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TABLE 3: Optical Parameters S of the Platinum Sols
Prepared by Method A at Different S/Pt Ratios during
Aging
S/Pt
aging time
0.1
0.3
0.5
0.7
1 hour
2 days
5 days
5 months
1.36
0.44
2.28
0.83
2.73
1.64
1.58
1.52
2.96
2.57
2.33
2.35
be caused by the change in the conduction electron density36
or the dielectric function37 of metal particles, but in our case,
according to the TEM observation shown in Figure 5, it is
plausible to attribute this red shift to the aggregation of particles,
which is mainly caused by the strong dipole-dipole interaction
in the aggregates.38 The disappearance of the plasmon band of
Pt particles after longer time of aging implies that this band
may be sensitive to the surface properties of the particles. As
we know, Pt has a strong affinity for oxygen;23b aging in air
may have resulted in the oxidation of surface Pt atoms.
The optical properties of the Pt sols have been depicted using
a parameter S (S ) - d (log extinction)/d (log wavelength))
derived by Furlong et al.12c This value was found to decrease
with the decrease in S/Pt ratio (Table 3), consistent with the
discussion that S values decreased with a stronger aggregation,
and, in turn, a decrease in particle sphericity of the platinum
sol.17b On the other hand, in the experiments of Figure 6, at the
initial particle formation steps, the S values were found to
decrease with the particle growth, i.e., S values are related to
the particle size. By comparing the final S values at different
NaBH4 concentrations, S values were found to decrease at a
higher NaBH4 concentration. Since the particle sizes under these
conditions are almost the same (Table 2), these decreases should
be related to the increased particle concentrations. This shows
that the S value is a complex parameter affected by several
factors including size, morphology, aggregation state, and even
particle concentration.
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Conclusion
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Water-dispersible platinum nanoparticles have been success-
fully synthesized by the reduction of chloroplatinic acid in the
presence of mercaptosuccinic acid. Because these particles are
stabilized by thiolate, they possess good properties including
air stability, isolability, and redispersibility. The well-separated
fcc single crystal particles of 2.5 nm are obtained in water when
initial molar ratio of mercaptosuccinic acid to Pt (S/Pt) is 0.7.
The average particle diameter can be changed to 4.7 nm by
decreasing the S/Pt ratio to 0.1; at the same time, particle
aggregation occurs due to the incomplete coverage of thiolate
molecules on the particle surface. The particle formation process
is found to be a thermodynamically controlled process.
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Acknowledgment. This work was supported in part by
Grants-in-Aid for Scientific Research on Basic Research (A:
09304068) and for JSPS fellows from Ministry of Education,
Science, Sports and Culture, Japan. S.C. thanks the Japan
Society for Promotion of Science for granting the postdoctoral
fellowship. We thank the help from Prof. K. Toriumi and Dr.
Y. Ozawa in thermogravimetric analysis.
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