Preparation and characterization of Cu(OH)2 and CuO nanowires by the coupling route of microemulsion with homogenous precipitation

Article abstract of DOI:10.1016/j.ssc.2009.12.039

Using CuSO₄ and NH₃{dot operator}H₂O as raw materials, one-dimensional (1D) Cu(OH)₂ nanowires were prepared by the coupling route of microemulsion with homogenous precipitation. The morphology and structure of products were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate that the as-prepared Cu(OH)₂ nanowires are of high purity and are single crystalline with orthorhombic structure. The diameter of Cu(OH)₂ nanowires ranges from 33?nm to 46?nm and the length ranges from 400?nm to 2000?nm. The effects of different conditions such as the molar ratio of water to surfactant (ω), reactant concentration and the aging time on the morphology and the size of the products were studied. 1D CuO nanowires could be simply obtained by the dehydration of Cu(OH)₂, while maintaining the wire-shaped architecture. The catalytic performances of CuO nanocrystals on the thermal decomposition of ammonium perchlorate (AP) were also studied.

Full text of DOI:10.1016/j.ssc.2009.12.039

Solid State Communications 150 (2010) 585–589
Contents lists available at ScienceDirect
Solid State Communications
journal homepage: www.elsevier.com/locate/ssc
Preparation and characterization of Cu(OH)2 and CuO nanowires by the coupling
route of microemulsion with homogenous precipitation
∗
Chengfa Li, Yidong Yin , Haige Hou, Naiying Fan, Fulong Yuan, Yanmei Shi, Qingling Meng
School of Chemistry, Chemical Engineering & Materials, Heilongjiang University, Harbin 150080, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Using CuSO4 and NH3 · H2O as raw materials, one-dimensional (1D) Cu(OH)2 nanowires were prepared
by the coupling route of microemulsion with homogenous precipitation. The morphology and structure
of products were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier
transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM) and transmission electron
microscopy (TEM). The results indicate that the as-prepared Cu(OH)2 nanowires are of high purity and are
single crystalline with orthorhombic structure. The diameter of Cu(OH)2 nanowires ranges from 33 nm to
46 nm and the length ranges from 400 nm to 2000 nm. The effects of different conditions such as the molar
ratio of water to surfactant (ω), reactant concentration and the aging time on the morphology and the size
of the products were studied. 1D CuO nanowires could be simply obtained by the dehydration of Cu(OH)2,
while maintaining the wire-shaped architecture. The catalytic performances of CuO nanocrystals on the
thermal decomposition of ammonium perchlorate (AP) were also studied.
Received 2 December 2009
Received in revised form
22 December 2009
Accepted 29 December 2009
by P. Sheng
Available online 7 January 2010
Keywords:
A. Cu(OH)2
A. CuO
C. Nanowires
D. Catalysis
© 2010 Elsevier Ltd. All rights reserved.
E. Microemulsion
E. Homogenous precipitation
1. Introduction
and superconductors [14,15]. The magnetic properties of Cu(OH)2
are remarkably sensitive to the intercalation of molecular anions
[16–18], making the material a candidate for sensor applications.
One-dimensional (1D) inorganic nanomaterials, such as nano-
wires, nanorods, and nanotubes, have attracted considerable at-
tention in recent years due to their potential applications in
constructing nanoscale electronic devices, optoelectronic devices,
and sensing devices [1–3]. Many methods have been used to pre-
pare 1D nanomaterials, such as laser ablation [4], the plasma
evaporation method [5], chemical vapor deposition (CVD) [6],
solvothermal synthesis [7], hydrothermal synthesis [8], using mi-
croemulsions [9], sol–gel processing [10] and coprecipitation [11].
Of those methods, the microemulsion method has become popu-
lar for the synthesis of 1D nanostructures [12]. But this method is
prone to cause a maldistribution of reactants: the size of the wa-
ter droplets, and the stability of the water droplets and the grains
change. The size distribution of particles thus becomes scattered.
So, the coupling method of microemulsion with homogenous pre-
cipitation was proposed [13].
More importantly, CuO can be obtained through the dehydration of
Cu(OH)2, and the original size and morphology of Cu(OH)2 nanos-
tructures can be retained. Copper(II) oxide (CuO) nanoparticles
are an important inorganic semiconductor with a direct bandgap
value of 1.85 eV [19]. This material has a great potential in differ-
ent technological applications such as gas sensors, magnetic phase
transitions, catalysts, superconductors, lithium ion electrode ma-
terials and field emission (FE) emitters [20–23]. Up to now, var-
ious methods have been developed for obtaining Cu(OH)2 and
CuO nanowires; however, no work has been published concern-
ing the preparation of Cu(OH)2 and CuO nanowires by the coupling
method of homogenous precipitation with microemulsion.
In this work, Cu(OH)2 and CuO nanowires were synthesized
by the coupling route of microemulsion with homogeneous
precipitation, and they characterized by TEM, SEM, XRD, TGA
and FT-IR spectroscopy. The effects of the ω value, aging time,
reactant concentration and the formation mechanism of Cu(OH)2
nanowires were studied and discussed. The results show that the
diameter of Cu(OH)2 nanowires ranges from 33 nm to 46 nm and
the length ranges from 400 nm to 2000 nm. 1D CuO nanowires
could be simply obtained by the dehydration of Cu(OH)2,
while maintaining the wire-shaped architecture. The catalytic
performances of CuO nanocrystals on the thermal decomposition
of ammonium perchlorate were studied by DTA method.
Layered materials, Cu(OH)2 and basic Cu(II) salts with an or-
thorhombic structure have received much attention because of
their potential applications in optoelectronic devices, catalysis,
∗
Corresponding address: School of Chemistry, Chemical Engineering & Materials,
Heilongjiang University, 74 Xuefu Street, Harbin 150080, China. Tel.: +86 451
86608616; fax: +86 451 86608616.
E-mail address: yin_yidong@sina.com (Y. Yin).
0038-1098/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2009.12.039

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C. Li et al. / Solid State Communications 150 (2010) 585–589
2. Experiments
2.1. Materials
The following chemicals were used: copper(II) sulfate pentahy-
drate (CuSO4 ·5H2O), ammonia (NH3 ·H2O), NaOH, CTAB (Hexade-
cyl trimethyl ammonium bromide), n-butanol, and cyclohexane.
All chemicals were analytical grade and used as received without
further purification.
2.2. Preparation of Cu(OH)2 and CuO nanocrystals
1 ml of 28 wt% ammonia was gradually added to 5 ml 0.2 mol/L
aqueous solution of CuSO4 with constant stirring, to prepare
aqueous solution of Cu(NH3)2+
.
4
Fig. 1. TEM image of Cu(OH)2 nanowires.
In a typical experimental procedure, 2 g CTAB and 3 g n-butanol
were added to 18 ml cyclohexane; after that 1.2 ml 0.2 mol/L
Cu(NH3)2+ aqueous solution was injected into this mixture, which
3.2. TGA and FTIR analysis
4
was stirred until a transparent microemulsion was obtained. Then,
the procedure was followed by gradually dropping 0.3 ml 2 mol/L
NaOH aqueous solution into the microemulsions; the solution
instantly turned blue, which implied that nanometer Cu(OH)2
particles had been produced. Then the mixture was constantly
stirred for 30 min and rested for 8 h at room temperature. The
resulting precipitate was obtained by centrifugation and washed
with ethanol and distilled water, respectively. After being dried at
The TGA test of the Cu(OH)2 was conducted simultaneously
◦
at a heating rate of 15 C/min between room temperature and
◦
500 C, and the result is shown in Fig. 2(a). According to the result,
◦
the major mass loss occurs in the temperature range of 140
C
◦
to 190 C, which is due to the hydroxide decomposition to give
CuO. The second mass loss occurs in the temperature range from
◦
◦
190 C to 400 C, which may be attributed to decomposition of the
◦
◦
60 C for 3 h, Cu(OH)2 nanowires were obtained. The Cu(OH)2 was
residual absorbed organic compounds. Thus 400 C is considered
◦
calcined at 400 C for 2 h to produce CuO nanowires. The procedure
as an appropriate calcination temperature for the CuO precursor.
The samples were further examined by FT-IR analysis. As seen
in Fig. 2(b), the FTIR spectrum indicated the existence of CTAB and
cyclohexane molecules in the Cu(OH)2 nanowires. The bands at
described above was repeated with different molar ratios (ω) of
water to surfactant and different salt concentration.
2.3. Catalytic performance on thermal decomposition of ammonium
perchlorate (AP)
−1
3573.3, 1385.2 and 939.7 cm can be assigned to the stretching
mode of the hydroxyl, C–H, and C–C of pure CTAB. The band at
−1
687.1 cm can be assigned to the C–H deformation vibration of
0.02 g CuO was blended with 0.98 g AP (2:98) in an agate
mortar; thermogravimetric analysis (TGA) of the mixture was
performed with a Beijing Optical Instrument Factory WCT-2A
TG-DTA apparatus under an atmosphere with a heating rate of
−1
cyclohexane. The bands at 3309.4 and 1631.2 cm correspond to
the stretching and bending modes of the hydroxyls of adsorbed
water. The band at 423.3 cm−1 can be assigned to Cu–O stretching
mode and may prove that Cu(OH)2 is formed. The CuO precursor
◦
◦
15 C/min from room temperature to 600 C.
◦
was obtained by calcining Cu(OH)2 at 400 C for 2 h. As can be
seen from the FT-IR spectrum, except for the bands at 594.3 and
537.1, which can be assigned to Cu–O stretching mode, all bands
corresponding to the CTAB and cyclohexane impurities disappear,
clearly demonstrating that the impurities have been removed.
2.4. Characterization
The composition and phase purity of the as-prepared products
were characterized by X-ray diffraction(XRD) on a D/Max-3B X-
ray diffractometer with monochromatized Cu Kα radiation (λ =
◦
−1
in a range from
0.15405 nm), at a scanning rate of 8 min
3.3. XRD analysis
◦
◦
10 to 70 ; scanning electron microscopy (SEM) images were
taken on a MX2600FE microscope at an accelerating voltage of
10 kV; transmission electron microscopy (TEM) was performed
on a JEM 2100 microscope with an accelerating voltage of
200 kV; thermogravimetric analysis (TGA) was carried out with
a Beijing Optical Instrument Factory WCT-2A TG-DTA apparatus
It is well known that orthorhombic Cu(OH)2 can be easily trans-
formed to monoclinic CuO upon heat treatment [14]. According
to the TGA analysis, the CuO nanowires were obtained by calcin-
◦
ing Cu(OH)2 at 400 C for 2 h; the corresponding XRD patterns
of the precursor Cu(OH)2 nanowires and the final product, CuO
nanowires, are shown in Fig. 3. Fig. 3(a) is the XRD pattern of the
precursor Cu(OH)2, in which all of the diffraction peaks can be in-
dexed to orthorhombic Cu(OH)2 (JCPDS 13-0420). The XRD pattern
shown in Fig. 3(b) suggests that the as-prepared final product is
pure monoclinic CuO (JCPDS 48-1548). No characteristic XRD peaks
arising from possible impurity such as Cu(OH)2, CTAB and cyclo-
hexane are detected, indicating the complete transformation from
Cu(OH)2 to CuO.
◦
with a heating rate of 15 C/min in an atmosphere from room
◦
temperature to 600 C. Fourier transform infrared (FT-IR) spectra
were recorded on the diluted samples in KBr pellets using a
Perkin Elmer spectrum 100 FT-IR spectrophotometer in the range
−1
4000–400 cm
.
3. Results and discussion
3.1. TEM analysis
The TEM image shown in Fig. 1 reveals the general morphology
of the as-produced Cu(OH)2 nanowires for a reactant concentra-
tion of 0.3 mol/L, ω = 15, and an aging time of 8 h. It can be seen
that the uniform nanowires have a narrow size distribution. Most
of the nanowires have fairly uniform diameters of around 37 nm
and lengths up to 2000 nm.
3.4. Effect of the molar ratio (ω) of water to surfactant
Fig. 4 shows the morphology and the size distribution of
Cu(OH)2 nanocrystals with different molar ratio (ω) of water to
surfactant, for a reactant concentration of 0.2 mol/L and an aging
time of 8 h. As can be seen from these images, when the ω value

C. Li et al. / Solid State Communications 150 (2010) 585–589
587
a
b
temperature (°C)
Fig. 2. TGA and FT-IR curves of Cu(OH)2 nanowires to CuO nanowires.
value is 15 and the aging time is 8 h. As can be seen from these
images, when the reactant concentration is 0.1, 0.2, and 0.3 mol/L,
the average length is 1400, 1650, and 2000 nm, respectively;
however the diameter shows no remarkable change.
As we know, when the ω value is fixed, the size and morphology
of the droplets do not change, but the number of particles that
take part in the reaction will increase when the concentration of
reactant increases; the formation rate of Cu(OH)2 crystal nuclei
becomes rapid and the size of the Cu(OH)2 nanowires increases.
In this work, the length of the nanowires changes obviously, but
the diameter shows no remarkable change. Chen et al. [24] also
observed similar phenomena in the synthesis of barium strontium
titanate nanorods in reverse microemulsion. They suggested that
the crystal nucleus forms and starts to grow along the droplet
morphology to form nanorods after the molecules of the anion
penetrate in the droplet. The transverse growth of the crystal
is prone to end quickly because the diameter of droplets is
much shorter. But it is difficult for the length of the nanorods
to reach that of the droplet, because the length of droplets is
much longer and there are not sufficient reactants to afford the
longitudinal growth of the crystal. This can explain why the length
of nanowires changes obviously, but the diameter does not when
the concentration of reactant increase.
Fig. 3. XRD patterns of Cu(OH)2 nanowires (a) and CuO nanowires (b).
is 5 (Fig. 4(a)), the majority of Cu(OH)2 nanoparticles have a small
size of 30–40 nm (mean size 35 nm) with sphere-like shape and
a narrow size distribution. When the ω value increases to 10
(Fig. 4(b)), 15 (Fig. 4(c)), and 20 (Fig. 4(d)), the morphology of
Cu(OH)2 changes to nanowires: the average diameter is 33, 40,
and 46 nm, respectively, and the average length is 400, 1000,
and 1650 nm, respectively. This indicates that the morphology of
droplets in the microemulsion is sphere-like when the ω value is
low; with the increase of the ω value, the morphology of droplets in
the microemulsion becomes wire-like and both the diameter and
the length of the nanowires increase simultaneously. When the
sizes of the nanowires reach that of the droplets, the surfactants
absorb on the surface of the nanowires and restrict their growth
further. Therefore, by changing the ω value, the microemulsion
method can effectively control the size and morphology of the
product.
3.6. Effect of aging time
Fig. 6 shows the morphology and the size distribution of
the Cu(OH)2 nanocrystals aged for 16 h; the other experimental
conditions are the same as those in Fig. 4(c). In contrast to the
nanowires shown in Fig. 4(c), there is no obvious change in
diameter and length. This shows that there is no significant effect
on the size of Cu(OH)2 nanowires on increasing the aging time.
3.7. Morphology analysis of CuO nanocrystals
3.5. Effect of precursor salt concentration
Figs. 5 and 4(c) show the morphology and the size distribution
of the Cu(OH)2 nanowires with different concentrations; the ω
The CuO nanocrystals were obtained by calcining Cu(OH)2 at
◦
400 C for 2 h, Fig. 7(a) shows the morphology and the size
a
c
b
d
Fig. 4. SEM images of products for different ω values: (a) 5, (b) 10, (c) 15, (d) 20; c = 0.2 mol/L; t = 8 h.

588
C. Li et al. / Solid State Communications 150 (2010) 585–589
325.7 °C
341.1 °C
a
b
304.6 °C
308.1 °C
c
b
a
455.5 °C
315.6 °C
243.6 °C
100
200
300
400
500
600
T / °C
Fig. 5. SEM images of products at different reactant concentrations: (a)0.1 mol/L,
(b)0.3 mol/L; ω = 20; t = 8 h.
Fig. 8. DTA curves for the decomposition of the AP catalyzed by different
morphologic CuO catalyst: (a) pure AP, (b) AP + CuO nanowires, (c) AP + CuO
nanoparticles.
the partial decomposition of AP and the formation of some inter-
mediate NH3 and HClO4 by dissociation and sublimation, and the
◦
high-temperature exothermic peak at 437 C is caused by the com-
plete decomposition to volatile products [26]. It is found that CuO
nanocrystals have no obviously effect on the both endothermic
peak and low-temperature exothermic peak. But all of the high-
temperature exothermic peaks of AP in the presence of different
CuO nanocrystals shift somewhat to lower temperatures; the de-
composition heat is obviously increased when CuO nanocrystals
are mixed with the AP, which indicates that CuO can promote the
thermal decomposition of AP. For nanowires and nanoparticles of
CuO, the temperature shifts for the high-temperature exothermic
peaks of AP are 114.4 C and 129.8 C, respectively. In this work,
the CuO nanoparticles exhibit better catalytic activity than CuO
nanowires. However, in many important fields such as construct-
ing nanoscale electronic devices, the CuO nanowires exhibit better
potential application than CuO nanoparticles.
◦
◦
Fig. 6. SEM images of products aged for 16 h. ω = 20; c = 0.1 mol/L.
a
b
3.9. Formation mechanism of the Cu(OH)2 nanowires
Most researchers consider that the formation of 1D nanostruc-
tures is closely related to the molar ratio (ω) of water to surfactant.
When the ω value is low, the droplets are small. There is lots of
surfactant on the interfaces of the droplets, the boundary strength
of the droplets is higher, the morphology of droplets is sphere-
like in shape, so nanoparticles are obtained. With the increase of
ω value, the volume of droplet increases, the sphere-like shape of
the droplets easily changes to a rod-like shape, and the 1D nanos-
tructure can be obtained.
Fig. 7. SEM image of CuO nanowires.
distribution of the CuO nanoparticles when the other experimental
conditions are the same as those in Fig. 4(a). Compared with
Fig. 4(a), no obvious change is observed in the morphology and
the size distribution. Fig. 7(b) shows the morphology and the size
distribution of the CuO nanowires when the other experimental
conditions are the same as those in Fig. 4(d). In contrast to
the samples without calcination (Fig. 4(d)), the wire-shaped
architecture was still maintained; the morphology and the size
show no remarkable change. This means the Cu(OH)2 has a high
In theory, without the assistance of an additive, the crysta-
llization of Cu(NH3)2+ and OH as source materials also shows
−
4
a tendency toward 1D nanostructures (Fig. 9(a)). Cu2+ prefers
−
−
−
square-planar coordination by OH . When OH is added, OH
2+
replaces NH3 in the[Cu(NH3)4]
complex to form square-
2−
planar [Cu(OH)4] units and a coordination self-assembly of >
Cu(OH)2 · · · Cu(OH)2Cu < chains takes place quickly [14].
In this work, we put Cu(NH3)2+ into the microemulsion; the
4
◦
thermal stability and is stable even after calcination at 400 C.
process of homogenous precipitation occurred in microemulsion.
Fig. 9(b) gives the reaction process that we suggest. At a lower ω
value (ω = 5), only particles are obtained. The reason may be
that the interfacial intensity of spherical droplets is higher, the
sphere-like morphology of droplets is hard to metamorphose, so
nanoparticles are obtained. When the ω value increases to 10–20,
the bulk of droplet becomes larger, the sphere-like morphology
of droplets easily changes to rod-like morphology because the
3.8. Catalytic effect
The catalytic performances of CuO nanoparticles and nanowires
on thermal decomposition of ammonium perchlorate were stud-
ied by the DTA method. Fig. 8 shows the DTA curves of AP de-
composition in the absence and presence of CuO nanoparticles
and nanowires. It can be seen that the decomposition of pure AP
mainly consists of three steps: the endothermic peak at a max-
−
interfacial intensity of the droplets becomes lower. As OH is
−
2+
added into microemulsion, OH replaces NH3 in the [Cu(NH3)4]
◦
2−
imum peak temperature of 243.6 C is due to the transition of
complex to form square-planar [Cu(OH)4] units. At the same
2−
AP crystals from the orthorhombic form to cubic form [25], the
time, the square-planar [Cu(OH)4]
will be absorbed to the
◦
+
low-temperature exothermic peak at 337 C can be attributed to
surface of CTA and arrange with the spatial location of CTAB to

C. Li et al. / Solid State Communications 150 (2010) 585–589
589
a
b
Fig. 9. The formation process of Cu(OH)2 nanowires.
form > Cu(OH)2 · · · Cu(OH)2Cu < chains. The transverse growth
of the crystal is prone to end quickly because the diameter of the
droplets is much shorter. But the longitudinal growth is difficult
to end because the length of the droplets is much longer. The
special properties of the droplets controls the necessary anisotropy
of enriching water molecules to generate the 1D nanostructured
inorganic morphology.
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Acknowledgements
This work was financially supported by the Heilongjiang
Provincial Nature Science Foundation of China (No. B200811) and
the Science and Technology Project of Education Department of
Heilongjiang province of China (No. 11521220), for which we are
very grateful.