Synthesis of porous silica structures with hollow interiors by templating nanosized calcium carbonate

Article abstract of DOI:10.1016/j.inoche.2004.01.003

Shape-controlled porous silica structures with hollow interiors were synthesized by using CaCO₃ nanoparticles with the corresponding morphologies and sizes as the inorganic templates. Through the hydrolysis of sodium silicate, a continuous coating of SiO₂ was deposited over the surface of CaCO₃ cores to form core-shell structures. Hollow silica structures were finally obtained after calcining process and dissolving CaCO₃ in HCl dilute solution. The as-prepared hollow products were characterized with TEM, SEM, EDS, BET, and FT-IR.

Full text of DOI:10.1016/j.inoche.2004.01.003

Inorganic Chemistry Communications 7 (2004) 447–449
www.elsevier.com/locate/inoche
Synthesis of porous silica structures with hollow interiors
by templating nanosized calcium carbonate
*
Jian-Feng Chen , Jie-Xin Wang, Run-Jing Liu, Lei Shao, Li-Xiong Wen
Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology,
No. 15 Bei San Huan Dong Road, Beijing 100029, China
Received 16 October 2003; accepted 8 January 2004
Published online: 4 February 2004
Abstract
Shape-controlled porous silica structures with hollow interiors were synthesized by using CaCO3 nanoparticles with the corre-
sponding morphologies and sizes as the inorganic templates. Through the hydrolysis of sodium silicate, a continuous coating of SiO2
was deposited over the surface of CaCO3 cores to form core–shell structures. Hollow silica structures were finally obtained after
calcining process and dissolving CaCO3 in HCl dilute solution. The as-prepared hollow products were characterized with TEM,
SEM, EDS, BET, and FT-IR.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Nanosized calcium carbonate; Shape control; Hollow silica; Core–shell structure
In recent years, there has been great interest con-
cerning the synthesis of hollow materials with specific
structures and different compositions [1–4]. Among
them, hollow spherical materials with various diameters
attract the most attentions since they can serve as small
containers for applications in catalysis, controlled re-
lease for drugs, dyes and perfumes, development of ar-
tificial cells, and protection of biologically active agents
such as proteins and enzymes [5,6]. Furthermore, hollow
spheres also exhibit some advantages over their solid
counterparts due to lower densities, higher surface area,
and more unique optical and chemical properties.
Several methods have been reported for the synthesis
of hollow spherical materials, such as nozzle reactor
approaches, emulsion/phase extraction techniques, and
self-assembly processes including layer-by-layer (LBL)
approaches [7,8]. Of these methods, self-assembly pro-
cesses, also called core–shell techniques, are probably
the most effective approaches to the formation of inor-
ganic hollow spherical particles. In such processes, the
assembly of the shell materials onto the core template is
carried out to obtain composite materials with the core–
shell structure, followed by removal of the templates via
calcination in air or exposure to appropriate solvents to
generate hollow structures with inner diameters deter-
mined by the size of the templates. The sacrificial tem-
plates that have been used are mainly organic templates,
including polystyrene latex spheres [9], resin spheres
[10], liquid droplets [11], and vesicles [12], etc., while the
utilization of some inorganic templates, such as silica
and CS2, to produce hollow spheres has also been re-
ported [13,14]. However, many of these templates are
organic or inorganic spheres with sizes of several hun-
dred nanometers, by which it is difficult to produce
nanosized hollow spheres and obtain controllable hol-
low morphologies.
In this communication, we present a simple and novel
pathway to fabricate shape-controlled hollow silica par-
ticles via core–shell structure of CaCO3/SiO2 by a sol–gel
route. Scheme 1 shows the overall procedure used to
synthesize the spherical hollow silica. Nanosized CaCO3
particles with cubic and needle-like morphologies, which
were synthesized by a high gravity reactive precipitation
method [15], were employed as novel inorganic structure
* Corresponding author. Tel.: +86-10-64446466; fax: +86-10-
64434784.
E-mail address: chenjf@mail.buct.edu.cn (J.-F. Chen).
1387-7003/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2004.01.003

448
J.-F. Chen et al. / Inorganic Chemistry Communications 7 (2004) 447–449
Scheme 1. The procedure for preparation of spherical hollow silica.
(a) Hydrolysis and condensation of sodium silicate occurred, then SiO2
coated CaCO3 cores to form core–shell structures. (b) After calcina-
tions, CaCO3 was removed by dissolving it in HCl dilute solution.
directing templates to produce the hollow particles, in-
stead of the normally used organic templates.
In a typical process, 500 ml of nanosized calcium
carbonate aqueous suspension (8 wt%) was heated and
kept at 80 °C under vigorous stirring. 180 ml of sodium
silicate (Na2SiO3 ꢀ 9H2O) solution (2 wt% SiO2) was
then added dropwise into the suspension to form core–
shell composites with the molar ratio of SiO2/
CaCO3 ¼ 1/10, while the system was maintained at pH
9.0 by simultaneously adding HCl dilute solution. After
the adding process was completed, the slurry was further
stirred at the same condition as above for 2 h and
subsequently filtered, rinsed with deionized water, dried
at 100 °C, and followed by calcination in air at 700 °C
for 5 h to yield a core–shell composite with CaCO3 as
the core and SiO2 as the shell. The as-prepared com-
posite was afterwards put into HCl dilute solution
maintaining pH < 1 for 12 h to remove the CaCO3
templates completely. Finally, the resulting gel was fil-
tered, rinsed with deionized water and ethanol in se-
quence, and dried in the oven at 90 °C to obtain hollow
silica powder.
Fig. 1 shows the transmission electron microscope
(TEM) (HITACHI-800) images of the as-synthesized
hollow silica structures. Fig. 1(a) consists of spherical
hollow silica nanoparticles with a diameter of 70–100
nm and wall thickness of approximately 10 nm, which
was fabricated via a CaCO3 template with cubic mor-
phology, while Fig. 1(b) shows tubular hollow silica
synthesized through a needle-like CaCO3 template. The
strong contrast between the dark edge and pale center is
Fig. 1. TEM images of hollow silica particles: (a) spherical particles by
cubic CaCO3 templating; (b) tubular particles by needle-like CaCO3
templating.
evidence for their hollow nature [16]. Some twin spheres
are also observed in Fig. 1(a). This is because the coa-
lescence of CaCO3 nanoparticles may result in conden-
sation and coating of SiO2 on the surface of aggregate
CaCO3 nanoparticles. Typical field emission scanning
electron microscope (FE-SEM) (JEOL-6301F) images
of spherical and tubular hollow silica are shown in
Fig. 2. Some broken spheres are observed in Fig. 2(a),
providing further proof of their hollow nature. Fig. 2(b)
also shows hollow tubes with openings consistent with
the TEM image in Fig. 1(b), indicating a tubular hollow
structure of silica.
Energy dispersive spectroscopy (EDS) (LINKISIS-
300) indicates the presence of Si and O and the absence
of Ca, confirming the complete removal of the CaCO3
templates. The XRD results are those expected for non-
crystalline materials. By IR spectroscopy (NEXUS-470)
(Fig. 3), there are three characteristic peaks (at 1081.15,
798.51 and 456.65 cmꢁ1), respectively, belonging to
three fundamental vibration bands for the silica struc-
ture in the wavenumber range of 450–1200 cmꢁ1 [17–19].
The typical nitrogen adsorption isotherm at 77 K
for hollow silica spheres and its pore size distribution
Fig. 2. SEM images of hollow silica particles: (a) spherical particles; (b) tubular particles.

J.-F. Chen et al. / Inorganic Chemistry Communications 7 (2004) 447–449
449
thickness of hollow silica by controlling the amount of
deposited SiO2 or the concentration of sodium silicate
solution are underway.
45
40
35
30
25
20
15
798.51
955.01
In summary, hollow silica materials with spherical
and tubular morphologies were successfully synthesized
using nanosized CaCO3 particles with the corresponding
shapes and sizes as inorganic templates in our approach.
This novel method may also be feasible to fabricate
hollow materials of other compositions. Since sodium
silicate and calcium carbonate can be produced with a
low cost, the preparation of hollow silica by this method
might benefit its commercialization. In addition, due to
its large specific area, porosity, hollow structure, and
compatibilities with other materials, the as-synthesized
hollow silica may find wide applications in many fields.
1637.65
456.65
3439.27
1081.15
1000
4000
3500
3000
2500
2000
1500
500
Wavenumber (cm-1)
Fig. 3. IR spectroscopy for hollow spherical silica.
Acknowledgements
determined by a Micromeritics ASAP 2010 Analyzer are
shown in Fig. 4. The resulting isotherm can be classified
as a type II isotherm according to the International
Union of Pure and Applied Chemistry (IUPAC) no-
menclature [20]. The corresponding size distribution
data calculated from the nitrogen adsorption isotherm
by the Barrett–Joyner–Halenda (BJH) method reveals
that the pores on the shell are mainly microporous with
very narrow pore size distribution centered at 0.9–1.0
nm. The BET specific surface area of the hollow silica
sphere is found to be as high as 725.2 m2/g. In the same
way, the BET surface area of hollow silica tube is
measured to be 516.5 m2/g.
Importantly, the inner diameter and the morphology
of the hollow silica materials prepared in this work may
be controlled by adopting appropriate nanosized
CaCO3 particles, whose size and morphology can be
tailored precisely by a high gravity reactive precipitation
method [15]. Further experiments to adjust the shell
This work was financially supported by National
Natural Science Foundation of China (No. 20236020),
National High Tech Program (‘‘863’’ plan, No.
2002AA327100) and Key R&D plan of the Ministry of
Education of China (No. KEY0202).
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