Photocatalytic Sonogashira reaction over silicon carbide supported Pd-Cu alloy nanoparticles under visible light irradiation

Article abstract of DOI:10.1039/c8cy00618k

The Sonogashira reaction is an important reaction for forming carbon-carbon bonds in organic synthesis, which is typically carried out under harsh reaction conditions. We herein report that PdCu alloy nanoparticles supported on SiC can efficiently catalyz

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Photocatalytic Sonogashira Reaction over Silicon Carbide
Supported Pd-Cu Alloy Nanoparticles under Visible Light
Irradiation
Received 00th January 20xx,
Accepted 00th January 20xx
Bing Wang,a,b Yingyong Wang,*a Jiazhou Li,a,b Xiaoning Guo,a Gailing Bai,a Xili Tong,a Guoqiang Jin,a
Xiangyun Guo*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Sonogashira reaction is an important reaction for forming carbon-carbon bond in organic synthesis, which is typically
carried out under harsh reaction conditions. We herein report that PdCu alloy nanoparticles supported on SiC can
efficiently catalyze the Sonogashira reaction by visible light irradiation under ligand-free and mild condition. The addition
of Cu enhanced the activity of PdCu alloy nanoparticles. Pd3Cu1/SiC catalyst exhibited the highest catalytic performance for
the Sonogashira reaction of iodobenzene with phenylacetylene, and the conversion of iodobenzene and selectivity of
diphenylacetylene were 99.7% and 98.7%, respectively. The Sonogashira reaction catalyzed over SiC supported PdCu alloy
is a light-driven process, and the reaction rate was greatly influenced by light intensity and wavelength. A possible reaction
mechanism for the photocatalytic Sonogashira reaction over Pd3Cu1/SiC was proposed.
developing PdCu bimetallic catalysts for Sonogashira
reaction.12-14 For example, Diyarbakir et al used PdCu alloy
1. Introduction
Bimetallic nanoparticles usually show superior catalytic
properties over their monometallic counterparts, which have
nanoparticles supported on reduced graphene oxide as
efficient catalysts for the Sonogashira reaction,13 which
showed good activity and applicability.
attracted
intensive
attentions.1-2 Among
bimetallic
nanomaterials, PdCu alloy catalysts exhibit high activity and
selectivity in many reactions, such as formic acid oxidation,3
photocatalytic nitrate reduction,4 hydrogenation of CO2,5
Photocatalysis uses solar energy to effectively trigger
chemical reactions under mild conditions, which has been
widely studied in water splitting, cancer therapy, CO2
reduction and so on.15 Cubic silicon carbide (SiC) with a band
gap of 2.4 eV can effectively absorb and harvest visible light.16
In our previous work, SiC supported Pd and Au nanoparticles
showed outstanding photocatalytic activity towards Suzuki-
Miyaura coupling reaction17 and hydrogenation of
cinnamaldehyde,18 respectively. From our previous work,
Pd/SiC catalyst showed photocatalytic activity towards the
Sonogashira cross-coupling reaction under copper-, and ligand-
free conditions.19 Under the visible light irradiation, the photo-
generated electrons on SiC were transferred to Pd
nanoparticles through the Mott-Schottky contact. Electron-rich
Pd nanoparticles can facilitate the cleavage of carbon-halogen
bonds in aryl halides. Then, the final products were achieved
by the substitution reaction and reductive elimination
processes. However, the photocatalytic Sonogashira process
catalyzed by Pd/SiC still needed high reaction temperature
6
electrocatalytic oxidation of methanol and so on. Wang et al
synthesized Pd-Cu nanoalloy catalysts supported on Vulcan XC-
72 carbon by a facile one-pot polyol reduction process for
formic acid oxidation (FAO),7 and found the Pd1Cu1/C catalyst
exhibited the best electrocatalytic performance toward FAO
among the alloy catalysts. The enhanced electrocatalysis
performance was due to the alloying of Pd with Cu. Generally,
the addition of Cu can lower the d-band center of Pd to
weaken the adsorption of reaction intermediates and increase
the thermal stability of PdCu alloy nanoparticles.8-9 Thus, PdCu
alloy catalysts are very potential in catalytic areas. Sonogashira
cross-coupling reaction is an important method for
constructing C(sp2)-C(sp) bonds.10 Traditionally, palladium
phosphine complexes with CuI (cuprous iodide) as co-catalyst
were employed for the Sonogashira cross-coupling reaction,
which always required usage of organic ligands and high
11
amount of copper. Recently, many studies are focused on
(120 ̊C). Considering the synergetic effect in PdCu alloy
nanoparticles, we prepared SiC supported PdCu alloy catalysts
through liquid-reduction method for photocatalytic
Sonogashira cross-coupling reaction. According to TEM, SEM-
EDS and XPS analysis, PdCu alloy nanoparticles are successfully
prepared and homogeneously dispersed on SiC support. SiC-
supported PdCu alloy nanoparticles can efficiently drive the
a.State Key Laboratory of Coal Conversion Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan 030001, China Email: xyguo@sxicc.ac.cn;
wangyy79@sxicc.ac.cn.
b.University of the Chinese Academy of Sciences, Beijing 100039, China.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Journal Name
Sonogashira
reaction
between
aryl
halides
and liquid which was analyzed by BRUKER SCION SQ 456 GC-MS.
phenylacetylene under visible light irradiation and mild The influence of light wavelength on catalytic performance
reaction condition.
was investigated by employing various low pass optical filters
to block light below specific cut-off wavelengths while keeping
the light intensity unchanged.
2. Experimental section
2.1 Catalyst preparation
3. Results and Discussion
SiC support was prepared by a sol-gel and carbonthermal
reduction process.20 SiC-supported PdCu alloy catalysts with
different molar ratios of Pd to Cu were prepared by
3.1 Catalyst activity
The common constitutions of PdCu alloy are Pd3Cu, Pd2Cu,
impregnation-reduction method. The Pd content of all SiC- PdCu, PdCu2 and PdCu3. For the Sonogashira cross-coupling
supported PdCu alloy catalysts was fixed at around 3 wt%. reaction, Pd is always used as main active components. Thus,
Taking Pd3Cu1/SiC for example, 200 mg SiC was added into the we fixed Pd content at 3 wt% and prepared Pd3Cu1/SiC,
mixture of 5.64 mL Pd(NO3)2 aqueous solution (0.01 M) and Pd2Cu/SiC and PdCu/SiC catalysts by adjusting the content of
1.88 mL Cu(NO3)2 aqueous solution (0.01 M). After stirring for Cu. For comparison, we also prepared Pd/SiC and Cu/SiC
30min, 20 mL lysine aqueous solution (0.53 M) was added catalysts, which had the same metal content as their
dropwise into the above mixture. Subsequently the mixture counterpart in Pd3Cu1/SiC catalyst, respectively. The
was stirred for another 30 min. Then 20 mL of NaBH4 solution Sonogashira reactions of iodobenzene with phenylacetylene
(0.35 M) was added drop by drop. After 24 h of stirring, the over SiC-supported PdCu alloy catalysts under visible light
mixture was washed, filtered with deionized water and irradiation were performed and the results are summarized in
ethanol, and then dried in a vacuum-drying chamber at 60 °C. Table 1. SiC support alone had no catalytic activity (entry 1). The
The dried mixture was reduced in a mixture of H2 (5 vol %) and iodobenzene conversion of Pd/SiC catalyst was only 28.6%
Ar at 400 °C for 0.5 h. The obtained powder was Pd3Cu1/SiC without Cu. The addition of Cu can evidently enhance the
catalyst. Pd2Cu1/SiC, PdCu/SiC, Pd/SiC and Cu/SiC catalysts photocatalytic activity of Pd/SiC for Sonogashira reaction
were prepared by the similar method.
(entries 3-5). Pd3Cu1/SiC catalyst showed highest iodobenzene
conversion among SiC-supported PdCu alloy catalysts, and
increasing Cu content caused the decrease in conversion
(entries 4-5). The decrease in catalytic activity may be due to
the fact that the presence of excess Cu could occupy the active
sites and shelter the interaction between Pd and reactants.
Cu/SiC catalyst showed no catalytic activity towards
Sonogashira reaction (entry 6). It can be noted that the
conversion of iodobenzene of Pd3Cu1/SiC was only 2% in the
dark (entry 7), and it means that Sonogashira cross-coupling
reaction over SiC-supported PdCu alloy catalysts is a light-driven
process.
2.2 Catalyst characterization
X-ray diffraction (XRD) spectra were collected on a Rigaku D-
Max/RB X-ray diffractometer using Cu Kɑ radiation. X-ray
photoelectron spectroscopy (XPS) was recorded on a AXIS
ULTRA DLD spectrometer using Al Kɑ radiation, and binding
energies were calibrated by the position of C 1s peak at 284.8
eV. The loading of metals was measured by inductively
coupled plasma-atomic emission spectrometry (ICP-AES). High
resolution transmission electron microscopy (HRTEM) pictures
and line profile analysis by an energy dispersion X-ray (EDX)
spectrum were observed on JEOL-2100 transmission electron
microscope at accelerating voltage of 200 kV. The elemental
mapping was examined by field emission scanning electron
microscopy (FESEM) on JSM-7100F coupled with energy
dispersion spectroscopy (EDS). The ultraviolet-visible (UV-vis)
Table 1 Photocatalytic Sonogashira reaction of iodobenzene with
phenylacetylene over SiC-supported PdCu alloy catalysts.a
Entry
Catalyst
Conv. of
Select. of
Diphenylacetylene c/%
absorption spectra were measured by
a
UV-3600
Iodobenzene/%
spectrophotometer with Al2O3 as a reference. The
1
2
3
4
5
6
7
SiC
-
-
photoluminescence (PL) spectra were detected by the
fluorescence spectrophotometer (F-700 FL ) under the 320 nm
excitation wavelength.
Pd/SiC
28.6
99.7
74.6
33.5
-
96.6
98.7
99.2
97.8
-
Pd3Cu1/SiC
Pd2Cu1/SiC
PdCu/SiC
Cu/SiC
2.3 Sonogashira cross-coupling reaction.
Sonogashira reaction was carried out in a 150 mL glass
reaction vessel under Ar atmosphere. 1 mmol of iodobenzene
or one of its derivatives, 1.2 mmol of phenylacetylene, 2 mmol
of Cs2CO3, 10 mL of N,N-Dimethylformamide (DMF), and 50 mg
of SiC-supported PdCu alloy catalyst were put into the glass
reactor, and the reaction temperature was kept at 60 °C by oil
bath. The reactant was irradiated under a 300 W Xe lamp for 8
h with light intensity of 0.7 W/cm2. After the reaction, the
mixture was collected and centrifuged to get supernatant
Pd3Cu1/SiC b
2
92.6
a Reaction conditions: 1 mmol iodobenzene, 1.2 mmol phenylacetylene,
2 mmol Cs2CO3, 50 mg of catalyst in 10 mL of DMF. The reactions were
conducted in an argon atmosphere at 60 °C under Xe-lamp irradiation
(400-800 nm) with light intensity of 0.7 W/cm2. The reaction time was
b
C
8
h.
Without visible light irradiation.
Among the generated
compounds, diphenyls were the main byproduct.
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nanoparticles.
3.2 Catalyst characterization
To better understand the synergistic effect of PdCu alloy
nanoparticles, Pd3Cu1/SiC catalyst was characterized in detail.
High resolution transmission electron microscopy (HRTEM) is
used to verify the surface morphology and structure of Pd3Cu1
alloy. As shown in Figure 1 A and B, the Pd3Cu1 alloy
nanoparticles are homogeneously dispersed on the SiC support
and an interplanar spacing of 0.22 nm corresponds to the (111)
crystal face of Pd3Cu1 alloy nanoparticles in the inset of HRTEM
image.8 Line profile analysis of Pd3Cu1/SiC catalyst by energy
dispersion X-ray spectroscopy indicates that Pd and Cu exist as
alloy nanoparticles in Figure 1 C.21-22 The elemental mapping of
Pd and Cu by field emission scanning electron microscopy
(FESEM) also confirms that Pd and Cu are homogeneously
dispersed on the SiC (Figure 1 D, E and F). In addition, the
atomic ratio of Pd and Cu in the Pd3Cu1/SiC catalyst is 2.89:1
calculated by ICP analysis, which is close to the theoretical
value of 3:1. The X-ray diffraction (XRD) patterns of Pd3Cu1/SiC
are shown in Figure 1 G. All strong diffraction peaks can be
indexed to SiC crystal and no diffraction peaks corresponding
to Pd3Cu1 alloy nanoparticles are detected. This phenomenon
may be due to the low loading of metals and high
homodisperse.
Figure 2 shows the X-ray photoelectron spectroscopy (XPS)
analysis of Pd/SiC, Cu/SiC and Pd3Cu1/SiC catalysts. The binding
energy (BE) values of metallic Pd are 334.8 eV for Pd 3d5/2 and
23
340.2 eV for Pd 3d3/2
.
Due to the electron transfer from SiC
to Pd,17 the BE values of Pd in Pd/SiC shift to lower values
(334.6 eV and 340 eV) in Figure 2 A. The BE values of Pd in
Pd3Cu1/SiC are 334.3 eV and 339.8 eV (Figure 2 C). Lower
binding energies of Pd in Pd3Cu1/SiC than those in Pd/SiC mean
more electrons are transferred to Pd. For Cu/SiC, there are two
peaks at 934 eV and 953.8 eV,24 which correspond to Cu 2p3/2
and Cu 2p1/2 of CuO, respectively (Figure 2 B). It suggests that
metallic Cu is unstable in air and easily oxidized to CuO. The
binding energies of Cu in Pd3Cu1/SiC are at 952 eV for Cu 2p1/2
and 932.6 eV for Cu 2p3/2, respectively (Figure 2 D). According
to the literature, the binding energies of Cu0 state are around
25
at 951.9 eV for Cu 2p1/2 and 932.4 eV for Cu 2p3/2
. Similarly,
binding energies of Cu+1 are at 952.4 eV and 932.7 eV.24 Due to
the fact that the Cu0 and Cu+1 cannot be distinguished by the
XPS spectra, we use Auger electron spectrum to confirm the
valance state of Cu in Pd3Cu1 catalyst. The inset of Figure 2 D
shows the Cu LMM Auger electron spectrum, and the binding
energy of the Auger peak is around at 568 eV which
corresponds to metallic Cu.26 The Cu in Pd3Cu1/SiC is slightly
positively charged due to electron transfer from Cu to Pd,
which is in accord with the negative shift of BE values of Pd in
Pd3Cu1/SiC as mentioned above. Based on these analyses, we
can conclude that PdCu bimetallic nanoparticles are
homogeneously dispersed over SiC in a form of alloying.
Fig. 2 XPS of (A) Pd in Pd/SiC, (B) Cu in Cu/SiC, (C) Pd in Pd3Cu1/SiC and
(D) Cu in Pd3Cu1/SiC. The inset in (D) is Auger electron spectrum of Cu
in Pd3Cu1/SiC.
Fig.1 (A, B) TEM images of Pd3Cu1/SiC, (C) Line profile analysis of
Pd3Cu1 alloy nanoparticles, (D) SEM image of Pd3Cu1/SiC for element
mapping, (E) Palladium mapping, (F) Copper mapping, and (G) XRD
pattern of Pd3Cu1/SiC. The inset in (B) is a HRTEM of Pd3Cu1 alloy
The ultraviolet-visible (UV-vis) absorption spectra and
photoluminescence (PL) spectra are used to measure the
optical properties of the catalyst. The UV-vis absorption
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spectra and PL spectra of SiC, Pd/SiC, Cu/SiC and Pd3Cu1/SiC
catalysts are shown in Figure 3 A and B. The pure SiC and
Cu/SiC have similar absorption curve. Pd/SiC exhibits a
stronger UV and visible absorption than pure SiC, which means
a better utilization of light energy. For Cu/SiC, the localized
surface plasmon resonance (LSPR) absorption of Cu at 560 nm
is not detected since metallic Cu is easily oxidized to CuO in the
air. Pd3Cu1/SiC shows the strongest light absorption, and it is
the consequence of the absorption of alloy nanoparticles in
the visible light region. All metal-supported catalysts have
obvious decrement of PL intensity compared with SiC.
Especially, the Pd3Cu1/SiC has the lowest PL intensity
suggesting that the recombination of photogenerated
electrons and holes has been effectively suppressed.
Fig. 4 Dependence of catalytic activity of Pd3Cu1/SiC for the Sonogashira on
the intensity (A) and wavelength (B) of the irradiation.
A series of photocatalytic Sonogashira reactions between
various aryl halides and phenylacetylene were performed to
investigate the general applicability of the Pd3Cu1/SiC catalyst.
Aryl iodides with electron-donating or electron-withdrawing
substitutions had high conversion and selectivity towards the
target products (entries 1-6, Table 2). Notably, Pd3Cu1/SiC
catalyst also showed superior catalytic performance for aryl
bromides, but a longer reaction time was required (entries 7-
11, Table 2). It is due to the fact that activation of C-Br bonds is
much more different than that of C-I bonds, which needs
higher energy and more reaction time.
Fig. 3 (A) UV-vis absorption spectra and (B) PL spectra of SiC, Pd/SiC,
Table 2 Photocatalytic Sonogashira reaction of various aryl halides with
phenylacetylene over Pd3Cu1/SiC catalyst. a
Cu/SiC and Pd3Cu1/SiC.
3.3 Photocatalytic performance
Enrty
1
Aryl halides
Product
Conv. /% Select. /%
For the photocatalysis, the light intensity and wavelength
have critical influence on the catalytic activity. The
dependence of the photocatalytic performance over
Pd3Cu1/SiC on the light intensity and wavelength was
investigated and shown in Figure 4 A and B. The iodobenzene
conversion had an almost linear growth with the increase of
light intensity (Figure 4 A). Higher light intensity can generate
more energetic electrons and lead to higher reaction rate and
increment of iodobenzene conversion. A series of optical pass
filters were employed to block light below a specific cut-off
wavelength and the results were shown in Figure 4 B. Without
any filter (400-800 nm), the conversion of iodobenzene
reached to 99%. Under the light with wavelengths in 450-800,
500-800 and 600-800 nm, the conversion of iodobenzene was
75%, 52% and 18%, respectively. After subtracting the
contribution of thermal effect (2%), the conversion of
iodobenzene within each wavelength range was 24% for 400-
450 nm, 23% for 450-500 nm, 34% for 500-600 nm and 16% for
600-800 nm. The light in the range of 400-600 nm made a
great contribution to the conversion of iodobenzene.
92
93
2
3
4
5
98
96
91
95
99
100
94
93
6
7
8
99
93
85
100
95
92
9
82
94
91
93
100
98
10
11
a
Reaction conditions: 1 mmol aryl halides, 1.2 mmol phenylacetylene, 2
mmol Cs2CO3, 50 mg of Pd3Cu1/SiC catalyst in 10 mL of DMF. The reactions
were conducted in an argon atmosphere at 60 °C under Xe-lamp irradiation
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(400-800 nm) with light intensity of 0.7 W/cm2. The reaction time was 8 h. halides. At the same time, the positively charged sites of Cu
For aryl bromides, the reaction time was 12 h.
react with phenylacetylene to form phenylethynylcopper
compound. Finally, the target products are achieved by cross-
coupling reaction.
For practical applications, fivefold and tenfold scale-up
experiments were performed for the photocatalytic
Sonogashira reaction of iodobenzene with phenylacetylene
over Pd3Cu1/SiC. The conversion of iodobenzene was 93.5%
and 85.1% for fivefold and tenfold scale-up experiments,
respectively. The selectivity of diphenylacetylene was almost
not changed. It can be seen that with the increase of reaction
scale the conversion has a slight decrease. This could be
relative to the depth that the light irradiation can pass through
the reaction suspension. In addition, the mass or heat transfer
may also influence the reaction after amplification. Therefore,
there are many experiments to do before the industrial scale-
up.
Fig. 5 Proposed mechanism for the photocatalytic Sonogashira reaction of
3.4 Proposed mechanism
aryl halides and phenylacetylene over Pd3Cu1/SiC.
Several controlled experiments were performed
to elucidate the reaction mechanism of the photocatalytic
Sonogashira. SiC had no catalytic activity for the Sonogashira
reation and the conversion of iodobenzene was 2% without
visible light irradiation for Pd3Cu1/SiC catalyst. This indicates
that Pd3Cu1 alloy nanoparticles and light irradiation are
indispensable for the photocatalytic Sonogashira reaction. To
investigate the roles of photogenerated electrons and holes in
photocatalytic reaction process, 5,5-Dimethyl-1-pyrroline N-
oxide (DMPO) and triethanolamine (TEA) were employed.
When DMPO was added to the reaction system as the
electron-trapping agent, the conversion of iodobenzene was
only 5%. It was very close to that in the dark reaction,
indicating the light-driven reaction was quenched. This is in
accordance with our previous work that photo-generated
electrons can facilitate the cleavage of carbon-halogen bond in
the Pd/SiC catalyzed Sonogashira reaction.19 While TEA was
added to trap the photogenerated holes, the conversion of
iodobenzene decreased to 77%. TEA lowered the catalytic
activity of the Pd3Cu1/SiC catalyst. It was much different from
the results of Pd/SiC reaction system, since the photo-
generated holes of SiC has no influence to the photocatalytic
Sonogashira reaction.19 According to the literature,27-28 Cu(I)
state easily reacts with phenylacetylene to form
phenylethynylcopper(I) compound. We supposed that the
addition of TEA quenched the positively charged sites at the
surface of Cu particles and thus lowered the catalytic activity
of Pd3Cu1/SiC catalyst. Since Pd nanoparticles on SiC can
catalyze Sonogashira reaction alone, the photocatalytic
Sonogashira reaction over Pd3Cu1/SiC could still proceed when
Cu failed to activate phenylacetylene in the presence of TEA.
Thus, a reaction mechanism for the photocatalytic Sonogashira
reaction over Pd3Cu1/SiC catalyst is proposed as shown in
Figure 5. Under the visible light irradiation, the
photogenerated electrons on SiC surface and excited electrons
of Cu in PdCu alloy are transferred to Pd. The electron-rich Pd
can facilitate the cleavage of carbon-halogen bond in aryl
3.5 Stability
The recyclability is an important parameter to evaluate
the heterogeneous catalyst. The Pd3Cu1/SiC photocatalyst was
reused for up to 5 cycles under the same reaction condition
and there was no evident loss in the catalytic activity,
suggesting the superior stability of this catalyst (Figure 6 A).
TEM image of the used catalyst indicates that PdCu alloy
nanoparticles are still in homogeneous distribution and have
no obvious morphology change after five cycles as shown in
Figure 6 B.
Fig. 6 (A) Recycle stability of the Pd3Cu1/SiC catalyst in five cycles, (B) TEM
image of Pd3Cu1/SiC catalyst after five cycles.
4. Conclusions
SiC supported Pd3Cu1 alloy nanoparticles showed excellent
performance for the Sonogashira cross-coupling reaction in
visible light irradiation under ligand-free and mild condition.
The alloying of Cu into Pd can increase the activity of PdCu
alloy nanoparticles. The photogenerated electrons of SiC
surface and excited electrons of Cu particles in PdCu alloy are
transferred to Pd under light irradiation. Due to the synergistic
effect of PdCu alloy nanoparticles, the electron-rich Pd can
activate carbon-halogen bond in aryl halides and the positively
charged sites of Cu react with phenylacetylene to form
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phenylethynylcopper compound where the final products are 20 G. Q. Jin and X. Y. Guo, Micropor. Mesopor. Mat., 2003, 60
achieved by reductive elimination processes. Besides, light 207-212.
intensity and wavelength had the great influence on the 21 M. Y. Liu, W. Zhou, T. Wang, D. F. Wang, L. Q. Liu and J. H. Ye,
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5
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DOI: 10.1039/C8CY00618K
Journal Name
ARTICLE
Photocatalytic Sonogashira Reaction over Silicon Carbide Supported Pd-Cu
Alloy Nanoparticles under Visible Light Irradiation
Bing Wang,a,b Yingyong Wang,*a Jiazhou Li,a,b Xiaoning Guo,a Gailing Bai,a Xili Tong,a Guoqiang Jin,a Xiangyun Guo*a
PdCu alloy nanoparticles supported on SiC can efficiently catalyze the Sonogashira reaction by visible light irradiation under ligand-
free and mild condition. Among the prepared alloy catalysts, Pd3Cu1/SiC catalyst exhibited the highest catalytic performance for the
Sonogashira reaction, and the conversion of iodobenzene and selectivity of diphenylacetylene were 99.7% and 98.7%, respectively.
The superior catalytic activity of PdCu alloy catalysts was caused by synergistic effect of PdCu alloy nanoparticles.
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