Single-crystal Pt nanorods with tunable lengths fabricated by a simple glycol-assisted vacuum impregnation method

Article abstract of DOI:10.1039/b900262f

Single-crystal platinum nanorods with high aspect ratios and tunable length were fabricated on a large scale by a glycol-assisted one-step vacuum impregnation method, using mesoporous inorganic silica SBA-15 as the host. The Royal Society of Chemistry 200

Full text of DOI:10.1039/b900262f

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www.rsc.org/dalton | Dalton Transactions
Single-crystal Pt nanorods with tunable lengths fabricated by a simple
glycol-assisted vacuum impregnation method†
Hongyang Liu, Ding Ma* and Xinhe Bao*
Received 6th January 2009, Accepted 15th January 2009
First published as an Advance Article on the web 6th February 2009
DOI: 10.1039/b900262f
Single-crystal platinum nanorods with high aspect ratios
and tunable length were fabricated on a large scale by a
glycol-assisted one-step vacuum impregnation method, using
mesoporous inorganic silica SBA-15 as the host.
the mesoporous silica matrix and had lengths tunable from 20–
100 nm. With proper modification, interconnected Pt nanorod
bundles composed of 3–7 Pt nanorods could be obtained. The
nanorod bundles have an average length of 100 nm and width of
50 nm.
Scheme 1 shows the schematic procedure for the fabrication of
the single-crystal Pt nanorods and nanorod bundles. The template-
free mesoporous silica (in this case SBA-15) was dehydrated at
Nanostructured materials exhibit novel properties that largely
differ from those of the bulk solids. This is due to the quantum
confinement effect from the changes in the density and effective
band gap of the electronic energy levels as well as their high ratio of
3
83 K for about 4 h under vacuum.
1
surface-to-bulk atoms. Among the noble metals, platinum plays
important roles in many applications in catalysis, electronics and
2–4
photonics, owing to its unique physical and chemical properties.
Therefore, the design and synthesis of size or shape controlled Pt
nanostructures have been intensely studied over the years.
5
Beside soft template approaches, protocols coupling porous
materials with electrochemical deposition, chemical vapour depo-
sition, thermal hydrogen reduction, photo-reduction and chemical
6–19
reduction have been studied. Anodic aluminum oxide (AAO)
is one of the most widely used templates for the generation
of well defined metal structures, but it is not suitable for the
fabrication of nanorods/nanowires smaller than 20 nm due to an
6
intrinsic limitation in AAO itself. For the preparation of platinum
nanostructures with a smaller diameter, mesoporous silicas have
been considered a good template, which was attributed to their
high BET surface areas, and uniform and tunable pore sizes.
Different Pt nanostructures synthesized using mesoporous silicas
such as MCM-41, SBA-15, FSM-16, MCM-48 and HMM-1
Scheme 1 Pt nanorods and nanorod bundles prepared by a vacuum
impregnation method.
After cooling to room temperature, it was immersed into a
7–19
etc. as templates have been reported in recent years.
However,
mixture of glycol, water and H
2
PtCl
6
·6H
2
O and was transferred
normally polycrystalline Pt nanowires or nanowire networks
have resulted from using these templates, thus the fabrication
of individual single-crystal Pt nanorods confined inside the
mesoporous silica remains a challenge. Lee et al. developed a
strategy to prepare single-crystal Pt nanowires using electrochem-
ical deposition, but with this method, it is critical to have the
channels of the silica run vertically on the substrate surface and
interconnections between nanowires in the template pore structure
very quickly into an oven at 413 K for 30 min. The obtained
materials were washed, filtrated and dried at 373 K to get Pt
nanostructures encapsulated inside the silica matrix. The silica
host was removed by HF etching, and unsupported Pt nanorods or
nanorod bundles were collected. Experimental details are available
in the ESI.†
Fig. 1A is a TEM image of the Pt-SBA-15. The loading of
Pt was 4 wt%. It can be seen that the channels of SBA-15 were
filled with many individual Pt nanorods while there were almost
no Pt nanoparticles inside the channels nor large aggregated Pt
nanoparticles on the external surface. The Pt nanorods have a
uniform diameter of 6–7 nm, which is equal to or a little smaller
than the pore size of SBA-15 (7 nm) (ESI, Fig. S1).† The as-
synthesized Pt nanorods were not connected to each other and
were oriented parallel to the channels of the SBA-15 (Fig. 1B).
The nanorods have a narrow length distribution, which is shown
in Fig. 1B, inset. This shows that the present improved vacuum
impregnation method has a better control over the nucleation and
crystallization of Pt crystallites, which leads to the formation of
uniform length Pt nanorods.
20
cannot be completely avoided. In this paper, a simple (one-pot)
method to fabricate individual single-crystal Pt nanorods with
high aspect ratios is reported. No hydrogen treatment at high
temperature was needed to obtain metallic Pt nanostructures. The
individual nanorods that resulted were evenly distributed inside
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics,
The Chinese Academy of Sciences, Dalian, 116023, P. R. China. E-mail:
xhbao@dicp.ac.cn, dma@dicp.ac.cn; Fax: +86 0411 8469444; Tel: +86 0411
8
4379253
†
Electronic supplementary information (ESI) available: Details of sample
preparation and characterization. See DOI: 10.1039/b900262f
1
894 | Dalton Trans., 2009, 1894–1896
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total pore volume (V
t
) of Pt-SBA-15 (4 wt%) decreased from 711
2
-1
3
-1
to 583 m g and from 0.92 to 0.73 cm g , respectively (ESI,
Table S1).† Fig. 2B shows the XRD patterns of the parent SBA-15
and that with grown Pt nanorods. In the low angle region, it can
be observed that the mesoporous structure of SBA-15 was well
preserved after the incorporation of Pt nanorods. The decrease of
the peak intensity is another indication of the encapsulation of Pt
7
nanorods inside the channels. Four peaks at 39.8, 46.2, 67.4 and
◦
8
1.2 corresponding to the [111], [200], [220] and [311] diffraction
of Pt, respectively, were observed in the high angle region (Fig. 2B
inset), which indicated that the Pt nanorods synthesized have a
face centered cubic (fcc) structure, similar to Pt bulk solid.
The silica host can be removed by a 5% HF–ethanol solution
to give unsupported Pt nanorods (Fig. 1C). It can be seen from
Fig. 1C that all the individual Pt nanorods have a uniform
diameter of 6–7 nm and average length of 30–40 nm. From the
high magnification image (Fig. 1D) of the selected nanorod, one
can see that the Pt nanorods are single crystalline in nature. The
nanorod grew along the [111] direction with a lattice spacing of
the [111] planes of 0.23 nm, which is in good agreement with that
of bulk Pt crystal and XRD results. The fast Fourier transform
Fig. 1 (A) and (B) TEM images of Pt-SBA-15 (4 wt%), inset of (B) is the
particle length distribution of the nanorods. (C) TEM image of individual
Pt nanorods after removing silica matrix. (D) HRTEM image of single
Pt nanorods (the insert shows the corresponding fast Fourier transform
image).
(
FFT) of the atomic lattice fringing, in the inset in Fig. 1D,
further demonstrates the crystallinity of the Pt nanorods. If SBA-
5 zeolites with different pore sizes were used as the template with
1
the same procedure, individual single-crystal Pt nanorods with
different diameters can be obtained (details not shown here). This
confirmed that the glycol-assisted one-step vacuum impregnation
method is an efficient and effective method to prepare individual
single-crystal Pt nanorods.
After the growth of the Pt nanorods, a type IV isotherm with
a H1 hysteresis loop was also obtained in a BET measurement,
which is similar to that of the parent SBA-15 (Fig. 2A). This
means that the parent mesoporous structure was maintained after
growth of the Pt nanorods, while the BET surface (SBET) and the
Interestingly, when the amount of Pt precursor in the H
2
PtCl –
6
glycol–water solution was changed, it was possible to prepare
individual Pt nanorods with different lengths. Fig. 3A and C,
displays the TEM images of the Pt nanorods grown in the SBA-15
matrix with different Pt weight loadings (2 and 8%). Clearly, when
the Pt loading was 2%, very short Pt nanorods in the SBA-15
channels were obtained. The diameter of these nanorods was the
same, i.e. around 6–7 nm, but the length of them had changed
to be 15–25 nm. Fig. 3A inset shows that a quite narrow length
distribution was obtained for the Pt nanorods hosted inside the
SBA-15 channels (2 wt%). The corresponding HRTEM image
demonstrated that these short Pt nanorods were single crystals,
showing the effectiveness of the present glycol-assisted one-step
vacuum impregnation method. With further increases in the
concentration of Pt (8 wt%), Pt nanorods with average lengths
of 60 nm were obtained (Fig. 3C). Again, these nanorods had a
fairly narrow length distribution (Fig. 3C inset), and were single
crystals (Fig. 3D and E). After acid etching to remove the silica
host by HF–ethanol solution, the morphology and size of the
nanorods were well maintained, and resulted in pure individual
Pt nanorods (ESI, Fig. S2).† Significantly, when the loading of
the Pt precursor was higher than 15%, very evenly distributed
Pt nanorod bundles were obtained that have an average length of
1
00 nm and average diameter of 50 nm (Fig 3F). From a close look
at these bundles, it was seen that they were 3–7 interconnected Pt
nanorod networks. Each nanorod was around 100 nm in length
and, again, 6–7 nm in diameter. It has been reported that there
are many mesopores in the wall of the hexagonal channel of
SBA-15, and the Pt nanorods were actually connected to each
Fig. 2 (A) Nitrogen adsorption isotherm of (a) parent SBA-15 and
(
b) Pt-SBA-15 (4 wt%). (B) Small-angle XRD pattern of parent SBA-15
18
and Pt-SBA-15 (4 wt%); inset is their wide-angle XRD patterns.
other by small bridges. After removing the silica host by acid
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Dalton Trans., 2009, 1894–1896 | 1895

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the lower nucleation rate may contribute to the formation of these
nanorod bundles.
In conclusion, a simple glycol-assisted one-step vacuum impreg-
nation method has been developed to prepare individual single-
crystal Pt nanorods in the channels of a mesoporous silica host.
The length of the Pt nanorods was tunable from 20–100 nm by
simply changing the concentration of Pt in the mixture solution
(
H
2
PtCl –glycol–water). It was also possible to prepare uniform Pt
6
nanorod bundles. Each bundle was composed of 3–7 Pt nanorods.
This method provides a facile approach to prepare individual
single-crystal metal nanorods that can be used in various fields,
e.g. as nanosensors, catalysts and in fuel cell applications.
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (20773121) and Chinese Academy of Sceience
(KJCX2.YW.H16). D. M. thanks the Chinese Academy of Science
for financial support through the Bairen project.
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been reported before,
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2
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1
896 | Dalton Trans., 2009, 1894–1896
This journal is © The Royal Society of Chemistry 2009