Controlled synthesis of water-dispersible faceted crystalline copper nanoparticles and their catalytic properties

Article abstract of DOI:10.1002/chem.201000354

We report a solution-phase synthetic route to copper nanoparticles with controllable size and shape. The synthesis of the nanoparticles is achieved by the reduction of copper(II) salt in aqueous solution with hydrazine under air atmosphere in the presence of poly(acrylic acid) (PAA) as capping agent. The results suggest that the pH plays a key role for the formation of pure copper nanoparticles, whereas the concentration of PAA is important for controlling the size and geometric shape of the nanoparticles. The average size of the copper nanoparticles can be varied from 30 to 80 nm, depending on the concentration of PAA. With a moderate amount of PAA, faceted crystalline copper nanoparticles are obtained. The as-synthesized copper nanoparticles appear red in color and are stable for weeks, as confirmed by UV/Vis and X-ray photoemission (XPS) spectroscopy. The faceted crystalline copper nanoparticles serve as an effective catalyst for N-arylation of heterocycles, such as the C-N coupling reaction between p-nitrobenzyl chloride and morpholine producing 4-(4-nitrophenyl) morpholine in an excellent yield under mild reaction conditions. Furthermore, the nanoparticles are proven to be versatile as they also effectively catalyze the three-component, one-pot Mannich reaction between p-substituted benzaldehyde, aniline, and acetophenone affording a 100% conversion of the limiting reactant (aniline).

Full text of DOI:10.1002/chem.201000354

FULL PAPER
DOI: 10.1002/chem.201000354
Controlled Synthesis of Water-Dispersible Faceted Crystalline Copper
Nanoparticles and Their Catalytic Properties
Yanfei Wang,[a, b] Ankush V. Biradar,[a, b] Gang Wang,[c] Krishna K. Sharma,[c]
Cole T. Duncan,[c] Sylvie Rangan,[d] and Tewodros Asefa*[a, b]
Abstract: We report a solution-phase
synthetic route to copper nanoparticles
with controllable size and shape. The
synthesis of the nanoparticles is
varied from 30 to 80 nm, depending on
the concentration of PAA. With a mod-
erate amount of PAA, faceted crystal-
line copper nanoparticles are obtained.
The as-synthesized copper nanoparti-
cles appear red in color and are stable
for weeks, as confirmed by UV/Vis and
X-ray photoemission (XPS) spectrosco-
py. The faceted crystalline copper
nanoparticles serve as an effective cata-
lyst for N-arylation of heterocycles,
ꢀ
such as the C N coupling reaction be-
tween p-nitrobenzyl chloride and mor-
pholine producing 4-(4-nitrophenyl)-
morpholine in an excellent yield under
mild reaction conditions. Furthermore,
the nanoparticles are proven to be ver-
satile as they also effectively catalyze
the three-component, one-pot Mannich
reaction between p-substituted benzal-
dehyde, aniline, and acetophenone af-
fording a 100% conversion of the lim-
iting reactant (aniline).
ACHTUNGTRENNUNGachieved by the reduction of copper(II)
salt in aqueous solution with hydrazine
under air atmosphere in the presence
of poly(acrylic acid) (PAA) as capping
agent. The results suggest that the pH
plays a key role for the formation of
pure copper nanoparticles, whereas the
concentration of PAA is important for
controlling the size and geometric
shape of the nanoparticles. The average
size of the copper nanoparticles can be
Keywords: heterogeneous catalysis ·
copper · nanoparticles · poly(acrylic
acid) · shape control · size control
Introduction
Copper nanostructures are expected to be essential compo-
nents in future nanodevices and nanocircuits due to their ex-
cellent electrical conductivity.[5–7] Copper nanoparticles are
also widely used as catalysts for various reactions including
water–gas shift[8] and gas detoxification reactions,[9] and as
electrocatalysts in solid oxide fuel cells.[10] On the other
hand, due to their good biocompatibility and surface-en-
hanced Raman scattering (SERS) properties, copper nano-
particles may have potential applications as nanoprobes in
medical diagnosis and biological analysis.[2b] Like many
other nanomaterials, the size and shape of copper nanoparti-
cles have a significant effect on their electronic, catalytic,
and optical properties. Therefore, the preparation of mono-
dispersed copper nanoparticles with controlled size and
shape along with the investigation of their catalytic proper-
ties remains to be of considerable interest.
A number of different methods have been reported for
the preparation of copper nanostructures. The first approach
is a surfactant-assisted reduction method involving the
chemical reduction of copper ions in an aqueous solution in
the presence of various capping agents.[11–19] For instance,
mixed reverse micelles as a template have been used to syn-
thesize and control the size and shape of copper nanoparti-
Noble metal nanoparticles have attracted considerable at-
tention owing to their potential applications in fields such as
catalysis, biology, electronics, and information technology.[1–4]
[a] Dr. Y. Wang, Dr. A. V. Biradar, Prof. T. Asefa
Department of Chemistry and Chemical Biology, Rutgers
The State University of New Jersey, 610 Taylor Road
Piscataway, NJ 08854 (USA)
Fax : (+1)732-445-5312
E-mail: tasefa@rci.rutgers.edu
[b] Dr. Y. Wang, Dr. A. V. Biradar, Prof. T. Asefa
Department of Chemical Engineering and Biochemical Engineering
Rutgers, The State University of New Jersey
98 Brett Road, Piscataway, NJ 08854 (USA)
[c] G. Wang, Dr. K. K. Sharma, C. T. Duncan
Department of Chemistry, Syracuse University, 111 College Place
Syracuse, NY 13244 (USA)
[d] Dr. S. Rangan
Department of Physics and Astronomy, Rutgers
The State University of New Jersey
136 Frelinghuysen Rd, Piscataway, NJ 08854 (USA)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000354.
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cles.[11] Furthermore, by using sodium dioctyl sulfosuccinate
(AOT)–water–oil reverse micellar systems, copper nanopar-
ticles, nanorods, and nanodisks, the sizes and shapes of
which were controlled by regulating the water content in the
oil phase and/or the concentration of reducing agent or salt,
were synthesized.[12–15] Other capping agents such as cetyltri-
methylammonium bromide (CTAB),[16] sodium dodecylben-
zene sulfonate (DBS),[17] ethylenediamine (EDA),[18] and
poly(vinylpyrolidone) (PVP)[19] were also successfully used
to synthesize copper nanoparticles with shapes including
spheres, cubes, and wires, from the reduction of Cu(OH)2
with hydrazine in the presence of these surfactants. The
second synthetic approach to generate copper nanoparticles
is performed in high-boiling-point organic solvents by de-
composing various types of copper complexes at an elevated
reaction temperature in the presence of capping agents. For
instance, copper nanoparticles with different shapes such as
rods and cubes were synthesized by decomposing [Cu-
faceted crystalline copper nanoparticles showed high catalyt-
ꢀ
ic activity towards N-arylation of heterocycles by C N cou-
pling between aryl halides and N-heterocycles under mild
and aerated reaction conditions. Furthermore, the Cu nano-
particles effectively catalyzed the Mannich coupling reaction
of three reactants (para-substituted benzaldehyde, aniline,
and acetophenone) in one pot.
Results and Discussion
Synthesis of copper nanoparticles by optimizing the pH of
the solution (effect of pH): Although the chemistry and
most of the procedures we used to make copper nanoparti-
cles and the ones previously reported for making copper(I)
oxide nanowires[30] are similar, the two syntheses resulted in
completely different types of nanomaterials. In the synthesis
of copper(I) oxide nanowires, the primary reaction involves
the reduction of copper(II) salts with hydrazine in PVP pro-
ducing yellowish-colored samples of Cu2O nanowires. Here
in our case, the key modification of using PAA as capping
agent resulted in faceted crystalline copper nanoparticles in-
stead of copper(I) oxide, although the same reduction step
was employed. In aqueous solution, the chains of the PAA
polymer extend from the copper nanoparticle surface and
provide the nanoparticles steric stabilization from aggrega-
tion. It is also worth mentioning that the synthetic route
with PVP capping agent[30] does not produce pure copper
nanoparticles even in a wide pH range. Although our syn-
thetic method is somewhat similar to the one described in
reference [30], the use of PAA as a capping agent made the
pH optimum enough to favor the formation of pure copper
rather than copper oxide. An additional advantage of the
PAA capping agent is the extension of its uncoordinated
carboxylate groups into aqueous solution conferring the par-
ticles a high degree of dispersion in water.
Figure 1 shows typical UV/Vis absorption spectra of
PAA-capped copper nanoparticles that were synthesized at
various pH values and a graph of their absorption maxima
versus the pH. The UV/Vis absorption spectra of the as-pre-
pared copper nanoparticles display a well-defined absorp-
tion peak at around l=565 nm (Figure 1A). The absorption
peak can be attributed to the excitation of plasmon reso-
nance or interband transition, which is a characteristic prop-
erty of the metallic nature of copper nanoparticles.[31] The
appearance of this absorption peak further indicates the re-
duction of copper ions into copper nanoparticles under our
synthetic conditions.
ACHTUNGTRENNUNG(acac)2] (acac=acetylacetonate) in octyl ether with oleic
acid or oleyl amine capping agent at different reaction tem-
peratures.[20] The formation of the shaped nanoparticles was
attributed to three sequential events: initial nucleation of
seed metal clusters, preferential adsorption of capping
agents on selected facets of the seed nanocrystal, and aniso-
tropic growth of the metal in the other facets. Similarly,
shaped Cu nanoplates with intense plasmon resonances
were also obtained in N,N-dimethylformamide (DMF) sol-
vent.[21] In the third synthetic approach, called the polyol re-
duction method, ethylene glycol serves as the solvent and
the reductant for copper ions with PVP capping agent to
produce copper nanomaterials.[22] The fourth reported
method involves bioinspired synthetic routes that use a histi-
dine-containing peptide backbone to synthesize and control
the size of copper nanomaterials.[23] The conformational
changes in the peptide backbone have been correlated to
the size and the degree of dispersity of the nanoparticles.[23]
Finally, physical vapor deposition,[24,25] chemical vapor depo-
sition,[26] g irradiation,[27] irradiation with UV light,[28] and
sonochemical methods[29] have been reported for the synthe-
sis of copper nanostructures. However, the development of
aqueous-phase synthetic methods to shaped copper nanopar-
ticles has been rarely explored because the propensity of
surface oxidation of copper makes it difficult to fabricate
chemically stable Cu colloids.
Herein, we report an aqueous-phase synthetic route to
pure faceted crystalline copper nanoparticles with tunable
size and shape. The synthesis involves the reduction of cop-
per(II) salts with hydrazine in presence of poly(acrylic acid)
(PAA) capping agent. By adding different amounts of PAA,
copper nanoparticles with tunable sizes and shapes were ob-
tained. For instance, the average size of the copper nanopar-
ticles was varied between 30 to 80 nm by simply changing
the concentration of PAA. Furthermore, the pH of the solu-
tion was determined to play a key role for the formation of
pure copper nanoparticles as opposed to copper oxide nano-
particles. By adjusting the amount of PAA, faceted crystal-
line copper nanoparticles were obtained. These as-prepared
Our studies indicated that setting the pH of the solution
to an appropriate value played an important role to obtain
pure copper nanomaterial as opposed to copper oxide nano-
material. Copper nanoparticles are only formed if the final
pH of the solution ranges between 9.2–10.5. From the UV/
Vis absorption spectra obtained at various pH values
(Figure 1), it was obvious that the intensity and position of
the absorption maxima changed at different pH values. At
lower pH (i.e., 9.2–10.3), the spectra exhibited an absorb-
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Copper Nanoparticles
FULL PAPER
maxima of the copper nanoparticles with respect to the pH
values is also consistent with that reported in reference [33]
for copper oxide and alkanethiol-capped copper nanoparti-
cles synthesized in supercritical water, except for a minor
difference. In the case described in reference [33], pH <6.0
favored the formation of copper nanoparticles and pH >6.0
resulted in oxidized products from the reactions of copper
ions with excess hydroxyl groups.[33] Whereas we have also
observed the formation of a mixture of polymer-capped
copper/copper oxide nanoparticles or pure copper oxide
nanoparticles from CuSO4 in the presence of PAA, this was
observed at significantly higher pH; that is, copper oxide
nanoparticles at pH > ꢁ10.5 and metallic copper nanoparti-
cles between pH 9.2–10.5. It should be emphasized again
that the pH range that produces copper versus copper oxide
varies for different surface ligands; that is, whether alkane-
thiol, PAA, or PVP are used as capping agent during the
synthesis of the nanoparticles. The latter observation can be
explained on the basis of the pKa values of these capping
agents. PAA has a pKa value of 4.0. In the range of pH
ꢁ9.5–10.5, where we obtained copper nanoparticles, PAA
exists predominantly in its negatively charged form because
of significant abstraction of its acidic protons by the basic
solution. Besides serving the purpose of reducing the hy-
droxyl ion concentration in the solution, the negatively
charged PAA polymer will cap the copper nanoparticles
better. However, as the ligands such as PVP and alkanethiol
do not react with or reduce the hydroxyl ions, all the hy-
droxyl groups will react with the copper ions to form copper
oxides, even at relatively lower starting pH. Our studies
demonstrated that the reduction of copper(II) ions with hy-
drazine in the presence of PAA capping ligand in an appro-
priate pH range of 9.2–10.5 yields polymer-coated and
water-dispersible pure copper nanoparticles, whose yield is
dependent on the pH of the solution as shown in Figure 1A.
The as-synthesized copper nanoparticles appear red in color
and are stable for weeks as also observed by UV/Vis spec-
troscopy. These nanoparticles were quite stable as their plas-
mon band barely changed even after eight weeks (see the
Supporting Information, Figure S1).
Figure 1. A) UV/Vis absorption spectra of copper nanoparticles synthe-
sized in 1.0 mm aqueous PAA solution at different pH: a) 9.2, b) 10.0,
c) 10.3, d) 10.5, and e) 11.0. B) Absorption maximum of copper nanopar-
ticles versus the pH value.
ance maximum at ꢁ565 nm, the characteristic plasmon ab-
sorption band for copper nanoparticles. The intensity of this
plasmon peak markedly increased when the pH increased
from 9.2 to 10.3, indicating the increase in the number or
yield of copper nanoparticles. At a slightly higher basicity
(or at pH 10.5), the absorption maximum appeared at l=
570 nm. When the pH of the solution was increased to 11.0
with the addition of more NaOH, the UV/Vis absorption
peak shifted to 575 nm, whereas its absorption intensity de-
creased. It has been reported that metallic copper nanopar-
ticles that are surrounded by a copper oxide shell are char-
acterized by a peak centered at about l=570 nm with a re-
sidual absorption peak at l=800 nm.[32] Thus, the absorption
maximum at 575 nm that we have observed for the sample
synthesized at pH 11.0 could be attributed to the formation
of copper nanoparticles with a copper oxide monolayer
around them.[32] However, it is worth noting that the absorp-
tion maximum at 800 nm was not observed in the latter
case, implying that the copper oxide shell is only a thin
XPS is widely used to confirm the chemical composition
of copper nanoparticles.[7] In particular, the peak corre-
sponding to the CuACTHNUTRGNEUNG(2p3/2) core level is commonly used to an-
alyze the degree of oxidation of Cu. Figure 2 shows the XPS
spectra of the copper nanoparticles synthesized with the
method presented here. The reference peak for C1s was set
at 284.7 eV. The major peak for CuACTHNUTRGNE(UNG 2p3/2) was observed at
932.8and 932.5 eV for the samples prepared with 1.0 and
2.0 mm PAA, respectively. This peak can be attributed to
zerovalent copper.[7a,e–n] Furthermore, a peak for Cu
ACHTUNGTRENNUNG(2p1/2)
mono
layer, if any. This was further confirmed by X-ray pho-
was observed at 952.7 and 952.3 eV, respectively, which also
correspond to Cu0. Peaks corresponding to Cu2+ were
barely observed, ruling out the possibility of the presence of
CuO or Cu(OH)2 in these samples. However, as shown in
toelectron spectroscopy (XPS; see below). It can be con-
cluded that in the pH range of 10.5–12.0, a very low propor-
tion of copper atoms on the copper nanoparticles form
copper oxide. However, upon raising the pH to 12.0 by
adding more NaOH, we observed the exclusive formation of
Cu2O nanoparticles. The obtained trend in the absorption
Table 1 and as reported by others,[7] the Cu
ACHTNUTGRNEUNG(2p3/2) peaks for
Cu+ and Cu0 overlap. Consequently, our result can not rule
out the presence of Cu2O. In fact, a small oxygen peak was
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T. Asefa et al.
Reference
Table 1. XPS data for Cu and for its oxidized forms.
Materials/compound
Binding energy [eV]
Cu(2p3/2 CuCATHUNGRNTE(NUG 2p1/2)
A
)
Cu0
932.7–932.8
932.6
932.9
932.5–932.8
932.6
933.5–933.6
933.8
[7e]
[7f,g–i]
[7m]
[7e]
[7h–i]
[7e]
952.7
Cu2O
CuO
[7f,h,j]
[7i]
934.0
Cu(OH)2
934.6
[7f]
934.7
[7k]
935.0
[7g,j,l]
also observed as described in references [7m] and [7n].
Based on previous results (ref. [7]), we have assigned the
small oxygen peak to be due to the presence of a small
Cu2O layer around the nanoparticles. Nevertheless, we wish
to emphasize that this oxide layer is a result of the unavoid-
able exposure of the samples to air and the necessity of
being in a dried form for the XPS measurement. In fact, we
noticed a slight color change of the sample to greenish color
during this measurement. Their reddish color and their UV/
Vis absorption maxima remained unchanged when they
were in solution suggesting that they were purely metallic
copper nanoparticles in solution. Upon deliberate exposure
of the dried copper nanoparticle samples to air for longer
time, a significant portion of the nanoparticles was oxidized
[7]
to CuO or Cu(OH)2 (Figure S1 in the Supporting Informa-
tion). Figure S2 in the Supporting Information shows a
major peak at 933.1 eV and some satellite peaks at 943.1
and 939.9 eV, which correspond to CuACTHNUTRGNEUNG
(2p3/2) of CuO,[7] for
copper nanoparticles left under air for a week.
Controlling the size and shape of PAA-capped copper nano-
particles: Controlling the nanoparticle size and shape can
provide an elegant strategy for tuning optical properties of
nanomaterials. Here, the size and shape of the copper nano-
particles were found to strongly depend on the concentra-
tion of PAA. The size of the nanoparticles can be decreased
from ꢁ80 to ꢁ30 nm simply by increasing the amount of
PAA while keeping all other parameters, including the pH,
fixed. Figure 3 shows the TEM images of the nanoparticles
obtained by using a higher amount of PAA protecting
agent. The TEM images further confirm the formation of
nanoscale copper material with varying diameter and inter-
esting morphologies. When the concentration of PAA was
low (1.0 mm), the nanoparticles were large and most of
them had spherical shape. The average diameter for these
copper nanoparticles was ꢁ80 nm. When the PAA concen-
tration was increased to 1.33 mm, faceted crystalline Cu
nanoparticles with edge length of approximately 50 nm were
formed (Figure 3B). At a PAA concentration of about
2.0 mm, the size of the nanoparticles decreased whereas
their polydispersity increased. The TEM images showed that
the size of the nanoparticles decreased from 80 to 30 nm
Figure 2. A,B) X-ray photoelectron spectra of copper nanoparticles pre-
pared with 1.0 mm PAA showing: A) all the main peaks and B) the peaks
corresponding to Cu
tra of copper nanoparticles prepared with 2.0 mm PAA showing: C) all
the main peaks and D) the Cu(2p3/2) and Cu(2p1/2) peaks.
ACHTUNGTRENNUNG(2p3/2) and CuAHCTUGNTERN(NUGN 2p1/2). C,D) X-ray photoelectron spec-
A
ACHTUNGTRENNUNG
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Figure 4. A) TEM images of faceted crystalline copper nanocrystals pre-
pared by using a 1.33 mm PAA solution. B) TEM image of an individual
faceted crystalline copper nanoparticle. C) Electron diffraction (ED) pat-
tern of an individual faceted crystalline nanoparticle.
rhombus shape was not observed, which excluded the possi-
bility of the octahedral shape. So the as-synthesized copper
nanoparticles must have cuboctahedral shape. Figure 4B
shows a representative TEM image of the hexagon, consis-
tent with the hexagonal projection of an ideal cuboctahe-
dron. Typical TEM images of a copper nanoparticle before
and after tilting by 308 are shown in Figure S3 in the Sup-
porting Information. The TEM images show that the shape
of the particle did not change when the particle was tilted
by 308, indicating that the copper nanoparticles were cuboc-
tahedral in shape. It is worth mentioning that in this cuboc-
tahedron projection, four {111} and two {100} facets are
placed on the edges of the hexagonal shape. Other facets
represent the projections from the cuboctahedral structure
along different orientations. The corresponding selected-
area electron diffraction (SAED) patterns of the nanoparti-
cles are shown in Figure 4C. The SAED patterns showed
diffraction rings corresponding to 111, 200, 220, and 311
characteristic reflections. Unfortunately, our attempts to get
scanning electron microscopy (SEM) images for the parti-
cles did not yield clear images because of the continuous
change of the shapes and morphologies of the copper nano-
particles in the electron beam, possibly due to melting, as
well as oxidation of the surface of the nanoparticles into a
non-conducting copper oxide. In fact, the nanoparticles con-
tinuously get fuzzier while under the beam for longer and
longer time during imaging. Coating the particleꢁs surface
with carbon and lowering the electron beam did not help
either. Nonetheless, some SEM images were obtained and
these are shown in Figure S4 in Supporting Information.
These SEM images still exhibit particles with shaped mor-
phologies, although the shapes are not quite clear.
Figure 3. TEM images of copper nanoparticles synthesized in aqueous
PAA solution with different concentration: A) 1.0, B) 1.33, C) 1.66, and
D) 2.0 mm.
upon increasing the PAA concentration from 1.0 to 2.0 mm.
This is consistent with the fact that the protecting effect of
the polymer increases with its concentration favoring the
formation of smaller nanoparticles.
In addition to tuning the sizes of nanoparticles, a certain
amount of PAA was also indispensable in controlling the
morphology of the copper nanoparticles. The TEM images
(Figure 3) showed that most of the nanoparticles synthesized
with lower PAA concentration (1.0 mm) had spherical
shape. The increase of the PAA concentration from 1.0 to
1.66 mm and 2.0 mm induced the formation of some smaller
spherical copper nanoparticles as well as some elongated
nanoparticles (or nanorods) with an aspect ratio of ꢁ2
(Figure 3).
Interestingly, with a moderate amount of PAA (1.33 mm),
nanoparticles with predominantly faceted crystalline struc-
ture that look more like cuboctahedral-shaped were formed
(Figure 3B). Hence, varying the PAA concentration not
only induces a change in the size of the copper nanoparti-
cles, but also tunes their shapes. An enlarged TEM image
for the sample prepared by adjusting the PAA concentration
to 1.33 mm is shown in Figure 4A. It reveals many well-trun-
cated cuboctahedron-shaped copper nanoparticles. These
cuboctahedral-shaped nanoparticles exhibit a projection of
hexagonal shape with an average edge length of 50 nm. It is
well known that a cuboctahedral particle lying on a flat sur-
face against one of its faces rather than its corner or edge
also shows a projection of hexagon. But the octahedral
nanoparticles usually exhibit two differently projected
shapes, namely, a rhombus and a hexagon, depending on
their orientation relative to the electron beam. Here, the
The size-controlled copper nanoparticles that were de-
scribed in Figure 3 were further characterized by UV/Vis ab-
sorption spectroscopy. Figure 5 shows the UV/Vis absorp-
tion spectra and digital images of the aqueous dispersions of
copper nanoparticles, which were synthesized by using dif-
ferent concentrations of PAA. As observed from the absorp-
tion spectra, the absorption bands of the copper nanoparti-
cles strongly depend on the concentration of PAA. At
higher PAA concentration (1.66 and 2.0 mm), the absorption
spectra of the copper nanoparticles showed plasmon peaks
centered at 562 and 560 nm, respectively. For the faceted
crystalline copper nanoparticles, which corresponds to a
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T. Asefa et al.
marked change in shape.[35] Therefore, based on this previ-
ous reports, the shift of the absorption maxima may also be
attributed to the different shapes of the copper nanoparti-
cles. Furthermore, shifts in absorption maxima are often at-
tributed to changes in the size of nanoparticles. For example,
in the case of Ag nanoparticles, a redshift of 14 nm was re-
ported as the particle size change,[36] which is consistent with
the general trend that the plasmon maximum shifts to
longer wavelength with an increase in particle size.[37]
Hence, the redshifts that we observed for the absorption
maxima of the copper nanoparticles in Figure 5B could be
attributed to the changes in size and/or shape of the nano-
particles. However, as both the sizes and shapes of the Cu
nanoparticles changed, it is not clear what may have contrib-
uted most to this shift of the absorption maxima.
The sizes of the nanoparticles were further analyzed by
dynamic light scattering (DLS) (Figure S5 and S6 in the
Supporting Information). Two samples of the copper nano-
particle that were prepared with PAA concentration of 1.33
and 2.0 mm were investigated. The results showed that the
nanoparticles prepared with a 1.33 mm solution of PAA
have an average particle size of 85.5 nm (Figure S2 in the
Supporting Information), whereas the nanoparticles pre-
pared with a 2.0 mm solution of PAA exhibited an average
particle size of 118.4 nm (Figure S3 in the Supporting Infor-
mation). These sizes were expectedly higher than those ob-
tained by TEM for the corresponding samples. This is be-
cause DLS measurements take the hydrodynamic radius of
the polymer around the nanoparticles into account.
Figure 5C displays digital images of aqueous solutions of
PAA-capped copper nanoparticles, which show colors rang-
ing from reddish-brown to deep wine-red, depending on the
PAA concentration used in the synthesis. It is also worth
noting that the greenish color associated with copper(I) and
copper(II) oxide nanoparticles was absent, further indicating
that the solutions possessed most likely pure copper nano-
particles.
Figure 5. A) UV/Vis absorption spectra of copper nanoparticles synthe-
sized in aqueous PAA solution of different concentration: a) 1.0,
b) 1.33 mm, c) 1.66, and d) 2.0 mm. B) Absorption maximum of the
copper nanoparticles versus the PAA concentration. C) Photograph of
solutions of copper nanoparticles synthesized with different PAA concen-
tration: a) 1.0, b) 1.33, c) 1.66, and d) 2.0 mm.
Figure 6 displays the powder X-ray diffraction (XRD)
pattern of the as-synthesized copper nanoparticles. The dif-
fraction peaks at 2q=43.5, 50.6, and 74.38 can be indexed as
PAA concentration of 1.33 mm, the absorption spectrum is
characterized by a maximum centered at 565 nm. At low
PAA concentration (1.0 mm), the absorption maximum is
centered at 570 nm. It is interesting to note that the optical
absorption of the copper nanoparticles synthesized from
PAA concentration of 1.0 mm exhibited another shoulder
around 430 nm. With an increase in the PAA concentration
from 1.0 to 2.0 mm, we clearly observed a shift of the ab-
sorption maxima of ꢁ10 nm (Figure 5B). It has been report-
ed that a peak centered at 560 nm can be attributed to
spherical and cuboctahedral copper nanocrystals.[34] On the
other hand, a resonance in the absorption spectrum at lower
energies was reported for copper nanocrystals having a
Figure 6. XRD pattern of copper nanoparticles synthesized with a 1.0 mm
PAA solution.
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Copper Nanoparticles
FULL PAPER
the [111], [200], and [220] planes of copper nanoparticles.
These results are also quite consistent with those reported
previously for metallic copper.[38] Furthermore, no diffrac-
tion peaks corresponding to copper oxides were detected,
confirming the purity of the
ture, we have conducted the three component Mannish reac-
tion (Scheme 2) by using our Cu nanoparticles as a catalyst.
In the presence of a slight amount of base (CsCO3), 53%
yield of the three-component Mannish product were ob-
copper nanoparticles obtained
by this synthetic method. This
result is also consistent with the
XPS data.
Catalytic activity: Metal-cata-
ꢀ
lyzed C N coupling reactions
Scheme 2. Nanoparticles-catalyzed Mannich reaction between acetophenone, benzaldehyde, and aniline.
of aryl halides and heterocycles
is an important reaction[39] as its
products have a wide range of
applications in medicinal,[40] biological,[41] and N-heterocyclic
carbene chemistry.[42] Copper, as well as Cu/Cu2O/CuO
nanostructures, are reported to be highly active catalysts for
tained in 24 h at room temperature; remaining reactants
ended up by forming the imine byproduct. After the reac-
tion, the nanoparticles were found to remain stable and did
not aggregate. As shown in the TEM images in Figure S11
in the Supporting Information, the sizes of the nanoparticles
were unchanged after catalysis, but the edges of the faceted
crystalline structure were slightly destroyed. Furthermore,
the UV/Vis spectra of the nanoparticles after the reaction
showed a change in the position of the absorption maxima,
indicating some oxidation after catalysis. The catalytic per-
formance of the nanoparticles was also lower, giving 3 times
less reactant conversion in the same period of reaction time
compared to the original sample.
[7c,43]
ꢀ
C N coupling and oxidation reactions.
On the other
hand, metal nanocrystals with large number of edge and
corner atoms are known to have improved catalytic perfor-
mance.[44] Furthermore, the reactivity and selectivity of a
nanocatalyst can depend on its shape, which influences the
number and type of atoms located at the edges or corners.[45]
Therefore, we have evaluated the catalytic performance of
the faceted crystalline copper nanoparticles that we have
synthesized for N-arylation of heterocycles (Scheme 1).[46]
Conclusion
In summary, we have successfully demonstrated the synthe-
sis of water-dispersible, cuboctahedron-shaped pure copper
nanoparticles by reducing Cu2+ with hydrazine at an appro-
priate pH in the presence of PAA as protecting agent. The
shape and diameter of the nanoparticles were tuned by vary-
ing the concentration of PAA in the solution. We have dem-
onstrated that cuboctahedral-shaped copper nanoparticles
can be obtained by using moderate concentration of PAA.
With increase in the size of the copper nanoparticles, a red-
shift in the UV/Vis absorption band was observed. The as-
synthesized cuboctahedral copper nanoparticles showed
ꢀ
Scheme 1. Nanoparticles-catalyzed C N coupling reaction.
The materials gave a complete conversion of the starting
material in 2.5 h reaction time with a selectivity of ꢁ99%
for the N-arylation product. The product was confirmed by
1H NMR and FTIR spectroscopy (see the Experimental Sec-
tion and Figures S9 and S10 in the Supporting Information).
These results prove that the cuboctahedron-shaped copper
nanoparticles might be used as catalysts for a very valuable
ꢀ
ꢀ
and difficult to do C N coupling reaction. The N-arylation
high catalytic activity in promoting C N coupling reactions,
of heterocycles is often performed in the presence of a base
such as cesium carbonate.[46] In the presence of a high con-
centration of cesium carbonate at 1208C, the copper nano-
particles could undergo some oxidation and a color change
from reddish to greenish is observed (Figure S7 and S8 in
the Supporting Information). The resulting oxidized copper
nanoparticles also show catalytic activity. The latter is not
surprising as many earlier reports have suggested that both
copper and copper oxide and even their combinations as
as demonstrated by the reaction between p-nitrobenzyl chlo-
ride and morpholine forming 4-(4-nitrophenyl)morpholine
in an excellent yield and good selectivity (Figure S9 and S10
in the Supporting Information). Furthermore, the nanoparti-
cles were proven to catalyze the three-component, one-pot
Mannich reaction giving selectively the Mannich product. In
addition, to stabilize the nanoparticles and offer them water
dispersibilty, the carboxylic acids of the PAA capping ligand
can potentially be used as an abundant anchoring point for
further attachment of other functional groups and bioactive
species for bioanalysis by using the surface enhanced
Raman scattering (SERS) property of copper nanoparticles.
Cu/Cu2O/CuO serve as active catalysts for various types of
C C coupling reactions.
These Cu nanoparticles were proven to be versatile in
their catalytic activities. As previously reported in the litera-
[7c,43c,47]
ꢀ
Chem. Eur. J. 2010, 16, 10735 – 10743
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10741

T. Asefa et al.
stirred at room temperature. The reaction product was characterized by
GC after 24 h and 48 h.
Experimental Section
Materials and reagents: Copper(II) sulfate, hydrazine hydrate solution
(N2H4·H2O solution; 78–82%), poly(acrylic acid) (PAA; Mw =1,800), p-
nitrobenzyl chloride, morpholine, aniline, benzaldehyde, acetophenone,
and cesium carbonate were obtained from Sigma–Aldrich. Sodium hy-
droxide (98.8%) and dimethyl sulfoxide (DMSO) were purchased from
Fisher Scientific. All the chemicals were directly used as received without
further treatment.
Acknowledgements
We gratefully acknowledge the financial assistance by the US National
Science Foundation (NSF), CAREER Grant No. CHE-0645348 and NSF
DMR-0804846 for this work. We thank Dr. Jonathan Shu at Cornell Uni-
versity for his help with the XPS measurement. We also thank Prof.
Qingrong Huang at Rutgers for allowing us to use the dynamic light scat-
tering instrument in his lab and for some valuable discussion on the re-
sults.
Synthesis of copper nanoparticles: In a typical synthesis of copper nano-
particles, a mixture of CuSO4 (0.10 mmol) and various amount of PAA
were completely dissolved in nanopure H2O (20 mL) under vigorous stir-
ring at 608C for 20 min, giving a transparent light-blue solution. The
PAA concentration varied from 1.0–2.0 mm. Then, 0.95–1.2 mL NaOH
(0.5m) was added dropwise to the solution to adjust the pH. Precipitation
of copper(II) hydroxide or copper oxides was not observed because the
pH was not high enough. After the solution was stirred for 20 min at
608C, a solution of N2H4·H2O (2.0 mmol) was added to the solution with
constant stirring. The reaction flask was kept in a water bath at 608C for
30 min, and the particles were allowed to age for 24 h at room tempera-
ture. It should be pointed out that both NaOH and N2H4·H2O can affect
the pH of the final solution. The metallic copper nanoparticles in this
work were produced following the redox reaction given in Equation (1)
under basic conditions.
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2 Cu2þ þ N2H4 þ 4 OHꢀ ! 2 Cu þ N2 þ 4 H2O
ð1Þ
Characterization: Samples for transmission electron microscopy (TEM)
were prepared by casting a drop of the as-prepared copper nanoparticles
suspension on a carbon-formvar-coated copper grid. The grid was then
placed under a JEOL 1200 EX and FEI Tecnai T12 TEM transmission
electron microscopes. The images were taken at an acceleration voltage
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a
Lambda 950 spectrophotometer (PerkinElmer). The X-ray diffraction
patterns (XRD) of the samples were measured by using a Bruker Scintag
powder diffractometer. GC-MS was measured with a HP 5971 GC-MS
spectrometer. The solution 1H NMR spectrum was measured by using a
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on a Nicolet IR 200 by dissolving the 4-(4-nitrophenyl)morpholine in eth-
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9000AT Digital autocorrelator.
ꢀ
Catalysis: In a typical C N coupling reaction, a 50 mL three-necked
round bottom flask was charged with p-nitrobenzyl chloride (1 mmol),
morpholine (1.1 mmol), cesium carbonate (2 mmol), and Cu nanoparti-
cles (0.025 mmol) in DMSO (5 mL). To prepare the Cu nanoparticles in
DMSO, an aqueous solution of Cu nanoparticles (5 mL) prepared from a
solution of PAA (1.33 mm) were centrifuged. After decanting the water
and washing the nanoparticles with DMSO once, the particles were dis-
persed in DMSO (5 mL). The reaction mixture was heated to 1208C for
2.5 h. Upon completion, the reaction mixture was extracted by using di-
chloromethane and the organic extract was dried over anhydrous sodium
sulfate to give the desired 4-(4-nitrophenyl)morpholine as a crystalline
solid. The formation of the reaction product was confirmed by GC–MS,
1H NMR, and FTIR spectroscopy and its yield was determined to be
1
99% by GC. H NMR (300 MHz, CDCl3): d=8.12 (d, 2H; aromatic CH-
C-NO2), 7.52 (d, 2H; aromatic CH), 3.86 (t, 4H; N-CH2), 3.35 ppm (t,
ꢀ
24H; O-CH2); FTIR (KBr): n˜ =2967, 2901 (aromatic C H stretching),
1205 cmꢀ1 (C N stretching mode); GC–MS: m/z: 208.
ꢀ
For the Mannich reaction, acetophenone (1 mmol), benzaldehyde
(1 mmol), and aniline (1 mmol) were mixed in methanol (10 mL) in a
50 mL round bottom flask and stirred under a nitrogen atmosphere at
room temperature. To this solution, Cu nanoparticle (10 mol%, 50 nm)
and cesium carbonate (200 mg) were added. The reaction mixture was
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10742
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 10735 – 10743

Copper Nanoparticles
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Received: February 10, 2010
Published online: July 20, 2010
Chem. Eur. J. 2010, 16, 10735 – 10743
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10743