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Chemistry Letters Vol.35, No.9 (2006)
Synthesis of Highly Monodispersed Core/Shell Mesoporous Silica Spheres
Kazuhisa Yanoꢀ and Tadashi Nakamura
Toyota Central R & D Laboratories, Inc., Nagakute, Aichi 480-1192
(Received June 5, 2006; CL-060644; E-mail: k-yano@mosk.tytlabs.co.jp)
Highly monodispersed core/shell mesoporous silica spheres
turned opaque in three min and resulted in white precipitate. Af-
ter 30 min the experiment started, 1.32 g of TMOS was added
again. This procedure was repeated. Then, the mixture was stir-
red for 6 h and aged overnight. The white powder was filtered
off, washed with distilled water at least three times, and dried
at 318 K for 72 h. The powder obtained was calcined in air at
823 K for 6 h to remove the surfactant. In case an organically
modified silane was used, surfactants were extracted by acidic
ethanol at 333 K for 3 h instead of calcination.
Figure 1 shows SEM images of particles obtained by differ-
ent number of TMOS addition times and its schematic illustra-
tion of the particle growth. The diameter of the particles became
larger upon the addition of TMOS while retaining its monodis-
persion characteristic (standard deviation in parenthesis). This
result indicates that added TMOS preferred to react with the
surface silanol of existing particles rather than generated new
particles by reacting between TMOS.
The above results support the assumption that core/shell
MMSS can be obtained by changing the type of additive silica
precursor from the original one as illustrated in Scheme 1.
Synthesis was conducted in which double the molar
amounts of mercaptopropyltrimethoxysilane (MPTMS)/TMOS
(¼ 20=80 mol/mol) mixture instead of TMOS was added to
the solution. Because the synthesis was conducted under a large
molar ratio of a surfactant (sur/Si = 1.27), the amount of surfac-
tant in the solution was sufficient for further mesoporous silica
with radially aligned pore structure have been successfully
obtained by adding different type of silica precursor to the
existed particles in the preparation.
Recent development in the synthesis of mesoporous silica
has opened up the door for the applications in the new types
of catalysis,1 adsorbents,2 and host materials.3 Morphological
control of these materials has been one of the main objectives
toward the industrial use. Among various shapes, spheres are
of great interest for applications in chromatography,4 cosmetics,5
and colloidal crystals. Recently, we have successfully synthe-
sized monodispersed mesoporous silica spheres (MMSS) pos-
sessing highly ordered hexagonal regularity from tetramethoxy-
silane (TMOS) and alkyltrimethylammonium halide in a metha-
nol/water mixture.6 Furthermore, we have fabricated colloidal
crystal films from MMSS and investigated their optical proper-
ties.7 In the process of investigation of the formation mechanism
of MMSS, it was found that primary generated small particles
grew to larger particles homogeneously, leading to the formation
of monodispersed spherical particles.8 On the basis of the data,
it is speculated that the addition of silica precursors after the
completion of the reaction enlarges particles of MMSS. Further-
more, by changing the type of additional silica precursors to the
original one, core/shell MMSS can be obtained.
There have been many reports conducting on the synthesis
of core/shell particles. The objectives of the researches are clas-
sified into the following three categories: 1. Protection of cores.
Magnetic particles were coated with silica shell to prevent from
coaggulation.9 2. Enhancing the properties of shell. Carbon core
increased the oxidation current of metal shell. This material was
expected for the methanol oxidation catalyst of direct methanol
(a)
(b)
(c)
fuel cell.10 By coating on Stober silica spheres, plasmon peak of
1.11 µm
1.21 µm (2.8%)
1.11 µm
0.61 µm (5.1%)
0.80 µm (3.1%)
1.09 µm
¨
Au was shifted.11 3. Preparation of core/shell particles from ma-
terials with different properties. The combinations of particles
with magnetic and optical properties were widely carried out.
Increase of fluorescence radiation decay was observed for Co/
CdSe core/shell particles.12 Plasmon peak red-shifted and coer-
civity increased for magnetite/Au core/shell nanoparticles.13
Incorporation of guest materials like metal,14 metal oxides,15
and chromophores,16 into mesoporous silica has been conducted
by many researchers. If core/shell mesoporous silica spheres are
obtained, different types of guests can be incorporated into their
core and shell, selectively, by designing the combination of core
and shell properties. Here, we report the synthesis of MMSS
with core/shell structure.
TMOS
addition
TMOS
addition
Figure 1. SEM images of particles obtained by the different
TMOS addition times: (a) 0, (b) 2, and (c) 4, and schematic illus-
tration of the particle growth.
MPTMS-TMOS
TMOS
TMOS
In a typical synthesis, 3.52 g of hexadecyltrimethylammoni-
um chloride and 2.28 mL of 1 M sodium hydroxide solution were
dissolved in 800 g of methanol/water (50/50 = w/w) solution.
Then 1.32 g of TMOS was added to the solution with vigorous
stirring at 298 K. After the addition of TMOS, the clear solution
MPTMS -TMOS
addition
Scheme 1.
Copyright ꢀ 2006 The Chemical Society of Japan