1
4,16
(
HAADF-STEM) images of the as-made Pt nanocubes
supported on SiO . Most of the particles are seen to have
straight edges with sharp corners. The high-resolution TEM
HRTEM) image of a single Pt nanocube shows lattice fringes
becoming the bounding surfaces of the final particles.
was also observed that CO alone, but not H , could reduce
Pt efficiently at 150 1C, suggesting the CO plays the primary
It
2
2
2
3
(
role in the reduction process.
parallel to its straight edges (Fig. 1c). The measured inter-
˚
planar spacing of 1.96 A matches that of Pt (200) planes for
The effects of support pretreatment were examined by
dehydrating SiO at different temperatures. Pt cubes with a
2
the face-centered cubic (fcc) structure and demonstrates that
the cubic Pt nanoparticles are primarily enclosed by (200)
surfaces. The fast Fourier transform (FFT) pattern of the
lattice image exhibits a four-fold symmetry, consistent with the
projection of the fcc structure from its [001] direction (see inset
of Fig. 1c). Statistical analysis of the sample shows that 87%
of the Pt particles are cubic in shape under the specified
reaction conditions (Fig. 1d). The byproducts consist of some
spheres, rectangular structures and twin-planed rods (Fig. S3,
ESIw). We believe that the growth of rectangular shaped
particles is attributable to the presence of rough end surfaces
that contain steps which facilitate faster crystal growth than
would be expected for perfect (200) planes (Fig. S3a, ESIw).
The appearance of Pt rods and spheres is possibly due to
formation of twin planes in the early stages of crystal growth,
which result in the exposure of (111) planes and thus a change
in the pattern of crystal growth (Fig. S3b, ESIw). The distribution
of edge sizes of the Pt particles follows a narrow Gaussian
distribution with an average edge size of 8.4 Æ 1.9 nm
2
smaller size were obtained when SiO was pretreated at
temperatures below 300 1C. Dehydration at this temperature
leaves a larger surface concentration of silanol groups than
pretreatment at 700 1C (Fig. S7a, ESIw), and suggests that the
silanol groups are involved in the Pt nucleation and growth.
Cubic Pt can also be produced on an amorphous carbon
support, which has abundant hydroxyl/carboxyl groups on
its surface (Fig. S7b, ESIw). In contrast, only small particles
were formed when non-protonated Al O and TiO supports
2
3
2
were used (Fig. S7c and d, ESIw). These results indicate the
importance of surface groups, which may work in concert somehow
2
with H to assure the efficient movement of Pt precursors on
the support.
2 2 2
The reduction of Pt(acac) /SiO to form cubic Pt/SiO was
studied by stopping the reaction at designated temperatures
and times and then characterizing the sample by TEM. Some
of the Pt had already been reduced, primarily in the form of
small nanoparticles, when the temperature was raised to 180 1C
(Fig. S8, ESIw). These small Pt entities appear to act as seeds
and quickly grew into cubes as the temperature was further
increased. Cubic Pt particles of similar size compared to the
final product were obtained shortly after the temperature was
raised to 200 1C, suggesting that reduction occurs very rapidly
at this temperature. In situ Fourier transform infrared (FTIR)
spectra of adsorbed CO were acquired as a function of time as
reduction proceeded. Fig. S9 (ESIw) and Fig. 2a show the
original FTIR spectra and those obtained after background
(Fig. 1d). The size is in a reasonable agreement with that
estimated from the (200) X-ray diffraction peak using the
Scherrer equation (Fig. S4, ESIw).
The formation of cubic Pt/SiO can be achieved in the
2
temperature range of 150 to 250 1C, but at lower temperatures
Pt cannot be reduced efficiently (Fig. S5, ESIw). The Pt cubes
produced at the temperature of 250 1C have round corners and
less sharp edges than those produced at lower temperatures.
When the reduction temperature is raised to 300 1C, the
particles become spherical. These findings suggest that the
cubic shaped particles are thermodynamically unstable at
À1
subtraction, respectively. The band at 2093 cm is due to
linearly chemisorbed CO on metallic Pt. This feature was not
observable until the sample had been heated to B180 1C,
indicating that only a minimal amount of Pt had been reduced
below this temperature. A significant growth in the intensity of
the band was observed with further increase in the temperature.
Concurrently, the peaks associated with acetylacetonate groups,
elevated temperatures. The concentrations of CO and H were
2
adjusted systematically to study the effects of reducing gas
composition on the final product. Both gases were found to be
crucial in the formation of Pt nanocubes (Fig. S6, ESIw). The
À1
CO/H
2
ratio did not have a significant effect on the final
located at 1553, 1526, and 1381 cm , diminished in intensity as
3
À1
product for flows in the range of 25/5 to 10/20 cm min ; in
each case cubic Pt particles of similar size were formed. Most
of the particles obtained in the presence of pure CO, however,
were dramatically different and much smaller in size (Fig. S6a,
ESIw). On the other hand, large spherical particles were
observed when H2 was used alone (Fig. S6d, ESIw). These
a consequence of the decomposition of the organic groups. The
absence of peaks at these positions after reaction is indicative of
complete removal of the acetylacetonates (Fig. S10, ESIw).
The amount of chemisorbed CO on Pt was used as a measure of
the extent of Pt reduction, and was calculated by integrating the
area under its peak. The normalized data reached its maximum
after B40 min of reaction at 200 1C, suggesting complete reduction
of Pt at this temperature and time (Fig. 2b). The extent of Pt
reduction, X , and the rate of reduction, dX /dt, over the course
findings suggest that H facilitates the transport of Pt precursors
2
to their growth sites. The small Pt particles formed in pure CO
are probably due to a restricted diffusion of the precursors
Pt
Pt
on the support in the absence of H
2
. The spherical shape and
suggests that CO
of reaction at 200 1C were estimated from Fig. 2b and are shown in
Fig. 2c. The extent of Pt reduction was calculated based on their
correlation between the amount of chemisorbed CO and the
assumption that Pt nanocubes are grown from cubic seeds after
a burst of nucleation (see Fig. S8 and ESIw for details). The curve
for dXPt/dt can be fitted to an exponential decay characteristic of a
diffusion-controlled reaction, suggesting that the diffusion of
non-uniform size of the formed Pt in pure H
2
mediates the growth of the Pt and shapes them into cube. The
formation of a cubic shape is very likely associated with
preferential chemisorption of CO on Pt (100), thereby modifying
2
3–25
the growth kinetics.
studies have shown a stronger binding of CO molecules to Pt (100)
Previous experimental and theoretical
24,26
than to other low-indexed facets.
The strongly adsorbed CO
Pt precursors on the SiO surface may be the rate-determining
2
2
7–29
will inhibit the growth of (100) planes, resulting in these surfaces
step in their transformation into Pt cubes (Fig. 2c).
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 1854–1856 1855
Peng, Zhenmeng
