Synthesis of a flower-like CuS/ZnS nanocomposite decorated on reduced graphene oxide and its photocatalytic performance

Article abstract of DOI:10.1039/c5ra04030b

A series of graphene-based materials composed of flower-like CuS/ZnS (CZS) microspheres and reduced graphene oxide (rGO) with different weight ratios have been fabricated via a two-step approach, in which CZS was synthesized first by a hydrothermal method, and then combined with rGO via a sonochemical process to form CuS/ZnS/rGO (CZS/rGO) composites. The photocatalytic activity was studied by photodegradation of methylene blue (MB) in the presence of hydrogen peroxide (H₂O₂). The results showed that all the graphene-based photocatalysts exhibited enhanced catalytic activity, compared to pure CuS/ZnS (CZS) composites. When the weight ratio of graphene in the composite materials was 20 wt%, the composite presented the highest photocatalytic efficiency, which could eliminate more than 99% of MB within 20 min and the photocatalytic efficiency could be kept at about 99% of the original photocatalytic efficiency after three cycles.

Full text of DOI:10.1039/c5ra04030b

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Synthesis of a flower-like CuS/ZnS nanocomposite
decorated on reduced graphene oxide and its
photocatalytic performance†
Cite this: RSC Adv., 2015, 5, 36185
a
a
b
b
*
Xiao-Hui Guan, Liu Yang, Xin Guan and Guang-Sheng Wang
A series of graphene-based materials composed of flower-like CuS/ZnS (CZS) microspheres and reduced
graphene oxide (rGO) with different weight ratios have been fabricated via a two-step approach, in
which CZS was synthesized first by a hydrothermal method, and then combined with rGO via a
sonochemical process to form CuS/ZnS/rGO (CZS/rGO) composites. The photocatalytic activity was
studied by photodegradation of methylene blue (MB) in the presence of hydrogen peroxide (H2O2). The
results showed that all the graphene-based photocatalysts exhibited enhanced catalytic activity,
compared to pure CuS/ZnS (CZS) composites. When the weight ratio of graphene in the composite
materials was 20 wt%, the composite presented the highest photocatalytic efficiency, which could
eliminate more than 99% of MB within 20 min and the photocatalytic efficiency could be kept at about
99% of the original photocatalytic efficiency after three cycles.
Received 6th March 2015
Accepted 16th April 2015
DOI: 10.1039/c5ra04030b
www.rsc.org/advances
the cubic zinc blende and hexagonal wurtzite phase respectively)
is too large to utilize visible light. In addition, the fast recombi-
Introduction
nation rate of electron–hole pairs dramatically restricts its
applications as photocatalyst.9 Copper sulde (CuS) is a p-type
semiconductor with narrow band gap, and simultaneously it
exhibits low reectance in the visible region and relatively high
reectance in the near-infrared region, which enables CuS to
effectively utilize the visible light energy.10,11 Hence, it is a facile
strategy to load ZnS on the CuS surface to obtain a novel hybrid
photocatalyst, CuS/ZnS (CZS), with enhanced UV and visible-
light-driven photocatalytic activity. CZS nanocomposite not only
can facilitate the separation of photoinduced electron–hole pairs,
which ensures more charge carriers migrate to the surface of the
photocatalyst to trigger redox reactions, but also shows extensive
adsorption range and improved adsorption intensity of solar
energy compared to the bare CuS and ZnS.12 Although the inte-
gration of CuS and ZnS has already achieved enormous progress
in improving the photocatalytic performance, more efforts and
attention should be devoted to enhancing the poor adsorption
capacity and further promoting the separation rate of photo-
generated electron–hole pairs, which substantially inhibit the
practical applications of the photocatalyst.
Graphene, a two dimensional carbon material, consists of a
at monolayer of sp2-hybridized carbon atoms that are tightly
packed into a honeycomb lattice.13 To investigate and apply
graphene and graphene-based nanomaterials more compre-
hensively, plenty of feasible approaches have been proposed
including physical and chemical methods to synthesize gra-
phene with remarkable properties, such as mechanical exfoli-
ation,14 chemical vapour deposition route,15 epitaxial growth,16
electrochemical exfoliation of graphite17 and reduction of
Semiconductor nanomaterials have gained considerable interest
in the applications in environmental remediation and energy
conversion, such as photocatalytic degradation of organic and
toxic pollutants in water and air, water splitting into H2 and
photoelectrochemical solar cells and so on.1–3 They have shown
many excellent properties, such as high photocatalytic efficiency,
great economic interest, harmlessness to the environment
without introducing other undesirable by-products, and the
capability to utilize solar energy, which is considered to be the
most important merit. In the past few decades, more and more
investigations have been conducted to fabricate new-type, cost-
efficient and clean semiconductor photocatalysts that response
to both UV and visible lights to eliminate organic contaminants,
especially non-biodegradable ones.4 Metal suldes have proven
to be excellent photo-reactive catalysts.5 Particularly, zinc sulde
(ZnS), one kind of nontoxic and abundant chalcogenide mate-
rials with a direct wide-gap, is a promising candidate as photo-
catalysts in environmental protection, which can be ascribed to
its unique properties: rapid generation of electron–hole pairs by
photo-excitation,6 highly negative reduction potentials of excited
elections,7 and prominent stability against the photo-corrosion;8
However, its relatively wide band gap (ꢀ3.72 eV or ꢀ3.77 eV for
aSchool of Chemical Engineering, Northeast Dianli University, Jilin 132000, PR China
bKey Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry
of Education, School of Chemistry and Environment, Beihang University, Beijing
100191, PR China. E-mail: wanggsh@buaa.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra04030b
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exfoliated graphene oxide (GO)18,19 and so on. Among these
synthetic routes, reduction of the exfoliated GO sheets is
thought to be of more importance and practical application
value, owing to its economical efficiency and high yield for the
mass production of graphene. What's more important, reduced
graphene oxide (rGO) possesses more reactive oxygen-
containing functional groups in comparison to the products
synthesized by other routes, which is favorable to fabricating
functionalized graphene-based nanomaterials.20,21 In recent
years, graphene-based semiconductor nanomaterials used in
photocatalytic eld have aroused tremendous attention due to
its outstanding properties: fast electron conductivity, large
specic surface area, high optical transparency, good mechan-
ical strength and high thermal conductivity. The roles of gra-
phene playing in the hybrid photocatalysts can be summarized
as follows. Firstly, the two-dimensional platform structure of
graphene makes it a suitable substrate for the uniform deco-
ration and immobilization of semiconductor particles.22,23
Evidently, the homogeneous dispersion of nanoparticles on the
graphene sheets can hinder their aggregation effectively.
Meanwhile, it is also able to inhibit the graphene sheets from
aggregating, which can ensure the large specic surface area of
the hybrid photocatalysts. Secondly, graphene, existing in the
hybrid photocatalysts, is benecial to transferring the photo-
generated electrons from the semiconductor to the surface of
the photocatalysts to participate in redox reactions, and sup-
pressing the recombination of the electrons and holes due to its
superior electrical conductivity.24,25 Finally, graphene possesses
great adsorption capacity for organic molecules because of its
large surface area and the strong p–p interaction between
graphene and organic molecules.26,27 Therefore, assembling
CZS particles on the graphene sheets to synthesize a novel
graphene-based photocatalyst, CZS/rGO, is of great signicance
for the improvement of photocatalytic performance and appli-
cation in environmental remediation.
Preparation of CZS/rGO composites
GO was prepared by a modied Hummer's method and then
reduced through a chemical reduction route using hydrazine as
the reducing agent.29 And one-pot hydrothermal approach was
utilized to synthesize CZS. In our previous studies, CZS with
Zn2+/Cu2+ molar feed ratio of 0.4 exhibited the most preeminent
photocatalytic reactivity, so we decide to synthesize CZS with
Zn2+/Cu2+ molar ratio of 0.4 and combine it with rGO conse-
quently in this research.28
CZS/rGO composites were fabricated via a sonochemical
approach. First of all, desired weight of GO and CZS were
dispersed in 60 ml double distilled water under ultrasonication
for 30 min at room temperature. Then hydrazine was added into
the solution and stirred gently for 5 min. Aerwards, the
obtained solution was heated in the oil bath with continually
magnetic stirring for 2 h at 90 ꢁC, leading to the reduction of GO
to rGO. At last, the solution containing CZS and rGO was
ultrasonicated again for 2 h to obtain the nal product. Then
the sample was centrifuged, washed with ethanol and double
distilled water, and nally dried in a vacuum oven at 50 ꢁC for 12
h. A series of CZS/rGO composites with different weight ratios of
rGO (5%, 10%, 15%, 20% and 30%) were synthesized under the
same condition, which were labeled as CZS/rGO-1, CZS/rGO-2,
CZS/rGO-3, CZS/rGO-4 and CZS/rGO-5, respectively. The sche-
matic illustrations for the process to synthesize CZS/rGO
composites are shown in Scheme 1.
Characterization
X-ray diffraction (XRD) characterization was conducted on a X-
ray powder diffractometer (Philips X'pert Pro Super) with Cu
KR radiation (l ¼ 0.154056 nm). The micro-morphology of the
photocatalysts was obtained by scanning electron microscopy
(SEM) at 20.0 keV on a JSM6510A microscope. Fourier-
transform infrared (FTIR) spectra were recorded from 4000
cmꢂ1 to 500 cmꢂ1 on an FTIR spectrometer-733 (iN10MX).
Raman spectra were performed on a LabRAM HR800 Laser
Raman spectroscopy with 632.5 nm wavelength incident laser
light. Thermal gravimetric analyses (TGA) were carried out in air
on a Pyris Diamond TG analyzer (PerklinElemer Inc., U.S.A).
The heating temperature range was from 50 ꢁC to 800 ꢁC with a
heating rate of 5 ꢁC minꢂ1. UV-vis adsorption spectra were used
to investigate the optical properties of the photocatalysts,
recorded on a UV-2550 spectrophotometer.
Herein, we present the fabrication of CZS/rGO composites
with different weight ratios of rGO via an ultrasound assisted
chemical method. CZS with novel hierarchical nanostructure
was synthesized via a hydrothermal process, reported by our
group.28 And GO was obtained through a modied Hummer's
method.29 Subsequently, the as-prepared photocatalysts were
used to photodegrade methylene blue (MB) in the presence of
hydrogen peroxide (H2O2). Furthermore, all the composites we
prepared were characterized carefully, and the possible mech-
anism of photocatalytic reaction was put forward in this paper
as well. According to the nal results, all the graphene-based
photocatalysts are of better photocatalytic performance than
pure CZS composite.
Photocatalytic experiments
The photodegradation of MB was carried out to evaluate the
photocatalytic performance of the as-prepared photocatalysts,
Experimental
Graphite powder was purchased from Alfa Aesar. The other
chemicals were of analytical grade and purchased from Tianjin
Guangfu Fine Chemical Research Institute. All the reagents
were used as received without any further purication. The
water we used in the research was double distilled water.
Scheme 1 Schematic description for the formation procedure of
CZS/rGO.
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using a homemade reactor and a 300 W Xe lamp as the light CZS/rGO-4 are shown in Fig. 2. Both of the spectra have two
source. First of all, 0.1 g sample was added into 100 ml MB strong peaks, D and G bands, which correspond to the vibration
solution of 30 mg Lꢂ1 under stirring. Subsequently, the solution of carbon atoms in disorder or defect sites and in-phase vibra-
was sonicated for 5 min and continuously stirred for 15 min in tion of sp2-bonded carbon atoms respectively.32 For the spec-
the dark, which would be of benet to the complete dispersion trum of GO (curve a in Fig. 2), the D and G bands appear at 1331
of photocatalyst powder in the solution. Then, under magnetic cmꢂ1 and 1581 cmꢂ1, while for rGO in CZS/rGO-4 (curve b in
stirring, 2 ml H2O2 was injected into the above solution, and Fig. 2), the two bands shi to 1335 cmꢂ1 and 1583 cmꢂ1
.
meanwhile the reaction was triggered by the irradiation (250 nm Apparently, the intensity ratio of D and G bands (ID/IG) for CZS/
< l < 780 nm) of the Xe lamp placed 66 cm away from the rGO-4 has increased to 1.72 from 1.22 compared to GO, due to
reactor. Finally, 1 ml of solution was taken out at specic time the creation of new and smaller graphitic domains.33 For the
intervals, diluted to 6 ml with double distilled water, and spectrum of GO, the intensity of D band is higher than that of G
centrifuged to remove the residual sample. The concentration band, which can be attributed to partial reduction of GO during
of MB was analyzed by a UV-vis absorption spectrometer at 663 the freeze-drying process. The result of Raman spectra implies
nm. In order to compare the photocatalytic activity of the the effective reduction of GO to rGO during the reaction, which
photocatalysts we synthesized with CZS, the experimental has also veried the result of FTIR spectra.
condition was similar as reported by our group previously.28
As displayed in Fig. 3, XRD analysis was applied to investi-
gate the crystalline structures and structural changes of CZS,
GO, and CZS/rGO-4. It is obvious that the XRD pattern of CZS
(curve a in Fig. 3) exhibits several diffraction peaks at 2q ¼
27.216ꢁ, 27.731ꢁ, 29.364ꢁ, 32.863ꢁ, 48.017ꢁ, 52.712ꢁ and 59.339ꢁ,
corresponding to (100), (101), (102), (103), (006), (110), (108),
and (116) planes of the hexagonal phase of CuS (JCPDS no. 06-
0464) respectively. Besides, the diffraction peaks at 2q ¼
28.650ꢁ, 48.017ꢁ, 56.518ꢁ are corresponded to the (111), (220),
and (311) planes of the cubic phase of ZnS (JCPDS no. 05-0566).
The sharp and narrow peaks manifest the high crystalline
structure of the sample. The sharp peak at 2q ¼ 9.96ꢁ assigned
to the (001) reection of GO is observed in Fig. 3(b), implying
the well-ordered lamellar structure of GO.34 The XRD pattern of
CZS/rGO-4 shows similar diffraction peaks with that of CZS,
indicating that CZS is a signicant component of CZS/rGO-4,
and the addition of rGO does not impede the crystal growth
of CZS. The characteristic peak of GO at 2q ¼ 9.96ꢁ does not
exist in Fig. 3(c), which can be attributed to the effective
reduction of GO to rGO in CZS/rGO-4. In addition, the absence
of the typical stacking peak of graphite at about 26ꢁ indicates
not only that the CZS particles homogeneously distribute on the
rGO sheets, it would also illustrate that the rGO sheets do not
stack and conglomerate aer reduction. Finally, there are no
relevant characteristic peaks for other impurities observed in
Results and discussion
As shown in Fig. 1, the molecular structure of GO and the
reduction of GO to rGO were affirmed by FTIR spectra.
According to the Fig. 1(a), several characteristic bands of GO
associated with oxygen-containing functional groups can be
recognized apparently. The band at 3400 cmꢂ1 belongs to the
O–H stretching vibration. The C]O vibration of carboxyl is
observed at 1726 cmꢂ1. The H–O–H bending vibration of
absorbed H2O molecules and skeletal ring vibration of graphitic
domains can be seen at 1620 cmꢂ1. Additionally, the peak at
1221 cmꢂ1 and 1050 cmꢂ1 can be attributed to the epoxy C–O
stretching vibration and alkoxy C–O stretching vibration,
respectively.30,31 Nevertheless, the FTIR spectrum of CZS/rGO-4
exhibits no characteristic peaks of GO as shown in Fig. 1(a)
except the weak C–O vibration at 1221 cmꢂ1, indicating that the
oxygen-containing functional groups on GO had been success-
fully reduced by hydrazine. In addition, CZS/rGO-4 shows
similar peaks to the result in the spectrum of CZS in cruve c,
demonstrating that the graphene-based composite contained
CZS.
Raman spectroscopy has already been widely applied to
investigate carbon materials. The Raman spectra of GO and
Fig. 1 FTIR spectra of (a) GO; (b) CZS/rGO-4; (c) CZS.
Fig. 2 Raman spectra of (a) GO; (b) CZS/rGO-4.
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The TGA curves of GO, CZS and CZS/rGO-4 are shown in
Fig. 5. The TGA pattern of GO (curve a in Fig. 5) exhibits a two-
step weight loss. The rst weight loss takes place before 300 ꢁC,
due to the loss of the oxygen-containing groups in GO structure.
ꢁ
The second one starts approximately at 450 C, which can be
attributed to most of the oxygen-containing functional groups
being removed from the GO and completely decomposed aer
about 540 ꢁC.37 According to the TGA curves of CZS (curve b in
Fig. 5) and CZS/rGO-4 (curve c in Fig. 5), the weight loss and
weight gain of CZS/rGO-4 occur at a lower temperature than that
of CZS, and the weight loss of CZS/rGO-4 is more, because of the
oxidation of carbon.38 The weight loss (150–330 ꢁC) is also
partially attributed to the transformation from CuS to Cu1.8S
and Cu2S. In the next step, the weight increase corresponds to
the formation of CuO$CuSO4 and CuSO4. CZS/rGO-4 contains
less content of CZS because of the introduction of rGO, which
induces the less weight gain of CZS/rGO-4 than that of CZS.39
The optical properties of the as-prepared samples were
characterized by the UV-vis adsorption spectra with the scan-
ning range from 200 nm to 800 nm (shown in Fig. S1†). It is
obvious that all the spectra have a broad adsorption region of
580–700 nm and a relatively small adsorption region occurred
below 400 nm. And the graphene-based samples have a broader
adsorption region in the visible range. It demonstrates that the
photocatalysts possess both UV and visible light response,
which allows them to be excellent candidates in photocatalystic
eld making full use of solar energy.40
The photocatalytic activity of as-synthesized composites was
evaluated by photocatalytic degradation of MB solution under
light irradiation with the presence of H2O2. As shown in Fig. 6 A,
in the rst twenty minutes, it was mainly the adsorption process
of photocatalysts without light irradiation and addition of
H2O2, during which all the composites adsorbed as much MB
molecules as they could to reach the adsorption–desorption
equilibrium. Evidently, the larger amount of rGO one photo-
catalyst contained, the higher adsorption efficiency it
possessed. The adsorption rates were 17.59%, 29.75%, 41.20%,
53.73%, and 80.92% respectively for the photocatalysts with
different weight ratios of rGO ranging from 5% to 30%.
However, the adsorption rate for CZS was only 7.21%, indicating
Fig. 3 XRD patterns of (a) CZS; (b) GO; (c) CZS/rGO-4.
the XRD pattern of CZS/rGO-4, indicating the well synthesis of
the sample.35,36
The direct information of structure and morphology of the
as-synthesized products can be easily received through the SEM
images. As can be seen in Fig. 4(a), the liquid graphite oxide
(GO) without performing any exfoliation treatment is of
lamellar structure and possesses relatively large supercial
area. In contrast, Fig. 4(b) presents that the rGO sheets origi-
nating from the reduction of graphite oxide are of stacked
structure. It is obvious to deduce that GO sheets are of more
excellent dispersibility and larger supercial area in aqueous
solution than rGO sheets. CZS particles, with hierarchical
structure, are displayed in Fig. 4(c). They are ower-like
microspheres, constructed by nanoplates, and their average
diameter is about 3 mm. It can be seen from Fig. 4(d) that CZS
particles are decorated between rGO sheets homogeneously
with intimate interfacial contact. Neither the CZS particles nor
the rGO sheets seriously conglomerate during the reaction,
because the loading of CZS particles inhibits the rGO sheets
from stacking together, and the large surface area of rGO sheets
serves as a two-dimensional supporting matrix for CZS particles
to spread out uniformly. The analysis of SEM images is in
accordance with the results of XRD analysis, which has further
affirmed the successful synthesis of CZS/rGO-4.
Fig. 4 SEM images of (a) GO; (b) rGO; (c) CZS; (d) CZS/rGO-4.
36188 | RSC Adv., 2015, 5, 36185–36191
Fig. 5 TGA curves of (a) GO; (b) CZS; (c) CZS/rGO-4.
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that the introduction of rGO could dramatically increase the
adsorption capacity of the photocatalysts and further facilitate
the removal efficiency of pollutants. Aer being exposed to light
irradiation for 20 min, most of the MB was eliminated in the
presence of H2O2 and graphene-based photocatalysts. For
example, the removal rates of MB were 99.05% and 98.92% for
CZS/rGO-4 and CZS/rGO-5, which were higher than 93.10% for
pure CZS. Compared to CZS/rGO-4, CZS/rGO-5 exhibited less
stable photocatalytic efficiency, and had higher preparation
cost because of the higher content of rGO. Furthermore, it
would take 80 min, 70 min and 40 min respectively for CZS/rGO-
1, CZS/rGO-2 and CZS/rGO-3 to eliminate more than 99% of MB
in the presence of H2O2. While for CZS composite, the MB
removal efficiency could hardly reach 99% in 90 min. According
to the experimental results, all the graphene-based composites
showed more outstanding photocatalytic performance than
CZS. In addition, utilizing CZS/rGO-4 to remove MB could
greatly shorten the reaction time compared with the other
prepared photocatalysts. Thus it would be quite reasonable to
infer that CZS/rGO-4 is the most suitable photocatalyst for
practical application of all. Fig. 6B shows the temporal evolu-
tion of UV-vis adsorption spectra of MB solution during the
photocatalytic reaction with H2O2 and CZS/rGO-4 presence. As
can be seen, the characteristic peak of MB at 663 nm decreased
gradually. In the rst twenty minutes, the great decrease of the
peak was ascribed to the adsorption of CZS/rGO-4 for MB. Aer
exposure to light irradiation for 15 min, the adsorption peak
almost disappeared, and no new peaks were observed, which
demonstrated that MB was successfully eliminated by CZS/rGO-
4. The results of blank experiments were displayed in Fig. 6C.
According to the adsorption experiment of CZS/rGO-4 (curve a
in Fig. 6C) performing in the dark, CZS/rGO-4 reached the
adsorption–desorption equilibrium within 20 min. The result of
photocatalytic reaction with only CZS/rGO-4 presence under
light irradiation (curve b in Fig. 6C) showed higher MB removal
rate than that in Fig. 6C(a), indicating that the degradation of
MB was mainly attributed to the photocatalytic activity of CZS/
rGO-4. As shown in Fig. 6C(c) and(d), H2O2 could only
degrade about 14% of MB under light irradiation in 70 min, and
about 75% of MB was eliminated with the presence of CZS/rGO-
4 and H2O2 under the dark condition, mainly owning to the
adsorption capacity of rGO and the oxidative capacity of H2O2.
The stability test of CZS/rGO-4 applied to photodegrade MB
under illumination was carried out and illustrated in Fig. 6D.
The adsorption efficiency of CZS/rGO-4 only decreased a little
aer reuse every time, indicating that the MB molecules
adsorbed on the surface of photocatalyst were nally decom-
posed. In addition, there was no signicant decrease in pho-
todegradation efficiency aer reuse for three cycles, suggesting
that CZS/rGO-4 is a promising photocatalyst for recyclable
utilization.
Fig. 6 (A) Degradation rate of MB in the presence of CZS and H2O2,
CZS/rGO-1 and H2O2, CZS/rGO-2 and H2O2, CZS/rGO-3 and H2O2,
CZS/rGO-4 and H2O2, and CZS/rGO-5 and H2O2 respectively; (B) UV-
vis adsorption spectra of MB aqueous solutions with CZS/rGO-4 and
H2O2 presence at different time intervals; (C) removal rate of MB in the
presence of (a) CZS/rGO-4 without light irradiation; (b) CZS/rGO-4
with light irradiation; (c) H2O2 with light irradiation; (d) CZS/rGO-4 and
H2O2 without light irradiation; (D) repetitive use of CZS/rGO-4.
The reduced graphene oxide loaded CuS/ZnS has rarely been
reported. Compared to another rGO-ZnS/CuS composite repor-
ted by Zeng,36 the as-prepared CZS/rGO-4 composite exhibits
better crystalline structure, morphology and photocatalytic
performance.
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The possible mechanism of photocatalytic degradation of the mobility of photogenerated electrons and inhibits the
MB using CZS/rGO-4 as the photocatalyst is depicted as shown recombination of charge carriers.44–46
in Scheme 2. According to the IFCT (interfacial charge transfer)
mechanism,41 electrons in VB of ZnS can be excited by light
irradiation and transfer to CuS clusters, leading to the partial
Conclusions
reduction of CuS to Cu2S. Because of the intimate contact of CZS
particles and rGO sheets, as well as the excellent conductivity of
In summary, a series of CZS/rGO composites with different
weight ratios of rGO were successfully synthesized via a sono-
rGO, the excited electrons in the CB of CuS/Cu2S clusters are
chemical process, using CZS and GO as the initial reagents. CZS
most likely to transfer to rGO that acts as the electron collector
microspheres were homogeneously decorated on the surface of
and transporter. O2 is able to scavenge photoinduced electrons
rGO sheets, which effectively hindered the aggregation of rGO
sheets. In addition, all the graphene-based composites were of
improved photocatalytic efficiency in the process of photo-
catalytic degradation of MB in comparison to the bare CZS
to form O2ꢂ, and then reacts with H2O to generate hydroxyl
radicals. During the whole photocatalytic process, H2O2 can
nally become hydroxyl radicals, which play an extremely
important role in MB degradation. The dyes adsorbed on the
microspheres. CZS/rGO-4 was able to eliminate more than 99%
surface of photocatalyst can be rapidly decomposed to CO2, H2O
of MB within 20 min, utilizing both UV and visible light, and
and small organic compounds by hydroxyl radicals. In addition,
exhibited no signicant decrease in photodegradation effi-
ciency aer reuse in three cycles. The introduction of reduced
graphene oxide to the hybrid photocatalysts has a critical effect
on the photocatalytic activity enhancement, it would not only
the holes in the VB of ZnS, on one hand, can destroy MB
molecules directly; on the other hand, they prefer to react with
adsorbed water or hydroxyl ions to generate hydroxyl radicals.
All the phenomena aforementioned are favorable to suppress-
facilitate the transportation of excited electrons and suppress
ing the recombination of electron–hole pairs, and consequently
the recombination of charge carriers, the giant two-dimensional
rGO sheets would also be capable of increasing the adsorption
capacity for organic molecules signicantly. The results
demonstrate that CZS/rGO composites used as novel photo-
catalysts have enormous potential for photodegradation of
organic pollutants.
improve the photocatalytic activity of CZS/rGO-4. At last, the
removal of MB molecules adsorbed on the surface of photo-
catalyst breaks the adsorption–desorption equilibrium, and
more MB molecules in the solution would adsorb on the surface
of photocatalyst to construct a new equilibrium and nally be
decomposed.42,43
The most critical factors associated with rGO that cause the
great enhancement of MB degradation efficiency are concluded Acknowledgements
as follows. Firstly, due to the large specic surface area, rGO
This project was nancially supported by the National Natural
sheets can not only act as an ideal matrix for the distribution of
Science Foundation of China (nos 51472012) and supported by
CZS particles, they would also provide a larger place for the
the Fundamental Research Funds for the Central Universities.
photodegradation reaction to take place. Additionally, the
effective adsorption of MB molecules on the surface of rGO can
enormously shorten the reaction distance of hydroxyl radicals
and MB molecules, which consequently results in the
Notes and references
improvement of photocatalytic efficiency. The adsorption can
be ascribed to strong electrostatic force and p–p stacking
interaction between rGO and MB molecules. Last but not least,
the extraordinary conductivity of rGO signicantly facilitates
1 L. J. Zhang, T. F. Xie, D. J. Wang, S. Li, L. L. Wang, L. P. Chen
and Y. C. Lu, Int. J. Hydrogen Energy, 2013, 38, 11811–11817.
2 H. S. Kim, D. Kim, B. S. Kwak, G. B. Han, M. H. Um and
M. Kang, Chem. Eng. J., 2014, 243, 272–279.
3 L. Chen, X. Li, Y. Wang, C. Gao, H. Zhang, B. Zhao, F. Teng,
J. Zhou, Z. Zhang, X. Pan and E. Xie, J. Power Sources, 2014,
272, 886–894.
4 J. Zhang, Y. Hu, X. Jiang, S. Chen, S. Meng and X. Fu, J.
Hazard. Mater., 2014, 280, 713–722.
5 Z. Yao, L. Wang, Y. Zhang, Z. Yu and Z. Jiang, Int. J. Hydrogen
Energy, 2014, 39, 15380–15386.
6 J. S. Chen, F. Xin, S. Y. Qin and X. H. Yin, Chem. Eng. J., 2013,
230, 506–512.
7 H. X. Sang, X. T. Wang, C. C. Fan and F. Wang, Int. J.
Hydrogen Energy, 2012, 37, 1348–1355.
8 J. S. Hu, L. L. Ren, Y. G. Guo, H. P. Liang, A. M. Cao, L. J. Wan
and C. L. Bai, Angew. Chem., 2005, 117, 1295–1299.
9 Y. C. Weng, Y. D. Chou, C. J. Chang, C. C. Chan, K. Y. Chen
and Y. F. Su, Electrochim. Acta, 2014, 125, 354–361.
10 Q. Wang, G. Yun, Y. Bai, N. An, Y. Chen, R. Wang, Z. Lei and
W. S. Guan, Int. J. Hydrogen Energy, 2014, 39, 13421–13428.
Scheme 2 The mechanism of photocatalytic degradation of MB by
CZS/rGO.
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RSC Advances
11 Q. Wang, N. An, Y. Bai, H. Hang, J. Li, X. Lu, Y. Liu, F. Wang, 28 X. H. Guan, P. Qu, X. Guan and G. S. Wang, RSC Adv., 2014, 4,
Z. Li and Z. Lei, Int. J. Hydrogen Energy, 2013, 38, 10739–
15579–15585.
10745.
29 D. Chen, L. Li and L. Guo, Nanotechnology, 2011, 22, 325601.
12 U. T. D. Thuy, N. Q. Liem, C. M. A. Parlett, G. M. Lalev and 30 W. F. Chen, L. F. Yan and P. R. Bangal, J. Phys. Chem. C, 2010,
K. Wilson, Catal. Commun., 2014, 44, 62–67. 114, 19885–19890.
13 X. Z. Zhou, X. Huang, X. Y. Qi, S. X. Wu, C. Xue, F. Y. C. Boey, 31 C. Park, T. Ghosh, Z. Meng, U. Kefayat, N. Vikram and
Q. Y. Yan, P. Chen and H. Zhang, J. Phys. Chem. Lett., 2009,
113, 10842–10846.
14 S. Akcoltekin, M. E. I. Kharrazi, B. Kohler, A. Lorke and
M. Schleberger, Nanotechnology, 2009, 20, 155601.
15 A. Reina, X. T. Jia, J. Ho, D. Nezich, H. B. Son, V. Bulovic,
M. S. Dresselhaus and J. Kong, Nano Lett., 2009, 9, 30–35.
W. Oh, Chin. J. Catal., 2013, 34, 711–717.
32 Y. Sun, Q. Wu and G. Shi, Energy Environ. Sci., 2011, 4, 1113–
1132.
33 C. Chen, W. M. Cai, M. C. Long, B. X. Zhou, Y. H. Wu,
B. Y. Wu and Y. J. Feng, ACS Nano, 2010, 4, 6425–6432.
34 H. Sun, L. Cao and L. Lu, Nano Res., 2011, 4, 550–562.
16 V. Borovikov and A. Zangwill, Phys. Rev. B: Condens. Matter 35 L. Jia, D. H. Wang, Y. X. Huang, A. W. Xu and H. Q. Yu, J.
Mater. Phys., 2009, 80, 121406. Phys. Chem. C, 2011, 115, 11466–11473.
17 N. Liu, F. Luo, H. Wu, Y. Liu, C. Zhang and J. Chen, Adv. 36 B. Zeng, X. Chen, C. Chen, X. Ning and W. Deng, J. Alloys
Funct. Mater., 2008, 18, 1518–1525. Compd., 2014, 582, 774–779.
18 H. J. Shin, K. K. Kim, A. Benayad, S. M. Yoon, H. K. Park, 37 Z. Y. Lin, Y. G. Yao, Z. Li, Y. Liu, Z. Li and C. P. Wong, J. Phys.
I. S. Jung, M. H. Jin, H. K. Jeong, J. M. Kim, J. Y. Choi and
Y. H. Lee, Adv. Funct. Mater., 2009, 19, 1987–1992.
19 M. Zhou, Y. Wang, Y. Zhai, J. Zhai, W. Ren, F. Wang and
S. Dong, Chem.–Eur. J., 2009, 15, 6116–6120.
20 L. Liu, J. Liu and D. D. Sun, Catal. Sci. Technol., 2012, 2,
2525–2532.
Chem. C, 2010, 114, 14819–14825.
38 X. J. Zhang, G. S. Wang, Y. Z. Wei, L. Guo and M. S. Cao, J.
Mater. Chem. A, 2013, 1, 12115–12122.
39 H. T. Zhu, J. X. Wang and D. X. Wu, Inorg. Chem., 2009, 48,
7099–7104.
40 Y. Zhang, J. Tian, H. Li, L. Wang, X. Qin, A. M. Asiri, A. O. Al-
Youbi and X. Sun, Langmuir, 2012, 28, 12893–12900.
21 X. X. Wei, C. M. Chen, S. Q. Guo, F. Guo, X. M. Li, X. X. Wang,
H. T. Cui, L. F. Zhao and W. Li, J. Mater. Chem. A, 2014, 2, 41 J. G. Yu, J. Zhang and S. W. Liu, J. Phys. Chem. C, 2010, 114,
4667–4675. 13642–13649.
22 X. Zhao, F. Zhang, D. Lu, Y. Du, W. Ye and C. Wang, Anal. 42 X. Liu, L. Pan, T. Lv, G. Zhu, Z. Sun and C. Sun, Chem.
Methods, 2013, 5, 3992–3998.
Commun., 2011, 47, 11984–11986.
23 J. Bai and X. Jiang, Anal. Chem., 2013, 85, 8095–8101.
24 H. Zhang, X. J. Lv, Y. M. Li, Y. Wang and J. H. Li, ACS Nano,
2009, 4, 380–386.
25 J. S. Lee, K. H. You and C. B. Park, Adv. Mater., 2012, 24,
1084–1088.
43 J. Qian, K. Wang, Q. Guan, H. Li, H. Xu, Q. Liu, W. Liu and
B. Qiu, Appl. Surf. Sci., 2014, 288, 633–640.
44 Y. Wang, L. Zhang, H. Jiu, N. Li and Y. Sun, Appl. Surf. Sci.,
2014, 303, 54–60.
45 H. N. Tien, V. H. Luan, L. T. Hoa, N. T. Khoa, S. H. Hahn,
J. S. Chung, E. W. Shin and S. H. Hur, Chem. Eng. J., 2013,
229, 126–133.
26 Q. Xiang, J. Yu and M. Jaroniec, Chem. Soc. Rev., 2012, 41,
782–796.
27 X. An and J. C. Yu, RSC Adv., 2011, 1, 1426–1434.
46 Z. Gao, N. Liu, D. Wu, W. Tao, F. Xu and K. Jiang, Appl. Surf.
Sci., 2012, 258, 2473–2478.
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