Preparation, characterization, and electrochemical application of mesoporous copper oxide

Article abstract of DOI:10.1016/j.materresbull.2009.08.001

Mesoporous CuO was successfully synthesized via thermal decomposition of CuC₂O₄ precursors. These products had ring-like morphology, which was made up of nanoparticles with the average diameter of 40 nm. The electrochemical experiments showed that the mesoporous CuO decreased the overvoltage of the electrode and increased electron transference in the measurement of dopamine.

Full text of DOI:10.1016/j.materresbull.2009.08.001

Materials Research Bulletin 45 (2010) 235–239
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Preparation, characterization, and electrochemical application of mesoporous
copper oxide
b
b
Liang Cheng a,b, Mingwang Shao a,b, , Dayan Chen , Yuzhong Zhang
*
a Functional Nano & Soft Materials Laboratory (FUNSOM), Soochow University, Suzhou 215123, PR China
b Anhui Key Laboratory of Functional Molecular Solids, and College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Mesoporous CuO was successfully synthesized via thermal decomposition of CuC2O4 precursors. These
products had ring-like morphology, which was made up of nanoparticles with the average diameter of
40 nm. The electrochemical experiments showed that the mesoporous CuO decreased the overvoltage of
the electrode and increased electron transference in the measurement of dopamine.
ß 2009 Elsevier Ltd. All rights reserved.
Received 21 May 2008
Received in revised form 30 May 2009
Accepted 3 August 2009
Available online 8 August 2009
Keywords:
A. Oxides
B. Chemical synthesis
D. Electrochemical properties
1. Introduction
Herein, we synthesized ring-like copper oxalate in large scale
via a simple reflux method, and further successfully prepared
Mesoporous materials, which have excellent catalytic [1,2],
electrical [3], and surface properties [4], have been focused on in
various fields. And numerous preparation methods have been
proposed, such as, sol–gel techniques [5], hydrolysis processes [6],
polyglycol-assisted routes [4], inorganic and organic templates
[7,8], ultrasonic ways [9], and co-precipitation approaches [10].
Among the various materials, copper oxide (CuO), as an
important p-type transition-metal oxide with a narrow band
gap (Eg = 1.2 eV), has attracted much attention due to its
photochemical and photoconductive properties [11,12], and
excellent catalytic activity [13–15].
In this work, copper oxalate template method was chosen,
because metal oxalates can be transformed to oxides or metals
without losing the ordered structure [16]. And numerous works on
the structure, properties and thermal behavior of metal oxalates
have been published [17–19]. Copper oxalate is a material with
polymer-like structure described as a stacking of Cu(C2O4)-
Cu(C2O4) ribbons [20]. The cushion- and rod-like shapes of copper
oxalate had been produced via a precipitation reaction, and a brick-
by-brick growth mechanism was proposed by Bowen and cow-
orkers [21].
mesoporous CuO composed of nanoparticles with average
diameter of 40 nm by thermal decomposition of copper oxalate.
The optical and electrical properties of the CuO materials were
investigated.
2. Experiment
2.1. Synthesis of CuC2O4 and CuO nanostructures
All the reagents in the experiments were analytically pure,
purchased from Shanghai Chemical Company, and used without
further purification. In a typical procedure, 20 ml Na2C2O4 (0.02 M)
was dropwise added into 20 ml CuSO4Á5H2O (0.02 M) aqueous
solution under magnetic stirring. Then the suspension was
transferred to a round-bottom flask, refluxed at 100 8C for 2 h,
and cooled to room temperature. The resultant was collected, and
washed several times with absolute ethanol and distilled water,
and dried under vacuum at 65 8C for 5 h. Finally the obtained
copper oxalate precursors were heated to 400 8C at a ramping rate
of 10 8C minÀ1 and calcined at 400 8C for 5 h to give copper oxide
products.
2.2. Characterization
The phase and crystallography of the products were character-
ized by a Shimadzu XRD-6000 X-ray diffractometer equipped with
* Corresponding author at: Functional Nano
& Soft Materials Laboratory
(FUNSOM), Soochow University, Suzhou 215123, PR China.
Tel.: +86 512 65880953; fax: +86 512 65882846.
E-mail address: mwshao@suda.edu.cn (M. Shao).
Cu K
applied to record the pattern in the 2
a radiation (l
= 0.15406 nm). A scanning rate of 0.05 sÀ1 was
u
range of 20–708. The
0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2009.08.001

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L. Cheng et al. / Materials Research Bulletin 45 (2010) 235–239
morphologies of copper oxalate and copper oxide were analyzed
with an S-4800 field emitting scanning electron microscopy
(FESEM). TG-DTA measurement of copper oxalate was performed
in SETARAM-TGA92 in the temperature range from room
temperature to 400 8C at a heating rate of 10 8C minÀ1. The
pore-size and distribution was measured on a Micrometrics ASAP-
2000 adsorption apparatus.
2.3. The properties of the products
The optical property of the products was investigated by a UV–
vis spectrophotometer (Shimadzu U-4100). A small amount of the
sample was dispersed in ethanol for UV–vis absorbance spectro-
scopy measurement in the wavelength range between 200 and
800 nm.
Electrochemical experiments were performed with electro-
chemistry work station (CHI 620B, ChenHua Instruments Co.) with
Fig. 1. XRD patterns of: (a) copper oxalate and (b) copper oxide.
a
conventional three-electrode system. A saturated calomel
electrode (SCE) and a platinum wire were employed as the
reference electrode and the counter electrode, respectively. The
CuO/GCE was used as working electrode, which was fabricated as
follows: mesoporous CuO powder (0.01 g) was added in 5 ml N,N-
dimethylformamide (DMF) under ultrasonic irradiation to get CuO
SEM image (Fig. 2a) indicates that there are ring-like products in
plenty. And the high magnification FESEM image (Fig. 2b) shows
that each ring is in the similar size with the average inner diameter
and outer diameter of 170 and 870 nm, respectively.
suspension. GCE (
F = 2 mm) was polished with 0.05 mm alumina
slurry and washed with 1:1 nitric acid, ethanol and water in an
ultrasonic bath for a few minutes. After washing with sonication,
Fig. 2c displays CuO products with mesoporous structure,
which keep the similar shape as CuC2O4. The high magnification
FESEM image (Fig. 2d) shows the ring-like CuO with the inner
diameter and outer diameter of 200 and 900 nm, which is made of
nanoparticles with the average diameters of 40 nm.
the GCE was coated with 10
temperature.
ml CuO suspension, and dried at room
A series of parallel experiments were performed to investigate
the influence of reaction time on the morphology of copper oxalate,
which was checked with the FESEM as shown in Fig. 3. Fig. 3a is the
image of discus-like products with refluxing time of 30 min: their
diameter is in the range of 800–1000 nm, and the center of the
product began to sink. When the reaction time was prolonged to
60 min, the center of the products sank ulteriorly, and some of the
ring-like began to appear, as shown in Fig. 3b. After refluxing for
120 min, the discus-like products were converted to ring-like
completely as shown in Fig. 2a. When the reaction proceeded for
3. Results and discussion
Fig. 1a is the XRD pattern of the resultants after refluxing. All the
diffraction peaks may be indexed as orthorhombic phase copper
oxalate (JCPDS No. 21-0297). Fig. 1b displays the XRD pattern of
CuO, obtained by calcining at 400 8C for 5 h. No other characteristic
peaks were observed except for those for CuO (JCPDS No. 41-0254),
indicating the as-obtained products has high-purity.
Fig. 2a and b shows the FESEM images of ring-like CuC2O4 under
low and high magnification, respectively. The low magnification
Fig. 2. FESEM images of the ring-like: (a) CuC2O4 under low magnification, (b) CuC2O4 under high magnification, (c) CuO under low magnification, and (d) CuO under high
magnification.

L. Cheng et al. / Materials Research Bulletin 45 (2010) 235–239
237
Fig. 3. Time-dependent evolution of the ring-like morphology of the CuC2O4 at different refluxing time: (a) 30 min, (b) 60 min, and (c) 180 min; And FESEM images of the CuO
obtained by calcined the copper oxalate precursors with the refluxing time: (d) 30 min, (e) 60 min, and (f) 180 min.
Fig. 4. TGA and DSC curves of the copper oxalate precursors obtained with refluxing
time of 120 min.
Fig. 5. The curve of the BJH pore-size distribution.
180 min, the morphology was changed greatly, as shown in Fig. 3c.
Only the square CuC2O4 with hole in the center was found, while the
ring-like was disappeared. The side length of the square CuC2O4 was
ca. 800 nm. And the morphology and the size of the products kept
unchanged with further increasing the refluxing time.
The corresponding copper oxalate precursors were calcined at
400 8C for 5 h. And Fig. 3d and e, and f show the SEM images of the
CuO nanoparticles resulting from copper oxalate with different
refluxing time (30, 60, and 180 min, respectively). All the images
show that the nanostructures are also made up of CuO
nanoparticles with the average diameter of 40 nm, which have
high specific surface area and interspaces and may find applica-
tions in the fields of sensor and catalysis.
In the growth of copper oxalate, the C2O42À acts as a bidentate
ligand in the aqueous solution to form the stable compound. It
decreases the rate of the combining process and favors for
controlling the morphology of the products as reported before [22],
which is agreed with the brick-by-brick growth mechanism [21].
The direct role of the copper oxalate precursors is assuredly crucial,
which serves as a template to form the CuO materials.
The TGA–DSC curves of the as-prepared precursors are shown
in Fig. 4. There is a weight loss step in the temperature range of
290–310 8C, which may be ascribed to the decomposition of the
oxalate. The weight loss is about 46.0%, which is close to the
theoretical value [23].
The pore-size and distribution of the mesoporous CuO was
measured on a Micrometrics ASAP-2000 adsorption apparatus
(Fig. 5). The product of the mesoporous CuO has a wide porous
diameter distribution with the central pore-size distribution of ca.
62 nm, which is probably caused by the two kinds of porous
architecture in the products: one within the microspheres and the
other from the interspaces of the constituent nanoparticles. And
the surface area of the mesoporous CuO is 35.6 m2/g. Such
mesoporous structure provides efficient transport pathways to
their interior voids, which is critical for catalyst and other
applications.
Fig. 6a shows the absorbance spectrum of the CuO materials,
which exhibited prominent features at ca. 257 nm. An evaluation
of the optical band gap is obtained using the following equation for
a semiconductor: (
the absorption coefficient and m equals 1 for a direct transition.
The energy intercept of a plot (a
Eplot)2 vs. Eplot yields Eg for a direct
n
) = A(h
n
/2 À Eg)m/2, where A is a constant,
a is
transition (Fig. 6b). The band gap of the as-prepared CuO nanorings
is calculated to be 3.43 eV from the UV–vis absorption spectrum,
which is larger than the reported value for bulk CuO (Eg = 1.85 eV)
[24]. Presumably, this blue shift is explained by two combined
factors: (1) the size of particles is in the intermediate confinement
regime, where the size of particles is comparable to their Bohr
exciton diameter and (2) spatial composition fluctuation can
produce atomically abrupt jumps in the chemical potential, which

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L. Cheng et al. / Materials Research Bulletin 45 (2010) 235–239
2
Fig. 6. (a) UV–vis absorption spectrum of the mesoporous CuO; (b) plot of the (
a
Eplot
)
vs. Eplot yields Eg for as-prepared mesoporous CuO.
Fig. 7. CVs of 1 Â 10À4 M DA in 0.1 M phosphate buffer solution (PBS pH 7.4) with
scan rate of 50 mV sÀ1: (a) CuO/GCE, (b) CuO/GCE and 1.0 Â 10À4 M DA, and (c) bare
GCE and 1.0 Â 10À4 M DA.
Fig. 8. SWV with CuO/GCE in a pH 7.4 phosphate buffer at different concentrations
of DA: (a) 3.98 M, (b) 9.91 M, (c) 13.81 M, (d) 42.14 M, (e) 125.87 M, and (f)
196.10 M, and the linear plot of peak current vs. concentration of DA (inset).
m
m
m
m
m
m
localizes the free exciton states [25]. It might be reasonably
speculated that the latter effect is more dominant in mesoporous
copper oxide than the bulk CuO samples.
4. Conclusions
In summary, uniform, ring-like and high-purity CuO nanopar-
ticles with the diameters of ca. 40 nm were successfully
synthesized via a simple method using the copper oxalate as
precursors. The refluxing time was found to have an influence on
the morphology of the precursors. The obtained mesoporous CuO
might find various potential applications in catalysis, electro-
chemistry, and optics because of the high specific surface area.
Fig. 7 shows the cyclic voltammograms of different electrodes in
0.01 M phosphate buffer solution (pH 7.4) at a scan rate of
50 mV sÀ1. It shows no redox peak was observed with mesoporous
CuO-modified GCE (curve a). While it using as the working electrode
to detect 1.0 Â 10À4 M dopamine (DA), a couple of distinct redox
peaks were displayed (curve b), and the anodic and cathodic peak
potentials are 145.6 and 88.36 mV, respectively, which has a
negative direction shift for the anodic peak potential and positive
shift for cathodic one comparing with the bare GCE (curve c). The
Acknowledgements
separation between the peak potentials (DEp) decreases from 121.8
Financial support from the National Natural Science Foundation
of China (20571001, 20675002), the Education Department (No.
2006KJ006TD) of Anhui Province and Anhui Provincial Natural
Science Foundation (070414185) are appreciated.
to 57.24 mV, illustrating a more reversible system. In addition, the
peak currents enhance remarkably. This phenomenon implies that
the as-prepared products can improve the electron transferbetween
DA and the GC electrode. It should be ascribed to mesoporous
morphology with high specific surface area, which results in a better
reversibility in the electrochemical detection.
References
[1] H.L. Cao, Y. Wang, X.L. Yu, S.R. Wang, S.H. Wu, Z.Y. Yuan, Appl. Catal. B 79 (2008)
26.
[2] S. Pavasupree, J. Jitputti, S. Ngamsinlapasathian, S. Yoshikawa, Mater. Res. Bull. 43
(2008) 149.
Because square-wave voltammogram (SWV) has a much higher
sensitivity, it was employed to estimate the lower detection limit
of DA. Fig. 8 shows the SWV recordings at various DA concentra-
tions at the CuO/GCE in PBS (pH 7.4). A series of well-defined peaks
are obtained owning to its good reversibility. After the correction of
the background current, the detection limit of DA is calculated as
1.0 Â 10À7 M, and the linear calibration graph (Fig. 8, inset) is
obtained in the DA concentration range 3.98 Â 10À6 to 1.96 Â
10À4 M. The linear equation is calculated as ipa = 0.3609 +
0.0054CDA with the correlation coefficient of 0.9976.
[3] Y.G. Li, B. Tan, Y.Y. Wu, Nano Lett. 8 (2008) 265.
[4] Y.Y. Xu, D.R. Chen, X.L. Jiao, K.Y. Xue, J. Phys. Chem. C 111 (2007) 16284.
[5] J. Bao, Z.L. Liu, Y. Zhang, N. Tsubaki, Catal. Commun. 9 (2008) 913.
[6] H. Li, H.L. Shi, H. Liang, X. Li, L. Li, M. Ruan, Mater. Lett. 62 (2008) 1410.
[7] W.C. Geng, N. Li, X.T. Li, X.Y. Lai, L.F. Wang, B.H. Long, J.J. Ning, J.C. Tu, S.L. Qiu,
Shilun, Mater. Res. Bull. 43 (2008) 601.
[8] S. Tanaka, Y. Katayama, M.P. Tate, H.W. Hillhouse, Y. Miyake, J. Mater. Chem. 17
(2007) 3639.
[9] M. Sivakumar, A. Gedanken, Z.Y. Zhong, L.W. Chen, New J. Chem. 30 (2006) 102.

L. Cheng et al. / Materials Research Bulletin 45 (2010) 235–239
[10] Y.J. Zhang, Q.X. Hu, B. Hu, Z.Y. Fang, Y.T. Qian, Z.D. Zhang, Mater. Chem. Phys. 96 [18] B.V. L’Vov, Thermochim. Acta 364 (2000) 99.
239
(2006) 16.
[19] M.A. Mohamed, A.K. Galwey, S.A. Halawy, Thermochim. Acta 429 (2004) 57.
[20] A. Michalowicz, J.J. Girerd, J. Goulon, Inorg. Chem. 18 (1979) 3004.
[21] N. Jongen, P. Bowen, J. Lemaitre, J.C. Valmalette, H. Hofmann, J. Colloid Interface
Sci. 226 (2000) 189.
[11] X.F. Yu, N.Z. Wu, Y. Xie, Y.Q. Tang, J. Mater. Chem. 10 (2000) 1629.
[12] X.C. Jiang, T. Herricks, Y.N. Xia, Nano Lett. 2 (2002) 1333.
[13] J.B. Reitz, E.I. Solomon, J. Am. Chem. Soc. 120 (1998) 11467.
[14] H. Wang, J.Z. Xu, J.J. Zhu, H.Y. Chen, J. Cryst. Growth 244 (2002) 88.
[15] P.E. de Jongh, D. Vanmaekelbergh, J.J. Kelly, Chem. Commun. (1999) 1069.
[16] L.C. Soare, P. Bowen, J. Lemaitre, H. Hofmann, J. Phys. Chem. B 110 (2006) 17763.
[17] S.S. Hayrapetyan, H.G. Khachatryan, Micropor. Mesopor. Mater. 89 (2006) 33.
[22] X.F. Zhao, J.G. Yu, J. Cryst. Growth 306 (2007) 366.
[23] X.J. Zhang, D.E. Zhang, X.M. Ni, H.G. Zheng, Solid-State Electron. 52 (2008) 245.
[24] K. Santra, C.K. Sarker, M.K. Mukherjee, B. Cosh, Thin Solid Films 213 (1992) 226.
[25] H.K. Lee, H.S. Yang, P.H. Holloway, J. Lumin. 126 (2007) 314.