Copper hydroxide nanoneedle and nanotube arrays fabricated by anodization of copper

Article abstract of DOI:10.1021/jp054350p

Cu(OH)₂ nanoneedle and nanotube arrays were electrochemically synthesized by anodization of a copper foil in an aqueous solution of KOH. The nanoneedles and nanotubes were constructed from nanosheets of Cu(OH) ₂. Controlling the electrochemical conditions can qualitatively modulate the lengths, amounts, and shapes of Cu(OH)₂ nanostructures. The composition of as-prepared Cu(OH)₂ nanostructures has been confirmed by X-ray diffraction and select-area electron diffraction. The influences of the KOH concentration of the aqueous electrolyte, the reaction temperature, and current density on the morphology of Cu(OH)₂ nanostructures were investigated, and the formation mechanism of the nanostructures is discussed. Furthermore, Cu(OH)₂ nanoneedles can be successfully transformed to CuO nanoneedles with little morphology change by heating. This work developed a simple, clean, and effective route for fabrication of large area Cu(OH)₂ or CuO nanostructured films.

Full text of DOI:10.1021/jp054350p

22836
J. Phys. Chem. B 2005, 109, 22836-22842
Copper Hydroxide Nanoneedle and Nanotube Arrays Fabricated by Anodization of Copper
Xufeng Wu, Hua Bai, Jiaxin Zhang, Feng’en Chen, and Gaoquan Shi*
Department of Chemistry and Bio-organic Phosphorous Laboratory, Tsinghua UniVersity,
Beijing 100084, People’s Republic of China
ReceiVed: August 4, 2005; In Final Form: September 18, 2005
Cu(OH)2 nanoneedle and nanotube arrays were electrochemically synthesized by anodization of a copper foil
in an aqueous solution of KOH. The nanoneedles and nanotubes were constructed from nanosheets of
Cu(OH)2. Controlling the electrochemical conditions can qualitatively modulate the lengths, amounts, and
shapes of Cu(OH)2 nanostructures. The composition of as-prepared Cu(OH)2 nanostructures has been confirmed
by X-ray diffraction and select-area electron diffraction. The influences of the KOH concentration of the
aqueous electrolyte, the reaction temperature, and current density on the morphology of Cu(OH)2 nanostructures
were investigated, and the formation mechanism of the nanostructures is discussed. Furthermore, Cu(OH)2
nanoneedles can be successfully transformed to CuO nanoneedles with little morphology change by heating.
This work developed a simple, clean, and effective route for fabrication of large area Cu(OH)2 or CuO
nanostructured films.
Introduction
attempt to synthesize 1D inorganic nanostructures using an
electrochemical method without any template and additives. In
addition, CuO is a p-type semiconductor with a narrow band
gap (1.2 eV). CuO-based materials have been used as high-
temperature superconductors, catalysts, anode electrodes for
batteries, and optical switches.23-26 In this work, we also report
that CuO nanoneedle arrays can be easily generated by heating
the Cu(OH)2 precursors in the temperature range 150-200 °C
without obvious morphological changes.
One-dimensional (1D) nanostructures (e.g., nanowire, nano-
tube, nanoribbon) have attracted a great deal of attention, mainly
due to their unique electronic, magnetic, mechanical, or chemical
properties.1 Many techniques including template-guide growth,2,3
vapor-liquid-solid (VLS) synthesis,4,5 epitaxial electrodeposi-
ton,6 and chemical vapor deposition7,8 have been developed to
synthesize these materials. However, the size and morphology
control of 1D nanomaterials is still one of the challenging issues,
because the control of nucleation and growth processes of
nanostructures is rather difficult.
Experimental Section
Typically, the growth of Cu(OH)2 nanoneedles and scroll-
like nanotubes was carried out at room temperature in a one-
compartment cell by the use of a model 263 potentiostat-
galvanostat (EG&G Princeton Applied Research) under computer
control. A copper foil (99.9%, Shanghai Chemical Reagent Co.
Ltd., Shanghai, China) was used as the working electrode
(surface area ) 1.5 cm2). The counter electrode was a stainless
steel sheet (AISI 304, surface area ) 1.6 cm2). The typical
electrolyte was an aqueous solution of KOH (g84%; Beijing
Fine Chemical Co. Ltd., Beijing, China). The solutions were
deaerated by a dry nitrogen stream and maintained at a light
overpressure during the experiments. Cu(OH)2 films were
electrochemically grown at a constant current density of 1.5
mA cm-2 in KOH aqueous solution with integrated charge
control. The electrochemical deposition conditions are sum-
marized in Table 1. After reaction, a faint-blue film was formed
on the copper foil surface. Then the copper foil with the product
was taken out from the solution and washed with distilled water
and dried under vacuum at room temperature before character-
izations. For the preparation of CuO nanoneedles, the Cu(OH)2
nanoneedle sample was heated to 150 °C for 3 h to complete
dehydration, then the temperature was evaluated to 200 °C and
kept for another 3 h to promote crystallization under nitrogen
protection. Finally, the sample was naturally cooled to room
temperature.
Basic Cu(II) salts and Cu(OH)2 with an orthorhombic
structure have been studied extensively in the past several years.
Matijevic´ and co-workers investigated the relationship between
the morphologies of homogeneously precipitated Cu(OH)2 and
the solution compositions.9-11 Polycrystalline Cu(OH)2 nano-
wires and nanoribbons were synthesized through a two-step wet
chemical route12,13 or by the interaction between a copper
complex and NaOH at the aqueous-organic interface14 or
prepared in aqueous solution of ammonia using Cu2S nanowires
as precursors.15 Single-crystalline Cu(OH)2 nanoribbon and
nanotubes were successfully synthesized by surface oxidation
of copper foil in alkaline solutions16,17 or anion exchange
reaction using NaOH solution.18 However, control of the size,
yield, and shape of the final nanomaterials has not yet been
achieved.
It is well-known that electrochemical reaction can be ac-
curately modulated by integrated charge passed through the
electrochemical cell, and so the size and yield of the product
may be easily controlled to some extent.19,20 Extensive work
has been reported on the synthesis of 1D nanostructures through
template-guided electrochemical deposition.21,22 Here, we report
that Cu(OH)2 nanoneedles and scroll-like nanotube film can be
simply grown by direct anodization of copper in an aqueous
solution of KOH. To the best of our knowledge, this is the first
Scanning electron micrographs (SEMs) were taken by the
use of a FEI Sirion 200 scanning electron microscope. X-ray
* To whom correspondence should be addressed. Phone: 86-10-
62773743. Fax: 86-10-62771149. E-mail: gshi@tsinghua.edu.cn.
10.1021/jp054350p CCC: $30.25 © 2005 American Chemical Society
Published on Web 11/03/2005

Nanoneedle and Nanotube Arrays
J. Phys. Chem. B, Vol. 109, No. 48, 2005 22837
Figure 1. SEM images of as-prepared Cu(OH)2 nanoneedles deposited at a constant current density of 1.5 mA cm-2 in 2.0 mol L-1 KOH solution
for different deposition times: (A) 300 s; (B) 500 s, insert contains a magnified view; (C) 1000 s; (D) 1500 s. T ) 28 ( 2 °C.
TABLE 1: The Products Formed by Anodization of a Copper Foil at a Constant Current Density of 1.5 mA cm-2 in the Media
with Different Concentrations of KOH (CKOH) and at Different Temperatures (T)
T (°C)
CKOH, 2 mol/L
CKOH, 2.5 mol/L
CKOH, 3 mol/L
CKOH, 3.5 mol/L
2 ( 2
15 ( 2
28 ( 2
Cu(OH)2 nanoneedles
Cu(OH)2 nanoneedles
Cu(OH)2 nanoneedles
Cu(OH)2 nanoneedles
Cu(OH)2 nanoneedles
Cu(OH)2 nanotubes
Cu(OH)2 nanoneedles
Cu(OH)2 nanotubes
Cu(OH)2 nanotubes
Cu(OH)2 nanoneedles
Cu(OH)2 nanotubes
CuO nanoparticles
diffraction (XRD) was carried out by using an X-ray diffrac-
tometer model D8 advance (Bruker) with Cu KR radiation
(λ ) 1.5418 Å). Transmission electron micrographs (TEMs)
and high-resolution TEMs were taken on a JEM 2010 (200 kV)
transmission electron microscope (JEOL).
copper substrate uniformly and compactly (Figure 1D). The
nanoneedles are 500-550 nm in diameter at their roots and
100-120 nm at their sharp tips.
The formation of crystalline Cu(OH)2 was confirmed by the
use of wide-angle X-ray diffraction. Figure 2A is the XRD
pattern of an as-prepared Cu(OH)2 nanoneedle film covered
copper foil. The indexed diffraction peaks except those marked
with stars (they are attributed to the copper substrate) can be
indexed to the orthorhombic phase of Cu(OH)2.27 The diffraction
lines are in good agreement with the corresponding literature
values: a ) 2.949 Å, b ) 10.59 Å, c ) 5.256 Å (JCPDS 13-
420). In comparison to those of the powder XRD pattern, the
intensities of (111) and (130) lines are significantly enhanced,
indicating that the crystalline Cu(OH)2 nanoneedles are on
average roughly perpendicular to the copper substrate with a
wide distribution of tilts.16 The weak diffraction peaks at 29.6,
42.3, and 61.4 can be assigned to cubic cuprous oxide (110),
(200), and (220) planes (JCPDS 78-2076), which is a product
of the side reaction
Results and Discussion
Figure 1 shows the typical SEMs of as-prepared Cu(OH)2
nanoneedle film formed by anodization a copper foil in 2.0 mol
L-1 KOH solution at a constant current density of 1.5 mA cm-2
for different times. We can find that the amount of integrated
charge passed through the cell during electrolysis process has
influences on the length and the amount of Cu(OH)2 nanon-
eedles. When the deposition time was less than 150 s, no needle
was observed on the copper surface. As the electrolysis time
was increased to 300 s, the Cu(OH)2 pinelike needles with
average length of ∼5 µm were grown sparsely on the copper
surface (Figure 1A). The inset of Figure 1A demonstrates that
the nanoneedle was composed of tightly arranged nanoribbons.
Figure 1B and its insert image also clearly show the formation
of Cu(OH)2 nanoneedles. Furthermore, as can be seen from parts
B-D of Figure 1, when the electrolysis time increased from
300 to 1500 s, the length of the needles increased from ∼5 to
∼15 µm and their amount was also markedly increased. Finally,
pine-needle-like Cu(OH)2 covered the whole surface of the
Cu + 2OH- f Cu2OV + 2e- + H2O
The structure of the nanoneedles was further investigated by
TEM. As shown in Figure 2B, a nanoneedle is composed of
layered nanoribbons. The widths of the ribbons are in the range

22838 J. Phys. Chem. B, Vol. 109, No. 48, 2005
Wu et al.
Figure 2. Typical XRD pattern and TEM images of as-prepared Cu(OH)2 nanoneedles deposited at a constant current density of 1.5 mA cm-2 in
2.0 mol L-1 KOH solution for 2.25 C cm-2 on a copper foil surface at 28 °C: (A) XRD pattern; (B, C) TEM images (insert, SAED).
of 80-100 nm (Figure 2B-C). These can be confirmed further
by the image inserted in Figure 1A. The selected area electron
diffraction (SAED) pattern of a 100 nm wide ribbon (inset of
Figure 2C) indicates that the ribbon is a single crystal. According
to the diffraction patterns, Cu(OH)2 ribbons are arranged parallel
to their (010) planes, and the preferential growth direction of
the single-crystal nanoribbons is their [100] direction. Since Cu-
(OH)2 had been transformed to CuO nanocrystallites rapidly
under the irradiation of a high-energy electron beam, we failed
to got a high-resolution transmission electron microscope (HR-
TEM) image of the Cu(OH)2 nanoribbon. Figure S1 shows a
TEM image of the root part of a nanoribbon (A) and a HR-
TEM image of a whole nanoribbon (B). According to Figure
S1-B, Cu(OH)2 was transformed to CuO nanocrystallites by the
irradiation of a high-energy electron beam. This also was
confirmed by the SAED pattern shown as an inset of Figure
S1-A.
above, when CKOH was increased to over 2.5 mol L-1, it was
interesting to find that the nanoneedles were disappeared and
scroll-like Cu(OH)2 nanotubes were formed on the copper foil.
Figure 3 shows the typical SEM images of as-prepared
Cu(OH)2 nanotube film formed by electrolysis of a copper foil
in a 3 mol L-1 KOH aqueous solution at 1.5 mA cm-2 for 2.7
C cm-2 at 28 °C. The low-magnified SEM image of the
nanotubes is similar to that of as-prepared Cu(OH)2 nanoneedles
(Figure 3A). However, the magnified view shown in Figure 3B
clearly demonstrates that the nanotubes have irregular opened
mouths with elliptical, forklike, and polygonal shapes. The
nanotubes are 80-550 nm in diameter and over 10 µm in length,
the thickness of the tube skin is only tens of nanometers. The
XRD pattern of the nanotubes (not shown here) is similar to
that of Cu(OH)2 nanoneedles. Parts C and D of Figure 3 are
TEM images of the as-prepared Cu(OH)2 nanotubes. From
Figure 3C, we realize that the nanotube seems to be formed by
rolling a sheet. The SAED pattern of a 400 nm wide tube (inset
of Figure 3D) demonstrates it is a single crystal, the tube is
lying on its (010) plane, and the preferential growth direction
As the other experimental parameters were kept unchanged,
the morphology of Cu(OH)2 nanostructures depends strongly
on the concentration of KOH (CKOH). In the system described

Nanoneedle and Nanotube Arrays
J. Phys. Chem. B, Vol. 109, No. 48, 2005 22839
Figure 3. Typical SEM images of Cu(OH)2 nanotubes prepared at a constant current density of 1.5 mA cm-2 for 2.7 C cm-2 in 3.0 mol L-1 KOH
solution at 28 °C: (A) overall view; (B) a regional magnified view of A; (C, D) TEM images of the sample shown in B (insert, SAED).
of the single-crystal nanotube is its [100] direction. Further
increase of CKOH to 3.5 mol L-1 will cause the dehydration of
Cu(OH)2 to form CuO nanoparticles as listed in Table 1.
To understand the growth mechanism of Cu(OH)2 nanostruc-
tures, we have studied the effects of temperature and KOH
concentration on the morphology of the nanostructures. Parts
A-C of Figure 4 are the SEM images of Cu(OH)2 nanoneedles
grown in 2.0 mol L-1 KOH solution at 1.5 mA cm-2 for 2.25
C cm-2 and at different temperatures. Comparing the nanon-
eedles grown at 2 or 15 °C to those grown at 28 °C, we can
find that the number density of nanoneedles grown at a lower
temperature is much higher than that formed at a higher
temperature, and the nanoneedles trend to grow more upright
(insert image of Figure 4C). Furthermore, as the temperature
was decreased from 28 to 15 or 2 °C, the average length of the
nanoneedles decreased from 15 to about 10 or 5 µm, cor-
respondingly. The decrease of the nanoneedles’ length is
reflected in the increase of number density of the nanoneedles,
because the total amounts of Cu(OH)2 on unit area are the same
in these three cases. The higher number density implies that
the nucleation of Cu(OH)2 crystals is easier at a lower
temperature, so that more nanoneedles grow within the same
time period. Parts D-F of Figure 4 show SEM images of a
copper surface anodized in a 3.5 mol L-1 KOH solution at 1.5
mA cm-2 for 2.7 C cm-2 and at different temperatures. As
shown in Figure 4D, at 28 °C, only black CuO irregular
nanoparticles were generated and covered the copper substrate
compactly. These results indicate that a high KOH concentration
and a high reaction temperature support the dehydration of Cu-
(OH)2 into CuO. However, it was interesting to find that Cu-
(OH)2 nanotubes were formed as the temperature was decreased
to 15 °C (Figure 4E). On further decrease of the temperature to
2 °C, Cu(OH)2 nanoneedles were generated, instead of nano-
tubes (Figure 4F).
The influences of current density (Id) on the resulting products
have also been investigated. Figure 5 illustrates the SEM images
and XRD patterns of the copper foil anodized in a 2 mol L-1
KOH aqueous solution at 28 °C and at different current densities
for 3 C cm-2 each. As shown in Figure 5A, in the case of Id )
0.5 mA cm-2, black nanosheets or flowers (composed of
staggered nanosheests) of CuO were generated, and they came
along with a certain amount of Cu2O (Figure 5D-a). This
phenomenon indicated that the dehydration of Cu(OH)2 to CuO
is a competition reaction to the growth of Cu(OH)2 nanocrystals
at the nucleation process. At a low current density of 0.5 mA
cm-2, the amount of as-formed Cu(OH)2 was not sufficient for
growing crystals. Thus, it tended to dehydrate into CuO for its
high reaction activity. Cuprous oxide was generated by the
competition oxidation of copper (Cu + 2OH- f Cu2OV + 2e-
+ H2O) as described above. When Id was increased up to over
1 mA cm-2, Cu(OH)2 nanoneedles were produced instead of
CuO (Figure 5B). Further increase of Id, the number density of
the nanoneedles, was increased due to the increase of nucleation
rate, while the needle length was decreased (Figure 5C). Figure
5D shows XRD patterns of as-prepared products, the peaks
marked with stars are diffraction lines of a copper substrate.
As can be seen from Figure 5D-a, the wide diffraction peaks at
35.5° and 38.7° reveal that the resulting CuO is polycrystalline.
The diffraction peaks at 29.6° (110), 36.4° (111), and 42.3° (200)
can be indexed to cubic cuprous oxide (JCPDS 78-2076). The
strong diffraction line at 61.5 is an overlapped peak of CuO
(-113) and Cu2O (220). The overall features of Figure 5D-b
are similar to those of Figure 2A except that there are no
diffraction peaks of Cu2O. This results revealed that the increase
of current intensity supports the formation of Cu(OH)2, and the
side product, Cu2O, can be effectively avoided. Figure 5D-c is
similar to that of the Cu(OH)2 powder diffraction pattern, which
is consistent with the SEM image shown in Figure 5C.

22840 J. Phys. Chem. B, Vol. 109, No. 48, 2005
Wu et al.
Figure 4. The surface morphology of a copper foil anodized in a 2 mol L-1(A-C) or a 3.5 mol L-1 (D-F) KOH solution at a constant current
density of 1.5 mA cm-2 for 2.25 C cm-2 (A-C) or 2.7 C cm-2 (D-F) and at different temperatures: (A) 28 ( 2 °C; (B) 15 ( 2 °C; (C) 2 ( 2
°C; (D) 28 ( 2 °C; (E) 15 ( 2 °C; (F) 2 ( 2 °C. Inserts show magnified views.
On the basis of the investigations described above, the
formation mechanism of Cu(OH)2 can be postulated as follows.
The growth of pine-needle-like and scroll-like Cu(OH)2 nano-
structure arrays is an electrochemical process and can be simply
represented by the following two electrochemical half reactions:
trolled mainly by current density. As the electrochemical
reaction was carried out in a medium with a KOH concentration
lower than 0.5 mol L-1, no Cu(OH)2 nanoneedle was produced.
Gas bubbles were generated at both working and counter
electrodes, and the whole electrochemical reaction is just the
electrolysis of water. This is mainly because a compact
Cu(OH)2 film was initially formed on copper surface which
prevented the substrate from further oxidation. The passivation
film could not be formed as KOH concentration was increased
to over 1 mol L-1 and nanoneedles were obtainable. The main
compete reaction to the growth of Cu(OH)2 nanostructures is
dehydration of Cu(OH)2 to CuO. This is a chemical process
and its reaction rate depends on the concentration of KOH and
the reaction temperature, while depending little on current
density. The Cu(OH)2 formed in the initial stage of electrolysis
is active in an alkaline solution.9-11,28 The increase of reaction
anode
Cu - 2e- f Cu2+, Cu2+ + 2OH- f Cu(OH)2V
cathode
2H+ + 2 e- f H2v
Gas bubbles were observed to be released from the counter
electrode surface during the electrolysis process, indicating the
formation of H2. The electrochemical reaction rate was con-

Nanoneedle and Nanotube Arrays
J. Phys. Chem. B, Vol. 109, No. 48, 2005 22841
Figure 5. Typical SEM images (A-C) and XRD patterns (D) of a copper surface after anodizing in a 2.0 mol L-1 KOH solution at 28 °C and at
different current densities for 3 C cm-2 each: (A) 0.5 mA cm-2; (B) 2 mA cm-2; (C) 6 mA cm-2; (D) XRD patterns at (a) 0.5 mA cm-2; (b) 2 mA
cm-2; (c) 6 mA cm-2
.
Figure 6. Typical SEM images (A, B), XRD pattern (C), and TEM image (D) of CuO nanoneedles converted from the as-prepared Cu(OH)2
nanoneedles: (A) overall view; (B) a magnified view; Insert of D: a SAED pattern.

22842 J. Phys. Chem. B, Vol. 109, No. 48, 2005
Wu et al.
temperature and KOH concentration accelerated the dehydration
rate of Cu(OH)2, while the increase of current density increases
the growth rate of Cu(OH)2 nanostructures. Thus, high temper-
ature and high concentration of KOH and a low current density
favor the formation of CuO nanoparticles film (T > 28 °C, CKOH
> 3.5 mol L-1 KOH, see Figure S2). The appropriate conditions
for the Cu(OH)2 nanoneedles and nanotubes are summarized
in Table 1. Accordingly, nanoneedles are formed in the systems
with lower KOH concentrations and at lower temperatures than
those used for growing nanotubes.
are summarized in Table 1. Furthermore, CuO nanoneedles, a
novel nanomaterial with various potential applications, can be
simply produced by heating their Cu(OH)2 precursors.
Acknowledgment. This work was supported by National
Natural Science Foundation of China (90401011, 20374034,
50225311) and 973 project (2003CB615700).
Supporting Information Available: TEM images of Cu-
(OH)2 nanoneedles and CuO film. This material is available
free of charge via the Internet at http://pubs.acs.org.
It is known that different growth rates of the crystal faces
determine the ultimate morphology of the nanomaterial.15 The
growth of Cu(OH)2 nanoneedles along the [100] direction can
be understood on the basis of the assembly of oblated chains
>Cu(OH)2Cu< in the plane (001), oriented along [100].9-11,27
According to the Bravais-Friedel-Donnay-Harker law,29,30
the growth rate of orthorhombic Cu(OH)2 crystal is normally
proportional to 1/dhkl. So the growth of Cu(OH)2 along [100] is
much faster than those along the other directions, leading to
the formation of ribbonlike nanostructures. The nanoneedle was
formed by stacking the nanoribbons with hydrogen bonding
interactions. In a solution with high KOH concentration or at
high reaction temperature, the interlayer hydrogen bond linkages
at the sheet edges were weakened, which caused stresses in the
layers. So, the ribbons were rolled to relieve the stresses, forming
the final scroll-like tubular structure.13,16,30
Dehydration of Cu(OH)2 nanoneedles at 150 °C for 3 h and
then further crystallized at 200 °C for another 3 h produced
CuO nanoneedles. As can be see from Figure 6A,B, the CuO
nanoneedles preserved the morphology of the Cu(OH)2 precur-
sors. The XRD pattern of as-prepared CuO nanoneedles (Figure
6C) reveals that the conversion of Cu(OH)2 to monoclinic CuO
is complete. The weak peaks of the XRD pattern indicate that
CuO nanoneedles are polycrystalline (the peaks marked with
stars are attributed to copper foil). Figure 6D is the TEM images
of a single CuO nanoneedle, the SAED image in the insert
reveals that the CuO nanoneedle was lying on the (001) plane
and axis direction is along [-110], which is consistent with
the result reported in the literature.13,17,30
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In summary, large area Cu(OH)2 nanoneedle and nanotube
arrays can be fabricated by simply anodization of a copper foil
in an aqueous solution of KOH without any template and
additive. The nanostructures are made of Cu(OH)2 nanosheets.
The advantages of our method for preparation of these inorganic
nanostructures lie in its simplicity, mild reaction conditions and
by the ability to control the lengths, number densities, and shapes
(needle or tube) of the nanostructues to some extent through
modulation the experimental parameters including temperature,
concentration of KOH, and current density. The optimum
conditions for producing Cu(OH)2 nanoneedles or nanotubes