Ultrasound assisted quick synthesis of square-brick-like porous CuO and optical properties

Article abstract of DOI:10.1016/j.materresbull.2012.05.034

Square-brick-like CuC₂O₄ intermediate was synthesized quickly by an ultrasound and microwave assisted solution route in the presence of sodium dodecyl benzene sulfonate (SDBS) surfactant using CuCl₂ and K₂C₂O₄ as raw materials. The CuC ₂O₄ was transformed into CuO with a preserved morphology by heating at 300°C for 2 h. The products were characterized by XRD, TG-DTA, XPS, SEM, TEM, HRTEM and N₂ adsorption-desorption measurement. The light harvesting capability, photoluminescence and surface photovoltage properties of the CuO were investigated. The width and thickness of the obtained porous CuO square bricks are ca. 0.5-1 μm and 200 nm, respectively. Its average primary particle size is 12 nm. The formation mechanism of this square-brick-like CuO was investigated on the basis of series of controlling experiments. The results show that SDBS, C₂O₄^2- and ultrasonic processing play important roles in the formation of this square-brick-like morphology.

Full text of DOI:10.1016/j.materresbull.2012.05.034

Materials Research Bulletin 47 (2012) 2437–2441
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Ultrasound assisted quick synthesis of square-brick-like porous CuO and
optical properties
*
Mo Qiu, Lianjie Zhu , Tongtong Zhang, Hongbin Li, Youguang Sun, Kun Liu
School of Chemistry & Chemical Engineering, Tianjin University of Technology, Tianjin 300384, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Square-brick-like CuC2O4 intermediate was synthesized quickly by an ultrasound and microwave
assisted solution route in the presence of sodium dodecyl benzene sulfonate (SDBS) surfactant using
CuCl2 and K2C2O4 as raw materials. The CuC2O4 was transformed into CuO with a preserved morphology
by heating at 300 8C for 2 h. The products were characterized by XRD, TG-DTA, XPS, SEM, TEM, HRTEM
and N2 adsorption–desorption measurement. The light harvesting capability, photoluminescence and
surface photovoltage properties of the CuO were investigated. The width and thickness of the obtained
Received 29 January 2012
Received in revised form 9 April 2012
Accepted 21 May 2012
Available online 30 May 2012
Keywords:
porous CuO square bricks are ca. 0.5–1
mm and 200 nm, respectively. Its average primary particle size is
A. Oxides
12 nm. The formation mechanism of this square-brick-like CuO was investigated on the basis of series of
controlling experiments. The results show that SDBS, C2O42ꢀ and ultrasonic processing play important
roles in the formation of this square-brick-like morphology.
C. X-ray diffraction
C. Electron microscopy
D. Optical properties
ß 2012 Elsevier Ltd. All rights reserved.
1. Introduction
of CuO with a desirable morphology. In the present article, porous
hierarchical CuO square bricks were fabricated quickly via CuC2O4
Cupric oxides have been extensively investigated due to their
various applications in solar energy materials, gas sensors, electron-
ics, heterogeneous catalysts, optical switches and magnetic storage
media [1–3]. Since the morphology of nanostructure materials was
found to influence their properties significantly, effectively control-
ling their morphologies has attracted much attention and become a
hot issue in recent years. The common morphologies of monoclinic
CuO nanostructures include nanoparticles, one-dimensional nano-
wires, nanorods and nanobelts [4–7], two-dimensional nanosheets,
nanoleaves and nanowiskers[8–10], three-dimensionalpeachstone-
like, boat-like and ellipsoid-like nanostructures [11,12]. However, a
porous square-brick-like hierarchical CuO has been hardly seen in
literature [13]. Recently, Aimable et al. have synthesized a similar
structure using copper acetate and sodium oxalate as raw materials
and hydroxypropylmethylcellulose as an organic additive. Lei et al.
[14,15] have prepared CuO nanostructures with novel morphologies
by calcining the intermediate products of Cu(OH)2 from Cu2(OH)3Cl
and Cu4SO4(OH)6 precursors, respectively. Malachite Cu2(OH)2CO3
has also been applied as a precursor to prepare CuO with the
preserved morphologies through a thermal conversion [16–18].
Although many raw materials and synthetic methods were tried
to prepare CuO with various morphologies, a quick facile synthesis
routeusingcheaprawmaterialsisstillachallengingtaskforsynthesis
intermediate, assisted by ultrasound and microwave processing, and
its formation mechanism was discussed. Compared to the work in
literature [13,19], where a thermal conversion of CuC2O4 was also
included,ourworkhasthefollowingcharacteristics:(1)thesynthesis
route is facileand fastwhereultrasoundandmicrowavewereusedto
assist the quick synthesis of CuC2O4 intermediate. Moreover, the
temperature for thermal conversion of CuC2O4 into CuO is low); (2) a
cheap and water soluble surfactant, sodium dodecyl benzene
sulfonate (SDBS), was applied as a directing agent, which could be
easily removed by rinsing after reactions; (3) the size of the obtained
CuO square bricks is much smaller than the reported one [13].
Especially it possesses hierarchical porous structure; (4) the CuO
product can emit purple-blue light and has rich surface states, which
makes it to be potential optics and catalyst materials.
2. Experimental
In a typical procedure, 50 mL of K2C2O4 aqueous solution with a
concentration of 0.0125 mol Lꢀ1 was dropped into 50 mL of CuCl2
aqueous solution (0.0125 mol Lꢀ1) under vigorous stirring and
kept stirring for 10 min to obtain an azury suspension. Then 0.02 g
of SDBS was added into the above suspension and continuously
stirred for another 10 min. The molecular structure of SDBS is
shown in the Scheme 1. After an ultrasonic processing for 30 min,
the suspension was transferred into an 800 W domestic micro-
wave oven and intermittently (5 min interval) heated with 20%
power for 30 min. The precipitate was separated by centrifugation,
* Corresponding author. Tel.: +86 22 60214259.
E-mail addresses: zhulj@tjut.edu.cn, zhulianjie71@yahoo.com.cn (L. Zhu).
0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.materresbull.2012.05.034

2438
M. Qiu et al. / Materials Research Bulletin 47 (2012) 2437–2441
Scheme 1. Molecular structure of sodium dodecyl benzene sulfonate (SDBS)
molecule.
followed by washing and drying processes. Finally the azury
powder was calcined in a muffle furnace at 300 8C for 2 h with a
heating rate of 1 8C minꢀ1 and a dark CuO powder was obtained.
The series of controlling experiments were performed in a similar
procedure under various reaction conditions.
X-ray powder diffraction (XRD) measurements were performed
on a Rigaku D/max-2500 diffractometer with Cu K
a radiation
(
l
= 0.154056 nm) at 40 kV and 100 mA. Thermal gravimetric
analysis (TGA) and differential thermal analysis (DTA) measure-
ments were performed on a TG-DTA 6300 (Japan Seiko) thermo-
analyzer using Al2O3 as a reference and heating at a rate of
5 K minꢀ1 under a dynamic Ar atmosphere. Scanning electron
microscope (SEM) images were taken by a JSM 6700F field-
emission scanning electron microscope. Transmission electron
microscope (TEM) and HRTEM images were obtained with a JEM-
2100 field emission electron microscope operating at 200 kV. The
UV–vis absorption spectra were recorded on a Hitachi/U-3900 UV–
visible spectrophotometer. The X-ray photoelectron spectroscopy
(XPS) measurement was performed on a PHI-5300 ESCA system. N2
adsorption–desorption isotherm was obtained with a V-Sorb
2800P surface area and pore size analyzer. The photoluminescence
(PL) emission spectrum was obtained from a FL3-2-IHR221-NIR-
TCSPC steady and time resolved spectrometer excited at 350 nm.
Surface photovoltage spectrum (SPS) was measured by a home-
made sandwich-type solid junction cell ITO/Sample/ITO with light
monochromator-lock-in detection technique. Details on this setup
have been described elsewhere [20]. The SPS was normalized to
make the light intensity of the lamp the same at each wavelength.
Fig. 1. XRD patterns of the as-prepared porous CuO square bricks and the
intermediate product CuC2O4 (the inset).
12
100
90
10
8
80
70
60
50
6
4
2
0
100
200
300
400
500
Temperature /oC
Fig. 2. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA)
curves for the as-prepared CuC2O4 square bricks.
3. Results and discussion
where a sharp exothermal peak appears and the corresponding
weight loss is 45%. This result implies that heating at 300 8C is
appropriate for full conversion of the intermediate to CuO without
bringing any impurity. According to the calculation based on the
decomposition reaction of the CuC2O4, the total theoretical weight
loss should be 47.4 wt.%. Therefore, we presume that the water
content in the intermediate product CuC2O4ꢁxH2O is almost zero. In
that case, the measurement error is a reasonable value, ca. 5%. Thus
the accurate expression for the intermediate product would be
CuC2O4. The possible reactions are as follows:
The crystal structures of the two products before and after
calcination were examined by XRD, as shown in Fig. 1. In the inset
of Fig. 1, the diffraction peaks except the weak peak at 318 are
indexed to orthorhombic CuC2O4ꢁxH2O (space group: Pnnm 58,
JCPDS No. 21-0297), which indicates that the intermediate product
before calcination are mostly CuC2O4ꢁxH2O. Thermal conversion of
the CuC2O4ꢁxH2O at 300 8C led to the formation of monoclinic CuO
(JCPDS No. 48-1548, space group: C2/c15) with lattice constants
a = 0.468, b = 0.342 and c = 0.513 nm (Fig. 1). The strong and sharp
diffraction peaks indicate a high purity and crystallinity of the CuO
square bricks. The average primary particle size of the CuO was
calculated by using Debye–Scherrer formula based on the 200
diffraction peak, which is 12 nm.
CuCl2 þ K2C2O4 ! CuC2O4 # þ 2KCl
(1)
(2)
D
CuC2O4ꢀ!CuO þ CO2 þ CO
The TG-DTA results (see Fig. 2) demonstrate that the phase
transition from CuC2O4ꢁxH2O to CuO occurs at around 294 8C
The purity of the CuO square bricks was further confirmed by
XPS results, as shown in Fig. 3. The peaks sitting at 933.2 eV and
3500
8000
7000
6000
5000
4000
Cu2p
3/2
O1s
Cu2p
3000
2500
2000
1500
1000
1/2
540
535
530
525
520
960
950
940
930
Binding energy /eV
Binding energy /eV
Fig. 3. XPS spectra of the CuO square bricks for Cu2p region (left) and O1s region (right).

M. Qiu et al. / Materials Research Bulletin 47 (2012) 2437–2441
2439
Fig. 4. SEM images of (a) CuC2O4, (b) panorama (c) front view (d) side view of the CuO square bricks; TEM (e) and HRTEM (f) images of the CuO square bricks.
953.4 eV for Cu2p region are attributed to Cu2p3/2 and Cu2p1/2
,
of the square. The inset of Fig. 4a shows that the square brick
composes of numerous self-assembled nanoparticles in the middle
and surrounded by thin nano-sheet layers at outer side surfaces.
This structural difference at the surrounding part of the square
bricks is also observed in Fig. 4c–e although the nanosheets also
become porous structures due to release of gases after thermal
decomposition of CuC2O4. One observes in Fig. 4c and e that the
primary particle sizes of the CuO micro-nanometer composite
structure are about 10–20 nm, which is consistent with the XRD
result. The porous nature of the CuO is clearly observed in Fig. 3e.
Apart from small pores, there exist some big pores due to collapse
of some CuO nanoparticles in the calcination process. This could
be confirmed by the N2 adsorption–desorption measurement in
the following text. Based on the HRTEM image, lattice spacings
of the CuO were measured to be 0.23 and 0.28 nm, corresponding
to the (1 1 1) and (1 1 0) planes, respectively, of monoclinic CuO.
The adsorption–desorption isotherm and pore size distribution
results, see Fig. 5, show that the BET specific surface area of the CuO
square bricks is 29.8 m2 gꢀ1, and the main pore sizes are in the
range of 2–50 nm. There is also a little macropores larger than
50 nm. The unclosed loop in Fig. 5 may be due to partial pore
structure collapse of the CuO, leading to irreversible adsorption of
N2 on the material surface. It is known that porous solids have
excellent adsorptive properties and the catalytic process would
occur more efficiently in materials with hierarchical pore size
distribution on the nanoscale [21,22]. Therefore, a good catalytic
activity could be expected for this hierarchical porous CuO.
In order to understand the formation mechanism of the square-
brick-like CuO nanostructures series of controlling experiments
were performed. The morphologies of these series of CuO products
are shown in Fig. S1 in the supporting information. The correspond-
ing reaction conditions and properties of these products are
summarized in Table 1. When CuCl2 and K2C2O4 were used as the
raw materials, CuO nanoparticles, nanosheets, large irregular
particles or conglomeration (sample a–e) were obtained at the
absence of SDBS. At the presence of SDBS, however, CuO square
bricks were hardly formed either without an ultrasound processing
respectively. The existence of strong satellite features at 942.1 eV
and 961.8 eV for Cu2p rules out the possibility of the presence of
Cu2O phase [19]. The peaks observed at 529.4 eV and 531.4 eV for
O1s region are identified as lattice binding of O2ꢀ with Cu and
oxygen adsorbed onto the surface of CuO square bricks,
respectively. The high peak intensity at 531.4 eV suggests rich
adsorbed oxygen on the CuO surface, which may result in rich
surface states and a high catalytic activity.
Morphologies and microstructures of the as-prepared CuO and
CuC2O4 square bricks were examined by SEM, as shown in Fig. 4a–
d. Obviously the square brick-like morphology of the CuC2O4
(Fig. 4a and the inset) was well reserved after heating at 300 8C for
2 h although the surface roughness and aperture of the CuO
became larger. The CuO square bricks are about 0.5–1
mm in width
and 200 nm in thickness with slightly convex surface in the middle
40
0.006
35
0.004
30
0.002
25
20
15
10
5
0.000
0
20
40
60
80
100
Pore diameter (nm)
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/Po)
Fig. 5. N2 adsorption–desorption isotherm and pore size distribution of CuO square
bricks (inset).

2440
M. Qiu et al. / Materials Research Bulletin 47 (2012) 2437–2441
Table 1
Series of CuO products prepared in various conditionsa.
Samples
Raw materials
Directing agents
Preparation methods
Morphologies of CuO products
a
b
c
d
e
f
CuCl2 + K2C2O4
CuCl2 + K2C2O4
CuCl2 + K2C2O4
CuCl2 + K2C2O4
CuCl2 + K2C2O4
CuCl2 + K2C2O4
CuCl2 + K2C2O4
Cu(NO3)2 + K2C2O4
CuCl2 + K2CO3
None
Ultrasound + microwave
Microwave
Nanoparticles
Triethylamine
PEG (2000)
Poly(acrylic acid)
Oleyl amine
SDBS
Nanosheets
Microwave
Irregular particles
Irregular particles
Agglomeration
Ultrasound
Ultrasound
–
Irregular particles
SBb + little particles
Particles + little SBb
Nanorod clusters
g
h
i
SDBS
Ultrasound
None
Ultrasound + microwave
Microwave
None
a
Note: All products above were calcined at 300 8C for 2 h.
Square bricks.
b
(sample f). When Cu(NO3)2 was used instead of CuCl2 in the absence
of SDBS, only little amount of square bricks was produced (sample
h). Completely different morphology, nanorod clusters and flowers
(sample i), was obtained when K2CO3 instead of K2C2O4 was used as
the raw material. The above results reveal that the morphology of
the CuC2O4 intermediate is significantly influenced by the structure
of a guiding agent and the ultrasound or microwave processing.
The ultrasonic processing is necessary to form the square-brick-like
CuC2O4, whereasthemicrowaveheatingcanimprovetheuniformity
molecules on the CuC2O4 surfaces could also change the
intermolecular forces in the reaction system under ultrasound
radiation.
The formation of the square-brick-like CuO possibly consists of
three steps: (i) formation of CuC2O4 nanoparticle seeds in terms of
chemical reaction described in Eq. (1); (ii) SDBS adsorbed on the
surface of CuC2O4 nanoparticles, followed by quick oriented self-
assembly of these nanoparticles under ultrasound and microwave
radiation to form square-brick-like structures. The nanoparticles
are interconnected with each other and the pores are the
interparticle voids. Then a few layers of nanosheet-like CuC2O4
aggregated around the square bricks via oriented attachment.
Since CuC2O4 is a nonpolar molecule, the aggregation of the
nanoparticles (without SDBS) may be driven by dispersive force,
and the orientation is random, which results in the formation of
irregular shape. After adsorption of the polar molecule SDBS on the
CuC2O4, the nanoparticles assemble along certain direction under
ultrasound induction. Here the driving forces could include
induction force, orientation force and dispersive force. Moreover,
the adsorption of SDBS could also change the surface energy of
CuC2O4 in certain crystal face. Consequently a square-brick-like
morphology was formed; (iii) thermal conversion from CuC2O4 to
CuO with preserved morphology, being accompanied by release of
gases to form more and larger pores.
The optical properties were investigated by UV–vis diffuse
reflection absorption spectrum and PL emission spectrum. As
shown in Fig. 6a, there is an abroad absorption band from 280 nm
to 800 nm. The band gap energy of the as-prepared CuO square
bricks was estimated by making a tangent line at the band edge,
referring to the method in the reference [24]. It is 1.5 eV, which is
much larger than the reported value for the bulk CuO (1.2 eV). This
blue shift in absorption could be attributed to the quantum size
effect since the primary particle size of the CuO square brick is as
small as 12 nm. The PL spectrum (Fig. 6b) demonstrates there is a
2ꢀ
of the product morphology. The existence of C2O4 is also a key
factor to formation of the square-brick-like morphology.
Supplementarymaterialrelated to this articlefound, in the online
version, at http://dx.doi.org/10.1016/j.materresbull.2012.05.034.
As we know, when a liquid is subjected to ultrasound bubbles
are formed, which grow and implode. This leads locally to
extremely high instantaneous temperatures and pressures, being
accompanied by shock wave and micro-jet flow [23]. The
cavitation effect induced by ultrasound could on the one hand
crumble object and increase its dispersivity. Thus, well-dispersed
CuC2O4 nanoparticles were possibly formed firstly under ultra-
sound. On the other hand, the cavitation effect could speed up the
diffusion of the CuC2O4 nanoparticles, which benefits the oriented
self-assembly of the CuC2O4 nanoparticles in the later reaction
stage after the adsorption of SDBS molecules on the CuC2O4
surfaces. The adsorption of the polar SDBS molecule may reduce
the surface energy of the CuC2O4 nanoparticle in certain crystal
face, which would result in epitaxial growth of CuC2O4. Combining
with the XRD result in Fig. 1, we found that the relative growth
speed along the (1 1 ꢀ1) plane is slow. Thus the crystal facet along
the thickness direction of the square-brick-like product is the
(1 1 ꢀ1) plane. In the orthorhombic crystal system of the precursor
phase, the adsorption of SDBS molecules decreases the growth
speed of the (1 1 ꢀ1) plane, which leads to the formation of the
square-brick-like morphology. The adsorption of the polar SDBS
a
b
400
500
600
700
300
400
500
600
700
800
Wavelength /nm
Wavelength /nm
Fig. 6. UV–vis diffuse reflectance absorption spectrum (changing light source at 370 nm) and photoluminescence emission spectrum of the as-prepared CuO square bricks.

M. Qiu et al. / Materials Research Bulletin 47 (2012) 2437–2441
2441
possesses hierarchical pores with sizes mainly in the range of
2–50 nm and its BET surface area is 29.8 m2 gꢀ1. The surfactant
SDBS, C2O42ꢀ, and ultrasonic processing are found playing crucial
roles on the formation of the square-brick-like CuC2O4. This
morphology was well preserved after heating at 300 8C for 2 h
except that the surface roughness and aperture of the CuO became
larger due to release of gaseous CO2 and CO. This square-brick-like
CuC2O4 nanostructure may be produced through an oriented self-
assembly mechanism driven by induction force, orientation force
and dispersive force under ultrasound radiation in the case of
adsorption of the polar molecules SDBS on the CuC2O4 surfaces.
The as-prepared CuO has good light harvesting capability in UV–
visible region and can emit purple-blue light. The SPS result
demonstrates that it has rich surface state, which could enhance
the separation of the photogenerated electrons and holes.
Consequently it could increase the photocatalytic activity of
CuO. This porous square-brick-like CuO could be potential catalyst,
optics and gas sensor materials.
8
6
4
2
0
300
400
500
600
700
800
Wavelength (nm)
Fig. 7. Surface photovoltage spectrum (SPS) of the as-prepared CuO square bricks.
single abroad emission band centered at 400 nm, which indicates
that the CuO square bricks can emit purple-blue light. This is
different from the result by Aslani and Oroojpour where green-
yellow emission was observed [25]. This PL emission band could be
attributed to the near-band-edge emission of the absorption band
at low wavelength.
Acknowledgements
This work is financially supported by National Natural Science
Foundations of China (grant no. 20871091) and the Open Project of
Key Lab of Adv Energy Mat Chem (Nankai Univ) (KLAEMC-
OP201201).
The surface photovoltage method is a well-established con-
tactless and nondestructive technique for semiconductor charac-
terization based on analyzing illumination-induced changes in the
surface voltage [20,26,27]. It has a very high sensitivity (ca. 108q/
cm2) which exceeds that of conventional spectroscopies such as
XPS and Auger spectroscopies by many orders of magnitude. SPS
can provide information about the properties of the sample surface
layer (several atomic layers). SPS method has been proven to be
one of the most effective methods to reflect the separation and
recombination situation of photogenerated electrons and holes
[28]. The SPS response of the as-prepared CuO square bricks (Fig. 7)
includes two bands centered at 331 nm and 463 nm, respectively.
The band from 300 nm to 360 nm is attributed to band-to-band
transition (from valence band to conduction band of CuO). The
other response band between 360 nm and 800 nm could be caused
by surface states. Comparing with the SPS response at 360 nm, the
stronger signal at 463 nm suggests the existence of rich surface
states for this CuO sample. The photogenerated electrons could
transfer to the surface state, which would enhance the separation
of the photogenerated carriers and consequently the photocata-
lytic activity of CuO.
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Porous square-brick-like CuO nanostructures were quickly
prepared via CuC2O4 intermediate by assistance of ultrasound and
microwave processing, combining with thermal conversion reac-
tion. The pure monoclinic CuO has high crystallinity with an
average primary particle size of 12 nm. The CuO square bricks are
about 0.5–1
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convex surface in the middle of the square. The as-prepared CuO
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