Yolk-shell hybrid materials with a periodic mesoporous organosilica shell: Ideal nanoreactors for selective alcohol oxidation

Article abstract of DOI:10.1002/adfm.201101900

This contribution describes the preparation of multifunctional yolk-shell nanoparticles (YSNs) consisting of a core of silica spheres and an outer shell based on periodic mesoporous organosilica (PMO) with perpendicularly aligned mesoporous channels. The new yolk-shell hybrid materials were synthesised through a dual mesophase and vesicle soft templating method. The mesostructure of the shell, the dimension of the hollow space (4～52 nm), and the shell thickness (16～34 nm) could be adjusted by precise tuning of the synthesis parameters, as evidenced by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nitrogen sorption investigations. Various metal nanoparticles (e.g., Au, Pt, and Pd) were encapsulated and confined in the void space between the core and the shell using impregnation and reduction of adequate metal precursors. The selective oxidation of various alcohol substrates was then carried out to illustrate the benefits of such an architecture in catalysis. High conversion (～100%) and excellent selectivity (～99%) were obtained over Pd nanoparticles encapsulated in the hybrid PMO yolk-shell structures.

Full text of DOI:10.1002/adfm.201101900

www.afm-journal.de
www.MaterialsViews.com
Yolk–Shell Hybrid Materials with a Periodic Mesoporous
Organosilica Shell: Ideal Nanoreactors for Selective Alcohol
Oxidation
Jian Liu, Heng Quan Yang, Freddy Kleitz, Zhi Gang Chen, Tianyu Yang,
Ekaterina Strounina, Gao Qing (Max) Lu,* and Shi Zhang Qiao*
1
. Introduction
This contribution describes the preparation of multifunctional yolk–shell
nanoparticles (YSNs) consisting of a core of silica spheres and an outer
shell based on periodic mesoporous organosilica (PMO) with perpendicu-
larly aligned mesoporous channels. The new yolk–shell hybrid materials
were synthesised through a dual mesophase and vesicle soft templating
method. The mesostructure of the shell, the dimension of the hollow space
So-called “yolk–shell” nanostructures or
rattle-type” nanomaterials containing a
“
void space between the core and the outer
shell of the material are emerging as an
interesting family of complex new hollow
[
1–4]
nanoarchitectures.
The
enclosed
void space of yolk–shell nanoparticles is
expected to be useful for chemical storage,
compartmentation, and confinement of
host–guest interactions. Furthermore,
their tunability and functionality in both
the core and the inner shell region can
endow yolk–shell nanoparticles (YSNs)
with new properties, thus rendering them
attractive for applications as nanoreac-
(
4∼52 nm), and the shell thickness (16∼34 nm) could be adjusted by precise
tuning of the synthesis parameters, as evidenced by X-ray diffraction (XRD),
scanning electron microscopy (SEM), transmission electron microscopy
(
(
between the core and the shell using impregnation and reduction of
adequate metal precursors. The selective oxidation of various alcohol sub-
strates was then carried out to illustrate the benefits of such an architecture
in catalysis. High conversion (∼100%) and excellent selectivity (∼99%) were
obtained over Pd nanoparticles encapsulated in the hybrid PMO yolk–shell
structures.
TEM) and nitrogen sorption investigations. Various metal nanoparticles
e.g., Au, Pt, and Pd) were encapsulated and confined in the void space
[5,6]
[7,8]
tors,
drug/gene delivery systems,
surface-enhanced
Raman
scattering
and energy
[9,10]
(SERS) technologies,
[
11–13]
storage media.
Most importantly, the
void space in the yolk–shell nanostructure
may provide a unique environment for
creating concerted actions between the
core and a permeable shell. Thus, porous shells with control-
lable permeability are particularly sought after.
Alongside numerous synthetic methods available to gen-
erate hollow nanostructures, several strategies for the prepara-
Dr. J. Liu, T. Y. Yang, Prof. G. Q.(M.) Lu, Prof. S. Z. Qiao
ARC Centre of Excellence for Functional Nanomaterials
Australian Institute for Bioengineering and Nanotechnology
The University of Queensland
QLD 4072, Australia
E-mail: maxlu@uq.edu.au; s.qiao@uq.edu.au
tion of a yolk–shell structure have been developed, including
[14]
the ship-in-bottle approach,
bottom-up or soft templating
Prof. H. Q. Yang
School of Chemistry and Chemical Engineering
Shanxi University
[15–18]
[19–23]
approaches,
selective etching approaches,
template-
[24,25]
[26]
free approaches,
ment processes,
Ostwald ripening or galvanic replace-
[
27]
and methods based on the Kirkendall
Taiyuan 030006, P. R. China
[2]
effect. Various yolk–shell colloidal particles and nanoparti-
cles with different chemical compositions, including metal
NPs@silica, metal oxide@silica, metal NPs@carbon, metal
NPs@metal oxide, metal NPs@polymer, silica@metal oxide,
silica@carbon, polymer@polymer, have all been obtained by
Prof. F. Kleitz
Department of Chemistry and Centre de recherche
sur les matériaux avancés (CERMA)
Université Laval
Quebec, G1V 0A6, QC, Canada
Dr. Z. G. Chen
[
1]
using the above synthesis methods.
Centre for Microscopy and Microanalysis and School of Engineering
The University of Queensland
QLD 4072, Australia
Soft templating approaches have recently been introduced
and these are considered as a simple, effective, and general
strategy to produce such yolk–shell structures, especially for
yolk–shell architectures possessing a permeable porous shell.
In particular, YSNs with mesoporous shell can exhibit unique
Dr. E. Strounina
Centre for Advanced Imaging
The University of Queensland
QLD 4072, Australia
[
7,17]
release properties,
or prevent sintering/aggregation of
DOI: 10.1002/adfm.201101900
entrapped catalytic species while promoting diffusion and mass
Adv. Funct. Mater. 2012, 22, 591–599
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com 591

www.afm-journal.de
www.MaterialsViews.com
transfer of reactants,[6] attributes which are
very useful in drug/gene delivery and nano-
reactor applications.
Although some progress in the synthesis
of yolk–shell particles with a nanoporous
shell has been achieved, the shells obtained
thus far consist essentially of disordered pore
structures, with poor level of compositional
diversity and functionality. More specifically,
the design of yolk–shell particles exhibiting
multiple cores with distinct properties, and
a functionalised ordered mesoporous shell
is still highly desirable in order to develop
versatile new nanoreactors and bioactive-
molecule delivery systems. Within this con-
text, the integration of both organic and
inorganic fragments within the pore walls of Scheme 1. Synthesis procedure and proposed structure of yolk–shell hybrid materials with a
periodic mesoporous organosilica (PMO) has PMO shell.
been a breakthrough in the area of functional
nanoporous materials.[
28–31]
Since then, a
great variety of PMOs exhibiting diverse mesostructures and
framework compositions, and controlled particle morpholo-
over Pd nanoparticles embedded in these hybrid PMO yolk–
shell structures.
[31]
[32–35]
[36]
gies (solid spheres,
hollow spheres,
single crystals,
helical[ and core–shell structures[38]) have been described.
These hybrid materials have wide perspectives of applications,
in areas such as catalysis, selective sorption and separation,
biomolecule immobilisation, low k-dielectrics, and so forth,
37]
2
. Results and Discussion
2
.1. Synthesis of Yolk–Shell Hybrid Materials with PMO Shell
owing to the great diversity of available functionalities and their
high thermal and chemical stability.[
28–31]
Therefore, in order to
develop optimised nanoreactors and nanocarriers, it could be
advantageous to produce YSNs constituted of a well-defined
functional PMO shell. This should enable the combination of
the benefits of both the yolk–shell architecture and organic–
inorganic hybrid PMO functionalities. In addition to this, the
use of the void space between the core and the shell in such a
system to selectively host some active catalytic species remains
to be demonstrated.
The synthesis of the yolk–shell PMO (YS-PMO) hybrid mate-
rials is schematically represented in Scheme 1. Here, solid
silica spheres prepared by the Stober method are selected as an
example of core materials. The first step of the material syn-
thesis involves the combination of the selected core materials
and the vesicles formed in aqueous solution by a fluorocarbon
+
surfactant [C F O(CFCF CF O) CFCF CONH(CH ) N (C H )
3
7
3
2
2
3
2 3
2
5 2
−
CH I ] (FC4). In the next step, hybrid organosilane-surfactant
3
With the aim of expending the range of composition and
functionality of the yolk–shell nanostructures, we report here
for the first time on the construction of multifunctional YSNs
consisting of a core of dense silica spheres, a hollow space
between core and shell, and an outer PMO-type shell with per-
pendicularly aligned mesoporous channels. The new yolk–shell
hybrid materials with PMO shell were synthesised through
combined mesophase and vesicle dual soft templating method.
In particular, we demonstrate that the mesostructure of the
shell, the dimension of the hollow space (4∼52 nm), and the
shell thickness (16∼34 nm) can all be well adjusted by precise
tuning of the synthesis parameters. Furthermore, the thus-
designed multifunctional YSNs possess both large void spaces,
enabling proper loading with guest molecules and confine-
ment of active metal nanoparticles, and an ordered organic-
functionalised mesopore shell allowing for fast diffusion of
reactants. The possibility of confining catalytic entities is clearly
evidenced by the selective encapsulation in the void space of
various metal nanoparticles (e.g., Au, Pt, Pd). To illustrate the
benefits of such architecture in catalysis, selective oxidation of
various alcohol substrates was then carried out. High conver-
sion (∼100%) and excellent selectivity (∼99%) were obtained
micellar aggregates, which are built of hydrolysed/oligomer-
ised bis(trimethoxysilyl)ethane (BTME) species and cetyltri-
methylammonium bromide (CTAB), assemble under basic
conditions while surrounding the core-vesicle complex eventu-
ally forming an ordered mesostructured organosilica shell via
cooperative self-assembly. Under these conditions, the growth
of the hybrid organosilica–CTAB mesophase remains confined
to the periphery of the vesicle region. Following further growth
and ripening process of the shell, induced by condensation
of the BTME oligomeric species, a PMO yolk–shell structure
can be obtained after removal of both the CTAB and FC4 tem-
plates by solvent extraction. The obtained material is denoted
as YS-PMO, which consists of a siliceous core, hollow shell,
and PMO shell. This dual mesophase and vesicle templating
ensures that the organosilica organises around the core vesicle
complex to generate the desired yolk–shell nanospheres exhib-
iting a PMO shell.
Figure 1 shows representative TEM images of the hybrid
yolk–shell material (YS-PMO-1) presenting a 160 nm silica
core and a mesoporous shell. The shell thickness is ∼20 nm,
the hollow void is ∼7 nm, resulting in an overall outer diameter
of ∼220 nm for a single yolk–shell particle (Figures 1a and b).
5
92 wileyonlinelibrary.com
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Funct. Mater. 2012, 22, 591–599

www.afm-journal.de
www.MaterialsViews.com
Figure 1. TEM images of the yolk–shell hybrid materials with a PMO shell (YS-PMO-1) at different magnifications. (Scale bars are 200 nm, 50 nm,
and 5 nm for (a), (b), (c), respectively).
A high-resolution transmission electron microscopy (HRTEM)
image of YS-PMO-1, as shown in Figure 1c, further confirms
the ordered organisation of the mesopores in the shell, with a
pore size of ∼3 nm. As illustrated in Figure 2a, low-angle XRD
pattern of YS-PMO-1 shows one strong peak in the 2θ range
the amount of the silica precursor, namely tetraethyl ortho-
silicate (TEOS) in that particular case. In the present work,
YS-PMO samples with varied shell thickness and void space,
designated as YS-PMO-2, YS-PMO-3, and YS-PMO-4, were
obtained similarly by tuning the concentration of the BTME
precursor, under otherwise similar experimental conditions to
those implemented for YS-PMO-1. As shown in Figure 3, with
increasing of the amount of BTME from 0.17 to 0.32 mL, the
shell thickness gradually increased from 16 to 28 nm, and the
void space gradually decreased from 18 to 4 nm, respectively,
while the mesopores size in the shell remained fairly constant
at 2∼3 nm (Table 1). However, at very high BTME concentra-
tion (0.32 mL), the presence of PMO nanoparticles coexisting
with YS-PMO-4 yolk–shell particles was observed. Furthermore,
with increasing the amount of BTME added to the synthesis
mixture, it was observed that BET surface area and total pore
volume of the resulting nanoparticles decreased gradually from
2
°–3°, confirming the mesoscopic ordering in the shell. In con-
trast to standard MCM41-type silica materials, the PMO shell
seems less ordered. Further evidence of the mesoporous struc-
ture of YS-PMO-1 could be obtained by N sorption analysis.
2
The adsorption–desorption isotherm obtained on YS-PMO-1
shows a type IV behaviour with a two-step capillary condensa-
tion in the relative pressure ranges of 0.1–0.3 and 0.7–0.9 (see
Figure 2b), in agreement with the presence of a hierarchical
porosity organisation consistent with the yolk–shell structure.
The corresponding Barrett–Joyner-Halenda (BJH) pore size dis-
tribution shows two peaks, centred at 3 nm and 16 nm, cor-
responding to the mesopores of the shell and the void between
the solid core and the shell, respectively (see Figure 2c). In
addition, the Brunauer–Emmett–Teller (BET) specific surface
area and total pore volume of YS-PMO-1 are calculated to be
2
−1
3
−1
262 to 82 m ·g , and 0.46 to 0.18 cm ·g , respectively (Table 1
and Figure S1 in the Supporting Information). Investigation
of all these samples using SEM indicated that their average
diameters agreed fairly well with the sizes observed by the TEM
images (i.e., ∼220 nm) (Figure S2).
2
−1
3
−1
1
47 m ·g and 0.32 cm ·g , respectively (Table 1).
For application of yolk–shell nanoparticles in catalysis and
drug delivery, a control of the shell thickness is desirable. Pre-
viously, we demonstrated the feasibility of the preparation of
similar YSNs with different silica shell thickness by controlling
The formation of such a yolk–shell structure is probably
a cooperative process of vesicle templating formed by FC4,
associated with ripening and shrinkage of the PMO shell. The
(A)
(B)
(C)
16 nm
200
150
100
3.0 nm
50
0
2
4
6
8
10
0.0 0.2 0.4 0.6 0.8 1.0
1
10
100
2
Theta/degree
Relative Pressure (P/P )
0
Pore Diameter (nm)
Figure 2. Low-angle XRD pattern (A), nitrogen adsorption-desorption isotherm (B) and respective pore size distribution obtained from the BJH model
applied to the adsorption branch (C) of the yolk–shell hybrid with PMO shell (YS-PMO-1).
Adv. Funct. Mater. 2012, 22, 591–599
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com 593

www.afm-journal.de
www.MaterialsViews.com
Table 1. Physicochemical parameters of the different yolk–shell structures.
Sample
BTME amount
mL]
Ethanol amount
[mL]
Void space
[nm]
Shell thickness
[nm]
BET surface area Shell pore diameter Total pore volume
2
−1
a)
3
−1
[
[m g
]
[nm]
3.0
2.3
2.3
2.3
2.5
2.5
2.5
2.5
[cm g ]
YS-PMO-1
YS-PMO-2
YS-PMO-3
YS-PMO-4
YS-PMO-5
YS-PMO-6
YS-PMO-7
YS-PMO-8
0.22
0.17
0.27
0.32
0.22
0.22
0.22
0.22
0
0
7
20
16
24
28
24
34
34
34
147
262
127
82
0.32
0.46
0.28
0.18
0.26
0.31
0.22
0.24
18
6
0
0
4
5
20
33
45
52
200
228
165
151
10
15
20
a)The pore size were calculated by BJH method which was probably underestimated according to previous report.[39]
addition of alcohol is believed to play an essential role in the
core–shell nanoparticles by using organosilane as an additive.
However, when ethanol (5∼20 mL) was used, the pore struc-
ture of the PMO shell was less ordered (Figure 4). The reason
for the reduced order may be attributed to the increasing crit-
ical micelle concentration (CMC) of CTAB in the presence of
ethanol. At high concentrations of ethanol, CTAB can interact
more strongly with ethanol than with water, which will even-
tually raise the critical micelle concentration value and thus
alter the formation of micelle. These effects are also in an
[
17,18]
formation of the vesicles, as it is generally the case.
To
further substantiate the influence of the different synthesis
parameter, we studied the role of ethanol present in the
synthesis medium on the formation of the yolk–shell PMO
materials. As presented in Figures 4 and S3, under similar
conditions as for YS-PMO-1 with a fixed BTME amount of
0
.22 mL, the addition of different amounts of ethanol led to the
formation of very uniform and well-dispersed hybrid YS-PMO
exhibiting different void space. By increasing the volume of
ethanol from 5 to 20 mL, the void space increased from 20 to
[
40–42]
agreement with previous reports,
which have shown
mesophase regression when the ethanol-to-surfactant ratio
exceeded 73.8. The lower order of the mesoporous shell of
these samples was also judged from low angle XRD (Figure 5a)
indicative of more distorted 2D hexagonal structures with
one broad and weak (100) diffraction peak. Furthermore,
the nitrogen sorption analysis results, depicted in Figure 5b,
show a type IV isotherm with H2/H4 hybrid hysteresis loop
5
2 nm (see also Table 1). Here, larger vesicles may form with
increasing volume of ethanol added. Surprisingly, the silica
core also becomes less regular, from dense to porous, and its
size was reduced with increasing the amount of ethanol. To
explain this, one can suggest that the core silica spheres are
presumably less dense under these conditions being built of
networks of disordered and discontinuous interconnection of
Si–O–Si and Si–OH bonds. Such silica spheres prepared by
the Stober process are formed through aggregation of small
nucleus particles, followed by growth via monomer addition.
In the presence of a large amount of ethanol, sequences of
Si–O–Si and Si–O–C H bond-breaking and bond-forming proc-
[
32–35]
typical of hollow structures with mesoporous wall.
The
mixed H2/H4 loops observed for all samples with parallel
and almost horizontal branches at a relative pressure close to
the saturation vapour pressure may thus be attributed to the
hollow voids between the core and shell. The capillary conden-
sation steps of the isotherms appearing in the relative pres-
2
5
esses in the presence of hydroxide ions could alter the nature
of the core, yielding a more porous structure in the presence
of BTME. A similar phenomenon was also described by Yang
et al.[ who synthesised yolk–shell hybrids starting from
sure interval P/P = 0.25–0.35 indicate that these materials
0
remained somewhat mesostructured. The pore-size distribu-
tions calculated by the BJH method using adsorption branch
are shown in Figure 5c for the samples synthesised with
25]
Figure 3. Representative TEM images of the yolk–shell hybrid materials with different thicknesses of PMO shell synthesised at different BTME molar
amounts: a) YS-PMO-2, BTME = 0.17 mL; b) YS-PMO-3, BTME = 0.27 mL; c) YS-PMO-4, BTME = 0.32 mL.
5
94 wileyonlinelibrary.com
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Funct. Mater. 2012, 22, 591–599

www.afm-journal.de
www.MaterialsViews.com
2
.2. Characterisation of the Chemical
Composition of Yolk–Shell Hybrid Materials
with PMO Shell
The chemical composition of YS-PMO was
characterised by Fourier-transform infrared
(
FT-IR) spectroscopy (Figure S4). The C–H
vibrations of –CH CH – appearing in the
2
2
−
1
−1
range 2918–2875 cm and at 1160 cm
are clearly observed in the FT-IR spectra of
YS-PMO. The FT-IR spectra of these samples
show the Si–C stretching modes at 696 and
−
1
7
68 cm , proving the integrity of the ethane
organic groups in YS-PMO. Solid-state NMR
spectroscopy was performed to further verify
the composition of YS-PMO. The resonance
at 4.4 ppm in the 13C CP NMR spectrum of
YS-PMO can be assigned to the C species of
the ethane moiety (Figure 6a). The signals
at 58.5, 29.8, and 17.3 ppm [labelled with an
asterisk (*)] are due to the surfactant resi-
dues and the carbons of the ethoxy groups
formed during the surfactant extraction
[
32,33,43]
13
process.
The result of solid-state
C
CP MAS NMR confirms the incorporation
and integrity of the ethylene organic groups
in the YSNs. 29Si NMR spectra of YS-PMO
n
n
show the existence of both Q and T sites as
expected (Figure 6b). The characteristic reso-
nances in the range of –90 to –110 ppm can
2
be assigned to (HO) Si(OSi) (Q δ -91), (HO)
2
2
3
4
Si(OSi) (Q δ -102), and Si(OSi) (Q δ -109)
3
4
silicon species. These Q species originate
from the pure silica core. The sharp signal at
–
67.6 ppm is attributed to Si atoms bridged
3
by the ethane groups [T , SiC(OSi) ]. The T/
3
(
T + Q) ratio calculated from the normalized
peak areas is 0.40, which corresponds to the
estimated shell/core ratio, which is in line
with the amount of BTME incorporated. The
combined results of NMR and FT-IR analysis
establish that the framework of YS-PMO is
built of O1.5Si-CH CH -SiO units.
2
2
1.5
2
.3. Encapsulation of Metal Nanoparticles
(
Au, Pt, Pd) into the PMO-based Yolk–Shell
Hybrid Materials
In the following, we will demonstrate the
successful incorporation of Au, Pt, and Pd
nanoparticles into the structure of YS-PMO-6
acting as a nanoreactor. The synthetic pathway
for preparing metal nanoparticles encapsu-
Figure 4. TEM images of the yolk–shell hybrid materials with PMO shells synthesised with
different volumes of ethanol: a,b) 5 mL; c,d) 10 mL; e,f) 15 mL; g,h) 20 mL.
addition of different volumes of ethanol. The physicochemical
properties of these samples are listed in Table 1. From these
data, it can be seen that all samples possess uniform pore size
lated into yolk–shell PMO nanoparticles is shown in Scheme 1.
After immersion of YS-PMO-6 in an aqueous solution of the
given metal salt, the resulting composite was reduced by triso-
dium citrate to generate metal-loaded yolk–shell nanoparticles
(denoted as metal–YS-PMO).
2
−1
around 3 nm, high surface areas (151∼228 m g ) and pore
3
−1
volume (0.22∼0.31 cm g ).
Adv. Funct. Mater. 2012, 22, 591–599
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com 595

www.afm-journal.de
www.MaterialsViews.com
the weight fraction of these metal nanopar-
ticles was increased with the increase of the
concentration of HAuCl . In the case of Pt-
4
YS-PMO, it is evident from the TEM and
HAADF-STEM images that Pt nanoparticles
with 3 nm coexist together with aggregates
of Pt nanoparticles exhibiting a more or less
dendritic shape. Finally, for the Pd-YS-PMO,
similarly to Au and Pt, small palladium parti-
cles (∼3 nm) together with larger aggregated
particles can be observed (Figure 7g–i). These
results are consistent with the well-docu-
mented aggregation of noble metal nanopar-
ticles through Ostwald ripening and forma-
tion of metal–Cl complex nanostructures in
silica shells as previously exploited by Lou
Figure 5. Low-angle XRD patterns (A), nitrogen adsorption-desorption isotherms (B), and pore
size distributions obtained from the BJH model applied to the adsorption branch (C) of the
yolk–shell hybrid materials with PMO shells synthesised with different volumes of ethanol:
a) 5 mL; b) 10 mL; c) 15 mL; d) 20 mL.
[
14]
et al. In this previous work, gold nanopar-
ticles with different morphologies including
From the TEM images taken on the metal-loaded yolk–shell
PMO samples (Figure 7 and S5), it can be seen that the metal
nanoparticles are mostly located inside the yolk–shell colloidal
particles. Furthermore, the monodispersity and yolk–shell archi-
tecture of the particles were not affected through the encapsula-
tion of the Au, Pd, and Pt nanoparticles. To directly image the
metal nanoparticles in the yolk–shell structures, high-angular
annular dark-field scanning transmission electron microscopy
triangular and rodlike shapes were formed through Ostwald
ripening in multicore Au–silica hollow particles.
2
.4. Selective Oxidation Reactions using Pd–YS-PMO
as a Catalytic Nanoreactor
The selective oxidation of alcohols to aldehydes remains an
important task in many important industrial and fine chemi-
cals production processes. Of particular current interest is the
use of molecular oxygen as the oxidant because it represents
a green chemical process, in contrast to the use of toxic and
expensive stochiometric oxidants based on metal oxides. Thus,
the selective oxidation profiles of a wide range of primary
and secondary alcohols were thus studied over Pd–YS-PMO
(
(
HAADF-STEM) was performed. The HAADF-STEM images
Figure 7c,f,i) provide a clear contrast of the core, the metal
nanoparticles, the hollow part and the porous shell. The metal
nanoparticles are visualised as bright spherical dots, evidence of
the incorporation of particles in the yolk–shell matrix. Energy-
dispersive X-ray (EDX) analyses confirmed the presence of Au,
Pt, Pd in the yolk–shell particles (Figure S6).
For Au–YS-PMO, two types of Au nanoparticles are visible,
coexisting inside the yolk–shell structure. One of the types of
Au nanoparticles exhibits a very narrow size distribution, with
an average size of about 2 nm, as estimated from the TEM
and HAADF-STEM images. The second type of Au nanopar-
ticles which have a size around 20 nm is probably formed by
sintering of adjacent small Au nanoparticles during synthesis.
The size of the incorporated Au nanoparticle can be controlled
catalysts (Pd loading of 5 mol%), in the presence of O . For
2
benzyl alcohol and methyl-substituted benzylic alcohol, conver-
sions of 94% and 92% were obtained within 8 h, respectively
(Table 2, entries 1 and 2). The selectivity for the target prod-
ucts was evaluated to be higher than 99%. In the case of CH O-
3
substituted benzylic alcohol, a total conversion with selectivity >
99% was also achieved over 8 h under O atmosphere (Table 2,
2
entry 3). Similarly, cinnamyl alcohol was totally converted to
cinnamaldehyde under the same conditions. The selectivity for
by tuning the initial concentration of HAuCl . At mean time,
4
Figure 6. Solid-state 13C CP MAS NMR spectra (a) and 29Si NMR spectra (b) of YS-PMO-1.
5
96 wileyonlinelibrary.com
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Funct. Mater. 2012, 22, 591–599

www.afm-journal.de
www.MaterialsViews.com
Figure 7. a),d,g,) TEM images, b,e,h,) HRTEM images, and c,f,i) HAADF-STEM images of Au–YS-PMO (a–c); Pt–YS-PMO (d–f); Pd–YS-PMO (g–i).
α,β-unsaturated aldehyde was again superior to 99% (Table 2,
entry 4). In the case of 3-methyl-2-butanol, as an example of typ-
ical aliphatic alcohol, the catalyst Pd–YS-PMO showed a mod-
erate activity with, nevertheless, an excellent selectivity (Table 2,
entry 5). As such, under the conditions used, the selectivity
to the corresponding aldehyde is in all cases superior to 99%.
No detectable carboxylic acid species were found among the
products, confirming the high selectivity to the aldehyde. The
excellent catalytic activity toward various oxygenate substrates
demonstrates that the mesoporous shell around the encap-
sulated metal catalyst allows the reaction molecules to freely
access to the encapsulated metal sites. Moreover, the protective
Table 2. Selective oxidation of various alcohols into aldehyde over Pd-YS-PMO nanoreactors.
Entrya)
1
Substrates
Products
RTb)
Time
[h]
Conversion
[%]
Selectivityc)
[%]
[
°C]
50
8
8
8
8
94
>99
>99
>99
>99
2
3
4
50
50
50
92
100
100
5
50
8
53
>99
a)The reaction was carried out in O2 atmosphere in the presence of 5 mol% Pd with water as solvent; b)RT: reaction temperature; c)Selectivity to aldehyde.
Adv. Funct. Mater. 2012, 22, 591–599
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com 597

www.afm-journal.de
www.MaterialsViews.com
mesoporous shells could prevent sintering of the metal nano-
A, solution B was prepared by dissolving 0.54 g of the core materials
in 18 mL water (30 mg mL ) and subject to ultrasonic treatment for
−1
particles.[44–46]
With almost identical Pd loading, Pd–YS-PMO
3
2
0 min. After that, solution B was added into solution A and stirred for
h, before the addition of 0.15 g of CTAB and 0.35 mL of 2 m NaOH. Then,
exhibits better catalytic behaviors than that of conventional Pd/
[
47]
SiO and Pd/C. This can be attributed to the yolk shell struc-
2
the system was kept at a constant temperature of 80 °C, and 0.17 mL
of BTME was introduced at once under vigorous stirring for 24 h. After
cooling down to room temperature, the white precipitates were collected
by centrifugation and dried at room temperature. Finally, the surfactant
CTAB and FC4 was extracted by refluxing 1.0 g of the as-synthesised
material in 200 mL of ethanol containing 1.5 g of concentrated HCl
aqueous solution for 24 h. The resulting powder products were denoted
as YS-PMO-n.
tures of support. In our system, the metal Pd nanoparticles are
thus stabilised while being freely accessible in the void between
the core and shell, which seems to be a substantial advantage
for exposing all of the active surfaces more efficiently. The high
activity of the catalyst suggests that the hollow space in the
yolk–shell catalysts could be beneficial for enhanced adsorption
of the organic reactant molecules.
Encapsulation of Metal Nanoparticles (Au, Pt, Pd) into the YS-PMO
Materials: Typically, 0.4 g of extracted YS-PMO-6 were impregnated with an
aqueous solution of the selected metal salts (14 mL). The concentration
of HAuCl , H PtCl , or Pd(NO) was 0.05 m. After sonication for 30 min
3
. Conclusions
4
2
6
3
in an ultrasonic bath, the particles were heated in an oven at 100 °C for
h. The dried particles were washed with water for several times, which
In conclusion, we have demonstrated that it is possible to fabri-
cate various yolk–shell nanostructures possessing well-defined
functional mesoporous inorganic–organic hybrid shells, with
tailored shell thickness and void space, using a simple soft tem-
plating method. Moreover, our contribution shows that Au, Pt,
and Pd nanoparticles can easily be encapsulated into the yolk–
shell PMO structure by the impregnation and reduction of
adequate metal precursors. This, in turn, enables the creation
of highly efficient nanoreactors for catalytic conversions. For
instance, the high conversion (∼100%) and selectivity (∼99%)
observed in the case of selective alcohol oxidation reactions per-
formed over Pd-loaded yolk–shell materials tend to confirm that
these Pd–YS-PMO could be promising catalysts for fine chem-
ical production. Here, it is clear that the presence of the hollow
space in the YSNs played a crucial role in promoting activity
and enhancing stability of the metal nanoparticles. Moreover,
it is expected that such metal nanoparticle yolk–shell nanoreac-
tors will serve as a useful platform to study reactivity in hetero-
geneous catalysis as the diffusion of reactants and morphology
of active catalysts can readily be manipulated through rational
catalyst design. The synthesis strategy reported in this paper
may open up new opportunities for preparing yolk shell struc-
tured nanomaterials with various compositions. Alternatively,
other promising prospects of applications of these materials
will be as nanocarriers for controlled delivery and release of
therapeutic and bioactive agents; the hierarchical organisation
of the pores enabling here co-delivery of multiple agents and
multi-step release processes.
6
was to ensure the removal of the nanoparticle outside of YSNs. Finally,
the resulting particles were collected by centrifugation and dried for 4 h
in a vacuum oven at ambient temperature.
Characterisation: XRD measurements were performed on a Rigaku D/
max-2550V diffractometer using Co Kα radiation at 30 kV and 15 mA.
SEM images of the samples coated with platinum were recorded on a
JEOL 6300 microscope operated at 5–10 kV. TEM images and HAADF-
STEM images were obtained by JEOL JEM 2100 and FEI Tecnai F20
electron microscope with an acceleration voltage 200 kV. The powder
samples for the TEM measurements were suspended in ethanol and then
dropped onto the Cu grids with holey carbon films. Nitrogen sorption
isotherms of samples were obtained with a Quantachrome Autosorb-1
and a Quadrasorb SI analyzer at –196 °C. Prior to the measurement, the
samples were out-gassed at 120 °C for 12 h. BET specific surface areas
were calculated using adsorption data in a relative pressure range of P/P0
= 0.05∼0.25. Pore size distributions were derived from the adsorption
branch using BJH method. The total pore volumes were estimated from
the amounts adsorbed at a relative pressure (P/P ) of 0.99. Solid-state
0
1
3
C (100.5 MHz) cross-polarisation magic angle spinning (CP-MAS)
and 29Si (79.4 MHz) MAS NMR experiments were recorded on a Bruker
MSL-300 spectrometer. The spectrometer was equipped with a 4-mm
double air bearing, magic angle spinning probe for MAS experiments.
The proton 90° pulse time used in the CP-MAS method was 5.5 μs,
the acquisition time was 45 ms, cross-polarisation time 2 ms and the
relaxation delay was 3 s. The spectrum width was 50 kHz, and 4000 data
points were collected over 2000 scans. The experimental parameters
were 8 kHz spin rate, 1ms contact time, repetition delay 3s, 20K scans
1
3
collected for C CP NMR experiments and 8 kHz spin rate, 5 ms contact
29
time, repetition delay 3 s, 20 K scans for Si MAS NMR experiments.
FT-IR spectra were collected with a Nicolet Nexus 470 IR spectrometer
with KBr pallet.
Catalytic Reaction: The given alcohol (1 mmol), water (2 mL) and
the Pd–YS-PMO catalyst were combined in a dry flask, and the mixture
was stirred at given temperature in O2 for a given time. Oxygen gas
was introduced into the flask from an O2 balloon under atmospheric
pressure. After the reaction, the liquid was extracted with diethyl ether.
4
. Experimental Section
Chemicals: All materials were analytical grade and used as received
The resulting organic layer was dried with Na SO4 and then analysed
without any further purification. CTAB, BTME (96%), TEOS (>99%), gold
chloride (HAuCl ·3H O), H PtCl ·6H O (99.95%), Pd(NO ) ·xH O, were
2
with GC to determine the conversion and selectivity.
4
2
2
6
2
3 2
2
purchased from Aldrich. Fluorocarbon surfactant, FC4, was bought from
Yick Vic Chemicals (Hong Kong). Water was purified by a Milli Q system
and had an electrical resistance of 18 MΩ·cm.
Supporting Information
Synthesis of Monodisperse Silica Spheres: Monodisperse silica spheres
Supporting Information is available from the Wiley Online Library or
from the author.
(
(
160 nm) were synthesised by mixing 5.5 mL of aqueous ammonia
NH OH, 28 wt%) with a solution containing 250 mL of absolute
4
ethanol (EtOH) and 20 mL of deionised water (H O). After stirring for
2
1
h, 15 mL of TEOS was added to the solution and stirred for 6 h at
[
17]
room temperature.
Acknowledgements
Synthesis of Yolk–Shell Hybrid Materials with Mesoporous Silica Core and
PMO Shell: In a typical procedure, 0.15 g of FC4 was dissolved in 30 mL
of water and stirred at room temperature for 1 h to prepare the solution
This work was financially supported by the Australian Research Council
(ARC) through Linkage Project program (LP0882681) and Discovery
5
98 wileyonlinelibrary.com
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Funct. Mater. 2012, 22, 591–599

www.afm-journal.de
www.MaterialsViews.com
Project program (DP1094070, DP1095861, DP0987969), and partly
financially supported by the National Natural Science Foundation
of China (No.21028003). J.L. gratefully acknowledges the award of
ARC Australian Postdoctoral Fellowship (APD), a UQ Foundation
Research Excellence Award, and a UQ Early-Career-Research Grant.
F.K. acknowledges financial support from the National Science and
Engineering Research Council, Canada.
[22] L. F. Tan, D. Chen, H. Y. Liu, F. Q. Tang, Adv. Mater. 2010, 22,
4885.
[23] Y. F. Zhu, E. Kockrick, T. Ikoma, N. Hanagata, S. Kaskel, Chem.
Mater. 2009, 21, 2547.
[
24] Y. J. Wong, L. F. Zhu, W. S. Teo, Y. W. Tan, Y. H. Yang, C. Wang,
H. Y. Chen, J. Am. Chem. Soc. 2011, 133, 9510.
[
25] Y. Yang, J. Liu, X. B. Li, X. Liu, Q. H. Yang, Chem. Mater. 2011, 23,
3
676.
Received: August 14, 2011
Revised: September 22, 2011
Published online: November 25, 2011
[
[
[
26] B. Liu, H. C. Zeng, Small 2005, 1, 566.
27] E. C. Cho, P. H. C. Camargo, Y. N. Xia, Adv. Mater. 2010, 22, 744.
28] A. Sayari, S. Hamoudi, Y. Yang, I. L. Moudrakovsk, J. R. Ripmeester,
Chem. Mater. 2000, 12, 3857.
[
29] Q. H. Yang, J. Liu, L. Zhang, C. Li, J. Mater. Chem. 2009, 19, 1945.
[
1] J. Liu, S. Z. Qiao, J. S. Chen, X. W. Lou, X. R. Xing, G. Q. Lu, Chem.
Commun. 2011, 47, in press, DOI:10.1039/c1cc13658e.
2] Y. D. Yin, R. M. Rioux, C. K. Erdonmez, S. Hughes, G. A. Somorijai,
A. P. Alivisatos, Science 2004, 304, 711.
[30] F. Hoffmann, M. Cornelius, J. Morell, M. Fröba, Angew. Chem. Int.
Ed. 2006, 45, 3216.
[31] V. Rebbin, R. Schmidt, M. Fröba, Angew. Chem., Int. Ed. 2006, 45,
5210.
[
[
[
[
[
[
3] Y. Zhao, L. Jiang, Adv. Mater. 2009, 21, 3621.
[32] J. Liu, S. Bai, H. Zhong, C. Li, Q. H. Yang, J. Phys. Chem. C 2010,
114, 953.
[33] J. Liu, Q. H. Yang, L. Zhang, H. Q. Yang, J. S. Gao, C. Li, Chem.
Mater. 2008, 20, 4268.
[34] H. Djojoputro, X. F. Zhou, S. Z. Qiao, L. Z. Wang, C. Z. Yu, G. Q. Lu,
J. Am. Chem. Soc. 2006, 128, 6320.
4] X. W. Lou, L. A. Archer, Z. Yang, Adv. Mater. 2008, 20, 3987.
5] K. Kamata, Y. Lu, Y. N. Xia, J. Am. Chem. Soc. 2003, 125, 2384.
6] J. Lee, J. C. Park, H. Song, Adv. Mater. 2008, 20, 1523.
7] Y. Chen, H. R. Chen, M. Ma, F. Chen, L. M. Guo, L. X. Zhang,
J. L. Shi, J. Mater. Chem. 2011, 21, 5290.
[
8] L. Zhang, S. Z. Qiao, Y. G. Jin, Z. G. Chen, H. C. Gu, G. Q. Lu, Adv.
Mater. 2008, 20, 805.
9] M. Roca, A. J. Haes, J. Am. Chem. Soc. 2008, 130, 14273.
[35] S. Z. Qiao, C. X. Lin, Y. G. Jin, Z. Li, Z. M. Yan, Z. P. Hao,
Y. N. Huang, G. Q. Lu, J. Phys. Chem. C 2009, 113, 8673.
[36] S. Guan, S. Inagaki, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc. 2000,
122, 5660.
[37] X. J. Meng, T. Yokoi, D. L. Lu, T. Tatsumi, Angew. Chem. Int. Ed.
2007, 46, 7796.
[38] S. Haffer, M. Tiemann, M. Froba, Chem. Eur. J. 2010, 16, 10447.
[39] M. Jaroniec, L. A. Solovyov, Chem. Commun. 2006, 2242.
[40] D. C. Niu, Z. Ma, Y. S. Li, J. L. Shi, J. Am. Chem. Soc. 2010, 132,
15144.
[41] A. M. El-Toni, M. W. Khan, M. A. Ibrahim, M. Abid, M. Al-Hoshana,
M. Al-salhia, Chem. Commun. 2010, 46, 6482.
[42] W. Q. Wang, J. G. Wang, P. C. Sun, D. T. Ding, T. H. Chen, J. Colloid
Interface Sci. 2009, 331, 156.
[
[
10] G. Chen, Y. Wang, M. X. Yang, J. Xu, S. J. Goh, M. Pan, H. Y. Chen,
J. Am. Chem. Soc. 2010, 132, 3644.
11] J. Liu, H. Xia, D. F. Xue, L. Lu, J. Am. Chem. Soc. 2009, 131, 12086.
12] X. W. Lou, C. Yuan, L. A. Archer, Adv. Mater. 2007, 19, 3328.
13] W. M. Zhang, J. S. Hu, Y. G. Guo, S. F. Zheng, L. S. Zhong,
W. G. Song, L. J. Wan, Adv. Mater. 2008, 20, 1160.
14] X. W. Lou, C. Yuan, E. Rhoades, Q. Zhang, L. A. Archer, Adv. Funct.
Mater. 2006, 16, 1679.
15] X. J. Wu, D. S. Xu, J. Am. Chem. Soc. 2009, 131, 2774.
16] X. J. Wu, D. S. Xu, Adv. Mater. 2010, 22, 1516.
[
[
[
[
[
[
[
17] J. Liu, S. Z. Qiao, S. B. Hartono, G. Q. M. Lu, Angew. Chem. Int. Ed.,
2
010, 49, 4981.
[43] W. P. Guo, F. Kleitz, K. H. Cho, R. Ryoo, J. Mater. Chem., 2010, 20,
8257.
[
[
[
[
18] J. Liu, R. Harrison, J. Z. Zhou, T. T. Liu, C. Z. Yu, G. Q. M. Lu,
S. Z. Qiao, Z. P. Xu, J. Mater. Chem. 2011, 21, 10641.
19] T. R. Zhang, J. P. Ge, Y. X. Hu, Q. Zhang, S. Aloni, Y. D. Yin, Angew.
Chem. Int. Ed. 2008, 47, 5806.
[44] J. Liu, S. Z. Qiao, Q. H. Hu, G. Q. Lu, Small, 2011, 7, 425.
[45] Y. H. Deng, Y. Cai, Z. K. Sun, J. Liu, C. Liu, J. Wei, W. Li, C. Liu,
Y. Wang, D. Y. Zhao, J. Am. Chem. Soc. 2010, 132, 8466.
[46] P. M. Arnal, M. Comotti, F. Schüth, Angew. Chem. Int. Ed. 2006, 45,
8224.
20] Q. Zhang, J. P. Ge, J. Goebl, Y. X. Hu, Z. D. Lu, Y. D. Yin, Nano Res.
2
009, 2, 583.
21] D. Chen, L. L. Li, F. Q. Tang, S. Qi, Adv. Mater. 2009, 21, 3804.
[47] M. Besson, P. Gallezot, Catal. Today 2000, 57, 127.
Adv. Funct. Mater. 2012, 22, 591–599
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com 599