrothermal
treatment, which is determined for controlling the shape of the
anatase TiO2 nanocrystals prepared using the hydrothermal
method. In the photodegradation of CV dyes, the anatase TiO2
nanocrystals have photocatalytic activity comparable to that of
commercial P25.
Acknowledgements
25 J. Yu, J. Fan and K. Lv, Nanoscale, 2010, 2, 2144–2149.
26 K. Hou, B. Tian, F. Li, Z. Bian, D. Zhao and C. Huang, J.
Mater. Chem., 2005, 15, 2414–2420.
27 W. Yang, F. Wan, Q. Chen, J. Li and D. Xu, J. Mater. Chem.,
2010, 20, 2870–2876.
B. Liu thanks the Central Universities for the Fundamental
Research Funds (Wuhan University of Technology, 2012-IV-
008).
28 Y. Zhang, Z. Xie and J. Wang, ACS Appl. Mater. Interfaces,
2009, 1, 2789–2795.
29 H. G. Yang, G. Liu, S. Z. Qiao, C. H. Sun, Y. G. Jin, S. C. Smith,
J. Zou, H. M. Cheng and G. Q. Lu, J. Am. Chem. Soc., 2009,
131, 4078–4083.
Notes and references
1 Y. Wu, D. Chu, P. Yang, Y. Dua and C. Lu, Catal. Sci. Technol.,
2015, 5, 3375–3382.
103392 | RSC Adv., 2015, 5, 103386–103393
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
30 G. Liu, L. Z. Wang, C. H. Sun, X. X. Yan, X. W. Wang, 36 Z. Zhao, Z. Sun, H. Zhao, M. Zheng, P. Du, J. Zhao and
Z. G. Chen, S. C. Smith, H. M. Cheng and G. Q. Lu, Chem.
Mater., 2009, 21, 1266.
31 G. Liu, H. G. Yang, X. Wang, L. Cheng, J. Pan, G. Q. Lu and
H. M. Cheng, J. Am. Chem. Soc., 2009, 131, 12868–12869.
32 K. Nakata and A. Fujishima, J. Photochem. Photobiol., C, 2012,
13, 169.
H. Fan, J. Mater. Chem., 2012, 22, 21965–21971.
37 Y. Dai, C. M. Cobley, J. Zeng, Y. Sun and Y. Xia, Nano Lett.,
2009, 9, 2455–2459.
38 J. S. Chen, Y. L. Tan, C. M. Li, Y. L. Cheah, D. Luan,
S. Madhavi, F. Y. C. Boey, L. A. Archer and X. W. Lou, J.
Am. Chem. Soc., 2010, 132, 6124–6130.
33 H. G. Yang, C. H. Sun, S. Z. Qiao, J. Zou, G. Liu, S. C. Smith, 39 C.-T. Dinh, T.-D. Nguyen, F. Kleitz and T.-O. Do, ACS Nano,
H. M. Cheng and G. Q. Lu, Nature, 2008, 453, 638. 2009, 3, 3737–3743.
34 B. Wu, C. Guo, N. Zheng, Z. Xie and G. D. Stucky, J. Am. 40 B. Liu, X. Wang, G. Cai, L. Wen, Y. Song and X. Zhao, J.
Chem. Soc., 2008, 130, 17563. Hazard. Mater., 2009, 169, 1112–1118.
35 D. Q. Zhang, G. S. Li, X. F. Yang and J. C. Yu, Chem. Commun., 41 L. Wen, B. Liu, X. Zhao, K. Nakata, T. Murakami and
2009, 4381.
A. Fujishima, Int. J. Photoenergy, 2012, 368750.
This journal is © The Royal Society of Chemistry 2015
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