Cu(OH)2 nanowires, CuO nanowires and CuO nanobelts

Article abstract of DOI:10.1016/j.cplett.2004.06.017

Cu(OH)₂ nanowires, CuO nanowires and CuO nanobelts have been prepared and their (micro)structure is characterized by X-ray diffraction, high-resolution transmission electron microscopy and electron energy loss spectroscopy. Cu(OH)₂ nanowires are formed by a self-assembly mechanism of nanocrystals. These nanocrystals tend to orient their [100] axis parallel or close to parallel with the growth direction. The diameter of the monoclinic CuO nanowires is 8-20 nm. CuO nanobelts are single crystalline; the growth direction is along the [010] direction and they lie on the (001) plane; the edge surfaces being the (100) crystal planes.

Full text of DOI:10.1016/j.cplett.2004.06.017

Chemical Physics Letters 393 (2004) 64–69
www.elsevier.com/locate/cplett
Cu(OH) nanowires, CuO nanowires and CuO nanobelts
2
G.H. Du, G. Van Tendeloo *
EMAT, Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
Received 28 April 2004; in final form 1 June 2004
Available online 20 June 2004
Abstract
Cu(OH)2 nanowires, CuO nanowires and CuO nanobelts have been prepared and their (micro)structure is characterized by X-ray
diffraction, high-resolution transmission electron microscopy and electron energy loss spectroscopy. Cu(OH)2 nanowires are formed
by a self-assembly mechanism of nanocrystals. These nanocrystals tend to orient their [1 0 0] axis parallel or close to parallel with the
growth direction. The diameter of the monoclinic CuO nanowires is 8–20 nm. CuO nanobelts are single crystalline; the growth
direction is along the [0 1 0] direction and they lie on the (0 0 1) plane; the edge surfaces being the (1 0 0) crystal planes.
Ó 2004 Elsevier B.V. All rights reserved.
1
. Introduction
coworkers [17] reported the synthesis of CuO nanowires
by heating copper substrates to temperatures between
400 and 700 °C. CuO nanofibers have been synthesized
by Hsieh et al. [18] using a process of self-catalytic
growth at 500 °C.
Here we describe a facile method to produce
Cu(OH)2 nanowires, CuO nanowires and nanobelts. No
organic template and catalyst are needed and the
method is suited for large-scale preparation. The micro-
structures of these nanoscale products are analyzed in
detail using transmission electron microscopy (TEM),
which allows us to propose a formation mechanism.
The study of one-dimensional nanostructures is one
of the most active areas due to their peculiar, fascinating
physical, chemical and mechanical properties different
from the bulk material. In particular, their important
role as both interconnect and active component for
fabricating nanoscale electronic and phonic devices has
attracted attention [1–3]. Cupric oxide (CuO) is a p-type
semiconductor with a narrow band gap (1.2 eV) and
exhibits a number of interesting properties [4]. Copper
oxide based materials have widespread applications [5–
7
], such as high-temperature superconductors, optical
switch, anode electrodes for batteries. Recent studies
indicate that CuO could exist in as many as three dif-
ferent magnetic phases [8,9]. CuO has also been ex-
ploited as a powerfully heterogeneous catalyst to
convert hydrocarbons completely into carbon dioxide
and water [10]. Several methods have been developed to
prepare one-dimension nanostructures. Among these,
chemical vapor deposition (CVD) [11], laser vaporiza-
tion, electrochemical techniques [12,13], hydrothermal
treatment [14,15] and the exfoliating method [16] have
been documented. However studies about CuO nano-
wires and nanobelts are still limited. Recently Xia and
2
. Experimental
In a typical synthesis, 1 g of Cu(NO ) is first dis-
3
2
solved in 100 ml distilled water. Then, 30 ml NH ꢀ H O
3
2
(0.15 M) solution is added to the Cu(NO3)2 solution
under constant stirring. A blue precipitate of Cu(OH)2 is
produced when NaOH (1 M) solution is added dropwise
to the above solution to adjust the pH value to 9–10. At
last the blue Cu(OH)2 precipitate is filtered and washed
several times to obtain a solid product. The solid
products are separated into three parts. One part is dried
at room temperature (denoted as sample A). The second
part is placed in an oven and heated at 130 °C for 10 h
(denoted as sample B). The third part is added to 30 ml
*
Corresponding author. Fax: +32-3-2653257.
E-mail addresses: staf.vantendeloo@ua.ac.be (G. Van Tendeloo).
0
009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2004.06.017

G.H. Du, G. Van Tendeloo / Chemical Physics Letters 393 (2004) 64–69
65
distilled water and the mixture solution is sealed in a
Teflon-lined stainless autoclave and kept at a tempera-
ture of 130 °C for 10 h. A black solid product is formed
in the solution and separated by filtration (denoted as
sample C).
Fig. 2a is a low magnification transmission electron
microscopy (TEM) image of sample A. It consists en-
tirely of a large number of nanowires with a fairly uni-
form diameter of 10–20 nm. The length of nanowires
varies from 500 nm to 2 lm and the purity of nanowires
is as high as 98% (estimated from XRD and TEM re-
sults). Fig. 2b is a selected area electron diffraction
(SAED) pattern recorded from an area containing a
large number of nanowires. The rings in the pattern can
be indexed as the (0 2 0), (0 2 1), (1 1 0), (1 1 1), (1 3 0),
(1 3 2), (2 0 0) and (2 2 2) rings of the orthorhombic
Cu(OH)2, in agreement with the XRD results. No extra
reflections are observed and we can therefore conclude
Powder X-ray diffraction (XRD) analysis was per-
formed using a Rigaku Dmax X-ray diffractometer with
Cu K a radiation (k ¼ 0:1541 nm). The morphology and
structure of the products was determined using high-
resolution transmission electron microscopy (HRTEM)
using a JEOL 4000EX and a Philips CM30FEG mi-
croscope. Electron energy loss spectroscopy (EELS) was
performed using a Gatan Image Filtering (GIF) system
attached to the Philips CM30FEG. TEM samples were
prepared by dispersing the powder in alcohol by ultra-
sonic treatment, positioning a drop onto a porous car-
bon film supported on a copper grid, and then drying in
air.
that sample A consists of pure Cu(OH) nanowires.
2
Cu(OH)2 nanowires prefer to grow into bundles,
which usually have a width of 40–100 nm (Fig. 2c). A
HRTEM image of a single Cu(OH)2 nanowire is dis-
played in Fig. 2d, revealing that a single Cu(OH)2
nanowire is actually assembled from dozens of nano-
crystals with an average size of 4 ꢁ 9 nm. The consti-
tuting nanocrystals have an elongated shape along the
direction of the longest dimension of the nanowire. In
the HREM image several interplanar spacings (0.226,
0.25 and 0.28 nm) are observed, corresponding to the
(1 3 0), (1 1 1) and (1 1 0) planes of Cu(OH)2. There
seems to be no fixed orientation relationship between the
different nanocrystals.
Single-crystalline nanowires in general have been ex-
tensively studied. Their formation is clearly related to
the fact that the growth rate along one crystallographic
direction is significantly faster than along the other di-
rections. The situation is different however for nano-
wires with a polycrystalline structure, which is usually
formed by the aggregation of many crystals [19]. Porous
single-crystalline CaCO3 have been prepared via nano-
crystals aggregation by Zhan et al. [20] and aggregation-
based crystal growth in natural iron oxyhydroxide
biomineralization products has been reported by Ban-
3
. Results and discussion
Fig. 1 shows the XRD profiles taken from the sample
A, B and C. The peaks in Fig. 1a can all be well indexed
in the orthorhombic Cu(OH)2 structure with lattice
ꢀ
ꢀ
ꢀ
constants a ¼ 2:951 A, b ¼ 10:59 A and c ¼ 5:273 A
JCPDS 35-0505). Figs. 1b,c show that both sample B
and C are pure CuO with a monoclinic structure
(
ꢀ
ꢀ
ꢀ
(
a ¼ 4:685 A, b ¼ 3:425 A, c ¼ 5:13 A, b ¼ 99:549,
JCPDS 45-0937). The relatively broad XRD peaks in
Figs. 1a,b indicate that the size of crystals in sample A
and B is small. The XRD profile of sample C (Fig. 1c)
shows narrow and sharp peaks in comparison with
Fig. 1a,b, indicating a larger crystal size and a better
crystallinity than sample A and B.
field [21]. For Cu(OH) nanowires we suggest the fol-
2
lowing mechanism. First, Cu2 cations in the Cu(NO3)2
þ
þ2
solution form a square-planar complex [Cu(NH3)4]
with the addition of NH3 ꢀ H2O. When NaOH is added,
the pH value of the solution increases and the stability
þ2
of the [Cu(NH3)4] decreases. The effects of pH and
NH3 ꢀ H2O on the Cu(OH)2 morphology have been
studied by Wang et al. [22]. Cu(OH) precipitates be-
2
þ2
cause it is more stable than [Cu(NH ) ] . It is con-
3
4
ceivable that the nucleation of Cu(OH)2 starts from
localized regions with relatively high concentrations of
ꢂ
2þ
OH where the [Cu(NH3)4] complex is unstable. Be-
cause a large number of nuclei are formed simulta-
neously, many Cu(OH)2 nanocrystals precipitate.
Cu(OH)2, however, is a layered structure (Fig. 2e) and
the growth rate is anisotropic. Therefore the shape of
the Cu(OH)2 nanocrystals is not spherical and they
prefer a morphology with one dimension longer than the
Fig. 1. XRD profiles of (a) sample A, (b) sample B and (c) sample C.
Sample A was indexed as pure Cu(OH)2 while sample B and C could
be indexed as pure CuO.

66
G.H. Du, G. Van Tendeloo / Chemical Physics Letters 393 (2004) 64–69
Fig. 2. (a) Low magnification TEM image of Cu(OH)2 nanowires, (b) SAED pattern taken from a region including a large number of nanowires, (c)
TEM image of bundled Cu(OH)2 nanowires, (d) HRTEM image of a Cu(OH)2 nanowire (the arrow indicates the growth direction) and SAED
pattern taken from a single nanowire (inset), (e) structure model of Cu(OH)2 and (f) EELS spectrum from a Cu(OH)2 nanowire.
others. At a certain point the nanocrystals aggregate and
assemble to form nanowires.
However one important question remains: why do
these nanocrystals assemble to form nanowires instead of
clusters or random particles? We can get some clue from
the SAED taken from a single Cu(OH)2 nanowire (inset in
Fig. 2d). The ring pattern confirms that the nanowire is
composed of many nanocrystals (polycrystalline struc-

G.H. Du, G. Van Tendeloo / Chemical Physics Letters 393 (2004) 64–69
67
ture). However the brightness of the diffraction ring is not
uniform, indicating a preferential orientation of these
nanocrystals. The angle between the two brighter spots on
the {1 1 1} diffraction ring is about 64° and they can be
indexed as (1 1 1) and (1 )1 )1) reflections. The detailed
SAED experiment and crystallographic analysis indicate
that there is indeed a preferential orientation and many
nanocrystals tend to orient their [1 0 0] direction parallel
to the growth direction of the Cu(OH)2 nanowire. This
can explain why the wire-like Cu(OH)2 are formed. The
aggregation and assembly process of Cu(OH)2 nano-
crystals is not random but selective. The nanocrystals of
similar orientation are easy to connect and grow together;
the aggregating rate along the [1 0 0] direction is faster
than along other directions, leading to the formation of
Cu(OH) nanowires. The diameter of the nanowires is
2
uniform mainly because the aggregating rate along these
directions is very low so that the difference between them
is small.
The composition of a single Cu(OH)2 nanowire was
analyzed using EELS. Fig. 2f is a typical EELS spectrum
taken from a single nanowire, clearly showing the O K
edge at 532 eV, the L2 and L3 edges of Cu at 951 and
9
is detected, indicating the absence of the complex
31 eV, respectively. No nitrogen (K edge is at 401.6 eV)
Cu(NH3)4]2
boundaries.
þ
þ
or NH4 along the nanocrystalline
[
TEM images of sample B are shown in Fig. 3; the
morphology is very similar to sample A. A large amount
of nanowires with a diameter of 8–20 nm are observed in
Fig. 3a. The rings in the SAED pattern (inset of Fig. 3a)
can be indexed as ()1 1 0), (0 0 2), (1 1 1), ()2 0 2), (2 0 2)
and (0 2 2) reflections of pure monoclinic CuO. The
HRTEM image reveals that the CuO nanowires are also
polycrystalline with a nanocrystal size around 3–8 nm
(
Fig. 3b). The boundaries between nanocrystals become
vague because of the crystallization process at 130 °C.
The formation mechanism of the CuO nanowires is easy
to understand. During the heating process, the Cu(OH)2
nanowires loose H O molecules and transform into CuO
2
while the nanowire morphology still remains. A detailed
transformation process from Cu(OH)2 to CuO was
suggested by Cudennec and Lecert [23] and can be de-
scribed according to the following scheme:
CuðOHÞ2ðsÞ ! CuOðsÞ þ H
2
ðgÞ
O :
The loss of water is performed by an oxolation
mechanism, which involves a dehydration process and
the formation of O–Cu–O bridges (see Fig. 3c). Bridges
are formed after the loss of water molecules followed by
a contraction of the structure along the [0 1 0] direction.
Simultaneously shifts of CuO4 groups or Cu atoms
along (0 0 1) direction are performed to promote the
evolution towards crystallized CuO.
Fig. 3. (a) TEM image of CuO nanowires; the SAED pattern is shown
as inset, (b) HRTEM image of a CuO nanowire and (c) structure
model of Cu(OH)2 showing the process to loose water molecules.
shows a different morphology: a lamellar and belt-like
morphology is clear. The width of the CuO belts ranges
from 120 to 350 nm and the length is over 1 lm. Fig. 4b is
an enlarged image of a single CuO nanobelt and the inset
A low-magnification TEM image of sample C is shown
in Fig. 4a. Compared with sample A and B, sample C

68
G.H. Du, G. Van Tendeloo / Chemical Physics Letters 393 (2004) 64–69
Fig. 4. (a) Low magnification TEM image of CuO nanobelts, (b) TEM image of a single CuO nanobelt with the corresponding SAED pattern as
inset, (c) HRTEM image of a CuO nanobelt imaged along the [0 0 1] direction and (d) projected structure model of CuO along the [0 0 1] direction.
is the corresponding SAED pattern. The electron beam is
along the [0 0 1] zone axis and the diffraction spots can be
indexed as the ()1 1 0), (1 1 0) and (0 2 0) reflections of
CuO. Detailed tilt experiment and SAED demonstrate
that the CuO nanobelt is single crystalline with the long
axis along [0 1 0]. This is reasonable because the close-
packed plane of the monoclinic CuO crystal is (0 1 0).
Fig. 4c is a HRTEM image of a CuO nanobelt and it
clearly shows sub unit cell detail; the projected unit cell
non, combined with a loss of two hydroxyl ions and one
water molecule, lead to the formation of chains of
square planar CuO groups and then to solid CuO. The
transformation can be described as the following
scheme:
4
CuðOHÞ2ðsÞ þ 2OHꢂ ! CuðOHÞ ðaqÞ
2ꢂ
ðaqÞ
4
ꢂ
! CuOðsÞ þ 2OH þ H
2
O
ðaqÞ
The presence of water facilitates the morphologic
transformation from nanowire to nanobelt.
ꢀ
ꢀ
(
a ¼ 4:6 A and b ¼ 3:4 A) is outlined. White dots in the
image correspond to the open channels between four Cu-
atoms. The corresponding projected structure model of
CuO is shown in Fig. 4d.
4. Conclusions
TEM results suggest that the CuO nanobelts lie on
0 0 1) planes and that the edge surface is the (1 0 0)
(
We report the preparation and structure investigation
of Cu(OH) nanowires, CuO nanowires and nanobelts.
Cu(OH)2 nanowires are formed by self-assembly of
nanocrystals with a size of 4 ꢁ 9 nm. These nanocrystals
tend to orient their [1 0 0] axis parallel to the growth
crystal plane. The formation of a CuO nanobelt is due to
the processes of thermal dehydration and re-crystalli-
zation of the Cu(OH)2 nanowires. It is a different
transformation mechanism for Cu(OH)2 in aqueous
solutions or in the solid state [23]. Divalent copper ions
are firstly dissolved under the form of complex anions
2
direction. The diameter of Cu(OH) and CuO nanowires
2
are in the range from 5–20 nm and their length varies
2
4
ꢂ
Cu(OH) , which result in the a square planar sur-
from 500 nm to 2 lm. The CuO nanobelts are single
crystalline and lie on the (0 0 1) plane with the edge
surfaces as the (1 0 0) crystal plane; the growth direction
rounding. This anion can be considered as the precursor
for the formation of CuO. A condensation phenome-

G.H. Du, G. Van Tendeloo / Chemical Physics Letters 393 (2004) 64–69
69
is the [0 1 0] direction. The growth mechanism of these
nanostructures is discussed. The preparation method
has the advantages of relatively low cost, high purity
and large-scale production, and may be extended to the
synthesis of other nanostructures.
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