Hydrothermal etching assisted crystallization: A facile route to functional yolk-shell titanate microspheres with ultrathin nanosheets-assembled double shells

Article abstract of DOI:10.1021/ja2055287

We report a facile "hydrothermal etching assisted crystallization" route to synthesize Fe₃O₄@titanate yolk-shell microspheres with ultrathin nanosheets-assembled double-shell structure. The as-prepared microspheres possess a uniform size, tailored shell structure, good structural stability, versatile ion-exchange capability, high surface area, large magnetization, and exhibit a remarkable catalytic performance.

Full text of DOI:10.1021/ja2055287

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Hydrothermal Etching Assisted Crystallization: A Facile Route to
Functional Yolk-Shell Titanate Microspheres with Ultrathin
Nanosheets-Assembled Double Shells
Wei Li, Yonghui Deng,* Zhangxiong Wu, Xufang Qian, Jianping Yang, Yao Wang, Dong Gu, Fan Zhang,
Bo Tu, and Dongyuan Zhao*
Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, and Laboratory of Advanced
Materials, Fudan University, Shanghai 200433, P. R. China
S Supporting Information
b
ABSTRACT: We report a facile “hydrothermal etching
assisted crystallization” route to synthesize Fe O @tita-
3
4
nate yolk-shell microspheres with ultrathin nanosheets-
assembled double-shell structure. The as-prepared micro-
spheres possess a uniform size, tailored shell structure,
good structural stability, versatile ion-exchange capabil-
ity, high surface area, large magnetization, and exhibit a
remarkable catalytic performance.
Figure 1. Schematic illustration of the formation process of the
Fe @titanate double-shelled yolk-shell microspheres.
3 4
O
ollow colloidal particles have recently been subjected to
extensive research for their unique characteristics such as
Therefore, it remains a grand challenge to develop a new strategy
for fabricating yolk-shell structures with both multishells, built-in
shell functionalities, and especially unique shell structures.
Here, we demonstrate a facile “hydrothermal etching assisted
H
high surface area, low density, and potential applications in
1
catalysis, controlled release, and lithium-ion batteries. More
recently, to endow hollow particles with multifunctionalities and
synergetic properties, considerable effort has been devoted to
rational design of the shells with desirable components and
unique structures, and functionalize the interior spaces through
crystallization” route to produce Fe O @titanate double-shelled
3
4
yolk-shell microspheres. To our knowledge, this is the first report
on synthesizing this unique structure with double shells con-
structed by 2D ultrathin nanosheets, which intrinsically possess
active acid sites. The solÀgel process of tetraethyl orthosilicate
2À6
encapsulating functional species,
namely, creating yolk-shell
structures with both hollow space, functional interior core and
useful outer shell.
(
TEOS) and tetrabutyl titanate (TBOT) was explored to coat
SiO and TiO on uniform Fe O nanoparticles in turn, giving
So far, numerous approaches based on template-assisted
2
2
3 4
3
,4
5
6
rise to a typical sandwich structure (Fe O @SiO @TiO ).
process, Kirkendall or Ostwald ripening effect, and one-step
3
4
2
2
2
During hydrothermal treatment in an alkaline solution, the SiO2
interlayer was first etched, followed by concurrent but separate
surfactant-involved strategy have been developed to synthesize
yolk-shell structures; however, most are composed of a single-
layer shell. Multishelled yolk-shell structures can provide a
chance to create sophisticated functionalities through introdu-
TiO layer-etching and epitaxial titanate nanosheets-growth. In
2
this case, the SiO layer not only promotes the coating of TiO ,
2
2
7
but also acts as the template for the first cavity. The TiO layer
cing different guest species into selective chambers. Lou and co-
2
provides two interfaces for the nucleation and discrete space for
simultaneously self-assembled nanosheets-growth along oppo-
site directions, and in situ acts as the template for the second
cavity. In this way, the yolk-shell structure with nanosheets-
workers have reported the synthesis of double-shelled SnO2
yolk-shell structures based on a hydrothermal shell-by-shell
8
deposition method. Wu and co-workers have demonstrated a
layer-by-layer method to produce multishelled SiO yolk-shell
2
9
assembled double shells is formed. The inclusion of Fe O cores
structures. To the best of our knowledge, almost all existing
3 4
allows the convenient separation and recycling by applying
methods for yielding multiple shells are exclusively based on the
10
1a
external magnetic fields. The designed yolk-shell microspheres
possess a uniform size, tailored shell structure, good structural
stability, versatile ion-exchange capability, high surface area, large
magnetization, and exhibit a remarkable catalytic performance.
The synthesis procedure involves three steps (Figure 1). First,
uniform magnetite particles were coated with silica through
classical St €o ber method using TEOS as a precursor. Second,
layer-by-layer strategy. Moreover, the shells are generally
composed of aggregated nanoparticles. Very little success has
been achieved to construct the shells with anisotropic building
blocks, such as one-dimensional (1D) nanowires, naotubes, and
3
f
2
D nanosheets. In addition, although remarkable progress has
been made in the synthesis of yolk-shelled catalysts with an active
core and a protective shell, there is rarely effective work to in situ
produce the active catalysis sites, especially acid sites, on the shell.
It is well-known that such heterogeneous acid catalysts are easy
to handle and applicable for designing “green” catalytic process.
Received: June 15, 2011
Published: September 09, 2011
r 2011 American Chemical Society
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|J. Am. Chem. Soc. 2011, 133, 15830–15833

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Figure 2. TEM images of the (A) Fe O @SiO and (B) Fe O @-
3
4
2
3 4
2 2
SiO @TiO microspheres. The inset in B is a magnification TEM image
and the scale bar is 100 nm.
the resulted Fe O @SiO microspheres were further coated with
3
4
2
titania through a facile solÀgel approach using TBOT as a
precursor. Third, the obtained Fe O @SiO @TiO micro-
3
4
2
2
spheres were hydrothermally treated in NaOH solution (1.0 M)
at 150 °C, leading to the formation of Fe O @titanate double-
3
4
shelled yolk-shell microspheres.
The magnetite particles were prepared via a solvothermal
10b
method as described previously. The obtained Fe O particles
3
4
possess uniformly spherical shape and a mean diameter of
180 nm (Figure S1). The particles show excellent dispersibility
∼
in polar solvents because of numerous citrate groups anchored
on the surface, which favors the subsequent coating with silica
and titania. Fe O @SiO microspheres obtained after the first
3
4
2
solÀgel process show a relatively smooth surface with a diameter
of ∼280 nm (Figure S2a). TEM image indicates that the
thickness of the SiO layer is ∼50 nm (Figure 2A). The further
2
solÀgel process leads to the formation of Fe O @SiO @TiO
3
4
2
2
microspheres (Figure S2b) with a diameter of ∼340 nm, in-
dicating the presence of ∼30 nm thick TiO layer. The Fe O @-
2
3 4
SiO @TiO microspheres possess rough surface compared with
2
2
Fe O @SiO microspheres (Figure S2 inset). It may be attrib-
3
4
2
Figure 3. The obtained Fe
3
O
4
@titanate double-shelled yolk-shell
uted to the fact that TBOT undergoes hydrolysis and condensa-
tion faster than TEOS, thus, yielding the large nanoclusters of
titania. TEM images (Figure 2B) further confirm the rough
surface and the typical sandwich structure, where obvious con-
trast of the rough titania-layer, gray silica-interlayer, and dark
magnetite core can be clearly visible.
microspheres: (a) FESEM images (inset is the magnification FESEM
image, which clearly shows the ultrathin-nanosheets self-assembled
structure), (b) the FESEM image of a single sphere, (c and d) TEM
images, (e) HRTEM images (the scale bar of the inset is 1 nm), and (f)
the STEM image; the scale bars in b, d, and f are all 200 nm. (gÀj) EDX
elemental maps of Ti, O, Na, and Fe, respectively, (k) line scanning
profiles of Ti, O, Na, and Fe recorded along the line shown in f.
After the hydrothermal treatment in NaOH solution (1.0 M)
at 150 °C, unique flower-like microspheres with a uniform size of
∼
560 nm are obtained (Figure 3a). The X-ray diffraction (XRD)
pattern of the obtained microspheres (Figure S3) shows the
of each nanosheet varies in the range of 2À6. A magnification
HRTEM image (Figure 3e inset) clearly reveals three mono-
characteristic broad diffraction peaks, which can be indexed to
layers, which are relative to the edge-sharing TiO octahedra
6
11,10b
the layered titanates and Fe O particles, respectively.
and the interlayer Na ions, endowing the versatile ion-exchange
3
4
1
2
It suggests the amorphous titania experiences a localized crystal-
lization process during the hydrothermal treatment. A magnifica-
tion FESEM image (Figure 3b) clearly reveals that the micro-
spheres are composed of numerous ultrathin nanosheets with
bottom edges pointing toward the center. The slight curl and
self-attachment of the nanosheets result in the highly open and
stable structure. TEM observations (Figure 3c,d) clearly in-
dicate that the microspheres possess a unique yolk-shell struc-
ture, where a dark particle encapsulated in two separated gray
layers and the self-assembled nanosheets can be clearly eluci-
dated. The mean shell thicknesses of the outer and inner layer
are estimated to be around ∼110 and ∼40 nm, respectively.
Furthermore, the double-shelled structure is also verified from
the broken microspheres (Figure S4). High-resolution TEM
capability. Generally, the lamellar nanosheets formed during
the alkaline hydrothermal treatment are very unstable and
spontaneously roll into tubular structures driven by excess sur-
1
2
face energy. The remarkable ultrathin nanosheets obtained in
this study are probably caused by the in situ crystallization and
self-attachment of them on the interface between TiO and the
2
alkali solution. The scanning TEM (STEM) image of a single
+
yolk-shell microsphere after the H -exchange, combined with
EDX elemental mapping and line scanning (Figure 3fÀk), clearly
reveals the double-shelled yolk-shell structure with a magnetite
core and titanate double shells. The integrated energy dispersive
X-ray spectroscopy (EDS) analysis (Figure S5) further confirms
the presence of Ti, Fe, O, and neglectable amount of Na and Si.
The atomic ratio of Ti and Fe is calculated to be ∼2.56, which is
consistent with the results (the atomic ratio of Ti/Fe = ∼2.44,
∼0.48 wt % for Na and ∼0.12 wt % for Si) determined from
the elemental analysis by inductively coupled plasma-atomic
(
HRTEM) image (Figure 3e) clearly shows that the nanosheets
are the lamellar structures with a d-spacing of ∼0.8 nm,
consistent with the XRD results (Figure S3). The layer number
1
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Journal of the American Chemical Society
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TEM images indicate that the diameters of the inner-shell and
inward-diameter of the outer shell (circled in Figure 4f) are ∼265
and ∼360 nm, consistent with the inner (280 nm) and outer one
(
340 nm) of the amorphous TiO layer (circled in Figure 4b),
2
respectively. That clearly indicates that the titanate-nanosheets
grow oppositely along the surface of TiO layer, accompanied by
2
some structure shrinkage. Further prolonging the reaction time
(
48 h), nanosheets tend to roll into nanotubes driven by the weak
attachment interactions and the excess surface energy (Figure
S11). With this interesting structural evolution, the entire micro-
sphere size increases from ∼340 to ∼560 nm (Table S1) with
increasing hydrothermal treatment time; however, the thickness
of the nanosheet shows no obvious change (Figure S9c,d and
Figure 3e). The intensities of diffraction peaks from titanate
(
Figure S10) gradually become stronger from 0 min to 24 h,
Figure 4. (a) Schematic illustration of the “hydrothermal etching
assisted crystallization” strategy for the formation of the Fe O @titanate
3 4
double-shelled yolk-shell microspheres. TEM images of a single sphere
further demonstrating that the nanosheets grow longer with
prolonging treatment time.
On the basis of the above observations, we propose a “hydro-
thermal etching assisted crystallization” strategy for the formation
of the Fe O @titanate double-shelled yolk-shell microspheres
synthesized at 150 °C with hydrothermal treatment time: (b) 0 min,
(c) 20 min, (d) 2 h, (e) 12 h, and (f) 24 h.
3
4
emission spectrometry (ICP-AES). The results indicate that Si
species are almost completely removed after the hydrothermal
treatment, resulting in Fe O @titanate double-shelled yolk-shell
(Figure 4a). The high concentration NaOH solution (1.0 M) first
can etch the amorphous silica layer at the beginning of the
reaction, resulting in hollow titania micrsopheres encapsulated
magnetite particles (the first cavity). The porous structure of the
3
4
microspheres with versatile ion-exchange capability.
The magnetic properties of the pristine Fe O particles and
TiO shells helps the alkali solution permeate conveniently and
3
4
2
Fe O @titanate microspheres were investigated using a super-
enables the formation of relatively high alkali concentration along
3
4
conducting quantum interference device (SQUID) magnet-
ometer (Figure S6). Both the samples show the typical
hysteresis loops with no detectable remanence or coercivity,
reflecting a superparamagnetic property. The magnetization
saturation values of the Fe O particles and Fe O @titanate
the inner and outer interface, which facilitates the consequent
nucleation. As the reaction progresses, the amorphous TiO layer
2
starts to be etched. Simultaneously, the epitaxial titanate na-
nosheets-growth which occurs as the result of heterogeneous
4
f
nucleation is energetically favored over the homogeneous one.
3
4
3 4
microspheres are 63.7 and 17.7 emu/g, respectively. The Fe O4
It should be noted that the two interfaces and separated space
make the nanosheets grow along the opposite directions, and the
3
content of Fe O @titanate microspheres is calculated to be ∼28
3
4
wt %, consistent with the EDX and ICP results. As a result of the
superparamagnetic property and high magnetization, the yolk-
shell microspheres in their homogeneous dispersion show fast
motion to the applied magnetic field. Once the magnetic field is
removed, they can be quickly redispersed into homogeneous
suspension upon a slight shake. This trait enables the efficient
separation and purification, and facilitates their wide applications.
TiO layer-dissolution in situ produces the second cavity. The
2
stable structure strongly relies on spontaneous self-assembly of
the 2D ultrathin nanosheets into 3D hierarchical spheres, which
reduces the surface energy and prevents them from wrapping or
collapse. The further titania-etching and titanate-growth may be
6
facilitated through Ostwald ripening. Obviously, as the hydro-
thermal reaction proceeds, the TiO layer can be eventually
2
N sorption measurements show that the yolk-shell micro-
dissolved and the titanate nanosheets grow longer along inde-
pendent directions, thereby creating the double-shelled yolk-shell
structure. However, further prolonging the reaction time could
increase the length of the nanosheets but decrease the self-
attachment interaction, resulting in nanotubular strcuture. There-
fore, by simply controlling the hydrothermal treatment time, the
shell structure can be effectively tuned from single layer to double
layers, meanwhile the building block of each layer varies from 0D
nanoparticles to 2D nanosheets and 1D nanotubes.
2
spheres have a nanoporous structure (Figure S7), derived from
the packing of the nanosheets in the shells. It exhibits a type-IV
isotherm with a type-H1 hysteresis loop. The pore size distribu-
tion derived from the adsorption branch by using the Bar-
rettÀJoynerÀHalenda (BJH) model indicates a mean pore size
of ∼7.5 nm. The BET surface area and total pore volume are
2
3
calculated to be ∼150 m /g and ∼0.27 cm /g, respectively.
To gain insight into the formation of the Fe O @titanate
3
4
double-shelled yolk-shell microspheres (Figure 4a), time-depen-
dent experiments were carried out. After the hydrothermal
treatment at 150 °C for 20 min, the typical sandwich structure
The magnetic separability and unique yolk-shell structure with
high surface area and highly open pores make the Fe O @tita-
3
4
nate microspheres ideal candidates for catalysis application. The
catalytic performance of the Fe O @titanate microspheres was
(
Figure 4b) was converted to a yolk-shell structure (Figure 4c,
3
4
Figure S8a) with no obvious change on the TiO layer (Figure
tested for the FriedelÀCrafts (FC) alkylation of toluene with
benzylchloride, and compared with that of conventional solid
acid catalysts (Table S2). The Fe O @titanate microspheres
2
S9a,b). Compared with the XRD patterns of the parent Fe O @-
3
4
SiO @TiO microspheres, the characteristic diffraction peak from
2
2
3 4
amorphous silica is absent in the obtained products (Figure S10).
After continuous hydrothermal treatment for 2 h, the shells of the
microspheres were found to be composed of ultrathin nanosheets
show a high conversion of 97.1% with a selectivity of 54% toward
p-benzyltoluene, which is better than other acid catalysts such as
2-
niobic acid, zeolites, and SO /ZrO , due to the lagre pore and
4
2
(
Figure4d, FigureS8b). As timewas prolonged to 12h (Figure4e,
high surface area. The result is comparable with protonated
titanate nanotubes. The excellent performance may be attrib-
uted to: (1) the high porosity and hollow structure, which are
13a
Figure S8c) and 24 h (Figure 4f), well-developed yolk-shell
structure with nanosheets-assembled double shells are formed.
1
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beneficial for the transportation and adsorption of large molecules
thus, promoting the catalytic process; (2) the ultrathin nanosheets
structures, which enable a maximized molecular diffusion and
make active sites exposed to reactants; (3) the synergic effect
between Lewis and Brønsted acid sites of titanates, which plays an
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3
4
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catalytic performance without visible reduction even after running
for more than 8 cycles (Figure S12). Interestingly, the Fe O @-
3
4
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3
4
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shell through simple annealing (Figure S13a). The double-shelled
yolk-shell structure were well retained (Figure S13b). The
Fe O @NS-TiO microspheres show an excellent photocatlytic
3
4
2
À1
activity toward Rhodamine B with a rate constant of 0.072 min ,
À1
which is faster than that of P25 TiO (0.042 min , Figure S13c).
2
Because of the magnetic core, the catalysts can be easily recycled
and exhibit approximately constant activity even after 10 cycles
(
Figure S13d). The high porosity and ultrathin nanosheets struc-
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tures of Fe O @NS-TiO microspheres offer more active sites,
which are responsible for the excellent performance.
3
4
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2
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In summary, we have demonstrated a facile “hydrothermal
(
etching assisted crystallization” strategy to synthesize Fe O @-
titanate yolk-shell microspheres with ultrathin nanosheets-as-
sembled double-shell structure. The obtained Fe O @titanate
3
4
(
3
4
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Crafts alkylation. The corresponding Fe O @NS-TiO deriva-
3
4
2
tives show excellent photocatalytic activeity. This facile synthesis
strategy can be easily extended to design other multifunctional
yolk-shell materials, such as Au@titanate, Fe O @ titanate and
2
3
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ASSOCIATED CONTENT
2
006, 18, 2807. (b) Bavykin, D. V.; Walsh, F. C. Eur. J. Inorg. Chem.
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(13) (a) Kitano, M.; Nakajima, K.; Kondo, J. N.; Hayashi, S.; Hara,
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Supporting Information. Detailed experimental proce-
b
dures, characterization methods, SEM images, XRD data, EDX
data, N sorption results, TEM images and catalytic performance
measurements. This material is available free of charge via the
Internet at http://pubs.acs.org.
M. J. Am. Chem. Soc. 2010, 132, 6622. (b) Li, S. H.; Zheng, A. M.; Su,
Y. C.; Zhang, H. L.; Chen, L.; Yang, J.; Ye, C. H.; Deng, F. J. Am. Chem.
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(
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’
AUTHOR INFORMATION
Kim, J. H.; An, K. J.; Yu, J. H.; Kwon, S. G.; Choi, S. H.; Kuk, Y.;
Hoffmann, R.; Hyeon, T. Angew. Chem., Int. Ed. 2009, 48, 6861.
Corresponding Author
yhdeng@fudan.edu.cn; dyzhao@fudan.edu.cn
(b) Son, J. S.; Yu, J. H.; Kwon, S. G.; Lee, J.; Joo, J.; Hyeon, T. Adv.
Mater. 2011, 23, 3214.
’
ACKNOWLEDGMENT
We greatly appreciate financial support from the National
Science Foundation (20890123 and 21073040) and the State
Key Basic Research Program of China (2009AA033701 and
2
009CB930400) and Science & Technology Commission of
Shanghai Municipality (08DZ2270500) and Shanghai Leading
Academic Discipline Project (B108).
’
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