.
Angewandte
Communications
DOI: 10.1002/anie.201107822
Metal Nanoparticles
Anti-Galvanic Reduction of Thiolate-Protected Gold and Silver
Nanoparticles**
Zhikun Wu*
Among the many techniques to engineer the compositions,
structures, and properties of metal nanostructures, galvanic
reduction (GR) received particular interest owing to its high
tunability and feasibility, in particular the possibility to
mechanistically study the properties of nanostructures.[1,2]
GR occurs spontaneously when the atoms of one metal
touch ions of another metal that has a higher electrochemical
potential in solution. The metal atoms are oxidized and enter
the solution phase, while the metal ions are reduced and
deposited on the surface of the metal template, in simple
terms, metals can reduce less reactive (or more noble) metal
ions in solution. This simple reaction can be used with a wide
variety of metal templates and salt precursors, thus providing
a straightforward and versatile route to access a broad range
of nanostructures (including simple and complex nanostruc-
tures).[3–9] However, the opposite of GR (anti-galvanic
reduction, AGR), that is, metal ions are reduced by less
reactive (or more noble) metals, is not recognized. Recently,
Murray and co-workers revealed that a well-studied gold
nanocluster, [Au25(SC2H4Ph)18], can react with silver ions,[10]
thus suggesting that AGR could occur. It is well-known that
Au25 has multiple charge states, even the anionic and neutral
species were unambiguously confirmed by single-crystal X-
ray diffraction analyses.[11,12] To exclude the possibility that
anionic [Au25(SC2H4Ph)18] has a particular reducing ability,
the reaction of neutral [Au25(SC2H4Ph)18] with silver ions was
tried. Furthermore, such replacement on several gold and
silver nanoparticles of different sizes was also investigated.
The positive results demonstrate the definite occurrence of
AGR for small nanoparticles that are protected by thiolates,
and these interesting results are presented herein.
the mass range from 4000 to 7000 Da, and the comparison of
experimental and theoretical isotopic patterns also confirms
the assignment of bimetal cluster ions (Figures 1A–C), thus
indicating that the replacement between the silver ions and
neutral Au25 occurs. X-ray photoelectron spectroscopy (XPS,
Figure 1D) further confirms such reduction: the binding
energies of 368.2 (assigned to Ag3d5/2) and 374.2 eV (assigned
to Ag3d3/2) match very well that of pure silver (Ag3d5/2: 368.2;
Ag3d3/2: 374.2 eV). Gold is less reactive (or more noble) than
silver, deduced from the electrochemical potential, and it is
anticipated that silver metal can reduce gold(III) ions, but the
reduction of silver ions by gold metal is difficult in ambient
environment according to the galvanic theory. Indeed it was
found that silver ions were reduced by Au25, and the gold
atoms in Au25 were replaced by silver. Such a finding is very
surprising, because it is against the classic galvanic theory and
was rarely reported before. For convenience, this reaction was
named anti-galvanic reduction (AGR). To exclude the
possibility that the ability to undergo AGR is a unique
property of Au25, the reactions of approximately 2 and 3 nm
sized gold nanoparticles with silver ions were also inves-
tigated. The approximately 2 nm sized nanoparticles were
separated from the residual in the synthesis of Au25, and the
approximately 3 nm sized nanoparticles were synthesized by
a modified Brust method[14] (for details see the Experimental
Section). Note that the approximately 2 nm sized nano-
particles do not show the plasma resonance absorption,
whereas the approximately 3 nm sized nanoparticles start to
exhibit the plasma peak less prominent than approximately
4 nm sized or even bigger nanoparticles; see Figure 2C and
our previous work[15] for a comparison. After the approx-
imately 2 and 3 nm sized nanoparticles were treated with
silver nitrate in a similar way as Au25 was treated, silver was
detected by XPS (see Figure 2D). The Au/Ag atom ratios
indicated by XPS analyses are 7.0:1 in the case of the
approximately 2 nm sized Au nanoparticles and 5.3:1 in the
case of the approximately 3 nm sized Au nanoparticles (the
starting nanoparticles/AgNO3 weight ratios were both 5.2:1).
The binding energies of 368.2 and 374.2 eV (assigned to
Ag3d5/2 and Ag3d3/2, respectively) indicate that the incorpo-
rated silver is neutral, that is, the silver ions can be reduced by
approximately 2 or 3 nm sized gold nanoparticles as well,
which again demonstrates the occurence of AGR. To further
test the universality, the approximately 3 nm sized silver
nanoparticles that have the same protecting ligand, phenyl-
ethanethiolate, were tested. The nanoparticles were obtained
by a method similar to that for the synthesis of [Au25-
(SC2H4Ph)18][16] (note: the unstability of silver nanoclusters
results in their growth to approximately 3 nm).
First, the reaction product of the reaction between neutral
[Au25(SC2H4Ph)18] and silver ions was investigated by laser
desorption and ionization (i.e. no matrix, LDI) mass spec-
trometry. In mass spectra of Au25 the most abundant species
detected is [Au25S12]À(author note: the charge state of gaseous
ion is -1).[13] However, for the product of the reaction of
[Au25(SC2H4Ph)18] with 9.0 equivalents Ag+, the mass spectra
show that bimetal cluster ions, such as [Au22Ag3S12]À and
[Au23Ag2S12]À instead of [Au25S12]À, are abundant species in
[*] Prof. Dr. Z. Wu
Key Laboratory of Materials Physics
Anhui Key Laboratory of Nanomaterials and Nanotechnology
Institute of Solid State Physics, Chinese Academy of Sciences
Hefei 230031(P.R. China)
E-mail: zkwu@issp.ac.cn
[**] This work is supported by the Hundred Talent Program of the
Chinese Academy of Sciences and the Natural Science Foundation
of China (Grant no. 21171170).
2934
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 2934 –2938