Integrated structural control of cage-type mesoporous platinum possessing both tunable large mesopores and variable surface structures by block copolymer-assisted Pt deposition in a hard-template

Article abstract of DOI:10.1039/b921016d

Cage-type mesoporous Pt with tunable large mesopores possessing smooth and rough pore surfaces were prepared selectively by the deposition of Pt in the absence and presence of a block copolymer in a hard-template, respectively. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b921016d

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Integrated structural control of cage-type mesoporous platinum
possessing both tunable large mesopores and variable surface structures
by block copolymer-assisted Pt deposition in a hard-templatew
Yoshiyuki Kuroda,a Yusuke Yamauchibcd and Kazuyuki Kuroda*ae
Received (in Cambridge, UK) 8th October 2009, Accepted 18th December 2009
First published as an Advance Article on the web 22nd January 2010
DOI: 10.1039/b921016d
Cage-type mesoporous Pt with tunable large mesopores possessing
smooth and rough pore surfaces were prepared selectively by the
deposition of Pt in the absence and presence of a block copolymer
in a hard-template, respectively.
Although this method is based on the well-known colloidal
templating to form macroporous metals,4 the size control in
mesopore range has recently been achieved for mesoporous
carbon,5 though the application to the preparation of
mesoporous metals has not yet been reported.
To control the surface structure of mesoporous metals
replicated with silica nanoparticles, we have focused on the
use of a block copolymer Pluronic P123 (EO20PO70EO20) as a
secondary template because it is effective to control the
Pt deposition process. Pluronic block copolymers have been
utilized for the syntheses of various metal nanoparticles
(e.g. Au,6 Pt,7 and Pd8) by a surface capping mechanism.
Therefore, the formation of pore walls consisting of roughly
aggregated Pt nanoparticles is expected by the Pt deposition
with the block copolymer. We have already revealed that some
mesoporous Pt and alloys prepared by a soft-templating
method have pore walls consisting of interconnected metal
nanoparticles,9 whereas the surface structures have not been
controlled.
Microstructural design of metals is important to provide
useful properties, such as catalytic, electronic, magnetic, and
optical properties, which leads to the applications including
electrodes, sensors, and electrooptical devices.1 Metal
nanoparticles generally show size-dependent properties. High
surface area resulting from a large amount of outer surfaces of
nanoparticles is essential for the applications to sensors and
catalysts. However, because most metal nanoparticles are
randomly aggregated, guest diffusion is not controllable.
In contrast, the formation of uniform mesopores in metals is
advantageous to the control of surface area, size selectivity,
and guest diffusion.2 The structure of mesoporous metals can
be varied by using different templates and deposition
processes. However, the integrated control of both pore size
and surface structure is still an important issue. The expansion
of pore size is certainly essential for efficient diffusion of large
guest species, and the control of surface structure is quite
influential to the increase in the surface area and also catalytic
activities.
In this study, a dual-template consisting of silica nano-
particle assembly and Pluronic P123 was used for the structural
control of mesoporous Pt (Scheme 1). The pore size was
controlled by the size of the silica nanoparticles. Mesoporus
Pt with smooth and rough pore surfaces were independently
prepared by silica nanoparticle assembly only (single-template
system) and the dual-template system, respectively.
Recently, the syntheses of mesoporous Pt with large
mesopores ca. 15 nm in size have been achieved by using
block copolymer templates.3 However, the pore size control in
a wider range has not yet been established. Silica nanoparticle
assembly is one of the ideal hard-templates because the pore size
is easily controllable depending on the size of nanoparticles.
Silica nanoparticles ca. 30 nm in size were prepared and
assembled according to literatures5a,c with a slight modification
and the details are described in the ESI.w The high-resolution
scanning electron microscopy (HRSEM) images of the silica
nanoparticle assembly is shown in Fig. S1 in the ESI.w
a Department of Applied Chemistry, Faculty of Science and
Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku,
Tokyo 169-8555, Japan. E-mail: kuroda@waseda.jp;
Fax: +81 3 5286 3199; Tel: +81 3 5286 3199
b World Premier International (WPI) Research Center, International
Center for Materials Nanoarchitectonics (MANA), National
Institute for Materials Science (NIMS), 1-1 Namiki,
Ibaraki 305-0047, Japan
c PRESTO, Japan Science and Technology Agency (JST),
5 Sanban-cho, Chiyoda-ku, Tokyo 102-0075, Japan
d Major in Nanoscience and Nanoengineering, Faculty of Science and
Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku,
Tokyo 169-8555, Japan
e Kagami Memorial Research Institute for Materials Science and
Technology, Waseda University, 2-8-26 Nishiwaseda, Shinjuku-ku,
Tokyo 169-0051, Japan
w Electronic supplementary information (ESI) available: Preparation
of silica nanoparticle assembly, Characterization techniques, HRSEM
and HRTEM images, WAXD data, and N2 adsorption data. See DOI:
10.1039/b921016d
Scheme 1 Formation of mesoporous Pt with smooth and rough pore
surfaces by single- and dual-template systems, respectively.
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The powdery nanoparticle assembly was dried under vacuum
at room temperature. For the preparation of mesoporous Pt
with smooth pore surfaces by a single-template system
(denoted as MP-S), 0.1 g mLÀ1 of H2PtCl6Á6H2O in ethanol
was used as a precursor solution. For the preparation of
mesoporous Pt with rough pore surfaces by a dual-template
system (denoted as MP-D), Pluronic P123 block copolymer
was dissolved (3 mg mLÀ1) in the same precursor solution as
that of MP-S. The following procedure is common for both
MP-S and MP-D. One gram of silica nanoparticle assembly
was infiltrated with 0.2 mL of the precursor solution. The
mixtures were placed in a closed vessel which contained
dimethylamineborane (DMAB).10 The reaction temperature
was optimized to be at 40 1C where DMAB became liquid. The
reaction was performed for three days. Unreacted H2PtCl6
and excess DMAB were washed with water and ethanol after
the reaction. The silica nanoparticles and P123 were removed
by a HF treatment and washing twice with water, respectively.
The EDX data of the products did not show the remaining of
Si. The characterization techniques are described in the ESI.w
The small angle X-ray scattering (SAXS) pattern of the
template (silica nanoparticle assembly only) shows several
peaks which can be interpreted as the overlapping of peaks
corresponding to a face-centered cubic arrangement of the
nanoparticles (Fig. 1A).5a,b The SAXS profiles of MP-S and
MP-D are similar to that observed for the template though the
resolution is lowered (Fig. 1B and C). Direct observation of
both MP-S and MP-D by HRSEM shows the highly ordered
pore arrangements, which are attributable to the formation
of the inverse structure of the template (Fig. 1a and b). The
broadening of the SAXS peaks (Fig. 1B and C) after the
replication is probably due to the short coherent length of
the ordered mesostructures, which is similar to that reported
previously using a soft-template.9 The better structural ordering
of MP-D than MP-S is possibly due to the flexible framework
consisting of roughly aggregated Pt nanoparticles. When silica
nanoparticles ca. 15 nm in size were used as templates, such a
replication was also confirmed (Fig. S2, ESIw).
Highly magnified HRSEM images of both MP-S and MP-D
(Fig. 2a and b) show spherical mesopores. The details of the
images display the outlines of the pore surfaces indicated by
blue lines, showing that MP-S and MP-D possess smooth and
rough pore surfaces, respectively. The wall thicknesses of
MP-S and MP-D are 2–4 and 3–8 nm, respectively, the
variation of which corresponds to their surface roughness.
Fig. S3 in the ESIw also shows smooth pore walls of mesoporous
Pt prepared by the single-template system with different size of
silica nanoparticles. The high-resolution transmission electron
microscopy (HRTEM) image of MP-S does not show any
specific features on the pore walls, though it seems that the
pore walls consist of irregular Pt nanoparticles (Fig. 2c). The
domain size of Pt crystallites observed in the high magnification
image of MP-S is typically 5–10 nm in size (Fig. S4, ESIw),
suggesting that the pore walls consist of many small Pt crystallites.
The selective area electron diffraction (SAED) pattern of
MP-S shows a ring-like pattern with many intense spots,
which corresponds to the presence of many small Pt domains
in the pore walls (Fig. 2e). Such pore walls can be formed by
continuous grain growth of Pt with a small amount of Pt
nuclei.9 In contrast, the pore walls of MP-D consist of roughly
aggregated spherical Pt nanoparticles 3–4 nm in size (Fig. 2d).
Each nanoparticle is a single crystal and the size and shape of
the Pt nanoparticles are uniform probably due to the surface
stabilization with P123. The SAED pattern of MP-D is
attributable to random crystal orientation of the Pt nano-
particles in the pore walls (Fig. 2f). Because both samples consist
of small Pt nanoparticles in the pore walls, little difference
was observed in the wide-angle X-ray diffraction (WAXD)
patterns (Fig. S5, ESIw). Low magnification TEM images
(Fig. S6, ESIw) show that similar features are present in a
wide range of samples.
The porosity of the mesoporous Pt reflects the difference of
the surface structures. The Brunauer-Emmett-Teller (BET)-
equivalent surface areas of MP-S and MP-D were 40 and
70 m2
g
À1, respectively.11 The calculated surface area of a
hypothetical Pt nanoparticle 3 nm in size is ca. 93 m2 gÀ1
,
therefore, many surfaces, where N2 molecules can access,
should be exposed inside the pore walls of MP-D. The N2
adsorption-desorption isotherm and Barrett-Joyner-Halenda
(BJH) pore size distribution of MP-S (Fig. S7A and C, ESIw)
show the presence of only large mesopores (ca. 30 nm). In
contrast, those of MP-D (Fig. S7B, D, and E, ESIw) show the
presence of large mesopores (ca. 30 nm) and possibly small
mesopores among Pt nanoparticles (ca. 5 nm).12 An uptake in
the low pressure region (P/P0 o 0.1), observed for the
isotherm of MP-D, is also attributable to the cavities. The
reason for the large hysteresis in the isotherm of MP-D is
unclear, which might be due to slow diffusion of N2 in the
cavities.
In the single-template system, a relatively highly crystalline
framework was formed by continuous grain growth of Pt.9,10
In the dual-template system, the adsorption of P123 on the
surface of Pt suppresses the continuous growth of Pt nano-
particles. Additionally, the spatial occupancy of polymer chain
among Pt nanoparticles causes the formation of the cavities.
We think that the possibility of micelle formation of P123 on
Pt nanocrystals in the voids of the silica nanoparticle assembly
Fig. 1 (left) SAXS patterns of (A) the template, (B) MP-S, and
(C) MP-D. (right) HRSEM images of (a) MP-S and (b) MP-D
recorded along [111].
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Spaces-Strategy in Ubiquitous Elements) and the Global
COE program ‘‘Practical Chemical Wisdom’’ from MEXT,
Japan. The A3 Foresight Program ‘‘Synthesis and Structural
Resolution of Novel Mesoporous Materials’’ supported by
the Japan Society for Promotion of Science (JSPS) is also
acknowledged. Y. K. was grateful for financial support via a
Grant-in-Aid for JSPS Fellows from MEXT.
Notes and references
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Fig. 2 HRSEM images of the pore walls of (a) MP-S and (b) MP-D.
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In conclusion, we have demonstrated the control of pore
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single- and dual-templates consisting of a silica nanoparticle
assembly and a block copolymer. High surface area of ca.
70 m2 gÀ1 was observed in spite of large mesopores ca. 30 nm
in size. The varied crystallinity and pore surfaces are essential
to the development of catalytic activity and optical property.
The concept, the block copolymer-assisted metal deposition in
a confined space of hard-templates, should be effective for
further control of crystal surfaces (e.g. formation of catalytically
active facets on the surface) by using proper capping agents.
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supported by a Grant-in-Aid for Scientific Research on Priority
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This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 1827–1829 | 1829