Fluoride-free self-templated synthesis of hollow TiO 2 nanostructures for hydrogen evolution{ Received 16th March 2012, Accepted 26th March 2012 DOI: 10.1039/c2ce25375e Hollow TiO2nanost
Trang 1Fluoride-free self-templated synthesis of hollow TiO 2 nanostructures for hydrogen evolution{
Received 16th March 2012, Accepted 26th March 2012
DOI: 10.1039/c2ce25375e
Hollow TiO2nanostructures have been fabricated by a facile
self-templated hydrothermal synthesis without the assistance of
any fluoride compound The hollow nanostructures with
diameters of approximately 2 mm are composed of mesoporous
shell walls of 200 nm in thickness On the basis of the
investigation result, it was found that the inside-out Ostwald
ripening was responsible for the hollowing process in which the
evacuation of solid cores was mainly driven by the
minimiza-tion of overall surface energy The hollow nanostructures
exhibited enhanced photocatalytic activity by means of
hydro-gen evolution from methanol solution which is attributed to
their unique structural features
Hollow inorganic nano- and microstructures are particularly
important owing to their versatile applications including
photo-catalysis, sensing, lithium-ion batteries, drug-delivery carrier and
solar energy conversion.1 Due to its unique mechanical, optical
and chemical properties such as low density, high surface area,
effective light-harvesting property, hollow nanostructures usually
exhibit a novel or optimal functional performance.2 Therefore,
efforts have been devoted for the fabrication of hollow structures
with controllable size, shell thickness, wall porosity, and structural
features, etc.1–3 Up to now, hollow nanostructures have been
successfully synthesized using removable hard templates including
polystyrene latex spheres,4,5 silica spheres,6 carbon spheres,7etc
The soft and sacrificial templates such as emulsion droplets,8
micelles/vesicles,9,10 and even gas bubbles,11,12 can also serve as
scaffolds to fabricate hollow structures However, the limitations
of templating methods such as low yields, labour consuming, lack
of structural robustness upon heat treatment, may prevent them
from being used for large-scale production Thus, the challenge to
explore a facile, rapid, efficient approach for the synthesis of
hollow structures still remains
Recently, interest in the synthesis of hollow nanostructures by self-templated methods has increased dramatically due to their cost-effectiveness and simple implementation In particular, efforts have been devoted to the development of a fabrication method for hollow nanostructures based on unique physical mechanisms such
as the Kirkendall effect,13–15or Ostwald ripening.16A template-free synthesis approach based on Ostwald ripening has proved to
be an advanced method for practical application due to its efficiency for the construction of various hollow nano-/micro-structures covering a wide range of inorganic materials such as metal,16metal oxide,17–24metal sulfide,25–27and other.28,29 Over the past two decades, TiO2 has attracted extraordinary research interest due to its unique intrinsic properties such as strong oxidative ability, stability, efficiency, long-term durability as well as a wide range of industrial applications.30–39The synthesis of hollow TiO2nanostructures by a self-templated method is mainly based on Ostwald ripening with a sophisticated solid evacuation process.30–39This technique, so-called inside-out Ostwald ripening, based on the self-etching or self-transformation of the inner solid core is mediated by a corrosive solution One critical condition of this method is to use fluoride compounds such as TiF4and NH4F which are able to create HF under hydrothermal reaction for the etching of solid cores.30–39Therefore, this approach still has many limitations related to the corrosion, toxicity accompanied with low yields, fluoride adsorbed surface, etc Thus, it is remains a challenge to discover a green fluoride-free method for the controlled synthesis of hollow TiO2structures by a self-templated route Herein, we propose a novel fluoride-free self-templated approach to construct titania hollow nanostructures The hollow-ing process was directed by inside-out Ostwald ripenhollow-ing without the assistance of any fluoride compound The hollow nanostruc-tures with mesoporous shell feananostruc-tures are expected to be superior photocatalysts Herein, we demonstrate an enhancement of the photocatalytic activity of the hollow nanostructures by means of hydrogen evolution from methanol solution which is attributed to their integrated structural properties
Hollow TiO2nanostructures were synthesized by hydrothermal treatment of titanium disulfate (Ti(SO4)2) in the presence of a small amount of hydrogen peroxide (H2O2) Typically, Ti(SO4)2solution (1.37 cm3, 2 mmol, 24%, J.T Baker, Germany) was diluted in
19 cm3distilled water One cm3solution consisting of hydrogen peroxide (30%, 10 mmol, J.T Baker, Germany) was dropped slowly into the above solution with continuous stirring to form a
a Department of Chemistry, National Tsing Hua University, Hsinchu,
30013, Taiwan
b Institute of Multidisciplinary Research for Advanced Materials, Tohoku
University, Sendai, 980-8577, Japan.
E-mail: tqduc@mail.tagen.tohoku.ac.jp; Fax: +81-22-217-5651;
Tel: +81-22-217-5651
c
Department of Chemistry, Vietnam National University, Hanoi,
Vietnam
{ Electronic Supplementary Information (ESI) available See DOI:
10.1039/c2ce25375e/
Cite this: CrystEngComm, 2012, 14, 4274–4278
www.rsc.org/crystengcomm COMMUNICATION
Trang 2characteristic red solution The obtained solution at pH 1.0 was
transferred to a Teflon-lined stainless-steel autoclave The
auto-clave was then heated at 473 K for 24 h (unless otherwise stated)
for hydrothermal treatment, after which it was allowed to cool to
room temperature The resultant powder was separated by
centrifugation and washed with distilled water until the pH of
the solution became neutral Finally, the obtained specimen was
dried at 353 K for 1 day
The crystalline phase was characterized using powder X-ray
diffraction (XRD; Rigaku D/MAX-IIB, 40 kV and 30 mA) with
Cu-Ka radiation (l = 1.5406 A˚ ) The morphology was examined
using scanning/transmission electron microscopy (SEM Hitachi
S4700, TEM Jeol-2010) N2adsorption and desorption isotherms
were measured at 77 K (Micromeritics ASAP 2010) to yield
Brunauer–Emmett–Teller (BET) specific surface area
Photocatalytic hydrogen evolution was carried out in a closed
gas circulation system Each sample contained 150 cm3of aqueous
methanol solution of 10/1 water/methanol (v/v) Typically, 0.1 g
TiO2particles were sonicated in 135 ml distilled water for 1 min,
then a solution containing 5 mmol H2PtCl6(1 wt% Pt/TiO2) was
injected directly to solution and finally methanol was used to fill to
150 ml Prior to photocatalytic H2 evolution, a mixture of
methanol solution, TiO2 and H2PtCl6 solution was degassed by
circulation of Ar gas and evacuated for 30 min A 300 W Xe lamp
employed as the light source was delivered from the top of the cell
through a Pyrex window The amount of hydrogen evolved was
measured using an online gas chromatograph (GC-8A, Shimadzu,
TCD) The experimental setup for photocatalytic production of
hydrogen has been described in detail by Kudo et al.40
Fig 1 shows the SEM and TEM images of the synthesized
particles From the SEM image in Fig 1a, it is evident that the
synthesized particles comprise interconnected self-assembled
spheres with an average diameter of 2 mm Fig 1a also displays
the hollow nanostructures which are composed of many sphere
particles with broken shell walls This image clearly shows the
presence of a hollow core structure The high magnification image
in Fig 1b reveals that the shell wall with a thickness of ca 200 nm
is composed of assembled spherical particles with porous hierarchical networks The TEM image (Fig 1c) indicates the presence of interconnected hollow nanostructures which are constructed from primary nanoparticles with diameters of about
20 nm From the TEM image in Fig 1c, the bright areas in the center of each sphere can be observed, which is a typical feature of hollow nanostructures The SAED pattern of a sphere shown in the inset of Fig 1c reveals the presence of a polycrystalline anatase phase The crystalline structure of the product was characterized
by powder XRD Fig 2 shows the XRD pattern of the synthesized particles which indicates the presence of tetragonal anatase TiO2
(JCPDS 21-1272) as the major phase The diffraction peaks can be indexed to {101}, {004}, {200} and {105} of anatase structure The high porosity of the synthesized nanostructures was further confirmed by N2gas adsorption analysis and the results are shown
in Fig 3 The BET specific surface area is 60 m2g21 The isotherm curve can be categorized as an IUPAC type III with a hysteresis loop in the range of 0.5–1 P/P0, indicating the presence of mesopores This was also confirmed by the pore-size distribution with a peak mesopore diameter of about 20 nm
In order to investigate the formation mechanism of the hollow nanostructures, the time-dependent experiments were performed Fig 4 displays the TEM images of products obtained by hydrothermal treatment for various ageing times The sample collected after 6 h consists of solid particles (Fig 4a) which were transformed into hollow structures after 10 h treatment (Fig 4b) Extending the ageing time to 24 h, the hollow particles with a uniform interconnected cavity were obtained (Fig 4c), suggesting that the hollowing process has occurred
In the present system, titanium disulfate reacted initially with hydrogen peroxide via a ligand-exchange reaction with O222ion generated under acidic solution.12The formation of intermediate complex was evident from the characteristic red color of the solution Under hydrothermal conditions, the complex decom-posed and crystallized into the anatase form It is well-known that anatase is formed under acidic solutions of sulfuric acid.41 It should be noted that a similar strategy using H2O2 as a coordination agent has been reported for synthesis of TiO2hollow structures.12According to their report, the H2O2amount is about
75 times with respect to titanium which is large enough to produce
O2bubbles to serve as soft templates for the formation of hollow
Fig 1 SEM images (a, b, d) and TEM image (c) of TiO 2 nanostructures
synthesized by hydrothermal treatment of Ti(SO 4 ) 2 in the presence of
H 2 O 2 Black arrows indicate large crystallites on the outer surface White
arrows indicate small crystallites in the inner cores.
Fig 2 XRD pattern of TiO 2 nanostructures synthesized with reaction times of 2, 6, 10, and 24 h, respectively.
Trang 3structures In the present case, the H2O2amount is only 5 times
compared to that of titanium which is not sufficient for the
production of gas templates To confirm this speculation, thermal
treatment at 353 K for 24 h was applied to the mixture of titanium
sulfate and hydrogen peroxide to remove excess reagent and O2
bubbles before hydrothermal treatment The obtained red complex
solution was then hydrothermally treated as for the typical
experiment The resulting powder was found to be composed of
hollow nanostructures (Fig S1{) This control experiment
indicates that generated O2 is not involved in the formation of
the hollow structures We, therefore, reached the conclusion that
the hollow nanostructures were formed following an Ostwald
ripening mechanism rather than gathering on a soft template
According to the above results, a possible mechanism was
proposed to illustrate the formation of microspheres and the
hollowing process as follows It is well-known that the primary
particles preferentially organize on spherical structures to minimize
their overall surface energy.30–39 Thus, the preformed primary
particles are likely to aggregate to each other to minimize their
total surface energy, resulting in the growth of microspheres
Besides acting as a coordination agent, H2O2also played another
important role, namely, stabilizing the particle’s interfaces through
coordination on them Thus, when O2was released, the particle
interface became vacant and unstable due to the presence of many
dangling bonds Consequently, particles have a strong tendency to
aggregate into assembled structures for the elimination of dangling
interfaces The control experiment also supports this speculation
In the absence of H2O2, the monodispersed nanoparticles were
obtained under the same hydrothermal treatment (Fig 4, inset)
Therefore, it can be concluded that H2O2 is crucial for the
aggregation of primary particles into hierarchical structures
On the basis of the above investigation, an inside-out Ostwald
ripening mechanism is proposed to account for the hollowing
process As solid spheres have been formed by the assembly of the
primary particles, the surface crystallites have grown into larger
and less-soluble crystalline phase due to its full contact with
supersaturated solution in the surrounding environment.42 The
microscopic observation shows that the crystallites on the outer
shells are significantly larger than ones in the inner cores as shown
in Fig 1d, giving a direct evidence for such growth It is well-known that smaller particles have a higher surface energy than larger ones Consequently, the particles in the inner sphere possess
a strong tendency to dissolve and diffuse out through the shell.30,31
In the next stage, recrystallization on the outer surface may occur because the supersaturation increases in the solution surrounding them.32 Finally, the shell wall increase in the thickness and the cores became depleted to produce hollow interconnected struc-tures The XRD investigation shown in Fig 2 indicates that the crystallinity of the obtained particles is gradually increased with the ageing time The crystallite sizes of the products calculated following the Scherrer equation are also increased These evidences imply that Oswald ripening is the mechanism responsible for the growth of nanostructures in this system In addition, as mentioned above, generated O2 itself could not serve as a scaffold for microsphere formation In contrast, O2bubbles may act as soft templates to form a narrow pore channel for crystallization of titania.43As a result, a uniform mesoporous shell wall has been produced together with the hollow nanostructures
To further investigate the role of H2O2, a series of control experiments were carried out systematically with tailoring the amount of additive It was found that the amount of hydrogen peroxide strongly affects the formation of the hollow particles The desired crystal shape can be achieved by adding an appropriate amount of hydrogen peroxide (10 mmol) The tailoring of the additive amount has a crucial effect on the morphology of the synthesized particles Particularly, with the addition of a smaller amount of hydrogen peroxide (4–6 mmol), the obtained particles comprise mainly regular agglomerations along with a few hollow nanostructures (Fig S2{) In the present case, H2O2may not be enough for saturated chelation with Ti4+, thus the growth of microspheres is restrained On the other hand, large amounts of additive were introduced (20–50 mmol), resulting in the formation
of regular nanoparticles without any assembly of hierarchical
Fig 3 Nitrogen adsorption–desorption isotherm of TiO 2 hollow
nanos-tructures and the BJH pore-size distribution curve (inset).
Fig 4 TEM images of particles obtained by hydrothermal treatment with increasing reaction time: (a) 6 h, (b) 10 h, and (c) 24 h.
Trang 4structures (Fig S3{) Therefore, it can be concluded that H2O2
plays versatile roles for the self-assembly of microspheres as well as
its transformation into hollow nanostructures It should be
mentioned that very few fluoride-free self-templated methods for
the synthesis of hollow TiO2nanostructures have been reported.44
The present approach, on the basis of its efficiency and simple
implementation, would be of significant advantage in terms of
environmental harmony and cost effectiveness for the production
of TiO2hollow structures on a large-scale
The photocatalytic activity of the synthesized hollow
nanos-tructures was evaluated by means of the hydrogen evolution
reaction The photocatalytic activity of the usual anatase
nanoparticles synthesized without H2O2by the same hydrothermal
method was also evaluated as a reference The data for the
catalysts is shown in Table 1 and Fig S4.{ Fig 5 shows the
hydrogen evolution rate on the synthesized particles It is obvious
that the hollow particles exhibit significant improvement of
photocatalytic production of hydrogen with a hydrogen evolution
rate of 525 mmol h21which is higher than that on an anatase-type
TiO2ST-21 (Ishihara Sangyo, Co., Ltd., Japan).45The hydrogen
evolution rate on the hollow nanostructures is also two times
higher than that on regular nanoparticles despite the lower surface
area of the former Recently, Ohtani et al found that large
secondary particles composed of large primary particles showed
enhanced photocatalytic H2 evolution from aqueous methanol
solution.45In the present study, the primary particle size of anatase
hollow structures is larger than anatase nanoparticles as concluded
from the measured crystallite sizes (Table 1, Fig S4{) At the same
time, the secondary particle size, defined as a volume-average
particle size, of interconnected hollow nanostructures is expected
to be higher than that of individual monodispersed nanoparticles
Thus, the increased photocatalytic activity of hollow
nanostruc-tures relative to that of anatase nanoparticles and ST-21 can be
explained either by the primary particle size or by its assembled
structures
Owing to the unique structural features, the hollow porous
nanostructures provide hierarchical channel network which may
accelerate the reaction velocity Besides that, the presence of an
interconnected structure with a continuous conducting pathway
could also enhance the photogenerated electron-hole separation
and charge transport, which is an important factor for the
remarkable photocatalytic activity of the hollow nanostructures
Furthermore, a small percentage of rutile was found on the
synthesized sample which may lead to a better charge carrier
separation and consequently improve the photocatalytic activity
In addition, the strong adsorption of sulfate ions on the surface of
the nanoparticles synthesized without H2O2might also be a reason for their low photocatalytic performance.46 The hollow nanos-tructures may avoid this effect due to the complete ligand-exchange reaction in the early stage of the reaction, thus, their photocatalytic activity is retained Regarding photocatalytic hydrogen evolution, current research has been focused on the effect of the crystal facet and the crystalline phase on the evolution rate.47–50 The effect of architectures has not been considered appreciably.51 The present work suggests that the hierarchical structures with interconnected features could also enhance the hydrogen production capacity
Conclusions
In summary, we have developed a facile green fluoride-free approach for the fabrication of hollow TiO2 nanostructures Coordination agents associated with the minimization of surface energy lead to the self-assembly of the primary nanoparticles into microsphere structures, while inside-out Ostwald ripening is responsible for the hollowing process The hollow nanostructures exhibited enhanced photocatalytic activity in terms of hydrogen evolution which is attributed to their unique structural features Because of its efficiency and simple implementation, the proposed route could be significantly advantageous in terms of environ-mental harmony and cost effectiveness and its potential for large-scale production
Acknowledgements Authors thank Prof Masato Kakihana and Dr Hideki Kato at Tohoku University for providing experimental facilities of hydrogen evolution reaction Author Truong is thankful to MEXT, Japan and National Science Council, Taiwan for
Table 1 Properties and photocatalytic performances of the TiO 2
catalysts
Sample
Crystallite size a (nm)
BET (m 2 g 21 )
Evolution rate (mmol h 21 )
Nanoparticles 10.3 83 250
Hollow structures 15.8 60 525
a Estimated according to the Scherrer equation: D (hkl) = (K l)/(b
cosh) where K is the shape factor, l the wavelength of the Cu-Ka
radiation, b the full width at half-maximum (fwhm) of the (hkl)
peak, and h the diffraction angle b Reference 45
Fig 5 Hydrogen evolution rate on the synthesized particles: (1) nanoparticles obtained without H 2 O 2 , (2) hollow particles prepared with
H 2 O 2
Trang 5financial support for this research We appreciate Dr Sudeshna
Ray at Tohoku University for reading the proof
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