Hydrothermal synthesis of three-dimensional hierarchical CuO butterfly-like architectures

Article abstract of DOI:10.1002/ejic.200800911

Uniform 3D hierarchical CuO butterfly-like architectures were fabricated by a surfactant-assisted hydrothermal oriented attachment route. This route included the formation of CuO butterfly-like architectures in a solution of cupric chloride (CuCl₂

Full text of DOI:10.1002/ejic.200800911

FULL PAPER
DOI: 10.1002/ejic.200800911
Hydrothermal Synthesis of Three-Dimensional Hierarchical CuO
Butterfly-Like Architectures
Yajing Zhang,[a,b] Siu Wing Or,*[b] Xiaolei Wang,[a] Tieyu Cui,[a] Weibin Cui,[a]
Ying Zhang,[a] and Zhidong Zhang[a]
Keywords: Hydrothermal synthesis / Copper oxide / Nanostructures / Self-assembly
Uniform 3D hierarchical CuO butterfly-like architectures
were fabricated by a surfactant-assisted hydrothermal ori-
ented attachment route. This route included the formation of
CuO butterfly-like architectures in a solution of cupric chlo-
ride (CuCl2·2H2O) and sodium hydroxide (NaOH) at 100 °C
for 15 h by using sodium dodecyl benzenesulfonate (SDBS)
as surfactant. The as-prepared CuO architecture was charac-
terized by X-ray diffraction, scanning electron microscopy
and transmission electron microscopy. The CuO butterfly-
like architectures, with lengths of about 6 µm and widths of
2–4 µm, were assembled from several tens of oriented attach-
ment rhombic nanosheets with a thickness of about 60 nm.
A growth mechanism for the formation of the CuO butterfly-
like architectures was proposed on the basis of time-depend-
ent experiments. The synthetic parameters such as reaction
temperature, the concentration of sodium hydroxide and re-
action time all affected the morphology of the CuO architec-
tures. The synthetic strategy could be extended to assemble
3D architectures of other materials.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
bandgap energies of CuO nanostructures and their shapes
and sizes, different synthetic routes including vapour–solid,
vapour–liquid–solid and solid–liquid routes were developed
for the synthesis of CuO.[14] Various CuO micro- and/or
nanomaterials, such as nanoparticles,[15] one-dimensional
(1D) nanotubes, nanorods, nanobelts and nanoneedles,[16,17]
two-dimensional (2D) nanoribbions, nanosheets, nano-
leaves and nanowiskers,[18] three-dimensional (3D) micro-
flowers and microbranches formed through the self-as-
sembly of nanometre-sized building blocks,[19] have been
synthesized by these routes. For example, CuO nanowires
supported on various substrates were obtained by the ther-
mal oxidation of these substrates in air.[20] CuO nanotubes
and nanorods were synthesized by controlling the concen-
tration of the precursor.[21] The formation of CuO rod-like,
flake-like and branch-like nanostructures was achieved by
adjusting the molar radio of sodium citrate to cupric
salts.[22] CuO nanoribbons and nanorings were synthesized
by a surfactant-assisted hydrothermal route for different re-
action times.[23] CuO nanoleaves were fabricated by hierar-
chical oriented attachment of Cu(OH)2 nanowires.[24] For
3D hierarchical CuO micromaterials with two or more
levels of structure, their nanometre-sized building blocks
provide a high surface area, whereas their whole microme-
tre-sized structure provides sufficient mechanical properties,
so the combination of these features will improve their elec-
trochemical performance and adsorption ability (removal
of heavy metal ions)[25,26] However, 3D hierarchical CuO
micromaterials have seldom been reported. Zeng et al. as-
sembled hollow dandelion-like CuO microspheres com-
Introduction
Synthesis of hierarchical micro- and/or nanomaterials,
self-assembled by ordered alignment of nanobuilding
blocks, has attracted much attention due to their novel
properties and potential application in optics, electronics,
magnetics and biology.[1] As a result, a number of micro-
and/or nanomaterials with hierarchical structures, including
metals, metal oxide, hydrate, sulfide, copolymers and other
materials,[2–6] have been fabricated by different driving
mechanisms, including surface tension, electric and mag-
netic forces, and hydrophobic interactions.[7–9] However,
synthesis of novel hierarchical structured micro- and/or
nanomaterials is still an interesting topic, because they can
exhibit new physicochemical behaviours and provide much
promise for the fabrication of materials with advanced func-
tionalities.
Cupric oxide, as an important p-type semiconductor with
a narrow bandgap (Eg = 1.2 eV), has been extensively inves-
tigated for its application in solar energy conversion, lith-
ium-ion batteries, gas sensors, field emission and cataly-
sis.[10–13] Because there is a close relationship between the
[a] Shenyang National Laboratory for Materials Science, Institute
of Metal Research and International Centre for Materials Phys-
ics, Chinese Academy of Sciences,
Shenyang 110016, P. R. China
[b] Department of Electrical Engineering, The Hong Kong Poly-
technic University,
Hung Hom, Kowloon, Hong Kong
E-mail: eeswor@polyu.edu.hk
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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Hydrothermal Synthesis of 3D Hierarchical CuO Butterfly-Like Architectures
posed of rhombic nanobuilding blocks through the oriented results confirm that the final product of CuO is trans-
aggregation of smaller nanoribbons by a two-step pro- formed from the initial precursor of Cu(OH)2. The energy-
cedure with the use of ethanol as solvent.[27] Xu et al. fabri- dispersive spectrum (EDS) result reveals that only the Cu
cated CuO microflowers composed of nanosheets in ammo- and O elements are contained in the final product, and the
nia solution.[28] Nevertheless, to the best of our knowledge, atomic ratio of Cu to O in the product is about 1:1 (49:51),
3D hierarchical CuO butterfly-like architectures have not as shown in Figure 2. The EDS result further confirms that
been reported.
the final product is only CuO (Au element originates from
Here we report the controlled synthesis of 3D hierarchi- the thin Au layer sputtering on the sample in the test).
cal butterfly-like CuO architectures by a surfactant-assisted
hydrothermal oriented attachment route. The effects of the
reaction parameters, such as the reaction temperature, reac-
tion time and the concentration of NaOH, on the mor-
phologies of CuO are investigated. The growth mechanism
of the 3D hierarchical CuO butterfly-like architectures is
proposed on the basis of the results of systematic time-de-
pendent experiments. The CuO butterfly-like architectures
provide a supplement of available 3D CuO architectures,
and it can be expected to extend for assembling new elec-
tronic and optoelectronic micro- and/or nanodevices.
Figure 2. EDS pattern of the as-prepared CuO butterfly-like archi-
tectures at 100 °C for 15 h.
Figure 3 shows the typical morphology of the final CuO
product with different magnifications. The low-magnifica-
Results and Discussion
tion scanning electron microscopy (SEM) (Figure 3a) image
implies that the final CuO product is highly uniform butter-
fly-like architectures, with a length of about 6 µm and width
of 2–4 µm. The magnified SEM image (Figure 3b) shows
that the CuO butterfly-like architectures are composed of
several tens of order-attached rhombic nanosheets, which
can be classified as a hierarchical structure. The nanosheets
are about 60 nm thick, as estimated from the side view of
the architecture (Figure 3c). It is also observed that the
CuO nanosheets attach together in the middle part of an
Synthesis and Characterization of Hierarchical CuO
Butterfly-Like Architectures
The X-ray diffraction (XRD) patterns of the precursor,
the intermediate and the final products are shown in Fig-
ure 1. Figure 1a is the XRD pattern of the precursor, in
which all the peaks can be indexed to orthorhombic
Cu(OH)2 (JCPDS No. 13-0420). Figure 1d shows the XRD
pattern of the final product obtained at 100 °C after 15 h
hydrothermal treatment, in which all the XRD peaks can
be indexed to monoclinic CuO, which is consistent with the
standard card JCPDS No.48–1548 (space group C2/c; a =
4.688 Å, b = 3.423 Å, c = 5.132 Å). Impurities were obvi-
ously not found in the final product, indicating it is pure
CuO. Figure 1b [the mixture of Cu(OH)2 and CuO] and
Figure 1c (CuO) are the XRD patterns of the products as-
prepared at 100 °C after 1 and 5 h, respectively. The XRD
Figure 3. (a) Low-magnification and (b) high-magnification SEM
images of the CuO butterfly-like architectures prepared at 100 °C
for 15 h; (c) side view and (d) top view of an individual CuO butter-
fly-like architecture.
Figure 1. XRD patterns of (a) the precursor of Cu(OH)2 and the
products collected at 100 °C for (b) 1 h, (c) 5 h and (d) 15 h.
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Y. J. Zhang, S. W. Or, X. L. Wang, T. Y. Cui, W. B. Cui, Y. Zhang, Z. D. Zhang
FULL PAPER
individual butterfly-like architecture, and they become curly
and separate at the two ends. Furthermore, the two ends
exhibit a symmetrical character, as displayed in the top-
view image (Figure 3d). The BET surface area for the CuO
butterfly-like architectures is about 17 m2 g–1, which is
smaller than that of the hollow dandelion-like CuO.[27] This
is attributed to the fact that the middle part of the butterfly-
like architectures attach densely together. It is worth men-
tioning that the hierarchical CuO butterfly-like architec-
tures are very stable and still maintain the morphology after
ultrasonication for 30 min.
Growth Mechanism for the Formation of the CuO
Butterfly-Like Architectures
To understand the growth mechanism of the CuO butter-
fly-like architectures, systematic time-dependent experi-
ments were carried out at 100 °C. The SEM and TEM
images (Figure 4) of the products obtained after 1, 5 and
10 h in the hydrothermal process illustrate the evolution of
the morphology of the butterfly-like CuO architectures.
When the reaction time is 1 h, 1D nanoribbons, with widths
of 50–60 nm and lengths of 1–2 µm, are observed as the
dominant product, although several 2D rhombic nano-
sheets are also detected (shown by white arrow in Fig-
ure 4a). An individual nanosheet is about 500 nm in width
and 1–2 µm in length. It can be obviously observed that the
nanosheet is assembled by the ordered attachment of sev-
eral nanoribbons. TEM results (Figure 4b) further confirm
the morphologies of the nanoribbons and rhombic nano-
sheets, and those monodispersive nanoribbons are very flex-
ible and tend to curve. The corresponding select area elec-
tron diffraction (SAED) of the nanosheet (the top square
part shown in Figure 4b) is shown in Figure 4c. The diffrac-
Figure 4. (a) SEM and (b) TEM images of the products prepared
at 100 °C for 1 h; (c) the corresponding SAED pattern of the top
part of the rhombic nanosheet (shown by top square in Figure 4b);
(d) the corresponding SAED pattern of the top part of the
nanowire (shown by bottom square in Figure 4b); (e) SEM images
of the products prepared at 100 °C for 5 h and (f) 10 h.
Figure 5. The formation of the architectures possibly con-
sists of four main steps: (1) Formation of Cu(OH)2 crystal
nuclei (step a). (2) CuO nuclei form by the decomposition
of Cu(OH)2 and grow into CuO nanoribbons (step b). (3)
The CuO nanoribbons self-assemble into CuO rhombic
nanosheets through an oriented attachment mechanism
(step c). (4) CuO nanosheets attach orderly and assemble
into hierarchical butterfly-like architectures (step d). The
oriented attachment in both steps c and d are driven by
eliminating higher surface energies.
¯
¯
tion pattern can be indexed to (020), (111) and (202) planes
of CuO, and the obvious symmetry arc-like diffraction
spots illustrate that the nanosheet is assembled by oriented
building blocks.[3b] Figure 4d is the SAED result of an indi-
vidual nanoribbon (the bottom square part shown in Fig-
ure 4b), revealing the single-crystal nature of the nanorib-
bon, and the SAED spots can also be determined to be
¯
¯
(020), (111) and (202) planes of CuO. The results further
indicate that the nanosheets are formed by oriented attach-
ment of CuO nanoribbons. This is in agreement with the
earlier reports.[23,24,27] No nanoribbons are found in the
product obtained after 5 h (Figure 4e), but 2D rhombic
nanosheets (shown by arrows) and 3D undeveloped butter-
fly-like architectures are observed. When the reaction is
prolonged for 10 h, more 3D perfect butterfly-like architec-
tures are yielded, whereas the number of 2D nanosheets
Figure 5. Schematic illustration of the formation procedure of the
3D CuO butterfly-like architectures.
To determine the role of SDBS in the formation process
becomes less, as shown in Figure 4f. It is found that 15 h is of CuO butterfly-like architectures, time-dependent experi-
sufficient for generating well-developed CuO butterfly-like ments were also carried out at 100 °C in the absence of
architectures, and nearly no individual 2D nanoleaves can SDBS, while keeping other reaction parameters unchanged.
be observed (Figure 2).
The results show that: (1) CuO nanoribbons did not form
On the basis of above experimental results and the litera- during the reaction process (Figure S1, Supporting Infor-
ture,[21,23,24] a growth mechanism for the formation of the mation). (2) The final CuO product consisted of a majority
CuO butterfly-like architectures is proposed, as schemed in of 3D shuttle-like architectures and a minority of 2D flat
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Hydrothermal Synthesis of 3D Hierarchical CuO Butterfly-Like Architectures
nanosheets. The results indicate that the SDBS surfactant
plays a critical role in the formation of CuO nanoribbons.
The nanoribbons tend to curve, because they are flexible
and thin, and hence, they are prerequisite building blocks
for assembling intermediate curved CuO nanosheets and
then final butterfly-like architectures. The group of Qian
proposed a crystal-splitting mechanism for forming CuO
nanoribbons, wherein SDBS directly interacts with the sur-
faces of CuO flakes and subsequently its intense Brownian
movement leading to CuO flakes split into CuO nanorib-
bons.[23] It is understood on the basis of our experimental
results that SDBS in our reaction system plays the same
role. In the presence of SDBS, the Cu2+ cations could di-
rectly attach to the DBS– anions and produce Cu(DBS)2,
then Cu(DBS)2 reacts with NaOH to form CuO. The whole
reaction process can be described by Equations (1) and
(2).[23]
Cu(DBS)2 + 4OH– Ǟ [Cu(OH)4]2– + 2DBS–
(1)
[Cu(OH)4]2– Ǟ CuO + H2O + 2 OH–
(2)
Figure 6. SEM images of the products prepared in the presence of
(a) SDS, (b) CTAB and (c) PVP.
SDBS was also substituted with other surfactants such
as sodium dodecyl sulfate (SDS), cetyltrimethylammonium
bromide (CTAB) and poly(vinylpyrrolidone) (PVP). When
SDS was used as surfactant, the butterfly-like architectures
were also obtained (Figure 6a). In the case of CTAB and
PVP, the products were poorly defined butterfly-like archi-
tectures with densely attached nanosheets and flat nano-
sheets, as shown in Figure 6b and c, respectively. The results
demonstrate that the anionic surfactants are more favour-
able for achieving well-defined butterfly-like architectures
in comparison to cationic and neutral surfactants. This may
be ascribed to the direct interaction of anionic surfactants,
such as SDBS and SDS, with cupric ions or to anion-selec-
tive adsorption on the CuO surface in solution. Cationic
and nonionic surfactants, such as CTAB and PVP, cannot
produce butterfly-like architectures possibly because of
their weak interaction with cupric ions.[29]
high enough (e.g., 5 ) in solution, and hence, the reaction
rate is excessively fast, but the flat nanosheets cannot per-
form oriented self-assembly at such a fast reaction rate, so
instead they attach randomly and form CuO flower-like
architectures.
In addition, the effect of the concentration of NaOH on
the morphology of the final CuO product was examined.
In the controlled experiments, when the concentration of
NaOH was 2 , it did not form any butterfly-like structures;
rather, shuttle-like architectures were formed by compact
Figure 7. SEM images of the products prepared at 100 °C with dif-
ferent concentrations of NaOH: (a) 2  and (b) 5 .
Apart from the concentration of NaOH, the reaction
attached nanosheets (Figure 7a). A further increase in the temperature affects the morphology of the final CuO prod-
concentration of NaOH to 5 , resulted in the formation of uct. The controlled experiments demonstrated that a high
spherical architectures composed of a number of nano- reaction temperature is not favourable for the self-assembly
sheets (Figure 7b). It can be concluded that a low concen- of CuO butterfly-like architectures, but favours the forma-
tration of NaOH is critical for the formation of CuO butter- tion of other 3D architectures. For instance, when the reac-
fly-like architectures. This can also be explained by the for- tion is carried out at 120 and 140 °C, irregular structures
mation model proposed in Figure 5. When the concentra- form by random aggregation of nanoribbons, instead of ori-
tion of NaOH in the solution is increased, the reaction rate ented attachment of nanosheets (Figure 8a and b). A fur-
is accelerated according to Equations (1) and (2), which ther increase in the reaction temperature (160 and 180 °C)
leads to the formation of much thicker and less flexible leads to the formation of isotropic flower-like architectures,
nanoribbons in a short time; subsequently, they self- which are created through the self-assembly of many nano-
assemble in an oriented manner into flat and thick nano- ribbons (Figure 8c and d). At a relatively low temperature,
sheets instead of curved nanosheets. Finally, the flat nano- such as 80 °C (Figure S2, Supporting Information), CuO
sheets self-assemble in an oriented manner into CuO butterfly-like architectures can also be obtained, but need a
shuttle-like architectures. The concentration of NaOH is relatively long reaction time. According to our proposed
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Y. J. Zhang, S. W. Or, X. L. Wang, T. Y. Cui, W. B. Cui, Y. Zhang, Z. D. Zhang
FULL PAPER
der constant stirring, and the precursor blue suspension was ob-
tained. For obtaining the precursor, the initial blue suspension was
placed static for several hours in air and then the blue precipitate
was separated, washed and dried for characterization. For prepar-
ing the 3D hierarchical butterfly-like CuO architectures, the initial
blue suspension was transferred into a 50-mL Teflon-lined stainless
steel autoclave. The autoclave was sealed and heated at 100 °C for
15 h and then cooled to room temperature naturally. The products
obtained after hydrothermal treatment were centrifuged, washed
with distilled water and ethanol several times and finally dried in
formation model in Figure 5, it is understandable that iso-
tropic spherical (or near-spherical) architectures are
achieved at higher reaction temperatures, because the reac-
tion rate rises with increasing temperature, and accordingly,
more intermediate building blocks of nanoribbons are pro-
duced in a shorter reaction time, and together with the in-
tense Brownian movement, this results in random aggrega-
tion instead of oriented attachment.[30] This is consistent
with previous reports.[2f,27] The results reveal that the mor-
phology of the product is strongly determined by the reac- vacuo at 60 °C for 4 h. Control experiments were carried out by
adjusting the reaction temperature (80–180 °C) and the amount of
NaOH (1–5 ), while other reaction parameters were left un-
changed.
tion temperature.
The phases of all the products were identified with a Rigaku D/
max 2500 pc X-ray diffractometer (XRD) with Cu-Kα radiation
(λ = 1.54156 Å) at a scan rate of 0.04°s–1. The morphology was
investigated with a Supra 35 field-emission scanning electron
microscope (FESEM) equipped with energy-dispersive spectrum
(EDS) operated at an acceleration voltage of 20 kV. The Brunauer-
Emmett-Teller (BET) surface area was measured by the nitrogen
adsorption–desorption method by using a Micromeritics (AS-
AP2010M) analyzer. Transmission electron microscopy (TEM) ob-
servation was carried out with a TECNAI 20 with an emission
voltage of 200 kV. For sample preparation for FESEM and TEM
observations, at first, a small amount of the as-prepared products
was dispersed in ethanol by ultrasonic treatment for 10 min, and
then one drop of the colloidal solution was deposited onto Si wafer
and Cu grids coated with a carbon layer, respectively.
Supporting Information (see footnote on the first page of this arti-
cle): SEM images of the products prepared at 100 °C without
SDBS for different reaction time; SEM image of the products pre-
pared at 80 °C for different reaction time.
Figure 8. SEM images of the products prepared at different reac-
tion temperatures: (a) 120 °C, (b) 140 °C, (c) 160 °C and (d) 180 °C
for 15 h.
Acknowledgments
This work was supported by the National Nature Science Founda-
tion of China (Projects No. 50331030 and No.50703046), the Re-
search Grants Council of the HKSAR Government (PolyU 5257/
06E) and the Central Research Grant of The Hong Kong Polytech-
nic University (A-PA3C).
Conclusions
3D hierarchical CuO butterfly-like architectures com-
posed of several tens of rhombic nanosheets were fabricated
by a hydrothermal oriented attachment route. Experimental
results suggest that the reaction temperature, the concentra-
tion of NaOH and the reaction time all influence the mor-
phology of CuO, and the SDBS surfactant plays a key role
in the formation of the CuO architectures. In addition, the
CuO butterfly-like architectures were found to self-assemble
through an oriented attachment mechanism. Our synthetic
approach can be employed to fabricate 3D hierarchical
architectures of other materials under appropriate condi-
tions.
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Received: September 11, 2008
Published Online: November 25, 2008
Eur. J. Inorg. Chem. 2009, 168–173
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