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Materials Chemistry and Physics 126 (2011) 847–852
Contents lists available at ScienceDirect
Materials Chemistry and Physics
journal homepage: www.elsevier.com/locate/matchemphys
Enhanced photocatalytic activity of flowerlike Cu2 O/Cu prepared using
solvent-thermal route
Bo Zhou a , Hongxia Wang b , Zhiguo Liu a,∗ , Yanqiang Yang a , Xiqiang Huang a , Zhe Lü a ,
Yu Sui a,c , Wenhui Su a,c
a
b
c
Center for Condensed Matter Science and Technology, Department of Physics, Harbin Institute of Technology, Harbin 150080, PR China
College of Chemistry & Chemical Engineering, Harbin Normal University, Harbin 150025, PR China
International Center for Materials Physics, Academia Sinica, Shenyang 110015, PR China
a r t i c l e
i n f o
Article history:
Received 9 April 2009
Received in revised form 8 November 2010
Accepted 13 December 2010
Keywords:
Nanocomposite materials
Chemical synthesis
Electron microscopy
Heterostructures
a b s t r a c t
Cu2 O/Cu nanocomposites (NCs) with flowerlike nano-architecture were prepared using template-free
stepwise solvent-thermal synthesis route with Cu(NO3 )2 ·3H2 O as a precursor. With the precursor concentration increasing gradually from 0.01 to 0.1 M, the morphology of the NCs evolves from nano-flower
to microsphere. The content of Cu in the NCs can be easily controlled by adjusting the concentration of
precursor and synthesis time. Using photocatalytic degradation of monoazo dye Procion Red MX-5B (PR)
and phenol as the probe molecules under visible-light illumination, we have investigated the influence
of Cu on the photocatalytic activity of Cu2 O. When the content of Cu lies in the range of 27–71 wt%, the
samples exhibit higher photocatalytic performance, indicating that these flowerlike Cu2 O/Cu NCs are
promising candidates for pollutant processing.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Environmental protection calls for efficient ways and means to
process waste and emissions, e.g., polluted water, which provides
the impetus for sustained fundamental and applied researches in
the field of environmental remediation. Semiconductor-based photocatalysts [1] have attracted much attention because of their low
cost, cleanliness, high stability, and easy availability. TiO2 and ZnO
are the most commonly used photocatalysts, however, they need
to be activated by ultraviolet (UV) light because of their broad
band gap (3.0–3.2 eV), hence only less than 5% solar energy can
be utilized [2–5]. On the contrary, cuprous oxide (Cu2 O), with a
direct band gap of ca. 2.2 eV, could be activated by visible light,
making it a promising candidate for better utilization of solar
energy, e.g., solar energy conversion [6], photocatalytic degradation of organic pollutants [7] and decomposition of water into
O2 and H2 [8]. Nevertheless, a pure-phase semiconductor exhibits
very low quantum efficiency due to the quick recombination of
photoelectrons and holes. An efficient way to improve the quantum efficiency is to add noble metal (e.g., Ag, Au, Cu and Pt) into
metal oxide, because the quick transfer of the photogenerated electrons by the metal can prevent the recombination. As a result, the
∗ Corresponding author. Tel.: +86 451 86402772; fax: +86 451 86418440.
E-mail addresses: zhoubo a@hit.edu.cn, liuzhiguo@hit.edu.cn (Z. Liu).
0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.matchemphys.2010.12.030
metal/semiconductor heterostructures exhibit an excellent photocatalytic activity [9–15]. Therefore, it is highly desirable to develop
a synthetic route for the production of metal/metal oxide composites.
On the other hand, the activity of a catalyst also depends
on its structural parameters, such as geometry and morphology.
Nanoparticles [16,17] and porous/hollow structures [18–20] effectively increase the area of surface and interface, which enhances
not only the adsorption and desorption of reactants (or products),
but also light harvesting, thereby improves the catalytic activity. In the past decade, much efforts have been devoted to the
shape-controlled synthesis of Cu2 O micro- and nanocrystals, such
as nanowires [21], nanocubes [22], hollow spheres [23], and mesoporous nanoflowers [24]. To the best of our knowledge, few results
were reported on the synthesis and photocatalytic property of
Cu2 O/Cu NCs.
In order to develop an efficient photocatalyst that can sufficiently utilize solar energy, Cu2 O/Cu NCs were prepared in this
study. The in situ synthesis of Cu2 O/Cu NCs was developed with
a template-free stepwise solvent-thermal route to control its morphology and phase composition. The effect of reaction time and
precursor concentration on its phase composition and morphology,
and the effect of the content of Cu in composite Cu2 O/Cu photocatalysts on photocatalytic activity was investigated. The flowerlike
Cu2 O/Cu NCs show high photocatalytic activity, indicating their
potential applications in the future.
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B. Zhou et al. / Materials Chemistry and Physics 126 (2011) 847–852
Table 1
Experimental conditions, phase composition and characterized parameters of different samples, where SBET , dP and VP are specific surface area, pore diameter and pore
volume, respectively.
2.1. Preparation of Cu2 O/Cu nanoflowers
Cu2 O/Cu nanoflowers were synthesized using Cu(NO3 )2 ·3H2 O as the precursor
by a typical two-step method similar to Ref. [25]. Cu(NO3 )2 ·3H2 O was dissolved
in an organic solvent N,N-dimethylformamide (DMF) with a concentration range
of 0.01–0.1 M. 35 mL of this transparent blue solvent was sealed in a Teflon-lined
stainless steel autoclave with a capacity of 50 mL, and heated at 150 ◦ C for 24 h in
electric oven, and then further heated at 180 ◦ C for 8–26 h. The autoclave was slowly
cooled down to room temperature in the oven. The precipitation was separated
from the solution by centrifugation, then washed with alcohol for several times, and
finally dried at 70 ◦ C for 5 h in a vacuum oven. The denotations of samples prepared
at different conditions are shown in Table 1.
2.2. Characterization
The samples were characterized by X-ray powder diffraction (XRD) using
a Rigaku 12 kW X-ray diffractometer with Cu K␣ radiation ( = 0.15418 nm).
Their morphology and size distribution were determined by a JEM-S4800
field-emission scanning electron microscope (FESEM) operated at 15 kV. N2
adsorption–desorption isotherms were obtained using a Quantachrome Autosorb1 apparatus. Barret–Joyner–Halenda (BJH) method was employed to determine the
pore diameter (dP ) and pore volume (VP ). The specific surface area (SBET ) was calculated using the Brunauer–Emmett–Teller (BET) method.
2.3. Adsorption and photocatalytic activity
The adsorption behavior and photocatalytic activity of the Cu2 O/Cu NCs were
evaluated by the degradation of the solution of monoazo dye Procion Red MX5B (PR, Aldrich) and phenol at a concentration of 10 mg L−1 . 0.030 g Cu2 O/Cu NC
powder was dispersed in a 50 mL probe molecular aqueous solution. Before illumination, the suspension was stirred in dark for more than 120 min to achieve an
adsorption/desorption equilibrium of organic molecules on the surface of Cu2 O/Cu
NCs. The photocatalytic reaction was carried out at room temperature by using
a 40 W tungsten lamp as a visible-light source. At different time intervals during
the experiment, 5 mL of solution was sampled for analysis. After centrifugation at
11,000 rpm for 10 min, the absorbance of solution was measured using a UV–visible
spectrophotometer.
3. Results and discussion
3.1. Phase composition
Fig. 1 shows the XRD patterns of a series of samples prepared
with different precursor concentrations at 180 ◦ C for 24 h. All the
samples are found to be composite materials, which are characterized by the two sets of diffraction peaks, i.e., Cu2 O (space group
Pn3m; a0 = 0.4252 nm; JCPDS 05-0667) and cubic Cu (space group
Fm3m; a0 = 0.3613 nm; JCPDS 04-0836). With increasing precursor
concentration, the relative intensity of Cu2 O (1 1 1) peak increases,
indicating the higher content of Cu2 O in the NCs. The mass fraction
of Cu2 O in the NCs can be calculated using the relative ratio [26]:
ICu2 O(1 1 1)
ICu2 O(1 1 1) + ICu(1 1 1)
16.4
16.9
9.6
4.2
5.5
29.1
8.6
12.2
12.5
1.888
3.774
3.704
3.750
3.748
1.888
3.774
2.993
2.143
0.0156
0.0152
0.0106
0.0048
0.0052
0.0334
0.0096
0.0111
0.0117
where ICu2 O(1 1 1) and ICu(1 1 1) are the height of the characteristic
diffraction peaks of Cu2 O (1 1 1) and Cu (1 1 1) planes, respectively.
The content of Cu2 O increases from 29 wt% to 89 wt% when the
precursor concentration increases from 0.01 M to 0.1 M, as shown
in Table 1.
3.2. Structure and morphology
Fig. 2 shows the relationship between the morphology and
the precursor concentration. At low precursor concentration of
0.01 M (S1, Fig. 2a) and 0.02 M (S2, Fig. 2b), flowerlike morphology is formed, whose “petals” are approximately 300–500 nm long
and 30–70 nm wide (the inset of Fig. 2a). At the concentration
of 0.03 M (S3), the petals become shorter and smaller and some
of them even aggregate into spheres of 500–800 nm in diameter
(Fig. 2c). When the precursor concentration increases to 0.05 M
(S5), the products become cubes or octohedra with a few small
nanobelts standing on them, whose edge length is 500–800 nm
(Fig. 2d). No flowerlike structures can be seen at the precursor
concentration of 0.1 M (S10, Fig. 2e), while the mixture of large
porous spheres and octahedra appears instead. The spheres are
of about 10 ␮m in diameter, whereas the octahedra are relatively
small, with an edge length of 300–600 nm. Highly magnified SEM
images of S10 are shown in Fig. 2f. The broken spherical particle (the inset at the top left corner of Fig. 2f) indicates that the
big spherical assemblies are not hollow, and the other magnified
image of the surface of sphere (at the right bottom corner of Fig. 2f)
indicates a mesoporous structure. It can be concluded that the precursor concentration has a significant effect on the morphology of
the products. This concentration-dependent self-assembly allows
S10
20
A: Cu2O
B: Cu
A (311)
2. Experimental
29
46
72
81
89
100
73
44
15
S5
S3
S2
S1
30
40
50
B (220)
71
54
28
19
11
–
27
56
85
VP (cm3 g−1 )
A (220)
24
24
24
24
24
8
14
24
26
dP (nm)
B (200)
0.01
0.02
0.03
0.05
0.1
0.01
0.01
0.01
0.01
SBET (m2 g−1 )
A (200)
S1 (S 1-24)
S2
S3
S5
S10
S1-8
S1-14
S1-20
S1-26
Cu2 O (wt%)
B (111)
Cu (wt%)
A (111)
Secondary reaction
time (h)
A (110)
Precursor
concentration (M)
Relative Intensiy (arb.units)
Sample
60
70
o
2θ ( )
(1)
Fig. 1. XRD patterns of S1, S2, S3, S5 and S10.
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B. Zhou et al. / Materials Chemistry and Physics 126 (2011) 847–852
849
Fig. 2. FESEM images of Cu2 O/Cu NCs prepared at different precursor concentrations: (a) 0.01 M, (b) 0.02 M, (c) 0.03 M, (d) 0.05 M and (e) 0.1 M. (f) Enlarged images of (e).
The inset of (a) is a magnified image. The inset on the top left of (f) shows a broken spherical particle, and the other one at the bottom shows a highly magnified image of the
surface of a sphere.
selective synthesis of nano-flowerlike architectures or mesoporous
spheres by changing precursor concentration, which is of great
importance for designing catalysts with high activity.
When the reaction time was prolonged from 8 h to 26 h at the
second step (180 ◦ C), the flowerlike structure obtained at 0.01 M
exhibits little change on morphology (Fig. 3) even though its phase
composition is changed (Table 1). As calculated from XRD results
(Fig. 4), the corresponding content of Cu increases from 27 wt% to
85 wt% when the time increases from 14 h (S1-14) to 26 h (S1-26).
In the experiments, DMF acts as the reducing agent. As described
by Zeng et al. [25,27], solid CuO forms only after reaction for 1.5–2 h
at 150 ◦ C, then it is reduced to Cu2 O, and further reduced into metal
Cu after longer reaction. Moreover, the physical and chemical processes can be accelerated at higher processing temperature. The
redox chemical process can be described as follows:
HCON(CH3 )2 + H2 O → HCOOH + NH(CH3 )2
(2)
Cu2+ + H2 O + 2NH(CH3 )2 → CuO + 2NH(CH3 )2+
(3)
CuO + HCOOH → Cu2 O + H2 O + CO2
(4)
Cu2 O + HCOOH → Cu + H2 O + CO2
(5)
In fact, formic acid HCOOH in Eq. (2) is generated through hydrolysis of DMF, and acts as a reducing agent. The small amount of water
comes from the starting regent Cu(NO3 )2 ·3H2 O. The reaction time
plays a crucial role on the ratio of Cu2 O to Cu in the NCs. Therefore, by controlling the reaction time at the second step, the phase
composition of the products can be adjusted without changing the
morphology.
3.3. N2 adsorption–desorption
Fig. 5 shows the N2 adsorption–desorption isotherm of S1,
which is representative for all the flowerlike samples. The isotherm
exhibits type IV hystersis, indicating S1 is porous. The corresponding parameters are listed in Table 1. As shown in the inset of Fig. 5,
the pore size of S1 ranges from 3 to 15 nm, which can be attributed
to the aggregation of primary nanoparticles.
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B. Zhou et al. / Materials Chemistry and Physics 126 (2011) 847–852
Fig. 3. SEM images of flowerlike Cu2 O/Cu NCs obtained in a 0.01 M Cu(NO3 )2 ·3H2 O solution at 180 ◦ C for (a) 8 h, (b) 14 h, (c) 20 h and (d) 26 h.
3.4. Photocatalytic activity
12
dv/dD (cm3/g/nm)
10
Volume (cm 3/g)
Fig. 6 shows the absorption spectra of PR solution with the presence of S1 at different visible light irradiation times. The strong
absorption peak that lies at about 512 nm is selected to monitor the photocatalytic degradation process. With longer irradiation
time, this characteristic peak becomes weaker. No new adsorption peaks appear, and no obvious shift of the characteristic peak
is observed, indicating the degradation of PR. After the irradiation
time of 60 min, PR is completely decolorized.
Shown in Fig. 7 is the photocatalytic activity of S1, S2, S3, S5
and S10, which is characterized by the initial concentration (C0 )
and the concentrations at different time intervals (C). Without catalyst, PR cannot be degraded under visible-light irradiation, whereas
0.0003
8
0.0002
0.0001
0.0000
6
0
5
10
15
20
Pore Diameter (nm)
Adsorption
4
Desorption
2
0
0.0
0.2
0.4
0.6
0.8
1.0
A: Cu2O
Fig. 5. N2 adsorption–desorption BET isotherms of sample S1. The inset shows the
pore size distribution.
B (220)
B (200)
B (111)
S1-26
B: Cu
0.14
S1-24
Absorbance
A (311)
A (220)
A (200)
A (111)
S1-14
S1-8
0 min
0.12
S1-20
A (110)
Relative Intensiy (arb.units)
Relative Pressure (P/P0)
PR
0.10
0.08
0.06
20 min
40 min
0.04
0.02
60 min
0.00
20
30
40
50
60
70
80
o
2θ ( )
Fig. 4. The XRD patterns of the flowerlike Cu2 O/Cu NCs. The Cu (2 2 0) is marked by
the dashed line.
400
500
600
700
800
Wavelength (nm)
Fig. 6. Absorption spectra of PR solution with the presence of S1.
B. Zhou et al. / Materials Chemistry and Physics 126 (2011) 847–852
(a)
1.0
0.6
0.4
Irradiation
0 min
Absorbance
C/C0
0.8
Phenol
0.6
Without catalyst
S1
S2
S3
S5
S10
851
20
0.4
40
60
0.2
80
100
0.2
120
0.0
0.0
240
0
20
40
60
80
100
260
Irradiation time (min.)
280
300
320
340
Wavelength (nm)
120
(b) 1.0
Fig. 7. Relative concentration (C/C0 ) of PR versus time under visible-light irradiation
with S1, S2, S3, S5 and S10 as photocatalysts.
it can be easily decolorized with the presence of Cu2 O/Cu NCs.
Among these NCs, the performance of S10 is relatively low, while
S1 exhibits the highest performance. As shown in Table 1, the specific surface areas of S10 and S1 are 5.5 m2 g−1 and 16.9 m2 g−1 ,
respectively. It is generally accepted that the catalytic process is
closely related to the adsorption and desorption of molecules on
the surface of the catalyst [28]. The large specific surface area of S1
enhances its adsorption ability and consists in its high performance.
It should also be noted that the mass ratio of Cu2 O to Cu in the
NCs plays an important role on the photocatalytic activity. The photocatalytic activities of S1-8, S1-14, S1-20, S1-24 (S1) and S1-26 are
shown in Fig. 8, which possess flowerlike morphology and different contents of Cu (Table 1). At an irradiation time of 120 min, PR
is completely degraded with the presence of S1-14, S1-20 and S124. On the contrary, the photocatalytic performances of S1-8 (pure
Cu2 O) and S1-26 (with a high Cu content) are relatively low.
As shown in Figs. 7 and 8, within the first 20 min of irradiation, a very high rate of dye degradation is observed, namely, more
than 90% for Cu2 O/Cu NCs. In fact, the absorption of PR lies in the
range of 440–580 nm with max = 512 nm. With the presence of PR,
a large amount of light becomes inaccessible to catalyst particles
[29,30]. A possible mechanism for the photocatalytic process can be
described as follows. The electrons are injected from photoexcited
PR molecules to the conduction band of Cu2 O, and the adsorbed O2
can scavenge the injected electrons, which prevents the recombination, hence the degradation of the electron-deficient PR is ensured.
1.0
S1-8
S1-14
C/C0
0.8
S1-20
S1-24
0.6
S1-26
0.4
0.2
0.0
0
20
40
60
80
100
120
Irradiation time (min.)
Fig. 8. Relative concentration (C/C0 ) of PR versus time under visible-light irradiation
with the presence of flowerlike Cu2 O/Cu NCs possessing different Cu contents.
C/C0
0.8
0.6
S1-8
S1-14
S1-20
0.4
S1-24
S1-26
0.2
0
20
40
60
80
100
120
Irradiation time (min)
Fig. 9. (a) Absorption spectra of phenol solution with the presence of S1. (b) Relative
concentration (C/C0 ) of phenol versus time under visible-light irradiation with the
presence of flowerlike Cu2 O/Cu NCs possessing different Cu contents.
When 90% of PR is decomposed, the decrease of degradation rate is
largely due to the low dye concentration.
Phenol is transparency for visible light and very hard to mineralize due to its resonance stability. Shown in Fig. 9(a) are the
absorption spectra of phenol solution with the presence of S1. The
strong absorption peak at about 269 nm is selected to monitor the
photocatalytic degradation process. With longer irradiation time,
this characteristic peak becomes weaker and weaker, indicating
the degradation of phenol. Fig. 9(b) shows the photocatalytic performance of Cu2 O/Cu NCs, whose Cu contents are different, on the
degradation of phenol. Compared with PR, the maximum degradation rate of phenol is only 40% within the first 20 min of irradiation.
For pure Cu2 O nanoparticles, only a degradation rate of 15% is
observed. Consisted in PR, Cu2 O/Cu NCs exhibit better photocatalytic performance than pure Cu2 O.
According to the theory of metal/semiconductor heterostructure catalysis [20,31–33], the photocatalytic activity
of semiconductor-based heterostructure depends greatly on the
concentration of heterostructure interface and defect, which can
increase the separation efficiency of photogenerated electron–hole
pairs. The metal on the surface of semiconductor acts as a sink for
the electrons, which promotes interfacial charge-transfer kinetics
between the metal and semiconductor, improves the separation of
photogenerated electron–hole pairs, thus increases photocatalytic
activity [31]. In Cu2 O/Cu NCs, the interfaces between Cu and
Cu2 O act as the sites where rapid separation of photogenerated
electrons and holes occurs. On the other hand, very high content
of Cu (S1-26, 85 wt%) does not favor high photocatalytic activity,
since Cu becomes recombination centers for electrons and holes
instead [34]. Therefore, the mass fraction of Cu in the NCs also
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B. Zhou et al. / Materials Chemistry and Physics 126 (2011) 847–852
influences photocatalytic activity. However, detailed degradation
mechanism has not been clear yet. Further work is needed for
better understanding of this issue.
4. Conclusions
Flowerlike Cu2 O/Cu photocatalysts have been synthesized by
following a two-step solvent-thermal synthesis route without
using any templates and additives. The precursor concentration has
a strong effect on the microstructure and phase composition of the
Cu2 O/Cu NCs. The flowerlike structure is formed in a precursor concentration range of 0.01–0.02 M. The content of Cu increases with
increasing reaction time or decreasing precursor concentration. As
visible light photocatalysts, Cu2 O/Cu NCs exhibit better photocatalytic performance than the pure-phase Cu2 O. Our stepwise route
offers an effective way to control the synthesis of Cu2 O/Cu NCs,
and may shed light on the design of other well defined complex
nanostructures.
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
Acknowledgments
[22]
This research work is funded by the National Natural Science
Foundation of China (No. 10504005 and No. 10674034), and Development Program of Outstanding Young Teachers in Harbin Institute
of Technology (Grant No. HITQNJS.2006.059).
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