Synthesis of platinum nanowheels using a bicellar template

Article abstract of DOI:10.1021/ja8047464

Disk-like surfactant bicelles provide a unique meso-structured reaction environment for templating the wet-chemical reduction of platinum(II) salt by ascorbic acid to produce platinum nanowheels. The Pt wheels are 496 ± 55 nm in diameter and possess thickened centers and radial dendritic nanosheets (about 2-nm in thickness) culminating in flared dendritic rims. The structural features of the platinum wheels arise from confined growth of platinum within the bilayer that is also limited at edges of the bicelles. The size of CTAB/FC7 bicelles is observed to evolve with the addition of Pt(II) complex and ascorbic acid. Synthetic control is demonstrated by varying the reaction parameters including metal salt concentration, temperature, and total surfactant concentration. This study opens up opportunities for the use of other inhomogeneous soft templates for synthesizing metals, metal alloys, and possibly semiconductors with complex nanostructures. Copyright

Full text of DOI:10.1021/ja8047464

Published on Web 08/26/2008
Synthesis of Platinum Nanowheels Using a Bicellar Template
,
†
†,‡
†,‡
†
†,‡
Yujiang Song,* Rachel M. Dorin, Robert M. Garcia, Ying-Bing Jiang, Haorong Wang,
†
,|
†,‡
†
†
,‡,§
Peng Li, Yan Qiu, Frank van Swol, James E. Miller, and John A. Shelnutt*
AdVanced Materials Laboratory, Sandia National Laboratories, Albuquerque, New Mexico 87106, Department of
Chemical and Nuclear Engineering, UniVersity of New Mexico, Albuquerque, New Mexico 87131, and Department
of Chemistry, UniVersity of Georgia, Athens, Georgia 30602
Received June 20, 2008; E-mail: ysong@sandia.gov; jasheln@unm.edu
Disc-like micelles called bicelles are a rare class of microstruc-
1
tures composed of mixed lipid and detergent molecules. Bicelles
have also been obtained by self-assembly of two oppositely charged
2
-5
single-chain surfactants or by using a block copolymer.
Inves-
tigations of bicelles have mainly focused on their alignment and
their use as biomimetic membrane models especially for reconstitu-
6
-10
tion of membrane proteins.
However, bicelles also provide a
unique inhomogeneous reaction environment possessing both low
curvature and high curvature regions within a bilayer disk. Although
the utilization of bicelles as soft templates for the synthesis of
2,4
nanomaterials has been suggested, no examples of their successful
use for this purpose have been reported. Herein, we show that
bicellar templates can be used to control the growth of platinum,
producing metal nanodisks and nanowheels.
Bicelles like that illustrated in Figure 1a can be assembled from
the surfactants cetyltrimethylammonium bromide (CTAB) and
Figure 1. (a) Illustration of the cross-sectional view of a bicelle composed
of two different surfactants, CTAB and FC7 in the present system. (b-e)
SEM images of platinum nanowheels at different magnifications
4,11
sodium perfluorooctanoate (FC7).
Jung and Zasadzinski reported
the presence of a small percentage of bicelles mixed with dominant
spherical vesicles and cylinders at a CTAB/FC7 molar ratio of 25:
4
7
5 and 2 wt% total surfactant. The CTAB/FC7 molar ratio of 50:
5
0 used in this work gives a higher yield of bicelles. For metal
12-14
growth,platinumwaschosenforitsdendriticgrowthcharacteristics
1
5,16
and catalytic activity.
metal nanostructures using lipid vesicles,
reaction employed 264 mg of ascorbic acid (AA) as a reducing agent
and 10 mL of 20 mM aqueous K PtCl added to 10 mL of CTAB (1
Following our previous syntheses of Pt
12-14
a typical Pt reduction
2
4
mM) and FC7 (1 mM) (0.08 wt% total surfactant); these were allowed
to react for at least 1 h at 25 °C to give a black precipitate. A transparent
and colorless supernatant was obtained after reduction, suggesting that
the reaction was complete. This was confirmed by the absence of
UV-visible bands of the Pt(II) complex in the supernatant (Figure
S1). Additional details of the synthesis are given in the Supporting
Information.
Scanning electron microscopy (SEM) reveals that the black
precipitate consists largely of circular platinum nanostructures, along
with a small percentage of Pt metal without a defined morphology
(
Figures 1b, S2). The average diameter of the Pt nanostructures is
uniform, 496 ( 55 nm, as determined by the measurement of 100
randomly selected nanostructures (Figure S3). The random packing
of the structures results in their being tilted at different angles thus
revealing their morphological details. In particular, the tilted circular
structure shown in Figure 1c shows the flaring at the edge that
produces the wheel-like morphology as well as the thin inner portion
of the disk. In Figure 1d,e for a platinum nanowheel lying flat on
the supporting substrate, the dendritic sheet features of the inner
Figure 2. TEM (a) and HAADF STEM (b-d) image of platinum
nanowheels (Inset: platinum density profile crossing selected region of a
wheel in (b)).
disk are discernible. In these SEM images, very small globular
14
dendrites are also seen growing outward from the dendritic sheet.
†
Sandia National Laboratories.
The transmission electron microscopy (TEM) images shown in
Figure 2 further elucidate the morphology and growth pattern. In
particular, the images reveal that the centers of the platinum nanowheels
‡
University of New Mexico.
§
University of Georgia.
Currently at Advanced Technology Development, Intel Corp., Chandler, AZ.
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2602 9 J. AM. CHEM. SOC. 2008, 130, 12602–12603
10.1021/ja8047464 CCC: $40.75  2008 American Chemical Society

C O M M U N I C A T I O N S
solution conditions of this complex mixture of diluted bicellar
2
,18,19
solution, Pt salt and aqueous AA, and the different pH (2.7).
In this mixture, the hydrodynamic size could also be increased by
bicellar aggregation and the presence of other mesophases. Nev-
ertheless, a bicellar diameter of 602 nm calculated from the
hydrodynamic diameter (details given in the Supporting Informa-
tion) is compatible with the diameter of the Pt nanowheels obtained
from electron microscopy (496 nm), especially given that Pt sheet
growth might shrink the bicelles.
To conclusively demonstrate that the flaring at the edge of the
Pt nanowheels is a consequence of nanosheet growth reaching the
edge of the bicelle, we lowered the Pt(II) concentration from 10 to
5
mM while holding the other parameters constant. For this case,
circular dendritic nanodisks with thickened centers are observed,
but without the flaring (Figures 3b and S6a). This was expected
because the Pt complex is consumed before the sheet reaches the
edge of the bicelle. This also provides further proof that the
nanowheel grows outward from a central nucleation site.
Figure 3. (a) DLS size distributions of bicelles in stock suspension
containing 1 mM of CTAB and FC7 (blue), and three repetions for the
reaction system containing bicelles, Pt(II) salt, and AA (0.5 mM CTAB
and FC7, 10 mM K2PtCl4, and 150 mM AA (red)). (b) TEM image of
platinum nanodisks.
Finally, when the original synthesis was conducted at 30 and 20
C, nanowheels with small and large flarings at the edges (Figure
are thickened (Figure 2a). The central thickening is possibly due to
the fact that dendritic growth is initiated at a centrally located seed
nanoparticle. Slow growth normal to the surface of the sheet occurs
throughout the reduction reaction and is thus most extensive near the
center of the sheet where growth occurs for the longest time. The reason
that the seed particles locate at the center of the bicelle may be due to
°
S6b,c) were observed, respectively. This is consistent with the
production of fewer bicelles containing seed particles at low
temperature, leaving more Pt complex per seed and thus more
extensive growth for the fewer seeded bicelles. Variation of total
surfactant concentration provides additional opportunities to direct
the growth of Pt to produce complex nanostructures. The metal
growth methods described here might also be used as a means of
imaging and discovering structural features of other types of
surfactant assemblies and other templating materials.
4
electrostatic forces caused by the high positive charge density at the
edge of the bicelles. A radial mechanical stress also likely exists in
the bicelle because the layers are bound together more tightly at the
highly curved edge than at the loose central region, which can thus
more easily accommodate the Pt seed particle.
The high-angle annular dark-field scanning TEM (HAADF
STEM) images (Figure 2b), for which the brightness is related to
the Pt density, confirm that the rim and the center are much more
dense than the intermediate region. Additionally, the Pt density
profile (Figure 2, inset) indicates that the disk-like part between
the center and the edge is roughly uniform in thickness. Consistent
with other dendritic Pt sheets formed in lipid bilayers, the thickness
is estimated to be about 2 nm. This can be seen by comparing
the size of dendritic tips and the width of bright lines originating
from the approximately vertically aligned dendritic branches in the
edge region (see Figure S4). In high-magnification images (Figure
Acknowledgment. Sandia is a multiprogram laboratory operated
by Sandia Corporation, a Lockheed Martin Company, for the U.S.
Department of Energy’s National Nuclear Security Administration
under Contract DEAC04-94AL85000.
Supporting Information Available: Experimental details; UV-vis
spectrum; SEM, TEM, and STEM images; size analysis plot; and XRD
pattern. This material is available free of charge via the Internet at
http://pubs.acs.org.
1
3
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JA8047464
J. AM. CHEM. SOC. 9 VOL. 130, NO. 38, 2008 12603