Low temperature hydrothermal synthesis of monodispersed flower-like titanate nanosheets Jaturong Jitputtia, Thitima Rattanavoravipaa, Surawut Chuangchotea, Sorapong Pavasupreeb, a Institu
Trang 1Low temperature hydrothermal synthesis of monodispersed flower-like titanate nanosheets
Jaturong Jitputtia, Thitima Rattanavoravipaa, Surawut Chuangchotea, Sorapong Pavasupreeb,
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
a r t i c l e i n f o
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
a b s t r a c t
Monodispersed flower-like titanate superstructure was successfully prepared by simple hydrothermal process without any surfactant or template N2-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 (SBET) of 350.7 m2g 1 Upon thermal treatment at 500 °C, the titanate nanosheets were converted into anatase TiO2with moderate deforma-tion of their structures The as-prepared flower-like titanate showed high photocatalytic activity for H2
evolution from water splitting reaction Moreover, the sample heat treated at 500 °C exhibited higher photocatalytic activity than that of commercial TiO2anatase powder (ST-01)
Ó 2008 Elsevier B.V All rights reserved
1 Introduction
Titania (TiO2) is an attractive semiconducting material due to its
characteristic photochemical properties and high chemical
stabil-ity[1] Applications of TiO2have 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 TiO2nanomaterials, 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 TiO2 nanomaterials are strongly depended
on their size, shape, and crystal structure, several attempts have
been paid on the preparation of special morphologies of TiO2 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 TiO2nanomaterials 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 TiO2 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 TiO2 nanosheets exhibit higher photocatalytic activity and efficiency for dye-sensitized solar-cell when compared to the commercial TiO2nanoparticles (Ishihara ST-01 and Degussa P25) However, the TiO2 prepared by this method were not homoge-neous in size with large diameter raging from 500 nm to 2lm
So it is meaningful to develop a simple method to synthesize the spherical TiO2nanosheets 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.
* Corresponding author Tel.: +81 774 38 3502; fax: +81 774 38 3508.
E-mail address: s-yoshi@iae.kyoto-u.ac.jp (S Yoshikawa).
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Catalysis Communications
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / c a t c o m
Trang 2be easily converted into spherical TiO2 nanosheets by thermal
treatment at 500 °C
2 Experimental
2.1 Preparation of monodispresed amorphous TiO2spheres
Monodispersed amorphous spherical TiO2 particles were
syn-thesized through controlled hydrolysis of titanium
tetraisopropox-ide (TTIP, Ti-(OC3H7)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
H2 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 H2O, 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 TiO2nanoparticles were formed through a con-trolled hydrolysis of TTIP in ethanol[23]
Fig 1shows 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
CuK α θ , 2 / Degree
(b)
(e) (d) (c)
(a)
(f)
010
211 105 200 004
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.
Trang 3According 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
(NH4)2Ti3O7 xNx[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 TiO2manolayers with the counter cations (NH4 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 2gives the morphologies
of AS As shown inFig 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 3shows 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
Trang 4After 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 inFig
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 inTable 1 The
spe-cific surface area of the products obtained at 120 °C, 12 was
estimated to be about 350.7 m2g 1 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 TiO2 (Fig 1f), with slightly
deformation of their structures (Fig 4) The anatase nanosheets
exhibited specific surface area about 31.7 m2g 1, where the
thick-ness of the sheets was estimated to be larger than 15 nm from SEM
images
Fig 5shows 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 TiO2anatase (Fig 1) shows an
absorption edge at around 400 nm
Preliminary measurement of photocatalytic activity was
evalu-ated with H2evolution 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
inFig 6
As shown inFig 6, the amount of H2evolved after 5 h irradia-tion for ST-01was estimated to be 92lmol g 1 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 1 Amount of H 2 evolved
lmol g 1 lmol m 2
FL TiO 2 (500 °C) Anatase 31.7 588 18.6
Wavelenght / nm
(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
Irradiation time / min
H2
(a) ST01
(b) AS
(d) Flowe r-like TiO2 (calcine d 500 oC)
(c) Flowe r-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.
Trang 5amount of H2evolved was estimated to be 72lmol g 1, 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 TiO2 photocatalyst The
amount of H2evolved was raised to be 342lmol g 1when
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 /h+recombination at both surface and bulk trap, contained to
some extent in the as-synthesized flower-like titanate resulted in
the decrease in H2evolution rate after 2 h of reaction (Fig 6)
The amount of H2evolved was raised to be 588lmol g 1when
the flower-like TiO2calcined at 500 °C was used This was due to
the spherical TiO2nanosheets calcined at 500 °C has anatase
struc-ture which show highest activity when compared to other TiO2
phase[20,21,26] Moreover, the UV–Vis spectra (Fig 5) shows that
the number of absorbed photon of flower-like TiO2 calcined at
500 °C is larger than other samples The results revealed that the
use of the flower-like TiO2with unique structure could promote
great H2evolution
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 TiO2 materials, such as TiO2
nanowires and TiO2nanowire 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 TiO2spheres 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 TiO2spheres 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 TiO2by 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 TiO2 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
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