Template synthesis of asymmetrically mesostructured platinum networks [9]

Full text of DOI:10.1021/ja003461t

1246
J. Am. Chem. Soc. 2001, 123, 1246-1247
Template Synthesis of Asymmetrically
Mesostructured Platinum Networks
†
,†
‡
Hyun June Shin, Ryong Ryoo,* Zheng Liu, and
Osamu Terasaki‡
Materials Chemistry Laboratory, Department of Chemistry
School of Molecular Science-BK21
Korea AdVanced Institute of Science and Technology
Taejon 305-701, Korea
Department of Physics and CREST, JST
Tohoku UniVersity, Sendai 980-8578, Japan
ReceiVed September 22, 2000
Recently there has been a growing attention to mesoporous
molecular sieves, due to the possibilities as a template for the
synthesis of nanostructured new materials. For example, a new
type of ordered mesoporous carbon molecular sieves designated
CMK-1 was obtained by using MCM-48 silica as a template and
sucrose as a carbon source.1 Platinum nanowires were synthe-
sized using MCM-41 or SBA-15 silica as templates.3 The
investigation with high-resolution transmission electron micros-
copy (TEM) and the energy-dispersive X-ray (EDX) spectroscopy
revealed that the Pt nanowires obtained after the removal of the
template were composed of Pt atoms with the face-centered cubic
structure.4
,2
-5
Here we report that the template synthesis technique can be
extended to the composition of Pt nanowires 3 nm in diameter in
a form of three-dimensionally ordered asymmetric networks, using
MCM-48 silica as the template. The asymmetically nanostructured
Pt material has been obtained by templating the Pt metal with
one of the enantiomeric pair of the chiral channel systems, which
are three-dimensionally interwoven, in the structure of MCM-
Figure 1. (a-e) TEM images of nanostructured Pt networks obtained
using MCM-48 silica as a template. (f) A structure model proposed by
Luzzati and Spegt12 for cubic Ia3d lipid mesophase. Note that the TEM
images in (d) and (e), taken at an extremely thin edge of Pt network,
shows a remarkable similarity with the dark-shaded chiral network in
the structure model shown in (f).
6,7
48. The Pt networks, obtained after complete removal of the
template with HF, exhibited the characteristic X-ray powder
diffraction (XRD) pattern and TEM images, indicating the highly
ordered nanostructure.
4
2
00 H O. The mixture was heated for 40 h in an oven at 373 K
and subsequently cooled to room temperature. Acetic acid
equivalent to 57% of the total Na content was added to the
mixture, drop by drop, with vigorous magnetic stirring. After
subsequent heating for 48 h at 373 K, product was filtered, washed
with distilled water, and dried at 393 K. The product was calcined
in air at 823 K after extraction of the surfactant with an ethanol-
HCl mixture.11
High-quality MCM-48 silica was obtained via a synthesis
method using cationic-nonionic surfactants mixture.8,9 The silica
source was a sodium silicate solution with Na/Si ) 0.5 (2.3 wt
%
Na
2
O, 9.0 wt % SiO
2
, 88.7 wt % H
2
O), which was prepared
, Aldrich) and
NaOH.10 Octadecyltrimethylammonium chloride (C18
CH Cl, 97%, TCI) and Triton X-100 (C O(C O)10H,
with the colloidal silica Ludox (40 wt % SiO
2
H
37N-
The template synthesis of the nanostructured Pt was carried
out as follows: aqueous solution of tetraammineplatinum(II)
(
3
)
3
H C
8 17 6
H
4
2 4
H
Aldrich) were dissolved in doubly distilled water. The surfactant
solution and the silica source were rapidly mixed using a high-
speed blender for 5 min at room temperature. The mixture was
transferred to a polypropylene bottle and continuously stirred for
nitrate (Pt(NH
calcined MCM-48 silica powder, so that the Pt content cor-
weight. If necessary, the
3 4 3 2
) (NO ) , Aldrich) was impregnated into the
responded to 30-60 wt % of the SiO
2
impregnation was repeated after evaporation of the solvent. After
being completely dried at 373 K, the MCM-48 sample impreg-
1
h with a magnetic stirrer. The molar composition of the mixture
was 5.0 SiO :1.25 Na O:0.92 C18 Cl:0.08 Triton X-100:
2
2
3 3
H37N(CH )
nated with Pt(NH
reactor equipped with two fritted disks. The Pt compound was
reduced to Pt(0) with H flow through the reactor while the
3 4 3 2
) (NO ) was mounted inside a Pyrex U-tube
*
To whom correspondence should be addressed. E-mail: rryoo@
mail.kaist.ac.kr.
†
2
Korea Advanced Institute of Science and Technology.
Tohoku University.
‡
temperature was changed from room temperature to 383 K over
1 h, maintained at 383 K for 1 h, increased from 383 to 573 K
over 2 h, and finally maintained at 573 K for 2 h. The resultant
Pt/MCM-48 composite sample was exposed to air after the reactor
was cooled to room temperature. The Pt/MCM-48 sample was
washed with 10 wt % hydrofluoric acid until the EDX analysis
indicated trace amounts of silica. The black-colored powder Pt
sample thus obtained was filtered, washed with distilled water,
and dried in a vacuum oven at room temperature. Centrifugation
was preferred to the filtration in the case of small amounts of
sample.
(
1) Ryoo, R.; Joo, S. H.; Jun, S. J. Phys. Chem. B 1999, 103, 7743.
2) Kruk, M.; Jaroniec, M.; Ryoo, R.; Joo, S. H. J. Phys. Chem. B 2000,
(
1
04, 7960.
(
3) Ryoo, R.; Ko, C. H. First IUPAC Workshop on AdVanced Materials:
Nanostructured Systems, Hong Kong, July 14-18, 1999; Abstract book p
4.
4
(
4) Liu, Z.; Sakamoto, Y.; Ohsuna, T.; Hiraga, K.; Terasaki, O.; Ko, C.
H.; Shin, H. J.; Ryoo, R. Angew. Chem., Int. Ed. 2000, 39, 3107.
(
(
(
5) Han, Y.-J.; Kim, J. M.; Stucky, G. D. Chem. Mater. 2000, 12, 2068.
6) Alfredsson, V.; Anderson, M. W. Chem. Mater. 1996, 8, 1141.
7) Carlsson, A.; Kaneda, M.; Sakamoto, Y.; Terasaki, O.; Ryoo, R.; Joo,
S. H. J. Electron Microsc. 1999, 48, 795.
(
8) Ryoo, R.; Joo. S. H.; Kim. J. M. J. Phys. Chem. B 1999, 103, 7435.
9) Kruk, M.; Jaroniec, M.; Ryoo, R.; Joo, S. H. Chem. Mater. 2000, 12,
(
1
414.
(11) Ryoo, R.; Jun, S.; Kim, J. M.; Kim, M. J. Chem. Commun. 1997,
(10) Ryoo, R.; Kim, J. M. J. Chem. Soc., Chem. Commun. 1995, 711.
2225.
1
0.1021/ja003461t CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/18/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 6, 2001 1247
after the complete removal of the silica template using HF, as
shown in Figure 2c. The XRD pattern in Figure 2d was obtained
to investigate if the diffraction changes were related to changes
in the silica template. The XRD pattern shows that the structure
of the silica template was retained after the removal of the Pt
network using aqua regia, despite the line broadening. On the
basis of these results and the aforementioned TEM data, the
difference between Figure 2a and b can be attributed to the
formation of the Pt networks that occupy exclusively one of the
two chiral channel systems in the MCM-48 structure. It is
therefore possible to infer that the Pt networks have a left- or
right-handed chiral structure of the cubic space group I4
I4 32, depending on the individual particles. Unfortunately, these
1
32 or
3
two space groups are not distinguished by normal XRD pattern.
In addition to the XRD lines observed in the small angle region,
the Pt nanowire networks exhibited two broad lines at 2θ ) 39.83°
and 46.32°. These diffractions corresponded to the (111) and (200)
diffractions of the face-centered cubic structure with the unit cell
dimension of 0.3920 nm, which was very close to 0.3923 nm of
the bulk metal. The Pt nanowires were completely crystalline as
indicated by the lattice fringe in Figure 1e. X-ray photoelectron
spectroscopy (XPS) was used to confirm the oxidation state of
the Pt atoms consisting of the nanowires. The XPS spectra of the
template-free Pt materials were taken with powder samples after
evacuation at room temperature, using an ESCA Lab-200R
instrument with an Mg KR X-ray source. The XPS spectra showed
a Pt 4f7/2 peak corresponding to the binding energy of 71.2 eV,
which was very close to 71.0 eV for the bulk Pt metal.
In obtaining the Pt network, it turned out that the most
important experimental factor was the control of the reaction
3 4 3 2
temperature during the conversion of Pt(NH ) (NO ) to Pt. The
present asymmetric networks were obtained when the reaction
temperature was slowly increased in the aforementioned mode.
On the other hand, separate Pt particles 3-5 nm diameters were
obtained inside the MCM-48 when the temperature was rapidly
Figure 2. XRD patterns of (a) MCM-48 silica, (b) Pt/MCM-48
composite, (c) template-free Pt obtained after removing silica from Pt/
MCM-48 with HF, and (d) MCM-48 silica recovered from Pt/MCM-48
after removing Pt with aqua regia.
The template-free Pt sample was characterized by TEM. The
TEM images were taken with a Philips CM20 transmission
electron microscope operated at 100 kV and JEOL JEM-4000EX
operated at 400 kV. Samples for the TEM measurement were
suspended in ethanol and supported on a carbon-coated copper
grid. Figure 1a-e shows representative TEM images for the
template-free Pt material. The TEM investigation revealed that
the Pt material consisted mainly of particles 50-400 nm in
diameter, which were composed of three-dimensional regular
networks of Pt wire 3 nm in diameter. The Pt material contained
very little impurities other than the ordered mesoporous networks.
Many of the Pt networks exhibited edges and corners characteristic
of typical crystal morphologies. The TEM images shown in Figure
increased. Measurement of H
tions showed that the major reaction of Pt(NH
around 383 K. At this temperature, the MCM-48 frameworks
contain moisture so that the impregnated Pt(NH (NO is mobile
2
uptake under the reaction condi-
3
)
4
(NO occurred
3 2
)
1d and e were taken from an extremely thin edge, so that images
)
3 4
3 2
)
of single nanowires could be obtained. The images show a
remarkable resemblance to the dark-shaded portion in Figure 1f,
which corresponds to a helical micelle in the structure model
proposed for the cubic Ia3d lipid mesophase.12 This is the first
direct observation of TEM image corresponding to the micelle
network system of the cubic Ia3d mesophase since the structure
model was proposed in 1967.12
inside the silica channels. On this ground, we believe that the
key to the success of the template synthesis is in the reaction
control so that the conversion of the Pt precursor to the metal
can occur catalytically at the growing tips of the Pt agglomerates.
It is reasonable that the reaction can be initiated at such catalytic
active sites as the solid-state defects located on the pore walls of
MCM-48 silica. Small Pt agglomerate formed at the active site
The formation of the Pt networks during the template synthesis
was investigated with XRD. Figure 2 shows the XRD patterns
obtained with a Rigaku D/MAX-RC (3 kW, Cu KR X-ray source)
diffractometer at room temperature. As the XRD pattern in Figure
3 4 3 2
should then catalyze the conversion of Pt(NH ) (NO ) , as Pt is
a well-known catalyst for hydrogenation. The growth of the Pt
agglomerate under this circumstance is believed to lead to the
formation of the nanostructured network inside the mesoporous
silica structure, similar to growth of a porous crystal. We also
believe that the present synthesis methodology using MCM-48
template may open a possibility for synthesizing various nanoscale
materials with chirality. Further, if it is possible to control the
nucleation of Pt so that agglomeration occurs within the same
kind of the chiral channels, the synthesis will be chiral-selective.
2a shows, the silica template prior to the incorporation of Pt gave
an XRD pattern of highly ordered MCM-48 sample. The XRD
pattern in Figure 2b shows that the Pt incorporation into the
MCM-48 silica brought about two distinct changes in the XRD
pattern. First, all of the diffraction peaks in the range of 2° < 2θ
<
6° decreased in intensity. Second, a new peak similar to the
110) diffraction of the MCM-48 silica phase appeared at 2θ )
.30°. However, this peak cannot be due to the (110) diffraction
(
1
Acknowledgment. This work was supported by Korea Ministry of
Science and Technology (R.R.) and by CREST, Japan Science and
Technology Corporation (O.T.).
since the diffraction is symmetry-forbidden for the cubic Ia3d
structure. It is noteworthy that this diffraction was retained even
(12) Luzzati, V.; Spegt, P. A. Nature 1967, 215, 701.
JA003461T