Simple template-free solution route for the controlled synthesis of Cu(OH)2 and CuO nanostructures

Article abstract of DOI:10.1021/jp046772p

The controlled synthesis of Cu(OH)₂ nanowires and nanoribbons in a solution phase has been realized with high yield at low cost by simply dropping KOH and ammonia solutions into an aqueous solution of CuSO₄ at ambient temperature. It is demonstrated that the morphology of nanostructured Cu(OH)₂ is significantly influenced by the feeding manner of the alkaline solutions. A rational mechanism based on coordination self-assembly and oriented attachment is proposed for the selective formation of the polycrystalline Cu(OH)₂ nanowires and single-crystalline Cu(OH) ₂ nanoribbons. In the presence of a polymeric additive, poly(acrylic acid) (PAA), ordered assemblies of Cu(OH)₂ nanorods can be readily obtained. Furthermore, well-defined CuO nanostructures, such as CuO nanoplatelets, nanoleaflets, and nanowires, were produced by thermal dehydration of the as-prepared Cu(OH)₂ nanostructures in solution or in the solid state. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were used to characterize the products.

Full text of DOI:10.1021/jp046772p

J. Phys. Chem. B 2004, 108, 17825-17831
17825
Simple Template-Free Solution Route for the Controlled Synthesis of Cu(OH)2 and CuO
Nanostructures
Conghua Lu, Limin Qi,* Jinhu Yang, Dayong Zhang, Nianzu Wu, and Jiming Ma
State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry,
Peking UniVersity, Beijing, 100871, People’s Republic of China
ReceiVed: July 21, 2004; In Final Form: August 29, 2004
The controlled synthesis of Cu(OH)2 nanowires and nanoribbons in a solution phase has been realized with
high yield at low cost by simply dropping KOH and ammonia solutions into an aqueous solution of CuSO4
at ambient temperature. It is demonstrated that the morphology of nanostructured Cu(OH)2 is significantly
influenced by the feeding manner of the alkaline solutions. A rational mechanism based on coordination
self-assembly and oriented attachment is proposed for the selective formation of the polycrystalline Cu(OH)2
nanowires and single-crystalline Cu(OH)2 nanoribbons. In the presence of a polymeric additive, poly(acrylic
acid) (PAA), ordered assemblies of Cu(OH)2 nanorods can be readily obtained. Furthermore, well-defined
CuO nanostructures, such as CuO nanoplatelets, nanoleaflets, and nanowires, were produced by thermal
dehydration of the as-prepared Cu(OH)2 nanostructures in solution or in the solid state. Scanning electron
microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray diffraction
(XRD), and X-ray photoelectron spectroscopy (XPS) were used to characterize the products.
Introduction
bers arranged along the outer contour of the tooth tip, leading
to an extraordinary resistance to abrasion.14 Recently, 1D
Cu(OH)2 nanomaterials have been fabricated through solution-
phase chemical approaches involving coordination self-as-
sembly. For example, polycrystalline Cu(OH)2 nanoribbons were
prepared in aqueous solution of ammonia using Cu2S nanowires
as precursors,12a whereas single-crystalline Cu(OH)2 nanoribbon12b
and nanotube16 arrays were prepared by surface oxidation of
copper foil in alkaline solutions. On the other hand, polycrys-
talline Cu(OH)2 nanowires were synthesized by a simple two-
step process in aqueous ammonia and NaOH solutions17 or by
the interaction between a copper complex and NaOH at the
aqueous-organic interface.18 However, it is noted that the
effective adjustment of 1D Cu(OH)2 nanostructures between
ribbon- and wirelike morphologies in one synthesis system has
not been achieved so far.
One-dimensional (1D) nanostructured materials, such as
nanotubes, nanorods/nanowires, and nanobelts/nanoribbons,
have attracted considerable attention due to their fundamental
importance and potential wide-ranging applications.1 Among
the various methods developed for the fabrication of 1D
nanomaterials, the solution-phase chemical synthesis has been
considered as the most promising route in terms of cost,
throughput, and the potential for large-scale production.2 Gener-
ally, the solution-phase chemical approach may be classified
into the template method, which employs hard3 and soft4-6
templates, and the template-free method, which uses surface-
capping reagents for anisotropic confinements7 or directly
utilizes the highly anisotropic crystal structure of solid materials
without capping reagents.8 Accompanying the advance in
synthesis method of 1D nanostructures, there is growing interest
in the morphological control of the 1D nanostructures. In
particular, as a new family of 1D nanostructures, nanobelts/
nanoribbons are receiving increasing attention because of their
unique properties and potential applications in building func-
tional nanodevices;9 however, there are few reports concerning
the wet chemical fabrication of inorganic nanobelts.10-12 It
remains a significant challenge to realize elaborate control of
the morphology of 1D nanostructures in the absence of template
in aqueous solution.
As a p-type semiconductor with a narrow band gap (1.2 eV),
CuO has been widely exploited for diverse applications as
heterogeneous catalysts,19 gas sensors,20 lithium ion electrode
materials,21 and field emission (FE) emitters.22 Many recent
efforts have been directed toward the fabrication of nanostruc-
tured CuO to enhance its performance in currently existing
applications. In particular, a variety of 1D CuO nanostructures
have been prepared by high-temperature approaches23 and low-
temperature wet chemical approaches.21,24 Notably, 1D Cu(OH)2
nanostructures have been frequently employed as the precursors
for the fabrication of 1D CuO nanostructures.12b,18,25 However,
a detailed study on the transformation from Cu(OH)2 to CuO
nanostructures is still lacking.
As well-known layered materials, Cu(OH)2 and basic Cu(II)
salts with an orthorhombic structure have received much
attention in recent years.12-18 Since its magnetic properties are
sensitive to the intercalation of molecular anions, Cu(OH)2 has
the potential application as sensors.13 Orthorhombic Cu2(OH)3-
Cl was recently found in living organisms as the first identified
copper-based biomineral, which is formed in the teeth of a
carnivorous marine worm and exists as polycrystalline nanofi-
In this work, wire- and ribbonlike Cu(OH)2 nanostructures
have been selectively prepared by a two-step, template-free, wet
chemical approach just through simply changing the feeding
order of two alkaline solutions. The transformation from the
1D Cu(OH)2 nanostructures to a variety of novel CuO nano-
structures (wires, platelets, leaflets, and flowers) has been
* To whom correspondence should be addressed. E-mail: liminqi@
chem.pku.edu.cn.
10.1021/jp046772p CCC: $27.50 © 2004 American Chemical Society
Published on Web 10/21/2004

17826 J. Phys. Chem. B, Vol. 108, No. 46, 2004
Lu et al.
realized by thermal dehydration of the as-prepared Cu(OH)2
nanostructures either in solution or in the solid state.
Experimental Section
Synthesis of Cu(OH)2 Nanostructures. Two different
methods were used for the two-step synthesis of Cu(OH)2
nanostructures, i.e., the KOH/NH3 route and the NH3/KOH
route. In a typical KOH/NH3 route to Cu(OH)2 nanostructures,
0.8 mL of 1 M KOH solution was first dropped into 2 mL of
0.1 M CuSO4 solution under rigorous stirring at ambient
temperature (∼19 °C), which was followed by the dropping of
0.3 mL of ammonia solution (12.5 wt %, ∼7.0 M). After 3 min
of further stirring, the mixed solution was kept static in a sealed
vessel for 12 h (unless otherwise stated). The resultant precipitate
was filtered off, washed thoroughly with deionized water, and
dried in a vacuum at 25 °C for 12 h. For the NH3/KOH route
to Cu(OH)2 nanostructures, the synthesis procedure was identical
to that for the KOH/NH3 route except for the feeding order of
the two alkaline solutions; i.e., the KOH solution was added
after the addition of the ammonia solution. Occasionally, a
polymeric additive, poly(acrylic acid) (PAA, Aldrich, Mn )
5100), was added to the solution before the addition of the
alkaline solutions for the investigation of its effect on the Cu-
(OH)2 nanostructures.
Transformation of Cu(OH)2 Nanostructures into CuO
Nanostructures. For the dehydration of Cu(OH)2 nanostructures
in solution, the Cu(OH)2 nanostructures formed immediately
after adding both alkaline solutions were heated at a given
temperature (e.g., 30 and 50 °C)for 12 h; alternatively, the as-
prepared Cu(OH)2 nanostructures were separated and redispersed
in deionized water, followed by heat treatment at 70 °C for a
given time (e.g., 1 and 12 h). For the dehydration of Cu(OH)2
nanostructures in the solid state, the as-prepared Cu(OH)2
nanostructures were heated at 120 °C for 2 h to dehydrate and
subsequently at 180 °C for another 3 h to promote crystalliza-
tion.
Characterization. Field-emission scanning electron micros-
copy (FE-SEM) images were taken on a FEI STRATA DB235
microscope at an accelerating voltage of 10 kV. X-ray diffraction
(XRD) was carried out on a Rigaku Dmax-2000 X-ray diffrac-
tometer equipped with Cu KR radiation. Transmission electron
microscopy (TEM) was performed on a JEOL JEM-200CX
microscope with an accelerating voltage of 160 kV. High-
resolution TEM (HRTEM) was performed on a Hitachi 9000HAR
microscope. Selected samples were also investigated by X-ray
photoelectron spectroscopy (XPS, Kratos Axis Ultra spectrom-
eter) with Al KR monochromatized X-ray source running at
250 W and the base pressure in the analytic chamber about 5
× 10-9 Torr.
Figure 1. SEM (a, b), TEM (c, d), and HRTEM (e) images of Cu-
(OH)2 nanowires prepared via the KOH/NH3 route aged for 12 h. Inset
shows ED pattern of the bundled nanowires. Scale bars: (a) 5 µm; (b)
500 nm; (c) 100 nm; (d) 50 nm; (e) 5 nm.
nanowire bundles up to several hundreds of nanometers in
diameters (Figure 1b). The TEM images (Figure 1c,d) suggest
that the 1D nanostructures exhibit relatively uniform width (∼10
nm), which is consistent with a cylindrical wirelike morphology
rather than a ribbonlike morphology. The HRTEM image
(Figure 1e) suggests that the nanowires are polycrystalline
nanowires consisting of orthorhombic Cu(OH)2 nanocrystallites.
However, the electron diffraction (ED) pattern corresponding
to a bundle of nanowires shows some diffusive, yet strong,
reflections, agreeing well with the orthorhombic structure of
Cu(OH)2, which suggests that orientations of the crystallites are
not random but coordinated with the [100] direction being the
preferential growth direction of the polycrystalline nanowires
or the growth direction on average. The XRD pattern of the
nanowires aged for 12 h is presented in Figure 2, which shows
sharp diffraction peaks attributed to the orthorhombic Cu(OH)2,
implying the formation of pure Cu(OH)2 nanowires. It is noted
that only (0kl) reflections are clearly observed, which is in good
agreement with the Cu(OH)2 nanowires with preferential
orientation along the [100] direction. The polycrystalline Cu-
(OH)2 nanowires with preferential orientation could form
through an oriented attachment mechanism,26 which is remi-
niscent of the polycrystalline, [100]-oriented Cu(OH)2 nanor-
ibbons obtained in aqueous solution by using Cu2S nanowires
as precursors.12a
Results and Discussion
Synthesis of Cu(OH)2 Nanostructures. The synthesis of
Cu(OH)2 nanostructures was simply achieved by successively
dropping KOH and ammonia solutions into an aqueous solution
of CuSO4 at ambient temperature (∼19 °C). According to the
feeding order of the two alkaline solutions, two different
synthesis routes were adopted: the KOH/NH3 route where KOH
was added first and ammonia was added subsequently, and the
NH3/KOH route where ammonia was added first followed by
the addition of KOH. Figure 1 shows typical SEM and TEM
images of the as-prepared Cu(OH)2 nanostructures produced via
the KOH/NH3 route and aged for 12 h. The product consists
entirely of very long (∼20 µm) wirelike nanostructures (Figure
1a), and it appears that the nanowires tend to assemble into
To understand the growth process of the Cu(OH)2 nanowires
in solution, we investigated the effect of aging time on the
product. It was observed that the solution precipitated im-
mediately after the addition of the KOH solution. As shown in
Figure 2, the XRD pattern of the obtained precipitates suggests
the precipitates consist of triclinic Cu(OH)2‚H2O crystals
(JCPDS 42-0746). The XRD patterns of the products obtained
at different aging time after the addition of the ammonia solution

Controlled Synthesis of Copper Nanostructures
J. Phys. Chem. B, Vol. 108, No. 46, 2004 17827
Figure 2. XRD patterns of Cu(OH)2 nanowires prepared via the KOH/
NH3 route aged for different periods. XRD pattern of Cu(OH)2‚H2O
precipitates obtained before adding ammonia is also displayed.
Figure 4. SEM (a, b) and TEM (c, d) images and ED (e) and XRD
(f) patterns of Cu(OH)2 nanoribbons prepared via the NH3/KOH route.
The ED pattern shown in plate e corresponds to the single nanoribbon
shown in plate d. Scale bars: (a) 5 µm; (b) 1 µm; (c) 200 nm; (d)
100 nm.
morphology was almost completely reserved (Figure 3d). This
result suggests that the nanowires aged for 1 h were still not
well-crystallized and continued to develop into the well-
crystallized nanowires probably through the Ostwald ripening
process, which has been believed to assist the 1D growth process
by oriented attachment.26c It is worth noting that oriented arrays
of polycrystalline Cu2(OH)3Cl nanofibers play a key role in
enhancing hardness and stiffness of the teeth of a marine
worm.14 Therefore, the current room-temperature solution
synthesis of bundles of polycrystalline Cu(OH)2 nanowires,
which has a similar orthorhombic structure, would have relevant
biological implications.
When the feeding order of the two alkaline solutions was
altered, a significant change in the morphology of the produced
1D Cu(OH)2 nanostructures occurred. Figure 4 presents typical
SEM and TEM images of the as-prepared Cu(OH)2 nanostruc-
tures produced via the NH3/KOH route along with their ED
and XRD patterns. The SEM images (Figure 4a,b) suggest that
the product consists of well-defined sheaves of nanoribbons
about 5 µm in length, and the TEM image (Figure 4c) shows
thinning in the smooth bend and wring sections indicated by
the arrows, which is characteristic of ribbonlike nanostructures.
The nanoribbons are typically 30-100 nm in width and several
nanometers in thickness. An enlarged TEM image showing a
single nanoribbon is presented in Figure 4d with its related ED
pattern shown in Figure 4e, which clearly shows spots associated
with the [010] zone axis of the orthorhombic Cu(OH)2,
indicating that the nanoribbon is a single-crystal grown along
the [100] direction and enclosed with the (010) and the (001)
as the top and side planes, respectively. The related X-ray
diffraction pattern (Figure 4f) suggests the nanoribbons consist
Figure 3. SEM images of Cu(OH)2 nanowires prepared via the KOH/
NH3 route aged for 2 min (a) and 1 h (b), and the Cu(OH)2 nanowires
obtained via the KOH/NH3 route aged for 1 h (c) and 12 h (d) and
sonicated for 10 min. Scale bars: 500 nm.
are also shown in Figure 2. After 2 min of aging, the product
shows the XRD pattern corresponding to pure orthorhombic
Cu(OH)2, indicating a quick transformation from Cu(OH)2‚H2O
to Cu(OH)2 accompanying the addition of ammonia. The
relatively broader diffraction peaks suggests the smaller crys-
tallite size for the nanowires formed at the early stage. The
typical SEM image of the sample is presented in Figure 3a,
which shows that a large number of separated nanowires ∼10
nm in diameter coexisted with some bundles of nanowires
(larger than 50 nm in bundle diameter). At an aging time of 1
h, the diffraction peaks were considerably narrowed, suggesting
an increase in the crystallite size. Meanwhile, all the individual
nanowires were assembled into bundles (Figure 3b), which is
very similar to the product obtained after 12 h of aging (Figure
1a,b). However, simple ultrasonic treatment using a normal
ultrasonic bath revealed a considerable difference in the tough-
ness between the nanowires aged for 1 and 12 h. Upon 10 min
of sonication, most of the long nanowires aged for 1 h were
broken into short nanorods around 200-500 nm in length
(Figure 3c). In contrast, the long nanowires aged for 12 h were
not broken at all upon 10 min of sonication and the original

17828 J. Phys. Chem. B, Vol. 108, No. 46, 2004
Lu et al.
SCHEME 1: Schematic Illustration of the Growth Process of 1D Cu(OH)2 Nanostructures in Solution
of pure orthorhombic Cu(OH)2 and the presence of reflections
other than the (0kl) reflections can be rationalized by considering
that the nanoribbons are not well-aligned parallel to the substrate
as they aggregate into sheaflike assemblies. The obtained [100]-
oriented Cu(OH)2 nanoribbons are reminiscent of the single-
crystalline Cu(OH)2 nanoribbons prepared by surface oxidation
of copper foil in ammonia solutions where Cu(OH)2 nanoribbons
were grown almost vertically on the copper surface to form
nanoribbon arrays.12b To the best of our knowledge, this is the
first realization of selective synthesis of Cu(OH)2 nanowires
and nanoribbons in an aqueous solution system through simple
alteration of the feeding order of two alkaline solutions. It is
noteworthy that for the current NH3/KOH route to the Cu(OH)2
nanoribbons, a considerable excess of ammonia with respect to
CuSO4 (e.g., [NH3]/[CuSO4] ∼ 10) was required; otherwise,
Cu(OH)2 nanowires may be formed. For example, it was
reported that only Cu(OH)2 nanowires were obtained at a
relatively low ammonia concentration ([NH3]/[CuSO4] ∼ 1)
when a similar NH3/NaOH route was employed.17
The exact mechanism for the selective formation of Cu(OH)2
nanowires and nanoribbons remains to be investigated in more
detail. As is well-known, the orthorhombic Cu(OH)2 with a
layered structure parallel to (010) consists of olated >Cu-
(OH)2Cu< chains oriented along [100] and characterized by
the square-planar coordination of the Cu2+ ions by OH- ions
with strong σx2-y2 bonds. According to the Bravais-Donnay-
Harker law, the growth of Cu(OH)2 along [100] is much faster
than along other directions and the [010] direction is the lowest
growth direction, leading to a tendency to form a ribbonlike
structure.12,16 Based on the recently discussed coordination self-
assembly mechanism for the formation of Cu(OH)2 nanoribbons,
a tentative mechanism (Scheme 1) involving coordination self-
assembly and oriented attachment was proposed for the forma-
tion of the current 1D Cu(OH)2 nanostructures. For the NH3/
KOH route to Cu(OH)2 nanostructures, a homogeneous
[Cu(NH3)n]2+ solution was formed during the first step. Upon
adding the KOH solution, OH- replaced NH3 in the
[Cu(NH3)n]2+ complex to form square-planar [Cu(OH)4]2- units
and a coordination self-assembly of the >Cu(OH)2Cu< chains
occurred quickly, resulting in the formation of single-crystalline
Cu(OH)2 nanoribbons. The nucleation of Cu(OH)2 could start
from localized regions with relatively high [Cu(NH3)n]2+
concentrations. Once the nuclei were formed, the growth of Cu-
(OH)2 nanoribbons started on it, leading to the formation of
sheaves of nanoribbons. The complex of [Cu(NH3)n]2+ may act
as a molecular transporter that transports Cu2+ to the growing
nanoribbon tips with OH- ligands attached; moreover, NH3 may
probably adsorb on the (010) surface to further slow the growth
along the [010] direction, favoring the formation of the
ribbonlike structure. When the feeding order of the NH3 and
KOH solutions was altered, i.e., the KOH/NH3 route was
employed, Cu(OH)2‚H2O precipitated during the first step. Upon
adding the ammonia solution, the Cu(OH)2‚H2O precipitate was
converted to Cu(OH)2 crystals, probably through a solid-
liquid-solid (SLS) process where the complex [Cu(NH3)n]2+
may also play an important role as a transporter. Although there
was still a tendency to form Cu(OH)2 nanoribbons by coordina-
tion self-assembly, during the SLS process, the [Cu(NH3)n]2+
concentration in the solution could be relatively lower, which
did not favor the formation of nuclei for the Cu(OH)2 nanor-
ibbons. Therefore, individual Cu(OH)2 nanoparticles rather than
long Cu(OH)2 nanoribbons were formed at the beginning. Then
Cu(OH)2 nanowires grew preferentially along the [100] direction
by the oriented attachment of the primary nanoparticles, which
was coupled with coordination self-assembly, resulting in
polycrystalline Cu(OH)2 nanowires. The individual nanowires
were further attached side by side to assemble into bundles on
further aging, accompanying an Ostwald ripening process
leading to larger crystallite sizes and tougher nanowires. The
obtained nanowire bundle structures are reminiscent of the
bundles of BaSO4/BaCrO4 nanofibers obtained in the presence
of polymeric additives;7d,e however, the current nanowire
bundles were produced in polymer-free processes, and the
driving force could partly come from the lateral capillary forces.
On the other hand, the nanobelt bundles obtained via the NH3/
KOH route may be formed due to the growth from the central
nucleation points, which is somewhat similar to the bundles of
BaSO4/BaCrO4 nanofibers formed by growth from individual
nucleation points under the direction of polymers.7d,e
Although the NH3/KOH route seems to be favorable for the
formation of Cu(OH)2 nanoribbons, such a tendency can be
restrained by polymeric additives present in the solution. Poly-
(acrylic acid) (PAA) is a typical crystal growth inhibitor for
many inorganic minerals.27 When 2.5 × 10-4 M PAA was
present in the CuSO4 solution before the addition of the two
alkaline solutions, the product obtained by the NH3/KOH route
consisted of relatively uniform assemblies of parallel-aligned
Cu(OH)2 nanorods 5-10 nm in width and ∼200 nm in length
(Figure 5a,b). From the HRTEM image (Figure 5c), it is
observed that the nanorod is a single-crystal grown along the
[100] direction, the fastest growth direction of the orthorhombic
Cu(OH)2. The amorphous area can be observed on both sides
of the nanorod, implying the adsorption of the polymer
molecules on the nanorod. The corresponding XRD pattern
(Figure 5d) is indicative of orthorhombic Cu(OH)2, confirming
the formation of Cu(OH)2 nanorod assemblies. The XPS result
(Figure 6) shows typical Cu 2p3/2 and Cu 2p1/2 peaks with the
measured binding energy of 934.5 and 954.2 eV, respectively,
as well as their concomitant shake-up lines at 942.4 and 962.4
eV, which is indicative of the paramagnetic chemical state of
Cu2+ 28
moreover, the well-resolved C 1s peak is observed at
;
284.8 eV, confirming the presence of PAA within the as-
semblies, i.e., the formation of PAA/Cu(OH)2 hybrid nanoma-
terials. It is therefore reasonable to believe that the adsorption
of PAA modifies the growth rate for special crystal planes to
some extent. Especially, it could restrain the preferential growth
along the [001] direction with respect to the [010] direction,
resulting in the formation of Cu(OH)2 nanorods rather than
nanoribbons. The presence of adsorbed PAA molecules may
also be contributed to the parallel attachment of the short

Controlled Synthesis of Copper Nanostructures
J. Phys. Chem. B, Vol. 108, No. 46, 2004 17829
Figure 7. SEM images (a, b) and XRD patterns (c) of CuO
nanoplatelets obtained by thermal dehydration of the Cu(OH)2 nanow-
ires prepared via the KOH/NH3 route immediately after the addition
of the KOH and ammonia solutions: (a) 30 °C; (b) 50 °C. Scale bars:
500 nm.
Figure 5. TEM (a, b) and HRTEM (c) images and the XRD pattern
(d) of Cu(OH)2 nanorod assemblies prepared via the NH3/KOH route
in the presence of PAA. Scale bars: (a) 1 µm; (b) 200 nm; (c) 3 nm.
at 30 °C. The corresponding XRD pattern (Figure 7c) shows
the appearance of reflections attributed to monoclinic CuO,
suggesting the part transformation of Cu(OH)2 to CuO. When
aged at 50 °C, the Cu(OH)2 nanowires were completely
converted to the irregular CuO nanoplatelets (Figure 7b), which
was confirmed by the corresponding XRD pattern (Figure 7c).
If the well-developed Cu(OH)2 nanowires after 12 h of aging
were separated and redispersed in water, the temperature for a
complete transformation to CuO within 12 h was increased to
∼70 °C. Figure 8 shows typical TEM and HRTEM images as
well as the XRD pattern of the products dehydrated at 70 °C
for different times. When heated for 1 h, rhombic, leafletlike
nanostrips appeared in addition to the original bundles of Cu-
(OH)2 nanowires accompanying the appearance of the XRD
peaks characteristic of monoclinic CuO; after 12 h of heating,
uniform nanoleaflets 0.7-0.8 µm in length, ∼250 nm in width,
and ∼50 nm in thickness were obtained, and the corresponding
XRD pattern suggests that the product consists of pure mono-
clinic CuO. This result suggests a gradual transformation of the
Cu(OH)2 nanowire bundles to the CuO nanoleaflets with heating
at 70 °C. The ED pattern of a single CuO nanoleaflet shows
spots due to the [001] zone axis of monoclinic CuO, indicating
that the CuO nanoleaflet is a single crystal elongated along the
[010] direction with the top surface of the (001) plane, which
is supported by the related HRTEM image.
It is noted that Matijevic´ et al. have prepared polycrystalline
ellipsoidal CuO colloids by a controlled double-jet precipitation
technique using NaOH and Cu(NO3)2 as the reactants, and they
believed that the polycrystalline CuO ellipsoids were formed
by aggregation of primary CuO nanoparticles.24a Recently, Zeng
et al. have prepared hierarchical dandelion-like architectures
constructed with single-crystalline rhombic CuO nanostrips as
building units through a hydrothermal synthesis in alkaline
ethanol solution, and they proposed that the CuO nanostrips
(larger than 600 nm in breadth) were formed by oriented
attachment of CuO nanoribbons (about 10-20 nm in breadth).24d
It should be pointed out that during the solution synthesis of
CuO, the final product composition and morphology are largely
dependent on the synthesis condition, such as basicity and
solvent, and the solid precursors including Cu(OH)2 may exist
only momentarily or may not exist at all. In the current situation,
Figure 6. X-ray photoelectron spectrum of Cu(OH)2 nanorod as-
semblies prepared via the NH3/KOH route in the presence of PAA.
nanorods, which leads to the formation of the novel assemblies
of Cu(OH)2 nanorods. Such Cu(OH)2-based organic-inorganic
hybrids with ordered nanostructures would find potential
application as ferromagnets.29 It is worth mentioning that the
Cu(OH)2 product obtained by the KOH/NH3 route in the
presence of PAA still exhibited bundles of nanowires, suggesting
that PAA would be favorable for the formation of cylindrical
1D nanostructures.
Synthesis of CuO Nanostructures. It is well-known that
orthorhombic Cu(OH)2 can be easily transformed to monoclinic
CuO upon heat treatment. The thermal dehydration of the
obtained 1D Cu(OH)2 nanostructures in the solution phase and
in the solid state was investigated but special effort was devoted
to the transformation in solution as the transformation of
Cu(OH)2 nanoribbons to CuO nanoribbons/nanowires in solid
state has been well-documented.12b,25 When the Cu(OH)2
nanowires obtained by the KOH/NH3 route were heated in the
original solution immediately after the addition of the two
alkaline solutions, a complete transformation to CuO occurred
at a temperature as low as 50 °C. As shown in Figure 7a, some
nanoplatelets showing irregular contours appeared in addition
to the bundles of Cu(OH)2 nanowires after being aged for 12 h

17830 J. Phys. Chem. B, Vol. 108, No. 46, 2004
Lu et al.
Figure 9. SEM images of CuO nanostructures obtained by thermal
dehydration of the as-prepared Cu(OH)2 nanoribbons in water at 70
°C for 12 h. Scale bars: (a) 2 µm; (b) 500 nm.
Figure 8. SEM (a, b), TEM (c), and HRTEM (d) images and XRD
patterns (e) of CuO nanoleaflets obtained by thermal dehydration of
the as-prepared Cu(OH)2 nanowires in water at 70 °C for different
times: (a) 1 h; (b-d) 12 h. Inset shows ED pattern of the single CuO
nanoleaflet. Scale bars: (a, b) 500 nm; (c) 100 nm; (d) 3 nm.
Figure 10. SEM (a) and TEM (b) images and ED (c) and XRD (d)
patterns of CuO nanowires obtained by thermal dehydration of the as-
prepared Cu(OH)2 nanowires in the solid state at 120 °C for 2 h. Scale
bars: (a) 500 nm; (b) 200 nm.
the CuO nanoleaflets may form by a reconstructive transforma-
tion from Cu(OH)2 in aqueous solution involving a dissolution
reaction followed by the crystallization of CuO.30 It is note-
worthy that at the early stage of the transformation, the
intermediate state of transformation can be observed, which
indicates that the CuO nanoleaflets were grown from some parts
of the Cu(OH)2 nanowires (Figure 8a). Moreover, when the
transformation was complete, some parallel linear traces on the
CuO nanoleaflets due to the sacrificial precursors of Cu(OH)2
nanowire bundles can be clearly observed (Figure 8c). Therefore,
it can be reasonably proposed that upon heating in water, some
parts of the Cu(OH)2 nanowires were preferentially dissolved
and then CuO nucleated on some parts of the undissolved Cu-
(OH)2 nanowires, which was followed by the gradual growth
of CuO nanoleaflets accompanying the dissolution of the Cu-
(OH)2 nanowires, both the bare nanowires and the nanowires
encapsulated within the CuO nanoleaflets.
For comparison purpose, the thermal dehydration of the
sheaves of Cu(OH)2 nanoribbons, which were obtained by the
NH3/KOH route, in solution was also investigated. As shown
in Figure 9, unique, flowerlike aggregates consisting of CuO
nanopetals were obtained after the as-prepared Cu(OH)2 nan-
oribbons were redispersed in water and heated at 70 °C for 12
h. This result supports the mechanism of reconstructive trans-
formation in solution involving a dissolution process followed
by the CuO crystallization.
original Cu(OH)2 nanowires, were obtained (Figure 10a,b). The
related ED and XRD patterns demonstrate the complete conver-
sion of Cu(OH)2 to monoclinic CuO. The relatively broad peaks
shown in the XRD pattern (Figure 10d) indicates that the CuO
nanowires are actually polycrystalline nanowires consisting of
primary nanocrystallites. This result suggests that the morphol-
ogy of the 1D Cu(OH)2 nanostructures obtained by the solution
route can be well-reserved upon a simple heating process in
the solid state, which is consistent with the result reported for
the Cu(OH)2 nanoribbons obtained by the surface oxidation of
Cu and solution-phase coordination self-assembly.12b
Conclusions
In summary, a facile, template-free, low-temperature synthesis
of Cu(OH)2 nanowires and nanoribbons has been realized by
simply dropping KOH and ammonia solutions into an aqueous
CuSO4 solution. According to the feeding order of the two
alkaline solutions, the KOH/NH3 route and the NH3/KOH route
were employed to selectively fabricate polycrystalline Cu(OH)2
nanowires and single-crystalline Cu(OH)2 nanoribbons, respec-
tively. It has been reasonably proposed that the Cu(OH)2
nanoribbons were formed by solution-phase coordination self-
assembly at a relatively high [Cu(NH3)n]2+ concentration,
whereas the Cu(OH)2 nanowires were formed by oriented
attachment coupled with coordination self-assembly through
a solid-solution-solid process, where a relatively low [Cu-
(NH3)n]2+ concentration was retained. Ordered assemblies of
Cu(OH)2 nanorods were readily synthesized under the direction
For the thermal dehydration of 1D Cu(OH)2 nanostructures
in the solid state, the Cu(OH)2 nanowires obtained by the KOH/
NH3 route were selected as a representative example. After
dehydration in the solid state at 120 °C for 2 h, bundles of CuO
nanowires, which essentially reserved the morphology of the

Controlled Synthesis of Copper Nanostructures
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Acknowledgment. Financial support from NSFC (Grants
20325312, 20233010) and FANEDD (Grant 200020) is grate-
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