Y. Xia et al.
precursors to the metals, respectively. The concentration of NVP in the
reaction for Ag, Pd, and Pt was 7.84m, 4.54m, and 3.25m, respectively.
The reactions were conducted at 1008C over 15 min for Ag, and 28 h for
both Pd and Pt.
Conclusions
We have successfully demonstrated a water-based protocol
for the preparation of noble-metal nanoparticles in aqueous
solutions. In a typical synthesis, an aqueous solution of the
salt precursor was quickly added to an aqueous NVP solu-
tion at 1008C and then reacted for a certain period of time.
NVP was shown to be a precursor to an effective reductant
and capping agent for generating uniform Au icosahedra of
about 18 nm in diameter in high yields. Other water-soluble
vinyl compounds such as NIPAAm and undecylenic acid
sodium salt also gave fast reduction of HAuCl4, but resulted
in Au nanoparticles with less controllable shapes. How does
the capping agent contribute to the formation of icosahedral
shape is still not very clear at the moment and related stud-
ies are currently underway. Most likely, the control of shape
was determined by the reduction kinetics. Also, the concen-
tration of HAuCl4 was found to play an important role in
the formation of well-defined Au icosahedra.
Characterization: For SEM and TEM characterization, a drop of the
nanocrystal suspension (in water) was placed on a piece of silicon sub-
strate (for SEM) or carbon-coated copper grid (Ted Pella, for TEM) and
dried under ambient conditions in the fume hood. After that, the sample
was transferred into a gravity-fed flow-cell and washed for 30 min with
deionized water to remove the remaining NVP. SEM images were taken
using an FEI field-emission microscope (Nova NanoSEM 230) operated
at an accelerating voltage of 15 kV. TEM images were recorded with FEI
Tecnai G2 Spirit transmission electron microscope operating at 120 kV.
High-resolution TEM (HRTEM) images were taken using a JEOL 2100F
microscope operated at 200 kV. For MS analysis, 0.5 mL of the final prod-
uct was injected into the Agilent 6850 series II network gas chromato-
graph (based on the Agilent HP-5 ms column), and analyzed using Agi-
lent 5975C inert mass spectrometry with triple-axis detector.
Acknowledgements
Our mechanistic study based on mass spectrometry
showed that the final products in the supernatant of a reac-
tion exhibited the formation of carboxylic acid containing
molecules. Therefore, in order to generate acid containing
final products, the water, the oxygen source in the reaction
media, is essential in the reduction of HAuCl4. The water
molecule attacks to the AuIII activated vinyl group in the
monomers. This nucleophilic addition of water to the double
bond leads to the generation of a AuI alcohol-based inter-
mediate. This intermediate then undergoes a redox reaction
in which AuI is reduced to Au atoms. We also extended the
reduction process to other noble metals such as Ag, Pd, and
Pt and obtained nanoparticles of these metals with various
morphologies.
This work was supported in part by a research grant from the NSF
(DMR-0804088) and a 2006 Directorꢁs Pioneer Award from the NIH
(DP1 OD000798–04). This work was performed in part at the Nano Re-
search Facility (NRF) a member of the National Nanotechnology Infra-
structure Network (NNIN), which is supported by the NSF under ECS-
0335765. NSF is part of th School of Engineering and Applied Science at
Washington University in St. Louis.
2, 657–668; b) S. R. Sershen, S. L. Westcott, N. J. Halas, J. L. West, J.
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1506; b) J. J. Storhoff, A. D. Lucas, V. Garimella, Y. P. Bao, U. R.
829–834; b) P. Fortina, L. J. Kricka, D. J. Graves, J. Park, T. Hyslop,
Experimental Section
Chemicals: N-vinyl pyrrolidone, N-isopropyl acrylamide, N-ethyl pyrroli-
done, and undecylenic acid sodium salt were all obtained from Acros and
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mura, S. Tsubota, M. Haruta, Angew. Chem. 2004, 116, 2181–2184;
Angew. Chem. Int. Ed. 2004, 43, 2129–2132; d) L. M. Molina, M. D.
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[9] M. Faraday, Philos. Trans. R. Soc. London 1857, 147, 145–181.
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used without further purification. Hydrogen tetrachloroaurateACHTNUTGRNEUNG(III) hy-
drate (HAuCl4), silver nitrate (AgNO3), hexachloroplatinic(IV) acid
(H2PtCl6), and sodium tetrachloropalladate(II) (Na2PdCl4) were all ob-
tained from Aldrich and used as received. The water used in all experi-
ments was deionized water with a resistivity of 18 MW, which was pre-
pared using an ultrapure water system (Aqua Solutions).
Synthesis of Au nanocrystals: In a typical reaction, 4 mL of the aqueous
solution (90 mm) containing N-vinyl pyrrolidone, N-isopropyl acrylamide,
or undecylenic acid sodium was added to a 25 mL vial equipped with a
teflon-coated magnetic stir bar. The vial was inserted into an oil bath (ca.
1008C) and preheated for 5 min at a stirring rate of 280 rpm. Then, 50 mL
of an aqueous HAuCl4 solution was rapidly added to the vial with a pip-
ette, and the concentration of the HAuCl4 in the final reaction solution
was 0.10 mm. The reaction mixture was stirred for another 15 min. After
that, the product was collected by centrifugation, and then washed twice
with water to remove excess vinyl compound and by-products. Finally,
the precipitate was re-dispersed in water for further characterization.
Synthesis of Ag, Pd, and Pt nanocrystals: The procedure was essentially
the same as the one used for synthesizing Au nanocrystals (see the above
section). Ag, Pd, and Pt nanocrystals were prepared with 26.14 mm
AgNO3, 15.12 mm Na2PdCl4, and 10.84 mm H2PtCl6 aqueous solutions as
13186
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Chem. Eur. J. 2009, 15, 13181 – 13187