Hierarchical Self-Assembly of Flower-Shaped CuO With High Photocatalysis Efficiency for Degrading Rhodamine B

Article abstract of DOI:10.1080/15533174.2015.1137018

Flower-shaped CuO with hierarchical microstructures was successfully synthesized through an ultrasound-assisted aqueous chemical method. The comparative experiment results show that the phase and morphology of as-obtained products are greatly affected by the volume of ammonia and the time for keeping ultrasonic irradiation, respectively. The prepared CuO microflowers assembled by nanosheets with needle-like tips exhibit a much higher activity than the commercial CuO for degrading Rhodamine B. Additionally, the photocatalytic performance of as-prepared hierarchical CuO microflowers was also evaluated through the degradation of Methylene blue and Methyl orange under the same conditions.

Full text of DOI:10.1080/15533174.2015.1137018

Synthesis and Reactivity in Inorganic, Metal-Organic, and
Nano-Metal Chemistry
ISSN: 1553-3174 (Print) 1553-3182 (Online) Journal homepage: http://www.tandfonline.com/loi/lsrt20
Hierarchical self-assembly of flower-shaped CuO
with high photocatalysis efficiency for degradating
Rhodamine B
Xinqing Ge, Hanmei Hu, Qiang Zheng & Mei Sun
To cite this article: Xinqing Ge, Hanmei Hu, Qiang Zheng & Mei Sun (2016): Hierarchical self-
assembly of flower-shaped CuO with high photocatalysis efficiency for degradating Rhodamine
B, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, DOI:
10.1080/15533174.2015.1137018
To link to this article: http://dx.doi.org/10.1080/15533174.2015.1137018
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Date: 15 March 2016, At: 09:45

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Hierarchical self-assembly of flower-shaped CuO with high photocatalysis efficiency for
degradating Rhodamine B
Xinqing Gea, Hanmei Hua,b*, Qiang Zheng a, Mei Suna
aKey Laboratory of Functional Molecule Design and Interface Process, Anhui University of
Architecture, Hefei, 230601, China
bSchool of Materials and Chemical Engineering, Anhui University of Architecture, Hefei,
230601, ,China
* Corresponding author. E-mail address: hmhu@ustc.edu (H.M. Hu)
Abstract
The flower-shaped CuO with hierarchical microstructures were successfully synthesized
through an ultrasound-assisted aqueous chemical method. The comparative experiment results
show that the phase and morphology of as-obtained products are greatly affected by the volume
of ammonia and the time for keeping ultrasonic irradiation, respectively. The prepared CuO
microflowers assembled by nanosheets with needle-like tips exhibit a much higher activity than
the commercial CuO for degradating Rhodamine B (RhB). Additionally, the photocatalytic
performance of as-prepared hierarchical CuO microflowers was also evaluated through the
degradation of Methylene blue (MB) and Methyl orange (MO) under the same conditions.
Keywords: Copper oxide; Hierarchical self-assembly; Microstructure; Organic pollutants;
Photocatalytic activity
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1. Introduction
Recently, many efforts have been focused on the hierarchical assembly of nanoscale building
blocks (nanoparticles, nanorods or nanosheets, etc.) into ordered superstructures or higher level
sophisticated architectures. [1-4] Superstructures or hierarchical assembly can exhibit collective
physical and chemical properties which are not found on the level of individual nanoblocks. [5-6]
Copper oxide (CuO), as an important p-type semiconductor with a narrow band gap of 1.2eV, [7]
has attracted a great interest in which not only its excellent physical and chemical properties but
also its potential applications in many fields, such as, gas sensor material,[8] anion material, [9]
catalyst, [10,11] lithium ion battery [12] and supercapacitor. [13]
Numerous efforts have been made to prepare CuO micro/nano hierarchical architectures
with special properties. Dendrite-shaped CuO hollow micro/nanostructures have been
synthesized via a Kirkendall-effect-based approach, acting as a high-performance anode material
for lithium-ion batteries.[14] Perpendicularly cross-bedded CuO microstructures with good gas
sensitivities targeting at ethanol and acetone have been prepared through a precursor-based
route.[15] Urchin-like core-shell CuO assembled by nanorods has been hydrothermally
synthesized in assistance of poly(ethylene glycol) (PEG), which exhibits wide biosensor
capability toward H2O2.[16] Flower-like CuO nanostructures with room-temperature
ferromagnetism were prepared by the coprecipitation method with postannealing in air at
different temperatures. [17] CuO microflowers composed of nanosheets have been fabricated in
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ammonia solution at 90-180 ºC, exhibiting effective catalytic activity on the decomposition of
ammonium perchlorate.[18] Recently, Uniform and well-defined CuO cauliflowers have been
prepared through a hydrothermal route, showing a much higher activity than the irregular CuO
for the degradation of Rhodamine B (RhB). [11]
However, many reported methods have ineluctable high-cost and time-consuming, is
unfriendly to the environment. Hence, it’s still a challenge to design a facile and green
preparation method to obtain CuO nano/micromaterials. In this work, well-defined hierarchical
CuO flower-shaped microstructures were successfully fabricated by an ultrasound-assisted
aqueous chemical method. Photocatalytic results indicate that the obtained CuO flowers exhibit
excellent photocatalytic activity for the degradation of RhB in the presence of H2O2 under
tungsten lamp irradiation.
2. Experimental
2.1 Materials
All chemicals used in this study were analytical grade and were not subjected to additional
purification. Copper acetate monohydrate (Cu(CH3COO)2·H2O), ammonia (NH3·H2O) and
hydoxylamine hydrochloride (NH2OH·HCl), which were purchased from Shanghai guoyao
chemical reagent LTD.
2.2 Synthesis of flower-shaped CuO
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In a typical procedure, 1mmol Cu(CH3COO)2·H2O and 0.5mmol NH2OH·HCl were
dissolved in 60 mL distilled water under stirring. After continuous stirring for 10 minutes, 0.5mL
ammonia was added to the mixed solution under stirring to form precursor solution. Afterwards,
the precursor solution was transferred into a 100 mL autoclavable bottle sealed with attached
screw cap to form a closed system, which was put into a sonication bath with the water
temperature of 60 ºC and irradiated for 40 minutes by ultrasonic wave. Subsequently, the bottle
was taken out carefully and cooled down to room temperature naturally. The black precipitate
was observed and filtered out, washed several times with distilled water and absolute ethanol,
and then dried in a vacuum at 60 ºC for 6h.
2.3 Photocatalytic experiments
Photocatalytic experiments were performed in a reflux system (shown in Fig. 1). A
self-made double walled glass vessel with cooling water was placed on a magnetic stirrer, in
which a beaker containing dye and catalyst suspension was put, so as to decrease the thermal
effect. A 500 W tungsten lamp was used as the light resource. Prior to photocatalytic studies, the
mixture of 25mg catalyst and 50mL organic dye (10mg/L) was stirred for 60min in dark to reach
the balance of adsorption and desorption on the surface of catalyst. Then the suspension was
exposed to tungsten lamp light with continuous stirring in the presence of 0.1mL hydrogen
peroxide (H2O2). At the specific time interval, the dispersion was sampled (4mL) and centrifuged
for 3 min to separate the catalyst. The supernatant of dyes were put into a quartz cell. The
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concentration of the corresponding dye was measured with a UV-5500PC spectrophotometer.
2.4 Instrument and characterization
The chemical reactions were implemented in an ultrasonic generator (Kunshan Co., KQ-50,
50 W, 40 kHz). The X-ray diffraction patterns of as-prepared products were obtained by X-ray
diffraction (XRD) using a Bruker D8 Advance X-ray diffractometer (40 kV, 40 mA) equipped
with graphite monochromatized Cu Kα radiation (λ=0.15406 nm). The surface morphology and
size of as-obtained samples were measured on a field emission scanning electron microscope
(JEOL JSM-6700F) and a transmission electron microscope (H-7650). UV-Vis diffuse
reflectance spectra (DRS) were recorded on a UV-Vis spectrophotometer (Solidspec-3700 DUV)
at room temperature.
3. Results and discussion
3.1 Characterization of flower-shaped CuO microstructures
The crystal structure and phase purity of the prepared products were determined by using
X-ray diffraction (XRD). Fig. 2 shows the XRD pattern of the as-prepared sample. All the peaks
can be clearly indexed to cubic CuO crystal planes, which were consistent with the standard data
for CuO (JCPDS 80-1916). In addition, no other obvious impurities are observed, such as
Cu(OH)2 and Cu2O. The intense and strong reflection peaks suggest that the products are highly
crystalline.
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Fig. 3(a) and (b) show the FESEM images of CuO samples prepared by ultrasonic radiation
for 40 min. It can be seen that the as-prepared CuO products are mainly composed of uniformly
elegant flower-like microstructures, which were produced in large quantities. The sizes of CuO
microflowers are estimated to be 2-3 μm. These flower-like architectures are self-assembled by
nanosheets with needle-like tips radiating from their center. The inset figure in Fig. 3(b) clearly
shows a typical peony-like particle. The thicknesses of nanosheet building units are in the range
of 10-20 nm and the widths of middle parts of these nanopetals are about 300-400nm. Fig. 3(c)
and (d) are the representative TEM images of the obtained CuO sample, which still preserve the
shapes of flower after the ultrasonic treatment. Fig. 3(d) is a high-magnification image of many
flower petals, indicating that the needle-like tips grow out from the middle region of CuO
nanosheets, and the width of each sheet-like nanopetal gradually decreases from the bottom
toward the tip.
3.2 Influencing factor and growth mechanism
In the present designed reaction system, the phases of obtained products are greatly affected
by the adding amounts of ammonia (NH3·H2O). The XRD patterns of comparative experiments
are presented in Fig. 4(a). The introduction of NH3·H2O and its dosage are crucial for the
formation of CuO phase. If no NH3·H2O is added into the reaction system keeping other
conditions unchanged, the obtained products are Cu(OH)2 (JCPDS 72-0140) as well as trace
amounts of Cu2O (marked by “&”). It is suggested that NH2OH·HCl mainly act as precipitation
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reagent for Cu2+ ions to generate Cu(OH)2 precipitate in the absence of NH3·H2O.
Simultaneously, a little bit of Cu2+ ions in the solution are reduced to Cu+ by NH2OH·HCl,
generating Cu2O impurity phase in the products. When the volume of ammonia is 0.3mL, the
obtained products are the mixture of Cu(OH)2, Cu2O and CuO (marked by “*”). While the
volume of ammonia is added up to 0.5 mL, pure CuO phase can be achieved (Fig. 1). In addition,
ultrasonic treatment is indispensable for growing CuO microflowers in the present system. In the
absence of ultrasonic irradiation, pure CuO can not be obtained. Fig. 4(b) present the XRD
pattern of product prepared without ultrasonic treatment keeping other condition constant, which
exhibit that the product is the mixture of CuO and Cu2O.
In the present reactive system, NH3·H2O is a weak base (Kb=1.810-5), but NH2OH·HCl is
an acid reagent (Ka=1.110-6). In the absence of ammonia, Ac- ions in the aqueous solution
hydrolyze to produce OH- [Eq. 1]. New-produced OH- ions react with NH2OH·HCl, generating
NH2OH [Eq. 2]. As a weak base (Kb=9.110-9), NH2OH continue to hydrolyze to produce OH-
[Eq. 3], which combine with Cu2+ to form Cu(OH)2 precipitate [Eq. 4]. When the appropriate
amount of ammonia is added, abundant OH- ions are produced owing to the dissociation of
2-
NH3·H2O [Eq. 5]. Then, the preformed Cu(OH)2 combine with OH- ions to form Cu(OH)4
2-
complexes [Eq. 6]. Finally, Cu(OH)4 coordinate compounds will dehydrolyze to generate CuO
products [Eq. 7]. The whole reaction process can be described as follows:
Ac- + H2O = HAc + OH-
(1)
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NH2OH·HCl + OH- = NH2OH + H2O + Cl-
NH2OH + H2O = [NH3OH]+ + OH-
Cu2+ + 2OH- = Cu(OH)2
(2)
(3)
(4)
(5)
(6)
(7)
+
NH3·H2O = NH4 + OH-
2-
Cu(OH)2 + 2OH- = Cu(OH)4
Cu(OH)42- = CuO + H2O +2OH-
To investigate the growth process of flower-like CuO. Time-dependent comparative
experiments were carried out while keeping other conditions unchanged, which was illustrated in
Fig. 5. Fig. 5(a) indicates the corresponding XRD patterns of products obtained when the
ultrasonic irradiation time is 0 min, 5 min and 20 min, respectively. Before the ultrasonic
irradiation, light blue Cu(OH)2 products (JCPDS 72-0140) as well as trace amounts of Cu2O
impurity phase (marked by “&”) have been detected. When the ultrasonic irradiation time is 5
min, the products are the mixture of Cu(OH)2, Cu2O and CuO (marked by “*”). While the
ultrasonic irradiation time is extended to 20 min or above, pure CuO phase can be obtained. The
morphology of Cu(OH)2 are nanobelts and belt-like aggregates (Fig. 5b). Fig. 5(c) give the
picture of products obtained when the ultrasonic irradiation time is 5 min, indicating that the
products are composed of Cu(OH)2 nanobelts and CuO shuttle-shaped nanostructures (indicated
by the white dashed circle in Fig. 5(c)). When the irradiation time is 10 min, large quantities of
CuO nanoshuttles are generated. It can be seen that some nanoshuttles have secondary generation
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branching off the central section of them (indicated by the white dashed circle in Fig. 5(d)). Fig.
5(e) present the morphology of products obtained when the ultrasonic irradiation time is 20 min,
uncompleted flower-like CuO microstructures are found. When the ultrasonic irradiation time is
prolonged to 40 min, elegant CuO microflowers are successfully achieved (Fig. 3).
Based on the above experimental facts, the growth process of flower-shaped CuO
architectures is described as Fig. 6. First, under the condition of ultrasonic irradiation, CuO
2-
primary nuclei are generated by the dehydration of preformed Cu(OH)4 and rapidly
developed into shuttle-like monomers, which can act as the body for the following secondary
nucleation and growth . Second, possibly due to existing high active sites at the central part of
CuO nanoshuttles, secondary nucleation and growth of shuttle-like nanosheets would firstly take
place on these active sites, and nanoshuttles with nanosheets standing on their surfaces were
produced. Third, with the prolongation of reaction time, secondary nucleation and growth of
shuttle-like nanosheets would also occur at the other places on the surfaces of shuttle-like body,
forming uncompleted flower-like CuO microstructures. Finally, in the later growth stage, the
concentration of reactant is decreased, which could not satisfy the growth of shuttle-like
nanostructures and lead to the formation of sheet-like petals with acicular tips. Through the
unceasing process of oriented aggregation growth and Ostwald ripening, beautiful peony-shaped
CuO architectures have been acquired.
3.3 Optical property
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Fig. 7a is the UV-vis diffuse reflectance spectrum of as-prepared CuO flower-shaped
microstructures, which indicates that there is a wide absorption from 200 nm to 800 nm.
According to the equation αEphoton=K(Ephoton-Eg)n/2, where α is the absorption coefficient, Ephoton
is the discrete photo energy, K is a constant and Eg is the band gap energy. [19] For the CuO, the
value of n is 1 for the direct conductor. Fig. 7b is the (αEphoton)2 ~ Ephoton curve of CuO
microflowers. The value of optical band gap Eg is estimated to be 2.3eV, similar to CuO
sheet-like nanostructures (2.31eV) reported by Wang’s group, [20] greater than the value for bulk
CuO (1.2eV). [7] Owing to the band gap energy in the visible region, the as-prepared hierarchical
CuO microflowers may have visible-light-driven photocatalytic activity on the degradation of
some organic pollutants.
3.4 Photocatalytic performance
The photocatalytic performance of as-prepared hierarchical CuO microflowers was
evaluated through the degradation of organic dyes in water under visible light irradiation.
Rhodamine B (RhB) was selected as a model organic pollutant for detail discussion. Fig. 8(a)
shows the typical absorption spectra of RhB solution detected at different intervals using CuO
microflowers as photocatalyst assisted by a small amount of hydrogen peroxide (H2O2). The
adsorption rate of RhB on as-prepared CuO is 5.8% in 60 min under dark condition. The
intensity of the characteristic absorption peak at 554 nm decreases rapidly within the initial 10
min irradiation, and the degradation rate of RhB is about 50.6%. The intense pink color of the
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starting RhB solution almost completely fades after 80 min, suggesting the efficient
decomposition of RhB into CO2 and H2O. For comparison, series of tests were carried out under
different conditions, and the results are showed in Fig. 8(b). For the blank experiment, direct
photolysis of RhB could almost be neglected. Only in the presence of H2O2, the degradation rate
is 9.7 % in 80 min. It is suggested that H2O2 may to some extent combine with photon and
generate ∙OH radicals, [21] which can oxide RhB molecules. Only in the presence of CuO, about
14.7% RhB was decomposed in 80 min. The lower decolorization rate of RhB only over CuO is
possibly due to the fast recombination of photogenerated electron-hole pairs in CuO crystals.
However, the degradation rate of RhB over as-prepared CuO microflowers can reach 94% within
80 min with the assistance of 0.1 mL H2O2. For comparison, the degradation process of RhB
over commercial CuO was also detected under the same conditions with as-prepared CuO. One
can see that the degradation rate of RhB is only 10.1% within the initial 10 min irradiation and
about 59.3% RhB was decomposed in 80 min. By comparing as-prepard CuO with commercial
CuO, a conclusion can be drawn that as-prepared hierarchical CuO microflowers have the
stronger ability to quickly photocatalytic degradation of organic dyes. We think the main reason
is that CuO microflowers built by thin nanosheets with needle-like tips may have more active
surface sites for photocatalytic reaction.
In addition, the photocatalytic performance of as-prepared hierarchical CuO microflowers
was also evaluated through the degradation of Methylene blue (MB) and Methyl orange (MO)
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under the same conditions, as shown in Figs. 8(c) and (d). It is found that the characteristic
absorption peak of MB at 665 nm almost disappeared after irradiation for 80min, but the
characteristic absorption intensity of MO at 464 nm is still stronger. The removal efficiency of
MB and MO is about 96% and 41%, respectively. The comparative experiment results indicate
that the as-prepared CuO catalyst obviously has selectivity of dyes, which show high
photocatalytic efficiency for RhB and MB.
On the base of above series of comparative experiments, we think the appropriate
combination of CuO and H2O2 is crucial to enhance the photocatalytic reaction efficiency.
According to report, H2O2 is suggested to be a better photogenerated electron acceptor than
[22]
oxygen to reduce the chances of electron-hole pairs recombination.
Under the light
illumination, the electrons in CuO crystals transit from the valence band (VB) to the conduction
band (CB) and leave the holes in VB, producing photogenerated electron-hole pairs. As an
outstanding electron-capturing agent, the dissolved H2O2 adsorbed on the surface of catalyst
rapidly combine with photogenerated electrons (eCB) and produce ∙OH radicals, prolonging the
lifetime of photo-induced carriers. Due to its stronger oxidation capacity, the new-produced ∙OH
radicals can effectively oxidize RhB molecules into carbon dioxide and water. The schematic
diagram of band structure in the CuO crystal is illustrated in Fig. 9. Additionally, It is reported
that the active sites of the surfaces of CuO may also catalyze the decomposition of H2O2 to
generate free radical species as·OH , ·OOH and O2·-,[23] also resulting in the enhancement of
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photocatalytic efficiency.
4. Conclusions
In conclusion, we have prepared hierarchical CuO microflowers via a simple
ultrasound-assisted aqueous chemical method. The sizes of CuO microflowers are estimated to
be 2-3 μm. These flower-like architectures are self-assembled by CuO nanosheets with
needle-like tips radiating from their center. The use of ammonia and reactive time has great
effects on the phase purity and morphology development of the products. The mechanism of
oriented aggregation growth and Ostwald ripening was suggested for explaining the formation of
CuO hierarchical microflowers. Photocatalytic performance results indicate that the as-prepared
CuO microflowers obviously have selectivity of dyes, showing high photocatalytic efficiency for
RhB and MB in short time.
Acknowledgements
This work was supported by the Natural Science Foundation of Anhui Educational
Committee (KJ2014ZD08) and the Fifth Science and Technology Foundation of Outstanding
Youth of Anhui Province (1308085JGD06, 10040606Y25).
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Fig. 1
Apparatus of photocatalytic experiment
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Fig. 2
XRD pattern of the as-prepared sample
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Fig. 3
FESEM images of CuO flowers products recorded at different magnification
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Fig. 4
(a) XRD patterns of samples obtained with different adding volume of ammonia under the
condition of ultrasonic irradiation, (b) XRD pattern of product prepared without ultrasonic
treatment keeping other condition constant
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Fig. 5
(a) XRD patterns of products obtained with ultrasonic irradiation time of 0, 5 and 20 min, (b-e)
FESEM images of the obtained sample at different growth stages: (b) 0min, (c) 5min, (d) 10min,
(e) 20 min
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Fig. 6
Schematic illustration of the growth process for flower-shaped CuO microstructure
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Fig. 7
(a) UV-vis absorption spectrum and (b) (αhυ)2 ~ hυ curve of CuO flower-shaped architectures
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Fig.8
(a) Photodegradation process of RhB over CuO microflowers in the presence of H2O2, (b)
Degradation rate of RhB at different intervals under different conditions, (c) Photodegradation
process of MB over CuO microflowers in the presence of H2O2, (d) Photodegradation process of
MO over CuO microflowers in the presence of H2O2
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Fig. 9
Schematic diagram of band structure in the CuO crystal
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