shows the single-crystal nature of particles, and {200} lattice
fringes are parallel to the edges of the cube.
nuclei in the reaction system, to induce the Pt atoms to grow
directly from the Pt nuclei, so as to form large particles (based
1
1
The advantage of this technique over other high-temperature
synthesis methods is that the formation of nanocubes under
alcohol occurs at a slow rate, facilitating the formation of
anisotropic and faceted particles by controlling the polarity of
solution with DMF and toluene. Furthermore, the shape
and size of nanocrystals depend on other different conditions,
such as the concentration of surfactant, reagents, synthesis
temperature and time. Among the various parameters, the
reaction conditions of polarity, temperature and stirred state
were found to be critical in controlling particle size and
morphology.
on LaMer and Dinegar’s theory). However, Pt nuclei seem
to form easily due to the high internal strain energy between Pt
6
atoms. In order to control the growth speed, keeping the
system in a static state is an efficient way to increase the
growth of the Pt nuclei, or to combine the nuclei while
1
2
reducing the number of nuclei formed. Furthermore, it is
easy for a static system to reach the equilibrium state of atoms
and solution. Coincident with the above assumptions, the
diameter of Pt nanocubes decreased from 35 nm to 22 nm,
as shown in Fig. 2b.
Temperature plays two roles in this system, (1) to induce
thermal decomposition of Pt precursor in the presence of
alcohol, and (2) to enhance the solubility of large nanocubes
in solution. For thermal decomposition, high temperature
tends to result in fast reduction. We have proved that Pt(acac)2
can be reduced in our system even at 145 1C. It is well known
that slow reduction benefits the formation of large nanoparticles.
Interestingly, after decreasing the reaction temperature from
185 1C to 155 1C, we could not get larger nanocubes, but some
attached nanostructures, where small nanocrystals attached to
the surface of large ones (Fig. 2c). A plausible reason is that
high temperature enhances the solubility of nanocubes in a
homogeneous system. However, the large Pt nanoparticles
precipitate out at low temperature, and the remaining Pt
precursor becomes new nuclei which grow into particles, then
adsorb on the previous ones, as shown in Fig. 2c.
According to our previous research, the morphology of
nanocrystals was significantly affected by solvent polarity,
which may change the interaction between nanoparticles and
9
protective agent and result in anisotropic growth. Without
DMF in the reaction system, while keeping other parameters
constant, we found that the size and shape are totally different
from those shown in Fig. 1. The nanocrystals formed
cuboctahedrons without DMF with diameter of 6.9 Æ 1.5 nm
(
Fig. 2a). DMF also can serve as a temporary protective agent
for Pt nanocrystals through the interaction between carbonyl
amide group and Pt. To clarify how the functional group and
the polarity affect the shape, we employed another similar
functional group with low polarity, N,N-dibutylformamide
(
DBF), to replace DMF in this reaction. As shown in the
1
0
TEM image (Fig. 2d), only multibranched particles with
diameter of ca. 500 nm were obtained. That is to say, polarity
plays a vital role in the formation of large nanocubes.
For large nanoparticles, their dispersion in solution remains
a big issue, because of strong interparticle interaction at room
temperature. Traditionally the dispersion of particles with
polymeric protective agent has been studied for over 50 years,
and was found to be a delicate balance among energies such as
The words ‘vigorous stirring’ frequently appear in
descriptions of the synthesis of monodispersed nanocrystals
in wet-chemical syntheses. Here we distinguish between two
systems: with and without stirring. For the synthesis of large
Pt nanocrystals, it is important to decrease the number of Pt
1
3
van der Waals force, electric double layers, entropy. PVP is
commonly used as protective agent for silver, palladium, and
platinum. The analysis indicates the carbonyl group can attach
to the Pt surfaces due to chelated interaction. In our experiment,
we use a 2.0% PVP (w/v) solution in ethanol to replace
oleylamine by sonication for 30 min, and the nanocubes
(
Pt@PVP) were precipitated by additional acetone. The
precipitate was redissolved into ethanol and precipitated by
acetone again. Thermogravimetry analysis shows the weight
loss of Pt@OA and Pt@PVP to be 2.46% and 6.14%,
respectively. The Pt nanocubes did not precipitate out in
À1
ethanol with a concentration of 0.1 mg mL after being kept
at room temperature for one month. The individual nanocubes
by TEM observation also indicated Pt@PVP does not aggregate
in polar solution after ligand exchange.
The thermal stability of Pt nanocubes on SiO2 has been
investigated using XRD and TEM. We directly fabricate Pt
nanocubes on silicon substrate by drop-casting. The large
nanocubes tend to have face-to-face deposition, because it is
energetically the most favorable. After annealing at 500 1C for
5
min by rapid thermal annealing (RTA) as shown in Fig. 3,
the peaks retain a high intensity ratio of (200) to (111) with the
same full width at half maximum (FWHM). The slightly
increased intensity at (111) originates from thermal wetting,
Fig. 2 TEM image of Pt nanocrystals prepared in the same
conditions as 35 nm Pt nanocubes except (a) without DMF; (b) with
stirring; (c) at 155 1C; and (d) replacing DMF with DBF.
1
as explained previously by Yang and co-workers. We used
TEM to check thermal stability. TEM samples were prepared
This journal is c The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 8606–8608 8607
Shen, Zhongrong
