Download Microwave Synthesis of Cuprous Oxide Micro

J. Mater. Sci. Technol., 2011, 27(4), 289-295.
Microwave Synthesis of Cuprous Oxide Micro-/Nanocrystals
with Different Morphologies and Photocatalytic Activities
Qingwei Zhu1) , Yihe Zhang1,3)† , Jiajun Wang2) , Fengshan Zhou1) and Paul K. Chu3)
1) State Key Laboratory of Geological Processes & Mineral Resources, National Laboratory of Mineral Materials, School
of Materials Science and Technology, China University of Geosciences (Beijing), Beijing 100083, China
2) Department of Powder Metallurgy and Special Materials, General Research Institute of Non-Ferrous Metal,
Beijing 100088, China
3) Department of Physics & Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon,
Hong Kong, China
[Manuscript received September 21, 2010, in revised form December 11, 2010]
Cuprous oxide micro-/nanocrystals were synthesized by using a simple liquid phase reduction process under microwave
irradiation. Copper sulfate was used as the starting materials and macromolecule surfactants served as the templates.
The morphologies phase and optical properties of them are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and ultraviolet-visible diffuse reflection absorptive spectra (UV-vis/DRS), respectively. The
crystals had four different shapes, namely spheres, strips, octahedrons, and dandelions. The photocatalytic behavior
of the cuprous oxide particles were investigated by monitoring the degradation of rhodamine B. In spite of the different morphologies, all of the cuprous oxide micro-/nanocrystals exhibited photocatalytic activities under visible light
irradiation in the following order: dandelions, strips, spheres, and octahedral crystals. The photocatalytic degradation
rates of rhodamine B are 56.37%, 55.68%, 51.83% and 46.16%, respectively. The morphology affects significantly the
photocatalytic performance.
KEY WORDS: Cuprous oxide; Synthesis; Morphology; Photocatalytsis
1. Introduction
As a transition metal oxide, cuprous oxide has
been studied extensively for its distinctive properties.
It is a p-type semiconductor with a direct band gap
of ca. 2.2 eV, which can be activated by irradiation of
visible light. Hence, it has been used as a visible-light
driven photocatalyst to break down organic contamination in water[1–5] as well as solar cells[6–10] . Moreover, cuprous oxide can be used in smoke suppression,
thermal degradation of poly-vinyl-chloride [11] , and
dissociation of nitrous oxide [12] due to its low toxicity
and good environmental acceptability. Cuprous oxide
crystals with different morphologies possess different
physical and chemical properties. Much effort has
been made to synthesize cuprous oxide crystals with
different shapes such as octahedron, whisker, cactus,
box, cube, sphere, wire, tube, rod, bi-pyramid, star,
† Corresponding author. Prof., Ph.D.; Tel./Fax: +86 10
82323433; E-mail address: [email protected] (Y.H. Zhang).
and flower[13–22] . The morphology and size often affect significantly the physical and chemical property
of crystal. Therefore, there have been some studies
on the photocatalytic activities of the crystals with
different shapes and/or sizes[17, 23–27] .
In this work, a simple method is designed
to synthesize four types of cuprous oxide micro/nanocrystals using copper sulfate as the starting materials, glucose and hydrate hydrazine as the reducing
agents, and macromolecule surfactants as the templates. The phase, morphologies and optical properties of four types of crystals were characterized by
XRD, SEM, and UV-vis/DRS. Their photocatalytic
activities are determined by monitoring the photocatalytic degradation of rhodamine B.
2. Experimental
2.1 Synthesis of samples
All the reagents except copper sulphate were
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Table 1 Reaction materials and heating time in the synthesis of the samples
and the specific surface areas (SBET ) of samples
No.
1
2
3
4
CuSO4
CuSO4
CuSO4
CuSO4
NH3 H2 O
NaOH
NaOH
NaOH
Reactants
glucose
PVP
N2 H4
P(VP-NVP-St)
N2 H4
P(NVP-St)
N2 H4
P(VP-NVP)
analytical grade and used without further purification. Copper sulfate was purified via recrystallization and water used in these experiments was
deionized. The macromolecule surfactants such as
poly-vinyl-pyridine(PVP), copolymer of vinyl pyridine, vinyl pyrrolidone and styrene{P(VP-NVP-St)},
copolymer of vinyl pyrrolidone and styrene {P(NVPSt)}, and copolymer of vinyl pyridine and vinyl
pyrrolidone{P(VP-NVP)} were synthesized in the
laboratory using radical polymerization with ammonium peroxydisulfate as initiator and their average
molecular weight are 7,000–14,000.
To synthesize the cuprous oxide microcrystals with
spherical structure, 50.0 mL of 0.5 mol/L copper sulfate solutions and 1.0 mL of 5% (M/V) PVP were
mixed in a beaker at room temperature. Under constant magnetic stirring, 15.0 mL of 5.0 mol/L ammonia solution was added to the above solution quickly.
Then, 20.0 mL of 2.5 mol/L glucose was added and
red. The beaker was heated in a microwave oven and
the solution boiled constantly for 30 min at 600 W. Finally, the precipitates were filtrated and washed with
deionized water and absolute ethanol and dried in a
vacuum dryer at 80o C for 2 h.
To synthesize the cuprous oxide crystals with the
other three types of shapes, 50.0 mL of 0.5 mol/L
copper sulfate solutions and 1.0 mL of 5% (M/V)
macromolecule surfactants were mixed in a beaker
at room temperature. For dandelion-shaped crystals,
50 mL of 1 mol/L tartaric acid was also added. Under constant magnetic stirring, after adding 50.00 mL
of 1.0 mol/L sodium hydroxide quickly, 5.0 mL of
1.0 mol/L hydrazine hydrate was added to the above
solution. Then the beaker was moved to a microwave
oven and heated at 600 W. Followed by boiling constantly for 5 min, the mixtures were separated by filtrating and the residues were washed with deionized
water and absolute ethanol and dried in a vacuum
dryer at 80◦ C for 2 h. Different crystal shapes were
obtained by changing the reactants and heating time
in the synthesis process as illustrated in Table 1 in
detail.
2.2 Characterizations of samples
The crystal phase of the synthesized particles was
identified by X-ray diffraction (XRD, Rigaku D/Max2000) using CuKα radiation (2 kV rotating anode,
λ=0.154056 nm). The samples were scanned from 10◦
to 110◦ at a scanning rate of 0.06◦ /s. The microstruc-
–
–
–
tartaric acid
Time
30 min
5 min
5 min
5 min
Morphology
sphere
strip
octahedron
dandelion
SBET /(m2 /g)
29.70
51.70
41.94
54.06
tures and morphologies of the particles were determined by scanning electron microscopy (SEM, Hitachi
S-4300). The samples were coated with gold to improve the conductivity during SEM observation. The
specific surface area of the particles was determined
by an automated surface area and pore size analyzer
(Autosorb-1, Quantachrome, USA) using the nitrogen adsorption. The ultraviolet-visible diffuse reflection absorptive spectra (UV-vis/DRS) were acquired
on a UV-visible spectrophotometer (Hitachi-UV3010)
equipped with an integration sphere at room temperature.
2.3 Photocatalytic degradation of rhodamine B
To assess the photocatalytic activity of various
cuprous oxide micro-/nanocrystals, the photocatalytic reaction was conducted in a 250 mL glass beaker
in a reactor equipped with a magnetic force stirrer
and circular cooling copper tube. A 500 W tungsten
halogen lamp without filter chip was used as the visible light source. 0.5 g of cuprous oxide powders were
dispersed in 200 mL of 5 mg/L rhodamine B aqueous solution. The suspension was sonicated for 1 h
in darkness to disperse the powders well in the solution before irradiation with visible light. At regular
intervals of 1 h, 25 mL of the suspension was sampled
and separated by centrifugation at 12,000 r/min for
10 min. The concentration of remaining rhodamine
B was determined by measuring their spectroscopy in
the wavelength of 250–700 nm using a UV-vis spectrophotometer (Hitachi UV-3010).
3. Results and Discussion
3.1 SEM images of the samples
Figure 1 illustrates the SEM images revealing four
kinds of crystal morphologies. And the specific surface areas of various cuprous oxide crystals are shown
in Table 1. The specific surface areas of the as synthesized cuprous oxide crystals with sphere, strip octahedron and dandelion shape are 29.70, 51.70, 41.94 and
54.06 m2 /g, respectively. In Fig. 1(a), the crystals are
spherical and the size is about 2∼12 µm. The spherical crystals are synthesized in an ammonia solution
with PVP as the template and glucose as the reducing
agent. Figure 1(b) discloses strip-shaped crystals with
a thickness of about 20 nm and width of 300∼400 nm.
The strip-shaped crystals are obtained from a sodium
hydroxide solution with P(VP-NVP-St) as the tem-
Q.W. Zhu et al.: J. Mater. Sci. Technol., 2011, 27(4), 289–295
291
Fig. 1 SEM images of cuprous oxide with different morphologies: (a) sphere, (b) strip, (c) octahedron, (d) dandelion
plate and hydrazine hydrate as the reductant. The
octahedral cuprous oxide particles shown in Fig. 1(c)
are obtained using P(NVP-St) as the template
whereas the dandelion-shaped cuprous oxide crystals
in Fig. 1(d) are synthesized using P(VP-NVP) as the
template. The SEM results suggest that it is possible
to control the morphology of the cuprous oxide crystals by changing reaction conditions, among which the
self-assembly-induced template is significantly important.
In the aqueous solution, as a result of their
amphiphilic nature, surfactant molecules acting as
templates often spontaneously self-assemble into microscopic structures shaped spheres, cylinders, vesicles, and membranes depending on outside factors[28] .
During the process of synthesis of materials with
nanometer or submicrometer scale, the particles growing on the self-assembly-induced template are often
affected by such factors as liquid interfacial tension,
capillary force, and different hydrophilic or hydrophobic forces. And selective adsorption of ions or molecules onto different surfaces also affects the particle
shape. Any of these factors can become the key controlling parameter for the resulting morphologies[29] .
In this study, the formation of the spherical crystals may be resulted from the effect of PVP template.
Because ammonia solution exists, cupric tetraammino ions would generate before they were reduced.
Cupric tetraammino ions combined with PVP to selfassemble into spherical microscopic structure in the
aqueous solution due to interfacial tension and hydrophilic and hydrophobic forces. During the process
of reducing, crystallographic planes of cuprous oxide
microcrystals were affected by PVP in the interface
to form spherical structure. For strip-shaped crystals, it is possible that the longitudinal growth of
cuprous oxide nanocrystals was postponed because
of self-assembly membrane spatial structure of P(VPNVP-St) in the aqueous solution. The formation of
Cu2 O octahedra can be explained as the corporate
effect coming from the growth units of anion coordinative polyhedra theoretical model and the polymer selective adsorption. According to the growth
units of anionic coordinative polyhedra theoretical
model, there are two anion coordinative polyhedra
in Cu2 O. These two coordinative polyhedra connect
each other and constitute the different growth unit
with the different dimension and the dimensions of the
growth units can vary with the change of the growth
condition[13] . In our experiments, when P(NVP-St)
was introduced, due to the selective interaction between P(NVP-St) and various crystallographic planes
of cuprous oxide, the growth along the [111] direction
could be greatly postponed but the growth along the
(100) direction could be enhanced. So the (111) faces
preferentially appear and the octahedra are obtained.
For dandelion-shaped structure, we suggest that it is
resulted from self-assembly of nanosized cuprous oxide crystallites with the assistance of P(VP-NVP) and
complexing agent tartaric acid. In aqueous solution,
copper ion can combine tartaric acid to form coordination complexes with polyhydroxy compounds. When
copper ions were reduced to cuprous oxide crystallites
by hydrazine hydrate, the growth of various faces of
these crystallites was controlled in the interface with
P(VP-NVP) to produce variform tiny polyhedral par-
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Q.W. Zhu et al.: J. Mater. Sci. Technol., 2011, 27(4), 289–295
o
*
*
*
o o
(b)
1.8
Copper oxide
Sphere
(a)
Cuprous oxide
Strip
1.6
*
Octahedron
* *
Absorbance / a.u.
Instensity / a.u.
(a)
*
o
(c)
Dandelion
1.4
1.2
1.0
0.8
(d)
0.6
10
20
30
40
50
2
60
70
80
90
100
0.4
/ deg.
300
Fig. 2 XRD patterns of (a) spherical, (b) strip-shaped,
(c) octahedral and (d) dandelion-shaped cuprous
oxide crystals
400
500
600
700
800
Wavelength / nm
12
2.0x10
12
(b)
1.8x10
Figure 2 depicts the XRD patterns of the products which possess a cubic structure which can be
indexed according to the JCPDS card No. 05-0667.
Five characteristic peaks are observed at 2θ values
of 29.6◦ , 36.5◦ , 42.4◦ , 61.5◦ , 73.7◦ , and 77.6◦ corresponding to the crystal planes of (110), (111), (200),
(220), (311) and (222) of crystalline cuprous oxide, respectively. The other characteristic peaks caused by
impurities of copper oxide, such as 35.5◦ , 38.7◦ , 48.7◦ ,
are found from the strip-shaped cuprous oxide crystals
(Fig. 2(b)). According to JCPDS card No.45-0937,
they correspond to the planes of (002), (111) and
(2̄02) of crystalline copper oxide, respectively. Because of the large specific surface area, a small quantity of cuprous oxide particles on the surface is oxidized to copper oxide during synthesis process. In
curves (a), (c), and (d) in Fig. 2, no noise peaks can
be observed and it can be inferred that the synthesized
spherical, dandelion-shaped, and octahedral cuprous
oxide crystals have high purity. These three kinds of
crystals were not oxidized during synthesis process,
which is possibly due to the protection of surfactants
coating in the surface of the particles, while the surfactants with membrane structure coating in the surface
of strip-shaped cuprous oxide nanocrystals were more
likely to be partially broken.
3.3 Optical properties of the samples
The optical response of the samples was characterized by using UV–vis diffuse reflectance spectrometry
(UV-vis/DRS). The UV-vis/DRS of the cuprous oxide samples are shown in Fig. 3(a). All of the samples
show a clear absorption edge at 650 nm. The maxi-
Sphere
-2
cm
Strip
12
1.4x10
Octahedron
2
) / eV
2
12
(
3.2 XRD patterns of the samples
12
1.6x10
h
ticles. And these variform tiny polyhedral particles
agglomerated into dandelion-shaped structure due to
their high surface free energy.
Dandelion
1.2x10
12
1.0x10
11
8.0x10
11
6.0x10
11
4.0x10
11
2.0x10
0.0
1.8
1.9
2.0
2.1
2.2
2.3
h
2.4
2.5
2.6
2.7
2.8
2.9
3.0
/ eV
Fig. 3 (a) UV-vis/DRS of the as prepared cuprous oxide crystals; (b) plot of (αhν)2 vs hν of various
cuprous oxide micro-nanocrystals
mum absorbance values of the spherical, strip-shaped,
octahedral, and dandelion-shaped cuprous oxide microcrystals are at 410 nm, 427 nm, 453 nm, and 453
nm, respectively and in the visible range. The cuprous
oxide crystals with different morphologies exhibit different sensitivities to visible lights and the dandelionshaped cuprous oxide crystal has the strongest absorbency and the octahedral one has the weakest one.
It is possibly because dandelion-shaped cuprous oxide
crystals have a porous structure and active crystallite
facets and thus light reflection is reduced. In comparison, the inactive facets of the octahedral cuprous
oxide crystals reflect most of the incoming irradiation
and the absorbency is subsequently the smallest. The
strip-shaped cuprous oxide crystals also show higher
absorbance than the spherical and octahedral ones because the particle is tiny and has larger specific surface. Our results also suggest that optical absorption
is often affected by the morphology of the crystals[30] .
Since cuprous oxide is a P-type direct gap
semiconductor[31] , the relationship between absorption coefficient (α) and photon energy (hν) can be
expressed as[32] :
1
αhν = B(hν − Eg ) 2
(1)
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Q.W. Zhu et al.: J. Mater. Sci. Technol., 2011, 27(4), 289–295
1.4
60
1.3
55.68
56.37
51.83
Degradation rate / %
Absorbance / a.u.
1.2
1.1
1.0
0.9
No photocatalyst
Octahedron
0.8
Sphere
0.7
Strip
46.16
40
30
20
10
Dandelion
0.6
50
1.07
0.5
0
0
1
2
3
4
No catalysts
Octahedron
Time / h
Fig. 4 Absorbance of remaining rhodamine B degraded
at different time in the wavelength of 553 nm. No
light irradiation in the first hour
In this equation, B is a constant which does not
depend on photo energy and Eg is the band gaps energy. The band gap can be estimated by the intercepts
of the tangents to the (αhν)2 vs photon energy (hν)
plots. Plots of (αhν)2 vs hν for cuprous oxide samples
with different morphologies are shown in Fig. 3(b).
The direct band gaps of spherical, strip-shaped, octahedral and dandelion-shaped cuprous oxide micro/nanocrystals are estimated to be 2.10, 2.06, 2.15 and
2.04 eV, respectively. It thus can be inferred that
cuprous oxide can be activated by visible light. The
red shift of the direct band gaps displays the effect
of the morphologies of crystals. The microcrystals
with different morphologies have different dominant
active facets and will response different excitation energy and consequently have different direct band gaps.
3.4 Photocatalytic activities of the samples
The photocatalytic activities of the cuprous oxide
crystals with different morphologies are evaluated by
monitoring the decomposition of rhodamine B in an
aqueous solution under visible light irradiation. Direct decomposition of rhodamine B without cuprous
oxide cannot almost be detected under visible light
irradiation in our control experiment. The decreasing absorbance of remaining rhodamine B degraded
directly is depicted in Fig. 4 which exhibits that the
absorbance does not almost change after being illuminated for 4 h. From Fig. 4, rhodamine B is degraded
to different degrees through visible-light driven photocatalysis by the various cuprous oxide crystals. On
the other hand, the absorbance of rhodamine B decreases slightly without visible light irradiation due to
the small absorption of rhodamine B by the cuprous
oxide crystals.
As rhodamine B has a strongest absorbance in
553 nm, it is rational to obtain the degradation rate
by the following equation.
Sphere
Strip
Dandelion
Sample
Fig. 5 Photocatalytic degradation rate of rhodamine B
by various cuprous oxide crystals
Di =
A0 − Ai
× 100%
A0
(2)
where Di (%) is the degradation rate when irradiation time is i hours, A0 the initial absorbance and Ai
is the absorbance when irradiation time is i hours in
the wavelength of 553 nm.
The degradation rates of rhodamine B by different cuprous oxide crystals when irradiation time is 4 h
was described in Fig. 5. It reveals that under visible
light irradiation, 56.37%, 55.68%, 51.83% and 46.16%
of rhodamine B (monitored at 553nm) can be decomposed by dandelion-shaped, strip-shaped, spherical
and octahedral cuprous oxide crystals, respectively.
It can be inferred that all the cuprous oxide crystals
synthesized have some photocatalytic activities which
are much higher than those reported in the previous
literature[33,34] and the shape factor seems to be of
overriding importance to photocatalytic activities.
The different photocatalytic activities of the
cuprous oxide crystals can be explained by reaction
activities. According to the literature reported by Xu
et al.[35] and Zhang et al.[36] , the (111) facets are more
active than (100) facets due to the dangling bonds
of (111) surfaces, while no dangling bands but saturated chemical bonds exist in the (100) facets. So
the cuprous oxide crystals with dominant (111) facets
have a higher adsorption and photocatalytic activity
than those with dominant (100) surfaces and (100)
surfaces. Due to their dominant (110) facet, the octahedral cuprous oxide crystals show the lowest photocatalytic activities among these four kinds of crystals though they have a larger specific surface area
(41.94 m2 /g) comparing to the spherical Cu2 O crystals (29.70 m2 /g). The dandelion-shaped Cu2 O crystals are composed of lots of tiny polyhedral crystals
with dominant (111) surfaces by aggregation and have
a largest specific surface area (54.06 m2 /g), so they
exhibit the best activities in the photocatalytic degradation of rhodamine B. For strip-shaped and spherical Cu2 O crystals, the dominant facets also are of
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Q.W. Zhu et al.: J. Mater. Sci. Technol., 2011, 27(4), 289–295
the (111) type, but they display somewhat lower photocatalytic activities than dandelion-shaped ones because of their lesser specific surface (51.70 and 29.70
m2 /g, respectively). Also, the stability of surface of
crystal is an important factor of photocatalytic activity. The strip-shaped Cu2 O crystal was oxidized
slightly on the surface resulting in its relatively lower
catalytic activity.
On the other hand, the shape of the crystals affects
the absorbencies and direct band gaps and then impacts on the photocatalytic performance due to the
different activity of the dominant facet. The specific surface dominated by the structural and crystal
size is an important factor affecting the reaction activity. High absorbency in the visible region results
in the visible-light induced photocatalytic activity[37] .
In general, the higher the absorbency of the photocatalyst, the higher the utilization of light as well
as electron-hole pairs generated by excited electrons.
The potential of the photocatalytic redox activity is
improved and the catalytic activity is increased. The
dandelion-shaped cuprous oxide crystals with the active dominant (111) facets have a much stronger absorbency and smaller direct band gap (∼2.04 eV), so
they exhibit much higher activity in the photocatalytic degradation of rhodamine B. The decreasing of
the absorbencies and direct band gaps of strip-shaped,
spherical and octahedral Cu2 O crystals in order result
in the diminution of their photocatalytic activities accordingly. As a result, the different morphologies of
cuprous oxide crystals lead to the different specific
surface areas, direct band gaps and UV-vis absorbencies which cause the different photocatalytic activities.
4. Conclusion
Spherical,
strip-shaped,
octahedral,
and
dandelion-shaped cuprous oxide micro-/nanocrystals
are synthesized via simple reduction of copper sulfate
using different reducing agents and macromolecule
surfactants under microwave irradiation. The macromolecule surfactants acting as the templates play an
important role in controlling the morphologies of the
resulting cuprous oxide crystals. Different reducing
agents and alkaline solutions also influence the morphology of the cuprous oxide crystals. It is possible
to control the morphology of the cuprous oxide crystals by changing the reducing agents, templates, and
alkaline solution. Investigation of the photocatalytic
activity shows that dandelion-shaped cuprous oxide
crystals have the highest activity and can degrade
56.37% of rhodamine B, the octahedral cuprous oxide
crystals exhibit the lowest activity and can only degrade 46.16% of rhodamine B, and the strip-shaped
and spherical cuprous oxide crystals can decompose
55.68% and 51.83% of rhodamine B, respectively. Different morphology results in different photocatalytic
activities of cuprous oxide crystals under visible light
illumination due to different active dominant facets,
direct band gaps, specific surface areas and absorbencies. It is suggested to improve the photocatalytic
activities by tuning the morphologies of cuprous oxide crystals.
Acknowledgements
This study was supported by the Open Foundation
of National Laboratory of Mineral Materials of China
University of Geosciences (Grant No. 08A006), the Key
Project of Chinese Ministry of Education (No. 107023),
Special Fund of Co-construction of Beijing Education
Committee, and City University of Hong Kong Strategic
Research Grant (SRG) No. 7008009.
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