Adding two active silver atoms on Au25 nanoparticle

Article abstract of DOI:10.1021/nl504477t

Alloy nanoparticles with atomic monodispersity is of importance for some fundamental research (e.g., the investigation of active sites). However, the controlled preparation of alloy nanoparticles with atomic monodispersity has long been a major challenge. Herein, for the first time a unique method, antigalvanic reduction (AGR), is introduced to synthesize atomically monodisperse Au₂₅Ag₂(SC₂H₄Ph)₁₈ in high yield (89%) within 2 min. Interestingly, the two silver atoms in Au₂₅Ag₂(SC₂H₄Ph)₁₈ do not replace the gold atoms in the precursor particle Au₂₅(SC₂H₄Ph)₁₈ but collocate on Au₂₅, which was supported by experimental and calculated results. Also, the two silver atoms are active to play roles in stabilizing the alloy nanoparticle, triggering the nanoparticle fluorescence and catalyzing the hydrolysis of 1,3-diphenylprop-2-ynyl acetate.

Full text of DOI:10.1021/nl504477t

Letter
pubs.acs.org/NanoLett
Adding Two Active Silver Atoms on Au25 Nanoparticle
Chuanhao Yao,† Jishi Chen,†,‡ Man-Bo Li,† Liren Liu,§ Jinlong Yang,§ and Zhikun Wu*,†
†Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanostructures, Institute of Solid State Physics,
Chinese Academy of Sciences, Hefei 230031, China
‡Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China
§Hefei National Laboratory for Physics Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui
230026, China
S
* Supporting Information
ABSTRACT: Alloy nanoparticles with atomic monodispersity
is of importance for some fundamental research (e.g., the
investigation of active sites). However, the controlled
preparation of alloy nanoparticles with atomic monodispersity
has long been a major challenge. Herein, for the first time a
unique method, antigalvanic reduction (AGR), is introduced to
synthesize atomically monodisperse Au25Ag2(SC2H4Ph)18 in
high yield (89%) within 2 min. Interestingly, the two silver
atoms in Au25Ag2(SC2H4Ph)18 do not replace the gold atoms in
the precursor particle Au25(SC2H4Ph)18 but collocate on Au25,
which was supported by experimental and calculated results.
Also, the two silver atoms are active to play roles in stabilizing
the alloy nanoparticle, triggering the nanoparticle fluorescence
and catalyzing the hydrolysis of 1,3-diphenylprop-2-ynyl acetate.
KEYWORDS: Alloy, nanoparticles, atomic monodispersity, antigalvanic reduction, fluorescence, active sites
reactive (i.e., more noble) metal atoms, that is, the more
ontrolling the nanoparticles with atom precision has long
Cbeen a major goal for nanoscientists and chemists due to reactive metal nanoparticles act as templates for the
substitution with the less reactive metal atoms in order to
prepare nanoalloys.43,44,46,47 Because of the instant and
spontaneous occurrence of GR, it is difficult to precisely
control the reaction process at the atomic level and, until now,
no monodisperse TAN has been reported by this method. We
recently revealed an unexpected antigalvanic reduction
(AGR),45,48 which provides a facile and mild way to synthesize
nanoalloys that are otherwise difficult to obtain. Herein we
demonstrate that AGR45,48,49 can be used to synthesize
atomically monodisperse TAN, Au25Ag2(SC2H4Ph)18 (denoted
as Au25Ag2) in gram scale and high yield (89%) within 2 min.
Surprisingly, only a single-component TAN is formed, that is,
no other compositional Au−Ag TAN such as Au25Ag-
(SC2H4Ph)18 with an 8-electron closing shell50,51 is observed
in the product through wide-ranged reaction conditions (note:
Au25Ag2(SC2H4Ph)18 is of 9-shell-closing electron count based
on the superatom concept50,51). The gold atoms in
Au25(SC2H4Ph)18 (denoted as Au25, counterion: [N-
(C8H17)4]+) are not replaced by the silver atoms, which is
supported by the inductively coupled plasma atomic emission
spectrometry (ICP-AES) together with other characterizations
the significance for achieving precise structure (composition)-
property correlations and understanding some essential issues
of nanostructures (e.g., active sites in catalysis).1−7 Since the
early work by Brust et al.,8 some important advances have been
achieved on the thiolated monometallic nanoparticles (e.g., Au,
Ag nanoparticles) with atomic precision through wet chemical
synthesis in the past decade.9−27 Alloy nanoparticles exhibit
unusual structures and properties28−37 and are long pursued in
research community; however, the synthesis of thiolated alloy
nanoparticles (TAN for short) with atomic monodisper-
sity38−40 is still a major challenge, which hampers the
subsequent research of TAN on the structure, mechanism,
and so forth. To the best of our knowledge, atomically
monodisperse Au−Ag TAN (silver as the doping metal) are
rarely attained by now. The main reasons are the diversity and
similarity of the products based on the current synthesis
methods (mainly the synchro-synthesized method, that is, the
nanoalloys were formed after the mixed metals or salts were
synchronously manipulated by physical or/and chemical
protocols).30,40,41 Galvanic reduction (GR)42−47 is another
well-recognized way to engineer the compositions, structures,
and properties of metal nanostructures, but it requires the
preparation of more reactive (i.e., less noble) metal nano-
particles initially, followed by the GR step in which the metal
atoms in the as-prepared nanoparticles are replaced by less
Received: November 21, 2014
Revised: January 1, 2015
© XXXX American Chemical Society
A
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(e.g., mass spectrometry (MS) analyses). The composition,
UV/vis/NIR, and MS analyses also indicate that the structural
framework of Au25 remains in Au25Ag2 and the two silver atoms
are just deposited on Au25; computer calculation provides
further support for this. Importantly, the as-prepared TAN
(with two silver atoms enclosed) exhibit enhanced ambient
stability, fluorescence, and catalytic activity compared with the
precursor particle Au25 and the synchro-synthesized Au25−xAgx
(SC2H4Ph)18 (x = 2, 3, 4, denoted as Au25−xAgx). Below we
present more detailed information and discussions.
A facile and mild approach is developed for obtaining
Au25Ag2 (details are seen in the Supporting Information).
Briefly, in a typical experiment (note: the entire experimental
process is conducted under ambient conditions), Au25
precursor particles (39.3 mg, 0.005 mmol) were dissolved in
acetonitrile, then a freshly prepared AgNO3 (1.7 mg, 0.01
mmol) solution was added drop by drop within 2 min. Over the
period of 2 min, the solution quickly turned from red to gray-
green and a large amount of black solid precipitated out (see
inset of Supporting Information Figure S1a), indicating the
occurrence of the AGR reaction. The collected solid was
washed by excess CH3CN, then recrystallized to yield tiny cubic
crystals (Supporting Information Figure S1a). It is noteworthy
that one-batch synthesis of Au25Ag2 particles on a scale of
grams has been achieved by scaling up the reaction (Supporting
Information Figure S2). As a comparison, the mixed Au25−xAgx
(Supporting Information Figure S3) nanoparticles are also
prepared by modifying a previous method.9,20 Compared with
the synthesis of Au25−xAgx, the synthesis of Au25Ag2 from Au25
is rather facile, time-saving, high-yield, and can be reproduced
in large scale.
nm), which is also confirmed by transmission electron
microscopy (TEM) measurement, Supporting Information
Figure S1b. The absorption spectrum of the product (Figure
1a) is distinctly different from that of the synchro-synthesized
Au25−xAgx (Figure 1b) but somewhat resembles that of Au25
(Figure 1c) although some differences do exist (e.g., Au25 has
two absorption bands centered at 445 and 400 nm in the short-
wavelength visible region, while the new product has only one
peak centered at 412 nm in the same region). The absorption
differences between the product and Au25−xAgx (or Au25)
indicate that the product is not identical with Au25−xAgx (or
Au25), however the resemblances between the product and Au25
remind us that they may have some structural similarities. As
for the structure of Au25, it can be viewed as a core−shell
structure composed of an icosahedral Au13 core and a shell of
12 Au atoms that are face-capped on the Au13 core, that is,
Au13(core)/Au12 (shell).13,15,52 As far as we know, for Au25 the
lowest energy band at 685 nm corresponds to the HOMO−
LUMO transition that is due entirely to the electronic and
geometric structure of the Au13 core in Au25 skeleton, and the
other two featured bands (associated with the exterior Au12
shell in Au25 skeleton) at 445 and 400 nm are assigned to mixed
intraband (sp ← sp) and interband (sp ← d) transitions, an
interband transition (sp ← d), respectively.1,15 Thereby, the
slight shift from 685 nm (Au25) to 700 nm (product) indicates
that the Au13 core in Au25 is not influenced greatly, while the
disappearance of absorption bands at 445 and 400 nm and the
emergence of a new band at 412 nm indicate that the exterior
Au12 shell in Au25 skeleton is distinctly effected.
Matrix-assisted laser desorption ionization mass spectrometry
(MALDI-MS), which is a very powerful and well-recognized
tool to identify metal nanoclusters, is employed to analyze the
product. Interestingly, it shows a distinct peak at ∼7610 Da
(highest intensity in the mass spectrum, Figure 2). The unity
spacing of the isotopes (Figure 2 inset) implies that the ionized
clusters bear a +1 charge (acquired in the positive ion mode);
the m/z value of the peak therefore represents the molecular
ion mass. It is known that the formula weight of
Au25(SC2H4Ph)18 is ∼7394 Da.20 The deviation between
The product (dissolved in toluene) shows two distinct
absorption bands centered at 700 and 412 nm, respectively
(Figure 1a). The UV/vis/NIR spectrum of the crude product is
Figure 1. UV/vis/NIR absorption spectra of Au25Ag2 before and after
purification (a), Au25−xAgx (b), and Au25 (c). (Insets: digital photos of
the corresponding products dissolved in toluene).
indeed almost superimposable with that of the pure clusters,
indicating an extraordinarily high purity of the as-synthesized
product, which was evidenced by further characterization such
as MS analyses and elemental analyses (see below). The
absence of plasma resonance peak at ∼520 nm implies that the
nanoparticles are smaller than ∼2 nm (often called nano-
clusters, opposite to plasmonic nanocrystals larger than ∼2
Figure 2. MALDI-MS spectra of Au25Ag2(SC2H4Ph)18 (a) and
Au25(SC2H4Ph)18 (b). (Inset a: comparison of the experimental and
calculated isotope patterns; resolution, 15 000; calcd for
Au25Ag2C144H162(M)+, 7608.7406 Da; found, 7608.7197 Da; error,
2.7 ppm). Note: the mass calibration was performed using
Au25(SC2H4Ph)18 as an internal standard, the error measurement
was referred to a previous method.53
B
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Figure 3. MALDI-MS spectra of Au25Ag2 (SC2H4Ph)18 under various laser intensity (a); MALDI-MS spectra of Au25Ag2(SC2H4Ph)18 with or
without further recrystallization for twice under identical laser irradiation (25%) (b). Inset: thin-layer chromatography of Au25 (1 and 1′), Au25Ag2 (2
and 2′) and the mixture of Au25 and Au25Ag2 (3 and 3′). Note: 2, without further recrystallization; 2′, with further recrystallization.
7394 Da and the 7610 Da peaks is 216 Da, which is equal to the
mass of two Ag atoms. Upon the basis of the exact mass, the
formula can be readily deduced, that is, Au25Ag2(SC2H4Ph)18,
which is supported by the excellent agreement of the isotopic
patterns between experiment and calculation (Figure 2a inset,
the more accurate molecule ion peak is observed at 7608.7197
Da after calibration with Au25 as an internal standard). Figure
2a also shows some minor peaks in the range of m/z lower than
7000 Da. By comparing with that of Au25 (Figure 2b), it is
rationally deduced that they are fragments from Au25. Of note,
herein Au25 is also a fragment of Au25Ag2 due to three facts: (i)
the peak intensity at 7394 Da is influenced by the laser intensity
(see Figure 3a); (ii) the peak intensity does not vary notably
under identical laser irradiation after further recrystallization of
the product for twice (see Figure 3b); and (iii) thin-layer
chromatography (TLC) results also exclude the coexistence of
Au25 (see Figure 3b, inset).
To further confirm the composition and purity, both
Figure 4. TGA of Au25Ag2(SC2H4Ph)18 (a), Au25−xAgx(SC2H4Ph)18
thermogravimetric analysis (TGA) and X-ray photoelectron
spectroscopy (XPS) were performed. The ratio of metals (Au
and Ag) to organic component (ligands) can be determined by
TGA, and the atomic number ratio of Au/Ag/S can be
determined by XPS. Note that these methods rely on the
nanoparticles’ purity. If the nanoparticles are pure, these
approaches readily give rise to a precise formula.12 But for a
size-mixed sample, such approaches are less useful in
nanoparticle formula determination because these methods
only provide a mean value of the total sample. First, TGA was
performed. A weight loss of 32.6 wt % indicates that the
nanoparticles contain 32.6 wt % organic component and 67.4
wt % metal (Au and Ag), see Figure 4a. This is in perfect
agreement with the theoretical values of Au25Ag2(SC2H4Ph)18
(organic, 32.5 wt %; metal, 67.5 wt %). For comparison, the
TGA of Au25−xAgx(SC2H4Ph)18 (x = 2, 3, 4) and
Au25(SC2H4Ph)18 were conducted as well (Figure 4b,c).
Au25−xAgx are mixed nanoparticles in terms of the x values (x
= 2, 3, 4), thus 38.7 wt % is a mean value of weight loss (the
theoretical losses: for Au23Ag2, 38.2 wt %; for Au22Ag3, 38.7 wt
%; for Au21Ag4, 39.1 wt %). Of note, both Au25 and Au25−xAgx
(b), and Au25(SC2H4Ph)18 (c).
involved. The weight loss of 36.5 wt % for Au25 is in good
agreement with the theoretical value (37.3 wt %). For Au25Ag2,
in case that the counterion-N(C8H17)4 was enclosed, the
+
expected weight loss would be 36.4 wt %, which has a large
deviation from the experimental value (32.6 wt %), thus
+
N(C8H17)4 should not be present in the as-prepared
nanoparticle, and indeed XPS analysis also excludes the
existence of element N. The absence of N(C8H17)4+ counterion
indicates that Au25Ag2 is not anionic as the case of Au25. The
fact that both Cl− and Br− are not detected in the XPS
spectrum and salt tests (see Figure S4 and the Experimental
Procedure in Supporting Information) indicates that Au25Ag2 is
not cationic (with Cl− or Br− as counterion), either. Therefore,
it should be charge neutral. The XPS analysis shows that the
Au/Ag/S atomic ratio is 14.67/1.17/10.68 (i.e., 25/1.99/18.1),
which is consistent very well with the expected ratio of 25/2/18
for Au25Ag2(SC2H4Ph)18. Besides, elemental analysis provides
another strong support for the purity and composition
(calculated for Au25Ag2S18C144H162: C, 22.73; H, 2.15; S,
7.58. Found: C, 22.51; H, 2.11; S, 7.46). With respect to the
neutral state of the silver atoms in the alloy nanoparticles, XPS
+
are anionic nanoparticles (counterion: N(C8H17)4 ). When one
+
calculates the weight loss, the mass of N(C8H17)4 should be
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shows that the binding energies of Ag 3d5/2 (368.2 eV) and Ag
3d3/2 (374.2 eV) are identical to those of standard neutral silver
(368.2 eV for Ag 3d5/2, 374.2 eV for Ag 3d3/2),45 see
Supporting Information Figure S5a. A slight negative shift of
the binding energy of Ag 3d5/2 similar to the previous report35
was not found and may related to the increase of electro-
positivity of Au in Au25Ag2, see Supporting Information Figure
S5b,c; a positive 0.2 eV shift of the binding energy of Au4f was
detected from Au25 to Au25Ag2 (note: other possibilities may
also exist). Moreover, salt test involving the addition of excess
I− to the nanoparticles solution (see the Supporting
Information for experimental details) did not result in any
precipitate, excluding the presence of Ag+ enclosed in the
nanoparticles. Taken together, these experimental results
unambiguously demonstrate that the nanoparticles are pure
with a molecule-like composition, Au25Ag2(SC2H4Ph)18. To be
noted, the preparation reaction involves adding two silver
atoms on Au25 but also changing the charge state from −1 to 0.
The monodispersity of the as-prepared alloy nanoparticles
without the coexistence of other compositional TAN (e.g.,
Au25Ag(SC2H4Ph)18 with a shell-closing electronic struc-
ture50,51) is indeed a surprise, considering the previous efforts
to make pure silver-doped gold nanoparticles.30,40,41 “Size-
focusing”20,54 is a recognized strategy to attain monodisperse
nanoparticles, however, herein it should not occur because the
product was collected immediately without aging after the
completion of the addition of silver ions. The starting Ag+/Au25
molar ratio in the reaction could be a nontrivial factor that
influences the composition of the product. To investigate this,
various Ag+/Au25 molar ratios were tested for the AGR reaction
(see Supporting Information). Very surprisingly, no matter how
to tune the starting Ag+/Au25 molar ratios (from 0.1 to 7), the
alloy product is only one with unchanged composition,
Au25Ag2(SC2H4Ph)18, which is evidenced by the UV/vis/NIR
spectrum and critical MALDI-MS analyses (see Supporting
Information Figure S6). Of note, the yield of the synthetic
reaction is lower in the cases of less than 2 equiv. of silver.
The exclusive composition is another surprise for it implies
that the two incorporated silver atoms do not replace the gold
atoms in the parent nanoparticle, which is supported by the
UV/vis/NIR spectral resemblance to Au25 (Figure 1), together
with the identification of the fragment ion peak at 7394 Da
(Figure 2). ICP-AES analyses provide further support. The
concentration of gold in the supernatant after AGR reaction is
below the detection limit of ICP-AES, while in the case where
two gold atoms were replaced by two silver atoms it would be
ca. 490 ppm as expected in the preset conditions. As mentioned
above, the UV/vis/NIR spectral resemblance to Au25
(especially the spectral shape similarity in the NIR region
that is assigned to the feature absorption of Au13 core15),
together with the identification of fragment ion peak at 7394
Da, indicates that Au25Ag2 might retain the structural
framework of Au25 and the two silver atoms could be deposited
on Au25, which is further supported by the absence of fragment
containing silver atom(s) in the mass spectrum (see Figure 2)
and the similar XRD patterns of Au25Ag2 and Au25, see Figure
5. Besides, the loss of two Ag atoms from independent sites
seems very unlikely without an intermediate M-Ag (i.e.,
Au25Ag(SC2H4Ph)18) being in evidence, and the present data
cannot rule out a structure in which two silvers are collocated
on the Au25. The emerging multiple peaks (in the range from
2.5 to 4.5 ppm) assigned to methylene protons of phenyl-
ethanethiolate in 1H NMR spectrum of Au25Ag2 provides
Figure 5. XRD patterns of Au25Ag2 and Au25..
another support for the structure, see Supporting Information
Figure S7 (as a comparison, the 1H NMR spectra of Au25−/0 are
also shown). Theoretical calculation verified this possibility: the
simulated UV/vis/NIR spectrum of the proposed structure
(after optimizations, see Figure 6b) is very close to the
Figure 6. Structure of Au25 (a) and Au25Ag2 (b) and the comparison of
the experimental and calculated spectrum of Au25Ag2 (c). (Color
labels: yellow, S; green, Au; red, Ag. All H and C atoms are omitted for
clarity) .
experimental one (Figure 6c). Of course, the exact structure of
Au25Ag2 remains to be attained by single-crystal X-ray
diffraction measurement in future efforts.
Structure (composition)-property correlation for nano-
particles is a puzzling but fundamental issue that has stimulated
enormous interests to nanoscientists. For relatively large
nanoparticles (>2 nm), due to the difficulties in both
controlling the nanoparticles with atomic precision and
accessing the single crystal structure it is very difficult to
achieve precise structure (composition)-property correlation,
while for the ultrasmall nanoparticles (<2 nm) with
monodisperse composition and well-defined structure such a
correlation becomes possible. The influence of the two
enclosed silver atoms on the properties of Au25 is an intriguing
issue. The close relations among Au25, Au25Ag2, and Au25−xAgx
in composition and structure inspire us to take a comparison of
some of their properties. First, the ambient stability, which is of
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great importance for the nanoparticle synthesis, character-
ization, and practical application, of the three nanoparticle
species in dichloromethane is evaluated by monitoring the UV/
vis/NIR spectral change of the solutions exposed to air and
light after standing still for various times. All of them show
rather ambient stability as no obvious spectral change is
detected for any of them after 1 week, Figure 7. However, some
Figure 8. Fluorescence spectra of the toluene (black, background),
Au25 (red profile), Au25−xAgx (blue), and Au25Ag2 (dark cyan).
Solvent, toluene; the excitation wavelength, 514 nm.
previous computation30 and experimental results41 indicated
that Ag atoms would preferentially occupy sites on the surface
of the cluster core, but occupation of the sites within the ligand
shell was found to be energetically unfavorable, which is
completely different from the case of Au25Ag2 obtained by
AGR.
It is well-known that the catalytic activity of nanoparticles is
size-dependent, while many industrial nanocatalysts have a
nonuniform particle size distribution, which makes it difficult to
relate the observed catalytic performance to the size and
structure of individual catalyst particles. To overcome this
obstacle, monodisperse nanocatalysts with well-defined struc-
tures are highly desirable.
Figure 7. Time-dependent UV/vis/NIR absorption spectra of
dichloromethane solutions containing Au25Ag2 (a), Au25−xAgx (b),
and Au25 (c). Spectra are vertically shifted for clarity.
Gold nanoparticles (large than 2 nm) are known to be good
catalysts in various organic reactions.61,62 Recently Jin and co-
workers revealed that thiolate-stabilized, well-defined
Au25(SR)18 nanoparticles also exhibit excellent catalytic
performance to the chemoselective hydrogenation (100%) of
α,β-unsaturated ketones and aldehydes because the electron-
rich Au13 core in Au25 facilitates adsorption and activation of
CO bond and H2.63 These intriguing results inspire us to
investigate the possibility for Au25Ag2 to catalyze the hydrolysis
of carboxylic esters, which is a classic organic reaction and plays
important roles in many biochemical and industrial pro-
cesses,64−66 such as the digestion of polysaccharides and the
preparation of soap. Lots of carboxylic esters can be hydrolyzed
smoothly under rather mild conditions, however, the hydrolysis
could be seriously retarded by the steric hindrance67 or the
stabilizing of adjacent group(s) of the ester bond, for example,
the hydrolysis of 1,3-diphenylprop-2-ynyl acetate is only with a
7% conversion after 2 h reaction at room temperature (other
conditions in Table 1 or Supporting Information). Interest-
ingly, Au25Ag2 can remarkably accelerate the hydrolysis of 1,3-
diphenylprop-2-ynyl acetate, and a 52% conversion is achieved
in the same conditions. What’s more, owing to the ultrastability
(see above), the catalyst can be recovered by centrifugation
without decomposing, which is evidenced by the monitored MS
spectrum and UV/vis/NIR spectra, see Supporting Information
Figure S8. As a comparison, the catalysis of Au25, Au25+, and
Au25−xAgx were also tested and all of them somehow showed
catalytic activity for that the conversion in hydrolysis under the
same conditions can be improved to 11, 15, and 17%,
respectively (see Table 1). As mentioned in our previous
differences between samples can be detected with longer time
(e.g., 15 days). Specifically, hardly any change is observed in the
spectrum of Au25Ag2 after 15 days, while for Au25−xAgx the 435
nm peak becomes more prominent and red shifts to 445 nm
and the absorption peaks at 705 and 470 nm almost disappear.
For Au25, the 400 nm peak becomes more prominent while the
450 nm peak turns to less prominent concomitantly, and the
800 nm shoulder peak that is characteristic of the anionic Au25
nanoparticles disappears, indicating that the anionic Au25
nanoparticles are oxidized to the neutral ones.55 These results
clearly demonstrate that Au25Ag2 possesses the highest ambient
stability among the three species.
The fluorescence of metal nanoparticles has long been an
intriguing topic that has drawn considerable research
interest.43,56−59 The well-defined ultrasmall nanoparticles
compared with their larger counterparts provide more
opportunities for the insightful investigation of some intriguing
issues. Our previous work explicitly showed that the surface
ligands play a major role in enhancing the fluorescence of gold
nanoparticles, and the electropositivity of the metal core also
greatly influences the fluorescence.60 Herein we additionally
find that the doping modes also distinctly impact the
fluorescence of nanoparticles. After ∼3 gold atoms in Au25
are replaced by silver atoms, the maximum emission peak of the
resulted nanoparticles (Au25−xAgx) red shifts by ∼15 nm and
becomes sharper compared to that of Au25, see Figure 8; when
two silver atoms are deposited on Au25, the maximum emission
peak of the resulted nanoparticles (Au25Ag2) appears at ∼760
nm. Also, the quantum yield (QY) of Au25Ag2 increases by
∼3.5-fold compared with that of Au25 (in the case of Au25−xAgx,
the QY is almost equal to that of Au25). These results clearly
demonstrate that the doping mode of silver atoms has a
different influence to the fluorescence of Au25. For Au25−xAgx,
work,60 Au25 possesses high electropositivity compared with
+
that of negative Au25, however, it does not exhibit any notable
increase in catalytic activity, indicating that the electropositivity
of the metal core is not a key in the catalysis. The replacement
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nanoparticles, the Au25Ag2(SC2H4Ph)18 nanoparticles exhibit
distinctive optical properties, superior ambient stability, and
enhanced catalytic activity. Herein, our research demonstrates
the possibility and significance of preparing atomically
monodisperse bimetal nanostucture and introducing active
sites for metal nanostructure by AGR. The importance and
novelty of our work lies in: (i) Antigalvanic reduction (AGR) is
first introduced to synthesize atomically monodisperse alloy
nanoparticles. (ii) For the first time, the atomically mono-
disperse alloy nanoparticles are obtained in large scale (gram)
and high yield (89%) within a very short time (2 min). (iii) The
as-prepared bimetal nanoparticles are of unique composition
and structure compared with those synthesized by other
methods (such as synchro-synthesized method and Galvanic
reduction method). (iv) The as-prepared bimetal nanoparticles
by AGR exhibit some promising properties (including optical
and catalytic properties) for practical application compared
with the synchro-synthesized bimetal nanoparticles or the
monometallic nanoparticles. (v) Our work illustrates that the
bimetal nanoparticles prepared by AGR may provide ideal
models for investigation on active sites. It is expected that our
work may trigger more future work to control nanoalloys at the
atomic level to achieve precise structure (composition)-
property correlations and to understand some fundamental
science of alloy nanostructures.
Table 1. Catalytic Performance of Some Gold (Silver)
Catalysts for the Hydrolysis of 1,3-Diphenylprop-2-ynyl
Acetate
a
b
entry
catalyst
none
additive
yield (%)
1
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
7
11
17
18
52
9
−
2
Au25
Au25
c
+
3
4
Au25−xAgx
Au25Ag2
AuCl
5
6
7
AgNO3
8
8
AuCl+AgNO3
Au(PPh3)Cl
HAuCl4
Ag152
10
10
8
9
10
11
12
13
14
34
12
45
15
Au38
∼3 nm Ag
∼3 nm Au
a
Reaction conditions: 1,3-diphenylprop-2-ynyl acetate (0.10 mmol),
catalyst (0.2 mol %), K2CO3 (0.05 mmol), solvent (2.0 mL,
b
c
DMF:H2O = 10:1), temperature (25 °C). Isolated yield. Obtained
by a previous method.45
ASSOCIATED CONTENT
* Supporting Information
■
of gold sites by silver atoms does not cause a dramatic
improvement of the catalytic activity, either (see Table 1). The
greatly enhanced catalytic activity of Au25Ag2 compared with
Au25−xAgx indicates the two silver atoms on Au25 play a critical
role in the catalytic hydrolysis of 1,3-diphenylprop-2-ynyl
acetate. To further confirm this and have a better under-
standing on what is the active species, more catalytic systems
including HAuCl4, AuCl, Au(PPh3)Cl, AgNO3, combination of
S
Experimental section, supplemental analytical data (MS, XPS
and UV/vis/NIR spectra, TEM and SEM images, and digital
photos). This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
AuCl and AgNO3 (molar ratio:1:1), Au38, ∼3 nm Au, Ag152
,
*E-mail: zkwu@issp.ac.cn.
and ∼3 nm Ag were tested. The results revealed that Au25Ag2
exhibits the highest catalytic activity among all the investigated
catalysts, see Table 1. The gold species in various charge state
(0, +1, +3) cannot work well in this reaction system, and the
leaching of active gold(0) species from Au25Ag2 are not
detected by MS and XPS analysis. Obviously, for silver the
neutral species (including Ag152 and ∼3 nm Ag particles) show
higher catalytic activity than the cationic one. All the
experimental facts unambiguously demonstrate that the
incubated two silver atoms in Au25Ag2 play very important
role in the catalysis and may act as the so-called “active sites”,
which is long-pursued but difficult to “touch” due to the
unraveled composition and structure of some previous
catalysts.68−71 Our work illustrated that AGR could be a
strategy to introduce active sites for nanocatalysts.
In this work, we have developed a unique strategy for facile
synthesis of molecule-like bimetal nanoparticle in gram scale
and high yield (89%) within 2 min. The formula is determined
to be Au25Ag2(SC2H4Ph)18 by MALDI-MS in conjunction with
XPS, TGA, and other detailed analyses. Surprisingly, no trace
amount of other compositional Au−Ag TAN including
Au25Ag(SC2H4Ph)18 (with shell closing electronic structure)
are observed under wide-ranged reaction conditions. UV/vis/
NIR, MS, ICP-AES, and computer calculation indicate that the
two silver atoms in Au25Ag2 do not replace the gold atoms in
the Au25 but instead deposit on Au25. In comparison with
Au25(SC2H4Ph)18 and Au25−xAgx(SC2H4Ph)18 (x = 2, 3, 4)
Author Contributions
C.Y., J.C., and M.-B.L. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Z.W. would like to thank National Basic Research Program of
China (Grant 2013CB934302), the Natural Science Founda-
tion of China (No. 21222301, 21171170), the Ministry of
Human Resources and Social Security of China, the Innovative
Program of Development Foundation of Hefei Center for
Physical Science and Technology (2014FXCX002), the CAS/
SAFEA International Partnership Program for Creative
Research Teams and the Hundred Talents Program of the
Chinese Academy of Sciences for financial support.
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