Ligand-stabilized Au13Cux (x = 2, 4, 8) bimetallic nanoclusters: Ligand engineering to control the exposure of metal sites

Article abstract of DOI:10.1021/ja402249s

Three novel bimetallic Au-Cu nanoclusters stabilized by a mixed layer of thiolate and phosphine ligands bearing pyridyl groups are synthesized and fully characterized by X-ray single crystal analysis and density functional theory computations. The three clusters have an icosahedral Au₁₃ core face-capped by two, four, and eight Cu atoms, respectively. All face-capping Cu atoms in the clusters are triply coordinated by thiolate or pyridyl groups. The surface ligands control the exposure of Au sites in the clusters. In the case of the Au₁₃Cu₈ cluster, the presence of 12 2-pyridylthiolate ligands still leaves open space for catalysis. All the 3 clusters are 8-electron superatoms displaying optical gaps of 1.8-1.9 eV. The thermal decomposition studies suggest that the selective release of organic ligands from the clusters is possible.

Full text of DOI:10.1021/ja402249s

Communication
pubs.acs.org/JACS
Ligand-Stabilized Au Cu (x = 2, 4, 8) Bimetallic Nanoclusters: Ligand
13
x
Engineering to Control the Exposure of Metal Sites
†
†
†
‡
‡
§
†
‡
Huayan Yang, Yu Wang, Jing Lei, Lei Shi, Xiaohu Wu, Ville Makinen, Shuichao Lin, Zichao Tang,
̈
∥
§
†
,†
Jian He, Hannu Hak
†
̈
kinen, Lansun Zheng, and Nanfeng Zheng*
State Key Laboratory for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials,
and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
‡
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian
16023, China
1
§
Departments of Physics and Chemistry, Nanoscience Center, University of Jyvas
̈
kyla,
Institute of Physics and Mechanical and Electrical Engineering, Xiamen University, Xiamen 361005, China
S Supporting Information
̈ ̈ ̈
FI-40014 Jyvaskyla, Finland
∥
*
structure is essentially an Au icosahedral cluster face-capped by
13
ABSTRACT: Three novel bimetallic Au-Cu nanoclusters
stabilized by a mixed layer of thiolate and phosphine
ligands bearing pyridyl groups are synthesized and fully
characterized by X-ray single crystal analysis and density
functional theory computations. The three clusters have an
icosahedral Au13 core face-capped by two, four, and eight
Cu atoms, respectively. All face-capping Cu atoms in the
clusters are triply coordinated by thiolate or pyridyl
groups. The surface ligands control the exposure of Au
1
2 Au atoms. An Au icosahedron has 20 triangular Au faces.
1
3
3
Structurally, the number of metal atoms face-capping the
triangular Au faces could be varied from 0 to 20. Very recently,
3
we have found that thiolated Ag nanoclusters do not necessarily
have RS(MSR) units on their surfaces due to the following two
x
2
9,30
structural features:
(1) Surface Ag atoms do not need to take
the linear bicoordination; and (2) when coordinated to Ag,
thiolate can bridge more than two metal atoms. We thus believe
that the combined use of Au and other metals would significantly
enrich the structure of thiolated Au-containing nanoclusters. The
goal of this work was to prepare thiolated metal nanoclusters
sites in the clusters. In the case of the Au Cu cluster, the
1
3
8
presence of 12 2-pyridylthiolate ligands still leaves open
space for catalysis. All the 3 clusters are 8-electron
superatoms displaying optical gaps of 1.8−1.9 eV. The
thermal decomposition studies suggest that the selective
release of organic ligands from the clusters is possible.
containing Au13 icosahedron with <12 triangular Au faces
3
capped by metal atoms. The achievement of this goal would
eventually provide an effective way to tune the electronic
properties and expose different number of Au sites for catalysis.
We report our success in preparing three Au-Cu bimetallic
nanoclusters, Au Cu , Au Cu and Au Cu , having icosahedral
13
2
13
4
13
8
1
ince the development of the Brust synthesis in 1994, thiol-
Au13 core decorated by two, four, and eight Cu atoms,
respectively. The total structures of the three clusters were
determined by X-ray single-crystal analysis. The number of face-
capping Cu atoms is determined by the available binding sites
offered by the 12 surface ligands. The surface ligands also control
the exposure of Au sites in the clusters and thus their catalysis.
Density functional theory (DFT) calculations were used to
explain the stability of the clusters, analyze the electronic
S
stabilized metal nanoparticles are a class of stable metal
nanomaterials that have found applications in many areas, such as
2
−7
biology, sensing, nanotechnology and catalysis.
For many
applications, the fine-tuning of composition and surface
4
,5
modification of the metal nanoparticles is important. Under-
standing the interfacial structure between metal core and thiol
ligands is thus crucial. With the structure determination of several
8
9,10
11
12
31
Au nanoclusters (i.e., Au102, Au , Au , and Au36 ) by X-
structure in terms of the superatom model, and correlate the
25
38
ray single crystal analysis, the past several years have witnessed
significant progress toward understanding the interfacial
structure of thiolated Au nanoclusters. It is found that
electronic structure to the measured UV/vis absorption spectra.
The syntheses of Au13Cu
reduction of Au and Cu species by NaBH
and Au13Cu
involved the chemical
using a two-phase
2
4
4
RS(AuSR) (x = 1, 2) oligomeric units are commonly present
method in the presence of both thiol and phosphine as surface
capping ligands (see Supporting Information (SI) for details). X-
x
13−15
on the surface of thiolated Au nanoclusters.
Based on such
ray single crystal analysis reveals that Au Cu and Au Cu are
motif, various structures of thiolated Au and even Ag containing
13
2
13
4
16−28
crystallized in monoclinic space groups P-1, and C2/c,
nanoclusters have been computationally predicted.
Au25
respectively. As illustrated in Figure 1, the compositions of
and Au38 represent the most successful examples whose
+
18−20
Au Cu and Au Cu clusters are [Au Cu (PPh ) (SPy) ] (Py
=
structures were predicted
firmed.
and later experimentally con-
13
2
13
4
13
2
3 6
6
+
9
−11
2-pyridyl) and [Au Cu (PPh Py) (SC H -tert-C H ) ] .
13 4 2 4 6 4 4 9 8
Both Au Cu and Au Cu have an icosahedral Au as their
However, modification of the metal-ligand interface of
13
2
13
4
13
stabilized gold nanoclusters to display other structural units
than RS(AuSR) may require the use of nonthiolate ligands and
Received: March 4, 2013
x
other metals than gold. Taking thiolated Au as an example, its
2
5
©
XXXX American Chemical Society
A
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Journal of the American Chemical Society
Communication
Figure 2. Local coordination structures of Cu atom in
+
[
Au Cu (PPh ) (SPy) ] (a) and [Au Cu (PPh Py) (SC H -tert-
1
3
2
3
6
6
13
4
2
4
6
4
+
C H ) ] (b). Color legend: gold sphere, Au; green sphere, Cu; yellow
4
9 8
sphere, S; pink sphere, P; gray stick, C; blue stick, N.
(
one each) bind to the six Au atoms from the two Au triangles,
3
leaving six pyridyl motifs available to hold two Cu atoms at the
opposite sides of the cluster. Each Cu in Au Cu is thus
1
3
2
coordinated by three pyridyl groups (Figure 2a). In comparison,
the surface Au atoms of the Au unit in Au Cu are protected by
+
Figure 1. Crystal structures of [Au Cu (PPh ) (SPy) ] (a,b) and
13
13
4
13
2
3
6
6
+
[
Au Cu (PPh Py) (SC H -tert-C H ) ] (c,d) clusters. (b,d) Struc-
four PPh Py and eight 4-tert-butylbenzenthiol ligands. Both thiol
2
13
4
2
4
6
4
4
9 8
tures showing only metal atoms. Color legend: gold sphere, Au; green
sphere, Cu; yellow sphere, S; pink sphere, P; gray stick, C; blue stick, N.
and PPh Py serve as bidentate ligands. While every PPh Py binds
2
2
to Au using its P site and Cu using its pyridyl motif, each 4-tert-
butyl-benzenthiolate is doubly bridged between Au and Cu
atoms. In Au Cu , each Cu capping on the Au core is triply
All H atoms in both clusters and tert-butyl groups in
+
[
Au Cu (PPh Py) (SC H -tert-C H ) ] are omitted for clarity.
13
4
2
4
6
4
4
9 8
13
4
13
bound by two thiolate motifs and by one pyridyl (Figure 2b). The
triple coordination of Cu atoms in both Au Cu and Au Cu is
quite different from those for the Ag atoms tethered to the
−
core, similar to the reported Au (SR) clusters. It should be
2
5
18
13
2
13
4
noted that although doping Au clusters with other metals (e.g.,
Pd, Pt, Ag, Cu) has been well-documented,
challenging to decorate the outer surface of Au13 clusters with
1
3
32−34
it still remains
underlying M facets found in phosphine-capped Au Ag and
3
18
20
3
6,37
Au Ag clusters.
The tethered Ag sites in those two clusters
18
19
nongold metal atoms to form well-defined bimetallic clusters. An
are tetrahedrally coordinated by four halide anions.
Au13 icosahedron has 20 triangular Au faces. In the reported
In short, Au Cu and Au Cu have the following two
3
13
2
13
4
−
Au (SR) cluster, 12 of the 20 Au faces are face-capped by Au
common structural features: (1) There are a total number of 12
phosphine and thiolate ligands binding to the 12 apex sites of the
25
18
3
atoms. In Au Cu , the Au core is face-capped by only two Cu
13
2
13
(
Figure 1b). The two Cu atoms in Au Cu are oriented along
icosahedral Au core through Au-P and Au-S interactions; and
1
3
2
13
one of the 3-fold axes of the icosahedral Au13 with Au-Cu
distances between 2.775 and 2.886 Å (averaged at 2.824 Å). In
comparison, each Au Cu has the Au core face-capped by four
(2) Cu atoms are face-capping triangular Au faces of the Au
3
13
core and tricoordinated by three pyridyl/thiolate motifs. The
number of face-capping Cu atoms depends on the available
coordination motifs from the 12 surface ligands. In Au Cu , the
1
3
4
13
Cu atoms (Figure 1d). The four Cu atoms in each Au Cu are
13
4
13
2
located in a tetrahedral configuration with Au-Cu distances
binding of six PPh and six 2-pyridinethiolate ligands on Au
3
1
3
between 2.785 and 3.165 Å (averaged at 2.973 Å). In the Au13
leads to six pyridyl motifs available for Cu. Such a ligand
arrangement yields two positions to hold face-capping Cu atoms
on the Au13 core. In Au Cu , however, the binding of four
−
core of Au (SR) cluster, it should be noted that the average
25
18
Au-Au distance from the central atom to the surrounding 12 Au
atoms is ∼3.8% shorter than the Au-Au distance (∼2.884 Å) in
bulk Au. Such a structural feature of Au-Au bond contraction is
also revealed in Au Cu and Au Cu , although their Au cores
13
4
PPh Py and eight 4-tert-butyl-benzenthiolate ligands leaves 12
2
pyridyl and thiolate coordinating motifs available for holding four
surface-capping Cu atoms.
1
3
2
13
4
13
are capped by different numbers of Cu atoms. In the Au cores of
Based on the above structural analysis on Au Cu and
13
13
2
Au Cu and Au Cu , the Au-Au distances from the central atom
Au Cu , it is expected that the bimetallic Au-Cu nanocluster
13
2
13
4
13 4
are averaged at 2.771 and 2.757 Å, respectively. Compared with
bulk Au, up to 4.4% of Au-Au contraction is observed in Au Cu .
with Au core face-capped by different number of Cu atoms can
13
be prepared by changing the surface ligands appropriately.
Experimentally, we have thus attempted to prepare Au-Cu
nanoclusters with more face-capping Cu atoms. By increasing the
Cu/Au ratio and using 2-pyridinethiol in the synthesis, we
13
4
The presence of RS(AuSR) (x = 1, 2) units is always revealed
x
in the determined structures of thiolated Au nanoclusters.
However, such structural units are not present in Au Cu and
13
2
+
Au Cu (Figure 1a,c). In both clusters, all of the 12 outer Au
successfully prepared [Au Cu (PPh Py) ] cluster (Au Cu )
13
4
13
8
2
12
13
8
atoms of the Au core are passivated by either a phosphine or a
having Au core face-capped by eight Cu atoms (Figure 3a). In
13
13
thiolate ligand. In Au Cu , the Au icosahedron is passivated by
each Au Cu , the eight Cu atoms are divided into two groups
1
3
2
13
13
8
six PPh and six 2-pyridinethiolates. The six PPh ligands
coordinate (one each) in a radial fashion to the six gold atoms
that are arranged in cyclohexane-like ‘chair’ configuration at the
(Figure3b), two oppositely oriented single Cu atoms across the
Au13 core (Au-Cu averaged at 2.768 Å) and three equatorially
distributed Cu dimers (Au-Cu averaged at 2.911 Å). In the Au
3
3
2
13
‘
[
equatorial’ positions of the Au core, similar to the structure in
cores of Au Cu , the Au-Au distances from the central atom are
1
3
13 8
3
5
(Ph P) Au Ag Pt(AgI ) ]. In an Au13 icosahedron, the
averaged at 2.768 Å. The Au13 unit is capped by 12 2-
pyridylthiolate ligands. Each 2-pyridylthiolate in Au Cu binds
3
6
6
6
3 2
cyclohexane-like unit is capped on top and below by two Au3
triangles. As bidentate ligands, the six 2-pyridylthiolate ligands
13
8
to one Au through Au-S and two Cu atoms through Cu-S and
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Journal of the American Chemical Society
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Figure 4. (a) UV/vis absorption spectra of Au Cu , Au Cu and
13
2
13
4
Au Cu in CH Cl . (b) Comparison of the measured spectra of
13
8
2
2
Au Cu with the theoretical absorption spectra calculated from the LR-
13
2
TDDFT. The spectra were calculated both for cationic and neutral
clusters. In the calculated spectra the individual optical transitions have
been smeared out by Gaussians of 0.075 eV width and the intensity is
scaled to correspond to the experiment at around 550 nm.
+
As shown in Figure 4, our UV/vis spectral measurements
reveal that Au Cu , Au Cu , and Au Cu in CH Cl exhibit
Figure 3. Crystal structure of [Au Cu (PPh Py) ] . (a) Overall
13
8
2
12
13
2
13
4
13
8
2
2
structure; (b) Distribution of eight Cu atoms on Au ; (c,d) Local
13
UV/vis absorption spectra with molecule-like optical transitions.
The optical gaps for Au Cu , Au Cu and Au Cu are
coordination structures of two types of Cu atoms in [Au Cu (PPh -
Py) ] . Color legend: gold sphere, Au; green sphere, Cu; yellow sphere,
13
8
2
+
12
13
2
13
4
13
8
S; gray stick, C; blue stick, N. All H atoms are omitted for clarity.
estimated from the measured absorption edge and are 1.88,
.87, and 1.80 eV, respectively. Comparison of the computed
spectra for neutral and cationic Au Cu to the measured
1
13
2
Cu-N, resulting in 24 binding sites (12 thiolates and 12 pyridyls)
available to have 8 Cu atoms face-capping the Au13 core. As
shown in Figure 3c,d, while each of the two oppositely oriented
Cu atoms is coordinated by three pyridyls, each of the six Cu
atoms that are equatorially distributed are coordinated by two
thiolates and one 2-pyridyl.
spectrum shows that the calculated spectrum for the cationic
Au Cu is in a very good agreement with the measured one,
13
2
reproducing all the four observed features in the experimental
data, while the spectrum of the neutral Au Cu does not
13
2
correspond to the experiment. Theory thus gives strong support
for the cationic state of Au Cu . Furthermore, the average Au-
13
2
As identified by the X-ray single-crystal analysis, each Au Cu
Cu distance in the computationally optimized cationic Au Cu
13
4
13 2
−
cluster has a +1 charge which is balanced by a ClO4
cluster corresponds much better to the measured value as
compared to the neutral Au Cu .
−
counteranion. ClO4 anions are located in the voids between
13
2
1
0
1
10
1
the clusters (Figure S1). Since each Au (5d 6s ) or Cu (3d 4s )
has one valence electron and the bonding to each thiolate anion
consumes one valence electron, each Au Cu cluster has 8 (17 ×
Having different surface ligands should create different
chemical properties among the three obtained bimetallic Au-
Cu nanoclusters. For example, compared with PPh , PPh Py and
1
3
4
3
2
3
1,38
−
1
-8 × 1-1) electrons and obeys the superatom rule.
Unfortunately, the unambiguous identification of counteranions
tert-C H -C H S , the size of 2-pyridylthiolate is smaller. As
4 9 6 4
clearly illustrated in their space-filling structures (Figure S6), the
presence of 12 2-pyridylthiolate ligands on the surface of
Au Cu still leaves open space for small molecules to access its
in the single crystals of Au Cu and Au Cu was not possible,
1
3
2
13
8
probably due to the heavy disordering of the anions. For this
reason, we have performed mass spectrometry (ESI-MS)
measurements to evaluate the charge state of Au Cu and
13
8
Au sites. In comparison, the surface ligands on Au Cu and
1
3
2
Au Cu completely block the Au sites for catalysis applications.
13
2
13
4
Au Cu . Both Au Cu and Au Cu were found to have a +1
charge and display clean and well-resolved peaks at 4922.3 and
Experimentally, the hydrogenation of 4-nitrophenol by NaBH4
to 4-aminophenol was chosen as a model reaction to evaluate the
accessibility of Au sites in the three clusters. The reduction
process was monitored by UV/vis absorption spectroscopy
(Figure S7). As shown in the reduction processes, in the presence
of Au Cu , the intensity of the absorption peak of 4-nitrophenol
13
8
13
2
13
8
4
390.8 m/z under positive ion mode, respectively (Figure S2).
DFT analysis of the atomic structure and charges and
electronic structure (Table S1 and Figures S3−S5) shows
unambiguously that Au Cu , Au Cu , and Au Cu clusters are
1
3
2
13
4
13
8
13
8
most stable as charged cations when all the systems are 8-electron
superatoms. The clusters have significant calculated HOMO-
LUMO gaps (of the order of 1−1.3 eV). While the P-superatom
orbitals are always fully occupied, they are in many cases close in
at 400 nm quickly decreased, and the absorption of 4-
aminophenol at 295 nm increased accordingly. The reduction
of 4-nitrophenol into 4-aminophenol was completed in 10 min
with the color change of bright yellow to colorless. A linear
relation of ln(C /C ) vs time, where C and C are 4-nitrophenol
+
energy and partially mixed with hybridized Cu /ligand states.
t
0
t
0
+
The degree of Cu /ligand mixing increases from x = 2 to 8. The
concentrations at time t and 0, respectively, was observed. In
superatom D orbitals are found either as LUMO states or close to
comparison, in the presence of Au Cu or Au Cu clusters, the
13
2
13
4
the LUMO states. Au Cu appears as a special case where both
reactions did not proceed. Moreover, carbon nanotube-
supported Au Cu clusters were also catalytically active in the
1
3
4
+
HOMO and LUMO states show strong hybridization of Cu /
ligand contributions. The strong interaction between Cu and
13
8
+
aerobic selective oxidation of benzyl alcohol, while [Au (SC -
25 2
−
nitrogen of the pyridyl ring is similar to coordination to
benzotriazolate (BTA), which has been previously discussed in
context of small copper clusters and Cu(111) surface protected
H C H ) ] clusters without removal of surface ligands
4 6 5 18
displayed negligible catalysis in the same reaction (Table S2).
Considering the copresence of phosphine and thiolate on the
surface of Au Cu and Au Cu , thermal gravimetric analysis
39,40
+
by BTA-Cu-BTA units.
part of the ligand layer.
Our interpretation is thus that Cu is
13
2
13
4
(TGA) was also performed to investigate the thermal
C
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Journal of the American Chemical Society
Communication
degradation characteristics of the ligands. TGA curves (in N ) of
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(
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capped by different number of metal atoms are expected to be
prepared by tuning surface organic ligands.
25) Pei, Y.; Pal, R.; Liu, C. Y.; Gao, Y.; Zhang, Z. H.; Zeng, X. C. J. Am.
Chem. Soc. 2012, 134, 3015.
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ASSOCIATED CONTENT
(27) Jiang, D.-e.; Walter, M.; Akola, J. J. Phys. Chem. C 2010, 114,
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5883.
*
S
Supporting Information
Experimental details and crystallographic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
(
(
28) Negishi, Y.; Iwai, T.; Ide, M. Chem. Commun. 2010, 46, 4713.
29) Yang, H. Y.; Lei, J.; Wu, B. H.; Wang, Y.; Zhou, M.; Xia, A. D.;
Zheng, L. S.; Zheng, N. F. Chem. Commun. 2013, 49, 300.
(30) Yang, H. Y.; Wang, Y.; Zheng, N. F. Nanoscale 2013, 5, 2674.
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31) Walter, M.; Akola, J.; Lopez-Acevedo, O.; Jadzinsky, P. D.; Calero,
AUTHOR INFORMATION
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G.; Ackerson, C. J.; Whetten, R. L.; Gronbeck, H.; Hak
Natl. Acad. Sci. U.S.A. 2008, 105, 9157.
(32) Copley, R. C. B.; Mingos, D. M. P. J. Chem. Soc., Dalton Trans.
996, 491.
̈
kinen, H. Proc.
Corresponding Author
nfzheng@xmu.edu.cn
1
Notes
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(33) Laupp, M.; Strahle, J. Angew. Chem., Int. Ed. 1994, 33, 207.
(34) Teo, B. K.; Zhang, H.; Shi, X. J. Am. Chem. Soc. 1993, 115, 8489.
(35) Teo, B. K.; Zhang, H. J. Organomet. Chem. 2000, 614-615, 66.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(36) Teo, B. K.; Zhang, H.; Shi, X. J. Am. Chem. Soc. 1990, 112, 8552.
37) Teo, B. K.; Hong, M. C.; Zhang, H.; Huang, D. B. Angew. Chem.,
Int. Ed. 1987, 26, 897.
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(
We thank MOST of China (2011CB932403, 2011CB201301,
009CB930703), the NSFC (21227001, 21131005, 21021061,
0925103, 20923004), the Fundamental Research Funds for the
Central Universities (2010121042) for the financial support. The
2
(
(
38) Hak
̈
kinen, H. Chem. Soc. Rev. 2008, 37, 1847.
2
39) Salorinne, K.; Chen, X.; Troff, R. W.; Nissinen, M.; Hak
̈
kinen, H.
Nanoscale 2012, 4, 4095.
computational work is supported by the Academy of Finland, and
(40) Chen, X.; Hakkinen, H. J. Phys. Chem. C 2012, 116, 22346.
̈
the computations were made in NSC (Jyvas
Stuttgart). V.M. and H.H. wish to thank L. Lehtovaara for his
help in the LR-TDDFT calculations.
̈ ̈
kyla) and in HLRS
(
D
dx.doi.org/10.1021/ja402249s | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX