Bull. Mater. Sci., Vol. 34, No. 4, July 2011, pp. 967–971. © Indian Academy of Sciences.
Flower-like CuO synthesized by CTAB-assisted hydrothermal method
YUNLING ZOU*, YAN LI, NAN ZHANG and XIULIN LIU
College of Science, Civil Aviation University of China, Tianjin 300300, P.R. China
MS received 9 December 2009
Abstract. Flower-like CuO nanostructures have been synthesized by cetyltrimethylammonium bromide
(CTAB)-assisted hydrothermal method. Here, CuCl2⋅2H2O was used as copper raw material, and sodium
hydroxide was used as precipitate. The resulting CuO powders were characterized by X-ray diffraction
(XRD) and field emission scanning electron microscopy (FESEM). X-ray diffraction (XRD) pattern exhibited
the nanocrystalline nature with monoclinic structure for the as-synthesized nanostructures. FESEM images
indicated that the flower-like CuO nanostructures are composed of many interconnected nanosheets in size of
several micrometres in length and width and 60–80 nm in thickness. The possible formation mechanism of
flower-like CuO nanostructures was discussed.
Keywords.
1.
CuO; flower; nanostructures; CTAB; hydrothermal method.
Introduction
Copper oxide (CuO) is a narrow band gap (E = 1⋅2 eV)
p-type semiconductor and has received considerable
attention due to its potential applications in many fields,
such as solar cells (Musa et al 1998), magnetic storage
media (Dar et al 2008), superconductors (MacDonald
2001), lithium batteries (Morales et al 2005), heterogeneous catalysts (Chen et al 2008), and so on. For the properties of semiconductors depending on their size, shape
and crystalline structure, the control of the shape and size
of the semiconductors has become more important.
Recently, much effort has been devoted to synthesizing
unique CuO nanostructures, such as rods (Xu et al 2002),
robbons (Liu and Zeng 2004; Gao et al 2009), wires (Su
et al 2007), belts (Zhang et al 2008b), sheets (Zheng et al
2007), platelets (Zarate et al 2007), needles (Dar et al
2008), and tubes (Cho and Huh 2008). As one of the
novel structures, flower-like CuO was expected to offer
some exciting opportunities for some potential applications on electrochemistry (Pan et al 2007), sensors (Teng
et al 2008), catalysis (Vaseem et al 2008), and field
emission (Yu et al 2008). So far, a variety of approaches
to fabricating flower-like CuO nanostructures have been
developed, such as hydrothermal (Yang et al 2007; Teng
et al 2008), solution-immersion (Pan et al 2007),
hydrolysis (Zhu et al 2007), microwave-hydrothermal
(Volanti et al 2007; Xia et al 2009), chemical precipitation (Zhang et al 2008a), thermal oxidation (Yu et al
2008) and solution-phase route (Yu et al 2009). Teng et al
(2008) synthesized the flower-like CuO nanostructures by
*Author for correspondence (zouyunling1999@126.com)
hydrothermal process using copper threads as precursor
and pointed out that the flower-like CuO nanostructures
are made of three structures: the nanocrystals, the petals,
and the assembly of the petals. Zhu et al (2007) reported
a facile route for synthesis of the flower-like CuO nanostructures composed of many interconnected needle-like
crystallites by hydrolyzing of Cu(OAc)2 solution without
any surfactants. Rose-like nanoarchitectures CuO composed of wide nanosheets have been prepared by a mild
solution-phase route without the utility of the templates,
additives or external magnetic field (Yu et al 2009).
Surfactant is conventionally used as morphologydirecting agent to obtain nanostructural-conducting polymers due to its ability to form thermodynamically stable
aggregates of inherently nanoscale dimensions (Jang and
Oh 2002; Andrew et al 2003). CTAB is a useful surfactant that has been widely used in fabricating the nanomaterials to control the morphology. Recently CTABassisted hydrothermal technique has emerged as an attractive technique to investigate the synthesis of zinc oxide
nanostructures. There are also some reports about the
preparation of CuO nanostructures by using CTAB assisted hydrothermal technique. Cao and co-workers (Cao
et al 2003) reported CTAB-assisted hydrothermal synthesis of CuO of various morphologies such as rod-like
spheroidal, hexahedron structures, and other irregular
structures. CuO shuttle-like nanocrystals were synthesized by the hydrolysis of cupric acetate (Cu(CH3COO)2⋅
H2O) via a CTAB-assisted hydrothermal route at low
temperature (Zhang et al 2006). In this work, we reported
the fabrication of flower-like CuO nanostructures by
CTAB-assisted hydrothermal method and its characterization by X-ray diffraction (XRD). The morphology of
flower-like CuO nanostructures was observed by field
967
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Yunling Zou et al
emission scanning electron microscopy (FESEM). The
possible formation mechanism of flower-like CuO nanostructures was discussed.
2.
Experimental
2.1 Chemicals
Analytical grade copper chloride dihydrate CuCl2⋅2H2O
and sodium hydroxide NaOH were used as precursors,
purchased from Tianjin Tianda Chemical Experiment
Factory. Cetyltrimethylammonium bromide (C19H42BrN)
of analytical grade was purchased from Kewei Company
of the Tianjin University. All the chemicals were directly
used without further purification. Deionized water was
used throughout.
hydrothermal method at 150°C for 12 h are shown in
figure 1. In the XRD pattern, compared with the standard
diffraction peaks from JCPDS card no. 80-1917, the
peaks located at 2θ values of 30–80° can be indexed to
the characteristic diffractions of monoclinic phase CuO
(a = 4⋅689 Å, c = 5⋅132 Å). The peak intensities and
widths clearly indicated that the sample was highly crystalline in nature. Compared with the standard diffraction
patterns, there are no other characteristic peaks observed
belonging to impurities indicating that all the products
were phase-pure.
Energy dispersive analysis of X-ray (EDAX) on the
obtained as-prepared CuO sample was performed using
the field emission scanning electron microscope (FESEM).
EDAX spectra (shown in figure 2) clearly demonstrates
2.2 Sample preparation
In a typical synthesis, the starting solution of copper
(0⋅25 mol l–1) was prepared by dissolving 0⋅8524 g
(5 mmol) CuCl2⋅2H2O in 20 ml deionized water. Subsequently, the CuCl2 solution was slowly dropped into the
50 ml of NaOH solution (3 mol l–1) under vigorous stirring, and a blue-coloured precursor was obtained. 1 g
CTAB (3 mmol) was added to the blue-coloured precursor
and stirred vigorously for 30 min at 50°C to ensure complete dissolution of CTAB. This reaction solution was
then transferred to a 100 ml Teflon-lined stainless steel
autoclave and heated at 150°C for 12 h in an electric
oven. After reaction, the autoclave was allowed to cool to
room temperature. The obtained black precipitate was
centrifuged and washed thoroughly with deionized water
and ethanol. Then, the precipitate was dried in drying
oven at 60°C for 24 h. Finally, the products were calcined
in a furnace with an air atmosphere at 500°C for 2 h.
Figure 1. XRD pattern of flower-like CuO nanostructures
synthesized at 150°C for 12 h.
2.3 Instrumentation
Power X-ray diffraction (XRD) was measured on a DX2000 X-ray diffractometer with CuKα radiation
(λ = 0⋅1542 nm), and the tests were under accelerated voltage of 30 kV and current of 25 mA. Field emission scanning electron microscopy (FESEM) images and energy
dispersive spectrum (EDS) were obtained by FEI
Nanosem 430 FESEM and Genesis XM2 APEX 60SEM
respectively.
3.
Results and discussion
3.1 The structural characterization of CuO
The structure and chemical composition of the sample
were confirmed by an X-ray diffraction. The typical XRD
patterns of the samples synthesized via CTAB-assisted
Figure 2. EDS spectra of flower-like CuO nanostructures.
Flower-like CuO synthesized by hydrothermal method
969
Figure 3. FESEM images of flower-like CuO nanostructures.
the presence of Cu and O peaks and quantitative analysis
reveals that Cu and O are in a stoichiometry with 1 : 1
ratio. Therefore, it was obvious that the sample is composed of a pure monoclinic phase CuO, which is consistent with the XRD pattern.
3.2 The morphology of product and formation
mechanism
The morphology of the as-prepared CuO sample was analysed by the FESEM as shown in figure 3. The lower
magnification image in figure 3a indicates that the
obtained CuO is composed of flower-like structure. The
diameter of the micro-flower is about 6 µm. A detailed
side view on the individual flower-like CuO particle can
be observed from the higher magnification image (figure
3b), which clearly shows that the CuO flowers with
spherical symmetry are composed of many interconnected wide nanosheets. The thickness of the nanosheets
is in average of 60–80 nm (shown in figure 3c). Figure 3d
shows a typical image of a piece of the leaves forming
the flower-like structures, which clearly shows that the
width of the nanosheets is about 932 nm and the length is
about 2⋅10 µm at the higher magnification. FESEM
images in figures 3b–d indicate that the nanosheets were
first formed in the beginning and then they interconnect
each other to form the flower-like CuO nanostructures.
From figure 3b, we can also see that the nanosheets are
aligned perpendicularly to the flower surface pointing
toward a common centre.
According to references, the formation mechanisms of
the flower-like CuO nanostructures were different when
different preparation methods were used. Zhu et al (2007)
obtained the flower-like CuO nanostructures composed of
many interconnected needle-like crystallites by hydrolysing of Cu(OAc)2 solution without any surfactants. They
analyzed that the morphology of CuO crystallites is
depended on different growing rates of various crystal
facets. The coordination number of Cu2+ generally keeps
six in the hydrolysis reaction. Each Cu2+ would be surrounded by six water molecules due to the solvating
action when copper salt dissolved in water, in which four
water molecules surrounded Cu2+ to form square structure, and other two water molecules located at its axis.
970
Yunling Zou et al
Teng et al (2008) synthesized the flower-like CuO nanostructures by hydrothermal process using copper threads
as precursor. They investigated the influences of hydrothermal temperature and hydrothermal time on the nanostructures and reported that the formation of the flowerlike structure was controlled not only by the growth
thermodynamics, but also by the growth kinetics. Yu et al
(2008) prepared the flower-like CuO nanostructures by
reaction between a Cu plate and a KOH solution at room
temperature. They speculated that the nanoflower was a
representative morphology of spherulite formed by radiating growth from a centre or a number of centres and the
[Cu(OH)4]2– complexes played a key role in the growth of
nanoflowers.
In our work, we reported the preparation of by cetyltrimethylammonium bromide (CTAB)-assisted hydrothermal method. CTAB has been systematically studied
in the synthesis of mesostructured materials and may
form spherical, cylindrical micelle, or even higher-order
phases depending on the solution conditions (Fendler and
Fendler 1975). Here, we think that CTAB serves as a
template in the formation of flower-like CuO nanostructures. The formation of flower-like CuO nanostructures in the reaction system could be represented by the
following reactions
Cu2+ + 4OH– f [Cu(OH)4]2–,
Cu(OH)42–
(1)
–
f CuO + H2O + 2OH .
(2)
The [Cu(OH)4]2– anion can be considered as a precursor
entity for the formation of CuO in this study. We believe
that inorganic precursor Cu(OH)24– and cationic surfactant
CTAB form CTA+-[Cu(OH)4]2– ion pairs at the beginning
of the CTAB-assisted hydrothermal process. Since CTAB
is a kind of strong-acid-weak-base salt, it can accelerate
the ionization of [Cu(OH)4]2–. The CTA+-[Cu(OH)4]2– ion
pairs form combination of CTAB and CuO. In addition to
the general confirmation of CuO phase, the XRD patterns
also provide information on crystal orientations (Chang
and Zeng 2004). From the XRD pattern of figure 1, we
know that low miller-indexed ((002) and (200))
reflections are the strongest. It indicated that the CuO
nanosheets were first formed with the oriented growth of
the [Cu(OH)4]2– along (002) and (200) direction in the
beginning (shown in figure 3d). Therefore a possible
formation process of the flower-like CuO nanostructures
could be purposely divided into several processes:
(1) formation of CTA+–[Cu(OH)4]2– ion pairs; (2) growth
of the CuO nanosheets (shown in figure 3d); and (3) formation of the flower-like nanostructures (shown in
figure 3b).
4.
Conclusions
Flower-like CuO nanostructures have been synthesized
by cetyltrimethylammonium bromide (CTAB)-assisted
hydrothermal method with CuCl2⋅2H2O and sodium
hydroxide as raw materials. X-ray diffraction (XRD) pattern exhibited the nanocrystalline nature with monoclinic
structure for the as-synthesized nanostructures. FESEM
images indicated that the flower-like CuO nanostructures
are composed of many interconnected nanosheets in size
of several micrometres in length and width and 60–80 nm
in thickness. The nanosheets were aligned perpendicularly to the flower surface pointing toward a common
centre. A possible formation mechanism is proposed: the
nanosheets were first formed in the beginning and then
the nanosheets interconnect each other to form the
flower-like CuO nanostructures.
Acknowledgements
This study was supported by the Natural Science Foundation of Tianjin (no. 09JCYBJC04200). We are also grateful to the Research Fund of Civil Aviation University of
China (no. 07KYS05) and the Centre of Analysis and
Measurement of Tianjin University.
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