Shape- and size-controlled synthesis in hard templates: Sophisticated chemical reduction for mesoporous monocrystalline platinum nanoparticles

Article abstract of DOI:10.1021/ja2058617

Here we report a novel hard-templating strategy for the synthesis of mesoporous monocrystalline Pt nanoparticles (NPs) with uniform shapes and sizes. Mesoporous Pt NPs were successfully prepared through controlled chemical reduction using ascorbic acid by employing 3D bicontinuous mesoporous silica (KIT-6) and 2D mesoporous silica (SBA-15) as a hard template. The particle size could be controlled by changing the reduction time. Interestingly, the Pt replicas prepared from KIT-6 showed polyhedral morphology. The single crystallinity of the Pt fcc structure coherently extended over the whole particle.

Full text of DOI:10.1021/ja2058617

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pubs.acs.org/JACS
Shape- and Size-Controlled Synthesis in Hard Templates:
Sophisticated Chemical Reduction for Mesoporous Monocrystalline
Platinum Nanoparticles
†
,‡
§
†
†
†
Hongjing Wang, Hu Young Jeong, Masataka Imura, Liang Wang, Logudurai Radhakrishnan,
||
^
§,^
†,‡,#,
Nobuhisa Fujita, Toen Castle, Osamu Terasaki, and Yusuke Yamauchi
*
†
World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA),
National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
‡
Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
§
Graduate School of EEWS, WCU Energy Science and Engineering, KAIST, Daejeon 305-701, Korea
Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Katahira 2-1-1,
Aoba, Sendai 980-8577, Japan
^
Inorganic and Structural Chemistry, Arrhenius Laboratory, Stockholm University, 10691 Stockholm, Sweden
#
Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST),
4
-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
S Supporting Information
b
7
templates. Through chemical or electrochemical reduction, ordered
mesoporous metal powders or films, respectively, can be synthesized.
However, the ordered arrangement of the rod self-assemblies in
the liquid crystals is often distorted during the metal deposition
process, and thus, long-range order of the mesoporous structures
ABSTRACT: Here we report a novel hard-templating strategy
for the synthesis of mesoporous monocrystalline Pt nano-
particles (NPs) with uniform shapes and sizes. Mesoporous Pt
NPs were successfully prepared through controlled chemical
reduction using ascorbic acid by employing 3D bicontinuous
mesoporous silica (KIT-6) and 2D mesoporous silica (SBA-15)
as a hard template. The particle size could be controlled by
changing the reduction time. Interestingly, the Pt replicas
prepared from KIT-6 showed polyhedral morphology. The
single crystallinity of the Pt fcc structure coherently ex-
tended over the whole particle.
8
in the final product is lacking. In comparison with the soft-
templating method, hard templating is a very attractive pathway
9
that is widely applicable to various compositions. Mesoporous silica
with a robust framework and high thermal stability is used as a
10
hard template to synthesize a metal replica. To date, various Pt
nanostructures such as 1D nanowires and 3D nanowire networks
have been prepared by using MCM-41 (p6mm), SBA-15 (p6mm),
10
KIT-6 (Ia3d), and MCM-48 (Ia3d) as hard templates. Chao and
11
co-workers used transmission electron microscopy (TEM) to
investigate the growth mechanism of Pt nanostructures in
mesoporous silica matrixes (MCM-48 and MCM-41).
ince the discovery of mesoporous materials in the early
1
S
1990s, various types of ordered mesoporous materials have
been synthesized under different conditions and extensively
studied with regard to mesostructural controllability, composi-
Despite these recent advances in soft and hard templates, the
obtained morphologies of mesoporous metals have been very
limited to only powders with irregular morphology or films on
conductive substrates. The lack of control over the particle sizes
and morphologies is a serious problem for further development
of mesoporous metals. The shape and size distribution of the
NPs are critical parameters of the function and utility for
applications. To bring out shape- and size-dependent physico-
chemical properties, it is extremely important to prepare uni-
form-sized particles with the same shapes in high yields.
2
tional diversity, and morphological flexibility. Currently, meso-
3
4
porous silica and carbon nanoparticles (NPs) with controlled
particle size have attracted great interest in many potential
applications of chromatography, catalysis, catalyst support, and
adsorption because the NPs can provide outstanding pore
accessibility of guest species from outside. Especially, mesopor-
ous NPs below submicrometer size (normally, below 500 nm)
5
can be utilized in biomedical applications. To date, many efforts
In this communication, we propose a new concept for the
shape- and size-controlled synthesis of mesoporous metals in
hard templates. We demonstrate the facile synthesis of uniformly
sized mesoporous Pt NPs by using mesoporous silica KIT-6
(Ia3d) and SBA-15 (p6mm) as the hard template [Figure S1 in
the Supporting Information (SI)] and ascorbic acid (AA) as the
reducing agent. The unit cell parameters and mesopore sizes of
have been made in the synthesis of mesoporous NPs.
In comparison with silica and carbon, mesoporous metals have
fascinating properties inherent to metal frameworks (e.g., high
electroconductivity, catalytic activities, etc.) along with the
6
general characteristics of mesoporous materials. Therefore, we
can anticipate various electrochemical applications that cannot be
realized by traditional mesoporous silica. Several approaches for
the preparation of mesoporous/mesostructured metals have
been reported. Lyotropic liquid crystals made of highly concen-
trated surfactants or block copolymers have been utilized as soft
Received: June 23, 2011
Published: August 30, 2011
r 2011 American Chemical Society
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|J. Am. Chem. Soc. 2011, 133, 14526–14529

Journal of the American Chemical Society
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Figure 2. (a) Low-magnification TEM image of meso-Pt prepared using
KIT-6. (b) ADF-STEM image of one meso-Pt. The inset shows an ED
pattern taken from this particle using an incident beam direction parallel
to [110] of the Pt fcc structure. (c) High-magnification TEM image
taken at the left square box in (b). (d) Energy-dispersive spectroscopy
elemental (Pt) mapping obtained at the right square box in (b).
Figure 1. (a) Low- and (b) high-magnification SEM images of the
obtained mesoporous Pt nanoparticles (meso-Pt) prepared with meso-
porous silica KIT-6. An enlargement of (a) is shown in Figure S2.
the original templates were 21 and 7.9 nm for KIT-6 and 9.2 and
7
.3 nm for SBA-15, respectively. Details are given in the SI.
First, mesoporous silica powders were immersed into an
materials. The BET surface area and BJH average pore size were
calculated to be 39.0 m /g and 2.7 nm, respectively. The
2
aqueous solution of K PtCl . Next, the composites were dried
mesopore size of 2.7 nm indicates void space between the Pt
nanowires. This value is in good agreement with the wall
thickness of the original KIT-6. TEM images and the elemental
mapping are displayed in Figure 2. The annular dark-field
scanning TEM (ADF-STEM) image of one meso-Pt (Figure 2b)
is well-consistent with the image of the original KIT-6 (Ia3d
space group) taken with the incident beam parallel to the Æ111æ
direction. Thus, meso-Pt corresponds to a perfect inversion
replica of the original highly ordered KIT-6. Notably, the
electron diffraction (ED) pattern (Figure 2b inset) taken from
this same particle showed that the Pt within the framework
possessed an almost single-crystalline nature of the Pt face-
centered cubic (fcc) structure. The wide-angle X-ray diffraction
(WAXRD) pattern of meso-Pt could also be assigned to the Pt fcc
crystal structure (Figure S6).
2
4
under reduced vacuum conditions, giving an orange powder.
Aqueous AA as a reducing agent was dropped on the powder.
After 12 h, the sample color had totally changed from orange to
black, giving a product hereafter called the meso-Pt/silica com-
posite. Finally, to remove the silica, the black powder was washed
with HF solution to give mesoporous Pt (meso-Pt). All of the
experiments were carried out at room temperature.
Figure 1 shows the morphology of the obtained meso-Pt after
the removal of the silica template (an enlarged image is shown in
Figure S2). The low-magnification scanning electron microscopy
(SEM) image confirmed that the NPs were isolated from each
other and had a narrow particle size distribution. Interestingly,
almost all of the NPs showed rhombic dodecahedral morphology
(as indicated by the yellow lines in Figure 1b), although some
appeared to be different in the SEM images because different
projections were viewed (Figure S3). The external surface was
bounded by 12 planes. The crystal morphology was commensurate
with cubic m3m point-group symmetry.
To understand the Pt deposition process, several types of
meso-Pt with different particle sizes were prepared by changing
the reduction time (10 min, 1 h, and 12 h) (Figures S7 and S8).
When the reduction time was 10 min, the deposited Pt particles
showed a polyhedral shape with an average diameter of 80 nm
(Figures S7a and S8a). With an increase in the deposition period
to 1 h, the average particle size gradually increased (up to 160 nm)
with retention of the polyhedral shape (Figures S7b and S8b).
After the complete reduction of the Pt sources (12 h), the average
particle size reached 180 nm (Figure 1 and Figure S8c). On the
basis of these results, at the beginning stage of the Pt deposition,
the nucleation occurs at the center of the particles. Next, Pt grain
growth from the nuclei increasingly proceeds, forming larger-
sized particles, while coherently retaining the Pt atomic crystal-
linity over the full region of each particle, as shown in Figure 2b,c.
With the increase in particle size, the single-domain size of the
“mesoporous crystals” became large. Actually, the intensities of
the SAXS profiles increased and the full width at half-maximum
(fwhm) decreased (Figure S9). Similarly, the WAXRD patterns
From high-magnification SEM images (Figure 1b and Figure S3b),
it was observed that Pt nanowires replicated from both side pores
of a double gyroid mesostructure (i.e., inverse double gyroid
structure). Also, the small-angle X-ray scattering (SAXS) peaks
were assignable to Ia3d, giving the same SAXS profile as for the
original KIT-6 mesoporous silica (Figure S4a). This is evidence that
the replication of Pt from KIT-6 retained the original symmetry.
However, a very weak 110 reflection that was not observed in the
profile of the original KIT-6 appeared in the SAXS pattern of
meso-Pt/silica (Figure S4a), meaning that formation of a lower-
symmetry material partially takes place at this stage. This results from
Pt deposition occurring in one side pores of the bicontinuous
structure, as indicated by the dotted ellipse in Figure S2.
The N adsorptionÀdesorption isotherm of meso-Pt was a typical
2
type-IV isotherm (Figure S5b), characteristic of mesoporous
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Figure S6) also showed the increase in peak intensities and
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(
decrease in fwhm, indicating the enlargement of Pt fcc crystal
domains with increasing particle size.
To clarify the reason why each particle shows a “single
mesoporous crystal”, the meso-Pt/silica composites before the
silica removal were carefully investigated by TEM. The deposited
Pt particles were distributed over the entire of the silica matrix
and isolated from each other. As clearly seen in Figure S10, the Pt
particles were deposited within the same single mesostructural
Ia3d domain of the original KIT-6. The domain sizes of
“
mesoporous crystals” with the same orientation in the KIT-6
template were submicrometer scale (Figure S11), which is much
larger than the particle sizes of the deposited meso-Pt particles.
Therefore, it is very reasonable that the obtained Pt particles were
single mesoporous crystals without defects (Figure 2b).
The strength of the reducing agent plays the most important
key role in the successful preparation of mesoporous Pt nano-
particles with uniform size. For comparison, two different kinds
of reducing agents, sodium borohydride (SBH) and dimethyla-
minoborane (DMAB), were used instead of AA. In both cases,
the deposited Pt showed irregular morphology (Figure S12).
Some particles had no ordered mesoporous structures as a result
of Pt deposition on the outer surface of the mesoporous silica.
The peak intensities in the low-angle range were much smaller
than for meso-Pt obtained using AA (Figure S13), exactly
consistent with the SEM images. These results prove that the
use of AA is vital for formation of Pt polyhedra with well-ordered
mesostructures. The Pt deposition rate is determined by the
strength of the reducing agent. The redox potentials of SBH and
DMAB are much lower than that of AA. Therefore, relative to
AA, SBH and DMAB are very strong reducing agents and can
reduce Pt sources very rapidly. Therefore, SBH and DMAB
molecules cannot enter the mesopores and reduce the Pt species
inside the mesoporous particles. Especially, SBH reduced the Pt
species immediately by a very severe self-decomposition reaction
with a large volume of gas, thereby depositing the Pt outside the
mesoporous particles (Figure S14). In contrast, the use of AA can
provide enough time to access the Pt sources located inside the
mesopores (Figure S10). In the initial stage of Pt deposition, the
AA reduced the Pt species embedded inside the mesopores to
deposit the Pt nuclei, and the grain growth proceeded continu-
ously from the primary Pt nuclei. Consequently, the reaction
kinetics was critical for high-quality synthesis.
Figure 3. (a-1, b-1) Low-magnification bright-field TEM images of
meso-Pt and (a-2, b-2) the corresponding ED patterns. Indices are based
on (a-1, b-1) the KIT-6 mesostructure and (a-2, b-2) Pt fcc.
is spent for the growth process. This idea is nicely supported by the
fact that (i) the density of the Pt particles (i.e., the number of Pt
particles per unit area) was almost constant irrespective of the
reaction time (Figure S10) and (ii) the particle size gradually
increased with the reaction time (Figures S7 and S8). Furthermore,
coalescence of the particles was not confirmed (Figure S2). Thus,
interference among Pt particles to produce their aggregation was
rather small. If the density of the nucleation sites were larger, such
interference would become relatively large, which would tend to
cause the condition of “partition of the space”. In this case, Pt ions
diffusing from the associated space to each nucleus (i.e., the Voronoi
cell) are likely to contribute to the growth of the Pt particle. The
envelope surface must therefore reflect the shape of the Voronoi cell.
In order to discuss further the present formation mechanism,
let us assume that the diffusion of the Pt sources as well as that of
the reducing agent is fast enough that it would not limit the
deposition rate of Pt at every growth front. The Pt network
would then grow by tracing the channels of the KIT-6 template
outward from the nucleation center with a uniform speed. Figure
S15 illustrates an early snapshot of the Pt network, assuming the
nucleation center is a single ternary junction of the gyroid
network. At a later stage, the Pt network would exhibit a
polyhedral envelope with 24 triangular faces (Figure S15b). This
shape is related to the rhombic dodecahedron observed experi-
mentally, as shown in Figure 1.
This slight discrepancy may be caused by a bias of the supply of
Pt sources along certain directions in the Ia3d channel network.
The double gyroid channels in KIT-6 are formed by connections
between the branched points that correspond to Wyckoff 16b
positions (site symmetry = 32) for Ia3d. As shown in Figure S16,
the gyroidal channels are orthogonal to the {110} faces of the
cubic structure. The formation of rhombic dodecahedra in this
study might indicate that the supply of Pt ions and AA can be
topologically delayed along the normal to the {110} planes.
Our synthetic concept reported here was also applied to a
different Pt replica system using mesoporous silica SBA-15. After
the silica removal, highly ordered mesoporous Pt NPs with an
olive shape were formed (Figure S17). The Pt NPs obtained
from SBA-15 showed olivelike morphology with an average
In general, fcc metals such as Pt and Au normally have
minimum surface energy for {111}, which is the reason for the
12
formation of typical mutiply twinned structures. However, we
could not observe such a special crystallographic surface on the
top of the Pt network (Figure 1 and Figure S2). In a careful TEM
investigation (Figure 3 and Table S1), we could not find any
special crystallographic orientation relationship between the
KIT-6 mesostructure (Ia3d) and the Pt fcc structure (Fm3m),
although the atomic coherency of the Pt crystal in meso-Pt was
confirmed (Figures 2b and 3). It is therefore natural to assume
that the rhombic-dodecahedral shape of the Pt replica synthe-
sized from KIT-6 is not an equilibrium form reflecting the crystal
morphology of the Pt fcc structure but originates from the KIT-6
mesostructure, that is, the geometry of the double gyroid
channels (Ia3d symmetry).
From the viewpoint of the electrochemical process, Pt nuclea-
tion on the silica walls of KIT-6 is thought to be less than Pt grain
growth from the Pt nuclei. In other words, after the nuclei are
formed in the early stage of the reaction, most of the reaction time
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length of around 200 nm. On the NPs surface, it was observed
that the several Pt nanowires were periodically arranged, forming
a negative replica of the 2D hexagonally ordered mesoporous
silica. The SAXS patterns (Figure S4b) before and after the silica
removal showed typical peaks assignable to the 10, 11, and 20
planes of a 2D hexagonal symmetry. The Pt replicas prepared
from SBA-15 also possessed the single-crystalline nature of
Pt fcc (Figure S18). The surface area and average pore size were
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
Rev. Phys. Chem. 2009, 60, 167.
Corresponding Author
Yamauchi.Yusuke@nims.go.jp
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