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Particle-Size Control and Surface Structure of the Cubic Mesocaged Material AMS-8.

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Monodispersed Materials
DOI: 10.1002/anie.200503253
Particle-Size Control and Surface Structure of the
Cubic Mesocaged Material AMS-8**
Alfonso E. Garcia-Bennett,* Kristina Lund, and
Osamu Terasaki
Mesoporous materials,[1] which are ordered porous structures
with siloxane-bridged amorphous walls, are a family of
nanoporous solids with several extremely attractive features
that allow them to host molecules of various shapes, sizes, and
functionalities.[2] Much research effort has been devoted to
extending the number of available mesostructures by varying
the templating surfactant and synthetic conditions as well as
to extending their compositional range and surface functionality.[3–5] Only recently, and as a result of the numerous
potential applications that have been reported for these
versatile silicates, has attention shifted towards the control of
morphological features, which can be achieved through an
understanding of crystal-growth mechanisms.
It has already been shown that the shape and form of
nanoparticles often determine their function and utility, often
with extreme changes in physicochemical properties.[6] Mesoporous silica morphologies that have been obtained include
fibers, toroids, spiral shapes, discoids, gyroids, and hollow and
solid spheres.[7] It has recently been beautifully demonstrated
by Lin et al.[8] that the cubic morphology of mesocaged solid
SBA-1 can be varied from cubes to rhombic dodecahedra and
[*] Dr. A. E. Garcia-Bennett,[+] K. Lund, Prof. O. Terasaki
Structural Chemistry
Arrhenius Laboratory, Stockholm University
10691 Stockholm (Sweden)
[+] Present address: Nanotechnology and Functional Materials
Department of Engineering Sciences
The ;ngstr<m Laboratory
Uppsala University
Box 534, 75121 Uppsala (Sweden)
Fax: (+ 46) 1850-0131
[**] We are grateful to Dr. Mika Linden for helpful discussions. This work
was partly supported by the Swedish Science Council (VR) and Core
Research for Evolutional Science and Technology (CREST) of JST
and BNRI, Japan (O.T.).
Supporting Information for this article is available on the WWW
under or from the author.
spheres simply by controlling the hydrolysis and condensation
rates of the silica source under acid conditions in an aqueous
synthesis. El-Safty and Hanaoka[9] have also reported the
synthesis of large-pore (ca. 10 nm), monolithic mesostructures, whilst thin films of mesoporous materials have been
prepared by the use of evaporation-induced self-assembly
(EISA) methods by Sanchez and co-workers.[10]
A novel synthetic route to mesoporous silicates using
anionic surfactants derived from N-acylamino acids has been
reported that involves the use of the co-structure-directing
agents (CSDA) N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride (TMAPS) and (3-aminopropyl)trimethoxysilane (APS) under alkaline conditions.[11] These mesoporous
structures are synthetically interesting not only due to the
interaction between the surfactants8 carboxylic acid moiety
and the CSDA, but also for their structural diversity, chiral
properties, and the opportunity to functionalize the pore
surface by removing the surfactant by solvent extraction.[12–14]
This family of materials has been named AMS-n (anionictemplated mesoporous silicas). The mechanism has been
generalized as S N+ I , where S is the surfactant, N+
represents the positively charged amine moiety in the aminosilane group, and I the condensing silica framework. Several
novel mesocaged structures have also been prepared using the
surfactant N-lauroylglutamic acid (C12GlutA), whose large
headgroup area favors the formation of spherical micelles.
The structure of the cubic mesocaged material AMS-8 can be
described as composed of bimodal spherical cages arranged
with Fd3̄m symmetry. Both large and small cages are
interconnected by small connecting windows, which makes
them ideal for the confinement of guest compounds.[15]
The control of morphology in mesoporous materials is
thought to be governed by kinetic effects as the self-assembly
of surfactant molecules and nucleation processes occur
simultaneously as a result of the hydrolysis of the silica
source, usually tetraethyl orthosilicate (TEOS), into small
silicate oligomers. The particle-growth process is dominated
by the condensation of these oligomers. Co-condensation with
organic functional groups in the form of organoalkoxysilanes
has been identified as an important parameter in the
formation of different particle morphologies in MCM-41,
and a proposed mechanism based on the intercalation of the
hydrophobic groups of organoalkoxysilanes between the
micellar headgroups has been proposed.[16] We recently
suggested that this proposed mechanism can be extended to
explain the structural transformations observed on addition
of increasing amounts of co-condensing agents and on
delayed addition of the silica source with respect to the
organoalkoxysilanes.[14, 17]
Alternative explanations for time-dependent structural
changes have been formulated that take into account
depletion or reduction in the concentration of co-surfactant
(co-structure-directing agents) in the synthesis gel not incorporated into the growing mesophase.[18] A double surfactant
system made up of a cationic and a nonionic polymeric
surfactant has been used by Imai et al.[19] for the controlled
growth of silica particles but resulted in poorly defined
morphologies and agglomerates. In the present study we
attempt to control the diffusion of these silicate oligomers in
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 2434 –2438
Table 1: Structural and porous properties of AMS-8(x) mesocaged solids.
the synthesis gel by addition of a triblock
copolymer surfactant (P123) at temperaAMS-8(x)
Total pore
tures above its cloud point, where interacx=
[cm3 g 1]
tion with silicate species is not expected.
This polymer acts by dispersing the nucle0
ation sites and therefore limits the diffusion
of silicate oligomers through the synthesis
gel. The structural and morphological prop6
erties of the mesoporous materials prepared
are discussed in terms of the “crystal”
[a] Determined by TEM. [b] Determined by XRD. [c] Determined by DFT.
growth mechanism and surface structure of
the cubic mesocaged solid.
The samples in this report are denoted AMS-8(x), where x
is the C12GlutA/P123 molar ratio in the synthesis gel. Powder
X-ray diffraction (XRD) patterns of calcined AMS-8 and
AMS-8(x) samples are shown in Figure 1. All spectra contain
Figure 1. XRD patterns of calcined samples of mesocaged silicate
AMS-8 prepared by addition of different amounts of P123 triblock
copolymer to the synthesis gel.
peaks at low angles consistent with a mesoscopic order. The
patterns can be indexed on the basis of a cubic unit cell with
Fd3̄m symmetry; however, it is evident that on addition of
further P123 surfactant the peaks broaden considerably and
less-resolved patterns are obtained. There is no apparent
variation in the unit-cell parameters on addition of further
surfactant, as calculated directly from the 222 reflection in the
XRD pattern. The structural and porous parameters of the
samples are summarized in Table 1.
Nitrogen adsorption–desorption isotherms of AMS-8 and
AMS-8(x) samples are shown in Figure 2. All curves are typeIV isotherms typical of mesoporous materials and are
characterized by a sharp capillary condensation step as a
result of filling of the mesocages in the relative pressure range
p/p0 = 0.3–0.4. There is no evidence of microporosity from the
t-plot curves (not shown).
Angew. Chem. Int. Ed. 2006, 45, 2434 –2438
[m2 g 1]
size [nm]
Figure 2. Nitrogen adsorption–desorption isotherms of calcined cubic
mesocaged material and related structures synthesized by addition of
P123. Pore-size distribution curves (inset; plotted against the increase
in pore volume) were derived with the NLDFT software provided by
Micromeritics and by assuming the Tarazona model for spherical
Pore-size-distribution curves, as derived by the NLDFT
method, show that there is little change in cage size on
addition of the polymeric surfactant P123 to the synthesis gel.
The BET surface areas for these materials are typical of
mesocaged solids, with the highest (720.0 m2 g 1) being that of
AMS-8(6) (Table 1). There is only a small increase in BET
specific surface area on addition of increased amounts of P123
to the synthesis gel; the increase for AMS-8(6) corresponds to
5.0 % relative to conventional AMS-8. However, there are
marked differences in the total pore volume (Vtot) adsorbed,
as can be seen by the sharp, hysteretic rise in nitrogen uptake
for AMS-8(x) in comparison with conventional AMS-8
(Figure 2). Samples synthesized in the presence of P123
show a lower initial nitrogen uptake in the pressure range p/
p0 = 0.2–0.6, followed by a rise in adsorption at higher
pressures, presumably as a result of adsorption in cavities or
pores between the particles. An increase of 40.6 % in Vtot is
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
observed for AMS-8(6) in comparison to conventional AMS8.
transmission electron microscopy (TEM) images of calcined samples of AMS-8(12.5), AMS-8(9), and AMS-8(6) are
shown in Figure 3. All samples, except AMS-8(6), consist of
highly ordered monodispersed particles. The Fourier transform diffractograms (not shown) confirm the structure and
symmetry of the samples as cubic Fd3̄m. The unit-cell
parameters derived from the TEM data are consistent with
those estimated from small-angle powder XRD patterns. The
average particle size, estimated from TEM observations,
decreased with increasing addition of P123 to the synthesis
gel. Monodispersed facetted particles, with an average
particle size of 100 nm, can be observed in sample AMS8(9). Particles with some degree of structural defects associated with the AMS-8 cubic Fd3̄m structure were observed
for all samples (see Figure 3). However, high mesoscopic
order is observed to the very edge of the particle. As the
particle size decreases, the structural “origins” of these
defects can be clearly observed by TEM analysis. Figure 3 C
shows a TEM image recorded along the [110] direction of
AMS-8(9). Structural defects of the stacking-fault type that
result in surface-morphology features parallel to the (110)
plane are indicated by arrows.
More interestingly, the stacking faults in this and other
particles can be traced to structural units consistent with a
different cubic structure (see legend for details), which can be
assumed to have Pm3̄n symmetry. The implications of this for
the growth mechanism are discussed below. Indeed, a few
particles have been obtained from these syntheses that show
an intergrowth structure composed of these two mesostructures (Figure 3 B). From these, it can be deduced that the
[110] and [100] directions of cubic Fd3̄m and Pm3̄n mesostructures are crystallographically related and may grow
epitaxially in this system. More specifically, the (00l) planes in
the Fd3̄m and Pm3̄n structures are equivalent, whilst the
(hk0) and (0k0) planes grow perpendicular to these for the
Fd3̄m and Pm3̄n phases, respectively. The size and connectivity of the cages in both structures is different, and such an
intergrowth material is likely to possess an interesting cage
network. Further studies to determine the precise phase
relationship are underway.
AMS-8(6) shows a disordered intraparticle arrangement
of pores that is consistent with the large overall adsorption
capacity of this sample. The point-group symmetry for
mesostructured particles observed in samples of AMS-8(12.5)
and AMS-8(9) is not consistent with that expected for the
Fd3̄m space group, that is m3̄m, where a 2mm plane-group
symmetry is expected along the [110] direction. A model
depicting the m3̄m point-group symmetry and a typical
particle of AMS-8(9) is shown in Figure 4.
Surprisingly, the particle displays a deviation from twofold
symmetry when viewed along the [110] axis. We propose an
explanation for this based on the presence of stacking faults
Figure 3. High-resolution TEM (HRTEM) images recorded at different magnifications of calcined AMS-8(12.5) (A and B), AMS-8(9) (C), and AMS8(6) (D). The images show typical particle morphologies and sizes obtained on addition of increasing quantities of P123 surfactant to the
synthesis gel. The images in (C) show facetted particles along the [211] and [110] orientations (top and bottom, respectively). Four stacking faults,
which result in steps in the surface of the particle, can be observed along the [110] direction. These are highlighted in the image. The images in
(B) show an HRTEM image of calcined AMS-8(12.5) with a repetitive series of stacking faults and intergrowths of Pm3̄n and Fd3̄m structures
viewed along the [100] and [110] directions, respectively. The fast Fourier transform (FFT) diffractograms taken from selected areas are also shown
and prove conclusively that these two structures grow epitaxially along these directions.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 2434 –2438
Figure 4. A model of the expected morphology for a cubic particle with
point-group symmetry m3̄m (left). The TEM image is taken along
[110], which is a twofold symmetry axis in the space group. The crystal
morphology does not show the expected symmetry because of the
existence of planar faults, as observed in a typical AMS-8(9) particle
and the layered growth of particles in this system. Figure 5
shows a speculative model of the self-assembly mechanism
occurring during the synthesis of AMS-8 nanoparticles. For
Figure 6. HRTEM image and enlarged areas of a typical particle
obtained from a calcined sample of AMS-8(70). The images were
recorded along the [110] orientation of AMS-8 and show the distinct
“rugged” {111} surface and features such as surface nucleation sites
in the form of follicle-like silica hairs surrounding the particle, growth
sites of different maturity, recurrent stacking faults and twinning
(dashed lines), surface termination sites (dashed circles), and Pm3̄n
units (dashed squares). The direction of growth (g) is observed to be
perpendicular to the stacking-fault planes (f).
Figure 5. A model of the self-assembly mechanism observed in the
preparation of AMS-8 nanoparticles. The different stages include:
1) surfactant–CSDA interaction, 2) partial hydrolysis of the CSDA
surrounding the micelle to form a silica shell, 3) and 4) self-assembly
of surfactant–CSDA units caused by TEOS hydrolysis and formation of
silica oligomers (small spheres), 5) TEOS hydrolysis and growth of a
silica wall around the nucleation sites, and 6) and 7) layered growth
and formation of surface features as a result of structural defects.
the purpose of this discussion, a typical TEM image of a
calcined sample of AMS-8(70) prepared with a synthesis time
of only 12 h at 80 8C is shown in Figure 6. The self-assembly
mechanism is thought to proceed in five stages. First (stage 1
in Figure 5), the amine moiety of the alkoxysilane CSDA
interacts electrostatically with the acid groups on the surface
of the micelle. Even though the hydrolysis rate of the CSDA is
relatively slow as a result of the presence of an organic group
bonded to the silica, under these synthetic conditions (pH of
about 9) partial hydrolysis and condensation of the groups
surrounding the micelle will occur. This leads to a negatively
charged, shell-like silica layer at the micelle–water interface
Angew. Chem. Int. Ed. 2006, 45, 2434 –2438
(stage 2 in Figure 5) and to an extension of the effective
headgroup area in the micelle. By considering the micellepacking parameter, g, the large repulsive headgroup, and the
headgroup separations at the micelle surface, it can be
concluded that spherical micelles must form in the CSDA–
surfactant system.[21] Self-assembly of spherical micelles will
then occur (stages 3 and 4). The formation of silicate
oligomers is well known under the synthetic conditions
employed here. Interaction with the micelle-surface oligomeric silica units will further self-assemble the CSDA–
surfactant and provide nucleation sites for further growth
(stage 5).
Overall, particle growth in this system appears to proceed
in a preferred direction perpendicular to the stacking-fault
planes and at the {111} surface, which accounts for the
elongation of the particles in this direction observed in the
TEM images shown in Figure 6. This is consistent with a
layered growth of particles where nucleation occurs at a
surface and growth proceeds along it. Indeed, several growth
sites and follicle-type surface features that can be interpreted
as nucleation sites on the surface can be seen and have been
highlighted in Figure 6. The formation of stacking faults in
this system is complex, and with the available data we can
only assume that they occur as a result of slight variations in
concentration, shape, and local environment of the CSDA–
surfactant micelle units, which aggregate and self-assemble in
the presence of silica oligomers on the growing surface at a
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
nucleation point. From the images shown it can be seen that
these features are more prominent perpendicular to the faulttwin planes on the particle. This is depicted in stages 6 and 7 in
Figure 5. Hence, when the particle size is small (below
200 nm), the rate of nucleation is faster perpendicular to the
(111) plane. Stacking faults are clearly responsible for the
morphology features observed and for the corresponding loss
of point-group symmetry by directing the growth of the
particle. Nonequilibrium growth is clearly visible in these
nanoparticles. However, with time, and hence with larger
particles, this effect disappears, that is, the growth of these
particles reaches equilibrium and the m3̄m point-group
symmetry is restored.
Monodispersed samples of AMS-8 mesocaged material
have been prepared using the polymeric surfactant P123 to
control the growth of the nanoparticles. These particles are
facetted and show structural and surface characteristics that
are typical of cubic AMS-8, as well as an increased adsorption
capacity as a result of the formation of intraparticle spaces.
Indeed, almost as much nitrogen is adsorbed at low pressures
in-between particles for this sample as there is within the
mesocages. However, unexpectedly, there is only a small
increase in surface area.
It is evident from this study that the addition of P123
surfactant under basic conditions at temperatures above the
cloud point of the polymeric surfactant has the effect of
controlling the size of the mesoporous particles. We suggest
that P123 acts as a dispersant of nucleation sites in the
synthesis of AMS-8. It can be seen from the TEM images that
the growth of this mesocaged solid occurs via a layeredgrowth mechanism and that stacking faults and other
structural defects play a decisive role in the final nanoparticle
morphology and surface structure.
Further studies are being conducted in order to determine
the degree of control that can be achieved using this
procedure in the synthesis of different AMS-n type mesostructures as well as with different polymeric surfactants.
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Received: September 13, 2005
Revised: January 9, 2006
Published online: March 9, 2006
Keywords: electron microscopy · materials science ·
nanostructures · surface analysis · surfactants
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Angew. Chem. Int. Ed. 2006, 45, 2434 –2438
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