Fig. 4 TEM images of Pt nanostructures grown on polystyrene beads
without SnOEP at low (a) and high (b) magnifications.
interval, seeds produced chemically at different times have varied
growth periods leading to a final broad size distribution. In
addition, because of the small number of seeds produced
chemically, the dendrites, although larger (up to 50 nm), do
not adequately bridge to adjacent dendrites to form complete
shells. A similar result is obtained in an experiment using SnOEP
decorated beads but without light irradiation, giving TEM
images (not shown) like those shown in Fig. 4. These experi-
ments demonstrate that the SnOEP-based photocatalytic seed-
ing is crucial for the preparation of high-quality hollow platinum
nanospheres. Additionally, it appears that platinum metal pre-
fers nucleating and growing on the hydrophobic surface of
polystyrene beads. This phenomenon agrees well with our
previous observations using liposomes to template platinum
growth, for which platinum metal preferably nucleates and
grows within the hydrophobic liposomal bilayer.12,17
Fig. 5 Bright-field TEM images of platinum nanostructures grown
on polystyrene beads in the presence of 8.8, 4.4, 2.2, 0.55, and 0.28 mM
platinum complex, respectively.
concentrations of 0.55 and 0.28 mM Pt salt, abundant Pt
nanoparticles are observed on the beads. However, to attain
intact Pt nanoshells thinner than 12 nm, many small dendrites
would be required to allow them to join with neighbouring ones
to form the shell. These thin shells might be obtained by using a
stronger light source to increase the seed density on the beads.
This work was partially supported by the Office of Basic
Energy of Sciences, US Department of Energy. Sandia is a
multiprogram laboratory operated by Sandia Corporation, a
Lockheed Martin Company, for the United States Depart-
ment of Energy’s National Nuclear Security Administration
under Contract DEAC04-94AL85000.
An important factor that influences the quality of the generated
hollow platinum spheres is the amount of SnOEP photocatalyst
coverage on the beads. In our typical synthesis, the optimum
surface area of the SnOEP molecules is calculated to be approxi-
mately 10 times that of the beads. This calculation compares the
geometrical area of the spheres with an average diameter of 99 nm
to the area of the SnOEP molecules, which are approximately
2 nm  2 nm assuming the square planar molecule lies flat on the
bead. Less SnOEP results in incomplete platinum coatings on the
surface of the beads, while excess SnOEP leads to the formation
of platinum not deposited on surface of the beads. The latter may
occur because some SnOEP molecules are not adsorbed and
function as isolated competing nucleation sites.
Notes and references
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Relying on the above understanding of the photocatalytic
reaction, the thickness of the shells should be controllable by
varying the amount of platinum complex available for growth
and the light exposure. Indeed, the coverage of platinum on
polystyrene beads can be controlled by simply reducing the
concentration of aqueous salt from 8.8 to 4.4, 2.2, 0.55, and
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decrease of Pt(II) concentration the coverage on the polystyrene
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ꢀc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 2535–2537 | 2537