Interfacial synthesis of dendritic platinum nanoshells templated on benzene nanodroplets stabilized in water by a photocatalytic lipoporphyrin

Article abstract of DOI:10.1021/ja0619859

Nanoscale metal shells have many potential uses and in some applications offer significant advantages over nanoparticles. The synthesis of platinum nanoshells using stabilized nanodroplets of benzene in water as growth templates is described; the nanodroplets are stabilized by a surfactant-like tin(IV)-porphyrin complex localized at the benzene-water interface. The porphyrin also acts as a photocatalyst that reduces the platinum complex and deposits metal onto the nanodroplets to form dendritic metal nanoshells. Below the solubility limit of benzene in water, the lipoporphyrin-stabilized nanodroplets have a reproducible number, size distribution, and surface area, which allows the thickness of the platinum shell walls to be controlled by changing the amount of platinum complex. Nanoscale platinum shells with magnetic interiors can be made by dispersing Fe₃O₄ nanoparticles in the benzene nanodroplets. Copyright

Full text of DOI:10.1021/ja0619859

Published on Web 06/28/2006
Interfacial Synthesis of Dendritic Platinum Nanoshells Templated on Benzene
Nanodroplets Stabilized in Water by a Photocatalytic Lipoporphyrin
Haorong Wang,†,‡ Yujiang Song,† Craig J. Medforth,† and John A. Shelnutt*,†,‡
AdVanced Materials Laboratory, Sandia National Laboratories, Albuquerque, New Mexico 87106, and Department
of Chemistry, UniVersity of Georgia, Athens, Georgia 30602
Received March 22, 2006; E-mail: jasheln@unm.edu
Platinum has extraordinary chemical and physical properties, and
Pt nanostructures have been used as sensors, molecular switches,
magnetic materials, and nanoreactors.1 Studies of platinum nano-
particles, nanorods, nanowires, nanotubes, nanodendrites, and shape-
controlled nanoparticles have been reported.2 Pt nanostructures with
hollow interiors (nanoshells) are rare,3 even though they have
advantages over nanoparticles for some applications. For example,
metallic shell structures have reduced material costs, lower densities,
and, in some cases, special optical properties. Metal nanoshells can
also be used as highly active catalysts and as capsules for other
functional nanomaterials.3b,4 Emulsion droplets have long been used
as templates for synthesizing hollow nanostructures composed of
glass, ceramics, or semiconductors,5 but we are not aware of the
droplet-templated synthesis of hollow metal spheres. Herein, we
describe the first synthesis of platinum nanoshells using stabilized
Figure 1. Structure of Sn(IV) tetrakis(N-octadecyl-4-pyridyl)porphyrin
nanodroplets as templates.
hexachloride (SnP18) and the proposed mechanism for formation of the Pt
The synthesis of platinum nanoshells uses a tin-lipoporphyrin
photocatalyst to initiate the reduction of platinum onto the surface
of benzene nanodroplets in water. Tin-porphyrin photocatalysts
have previously been used by our group to synthesize nanostruc-
tured platinum metal using micelles or liposomes as templates.2d
The structure of the lipoporphyrin, Sn(IV) tetrakis(N-octadecyl-4-
pyridyl)porphyrin hexachloride (SnP18), used in the synthesis is
given in Figure 1. The porphyrin ring acts as the hydrophilic
headgroup due to the charged pyridinium groups surrounding the
macrocycle, and it is expected to localize at the benzene-water
interface while the alkyl tails reside within the benzene droplet
(Figure 1a). These surfactant-like properties are found to be essential
for the formation of the stable nanodroplets that serve as the
nanoshell templates. Remarkably, the tin-lipoporphyrin stabilizes
the benzene droplets even as the benzene concentration is lowered
below the solubility limit in water. The net result is nanoscale
benzene droplets with reproducible droplet number, size distribution,
and thus total surface area.
nanoshells: porphyrins localized at the benzene-water interface (a)
photocatalytically grow platinum seeds and continue to grow autocatalyti-
cally (b) to form Pt dendrites that join to form a nanoshell (c).
In the presence of a sacrificial electron donor (ascorbic acid),
SnP18 acts as a photocatalyst that is capable of reducing a Pt(II)
complex to platinum metal (Figure 1a). When irradiated with visible
light, Pt nanoparticles deposit evenly over the surface of the
nanodroplets, as illustrated in a proposed mechanism shown in
Figure 1, which is based on earlier studies of Pt growth on micelles
and liposomes.2d The Pt seeds subsequently grow autocatalytically
(Figure 1b) and ultimately complete the coverage of the benzene
nanodroplets to form the Pt nanoshells (Figure 1c). Because the
total surface area is constant below the solubility limit, the thickness
of the Pt deposited onto the droplets can be controlled by varying
the amount of Pt complex.
Figure 2. SEM images of Pt nanoshells synthesized under different
conditions: (a) 0.5% benzene/SnP18 in water and 1 mM Pt complex, (b)
0.25% and 1 mM Pt complex, (c) 0.125% and 0.5 mM Pt complex. Size
distribution of the Pt nanoshells (d) obtained from four independent
preparations of (c) by measuring 300 shells in each TEM image.
reaction, 2 mL of the diluted emulsion is transferred to a 4 mL
reaction vial, and aged K2PtCl4 solution2d (20 mM) is added to
give final Pt concentrations of 0.025-1.00 mM (depending upon
the desired coverage). Freshly prepared ascorbic acid (AA) solution
(150 mM) is then added to produce an AA:Pt ratio of 7.5:1, and
the vial is sealed and irradiated with incandescent light (800
nmol‚cm-2‚s-1) for 30 min during which time a black suspension
gradually forms as the Pt(II) complex is reduced.
The preparation of benzene nanodroplets containing the SnP18
photocatalyst is described below.6 For the Pt nanoshell forming
Figure 2a-c shows SEM images of Pt shells made using the
SnP18-containing nanodroplets prepared with 0.5, 0.25, and 0.125%
v/v benzene in water.7 With 0.5% v/v benzene in water as the
† Sandia National Laboratories.
‡ University of Georgia.
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10.1021/ja0619859 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
containing suspended 5 nm diameter Fe3O4 particles (see details
in the Supporting Information). Figure 3d shows an STEM image
of the Fe3O4-containing Pt nanoshells. The edge of the spheres
appears brighter than their center region in these dark field images,
consistent with an encasing electron-dense Pt shell. EDS analysis
of the shell edge area (Figure S6) confirms that it is mainly Pt
(Pt:Fe, 93:7), and EDS of the center area indicates that it is mainly
Fe (Pt:Fe, 33:67). The magnetite-filled shells are clearly magnetic,
as shown by their attraction to a magnetic stirring bar placed on
the sidewall of the vial (Figure 3e).
In conclusion, we have developed a method for using lipopor-
phyrin-stabilized nanodroplets of an organic solvent in water as
templates to synthesize dendritic Pt nanoshells. A key feature of
the method is the use of a tin-porphyrin complex both as a
surfactant that stabilizes the nanoscale benzene droplets and as a
photocatalyst to initiate Pt reduction onto the surfaces of the
nanodroplets. At benzene concentrations below its solubility limit
in water, the thicknesses of the Pt nanoshells can be controlled
continuously from ∼5 to over 50 nm by varying the amount of the
Pt complex. The organic phase can also be used as a medium for
placing functional nanomaterials inside the nanoshells. The pho-
tocatalytic interfacial synthesis of nanoshells composed of other
metals is currently being studied in our laboratory.
Figure 3. TEM images of Pt nanoshells prepared using SnP18 in 0.125%
benzene in water and various concentrations of Pt complex: (a) 1 mM, (b)
0.5 mM (same as Figure 2c), (c) 0.25 mM. Panel (d) is an STEM dark
field image of Pt nanoshells prepared using 0.25% benzene/SnP18 containing
∼5 nm Fe3O4 nanoparticles. Panel (e) is a photograph showing a magnetic
stirring bar attracting the nanoshells to the side of the reaction vessel.
template, the product consists of hollow Pt structures with a wide
range of sizes with a significant quantity of micron-sized structures
present (Figure 2a). At 0.25%, relatively few micron-scale structures
are seen, mostly consisting of spherical Pt shells in the range of
100-400 nm (Figure 2b). Finally, at 0.125% (Figure 2c), the size
of the Pt spheres is further reduced and most are less than 100 nm
in diameter (Figure 2d). The reduction to a restricted size range
upon dilution below the solubility of benzene in water (0.22%)
indicates a limited loss of benzene from the droplets with SnP18
behaving as an unexpectedly good surfactant for stabilizing the
nanodroplets. The size distribution of the nanodroplets is reproduc-
ible at concentrations below 0.22% (see Figure S2), giving a
repeatable surface area. Consequently, the Pt concentration can be
varied to produce the desired thickness of the Pt nanoshells.
TEM images (Figure 3b) show that the surfaces of the shells
have a “furry” appearance consistent with dendritic Pt growth.2d
In addition, XRD spectra (Figure S3) show peaks characteristic of
nanocrystalline platinum in agreement with the nanoscale structural
features of Pt dendrites. Since the total surface area of the
nanodroplets is nearly the same for similarly prepared 0.125% v/v
benzene in water templates, the thickness of the shells can be
modified by simply altering the concentration of the Pt complex.
Figure 3 shows the effect of decreasing the Pt concentration; the
average wall thickness decreases from approximately 40 nm at 1
mM Pt complex (Figure 3a) to 20 nm at 0.5 mM (Figure 3b); the
shells are thin and porous at 0.25 mM Pt (Figure 3c) and consist
of particles and small dendrites with sufficient connectivity to
preserve the shell structure (see Figure S4).
Acknowledgment. Sandia is a multiprogram laboratory operated
by Sandia Corporation, a Lockheed Martin Company, for the United
States Department of Energy’s National Nuclear Security Admin-
istration under Contract DEAC04-94AL85000. Research partially
supported by the DOE Division of Chemical Sciences, Geosciences
and Biosciences (DE-FG02-02ER15369).
Supporting Information Available: Synthesis details, UV-visible
spectra, XRD spectra, EDS and TEM data. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
(1) (a) Matkoa, I.; Gaidia, M.; Hazemannb, J. L.; Cheneviera, B.; Labeaua,
M. Sens. Actuators, B 1999, 59, 210. (b) De, S.; Pal, A.; Pal, T. Langmuir
2000, 16, 6855. (c) Carpenter, E. E.; Sims, J. A.; Wienmann, J. A.; Zhou,
W. L.; O’Connor, C. J. J. Appl. Phys. 2000, 87, 5615. (d) Joo, S. H.;
Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature 2001,
412, 169.
(2) (a) Han, Y. J.; Kim, J. M.; Stucky, G. D. Chem. Mater. 2000, 12, 2068.
(b) Shin, H. J.; Ko, C. H.; Ryoo, R. J. Mater. Chem. 2001, 11, 260. (c)
Luo, Y.; Lee, S. K.; Hofmeister, H.; Steinhart, M.; Gosele, U. Nano Lett.
2004, 4, 143. (d) Song, Y. J.; Yang, Y.; Medforth, C. J.; Pereira, E.; Singh,
A. K.; Xu, H. F.; Jiang, Y. B.; Brinker, C. J.; van Swol, F.; Shelnutt, J.
A. J. Am. Chem. Soc. 2004, 126, 635. (e) Ahmadi, T. S.; Wang, Z. L.;
Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (f)
Herricks, T.; Chen, J. Y.; Xia, Y. N. Nano Lett. 2004, 4, 2367.
(3) (a) Sun, Y. G.; Mayers, B. T.; Xia, Y. N. Nano Lett. 2002, 2, 481. (b)
Liang, H. P.; Zhang, H. M.; Hu, J. S.; Guo, Y. G.; Wan, L. J.; Bai, C. L.
Angew. Chem., Int. Ed. 2004, 43, 1540.
In the absence of light exposure, the Pt structures obtained are
irregular and the platinum coverage is uneven (Figure S5) since
only a few seeds are spontaneously produced, as observed previ-
ously for templating surfactant assemblies.2d The importance of
photocatalytic seeding in producing the more evenly coated shell
structures shown in Figures 2 and 3 is that a large population of
seed particles is distributed uniformly over the surface of the
droplets, and most of these seeds are formed at the beginning of
the reaction giving them equal autocatalytic growth times. The equal
growth times lead to more consistent dendrite size and more uniform
wall thickness.2d
(4) (a) Kumar, R.; Maitra, A. N.; Patanjali, P. K.; Sharma, P. Biomaterials
2005, 26, 6743-6753. (b) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T.
J. Am. Chem. Soc. 2002, 124, 7642.
(5) (a) Nakashima, T.; Kimizuka, N. J. Am. Chem. Soc. 2003, 125, 6386. (b)
Fowler, C. E.; Khushalani, D.; Mann, S. J. Mater. Chem. 2001, 11, 1968.
(c) Huang, J. X.; Xie, Y.; Li, B.; Liu, Y.; Qian, Y. T.; Zhang, S. Y. AdV.
Mater. 2000, 12, 808.
(6) A milky stock emulsion was prepared by sonicating a 1:99 mixture of
benzene (containing 1 mM SnP18) and water. This was diluted with water
for the nanoshell syntheses. UV-visible spectra (Figure S1) show that
SnP18 is negligibly soluble in water or benzene, but the porphyrin Soret
band of the diluted emulsion (0.25%) indicates that SnP18 is partially
dissolved and probably resides at the benzene-water interface (Figure
1a).
(7) Under imaging conditions, benzene probably evaporates from within the
shells, while in aqueous suspension some benzene likely remains.
The benzene droplets can also carry other nanomaterials that will
be subsequently enclosed inside the Pt shells. For example, magnetic
Pt nanoshells were synthesized using templating nanodroplets
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