Facile fabrication of noble metal nanoparticles encapsulated in hollow silica with radially oriented mesopores: Multiple roles of the N-lauroylsarcosine sodium surfactant

Article abstract of DOI:10.1039/c1cc12823j

A facile one-pot method was reported to fabricate noble metal nanoparticles encapsulated in silica hollow nanospheres with radially oriented mesopores, and the anionic amino acid surfactant, N-lauroylsarcosine sodium, played multiple roles: reducing agent, stabilizer, emulsion droplets and mesopore template.

Full text of DOI:10.1039/c1cc12823j

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Cite this: Chem. Commun., 2011, 47, 7680–7682
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COMMUNICATION
Facile fabrication of noble metal nanoparticles encapsulated in
hollow silica with radially oriented mesopores: multiple roles
of the N-lauroylsarcosine sodium surfactantw
Hu Wang, Jin-Gui Wang, Hui-Jing Zhou, Yu-Ping Liu, Ping-Chuan Sun and
Tie-Hong Chen*
Received 13th May 2011, Accepted 24th May 2011
DOI: 10.1039/c1cc12823j
A facile one-pot method was reported to fabricate noble metal
nanoparticles encapsulated in silica hollow nanospheres with
radially oriented mesopores, and the anionic amino acid surfactant,
N-lauroylsarcosine sodium, played multiple roles: reducing
agent, stabilizer, emulsion droplets and mesopore template.
Metal nanoparticles have attracted broad interest in recent years
because of their unique properties and potential applications in
catalysis, biodetection, optics, photonics and electronics. The
chemical synthesis of metal nanoparticles generally involves the
reduction of metal ions by reducing agents such as hydrazine,
sodium borohydride and alcohols, which are highly reactive but
Scheme 1 Fabrication procedure of the Au@MSHS structure.
would have potential environmental and biological problems.
For the green synthesis of metal nanoparticles, green solvent
medium, an environmentally benign reducing agent and the
nontoxic stabilizers of the nanoparticles are required.1
metal nanoparticles encapsulated in a mesoporous hollow silica
shell was also presented.
The anionic amino acid surfactant, Sar-Na (Fig. S1, ESIw),
is commercially available and is commonly used in cosmetic
and pharmaceutical industries due to its biodegradability and
low toxicity. As shown in Scheme 1, when the solution
Metal nanoparticles encapsulated in hollow spheres, or the so
called yolk–shell or rattle type particles, have attracted more and
more attention.2–5 The nanorattle structures exhibited unique
optical and electrical properties and were advantageous in the
fields of drug delivery and catalysts.6–9 A variety of methods have
been developed to fabricate rattle type structures, including
template-assisted etching approach,2,3,10 selectively core-
etching,5,11 the surface-protected etching process,12 Kirkendall
effect13 or Ostwald ripening processes,8 vesicle-based method14
and water-in-oil microemulsion method.15 Although progress on
the synthesis of rattle type particles has been achieved, there are
disadvantages in some methods, such as tedious procedures and
employment of toxic and volatile organic solvents. In this work,
the amino acid anionic surfactant N-lauroylsarcosine sodium
(Sar-Na) was used as both a green reductant and stabilizer to
synthesize noble metal nanoparticles in aqueous solution.
Furthermore, based on the acid emulsification and templating
effects of the Sar-Na, a facile one-pot method to fabricate noble
À
containing Sar-Na and AuCl4 was stirred at 353 K for 1 h
(for the detailed experimental process see ESIw), the color of
the solution changed from bright yellow to purple (Fig. 1),
implying that Au nanoparticles were formed and suspended in
the solution. The average size of Au nanoparticles was 7.9 nm
(with a standard deviation of 2.6 nm) as shown in TEM images
(Fig. S2, ESIw).
Institute of New Catalytic Materials Science, Key Laboratory of
Advanced Energy Materials Chemistry (MOE),
College of Chemistry, Nankai University, Tianjin 300071,
P. R. China. E-mail: chenth@nankai.edu.cn
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c1cc12823j
Fig. 1 Photographs of Au, Pd and Pt nanoparticles in Sar-Na
solution.
c
7680 Chem. Commun., 2011, 47, 7680–7682
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As there was no other reducing agent used, it was concluded
that the surfactant Sar-Na acted as a reducing agent of the Au
nanoparticles and the tertiary amine group in Sar-Na would
be responsible for the reduction.16 Meanwhile, the Au nano-
particles were stabilized by the Sar-Na surfactant to form a
suspension. As shown in Scheme 1, the acid in the solution
could convert a portion of the Sar anions to Sar-H, which was
an amphiphilic polar oil and acted as emulsion droplets. Then
the Au nanoparticles stabilized by the surfactant would be
hosted and dispersed in the Sar-H polar oil droplets. When the
silica precursors of 3-aminopropyltrimethoxysilane (APMS)
and TEOS were added to the solution, a silica shell was
formed following the SÀN+BIÀ pathway, where S stands
for a surfactant, N stands for a co-structure directing agent
(CSDA), and I stands for inorganic precursors.17 During the
succedent reaction, the initially formed disordered silica/
surfactant hybrid mesophases in the shell underwent a structural
transformation and finally an ordered shell with radially
oriented mesochannels was obtained.18,19 Finally the rattle
structures with mesoporous shells were obtained after the
calcination to remove the surfactants. In the process, the
Sar-Na surfactant played multiple roles: reducing agent,
stabilizer, emulsion droplets after acidification and mesopore
template, and by this one-pot method the mesoporous silica
hollow spheres with encapsulated Au nanoparticles (Au@MSHS)
could be facilely fabricated.
Fig. 3 Nitrogen adsorption–desorption isotherms (a) and the pore
size distribution (b) of Au@MSHS.
The adsorption uptake at a relative pressure of 0.95 was
attributed to the aggregation voids between the silica nano-
spheres. The isotherms exhibited a H2 hysteresis loop in the
p/p0 range of 0.5–0.95, which had also been observed for other
hollow structures with mesoporous walls, and was ascribed to
the delay of nitrogen evaporation from the hollow voids
blocked by the surrounding mesopores during the N2 desorption
process.18,20 The BET surface area, total pore volume and
pore size were 330 m2 gÀ1, 0.39 cm3 gÀ1 and 2.8 nm, respectively
(Fig. 3b). The XRD pattern of Au@MSHS displayed
diffraction peaks of Au(0) (Fig. S4, ESIw). The UV-vis spectrum
(Fig. S5, ESIw) of Au@MSHS displayed an absorption peak
at 520 nm, corresponding to excitation of surface plasmon
vibrations in the gold nanoparticles.
In the anionic surfactant templated synthesis, the pH value
is an important factor to control the self-assembly of the
surfactant, co-structure directing agent (APMS) and TEOS.21
Here in our case, the pH value change would influence
the formation of oil droplets, the charge state of APMS
and surfactant, as well as the hydrolysis and condensation
rate of silica precursors, and it was reasonable that hollow
spheres could only be obtained within a certain pH range
(Fig. S6–S9, ESIw).
SEM images (Fig. 2a and b) display that the Au@MSHS
sample consisted of dispersed spheres. TEM images of
Au@MSHS (Fig. 2c and d) show that the particles were
hollow spheres, which were intact without aggregation. By
measuring the diameter of the particles from the TEM images,
it was found that the particle size was mostly in the range of
150–250 nm. It is interesting that all the hollow spheres possess
a uniform shell thickness of 25–35 nm independent of their
sizes. The significant feature of the hollow spheres was the
radially oriented mesochannels in their shells (Fig. 2d and
Fig. S3, ESIw) and each silica hollow sphere contained several
monodispersed Au nanoparticles inside.
The Sar-Na solution can also be used to reduce other noble
precursors, such as H2PdCl4. From TEM images (Fig. S10,
ESIw), the average size of the Pd nanoparticles in Sar solution
was 3.0 nm (with a standard deviation of 0.5 nm). Alloy
nanoparticles could also be obtained by this method and
different alloy particles could be prepared by altering the order
of addition of metal precursors to Sar-Na solution. For
instance, Pd–Au alloy nanoparticles were prepared by reducing
mixed HAuCl4 and H2PdCl4 solution simultaneously (denoted
as S-Pd–Au). If H2PdCl4 was reduced by Sar solution firstly,
and then HAuCl4 was added and reduced again, the alloy was
denoted as F-Pd–Au. With swap of the reduction order, an
alloy of F-Au–Pd was also prepared. Photographs of alloy
nanoparticles in Sar solution and TEM images of alloy
nanoparticles are shown in ESIw (Fig. S11–S14). Those three
alloy nanoparticles exhibited different shape and size distribution.
The fabrication route shown in Scheme 1 could be applied to
encapsulate different noble nanoparticles such as Pd, Pt, and
their bimetallic alloy nanoparticles into the mesoporous silica
hollow spheres. For instance, as shown in Fig. 4, Pd nano-
particles were encapsulated inside silica hollow spheres, and
the radially oriented mesopores in the silica shell could be seen.
After calcination the Pd nanoparticles were actually oxidized
to PdO as shown by the XRD pattern (Fig. S15, ESIw). The N2
The N2 adsorption/desorption isotherms of Au@MSHS
(Fig. 3a) exhibited a type IV isotherm demonstrating
the mesoporous characteristics of the hollow silica spheres.
Fig. 2 SEM (a, b) and TEM (c, d) images of Au@MSHS.
c
This journal is The Royal Society of Chemistry 2011
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In conclusion, we report a facile method for the one-pot
fabrication of mesoporous silica hollow spheres with encapsulated
noble metal nanoparticles. The green amino acid surfactant
Sar-Na serves as the reducing agent, stabilizing agent, polar oil
droplets after acidification and mesopore template. This kind
of rattle type particles was successfully applied in the catalytic
reduction of 4-nitrophenol as a model reaction. In addition,
this strategy can be easily extended to fabricate hollow silica
nanospheres encapsulating other noble metal nanoparticles,
which would be applicable in different aspects.
Fig. 4 SEM and TEM images of Pd@MSHS.
This work was supported by NSFC (20873070, 20973095
and 81071260), National Basic Research Program of China
(2009CB623502), NCET-07-0448 and MOE (IRT-0927).
Notes and references
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7682 Chem. Commun., 2011, 47, 7680–7682
This journal is The Royal Society of Chemistry 2011