1. No category

Nano Research
Nano Res
DOI
10.1007/s12274-014-0647-0
Magnetic Yolk-Shell Structured Anatase-based
Microspheres Loaded with Au Nanoparticles for
Heterogeneous Catalysis
Chun Wang1, Junchen Chen1, Xinran Zhou1, Wei Li1, Yong Liu1, Qin Yue1, Zhaoteng Xue1, Yuhui Li1,
Ahmed A. Elzatahry2,3, Yonghui Deng()1, Dongyuan Zhao1
Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0647-0
http://www.thenanoresearch.com on November 24 2014
© Tsinghua University Press 2014
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1
TABLE OF CONTENTS (TOC)
Magnetic
Yolk-Shell
Structured
Anatase-based
Microspheres Loaded with Au Nanoparticles for
Heterogeneous Catalysis
Chun Wang1, Junchen Chen1, Xinran Zhou1, Wei Li1,
Yong Liu1, Qin Yue1, Zhaoteng Xue1, Yuhui Li1, Ahmed
A. Elzatahry2,3, Yonghui Deng*1, Dongyuan Zhao1
1Department
of Chemistry, Laboratory of Advanced
Materials, State Key Laboratory of Molecular Engineering
of Polymers, and Shanghai Key Laboratory of Molecular
Catalysis and Innovative Materials, Fudan University,
Shanghai 200433, P. R. China.
2,3Department
of Chemistry-College of Science, King
Saud University, Riyadh 11451, Saudi Arabia; Polymer
Materials Research Department, Advanced Technology
and New Materials Research Institute, City for Scientific
Research and Technology Applications, New Borg
El-Arab City, Alexandria 21934, Egypt.
Magnetic yolk-shell structured anatase-based microspheres
were fabricated through successive and facile sol-gel coating on
magnetite particles, followed by annealing treatments. Upon
loaded with gold nanoparticles, the obtained functional
magnetic microspheres as stable heterogeneous catalysts
showed superior performance in catalyzing the epoxidation of
styrene with extraordinary high conversion (89.5 %) and
Page Numbers.
selectivity (90.8 %) towards styrene oxide after reaction for
33 h.
1
Nano Res
DOI (automatically inserted by the publisher)
Research Article
Magnetic Yolk-Shell Structured Anatase-based Microspheres
Loaded with Au Nanoparticles for Heterogeneous Catalysis
Chun Wang1, Junchen Chen1, Xinran Zhou1, Wei Li1, Yong Liu1, Qin Yue1, Zhaoteng Xue1, Yuhui Li1, Ahmed
A. Elzatahry2,3, Yonghui Deng()1, Dongyuan Zhao1
1
Department of Chemistry, Laboratory of Advanced Materials, State Key Laboratory of Molecular Engineering of Polymers, and
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, P. R. China.
2,3Department of Chemistry-College of Science, King Saud University, Riyadh 11451, Saudi Arabia; Polymer Materials Research
Department, Advanced Technology and New Materials Research Institute, City for Scientific Research and Technology
Applications, New Borg El-Arab City, Alexandria 21934, Egypt.
Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher)
© Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011
ABSTRACT
Magnetic yolk-shell structured anatase-based microspheres were fabricated through successive and facile
sol-gel coating on magnetite particles, followed by annealing treatments. Upon loaded with gold nanoparticles,
the obtained functional magnetic microspheres as heterogeneous catalysts showed superior performance in
catalyzing the epoxidation of styrene with extraordinary high conversion (89.5 %) and selectivity (90.8 %)
towards styrene oxide. It is believed that the constructing process of these fascinating materials features many
implications for creating other functional nanocomposites.
KEYWORDS
Magnetic microspheres, titania, yolk-shell structure, gold nanoparticles, heterogeneous catalysis
1. Introduction
Due to their attractive apparent nanoarchitectures,
tailorable diverse physicochemical properties and
functionalities, yolk-shell nanoparticles have emerged
as a favorite kind of nanomaterials among scientist
community [1-4]. Till now, yolk-shell nanoparticles
have been found versatile in a series of advanced
applications including lithium-ion batteries [5-6],
nanocatalysis [7-9], drug delivery systems [10-12], and
so on. Besides, considering the chemical composition
of nanomaterials, TiO2 is one of the most appealing
chemical species [13-15], showing great promise in the
fields of photocatalysis [16-19] and electrochemistry
[20,
21].
Particularly,
magnetically
separable
nanocomposites have recently been developed in order
to recover and reuse of expensive precious metallic
catalysts and absorbents [22-30]. Though great
progress has been achieved in addressing
above-mentioned respective standpoints, however,
seldom work has been done based on systematic
consideration of these viewpoints and the
composition-structure-characteristics relationship of
complex multifunctional nanomaterials is still
————————————
Address correspondence to Yonghui Deng, email: [email protected]
2
sophisticated. Also, few papers about magnetic
yolk-shell structured anatase-based microspheres have
been published.
Herein, we report magnetic yolk-shell structured
anatase-based microspheres by a successive and facile
sol-gel coating on initial solvothermal reaction-derived
magnetite particles, followed by annealing treatments.
After loading with gold nanoparticles, the functional
magnetic microspheres as heterogeneous catalysts
showed superior performance in catalyzing the
epoxidation of styrene with extraordinary high
conversion (89.5 %) and selectivity (90.8 %) towards
styrene
oxide.
The
obtained
multifunctional
nanomaterials are an ideal platform for us to better
understand the composition-structure-characteristics
relationship of complex multifunctional nanomaterials.
It is believed that the constructing process of these
fascinating materials features many implications for
creating other functional nanocomposites and the
obtained nanomaterials should find more diverse
potential applications such as bioseparation and drug
delivery in the near future.
2. Results and Discussion
Magnetite particles were first synthesized via a
solvothermal reaction of ferric chloride and ethylene
glycol in the presence of trisodium citrate [31].
Scanning electron microscopy (SEM) image (Figure S1a)
exhibits that the obtained magnetite particles possess a
uniform size of ~ 120 nm in diameter and nearly
spherical shape. Transmission electron microscopy
(TEM) image (Figure S1b) further validates their size
and spherical morphology with rough surface, which
can be ascribed to packing of many nanocrystals as
subunits. The magnetite particles were modified with
citrate groups and thus possess excellent dispersibility
in polar solvents such as water and ethanol,
establishing a solid foundation for the subsequent
coating or modification with other oxides or polymers
[31-32]. Employing a sol-gel method by the hydrolysis
and condensation of tetraethyl orthosilicate (TEOS) in
ammonia/ethanol solution, uniform silica layer (~ 50 or
10 nm in thickness) was formed on initial solvothermal
reaction-derived magnetite particles, giving rise to
core-shell Fe3O4@SiO2 microspheres. As revealed by the
SEM (Figure S2-3) and TEM (Figure S4) images, the
obtained Fe3O4@SiO2 microspheres exhibit uniform
size (~ 220 or 140 nm) and regular spherical shape with
smooth surface as a result of the deposition and
subsequent growth of silica on a molecular scale
during the sol-gel process [32-34]. Sol-gel coating of
Fe3O4@SiO2
microspheres
with
resorcinol-formaldehyde resin was subsequently
performed under the alkaline condition [35-40]. SEM
(Figure S5) and TEM images (Figure S6) prove that the
sandwich-like core-shell structured microspheres are
uniform with a mean size in diameter (~ 300 or 270 nm,
the
thickness
of
the
outer
layer
of
resorcinol-formaldehyde (RF) resin is ~ 40 or 65 nm,
respectively). It is also proved that the thickness of the
shell layer of resorcinol-formaldehyde (RF) resin can
be well controlled in a rather broad range by tuning
the mass of resorcinol/formaldehyde, which can be
further converted into carbon for carbon-based
functional nanomaterials or utilized as sacrificial
template for diverse hollow nanostructures. Then,
uniform
magnetic
core-shell
structured
Fe3O4@SiO2@RF@TiO2 microspheres with a diameter of
~ 360, 500, and 350 nm (the thickness of the outer layer
of amorphous TiO2 is ~ 30, 100, and 40 nm, respectively)
were obtained via the reported sol-gel process of
hydrolysis and condensation of tetrabutyl titanate
(TBOT) in ethanol/ammonia mixtures (Figure S7,
Figure 1) [17]. TEM images (Figure 1a-b, d-e, and g-h)
reveal that individual Fe3O4@SiO2@RF microspheres
are uniformly coated with porous TiO2 shell, which is
derived from numerous aggregated nanoparticles.
According to the previous report [17], the thickness of
TiO2 outer layer under the current ammonia-ethanol
system using the extended Stöber sol-gel method was
somewhat limited. However, by tuning the volume
and multistep addition of TBOT, the thickness can be
further tailored in a large range (Figure 1), which
paves a solid base for constructing diverse TiO2-based
nanostructures meanwhile retaining their well-defined
porous morphologies. Utilizing a two-step annealing
treatment,
magnetic
yolk-shell
structured
anatase-based microspheres (Fe3O4@SiO2@Void@TiO2)
were obtained. TEM images (Figure 1c, f, and i)
confirm that uniform microspheres are retained with a
slight decrease in diameter (~ 350, 450, and 340 nm,
and the thickness of anatase-based TiO2 outer layer is
shrunk to ~ 25, 75, and 35 nm, respectively) and an
aggregated nanocrystals-derived porous anatase-based
shell. The uniform spherical morphology is also
validated by SEM observation shown in Figure S8, and
the specific yolk-shell structure is verified by some
purposely broken microspheres. HRTEM image (the
inset of Figure 1f, i) show that the TiO2 nanoparticles
are well crystallized with a mean size of ~ 5 nm and a
d-spacing of 0.35 nm, which can be well matched to the
d101 of anatase TiO2. The XRD patterns of the calcined
samples (Figure S9) exhibit the characteristic
diffraction peaks that can be well indexed into anatase
TiO2 and Fe3O4. Control experiment for magnetic
3
core/shell-type microspheres (Fe3O4@SiO2@TiO2) was
also performed (Figure S10-11).
Figure 1 TEM images of magnetic core-shell structured
microspheres (Fe3O4@SiO2@RF@TiO2) and the corresponding
magnetic yolk-shell structured anatase-based microspheres
synthesized via a successive sol-gel coating process. Panels (a-b),
(d-e) and (g-h) correspond to Fe3O4@SiO2@RF@TiO2-0,
Fe3O4@SiO2@RF@TiO2-1,
and
Fe3O4@SiO2@RF@TiO2-2,
respectively. Panels (c), (f), and (i) correspond to M-0, M-1, and
M-2, respectively. Insets in panels (f, i) are the corresponding
magnified images. The scale bars are all 50 nm.
X-ray photoelectron spectroscopy (XPS) survey
spectra (Figure S12-13, Table S1) of magnetic
yolk-shell structured anatase-based microspheres
show three well-resolved peaks of C 1s, Ti 2p, and O
1s with negligible peak of Fe element, suggesting that
the magnetite core is uniformly coated. The relative
high content of carbon in the microspheres indicates
the presence of abundant alkoxy moieties in the
matrix of the anatase-based shell, suggesting the
incompletely hydrolyzation of TBOT under the
current condition (Scheme 1). The O 1s
high-resolution XPS spectra can be further split into
three single peaks corresponding to Ti-O (530.0 eV),
O-H (531.6 eV), and C-O (533.0 eV) functional groups
(Figure S14). Nitrogen sorption isotherms (Figure S15,
Table S2) of the magnetic core-shell structured
microspheres
(Fe3O4@SiO2@RF@TiO2-1),
Fe3O4@SiO2@RF@TiO2-1-N2-500 (the sample after
annealing treatments under N2 at 500 C), and
magnetic
yolk-shell
structured
anatase-based
microspheres (M-1) show typical type IV curves with
distinct hysteresis loops that are close to H1 type. The
BET surface area and pore volume of the as-made
microspheres are calculated to be as high as 387 m2/g
and 0.24 cm3/g, respectively, suggesting highly porous
structures. After calcination at 500 C under N2 and
further under air, the BET surface area and pore
volume are decreased from 96.0 to 50.8 m2/g and from
0.11 to 0.09 cm3/g, respectively. This is probably due to
the densification of the TiO2 networks and the growth
of nanocrystals [17]. Typically, the pore size
distributions before and after removal of the in-situ
formed carbon were almost unchanged. From a
combination of data from TEM and BET analysis, it
can be concluded that the wall of the TiO2 outer shell
is retained well under the calcination process, giving
rise to yolk-shell anatase-based microspheres. The
formation of magnetic yolk-shell structured
anatase-based microspheres with intact anatase-based
outer shell should be attributed to this
carbon-protected calcination process (Scheme 2).
Direct calcination of the as-made magnetic core-shell
structured microspheres under air only gives rise to
some microspheres with shattered anatase-based shell
(Figure S16). The morphology loss might be caused by
the significant structural rearrangement in the outer
TiO2 shell associated with the massive crystallization
and grain growth during calcination. The in-situ
formed carbon can support the pre-crystallized TiO2
outer shell during annealing treatments under N2 and
be further burn out to form yolk-shell structure while
retaining the whole spherical morphology during
subsequent annealing treatments under air, which is
similar to that for some previous reports [18, 41-42].
Also, the nitrogen sorption isotherms and
corresponding pore size distribution curves of M-0
and M-2 and shown in Figure S17-18. Their textural
properties are summarized in Table S2.
Scheme 1 The illustration for the formation of the magnetic
core-shell structured microspheres (Fe3O4@SiO2@RF@TiO2)
through a successive sol-gel process, using TEOS,
resorcinol-formaldehyde, and TBOT as precursors, respectively.
4
To test the feasibility of these multifunctional
magnetic microspheres, gold nanoparticles were
loaded into their cavities through a unique
deposition-precipitation (DP) method mediated by a
cationic complex precursor ([Au(en)2]3+, en =
ethylenediamine) as reported previously [43-44]. TEM
images (Figure S19) reveal that small Au nanoparticles
(ca. 4.0 nm) are well confined and the corresponding
enlarged image shows the lattice fringes with a
spacing of about 0.23 nm, which can be assigned to
the spacing of the (111) planes of single crystalline Au
(inset of Figure S19b). Nitrogen sorption isotherms
and the corresponding pore size distribution of the
sample Au@M-0 (Figure S20) indicates a similar
curves with that of the parent magnetic yolk-shell
structured anatase-based microspheres, associated
with slightly reduced values of BET surface area and
pore volume (Table S2).
Scheme 2 The formation of the magnetic yolk-shell structured
anatase-based microspheres (M) derived from magnetic core-shell
structured microspheres (Fe3O4@SiO2@RF@TiO2) through a
carbon-protected calcination process.
The obtained Au@M-0 was then employed as a
catalyst for the epoxidation of styrene using t-butyl
hydroperoxide (TBHP) as an oxidant under argon
atmosphere. As shown in Figure 2, the conversion of
styrene increases rapidly and reaches about 90.2 % at
36 h, suggesting that the catalyst has a rather high
catalytic activity. The selectivity of epoxidation
maintains at a relatively high level and reaches 88.5 %
at 36 h. Considering both conversion and selectivity, it
can be concluded that the Au-loaded multifunctional
magnetic microspheres can achieve high conversion
(89.5 %) and selectivity (90.8 %) after reaction for 33 h,
which is higher than the previous results of silica or
carbon-based Au or Ag-loaded catalysts [32, 45-50].
Particularly, the magnetic catalyst can be easily
separated from the reaction system with a magnet,
and no significant decrease in both conversion and
selectivity is observed after cycling for 3 times (Figure
S22). Control experiment based on the parent
magnetic
yolk-shell
structured
anatase-based
microspheres (M-0) was performed and only
negligible catalytic activity was found (Figure S21).
We attribute the excellent performance of these
multifunctional magnetic yolk-shell structured
anatase-based microspheres to the possible interplay
of microenvironment confinement effect and
synergistic effect between confined Au nanoparticles
and anatase-phase TiO2.
Figure 2 Epoxidation of styrene was carried out by using
Au-loaded magnetic microspheres (Au@M-0) as the heterogeneous
catalysts. The conversion of styrene and selectivity towards styrene
oxide were functioned with the reaction time. The inset is the
corresponding illustration of the 3-D mesostructure of Au-loaded
magnetic microspheres (Au@M-0).
3. Conclusions
In
summary,
magnetic
yolk-shell
structured
anatase-based microspheres were fabricated through
successive and facile sol-gel coating on initial
solvothermal reaction-derived magnetite particles,
followed by annealing treatments. Upon deposition of
gold nanoparticles, the obtained functional magnetic
microspheres as heterogeneous catalysts showed
superior performance in catalyzing the epoxidation of
styrene with extraordinary high conversion (89.5 %)
and selectivity (90.8 %) towards styrene oxide. It is
believed that the constructing process of these
fascinating materials features many implications for
creating other functional nanocomposites.
Acknowledgements
This work was supported by the State Key 973
Program of PRC (2013CB934104 and 2012CB224805),
the NSF of China (51372041, 51422202), the specialized
5
research fund for the doctoral program of higher
education of China (20120071110007), the innovation
program
of
Shanghai
Municipal
Education
Commission (13ZZ004), and the Program for New
Century
Excellent
Talents
in
University
(NCET-12-0123). We extend our appreciation to the
Deanship of Scientific Research at King Saud
University for funding the work through the research
group project No RGP-227.)
Electronic Supplementary Material: Supplementary
materials (details of the experimental, SEM images,
TEM images, XRD patterns, XPS spectra, N2 sorption
isotherms and pore size distribution) are available in
the
online
version
of
this
article
at
http://dx.doi.org/10.1007/s12274-***-****-*
(automatically inserted by the publisher).
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7
Electronic Supplementary Material
Magnetic Yolk-Shell Structured Anatase-based Microspheres
Loaded with Au Nanoparticles for Heterogeneous Catalysis
Chun Wang1, Junchen Chen1, Xinran Zhou1, Wei Li1, Yong Liu1, Qin Yue1, Zhaoteng Xue1, Yuhui Li1, Ahmed A.
Elzatahry2,3, Yonghui Deng()1, Dongyuan Zhao1
1
Department of Chemistry, Laboratory of Advanced Materials, State Key Laboratory of Molecular Engineering of Polymers, and
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, P. R. China.
2,3Department of Chemistry-College of Science, King Saud University, Riyadh 11451, Saudi Arabia; Polymer Materials Research
Department, Advanced Technology and New Materials Research Institute, City for Scientific Research and Technology Applications,
New Borg El-Arab City, Alexandria 21934, Egypt.
Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)
Experimental section
Chemicals. FeCl3·6H2O, trisodium citrate, sodium acetate, tetraethylorthosilicate (TEOS), resorcinol,
formaldehyde, tetrabutyl titanate (TBOT), concentrated ammonia solution (28 wt %), HAuCl 4·4H2O, NaOH,
styrene and the solvents of ethylene glycol, ethylenediamine, acetonitrile and ethanol were purchased from
Shanghai Chemical Corp. t-butyl hydroperoxide (70 wt % in water) was purchased from Aldrich. Prior to use,
the inhibitor in the as-received styrene was eliminated by passing through a column of alumina. All other
chemicals were used without further purification and deionized water was used for all experiments.
Synthesis of magnetite Fe3O4 Particles. The magnetite Fe3O4 particles were synthesized via a modified
solvothermal reaction. 1 Typically, FeCl3·6H2O (3.25 g), trisodium citrate (1.3 g), and sodium acetate (6.0 g)
were respectively dissolved in 20 mL, 40 mL, and 40 mL of ethylene glycol under magnetic stirring until
forming clear solutions. Then, the mixture of these three different solutions was stirred for 24 h and
subsequently transferred and sealed in a Teflon-lined stainless-steel autoclave (150 mL in capacity). The
autoclave was heated at 200 C for 10 h, and subsequently allowed to cool down to room temperature. The
black product was purified by three times of separation assisted by a magnet, decantation, and redispersion
in ethanol, and was finally redispersed into ethanol (170 mL) for further uses.
Synthesis of Fe3O4@SiO2 Microspheres. The core-shell structured Fe3O4@SiO2 microspheres were
prepared via a modified Stöber sol-gel method as follows.2 As a typical run for the synthesis, ethanolic
solution of magnetite particles (25 mL) was added to a three-neck round-bottom flask (500 mL in capacity)
charged with ethanol (250 mL), deionized water (70 mL), and concentrated ammonia solution (5.0 mL, 28
wt %) under mechanical stirring (ca. 200 rpm) for 15 min at 30 C. Then, 4.0 mL of TEOS was dropwise added.
The reaction was allowed to proceed for 12 h under continuous mechanical stirring (ca. 200 rpm). The
resultant product was collected by three times of separation by using a magnet, decantation, and
redispersion in ethanol, and was finally redispersed into ethanol (30 mL) for further uses. Control synthesis
was carried out by using 2.5 mL of concentrated ammonia solution, 2.0 mL of TEOS, and under all other
identical conditions. The derived samples were denoted as Fe 3O4@SiO2-1 and Fe3O4@SiO2-2, respectively.
————————————
Address correspondence to Yonghui Deng, email: [email protected]
8
Synthesis of Fe3O4@SiO2@RF Microspheres. The sandwich-like core-shell structured Fe3O4@SiO2@RF
microspheres were prepared through a modified sol-gel coating method as reported before.3 As a typical run
for the preparation, ethanolic solution of Fe 3O4@SiO2 microspheres (4.0 mL) (Fe3O4@SiO2-1 or Fe3O4@SiO2-2)
was mixed with ethanol (40 mL), water (20 mL), and concentrated ammonia solution (2.0 mL, 28 wt %) in a
three-neck round-bottom flask (100 mL in capacity) under ultrasonic treatment for 15 min and further
mechanical stirring (ca. 200 rpm) for 0.5 h at 30 C. Then, a certain amount (0.1 g for Fe3O4@SiO2-1 or 0.15 g
for Fe3O4@SiO2-2) of resorcinol (dissolved in 3.0 mL of ethanol) was added. After stirring for 0.5 h, a certain
volume (0.2 mL for Fe3O4@SiO2-1 or 0.30 mL for Fe3O4@SiO2-2) of formaldehyde was dropwise added. The
reaction was allowed to proceed under continuous mechanical stirring (ca. 200 rpm) for 12 h. The resultant
product was purified by three times of separation using a magnet, decantation, and redispersion in ethanol,
and was finally redispersed into ethanol (40 mL) for further usage. The corresponding samples were denoted
as Fe3O4@SiO2@RF-1 and Fe3O4@SiO2@RF-2, respectively.
Synthesis of Fe3O4@SiO2@TiO2 Microspheres. The core-shell sandwich-like structured Fe3O4@SiO2@TiO2
microspheres were obtained through the extended kinetics-controlled coating method as reported before.4
Typically, the above ethanolic solution of Fe 3O4@SiO2 (Fe3O4@SiO2-1) microspheres (4.0 mL) was mixed with
ethanol (116 mL) and concentrated ammonia solution (0.40 mL, 28 wt %) in a three-neck round-bottom flask
(250 mL in capacity) under ultrasonic treatment for 15 min and further mechanical stirring (ca. 200 rpm) for
0.5 h. After that, 0.75 ml of TBOT was dropwise added and the reaction was allowed to proceed for 24 h at 45
C under continuous mechanical stirring (ca. 200 rpm). To increase the thickness of the TiO2 outer shell, 0.75
mL of TBOT was subsequently added 12 hours after the first introduction. The resultant product was
collected assisted by utilizing a magnet, followed by washing with ethanol for 3 times. After being dried in
vacuum at 40 C for 24 h, the as-prepared Fe3O4@SiO2@TiO2 solid sample was put into a porcelain boat and
calcined at 500 C with a heating rate of 1 C/min in air for 10 h to eliminate the residual organic species and
improve crystallinity. The corresponding sample was denoted as Fe3O4@SiO2@ TiO2.
Synthesis of Fe3O4@SiO2@RF@TiO2 Microspheres. The magnetic core-shell structured
Fe3O4@SiO2@RF@TiO2 microspheres were prepared via the extended kinetics-controlled coating method as
follows.4 As a typical synthesis, ethanolic solution of Fe3O4@SiO2@RF (40 mL) was mixed with ethanol (80 mL)
and concentrated ammonia solution (0.40 mL, 28 wt %) in a three-neck round-bottom flask (250 mL in
capacity) under ultrasonic treatment for 15 min and further mechanical stirring for 0.5 h. Then, 0.75 mL of
TBOT was dropwise added and the reaction was allowed to proceed for 24 h at 45 C under continuous
mechanical stirring (ca. 200 rpm). To increase the thickness of the TiO 2 outer shell, 0.75 mL of TBOT was
subsequently added 12 hours after the first introduction. The resultant product was purified by three times
of separation using a magnet, decantation, and redispersion in ethanol, and the solid sample was finally put
into a porcelain boat. After being dried in vacuum at 40 C for 24 h, the as-prepared solid sample was
calcined at 500 C under N2 for 5 h, followed by calcined at 500 °C in air for 5 h to remove the organic species
and improve crystallinity. The corresponding as-prepared samples were denoted as Fe3O4@SiO2@RF@TiO2-1
and Fe3O4@SiO2@RF@TiO2-2, respectively. The thus derived magnetic yolk-shell structured anatase-based
microspheres (Fe3O4@SiO2@Void@TiO2) were denoted as M-1 and M-2, respectively. Control synthesis was
performed based on Fe3O4@SiO2@RF-1 by introducing of TBOT (0.75 mL) for once. The corresponding
as-made and calcined samples were denoted as Fe3O4@SiO2@RF@TiO2-0 and M-0, respectively.
Synthesis of Au-loaded Fe3O4@SiO2@Void@TiO2 (Au@M) Microspheres. Uniform gold nanoparticles
were loaded onto the cavities of M (Fe3O4@SiO2@Void@TiO2) using a unique deposition-precipitation (DP)
method mediated by a cationic complex precursor ([Au(en) 2]3+, en = ethylenediamine) as reported before. 5 In
a typical procedure, ethylenediamine (en) (50 mg) was added into 1.0 mL of HAuCl 4•4H2O aqueous solution
(0.10 g in 1.0 mL of H2O) under stirring until a clear aurantia solution was formed. Then, ethanol (8 mL) was
added and the precipitate of AuCl 3(en)2 was formed. The precipitation was subsequently collected by
9
centrifugation, washed with ethanol, and dried in vacuum at 40 C. Then, AuCl3(en)2 (15 mg) was added into
water (5 mL), and the pH value was adjusted to 10.0 by adding NaOH aqueous solution (2 M). Then, 40 mg
of the above magnetic yolk-shell structured anatase-based microspheres (Fe3O4@SiO2@Void@TiO2) sample
(M-0) was dispersed in the AuCl3(en)2 aqueous solution under ultrasound for 15 min. The solid was collected
by centrifugation, washed with deionized water, dried in vacuum at 40 C for 24 h, and then reduced under a
flowing H2/Ar (5.0 %) at 150 C for 1 h, yielded Au@M-0 composite catalyst.
Catalytic Epoxidation of Styrene. The obtained Au@M-0 (0.07 g) composite catalyst was added into a
mixture of acetonitrile (10.0 mL) and styrene (2.5 mL). The solution was then bubbled with Ar gas for 0.5 h
under magnetical stirring at ambient temperature. After injecting 10.0 ml of t-butyl hydroperoxide (70 wt %
in water), the reaction vessel was immersed in an oil bath and heated at 82 C under continuous magnetical
stirring. A minor amount of reaction solution (~ 20 μL) was withdrawn after a certain time for gas
chromatography-mass spectrometer (GC-MS) measurements. Prior to each sampling, the stirring was
temporarily stopped to induce attraction of the magnetic microspheres on the Teflon-coated magnetic stir bar
for avoiding loss of the catalyst during sampling. The reaction system was then cooled down after reaction
for 40 h.
Characterization. Transmission electron microscopy (TEM) measurements were carried out on a JEOL
JEM-2100 F microscope (Japan) operated at 200 kV. The samples were suspended in ethanol and supported
onto a holey carbon film on a Cu grid. Powder X-ray diffraction (XRD, D8 Diffractometer with Ni-filtered Cu
Kα radiation, Bruker, Germany) was used to determine the crystalline phase of the products. The
diffractometer was set at 40 kV working voltage and 40 mA working current, scanning from 5 to 80° in 2θ at
a step of 0.02° and a scan-step time of 4 s. Nitrogen sorption isotherms were measured at 77 K with a
Quantachrome Autosorb-1-MP (Quantachrome, USA). Prior to the measurements, all of the samples were
pretreated on a vacuum line at 180 °C for at least 8 h. The standard multipoint Brunauer-Emmett-Teller (BET)
method was employed to calculate the specific surface areas using the adsorption data in a relative pressure
range from 0.05 to 0.20. The pore size distributions (PSD) were calculated employing the equilibrium model
of non-local density functional theory (NLDFT method). The total pore volume Vt was estimated from the
adsorbed amount at p/p0 = 0.995. Gas chromatography-mass spectrometer (GC-MS) measurements were
conducted on a Thermo Focus-DSQ using the column of VF-5 MS (30 m, 0.25 mm, 0.25 μm, Agilent, USA).
Helium (99.999 %) was used as the carrier gas with a flow rate of 1.0 mL/min. The temperature of injection
was programmed at 250 C, and 1 μL of sample was injected at split mode (the split ratio is 150: 1). The
column oven temperature was programed with an initial value of 60 C for 2 min, and then it was raised up
to 250 C with a heating ramp of 20 C/min, keeping for 1 min at the final temperature.
Reference
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Chem. Int. Ed. 2009, 48, 5875.
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50, 329. b) Wang, J. X.; Li, W.; Wang, F.; Xia, Y. Y.; Asiri, A. M.; Zhao, D. Y. Nanoscale, 2014, 6, 3217. c) Chen, J.
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Zang, J.; Xu, C. F.; Zheng, M. -S.; Dong, Q. -F.; Sun, D. H.; Zheng, N. F. Nanoscale, 2013, 5, 6908. e) Zhang, X.
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Figure S1 SEM (a) and TEM (b) images of the magnetite Fe3O4 particles.
Figure S2 SEM image of the core-shell structured Fe3O4@SiO2 microspheres (Fe3O4@SiO2-1).
11
Figure S3 SEM image of the core-shell structured Fe3O4@SiO2 microspheres (Fe3O4@SiO2-2).
Figure S4 TEM images of the core-shell structured Fe3O4@SiO2 microspheres. Panels (a-b) and (c-d) correspond to
Fe3O4@SiO2-1 and Fe3O4@SiO2-2, respectively.
12
Figure S5 SEM image of the core-shell structured Fe3O4@SiO2@RF microspheres (Fe3O4@SiO2@RF-1).
Figure S6 TEM images of the sandwich-like core-shell structured microspheres (Fe3O4@SiO2@RF) synthesized by a modified
two-step sol-gel method with silica as the interlayer. Panel (a) and (b) correspond to Fe3O4@SiO2@RF-1 and Fe3O4@SiO2@RF-2,
respectively. Inset in panel (a) is the enlarged image.
13
Figure S7 SEM image of the core-shell structured Fe3O4@SiO2@RF@TiO2 microspheres (Fe3O4@SiO2@RF@TiO2-0).
Figure S8 SEM image of the purposely grinded magnetic yolk-shell structured anatase-based microspheres (M-0).
14
Figure S9 XRD pattern of magnetic yolk-shell structured anatase-based microspheres (M-1).
Figure S10 TEM image of the magnetic core-shell structured Fe3O4@SiO2@TiO2 microspheres (as-made Fe3O4@SiO2@TiO2)
prepared by an extended kinetics-controlled coating method for deposition of TiO2 outer layer.
15
Figure S11 TEM images of the magnetic core-shell structured Fe3O4@SiO2@TiO2 microspheres (calcined Fe3O4@SiO2@TiO2).
Inset is the corresponding enlarged image.
Figure S12 XPS spectra of the magnetic yolk-shell structured anatase-based microspheres (M) and the corresponding Au
nanoparticles-loaded magnetic yolk-shell structured anatase-based microspheres (Au@M-0). All the XPS data were calibrated with
the binding energy of C 1s at 284.6 eV.
16
Figure S13 XPS spectra of the magnetic core-shell structured Fe3O4@SiO2@TiO2 microspheres (calcined Fe3O4@SiO2@TiO2).
The XPS data was calibrated with the binding energy of C 1s at 284.6 eV.
Table S1 Contents of C, O, Ti, Si, Fe, and Au on the surface of the magnetic yolk-shell structured anatase-based microspheres (M)
and the corresponding Au nanoparticles-loaded magnetic yolk-shell structured anatase-based microspheres (Au@M-0).
Samples
C (%)
O (%)
Ti (%)
Si (%)
Fe (%)
Au (%)
M-0
52.28
36.36
9.06
2.25
0.04
0.00
M-1
41.99
43.37
13.12
1.43
0.00
0.00
M-2
49.45
37.75
10.98
1.65
0.17
0.00
Au@M-0
48.82
38.81
8.99
2.83
0.09
0.47
57.94
32.74
6.47
2.79
0.06
0.00
Calcined Fe3O4@SiO2@TiO2
17
Figure S14 O 1s high-resolution XPS spectra of the magnetic yolk-shell structured anatase-based microspheres (M-0). All the XPS
data were calibrated with the binding energy of C 1s at 284.6 eV.
Figure S15 Nitrogen adsorption-desorption isotherms (A) and the corresponding pore size distribution curves (B) of as-prepared
Fe3O4@SiO2@RF@TiO2-1 (a), Fe3O4@SiO2@RF@TiO2-1-N2-500 (b), and magnetic yolk-shell structured anatase-based
microspheres (M-1) (c). The isotherm (A) of as-prepared Fe3O4@SiO2@RF@TiO2-1 (a) is offset vertically by -25 cm3/g.
18
Figure S16 TEM image of the magnetic yolk-shell structured anatase-based microspheres prepared by direct calcination of the
as-made microspheres (as-prepared Fe3O4@SiO2@RF@TiO2-2) under air.
Figure S17 Nitrogen adsorption-desorption isotherms (A) and the corresponding pore size distribution curves (B) of the magnetic
yolk-shell structured anatase-based microspheres M-0 (a) and M-2 (b). The isotherm (A) of M-2 (b) is offset vertically by 20 cm3/g.
19
Table S2 Textural properties of the magnetic yolk-shell structured anatase-based microspheres (M) and the corresponding Au
nanoparticles-loaded magnetic yolk-shell structured anatase-based microspheres (Au@M-0).
Samples
BET surface area
Pore volume
Pore size
(m2/g)
(cm3/g)
(nm)
M-0
74.3
0.14
5.1
M-2
95.6
0.24
4.8
as-prepared Fe3O4@SiO2@RF@TiO2-1
387.0
0.24
1.5/2.6
Fe3O4@SiO2@RF@TiO2-1-N2-500
96.0
0.11
4.0
M-1
50.8
0.09
4.0
Au@M-0
61.1
0.12
4.8
as-made Fe3O4@SiO2@TiO2
336.4
0.25
1.8/2.6/3.6
calcined Fe3O4@SiO2@TiO2
55.7
0.10
5.6
Figure S18 Nitrogen adsorption-desorption isotherms (A) and the corresponding pore size distribution curves (B) of the magnetic
core-shell structured TiO2-based microspheres (as-made Fe3O4@SiO2@TiO2 sample (a) and the corresponding calcined sample (b)).
The isotherm (A) of (b) is offset vertically by 30 cm3/g.
20
Figure S19 TEM images of Au-loaded Fe3O4@SiO2@Void@TiO2 microspheres (Au@M-0). Inset in panel (b) is the corresponding
enlarged image.
Figure S20 Nitrogen adsorption-desorption isotherms (A) and the corresponding pore size distribution curves (B) of the Au-loaded
Fe3O4@SiO2@Void@TiO2 microspheres (Au@M-0).
21
Figure S21 Epoxidation of styrene by using t-butyl hydroperoxide (TBHP) as an oxidant over the magnetic yolk-shell structured
anatase-based microspheres (M-0) at 82 C (as the control experiment). The conversion of styrene and selectivity of styrene oxide
were functioned with the reaction time. The inset is the corresponding illustration of the 3-D mesostructure of magnetic yolk-shell
structured anatase-based microspheres (M-0).
Figure S22 Cycling catalytic performance for epoxidation of styrene by using Au-loaded magnetic microspheres (Au@M-0) as the
heterogeneous catalysts. The conversion of styrene and selectivity towards styrene oxide were functioned with the cycles. All the data
was obtained at reaction for 33 h for each cycle.
22