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Z. Yang, K.J. Klabunde / Journal of Organometallic Chemistry 694 (2009) 1016–1021
3.4. The cooperative protecting effects of oleylamine
As mentioned previously, the main function of oleylamine in
this system was to reduce Pd(II) ions. Without TOP added, only
aggregated Pd nanoparticles were obtained even when the molar
ratio of oleylamine to Pd(II) reached to 150. But further studies
indicated that, addition of an appropriate amount of oleylamine
as a supplementary capping ligand was very important and neces-
sary even when TOP was added. The experiments were carried out
according to Sample (0.6ꢀ) except changing the concentration of
oleylamine. In Sample (0.6ꢀ), the molar ratio of oleylamine to
Pd(II) was set as 15 arbitrarily. When the molar ratio was de-
creased to 3, only aggregated Pd(0) particles were generated
(Fig. 7a). However, when the molar ratio was increased to 60,
well-protected Pd(0) nanoparticles were obtained with a little big-
ger size but narrower distribution (size: 11.6 1.4 nm, SD = 12%)
compared with Sample (0.6ꢀ) (Fig. 7b. Also see Supplementary
material, Fig. S5 for the histogram of the size distribution). Further-
more, this sample has been found to be quite stable toward long-
term digestive ripening. After boiling for 17 h, a typical TEM pic-
ture shows Pd nanoparticles with an average diameter of
14.9 1.2 nm (SD = 8%). A close-up TEM picture indicates that,
when the size increases, it is more likely to form quasi-spherical
particles with certain crystal facets (Fig. 8. Also seeSupplementary
material, Fig. S5 for the histogram of the size distribution).
4. Conclusions
In this paper, we report the influence of TOP ligand on the syn-
thesis of Pd nanoparticles by oleylamine-induced reduction of
Pd(II) ions. Without TOP, only aggregated Pd particles were gener-
ated. When an appropriate amount of TOP was added, nearly
monodisperse Pd nanoparticles could be obtained. But further
increasing the molar ratio of TOP/Pd(II) to 2 resulted in the forma-
tion of Pd(II)–TOP coordination complexes. Also, the function of
oleylamine as a supplementary ligand was discussed, which will
be helpful for understanding the cooperation of mixed protecting
ligands in certain systems.
Fig. 8. TEM images of the sample prepared according to Sample (0.6ꢀ) except
increasing the molar ratio of oleylamine to Pd(II) to 60 and heating for 17 h. (A)
Small magnification to show the hexagonal close-packing pattern in short range. (B)
Large magnification to show the faceted quasi-spherical shape. Scale bar = 100 nm.
Acknowledgements
inadequate, some Pd(II) ions were consumed to form Pd(II)–TOP
complexes, while the remaining Pd(II) ions would still be reduced
by oleylamine. Since well-protected particles were generated in
these cases, the chemical binding of TOP (from the free Pd(II)–
TOP complexes in solution) on the as-synthesized Pd(0) particles
is assumed. So, some TOP ligands were consumed to protect the
Pd nanoparticles, while the others still remained in solution as
Pd(II)–TOP complexes. Fig. 6 shows the UV–Vis absorbance of the
solutions after heating. A peak at 318 nm was found for all the
samples except Sample (0ꢀ).
We thank the Biology Department of Kansas State University for
the TEM facilities. We gratefully acknowledge the partial support
of NSF, as well as the United States Marine Corps System Command
to M2 Technologies, Inc., KY.
Appendix A. Supplementary material
The different appearance of the as-synthesized samples; the
histograms indicating the size distribution; the UV–Vis absorbance
of Sample (1.0ꢀ) after wash; the UV–Vis absorbance to show the
generation of Pd(II)–TOP complexes at room temperature. Supple-
mentary data associated with this article can be found, in the on-
According to the above discussion, the influence of different
amounts of TOP on the final products is systematically shown in
Scheme 1. It should be pointed out, for collecting Pd(0) nanoparti-
cles, the Pd(II)–TOP complexes could be removed by washing with
acetone.
References
A similar influence of thiol ligands on a Pd(II)/octylamine sys-
tem was reported previously [24]. With increasing thiol concentra-
tion, the final products changed from sulfur-rich Pd nanoparticles
to a tiara-like Pd(II)–thiolate complex. Another similar example
was reported regarding the influence of oleic acid on the prepara-
tion of Co(0) nanocrystals [27]. Increasing the concentration of
oleic acid also resulted in the transformation of Co(0) nanoparticles
to Co(II) cluster compounds. The results described herein add to
our understanding of these sensitive chemical processes that are li-
gand-controlled.
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