low temperature hydrothermal synthesis of monodispersed flower - like titanate nanosheets

Low temperature hydrothermal synthesis of monodispersed flower-like titanate
nanosheets
Jaturong Jitputti
a
, Thitima Rattanavoravipa
a
, Surawut Chuangchote
a
, Sorapong Pavasupree
b
,
Yoshikazu Suzuki
a
, Susumu Yoshikawa
a,
*
a
Institute of Advanced Energy, Kyoto University, Uji, Kyoto, 611-0011, Japan
b
Department of Materials and Metallurgical Engineering, Faculty of Engineering, Rajamangala University of Technology Thanyaburi, Klong 6, Pathumthani, 12110, Thailand
article info
Article history:
Received 23 April 2008
Received in revised form 25 September
2008
Accepted 26 September 2008
Available online 2 October 2008
Keywords:
Titania
Flower-like
Sphere

Nanosheets
Template-free
Hydrothermal
abstract
Monodispersed flower-like titanate superstructure was successfully prepared by simple hydrothermal
process without any surfactant or template. N
2
-sorption analysis, scanning electron microscopy (SEM),
and X-ray diffraction (XRD) observation of as-synthesized product revealed the formation of flower-like
titanate with diameter of about 250–450 nm and BET surface area (S
BET
) of 350.7 m
2
g
À1
. Upon thermal
treatment at 500 °C, the titanate nanosheets were converted into anatase TiO
2
with moderate deforma-
tion of their structures. The as-prepared flower-like titanate showed high photocatalytic activity for H
2
evolution from water splitting reaction. Moreover, the sample heat treated at 500 °C exhibited higher
photocatalytic activity than that of commercial TiO
2
anatase powder (ST-01).
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
Titania (TiO
2
) is an attractive semiconducting material due to its

characteristic photochemical properties and high chemical stabil-
ity [1]. Applications of TiO
2
have been explored in different fields
including optoelectronics [2], photocatalysts [3], photoelectrodes
[4], lithium rechargeable batteries [5], and sensor devices [6].
Many studies in the fundamental and practical fields have been
performed using nanoscale materials, because a high specific sur-
face is generally required for their applications [1]. In addition,
many different methods and techniques have been developed for
the preparation of TiO
2
nanomaterials, including sol–gel technique
[7], micelle and inverse micelle methods [8], solvothermal method
[9], chemical vapor deposition [10], electrodeposition [10], hydro-
thermal method [11], etc. In contrast to other techniques, the
hydrothermal method offers an inexpensive and environmentally
friendly route and the ability to control chemical, homogeneity,
purity, morphology, shape, and phase composition of the powder
under moderate conditions [12–14].
Since it is known that the structural, thermal, electronic, and
optical properties of TiO
2
nanomaterials are strongly depended
on their size, shape, and crystal structure, several attempts have
been paid on the preparation of special morphologies of TiO
2
nano-
structure, such as nanotube [15], nanowire [16], hollow nano-
sphere [17], nanosphere [18], nanosheets [1,19], due to the fact

that the synthesis of these TiO
2
nanomaterials offer fundamental
scientific opportunities for investigating the influence of size and
dimensionality of materials with respect to their properties. In
our previous work, we have reported the synthesis of spherical
TiO
2
nanosheets (flower-like morphology composed of nano-
sheets) via simple hydrothermal synthesis using titanium butoxide
as starting material [19]. Our results showed that this prepared
spherical TiO
2
nanosheets exhibit higher photocatalytic activity
and efficiency for dye-sensitized solar-cell when compared to the
commercial TiO
2
nanoparticles (Ishihara ST-01 and Degussa P25).
However, the TiO
2
prepared by this method were not homoge-
neous in size with large diameter raging from 500 nm to 2
l
m.
So it is meaningful to develop a simple method to synthesize the
spherical TiO
2
nanosheets with homogeneous size at rather low
temperature.
In this work, we report a simple hydrothermal approach for

preparation of spherical titanate nanosheets by a novel 2-step
method combining the synthesis of spherical amorphous titania
using controlled hydrolysis and formation of spherical titanate
nanosheets by hydrothermal treatment in ammonia aqueous
solution. The effects of hydrothermal treatment temperature and
time are also investigated. The spherical titanate nanosheets can
1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2008.09.026
* Corresponding author. Tel.: +81 774 38 3502; fax: +81 774 38 3508.
E-mail address: (S. Yoshikawa).
Catalysis Communications 10 (2009) 378–382
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
be easily converted into spherical TiO
2
nanosheets by thermal
treatment at 500 °C.
2. Experimental
2.1. Preparation of monodispresed amorphous TiO
2
spheres
Monodispersed amorphous spherical TiO
2
particles were syn-
thesized through controlled hydrolysis of titanium tetraisopropox-
ide (TTIP, Ti-(OC
3
H
7

)
4
, 97%, Aldrich) in ethanol [13]. Typically,
100 mL of ethanol was mixed with 0.4 mL of 0.1 M aqueous potas-
sium chloride, followed by the addition of 2.2 mL of TTIP at ambi-
ent temperature under stirring condition. The solution was mixed
until a white precipitate appeared. The suspension was aged in a
static condition for 24 h in a closed container at room temperature
(25 °C). The powder (denoted as AS) suspended in the vessel was
collected by filtration and dried at 60 °C in air.
2.2. Preparation of titanate nanosheets
Monodispersed spherical titanate nanosheets were prepared by
hydrothermal treatment in ammonia solution. An amount of 0.3 g
of AS was suspended in 60 ml of ammonia solution (20 ml of 28%
ammonia solution, 40 ml of distilled water, pH 12) in 80-ml Tef-
lon-line stainless autoclave at 60, 80, 100, and 120 °C with stirring
condition for 12 and 24 h. The obtained powder was then washed
by distilled water and ethanol.
2.3. Characterization
X-ray diffraction (XRD) analyses were performed on a powder
diffractometer (Rigaku RINT-2100). The microstructure of the pre-
pared materials was analyzed by scanning electron microscopy
(SEM, JEOL JSM-6500FE) The nitrogen sorption isotherm and Bru-
nauer–Emmett–Teller (BET) specific surface area were determined
by nitrogen adsorption method (BEL Japan, BELSORP-18 Plus).
2.4. Photocatalytic measurement
The photocatalytic activity measurement was evaluated with
H
2
evolution from water splitting reaction. The procedure was

basically the same as described in previous work [20,21]. The
reaction was carried out in a closed gas-circulation system. The
amount of 1 g of sample was suspended in aqueous methanol
solution (800 mL distilled H
2
O, 80 mL methanol) by means of
magnetic stirrer in an inner irradiation-type photoreactor made
of Pyrex glass. The mixture was purged by Argon gas (Ar) to re-
move the air dissolved in solution. The suspension was irradiated
by a 450 W high-pressure Hg lamp (Ushio UM-452) and cooling
water was circulated through a cylindrical Pyrex jacket located
around the UV light source to maintain the reaction temperature.
The evolved gasses were periodically analyzed by an on-line gas
chromatograph (Shimadzu GC-8A, Molecular sieve 5A, TCD, Ar
Carrier).
3. Results and discussion
Generally, transition metal alkoxides are very reactive with
moisture, heat, and light due to the presence of highly reactive alk-
oxy (OR) groups. In sol-gel processing of metal alkoxides, hydroly-
sis and condensation reactions occur so rapidly that uniform and
fine particles are difficult to obtain. However, the use of bulky,
branched alkoxy groups (such as TTIP) could reduce the hydrolysis
rates as reaction rates decrease with the steric bulk of alkoxy li-
gands to favor the formation of fine particles with a more uniform
size distribution. Another approach to retard the hydrolysis and
condensation rates is the chemical modification of metal alkoxides
with modified agent, such as alcohols and acids or bases [22].In
this work, spherical TiO
2
nanoparticles were formed through a con-

trolled hydrolysis of TTIP in ethanol [23].
Fig. 1 shows the XRD patterns of AS and the samples hydrother-
mally prepared in ammonia solution for 24 h at 60 to 120 °C. No
XRD pattern of AS was found, indicating the amorphous nature
of the samples obtained by hydrolysis of TTIP.
5 10152025303540455055 6
0
CuK
α θ
, 2 / Degree
Intensity / a.u.
(b)
(e)
(d)
(c)
(a)
(f)
010
211105
200
004
Fig. 1. XRD patterns of (a) sample obtained by hydrolysis of TTIP with 0.1 M KCl
solution (AS), and samples hydrothermally treated in ammonia for 24 h at (b) 60 °C,
(c) 80 °C, (d) 100 °C, (e) 120 °C and (f) sample hydrothermally treated in ammonia
at 120 °C, 12 h followed by thermal treatment at 500 °C for 1 h.
Fig. 2. (a) SEM image and (b) high magnification SEM image of samples obtained by hydrolysis of TTIP.
J. Jitputti et al. /Catalysis Communications 10 (2009) 378–382
379
According to XRD measurement, the XRD data of hydrother-
mally treated samples prepared at P80 °C show a poorly crystal-

line titanate. The peak at around 2h % 9° indicated that titanate
has a layer structure with an interlayer distance of ca.1.0 nm. The
broad peaks in the high-angle region were similar to the
theoretically estimated diffraction peaks for layered titanate
(NH
4
)
2
Ti
3
O
7Àx
N
x
[1,24]. However, the peak cannot be clearly ob-
served for the sample prepared at 60 °C.
Recently, Takezawa and Imai have reported the formation of
layered titanate in the basic solutions [1,25]. According to their
work, the layered titanates can be constructed by stacking of anio-
nic TiO
2
manolayers with the counter cations (NH
4
+
or Na
+
). How-
ever, almost the same amount of anionic species and cationic
agents, and a basic condition above pH 8 are required for the for-
mation of the titanates [25]. As mentioned above, the pH of ammo-

nia solution used in our experiments was pH 12. Therefore, the pH
of ammonia solution used was sufficient for the formation of the
layered titanates.
The morphologies of resulting samples were investigated by
scanning electron microscopy (SEM). Fig. 2 gives the morphologies
of AS. As shown in Fig. 2a, uniform, solid spherical particles were
obtained after 24-h. The high magnification SEM image (Fig. 2b)
shows that the surface of these spheres is relatively smooth with
size of 250 to 450 nm.
Fig. 3 shows the high magnification SEM images of hydrother-
mally prepared samples prepared for 12 and 24 h at 60 to 120 °C.
Fig. 3. SEM images of samples hydrothermally prepared at (a) 60 °C, 12 h, (b) 60 °C, 24 h, (c) 80 °C, 12 h, (d) 80 °C, 24 h, (e) 100 °C, 12 h, (f) 100 °C, 24 h, (g) 120 °C,12 h, and
(h) 120 °C, 24 h.
380 J. Jitputti et al. /Catalysis Communications 10 (2009) 378–382
After hydrothermal treatment in ammonia solution, the size of
these spheres was not changed too much. However, their surfaces
became rough. Obviously, nanosheets structure could be observed
for all samples prepared at temperature P80 °C(Fig. 3c–h).
Our results showed that, for this preparation method, the
hydrothermal temperature was a critical factor. As seen in Fig.
3c–h, nanosheet structure could be obtained for samples prepared
at P80 °C. At 60 °C, the nanosheet morphology could not be ob-
tained in the sample prepared for 12 h (Fig. 3a). However, the
nanosheet structure could be observed for sample prepared for
24 h (Fig. 3b). Therefore, it can be preferably concluded that the
longer time or higher temperature was required in order to obtain
nanosheet structure.
Some physical properties of AS and flower-like titanate super-
structure prepared at 120 °C for 12 h are listed in Table 1. The spe-
cific surface area of the products obtained at 120 °C, 12 was

estimated to be about 350.7 m
2
g
À1
. Such a high value of the specific
surface area suggests that the nanosheet structure provide high sur-
face area. In addition, it has been reported that the interlayer spac-
ing can provide an effective surface for the adsorption of nitrogen
molecules [1]. The sample prepared at 120 °C, 12 h was then heat
treated at 500 °C to investigate the effect of treatment temperature.
Upon thermal treatment at 500 °C for 1 h, the titanate nano-
sheets were converted into anatase TiO
2
(Fig. 1f), with slightly
deformation of their structures (Fig. 4). The anatase nanosheets
exhibited specific surface area about 31.7 m
2
g
À1
, where the thick-
ness of the sheets was estimated to be larger than 15 nm from SEM
images.
Fig. 5 shows the UV–Vis spectra of AS, flower-like titanate and
flower-like titanate calcined at 500 °C which are compared with
that of commercial Ishihara ST-01 (ST-01). The AS shows an
absorption edge at around 380 nm. This absorption value is signif-
icantly lower than that of ST-01 (390 nm). The flower-like titanate
sample exhibits a main absorption edge at 385 nm and, in addition,
a shoulder that extends above 430 nm. Upon calcination of flower-
like titanate at 500 °C, the obtained TiO

2
anatase (Fig. 1) shows an
absorption edge at around 400 nm.
Preliminary measurement of photocatalytic activity was evalu-
ated with H
2
evolution from water splitting reaction under UV light
irradiation. For comparison, the same measurement was also car-
ried out on ST-01 sample as the reference. The results are shown
in Fig. 6.
As shown in Fig. 6, the amount of H
2
evolved after 5 h irradia-
tion for ST-01was estimated to be 92
l
mol g
À1
. In case of AS, the
Table 1
The physical properties of AS, flower-like TiO
2
superstructure prepared at 120 °C for
12 h, and flower-like TiO
2
superstructure calcined at 500 °C.
Materials Crystal structure S
BET
/m
2
g

À1
Amount of H
2
evolved
l
mol g
À1
l
mol m
2
AS Amorphous 128.2 72 0.56
FL TiO
2
Titanate 350.7 342 0.97
FL TiO
2
(500 °C) Anatase 31.7 588 18.6
Fig. 4. SEM images of (a) sample hydrothermally prepared at 120 °C,12 h, and (b) followed by calcination at 500 °C for 1 h.
300 350 400 450 500 550 600
Wavelenght / nm
Adsorbsion (arb unit)
(b)
(c)
(d)
(a)
Fig. 5. UV–Vis diffuse reflectance of (a) ST-01 (anatase powder), (b) AS, (c) as-
prepared flower-like titanate, and (d) flower-like titanate calcined at 500 °C.
0
100
200

300
400
500
600
700
0 50 100 150 200 250 300
Irradiation time / min
H
2
evolved /
μ
mol g
-1
(a) ST01
(b) AS
(d) Flower-like TiO
2
(calcined 500
o
C)
(c) Flower-like titanate
Fig. 6. Photocatalytic H
2
evolution from water splitting reaction over (a) ST-01
(anatase powder), (b) AS, (c) as-prepared flower-like titanate, and (d) flower-like
titanate calcined at 500 °C.
J. Jitputti et al. /Catalysis Communications 10 (2009) 378–382
381
amount of H
2

evolved was estimated to be 72
l
mol g
À1
, which was
lower than that of ST-01. This was expected because the AS possess
amorphous phase which generally show lower photocatalytic
activity when compared to crystalline TiO
2
photocatalyst. The
amount of H
2
evolved was raised to be 342
l
mol g
À1
when as-syn-
thesized flower-like titanate was used. However, the imperfect
crystallization from XRD spectra (Fig. 1) accompanying with amor-
phous phase, which favorably increase the probability of mutual
e
À
/h
+
recombination at both surface and bulk trap, contained to
some extent in the as-synthesized flower-like titanate resulted in
the decrease in H
2
evolution rate after 2 h of reaction (Fig. 6).
The amount of H

2
evolved was raised to be 588
l
mol g
À1
when
the flower-like TiO
2
calcined at 500 °C was used. This was due to
the spherical TiO
2
nanosheets calcined at 500 °C has anatase struc-
ture which show highest activity when compared to other TiO
2
phase [20,21,26]. Moreover, the UV–Vis spectra (Fig. 5) shows that
the number of absorbed photon of flower-like TiO
2
calcined at
500 °C is larger than other samples. The results revealed that the
use of the flower-like TiO
2
with unique structure could promote
great H
2
evolution.
In summary, this study provides a simple route for fabrication
of flower-like titanate superstructure with uniform size under mild
condition. In addition, we found that the hydrothermal treatment
in ammonia solution used in this study can also be applied to pre-
pare other nanosheets-composed TiO

2
materials, such as TiO
2
nanowires and TiO
2
nanowire arrays. The results will be reported
and discussed in the forthcoming work. Furthermore, it is expected
that the hydrothermal treatment in ammonia solution could also
be applied to other nanostructured materials.
4. Conclusion
Monodispersed amorphous TiO
2
spheres were prepared by con-
trolled hydrolysis of titanium isopropoxide in ethanol. By applying
the hydrothermal treatment in ammonia solution, flower-like tita-
nate of desired morphology could be easily prepared from amor-
phous TiO
2
spheres. In addition, it was obvious that temperature
exerts a great influence on the structure of the products prepared
by this method. Moreover, the titanate nanosheets could be easily
converted into anatase TiO
2
by calcination at 500 °C.
Preliminary photocatalytic activity measurement showed that
the flower-like titanate superstructure show high photocatalytic
activity due to their unique structure. Moreover, the sample
heat treated at 500 °C, which show anatase phase, exhibit higher
photocatalytic activity than that of commercial TiO
2

anatase
powder (ST-01).
Acknowledgements
The authors would like to express their gratitude to Prof. T.
Yoko, Assoc. Prof. M. Takahashi, and Asst. Prof. Y. Tokuda, Institute
for Chemical Research, Kyoto University for the use of XRD
equipment.
References
[1] Y. Takezawa, H. Imai, Small 2 (2006) 390.
[2] Y. Wang, L. Li, K. Yang, L.A. Samuelson, J. Kumar, J. Am. Chem. Soc.
(Communication) 129 (2007) 7238.
[3] A. Fujishima, K. Honda, Nature 238 (1972) 37.
[4] B. O’Regan, M. Gratzel, Nature 353 (1991) 737.
[5] L. Kavan, A. Attia, F. Lenzmann, S.H. Elder, M. Gratzel, J. Electrochem. Soc. 147
(2000) 2897.
[6] X. Du, Y. Wang, Y. Mu, L. Gui, P. Wang, Y. Tang, Chem. Mater. 14 (2002) 3953.
[7] Y. Bessekhouad, D. Robert, J.V. Weber, J. Photochem. Photobiol. A 157 (2003)
47.
[8] S.S. Hong, M.S. Lee, S.S. Park, G.D. Lee, Catal. Today 87 (2003) 99.
[9] B. Wen, C. Liu, Y. Liu, J. Phys. Chem. B 109 (2005) 12372.
[10] J.A. Ayllon, A. Figueras, S. Garelik, L. Spirkova, J. Durand, L. Cot, J. Mater. Sci.
Lett. 18 (1999) 1319.
[11] J. Yang, S. Mei, J.M.F. Ferreira, J. Am. Ceram. Soc. 83 (2000) 1361.
[12] Z. Wu, N. Kumagai, M. Yoshimura, Chem. Mater. 12 (2000) 3356.
[13] Y. Wang, H. Xu, X. Wang, X. Zhang, H. Jia, L. Zhang, J. Qiu, J. Phys. Chem. B 110
(2006) 13835.
[14] J. Jitputti, P. Charoensirithavorn, S. Yoshikawa, Chem. Lett. 36 (2007) 1508.
[15] Y. Ding, Z.L. Wang, J. Phys. Chem. B 108 (2004) 12280.
[16] H. Li, Z. Bian, J. Zhu, D. Zhang, G. Li, Y. Huo, H. Li, Y. Lu, J. Am. Chem. Soc.
(Communication) 129 (2007) 8406.

[17] J.L. Look, C.F. Zukoski, J. Am. Ceram. Soc. 75 (1992) 1587.
[18] X. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891.
[19] S. Pavasupree, S. Ngamsinlapasathian, Y. Suzuki, S. Yoshikawa, Mater. Lett. 61
(2007) 2973.
[20] J. Jitputti, S. Pavasupree, Y. Suzuki, S. Yoshikawa, J. Solid State Chem. 180
(2007) 1743.
[21] J. Jitputti, S. Pavasupree, Y. Suzuki, S. Yoshikawa, Catal. Commun. 9 (2008)
1265.
[22] M. Pal, J.G. Serrano, P. Santiago, U. Pal, J. Phys. Chem. C 111 (2007) 96.
[23] S. Qourzal, A. Assabbane, Y. Ait-Ichou, J. Photochem. Photobiol. A: Chem. 163
(2004) 317.
[24] C.H. Rhee, S.W. Bae, J.S. Lee, Chem. Lett. 34 (2005) 660.
[25] Y. Takezawa, H. Imai, J. Cryst. Growth 308 (2007) 117.
[26] T. Sreethawong, Y. Suzuki, S. Yoshikawa, J. Solid State Chem. 178 (2005) 329.
382 J. Jitputti et al. / Catalysis Communications 10 (2009) 378–382