Economical and green synthesis of Cu nanowires and their use as a catalyst for selective hydrogenation of cinnamaldehyde

Article abstract of DOI:10.1039/c4ra06680d

The reduction of Cu(OH)₂by hydrazine in a concentrated NaOH solution containing ethylenediamine at 70 °C has been employed to prepare Cu nanowires. The net change during nanowire synthesis is that the consumption of Cu(OH)₂and hydrazine gives rise to the formation of Cu nanowires, N₂and H₂O, which make it possible for us to endlessly reuse the produced wastewater after the removal of Cu nanowires. By controlling the re-added amounts of hydrazine and ethylenediamine in the case of reusing the wastewater, we have found that the morphology of Cu nanowires is kept almost unchanged even though the wastewater is recycled more than ten times. The recycle of the endlessly reusable wastewater not only significantly reduces the cost of Cu nanowires but also makes this approach environmentally friendly. In addition, as a catalyst for selective hydrogenation of cinnamaldehyde, the as-formed Cu nanowires show different catalytic performance in comparison with the unsupported Cu nanoparticles. This journal is

Full text of DOI:10.1039/c4ra06680d

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Economical and green synthesis of Cu nanowires
and their use as a catalyst for selective
hydrogenation of cinnamaldehyde†
Cite this: RSC Adv., 2014, 4, 41683
*
Yifan Zheng, Jihong Liang, Yuanyuan Chen and Zongjian Liu
The reduction of Cu(OH)2 by hydrazine in a concentrated NaOH solution containing ethylenediamine at
70 ꢀC has been employed to prepare Cu nanowires. The net change during nanowire synthesis is that
the consumption of Cu(OH)2 and hydrazine gives rise to the formation of Cu nanowires, N2 and H2O,
which make it possible for us to endlessly reuse the produced wastewater after the removal of Cu
nanowires. By controlling the re-added amounts of hydrazine and ethylenediamine in the case of
reusing the wastewater, we have found that the morphology of Cu nanowires is kept almost unchanged
even though the wastewater is recycled more than ten times. The recycle of the endlessly reusable
wastewater not only significantly reduces the cost of Cu nanowires but also makes this approach
environmentally friendly. In addition, as a catalyst for selective hydrogenation of cinnamaldehyde, the as-
formed Cu nanowires show different catalytic performance in comparison with the unsupported Cu
nanoparticles.
Received 5th July 2014
Accepted 27th August 2014
DOI: 10.1039/c4ra06680d
www.rsc.org/advances
production of Cu nanowires mainly include electrodeposi-
tion,11–13 hydrothermal reduction method,14–17 and low-temper-
ature solution reduction method.6,7,18–20 Among these methods,
Introduction
Due to their high aspect ratio and large specic surface area,
one-dimensional (1D) materials, e.g. nanowires and nanotubes,
oen exhibit physical, chemical, and/or mechanical properties
that differ from those of their bulk counterparts, and thus are of
great importance from both theoretical and practical points of
view.1–4 1D Cu nanowires have drawn increasing attention in
recent years because of their excellent properties and promising
applications in many elds, such as electronics and catalysis.5–10
Owing to their superior electrical properties, for example, Cu
nanowires can be used as interconnects in computer chips.5 In
addition, Cu nanowires are excellent building blocks for
the reduction of Cu(NO3)2 by hydrazine in a concentrated NaOH
solution containing ethylenediamine (EDA) at low temperature
(60–80 C), rst developed by Zeng and co-workers,18 is partic-
ꢀ
ularly attractive due to its simplicity and robustness. Recently
this method has been modied by Wiley and co-workers and the
as-formed Cu nanowires were used for fabrication of trans-
parent conducting lms.6,7 More recently, we have found that
other linear alkyl diamines with short carbon-chain length can
also be used in the growth of Cu nanowires.20 In this diamine-
mediated method, the cost of Cu nanowires is dominated by the
consumption of NaOH and EDA.5 Furthermore, how to treat the
produced wastewater which contains large amount of NaOH
and EDA is crucial for its practical application. In the present
work, we will show that, by replacing the Cu source (namely
Cu(NO3)2) with Cu(OH)2 and recycling the produced wastewater,
this diamine-mediated method can be turned into a highly
economical and green way for synthesis of Cu nanowires.
Moreover, we have found that the as-prepared Cu nanowires
show much higher activity and selectivity for hydrogenation of
cinnamaldehyde to cinnamylalcohol compared with unsup-
ported Cu nanoparticles.
producing cheap and exible transparent conducting lms,6–8
a
promising alternative to indium tin oxide-based transparent
conducting lms used for exible electronics. As reported by
Yao et al., Cu nanowires also possess good thermal conductivity
and can be applied in pool boiling enhancement.9 In the case of
catalytic application, Alia et al. have found that platinum-coated
copper nanowires show high activity for hydrogen oxidation
reaction in a basic solution.10
From a practical point of view, the search for a cheap, envi-
ronmentally friendly way for production of Cu nanowires is
essential to their applications. So far, the methods for
Experimental
Synthesis of Cu nanowires
Institute of Industrial Catalysis, College of Chemical Engineering and Materials
Science, Zhejiang University of Technology, Hangzhou, 300014, P.R. China. E-mail:
zjliu@zjut.edu.cn
The synthesis of Cu nanowires was conducted in a glass
container with the preparation procedure being illustrated in
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra06680d
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Fig. 1. In a typical synthesis, about 0.1 g of Cu(OH)2 was rst
placed in the container. Then a concentrated NaOH solution
obtained by dissolving 120 g of NaOH into 200 ml of deionized
water was added to form a mixture (Step I in Fig. 1). Aer stir-
ring for several minutes, 1.5 ml of EDA and 0.25 ml of reducing
agent (namely 35 wt% hydrazine solution) were successively
added into the above mixture (Steps II and III in Fig. 1. Note that
the amount of EDA or hydrazine could be varied to investigate
its effect on the morphology of Cu products). The obtained
mixture was placed in a 70 ꢀC waterbath without stirring for 1 h
(Step IV in Fig. 1). Aer the reaction, the Cu product was
removed from the solution by centrifugation and then stored in
a container containing 3 wt% hydrazine solution at room
temperature (for catalytic application, the product was washed
thoroughly with ethanol and followed by vacuum drying) (Step V
in Fig. 1). The solution le (namely the wastewater) was reused,
Fig. 2 SEM images of the products formed at 70 ꢀC for 1 h in the
and about 0.1 g of Cu(OH)2 was added into it to initiate next presence of different amounts of hydrazine with the amount of EDA
being fixed at 1.5 ml: (a) 0.125 ml, (b) 0.25 ml, (c) 0.50 ml, and (d)
0.75 ml. Insets of (a and d) are high-magnification SEM images of the
corresponding products.
round of nanowire synthesis (Step VI in Fig. 1). Since both EDA
and excess hydrazine were present in the wastewater, only small
amount of EDA or hydrazine were re-added when the waste-
water was recycled (Steps VII and VIII in Fig. 1). As indicated by
the red arrows, the repeat of Steps VI, VII, VIII, IV, and V rep-
the fresh and spent Cu nanoparticles were obtained from a
resented a cycle. For comparison of catalytic activity, Cu nano-
Micromeritics ASAP 2010 sorptometer. The catalytic activity of
particles were prepared by the reduction of Cu(NO3)2 (the
Cu nanowires or Cu nanoparticles was tested in an autoclave
solvent was ethylene glycol) at 60 ꢀC for 30 min with an alkaline
(100 ml) under 4.5 MPa of H2 at 120 ꢀC for 4 h, where 0.05 g of
catalyst was added in a mixture of 2 ml cinnamaldehyde, 38 ml
hydrazine solution (1 mol Lꢁ1 NaOH) as reducing agent. Aer
being removed from the solution by centrifugation, Cu nano-
isopropanol and 0.30 mmol HAc. The stirring rate during the
particles were washed thoroughly with ethanol and then dried
reaction was 800 rpm. The concentrations of the reactant and
under vacuum.
products aer the reaction were measured by gas chromatog-
raphy (Shimadzu GC-2014) with a FID detector and a HP-5
column.
Characterization and catalytic test
The morphology of the Cu nanowires was examined using a
scanning electron microscope (SEM, Hitachi S-4700) operating
at 15 kV. The phase composition of the samples was analyzed by
X-ray diffraction (XRD) on a Thermo ARL XTRA X-ray diffrac-
Results and discussion
Preparation of Cu nanowires via recycling wastewater
tometer using Cu Ka X-ray source. The microstructure investi- When Cu(NO3)2 is used as the Cu source, the reaction between
Cu(NO3)2 and N2H4 in a NaOH solution can be expressed as
gations were performed with
a Tecnai G2 F30 S-Twin
Cu(NO3)2 + 2NaOH + 1/2N2H4 ¼ Cu + 2H2O + 2NaNO3 + 1/2N2.
transmission electron microscopy (TEM) operating at 300 kV.
The nitrogen adsorption and desorption isotherms at 77 K for The equation hints that the consumption of N2H4, Cu(NO3)2
and NaOH gives rise to the formation of Cu, H2O and NaNO3.
Aer the solid Cu is removed, the generated wastewater
contains EDA, NaOH, NaNO3, and excess N2H4. As shown in
Fig. S1 in the ESI,† the presence of large amount of NaNO3 in
the reaction mixture may affect the morphology of Cu nano-
wires. This indicates that, in the case of continuous recycling of
NaNO3-containing wastewater, the accumulation of NaNO3 in
the reaction mixture might change the morphology of the Cu
products when the amount of NaNO3 is high. In contrast, if
Cu(OH)2 serves as the Cu source, the net reaction can be written
as Cu(OH)2 + 1/2N2H4 ¼ Cu + 2H2O + 1/2N2. Aer the removal of
Cu nanowires, the resulting wastewater contains EDA, NaOH,
and excess N2H4, all of which can be reused. This is signicantly
different from the case when Cu(NO3)2 is used as the Cu source,
where NaNO3 is present in the wastewater. The analysis given
above suggests that if we replace Cu(NO3)2 by Cu(OH)2, the
produced wastewater can be recycled endlessly.
Fig.
1 Scheme for preparing Cu nanowires and recycling the
produced wastewater.
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Previous studies have shown that EDA serves as both com-
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plexing agent and capping agent in the growth of Cu nano-
wires.6,18,20 These results indicate that the removal of Cu from
the solution will lead to a decrease in the amount of EDA in the
wastewater because some EDA molecules are adsorbed on the
surface of Cu nanowires. Since the amount of hydrazine in the
wastewater also decreases aer the reaction, how to control the
re-added amounts of EDA and hydrazine is crucial for
producing Cu nanowires of good quality via wastewater recy-
cling. To resolve this problem, we rst need to understand the
effect of the amount of hydrazine or EDA on the morphology of
the Cu nanowires formed via this diamine-mediated method.
Fig. 2 presents the SEM images of the Cu products formed in the
presence of different amounts of hydrazine. As shown in Fig. 2a,
the Cu product grown in the presence of 0.125 ml of hydrazine
exhibits nanowires-like morphology with some nanowires pos-
sessing irregular surfaces (inset of Fig. 2a). If the amount of
hydrazine is raised to about 0.25 or 0.50 ml, the product is still
composed of nanowires but their size becomes more uniform
(Fig. 2b and c). As we further increase the amount of hydrazine
to 0.75 ml, however, many particles appear (Fig. 2d). The
formation of these particles might be related to the fact that the
addition of large amount of hydrazine will lead to a rapid
reduction of Cu(II) to Cu and thus a high supersaturation for the
growth of Cu crystals, which is not favourable for the formation
of the nanowires. The effect of the amount of EDA on the
morphology of the Cu products is shown in Fig. 3. Our early
study has shown that, in the case where Cu(NO3)2 serves as the
Cu source, the product formed in the presence of 4.5 ml EDA
contains large amount of nanoparticles.20 Therefore, the
amount of EDA less than 4.5 ml is used in this case. As can be
seen from Fig. 3, the morphology of the product does not
signicantly change when the amount of EDA is increased from
0.5 to 3.0 ml.
Fig. 4 SEM images of the products formed at different recycle times:
(a) 2, (b) 4, (c) 6, and (d) 8. The amount of EDA and hydrazine added at
each recycle times are 0.5 ml and 0.06 ml, respectively. Insets of (c and
d) are high-magnification SEM images of particle-like materials in the
corresponding products.
range. This implies that, when we reuse the wastewater, a slight
deviation from the originally-used amount of hydrazine or EDA
will not make a signicant change in the quality of Cu nano-
wires. Therefore, we use 1.5 ml of EDA and 0.25 ml of hydrazine
to start the nanowire synthesis, and re-add 0.5 ml of EDA and
0.06 ml of hydrazine into the wastewater at each recycle times to
show the possibility of production of high-quality Cu nanowires
via recycling of the wastewater. Fig. 4 shows the SEM images of
The above results demonstrate that, in the case of preparing
Cu nanowires via this diamine-mediated method, the amount
of hydrazine or EDA is allowed to vary within an appropriate
Fig. 5 (a) SEM image of the nanowires grown in the presence of 1.5 ml
Fig. 3 SEM images of the products formed at 70 ꢀC for 1 h in the EDA and 0.25 ml hydrazine at 70 ꢀC for about ꢂ16 min, and (b) TEM
presence of different amounts of EDA with the amount of hydrazine image of one nanoparticle observed in the product. Inset of (a) is high-
being fixed at 0.25 ml: (a) 0.5 ml, (b) 1.0 ml, (c) 2.0 ml, and (d) 3.0 ml. magnification SEM image of the product.
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the products formed at different recycle times. As shown in from 0.06 to 0.08 ml. A comparison between the results shown
Fig. 4a and b, the product is composed of nanowires if the in Fig. 4 and 6 clearly demonstrates that such an experimental
recycle times is not larger than 4. When the recycle times rea- modication can greatly improve the quality of the Cu products.
ches 6, however, we can nd many particle-like material present As can be seen from Fig. 6d, for example, the product is still
in the product (Fig. 4c). This phenomenon becomes more composed of nanowires even though the recycle times reaches
prominent when the recycle times is raised to 8 (Fig. 4d). A 11. The high magnication SEM (see inset of Fig. 6d) reveals
closer examination of these particles reveals that each particle that the diameter of these Cu nanowires is in the range of tens
contains some short nanowires protruding from it surface, of nanometers to about 200 nm. The XRD pattern of the as-
which is different from the particles shown in Fig. 2d. However, formed nanowires is presented in Fig. 7a. The peaks at 43.2ꢀ,
we have found that these particles are similar to those formed in 50.3ꢀ and 74.0ꢀ can be indexed to Cu with a face-centered cubic
the early growth stage where the wastewater is not reused. As (fcc) structure (JCPDS 04-0836) and correspond to the diffrac-
shown in Fig. 5a, the product grown for about 16 min also tions of (111), (200) and (220) planes, respectively. These sharp
contains many particles. The high-magnication SEM image diffraction peaks suggest that the Cu nanowires are highly
(inset of Fig. 5a), along with the TEM image (Fig. 5b), conrms crystallized. The TEM images shown in Fig. 7b and c conrm
that short nanowires are present on the surface of the particles. that the product is nanowire, rather than nanotube. The
In fact, Cu particles with short nanowires protruding from their reduced fast Fourier transform (FFT) image (inset of Fig. 7c)
surfaces have also been observed in the case where Cu(NO3)2 is suggests that the nanowires grow along the h1ꢁ10i direction,
used as the Cu source.20 The formation of these particles is via which is consistent with the result obtained in the case of
three steps:20,21 homogenous nucleation and growth to produce
Cu nanocrystals of small size, aggregation of these Cu nano-
crystals to form large-sized polycrystalline particles, and
oriented growth (via heterogeneous nucleation) along h1ꢁ10i
direction of some nanocrystals to generate short nanowires. The
observation of large amount of these particles at high recycle
times hints that in these cases the homogenous nucleation and
growth of Cu might also occur in the late growth stage (nor-
mally, the oriented growth via heterogeneous nucleation that
will generate long nanowires is dominant in the late growth
stage). We speculate that the concentration of Cu ions might
still be high in the late growth and thus lead to a high super-
saturation which favours the homogenous nucleation and
growth of Cu. To reduce the concentration of Cu ions in the late
growth stage, we modify our experiment by reducing the
amount of EDA but raising that of hydrazine at each recycle
times. Fig. 6 presents the SEM images of the products formed at
different recycle times with the re-added amount of EDA being
reduced from 0.5 to 0.3 ml and that of hydrazine being raised
Fig. 6 SEM images of the products formed at different recycle times: Fig. 7 XRD pattern and TEM images of the formed nanowires: (a) XRD,
(a) 2, (b) 4, (d) 7, and (d) 11. The amount of EDA and hydrazine added at (b) low-magnification TEM image, and (c) high-magnification TEM
each recycle times are 0.3 ml and 0.08 ml. respectively.
image. Inset of (c) is the corresponding reduced FFT image.
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Fig. 9 Comparison of catalytic performance between Cu nanowires
and unsupported Cu nanoparticles.
nanometers to about 180 nm, which is close to the diameter of
Cu nanowires (see inset of Fig. 6d). From the inset of Fig. 8a,
however, we can nd that these Cu nanoparticles are actually
composed of many small-sized nanocrystals. This is supported
by their XRD pattern (see Fig. 8b), where a larger value of the
peak width at half height can be observed compared to that of
Cu nanowires (see Fig. 7a).
The catalytic performances of both Cu nanowires and
unsupported nanoparticles are presented in Fig. 9. It is obvious
that over the Cu nanoparticle catalyst the hydrogenation of
C]C is favorable, with a selectivity to hydrocinnamaldehyde
being about 80% and that to cinnamylalcohol being only 19%.
It has been reported that in the case of selective hydrogenation
Fig. 8 SEM image (a) and XRD pattern (b) of as-prepared Cu nano-
particles. Inset of (a) is the high-magnification SEM image of the
nanoparticles.
Cu(NO3)2 used as the Cu source.19,20 Although in our experi-
ments we have only recycled the wastewater 11 times, we believe of cinnamaldehyde over supported copper catalysts, specic
that the wastewater can be endlessly reused. Furthermore, since sites resulting from the metal–support interaction were the
the cost of Cu nanowires is dominated by the consumption of main origin of the cinnamylalcohol selectivity.24 Therefore, it is
NaOH and EDA,6 the recycling of the wastewater can signi- not surprising that a low cinnamylalcohol selectivity is observed
cantly cut the cost of Cu nanowires by reducing the consump- over unsupported Cu nanoparticles due to the absence of the
tion of NaOH, EDA, and N2H4 (if the prices of reactants given in metal–support interaction. However, it is interesting to note
ref. 5 is used for calculation, the recycling of the wastewater can
reduce the cost of Cu nanowires by about 80%).
that the selectivity to cinnamylalcohol over Cu nanowires is
about 59%, much higher that over Cu nanoparticles. The result
implies that over Cu nanowires the hydrogenation of C]O is
favorable, different from the case where Cu nanoparticles are
used. Since no supports or promoters are involved and the
Catalytic activity for hydrogenation of cinnamaldehyde
The selective hydrogenation of a,b-unsaturated aldehydes to diameters of both Cu nanowires and nanoparticles are in the
their corresponding unsaturated alcohols, e.g. cinnamaldehyde similar range, we speculate that the difference in cinnamy-
to cinnamylalcohol, is an important process in the fabrication lalcohol selectivity between two kinds of catalysts might be
of various ne chemicals. Traditionally, supported noble metal related to their morphological features. It is reported that the
catalysts, e.g. Ir22 and Pt,23 have been employed due to their adsorption mode of cinnamaldehyde on the surface of Pt cata-
relatively high selectivity. As a non-noble metal catalyst, Cu has lysts depends on the type of the crystal plane.29–31 Cinna-
also drawn some attention because of its low cost. However, in maldehyde prefers to adsorb on the Pt(111) plane through
comparison with noble metal catalysts, copper catalysts do not h1(C]O) coordination, whereas on the Pt(100) plane the
exhibit a high selectivity to unsaturated alcohols. Methods for adsorption is via both h2(C]C) and h1(C]O) modes. This
improving the catalytic properties of copper mainly include indicates that Pt catalysts with more exposed (111) plane benet
modifying the interaction between Cu and its support,24–27 the hydrogenation of C]O and thus favor the formation of
poisoning the catalysts with chlorine or sulfur compounds,28 cinnamylalcohol. In the case of Cu catalysts, it is expected that
and adding the promoters.24,27 Here, we use the as-formed Cu the exposed crystal plane may also affect the selectivity to cin-
nanowires as catalyst for hydrogenation of cinnamaldehyde to namylalcohol. Unlike the Cu nanoparticles which are composed
cinnamylalcohol, which aims to utilize the features of their 1D of small-sized crystals with random orientation, Cu nanowires
morphology. For comparison, Cu nanoparticles with their size grow along h1ꢁ10i direction and thus possess preferentially
similar to that of Cu nanowires were also prepared. As shown in exposed crystal planes. It is possible that the hydrogenation of
Fig. 8a, the size of Cu nanoparticles is in the range of tens of C]O on some exposed planes is favorable, thus resulting in an
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Table 1 BET surface area and pore size of the fresh and spent Cu
nanoparticle catalysts
SBETa (m2 gꢁ1
)
Rporeb (nm)
Vpore (cm3 gꢁ1
)
c
Sample
Fresh
spent
13.1
4.3
18.9
14.2
0.062
0.015
a
b
c
BET surface area. Adsorption average pore diameter. Single point
adsorption total pore volume of pores.
derived from the nitrogen adsorption and desorption isotherms
for both the fresh and spent Cu nanoparticles (see Fig. 11 and
Table 1). The spent Cu nanoparticles possess a Brunauer–
Emmett–Teller (BET) surface area of ꢂ4.3 m2 gꢁ1 and a pore
volume of ꢂ0.015 cm3 gꢁ1, which are much lower than the
corresponding values of the fresh Cu nanoparticles (namely
ꢂ13.1 m2 gꢁ1 and ꢂ0.062 cm3 gꢁ1, respectively).
Conclusions
Fig. 10 SEM images of the spent Cu catalysts: (a) Cu nanowires and (b)
Cu nanoparticles.
In summary, we have replaced Cu(NO3)2 with Cu(OH)2 as the Cu
source to produce Cu nanowires. The resulting wastewater only
contains NaOH, EDA, and N2H4 and can be reused endlessly. By
controlling the amount of EDA and N2H4 added at each recycle
times, Cu nanowires of good quality can be produced even
though the recycle times reaches 11. The present method is not
only a green way but also a highly economic one for production
of Cu nanowires. Moreover, the as-prepared Cu nanowires show
much higher activity and selectivity for hydrogenation of cin-
namaldehyde to cinnamic alcohol in comparison with unsup-
ported Cu nanoparticles.
Fig. 11 Nitrogen adsorption and desorption isotherms for the fresh (a)
and spent (b) Cu nanoparticle catalysts.
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that over Cu nanoparticles (ꢂ11%). This observation surprises
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