A triblock-copolymer-templating route to carbon spheres@SBA-15 large mesopore core-shell and hollow structures

Article abstract of DOI:10.1039/c4ra08850f

The fabrication of mesoporous core-shell and hollow spheres with ordered mesostructures and tunable large pore sizes is highly desirable for fundamental research and practical applications. A direct triblock-copolymer-templating coating approach has been provided for the synthesis of carbon sphere@mesoporous silica core-shell (CS@SBA-15) and hollow structures in an acidic medium at room temperature. These CS@SBA-15 core-shell structures possess large mesopores (6.3-8.4 nm), high surface areas (318.1-438.4 m² g^-1) and large pore volumes (0.31-0.36 cm³ g^-1). The corresponding hollow mesoporous silica spheres (HMSS) with controllable mesopores (6.0-8.2 nm), high surface areas (239.9-326.5 m² g^-1) and pore volumes (0.37-0.62 cm³ g^-1) can be obtained after the removal of the carbon sphere cores through calcination at 700 °C in air. The current research results provide an effective methodology for the synthesis of other mesoporous silica core-shell structures with large pore sizes and multilayer core-shells, and even nanorattle structures. This journal is

Full text of DOI:10.1039/c4ra08850f

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PAPER
A triblock-copolymer-templating route to carbon
spheres@SBA-15 large mesopore core–shell and
hollow structures†
Cite this: RSC Adv., 2014, 4, 48676
Jianping Yang,*abc Xufang Qian, Minjun Chen, Jianwei Fan, Hua Kun Liu
and Wei-xian Zhang
b
a
a
c
a
The fabrication of mesoporous core–shell and hollow spheres with ordered mesostructures and tunable
large pore sizes is highly desirable for fundamental research and practical applications. A direct triblock-
copolymer-templating coating approach has been provided for the synthesis of carbon
sphere@mesoporous silica core–shell (CS@SBA-15) and hollow structures in an acidic medium at room
temperature. These CS@SBA-15 core–shell structures possess large mesopores (6.3–8.4 nm), high
2
ꢀ1
3
ꢀ1
surface areas (318.1–438.4 m
g
) and large pore volumes (0.31–0.36 cm
g
). The corresponding
hollow mesoporous silica spheres (HMSS) with controllable mesopores (6.0–8.2 nm), high surface areas
Received 18th August 2014
Accepted 16th September 2014
2
ꢀ1
3
ꢀ1
(239.9–326.5 m
g
) and pore volumes (0.37–0.62 cm g ) can be obtained after the removal of the
ꢁ
carbon sphere cores through calcination at 700 C in air. The current research results provide an
effective methodology for the synthesis of other mesoporous silica core–shell structures with large pore
sizes and multilayer core–shells, and even nanorattle structures.
DOI: 10.1039/c4ra08850f
www.rsc.org/advances
1
1
12
13
Au@mSiO
2
,
Pt@mSiO
2
,
Fe
3
O
4
/SiO
2
/Au@mSiO
2
,
g-Fe
2
-
1
. Introduction
14
10
O @nSiO @mSiO ,
b-NaYF @SiO @mSiO ,
and SiO
carbon
3
2
2
4
2
2
15,16
Recently, core–shell structures have attracted considerable nanotubes@mSiO
@mSiO , have been reported.
2
2
2
interest owing especially to their nanoarchitectures, alterable Moreover, these core–shell nanocomposites present excellent
1
2,17
18
19
compositions and sizes, which endow these materials with properties in catalysis,
adsorption,
drug delivery,
20–22
multifunctional properties, such as magnetic, electrical, optical, biological imaging and photodynamic therapy.
Although
thermodynamics and catalytic properties, and thus have much effort has been focused on the synthesis approaches for
extensive applications in areas such as biology, physics, mate- mesoporous silica coated core–shell structures with n-octade-
1
–4
23
rial science and catalysis. At present, core–shell materials cyltrimethoxysilane
with functional core and different shell compositions such as trimethylammonium bromide (CTAB) as surfactants,
polymer, carbon, metal, silica and mesoporous silica have been general pore size of the mesoporous silica shell is around 2–3
(C18TMS)
and
hexadecyl-
1
7,18,20
the
5–9
successfully synthesized. Regarding the diverse kinds of core– nm. Consequently, one of the obstacles toward practical appli-
shell structures, mesoporous silica coated inorganic nano- cations is the small pore size of mesoporous silica, due to the
materials have attracted considerable attention in recent years tedious synthesis methods. Therefore, the development of core–
because the mesoporous silica shells can serve as an initial shell structures with large pore size mesoporous silica shells is
isolated interface and easily modiable surface, and also necessary.
provide a high surface area, large pore volume and readily
In addition, hollow mesoporous silica spheres, possessing
accessible pore channels. Until now, mesoporous silica coated accessible chambers, large surface areas, uniform pore sizes,
core–shell structures with diversied functional cores, such as good biocompatibilities and surface functionalities, have
revealed signicant applications in many elds, e.g., adsorp-
24–26
10
27–30
tion, drug-delivery carriers, sensors, and catalysis.
tionally, hollow mesoporous silica spheres are fabricated by an
Tradi-
a
College of Environmental Science and Engineering, State Key Laboratory of Pollution
Control and Resources Reuse, Tongji University, Shanghai 200092, P. R. China. E-mail:
zcjpyang@gmail.com; Tel: +86-21-65985885
b
organic so template and a sacricial hard template. The so
Department of Chemistry, Laboratory of Advanced Materials, Fudan University, template method usually requires strict control of the reaction
Shanghai 200433, P. R. China
conditions because the so templates, such as micelles, emul-
c
Institute for Superconducting
& Electronic Materials, Australian Institute of
sions, polymers and bubbles, are sensitive to the reaction
Innovative Materials, University of Wollongong, Innovation Campus, Squires Way,
North Wollongong, NSW 2500, Australia
29–33
environment.
Particularly, polymer lattices, carbon spheres
and metal oxides can serve as hard templates with various
morphologies, controllable sizes and easily removable
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra08850f
1
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properties.
Therefore, the hollow mesoporous silica spheres
2.2.2 Synthesis of CS@SBA-15 and hollow mesoporous
can be prepared with changeable particles sizes, tunable cavi- silica spheres. CS@SBA-15 core–shell structures were synthe-
ties and morphologies via the hard template methods.
sized using a nonionic copolymer as a surfactant and TEOS as
Since Li and coworkers reported a simple hydrothermal the silica source in an acidic medium. In a typical procedure,
method to synthesize carbon spheres with different sizes using 0.06 g of P123 and 0.18 g of anhydrous magnesium sulfate were
36
glucose as the precursor, a variety of metal oxide hollow dissolved in 30 mL of 2 M hydrochloric acid. Then, 0.15 g of
spheres have been prepared by utilizing carbonaceous micro- carbon spheres was added into the above transparent solution
spheres as a removable template, attributing to the carbon with ultrasonication for 30 min. Finally, 0.15 g of TEOS was
spheres' hydrophilic surface and functional groups, i.e. OH and added dropwise to the mixture and stirred for 24 h before being
3
7,38
ꢁ
C]O.
Although carbon spheres have been generally applied transferred to an autoclave and heated at 100 C for 24 h. The
39
in the synthesis of all types of hollow structures, there are few precipitate was obtained by centrifugation and washed with
studies on the synthesis of large mesopore silica encapsulating water and ethanol for three cycles. Then, the black powder was
carbon spheres.
40,41
ꢁ
Moreover, from the viewpoint of method- dried in a vacuum oven at 40 C for more than 6 h. The large
ology, there are few reports on the controllable approach for the mesopore silica coated carbon sphere samples (CS@SBA-15)
ꢁ
synthesis of mesoporous silica core–shell and hollow structures were obtained aer calcination at 350 C for 5 h under a
ꢁ
ꢀ1
with large pore sizes and changeable cores by using triblock nitrogen atmosphere with the heating rate of 1 C min . The
copolymers as the surfactant.
24
ꢁ
carbon sphere core was removed at 700 C for 3 h in air, and
Herein, we report a direct triblock-copolymer-templating then the hollow mesoporous silica spheres with a large pore size
coating strategy to synthesize the large mesoporous silica- (HMSS) were formed. The CS@SBA-15 100H and HMSS 100H
ꢁ
coated carbon spheres core–shell structure (CS@SBA-15) and were prepared with the hydrothermal treatment at 100 C for 24
hollow mesoporous silica spheres (HMSS) by using Pluronic h without the addition of anhydrous magnesium sulfate. The
P123 (PEO20–PPO70–PEO20) as a surfactant template, tetraethyl CS@SBA-15 130H-MgSO4 and HMSS 130H-MgSO4 were
orthosilicate (TEOS) as the silica source, and anhydrous synthesized with anhydrous magnesium sulfate as the inor-
ꢁ
magnesium sulfate as the inorganic salt in HCl solution at room ganic salt and hydrothermal treatment at 130 C for 24 h. The
temperature. These CS@SBA-15 and HMSS present ordered traditional mesoporous silica hollow spheres were obtained by
mesostructures, high surface areas and large pore size distri- using carbon spheres as a core, CTAB as the surfactant, TEOS as
butions. The pore size of the mesoporous silica shell can be the silica source in ammonia–ethanol–water solution and then
ꢁ
adjusted in the range of 6.3–8.4 nm via changing the reaction calcined at 700 C in air.
conditions, including the hydrothermal temperature and the
amount of anhydrous magnesium sulfate.
2.3. Characterization
Scanning electron microscopy (SEM) images were obtained on a
Philip XL30 microscope. A thin lm of gold was sprayed on the
sample before characterization. Transmission electron micros-
copy (TEM) measurements were achieved on a JEOL 2011
microscope operating at 200 kV. The samples were suspended
in ethanol and dried on a holey carbon lm on a Cu grid for
TEM measurements. Nitrogen sorption isotherms were
measured at 77 K with a Micromeritics Tristar 3020 analyzer
2
. Experimental section
2.1. Chemicals
All chemicals were of analytical grade and used without further
purication. Glucose, hydrochloric acid, anhydrous magnesium
sulfate, tetraethyl orthosilicate (TEOS) and hexadecyl-
trimethylammonium bromide (CTAB) were purchased from
Sinopharm Chemical Reagent Co. (China). Triblock poly
(
USA). Before measurement, the samples were degassed in a
(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide)
ꢁ
vacuum at 180 C for at least 6 h. The Brunauer–Emmett–Teller
copolymer Pluronic P123 (PEO –PPO –PEO ) was purchased
from Acros Corp. Note that deionized water was used in all
experiments.
2
0
70
20
(
(
BET) method was utilized to calculate the specic surface areas
BET) by using the adsorption data in a relative pressure range
S
from 0.04 to 0.2. The pore volume and pore size distributions
were derived from the adsorption branches of the isotherms by
using the Barrett–Joyner–Halenda (BJH) model. The total pore
2
.2. Synthetic route
.2.1 Preparation of carbon spheres. The carbon spheres
CS) were prepared by the hydrothermal polymerization of
volume V was estimated from the amount adsorbed at a relative
t
2
pressure P/P
patterns were recorded on a Bruker D4 X-ray diffractometer
Germany) with Ni-ltered Cu Ka radiation (40 kV, 40 mA).
0
of 0.995. The small angle X-ray diffraction (XRD)
(
36
glucose according to a previously reported method. Briey,
.73 g of glucose was dissolved in 25 mL of deionized water and
(
2
stirred for 20 minutes to form a clear solution. Subsequently,
the solution was transferred into a 30 mL of Teon-sealed
autoclave for the hydrothermal treatment at 180 C for 8 h. The
3
. Results and discussion
ꢁ
black samples were obtained by continuous centrifugation at The large mesopore silica coated carbon spheres with a core–
5

000 rpm for 5 min and washed with water and ethanol. The shell structure (CS@SBA-15) and the corresponding hollow
nal products were obtained aer vacuum-drying at 40 C for mesoporous silica spheres (HMSS) are fabricated via three
ꢁ
12 h.
steps. As described in Scheme 1, rst, carbon spheres are
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prepared as the core. Then, the large mesopore silica is coated surfactant, the CS@SBA-15 core–shell structure is obtained. As
on the surface of the carbon spheres by using Pluronic P123 as shown in Fig. 1B, the CS@SBA-15 products are all spherical on a
the surfactant template under an acidic system. Subsequently, large-scale, indicating the uniform coating process. Interest-
the removal of surfactant P123 and the carbon sphere core ingly, the surface of these spherical samples become rough and
yielded CS@SBA-15 and HMSS, respectively.
tend to aggregate, as well as some strip-like structure can be
The carbon spheres are prepared with a hydrothermal observed, revealing the successful coating of the mesoporous
method by using glucose as the carbon species. Fig. 1A shows a silica. The morphology and microstructure of the obtained
typical scanning electron microscopy (SEM) image of carbon CS@SBA-15 are further characterized by transmission electron
spheres with an average diameter of about 550–650 nm. These microscopy (TEM). The TEM image shows that the carbon
uniform spheres with smooth surfaces possess hydrophilic spheres have been all coated by the homogeneous mesoporous
properties and can resist the etching under a strong acid silica with a thickness of 120–150 nm (Fig. 2A). The magnied
system; therefore, the carbon spheres can interact with the tri- TEM image further reveals the typical core–shell structure.
block copolymer P123 and silica source TEOS in the acidic Moreover, the shell is composed of many mesopore channels
medium. Aer sintering the as-made samples under a nitrogen with a pore size of 6.5 nm and wall thickness of about 6 nm
ꢁ
atmosphere at 350 C to remove the triblock copolymer (Fig. 2B). It is noteworthy that the mesoporous silica shell shows
a parallel mesoporous regional trend and some stripe-like
mesochannels in the [110] direction, thus demonstrating a 2-D
hexagonal ordered mesostructure (Fig. 2B). This result is in
good agreement with the typical TEM images of SBA-15 meso-
42
porous silica reported elsewhere.
The small-angle X-ray diffraction (XRD) pattern for CS@SBA-15
ꢁ
shows an obvious diffraction peak at 2q ¼ 0.86 , which is
attributed to the (100) reection associated with the p6mm
hexagonal symmetry (Fig. 1D), indicating an ordered mesostruc-
ture. The N sorption isotherms of CS@SBA-15 (Fig. 3A) present
2
typical type IV curves with a capillary condensation step at the
broad relative pressure (P/P ) of 0.5–0.7, suggesting the uniform
0
large pore size. The hysteresis loops are classied to be the
Scheme 1 The preparation procedure for the large mesopore coated
carbon sphere core–shell structure (CS@SBA-15) and the corre- H1-type, assigned to narrow pore size distributions and 1D
sponding hollow mesoporous silica spheres (HMSS).
cylindrical channels. The pore size calculated using the Barrett–
Joyner–Halenda (BJH) analysis with the adsorption branch shows
a narrow distribution at 6.7 nm (Fig. 3B). This result agrees very
well with the TEM images (Fig. 2A and B); moreover, the CS@SBA-
1
5 core–shell structure has a high BET surface area of
2
ꢀ1
3
ꢀ1
438.4 m g and large pore volume of 0.36 cm g (Table 1).
Fig. 1 SEM images of (A) carbon spheres prepared through a hydro-
thermal method by using glucose as the carbon species, (B) large
mesopore silica coated carbon spheres (CS@SBA-15) core–shell
structures prepared by using carbon spheres as a core, Pluronic P123
as the surfactant template, TEOS as the silica source, anhydrous
magnesium sulfate as the inorganic salt in HCl solution with hydro-
ꢁ
ꢁ
thermal treatment at 100 C for 24 h and calcination at 350 C for 5 h
under a nitrogen atmosphere, (C) hollow mesoporous silica spheres Fig. 2 TEM images of (A) and (B) CS@SBA-15 core–shell structures
ꢁ
ꢁ
(
HMSS) after calcination at 700 C for 3 h in air, (D) small-angle X-ray after calcination at 350 C for 5 h under a nitrogen atmosphere; (C) and
diffraction pattern of CS@SBA-15 and HMSS.
ꢁ
(D) HMSS after calcination at 700 C for 3 h in air.
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Fig. 3 (A) N sorption isotherm curves and (B) pore size distribution
plots of CS@SBA-15 and HMMS.
Since the triblock copolymer surfactant and carbon spheres
can be easily removed with calcination treatment, which results
in the formation of mesoporous silica shells with hollow inte-
rior structures. Signicantly, the spherical shape is preserved
for these hollow mesoporous silica spheres (HMMS) (Fig. 1C).
As revealed by the SEM image, a number of spheres have a hole
on the surface, indicating hollow structures (Fig. 1C). The TEM
images further conrm that the hollow interiors of the HMMS
are approximately 600 nm, originating from the carbon sphere
Fig. 4 SEM images of (A) CS@SBA-15 100H, without the addition of
anhydrous magnesium sulfate and hydrothermal treatment at 100 C;
ꢁ
(
B) the corresponding hollow mesoporous silica spheres (HMSS 100H)
ꢁ
after calcination at 700 C for 3 h in air. (C) CS@SBA-15 130H-MgSO
4
,
with the anhydrous magnesium sulfate as the inorganic salt and
ꢁ
core and mesoporous silica shell, which are about 120–150 nm hydrothermal treatment at 130 C; and (D) the hollow mesoporous
4
silica spheres (HMSS 130H-MgSO ).
(Fig. 2C and D); i.e. the regular stripe-like mesochannels
encircle the cavity.
ꢁ
A wide weak peak still appears at 2q ¼ 0.90 for the small-
angle XRD pattern, suggesting the retention of the mesoporous
structure (Fig. 1D). It should be noted that the diffraction peak
removal of the carbon sphere core in air maintained the hollow
structure (Fig. 4B). Furthermore, the CS@SBA-15 100H core–
shell structure and HMSS 100H hollow sphere presents a type IV
isotherm (Fig. S1†), and the BET surface area, pore size and pore
ꢁ
ꢁ
shied to the higher angle region (from 2q ¼ 0.86 to 0.90 ),
indicating the shrinkage of the mesoporous silica shell to a
certain extent. The N adsorption–desorption isotherms of the
HMSS shows a representative typical type-IV curve, consisting of
an H1-type hysteresis loop in the high relative pressure (P/P )
0
range of 0.5–0.7, indicating the existence of a large mesopore.
2
ꢀ1
3
ꢀ1
2
ꢀ1
2
volume are 376.1 m g , 6.3 nm, 0.31 cm g and 239.9 m g
,
3
ꢀ1
6.1 nm, 0.37 cm g , respectively (Table 1), which are lower
than those of samples with the addition of MgSO . Further-
4
more, the XRD patterns of the core–shell structures and hollow
spheres both display a distinct diffraction peak (Fig. S2†). These
The surface area and pore size distribution of the HMSS are
2
ꢀ1
3
26.5 m g and 6.0 nm, which is slightly lower than that of
results reveal that the introduction of the inorganic salt MgSO
4
3
ꢀ1
CS@SBA-15 (Table 1). The increased pore volume (0.42 cm g
is derived from the hollow interiors of the HMSS.
)
can result in the uniform coating of mesoporous silica due to
the enhanced electrostatic interactions between the triblock
To explore the role of MgSO4 in the coating process, the
CS@SBA-15 100H sample is synthesized without the addition of
MgSO . It is found that the CS@SBA-15 100H sample still
4
possesses a spherical morphology with a diameter of about 900
nm, indicating that the mesoporous silica was coated on the
surface of the carbon spheres (Fig. 4A). However, some sticky
particles with irregularly rounded shapes are observed on the
surface of the spheres, resulting in a rough surface and
nonuniform coating. The obtained HMSS 100H aer the
24,26
copolymer surfactant and silica precursor.
For the traditional synthesis approach of SBA-15, the pore
size distribution is sensitive to the hydrothermal temperature.
Generally, a higher hydrothermal temperature treatment favors
42
a larger pore size distribution. Therefore, in order to obtain
large pore size mesoporous silica shell during the coating
ꢁ
process, an elevated hydrothermal temperature at 130 C is
chosen. It is noticeable that the carbon spheres are coated by
the mesoporous silica shell (CS@SBA-15 130H-MgSO ) at such
4
high hydrothermal temperature with an average size of 900 nm.
From the broken spheres, the core–shell structure is apparently
Table 1 Physicochemical properties of CS@SBA-15 and HMSS
observed (Fig. 4C). The hollow spheres (HMSS 130H-MgSO
which clearly appeared aer the removal of the carbon core
Fig. 4D), further demonstrate the successful coating aer the
high hydrothermal treatment. Furthermore, the N adsorption–
desorption isotherms of CS@SBA-15 130H-MgSO and HMSS
4
),
2
ꢀ1
3
ꢀ1
Sample
S
BET (m g
)
t
Pore size (nm) V (cm g )
(
CS@SBA-15
438.4
376.1
318.1
326.5
239.9
277.7
6.7
6.3
8.4
6.0
6.1
8.2
0.36
0.31
0.33
0.42
0.37
0.62
2
CS@SBA-15 100H
CS@SBA-15 130H-MgSO
HMSS
HMSS 100H
HMSS 130H-MgSO
4
4
1
30H-MgSO exhibit typical IV curves with condensation steps
4
at a high relative pressure (P/P ) of 0.7–0.8, indicating the larger
0
pore size distribution (Fig. S1†). As shown in Table 1, the pore
size distributions are centered at ꢂ8.4 and 8.2 nm, respectively.
4
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Based on the aforementioned results, the mesoporous silica
SBA-15 can be coated on the surface of carbon spheres under a
low concentration of triblock copolymer and strong acidic
system at room temperature. The hydrogen ion interacts with
the oxygen derived from the hydrophilic chain of the block
copolymer, and then reacts with the silica precursor through
electrostatic interactions (Scheme 1). The micelles assemble to
intermediate mesostructures, and thus are subsequently
adsorbed on the surface of carbon spheres via polar interactions
and hydrogen bonding. Because many OH and C]O groups
exist on the surface of carbon spheres and present a hydrophilic
property, which is conrmed by FTIR (Fig. S3†), the carbon
spheres can also interact with the cationic surfactant hexa-
decyltrimethylammonium bromide (CTAB) and fabricate
hollow mesoporous silica spheres with a small pore size
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1
1
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.3 to 8.4 nm by changing the hydrothermal temperature and
ionic strength. This elegant design with carbon spheres as the
core and large pore mesoporous silica as the shell renders
fascinating features and advantages. First, these large pore, i.e.
CS@SBA-15 and HMSS, possess diversied applications in
adsorption, drug-release and catalyst carriers. Second, carbon
spheres as a benign and hydrophilic surface can offer a site for
the adsorption of metal ions and organic molecules to
construct other functionalized core–shells and hollow struc-
tures. Third, the carbon spheres are easily coated on the
surface of other nanocrystals and protected cores, which is
benecial for the further formation of multilayer large pore
mesoporous silica encapsulating core–shell and yolk–shell
structures. At last, these large mesopore channels afford a
location for loading and as a nanocasting template for other
mesoporous shells. Therefore, for fundamental interest and
methodology outlook, the current work is not limited to the
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sizes; however, it provides extensive directions for the
synthesis of other large pore mesoporous core–shell, yolk–
shell and hollow structures.
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Acknowledgements
We gratefully acknowledge the nancial support from the State
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Foundation (PCRRF12001) and the China Postdoctoral Science
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